GSM-CDMA Comparisons (Nokia Proprietary Information 1999 January 22) page 1 of 64 gsmcdma.doc Comparison of Major Properties of GSM/PCS-1900 with CDMA by Richard C. Levine, Sc. D., P. E. Principal Consulting Engineer Beta Scientific Laboratory, Inc. and Adjunct Professor of Electrical Engineering Southern Methodist University 1999 January 22 Notice: This document contains several trademarked product names, each of which is the property of the respective trademark holders.
Transcript
GSM-CDMA Comparisons (Nokia Proprietary Information 1999 January 22) page 1 of 64
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Comparison of Major Properties ofGSM/PCS-1900 with CDMA
by
Richard C. Levine, Sc. D., P. E.
Principal Consulting Engineer
Beta Scientific Laboratory, Inc.
and
Adjunct Professor of Electrical Engineering
Southern Methodist University
1999 January 22
Notice: This document contains several trademarked product names,
each of which is the property of the respective trademark holders.
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Table of Contents
Glossary of Terms and Symbols .........................................................................................4
MOS Mean Opinion Score (rating 1 to 5 for speech quality)
n Noise (signal) strength
NRZ Non-Return to Zero (bipolar waveform)
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PCS Personal Communication System(s)
PN-PRBS Pseudo Noise- Pseudo Random Bit Stream
RSSI Radio (or Received) Signal Strength Indication
S spectral-economic-geographic performance, given by conversations/(total kHz × $ equivalent cost × system area)
SIM Subscriber Identity Module
TASI Time Assignment Speech Interpolation
TDMA Time Division Multiple Access
Uplink Mobile transmit and base receive direction
WLL Wireless Local Loop, fixed wireless
3G Third generation cellular and PCS technology proposals
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1 Executive Summary:CDMA (IS-95) and the GSM technology for cellular/PCS access (such as PCS-1900 in NorthAmerica) are compared with regard to coverage and capacity, which both affect the cost persubscriber, and for applications in both cellular systems with mobility and in wireless local loop(WLL).
Contrary to the claims of its proponents, CDMA does not have a capacity advantage over fully
developed GSM or other TDMA technologies. Two methods are used in fully developed GSM
systems. First, GSM (utilizing frequency hopping) can exploit the capacity advantage afforded
by silent intervals in speech to increase the system capacity, just as CDMA does. But only
GSM technology can fully exploit the second method, overlay/underlay (tiered cells) for
providing extra channels in high traffic areas. Single frequency installations of IS-95 CDMA
cannot fully exploit this method. Both of these methods increase the capacity by amounts
which are quite significant (typically 20 to 70%) but which give different quantitative
improvements in different installations. Exploitation of silence increases the system capacity by
an amount which is dependent on individual user speech patterns of silence, but has no value
for continuous data or other continuous transmissions. Capacity increases from overlay
methods are site specific as well. The claimed capacity advantage of CDMA installations does
not appear when a fully optimized GSM installation makes proper use of the optional
capabilities of frequency hopping, discontinuous transmission (exploiting silence), and
optimally overlaid cell design. The highest system capacity can be achieved by means of
optimizing a GSM installation, using particularly site-specific adaptive or “intelligent” overlay
methods which are not available in an IS-95 CDMA system. These systems have from 10% to
35% more capacity than CDMA systems, and at lower cost.
Even in its most advantageous capacity configuration, equivalent operating cost per subscriberis at least 30% (and sometimes as much as 60%) higher for CDMA than GSM for voice or fordata. Part of the economic disadvantage of CDMA arises from greater system complexity. Partof this cost is due to the excessive size and cost of existing CDMA base station modules vis-à-vis optimal module sizes in smaller cells. Another part of the high CDMA cost, which cannot bealleviate by system redesign, is primarily due to excessive hardware, cell to switch digital links,and operational costs partially related to CDMA soft handover (handoff). In a adequatelyprovisioned system installation with proper RF cell coverage, seamless handover in TDMAsystems (such as the GSM family of technologies) produces no perceptible gap incommunication and no increased probability of dropping a call in progress, contrary to CDMAproponents’ claims. We conclude that, on balance, CDMA does not produce better systemperformance nor better price/performance ratio in these areas than does GSM.
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Among existing available technologies discussed here, GSM/PCS-1900 technology ispresently a superior economic choice for voice and data systems for mobile and fixed wirelessservice, giving a lower cost per subscriber and equal or better quality of service.
GSM has a superior speech CODEC at present, but neither of the two technologies is superiorto the other for data transmission accuracy or capacity. GSM is also ahead at present in datacommunications capabilities and as a platform for numerous competitive data processingapplications.
For WLL, which does not require handover, CDMA has the theoretical possibility to achieveequal or better overall performance, but this theoretical possibility is not realized with existingIS-95 equipment. Costs are still disproportionately high compared to competitive technologies.Both CDMA and GSM-based WLL technologies are more expensive than traditional wired(non-radio) technologies, but GSM/PCS-1900 can be more useful as a transition businessstrategy when rapid installation and shared mobile/WLL use is a significant businessconsideration. When planning fixed local loop equivalent service with lowest long-term cost,WLL can be used temporarily for rapid product roll out, and then followed up with a lower costtechnology (such as traditional wire subscriber loops) for the long run.
In the past, the high cost of marketing and promotion disguised the fundamental costdifferences between the base system technologies now operating in the North American PCSmarket. Recent pricing changes in the cellular/PCS industry are pointing to a “commodity”market where price is the most significant selling feature. In that predicted marketingenvironment, low cost of technology will become the dominant economic factor in theprofitability of cellular or PCS operation.
2 Background:Many extreme claims have been made in the ongoing battle of words regarding CDMA versusother cellular/PCS access technologies such as IS-136 TDMA and GSM/PCS-1900. Many ofthe most extreme early claims were based primarily on theoretical considerations orsimulations. These were made in the early 1990s before all the competitive technologies wereinstalled and working. In many cases the proponents of one access technology reachedconclusions which were diametrically opposite to their opponents – clearly a situation in whichboth sides could not be correct! These claims generally come from persons having a vestedinterest in only one side of the dispute between the different technologies1. The contradictoryclaims require independent analysis by a disinterested expert, and a view of recentexperimental results in the field. This brief document summarizes some of the importantdistinctions between GSM (and its North American embodiment, PCS-1900), on the one hand,and CDMA (IS-95) on the other hand2.
Although proponents (particularly CDMA proponents) of both technologies have made very few"official" statements regarding system capacity, knowledge of unpublished actual system
1 The present author has no financial interest in any vendor or operator who uses one of these technologiesexclusively, and has done consulting for vendors and operators who use all the relevant technologies mentioned:both GSM/PCS-1900 and IS-136 TDMA, and also “cdmaOne” IS-95 CDMA. He has no financial interest in theoutcome of competition between these technologies.2 We will refer to the "GSM technology" when a particular statement is equally applicable to 900 MHz GSM (usedprimarily in Europe, Asia, and Australia), PCS-1900 (used in North America) and DCS-1800 (used in the UK andsome parts of Europe). When a comment applies only to one of these, the specific name will be used.
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capacities have become widespread in the industry, and these unpublished capacities arequoted here without attribution.
3 Brief Description of GSM TDMA and IS-95 CDMA Technology:
3.1 GSM:
Time Division Multiple Access (TDMA3), is used in GSM technology to multiplex 8 full ratedigitally coded conversations (or channels) on the same modulated carrier frequency. Theradio bandwidth of a GSM technology signal is 200 kHz. Up to 16 channels can be multiplexedon a carrier using half rate speech coding as well, but this is not widely used today due tolower speech quality of the presently available half rate speech coder. Further discussions inthis report will consider only GSM family full rate or enhanced full rate speech coders.
Each mobile transmitter in the cell produces a synchronized repeating sequence of radiotransmission bursts. The duration of each burst is called a time slot (546 microseconds forGSM technology) and the time interval of 8 time slots (a total of 4.615 milliseconds), therepetition pattern time of the GSM technology, is called a time frame. Each mobile transmittertransmits a burst during only one assigned time slot of the 8 time slots.
The GSM base receiver receives continuously, and separates the portions of the receiveddigital bit stream occurring in each time slot so that portion is used for the proper conversation.The mobile receiver can receive during 7 of the 8 time slots, although only one of these carriesthe downlink signal for that particular mobile receiver. The mobile receiver is inactivated duringthe one time slot when the mobile set transmits. The two corresponding time slots (transmitand receive) used at the mobile station for one conversation have the same number label inthe documentation but are physically different time slots, because the number labeling of thetransmit and receive time slots is appropriately time offset. Therefore the GSM mobile set doesnot transmit and receive simultaneously, which removes the need for a frequency duplex filterin the GSM technology mobile set.
The mobile receiver processes the digital information received during its own assignedconversation time slot. It also scans other assigned radio carrier frequencies during theremaining 6 receive time slots for the purpose of measuring the signal strength and digital biterror rate of other nearby base station transmitters. The mobile receiver has the capability ofrapidly and accurately re-tuning to a different carrier frequency during each time slot (so called“frequency agility”). This capability is important for changing carrier frequencies for handover,scanning other carrier frequencies, and performing optional frequency hopping. Frequencyhopping is done by changing the carrier frequency used for each time slot of transmission andreception, during a single conversation, according to a pre-arranged schedule.
The use of mandatory error protection codes and the optional use of frequency hopping allowsGSM technology to operate at a relatively low signal level compared to the ambientinterference level (that is, at a low C/(I+n) ratio), without needing excessive channel bandwidth.The error protection process also utilizes bit interleaving, which is a process of re-arranging thetime order of the digital bits before transmitting over the radio, and then re-arranging thereceived bits in the receiver to put them back in proper consecutive order. The actual processspreads 114 consecutive data bits over 8 different radio bursts (a total time interval of about 36
3 TDMA technology is the basis of both IS-136 and GSM/PCS-1900 standards. Confusingly, the term TDMA issometimes used as a synonym for only IS-136. In this report, TDMA refers to both IS-136 and GSM/PCS-1900.
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milliseconds) and then re-arranges them in their original consecutive order. This does causesome increase in time delay, but it minimizes the chance of consecutive bit errors in the finalre-arranged bit stream and enhances the performance of other error correction coding, eventhough a typical fading pattern causes consecutive bit errors on the radio channel.
The use of optional frequency hopping improves performance (that is, it reduces the BER at agiven level of radio interference) in two ways: 1) reduction in fast fading and 2) reducedaverage co-channel interference when used in conjunction with discontinuous transmit. Radiochannel fading occurs at locations where certain multiple radio wave rays, which have beendelayed via different geometrical paths through the air, arrive at the same receiving antennalocation. Consider the special case of only two rays of equal power level, but with one raygoing directly by a straight line path from the transmitter and the other ray delayed more due toa longer zig-zag geometric path involving reflections. At a location where the carrier frequencyoscillations of the two rays are exactly out of phase (180 degree phase difference), one ray ispushing the electrons in the antenna up, while the other ray is pushing them down with anequal but opposite force. As a result, there is a complete radio fade at this location.Meanwhile, at another location about a quarter wavelength away (the precise distancedepends on the geometric angle between the two radio rays) the two rays are precisely inphase. This results in a double force on the electrons in the antenna from the combination ofthe in-phase electric field from the two rays. Thus the signal strength in a region subject tofading has many nearby locations having short term received signal strength which arestronger or weaker than the average received signal strength. As the receiving antenna movesabout in such a multi path radio field, the signal strength is continually changing (experiencingfast fading).
Because of the double strength electric field at an “in phase” location, the current is alsodouble, and the received power level is thus 4 times as high (6 dB greater) at this locationcompared to the power delivered by a single ray with no multi path conditions. Thus, in thisspecial case, as we move around from place to place, the power at various locations variesfrom zero (a negative infinity dB fade, since the logarithm of zero is negative infinity) to amaximum of 4 times (6 dB over) the average power level.
In a real multi path situation where more than two rays are present with different amplitudes,we seldom get exactly a +6dB local maximum, nor do we get such a deep fade because thetotal cancellation of multiple rays seldom occurs. Instead we get deep fades which are typically10 to 20 dB below average received power. Furthermore, with multiple rays arriving fromdifferent directions, the locations of the fades are not precisely a half wavelength apart, butare randomly distributed with some instances of fades separated by a half wavelength butsome others with larger separations as well. Fading affects both GSM and CDMA.
Both GSM and other technologies are subject to fast fading, and these different systemdesigns deal with the problem of fading in various ways. All system designs use errorprotection codes along with bit interleaving to maximize the effectiveness of the errorprotection codes, which work best with isolated bit errors rather than bursts of consecutive biterrors. The inherent bandwidths of different technology signal waveforms has somerelationship to the sensitivity to fast fading, with wider bandwidth signals somewhat lessaffected than narrower bandwidth signals in some case. We will discuss this topic more below.
3.2 CDMA (IS-95):
The modulated carrier frequency in CDMA utilizes approximately a 1.28 MHz bandwidth. Thetechnique used to multiplex several conversations onto this single carrier by CDMA is called
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direct spread spectrum (DSS) coding. It is implemented by combining the low bit rate (typically9.6 kb/s in the current commercial “cdmaOne” system) digital bit stream from each speechcoder with a distinct 1.28 Mbit/s pseudo-random high bit rate bit stream (usually called apseudo-noise pseudo-random bit stream – PN-PRBS) for each conversation.
The term "pseudo-random" means that the bit pattern appears to be random, like so-called"white" noise, but in fact it is generated by a deterministic digital structure (such as a re-circulating feedback shift register) which repeats the bit pattern after a relatively long interval oftime. For purposes of illustration, we will show the two bit streams in the form of non-return-to-zero (NRZ) bipolar electrical waveforms (having voltages which alternate between +1 and -1volts) and which are then multiplied with each other. (CdmaOne uses unipolar binary signals.)A block diagram of a structure designed for this purpose is shown in Figure 1.
In a base transmitter, many different high-bit-rate PN-PRBS bit streams are generated in asimilar way, by multiplying the output bit stream of each digital speech coder from eachconversation channel with a distinct PN-PRBS waveform. Then each of these bit streams isused to phase modulate the same carrier frequency, and these phase modulated radio signalsare combined. There is also a special PN-PRBS channel which carries setup and callprocessing messages, and which has its own particular PN-PRBS encoding bit stream, knownto all the mobile receivers in advance.
Figure 2 illustrates the mechanism of the receiver. To extract one particular channel from theoverall received radio signal, a properly synchronized duplicate of that one channel’s PN-PRBS bit stream is multiplied with the bipolar waveform from the detector stage of a wide bandradio receiver, and ideally the result is a replica of the original data bit stream.
Figure 3 illustrates the relevant waveforms. For purposes of illustration, we use bit rates of 10kb/s for the data and 100 kb/s for the PN-PRBS. It is difficult to illustrate the actual 1.28 Mb/sPN-PRBS bit rates since there would be too many PN-PRBS bits per data bit to distinguishclearly in a drawing. When these two waveforms, labeled a and b in Figure 3, are multipliedtogether, the result is a third bipolar voltage waveform c. Waveform c has the same bit rate aswaveform b, and has a comparable bandwidth (about 100 kHz in the figures or 1.28 MHz incdmaOne).
N R Z D a t aS t r e a m
e.g . 10 kb/sc o d e d s p e e c h
O n ePart icular P N - P R B S Other input channels
are added at basesystem. Only one
channel used in MS.
to RFtransmitter
(us ing phasemodulat ion)
a c
b
Figure 1: Simplified CDMA Coder
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In the receiver block diagram, Figure 2, a second channel or a third channel (not shown) maybe extracted from the detected radio waveform by multiplying that detector waveform with asecond or third properly synchronized PN-PRBS waveform as well.
In Figure 3, note that 10 bits of the PN-PRBS waveform b occur during each one bit of thedata waveform, a. (In the actual CDMA system the ratio is approximately 128.). The bit rate ofthe PN-PRBS stream is sometimes called the CHIP rate, and the PN-PRBS waveform issometimes called the CHIP or CHIPPING waveform, since it cuts up or "chips away at" thedata waveform by reversing the polarity of small "chips" of the waveform.
During those intervals where the data waveform voltage is +1, the composite signal c isidentical to the PN-PRBS waveform. During the interval where the data waveform voltage is -1,the composite waveform c is an inverted replica of the PN-PRBS waveform.4
To understand the decoding action at the receiver, consider the case where the transmittergenerates only one composite waveform. In that simplified example, the transmitted waveformis just waveform c, with no other channels involved. In that case, multiplying waveform c with aproperly synchronized copy of waveform b (the PN-PRBS waveform) to produce waveform d,which is, under ideal conditions with no other PN-PRBS codes or noise or interferencepresent, identical to the original waveform a.
4 4 In the actual structure of a CDMA system, the mathematically equivalent result of combining the data bit streamand the PN-PRBS bit stream is not done by multiplying bipolar waveforms, but is achieved by performing theexclusive OR logical function with two uni-polar bit streams.
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In this example there are 10 PN-PRBS bits for each data bit. This implies that we couldtheoretically multiplex up to 10 different 10 kb/s data bit streams by using 10 different PN-PRBS codes (although some of these codes – called "weak codes" – are not useful for variouspractical reasons). For optimal performance, the 10 PN-PRBS codes used should beorthogonal to each other. Two codes are mutually orthogonal to each other if their productwaveform (computed over the 10 bit interval which comprises one data bit value) has an equalnumber of +1 and -1 pulses in it. The result of averaging the voltage of such a productwaveform over one data bit time interval is an average voltage of zero volts. One example ofa set of orthogonal binary codes are the Walsh codes. Table 1 shows eight binary Walshcodes, each having a 8-bit duration:
Table 1 Walsh Code Example
Walsh code sequence number Walsh code binary value
0 11111111
1 10101010
2 11001100
3 10011001
4 11110000
5 10100101
6 11000011
7 10010110
The reader can verify that each one of the eight Walsh codes in Table 1 is orthogonal to all theother codes. For example, the product of code 0 with any other code is merely a replica of theother code (for example, the product of code 0 with code 3 is just another copy of code 3,namely binary 10011001). Since each of these product results contains four binary 1 and fourbinary 0 bits, (1 and 0 binary represent +1 and -1 volts respectively), the overall time averagevoltage of the 8-bit product waveform is zero. The same is true for the product of any other twocodes. Another example: the product of code 3 with code 7 is 00001111, which also has anaverage value of zero volts in NRZ waveform representation.
CDMA exploits the use of orthogonal (or nearly orthogonal) PN-PRBS code sequences topermit multiplexing and separation of the various different data bit streams which all aretransmitted on the same radio frequency carrier. Consider the case of two different orthogonalPN-PRBS codes, each one used with a different data bit stream. When two signals are addedin our simple NRZ waveform representation, the instantaneous voltage before modulation, orat the output of the demodulator, can be at any of the voltage values +2, +1, 0, -1 or -2 volts.Consider the case of a data bit value of +1 volt. When we look at the output waveform d fromthe matching channel decoder in Figure 2 during this one data bit time interval, we see adecoded waveform which jumps back and forth between +2 and 0 volts, with an average valueof +1 volt over the data bit interval. A simple time averaging operation (such as a suitable low-pass filter) removes these variations and produces only the average value of +1 volt.
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a NRZ (non-return tozero) datastream
b PRBSNRZstream
c Productwaveform
time
time
time
exampleshows only10 PRBS bitsper data bit
Notice the inversion of the NRZ polaritywhile the data bit is zero.
1 1 0 1Waveform d also is a replica of a after error free decoding
{
1100011110,1111001000,0011001111,...
Figure 3: CDMA Waveforms
The particular system design of the IS-95 standard uses 64 Walsh codes, each code having a64 bit duration (combined with secondary codes) as the basis of the downlink direction PN-PRBS code bit sequences. Two of these 64 Walsh codes are reserved respectively fortransmitting a clock signal from the base to the mobiles, and for a setup channel, shared by allthe mobile stations, for the exchange of call setup messages prior to the conversation phaseof a connection. As a part of the design strategy to support soft handover (described below),the uplink PN-PRBS codes are approximately orthogonal. Distinct uplink codes are generatedby an internal 42 bit feedback shift register in each mobile station. There are 242 (=4.398trillion) different unique mobile station identity codes which may be used to initialize this shiftregister, so there is no problem providing a unique identifier and a distinct uplink PN-PRBScode sequence for each mobile station ever to be manufactured. Downlink codes alsoincorporate the distinct output of such a specific shift register as well.
When the PN-PRBS codes for two channels are not orthogonal over a data bit time interval,the product waveform does not "average out" to produce exactly a +1 or -1 volt data bit value.The average voltage may have some positive or negative error. If the deviation of the averagevoltage from the desired +1 or –1 volt level is big enough, the result may be a data bit error atthe decoder.
Bit errors can also occur when the PN-PRBS codes are perfectly orthogonal, but the signalfrom an undesired channel is much stronger than the desired channel. Consider the case oftwo different radio transmitters, one powerful and the other weak. In our NRZ waveformrepresentation, one channel's NRZ waveform alternates between +1 and -1 volt at thedetector, while the other channel's NRZ waveform alternates between +0.01 and -0.01 volts. Inthis case, the stronger signal dominates the detection process and it is difficult or impossible to
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detect the correct binary value of the weak signal. This can occur in a base receiver when astrong signal arrives from a powerful nearby mobile transmitter and a weak signal arrives froma more distant, and/or lower power, mobile transmitter. In general, regardless of the cause ofthe disparity of the received power level from the two channels, this is known historically as the"near-far" problem in CDMA system design. This "near-far" problem is addressed in IS-95 byfeedback power control of the mobile transmitters. The objectives of this feedback control areto compensate for both differences is distance, and also for short term changes in signalstrength due to fast fading. The former objective is also achieved in GSM and other systemsby a similar method of feedback control, and it works with adequate accuracy in alltechnologies. The latter objective is only partially achieved in CDMA, and is still a problem areafor that technology.
All cellular systems make use of some type of power control of the mobile transmitter viamessages or commands transmitted from the base station. These messages are based uponthe measurement of the received signal strength indication – RSSI – at the base receiver. Inthe IS-95 design, one of the patented features of the system design is a very frequent(approximately 10 commands per second) and very small step (1 dB adjustments) feedbackpower control signal. CDMA is known to be very sensitive to slight differences in the RSSI ofdifferent code channels. A difference of as little as 2 dB between two received code channelscauses apprecciable degradation of the BER of the weaker channel. Use of these frequentpower adjustments produces a measurable improvement in the constancy of received powerlevel. However, the performance (BER) of the CDMA digital channel is extremely sensitive tothe precision of this power control, and the inability of existing CDMA systems to achieve theirtheoretical capacity may be partly attributed to imperfections in this feedback power control, inthe view of several independent analysts.
Even with perfect power control, and absolutely orthogonal PN-PRBS codes, CDMA bit errorsstill occur. The most significant cause occurs when the total number of received signals is suchthat the sum of all the signals and the random noise (from thermal noise and other sources,not previously discussed in this section of the report) is just too large compared to one desiredsignal. The combination of all these un-correlated CDMA and noise signals approaches arandom (Gaussian probability) amplitude distribution. The result is that the average of the totalvoltage waveform from all these many signal sources, over the data bit interval, is too close tozero to accurately classify the average voltage as either +1 or -1 volt. This problem is moresignificant in a system with a higher PN-PRBS bit rate and a higher number of codes.
To summarize the theoretical operation of a CDMA system: Each data bit stream (such asdigitally coded speech) is encoded by combining it with a distinct PN-PRBS code. The PN-PRBS codes for each such channel are approximately orthogonal, and thus they can beseparated from each other at the receiver by the multiplication and time averaging process(called cross correlation) which extracts the desired data bits and averages out the erroneousfluctuations caused by other undesired signals. However, in many cases these fluctuations areso large that they produce intolerable bit errors.
4 Historical Notes Regarding CDMA:The early promises regarding extreme CDMA capacity and performance were made in 1989-1990, when limited traffic capacity of 800 MHz analog cellular was perceived as the mainproblem of the day. Initial very approximate analysis by CDMA proponents suggested thatCDMA could offer capacity of up to 40 times that of 7-cell 30 kHz analog cellular. At that time,the TR-45 standards committee had just completed two years of divisive debate on the relative
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merits of TDMA versus frequency division multiple access, finally agreeing on the developmentof IS-54 (later IS-136) TDMA digital cellular. They were not initially receptive to opening officialdiscussion of yet another alternative. However, the arguments of Qualcomm proved to bepersuasive to several of the largest cellular operators who were suffering the most from 800MHz band capacity problems at that time. These influential cellular operators persuaded theTR-45 committee to set up a TR-45.5 working group to ratify the Qualcomm CDMA design asthe IS-95 standard. The IS-95 (cdmaOne) standard was later revised to cover 1.9 GHz bandequipment and dual band equipment, in addition to the original 800 MHz band design.
Like many completely new systems, CDMA took much longer to complete than originallypromised. Working systems (as opposed to test systems) did not start operating until the lastmonth of 1997, and then only on the 1.9 GHz band. While many scattered 800 MHzinstallations are in place, and while some are described by their respective system operatorsas commercial operations, many of these 800 MHz installations are still described in 1999 bytheir system operators as test systems. Although handover from analog cellular mode toCDMA mode is permitted in a dual mode 800 MHz band IS-95 handset, the reverse is notsupported.5 Once a mobile call is in CDMA mode in an 800 MHz dual-mode analog-CDMAsystem, all the cells in the system must have CDMA base equipment or the call will disconnectwhen the subscriber drives into a cell without CDMA base equipment. Most 800 MHz operatorshave stopped far short of a full 800 MHz CDMA system installation in every cell. They describethis incomplete installation as a test installation, and often restrict the use to a limited numberof specially recruited 800 MHz subscribers, who must agree to conditions of use which includeacknowledging that CDMA service is not available in all cells, and that unexpecteddisconnection may occur in some locations. Almost all of the commercially functioning CDMAinstallations in North America are on the 1.9 GHz band.
CDMA has been dogged by controversy. Several competitive manufacturers of other cellularand PCS technologies, particularly Ericsson Radio Systems, have engaged in a ratheracrimonious debate with Qualcomm about the relative merits of their technologies, and someof this debate has gone past the boundary of politeness and carefully considered scientificdialog. CDMA proponents have not been able to demonstrate system capacity as large asoriginally promised, and the continuing problems with 800 MHz installations have been asource of embarrassment (see further technical comments below). Many industry observerssay that the 1.9 GHz band has given CDMA a new lease on life, and without it there might stillnot be any commercially operating CDMA systems in North America.
4.1 Significance (if any) of “Third Generation” (3G)
Many cellular and PCS researchers are striving to design a so-called third-generation (3G)cellular and PCS system. However, the crystal ball which some industry pundits gaze into tosee the future of 3G is very murky, indeed. At the present time, 3G systems are the subject ofdiscussion and “paper tiger” designs, which will presumably give everyone full mobility andaccess to very high bit rates for voice, data, video, Internet access, and a variety of otherplanned services. One of the implied requirements of 3G systems is the agreement of theindustry on one technology, rather than today’s hodge podge of different incompatibletechnologies. Unfortunately, the telecommunications industry is not close to any agreement on
5 Several technological methods to upgrade the design of 800 MHz cdmaOne, so that CDMA-to-analog handoff canbe supported, have been suggested and some have been field tested. This capability may become available in the“Rate 2” redesign noted later in this report.
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a single radio interface technology for 3G. Not all 3G proposals involve CDMA, as some CDMAproponents claim. Several 3G proposals use TDMA. Even some opponents of existing CDMA,such as Ericsson and others, have proposed wide-band CDMA (or W-CDMA, using a 5 MHzor higher bit rate for the PN-PRBS bit stream) as part of a 3G system. An important pointregarding even the CDMA 3G proposals is that, except for the CDMA-2000 proposal ofQualcomm, none of the other CDMA-based 3G system proposals are backward compatiblewith present IS-95 equipment or technology. Qualcomm has just recently (November 1998)complicated the situation even further by claiming that these incompatible CDMA plans are allin violation of Qualcomm’s patents, and stated their intent to block development of allproposed 3G technologies using CDMA in any form, other than their own proposal. Ericssonhas countered with their own claims regarding other patents covering the same types oftechnology. At the time of publication of this report, the ITU-T standards organization hasthreatened to exclude all CDMA technologies from further consideration for 3G standards, ifthe various combatants cannot agree to reasonable and non-discriminatory licensing of anyapplicable patents, which is the normal practice for patents needed to implement standards.
In any case, it is far from certain what technology will actually exist in 3G, or whether aradically different 3G technology is really needed rather than a reduction in the retail price andfurther already-planned development of some existing technology such as GSM. Severalstakeholders in the cellular and PCS industry feel that the present assortment of competingtechnologies is still undergoing continual changes and improvements, and has not beensufficiently field tested and optimized to fully evaluate their relative suitability as the basis of a3G technology. Furthermore, the emphasis in several published 3G plans on extremely high bitrate digital data streams, video, and other exotic capabilities may not have a payoff in themarketplace. The present author tends to view most existing 3G activities as premature basedon this same evaluation.
4.2 Critique of CDMA and IS-95:
Qualcomm must be awarded high marks for their organizational ability to pull together adevelopment team and develop all the elements of a working system during 1989-1997 underthe difficult circumstances of the current PCS industry. At the same time, a neutral observercan make many justified criticisms of CDMA in general, and of the IS-95 design in particular,which call into question its ultimate competitive future. These appear below.
A number of problems have emerged with CDMA. Some of these are peculiar to the design ofthe IS-95 system, and theoretically could be eliminated by a system re-design6. Otherproblems are more fundamental, and will exist in any CDMA system, regardless of designdetails. Furthermore, some very productive methods for increasing cellular/PCS systemcapacity, such as overlay/underlay, are not feasible with CDMA. In many cases, thedisappointment expressed by some people about CDMA is not based on ranking itsperformance on an absolute scale, but rather on comparison of the actual performance(particularly the capacity) compared to early unfulfilled promises. In other cases, the concernscenter on economics rather than technological performance.
6 Qualcomm is engaged in at least a partial re-design of the IS-95 system, known as Rate 2 CDMA. The newdesign is intended to be implemented in dual-mode CDMA base equipment so that both existing and new Rate 2handsets will continue to operate on all base stations. While a few of the objectives and system changes havebeen made public, much of the redesign is not yet known to the public, and it is not clear to what extent Rate 2 willresolve the issues raised here.
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5 Unsubstantiated GSM Claims:As noted earlier, both sides in the GSM-CDMA dispute have made unsubstantiated claims.Rather than performing a systematic technological analysis of the total operation of CDMA andGSM, several specific claims will be addressed in this report. We will give conclusions statedwith a minimal summary of the background reasoning. First we admit that there is one GSMclaim which has not been fully substantiated.
The most significant unsubstantiated GSM claims relate to statements that GSM, usingoptional frequency hopping, is a “spread spectrum” technology, and out-does CDMA becauseof a wider range of radio frequencies it uses. In other words, these GSM proponents (primarilyEricsson) claim that the 15 MHz range of frequency hopping used in most 1900 MHz bandPCS-1900 installations always produces better signal quality and BER performance than IS-95CDMA. In practice, a GSM system using frequency hopping has superior BER performance,compared to a GSM installation with no frequency hopping. This is particularly important whenmany of the subscribers are stationary (as in a WLL installation) or are slow movingpedestrians. Frequency hopping combined with discontinuous transmit (described below) isalso beneficial for reducing the average level of mutual co-carrier cell-to-cell interference andincreasing the system capacity. The improved BER level arising from use of GSM frequencyhopping facilitates the installation of n=4 frequency plan GSM systems, and even “tighter”GSM frequency plans, which have a higher capacity than n=7 or other larger cell clusterfrequency plans.
Installation-optional frequency hopping (combined with mandatory bit interleaving) in GSMprimarily helps to reduce the BER due to a fade at the current location of a stationary mobileantenna at a particular frequency. When frequency hopping is operative, most of the TDMAradio bursts occur at frequencies which are not affected by a particular frequency fade at thatantenna location. The bandwidth over which a fade affects the radio signal (the so-calledcoherence bandwidth, explained in more detail in the appendix) is dependent upon the timedelay spread between the earliest and latest arriving multi-path rays. Because delay spread inurban areas can be as large as 20 microseconds, the coherence bandwidth can be as small as0.05 MHz (50 kHz). However, when the delay spread is less than a microsecond, which isoften the case in clutter-free rural areas with few reflecting surfaces, the coherence bandwidthis much greater than a megahertz. Furthermore, there may be more than one frequency atwhich a fade occurs in a particular location. This is a property of the local geography ofreflecting surfaces (from buildings, cliffs, etc.). A region with many reflecting surfaces mayhave fades at many different radio frequencies. Therefore, we cannot state with certainty thatany signal which has greater bandwidth (whether it is a CDMA or a GSM signal) will always bebetter quality (lower BER, for example) than another signal of narrower bandwidth.
In a region with a single fade frequency having a coherence bandwidth of 1 MHz, a GSMsignal using 15 MHz of frequency hopping range will be superior to a 1 MHz bandwidth CDMAsignal. However, in a region with several different fading frequencies, each one having a 50kHz coherence bandwidth, a CDMA signal may be better than the previously described GSMfrequency hopping signal. When extremely precise estimates are desired, the exact signalquality will depend on the particular frequencies of the fades vis-à-vis the frequencies used inthat cell for GSM frequency hopping. In short, the result is dependent on local cell multi pathcharacteristics and one cannot make a sweeping generalization. There is no quantitativeexperimental basis for claiming that a GSM system with a wider frequency hopping range willalways have proportionately superior performance (for example, 15 times as good because of15 MHz vs. 1 MHz frequency range) compared to a CDMA system.
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Analogies are sometimes made in the literature to the Discrete Multi-Tone (DMT) method ofdata transmission which was adopted, due to its superior performance, as the standard forAsymmetrical Digital Subscriber Loop (ADSL) high bit-rate digital transmission over coppertelephone wires. DMT is indeed partly analogous to frequency hopping, but DMT transmitsdigital information on several different carrier frequencies at the same time, instead of in asequential frequency hopping pattern. DMT systems continually evaluate the noise,interference, and multipath fading (which in wire transmission occurs due to unterminated wirestubs which are left attached to some telephone wires due to previous connections which areno longer in service). Carrier frequencies which have inferior performance are assigned alower modulation bit rate or are, in the extreme case, completely shut off.
Certain frequency hopping GSM installations (such as Nokia’s IFH, described below) operateaccording to similar principles. Idle carrier frequency noise is measured before assigning eachnew or handed-in conversation in each cell, so that bad frequencies are not used whenfeasible. When long term inferior performance is identified on certain frequencies (perhaps dueto external radio interference sources which the system operator cannot control), amodification is effected in the frequencies assigned to that cell. Only frequencies which do notfall in the interfered parts of the spectrum are used. When all of this is done, and the degradedfrequencies are predictable to a sufficient degree, frequency hopping GSM technology canindeed demonstrate superior performance to direct spread spectrum CDMA. However, whenall the degradation is effectively random for each mobile user, and has a coherence bandwidthwhich is very wide, and there is no dynamic adjustment of the system to respond to changingconditions, then no general claim of superiority can be substantiated merely because ofdifferent overall operative bandwidth. There is only an improvement when the system iscontinually optimized in response to actual operating conditions, as in Nokia IFH.
5.1 Supportable GSM Claims:
When GSM was still in the pre-production stage, a number of its planned features werechallenged by opponents on various grounds, but all of these GSM system capabilities haveproven to be workable and useful. The claims made for superiority of GSM with regard to easeof system design and installation, flexibility and availability of alternative bearer capabilities(half rate speech coder, fax and data, use of higher data bit rates, etc.) have been borne outby experience. The frequency planning and system engineering of a GSM system is much lessdependent on site-specific or subscriber-specific parameters than CDMA. A relatively firmestimate of best and worst case GSM system characteristics can be made with confidence. Incontrast, estimates of system capacity for CDMA are well known to be extremely sensitive tounique local cell-site characteristics. Two of these characteristics are multi-path radio wavepropagation (related to the presence and location of reflective surfaces from buildings, cliffsand other objects) and also the average ratio of sound to silence for the users of the voicechannels. (In contrast, local multi path delay spread does not affect the capacity estimates forGSM significantly.) The difference between best and worst case CDMA capacity estimates isvery large (factors of 3 to 1 or more occur) and are not easily predicted without extensive andcareful field testing. Changes in the base station transmitter power in one CDMA cell increasesthe interference level of all its neighboring cells which use the same carrier frequency. Thisusually requires careful re-balancing of the base station power throughout all the cells of theCDMA system. In contrast, the radio coverage of each GSM cell can be designed andoptimized independent of its neighbors. The bottom line of this aspect of GSM is a much fasterinstallation of new systems, and lower costs to maintain the proper radio coverage in anexisting system, or to predictably modify the cell radio coverage as a system grows.
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GSM has been first to implement a number of alternative bearer services, and has remainedahead of most other technologies with the implementation of short message service (SMS),broadcast messages, and other innovative features as well. There are several reasons for this.First, GSM was designed from the beginning with a long range plan in mind. This plan includedsupporting every type of digital signal which can be carried on the traditional wire telephonenetwork, as well as some others. Second, a large body of skilled technical people (from mostof the electronics manufacturing and academic research organizations of Europe, plus othersas well) are working continually to upgrade the features of GSM. This latter point may beviewed as a self-fulfilling prophecy, which is the result of a business arrangement betweenthese firms in the European Union, and not related to the technological issues underlying theGSM system design. However, it is a fact, and every GSM user benefits from this activity.
6 Relevant Measures of Capacity and Cost:Sometimes the misleading term "spectral efficiency" (the ratio of conversations per kHz ofbandwidth for a single base station) is used to compare among cellular or PCS technologies,without regard to service area coverage. To make valid comparisons between two cellular orPCS technologies, we must consider two system installations in the same total service area,where each installation is done with the maximum capacity of each technology installed in thelegally allocated total system bandwidth (kHz). The proper measure of system capacity for acellular or PCS system is the geographic-spectral capacity, G, given by
G= conversations/ (total kHz × system area). Eq. 1
The term “conversations” refers to the number or quantity of all circuit switched connections,whether carrying voice (traditional conversations) or data. This expression for capacity is moremeaningful than spectral efficiency, because cellular systems do not use all the frequencybandwidth existing in every cell. In a GSM system, only a fraction of the licensed frequenciesare used in each cell. In a CDMA system, only a fraction of the designed CDMA codes canactually be used in one cell of a multicell system. Many presentations of CDMA mislead thereader because they present the theoretical capacity of a single isolated CDMA cell (62 codechannels), but in a multi cell system one can, in fact, seldom exceed 18 useable codechannels. This is as misleading as presenting a single GSM cell with all the licensed carrierfrequencies operating in that one cell. This is theoretically possible in a one cell system, but itdoes not correspond to reality.
When comparing different technologies, it is necessary to use different values for theequivalent number of conversations in CDMA when comparing voice (which benefits fromexploiting intervals of silence as explained below) versus circuit switched or synchronous data(which does not benefit by exploitation of pauses in the signal flow). CDMA has an effectivelyfar lower number of simultaneous permitted circuit switched “conversations” for data,sometimes as low as 50% of the number of simultaneous voice conversations. GSM can alsobe configured to provide more capacity for voice than for data, and many past publishedcomparisons have not taken this into account.. Most early and simple GSM capacity estimateswere based on continuous transmission capacity. Packet data transmission via eithertechnology can also exploit silent intervals between packets, but many prior publishedcomparisons have unfairly computed the capacity of GSM and other TDMA technologies whencarrying continuous transmission voice or data, while computing the capacity of CDMA whilecarrying discontinuous transmission of voice.
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In this report, the unidirectional bandwidth (that is, uplink bandwidth alone of 25 kHz perconversation for GSM) will be used. This is done in the majority of other published studies aswell. Because both systems under consideration have a corresponding uplink and downlinkbandwidth, the total system bandwidth actually used is double the number described in thisreport.
A more significant measure than G is spectral-geographic-economic performance, S, given by:
Where the symbolic $ equivalent cost may be expressed either of two ways, so long as theselected method of expression is used consistently. It may be expressed as equivalentcapitalization, which is true capital installation cost plus a fictitious initial capital amountnecessary to cover equivalent monthly operational costs (rent, salaries, electric power, etc.) atprevailing interest rates. Alternatively, it may be expressed as equivalent operational cost permonth (or per year). This is a combination of true recurring operational costs plus theequivalent amortization cost of the true capital investment at prevailing interest rates. Thechoice of expressing this per month or per year is also arbitrary, provided that all calculationsare consistent. For WLL installations this cost will include the operator-furnished customerpremise equipment (such as a customer premise fixed antenna and signal converterequipment), but usually not the telephone set, fax machine, or other customer-providedequipment. In contrast, for mobile cellular/PCS installations, subscriber mobile set costs arenot included, when they are fully paid by the subscriber. When a part of the “true” cost of thehandset is subsidized by the system operator, this subsidy amount must be included in thesystem operational or capital cost.
In general, CDMA handsets cost about 40% more than a comparable GSM handset. At thistime the retail price of either CDMA or GSM mobile/portable handsets to the consumer areabout the same, but this is a misleading retail situation due to a higher operator-providedsubsidy of the CDMA sets. This difference may be smaller in the future, but most sourcesexpect the cost of CDMA handsets to remain higher than GSM sets for the foreseeable future.
In general, different cellular or PCS technologies may be compared by means of parameters Gand S. When only technology and not economics is of interest, the system with the largestvalue of G is superior. When economics as well as technology is important, the system withthe largest value of S is superior. The technology with the highest value of G, does notnecessarily have the largest value of S. A useful business comparison is to examine thevalues of S for two different technology installations which have been intentionally configuredto have the same total conversation or traffic capacity (at same or similar quality of voicecoding, etc., discussed below) in the same area and total system bandwidth. This type ofcomparison emphasizes distinctions between the number of base stations required to coverthe area and the cost per base station. This comparison of two equivalent capacity installationsis the best "apples to apples" comparison.
Looking ahead to the conclusion of this report, GSM can be configured to have a superiorvalue of both G and S, by using all of the system design optimization methods available forGSM. In the past, several published comparisons indicate an advantage for CDMA in measureG compared to GSM. In fact, the only cases where CDMA has such an advantage occur when:
1. The GSM design has not been optimized (frequency hopping with discontinuous transmitand overlaid cells are not used), but
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2. The comparison CDMA system makes used of discontinuous transmission and only in thesingle frequency configuration (n=1) for all cells. In contrast, CDMA frequency plansinvolving n=2 or more are inferior.
When we avoid this special and unrealistic case, GSM has a clear advantage in measure Scompared to CDMA in all configurations. Therefore, in a situation where strong pricecompetition is a major vendor differentiation in a competitive market, as some industryobservers predict for the near future, a GSM system is both economically and technicallysuperior to CDMA. In an extreme competitive situation with heavy price cutting, the GSMsystem could survive and retain profitability when its CDMA competitor may not.
We will consider the details of the various optimization methods and how they affect G, butfirst we make some comments about the relationship of measure S to some of the hardwaredesign aspects.
One reason why GSM systems are superior in measure S is the effect of the differentmodularity step size for the two technologies. GSM base station capacity can be selectivelyincreased or decreased (within the maximum capacity limits of the technology) in smaller stepsby provisioning more or less base radio transceivers in that cell. During the growth life cycle ofa GSM base station, the capacity can be increased when required by installation of oneadditional base transceiver having 8 channels. For a short time this will be under-utilized (orover-provisioned), but relatively soon the traffic will increase so that the capacity is fullyutilized.
CDMA base installations are less modular, and therefore the actual cost of the base stationincreases by large steps when hardware is installed for added CDMA capacity. During thegrowth life cycle of a base station the system cost will have a large jump when new baseequipment is added. This increase is typically a complete sector or cell assembly whichnominally adds 62 channels (in practice, approximately 18 of these channels will be useable inmost installations). Because the modular traffic increase step is larger (more than twice aslarge), there will be a longer time interval of under-utilization or over-capacity, even with thesame traffic growth rate as the GSM system. There will be a longer interval of higher cost perincome-producing channel. The cost per technically available channel is also larger for CDMAas well, due to higher overall hardware costs. This larger module size and cost also makes theuse of CDMA for micro-cells or pico-cells (geographically small cells with very low trafficcapacity but also very low cost) impractical with presently available CDMA equipment.
Expense:LargeModularSteps
Expense:SmallModularSteps
Income
IncomeProfit
ProfitLoss
time time
$ $
Figure 4: Profitability Comparison of Hypothetical Large and Small Expense Steps
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The cost of this temporary over-capacity is not considered in equation 2, because equation 2deals with technically available capacity, whether it is utilized or not. The effect of large modulesteps in capacity can be better appreciated by computing the time history and then cumulativeestimated income and expense for continually growing revenue producing traffic. Comparedto the higher total monthly operating costs which persist for several months after theinstallation of added base station capacity, income may catch up with increased expense in amonth or less with a system having a smaller expense step size. Even if two technologies hadthe same cost per channel at the same final maximum capacity installed configuration, thesystem with the smaller modules will be more profitable over the growth life cycle of thesystem. This is illustrated in Figure 4, where the system operator must continually providesome over-capacity to minimize traffic blockage. However, unlike the example in Figure 4, theequipment costs per channel of CDMA and GSM are not equal. CDMA equipment cost isactually 30 to 60% higher than GSM, even when fully utilized. In fairness to CDMA, we mustnote that CDMA vendors are already aware of this deficiency, and many are promising newhardware designs in the future which will permit smaller and less costly modules to providetraffic capacity step increases of less than the full set of code channels.
The measures S and G are related by the equation S = G/($ equivalent cost). Furthermore, themeasure S can also be expressed in the alternative form:
where M is the total number of base stations, and "cell area" is the average radio coveragearea of a single base station. (In a real system, not all cells have the same size.) The 3 distinctterms in Eq. 3, each one shown in square brackets, can be identified as follows: The first termis the traditional spectral efficiency or capacity for one base station. The second term is thereciprocal of the total cost of all base stations. In a complete analysis, this figure shouldinclude their directly associated costs, such as real estate, tower rental cost, cost oftransmission links between a cell and the central switch, signal conversion and processingequipment added at the switch to interface to the transmission links between switch and cellsite etc. The third term, as noted, is the reciprocal of the (average) cell area.
In a more comprehensive economic analysis, we should also add the cost of the centralswitching equipment – the Mobile-service Switching Center or MSC – to the denominator ofthis second term. The cost of an MSC is lower for a GSM system for two reasons. The firstreason is the result of the highly competitive market for GSM switches, since the interfaces areopen (fully standard) on both the PSTN side and the base station side (the so-called A-interface). Because of these open interfaces, competitive market forces between suppliers ofbase stations and switches benefit the system operator by reducing the cost of a GSM MSC.In contrast, although there are several vendors of CDMA base systems, a CDMA system todayis a closed system with regard to competition, since the system operator must buy all basesystem equipment from one vendor. The second reason is that the present design of all CDMAsystems uses a basic MSC switch plus a large and complex auxiliary switching assembly (asignal converter module and controller) between the MSC and each base station, which addssignificantly to the cost of the CDMA MSC hardware. Again future designs may combine thismodule with a switch to achieve reduced costs, but this costly auxiliary CDMA module is theonly way to make a CDMA base system today. Although CDMA proponents have longacknowledged that there was significantly higher cost in every part of the CDMA system, theirprevious argument was that the alleged higher capacity of CDMA compensated for this. In ourterms, they claimed that S for CDMA was larger because the G factor in the numerator term ofS was so much “larger” than the higher cost figures in the denominator that everything would
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come out better economically for CDMA. In fact, this does not appear to be the case in currentand expected future IS-95 technology.7
Not all the terms and expressions in Eq. 3 are independent of each other. Certain fixed costsrelated to real estate and structure (buildings and towers) are present for each base station.Therefore, a system which covers the total service area using less base stations and more cellarea per base station is usually economically desirable. The first two terms are not necessarilyindependent, since a radio modulation and coding technology with greater spectral efficiencyusually is also more susceptible to interference and noise, and thus the cell area is reducedand the required number, M, of base stations is increased.
The way that M appears in these equations suggests that the technological system capacity(G, but not S) can be improved by using more base stations (larger M) each with a small area.This is in fact the basis of the concept of cell splitting, the replacement of one large cell by acluster of smaller cells, thus giving more total system capacity. Cell splitting cannot continue tothe extreme case of an arbitrarily large number of arbitrarily small cells, for both technical andeconomic reasons. The most important technical limit is the severe reduction in power of bothbase and mobile transmitters required in smaller cells. It is extremely difficult to make a radiotransmitter which radiates controllable power levels at less than about 5 to 10 milliwatts, sincethe radio frequency (RF) "leakage" from the circuits operating inside a typical transmitter isalready at that level without any RF power amplifier. Thus the smallest cell size used in publicwireless systems in North America is typically about 1 km (0.6 mi) diameter. (Various types ofin-building and short range systems do use smaller microcells as well.) The economic limitationto cell splitting arises from the non-equipment costs (real estate, towers, buildings, power andair conditioning, etc.) and mounting of antennas, etc., for a base station. Costs related to realestate do not go down proportionately as the power level and size of the base stationdecreases and the number of base stations increases. Also, as noted before, CDMA baseequipment is not presently available in modules as small as GSM base stations, so so-calledmicro-cells or pico-cells are not presently as economically feasible as with GSM.
The optimal way to minimize the economic measure S occurs when the designed radiocoverage area per base station (sometimes called the cell coverage) is increased, thus givinga smaller M, since the product of the two, which represents the total system area, must remainconstant. The complex way in which these three quantities are inter-related to each other andto site-specific parameters, particularly for CDMA, has led to considerable confusion and someobfuscation of reality in previous comparisons. Furthermore, looking ahead to the conclusions,there is no consistent advantage in cell coverage area for CDMA compared to GSM. ActualCDMA cell coverage is, on average, about the same as GSM technology cell coverage, anduseful CDMA coverage is much more influenced by site-specific properties and changesaccording to overall system traffic in a way which does not occur in GSM technology.
7 Some examples of comparative costs of GSM vs. AMPS and IS-136 are available in the book GSM Superphones,by L. Harte, R. Levine, and G. Livingston, published by McGraw-Hill Publishing Co., New York, 1999, in chapter 7(e.g. page 221. The emphasis there is on the benefits of multiplexing more channels on the same carrier.) CDMAwas not treated in this book because the authors could not obtain useable published cost figures for CDMA.
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7 Capacity
7.1 Technological Factors Affecting System Capacity:
There are two technology factors which most affect system capacity (parameter G describedabove). The first parameter is the so-called "capture ratio," which is the minimum carrier tointerference (plus noise) ratio at which the radio system can operate with an output signalwhich is substantially free from error. The second parameter is the spectral bandwidth perconversation.
In algebraic symbols, the capture ratio is expressed as the minimum useable value of C/(I+n).A familiar benchmark of this type is the ratio 63/1 or 18 dB for analog cellular 30 kHz AMPS.Thus, an analog AMPS cellular system may be installed with a 7-cell frequency re-use patternusing omni-directional base station antennas. A radio technology with a lower capture ratiopermits the designer to re-use the same carrier frequency in cells which are much closer toeach other than a radio technology with a high capture ratio, and to install more carrierfrequencies in each cell.
This 7-cell arrangement (and also a 3 cell and 4 cell arrangement as well) is illustrated inFigure 5, where each of the 7 cells has an identifying number. This figure uses the familiarhexagon as a simplification for the useable area boundary of the cell, since the real shape ofeach cell is approximately circular but irregular in a manner dependent on local topography.For simplicity this figure also illustrates only omni-directional cells, and all cells of the samesize, although most modern systems use sectored cells exclusively, and also use smaller cellsin portions of the city having higher traffic density. All the cells in the 7-cell cluster part ofFigure 5 which are labeled 1 use carrier frequencies number 1, 8, 15, … etc. All cells labeled2 use carrier frequencies 2, 9, 16, … etc., and so on. Consider the cell labeled 1. Althoughthere are 6 cells near (but not touching) it which also use the same carrier frequencies, theratio of the desired signal in this cell to the sum of these interfering signals from the distantcells is greater8 than the capture ratio. Only 4 of these 6 interfering cells are illustrated in the 7-cell part of Figure 5. In a 7-cell cluster, 1/7th of the carrier frequencies can be installed in eachcell. For the case of 416 carrier frequencies legally assigned to one system operator in 800MHz AMPS, 59 carrier frequencies may be installed in each cell9.
8 This statement applies to the outer boundary of a cell, where the signal from the central base station antenna isweakest, and the interference from one of the interfering cells is strongest. This example is also the case where theradio signal strength, RSSI, decreases with distance r from the base station according to the formula RSSI(watts)= Po/r4 or RSSI(dB)=10×log Po -40×log(r). This decrease of 40 dB per decade (or factor of 10) in distance isreasonably accurate for most large city radio wave propagation estimates. If we look further away from the centralcell in a cellular system, there are even more than 6 interfering cells using the same frequency. However, with largecells in a public cellular system, this "second rank" of interfering cells is usually sufficiently distant to be over thehorizon, so there is insignificant interference from them at UHF band frequencies.9 The actual value of 416/7 is 59.42, and therefore we actually install 59 carriers in some cells and 60 carriers inothers. There are some special practical problems as well in real systems. Some frequencies are used for callsetup and not for voice traffic. Because most of the existing analog cellular radio equipment does not have the goodmodern radio filters used in new production radios, we cannot actually put two adjacent carrier frequency numbersin two geographically adjacent cells when we use omni-directional antennas. Therefore, early omni-directional cellanalog cellular installations had to omit many of the frequencies in each cell. The practical result was that actuallyonly 1/9th of all the frequencies were useable in each cell (only 46 carriers instead of 59). However, by usingsectored base station antennas, it is possible to use all 59 carrier frequencies with some other complications suchas reduction in trunking efficiency which, if discussed in detail, would take us too far from the main topic of thisreport.
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The second parameter which affects capacity G is the spectrum efficiency of the modulationand coding. In general, most modulation choices which reduce the effective bandwidth of aradio signal also have the undesirable effect of increasing the required capture ratio.
A striking example can be drawn from the cases of AM or from the narrow band FM system N-AMPS (IS-88), which otherwise have no other technical bearing on the current discussion. N-AMPS uses narrow-band low-deviation analog FM, so its radio bandwidth is only 10 kHzinstead of the 30 kHz of standard AMPS. Consequently it requires a greater capture ratio towork effectively10, about 200/1 (23 dB) compared to the 63/1 (18 dB) of AMPS. As a result, N-AMPS does not have a greater overall system capacity G compared to AMPS. This property ofN-AMPS was misunderstood for several years in the cellular industry, and it was consequentlyincorrectly promoted in the past as an alternative to IS-136 for higher overall system capacity.
If N-AMPS is installed throughout all the cells of a system and all the potentially interferingradio carrier frequencies are turned on in all cells, a 20-cell group (not illustrated in Figure 5) isneeded for N-AMPS. This separates the mutually interfering cells sufficiently to reduce theinterference. But the result is typically only 20 sets of 3 narrow bandwidth carrier frequencies ineach cell, for a total of 60 conversations in each cell, which is almost the same as the 59 or 60channels using 30 kHz AMPS.
The result is that the geographical spectral efficiency G of N-AMPS is about the same asAMPS. Although not useful for an overall system capacity increase, N-AMPS is useful incertain city installations, such as Las Vegas, NV, where there is high cellular traffic demand inthe downtown area, and very little cellular traffic in the residential suburbs of the city. In thatsituation, N-AMPS can be used only in the downtown cells to give more traffic capacity, andregular AMPS can be used in the suburban cells, with the potentially interfering AMPSfrequencies not installed in the suburbs. (A special dual bandwidth mobile station is required,and today only Motorola, the proponent of N-AMPS makes such a set.) The result is that somedowntown cells have 80 to 100 narrow-band N-AMPS carrier frequencies, while somesuburban cells have only 20 ordinary AMPS carrier frequencies installed (instead of the usual60).
Similarly, certain things which a designer can do to improve the capture ratio of a radio signal,such as adding more bits of error protection code to the fundamental data bits, also have theundesirable effect of increasing the radio bandwidth. The problem in choosing an optimumradio technology is simultaneously achieving both low signal bandwidth and low capture ratiosimultaneously.
It is well known that certain types of modulation and coding, for example amplitude modulation(AM), do not exhibit the so-called "capture effect." If we examine the signal-to-noise ratio (S/N)at the detector or demodulator stage inside a radio receiver, we find that the demodulated S/Nratio for AM is substantially the same as the radio C/(I+n) ratio. The only way to get a goodaudio signal from AM radio is to separate the two interfering “cells” (or transmitters) which usethe same AM frequency by an extremely large distance. If one listens to an AM band (550 to1600 kHz band) broadcast radio receiver, particularly in the evening or night, the effect ofdistant radio transmitters operating on the same carrier frequency is more pronounced. This ispartly a result of radio wave skip from ionized reflecting layers high in the evening sky. Oneoften hears a distant AM broadcast radio transmitter signal "in the background" of a local AM
10 The analysis of the relationship between analog FM deviation or bandwidth and its capture ratio is explained inseveral textbooks, such as FM by R. Argimbau and R.B. Adler.
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broadcast signal. The interference is most noticeable when the local station’s audio ismomentarily quiet11. FM is very different in this regard. When the C/(I+n) level of FM exceedsthe "capture" or threshold value, the audio S/N becomes much better than the radio channelC/(I+n), as shown in Figure 6. The nominal capture ratio, which is marked with a small squareon each curve, depends on the bandwidth of the FM signal.
Frequency ClustersIdeal hexagon pictures of n=3,4,7, omnidirectional clusters
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Figure 5: Ideal Cell Cluster Illustrations
We can display a similar graph for digital cellular and PCS systems but the relevant measureof signal quality is not the analog S/N ratio at the detector but the bit error rate (BER), the ratioof erroneous received data bits to the total number of received data bits. We display this inFigure 7 for ordinary GSM and optimized GSM, by adding an intentionally non-uniform scalewhich shows the typical BER. This is an approximation for illustration purposes only, becausethe details of the relationship between BER and S/N depends on the time pattern of the biterrors (consecutive clusters of errors vs. isolated errors). The error pattern affects the quality ofthe digital signal and its usefulness for error protection coding, and is not a constant factor.This and all similar figures are therefore illustrative only and should not be used for designpurposes.
11 It is traditional to add together the power of all the undesired signals at a radio detector and call this the "Noise,"which is represented by the capital letter N. In Cellular and PCS radio systems it is customary to distinguish theinterference power (capital I) from the (mainly thermal) noise (represented by a lower case n). The thermal noisecomponent n, of the total noise N, is due to the thermal kinetic motion of individual electrons, and is proportional tothe radio receiver bandwidth and the absolute (Kelvin) temperature. Another component part of the total noise is theinternally generated noise in the radio amplifiers. The ratio of this internal noise to the thermal noise is the so-called "noise figure" of the receiver. In the discussion of this section of the report, the noise figure is taken to be 1/1or 0 dB to simplify the discussion.
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Figure 7: Approximate Bit Error Rate (BER) vs. C/(I+n) for two GSM Installations
Within the approximations implied by the previously stated qualifications, Figure 7 indicates theapproximate ranges of BER for various quality levels of digitally coded speech by means of the
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corresponding color shading of the vertical BER axis. The two new curves, which represent theapproximate relationship between C/(I+n) and BER, and ultimately speech quality, are shownin black, while the related curves for analog cellular remain in pale gray for comparison.
The difference between ordinary GSM and optimized GSM is primarily the use of frequencyhopping in optimized GSM (explained further in other sections). Again the points representingthe capture ratio are marked with a small square, and this graph displays the fact thatoptimized GSM can operate with a C/(I+n) ratio of approximately 9 dB (corresponding to an 8/1power ratio between desired signal and I+n level). This low capture ratio allows closer spacingof cells using the same carrier frequencies (for example, an n=3 instead of n=7 frequencyplan), more carrier frequencies in each cell, and thus more system capacity (at lower cost).
Another technique for optimizing the capacity of GSM is the use of overlaid cells (describedbelow). This provides additional capacity in the central area of each cell, and is extremelyuseful when the base antennas are intentionally installed at locations with high local trafficdensity compared to the traffic density near the outer boundaries of each cell.
When sectored antennas are used, GSM installations may utilize n=4 or even n=3 cell clusters(see Figure 5). This permits either one quarter or one third (respectively) of all the carrierfrequencies to be installed in each cell. The pure spectral efficiency of GSM is designed to beabout the same as the prior European analog FM spectral efficiency, namely 25 kHz perconversation channel, but the geographic-spectral capacity G of GSM is much better thananalog cellular.12
7.2 Methods to Further Increase GSM Capacity
When the first GSM systems were installed in 1991, only the simplest configurations wereattempted. This was necessary to simplify the debugging of the early installations byminimizing the number of different control parameters which might affect the systemperformance. For example, most of the earliest 900 MHz GSM installations did not use theoptional frequency hopping feature of GSM. Most of the descriptions of GSM system capacityin the published literature of the early 1990s were based on these somewhat “timid”approaches to system design. As the system designers have gained experience and fullyoptimized these early installations, many of the designed-in optional capabilities of GSMtechnology have been utilized to increase the capacity of GSM. In many cases, innovativesystem designers have used these optional capabilities in ways which go beyond even whatthe original system architects thought was possible.
Two of the most significant options which have the most significant effects of increasing GSMcapacity are, first, the combined use of discontinuous transmit with frequency hopping, andsecond, the use of overlaid cells.
7.2.1 Discontinuous Transmit with Frequency Hopping
Discontinuous transmit (DTx) is a capability which is available in all GSM mobile and basestations. When DTx is activated on a particular radio channel, the mobile (or base) transmitterdoes not transmit RF bursts (with some exceptions explained below) when there is no audio atthe microphone (or the audio at the microphone is below some preset level chosen to excludebackground ambient audio noise). During intervals of substantial audio silence, the GSM
12 North American AMPS analog bandwidth is 30 kHz, in distinction to European 25 kHz analog bandwidth.
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system is designed to do several clever things to prevent negative impact on system operation.The mobile station occasionally transmits a few burst of radio energy from time to time for thenormally required periodic transmission of call processing information contained in the so-called GSM slow associated channel. The speech coder is designed to continue to produceaudio output (a low level audio noise signal called “comfort noise”) similar to the actualmicrophone’s low audio ambient background noise level. Therefore the people engaged in theconversation do not hear a disturbing change in background sound when the other personspeaks, compared to intervals of silence.
DTx has several system benefits. First, in the mobile station, it helps to conserve battery powerby reducing the amount of time that the transmitter operates during a conversation. Second, itreduces the average amount of co-channel radio interference to other receivers in other cells.All GSM systems can take advantage of the first benefit. However, it was customary in earlyGSM installations to configure the frequency plan for the worst case of continuoustransmission by all mobile stations and all mobile station uplink channels transmittingsimultaneously.
In early GSM installations, mobile DTx was used almost everywhere, but base DTx was not.Furthermore, the configuration of the frequency plan was based on the assumption that allmobile stations transmit continuously. Although this would be a correct assumption of they alltransmit continuous digital data, it was an exercise in overly conservative design for the caseof voice, where the interfering transmitter is off for almost 50% of the time for most speakers.Many GSM installations today are now configured to take advantage of DTx in both directions,uplink and downlink, and the lower level of co-channel interference in both radio directions isexploited by configuring a higher capacity frequency plan such as n=4 or n=3 instead of n=7.
Note that this exploitation of the lower average radio interference level due to DTx is extremelysimilar to what CDMA does as well. IS-95 mobile stations effectively make use of DTx,lowering the average transmit power when there is audio silence. Like CDMA, the quantitativeincrease in capacity due to use of DTx in GSM is dependent on the statistical properties of thespeakers who use the system. Cellular or PCS users who speak very rapidly, who seldompause, or who converse in locations with extremely high audio ambient noise, will all reducethe system capacity, for example. Mobile stations that transmit data continuously cannotbenefit in capacity from DTx, since there are no significant pauses in the operation of thetransmitter.
IS-136, the North American TDMA technology, also uses DTx at the mobile station. However,the IS-136 standard does not require DTX or separate transmit power control for eachindividual channel (time slot) at the base transmitter, and most (possibly all) existing IS-136base equipment in fact transmits using the same power level in all time slots.
Although DTx can produce a significant increase in capacity all by itself, it has an importantsynergistic effect when used in conjunction with frequency hopping.
Frequency Hopping.
As previously mentioned (and also described in the appendix), one of the main benefits offrequency hopping in GSM is to reduce the degradation of BER due to frequency selectivemultipath fading. Frequency hopping can also reduce the degradation in BER due to radiointerference as well. When the level of radio interference is different on different frequencies,the degrading effect of the worst interference channel can be “averaged” over all other radioconversations which it interferes with. The mechanism of this “averaging” process is due to thesame operations described in other parts of this report with regard to fading. First, the use of
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frequency hopping prevents one conversation from remaining on a particular frequency wherethe interference level is worse than other frequencies. Second, even when consecutive biterrors occur on the radio channel at one particular frequency, the mandatory process of bitinterleaving reduces the occurrence of strings of consecutive erroneous data bits. Therefore,the GSM error protection codes can more easily correct or detect the remaining errors.
Again, there is a significant synergistic effect when frequency hopping is used in conjunctionwith both mobile and base DTx. The level of radio interference varies, but is seldom at thehighest value. Therefore, the probability of a long string of consecutive bit errors on the radiochannel is reduced, which is important for good system performance.
IS-95 and IS-136 base and mobile stations cannot engage in frequency hopping. Therefore,they cannot take advantage of this technique as GSM systems can.
Nokia has developed a proprietary adaptive control method, named Intelligent FrequencyHopping (IFH), for optimizing system capacity by means of combined DTx and frequencyhopping. It is normally used in conjunction with Underlay/Overlay (described below). It is called“intelligent” because it uses continual measurements of interference levels on all channels tocontrol the frequency hopping pattern and the assignment of new conversations in each cell tothe optimal channel. This measurement capability is inherent in the design of GSM base andmobile receivers. The result is that interference is minimized and signal quality is optimized forhigh capacity. In this way the IFH system dynamically takes advantage of the actual measuredproperties of each cell and user characteristics. Its effect on capacity is described in the nextsection.
Frequency hopping is not used together with CDMA, since they may be viewed as twoalternative methods of producing a so-called “spread spectrum” radio signal. If there is a desireon the part of a system designer to further increase the spectrum bandwidth of CDMA (toreduce the degradation from fading or from narrow band interference, for example), mostengineers agree that this can be accomplished much more simply and directly by merelyincreasing the chip rate of the PN-PRBS code. This is one of the objectives of wider-band 3GCDMA proposals, such as the proposal to use a 5 MHz bandwidth in the future instead of thepresent 1.28 MHz IS-95 bandwidth. Combining DSS and frequency hopping together in thesame system would be highly complex (and therefore costly) to implement, yet is very unlikely,on theoretical grounds, to produce a synergistic combination of effects.
7.2.2 Underlay/ Overlay
Overlaid cells (also called Underlaid/Overlaid cells, or Tiered Cells) provide extra trafficcapacity in the central portion of a cell’s area by means of additional carrier frequenciesinstalled in the base station of a cell but operating with lower transmit power levels than the“normal” (full cell coverage) carrier frequencies. The basic concept of overlaid cells has been inuse in cellular systems since about 1985, but its versatility and performance has recently beenenhanced significantly as a result of a new development by Nokia.
Nokia has developed a proprietary adaptive method for optimizing the capacity andperformance of a GSM system using overlaid channels. It is named IUO (Intelligent UnderlayOverlay). This method can be used by itself, without frequency hopping and DTx, or it can beused in conjunction with those methods. Again there appears to be a significant synergisticeffect when all of these methods are used together.
A small portion of a simple fixed channel overlaid cell plan is shown in Figure 8: Example ofOverlaid Cells. Like the previous frequency plan diagram, Figure 5, the shape of these cells is
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approximated by hexagons, although real cells are often irregularly shaped, like the outline ofa potato. Overlaid cell technology can be (and usually is) used in sectored cells, but thisdiagram shows non-sectored omni-directional cells for the sake of simplicity. This figureillustrates a full cell frequency plan using n=7 cell clusters. The number shown at the left sideof each full cell is the lead number of a sequence of carrier frequencies used in that cell. Forexample, a full cell labeled 1 contains carrier frequencies 1, 8, 15, 22, 29, etc., since the n=7cluster size implies that each 7th carrier frequency from the overall carrier frequency list is usedin that cell. These frequencies are useable in both the inner and outer parts of the cell.
In the inner part of each cell are additional carrier frequencies which are operated at lowerpower and thus do not cause as much co-channel interference to other cells at the samedistance away as do the full coverage full power carriers. Therefore the frequency plan usedonly in the overlaid portions of each cell could be arranged in n=3, n=2 or even n=1 clusters insome circumstances, depending upon the path loss of the RF co-channel signals arising fromother cells in the vicinity. The figure illustrates an n=3 overlay. In the original historical designof overlaid cells, a fixed frequency plan was used for each overlaid cell. The inner overlaid celllabeled 3 contains carrier frequencies 3, 6, 9, 12, 15, 18, 21, 24, 27,etc. Under this fixedfrequency plan. One of each 3 carrier frequencies is used because of the n=3 frequencycluster size for the inner overlaid cells only. From this, we can determine that the cell having alabel 1 on the full cell and 3 on the inner overlaid cell has use of carrier frequencies 1, 3, 6, 8,9, 12,15, 18, 21,22, 24, 27, 29, 30, etc. in the inner overlaid part. This supports much highertraffic density in the central part of the cell.
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In this figure, there is a cell where the full cell lead number and the overlaid inner cell leadnumber are both 3 (in the lower right). In this particular cell, the useable frequencies in theinner part are 3, 6, 9, 10, 12, 15,17, 18, 21, 24, 27, 30, 31, etc. There are a few lessfrequencies available here than in the inner part of an overlaid cell having a distinct outer and
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inner lead number. Carrier frequencies 3 and 24 are only usable once (each) in this inner cell,even though they appear on the frequency lists for both the inner overlaid cell and also the fullcell.
The example just shown in this figure shows a fixed assignment of carrier frequency. Incontrast, the Nokia IUO system may put only a portion of the indicated frequencies into actionin some cells, where it determines that the interference level for some potential channels is toohigh. Or it could automatically reduce the transmit level of co-channel base and mobiletransmitters in other cells while still retaining adequate signal quality, and then put into action acarrier frequency in the overlaid portion of another cell which is not on the lists shown in thesimple fixed frequency plan here.
Overlaid frequency plans provide very significant extra capacity when used intelligently. Onerequirement is that the inner part of each cell must be geographically aligned with areas ofhigh traffic, and the outer “doughnut” portion of the cell must be aligned with areas of lowertraffic. Most cities do not have geographically uniform traffic density (Erlangs per square km) atevery location in the city. Areas of higher traffic (sometimes called “hot spots”) are eitherpredictable (places where vehicular traffic slows down, such as highway interchanges orentrances to tunnels and other bottlenecks) or can be discovered easily from experience asthe network grows. When these hot spots have been identified, the system designerpreferentially locates base antennas at or near these “hot” locations.
Of course, when a mobile station using a carrier frequency which is only operating in thecentral part of the cell moves from the inner cell area to the outer “doughnut” during aconversation, a handover must be made between two different carrier frequencies. Theoptimal method of determining the time and place of handover is very important to safelysqueeze every bit of extra capacity out of an overlaid configuration. If the handover is donewhen the mobile set is too close to the center of the cell, there will be little extra capacityachieved. Two kinds of improper handovers can occur in these older overlaid installations. Ifthe system attempts incorrectly to hold the mobile station on an inner part carrier frequencywhen that mobile set is too far from the center of the cell, the signal quality of the conversationwill be degraded, and in some cases dropped calls will be the sad result. This is why theadaptive or “intelligent” control of these process in the Nokia IUO system is most significant.
When overlaid cells were first used in analog cellular systems in the mid 1980s, and in mostother present applications of this same basic method, optimal control of this intra-cell handoverproved to be the most complex and difficult aspect of the problem. In analog systems, thishandover was triggered by the received signal strength indication (RSSI). However, in overlaidsystems with aggressive close-packed designs to achieve high traffic capacity, high co-channel interference can often produce false readings of RSSI, leading to improper handoversof both types. In short, the system operators did not have accurate site-specific control of theboundaries between the inner and outer parts of the cell.
The Nokia IUO software control system simultaneously measures and controls severaldifferent parameters to achieve higher capacity without degradation of the signal quality. First,Nokia IUO controls the mobile transmitter power to achieve an adequate signal perceived atthe base receiver. However, this by itself would not be distinctive, since all other cellular andPCS technologies also control the mobile transmitter power. The novel capabilities of NokiaIUO include adaptive control of these additional parameters: Second, the base transmitterpower on each channel is adjusted to provide an adequate perceived signal at the mobilereceiver, and at the same time, prevent undue interference to other co-channel mobile sets inother cells in the system. This implies that the base and mobile transmit power of the
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conversations in all other cells are also optimized in the same way as well. Third, when there isa better set of available channels which can be used for the various conversations in progress,the IUO software will automatically hand over some conversations to other channels so thateach conversation is on the optimal channel with regard to the level of mutual co-channelinterference in the entire system. Thus the Nokia IUO system achieves the optimalcombination of overlaid channel frequency and time slot assignments in each specificinstallation, for each specific combination of local cell traffic, at all times. The increase incapacity which results from this is far superior to a fixed frequency overlaid channelassignment.
Nokia IUO draws on the special built-in capabilities of GSM technology. First, both GSM baseand GSM mobile receivers are able to measure RSSI and also estimate the level of BER onboth the present conversation channel and also the other frequencies and time slots which arein action in the cell of interest and in its neighboring cells and sectors as well, regardless of theparticular frequencies used in those neighboring cells. The BER estimation arises from theresults of the error protection codes used in GSM. While IS-136 technology can also performsimilar measurements, analog cellular systems are very limited because they can onlymeasure RSSI, and they can only make this measurement on the current conversationchannel. IS-95 technology can measure the RSSI and estimate the BER on the currentconversation channel and also in adjacent cells when they are configured all using the samecarrier frequency. IS-95 mobile sets cannot measure or estimate the signal quality on otherfrequencies (when other carrier frequencies are used in adjacent cells), which is the situationin most 1900 MHz band IS-95 installations today.
In most cases, IUO is in use synergistically with DTx and frequency hopping. The capacity of asystem using IUO alone is typically 40% greater than an installation using only “plain vanilla”fixed frequency plans. When IFH is combined with IUO, the typical increase in capacity is 70%.Several important points must be emphasized.
• First, these 70% typical increases in capacity have all been demonstrated experimentally inreal installations – they are not merely theoretical or conceptual goals which have neverbeen achieved in the real world.
• Second, these increases in capacity are average or typical, because the exact results aresite specific and, to a certain extent, are also affected by typical speaker characteristics,amount of audio background ambient noise in the conversation, and other cell uniqueparameters. Keep in mind that some installations may have more, but some may have less,quantitative improvement than these experimental values.
• Third, while IFH can produce increased traffic capacity throughout the cell, IUO producesits increased capacity in the inner overlaid part of the cell only. Of course, in a properlydesigned installation, this is planned to be just where it is needed.
Also, IUO only works optimally when both the base and the mobile transmit power can beoptimally controlled for individual channels. GSM is superior in this regard to both IS-136 andIS-95. IS-136 specifications allow the mobile transmit power to be changed dynamically, but allthe time slots on the same base transmitter carrier frequency must have the same transmitpower. It is possible to attempt to optimize channel assignments to a limited degree bygrouping together all the conversations which require a lower transmit power on one carrierfrequency, and grouping together all the conversations which require a higher transmit poweron an different frequency. However, this is not as flexible as the capability of GSM to usedifferent transmit power for each time slot. Because of this limitation, IS-136 can use IUO to a
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limited extent, but when more different base transmit power levels are really needed than thereare available carrier frequencies, the performance is sub-optimal. Some conversations must beassigned to a carrier frequency with a base transmit power which is higher or lower than theoptimal level. Furthermore, IS-136 does not include frequency hopping, and therefore cannotutilize IFH.
IS-95 technology does permit limited changes in the base and mobile transmitter power level ofindividual code channels, for the purpose of trying to compensate for fast multipath fading.However, the base transmit power level (other than these intentional short term fluctuations) ofall codes in a single frequency IS-95 cell is the same. The base power level can be increasedwith all code channels at the same nominal power level, in order to increase or decrease thecell coverage, but the coverage for one code cannot be reduced while other codes cover alarger area. Because of this, IUO technology and its resultant increases in inner cell capacitycannot be applied to CDMA.
This is not just a present limitation of IS-95 technology, but is a more general property ofCDMA. Remember that the purpose of the intentional short term fluctuations in CDMA transmitpower, and the purpose of long term adjustments in the mobile CDMA transmitter power, havethe objective of producing (as accurately as possible) the exact same instantaneous RSSI fromall the different codes at the receiver. It is known that when one of these code signals is moreor less powerful than the others, even by so little as ±2 dB (equivalent to +58%, -37%above/below nominal RSSI), serious degradation in BER occurs. In effect, unless all thereceived code signals have the same instantaneous power, there is a degradation of the “near-far” type. The fact that existing IS-95 installations cannot always achieve this ±2 dB target forRSSI accuracy with their existing closed loop feedback control of transmitter power is one ofthe reasons why the theoretical capacity levels cannot be achieved in practice.
For the sake of argument, if we built a special CDMA base station which transmitted differentcodes at different power levels (all on the same frequency), some codes transmitted at lowerpower for the intended purpose of providing an inner cell overlay, a problem would arise.Consider a typical urban cell with a path loss of 40 dB per decade of distance. In order totransmit two different code channels which would cover, respectively, half the cell radius andthe full cell radius, their power ratio must be 12 dB. That is, the code channel which is intendedto cover only the inner overlaid part of the cell must be transmitted from the base transmitter ata level 12 dB lower than the code channel intended to cover the entire cell. This implies thatthe ratio of mobile receiver RSSI for these two received codes is 12 dB at all distances fromthe base antenna. The stronger CDMA code signal will “swamp” the weaker code signal in thissituation, making the weaker code signal unuseable. Alternatively, by using different carrierfrequencies for inner cell CDMA codes vs. full cell CDMA codes, it is theoretically possible toinstall an overlaid/underlaid cell. However, the overall spectral geographic capacity G will stillbe inferior to a corresponding GSM overlaid/underlaid cell pattern (as will S also), because ofthe greater amount of radio spectrum used. (The problem of undesired interference betweeninner cell and full cell carrier frequencies does not occur in GSM IUO because the differentchannels are separated by using different carrier frequencies and/or time slots, and thusgreatly different RSSI levels can coexist at the receiver without causing degradation.)
7.3 CDMA Capacity:
CDMA proponents originally made claims of capacity, corresponding to measure G, up to 40times greater capacity when compared to North American omnidirectional cell analog (AMPS,n=7) cellular as the basis of comparison. Currently, the most enthusiastic proponents such as
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Qualcomm are still claiming 20 times analog capacity, while the very few quantitativestatements by system operators such as AirTouch (formerly PacTel) Cellular have claimed only8 to 12 times analog capacity at best.
CDMA has a capacity advantage (when compared to some other non-optimized digital cellularor PCS systems) only for voice, and then only for the n=1 cell frequency plan. When otherfrequency plans are used, when the advanced methods of IUO and IFH are used with GSM, orwhen data is transmitted instead of voice, there is no comparative capacity advantage13. Thisis probably a surprising statement to many readers, since the impression conveyed by theongoing propaganda of most CDMA proponents is that there is some inherent increase incapacity due to CDMA DSS encoding, and innovative methods of error protection coding, etc.The importance of the silent intervals in speech, although not ignored by CDMA proponents, isnot given its due significance. Use of the n=1 CDMA frequency plan also produces otherproblems as well, discussed in other sections.
When other factors (type of modulation, bit rate of speech coder, antenna and cell frequencyplan, etc.) are otherwise the same, the only remaining substantiated reason for the capacity ofIS-95 CDMA, in a one frequency (n=1) configuration, is voice-controlled discontinuous radiotransmit. A CDMA system is designed to operate with an average level of intra and inter cellinterference arising from transmitters which are turned off for about 40% to 50% of the time ina statistically random way, due to normal pauses in speech. This allows more channels tooperate in the cell than the case of continuous transmission. When circuit switched orsynchronous data transmission occurs, the transmitter operates continuously, so the averageinterference is higher and there is no extra capacity to be had from a statistically lower averageinterference level. Furthermore, in installations with other frequency plans (n=2,3,4,etc.), thevalue of G achieved by CDMA does not exceed GSM for either voice or data.
GSM also uses voice control of the mobile station transmit power, but unlike CDMA, the overallradio interference design of most early GSM systems (not IFH) was based on the worst casesituation where all co-carrier frequency radios in other cells with the same carrier frequencyassignments are transmitting continuously. It is possible to exploit the lower averageinterference due to GSM discontinuous transmit to get more capacity as well. To do this, onemust do three things: 1) Equip each GSM cell with extra base transceivers. 2) Use frequencyhopping to help average out the effects of intermittent cell-to-cell interference arising fromdiscontinuous transmit. 3) In addition to standard GSM mobile discontinuous transmit, optionaldiscontinuous transmit must be activated at the GSM base stations as well. The result of thesethree steps is still more capacity for such GSM installations, compared to CDMA, as shown inTable 2.
Statistically, occasional bursts of worse-than-average BER will occur in a system which isconfigured for a statistically varying level of radio interference. If these occasional error burstsare worse than the error protection coding can handle, there will be perceptable degradation of
13 Several published analyses of CDMA capacity have expressed this same conclusion:
1. Paul Newson, Mark R. Heath, "The Capacity of a Spread Spectrum CDMA System forCellular Mobile Radio with Consideration of System Imperfections," IEEE Journal onSelected Areas in Communications, V. 12, No.4, May 1994, pp.673-684.
2. P. Jung, P.W. Baier, A. Steil, "Advantages of CDMA and Spread Spectrum Techniquesover FDMA and TDMA in Cellular Mobile Radio Applications," IEEE Transactions onVehicular Technology, V. 42, No. 3, August 1993, pp. 357-364.
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voice quality from time to time. In a GSM system of this type, there will be occasional intervalsof degraded speech coder output for individual conversations. In a CDMA system, thisdegradation of speech quality will occur on all conversations in the cell when more subscribersare momentarily speaking simultaneously than the expected average number. In some cases,this error burst is smoothed over by the frame interpolation capability of the LPC speech codertechnology. With a small number of conversations, occasional bursts of peak interference willstatistically occur more frequently. With a larger number of conversations in the same cell, thepeak to average RF power interference ratio is smaller and these error bursts do occur lessoften.
The exploitation of silent intervals in speech has been done for many decades, ever since theinstallation of the Atlantic undersea telephone cable14 in the late 1950s. DSI (digital speechinterpolation) is used today on digital undersea cables and for digital telephone satellitechannels. In a classical digital DSI15 system, each channel is reassigned (by means of specialcontrol signals) to another ongoing conversation when the earlier speaker falls silent, even fora short time. CDMA channels are not explicitly reassigned to another conversation during briefintervals of speech silence, but the result is much the same, namely greater conversationcapacity due to exploiting intervals of speech silence.
This greater CDMA capacity is shown in Table 2, column 3 (of 6), is 25% greater than ordinaryGSM, but it is less than GSM enhanced with IFH and IUO. Like IFH in GSM, the basic capacityof CDMA is dependent upon unique speaker characteristics and other properties such as localradio multipath delay spread and other site specific parameters.
7.4 Tabulated Capacity Estimates:
The design of IS-95 provides up to 62 distinct CDMA code channels in a cell, occupying aradio bandwidth of 1.28 MHz (actually somewhat more bandwidth is used due to requirementsfor radio guard band). Based on the limited unpublished16 information emerging from existinginstallations, a CDMA installation using an n=1 frequency plan can support only 12 to 18 codechannels (for voice conversation traffic) per cell, or 8 to 11 data channels when no voice isused. An n=1 frequency plan uses the same carrier frequency in all cells. This is done in the800 MHz band CDMA test installations, but most 1900 MHz band CDMA systems use an n=7
14 Bullington, K. and Fraser, J.M., Engineering Aspects of TASI [Time Assignment Speech Interpolation – analogpredecessor of DSI], Bell System Technical Journal, v.38, 1959, p. 353.15 DSI is already employed in undersea telephone cables and satellite transmission systems to increase capacity.Hughes Network Systems (HNS) patented and demonstrated a TDMA explicit digital speech interpolation (DSI)system in 1989, which increased the traffic capacity of IS-136 (the demonstration used IS-54 technology, thepredecessor to IS-136) by 25 to 80% over standard TDMA. This technology has not been further developedcommercially for both technical and competitive reasons. First, IFH already takes great advantage of silent intervalsin speech, and DSI requires changes in existing signaling and control standards and may not provide a greatercapacity. Second, all DSI systems work best statistically (very few brief blockages due to instantaneous trafficpeaks much larger than the average) with a larger traffic pool, and the use of the so-called "half rate" speech coderprovides a larger number of conversations in the same cell. HNS or others who may wish to exploit DSI technologyare apparently waiting for the official approval of a half-rate coder in the industry which is better in quality than thepresent GSM half-rate coder. Third, HNS is apparently contemplating using this DSI technology as a lever to enterthe base station market, and has not licensed it to the major incumbent base equipment manufacturers norsubmitted it to standards organizations.16 To date, Qualcomm and various system operators of CDMA have not released any "published" experimentalsystem capacity figures, although a number of unofficial sources such as training materials for system installersindicate the numbers quoted here.
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frequency plan, according to industry observers. Use of n=7 reduces the system capacity, butalso greatly reduces the complexity of adjusting CDMA radio coverage when needed. Only in asingle cell CDMA system has the theoretical maximum design capacity case of 62 codechannels per cell been achieved. It has never been achieved in real multicell systems, and olythen would CDMA achieve a theoretical capacity 13.9 times the capacity of analog cellular.
In an n=7 system (primarily used on the 1900 MHz band), more CDMA channels can be usedin each cell due to lower inter-cell radio interference. In these cases up to 40 channels are inuse in some systems. The corresponding per cell spectral capacity (corresponding to the firstterm only in Eq. 3 for S) for both theoretical and two real CDMA systems, is shown in Table 2,where the corresponding values for an n=7 AMPS and real un-enhanced and optimized GSMsystems are also shown for comparison.
Based on the real capacity of analog omni-directional cells (explained in note 1 of Table 2),“plain vanilla” GSM has at least 2.8 times the spectral-geographic capacity of analog cellular,and real IS-95 CDMA installations have spectral-geographic capacity which ranges from 2.26to 3.62 times analog cellular capacity. But optimized GSM systems using IUO, or IUO togetherwith IFH have capacity ratios of 4 times analog and 4.9, respectively. There are otherimprovements which are avaiable for GSM as well, which are not quoted in this table.
With regard to Table 2, please keep the following points in mind. All the capacity figures statedare based on 100% voice traffic. Increases in traffic capacity due to trunking efficiency havenot been considered in this table – the capacity stated relates only to the number of channelssupported by the base system. The comparisons in this table are meaningful when the cell sizeof each technology is the same. This point is justified in the next section, which notesexperimental evidence that cell size is the same or closely comparable in both GSM andCDMA installations.
Table 2: Theoretical and Real System Capacity Comparisons
System Configuration Geo-Spectralcapacity figure:conversations/cell/MHz
Real GSM n=4 with Nokia IUO (note 2) 11/800 kHz=13.7 4
Real GSM n=4 with Nokia IFH and IUO 14/800 kHz=17.5 4.9
Note 1: Due to the omission of certain carrier frequencies to prevent adjacent carrier frequencyinterference in omni-directional adjacent cells, the actual frequency occupancy in an analogn=7 frequency plan corresponds to one of each nine frequencies used in each cell, rather thanthe one of 7 expected with full utilization. This 3.45 conversations/cell/MHz is used for furthercomparisons in the text. In an analog sectored cell system, the full one of seven frequenciescan usually be used in a cell.
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Note 2: Because IUO and other overlay methods only give increased capacity in the inner partof the cell, this number can be taken at face value only on the assumption that the designerhas properly aligned the extra capacity “hot spots” with those inner portions of the cells whichrequire it. In contrast, IFH adds capacity throughout the entire cell.
8 Coverage
8.1 CDMA Coverage:
Claims have been made that a CDMA cell can provide greater RF coverage area per cell (atequal RF transmitter power), and thus the number of cells (parameter M in Eq. 3) can besmaller. This would lead to smaller CDMA system cost compared to current experience, if true.This claim has been attacked on theoretical grounds, particularly by Ericsson spokesmen, whoconclude that the coverage for a properly functioning CDMA system is no larger, and isperhaps in many cases smaller, than a corresponding GSM cell. The present author tends toagree with their view, and the little experimental information made public about working GSMsystems also appears to agree with Ericsson's analysis.
One important factor in this analysis is the extra power level needed at the outer edge of a cellto permit the IS-95 closed loop feedback power control to operate for the purpose ofcompensating for multipath fading. Proper operation of a CDMA system requires continual fineadjustments of the mobile transmit power to compensate for fading. If this does not operateaccurately, even on just one channel, the BER of all the channels in the cell is degraded.Consider the case of a CDMA mobile set operating at the very outer edge of the cell atmaximum transmit power. It is then given a command to briefly increase power (for somemilliseconds) to compensate for a fade, and it cannot do so because it is already at itsmaximum transmit power level. As a result, the digital signal received at the base station willthen experience a high momentary BER and may be unusable. This demonstrates thesignificance and necessity of the closed loop CDMA power control for the purpose ofproducing clear error-free digital transmission on all the channels in a CDMA system. Analysisof this situation by Ericsson and others indicates that useable cell size for CDMA is limitedmore severely by this phenomenon than originally claimed by CDMA proponents.
Because of this, the actual cell size coverage for CDMA does not exceed similar cell coveragefor GSM or IS-136. In fact, for mobile CDMA service, the cell coverage is often smaller than aGSM cell with similar base station transmitter power. This is in agreement with unpublishedinformation coming from real CDMA systems.
There is a large quantity of published material on the estimation of CDMA cell coverage, mostof it based on theoretical predictions before real systems were constructed. Much of it isclearly inaccurate in view of subsequent experimental and field data. To briefly summarize thepresent situation based on experimental and field testing, one can say that many differentfactors affect CDMA cell size coverage. These factors include traffic and interference level inadjacent cells, specific percentage use of each channel by users in each cell (individualspeaker activity factor), and site-specific multipath conditions. In general, one cannot concludethat CDMA coverage from a base station with the same type of antenna is greater than GSMtechnology coverage. For overall average comparisons with GSM or other technologies, it ismost reasonable to use the same cell size in any design estimates or comparisons.
CDMA proponents have stated that CDMA cell coverage could be greater than GSM for a WLLsystem with fixed subscriber antennas. This argument is based on the fact that the range of
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instantaneous power observed in fading is less extensive for a fixed antenna, and thereforethe degradation noted by Ericsson publications is no longer so large a factor. It is true that fastfading on a mobile radio channel is primarily due to movement of the mobile radio, so that itencounters both faded locations and peak power locations. In wireless local loop (WLL)installations, there is no movement of the subscriber antenna (since it is fixed to the home orother building) and thus short-term power control is less important to compensate for fading.However, the radio coverage range for CDMA is not increased compared to GSM. Somefading can also arise from the motion of vehicles such as trucks (which are reflectors of radiowaves) and other vehicles in the vicinity of a stationary subscriber antenna. The major factoraffecting radio coverage in WLL systems of all technological types is the radio signal strengthat the outer edges of the cell. The path loss (relationship of signal strength to distance from thebase station) is the same for CDMA and GSM signals, and thus the cell radius and coverageare the same for both.
Because the reflecting surface area of vehicles is smaller than the surface area of buildingsand cliffs, the quantitative amount of fading (peak to valley fade ratio) is smaller and the fadingis not so severe for WLL compared mobile/portable systems. But the problem of fading andthe need for some extra RF maximum power at the outer edge of the cell is still there, and noclear increase in cell coverage over cells using GSM or other access technologies has beendemonstrated. In different environments (urban vs. rural, etc.) where there are differentamounts of radio obstacles (such as trees and buildings, etc.) and thus different types of pathloss, the losses and thus the cell size again are the same for both types of signal (GSM vs.CDMA), and no advantage in cell size arises for either type of system.
In view of all of these points, we have assumed all cells for systems of different technology tohave the same size in Table 1Table 2.
9 Data Capacity
9.1 Data Protocol Support:
Both GSM and CDMA systems have and are continually improving protocol support for varioustypes of data and fax transmissions. There is no long term advantage of one technology, ingeneral, over the other for data or fax, although working GSM systems have more types ofdata protocols, different data rates, and fax service available at the present time and likely forthe next few years at least. Ultimately, 5 or more years in the future, all technology types willpossibly support all data protocols equally well
9.2 CDMA Data Capacity
With present version of CDMA, the maximum user data bit rate is 9.6 kb/s. A 13 kb/s channelrate is also supported in newer CDMA equipment, but at the cost of a somewhat worsechannel BER. It is possible to obtain a greater user bit rate by reducing the number ofchannels drastically and transmitting only a limited number of codes. Using a 1.28 MHz PN-PRBS clock rate, it is not feasible to encode a single user channel having a bit rate muchhigher than about 100 kb/s without losing the actual advantages of direct spread spectrumaltogether. Of course, providing one subscriber with a higher bit rate reduces the total systemcapacity for other subscribers as well. If one wishes to use the entire reserved bandwidth forone subscriber, of course one may transmit a data bit rate of 1.28 Mb/s, but then one is not
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using spread spectrum at all, but merely encrypting the subscriber data with the PN-PRBS bitstream and then phase modulating it onto the radio frequency carrier. Of course, using 1.28Mb/s for one user in the cell implies that no other user on that carrier frequency can have anychannel capacity in that cell whatever.
DSS encoding of a low bit rate data signal with a high bit rate PN-PRBS bit stream provides aso-called “spreading gain” (described in more detail below) which is approximately equal to theratio of the two bit rates. When we encode a 10 kb/s data bit stream with a 1000 kb/s PN-PRBS bit stream, this ratio is 100/1 or 20 decibels. The improvement of the received signalstrength due to cross correlation at the receiver with a replica of the particular PN-PRBSincreases the decoded signal strength because all the encoding rate product bits have thesame sign during one data bit (all positive during a binary 1 bit interval, and all negative duringa binary zero interval). Meanwhile, the effect of cross correlation with signals from otherchannels or with noise (provided that they are not significantly stronger than the desired signal)is, on average, approximately zero and they do not degrade the decoded signal. If we increasethe data bit rate compared to the PN-PRBS bit rate, this spreading gain becomes smaller,because we do not add as many pulses together during the time interval corresponding to onedata bit.
Some 3G CDMA proposals have suggest the use of a 4 or 5 MHz PN-PRBS clock rate, to beused in conjunction with single subscriber data bit rate such as 1 Mb/s. But this is a muchlower spreading gain system than even present CDMA. In this proposal, the spreading gain isonly 5/1, or 7 dB. The BER effect of interference from other channels and from radio noise willbe much worse, so fewer channels can be used in each cell and the system will have lowercapacity. The example in the previous paragraph suggesting a 1.28 Mb/s data rate for presentday 1.28 Mb/s PN-PRBS is the extreme case of a 1/1 ratio of PN-PRBS bit rate to data rate,corresponding to zero dB spreading gain. Because of this, a user of high bit rate data in oneCDMA cell may only be able to communicate when there are no radio channels operating inthe six cells surrounding that cell. This goes counter to the basic idea of cellular design.However, this again raises many issues about the use of cdmaOne or CDMA-2000 in 3Gdesign for high bit rate digital data as noted elsewhere. There is no capacity advantage, and itis not clear that CDMA provides any other system level advantage for continuous datatransmission. Consequently the suitability of CDMA for the proposed heavy data applicationsof 3G systems is not accepted by all the players in the 3G scene.
9.3 No CDMA Capacity Advantage for Data Users:
An important related fact is that there is no corresponding increase in system capacity, due tosilent intervals, when most of the subscribers are transmitting substantially continuous bitstreams, such as fax or data transmissions. In the case of IS-95, only 8 continuous circuit-switched data connections rather than 12 to 16 voice conversations can be supported in asingle cell. More than 8 packet data conversations may be supported as well, but the detailsare not described here. This difference occurs because there are no silent intervals duringcontinuous data transmissions, and the system therefore cannot “time share” the use of thechannel to increase the number of separate connections or conversations. This reduction inCDMA channel capacity is about 50% since the apparent higher capacity due to voice is theresult of typically about 40% to 50% silence during speech.
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9.4 GSM Data Capacity: Roadmap to higher data rates
The earliest data support available with GSM covered data rates up to 9.6 kb/s using a normalsingle TDMA time slot (corresponding to a full rate voice channel). The gross bit rate of a fullrate channel is 22.8 kb/s, but 13.2 kb/s of that total capacity is used for error protection code toensure accurate transmission of the 9.6 kb/s data across the fading radio channel. Higher bitrates can be given to a single GSM customer (for example, for extremely fast data file transfer,or digitally coded video). The present version of GSM is actually better adapted to providingthe high bit rate by linking several channels for use of one subscriber. Of course, this reducesthe number of channels used by other subscribers. This has been demonstrated in Europeand is now becoming available in North America as well. Many North American GSMinstallations already support 14.4 kb/s data rates and higher, for example, by using 2 time slotson the same carrier frequency. Linking seven of the time slots on one GSM carrier frequencycan readily provide a unidirectional data rate capability of approximately 64 kb/s or more. Thiscan be done without affecting the total system capacity and use of the other carrierfrequencies in this or other cells.
Conversely, when a low data rate is desired for applications such as remote monitoring, GSMcan provide either a full rate or a half rate (gross bit rate 11.4 kb/s) channel for this purpose.GSM also has a further digital channel called the Standalone Dedicated Control Channel(SDCCH) which is currently used for call setup messages and short message service (SMS).The gross bit rate of the SDCCH is 2.85 kb/s, and up to eight different users can share a singletime slot (that is, the equivalent of a single full rate voice channel) on a carrier when using theSDCCH channel protocol. In these two applications, the SDCCH channel is assigned and usedfor only a brief time, but there is no restriction in the GSM system design to prevent a longer ora scheduled intermittent use of that channel for various special purposes.
Because of the versatile original system design of GSM, user data rates ranging fromintermittent 1 kb/s at the low end to 64 kb/s or more at the high end are readily available withextremely simple modifications of the present GSM system design. Many of thesemodifications are backward compatible with existing hardware, and only involve softwaremodifications which can be downloaded to a mobile station. Multiple frequency GSM radioreceivers can also be used to get even more data rate, such as 128 kb/s or more. GSM istherefore seen as the most flexible data platform and is expected to retain a lead for all typesand rates of data services for at least half of the next decade.
10 QualityQuality of the information (speech or data) is the result of several factors. Error freetransmission over the fading radio channel is important. A speech coder which producesnatural sounding speech and is tolerant of digital channel error is another.
10.1 GSM Frequency Hopping and IFH
GSM frequency hopping reduces the total BER, compared to a non-frequency-hoppinginstallation, because the coordinated frequency hopping allows the base and mobile to usedifferent frequencies for each successive frame of digital information. In a typical installationthe choice of carrier frequencies can range over 5 to 20 MHz, using all the installed carrierfrequencies in a particular cell.
In a PCS-1900 installation there are typically about 15 carrier frequencies in a cell, thussupporting about 115 conversation channels and 5 channels reserved for call setup and short
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message service. With typical delay spreads in the range of 1 to 5 microseconds for the smallcells17 used in PCS-1900, the fade coherence bandwidth ranges from 200 kHz to 1 MHz. If theenvironmental clutter profile is simple, multiple fades at different frequencies due to differentgeometric ray paths are less likely. Therefore, it is likely that only one or two out of the 15carrier frequencies in that cell is subject to a fade at a particular place and time.
In such a situation, GSM frequency hopping, together with the pre-existing bit interleaving anderror correction codes of GSM, will achieve accurate digital transmission. Frequency hopping isparticularly effective for hand held portable mobile sets, of the type used almost universally inPCS-1900 installations, because these sets are normally stationary or moving only atpedestrian speeds (0 to 5 km/h, corresponding to 0-3 mi/h, the speed of vigorous walking).
If a non-hopping handset is stationary at the location of a fade on its operating carrierfrequency, or if it is subject to strong radio interference on that particular frequency, all thedigital bits may be corrupted (that is the worst case of a 50% BER). In contrast to that,frequency hopping avoids that bad carrier frequency most of the time (perhaps 14 of each 15radio bursts). Thus the level of un-correctable bit errors can be reduced to a fraction of apercent. In contrast to this, fast moving vehicle-mounted mobile stations normally experiencefast fading at all times (except when the vehicle stops!). In this latter situation, the use offrequency hopping still makes a small improvement, but does not make the dramaticimprovements which are apparent for stationary handset or single frequency interferencesituations.
Some further information about fading is given in the Appendix.
10.2 CDMA Speech Quality:
Repeated claims have been made that CDMA has better perceived speech quality, measuredby such things as mean objective score (MOS) speech quality evaluations produced by a panelof listeners. In general, due to the rapid and continuing advance in speech coder technology,at every date one may say that the most recent speech coder design is the best quality speechcoder design. When CDMA first was demonstrated using a coder designed in 1990, it hadbetter quality than digital speech coders used in competitive systems, which had beendesigned in 1986 and 1984 respectively. However, all of these were adequate for theirpurpose. In practice, CDMA has not demonstrated any significant advantage over GSM withregard to this item, and recent introduction of the GSM EFR speech coder has reversed theprevious speech quality rankings of GSM and CDMA. GSM now has the best quality speechcoder.
The GSM EFR speech coder sets a standard so close to standard wired telephone quality thatmajor differences in the quality of future speech coders are not expected, although gradualimprovement in the error sensitivity is expected. In the long term, one can expect each systemto be upgraded to the latest and best available speech coder, so that this should not be viewedas a long-term distinguishing feature of either technology.
17 Because of the low power handsets used in PCS-1900 and DCS-1800, cell diameter seldom exceeds 4 to 5 km.On the 900 MHz GSM band, much higher power mobile stations may be used in vehicles, and cells can be 30 kmotr more diameter, particularly in sparsely populated rural areas.
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10.3 CDMA Sensitivity to Intermodulation (IM):
Because a CDMA receiver has a large bandwidth and must provide more amplification of thelow spectral power density of the wider bandwidth CDMA signal, it is also more sensitive tointer-modulation (IM) than a narrow band receiver. IM is a non-linear interaction and distortionof the desired signal waveform due to a strong interference signal. IM sensitivity is afundamental problem for CDMA in any environment where strong undesired interferencesignals are present. It is not easily corrected by a simple modification of the hardware design.Many avenues of research are underway using active signal cancellation and other measuresto try to find a novel and practical way to reduce IM in broadband CDMA receivers.
IM has proven to be a serious problem for CDMA installations on the 800 MHz cellular band,and is reputed to be the main problem holding back the widespread implementation of CDMAon that band. Presence of a narrow bandwidth high power interference signal preventsreception on all the CDMA code channels present at that frequency. It is true that these narrowband interference signals which plague such CDMA installations arise from faults in the pre-existing analog cellular equipment, such as narrow-band IM from nearby base stations and thelike. However, the fact that this interference should not be present on legal grounds does notforgive the real practical problems it produces for CDMA.
In contrast, TDMA systems such as IS-136 or GSM can avoid such narrow band interferencesignals by merely using a frequency plan which avoids the interference signals in the cellswhere they occur, or in the worst situation, omitting one frequency in a particular cell, wherenecessary. Occasionally such problems may arise on other bands as well, typically due toother narrow band radio sources such as police or microwave radio signals. CDMA sensitivityto IM is not a constant problem affecting all installations, but it is a risk which may causesignificant delay and cost in the installation and debugging of some CDMA system, and in theworst cases prevent the successful operation of CDMA on some bands
10.4 CDMA C/(I+n) and Spreading Gain:
When we wish to express the relationship between BER and C/(I+n) for CDMA, it is useful toconsider two kinds of interference. One type of interference is completely un-correlated with(that is, orthogonal to) the PN-PRBS code component of the desired CDMA signal. When twocompletely uncorrelated waveforms are multiplied together, the product waveform has a zerotime average value. That is another way to say that the two waveforms are orthogonal orcompletely uncorrelated. We describe this by saying that the cross correlation coefficientbetween the two waveforms is zero.
The second type of interference is completely correlated. That is, it has exactly the same PN-PRBS code as the desired signal – a case which should only theoretically arise from mutualinterference of two channels using the same PN-PRBS code in two different cells. Acompletely correlated pair of signals are either exactly identical or one is the exact inverse (inNRZ waveforms, one waveform is the polarity inverse of the other.) These two cases havecross correlation coefficients of +1 or –1 respectively.
Other cases of interference may be viewed as intermediate in absolute magnitude of thecorrelation coefficient between these two extremes of 1 and zero. It is also informative toconsider the intermediate case of a PN-PRBS code which is 50% correlated with the desiredsignal's PN-PRBS code, and various other amounts of partial correlation. In a 50% correlatedPN-PRBS interfering signal, some (but not all) of the bits are exactly like the corresponding bitsof the desired signal, and other bits are opposite the corresponding bits of the desired signal.
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The cross correlation coefficient is either +0.5 or -0.5, depending on the overall polarity of onewaveform relative to the other. Given a first specific bit stream, only one specific second bitstream has a correlation coefficient of +1 relative to that first specific bit stream. Its inverse is inturn the only waveform which has a correlation coefficient of -1 relative to the first specific bitstream. On the other hand, many different PN-PRBS bit streams are half correlated or un-correlated with one particular PN-PRBS bit stream, because these matching bits do not needto occur in any particular time order or grouping. These intermediate cases are interesting toexamine because the actual set of PN-PRBS codes used in IS-95 are effectively not alwaysperfectly orthogonal over every data bit interval, particularly due to such conditions as radiomultipath propagation, and the presence of signals from other cells or excessive number ofchannels temporarily in use in the same cell.
30
20
10
0
-10
-20 -10 0 10 20 30 dBC/(I+n) Radio channel
dB S/N at detector
This illustration isnot based on precisedata. Do not usefor design.
Approx BER
50%
10%
1%
0.1%A CDMA with PN-PRBS clock 100 timesdata clock,uncorrelatedinterference
B CDMA, approx.50% correlatedinterference
C CDMA, 100%correlatedinterference
Figure 9: CDMA C/(I+n) for Various Degrees of Interference Correlation
Figure 9 shows that the C/(I+n) ratio for a completely un-correlated (orthogonal) PN-PRBSencoded interfering channel (curve A) can actually be 20 dB lower than the acceptable C/(I+n)for a fully correlated interfering channel (curve C) having the same PN-PRBS code. Thereason is, of course, that the effect of the un-correlated or orthogonal code is "averaged out" inthe decoding process. This apparent 20 dB increase in performance is called "spreading gain"and is the result of using 100 times as much bandwidth with CDMA as the signal data ratewould require with ordinary coding and modulation.
The spreading gain is the same number as the ratio of the PN-PRBS bit rate to the data clockbit rate. This ratio is approximately 100 to 1 for real IS-95 CDMA (corresponding to 20 dBsince 10*log(100) = 20), but the ratio is 10 to 1 (only 10 dB) for the example waveforms inFigure 3. So long as we use other CDMA signals having PN-PRBS codes which are allorthogonal to each other, we are able to operate with very low mutual interference, asillustrated by curve A in Figure 9.
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One problem with real CDMA systems is that, for various practical reasons, we cannot useperfectly orthogonal encoding waveforms. Furthermore, other non-orthogonal interferingsignals are produced due to multipath delay (although this is somewhat ameliorated by theRAKE receiver described below). Interference from other cells which use non-orthogonal PN-PRBS signals, and other true noise sources increase the effective “noise floor” The noise flooris the effective amount of background noise from which the signal must be extracted. Also, inreal CDMA systems, all the different mobile transmitted channel signals are notinstantaneously at the same receive signal strength at the base receiver, and the signals whichmomentarily have lower received signal levels experience worse BER than the good signals.They therefore are better described by an intermediate curve such as curve B.
10.5 So-called "Graceful Degradation" and "Breathing Cells":
CDMA proponents have described the mutual interference effect between all the encodedchannels in a cell or between encoded channels in adjacent cells in some rather unique ways.First, they make the point that the over-provisioning of a cell with more than the theoreticalmaximum number of orthogonal PN-PRBS codes will lead to a gradual or "graceful"degradation of the BER as more traffic is carried in the cell. This is in contrast to theinterference which occurs on only one carrier frequency in GSM when only one significantinterfering radio signal penetrates a cell.
The clear implication of this approach by CDMA proponents is that there will be a slightdegrading effect on all users, rather than a serious or debilitating interference effect on oneuser. Although the effect of over-provisioning interference is indeed gradual and is indeedspread over all users when expressed in terms of BER, there is a widespread qualitativemisunderstanding of its effect on voice and data transmission.
The error protection coding used with digitally coded speech and for digital subscriber data orfax works in this way: The error protection code, and the capability of the voice coder to bridgeover short gaps in accurate speech coder data, allows the channel to continue to operate moreor less satisfactorily up to a maximum BER of about 4 or 5% (depending upon the details ofthe time pattern of the bit errors). Above this level of BER, the speech coder produces silence(and the radio channel will ultimately automatically disconnect due to continuous erroneoussignals) and similar loss of data or fax communication will occur on all channels. Thus, theultimate effect of serious interference in a CDMA system may, in fact, be more widespreadthan in a GSM technology system, affecting all the users in a cell rather than just a few. This isan inherent system property of CDMA, and is a manifestation of the so-called “gracefuldegradation” of CDMA, which in fact, due to the properties of digital speech coders, is not so“graceful” after all! The bit error rate does degrade gradually and about equally on all codechannels, but the speech coder still has an abrupt cutoff point where it cannot continue tofunction.
CDMA proponents also state that, under conditions of high radio interference in one cell, aCDMA system can handover some of its users from that cell to another immediately adjacentcell which is free of such interference. (This discussion applies to n=1 frequency plans.) Thisability of a CDMA system to effectively shrink or increase the short term coverage area byhanding over the mobile stations in the outer part of a cell, is described as "breathing" cells.Cells are said to expand or contract, like the breathing lungs of a person, as required by localinterference conditions.
Again, the details of field tests indicate that there are significant complications with thisprocess. In order to provide a large area of overlapping radio coverage between adjacent cells
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to facilitate "breathing" cells, the base radio transmitter power must be increased in bothadjacent cells. This increases the interference between code channels in the two cells, thusreducing the initial capacity of each cell. If an “ordinary” cell can accommodate 18conversations, a cell which is involved in this “high overlap” situation can accommodateperhaps only 7 conversations. This is illustrated in Figure 10(a). In the crescent shapedoverlapping area, there mutual radio interference from the two base stations is possible underthe following conditions. Any momentary correlation between the PN-PRBS codes of twochannels in these two cells, respectively, will produce a momentary high BER condition. Incontrast, one can configure the system by reducing the base transmit power at the two cells,thus reducing the inter-cell interference and allowing more channels to operate withoutinterference in each cell, but sacrificing the so-called "breathing" capability. The latter casealso leaves the region where two neighboring cells meet with minimal signal strength andquality from either of the two base stations. As a result, even the “soft handover” methoddescribed below will not produce good communication, and users in the mutual boundaryregions are likely to be unexpectedly disconnected. This situation is illustrated in Figure 10 (b).
Region where highmutual interferenceis possible.
Region where callmay be lost duringhandover.
(a) Cells configured with large overlap to facilitate “breathing.”
(b) Cells configured with no overlap to maximize channelcapacity in each cell.
Figure 10: Cells configured to permit “Breathing,” or conversely, Higher Capacity
It should also be noted that exactly the same relationship between adjacent cell radio overlapand the ability to perform inter-cell handover to respond to unexpected interference occurs inGSM and other technologies as well, with the same undesirable increase in co-channelinterference between different cells due to larger base station transmitter power.
So-called "breathing" cells have been presented by CDMA proponents as the answer to thepossible (but actually very rare) problem of a "rogue" CDMA mobile station whosemalfunctioning transmitter transmits at an excessive power level and which therefore causes"near-far" interference with all the other CDMA channels in the cell. The "rogue mobile"problem is more serious for CDMA than the related rogue mobile problem for GSM or other
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technologies, since the CDMA rogue mobile kills all other channels, while the GSM roguemobile affects only one carrier frequency.
On the whole, the problem itself is so seldom encountered that no real CDMA system is set upto provide "breathing" cells, since it would reduce the capacity so severely throughout thesystem. There is effectively a “domino effect” in which excessive traffic in one cell causeshandovers to adjacent overlapping cells, and the resulting excess traffic in each of theadjacent overlapping cells causes further handovers, and so forth. This is more of a point inthe so-called "theological debate" between CDMA proponents and other technologyproponents than a realistic issue, but it is mentioned so frequently in the literature that it isnecessary to comment on it.
10.6 CDMA Sensitivity to Multipath Delay Spread:
Early in the development of CDMA, proponents made early claims that CDMA, unlike GSM andother TDMA systems, was immune to inter-symbol interference (ISI), which is anotherconsequence of multipath propagation (in addition to multipath fading). The multipath delayspread in most cells is longer in duration than the time interval of one PN-PRBS bit. Therefore,the delayed multipath copies of the same signals are uncorrelated with the desired receivedsignal, and appear to a CDMA receiver like other CDMA signals, having a different PN-PRBSbit sequence. They therefore add to the un-correlated or partly correlated interference level(the “noise floor”) in the cell, just like having additional transmitters present using other codechannels.
Because of the time delay, these multipath signals do not have their PN-PRBS bit streamproperly synchronized with the decoder, but they do add to the variation of the detector outputabove and below the voltage level for the correct data bit value. Their presence thereforeincreases the BER of all other receivers. The practical solution to this problem due tomultipath propagation for CDMA is the use of an adaptive equalizer in both mobile and baseCDMA receivers. This was perhaps a bitter pill for the CDMA proponents, because theyinitially claimed that CDMA, unlike TDMA systems like GSM, would not need an equalizer.
The particular type of adaptive equalizer used in IS-95 CDMA has the special name, "RAKE"receiver, because of its similarity to an adaptive equalizer of the same type used in radarsystems. However, when one examines the operations and complexity of a CDMA RAKEreceiver, it is clear that it does the same things as a GSM adaptive equalizer, and is similar incomplexity, power consumption, and other significant parameters. A RAKE receiver is anequalizer which generates multiple delayed copies of the received radio waveform. In CDMAreceivers this delay is accomplished by sampling and digitizing the received waveform at anappropriate sample rate, and then storing the samples in digital memory. The sample valuesare retrieved at a later time, when needed, to produce the desired amount of delay. Thesedelayed digital samples can then be added or subtracted with each other to cancel (or at leastreduce the amplitude of) delayed components of the signal which appear in the receivedwaveform due to multipath delays.
The standard CDMA design calls for only 4 delayed copies of the received signal (each copy iscalled a "finger" of the RAKE, for historical reasons). The amount of delay of each finger isadjustable, and the value of each finger signal may be added or subtracted after multiplying itby a coefficient adjusted to optimize the resulting output signal. A properly functioning RAKEreceiver applies large delays to the finger signals which contain “early” copies of the desiredradio waveform. It applies short delays to the finger signals which contain “later” delayedcopies of the desired radio waveform. The result is that each of the finger signals then have
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the major component of the desired signal in good time synchronization vis-à-vis the chip rate.If a particular signal has the wrong polarity, it is subtracted. If it has the correct polarity, it isadded. The result is a re-enforcement of the desired signal despite the multipath delay whichoccurs in space between the transmitting and receiving antennas. This is very similar to theoperations which occur in certain types of adaptive equalizers used on other radiotechnologies, despite the different names “adaptive equalizer” versus “RAKE receiver.”
Unfortunately, one of the undesirable results of using the RAKE receiver during soft handover(described below) is that at least 2 and in some case 3 of its fingers are occupied with delayedsignals which are not the result of multipath radio propagation, but are signals from a secondor third base station participating in the soft handover. During this interval, which may be alarge portion of the total conversation, the equalizer is not able to correct for multipathpropagation and multi-cell delayed signals at the same time, so the signal to noise ratio andthe BER performance both deteriorate somewhat. This is one of several cases in which thedesign of CDMA in IS-95 incorporates complex equipment which is not always utilized to fulltechnical or economic advantage.
The bottom line is that there is no particular inherent advantage for either CDMA nor GSM withregard to its sensitivity to multipath and ISI, and with regard to the complexity or cost of theequipment which the system uses to combat this problem.
10.7 Error Protection Code Equally Good for CDMA and GSM:
The second most effective method to address the problem of fading is to design the digitalencoding for transmission with both extra bits used for error protection codes and bitinterleaving to prevent long sequences of erroneous bits. Both GSM and CDMA do thesethings rather well, but neither technology is significantly better than the other with regard toanti-fade or interference caused bit errors.
11 CDMA Soft Handover:Soft handover is used only in CDMA systems having n=1 frequency plans. It is less often usedin the 1900 MHz band because most of these installations do not use n=1 frequency plans.Instead of abruptly disconnecting the radio connection to the old cell and continuing with aradio connection to the target handover cell only, the mobile station engaged in soft handoverremains in communication with both base stations (or in some special geographical cases,three base stations) for some interval of time. Eventually the old base station stopscommunicating with the mobile station and the handover is complete. More details aredescribed below in the section on Penalties of Soft Handover.
Soft handover, according to the designers of the Qualcomm IS-95 standard, was intended toprovide special protection against loss of a connection at handover time, and to prevent a gapin the audio during handover.
Soft handover is open to several criticisms, most of which have been amply repeated by thesevere critics of CDMA:
1. It is an overly complicated solution to a problem which is not significant in other systemssuch as GSM. It is an over-reaction to a misunderstood cause of a minor problem, or it is acomplicated solution to a problem which is already solved.
2. It adds significantly to the cost of the system hardware, and particularly to the cost of digitallinks between individual CDMA base stations and the central CDMA MSC switch.
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3. It does not increase the performance of the system, and in fact it decreases theperformance of some of the other aspects of the CDMA system (for example, the RAKEreceiver equalizer) during soft handover.
There was and is a genuine “gap” problem with analog cellular handovers. There is an audibleloss of speech for about 0.2 seconds or more in analog cellular systems at handover time,which is sometimes perceived as a click. Also, in analog systems with faulty RF coverage, callsmay be lost when a handover is attempted. This occurs because the radio signal coverage isnot good in some parts of some installations. Therefore the cellular system will automaticallydrop a call if the analog supervisory tone (inaudible to the human subscriber) or thecorresponding identification data (CDVCC code in IS-136 or Synch-training bit pattern in GSM)is wrong or missing for 5 seconds or more. Handovers can also fail if there is no availabletraffic channel in the target handover cell. In view of the system resources devoted to softhandover in CDMA, some industry observers have surmised that the designers of IS-95 CDMAperceived the problem of dropped calls as a problem with handover. The underlying causes ofdropped calls, whether occurring at handover or not, are in fact:
1) Inadequate radio coverage at the outer edges of the cell, or bad coverage areas due toshadowing by hills, buildings, etc.
2) Inadequate traffic capacity in a target cell.
The correct solution to problem 1 is to provide adequate radio coverage wherever necessary.The correct solution to problem 2 is to provide adequate traffic channel capacity in all cells ofthe system.
In a properly configured system with proper RF coverage (and any access technology, whetheranalog, CDMA or GSM) there are no significant dropped calls due to handovers. Handover in aGSM (or other TDMA) system has no inherent gap in the audio, because the carrier frequencychange occurs during the idle time slots of the TDMA frame, between two regularly scheduleduses of the mobile station’s assigned time slot. Therefore the digitally coded bit stream isusually received without interruption. This is called "seamless" handover.
A GSM handover between the old base station and the target base stations which differ indistance from the mobile station by over approximately 1 km theoretically requires some loss ofdigital data. This happens because the mobile must transmit a special short radio burst justafter changing to the new carrier frequency. The purpose of this short burst is to let the basestation measure the radio signal propagation delay and then send an appropriate timingadjustment signal to the mobile station. Theoretically, a frame of speech coding may be lostwhile the mobile station is engaged in this synchronization adjustment. The time duration ofeach data bit in GSM is about 3.7 microseconds. A timing change of less than 1.5microseconds is too small to affect the receiver timing adjustment. Since the radio signal delaydue to the speed of light is 3 microseconds per km, a timing accuracy specifying the delayaccurate to 1.5 microseconds, corresponding to an accuracy of ± 0.5 km, is adequate for thispurpose.
In practice, this theoretical gap in the data stream seldom if ever occurs for GSM or otherTDMA technologies. When the distance from the handover point18 (or the cell boundary line) tothe target base station is known by the system operator, a preset timing adjustment parameter
18 Some system operators set different handoff distances for different power classes of GSM mobile stations, sothat a different timing adjustment parameter may be used for these different power classes of mobile stations.
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can be used in the handover command sent to the mobile station. This parameter causesimmediate adjustment of the mobile transmitter burst timing during handover, andconsequently the transmission of the bit stream continues without interruption. This is the usual“seamless” handover for GSM. This occurs for most handovers in a properly engineered GSMsystem. The system engineers can easily find the proper value of the preset timing adjustmentparameter by performing a few trial handovers of the short burst type at several points alongthe mutual boundary between each two cells in the system. The timing adjustment determinedby the system in this case is the proper value to send to other mobile stations located at thisboundary when sending the handover command, to achieve a seamless handover.
When a handover occurs between the inner and outer part of an overlaid or tiered cell, thedistance from the base station is exactly the same both before and after the handover.Therefore, the handover command is preset to leave the timing adjustment unchanged, andthere is no possible gap in the received channel bit stream.
What about the worst GSM case? There are some cases in which short radio bursts must besent to establish synchronization. This occurs where the system operator has not sent apreset timing parameter because the boundary between two cells is not a relatively uniformdistance from the target cell. In particular, the distance to the target base station varies bymore than a kilometer at different places along the handover line. Examples of this occuralmost exclusively in very large rural cells in European or Australian 900 MHz GSMinstallations. Because the cells are smaller than 3 km radius in PCS-1900 systems, thissituation is practically unheard of in North America. Therefore, from a practical point of view,every handover which one will encounter in a North American PCS-1900 system will be aseamless (preset timing adjustment) handover. The “worst case” is almost never seen.
However, in such a worst case, some data is indeed lost, perhaps as much as a full speechdata frame of 20 milliseconds. But the speech coding system can bridge over relatively short(approximately 20 to 100 milliseconds) gaps in speech coder data. Audio sound is repeatedfrom the previously received accurate 20 ms speech coder data frame. The treatment of asingle 20 millisecond gap is almost imperceptibly good. Longer gaps produce sometimesperceptible effects on the speech, and sometimes imperceptible. There is no “click” or any ofthe artifacts which occur in an analog handover. In the worst case the continuity of speech isnot quite as good as a soft handover, but the worst case rarely occurs in a well engineeredsystem.
The problem of dropped calls due to lack of traffic capacity in the target cell or faulty RFcoverage can occur also in a CDMA system suffering from these flaws, and restoringperformance of such a CDMA system requires revising the RF coverage and cell channelcapacity of the system. Soft handover does not solve nor prevent this problem for CDMAsystems.
11.1 Penalties of Soft Handover:
Soft handover is an interesting and clever concept, but many critics view it as a excessivelyexpensive and complicated means (in terms of both operational resource and dollar cost) ofsolving an actually in-significant problem of competing digital technologies. It also produces acomplex system-wide interaction between cell power and RF coverage and cell capacity, whichis very difficult to optimize in real systems.
In order to perform soft handover, the total channel processing hardware and data linkhardware between all base stations and the central switch of a CDMA system must be
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provisioned with much greater capacity than for a hard handover system. In a typical CDMAinstallation, between 30% and 80% of all conversations at any time are engaged in a 2-base(or occasionally a 3-base) communication. That is, they are in the process of soft handover butthe connection to the original cell has not yet been broken. This is another one of the reasonswhy the cost of a CDMA system is typically 30% to 60% higher per conversation channel thanthe cost of GSM.
Thus, in an n=1 CDMA frequency plan, the cost of each base station can be as much as 60%to 100% greater than a GSM system. These cost comparisons are even less favorable toCDMA when data transmission is compared, rather than voice. Soft handover does not occurin CDMA installations having other frequency plans (for example n=4 or n=7) or for a WLLinstallation. In such installations, it is not necessary to install as much link hardware betweenthe base stations and the central switch. However, the cost of the base stations alreadyincorporates the extra hardware which was designed to handle soft handover, even though it isnot exercised in these latter installations, and the capacity is no longer superior to GSM.Therefore the cost of the system is still at least 30% greater (and often much greater indeed)per conversation channel than GSM or other technologies.
Another difficulty with soft handover arises from the interaction of the short term power controlfeedback loop. The base station is continually sending power control commands to the CDMAmobile station to attempt to compensate for fast fading. These power commands are based ona feedback control loop which orders the mobile to increase its power when there is a drop inreceived signal strength perceived at the base receiver, and vice versa. This is done essentialin order to minimize the near-far problem with signals from many different mobile stationsarriving at the base receiver.
Any inaccuracy in this control process is well known to cause a serious degradation of channelBER. However, when the mobile station is in a soft handover, it is communicating with two(and in some cases, three) different base stations. These two base stations are so far apartthat in general the time pattern of fading at the two base receivers is not correlated (that is,only 50% correlation or 0.5 magnitude of the cross correlation coefficient of the two receivedsignal strength profiles19). One base station receiver may experience a short term fade for thischannel, and thus will request that the feedback command should command the mobile stationto increase its power. At the same time the other base station may simultaneously experiencea brief peak of received power, and therefore wants to see a decrease in mobile transmitterpower. There are also moments in time when both base station experience simultaneousfades, and also moments when both experience simultaneous peaks of received power. Thesystem design of CDMA resolves these conflicting requests when they occur by commanding amobile transmitter power increase only when both base stations simultaneously request anincrease. In the other cases, when either one requests a decrease in mobile transmit power,the mobile station is commanded to decrease its transmitter power. This first requires that thecontrol signals be brought from both (or from three) base stations to the central controller atthe MSC, and then a power control decision is made there. The resulting command is sent outto both (or three) base stations, where it is transmitted via radio to the same mobile station.This requires low-delay high bit rate control links between all the base stations and the MSCmodules.
19 This is not the cross correlation of the two PN-PRBS bit streams, which is a completely different waveform. Thefluctuations in the received signal strength profile varies at a rate dependent on the motion (if any) of the mobilestation, but is typically about 2 or 3 fades per second for a pedestrian. The waveform fluctuations in the PN-PRBSoccur at 1.28 Mb/s as described previously.
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In addition, the outgoing digitally coded speech must be transmitted from the MSC to both (orthree) base stations. Furthermore, the incoming (uplink) digitally coded speech signal which isreceived at both base stations must be transmitted from there via digital channels to the CDMAcontrol module located at the MSC. At that location, the BER of each of the two receiveddigitally coded speech signals is evaluated, based on the error detection digital codes whichare normally included with the basic coded speech bits. If the two (or three) base receiversdiffer in estimated BER, only the better (or best) of the two (or three) is processed through thespeech decoder to be heard by the person at the other end of the conversation. This requiresadditional low delay high bit rate digital traffic links between all the base stations and the MSCmodules.
CDMA proponents claim that the selection of the better or best signal in this way is a form ofreceive diversity. Receive diversity is a method of increasing the received signal quality in thepresence of fading by selecting the better signal from two different receiving antennas, whichare located at different places and therefore seldom both have simultaneous fading. CDMAproponents claim that this compensates for the degradation of signal quality which occurswhen one of the base stations asks for a mobile power increase but instead gets a decreaseduring soft handover. CDMA opponents point out that this two-base-site diversity is not aseffective as the simpler switching diversity used in some GSM base station receivers. It is alsonot amenable to other even more effective types of receive diversity, such as equal gaindiversity or maximal ratio diversity.
There are several important differences between the two diversity systems which CDMAproponents try to present as equivalent. Here is a brief and simplified summary of the moreimportant differences:
In standard switching diversity, two receive antennas are located at the same base station.They are separated by typically 8 wavelengths or more, to produce un-correlated fading. Thestronger signal is selected. Nothing is done in this process to intentionally degrade the signalof either of the two receivers. No signals from a different cell site are involved.
In contrast to that, during CDMA soft handover, two receive antennas are located at a verylarge distance corresponding to two separate base stations. (In most installations, eachreceiver has two antennas and makes a pre-selection of the best signal before the furtherprocesses. We will ignore that for the moment.) The cross correlation coefficient of the fadingprofile for these two antennas is about the same as the cross correlation coefficient for twoantennas on the same base station. Further separation of the receiving antennas – aseparation of many km as opposed to several meters -- does not change the cross correlationcoefficient of the fading profiles. It is about 0.5 in either case. In contrast to standard switchingdiversity, the better signal from the two base stations is chosen based on estimated BERrather than signal strength. There is little argument about this aspect of the process. However,it does not increase the performance of the uplink channel significantly compared to usingstandard receive diversity at one base station. One reason for this is that on some occasionsthe signal strength at one of the two base receivers is degraded because of the design choicewhich forbids increasing the mobile transmitter power unless both base stations request thissimultaneously. There is a valid reason for this design choice. By always using a lower mobiletransmit power in case of conflicting fade conditions at different base stations, the level of RFinterference with other channels, which would occur in some cases when the mobile transmitpower is temporarily increased. But the final result is that there is a very slight improvement ofperformance and a very large increase in the complexity and cost of the system.
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Without going into all the details, there are valid cost and complexity reasons why the othermore effective diversity algorithms mentioned earlier (equal gain or maximal ratio) are not usedin a soft handover configuration. Their use would improve the receiver performance onlyslightly, but would increase the system cost far more. Upon reviewing all of these alternatives,one naturally questions the added system cost in terms of its payoff in system performance.This is one of the most questionable aspects of soft handover in the views of severalobservers.
Furthermore, if conventional diversity were not used at each cell, the performance of thesystem when a mobile set is not engaged in soft handover would be far inferior to competitivesystems. CDMA systems operating on the 1900 MHz band do not use n=1 frequency plans, sothey all use hard handover with no apparent problems. When everything is considered, itappears that a CDMA system designed specifically without soft handover (and thus without allthe extra hardware and processing capacity used exclusively by soft handover) would be adesirable modification of CDMA infrastructure. However, at this time all IS-95 CDMA systemshave soft handover built in to be compatible with the existing standard.
Finally, we should mention among the other penalties of CDMA soft handover, that in the 800MHz band, it is necessary to install CDMA base equipment in all the cells of a city, eventhough extra capacity may be needed in only a few cells of the system and dual mode (analog-CDMA) handsets are used. In contrast, in a 800 MHz band installation, IS-136 TDMA baseequipment may be installed in only those cells which need extra capacity, and standard dualmode analog-TDMA handsets may be used. The capital cost to the system operator is muchlower in the TDMA case. (Of course, there are no systems on the North American 800 MHzband using dual mode analog-GSM handsets with GSM base stations, so this point is notdirectly applicable to GSM.)
12 Implementation & Hardware
12.1 CDMA Technical and Engineering Effort:
Early on, claims were made by CDMA proponents that a CDMA system could be installed with"no frequency planning" (for n=1 frequency plans) and the claim was repeatedly made thatsignificantly less staff would be required since soft handover was claimed to compensate forsloppy or incomplete RF coverage. In fact, experience and field testing has shown that theeffort and complexity of adjusting RF coverage, particularly in an n=1 frequency plan, is equalto or far greater than the corresponding work to properly adjust RF coverage in a GSM system.
This is measured by the number of engineers and technicians needed, the need for detailedfield measurements of RF signal strength and quality, and the time required to make thenecessary adjustments. The cost of this effort, devoted to staff salaries, test equipment, andrelated support, is one important reason why the true operational cost of CDMA system ishigher than or equal to that of GSM systems.
12.2 Data Communication and Related Features
Both GSM and CDMA have certain basic data communication capabilities today. The mostsignificant of these is SMS. However, GSM already has circuit switched data communicationcapabilities including interworking with land-based data terminals and fax machines, whilethese capabilities are still in the works or in testing stages for CDMA. Industry standard packetswitched data communication is also appearing already for GSM.
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GSM system developments have included a number of specific improvements which allow fornon-proprietary interworking with various data bases and data systems. GSM permits thedownloading of software upgrades to the mobile set or to the SIM card which identifies thesubscriber. Sophisticated GSM handsets such as the Nokia 9000 permit one to carry a pocketcomputer with telephone and data communications capabilities. In general GSM is ahead ofCDMA in both the variety and the level of multi-vendor standardization of data communicationcapabilities.
Qualcomm announced in November 1998 a proprietary data communications packagedeveloped jointly with Microsoft, which requires a special handset and will be available onmany base systems in the coming months. This software package provides features similar toMicrosoft Outlook Explorer, such as a calendar, e-mail, contact manager, and the like, andmust be used with Microsoft software in other systems as well.
On a long term basis, the raw data communications capability for both circuit switched andpacket switched data communication will undoubtedly be available in both technologies. Thedifference in the design approaches of the two camps appears to be developing in thefollowing way. GSM developers appear to concentrate on industry standard datacommunications capabilities which act as platforms, on which many developers can use tosupport a variety of competitive software offerings. CDMA development appears to be focusedtoday on a complete vertically integrated software package. Although this discourages avariety of independent software developers to supply software to work with it, their hope isapparently that they can build on the near market monopoly of Microsoft in some areas. Whileit is true that Microsoft has a near monopoly on operating systems, the do not have the onlypopular schedule, e-mail and contact manager software, so the correctness of this partneringstrategy remains to be seen.
A further open question, in the mind of this author, is the actual attractiveness of elaboratedata communication and other features in a radio handset. Which data communicationcapabilities are useful and which are un-necessary? Due to its small physical size and the factthat most users are continually in motion when using their handset, many applications requiringa large display screen are impractical. Video as an entertainment and two-way conferencemedium in this context is a highly questionable product. Therefore, the more grandioseschemes in this area, and similar schemes for multi-media capability in the context of variousproposed 3G systems should be viewed with some care and even some skepticism. Perhaps asimple feed through of data communication to a separate device, such as a laptop computer,is all that is desired or needed. This would imply that the GSM approach is closer to the needsof the end user than the present CDMA approach.
12.3 Inherent vs. Specific Implementation Aspects:
Certain distinctive characteristics of the two technologies are inherent and difficult to surmountwithout great effort. One example of this is the sensitivity of CDMA to intermodulation (IM) fromnarrow-band interference. Another example is the incompatibility of overlaid cell technologywith CDMA. Other characteristics are not inherent in the technology but are the result of designchoices which, while not changeable in the present generation of equipment, could in theorybe modified in future generations of equipment.
One example of this is the high hardware cost of soft handover in CDMA, including the highoperational cost for renting or buying additional T-1 links between base stations and centralswitch. The future redesign effort for CDMA could in theory address these cost issues bydesigning a system with no hardware for soft handover in the base and mobile stations. This
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would still be applicable to frequency plans other than n=1 and for WLL or other fixed wirelessapplications. It is also possible that a redesign could address the cost of extra processinghardware which is present in CDMA base stations, but which is not needed in real installations.Since only about 18 channels can be supported per cell in a n=1 frequency plan, and onlyabout 40 channels for other frequency plans, provisioning hardware to process 64 totalchannels is excessive. It is possible that some of the hardware such as digital signalprocessors could be omitted in order to match the capability of the base equipment to theactual radio channel capacity. On the other hand, the clock rate of the digital signal processingequipment must still be high enough to handle the 1.28 Mb/s chip rate. The high clock rate alsoaffects power consumption as well. So it is not clear what cost reductions could be producedby design of a lower capacity base station. Design changes of this type, if feasible, wouldimprove the economic picture for such CDMA installations. In this connection, bear in mind,that when a frequency plan other than n=1 is used, the capacity advantage of CDMA over un-enhanced GSM (although it only exists when comparing to certain specific non-optimizedinstallations) is also is gone as well. However, for backward compatibility with many existinginstallations this is not currently a feasible alternative, since IS-95 compatibility demands allthis extra hardware and capacity.
Few aspects of the IS-95 Rate 2 redesign have been made public. One aspect of Rate 2redesign is to reduce the RF interference with hearing aids and broadcast radio and TVreceivers caused by CDMA mobile sets. Both CDMA and GSM sets cause some hearing aidinterference, but GSM "buzz" is simpler to remove by means of a simple frequency filter whilepresent CDMA "buzz" is wideband audio and difficult to filter out.
13 WLL Systems:Various types of cellular and PCS technologies have been used for WLL applications fromtime to time. The typical cost of traditional wired telephone installation (not including thetelephone switch, which is needed in either case) is about $400 capital cost per subscriber. Incontrast, most so-called "high tier" cellular and PCS systems such as CDMA and GSM have a$1000 to $5000 capital cost per subscriber for the base radio related equipment.
This high capital cost prevents these technologies from competing directly with wired serviceon a long term basis. Radio WLL systems are primarily useful in special situations where thelong installation time of traditional wire is a major political or public relations problem. Anotherspecial case for WLL occurs where there is a topographic problem which prevents wireinstallation, such as a canyon between the central switch location and the subscribers. A thirdapplication for WLL is to temporarily provide telephone service in an emergency, when regularwired service is disrupted by disasters. In many cases where topography is not the problem,WLL equipment has been installed first to give subscribers immediate service, and thentraditional wire has been installed later and service switched over to the wire. This has beendone repeatedly in so-called “third world” nations, such as East Germany immediately after re-unification.
Where permanent WLL service is desired, other technologies not mentioned in this reporthave a much lower cost per subscriber, and also a very limited radio range as well. In thesummary charts below, CDMA and GSM are considered relative to each other, in the intendedapplication of temporarily providing WLL service until traditional wire service can be provided.Neither one is economically recommended, in their presently available embodiments, as
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permanent WLL service vehicles. In fact, at the present time, the least costly WLL system onthe market is a TDMA system20.
14 Costs
14.1 Higher Cost of Extra CDMA Processing and Link Hardware:
To handle temporary traffic overloads and also to handle soft handover, a CDMA system isdesigned to also have two other properties which each have both benefits and undesirablecollateral effects. First, the base station is equipped with much extra processing capacity (morehardware using faster computer clock rates) and many more communication links (sometimesas much as 80% extra) between the base station(s) and the central switch. However, theseresources come at a high price, which is judged by many in the industry to be disproportionateto the performance gained. This high cost is primarily related to soft handover, and onlysecondarily related to more signal processing capacity than is actually useable. Unpublishedstatements by various operators in the industry indicate that the cost of GSM base equipmentis typically 30% to 60% more costly, per subscriber, than GSM or other technologies, evenwhen fully loaded with paying traffic.
14.2 Indirect Inferences Regarding Total Costs
The total capital and operational costs of CDMA (per end user) have not proven to be lowerthan GSM or other TDMA systems. This can also be inferred both directly and indirectly fromseveral secondary sources. One source is the training and provisioning documents for existingsystems, and the bulletin costs of hardware. This source is not unambiguous, because mostvendors make special price concessions to system operators based on gross purchasingvolume, delivery adjustments, and whether the equipment is part of the initial installation(usually at a lower unit price) or part of later added equipment (usually at a higher unit price).One may also infer from the number of engineers and technicians employed by CDMAoperators that the salaries and equipment costs for ongoing system engineering is usuallyhigher for CDMA than for GSM or other TDMA systems of similar size. In addition, at theconsumer pricing level, the price of CDMA service has, until recent pricing shakeups in thecellular and PCS business, been the highest in most markets. Consumers Reports magazine,a well-known neutral consumer watchdog publication, evaluated the cost of cellular service inthe 15 largest US cities in the summer of 1998. Using an average user’s activity profile,Consumers Reports magazine compared the most economical pricing package for all theservice providers in each city. The results indicated that CDMA service had the highest price inall but two of these cities. Most prices were in the range of $50 per month for 80 minutes ofmostly local air time (thus about 62 cents per minute). Most other features and capabilities,including roaming, short message service, etc., were priced extra with usage sensitive pricing.
Contemporaneous with the publication of that report, high volume customer cellular/PCSindustry pricing in the US was been strongly affected by the new Digital One Rate service ofAT&T Wireless, introduced in June, 1998. This plan is only sold to customers who reside in anarea served by AT&T Wireless where IS-136 base equipment is already installed. This pricingplan provides so-called “bucket” pricing at three levels: 600 minutes per month for $89.99,1000 minutes per month for $119.99, or 1400 minutes per month for $149.99. When fully
20 The Nortel-Ionica system is the current lowest price WLL system, although this status could change at any time.
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utilized, these give the consumer a price ranging from approximately 14 to 11 cents perminute. AT&T Wireless provides service via North American TDMA (IS-136 on both the 800MHz and 1900 MHz bands) where available, with backup service using 800 MHz band AMPSanalog cellular in areas where IS-136 base equipment is not installed. The preferred dual-banddual-mode IS-136 handset for this service is the Nokia model 6160. This service provides themost widespread geographical coverage of any bucket pricing plan in North America. Serviceis also provided in Canada and Mexico, although at higher prices (60 cents per minute21).These price choices all include long distance and roaming, short message service, voice mail,e-mail messages, caller ID, which are services otherwise offered for extra cost or usage-sensitive pricing under other pricing plans by AT&T Wireless and various other cellular/PCSservice providers.
This bucket pricing has been very successful for AT&T Wireless, bringing in a very largenumber of high volume customers, many taken away from the CDMA competitors. In August1998 both 1900 MHz CDMA operators, PCS Primeco and Sprint, responded by offering similar“bucket” pricing at 10 cents per minute, beginning at a $50 per month pricing level. The CDMAprice offerings also include extra services such as long distance, roaming, SMS, voice mail,etc. This competitive price offering from the CDMA operators has only partially stemmed theloss of high volume customers from the CDMA providers to AT&T, but it is only attractive tothose customers who confine their travel to the much more limited CDMA service areas ofthese two operators. Incidentally, Bell Atlantic Mobile Systems, another IS-136 serviceprovider, has also recently announced a similar bucket pricing plan at 10 cents per minute withnationwide coverage.
More central to the comparisons in this report, Omnipoint, a PCS-1900 service provider in NewYork, Florida, and part of southern New England, announced their bucket pricing plan in NewYork in September and modified it in December. They offer 6 price levels, with the longdistance component optional. Like other offerings, SMS and other traditionally “vertical”services are included. Their usage cost thus ranges from 63 to an industry low of 9.6 cents perminute, depending upon total monthly usage. Omnipoint has a much smaller total service area,mostly on the east coast.
Obviously, direct comparison of these different pricing plans is complicated by the fact thatonly AT&T has virtually complete North American coverage. Sprint and PCS Primeco andOmnipoint each have limited coverage areas in a few large cities. One of the ironies of theintentional non-interworking of CDMA handsets sold by Sprint and Primeco is that these twocarriers cannot back each other up in those areas where only one of them operates, thuspreventing them from offering a slightly larger coverage area. Perhaps they feel that they mustoffer a lower price than AT&T Wireless due to this disparity in national service coverage.
Meanwhile, the complicated mix of local service pricing plans mentioned earlier, which were ineffect before the introduction of bucket pricing, are still very much in use by customers whohave small call volume or who do not travel outside their home service area. With regard tothese prior pricing plans, the findings of Consumer Reports magazine is still true: CDMA is themost expensive service in most cities.
21 AT&T Wireless introduced an optional pricing modification for subscribers with heavy traffic to or from Canada inDecember, 1998. For a fixed supplement of $19.99 per month, a subscriber can make all or part of their calls underany one of the three aforementioned pricing plans to or from Canada, in addition to the USA.
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This history of pricing competition must be interpreted with care, because there are obviouslymany other factors which determine price, aside from the access technology used by thesystem. One confounding factor is clearly the higher capital cost paid by CDMA operators fortheir 1900 MHz band PCS licenses. The licenses for the 800 MHz band were issued in the1980s before the FCC commenced auction sales of licenses, and were substantially “free” tolicensees such as AT&T wireless. Although AT&T Wireless does use IS-136 on the 1900 MHzband in a few cities, this is a very minor factor in their capital costs. In most cities or areaswhere AT&T Wireless does not own the “A” band 800 MHz cellular license, they provideroaming service via billing agreements with the local “A” band 800 MHz licensee. There are avery few (mostly rural) areas where the local licensee (who in such cases often has onlyanalog base stations) charges higher roaming fees than AT&T’s retail price! It is widelyacknowledged that AT&T Wireless is actually losing money for such roaming service in thesefew areas. However, this is definitely an exceptional case.
It is true that the two CDMA providers have the added burden of the high FCC license auctionfee in their capital cost structure. However, one of the claims made in the early 1990s forCDMA was that its system cost per customer would be so much lower than other technologiesthat this would compensate for the license costs on the 1900 MHz band. This claim has notbeen heard in many years, and in fact it is certainly not true because the capital cost of CDMAbase equipment is significantly higher, per subscriber, than other technologies.
Another complicating factor in interpreting the inferred costs behind bucket pricing is that allsuch plans include nationwide long distance calling in the base rate. Omnipoint does not havea business affiliation with a nationwide long distance carrier, yet Omnipoint offers a lowerbucket price at the highest usage level than any other bucket pricing plan! Clearly AT&TWireless was able to exploit its position as a long distance network provider by folding longdistance costs, which are under its own control, into the total pricing package. However, Sprintis also a nationwide long distance network provider, and PCS Primeco is owned by aconsortium of telephone operating companies which have access to long distance rates whichare as low as any other in the industry. The Bell Atlantic (again an IS-136 technology provider)bucket price offering also includes nationwide long distance calling at the same 10 cent perminute price equal to the CDMA providers, and they are not a long distance provider.
One significant factor to note from the emergence of bucket pricing is that it was started by acarrier who uses TDMA technology, and that CDMA operators only responded to this changein pricing, albeit with a slightly lower price per minute. Industry observers are now watching fora further significant change in marketing. In the past, cellular and PCS services were offeredfrom a complicated menu of local pricing plans with many usage-sensitive options, free air timeat different off-peak days and hours, etc. Some industry observers stated sarcastically that oneobjective of this varied menu was to prevent direct pricing comparisons for equivalent servicemixes. The marketeers stressed such intangibles as voice quality, large radio area coverage,and reliability and reputation of the service provider. Price was not the selling point.
Bucket pricing is leading to highly price-competitive marketing of directly equivalent commodityservices. Local service offerings (no roaming) of the bucket pricing type, such as 300 minutesfor $24.95 (corresponding to 8.3 cents per minute) with all the optional features included,appear daily in local advertising by cellular and PCS carriers in many cities. If the industryraces into a true price competitive market with all features included, what will be theimportance of low cost radio access technology to the operating companies?
Some industry observers have recently stated that differences in technology costs were notthe determining factor of total costs for many system operators in the past, because retail
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cellular/PCS service prices included large amounts for sales promotions and customer support.Sales and billing costs still are extremely high in the cellular/PCS industry compared to wiredtelephone or other utilities. These observers have predicted that strong price competition in thefuture may lead to extensive cost cutting in the cellular/PCS industry. The high salescommissions and purchase subsidies of yesterday will be severely cut back, and lower costdirect marketing (order taking) will be used most extensively. Customers will be sold radiohandsets without subsidy. The number of service and billing options offered will be drasticallyreduced, and standard packages of features will all be included. In this predicted “bare bones”operation, the cost of operations, administration, maintenance and provisioning of the basetechnology system will become a much more significant part of the remaining total operatingcosts. This is where the cost differences between CDMA and TDMA will be most important. Itwill then be extremely interesting to observe which access technology can retain profitability iftheir competitive retail sales price continues to drop. Will the CDMA operators initiate the nextround of price competition, or will they be seriously injured by it?
15 Review of Conclusions:In the opinion of the present author, CDMA is not the primary technological (capacity related)nor the primary economic choice for cellular and PCS. The claims for higher system capacity inCDMA have not been proven in the field. The same factors which make CDMA moreexpensive for mobile systems also affect fixed WLL use. Although a redesign of existingCDMA equipment to eliminate soft handover could greatly reduce the costs, this is unlikely inthe near term due to requirements in the mobile cellular/PCS market for backward compatibilitywith existing IS-95 product already in the field. It is more realistic to anticipate a "strippeddown" CDMA WLL design in the more distant future, but even then, comparison of existingWLL systems of both the CDMA and TDMA technologies show equal quality of speech anddata but do not show a cost advantage for CDMA.
In the opinion of the present author, the numerous alternative technologies proposed for 3Gsystems, and their associated legal and business uncertainties, should not be significant indecisions regarding radio system technology at this time. Eventual de facto convergence onone of the existing technologies (with slight modifications and improvements) appears to be amore likely scenario than the introduction of a totally new 3G technology worldwide. GSMtechnology appears to be a good candidate for this role, based on the fact that it hasperformance and data communication capabilities equal to or better than other technologies,existing world wide installations (although admittedly rather thinly dispersed in North and SouthAmerica) and better capacity and cost figures than other technologies as well.
The major points of discussion are summarized in Table 3.
Present equipmentconfigurations noteconomicallyfavorable for smalltraffic cells.
FundamentalTechnicalProblems
No fundamentaltechnicalproblemsknown.
IM sensitivity canbe evaded in somebands where nostrong interference
IM sensitivity ofwide band low levelreceiver is aproblem in somebands. No
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is present. fundamental fix yetavailable. Notuseable with IUO.
Applicabilityfor WLL
Lowest costchoice.
Not low enoughcost forpermanentcompetitionwith wiredtelephone.
More costly forWLL than othertechnologies. Notlow enough cost forpermanentcompetition withwired telephone.
Cost Lowest cost percustomer, partlydue to highcapacity, partlyto competitivevendormarketplace.
Most expensiveequipment cost percustomer. Futuredesignimprovements andelimination of softhandover couldreduce cost, but notobvious thatreduction would becompetitive toGSM.
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Appendix 1: Frequency Dependence of FadingWhen a radio antenna is located in a multi path radio propagation zone, fading may occur atthis location over a range of frequency dependent on the multipath signal characteristics. Thedistribution of such fading frequencies in the radio spectrum is dependent on the particulardelay and amplitude of each component ray of various multi path radio rays in the vicinity ofthis antenna. The geometry and geography of each particular cell’s various radio reflectivesurfaces often produce significantly different spectral fading patterns in each part of the samecell. While there are many places in the cell area which experience fading at this particularfrequency, there are also other locations where there is a strong signal instead of a fade. Thishappens because the various multi path rays are in phase at these other locations, and theycombine there to produce a strong composite radio signal instead of a fade.
When there is a range of different delay times for the various rays, the fading occurs within aspectral range of frequencies called the coherence bandwidth. The coherence bandwidth of afade is approximately equal to the reciprocal of the delay spread. The depth of the fade (in dB)is greatest near the center of the coherence bandwidth and gradually decreases towards theedges of the coherence bandwidth. We state this fact without further mathematical proof. Forfrequencies outside of the coherence bandwidth, the various delayed radio rays are no longerclose enough to the particular phase angles which have combined at the central fadefrequency, and consequently they no longer cancel each other out. In the case where thedelay spread is very large (and thus the coherence fading bandwidth is small), using a radiosignal whose bandwidth is significantly greater than the bandwidth occupied by the fade willminimize the impact of fading on the power level of the signal. Only a fraction of the radiospectral power is affected by the relatively narrow band fade. However, even when the fadeoccurs over only a small part of the bandwidth of the signal, it may still produce a deleteriouseffect on the radio waveform, which then causes serious bit errors. In the case where the delayspread is very small (and thus the coherence fading bandwidth is very large), use of a wideband radio signal does not improve the performance, because the wide range of fadingfrequencies affects all the spectral components of the signal. Frequency hopping is often abetter and more practical way to deliver a radio signal in a bad multipath zone than designing atechnology with a wider signal bandwidth.
The delay (time) spread is measured in the following way. In a multi path radio environment,we generate a pulse (or, for experimental convenience, a step waveform) modulated radiocarrier waveform at the transmit location (such as the base transmitter). We then measure thetime delay for each multi path replica of the radio signal when it arrives at the receiver. Pulsesarrive with different amplitudes and delays, and a typical graphical representation of the powerlevel of several such delayed pulses is illustrated in Fig. A.1 (a).
The time scale in Figure A.1(a) does not include the initial time delay for a wave to travelbetween the transmitter and the location of the receiver, so the zero on this time scale doesnot represent the time the wave emerged from the transmit antenna. There is more than onechoice in the literature for a parameter to represent the amount of time delay spread. Themathematical standard deviation about the average (mean) delay time is frequently used as ameasure of delay spread, and this method is illustrated in Figure A.1. The received signal atthis location will have a rather deep fade at a particular frequency corresponding to the partialcancellation of several of the delayed waves. Although the power level of each wave is shownpositive in Fig A.1.a, some of these waves have inverted amplitude polarity, which isequivalent to a 180 degree phase shift. Inverted polarity occurs each time an electromagnetic
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wave is reflected from a conductive or dielectric (insulating) reflecting surface22. Therefore, anyray which has experienced an odd number of reflections is inverted in polarity.
As an example, the delay spread illustrated in Figure A.1.a is approximately 5 microseconds.This is typical of a relatively large cell in an urban environment, where delay spreads of asmuch as 16 microseconds have been occasionally observed. The corresponding fadecoherence bandwidth, shown in Fig. A.1.b, is 1/5 of a megahertz, or 200 kHz. A large delayspread indicates a fade which occurs within only a narrow coherence bandwidth within thespectrum, since the use of other frequencies away from the center of the coherence bandwidthwill not produce the precise combination of ray amplitudes and polarities which cause the fade.
Conversely, when there is very little delay spread, the coherence bandwidth is very wide.Consider the example with just two rays of equal amplitude but with one ray delayed by a halfcycle, or 180 degree phase difference, at the antenna location. At exactly 850 MHz, a 180degree phase difference between two carrier waves can be produced by a delay of only 0.59nanoseconds (0.00059 microseconds), which is half of the period of one cycle of an 850 MHzcarrier frequency oscillation. This hypothetical case has a very small delay spread, indeed, andtherefore it has a very large coherence bandwidth which is more than 1600 MHz. The fadethus can be observed at a single location over a range of frequencies almost from zerofrequency to about double the 850 MHz carrier frequency of interest. Such a small delayspread with two rays does not occur frequently in a cluttered urban area with many smallbuildings, but it can occur in an environment with a single large nearby reflecting surface, like abuilding or the side of a cliff. In such cases, the fade at that location occurs over such a widerange of frequencies that we cannot prevent the fade by designing a different radio technologywith a practical wider bandwidth. The use of error protection codes, frequency hopping and thelike are better for delivering a good signal.
Frequency hopping can be used to correct a fade like the previous example, because fades donot occur everywhere simultaneously. When a fade occurs on a particular carrier frequency ata particular location, we can get a stronger signal by making two types of changes. First, wecan move the antenna (typically about 1 /4, 3 /4, 5/4, or some other odd number of quarterwavelengths at that frequency) away from the location of the fade. Second, we can leave theantenna in the same location, but change to a different carrier frequency, which will producefades and also regions of higher received power at different locations in the cell. This secondmethod is one of the reasons for preferring frequency hopping to the use of a single widebandsignal in many applications. Frequency hopping combined with bit interleaving will generallyproduce a good overall received signal even when the antenna is stationary or moving veryslowly. This is important for the case of a hand held mobile set which moves only at pedestrianspeed or less.
When the GSM system operator uses optional frequency hopping, the mobile and basestations change the frequency of the channel used for their conversation in a pre-determinedmanner. The frequency hopping pattern typically goes through all of the different carrierfrequencies used in that cell, in a cyclic repeating pattern. There can be 20 or more suchcarrier frequencies in a high traffic cell. Even if there is a fade on one of these frequencies inone location that is so severe that 50% of the received digital bits are incorrect23, the result is
22 Only a wave which has reflected from a surface with high magnetic permeability would be non-inverted in electricfield polarity. This is a very rare circumstance in a natural environment.23 A 50% BER is the worst possible value. The hypothetical case of 100% BER implies that all error can be fullycorrected by merely replacing all binary 1s with binary zeros, and vice versa!
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approximately a 2.5% BER, which is at the upper end of the usable range. But GSM errorprotection coding can often support an acceptable voice conversation at this error level. Thereader should understand that some cells have a smaller quantity of carrier frequencies, orhave a different spectral pattern of fading, and therefore such a result cannot be guaranteed inall circumstances.
In contrast, use of a wideband signal like CDMA in a comparable wideband fade at a fixedlocation, would have at least part of its radio frequency spectrum wiped out. Depending uponthe bandwidth of the fade and the relationships of the faded frequencies at that location to thefrequency components of the CDMA signal, there could be a severe increase in BER, beyondthe level of the designed-in error protection coding. Again, this effect is very site dependent,and this description does not imply that the performance of CDMA is never satisfactory in thepresence of fading with a wide coherence bandwidth. An important factor in this comparison isthat this type of wideband CDMA fade may produce a very long string of consecutive bit errorswhile the corresponding coherence bandwidth might not produce this undesired result withfrequency hopping GSM.
Radio Signal Power
Time (microseconds)
0 10
Received Spectral Radio Power
Frequency (MHz)
Coherence bandwidthDelay time spread
5 840 841
a b
Figure A.1: Relationship Between Delay Spread and Fade Coherence Bandwidth
