Introduction CDMA

7/29/2019 Introduction CDMA

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1.0. Introduction

Mobile communications are rapidly becoming more and more necessary for everyday activities.

With so many more users to accommodate, more efficient use of bandwidth is a priority among

cellular phone system operators. Equally important is the security and reliability of these calls.

One solution that has been offered is a CODE DIVISION MULTIPLE ACCESS SYSTEM.

CDMA is one method for implementing a multiple access communication system. MULTIPLE

ACCESS is a technique where many subscribers or local stations can share the use of the use of a

communication channel at the same time or nearly so despite the fact originate from widely

different locations. A channel can be thought of as merely a portion of the limited radio resource,

which is temporarily allocated for a specific purpose, such as someones phone call. A multiple

access method is a definition of how the radio spectrum is divided into channels and how the

channels are allocated to the many users of the system.

Since there are multiple users transmitting over the same channel, a method must be established

so that individual users will not disrupt one another. There are essentially three ways to do this.


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2. History

The first generation of cellular systems, which include the AMPS (Advanced Mobile Phone

Systems), was introduced in the early 1980s. These systems used analog frequency modulation(FM) and have a frequency division multiple access (FDMA) based media access control (MAC)

architecture. Within a few years, market demands and capacity requirements began to grow

hitting the practical limitations. These limitations motivated the development of the second

generation cellular systems, which improved compatibility and accommodated higher capacity

than the first generation systems. These systems use digital modulation and processing

techniques. TDMA (Time Division Multiple Access), GSM (Global System for Mobile

Communication) and (narrowband) CDMA belong to the second generation systems. CDMA

was introduced in 1994, by Qualcomm, Inc. Using direct sequence code division multiple access,

it claimed to provide 10 times more capacity than analog systems far more than TDMA or GSM.

Today, CDMA is the basis to the third generation market in the United States and other places in

the world.

3. Back ground

A cellular system is called so because it divides the service area into small transmission areas

called cells. Each cell contains a base station (BTS), which consists of a transceiver and a

receiver in order to connect to mobile phones in the cell. Each cell is assigned a group of radio

channels (frequencies).Covering the area with small cells improves capacity and serves more

users. Since the transmitters need to serve only a small area, low power transmitters are suffice.

The low transmission power enables reuse of frequencies in cells with sufficient distance. As the

mobile user goes through one cells boundaries, he is handed off to another cell area. This is done

without interrupting to the mobile call.

The BTS in each cell is connected to the Mobile Telephone Switching Office (MTSO), which is

responsible for switching calls between cell sites and wire line central offices.

For mobile to wire line calls, as for mobile to other networks mobile calls, the path goes from the

MTSO to the PSTN (Public Switched Telephone Network), which is made up of local networks,

land or cellular.


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3. Multiple Access

Cellular systems divide a geographic region into cells where a mobile unit in each cell

communicates with a base station. The goal in the design of cellular systems is to be able to

handle as many calls as possible (this is called capacity in cellular terminology) in a given

bandwidth with some reliability. There are several different ways to allow access to the channel.These include the following.

frequency division multiple-access (FDMA) time division multiple-access (TDMA) time/frequency multiple-access random access code division multiple-access (CDMA)

o frequency-hop CDMAo direct-sequence CDMAo multi-carrier CDMA (FH or DS)

As mentioned earlier, FDMA was the initial multiple-access technique for cellular systems. In

this technique a user is assigned a pair of frequencies when placing or receiving a call. One

frequency is used for downlink (base station to mobile) and one pair for uplink (mobile to base).This is called frequency division duplexing. That frequency pair is not used in the same cell or

adjacent cells during the call. Even though the user may not be talking, the spectrum cannot be

reassigned as long as a call is in place. Two second generation cellular systems (IS-54, GSM) use

time/frequency multiple-access whereby the available spectrum is divided into frequency slots(e.g., 30 kHz bands) but then each frequency slot is divided into time slots. Each user is then

given a pair of frequencies (uplink and downlink) and a time slot during a frame. Different users

can use the same frequency in the same cell except that they must transmit at different times.

This technique is also being used in third generation wireless systems (e.g., EDGE). FDMA andTDMA are narrowband technologies, and CDMA is wideband.

In the FDMA technology, signals from various users are assigned different frequencies. When a

frequency channel is assigned to a user, no other user of the same cell or in the neighboring cell

can use it at the same time.

In the TDMA technology, the information from each user is conveyed in time intervals called

time slots. A few users using a different time slot might share the same frequency. When all the

available time slots in a given frequency are used, a new user connecting to the system must be

assigned a time slot on a different frequency.

In a way, TDMA is very similar to a computer with only one processor that seems to run

multiple processes simultaneously. Only one person is actually using the frequency channel at

any given moment, and then has to give up the channel to allow other users to use it


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3.1.FREQUENCY DIVISION MULTIPLE ACCESS

In this technique, the available bandwidth is split up into non-overlapping frequency bands and

these disjoint sub bands of frequency are allocated to the different users on a continuous time

basis. In order to reduce interference between users allocated adjacent channel bands, channelbands are used to act as buffer zones, as illustrated in figure(1). These guard bands are necessary

because of the impossibility of achieving ideal filtering for separating the different users. It could

be compared to AM or FM broadcasting radio where each station has a frequency assigned.

3.2. TIME DIVISION MULTIPLE ACCESS

In this technique, each user is allocated the full spectral occupancy of the channel, but only for a

short duration of time called time slot. Buffers zones are in the form of guard times are inserted

between the assigned time slots. This is done to reduce interference between users by allowing

for time uncertainty that arises due to system imperfections, especially in synchronization

scheme.


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4. CDMA

This is a hybrid combination of FDMA and TDMA. For example, frequency hopping may be

employed to ensure during each successive time slot, the frequency bands assigned to the users

are recorded in random manner. During time slot 1, user 1 occupies frequency band 1, user 2

occupies frequency band 2, user 3 occupies band 3 and so on. During time slot 2, user 1 hops to

frequency band 3, user 2 hops to band 1, user 3 hops to band 2, and so on. An important

advantage of CDMA over FDMA and TDMA is that it can provide for secure communication.

4.1. MEANING OF CDMA:

Here, the users are spread across both frequency and time in the same channel. Here, unique

digital codes, rather than separate RF frequencies or channels are used to differentiate

subscribers. The codes are shared by both the mobile stations (cellular phone) and the base

station, and are called pseudo random code sequences or pseudo-noise code sequences.

4.2. PNSEQUENCE:

A PN sequence is a periodic binary sequence with a noise like waveform that is usually

generated by means of a feedback shift register. pseudo word is used, as these are not real

noise. These are noise like.

4.3. BASIS OF CDMA:

Basis of CDMA is the spread spectrum technology.

SPREAD SPECTRUM is a means of transmission in which the data sequence occupies a

bandwidth in excess of the minimum bandwidth necessary to send it. Spread spectrum is

accomplished before transmission through the use of a code that is independent of the datasequence (PN).

It can provide secure communication in hostile environment such that the transmitted signal is

not easily detected or recognized by unwanted listeners. It can reject interference whether it is

the unintentional interference by another user simultaneously attempting to transmit through the

channel, or the intentional interference by a hostile transmitter attempting to jam the


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transmission. Another application is in multiple access communication in which a number of

independent users can share a common channel without an external synchronizing mechanism.

4.3.1. GENERAL THEORY:

The highest energy components occupy lowest part of the spectrum. Therefore, the signal can be

treated as band limited signal and the higher frequency components can be disregarded.

For the 44.1 kHz sampling rate the signal cannot exceed 7.4 kHz. Increasing the data symbol

duration is one way to squeeze the frequency spectrum; another is limiting the number of PN -

sequence chips per data symbol. However, it needs to be taken into account that the less PN

chips per data symbol the harder it would be to synchronize the PN-sequences in the receiver. At

this point would be rather difficult for designer to use specific formula to determine all of the

crucial variables. Rather designer has to intuitively pick best combination of data symbol

duration, number of PN sequence chips per data symbol and carrier frequency.

If the data symbol duration 64 samples then it is going to occupy frequency band of 689

Hz

442000 Hz/64 =690 Hz

If theres eight chips of PN-sequence per data symbol, the freq. spectrum is going to spread by

the factor of 8. Thus, the transmitted signal will occupy band of 5525 Hz, which is well below

the limit. The difference provides comfortable margin, which allows more flexibility in

designing low pass filter.

The safest way to choose the carrier frequency is to make it equal exactly to one third of the

highest frequency. Therefore carrier freq. was chosen to be 7.3 kHz.

Once those values are determined the actual coding can begin.

4.3.2. TRANSMITTER:

Modulating Scheme

Probably the most important issue in building transmitter and receiver is the choice of

modulating scheme. There are many different ways in which modulation can be done. Each ways

has its advantages and disadvantages. Among primary concerns are the following: speed of the

transmission, spectral efficiency, and energy required to transmit the signal, sensitivity to


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disruption form background noise and cost efficiency. Since both transmitter and receiver will be

supplied implemented on the TX-54 DSP card, the energy factor does not have to be taken into

consideration Due to the nature of the CDMA the spectral efficiency are not relevant to this

project. The speed of the transmission is also a non-essential factor in this project. What remains

are the resistance to noise and the simplicity/cost effectiveness of the design.

Carrier Generator

Another important decision to be made is the implementation of the carrier generator. This can

be done in two ways: either the cosine function can be generated externally and inputted into the

transmitter or it can be generated internally by reading table of values. The latter solution

provides more flexibility; it provides possibility of internal adjustment of the carrier frequency

by the program itself.

4.3.3. GENERATING THE CDMA SIGNAL:

In the CDMA technique the signal transmissions among the multiple users completely overlap in

both time and frequency. The separation between the users is made by assigning each user a

unique code. Generally, CDMA converts analog voice signal to a digital signal, encodes the

digital signals, and separates voice and control data into data streams called channels.

Generating a CDMA signal is a four steps process:

The first step is analog to digital conversion or A/D. The incoming voice signal is an analog

signal meaning that it is changing constantly, taking on all possible values of amplitude range.

The CDMA uses a digital signal for its further manipulations. That digital signal is characterizedby discrete states. In that step the analog voice signal is quantized to form a digital signal

consists of a few levels.

The second step is voice coding or Vocoding. Voice encoding is the process of compressing the

audio into as small a stream of bits as possible. The vocoder takes advantage of the pauses in

speech to accomplish maximum compression. The Vocoders rate must be variable to fit the rate

of the users speech activity.

The third step is encoding and interleaving. This step purpose is to reduce the errors when

receiving the signal.

Interleaving is a method of reducing the effects of burst errors and recovering lost bits. Thesymbols are interleaved such that originally neighboring symbols will be transmitted far away

from each other. In addition to that, the various encoding methods add redundancy to the signals

to help the recovery of information at the receiver in case of errors.


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The forth step is channelizing.

The signal of each user if further encoded to create a separation between different users. A

unique identification code is given to each user and the signals of all users are transmitted

together, sharing the same frequency and time. The CDMA receiver decodes the signal bymultiplying it by a decoding sequence of the desired user.

Two common codes types are Walsh code and PN (pseudo random noise) code:

The Walsh code is used for forward CDMA channel (e.g. cell to mobile direction of

communication). Walsh codes are orthogonal, meaning that the code of each user can be decoded

at the receiver only by using the same Walsh code used to transmit the signal.

The PN code is used for reverse CDMA channel (e.g. mobile to cell direction of

communication). A series of digital signals 0s and 1s goes into an antipodal mapping device to

produce bit stream of negative and positive 1s. Each user signal is then multiplied by the PN

code series. At this moment the signals occupy a wide frequencies spectrum. The PN sequence

rate is much higher than that of the original signal. It is generated in a deterministic manner, and

is repetitive. However there are about 4.4 trillion combinations of this code, and for practical

purposes we may assume that this sequence is truly random.


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5. TYPES OF SPREAD SPECTRUM

Different spread-spectrum techniques are distinguished according to the point in the system at

which a PRN is inserted in the communication channel. This is very basically illustrated in the

RF front-end schematic in Figure 1

Figure 1. Several spreading techniques are applied at different stages of the transmit chain.

If the PRN is inserted at the data level, this is the direct-sequence form of spread spectrum

(DSSS). (In practice, the pseudo-random sequence is mixed or multiplied with the informationsignal, giving an impression that the original data flow was "hashed" by the PRN.) If the PRN

acts at the carrier-frequency level, this is the frequency-hopping form of spread spectrum(FHSS). Applied at the LO stage, FHSS PRN codes force the carrier to change or "hop"

according to the pseudo-random sequence. If the PRN acts as an on/off gate to the transmitted

signal, this is a time-hopping spread-spectrum technique (THSS). There is also the "chirp"

technique, which linearly sweeps the carrier frequency in time.

One can mix all the above techniques to form a hybrid spread-spectrum technique, such as DSSS

+ FHSS. DSSS and FHSS are the two techniques most in use today.

5.1. Direct-Sequence Spread Spectrum (DSSS)

With the DSSS technique, the PRN is applied directly to data entering the carrier modulator. The

modulator, therefore, sees a much larger bit rate, which corresponds to the chip rate of the PRN

sequence. Modulating an RF carrier with such a code sequence produces a direct-sequence-

modulated spread spectrum with ((sin x)/x) frequency spectrum, centered at the carrier

frequency.


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The main lobe of this spectrum (null to null) has a bandwidth twice the clock rate of the

modulating code, and the side lobes have null-to-null bandwidths equal to the code's clock rate.

Illustrated in Figure 13 is the most common type of direct-sequence-modulated spread-spectrum

signal. Direct-sequence spectra vary somewhat in spectral shape, depending on the actual carrierand data modulation used. Below is a binary phase shift keyed (BPSK) signal, which is the most

common modulation type used in direct-sequence systems.

Figure 13. Spectrum-analyzer photo of a DSSS signal. Note the original signal (non spread)

would only occupy half of the central lobe.

DS sequence allows each station to transmit over the entire frequency

Spectrum all the time. Multiple simultaneous transmissions are separated using some sort ofcoding technique that is each user is assigned a chip sequence. The sender and receiver

synchronize by the receiver locking into the chip sequence and the sender and receiver locking

into the chip sequence of the sender. All the other (unsynchronized) transmission is then seen as

random noise. So with CDMA each user uses the full frequency spectrum. They employ a high

speed code sequence along with the basic information being sent, to modulate their RF carriers.

The high speed code sequence is used directly setting the transmitted RF bandwidth. Binary


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phase shift keying (BPSK) is the most common technique used in DS system. Direct sequence is,

in essence, multiplication of a more conventional communication waveform by PN sequence in

the transmitter.

5.2. FREQUENCY HOPPING SPREAD SPECTRUM

FHCDMA is a kind of spread spectrum technology that enables many users to share the same

channel by employing a unique hopping pattern to distinguish different users transmission. The

type of spread spectrum in which the carrier hops randomly from one frequency to another is

called FH spread spectrum. A common modulation format for FH system is that of M-ary

frequency shift keying (MFSK).the combination is referred to as FH/MFSK.

A major advantage of frequency hopping is that it can be implemented over a much larger

frequency band than it is possible to implement DS- spreading, and the band can be

noncontiguous. Another major advantage is that frequency hopping provides resistance to

multipleaccess interference while not requiring power control to prevent nearfar problems.

In DS systems, accurate power control is crucial but becomes less effective as the carrier

frequency is

increased.

Frequency hopping does not cover the entire spread spectrum

Instantaneously, we are led to consider the rate at which the hops occur. So, we may identify two

basic characterizations of frequency hopping.

1. Slow frequency hopping, in which the symbol rate Rs of MFSK signal is an integrator multiple

of the hop rate Rh . That is, several symbols are transmitted on each frequency hop

2. Fast frequency hopping, in which the hop rate Rh is an integrator multiple of the MFSK symbol

rate Rs. that is, the carrier frequency will change or hop several times during the transmission of

one symbol.

The FHSS method does exactly what its name impliesit causes the carrier to hop fromfrequency to frequency over a wide band according to a sequence defined by the PRN. The speed

at which the hops are executed depends on the data rate of the original information. One can,

however, distinguish between fast frequency hopping (FFHSS) and low frequency hopping

(LFHSS). The latter method, the most common, allows several consecutive data bits to modulate

the same frequency. FFHSS is characterized by several hops within each data bit.
http://www.maximintegrated.com/glossary/definitions.mvp/term/DSSS/gpk/96http://www.maximintegrated.com/glossary/definitions.mvp/term/DSSS/gpk/96


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The transmitted spectrum of a frequency-hopping signal is quite different from that of a direct-

sequence system. Instead of a ((sin x)/x)-shaped envelope, the frequency hopper's output is flat

over the band of frequencies used The bandwidth of a frequency-hopping signal is simply N

times the number of frequency slots available, where N is the bandwidth of each hop channel.

Figure. Spectrum-analyzer photo of a FHSS signal.

Time-Hopping Spread Spectrum (THSS)


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Figure. THSS block diagram.

Figure illustrates THSS, a method not well developed today. Here the on and off sequences

applied to the PA are dictated according to the PRN sequence.


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6. Handoffs in CDMA

CDMA systems support handoffs of the mobile from one cell to another while the mobile is in

the idle state, the Access state, or the Traffic Channel state:

1. IdleTransition from one cell to another while in the idle state must be a hard handoff.2. AccessHandoffs during Access are permitted only in TIA/EIA-95, but not in IS-95A.3. Trafficthe in-traffic transition from one cell to another can be either a soft handoff or a

hard handoff.

6.1 Idle Handoff

While in the idle state, the mobile may move from one cell to another. Idle handoff arises from

the transition between any two cells. Idle handoff is initiated by the mobile when it measures a

Pilot signal significantly stronger (3 dB) than the current serving Pilot.


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6.2 Handoff during Access

Handoff in the Access state is specifically prohibited in IS-95A. This prohibition made access

processes easier to implement during the initial development of the early CDMA systems.

Performance was sacrificed for simplicity.

However, Access failures in the handoff region were a significant performance deficiency, andTIA/EIA-95 includes the following handoff techniques to improve performance:

Access entry handoff

Access probe handoff

Access handoff

Channel assignment into soft handoff1.3 Traffic Channel Handoffs
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6.3. Types of Handoff:

6.3.1 Mobile assisted Soft Handoff

Soft handoff is the process of establishing a link with a target cell before breaking the link with a

serving cell. Mobiles continuously search for Pilot Channels on the current frequency, to detect

potential candidates for handoff.

Both Cells must be on the Same Frequency

The mobile typically contains only one RF receiver. Therefore soft handoff requires that both the

serving and the target cells be transmitting on the same CDMA frequency.

All Cells Deliver Vocoded Frames to the BSC

Each Base Transceiver Station (BTS) participating in a soft handoff transmits identical frames.

The mobile combines these frames and then forwards a single frame to the vocoder. On the

Reverse link, each BTS independently decodes and then delivers vocoded frames to the Base

Station Controller (BSC).
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6.3.2 Softer Handoff

Softer handoffis a soft handoff between two sectors of the same cell. Signals received by

different sectors can be combined by the rake receiver in the BTS. It should be noted, however,

that only one voice frame is eventually forwarded to the BSC. Softer handoff enables greater

efficiency in the use of hardware since only one Channel element is used to support such a

handoff.

6.3.3 Soft-Softer Handoff:

Multiple cells and sectors may be involved in a handoff in a variety of ways. The figure depicts a

scenario where a mobile is in softer handoff with two sectors of one cell and is also in softhandoff with another cell. The BSC will receive a vocoded frame from each cell and choose the

error-free one.
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6.3.4 Hard handoff

A hard handoffentails a brief disconnection from a current serving cell prior to establishing a

connection with a target cell. Hard handoffs can occur for several reasons.

The figure illustrates a hard handoff from a CDMA system to an analog system. Hard handoffs,

however, may also occur between CDMA cells. CDMA-to-CDMA hard handoffs are due to

frequency mismatches, frame offset misalignment, or disjoint cells (cells served by different

BSCs).
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6.4. Pilot Searching Process

6.4.1 The Mobile Searches for Strong Pilot Signals

The searching process is continuous and is conducted not only to find handoff candidates, but

also to identify usable multi path arrivals from the serving cell.

6.4.2 The Mobile Reports

The handoff process is mobile-assisted, meaning that when the mobile detects a Pilot of

sufficient strength, it reports the event to the Base Station.

6.4.3 The Base Station Directs

When the Base Station receives a report from the mobile, a handoff decision is made and

directions are sent to the mobile to perform the handoff.
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6.5 Pilot Set

Pilots are grouped into four sets, which prioritize them and increase the efficiency of searching.

The searching process is not standardized, but generally Pilots are searched in the following

order:

6.5.1 Active SetPilot Channels associated with forward Traffic Channels currently assigned to

the mobile. This is a search for additional multi paths of the same Pilot Channels.

6.5.2 Candidate SetPilot Channels whose strength, as measured by the mobile, exceeds a

given threshold.

6.5.3 Neighbor SetPilot Channels transmitted by cells in the vicinity of the cells currently

transmitting to the mobile. The contents of the Neighbor Set are normally configured by the

system operator, by means of the Neighbor List Message.

6.5.4 Remaining SetAll other Pilot Channels that are possible within the current system.

This search is conducted to allow the system to configure itself as well as to account for special

coverage spots within the cell.

Search WindowsThe system operator determines the size of the search windows used by the mobile. Searching

over a window of chips accommodates unpredictable changes in propagation delay due tovarying multipath conditions and propagation delay differences between the servingcells and

other cells that may be useful in the future.
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Multi path Arrivals:

The figure depicts the signals arriving from three different cells. The horizontal axis is time, in

PN chips. The vertical axis is the Pilot signal-to-noise ratio, Ec/I0, in dB.
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6.6 Handoff Signaling Messages

Pilot Strength Measurement Message (PSMM)

Handoff Direction Message (HDM)

Handoff Completion Message (HCM)

6.7 Transition between Pilot Sets

This graph illustrates the soft handoff process. The steps shown in this diagram are:

1. Pilot 2>T_ADD.MS sends PSMM (Pilot Strength Measurement Message) and addsPilot 2 to the Candidate Set.

2. Pilot 2>Pilot1+T_COMP*0.5. MS sends another PSMM. BTS decides to add Pilot

2 to the Active Set and sets up the soft handoff.3. MS receives message and moves Pilot 2 to the Active Set.

4. Pilot 1


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6.7 HandoffProblems: Window Dropped Calls

Calls are dropped during Hand off is due to:

Calls often drop when strong neighbors suddenly appear outside the neighbor

search window and cannot be used to establish soft handoff.

Neighbor Search Window SRCH_WIN_N should be set to a width at least twice the

propagation delay between any site and its most distant neighbor site

Remaining Search Window SRCH_WIN_R should be set to a width at least twice

the propagation delay between any site and another site which might deliver occasional

RF into the service area


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7. Power in CDMA

CDMA is interference limited multiple access system. Because all users transmit on the same

frequency, internal interference generated by the system is the most significant factor in

determining system capacity and call quality. The transmit power for each user must be reduced

to limit interference, however, the power should be enough to maintain the required Eb/No

(signal to noise ratio) for a satisfactory call quality. Maximum capacity is achieved when Eb/No

of every user is at the minimum level needed for the acceptable channel performance. As the MS

moves around, the RF environment continuously changes due to fast and slow fading, external

interference, shadowing, and other factors. The aim of the dynamic power control is to limit

transmitted power on both the links while maintaining link quality under all conditions.

Additional advantages are longer mobile battery life and longer life span of BTS power

amplifiers.

Open loop power control is the ability of the UE transmitter to sets its output power to a specific

value. It is used for setting initial uplink and downlink transmission powers when a UE is

accessing the network. The open loop power control tolerance is 9 dB (normal conditions) or

1 conditions)

Inner loop power control (also called fast closed loop power control) in the uplink is the ability

of the UE transmitter to adjust its output power in accordance with one or more Transmit Power

Control (TPC) commands received in the downlink, in order to keep the received uplink Signal-

to-Interference Ratio (SIR) at a given SIR target. The UE transmitter is capable of changing theoutput power with a step size of 1, 2 and 3 dB, in the slot immediately after the TPC_cmd can be

derived. Inner loop power control frequency is 1500Hz.


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8. Conclusion

In this technical we have stated about the CDMA technology and its functionality

in the real world. The rapid and efficient deployment of new wireless data andInternet services has emerged as a critical priority for communications equipment

manufacturers. Network components that enable wireless data services are

fundamental to the next-generation network infrastructure. Wireless data services

are expected to see the same explosive growth in demand that Internet service and

wireless voice services have seen in recent years.


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9. Bibliography

Google.com Scribd Wikipedia CDMA technology.com UMTS

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