HISTORY OF CDMA DEVELOPMENT

In the late 1800's, a Scottish physicist named James Clerk Maxwell formulated a principle that

would forever change our world. Maxwell was able to show that the generalized forms of the

laws of electricity and magnetism (the laws of Coulomb, Gauss, Biot-Savart, Ampere, and

Faraday) suggested the existence electromagnetic (EM) waves. Electromagnetic waves have

both an electric and magnetic field component that propagate through space, similar to how a

sound wave propagates through air or water. Maxwell's theory has since proven true and has

been put to great use. His work catalyzed the development of EM wave transmitters and

receivers, eventually leading to the creation of mobile phones.

Today, there are two major systems which play an important role in both current and future

mobile technologies. Those systems are the Global System for Mobile Communications (GSM),

originated in Europe and the Code Division Multiple Access (CDMA) Scheme, developed in the

United States.

CDMA became commercially available only in the mid-1990s. However, the origin of CDMA can

be traced back to 1940. The roots of CDMA technology are in the military field and navigation

systems. Originally developed to counteract intentional radio jamming, it was later proved to be

suitable for cellular communications.

CDMA has its roots in pre World War 2 America. In 1940, Hollywood actress turned inventor

Hedy Lamer and co-inventor George Antheil, with WW2 looming, co-patented a way for

torpedoes to be controlled by sending signals over multiple radio frequencies.

In 1949, John Pierce wrote a technical memorandum that described a multiple access system

that used a common medium that carries a coded signal that didn't need to be synchronized.

Later that year, Claude Shannon and Robert Pierce developed the basic operational ideas for the

CDMA scheme.

Despite all efforts by inventors to advance this technology from experiment to implementation,

the US Navy discarded their work as architecturally unfeasible.

The idea which was known as frequency hopping and later as frequency hopping spread

spectrum technique remained dormant until 1957 when engineers at the Sylvania Electronic

Systems Division in Buffalo, NY took up the idea and the Lamarr-Antheil patent expired, used it

to secure communication for the US during 1962 Cuban missile crisis.

After becoming an integral part of government security technology, the US military, in the mids-

80, declassified what's has now become CDMA technology, a technique based on Spread

Spectrum technology.

But CDMA's road was not a simple one. In fact, a panel of the world's leading engineers

reportedly met in Japan in the early 1990s to discuss the development of wireless CDMA

technology as a standard. They concluded, however, that it was impossible.

To the founders of Qualcomm, however, "impossible" simply meant that several thorny technical

issues needed to be overcome. With unstoppable entrepreneurial spirit, Qualcomm's team

solved them all, establishing CDMA as a legitimate wireless communications standard (and

patenting it to ensure their ownership).

Following these developments, other theoretical and technological discoveries were made that

led to Qualcomm's investigation into the use of CDMA techniques, beginning with the

introduction of narrow band CDMA IS-95 standards in July of 1993

Launched commercially in 1995, the first CDMA networks provided roughly ten times more

capacity than analog networks, and far more than TDMA or GSM. Besides supporting more

traffic, CDMA brought mobile carriers and consumers better voice quality, broader coverage

and stronger security, among other benefits.

CDMA EVOLUTION

CDMAOne

This describes a complete wireless system based on the TIA/EIA IS-95 CDMA standard,

including IS-95A and IS-95B revisions. It represents the end-to-end wireless system and all the

necessary specifications that govern its operation. CdmaOne provides a family of related

services including cellular, PCS and fixed wireless.

IS-95A: TIA/EIA IS-95 (Telecommunications Industry Association / Electronic Industries

Association Interim Standard - 95) was first published in July 1993. The IS-95A revision was

published in May 1995 and is the basis for many of the commercial 2G CDMA systems

around the world. IS-95A describes the structure of the wideband 1.25 MHz CDMA

channels, power control, call processing, hand-offs, and registration techniques for system

operation. In addition to voice services, many IS-95A operators provide circuit-switched

data connections at 14.4 kbps. IS-95A was first deployed in September 1995 by Hutchison

IS-95B: The IS-95B revision, also termed TIA/EIA-95, combines IS-95A, ANSI-J-STD-008

and TSB-74 into a single document. The ANSI-J-STD-008 specification, published in 1995,

defines a compatibility standard for 1.8 to 2.0 GHz CDMA PCS systems. TSB-74 describes

interaction between IS-95A and CDMA PCS systems that conform to ANSI-J-STD-008.

CDMA2000

CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes

CDMA2000 1x and CDMA20001xEV technologies. They deliver increased network capacity to

meet growing demand for wireless services and high-speed data services.

CDMA2000 is considered a 2.5G (or 2.75G) technology when the 1xRTT access network is used

and a 3G technology when the EV-DO access network is used.

CDMA2000 was the world's first 3G technology commercially deployed October 2000.

Shannon’s Capacity Equation

The core idea that makes CDMA possible was first explained by Claude Shannon. Shannon's

work relates amount of information carried, channel bandwidth, signal-to-noise-ratio, and

detection error probability; it shows the theoretical upper limit attainable.

 MODULATION USED IN CDMA SYSTEMS

In CDMA each user (the MS) is assigned a unique code (in radio environment terms, it is unique

spreading waveform). This unique code distinguishes different users in the radio environment.

We can draw an analogy between code in CDMA and frequency in FDMA and timeslot in TDMA.

Multiple-access is possible by spreading (simple multiplication) the information signal of the

user with his unique code. Each user in the environment transmits by spreading his information

with his unique code.

All users now transmit asynchronously in the same bandwidth achieving simultaneous access

on a shared communication channel. These codes are special and agree with the orthogonal

property of vectors. The receiver uses this property to differentiate the signals; this forms the

basis of the IS-95A standard.

This technique of spreading the user waveform (small bit rate) with code (fast bit rate) is called

Spread Spectrum, which otherwise would have just occupied very little bandwidth.

Spread Spectrum

A complete definition to Spread Spectrum is the one given by Haykins given below.

His definition is in two parts.

1.      Spread Spectrum is a means of transmission in which the data sequences occupy a

bandwidth in excess of the minimum bandwidth necessary to send it.

2.      Spread Spectrum is accomplished before transmission through the use of a code that is

independent of data sequences .The same code is used at the receiver to de-spread the received

signal so that the original data sequence may be recovered.

Concept of Spread Spectrum

When the information bearing signal and a PN sequence is multiplied at a multiplier we obtain

the desired modulation. The question is how do we get the increased spectrum? This is a simple

Fourier Transform property. Multiplication in time domain is convolution in frequency domain.

Hence by multiplying a narrow band information signal and a wideband code sequence, the

multiplied signal will have the spectrum similar to the wideband PN code sequence.

Spread Spectrum is typically of 2 types:

Direct Sequence (DS) and

Frequency Hopping (FH).

In Direct Sequence the information is spread over the transmit frequency, and in Frequency

Hopping the information hops across multiple carriers in the transmit spectrum in a pseudo-

random manner. IS-95A and the other standards use DS-CDMA. A DS-Spread Spectrum is shown

in figure 1.1 below.

Figure1.1: DS-Spread Spectrum   

Some points to be noted from the above diagram:

All users share the same BW.

Users are separated by a code, not a timeslot or frequency.

Each user is spread in the frequency domain

At the receive end, users are de-spread using their own unique code.

The user axis shows the strength of the cumulative addition of signals transmitted by all

users.

Major advantages of Spread Spectrum

1. Multipath rejection: ability to reject interference; be it intentional (some jamming

transmission) or unintentional (in this technique signal of one user interferes with another

user)

2. Multipath access: a number of users use a common channel for communication.

Synchronization

The success to a CDMA system is proper synchronization. To de-spread a spread spectrum signal

we need to use the same code used for spreading the signal. The operation takes place in two

stages acquisition and tracking. First we have an acquisition or coarse synchronization is done,

where we try to align the code in chips away from each other. In tracking or fine

synchronization we measure the correlation and bring the receiver code in synchronism with

the transmitted code.

Principles of CDMA Radio Technology DSSS

CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread

Spectrum communications is distinguished by three key elements:

The signal occupies a bandwidth much greater than that which is necessary to send the

information, which results in immunity to interference and jamming and multi-user access;

The bandwidth is spread by means of a code which is independent of the data;

The receiver synchronizes to the code to recover the data. The use of an independent code

and synchronous reception allows multiple users to access the same frequency band at the

same time. The same code is used at the receiver to de-spread the received signal so that the

original data sequence maybe recovered. In order to protect the signal, the code used is

pseudo-random. It appears random, but is actually deterministic, so that the receiver can

reconstruct the code for synchronous detection. This pseudo-random code is also called

pseudo-noise (PN).

Concept of Direct Sequence Spread Spectrum in CDMA Technology

There are three ways to spread the bandwidth of the signal:

Frequency hopping: The signal is rapidly switched between different frequencies within

the hopping bandwidth pseudo-randomly, and the receiver knows beforehand where to

find the signal at any given time.

Time hopping: The signal is transmitted in short bursts pseudo-randomly, and the receiver

knows before hand when to expect the burst.

Direct sequence: The digital data is directly coded at a much higher frequency. The code is

generated pseudo-randomly, the receiver knows how to generate the same code, and

correlates the received signal with that code to extract the data.

Figure1.2: Direct Sequence Spread Spectrum System

Signal transmission consists of the following steps:

A pseudo-random code is generated, different for each channel and each successive

connection.

The Information data modulates the pseudo-random code (the Information data is

“spread”).

The resulting signal modulates a carrier.

The modulated carrier is amplified and broadcast.

Signal reception consists of the following steps:

The carrier is received and amplified.

The received signal is mixed with a local carrier to recover the spread digital signal.

A pseudo-random code is generated, matching the anticipated signal.

The receiver acquires the received code and phase locks its own code to it.

The received signal is correlated with the generated code, extracting the Information data.

Implementing CDMA Technology

The following section describe how a system might implement the steps illustrated in Figure

1.2.

Input data

CDMA works on Information data from different possible sources with different data rates, such

as digitized voice or ISDN channels.

The system works with 64 Kbits/sec data, but can accept input rates of 8, 16, 32, or 64

Kbits/sec. Inputs of less than 64 Kbits/sec are padded with extra bits to bring them up to 64

Kbits/sec. For inputs of 8, 16, 32, or 64 Kbits/sec, the system applies Forward Error Correction

(FEC) coding, which doubles the bit rate, up to 128 Kbits/sec. The Complex Modulation scheme

(which will be discussed in more detail later), transmits two bits at a time, in two bit symbols.

For inputs of less than 64 Kbits/sec, each symbol is repeated to bring the transmission rate up

to 64 Ksymbols/sec. Each component of the complex signal carries one bit of the two bit

symbol, at 64 Kbits/sec, as shown in figure 1.3 below.

Figure1.3: Complex Modulation scheme

Generating Pseudo-Random Codes

For each channel the base station (BS) generates a unique code that changes for every

connection. The base station adds together all the coded transmissions for every subscriber.

The subscriber unit correctly generates its own matching code and uses it to extract the

appropriate signals.

In order for all this to occur, the pseudo-random code must have the following properties:

It must be deterministic; the subscriber station must be able to independently generate the

code that matches the base station code.

It must appear random to a listener without prior knowledge of the code (i.e. it has the

statistical properties of sampled white noise).

The cross-correlation between any two codes must be small.

The code must have a long period (i.e. a long time before the code repeats itself).

Code Correlation

In this context, correlation has a specific mathematical meaning. In general the correlation

function has these properties:

It equals 1 if the two codes are identical

It equals 0 if the two codes have nothing in common

Intermediate values indicate how much the codes have in common. The more they have in

common, the harder it is for the receiver to extract the appropriate signal.

There are two correlation functions:

Cross-Correlation: The correlation of two different codes. This should be as small as possible.

Auto-Correlation : The correlation of a code with a time-delayed version of itself. In order to

reject multi-path interference, this function should equal 0 for any time delay other than zero.

Note: The receiver uses Cross-correlation to separate the appropriate signal from signals meant

for other receivers, and Auto-correlation to reject multi-path interference.

Pseudo-Noise (PN) Spreading

The FEC coded Information data modulates the pseudo-random code, as shown in Figure 1.4a.

Figure 1.4a Pseudo-Noise Spreading

Figure 1.4 b Frequency Spreading

Some terminology related to the pseudo-random code:

Chipping Frequency (fc): the bit rate of the PN code.

Information rate (fi): the bit rate of the digital data.

Chip: One bit of the PN code.

Epoch: The length of time before the code starts repeating itself (the period of the code).

The epoch must be longer than the round trip propagation delay (The epoch is on the

order of several seconds).

Figure 1.4b, shows the process of frequency spreading. In general, the bandwidth of a digital

signal is twice its bit rate. The bandwidths of the information data (fi) and the PN code are

shown together. The bandwidth of the combination of the two, for fc>fi, can be approximated by

the bandwidth of the PN code.

Processing Gain

An important concept relating to the bandwidth is the processing gain (Gp). This is a theoretical

system gain that reflects the relative advantage that frequency spreading provides. The

processing gain is equal to the ratio of the chipping frequency to the data frequency:

There are two major benefits from high processing gain:

Interference rejection: the ability of the system to reject interference is directly

proportional to Gp.

System capacity: the capacity of the system is directly proportional to Gp.

Therefore the higher the PN code bit rate (the wider the CDMA bandwidth), the better the

system performance.

Transmitting Data

The resultant coded signal next modulates an RF carrier for transmission using Quadrature

Phase Shift Keying (QPSK). QPSK uses four different states to encode each symbol. The four

states are phase shifts of the carrier spaced 90_ apart.

Figure 1.5a Complex Modulator

Figure 1.5b Complex Modulation

By convention, the phase shifts are 45, 135, 225, and 315 degrees. Since there are four possible

states used to encode binary information, each state represents two bits. This two bit “word” is

called a symbol. Figure 1.5a&b shows in general how QPSK works.

Complex Modulation

Algebraically, a carrier wave with an applied phase shift, Y(t), can be expressed as a sum of two

components, a Cosine wave and a Sine wave, as:

I(t) is called the real, or In-phase, component of the data, and Q(t) is called the imaginary, or

Quadrature-phase, component of the data. This will result in two Binary PSK waves superimposed.

These are easier to modulate and later demodulate.

This is not only an algebraic identity, but also forms the basis for the actual

modulation/demodulation scheme. The transmitter generates two carrier waves of the same

frequency, a sine and cosine. I(t) and Q(t) are binary, modulating each component by phase shifting

it either 0 or 180 degrees. Both components are then summed together. Since I(t) and Q(t) are

binary, they will be denoted as simply I and Q.

The receiver generates the two reference waves, and demodulates each component. It is easier to

detect 180_ phase shifts than 90_ phase shifts. The following table summarizes this modulation

scheme. Note that I and Q are normalized to 1.

Table 1.1 Modulation scheme

For Digital Signal Processing, the two-bit symbols are considered to be complex numbers, I +jQ.

Symbol I Q Phase shift

00 +1 +1 45°

01 +1 -1 315°

10 -1 +1 135°

11 -1 -1 225°

Working with Complex Data

In order to make full use of the efficiency of Digital Signal Processing, the conversion of the

Information data into complex symbols occurs before the modulation. The system generates

complex PN codes made up of two independent components, PNi +jPNq. To spread the Information

data the system performs complex multiplication between the complex PN codes and the complex

data.

Summing many Channels Together: Many channels are added together and transmitted

simultaneously. This addition happens digitally at the chip rate. Remember, there are millions of

chips in each symbol. For clarity, let each chip be represented by an 8 bit word.

At the Chip Rate

Information data is converted to two bit symbols.

The first bit of the symbol is placed in the I-data stream; the second bit is placed in the Q-

data stream.

The complex PN code is generated. The complex PN code has two independently generated

components, an ‘I’ component and a ‘Q’ component.

The complex Information data and complex PN code are multiplied together.

For each component (I or Q):

Each chip is represented by an 8 bit word. However, since one chip is either a one or a zero,

the 8 bit word equals either 1 or -1.

When many channels are added together, the 8-bit word, as the sum of all the chips, can

take on values from between +128 to -128.

The 8-bit word then goes through a Digital to Analog Converter, resulting in an analog level

proportional to the value of the 8-bit word.

This value then modulates the amplitude of the carrier (the I component modulates the

Cosine, the Q component modulates the Sine)

The modulated carriers are added together.

Since I and Q are no longer limited to 1 or -1, the phase shift of the composite carrier is not limited

to the four states; the phase and amplitude vary as

A2 = I2 + Q2

Tan(Y) = Q/I

At the Symbol Rate

Since the PN-code has the statistical properties of random noise, it averages to zero over long

periods of time. Therefore, fluctuations in I and Q, and hence the phase modulation of the carrier,

that occur at the chip frequency, average to zero. Over the symbol period the modulation averages

to one of the four states of QPSK, which determine what the symbol is.

The symbol only sees the QPSK, and obeys all the statistical properties of QPSK transmission,

including Bit Error Rate.

Receiving Data

The receiver performs the following steps to extract the Information:

Demodulation

Code acquisition and lock

Correlation of code with signal

Decoding of Information data

Demodulation: The receiver generates two reference waves, a Cosine wave and a Sine wave.

Separately mixing each with the received carrier, the receiver extracts I(t) and Q(t). Analog to

Digital converters restore the 8-bit words representing the I and Q chips.

Code Acquisition and Lock: The receiver, as described earlier, generates its own complex PN code

that matches the code generated by the transmitter. However, the local code must be phase-locked

to the encoded data. The Radio Carrier Station (RCS) or Base Station (BS) and a Fixed Subscriber

Unit (FSU) or Mobile Station (MS) each have different ways of acquiring and locking onto the

other’s transmitted code.

Correlation and Data Dispreading: Once the PN code is phase-locked to the pilot, the received

signal is sent to a correlator that multiplies it with the complex PN code, extracting the I and Q data

meant for that receiver. The receiver reconstructs the Information data from the I and Q data.  

BAND OF OPERATION

There are 2 CDMA common air interface standards: Cellular (824-894 MHz) - IS-95A and

PCS (1850-1990 MHz) - Joint-STD-008

1. Cellular Band

45 MHz spacing for forward & reverse channel

Frequency assignments are on 30 kHz increments

Figure1.6: Cellular/PCS Spectrum

2. PCS Band

80 MHz spacing for forward & reverse channel

Frequency assignments are on 50 kHz increments

Cell Configuration in CDMA

In Wireless communication we divide a whole geographical area into smaller chunks called cells. A

single BS services each of these cells. These cells are grouped into groups of 3 –7 cells to form a

cluster. Groups of clusters are put under a single BSC. All the BSCs in PLMN are controlled by a MSC.

By repeating the 7-cell cluster over a city we can service the entire area by planning just one

cluster. 

Figure 1.7: seven Cells forming a cluster

Planning cells for a city is a lot more complicated and elaborate task but on paper in ideal

conditions, this is a simple concept.

This hierarchy of cells helps increasing capacity and easy of routing calls apart from other

operational advantages. But the final number of cells in an area is comprised of factors like the

density of calls ,number of BSs , size of each cell ,capacity, the budget in hand etc.

Comparison of Frequency Planning between FDMA/TDMA and CDMA

In the case of FDMA/TDMA a given spectrum would be divided into smaller chunks, each of these

chunks uniquely assigned to a cell in a cluster. By repeating this pattern (frequency re-use

technique) a big city is serviced by the limited spectrum as illustrated in figure 1.7. Smaller

bandwidth for transmission compromises on quality of signal. This planning of frequencies is called

Frequency Planning. This distribution of frequencies in a cluster is important to combat co-channel

interference and adjacent channel interference between repeating clusters.

Figure

1.7: Frequency re-use pattern; FDMA/TDMA VS CDMA

In CDMA, frequency planning is minimal; the entire spectrum can be assigned to all the cells in the

cluster as shown in Figure 1.7. This is possible because of the orthogonal property of the unique

codes used for transmission. As a consequence, usage of the entire spectrum enhances the quality of

voice. Adjacent channel interference is combated by power control (we will discuss this later) and

planning of Walsh codes in use at the BSC level.

ENVIRONMENT AROUND THE MOBILE STATION

Information flows from the BS to the MS via the forward channel or the forward link and from the

MS to the BS via the reverse channel or reverse link.

Walsh, Short PN and Long PN Codes

Walsh Codes: We first came across the unique codes for spreading. These special codes are called

Walsh Codes. In each cell, a user has a dedicated Walsh Code. These codes follow the orthogonal

property of vector i.e. auto-correlation of a code is 1 and correlation with any other code is 0. In IS-

95A and IS-95B we use 64 orthogonal codes and in CDMA-2000 we use 128 orthogonal codes.

These codes are also used for spreading on a forward link. Hence the understanding now is that the

forward link is divided into as many Walsh Codes and called a Code Channel. On the reverse link the

Walsh Codes are not used to differentiate users but for 64-ary modulation.

Short PN Code: This is a 16 bit short PN Code used to identify the BS and hence the cell.

Distinguishing of the different BS is done by assigning an offset of this code to a common time

reference to each BS in the network. On the reverse link the mobile uses the code for extra signal

robustness, but without any offset. Services of the GPS (Global Positing System) are used in

synchronizing the various offsets of BS in the network.

Long PN Code: This code on the reverse link is used for spreading, meaning identifying the mobile

station. It is 42-bit code. On the forward link it is used for data scrambling

CDMA Logical Channels

FORWARD LINK

The Forward CDMA link consists of up to 64 logical channels (code channels). A code channel is one

of a set of 64 so-called Walsh functions. The Walsh makes the channels completely separable in the

receiver. Each forward code channel is spread by the Short Code (short PN code) , which has I- and

Q-components. The two coded, covered, and spread streams are vector-modulated on the RF

carrier. The spreading modulation is thus QPSK, superimposed on a BPSK code symbol stream.

 The Forward Link is divided into 64 code channels. The logical structure is described below.

Pilot Channel: This channel is all zeros – carrying no data information. This channel is the

beacon channel that defines the radius of the cell and hence is transmitted with the largest

power. It is used as a timing source in system acquisition and as a measurement device during

handoffs (MAHO). pilot channel is assigned W0.The period of the pilot short code, 215= 26.67

ms at the 1.2288 MHz chip rate. The pilot phases are assigned to BS in multiples of 64 chips,

giving a total of 215/ 64= 512 possible assignments. Hence this 9-bit number (512 assignments)

identifies the pilot phase assignment is called the Pilot Offset.

Synchronization Channel: Used by the mobile during system acquisition to receive the system

time, system identification and parameter information and state of the Long Code. Sync

Channel is W32. This operates at 1200 bps.

Paging Channel: This channel carries overhead messages, pages, call setup messages and

orders. The bps (4800 or 9600bps) of this channel is got from the Synchronization Channel.

The paging channel is assigned Walsh codes W1-W7. W1 is called the primary paging channel

and overhead messages are always transmitted on the primary PCH. It operates in slotted-

mode (mobiles ‘sleep’ and ‘wakeup’ when it’s time to listen).

Traffic Channel: The traffic channels are assigned to individual users to carry call traffic. All

the remaining Walsh codes are available, subject to overall capacity limited by noise.

REVERSE LINK

Reverse CDMA Channel consists of 2 42-1 logical channels. One of the logical channels is permanently

and uniquely associated with each MS. The channel does not change upon handoff.

The reverse CDMA Channel does not follow the strict orthogonal rule strictly uses a very long

period spreading code, in distinct phases. The correlations between mobile stations are not zero,

but they are acceptably small.

Access Channel: Access channels are used by mobiles not yet in a call; to transmit

registration requests, call setup requests, page responses, order responses, and other

signaling information. An access channel is really just a public long code offset unique to the

BTS sector. Access channels are paired to Paging Channels. Each paging channel can have up

to 32 access channels. These channels operate at 4800 bps.

Reverse Traffic Channel: The reverse traffic channels are used by individual users during

their actual calls to transmit traffic to the BTS. A reverse traffic channel is really just a user-

specific public or private Long Code mask

POWER CONTROL IN CDMA SYSTEMS

The success of the system lies in controlling the total power in the CDMA system.

In a CDMA environment every MS (i.e. a handset) is a source of noise to the other. At the receiver at

the MS sees the radio environment around it as a cumulative addition of information for itself and

Interference. The interference is information for other MSs plus noise from others sources. Hence if

the interference is more, the information signal cannot be retrieved.

A mobile has a special receiver called a RAKE receiver that can make estimates of multipath fading

and retrieve the information for a particular mobile. The simple mathematical steps outline how the

receiver works.

SI(t) =Information signal for Ith mobile

WI(t) =Walsh Code for Ith mobile

R (t)= Received signal at RAKE receiver

Hence at any RAKE receiver the received signal will be (assuming no multipath fading)

R (t) = S SI(t)Å WI(t)

Example; retrieving the signal for user1

W1(t) Å R(t) = W1(t) Å S SI(t)Å WI(t)

= S1(t) + 0

The orthogonal property of Walsh Codes makes zero the noise and retrieves the signal.

Hence if noise or interference is more it will blind the receiver.

Generally the BS gets bombarded by signals from many different MS. Some of these MS are close

and their signals are much stronger than MS farther away. This results in the Near/Far problem

inherent in CDMA communications. System Capacity is also dependent on the signal power. For

these reasons, both the BS and MS measure the received power and send signals to control the

others transmit power.

Characteristics of Power Control

Power control on both the forward and reverse link.

It Increases CDMA system capacity

Power Control prevents Near/Far problem

Automatic power control on both the Forward/Reverse Link

Forward/Reverse Link: The BS uses Closed Loop Power Control on the forward link. The mobile

station periodically informs the BS to increase or decrease its power.

Reverse Link Power Control: two types exist;

Open Loop Power Control: The Open Loop Power Control is used during access attempts. It

increases the power during each attempt. The Communication process is not affected by

increasing power at the BS receiver, since BS has already informed the power increment

step on each attempt to the MS on a broadcast mobile.

Closed Loop Power Control : In Closed Loop Power Control there is a feedback procedure.

This type of power control is used when the MS is using the traffic channel resources i.e.

when active. The BS is continuously monitoring the reverse link. If it finds the quality of the

reverse link poor, then it will instruct the mobile to increase its power by inserting power

control bits in traffic data. This insertion of power bits for power control is called Bit

Puncturing. The BS does this every 800 b/s.

System Capacity

The capacity of a system is approximated by:

The capacity is directly proportional to the processing gain and also inversely proportional to the

signal to noise ratio of the received signal. Therefore the smaller the transmitted signal, the larger

the system capacity (as long as the receiver can detect the signal in the noise). Both the BS and MS

control the power transmitted by the other so that the received signal is as small as possible while

maintaining a minimum signal to noise ratio. This maximizes system capacity

Near / Far Problem

A user close to a cell would saturate the receiver and eliminate all users further away, unless the

power is controlled. This is referred to as Near /Far problem.

Because the cross-correlation between two PN codes is not exactly equal to zero, the system must

overcome the Near/Far problem.

The output of the correlator consists of two components:

The autocorrelation of the PN code with the desired coded signal

The sum of the cross-correlation of the PN code with all the other coded signals.

Mathematically, if we are trying to decode the kth signal, we have:

Where:

Aj is the amplitude of the jth signal,

rjk is the cross-correlation between the kth and jth signal, and

S is the sum over all the j signals (excluding k).

Since the cross-correlation is small (ideally, it is zero), the sum of cross-correlation terms should be

much less than the amplitude of the desired signal. However, if the desired signal is broadcast from

far away, and undesired signals are broadcast from much closer, the desired signal may be so small

as to be drowned out by the cross-correlation terms.

Note: This problem only exists in the reverse direction. The BS is receiving signals from many MS at

different distances, but the MS is receiving all signals from one BS. The BS controls the power of

each MS so that the signals received from all MS are the same strength.

Interference Rejection

CDMA technology is inherently resistant to interference and jamming. A common problem with

urban communications is multi-path interference.

Multi-path interference is caused by the broadcast signal traveling over different paths to reach the

receiver. The receiver then has to recover the signal combined with echoes of varying amplitude

and phase. This results in two types of interference:

Inter-chip interference: The reflected signals are delayed long enough that successive bits (or

chips, in this case) in the demodulated signals overlap, creating uncertainty in the data.

Selective fading: The reflected signals are delayed long enough that they are randomly out of

phase, and add destructively to the desired signal, causing it to fade.

Combating Interference

Two methods are commonly used to combat multi-path interference:

Rake filter: Correlators are set up at appropriate time intervals to extract all the echoes. The

relative amplitude and phase of each echo is measured, and each echo signal is phase corrected

and added to the signal.

Adaptive Matched Filter: This filter is “matched” to the transfer function (i.e. the propagation

characteristics) of the signal path. It phase shifts the echo signals and adds them to maximize

the received signal.