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PERFORMANCE EVALUATION OF IS-95A CDMA TRAFFIC

CHANNELS USING SIMULINK

Muhammad Umair Siddiqui, Qazi Safiullah Shahrukh, Muhammad Usman

Supervised by Associate Professor Dr. Zainab Zaidi

Department of Electronic Engineering, NED University of Engineering & Technology, Karachiumairsiddiqui@engineer.com, le.reaver.de.soul@gmail.com

ABSTRACT

The paper before you is an evaluation of the IS-95A

CDMA forward link and reverse link performance. Wehave devised some numerical results based on the real

time simulation of the forward link and the reverse link

under constraints imposed by the regular AWGN and

REYLEIGH faded channels along with the mobile users

imposed limitation. This simulation can also be used todeduce results for practical CDMA system designing.

We have come up with graphical relationship betweenthe BER (Bit Error Rate) and SNR (signal to noise

ratio).

Key words: CDMA, Rate Set, Convolutional Encoder,Orthogonal Spreading Codes, Pseudonoise (PN), CRC

(Cyclic Redundancy Check), Vocoder, Chip.

1. INTRODUCTION

Code Division Multiple Access (CDMA), as defined inInterim Standard 95 (IS-95), describes a digital airinterface standard for mobile equipment that enhanced

the capacity of older analog methods with greatly

improved transmission quality. cdmaOne is the brandname1 for the complete wireless telephone system that

incorporates the IS-95 interface. CDMA systems were

serving over 65 million subscribers worldwide by June,

2000. CDMA has proven itself as a successful wirelessaccess technology in 2nd generation networks.

Furthermore, the evolving third generation systems will

rely on CDMA techniques for radio access. However,

the structure of the physical layer of a cdmaOne

network is significantly different than its GSM or IS-136 counterparts.

Figure 1 gives you a brief idea.

Figure 1 Basic overview of CDMA

2. FORWARD AND REVERSE TRAFFIC

CHANNELS

2.1 Forward Traffic Channel For IS-95A CDMA

The forward channel is the link between the basestation and the mobile user.

Data is sent on RATE SET 1 (9600 bps).

The data or voice generated by the vocoder ispassed in to the CRC generator which attaches

its generator bits along with the data or voice

streams.

The baseband data plus the generator bits fromCRC is convolutionally encoded for further

strengthening error protection. Since rate set 1

is used therefore rate of encoder is .

After convolutional encoding the symbol arerepeated by the symbol repetition block onlywhen data rate is below FULL RATE only to

reduce interference power. After symbol repetition the data is interleaved

to combat fading.

Then the interleaved data is scrambled by adecimated long PN sequence which isgenerated by a ling code generator at 1.288

Mcps.

After this the PCBs (power control bits) aremultiplexed with the scrambled scheme at 19.2

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Ksps and can be punctured into any one of the

first 16 bits position of a PCG (which contains

24 bits).

After this the data is orthogonally spread bythe assigned Walsh function which is usedhere for channelization.

The data stream is further spread by theassigned short PN sequence of transmittingsector.

The output of the logical channel thus formedis then fed into a QPSK modulator. The gain

of each channel is dynamically adjusted and

indicates the amount of power transmitted for

that channel.

After this on the receiver end the signal afterdemodulation is passed into the rake receiverwhich due to diversity advantage turns multi-

path signals into one strong signal.

After this in the descrambling section, theshort PN sequence is descrambled first, then

the Walsh code and in the end the long PNsequence.

Then follows the de-inter-leaver and the de-repeater which de-repeats only when the data

rate is below FULL rate.

For decoding the VITERBI algorithm is usedand then the frame quality is checked forevaluating power handling instructions.[1,2,3]

Figure 2.1 Forward Link

2.2 Reverse Traffic Channel For IS-95A CDMA

The reverse link indicates the link between the mobile

user and the base station usually this is the limiting link.

The data or voice from the mobile user has topass through the same stages in the reverselink as in the forward link. The difference lies

in the modulation scheme and the spreading.

Here the rate of convolutional encoder islowered from to 1/3 which makes the error

protection more robust on the reverse link

Here channelization is done by long PNsequence instead of Walsh codes.

Here Walsh codes are used for orthogonalmodulation which is sending a Walsh code

instead of a group of six symbols or bits which

makes detection at the receiving end easier.

As the reverse link uses OQPSK modulationthe data is further scrambled into I and Q pathsby short PN sequences which is running at

1.2288 Mcps. This is done for increasingmobiles battery life.

The receiving end operates in the same manneras the forward link.[1,2,3]

Figure 2.2 Reverse Link

3. SIMULATION TOOLS

Simulation is performed with the help of MATLAB and

SIMULINK.

CDMA Reference Blockset, Communication Blockset

and Signal Processing Blockset are used to build theForward and Reverse Traffic Channel models.

3.1 What Is SIMULINK?

Simulink is a software package for modeling,

simulating, and analyzing dynamic systems. It supports

linear and nonlinear systems, modeled in continuoustime, sampled time, or a hybrid of the two. Systems can

also be multirate, i.e., have different parts that are

sampled or updated at different rates.

For modeling, Simulink provides a graphical userinterface (GUI) for building models as block diagrams,

using click-and-drag mouse operations. Simulink

includes a comprehensive block library of sinks,sources, linear and nonlinear components, and

connectors. You can also customize and create your

own blocks. Models are hierarchical, so you can buildmodels using both top-down and bottom-up

approaches. After you define a model, you can simulate

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it, using a choice of integration methods, either from

the Simulink menus or by entering commands in the

MATLAB Command Window.

3.2 What Is the CDMA Reference Blockset?

The CDMA Reference Blockset is a collection of

Simulink blocks designed to help you develop andsimulate CDMA wireless communication systems,

based on the current North American IS-95A CDMA

(code division multiple access) standard. With theCDMA Reference Blockset, you can construct block

diagram models of wireless systems quickly and easily

using click-and-drag mouse operations. You can then

run simulations on those models and change parameters

as needed. The blocks in the CDMA ReferenceBlockset encompass the complete functionality required

by the IS-95A standard.

3.3 What Is the Communications Blockset?

The Communications Blockset extends Simulink with acomprehensive library of blocks to design and simulate

the physical layer of communication systems and

components. The blockset helps you design

communications systems and their semiconductor

components, such as commercial or defense wirelessand wireline systems. The key features of the blockset

are: [4]

Blocks for designing and simulating thephysical layer of communications systems,including modulation, source and channel

encoding, channels, and equalization.

Graphical user interface for dynamicallytuning models and visualizing the results.

Hierarchical, block-based models for visuallyconveying complex designs.

3.4 What Is the Signal Processing Blockset?

The Signal Processing Blockset is a tool for digital

signal processing algorithm simulation and codegeneration. It is made up of blocks contained within

block libraries. You can interconnect these blocks to

create sophisticated models capable of operations suchas speech and audio processing, wireless digital

communications, radar/sonar, and medical electronics.[4]

4. SIMULATION MODEL

4.1 Forward Traffic Channel Model

The Simulink model of IS-95A Forward Traffic

Channel is given in figure 4.1 (see last page).

The IS-95A Forward Traffic Channel Model shows the

channel coding, modulation and spreading of the data

symbols at the base station and the corresponding

decoding as well as despreading and noncoherentdemodulation at the mobile station receiver. The

transmitter encodes the data, and then symbols are

modulated by a Walsh modulator and spread by a PN

sequence. The channel model adds noise to simulateerrors in transmission. The receiver side retrieves the

information bits by performing the decoding and

demodulation. The bit error rate for the data isdisplayed in the simulation.

The IS-95A Forward Traffic Channel Model uses these

library blocks from the CDMA Reference Blockset

IS-95A CRC Generator

IS-95A Frame Quality Detector

IS-95A Fwd Ch Convolutional Encoder

IS-95A Fwd Ch Interleaver/Deinterleaver

IS-95A Fwd Ch Repeater/Derepeater

IS-95A Fwd Ch Viterbi Decoder

IS-95A Fwd Ch Base Station TransmitterInterface

IS-95A Fwd Ch Detector

IS-95A Long Code Generator

IS-95A Short Code Generator

IS-95A Walsh Code Generator

IS-95A Fwd Ch Scrambler

Two library blocks from the Communication Blockset

AWGN Channel

Rayleigh Multipath Fading Channel

and two from the Signal Processing Blockset

Transmit Filter

Receive Filter

The base station transmitter section performs the CRC

(cyclic redundancy check) generation, convolutional

encoding, symbol repetition, and interleaving. TheRandom Binary Frame Generator masked subsystem

generates random data that act as information bits. The

Base Station Transmitter Data Rate masked subsystem

provides the selection of the data rate. The IS-95A CRC

Generator library block appends the CRC bits to the

information bits. These CRC bits are used to detecterrors in the data frame at the receiver. The IS-95A

Fwd Ch Convolutional Encoder library block

convolutionally encodes the data using a 1/2-rateencoder for protection against channel errors. Because

IS-95A supports variable data rate operation, the data

frame at this stage can have a number of different sizes.Depending on the data rate, the IS-95A Fwd Ch

Repeater library block may repeat the bits it receives to

create a data frame of 384 symbols. Then the IS-95A

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Fwd Ch Interleaver library block interleaves the data

frame for protection against the localized error bursts

that can occur in fading channel conditions.

The IS-95A Long Code Generator library block

generates the long code used to scramble the data. The

IS-95A Fwd Ch Scrambler library block uses the

decimated long code input to scramble the input trafficframe and insert power bits. The scrambled traffic

channel data and the data symbols from the various

other types of channel sources are input to theSpreading and Modulation subsystem. These inputs are

orthogonally encoded by their respective Walsh codes,

added, and spread with the in-phase and quadrature

components of the short PN sequence. Part of this is

accomplished by the IS-95A Fwd Ch Base StationTransmitter Interface library block, which is inside the

Spreading and Modulation subsystem. The signal

generated is processed by the pulse shaping TransmitFilter block, which generates the modulated I and Q

waveforms.

The Rayleigh Multipath and AWGN Channel

subsystem simulates the propagation through multiple

paths of a Rayleigh fading channel. Complex white

Gaussian noise is added to the channel output. This

noise represents the interference generated by otherbase stations that are using the same frequency band.

The interfering signal subsystem simulates interference

generated by other users in the same base station.

The receiver section of the system is responsible for the

recovery of the data symbols transmitted on the traffic

channel. The operations performed in this sectioninclude the receive filtering, the rake correlator, the

rake demodulator, and descrambling. The Receive

Filter block performs FIR [5] filtering on the I and Q

sample streams with a filter that is matched to the

transmit filter to maximize the in-band signal-to-noiseratio.

The IS-95A Fwd Ch Detector library block is a maskedsubsystem with several components inside. The rake

receiver computes symbol duration correlations for the

Traffic data and Pilot symbols. These correlation valuesare used by the IS-95A Fwd Ch Rake Demodulator to

recover the Traffic channel symbols. The Trafficsymbols are further processed by the IS-95A Fwd Ch

Descrambler to obtain the decision values for the

original transmitted data symbols.

The IS-95A Fwd Ch Deinterleaver library block

deinterleaves the input data to restore the originalsymbol ordering. The IS-95A Fwd Ch Derepeater

library block derepeats the symbols depending on the

symbol rate, which involves averaging the symbols that

were repeated. The resulting frame is then provided as

input to the IS-95A Fwd Ch Viterbi Decoder library

block, which retrieves the information that was

previously encoded. The decoded information bits andthe CRC bits are provided to the IS-95A Frame Quality

Detector library block. The final metrics from the IS-

95A Fwd Ch Viterbi Decoder block are also input to the

IS-95A Frame Quality Detector block, which decideswhether the frame was correctly received. The IS-95A

Frame Quality Detector block outputs the Quality

Indicator signal, as well as the information bits withoutthe CRC bits. One Error Rate Calculation block

compares the information bits to the bits generated at

the source. Finally, the resultant bit error rate is

displayed.

This simulation uses the raw BER as the measure of the

performance under the channel and noise conditions

selected. [4]

4.2 Reverse Traffic Channel Model

The Simulink model of IS-95A Reverse Traffic

Channel is given in figure 4.2 (see last page).

The IS-95A Reverse Traffic Channel Model shows the

channel coding, modulation and spreading of the datasymbols at the mobile station and the corresponding

decoding as well as despreading and noncoherent

demodulation at the base station receiver. The

transmitter encodes the data, then symbols aremodulated by a Walsh modulator and spread, and a

randomized gating is applied to the transmit bursts. The

channel model adds noise to simulate errors intransmission. The receiver side retrieves the

information bits by performing the decoding and

demodulation. The bit error rate for the data is

displayed in the simulation.

The IS-95A Reverse Traffic Channel Model uses these

library blocks from the CDMA Reference Blockset:

IS-95A CRC Generator

IS-95A Frame Quality Detector

IS-95A Rev Ch Convolutional Encoder

IS-95A Rev Ch Interleaver/Deinterleaver

IS-95A Rev Ch Repeater/Derepeater IS-95A Rev Ch Viterbi Decoder

IS-95A Rev Ch Detector

IS-95A Rev Ch Walsh Modulation andSpreading

IS-95A Short Code Generator

Two library blocks from the Communication Blockset

AWGN Channel

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Rayleigh Multipath Fading Channel

and two from the Signal Processing Blockset

Transmit Filter

Receive Filter

The mobile station transmitter section performs theCyclic Redundancy Check (CRC) generation,

convolutional encoding, symbol repetition, and

interleaving. The Random Binary Frame Generatormasked subsystem generates random data that act as

information bits. The Mobile Station Transmitter Data

Rate masked subsystem provides the selection of the

data rate. The IS-95A CRC Generator library block

appends the CRC bits to the information bits. TheseCRC bits are used to detect errors in the data frame at

the receiver. The IS-95A Rev Ch Convolutional

Encoder library block convolutionally encodes the data

using a 1/2-rate encoder for protection against channelerrors.

Because IS-95A supports variable data rate operation,the data frame at this stage can have a number of

different sizes. Depending on the data rate, the IS-95A

Rev Ch Repeater library block may repeat the bits it

receives to create a data frame of 384 symbols. Then

the IS-95A Rev Ch Interleaver library block interleavesthe data frame for protection against the localized error

bursts that can occur in fading channel conditions.

The short pseudonoise (PN) code is used for the in-

phase and quadrature spreading of the signal. The IS-

95A Short Code Generator library block generates the

short PN code.

The Spreading and Modulation subsystem contains

several blocks that are responsible for the Walsh

modulation, and the spreading with the long and shortPN codes. The IS-95A Rev Ch Walsh Modulation and

Spreading library block contains the IS-95A Rev Ch

Burst Randomizer library block, which processes the

long code and generates a gating signal based on thelong code and the data rate of the input frame. The IS-

95A Rev Ch Walsh Modulation and Spreading library

block also groups the input data in 6-bit groups, mapseach group of 6 bits to a 64-symbol Walsh code,

upsamples these 64 symbols by a factor of 4 to bringthe result to chip rate, and then spreads the upsampled

symbols with the gated long code. This gating ensures

that the transmission is only performed for a fraction ofthe frame duration (half the time for half rate, and so

on). Finally, other portions of the Spreading and

Modulation subsystem spreads the data in quadratureby the PN code. The Transmit Filter block generates the

I and Q waveforms. The Q waveform is delayed by a

1/2-chip duration relative to the I waveform.

The channel used is the same as that in the forward link

which is implicit as the user and the base station are in

the same environment.

In the receiver section, the incoming signal is first

filtered by the Receive Filter block, which implements

a filter matched to the transmit filter. The filters in this

model are designed to maximize the signal powerwithin the desired frequency band. Then the filtered

signal is sent to the IS-95A Rev Ch Detector library

block, which contains the reverse channel rake receiver.The rake receiver consists of three rake fingers that are

set to different delays to handle up to three multipaths.

Each active rake finger performs the despreading of the

input data with the short PN sequence, followed by

despreading with the long code. This is followed by thecorrelation with the entire set of 64 Walsh codes. The

energies in the I and Q components are added, and the

results from the fingers are added together. This isprocessed by the Walsh demodulator, which generates

decisions in groups of 6 bits, the size used formodulation. The Walsh demodulator outputs both softdecisions and bipolar-valued hard decisions. Both are

gated by the data burst randomizer signal.

The IS-95A Rev Ch Deinterleaver library block

deinterleaves the input data to restore the originalsymbol ordering. The IS-95A Rev Ch Derepeater

library block derepeats the data depending on the

symbol rate, which involves averaging the symbols that

were repeated. The resulting frame is then provided asinput to the IS-95A Rev Ch Viterbi Decoder library

block, which retrieves the information that was

previously encoded. The decoded information bits andthe CRC bits are provided to the IS-95A Frame Quality

Detector library block. The final metrics from the IS-

95A Rev Ch Viterbi Decoder block are also input to the

IS-95A Frame Quality Detector block, which decides

whether the frame was correctly received. The IS-95AFrame Quality Detector block outputs the Quality

Indicator signal, as well as the information bits without

the CRC bits. Error Rate Calculation block comparesthe information bits to the bits generated at the source.

Finally, the resultant bit error rate is displayed.

5. OBSERVATIONS AND RESULTS

The basic properties that are studied are:

Noise Effect in the channel

Multipath Fading Effect in the channel

Effect of Multiuser interference

All these properties are studied by measuring BERunder different conditions.

The simulation is done for 50 frames of data.

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5.1 Noise Analysis with AWGN ChannelBER [6] is measured under different SNR [6]

environments. The following graph is plotted with the

results obtained.

-20 -19 -18 -17 -16 -15 -14 -13 -12 -11 -100

5

10

15

20

25

30

35

40

45

50

SNR in dB

BERin%

BER VS S NR WITH AWGN

Figure 5.1 BER Vs SNR (AWGN)

The results obtained have been accurate and effective indemonstrating the effect of noise in affecting the BER

performance of the communication system.

As shown by the results, the simulated system is able to

achieve 0 BER at SNR>=-13.5 dB and starts to degradegradually from SNR=-15 dB.

5.2 Multipath Faidng Effect with Rayleigh Fading

Channel

Two tests are performed in this simulation1. BER versus Number of Rake Receiver Fingers2. BER versus SNR

The following graphs are plotted from the results

obtained:

1 2 30

5

10

15

20

25

30

No. of Rake Receiver Fingers

BER

in%

BER VS. NO. OF RAKE FINGERS

SNR = -10 dB

Figure 5.2 BER Vs No. of Rake Fingers

As shown in the graph, the BER performance of the

system increases dramatically with the increase in

number of fingers involved in the Rake receiver.

The improve in BER performance according to increase

in the number of Rake fingers is due to the improved

combined received signal quality after maximalcombining as the number of fingers in the rake receiver

increases.

Also maximal combing improves the strength of the

received signal by combining all the correlators output

according to their signal strength, and also, the received

signal will contain much less error than many of the

multipath signals received.

In common IS-95 CDMA forward link, 3-finger Rake

receiver is widely adopted and is still capable indelivering high received signal quality in extremely

poor SNR environments like SNR = -10 dB, as shownby the graph.

-20 -18 -16 -14 -12 -10 -80

5

10

15

20

25

30

35

40

45

50

SNR in dB

BERin%

BER VS. SNR WITH RAYLEIGH FADING CHANNEL

Figure 5.3 BER Vs SNR (Reyleigh Fading)

BER starts to degrade with increasing SNR and

becomes zero at SNR>=-9.5 dB. Comparing with the

result of noise analysis with AWGN, we see that in thepresence of multipath signals, the BER is higher at the

same value of SNR. With AWGN BER=0 at SNR=-13

dB but with Rayleigh Multipath BER=0 at SNR=-9.5dB.

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-20 -18 -16 -14 -12 -10 -80

5

10

15

20

25

30

35

40

45

50

SNR in dB

BERin%

COMPARISION OF AWGN AND RAYLEIGH FADING CHANNEL

Rayleigh Fading Channel

AWGN Channel

Figure 5.4 Comparision Of AWGN And Reyleigh

Faded Channel

5.3 Multiuser Interference Analysis

0 5 10 15 20 25 30 35 40 45

0

2

4

6

8

10

12

14

16

18

20

22

No. of users

BERin%

BER VS> NO. OF USERS

Figure 5.5 BER Vs No. Of Users

It is shown from the graph that the simulated IS-95

CDMA communication system is immune to multiuserinterference in SNR environments with SNR>=-5 dB. It

can support 25 users with BER=0 and approximately 40

users BER

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Bit Error Rate

Number of Errors

Number of Bits

IS-95A Forward Traffic Channel (From BS to MS)Model

Transmit Filter

Combine with

Sync & Paging

---

SS Modulate

Spreading and Modulation

(0 0)

Short PN Mask

Receive Filter

Random Power

Bits

Multipath

Rayleigh Fading

Multipath Rayleigh

Fading Channel

Users

Interfering Signal

from other users

[ 96 1 116 1 239 1 0 0]

Initial Phasesand

Finger Enables

IS-95A Walsh

Code Generator

IS-95A Long

Code Generator

IS-95A Long

Code Generator

Walsh Seq

Rx Signal

Path Delay/Enable

Short PN Mask

IS-95A Fwd Ch Detector

Rate

Frame InMetric

Frame Out

IS-95A Fwd Ch

Viterbi Decoder

Rate

Data In

Long Code

Power Bits

IS-95A Fwd Ch

Scrambler

Rate

Frame In

IS-95A Fwd Ch

Repeater/Derepeater

IS-95A Fwd Ch

Interleaver/

Deinterleaver

IS-95A Fwd Ch

Interleaver/Deinterleaver

Rate

Frame In

IS-95A Fwd Ch

Derepeater

IS-95A Fwd Ch

Interleaver/

Deinterleaver

IS-95A Fwd Ch

Deinterleaver

Rate

Frame In

IS-95A Fwd Ch

Convolutional Encoder

Rate

Frame In

Metric

Quality Indicator

Frame Out

IS-95A Frame Quality

Detector

Rate

Frame In

IS-95A CRC Generator

[Rate]

[Rate]

[Rate]

To

Frame

To

Frame

Error Rate

Calculation

Tx

Rx

Sel

Error Rate Calculation

Rate Idx

Determine Data Index

Rate

Raw Data

Data Source

Buffer

0

BER

AWGNAWGN

Channel

Figure 4.1 IS-95A Forward Traffic Channel Model in Simulink

IS-95A Reverse Traffic Channel

(From MS to BS) Model

Bit Error Rate

Number of Errors

Number of Bits

(0 0)

Short PN Mask

Random Binary

Frame GeneratorPulse Shaping Transmit Filter

Pulse Shaping Receive Filter1

[0 0]

PN Mask MultipathRayleigh Fading

Multipath

Rayleigh Fading

Users

Interfering Signal

from other users

[ 96 1 116 1 0 0]

Initial Phasesand

Finger Enables

PN I

PN Q

IS-95A Short Code

Generator

Rate

Rx Signal

Path Delay/Enable

Short PN Mask

Hard Dec

Soft Dec

IS-95A Rev Ch Detector

Rate

Frame In

IS-95A Rev Ch

Repeater

Rate

Frame In

IS-95A Rev Ch

Interleaver

Rate

Frame In

IS-95A Rev Ch

Convolutional Encoder

Rate

Frame In

IS-95A Rev Ch

Derepeater

Rate

Frame In

IS-95A Rev Ch

Deinterleaver

Rate

Frame InMetric

Frame Out

IS-95A Fwd Ch

Viterbi Decoder

Rate

Frame In

Metric

Quality Indicator

Frame Out

IS-95A Frame Quality

Detector

Rate

Frame In

IS-95A CRC Generator

[Rate]

[Rate]

[Rate]

Error Rate

Calculation

Tx

Rx

Sel

Error Rate Calculation

Rate I dx

Determine Data Index

Mobile Station

Transmitter

Data Rate

0

BER

AWGNAWGN

Channel

Rate

Frame In

PN I

PN Q

Out

Spreading and

Modulation

Figue 4.2 IS-95A Reverse Traffic Channel Model in Simulink