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VIRGINIA POLYTECHNIC INSTITUTE & STATE UNIVERSITY

MOBILE & PORTABLE RADIO RESEARCH GROUP

MPRG

VIRGINIA POLYTECHNIC INSTITUTEAND STATE UNIVERSITY

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A Simulation Tool for ThirdGeneration CDMA Systems

Presentation to IEEE Sarnoff Symposium

March 22, 2000

Fakhrul Alam, William Tranter, Brian Woerner

Mobile and Portable Radio Research Group (MPRG)

Virginia Tech

e-mail: [email protected], URL: www.mprg.ee.vt.edu

This work was carried out with sponsorship of MPRGindustrial affiliates and LGIC



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Outline of Presentation

• Introduction : Evolution of Third Generation (3G)Systems

• Description of WCDMA Physical Layer

• Description of the Simulator

• Simulation Results

• Performance Enhancing Features

• Conclusion & Extension of Research



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Introduction

• The goal for Third Generation Systems: Communication toanybody, anywhere, anytime.

• Intended Services• High Speed Data

• Video and Multimedia Traffic

• Voice Signals

• 3G communicators are oriented towards multimedia messagecapability marking a significant leap from current standards.



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Earlier Generations of CellularSystems

• First Generation• Analog Frequency Modulation (FM)

• Frequency Division Multiple Access (FDMA)

• Voice Traffic

• Advanced Mobile Phone System (AMPS): most popular 1st Generationsystem

• Second Generation• Digital Counterpart of First Generation

• Digital Modulation Schemes with compression and coding

• Time Division Multiple Access (TDMA) and Code Division MultipleAccess (CDMA) along with FDMA

• Voice Traffic

• USDC standards IS-54 & IS-136, GSM, PDC, cdmaOne



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Third Generation Systems

• Mobile phone meets Internet

• A picture tells a thousand words : but it needs a lot ofbandwidth



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Primary Requirements of ThirdGeneration Systems

• Voice quality comparable to Public Switched Telephone

Network (PSTN)

• Support of high data rate• Vehicular : 144 kbps

• Pedestrian : 384 kbps

• Indoor Office : 2 Mbps

• Backward compatibility with pre-existing networks andflexible introduction of new services and technology

• More efficient usage of the available radio spectrum

• Adaptive radio interface suited to the highly asymmetric natureof most Internet communications



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Evolution of 3 G

AMPS

PDC

GSM

IS-54

cdmaOne

ANSI-136

136 HS

136+

GPRS

ARIB (WCDMA)

UTRA (WCDMA)

EDGE

IS-95B

UWC-136

cdma2000

WCDMA

TDD

MulticarrierMulticode

FDD

This Figure is condensed from Figure 4,page 28 of Malcom W. Oliphant, " TheMobile Phone Meets the Internet," IEEESpectrum, pp. 20-28, August 1999.



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WCDMA Key TechnicalCharacteristics

Multiple Access Scheme DS-CDMA

Duplex Scheme FDD/TDD

Multirate/Variable rate scheme Variable spreading factor and multi-code

Chip Rate 3.84 Mcps

Carrier Spacing 4.4-5.2 MHz (200 kHz carrier raster)

Frame Length 10 ms

Inter Base Station synchronization FDD: No accurate synchronization needed

TDD: Synchronization required

Channel Coding Scheme Convolutional Code (rate 1/2 and 1/3, const.length 9)

Turbo code



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WCDMA Physical ChannelStructure

• Dedicated Channels : Connection dedicated• Dedicated Physical Data Channel (DPDCH)

• Dedicated Physical Control Channel (DPCCH)

• Common Channels : Shared among users• Primary and Secondary Common Control Physical Channel (CCPCH)

at the Downlink

• Synchronization Channel (SCH) at the Downlink.



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Uplink Frame Structure (DPDCH & DPCCH)

Data

Pilot

Tslot=2560 chips, 10×2k bits (k = 0..6)

Frame 1 Frame i Frame 72

Tf=10 ms

Tsuper=720 ms

TFCI FBI TPC

Slot 1 Slot i Slot 15

SFk

=256

2

• Pilot for Coherent Demodulation and Channel Estimation

• TFCI (Transport Format Combination Indicator) to Indicate and Identify Several Simultaneous Services

• FBI (Feedback Information) to support techniques requiring feedback

• TPC (Transmit Power Control) for power control purposes



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Downlink Frame Structure (DPDCH / DPCCH)

Tslot=2560 chips, 10×2k bits (k = 0..7)

Tf=10 ms

Tsuper=720 ms

TFCI Data 1 TPC Data 2 Pilot

Slot 1 Slot i Slot 15

Frame 1 Frame i Frame 72

SFk

=512

2



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Uplink Spreading & Modulation(DPDCH & DPCCH)

j

Channelization Code(CD)

Scrambling Code(CSC)Data (DPDCH)

Control (DPCCH)

Channelization Code(CC)

p(t)

p(t)

cos(ωct)

sin(ωct)

• Mobile Station Specific (MS) Complex Scrambling Code• MS has one DPDCH and one DPCCH



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Downlink Spreading &Modulation (DPDCH / DPCCH)

Serialto

Parallel

j

Channelization Code(Cch)

Scrambling Code(CSC)

DPDCH/

DPCCH

p(t)

p(t)

cos(ωct)

sin(ωct)

• Base Station (BS) Specific Complex Scrambling Code• Each user has one DPDCH and one DPCCH



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OVSF Codes

• Orthogonal Variable SpreadingFactor (OVSF) codes preserveorthogonality between channels

• Each level in the code treedefines channelization codes

of length SF

• All codes of the same levelconstitute a set : orthogonal toeach other

• Any two codes of differentlevels are orthogonal if one codeis not the mother of the other

c1(1)= (1)

c2(1)=(1,1)

c2(2)= (1,-1)

c4(1)= (1,1,1,1)

c4(2)= (1,1,-1,-1)

c4(3)= (1,-1,1,-1)

c4(4)= (1,-1,-1,1)

SF = 1 SF = 4SF = 2



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OVSF Codes (cont’d)

• All codes within the codetree cannot besimultaneously used withinone BS/by one MS

• Perfect orthogonality ismaintained only at zero lag

• Auto-correlation peak is not

narrow : difficulty insynchronization

Auto-correlation of the 3rd code (SF=256)



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Scrambling Codes

• Maintain separation among differentMS/BS

• Two types• Short : Advanced receivers

employing Multi User Detection(MUD) or Interference Cancellation

• Long:Employed in the Simulator

C C w jw Csc = + ¢1 1 2 2( )

C k C k C k2 2 22 2 1 2¢ = ¢ + =( ) ( ) ( )

C1

jC2

↓2C2´

w2

w2 = {1 -1 1 -1…}

CSC

C C jw C Csc = + ¢1 2 1 2

(w1 is a repetition of {1,1,…} at the chip rate )



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Scrambling Codes (Cont’d)

• Uplink GeneratorPolynomials:

3 +

3 2 1+ + +

• Generate sequences x and y

• Generate Gold Code sequences C1 and C2 from x and y

Cross-correlation Distribution

• Downlink Generator Polynomials:

X X18 7 1+ +

X X X X18 10 7 5 1+ + + +



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Summary of WCDMAModulation

Spreading Modulation Dual Channel QPSK for UL

Balanced QPSK for DL

Data Modulation BPSK for UL

QPSK for DL

Spreading OVSF codes.

4-256 spreading factor for UL

4-512 spreading factor for DL

Scrambling Complex Scrambling

Frame Length 10 ms

Chip Rate 3.84 Mcps

Pulse Shaping Raised Cosine with 0.22 roll off



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WCDMA Simulator

• Implements the physical layer channels of the WCDMAsystem

• Multipath time varying channel

• Rake diversity combining at the receiver

• Structured Multiple Access Interference (MAI)

• AWGN at the receiver front end

• Study of BER performance under different practical channelcondition is possible



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WCDMA Simulator(Cont’d)

• Flexible: Can be very easily modified to include

• error correction coding

• antenna diversity

• Modular: Can be modified to incorporate

• different receiver schemes

• different channels



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Uplink Simulator

+Raised CosineFilter

TimeVaryingChannel

Nth Interferer

Raised CosineFilter

TimeVaryingChannel

2nd Interferer

Raised CosineFilter

TimeVaryingChannel

1st Interferer

Root-Raised CosineFilter

GaussianNoise

Generator

Rake Receiver+

Raised CosineFilter

TimeVaryingChannel

DesiredMS

BERCounter

MAI



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Downlink Simulator

Frame generator of the

desired user +Raised CosineFilter

TimeVaryingChannel

Rake Receiver+

Root-Raised CosineFilter

GaussianNoise

Generator

BERCounter

MAI


Nth interferer


1st interferer


2nd interferer

+



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Channel Parameters(Proposed by ETSI)

Relative Dealay (ns) Avg. Power (dB)0 0

310 -1710 -91090 -101730 -152510 -20

Relative Dealay (ns) Avg. Power (dB)0 050 -3

110 -10170 -18290 -26310 -32

Relative Dealay (ns) Avg. Power (dB)0 0

110 -9.7190 -19.2410 -22.8

Indoor Channel Indoor to Outdoor Channel

Vehicular A Outdoor Channel



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Time Varying Channel

Delay of the 2nd path

RayleighWaveform

RayleighWaveform

RayleighWaveform

Delay of the Nth path

TransmittedFrame



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Implementation of Delay

N samples = 1 Frame

N-τ samplesτ

samples

N-τ samplesτ

samples

N samples = 1 Frame



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Conversion of Tabulated ChannelParameters with Ray Splitting

0 0.2 1Splitting of ray 1

0 21Addition of the two rays

1.251 2Splitting of ray 2

Sample

Power



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Frequency Response of theIndoor Channel



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Frequency Response of theOutdoor Channel



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Addition of Noise and PulseShaping

Square Root Raised Cosine

FilterChannel


Filter

Gaussian Noise Generator


Filter


FilterChannel



Filter

Raised CosineFilter

Channel


Filter




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Rake Receiver

Descrambling

e-jϕ2

e-jϕN

FrameAlignment

Resampling

Received

Frame

zNDespread

MRC

e-jϕ1

DescramblingFrame

AlignmentResampling

z1Despread

DescramblingFrame

AlignmentResampling

z2Despread



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Graphic User Interface (GUI)

• GUI Implemented in MATLAB

• Type wcdma at the MATLABprompt

• Choose Between Uplink andDownlink from the Main Menu



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Uplink Menu

• Run the simulator with

default parameters• Eb/N0 : 5 dB

• Spreading Factor : 32

• Samples per Chip :1

• Pulse Shape : RC

• Channel : Indoor

• Interferer Number : 0

• Rake Finger : 4

• Number of Frame : 1

• Define Parameters



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Final Uplink Menu

• Display BER plot

• Display BER as a text file

• Plot the multipath profile

• Plot the data



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Uplink BER vs Eb/N0 at IndoorChannel (User SF=32)

0 2 4 6 8 10 1210

-6

10-5

10-4

10-3

10-2

10-1

100

E b/N0 in dB

BE

R

0 interfe re r 2 interfe re rs 4 interfe re rs 8 interfe re rs 12 interfe re rs



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Uplink BER vs Eb/N0 at OutdoorChannel (User SF=32)

0 2 4 6 8 10 1210

-4

10-3

10-2

10-1

100

E b/N0 in dB

BE

R

0 inte rfe re r 2 inte rfe re rs 4 inte rfe re rs 8 inte rfe re rs 12 interfe re rs



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Uplink BER vs Interferers atIndoor Channel (Eb/N0 =12 dB)

0 2 4 6 8 10 1210

-4

10-3

10-2

10-1

Number of Interfe re rs

BE

R

s f=32s f=16s f=8 s f=4



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Uplink BER vs Interferers atOutdoor Channel (Eb/N0 =12 dB)

0 2 4 6 8 10 1210

-3

10-2

10-1

100


BE

R

s f=32s f=16s f=8 s f=4



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Downlink BER vs Eb/N0 at IndoorChannel (User SF=32)

0 2 4 6 8 10 1210

-5

10-4

10-3

10-2

10-1

100

E b/N0 in dB

BE

R




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Downlink BER vs Eb/N0 atOutdoor Channel (User SF=32)

0 2 4 6 8 10 1210

-3

10-2

10-1

100

E b/N0 in dB

BE

R




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Downlink BER vs Interferers atIndoor Channel (Eb/N0 =12 dB)

0 2 4 6 8 10 1210

-5

10-4

10-3

10-2

10-1


BE

R

s f=32s f=16s f=8 s f=4



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Downlink BER vs Interferers atOutdoor Channel (Eb/N0 =12 dB)

0 2 4 6 8 10 1210

-3

10-2

10-1


BE

R

s f=32s f=16s f=8 s f=4



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Summary of Results

• The system is interference limited at both the links for highernumber of users

• As the system load approaches 50%, the performance for theuncoded system becomes unacceptable

• Orthogonality among channels is preserved better at thedownlink



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Implementation of ConvolutionalCoding at the Uplink

• Voice application at the Uplink

• Data rate 8 kbps : 80 bits per 10ms frame

• Add 16 bits of CRC : 96 bits per 10ms

• Resultant data rate : 9.6 kbps

• Apply rate 1/3, constraint length 9 convolutional coding.

• 9 - 1 = 8 tail bits : 96 + 8 = 104 bits per 10ms

• 104 × 3 = 312 bits per 10ms

• Nearest matching : 300 bits per 10ms

• Puncture 12 bits

• 300 bits per 10ms corresponds to a spreading factor of 128



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BER/FER vs Eb/N0 for the 9.6kbps Uplink Service with Coding

Indoor Channel

0 0.5 1 1.5 2 2.510

-4

10-3

10-2

10-1

100

Eb

/N0

in dB

BE

R/F

ER

BERFER



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BER/FER vs Eb/N0 for the 9.6kbps Uplink Service with Coding

Outdoor Channel

0 1 2 3 4 5 6 710

-4

10-3

10-2

10-1

100

Eb

/N0

BE

R/F

ER

BERFER



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Performance Improvement withCoding

• Coding gain of approximately 4.7 dB

Target BER : 10-3 (No MAI )

Channel Required SNR for Uncoded System Required SNR for Coded System

Indoor 6.8 dB 2.1 dB

Vehicular A 11.5 dB 6.75



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Performance Enhancing Features

• Adaptive Antennas (at the Receiver)• Connection dedicated pilot bits as training sequence

• Increased capacity and coverage

• Transmit Diversity (Downlink)• Transfer processing burden to the base station

• Open Loop• Time Switched Transmit Diversity (TSTD)

• Space-Time Transmit Diversity (STTD)

• Closed Loop• Selection Transmit Diversity (STD)

• Feedback Mode Transmit Diversity



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Performance Enhancing Features(Cont’d)

• Advanced Receivers• Multi User Detection and Interference Cancellation

• Use Short Scrambling Codes for lower complexity

• Space-Time Rake or 2 D Rake : Combine spatial diversity with

temporal diversity



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Conclusion

• The research has produced a study of the physical layer ofWCDMA

• BER performance has been investigated for different channelconditions

• Channel coding was implemented

• The developed simulator can be a useful tool to evaluateperformance of WCDMA systems

• Paper and Presentation will be available at:• www.mprg.ee.vt.edu/people/woerner/sarnoff.html



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Extension of Work

• The Simulator is being used to investigate• Transmit Diversity Schemes

• Performance of Adaptive Antenna Array at the Receiver

• Space-Time Rake Receivers

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