19MPRG

CDMA Receivers for HighSpectral Utilization

CDMA Receivers for HighSpectral Utilization

20MPRG

Types of CDMA ReceiversTypes of CDMA Receivers

l Conventional Matched Filter

l Single User Receivers

l Multiuser Receivers

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l CDMA is interference limited

l CDMA subject to near/far problem on reverse channel

l Interference and near/far problem limit system capacity

l These problems are not inherent to CDMA , but ratherthe result of using conventional correlator

l Multiuser and Single User receivers mitigate theseproblems

Why Use Advanced Receivers?Why Use Advanced Receivers?

22MPRG

Receiver ArchitecturesReceiver Architectures

Multiuser Multistage RAKE Receiver

l Estimate the interference and subtract it out.l Number of stages : 2-3l Number of RAKE fingers : 2l Conventional RAKE correlators employed at each stage

Single User Mobile Receiverl Linear fractionally spaced adaptive filter of length 45l Adaptive algorithms: Normalized LMS, Linearly Constrained

CMA, Griffiths’ algorithm, Soft Decision Directed NormalizedLMS, etc.

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Single User Receiver ObjectivesSingle User Receiver Objectives

l Design and implement a mobile receiver capable ofoperating in heavily loaded and multipathenvironments

l Compare existing receiver structures andalgorithms on a common simulation testbed

l Develop novel blind algorithms for CDMAinterference rejection

l Implement promising algorithms on a DSPl Map the algorithms from the DSP to the COLT

reconfigurable FPGA processor

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Single User Receiver StructuresSingle User Receiver Structures

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Receiver StructuresReceiver Structures

Fractionally-spacedtransversal filter

Adaptationalgorithm

r(t) r(n) y(n)

d(n)

)d(n)

e(n)

DecisionDirected

Training

The Complex LinearFractionally-SpacedReceiver(CLFSR)

The Complex LinearFractionally-SpacedReceiver(CLFSR)

Ts Td

Ts Ts Ts Ts

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What Single User Receivers DoWhat Single User Receivers Do

Cross correlation of filter weight vector with the spreading code of eachuser

CLFSR

MFR

Cor

rela

tion

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Algorithms: TrainedAlgorithms: Trained

l Algorithms based on MSE cost functionl Adaptive receivers offer significant capacity gains

Capacity curves: Synchronous AWGN channel, Eb/N0 = 7.5 dB

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Algorithms: TrainedAlgorithms: Trained

l The adaptive receivers are near-far resistant

Capacity curves: Asynchronous AWGN channel, Eb/N0 = 7.5 dB

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ResultsResults

Capacity curves: Urban channel (COST 207 model), Eb/N0 = 23.5 dB

l Can adaptive receivers exploit multipath?

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Hardware ArchitectureHardware Architecture

Q arm

fixedpointDSP

SystemExecutive

I arm

Digital Matched Filter

Dual PortSRAM

A/D

NCOAGC

floatingpointDSP

Adaptive Filter

BPF

SRAM

Signalat IF

Data Bus

Control Bus

I DDC Q

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Multiuser Receiver ObjectivesMultiuser Receiver Objectives

l Design and implement a multiuser multistage RAKEreceiver for interference cancellation and multipathmitigation at the base station.

l Provide significant capacity gains and near/far resistancecompared to current CDMA receivers with a complexitythat makes implementation feasible.

l Development of analytical and simulation tools to studymultiuser interference cancellation for CDMA.

l Address implementation issues not considered in theliterature.

32MPRG

CDMA ReceiversCDMA Receivers

Conventional AdaptiveFilters

Single User

Optimal(MLSE)

Decorrelator MMSE

Linear

ConventionalFirst

Stage

DecorrelatingFirst

Stage

Succ. Canc.First

Stage

Multistage ReducedTree Search

NeuralNetworks

DecorrelatingDecisionFeedback

Successive Cancellation

DecisionFeedback

Non-linear

Sub-optimal

Multiuser

CDMAReceivers

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Multiuser ReceiversMultiuser Receivers

• Offer significant capacity gains over conventional receiver

• Provide robustness in near/far situations

• Parallel cancellation provides excellent tradeoff between complexity and performance

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ApproachApproach

R(t)

ImprovedEstimates

InitialMultiuser

Estimation

MAIRegeneration

MultiuserEstimation+

-

MatchedFilterUser 1

MatchedFilterUser 1

MatchedFilterUser K

Z1,i

ZK,i

Synch 1

Synch KZK,i

x

2a1(t-t1)T

x

2ak(t-tk)

+

MatchedFilterUser 1 Z1,iSynch 1

MatchedFilterUser K Synch K

T

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PerformancePerformance

• Significant improvements are observed with few stages of interference cancellation for different channel conditions.

• Multistage cancellation tends to the single user bound as the number of stages increases.

AW

GN

Ray

leig

h Fa

ding

MPRG

Interference Cancellation SchemeInterference Cancellation Scheme

User 1Matched

Filter

User 2Matched

Filter

User 3Matched

Filter

X

User 4Matched

Filter

X

X

X

Σ

C2

C3

C4

C1

+

+

+

+

X

c

Σ+-

Σ

X

c

+

+

User 1Matched

Filter

User 2Matched

Filter

User 3Matched

Filter

X

User 4Matched

Filter

X

X

X

Σ

C2

C3

C4

C1

+

+

+

+

X

c

Σ+-

Acquisition& Tracking

C = 0.75

DifferentialDecoder

User BitStreams

Colt-basedMatchedFilters

Colt-basedMatchedFilters

FPGA-basedInterferenceCancellation

FPGA-based Interference Cancellation andMultiuser Receiver

Receiver Stage 1 Receiver Stage 2