cdma receivers for high spectral utilizationsss-mag.com/pdf/glomo_transitionii.pdf · receiver for...

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CDMA Receivers for HighSpectral Utilization

CDMA Receivers for HighSpectral Utilization

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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?

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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.

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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

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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