VIRGINIA POLYTECHNIC INSTITUTEAND STATE UNIVERSITY

TechVirginia

1 8 7 2

VIRGINIA POLYTECHNIC INSTITUTE & STATE UNIVERSITY

MOBILE & PORTABLE RADIO RESEARCH GROUP

MPRG

Performance of Turbo Codesin Interleaved Flat Fading Channels

with Estimated Channel State Information

48th Annual Vehicular Technology ConferenceOttawa, Canada May 19, 1998

Matthew Valenti and Brian D. WoernerMobile and Portable Radio Research GroupVirginia TechBlacksburg, Virginia

5/19/98

Intr

oduc

tion

Introduction Turbo codes have been shown to achieve

remarkable performance over Rayleigh flat fading channels.

Typical assumptions in the literature: Fading is Rayleigh distributed. The channel is “fully-interleaved”. Perfect channel estimates are available.

The purpose of this presentation is to investigate the validity of these assumptions.

5/19/98

Intr

oduc

tion

Prior Work Effect of channel estimation.

Jordan and Nichols, MILCOM ‘96 Noise variance estimation errors.

Hoeher, Int. Symp. on Turbo Codes ‘97 Set the variance estimate equal to a constant.

Summers and Wilson, Trans. Comm. Apr. ‘98 Proposed an SNR estimator

Correlated fading and channel interleaving. Hall and Wilson, JSAC Feb. 98

Exponentially correlated channel. Block and systematic channel interleaving.

5/19/98

Intr

oduc

tion

Outline of Talk General channel model.

Fading can be Rayleigh or Rician. Fading is correlated using Clarke’s model.

Channel estimator Estimates fading amplitude and noise variance. Based on an FIR filter.

Simulation study SOVA and MAP decoding algorithms Correlated Rician/Rayleigh fading with channel

interleaving.

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Syst

em M

odel

System Model

TurboEncoder

ChannelInterleaver

BPSKModulator

TurboDecoder

De-interleaver

BPSKDemod.

De-Interleaver

ChannelEstimator

ka

kn

nm

nm

ka

kx

ky

ks

kr

kx

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Syst

em M

odel

Channel Model Multiplicative fading amplitude:

xk and yk are i.i.d., Each has autocorrelation

Ratio of specular to diffuse energy:

Rayleigh fading: = 0 Rician fading: > 0

kkk jyxa

),0( 2f

2

2

2 f

)2(1)( kBTJBT

kR sos

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Syst

em M

odel

Encoder and Decoder Encoder

Constraint length K=3 RSC encoders. Frame/interleaver size of 1,024 bits. Randomly designed interleaver. Rate r=1/2. Odd/even puncturing.

Decoder 8 decoder iterations. (Improved) SOVA, Papke et al ICC ‘96 Log-MAP, Robertson et al, Euro. Trans

Telecomm. Mar ‘97

Effect of correlated fading and interleaving

Turbo Code Performance in flat Rayleigh fading. Parameterized by type of

interleaving and BT 32 by 64 channel

interleaver. 8 iterations of Improved

SOVA decoding. Poor performance for all BT

with no channel interleaver. Performance degrades with

channel interleaver as BT decreases.

0 1 2 3 4 5 6 7 8 9 1010-6

10-5

10-4

10-3

10-2

10-1

100

101

Eb/No in dB

BE

R

BT = .0025, no interleaving BT = .005, no interleaving BT = .01, no interleaving BT = .0025, block interleavingBT = .005, block interleaving BT = .01, block interleaving fully interleaved

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Chan

nel E

stim

atio

n

Proposed Channel Estimator Fading amplitude estimator

FIR filter, order N=32. Lowpass with cutoff at fd.

FIRLPF

absolutevalue

ComputeSampleVariance

2ˆn

kakr kr

kn2ˆ n

21 cc

Noise variance estimator Take sample variance of estimated noise

magnitude. Constant required to unbias the estimates.

Effect of channel estimation in Rayleigh fading

Performance in flat Rayleigh fading. BT = .005 Block channel interleaving 8 iterations of decoding:

Improved SOVA. log-MAP.

MAP performs 2.0 dB better than SOVA.

Slight penalty for fade estimates:

0.25 dB for SOVA 0.75 dB for MAP

No penalty for noise variance estimates.0 1 2 3 4 5 6 7 8

10-6

10-5

10-4

10-3

10-2

10-1

100

Eb/No in dB

BE

R

SOVA, noise and fade estimatesSOVA, fade estimates only SOVA, perfect channel info MAP, noise and fade estimates MAP, fade estimates only MAP, perfect channel info

Effect of channel estimation in Rician fading

Performance in flat Rician fading. BT = .005 and = 1. Block channel interleaving. 8 iterations of decoding:

improved SOVA log-MAP

MAP performs 1.5 dB better than SOVA.

Slight penalty for fade estimates:

0.25 dB for SOVA 0.5 dB for MAP

No penalty for noise variance estimates.0 1 2 3 4 5 6 7 8

10-6

10-5

10-4

10-3

10-2

10-1

100

Eb/No in dB

BE

R

SOVA, noise and fade estimatesSOVA, fade estimates only SOVA, perfect channel info MAP, noise and fade estimates MAP, fade estimates only MAP, perfect channel info

5/19/98

Conc

lusi

on

Conclusion It is important to incorporate the effects of

channel correlation and interleaving when simulating turbo codes over fading channels.

A simple FIR filter can be used to estimate the fades with only slight loss in performance.

Performance is insensitive to noise variance estimates.

MAP algorithm is considerably superior to SOVA in severe fading. MAP is more sensitive to estimation.

5/19/98

Conc

lusi

on

Future Work The fading amplitude estimator could be

improved. Requires knowledge of Doppler frequency. A Kalman filter could be used instead.

Effects of estimating carrier phase should be considered.

Estimation could be absorbed into the turbo decoding algorithm. Estimate channel after each iteration. Use new estimates during next iteration.