VIRGINIA POLYTECHNIC INSTITUTE & STATE UNIVERSITY MOBILE & PORTABLE RADIO RESEARCH GROUP MPRG Performance of Turbo Codes in Interleaved Flat Fading Channels with Estimated Channel State Information 48th Annual Vehicular Technology Conference Ottawa, Canada May 19, 1998 Matthew Valenti and Brian D. Woerner Mobile and Portable Radio Research Group Virginia Tech Blacksburg, Virginia

5/19/98 Introduction n Turbo codes have been shown to achieve remarkable performance over Rayleigh flat fading channels. n Typical assumptions in the literature: u Fading is Rayleigh distributed. u The channel is “fully-interleaved”. u Perfect channel estimates are available. n The purpose of this presentation is to investigate the validity of these assumptions.

5/19/98 Introduction Prior Work n Effect of channel estimation. u Jordan and Nichols, MILCOM ‘96 F Noise variance estimation errors. u Hoeher, Int. Symp. on Turbo Codes ‘97 F Set the variance estimate equal to a constant. u Summers and Wilson, Trans. Comm. Apr. ‘98 F Proposed an SNR estimator n Correlated fading and channel interleaving. u Hall and Wilson, JSAC Feb. 98 F Exponentially correlated channel. F Block and systematic channel interleaving.

5/19/98 Introduction Outline of Talk n General channel model. u Fading can be Rayleigh or Rician. u Fading is correlated using Clarke’s model. n Channel estimator u Estimates fading amplitude and noise variance. u Based on an FIR filter. n Simulation study u SOVA and MAP decoding algorithms u Correlated Rician/Rayleigh fading with channel interleaving.

5/19/98 System Model Turbo Encoder Channel Interleaver BPSK Modulator Turbo Decoder De- interleaver BPSK Demod. De- Interleaver Channel Estimator

5/19/98 System Model Channel Model n Multiplicative fading amplitude: u x k and y k are i.i.d., u Each has autocorrelation u Ratio of specular to diffuse energy: F Rayleigh fading:  = 0 F Rician fading:  > 0

5/19/98 System Model Encoder and Decoder n Encoder u Constraint length K=3 RSC encoders. u Frame/interleaver size of 1,024 bits. u Randomly designed interleaver. u Rate r=1/2. Odd/even puncturing. n Decoder u 8 decoder iterations. u (Improved) SOVA, Papke et al ICC ‘96 u Log-MAP, Robertson et al, Euro. Trans Telecomm. Mar ‘97

Effect of correlated fading and interleaving n Turbo Code Performance in flat Rayleigh fading. u Parameterized by type of interleaving and BT u 32 by 64 channel interleaver. u 8 iterations of Improved SOVA decoding. u Poor performance for all BT with no channel interleaver. u Performance degrades with channel interleaver as BT decreases. E b /N o in dB BER BT =.0025, no interleaving BT =.005, no interleaving BT =.01, no interleaving BT =.0025, block interleaving BT =.005, block interleaving BT =.01, block interleaving fully interleaved

5/19/98 Channel Estimation Proposed Channel Estimator n Fading amplitude estimator u FIR filter, order N=32. u Lowpass with cutoff at f d. FIR LPF absolute value Compute Sample Variance n Noise variance estimator u Take sample variance of estimated noise magnitude. u Constant required to unbias the estimates.

Effect of channel estimation in Rayleigh fading n Performance in flat Rayleigh fading. u BT =.005 u Block channel interleaving u 8 iterations of decoding: F Improved SOVA. F log-MAP. u MAP performs 2.0 dB better than SOVA. u Slight penalty for fade estimates: F 0.25 dB for SOVA F 0.75 dB for MAP u No penalty for noise variance estimates E b /N o in dB BER SOVA, noise and fade estimates SOVA, 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 n Performance in flat Rician fading. u BT =.005 and  = 1. u Block channel interleaving. u 8 iterations of decoding: F improved SOVA F log-MAP u MAP performs 1.5 dB better than SOVA. u Slight penalty for fade estimates: F 0.25 dB for SOVA F 0.5 dB for MAP u No penalty for noise variance estimates E b /N o in dB BER SOVA, noise and fade estimates SOVA, fade estimates only SOVA, perfect channel info MAP, noise and fade estimates MAP, fade estimates only MAP, perfect channel info

5/19/98 Conclusion n It is important to incorporate the effects of channel correlation and interleaving when simulating turbo codes over fading channels. n A simple FIR filter can be used to estimate the fades with only slight loss in performance. n Performance is insensitive to noise variance estimates. n MAP algorithm is considerably superior to SOVA in severe fading. u MAP is more sensitive to estimation.

5/19/98 Conclusion Future Work n The fading amplitude estimator could be improved. u Requires knowledge of Doppler frequency. u A Kalman filter could be used instead. n Effects of estimating carrier phase should be considered. n Estimation could be absorbed into the turbo decoding algorithm. u Estimate channel after each iteration. u Use new estimates during next iteration.