Analysis and performance of bidirectional decoding of convolutional codes over fading channels

The performance of suboptimal convolutional decoding over fading channels is explored. The suboptimal decoding algorithm used is the bidirectional algorithm. By estimating a “decoder weight spectrum” for the decoder, an “equivalent free distance” may be observed. Furthermore, by using this “decoder weight spectrum”, useful estimations of the error probabilities are obtained and compared to computer-simulation results in the case of very slow and very fast fading. The resultant curves are shown to be very tightly related. Computer-simulation results are also shown for various signal-to-noise ratios, normalized Doppler spreads, and frame length on three typical fading channels: the Rayleigh fading channel with exponential and Bessel autocorrelation functions and the Rician fading channel with exponential autocorrelation function. We show that considerable gains (up to 4 dB) can be obtained with respect to a similar-complexity Viterbi decoder at a frame error probability P_e =10^-3 and a slightly smaller gain (up to 1.8 dB) at a bit error probability P_b=10^-5