Avoiding Cycles in Combined Turbo Decoding and Channel Estimation for Correlated Fading Channels

We consider combined turbo decoding and channel estimation using pilot symbol assisted modulation (PSAM). When transmitting information on correlated fading channels, a channel interleaver is required to break up deep channel bursts. Our primary concern in this work is not the estimation scheme, but the interleaver pair (consisting of the channel interleaver and the turbo code interleaver), and its effect on the overall system performance. If no precautions are taken, the interleaver pair will generate short cycles in the system’s graph description. In this report, different types of cycles are described, and the expected length distribution of primary combined cycles in a uniform channel-coding scheme is derived analytically. We further discuss how to design interleavers that avoid, or reduce the number of (short) primary combined cycles. Simulation results indicate that these cycles to some extent degrade the overall system performance in the high signal-to-noise ratio (SNR) region. Both low and high rate turbo codes are considered in the simulation study.