Noncoherent coded modulation for fading channels

Coded modulation for noncoherent transmission over slowly time-variant flat fading channels without channel state information is considered. We focus on differentially encoded M-ary PSK with multiple-symbol differential detec tion (MSDD). Interleaving of blocks of symbols is used as a compromise betw een the conflicting requirements of exploiting the channel coherence time a nd providing diversity for decoding. We study multilevel coding (MLC) which is perfectly matched to the overall vector channel. As already well-known for coherent transmission, properly d esigned MLC is proved to be asymptotically optimum in the noncoherent case, too. A favorable strategy for labeling of signal points is given. As a pro mising alternative to MLC, bit-interleaved coded modulation (BICM) is addre ssed. But since no Gray labeling is possible, BICM can only marginally bene fit from MSDD. Conversely, in case of strict decoding delay constraints, ML C suffers from component codes with very short code length. To overcome thi s drawback, we propose hybrid coded modulation schemes, which are able to c ombine the advantages of MLC and BICM, respectively. For performance assess ment, we evaluate both the achievable channel capacity and the random codin g exponent associated with noncoherent coded modulation. Moreover, we show that in multilevel coding and multistage decoding (MSD) t he complexity of MSDD can be reduced significantly. Remarkably, the perform ance gain of MSDD can be exploited almost completely with practically no in crease in computational complexity compared to conventional differential de tection.