On coding for fading channels

We discuss the coding design criteria for some classical fading channel models. If the channel is not stationary, as it happens for example in a mobile-radio communication system where it may fluctuate in time between the extremes of Rayleigh and AWGN, then a code designed to be optimum for a fixed channel model might perform poorly when the channel varies. Therefore, an optimal code for a given channel may be actually suboptimum for a substantial fraction of time. In these conditions, the issue of robustness plays an important role: a good code should provide acceptable performance over a wide range of channel models. A different approach to efficient communication over fading channels consists of “changing the game´s rules” and modifying the propagation channel by introducing diversity and/or exploiting transmitter channel state information. For example, it has been shown that antenna reception diversity with maximal-ratio combining turns asymptotically a fading channel into an AWGN channel. Then, optimal codes for the latter will provide good performance over wireless channels if used jointly with antenna reception diversity. More recently, joint antenna transmission diversity and coding has gained a great attention, because of its capability of providing large spectral efficiency. Finally, in some applications the transmitter can be provided with some channel state information. Then, it can counteract the effect of fading by dynamically selecting the appropriate coding scheme and/or by allocating the transmitted power