Rateless Coding for Block Fading Channels Using Channel Estimation Accuracy

The design of efficient rateless coding schemes for multicast applications in wireless environments is investigated. First, the digital fountain paradigm for non-ergodic channels is introduced by making use of the dynamic-decoding nature of rateless codes that allows them to adapt opportunistically the code rate to the channel realization (assumed unknown at the transmitter). The information theoretical limits of such codes can be interpreted in terms of the notion of outage capacity. Then, we consider a quasi-static Rayleigh-fading channel with perfect and imperfect channel state information (CSI) at the receiver. We show that the optimal consistent measure of information for decoding with imperfect CSI, is given by the log-likelihood ratio (LLR) of the received bits via a composite (more noisy) channel. The optimization of Raptor codes, which depends on the delay requirements of decoding, is obtained by using Information content evolution under Gaussian approximation. Simulation results show that optimized Raptor codes can operate very close to the theoretical limits on a wide range of delay requirements.