Variable-Redundancy Error Control Schemes

Conventional ARQ schemes employ a high-rate code for error detection only. This maximizes the throughput during good channel conditions. However, their throughput degrades rapidly as the channel conditions deteriorate. This degradation can be postponed by embedding FEC in the protocol (e.g. hybrid-ARQ) to correct common errors, and by requesting retransmissions only when uncorrectable errors are detected. The net effect of using a more powerful code for both error detection and correction is an increase in the amount of redundancy in the transmitted message, which reduces the maximum throughput achievable. Conventional ARQ, FEC, as well as hybrid-ARQ are essentially fixed-redundancy schemes. Nonstationary channels such as those encountered in digital mobile communications require adaptive error control. Fixed-redundancy schemes are not suitable for use on these channels since the the coding redundancy is either “too much” under good channel conditions, or “too little” under bad channel conditions, wasting bandwidth in the former case and increasing retransmission frequency in the latter.