Fixed-Redundancy Error Control Schemes

Abstract

Channel coding is implemented in a variety of ways in a digital communication system. A simple approach, known as automatic-repeat-request (ARQ), is to perform error detection at the receiver, and request retransmission of the packet if errors are detected [35, 36, 63, 286]. Conventional ARQ schemes require a feedback channel to send acknowledgements (positive or negative). Under good channel conditions, the throughput of ARQ schemes is very high, limited only by the rate of the error-detecting code. However, as the channel noise increases, the throughput degrades rapidly as a result of an increase in retransmission frequency. In the absence of a feedback channel, error control has to rely on forward error correction (FEC) alone. Various convolutional coding and decoding techniques discussed in this book so far are FEC schemes. FEC schemes offer a constant throughput, equal to the coding rate, but their reliability degrades as the channel noise increases.