Bandwidth Efficient Coding

Channel coding and interleaving techniques have long been recognized as an effective technique for combating the deleterious effects of noise, interference, jamming, fading, and other channel impairments. The basic idea of channel coding is to introduce controlled redundancy into the transmitted signals that is exploited at the receiver to correct channel induced errors by means of forward error correction. Channel coding can also be used for error detection in schemes that employ automatic repeat request (ARQ) strategies. ARQ strategies must have a feedback channel to relay the retransmission requests from the receiver back to the transmitter when errors are detected. ARQ schemes require buffering at the transmitter and/or receiver and, therefore, are suitable for data applications but are not suitable for delay sensitive voice applications. Hybrid ARQ schemes use both error correction and error detection; the code is used to correct the most likely error patterns, and to detect the more infrequently occurring error patterns. Upon detection of errors a retransmission is requested. Although ARQ schemes are essential for data transmission over wireless channels they nevertheless employ error correction codes. Therefore, the emphasis in this chapter is on error correction coding.