es 14/9 Encryption 14/11 Spread spectrum systems 14/12 14.9 References 14/13 This page intentionally left blank The need for error control coding 14/3 The need for error control coding simplest decoder can simply extract the unmodified input digits from the coded digit stream, ignoring the parity digits. A more In a digital (discrete) communication system, information is sent as sophisticated decoder may use the parity digits for error detection, a sequence of digits, which are first converted to an analogue and a full decoder for error correction. Unsystematic codes also (continuous) form by modulation at the transmitter, and then con- exist, but are less commonly used. verted back into digits by de-modulation at the receiver. An ideal communication channel would transmit information without any 14.2.2 Feedforward and feedback error form of corruption or distortion. Any real channel introduces noise correction and distortion, however, and these cause the corruption or loss of some digits at the receiver. The system designer can try to reduce In feedforward error correction (FEC) the decoder applies error the probability of digit errors by appropriate design of the analogue correction to the received codeword, and it may also detect some parts of the communication system, but it may be either impossible uncorrectable errors. However, no return path from the receiver to or too costly to achieve a sufficiently small probability of error in the sender is assumed. Either block or convolutional codes may be this way. A better solution will often be to use error control coding used for feedforward error correction. (ECC). In feedback error correction, for which only block codes can be used, the receiver only attempts to detect errors, and sends return 14.2 Principles of ECC messages to the sender which cause repeat transmission if any errors are detected in a received block. In the OSI model for packet data networks, this function is carried out within the data link layer, by Error control coding is the controlled addi