Forward Error Correction: Forward error correction is applicable in digital transmission applications: those in which the transmitted signal carries digital data or digitized voice or video data. The term forward refers to procedures whereby a receiver, using only information contained in the incoming digital transmission, cor- rects bit errors in the data. This is in contrast to backward error correction, in which the receiver merely detects the presence of errors and then sends a request back to the transmitter to retransmit the data in error. Backward error correction is not practical in many wireless applications. For example, in satellite communications, the amount of delay involved makes retransmission undesirable. In II obile communica- tions, the error rates are often so high that there is a high probabib y that the retrans- mitted block of bits will also contain errors. In these applicati ns, forward error correction is required.

Forward Error Correction can be achieved as follows:

• Using a coding algorithm, the transmitter adds a number of additional, redundant bits to each transmitted block of data. These bits form n error-correcting code and are calculated as a function of the data bits.

• For each incoming block of bits (data plus error-correcting code), the receiver calculates a new error-correcting code from the incoming d ta bits. If the calcu- lated code matches the incoming code, then the receiver assumes that no error has occurred in this block of bits.

• If the incoming and calculated codes do not match, then one or more bits are in error. If the number of bit errors is below a threshold that depends on the length of the code and the nature of the algorithm, it is pos 'ible for the receiver to determine the bit positions in error and correct all error.