Forward Error Correction (FEC)

In telecommunication, forward error correction (FEC) is a system of error control for data transmission. It differs from standard error detection and correction in that the technique is specifically designed to allow the receiver to correct errors in the currently received data without having to wait for the rest of the message to come in. The important distinction between forward correction and other detection & correction methods, such as CRC or other codes is that in CRC, you need to have all of the bits protected by the checksum received, so that you can compute the CRC of the whole. The difference between forward correction and CRC correction is that in forward correction, it is only the bits already received that are used to correct the "current" bit. In other correction codes, you may need to wait for bits that have not yet been received to determine the correct message.

In general, FEC codes tend to require greater bandwidth than other error-correcting codes but FEC codes are more appropriate for correcting errors "on the fly", as data comes in. FEC devices are often located close to the receiver of an analog signal, in the first stage of digital processing after a signal has been received. That is, FEC circuits are often an integral part of the analog-to-digital conversion process. Many FEC coders can also generate a bit-error rate (BER) signal which can be used as feedback to fine-tune the analog receiving electronics.

The maximum fraction of errors that can be corrected is determined in advance by the design of the code, so different forward error correcting codes are suitable for different conditions.

How it works
FEC is accomplished by adding redundancy to the transmitted information using a predetermined algorithm. Each redundant bit is invariably a complex function of many original information bits. The original information may or may not appear in the encoded output; codes that include the unmodified input in the output are systematic, while those that do not are nonsystematic.

An extremely simple example would be an analog to digital converter that samples three bits of signal strength data for every bit of transmitted data. If the three samples are mostly all zero, the transmitted bit was probably a zero, and if three samples are all one, the transmitted bit was probably a one. The simplest example of error correction is for the receiver to assume the correct output is given by the most frequently occurring value in each group of three.

http://en.wikipedia.org/wiki/Forward_error_correction

Resources