4.8: Advanced Codes

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Paul Penfield, Jr.
Massachusetts Institute of Technology via MIT OpenCourseWare

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Block codes with minimum Hamming distance greater than 3 are possible. They can handle more than single errors. Some are known as Bose-Chaudhuri-Hocquenhem (BCH) codes. Of great commercial interest today are a class of codes announced in 1960 by Irving S. Reed and Gustave Solomon of MIT Lincoln Laboratory. These codes deal with bytes of data rather than bits. The (256, 224, 5) and (224, 192, 5) Reed-Solomon codes are used in CD players and can, together, protect against long error bursts.

More advanced channel codes make use of past blocks of data as well as the present block. Both the encoder and decoder for such codes need local memory but not necessarily very much. The data processing for such advanced codes can be very challenging. It is not easy to develop a code that is efficient, protects against large numbers of errors, is easy to program, and executes rapidly. One important class of codes is known as convolutional codes, of which an important sub-class is trellis codes which are commonly used in data modems.