Fundamentals of Turbo Codes

Concatenated coding schemes were first proposed by Forney [1] as a method for achieving large coding gains by combining two or more relatively simple buildingblock or component codes (sometimes called constituent codes). The resulting codes had the error-correction capability of much longer codes, and they were endowed with a structure that permitted relatively easy to moderately complex decoding. A serial concatenation of codes is most often used for power-limited systems such as transmitters on deep-space probes. The most popular of these schemes consists of a Reed-Solomon outer (applied first, removed last) code followed by a convolutional inner (applied last, removed first) code [2]. A turbo code can be thought of as a refinement of the concatenated encoding structure plus an iterative algorithm for decoding the associated code sequence.