Soft Decision Decoding of Block Codes

Abstract

The use of soft decisions with block codes provides the engineer with another degree of freedom when designing a communication system. The additional information provided by the soft decisions in most instances can provide about 2 dB of additional coding gain and can, therefore, significantly increase the usefulness of a particular code. Soft decision decoding is particularly effective when used with moderate length block codes and provides substantial performance benefits over a broad range of signal-to-noise ratios. It is also effective in concatenation schemes (see Chapter 8) which utilize two or more levels of encoding and decoding. In this case the use of soft decisions is restricted to the innermost coder/decoder, and it is necessary for the soft decision decoding method to provide near optimum performance at word error rates in the range of 10⁻² to 10⁻³. This is in contrast to the usual system requirement that the decoding scheme be effective at error rates less than 10⁻⁵.