A PRACTICAL RELIABILITY METRIC FOR BLOCK-CODES USED ON BINARY-INPUT CHANNELS

A reliability metric (RM) for a block code is defined to be a function that operates on both the decoder input Y (a block of channel output) and the decoder output X (the codeword estimate) and produces, for Y =y and X = x, a real number Q(x,y) as a measure of the reliability of the decoder decision. The best RM, Px|y(x|y), has the disadvantage of depending on the codeword probabilities. Thus, the ideal RM is defined as the value of Px|y(x|y) that would be computed for equally likely codewords. The implementation of the ideal RM is too costly for most applications. This paper proposes an easily implemented RM for binary-input memoryless channels (or for stale-observable channels with a freely evolving state such as some fading channels) when the codewords consist of n 2m -ary symbols, each of which is transmitted serially by m uses of the channel. Simulation results for some BCH codes and some Reed-Solomon codes used in a simple ARQ system show that the proposed RM performs nearly as well as the ideal RM and much better than a previously proposed practical RM. © 1990 IEEE