Sequential decoding for multiple access channels

The use of sequential decoding in multiple access channels is considered. The Fano metric, which achieves all achievable rates in the one-user case, fails to do so in the multiuser case. A new metric is introduced and an inner bound is given to it achievable rate region. This inner bound region is large enough to encourage the use of sequential decoding in practice. The new metric is optimal, in the sense of achieving all achievable rates, in the case of one-user and pairwise-reversible channels. Whether the metric is optimal for all multiple access channels remains an open problem. It is noted that even in the one-user case, the new metric differs from the Fano metric in a nontrivial way, showing that the Fano metric is not uniquely optimal for such channels. A new and stricter criterion of achievability in sequential decoding is also introduced and examined