Mismatched decoding and the multiple-access channel

An achievable region is derived for the multiple-access channel under decoding mismatch conditions. It is shown that achievable rates higher than the random coding capacity of the single-user mismatched channel can sometimes be demonstrated by treating the single-user channel as a multiple-access channel. Refining these ideas we derive a lower bound on the capacity of the mismatched single-user channel, which is tighter than previously published bounds. Using this bound, we are able to answer in the negative the question raised by Csiszar and Narayan (see ibid., vol.41, no.1, p.35, 1995) as to whether equality between the mismatch capacity and the matched capacity implies that the random coding lower bound to the mismatch capacity is tight.