We derive a single-letter upper bound to the mismatched-decoding capacity for discrete memoryless channels. The bound is expressed as the mutual information of a transformation of the channel, such that a maximum-likelihood decoding error on the translated channel implies a mismatched-decoding error in the original channel. In particular, a strong converse is shown to hold for this upper-bound: if the rate exceeds the upper-bound, the probability of error tends to 1 exponentially when the block-length tends to infinity. We also show that the underlying optimization problem is a convex-concave problem and that an efficient iterative algorithm converges to the optimal solution. In addition, we show that, unlike achievable rates in the literature, the multiletter version of the bound does not improve. A number of examples are discussed throughout the paper.

翻译：我们为离散的无记忆频道得出一个不匹配解码能力的单字母上限。 约束的表达方式是频道转换的相互信息, 从而在翻译的频道上出现最大似解码错误意味着原始频道出现不匹配解码错误。 特别是, 显示一个强烈的反差可以维持这个上限: 如果利率超过上限, 当整块长度倾向于无限时误差的可能性会成1 倍。 我们还表明, 潜在的优化问题是一个螺旋形连接问题, 高效的迭接算法会与最佳解决方案汇合。 此外, 我们还表明, 与文献中可实现的速率不同, 约束的多字母版本不会改善。 在整个纸张中都讨论一些实例 。