A single-letter upper bound on the feedback capacity of a unifilar finite-state channel is derived. The upper bound is tight for all cases where the feedback capacity is known. Its efficiency is also demonstrated by direct application of the bound on the dicode erasure channel, which results in a new capacity result. The bound is based on a new technique, called the Q-contexts mapping, where the channel outputs are recursively quantized to a finite set, called the contexts set.
['Oron Sabag', 'Haim H. Permuter', 'Henry D. Pfister']
A single-letter upper bound on the feedback capacity of unifilar finite-state channels