Capacity of a POST channel with and without feedback

Abstract

We consider finite state channels where the state of the channel is its previous output. We refer to such channels as POST (Previous Output is the STate) channels. Our focus is on a simple binary POST channel, with binary inputs and outputs where the state determines if the channel behaves as a Z or an S channel (of equal capacities). We show that the non feedback capacity equals the feedback capacity, despite the memory in the channel. The proof of this surprising result is based on showing that the induced output distribution, when maximizing the directed information in the presence of feedback, can also be achieved by an input distribution that is ignorant of the feedback. Indeed, we show that this is a necessary and sufficient condition for the feedback capacity to equal the non feedback capacity for any finite state channel.