Feedback Does Not Increase the Capacity of Compound Channels with Additive Noise

TL;DR

This paper demonstrates that for a broad class of discrete compound channels with additive noise, neither feedback nor full transmitter channel state information increases capacity, extending previous results and identifying conditions for the strong converse.

Contribution

It extends the understanding of capacity limits in compound channels with additive noise, showing feedback and CSI do not increase capacity under certain conditions.

Findings

Feedback does not increase capacity in the considered setting.

Full transmitter CSI does not increase capacity under mild conditions.

Conditions for the strong converse to hold are characterized.

Abstract

A discrete compound channel with memory is considered, where no stationarity, ergodicity or information stability is required, and where the uncertainty set can be arbitrary. When the discrete noise is additive but otherwise arbitrary and there is no cost constraint on the input, it is shown that the causal feedback does not increase the capacity. This extends the earlier result obtained for general single-state channels with full transmitter (Tx) channel state information (CSI) to the compound setting. It is further shown that, for this compound setting and under a mild technical condition on the additive noise, the addition of the full Tx CSI does not increase the capacity either, so that the worst-case and compound channel capacities are the same. This can also be expressed as a saddle-point in the information-theoretic game between the transmitter (who selects the input distribution) and the nature (who selects the channel state), even though the objective function (the inf-information rate) is not convex/concave in the right way. Cases where the Tx CSI does increase the capacity are identified. Conditions under which the strong converse holds for this channel are studied. The ergodic behaviour of the worst-case noise in otherwise information-unstable channel is shown to be both sufficient and necessary for the strong converse to hold, including feedback and no feedback cases.