Bits through ARQs

TL;DR

This paper models a cognitive radio system that adapts its transmission based on primary ARQ feedback to ensure primary performance while optimizing its own capacity under interference uncertainty.

Contribution

It introduces adaptive strategies for cognitive radios using primary ARQ feedback, guaranteeing primary rate targets and achieving capacity in static interference scenarios.

Findings

Simple threshold-based strategy guarantees primary rate target.

Adaptive approach performs well under unknown interference.

Capacity-achieving strategy for static interference.

Abstract

A fundamental problem in dynamic frequency reuse is that the cognitive radio is ignorant of the amount of interference it inflicts on the primary license holder. A model for such a situation is proposed and analyzed. The primary sends packets across an erasure channel and employs simple ACK/NAK feedback (ARQs) to retransmit erased packets. Furthermore, its erasure probabilities are influenced by the cognitive radio's activity. While the cognitive radio does not know these interference characteristics, it can eavesdrop on the primary's ARQs. The model leads to strategies in which the cognitive radio adaptively adjusts its input based on the primary's ARQs thereby guaranteeing the primary exceeds a target packet rate. A relatively simple strategy whereby the cognitive radio transmits only when the primary's empirical packet rate exceeds a threshold is shown to have interesting universal properties in the sense that for unknown time-varying interference characteristics, the primary is guaranteed to meet its target rate. Furthermore, a more intricate version of this strategy is shown to be capacity-achieving for the cognitive radio when the interference characteristics are time-invariant.