Link adaptation in retransmission-based cognitive radio systems

In this paper we consider cognitive radio systems with automatic repeat request (ARQ)-based error control and propose a throughput-maximizing transmission strategy at the cognitive transmitter which takes advantage of opportunities that arise during ARQ retransmissions. In particular, we present a cross-layer approach to link adaptation design which integrates adaptive modulation and coding (AMC) at the physical layer with packet retransmission at the data-link layer. In this formulation, the cognitive throughput performance is maximized subject to (i) a packet-loss-rate (PLR) constraint at the cognitive link, and (ii) a minimum required throughput and a PLR constraint at the primary link. The results indicate that the maximum cognitive throughput is determined based on a tradeoff between the primary PLR and delay requirements.