Coexistence through ARQ retransmissions in fading cognitive radio channels

This paper considers the problem of user coexistence in cognitive radio networks operating under fading. The core idea of this work is to exploit the opportunities occurring during retransmissions of a primary system to allow the coexistence of cognitive radios. Several protocols are proposed depending on the amount of the channel knowledge available at the cognitive transmitter about the network links while avoiding the assumption of non-causal knowledge of the primary user data that appears in some existing works. The protocols presented allow the cognitive link to achieve non-trivial throughput while causing minimal effects on the throughput of the primary link. Performance analysis of the protocols is presented and their effectiveness is verified via simulations.