Cognitive relay selection with incomplete channel state information of interference links

The availability of channel state information (CSI) about the interference links from the secondary transmitters to the primary receivers is widely assumed in the literature on underlay cognitive radio (CR) in order to control the interference caused to the primary network. However, when multiple primary receivers are present, acquiring such CSI about all the interference links in a timely and scalable manner is practically challenging. We study an underlay cooperative relay system, in which the channel gains of only a subset of the interference links are available at the source and relays. Based on such incomplete CSI, the source and relays back-off their transmit powers in order to satisfy an interference outage constraint. We derive a tight upper bound on the outage probability of the secondary system for the rate-optimal relay selection rule. Our numerical results show the effect of incomplete CSI on the secondary system performance and how its impact can be ameliorated.