Rate-Optimal Relay Selection for Average Interference-Constrained Underlay CR

Cooperative relaying combined with selection exploits spatial diversity to improve the performance of interference-constrained secondary users in an underlay cognitive radio (CR) network. While a relay improves the signal-to-interference-plus-noise ratio (SINR) of the secondary network, it requires two hops and also generates interference to the primary network. We present a novel, optimal relay selection rule that maximizes the fading-averaged transmission rate of an average interference-constrained underlay secondary network. It differs from the several ad hoc incremental relaying schemes proposed in the literature, while requiring a feedback overhead that is comparable to them. We then analyze the average rate of the optimal rule. We also present insightful high and low SINR asymptotic analyses, which bring out the extent to which the use of the relays improves the average rate as a function of the system parameters. Our numerical results show that the proposed rule outperforms several known relay selection schemes for CR, and also characterize the regimes in which some of these schemes are near-optimal.