Revisiting Incremental Relaying and Relay Selection for Underlay Cognitive Radio

Cooperative relaying combined with selection exploits spatial diversity to improve the performance of interference-constrained secondary users in an underlay cognitive radio network. While a relay improves the signal-to-interference- plus-noise ratio, it requires two hops and also generates interference to the primary. Therefore, in underlay cognitive radio, new criteria are needed to determine which relay to select. We present an optimal relay selection rule that maximizes the fading-averaged transmission rate of an average interference-constrained underlay secondary network. It differs from the many rules proposed in the literature. We then analyze its fading-averaged channel capacity. Numerical results show that the proposed rule outperforms direct transmission and several other rules, such as incremental relaying, proposed in the literature.