A combined underlay and interweave strategy for cognitive radios

This paper addresses a hybrid setup for cognitive radio based on Gaussian interference channel where the secondary user can use both interweave and underlay strategies. The primary user does not cooperate or adapt since it is using legacy hardware. We show that these assumptions lead to a non-convex achievable rate region for various types of hybrid interweave-underlay strategies and that the rate optimization problem for the secondary user is in general non-convex and non-smooth. We analyze the structure of this optimization problem to reduce it to a number of tractable subproblems in various interference regimes. Numerical simulations are also presented to give insight into the performance of the proposed schemes.