Optimal Distributed Beamforming for Cooperative Cognitive Radio Networks

In this paper, we consider the problem of optimal distributed beamforming at the relay nodes of a cognitive radio (CR) network. We assume that the CR network is a cooperative relay network with amplify-and-forward (AF) relaying scheme and is composed of single-antenna source, relay and destination nodes. We further assume that the direct link between the secondary source and destination is absent. Based on these assumptions, we first formulate the signal-to-interference-plus-noise ratio (SINR) at the primary and the secondary destinations as a function of beamforming vector at the secondary relay nodes. We then develop an SINR maximization problem at the secondary destination subject to satisfying a minimum received SINR at the primary destination to guarantee a minimum quality-of-service (QoS) for the primary network. The proposed optimization problem is not generally convex and we relax it to a convex feasibility problem to be able to solve it numerically. Numerical results show that by using a simple bisection algorithm, we can efficiently find the solution to the constrained SINR maximization problem. We numerically study the performance of the proposed scheme in terms of the received SINR in several different scenarios. The proposed method enables a secondary network to use the licensed spectrum of a primary system while imposing a controlled interference and thereby guaranteeing a minimum QoS to the primary network.