Beamforming in Relay-Assisted Cognitive Radio Systems: A Convex Optimization Approach

Cognitive radio (CR) has been recently proposed as a promising technology to improve the spectrum utilization. In this paper, we consider the spectrum sharing between CR users and a licensed primary user (PU) in order to enhance the spectrum efficiency. In the considered scenario, the PU is located at a far position from the primary base station (PBS) which makes it difficult to obtain a successful transmission. In this case, the cognitive base station (CBS) at the vicinity of the primary network, offers its help to transmit the primary data. As a reward, the CR system will be able to share the spectrum with the primary system. With the deployment of multiple antennas at the CBS, quality-of-service (QoS)-aware spectrum underlay CR network is considered. In particular, we formulate a "fair'' opportunistic spectrum sharing approach that determines the optimal beamforming weights in order to maximize the overall minimum throughput of the CRs while guaranteeing the QoS of the PU. The original optimization problem is non-convex and does not have any closed-form solution. However, using a convex optimization approach, we transform it to a convex form and find an approximate solution using standard numerical methods. Simulation results will demonstrate the effectiveness of the proposed approach.