Beamforming and Power Control for Multi-Antenna Cognitive Two-Way Relaying

This paper considers a cooperative and cognitive radio (CCR) system where two secondary users (SUs) exchange their information through a "cognitive" relay station (RS) via the two-way relaying. The SUs share the same spectrum with a primary user (PU) while maintaining the interference power at the PU under a certain level. In order to enhance the achievable sum rate, the cognitive RS exploits the channel state information (CSI) of channel links from the RS to PU and from the RS to SUs to determine the relay beamforming (BF) and the transmit power levels for the RS and SUs. The structures of the optimal relay BF and suboptimal BF schemes based on the subspace projection are presented. Both orthogonal and non-orthogonal projection are considered. In addition, power control algorithms are proposed to satisfy the transmit power constraints as well as the interference power constraints. Numerical results are provided to compare the performance of the optimal relay BF with that of the suboptimal ones and also to justify the benefit of the optimal power allocation.