Optimal beamforming in spectrum sharing cognitive non‐regenerative multi‐relay networks

Abstract

An optimal beamforming design for spectrum sharing cognitive non‐regenerative multi‐relay networks that maximises the signal‐to‐noise ratio of the secondary user subject to the total transmit power constraint at the relays and the interference power constraint at the primary user is proposed. In the power‐limited regime and the interference‐limited regime, closed‐form beamformers can be found. While in the intermediate operating regime, the beamforming problem is first transformed to a convex semidefinite programming (SDP) via rank‐one relaxation and Charnes‐Cooper transformation, and then the optimal beamformers are found by applying the rank‐one decomposition theorem to the solution of the established SDP. Simulation results show that the proposed solution performs better than existing schemes.