Beamforming strategies for multi‐antenna relay‐assisted networks

Abstract

Physical layer security has received considerable attentions in recent years. Moreover, cooperative communications using some nodes of network as relays make use of the benefit of multi-antenna systems such as increasing the secrecy capacity. This study studies a communication network incorporating some multi-input–multi-output amplify and forward relays in the presence of some eavesdroppers. The objective persuaded in the current work is to find the beamforming matrices at the relays to maximise the achievable secrecy rate under either individual or total power constraint at these nodes. Accordingly, three main approaches including general rank beamforming (GRBF), rank-1 beamforming (RBF) and null-space beamforming (NSBF) are developed. For the NSBF under the total power constraint, a closed-form solution is obtained, while for the remaining scenarios the corresponding optimisation problem is expressed as a semi-definite programming problem. Numerical results indicate that the RBF and NSBF methods follow GRBF in some cases and degraded from GRBF in the others. However, it is shown that these two methods have much lower complexity than GRBF.