Toward Green and Secure Communications over Massive MIMO Relay Networks: Joint Source and Relay Power Allocation

In this paper, we address the problem of green and secure communications under adverse conditions, i.e., no instantaneous eavesdropper channel state information (CSI) and imperfect instantaneous legitimate CSI. The benefits of a massive multiple-input multiple-output relay are exploited to significantly improve the secrecy energy efficiency (bit per Joule), which is defined as the ratio of secrecy outage capacity and total power consumption. The focus of this paper is on joint power allocation at the source and the relay to maximize the secrecy energy efficiency. Accordingly, two energy-efficient power allocation schemes are proposed under joint and distributed source and relay power constraints. We show that the two schemes asymptotically approach the same saturated energy efficiency. Finally, simulation results validate the advantage of the proposed energy-efficient schemes compared with several baseline schemes.