Securing Downlink Non-Orthogonal Multiple Access Systems by Trusted\n Relays

A downlink single-input single-output non-orthogonal multiple access system\nis considered in which a base station (BS) is communicating with two legitimate\nusers in the presence of an external eavesdropper. A group of trusted\ncooperative half-duplex relay nodes, powered by the BS, is employed to assist\nthe BS's transmission. The goal is to design relaying schemes such that the\nlegitimate users' secrecy rate region is maximized subject to a total power\nconstraint on the BS and the relays' transmissions. Three relaying schemes are\ninvestigated: cooperative jamming, decode-and-forward, and amplify-and-forward.\nDepending on the scheme, secure beamforming signals are carefully designed for\nthe relay nodes that either diminish the eavesdropper's rate without affecting\nthat of the legitimate users, or increase the legitimate users' rates without\nincreasing that of the eavesdropper. The results show that there is no relaying\nscheme that fits all conditions; the best relaying scheme depends on the system\nparameters, namely, the relays' and eavesdropper's distances from the BS, and\nthe number of relays. They also show that the relatively simple cooperative\njamming scheme outperforms other schemes when the relays are far from the BS\nand/or close to the eavesdropper.\n