Securing Downlink Non-Orthogonal Multiple Access Systems by Trusted Relays
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A downlink single-input single-output (SISO) non-orthogonal multiple access (NOMA) system is considered where a base station (BS) is communicating with two legitimate users in the presence of an external eavesdropper. A number of trusted cooperative half-duplex relay nodes, that are powered by the BS, are introduced to assist the BS's transmission. The goal is to design relaying schemes such that the legitimate users' secrecy rate region is maximized subject to a total power constraint on the BS and the relays' transmission. We investigate three relaying schemes: cooperative jamming, decode-and-forward, and amplify-and-forward. Depending on the scheme, we carefully design a transmit beamforming signal at the relay nodes that either diminishes the eavesdropper's rate without affecting that of the legitimate users, or increases the legitimate users' rates without increasing that of the eavesdropper. The results show that there is no relaying scheme that fits all conditions; the best relaying scheme depends on the system parameters, namely, the relays' and eavesdropper's distances from the BS, and the number of relays. They also show that the relatively simple cooperative jamming scheme outperforms other schemes when the relays are far away from the BS and/or close to the eavesdropper.
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