A downlink single-input single-output non-orthogonal multiple access setting is considered, in which a base station (BS) is communicating with two legitimate users in two possible scenarios of unsecure environments: existence of an external eavesdropper and communicating through an untrusted relay. For the first scenario, a number of trusted cooperative half-duplex relays is employed to assist with the BS's transmission and secure its signals from the external eavesdropper. Various relaying schemes are proposed and analyzed for that matter: cooperative jamming, decode-and-forward, and amplify-and-forward. For each scheme, secure beamforming signals are devised at the relays to maximize the achievable secrecy rate regions. For the second scenario, with the untrusted relay, achievable secrecy rate regions are derived for two different relaying schemes, compress-and-forward and amplify-and-forward, under two different modes of operation. In the first mode, coined passive user mode, the users receive signals from both the BS and the untrusted relay and combine them to decode their messages. In the second mode, termed the active user mode, the users transmit a cooperative jamming signal simultaneously with the BS's transmission to further confuse the relay. Focusing on half-duplex nodes, the users cannot receive the BS's signal while jamming the relay, i.e., while being active, and rely only on the signals forwarded to them by the relay. It is shown that the best relaying scheme highly depends on the system parameters, in particular the distances between the nodes, and also on the part of the secrecy rate region at which the system is to operate.

中文翻译：

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非正交多路访问中的安全中继：可信和不可信场景


考虑下行链路单输入单输出非正交多址设置，其中基站（BS）在两种可能的不安全环境场景中与两个合法用户通信：存在外部窃听者和通过不可信中继进行通信。对于第一种情况，采用多个可信协作半双工中继来协助BS的传输并确保其信号免受外部窃听者的侵害。为此，提出并分析了各种中继方案：协作干扰、解码转发和放大转发。对于每种方案，在中继处设计安全波束形成信号以最大化可实现的保密率区域。对于第二种情况，使用不可信中继，在两种不同的操作模式下针对两种不同的中继方案（压缩转发和放大转发）导出可实现的保密率区域。在第一种模式中，即创造的被动用户模式，用户从基站和不受信任的中继接收信号，并将它们组合起来解码他们的消息。在被称为活动用户模式的第二种模式中，用户在BS传输的同时发送协作干扰信号，以进一步迷惑中继站。着眼于半双工节点，用户在干扰中继时（即，在活动时）无法接收BS的信号，而仅依赖于中继转发给他们的信号。结果表明，最佳中继方案很大程度上取决于系统参数，特别是节点之间的距离，并且还取决于系统运行的保密率区域的部分。