This paper investigates the impact of physical layer secrecy on the performance of a unified non-orthogonal multiple access (NOMA) framework, where both external and internal eavesdropping scenarios are examined. The spatial locations of legitimate users (LUs) and eavesdroppers are modeled by invoking stochastic geometry. To characterize the security performance, new exact and asymptotic expressions of secrecy outage probability (SOP) are derived for both code-domain NOMA (CD-NOMA) and power-domain NOMA (PD-NOMA), in which imperfect successive interference cancellation (ipSIC) and perfect SIC (pSIC) are taken into account. For the external eavesdropping scenario, the secrecy diversity orders by a pair of LUs (the $n$ -th user and $m$ -th user) for CD/PD-NOMA are obtained. Analytical results make known that the diversity orders of the $n$ -th user with ipSIC/pSIC for CD-NOMA and PD-NOMA are equal to zero/ $K$ and zero/one, respectively. The diversity orders of the $m$ -th user are equal to $K$ /one for CD/PD-NOMA. For the internal eavesdropping scenario, we examine the analysis of secrecy diversity order and observe that the $m$ -th user to wiretap the $n$ -th user with ipSIC/pSIC for CD-NOMA and PD-NOMA provide the diversity orders of zero/ $K$ and zero/one, respectively, which is consistent with external eavesdropping scenario. Numerical results are present to confirm the accuracy of the analytical results developed and show that: i) The secrecy outage behavior of the $n$ -th user is superior to that of the $m$ -th user; ii) By increasing the number of subcarriers, CD-NOMA is capable of achieving a larger secrecy diversity gain compared to PD-NOMA.

中文翻译：

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统一非正交多址接入框架中的安全通信

本文研究了物理层保密对统一非正交多址 (NOMA) 框架性能的影响，其中检查了外部和内部窃听场景。合法用户 (LU) 和窃听者的空间位置通过调用随机几何建模。为了表征安全性能，针对码域 NOMA (CD-NOMA) 和功率域 NOMA (PD-NOMA) 推导出了新的保密中断概率 (SOP) 的精确和渐近表达式，其中不完美的连续干扰消除 (ipSIC) ) 和完美 SIC (pSIC)。对于外部窃听场景，保密分集由一对 LU（ $n$ -th 用户和 百万美元 -th 用户）获得 CD/PD-NOMA。分析结果表明， $n$ - 具有 ipSIC/pSIC 的 CD-NOMA 和 PD-NOMA 的第一个用户等于零/ $K$ 和零/一，分别。多样性的命令 百万美元 -th 用户等于 $K$ / 一个用于 CD/PD-NOMA。对于内部窃听场景，我们检查了对保密多样性顺序的分析，并观察到 百万美元 -th 用户窃听 $n$ - 具有用于 CD-NOMA 和 PD-NOMA 的 ipSIC/pSIC 的第一个用户提供零/ $K$ 和零/一，这与外部窃听场景一致。数值结果证实了所开发的分析结果的准确性，并表明：i) 保密中断行为 $n$ -th 用户优于 百万美元 - 第一个用户；ii) 通过增加子载波的数量，CD-NOMA 能够实现比 PD-NOMA 更大的保密分集增益。