This letter proposes a novel hybrid-domain (HD) non-orthogonal multiple access (NOMA) approach to support a larger number of uplink users than the recently proposed code-domain NOMA approach, i.e., sparse code multiple access (SCMA). HD-NOMA combines the code-domain and power-domain NOMA schemes by clustering the users in small path loss (strong) and large path loss (weak) groups. The two groups are decoded using successive interference cancellation while within the group users are decoded using the message passing algorithm. To further improve the performance of the system, a spectral-efficiency maximization problem is formulated under a user quality-of-service constraint, which dynamically assigns power and subcarriers to the users. The problem is non-convex and has sparsity constraints. The alternating optimization procedure is used to solve it iteratively. We apply successive convex approximation and reweighted $\ell _1$ minimization approaches to deal with the non-convexity and sparsity constraints, respectively. The performance of the proposed HD-NOMA is evaluated and compared with the conventional SCMA scheme through numerical simulation. The results show the potential of HD-NOMA in increasing the number of uplink users that can be supported in a given time-frequency resource.

中文翻译：

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一种新型光谱高效上行链路混合域 NOMA 系统

这封信提出了一种新颖的混合域 (HD) 非正交多址 (NOMA) 方法，以支持比最近提出的码域 NOMA 方法（即稀疏码多址 (SCMA)）更多的上行链路用户。HD-NOMA 通过将用户聚集在小路径损耗（强）和大路径损耗（弱）组中，结合了码域和功率域 NOMA 方案。这两个组使用连续干扰消除进行解码，而组内用户使用消息传递算法进行解码。为了进一步提高系统性能，在用户服务质量约束下制定了频谱效率最大化问题，该问题动态地为用户分配功率和子载波。该问题是非凸的，并且具有稀疏性约束。交替优化程序用于迭代求解。我们分别应用连续凸逼近和重新加权的 $\ell_1$ 最小化方法来处理非凸性和稀疏性约束。通过数值模拟评估了所提出的 HD-NOMA 的性能并与传统的 SCMA 方案进行了比较。结果显示了 HD-NOMA 在增加给定时频资源中可支持的上行链路用户数量方面的潜力。