Efficient Non-Orthogonal Multiple Access Scheme Based on Modified Alamouti Code Design
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摘要: 为抑制非正交多址接入(Non-Orthogonal Multiple Access, NOMA)方案多用户组内干扰导致的可靠性差问题，通常是引入空时块编码(Space-Time Block Coding, STBC)技术中最经典的Alamouti编码设计来实现分集提升。然而，现有基于Alamouti辅助的NOMA方案是一种中心与边缘用户需求符号采用等额发送的编码模式。这种模式传承的是传统正交方案所需的Alamouti编码风格，它忽视了非正交传输下中心与边缘用户存在的显著信道差异的事实，不仅浪费中心用户天然信道好的优势，还无形中削弱了STBC技术抑制用户组内干扰的性能。基于此，本文根据用户信道差异显著的特点，提出了非正交通信场景下边缘与中心用户需求符号采用不等额发送的Alamouti编码设计，即构造Alamouti分组发送码的边缘用户需求符号通常比中心用户少。这种通过减少发送符号的做法，一方面对本身信道质量就差的边缘用户来说非常有好处，它可以极大地降低了组内的多用户干扰；另一方面，边缘用户自身符号发送数量的减少会带来它单位发送符号所获得功率的增大，在极大地提升接收端的信干噪比的同时提供更宽的功率优化空间，使得低信噪比场景下还可以获得更高的系统和速率。我们还推导了所提方案在可达和速率与中断概率的精确闭式解。数值仿真显示了所提方案的有效性。Abstract:
Objective Existing Alamouti-coding-based Non-Orthogonal Multiple Access (NOMA) schemes adopt an equal-number symbol transmission mode for both cell-center and cell-edge users, which overlooks the significant channel disparity between the two types of users. This leads to two drawbacks: on one hand, the superior channel condition of cell-center users is not fully exploited, resulting in wasted transmission resources; on the other hand, the equal-number transmission inevitably weakens the performance of Space-Time Block Coding (STBC) in suppressing intra-group multi-user interference. Therefore, a novel design that adapts to user channel differences is urgently needed to improve spectral efficiency and interference mitigation capability. Methods In light of the significant channel disparity between cell-center and cell-edge users, this paper proposes an unequal-number symbol transmission scheme for Alamouti coding in NOMA. Specifically, when constructing Alamouti group transmission codes, the number of symbols required for the cell-edge user is made smaller than that for the cell-center user. By reducing the number of transmitted symbols for the cell-edge user, two benefits are achieved: first, the multi-user interference imposed on the cell-center user is directly reduced; second, under a total power constraint, reducing the number of symbols for the cell-edge user equivalently increases the per-symbol transmission power, thereby significantly improving the received signal-to-interference-plus-noise ratio (SINR). Furthermore, the paper derives perfect closed-form solutions for the achievable sum rate and outage probability of the proposed scheme, and validates its effectiveness via numerical simulations. Results and Discussions The theoretical derivations yield closed-form analytical expressions for the achievable sum rate and outage probability, providing an accurate basis for system performance evaluation. Numerical simulation results demonstrate that, compared with existing equal-symbol Alamouti coding schemes, the proposed unequal-symbol Alamouti coding scheme effectively reduces the intra-group interference from the cell-edge user to the cell-center user, while significantly enhancing the SINR of the cell-edge user through power reallocation. Under typical channel parameters, the system achieves a notable sum-rate gain and a substantial reduction in outage probability, confirming the high efficiency of the proposed scheme. Conclusions The proposed unequal-number symbol transmission scheme based on Alamouti coding for cell-edge and cell-center users fully exploits the potential benefits arising from channel disparity. By reducing the number of symbols transmitted by the cell-edge user, the scheme achieves both suppression of intra-group interference and enhancement of the edge user’s transmission power. The theoretical closed-form solutions and simulation results consistently show that the proposed scheme outperforms conventional equal-number transmission schemes, providing an effective new approach for mitigating multi-user interference and optimizing resource allocation in NOMA systems. -
表 1 所提非等额P-NOMA方案的性能表现(相比传统等额C-NOMA方案的基准)
码率 分集增益 用户公平性 中断概率 和速率 性能表现 3/2，降低1/2 2，不变 无误码传播降低，有则提升 显著提升 低信噪比区域高 
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