The use of directional transmission in millimeter-Wave (mmWave) frequencies results in limited channel coherence time. In this paper, we take the limited channel coherence time into account for non-orthogonal multiple access (NOMA) in mmWave hybrid beamforming systems. Due to the limited coherence time, the beamwidth of the hybrid beamformer affects the beam-training time, which in turn directly impacts the data transmission rate. To investigate this trade-off, we utilize a combined beam-training algorithm. Then, we formulate a sum-rate expression which considers the channel coherence time and beam-training time as well as users’ power and other system parameters. Further, a joint power and beamwidth optimization problem is solved by iterating between the power allocation and the beamwidth optimization. When allocating the power, we use the log-exponential reformulation and the sequential parametric convex approximation (SPCA) methods to solve the non-convex problem. Since beamwidth optimization involves too many variables, we propose an algorithm which iterates between clusters of users. Numerical results show that the optimized mmWave hybrid beamforming-NOMA system can achieve much higher sum-rates compared to NOMA with analog beamforming and traditional multiple access techniques.

中文翻译：

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MmWave混合波束成形-NOMA系统中的联合波束宽度和功率优化

在毫米波（mmWave）频率中使用定向传输会导致有限的信道相干时间。在本文中，我们考虑了毫米波混合波束成形系统中非正交多路访问（NOMA）的有限信道相干时间。由于相干时间有限，混合波束形成器的波束宽度会影响波束训练时间，进而直接影响数据传输速率。为了研究这种折衷，我们利用组合的波束训练算法。然后，我们制定一个求和率表达式，该表达式考虑了信道相干时间和波束训练时间以及用户的功率和其他系统参数。此外，通过在功率分配和波束宽度优化之间进行迭代来解决联合的功率和波束宽度优化问题。分配电源时 我们使用对数指数重构和顺序参数凸逼近法（SPCA）来解决非凸问题。由于波束宽度优化涉及太多变量，因此我们提出了一种在用户群之间进行迭代的算法。数值结果表明，与采用模拟波束形成和传统多址技术的NOMA相比，优化的mmWave混合波束形成-NOMA系统可获得更高的求和率。