Abstract
In order to further improve the system capacity, we explore the integration of non-orthogonal multiple access (NOMA) in millimeter-wave communications (mmWave-NOMA) for future B5G and 6G systems. Compared with the conventional NOMA, the distinguishing feature of mmWave-NOMA is that, it is usually characterized by transmit/receive beamforming with large phased arrays. In this paper, we focus on the design challenges of mmWave-NOMA due to beamforming. Firstly, we study how beamforming affects the sum-rate performance of mmWave-NOMA, and find that with conventional single-beam forming, the performance may be offset by the relative angle between NOMA users. Then, we consider multi-beam forming for mmWave-NOMA, which is shown to be able to achieve promising performance enhancement as well as robustness. Next, we investigate the challenging joint design of the intertwined power allocation and user pairing for mmWave-NOMA. Relevant challenges are discussed and some potential solutions are proposed in detail. We further consider hybrid spatial division multiple access (SDMA) and NOMA in mmWave communications, where some possible system configurations and the corresponding solutions are discussed to address the multi-user issues including multi-user precoding and multi-user interference mitigation. Finally, we present future directions in mmWave-NOMA and summarize the paper.
Highlights
With the rapid development of the wireless communications, large aggregate capacity has always been one of the most critical issues [1]–[4]
The beam gains G1 for User 1 and G2 for User 2 can be different, where Gi = |wHa(φi)| for i = 1, 2, w is the antenna weight vector (AWV), and a(φi) is a given steering vector towards the direction φi [17]. This feature is very important for mmWave-non-orthogonal multiple access (NOMA), because in addition to the degree of freedom in the power domain, it provides another degree of freedom, i.e., beamforming, to improve the performance of mmWave-NOMA, which will be discussed in detail later
We discussed multi-beam forming for mmWave-NOMA, which is shown to be able to achieve improved sum-rate performance and robustness
Summary
With the rapid development of the wireless communications, large aggregate capacity has always been one of the most critical issues [1]–[4]. What further intensifies the unique challenge of mmWave-NOMA is that the multi-directional beamforming is usually entangled with power/beam gain allocation and user pairing in mmWaveNOMA These considerations differ from the existing technical studies on mmWave-NOMA [19], [20], where either random single-beam forming was adopted [19] or a lens array instead of a phased array was used [20]. This feature is very important for mmWave-NOMA, because in addition to the degree of freedom in the power domain, it provides another degree of freedom, i.e., beamforming, to improve the performance of mmWave-NOMA, which will be discussed in detail later. Both users can achieve a higher beam gain
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