Joint User Pairing and Power Allocation Scheme Based on Transmission Mode Switching between NOMA-Based Maximum Ratio Transmission and MMSE Beamforming in Downlink MISO Systems

This paper investigates the design of the joint user pairing and power allocation scheme with transmission mode switching (TMS) in downlink multiple-input-single-output (MISO) systems. Firstly, the closed-form expressions of the boundary of achievable rate region of two candidate transmission modes, i.e., non-orthogonal multiple access based maximum ratio transmission (NOMA-MRT) and minimum mean square error beamforming (MMSE-BF), are obtained. By obtaining the outer boundary of the union of the achievable rate regions of the two transmission modes, an adaptive switching method is developed to achieve a larger rate region. Secondly, based on the idea that the solution to the problem of weighted sum rate (WSR) optimization must be on the boundary of the achievable rate region, the optimal solutions to the problem of WSR optimization for NOMA-MRT and MMSE-BF are obtained for the two-user case, respectively. Subsequently, by exploiting the aforementioned optimal solutions for two transmission modes and the high efficiency of TMS, a suboptimal user pairing and power allocation algorithm (JUPA) is proposed to further improve the sum rate performance for the multiuser case. Compared with the exhaustive search-based user pairing and power allocation algorithm (ES-PPA), the proposed JUPA can enjoy a much lower computational complexity and only suffers a slight sum rate performance loss, and it outperforms other traditional schemes. Finally, numerical results are provided to validate the analyses and the proposed algorithms.