An Adaptive Switching Method for Sum Rate Maximization in Downlink MISO-NOMA Systems

摘要

In this paper, the problem of adaptive transmission mode switching between minimum mean square error beamforming (MMSE-BF) and non-orthogonal multiple access (NOMA)-based maximum ratio transmission (MRT) for downlink weighted sum rate maximization is investigated. In a downlink system with one multi-antenna base station and two single-antenna users, maximizing the weighted sum rate is equivalent to enlarging the corresponding achievable rate region as much as possible. The transmission mode switching is therefore conducted by finding the outer boundary of the union of the achievable rate regions of the two transmission modes. The expressions of the achievable rate region boundaries of MMSE-BF and NOMA-based MRT(NOMA-MRT) are derived, by which the mode switching points are obtained. It is observed from analyses that when the channel vectors of the two users are highly correlated, the achievable rate region of MMSE-BF is completely within that of the NOMA-MRT. Conversely, when the two channel are almost orthogonal, MMSE-BF outperforms NOMA-MRT. Numerical results are presented under various channel conditions to validate the analyses and the proposed adaptive switching method.