Joint mode selection and resource allocation to flexibly guarantee quality of service for downlink MISO-NOMA systems

Non -orthogonal multiple access (NOMA) with successive interference cancellation (SIC) detection, known as SIC NOMA, enjoys higher spectral efficiency (SE), but suffers increased processing complexity and additional error propagation compared to Non -SIC NOMA, in which the receivers directly detect their own signals from the superposed signal without SIC detection. We propose a joint Mode Selection and Power Allocation (MSPA) scheme that can flexibly realize different kinds of tradeoffs while guaranteeing minimum rate requirements (MRRs) for two -user MISO-NOMA downlink. Specifically, we first obtain rate region boundaries that guarantee two users' MRRs for two candidate modes: maximum -ratio transmission (MRT)/SIC NOMA and minimum mean square error beamforming (MMSE-BF)/Non-SIC NOMA. We then construct an expression of power allocation factors (transmit power factors) that can satisfy the two users' MRRs for MRT/SIC NOMA (MMSE-BF/Non-SIC NOMA) by introducing a parameter lambda. Finally, we achieve outer boundary of the union of the two modes' rate regions by mode selection and derive expected users' rates on it. Numerical results demonstrate that the proposed MSPA can improve SE and flexibly achieve different kinds of tradeoffs by adjusting lambda, while guaranteeing the two users' MRRs simultaneously for high signal to noise ratio (SNR) scenario and alternately guaranteeing the two users' MRRs for low SNR scenario.