Spectrum Efficient Resource Allocation of NOMA Downlink System With MMSE Receiver

Non-orthogonal multiple access (NOMA) with the effective successive interference cancellation (SIC) is proved as a promising solution to improve the spectrum efficiency (SE). However, since the effective SIC usually requires complex algorithm and leads to higher processing delay, it may not be accepted by the B5G wireless communication due to the limited hardware resource of the mobile terminal and strict real-time request of service. Therefore, whether the NOMA with simple receiver could keep performance advantage directly relate to the promotion and application of NOMA in future. In this paper, we mainly investigate the SE performance of the NOMA downlink system with Minimum Mean Squared Error (MMSE) receiver (MMSE-NOMA) under the imperfect SIC (ipSIC) and CSI (ipCSI) assumption. Based on the analysis of residual interference caused by the ipSIC and ipCSI, the alterative first-order approximation iteration (A-FOAI) algorithm is proposed to optimal the SE performance of MMSE-NOMA. To make the research more objective and comprehensive, we evaluate the SE performance of NOMA with perfect SIC (pSIC) and CSI (pCSI) by adopting the first-order approximation iteration (FOAI) algorithm firstly. Numerical results show that the performance advantage of NOMA heavily depends on the differential degree among various channel gains. Especially, under the ipSIC and ipCSI conditions, the SE of MMSE-NOMA may even be lower than that of the traditional orthogonal multiple access (OMA) system with MMSE receiver when the differential degree among various channel gains is relatively insignificant. This conclusion can help us correctly adopt MMSE receiver to avoid high hardware resource consumption and processing delay in the future NOMA system.