Power Allocation for Maximum MIMO-NOMA System User-Rate

This paper studies the sum-rate of downlink multiple-input multiple-output non-orthogonal multiple access (MIMO-NOMA) systems. The maximum number of users is sought, under a total transmission power constraint and a minimum rate constraint per user. We propose a simplified interference alignment (IA) scheme which does not require global channel state information (CSI); instead, each user has its own CSI, whereas the base station has the statistical information (SI) of all the users. Using the proposed scheme, the users are divided into clusters, where the inter-cluster interference is canceled using IA; whereas the conventional intra-cluster NOMA interference is mitigated using successive interference cancellation (SIC). A closed-form expression is obtained for the achievable ergodic sum rate. Additionally, a power allocation scheme is proposed to maximize the sum rate subject to the aforementioned constraints. Finally, an efficient algorithm is developed to obtain the maximum number of users that can be admitted in the network. The performance is compared to orthogonal multiple access (OMA) schemes. Results show that the proposed scheme efficiently computes the number of users and provides significant gain compared to OMA schemes.