User Capacity in Downlink MISO-NOMA Systems

Non-orthogonal multiple access (NOMA) emerges as a strong candidate for the next wireless generation as it promises large spectral efficiency. However, as the number of the users increases, the interference increases. This limits the rate per user. In this paper, the maximum number of users that can be admitted in a NOMA system, under a total power constraint and a minimum rate requirement per user, is sought. A lower bound for the achievable sum-rate is first obtained. A power allocation scheme that maximizes the sum-rate, under the aforementioned constraints, is then proposed. Using the proposed scheme, an efficient algorithm to obtain the maximum number of users is developed. The performance is compared with the orthogonal multiple access (OMA) schemes. Our results show that the proposed scheme efficiently computes the number of users and provides significant gain compared to the OMA schemes.