An approach for user grouping and power allocation for downlink NOMA in a cellular wireless system

This research focuses on how to improve benefit of downlink non-orthogonal multiple access (NOMA) concept in a cellular wireless system. The issues related with a user grouping in NOMA and an allocation of power among users are crucial to maximize the system performance and efficiency. Power allocation at transmitter side, as well as successive interference cancellation (SIC) at receiver side, are key operations which time consumption and computational complexity increase with increase of number of users in a NOMA group. In addition, accommodating more users in a group requires an increase in transmit power. As a result of trade-off between practicability and capacity gain provided by NOMA, we examine a user grouping scheme in which the number of users in a group does not exceed three. Namely, the idea is to avoid orthogonal multiple access (OMA) users and additional communication resources employing hybrid three-users NOMA grouping and NOMA pairing. The application of power allocation algorithm maximizes achievable sum rate (ASR) of NOMA group with restriction that individual user rates in NOMA must be higher than in the case of OMA system providing user fairness. Under these constraints, expressions for optimal values of power allocation coefficients are obtained in closed-form. Computer simulations are carried out for the system with perfect SIC over Rayleigh fading channels taking into account the influence of interferences from nearby base stations (BSs). The results demonstrate the effectiveness of the proposed framework for user grouping and power allocation compared with conventional NOMA pairing algorithm and OMA transmission.