On Optimal Power Allocation in Multibeam Multicast NOMA for Satellite Communication Systems

Abstract

This paper addresses two key challenges in the multibeam multicast non-orthogonal multiple access (MB-MC-NOMA) scheme and respective beamforming design problems in satellite systems. Achieving the max-min fairness (MMF) and maximum sum-rate among multiple multicast groups of users are jointly considered in a theoretical information framework. It is assumed that each frame contains information of multiple users in multicast transmission. Therefore, contrary to the unicast linear precoding, we have developed the multicast linear precoding with mapping function considering trade-offs to deal with the lack of the spatial degree of freedom. In our proposed scheme, each beamforming vector conveys information to more than one group of users in a NOMA framework, relying on the superposition techniques at the transmitters and successive interference cancellation (SIC) at the receivers. We have derived the capacity rates achievable in each beam, proposing the methods to maximize the minimum rate and weighted sum-rate. Considering the dependency of the broadcasting power and the respective achievable rates, the equivalent channel and water-filling algorithm for the MB-MC-NOMA is developed; as such, the optimal transmit power density for the groups of users within multiple beams are efficiently computed. The extensive simulation results confirm the proposed theoretical findings, providing a considerable boost in both minimum-rate and sum-rate with respect to state-of-the-art multibeam multicast satellite systems.