Earth-Fixed Multicast User Subgrouping for NGSO Satellite With Phased Array Antenna

In nongeostationary orbit (NGSO) satellite networks, multicast transmission is increasingly important to improve the system performance. Making full use of the beams to cover numerous nonuniform distributed users is a new challenge. User subgrouping techniques are utilized to optimize the coverage of beams and maximize the data rate. However, due to the high-speed movement of NGSO satellites, frequent handovers occur and the user subgroups should be updated frequently, aggravating network control overheads. This article proposes an Earth-fixed multicast user subgrouping scheme for NGSO satellite constellations to reduce network control overheads and improve network throughput (NT). In our proposed scheme, users are partitioned into multiple subgroups according to their geographical locations, and these subgroups remain the same while satellites keep moving along the orbit. First, we formulate a joint user subgrouping and beam optimization problem and decompose it into three subproblems by the approximation of models. An iterative greedy Earth-fixed user subgrouping algorithm is presented to obtain the user subgroups. Each user is chosen into a user subgroup which can maximize the NT, and then the position of each user subgroup is updated according to the positions of users in this subgroup. Next, the subgroup-satellite association relationship is optimized according to the maximum elevation angle criterion. Finally, a simple algorithm is proposed to optimize the direction and beamwidth of the beam serving each subgroup in each satellite. Simulation results demonstrate that our proposed scheme can reduce the update frequency significantly compared with the existing satellite-fixed user subgrouping scheme, and the NT achieved by our proposed scheme is improved by 1~3 times than conventional beam coverage scheme.