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This paper deals with user selection, considering accessed users and the number of ports. Due to the unstable channel condition caused by the mobility of the satellite, the phased array antenna is applied to adjust the direction of the beam timely to improve the system-level performance. For the multi-beam satellite system, the antenna array must be of a specific size to generate spot beams, which leads to limited co-channel interference between beams. Calculating complexity for the beamforming is related to the scale of the array. Thus, considering the burden caused by the high dimensional operation, practical user selection and low complexity beamforming are applied. The Channel State Information (CSI) quality will influence the beamforming performance. However, all the proposed methods are based on statistical CSI because instantaneous CSI is intractable in the delay-unlimited system. At the same time, we suggest that on-board processing suffers from outdated CSI with a relatively moderate delay compared with on-ground processing. Thus, the on-board system can obtain the statistical CSI with less error because the number of served users is usually larger than the number of antennas. User selection is necessary for improving beamforming performance. In this paper, user selection is divided into two stages. The first one guarantees that beam cells with identical frequency have limited co-channel interference and that users in the same cell have high channel correlation and gain. Each beam cell contains a fixed number of users assigned in different frequencies to avoid predominate intra-cell interference. The second one chooses the proper number of beams shaping vectors from the codebook based on a defined parameter to reduce the complexity of beamforming and the number of used ports. In conclusion, the proposed selection method has two advantages: (1) it enhances the system throughput; (2) it reduces the complexity of the beamforming. Lastly, simulation results will prove the effectiveness of the proposed schemes.
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