IRS-Assisted Multi-user Downlink Satellite Communication Systems: Joint Optimization with Spreading Designs

In this paper, we investigate the issue using intelligent reflecting surface (IRS) to assist multi-user downlink satellite communication (SatCom) systems with spreading designs. Two transmission strategies for the public data and private data have been discussed, employing time division multiple access (TDMA) and code division multiple access (CDMA). The goal is to maximize the weighted sum rate (WSR) by optimizing the IRS phase shift matrix and spreading code vector (SCV). An optimization framework is designed based on the WMMSE algorithm and Taylor approximation to transform non-convex optimization problems equivalently. And we solve them using alternating iterations. The simulation results show: 1) the proposed optimization approach achieves at least a improvement in WSR over conventional methods at the lowest received signal-to-noise ratio (SNR), 2) WSR shows an upward trend with the increase in the number of IRS reflecting elements increases, 3) WSR shows a downward trend with the increase in the length of SCV.