Channel Estimation and Tracking for Wideband mmWave Satellite Communications With Reconfigurable Intelligent Surface

In this paper, we investigate channel estimation and tracking for wideband millimeter wave (mmWave) satellite communications (SatComs) with reconfigurable intelligent surface (RIS). Different from the existing schemes that separately estimate the uplink channels from the user to the RIS and from the RIS to the satellite, we directly estimate the cascaded channel by proposing two schemes. In the first scheme with two stages, we power off different antennas in each stage and estimate the channels based on the estimating-signal-parameter-via-rotational-invariance-techniques (ESPRIT). In the second scheme based on the null space projection (NSP), we estimate the equivalent channel matrix through projecting the dictionary steering vectors to the null space of the received signal covariance matrices. The NSP-based scheme does not power off any antenna and needs only one stage. In addition, we propose a gradient descent (GD)-based channel tracking scheme for the moving user. We first obtain a rough estimation of the user channel based on geometry relationships and then make channel refinement using the GD. Simulation results show that the NSP-based scheme needs fewer pilots than the ESPRIT-based scheme but at the cost of some performance sacrifice. The GD-based channel tracking scheme outperforms the existing schemes.