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Beamforming design with partial channel estimation and feedback forfrequency-division duplexing (FDD) reconfigurable intelligent surface (RIS)assisted systems is considered in this paper. We leverage the observation thatpath angle information (PAI) varies more slowly than path gain information(PGI). Then, several dominant paths are selected among all the cascaded pathsaccording to the known PAI for maximizing the spectral efficiency of downlinkdata transmission. To acquire the dominating path gain information (DPGI, alsoregarded as the path gains of selected dominant paths) at the base station(BS), we propose a DPGI estimation and feedback scheme by jointly beamformingdesign at BS and RIS. Both the required number of downlink pilot signals andthe length of uplink feedback vector are reduced to the number of dominantpaths, and thus we achieve a great reduction of the pilot overhead and feedbackoverhead. Furthermore, we optimize the active BS beamformer and passive RISbeamformer by exploiting the feedback DPGI to further improve the spectralefficiency. From numerical results, we demonstrate the superiority of ourproposed algorithms over the conventional schemes.

Beamforming Design with Partial Channel Estimation and Feedback for FDD RIS-Assisted Systems