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Reconfigurable intelligent surface (RIS) is a promising technology for futurewireless communications due to its capability of optimizing the propagationenvironments. Nevertheless, in literature, there are few prototypes servingmultiple users. In this paper, we propose a whole flow of channel estimationand beamforming design for RIS, and set up an RIS-aided multi-user system forexperimental validations. Specifically, we combine a channel sparsificationstep with generalized approximate message passing (GAMP) algorithm, and proposeto generate the measurement matrix as Rademacher distribution to obtain thechannel state information (CSI). To generate the reflection coefficients withthe aim of maximizing the spectral efficiency, we propose a quadratictransform-based low-rank multi-user beamforming (QTLM) algorithm. Our proposedalgorithms exploit the sparsity and low-rank properties of the channel, whichhas the advantages of light calculation and fast convergence. Based on theuniversal software radio peripheral devices, we built a complete testbedworking at 5.8GHz and implemented all the proposed algorithms to verify thepossibility of RIS assisting multi-user systems. Experimental results show thatthe system has obtained an average spectral efficiency increase of 13.48bps/Hz,with respective received power gains of 26.6dB and 17.5dB for two users,compared with the case when RIS is powered-off.

Multi-user beamforming in RIS-aided communications and experimental validations