Low-Latency Retro-Reflective Beam Training for RIS-Assisted Cellular Systems

To compensate for high pathloss and improve coverage in millimeter-wave (mmWave) frequencies, highly directional transmit (Tx) and receive (Rx) beamforming is required. In addition, mmWave frequencies are extremely susceptible to blockages, and when blockage occurs, the link between Tx and Rx may be terminated and an alternate path is required to retain communication. Reconfigurable intelligent surface (RIS) can be considered as a possible solution to not only solve the blockage problem but also to cover dead spots. However, using conventional exhaustive search protocol, the overhead of beam training increases exponentially with the number of RIS reflective beams, as it requires finding the optimal beam pair in the Tx-RIS-Rx link. In this paper, a low-latency retro-reflective (LLRR) beam training protocol for RIS-assisted cellular system is proposed to reduce the overall beam training time by reflecting the RIS beams to the source. Furthermore, the feasibility of the RIS element to provide 180° phase shift is analyzed. Simulation results show that the proposed LLRR beam training significantly reduces the training time compared to the state-of-the-art and achieves gains of ≈ 1.5 bps/Hz and ≈ 9.8 bps/Hz at signal-to-noise ratio of 15 dB when the number of beams at RIS is 64 and 256, respectively.