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Reconfigurable intelligent surfaces (RISs) are able to provide passivebeamforming gain via low-cost reflecting elements and hence improve wirelesslink quality. This work considers two-way passive beamforming design inRIS-aided frequency division duplexing (FDD) systems where the RIS reflectioncoefficients are the same for downlink and uplink and should be optimized forboth directions simultaneously. We formulate a joint optimization of thetransmit/receive beamformers at the base station (BS) and the RIS reflectioncoefficients. The objective is to maximize the weighted sum of the downlink anduplink rates, where the weighting parameter is adjustable to obtain differentachievable downlink-uplink rate pairs. We develop an efficient manifoldoptimization algorithm to obtain a stationary solution. For comparison, we alsointroduce two heuristic designs based on one-way optimization, namely,time-sharing and phase-averaging. Simulation results show that the proposedmanifold-based two-way optimization design significantly enlarges theachievable downlink-uplink rate region compared with the two heuristic designs.It is also shown that phase-averaging is superior to time-sharing when thenumber of RIS elements is large.

Two-Way Passive Beamforming Design for RIS-Aided FDD Communication Systems