Intelligent Reflecting Surface Aided High-Speed Train Communications with Relaying: Joint Active and Passive Beamforming Design

Due to the high mobility of a high-speed train (HST), a fast time-varying channel typically exists between the static base station (BS) and high-mobility relays/
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sers, resulting in significant degradation in the communication performance. To address this issue, we propose an intelligent reflecting surface (IRS)-aided HST relaying system, where an IRS is integrated with a full-duplex decode-and-forward (FDDF) relay to aid its data relaying from a BS to multiple users inside the train. Specifically, by jointly optimizing the active and passive beamforming at the BS and relay/IRS sides, a high beamforming gain is achieved at the relay, which copes with the fast time-varying channel between the BS and relay sides in a cost-effective manner. We formulate a problem to minimize the total transmit power of the BS and relay by jointly optimizing the active transmit beamforming at the BS, the active transmit/receive beamforming at the relay, as well as the passive reflect beamforming at the IRS, subject to the rate constraints of the relay and users. To solve this problem, we first decouple it into two independent subproblems. Then, we propose an efficient beamforming design to solve the first subproblem in closed-form by exploiting the two low-dimensional vectors decomposed by the originally high-dimensional BS-relay channel matrix, and transform the second subproblem into a convex second-order cone program that can be efficiently solved via existing convex solvers. Simulation results validate the performance gains of the proposed IRS-aided HST relaying system, as compared to the conventional FDDF relaying system without IRS.