Passive Beamforming for RIS-Assisted OTFS Systems

Orthogonal time frequency space (OTFS) has emerged as a promising physical layer technnique to reduce the influence caused by relative movement in a mobile scenario. However, system performance of OTFS dramaticlly declines, when the transmission path is obstructed by obstacles. Reconfigurable intelligent surfaces (RIS) is a novel scheme, which can construct the reflection path and realize passive beamforming by adjusting the reflection coefficient of the reflector unit. Therefore, many researches have focus on kinds of have emerged as promising physical layer technniques. For the RIS-assisted OTFS system, the optimization function is constructed with the goal of maximizing the received signal-to-noise ratio (SNR), and the traditional method of multiple scaling to solve the reflection coefficient will cause a loss of system performance. In order to further improve the system performance, in this paper, we use the alternating iterative method to solve the problem. Since the phase-shift optimization problem in this system is a non-convex optimization problem, we solve each reflection coefficient iterately, and give a closed-form solution in each iteration. Simulation results show that the proposed method can achieve a higher received SNR and has good convergence. The simulation results also verify that RISassisted OTFS system has better performance than RIS-assisted orthogonal frequency division multiple (OFDM) system.