RIS-Assisted OTFS Enabled Reliable Communication in High Speed Railway Scenario

Orthogonal time frequency space (OTFS) has emerged as a promising 6G wireless communication technique, which is expected to tackle the Doppler effect caused by high mobility vehicles. Reconfigurable intelligent surface (RIS)-assissted system is recently prevalent due to its ability to reconstruct wireless channel. In this letter, a RIS-OTFS system is considered to mitigate the impact of large Doppler shifts on train-to-ground communications in high speed railway (HSR) scenarios. The optimal RIS phases are obtained by constructing the Lagrangian function and applying the gradient update rule, which is called augmented lagrangian multiplier (ALM) method. This approach maximizes the signal-to-noise ratio (SNR) of the communication system. Simulation results show that the proposed ALM method in the RIS-OTFS system outperforms the traditional ones in balanced trade-off between computational complexity and optimization performance when utilizing real-world HSR channel parameters, highlighting the substantial advantages of RIS in signal recovery and enhancement.