Delay-Doppler modelling and performance analysis of RIS-assisted OTFS systems with modified Zak receiver for 6G wireless communications

Emerging wireless communication systems must function reliably in high-mobility environments where channels exhibit rapid time variations. Orthogonal Time Frequency Space (OTFS) modulation has recently emerged as a promising solution for its robustness in time-varying channels by representing signals in the delay-Doppler domain, while Reconfigurable Intelligent Surfaces (RIS) enable adaptive control of wireless propagation through passive beamforming. This paper proposes a joint RIS-assisted OTFS communication framework employing practical rectangular pulse shaping and incorporating fractional delay and Doppler effects to accurately model realistic wireless channels. The end-to-end input–output relationship is derived in the delay-Doppler domain, and two signal processing approaches are investigated: a conventional two-step transformation (time to frequency to delay-Doppler) and a direct one-step transformation using the Zak transform. Simulation results validate that integrating RIS with OTFS significantly improves communication performance in high-mobility scenarios (carrier frequency: 4 GHz, maximum Doppler shift: 1.2 kHz). The Zak transform-based method simplifies receiver design while enhancing detection accuracy, achieving a Bit Error Rate (BER) improvement from 9.4 × 10⁻³ to 3.1 × 10⁻³ at 10 dB SNR. The inclusion of fractional delay and Doppler components along with rectangular pulse shaping provides a realistic, implementable system model. This paper highlights the effectiveness of combining RIS and OTFS for robust, energy-efficient wireless communication in dynamic environments.