Comparison of OTFS and OFDM for RIS-aided Systems in the Presence of Phase Noise

TL;DR

The paper addresses the challenge of oscillator phase noise in RIS-aided communications and compares OTFS against OFDM under PN. It introduces a two-stage Wiener-filter-based joint RIS channel and phase-noise estimation framework that leverages Doppler and PN statistics, applicable to both OTFS and OFDM, with offline computation of the estimator. Key contributions include an explicit PN model in the delay-Doppler domain, a practical two-stage estimation method, and extensive simulations showing OTFS substantially outperforms OFDM under PN (up to $2$ orders of magnitude BER gains) and the proposed estimator yielding up to $\sim$3 dB BER gains over existing methods. The work demonstrates the robustness of RIS-aided OTFS for high-mobility and PN-prone scenarios and provides a scalable, offline estimator whose complexity does not grow with the number of RIS elements.

Abstract

In this paper, we investigate the performance of RIS-aided orthogonal time frequency space (OTFS) and orthogonal frequency division multiplexing (OFDM) systems in the presence of oscillator phase noise. OFDM is known to be sensitive to phase noise, which could limit the potential gains promised by RIS systems. OTFS, on the other hand, is a compelling potential waveform for RIS-aided systems in the presence of phase noise due to it's resilience to time-varying channels. However, the effect of phase noise on OTFS has not been fully analyzed in the literature as of yet. Additionally, no existing works in the literature consider the effect of phase noise on an RIS-aided OTFS system. Hence, we propose a joint RIS channel and phase noise estimation technique using a Wiener filtering approach. Our proposed method exploits the statistical nature of both the phase noise and the Doppler spread channel in a setup with RIS. Our numerical analysis demonstrates the significant gain of RIS-aided OTFS offers compared to RIS-aided OFDM in the presence in the presence of phase noise. Additionally, our results demonstrate the superiority of our proposed estimation technique, with gains of up to 3~dB in terms of bit error rate (BER), over existing methods in the literature.

Figures (6)

• Figure 1: Illustration of the delay-time domain pilot patterns used for obtaining the initial estimate of the channel and phase noise for OTFS and OFDM systems.
• Figure 2: Block diagram illustration of a first order continuous-time PLL diagram
• Figure 3: NMSE performance of RIS-OTFS and RIS-OFDM for different phase noise levels.
• Figure 4: BER performance of RIS-OTFS and RIS-OFDM for 4-QAM for different phase noise levels.
• Figure 5: BER performance of RIS-OTFS and RIS-OFDM for 4-QAM for different number of RIS elements.