Synchronization for Multiuser Uplink OTFS

TL;DR

This paper introduces novel time and frequency synchronization methods for multiuser OTFS uplink in high-mobility environments, improving accuracy and efficiency in estimating timing and frequency offsets.

Contribution

It proposes a spectrally efficient pilot pattern, a threshold-based TO estimation, and a reduced-complexity CFO estimation using CPF-BEM for MU-OTFS uplink.

Findings

Accurate TO and CFO estimation in high mobility scenarios.

Enhanced channel estimation performance.

Validation through numerical simulations.

Abstract

In this paper, we propose time and frequency synchronization techniques for the uplink of multiuser orthogonal time frequency space (MU-OTFS) in high-mobility scenarios. We introduce a spectrally efficient and practical pilot pattern where each user utilizes a pilot with a cyclic prefix (PCP) within a shared pilot region on the delay-Doppler plane. At the receiver, a bank of filters is deployed to separate the users' signals and accurately estimate their timing offsets (TOs) and carrier frequency offsets (CFOs). Our technique employs a threshold-based approach that provides precise TO estimates. Our proposed CFO estimation technique reduces the multi-dimensional maximum likelihood (ML) search problem into multiple one-dimensional search problems. Furthermore, we apply the Chebyshev polynomials of the first kind basis expansion model (CPF-BEM) to effectively handle the time-variations of the channel in obtaining the CFO estimates for all the users. Finally, we numerically investigate the error performance of our proposed synchronization technique in high mobility scenarios for the MU-OTFS uplink. Our simulation results confirm the efficacy of the proposed technique in estimating the TOs and CFOs which also leads to an improved channel estimation performance.