Time and Frequency Synchronization for Multiuser OTFS in Uplink

This paper presents novel time and frequency synchronization techniques for uplink multiuser orthogonal time frequency space (MU-OTFS) systems in high-mobility scenarios. This work focuses on accurately estimating and correcting timing offsets (TOs) and carrier frequency offsets (CFOs), which are essential for reliable user pilot localization and channel estimation, respectively. A TO estimation method is first developed for an existing MU-OTFS pilot structure by replacing the conventional impulse pilot (IMP) with a more practical pilot with a cyclic prefix (PCP), referred to as the single-user-inspired PCP (SU-PCP). This structure employs different Zadoff-Chu (ZC) sequences, enabling pilot separation via correlation at the receiver and facilitating a correlation-based TO estimation technique for uplink MU-OTFS using this pilot structure. A multiuser PCP (MU-PCP) pilot pattern is subsequently proposed, where each user transmits a PCP within a shared pilot region on the delay-Doppler plane. The second TO estimation technique utilizes a bank of filters to separate user signals and accurately estimate their TOs. To further improve estimation accuracy, a mathematical threshold range is derived to identify the first major peak in the correlation function rather than relying solely on the maximum peak. Following TO estimation, a CFO estimation technique is proposed that transforms the multidimensional maximum-likelihood (ML) search into multiple one-dimensional searches. By employing the Chebyshev polynomials of the first-kind basis expansion model (CPF-BEM), the method effectively accounts for channel time variations in estimating CFOs for all users. Simulation results verify high-accuracy TO and CFO estimation and improved channel performance in high-mobility MU-OTFS uplinks.