Downlink Synchronization for OTFS-Based Cellular Systems in High Doppler Environments

Mohammed Saquib Khan,Qasim Sultan,Yeong Jun Kim,Yong Soo Cho,Jingon Joung

doi:10.1109/access.2021.3079429

Abstract

This paper proposes a downlink synchronization and cell identity (CID) estimation technique for cellular systems based on orthogonal time-frequency space (OTFS). In the proposed technique, each base station (BS) transmits one detection preamble (DP) and two OTFS symbols (pilot and secondary-CID (SCID) signal (SS)). The DP based on a linear frequency-modulated (LFM) waveform, called proposed-LFM (pLFM), carries the primary-CID (PCID) for symbol timing (ST) synchronization, whereas the SS based on the Zadoff-Chu (ZC) sequence mapped in the delay-Doppler domain carries the SCID. The pLFM is obtained by discretizing the LFM waveform, increasing the frequency sweeping parameter beyond the limit of the operational bandwidth, and incorporating the PCID into the LFM waveform. The autocorrelation and cross-correlation functions of the pLFM are analyzed to examine its correlation properties under the influence of a high Doppler shift. To reduce the undesirable correlation properties (side peak and time ambiguity), a receiver processing (RP) scheme is developed for the pLFM. It is found that the pLFM with RP is suitable for the DP design because it can provide an accurate ST in high-mobility scenarios. In addition, the influence of OTFS modulation on the ZC sequence is derived and a low-complexity detection algorithm based on message passing is applied to detect SCID at the user equipment. The simulation results demonstrate that the proposed downlink synchronization and CID estimation techniques are suitable for OTFS-based cellular systems in high-Doppler environments.

Highlights

Future wireless communication systems are expected to accommodate numerous diverse requirements and usage scenarios, including high-mobility scenarios with a maximum velocity of 500 km/h [1], [2]
The autocorrelation and cross-correlation functions of pLFM are examined to investigate their correlation properties when subjected to a high Doppler shift
To reduce the undesirable properties of the autocorrelation function, an receiver processing (RP) scheme is developed for pLFM

Summary

INTRODUCTION

Future wireless communication systems are expected to accommodate numerous diverse requirements and usage scenarios, including high-mobility scenarios with a maximum velocity of 500 km/h (high-speed trains) [1], [2]. The delay-Doppler channel estimation, MIMO, multiple access, detection, and equalization techniques of OTFS described in the abovementioned studies considered an ideal synchronization condition or synchronization with the conventional OFDM-based technique to detect OTFS information symbols. This paper proposes a downlink synchronization and CID estimation technique for OTFS-based cellular systems in a high-Doppler environment. A proposed-LFM (pLFM)-based DP design technique and receiver processing technique is presented to provide accurate ST estimation and PCID detection in a high-Doppler environment. The performance of the proposed downlink synchronization technique is examined with the detection probability in low- and high-mobility cellular environments and compared with the results obtained using the OFDM system.

PRELIMINARIES

SIMULATION

Findings

CONCLUSION