Primary Synchronization Signal Design and Low-Complexity Detection for OTFS Cellular Systems

In this paper, a downlink synchronization technique in an orthogonal time-frequency space (OTFS) cellular system at the physical layer is proposed. The primary synchronization signal (PSS) detection is the most computationally complex algorithm during synchronization and cell search. Therefore, a faster and low-complexity algorithm based on the overlap-add method is proposed. Here, PSS is a Zadoff-Chu sequence (ZCS) carrying a primary cell identity (PCID) mapped in the two dimensional (2D) delay-Doppler domain is broadcast by the base station (BS) using OTFS modulation, which is non-coherently detected at the user equipment (UE). Further, the effect of OTFS modulation on 2D delay-Doppler ZCS is investigated and analyzed to show that the duality property of ZCS is conserved between delay-Doppler to time-frequency domains and between time-frequency and time domains. Finally, the detection probabilities with multiple UE velocities are shown in multi-cell environment to show the robustness of the proposed technique. In addition, the OTFS performance is compared with OFDM for third generation partnership project (3GPP) defined tapped delay line (TDL) channel model. Simulation results show that when the UE speed is 540 km/h (
$\equiv \mathbf{14}\mathbf{kHz}$
Doppler frequency), a 99% detection probability is achieved at -13 dB SNR using the proposed technique, whereas the detection probability for OFDM drops to zero.