Downlink Secondary Synchronization Signal Design for OTFS Cellular Systems

In this paper, a non-coherent downlink synchronization technique in an orthogonal time-frequency space (OTFS) cellular system at the physical layer is proposed. Here, a Zadoff-Chu sequence (ZCS) carrying a secondary cell identity (SCID) is 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). Unlike OFDM-based fourth (4G) or fifth generation (5G) systems, the detection of SCID with the proposed technique does not require channel estimation and equalization. This enables the UE to detect the SCID from multiple BSs with lower computational complexity. 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. Unlike conventional 1D correlation properties, 2D correlation properties of ZCS are evaluated. Finally, to show the robustness of the proposed technique, the detection probabilities with multiple UE velocities are shown. Simulation results show that when the UE speed is 150 m/s (= 14 kHz Doppler frequency), a 100% detection probability is achieved at -21 dB SNR using the proposed technique, whereas the detection probability for OFDM drops to zero.