MAVIS: Map-Aided Vehicular ISAC Synchronization

Non-line-of-sight (NLoS) radar has the potential to significantly improve road safety. Signals in the sub-6 GHz bands are well known to provide good signal strength in such scenarios. This paper considers a bistatic radar that uses a 5G base station (BS) as transmitter (TX), while a user equipment (UE) integrated into an ego vehicle serves as receiver (RX); the goal is to identify target vehicles that may not have line-of-sight (LoS) to the ego vehicle. However, a key challenge is the oscillator/clock synchronization of TX and RX since traditional synchronization algorithms designed for communications purposes are insufficient for radar sensing. We address this issue by developing an algorithm that leverages map priors to identify significant reflected multipath components (MPCs) whose delay and Doppler is leveraged for determining and tracking the timing offset (TO) and carrier frequency offset (CFO) of the RX clock. We also develop a new clutter suppression method, as traditional background subtraction does not work in scenarios with moving RX. We demonstrate the practical feasibility of our proposed techniques via extensive real-world experiments. Our results show absolute synchronization with mean absolute error (MAE) of TO < 4 ns and CFO < 2 Hz, and successful identification of NLoS targets.