Bidirectional CFO Separation for Radial Velocity Estimation In 5G NR TDD V2X Links

Abstract

High-accuracy velocity estimation over orthogonal frequency division multiplexing (OFDM) links is challenging because Doppler shifts and local oscillator (LO) drift contribute the same way to the observed carrier frequency offset (CFO). We propose a bidirectional CFO separation method for a frequency range-1 (FR1) time division duplex (TDD) Uu link between a roadside gNB/TRP and a vehicle user equipment (UE). Downlink (DL) and uplink (UL) CFO estimates are obtained from the physical downlink and uplink shared channel phase tracking reference signals (PT-RS), respectively, their sum isolates the radial Doppler and allow for the extraction of the radial velocity, while their difference recovers the relative LO offset. The estimates are obtained in closed form, without any iterative processing, using only a compact scalar CFO report fed back on the UL, the method can be co-scheduled with an new radio (NR) round trip time (RTT) positioning event to enable simultaneous ranging measurements along velocity estimates. We derive an idealized Cramér-Rao lower bound (CRLB), show that a linear phase slope estimator attains it at high signal to noise ratio, evaluate sensitivity to angle of arrival uncertainty and multipath, and discuss RF chain mismatch. The resulting link level implementation is aligned with NR PDSCH/PUSCH PT-RS procedures and is suitable for network assisted vehicle to everything (V2X) integrated sensing and communication (ISAC).