Carrier-aided clock skew estimation for ToA ranging with minimal overhead

The distance between two wireless nodes can be obtained by taking time-of-arrival (ToA) timestamps during a round-trip measurement. Such a two-way ranging approach does not depend on accurate clock synchronization between the nodes. However, the timestamps in each node are taken with respect to the local oscillator which is subject to time-variable clock skews influencing the round-trip time and distance estimate. The symmetric double-sided ranging protocol is able to minimize this ranging error, but requires a second round-trip measurement, thus increasing the energy consumption of the nodes. This paper presents a novel carrier-aided clock skew estimation approach, which estimates the skew based on the carrier frequency estimate of uplink frames, thus removing the necessity for a second round-trip measurement. We show the feasibility of our approach by an FPGA-based prototype for IEEE 802.11 WLAN. The measurements show that the skew estimation error is about 40 ppb for a 1 ms frame. Given that the mobile devices requires 30 ms to respond to a ranging frame, the skew estimation error and the oscillator instability account for a ranging error of below 13 cm for the 80 % percentile.