Base Station Clock Offset of Cellular Measurements and Its Impact on Positioning

Positioning with cellular networks can serve as a backup or as a complement to the Global Navigation Satellite System (GNSS), making it relevant for autonomous vehicle navigation with integrity monitoring. The position is computed using code phase and carrier phase measurements that are obtained by tracking the downlink cellular signals in the receiver. Integrity monitoring requires rigorous characterization of cellular measurements and modeling errors. An important component of the downlink code phase and carrier phase measurements is the base station (BS) clock offset, which represents the deviation of the BS clock from a reference clock. Leaving the BS clock offset uncorrected or using a clock prediction model that does not fit the clock dynamics properly when computing the position can result in an unacceptable degradation of the positioning performance. The BS clock offset is studied using cellular measurements collected by tracking the cell-specific reference signal (CRS) of two commercially operated long-term evolution (LTE) BS uninterruptedly for eleven days. To avoid cycle slips or losses of the signal lock during high clock dynamics, the optimal bandwidths of the receiver tracking loop filters are first determined via simulations. The analysis of the carrier phase measurements shows that the main contributor to the BS clock offset are regular oscillator frequency jumps. The oscillator frequency jump is a sudden change of the clock oscillator frequency that might occur when the BS steers the clocks toward the primary reference clock. The results of the code-minus-carrier (CMC) analysis show that both monitored stations adjust the code phase and carrier phase clocks coherently. The impact of the BS clock offset on positioning performance is evaluated in a terrestrial cellular scenario, in which the cellular measurements are used to coast during GNSS unavailability. The results show that a clock prediction model can significantly reduce the horizontal position error (HPE) when compared to an uncorrected BS clock offset.