Calibrating Primary Reference Time Clocks for ITU G.8272 with GNSS Simulators

Richard Chan and Ajay Vemuru, Tim Frost

Abstract:	Time synchronization of the clock is a problem for telecom carriers and network equipment manufacturers migrating to next-generation networks. An example of this is in LTE-TDD networks where, typically, 1 µs time accuracy is needed at base stations. The reference time signal to the network is provided by a primary reference time clock (PRTC). The majority of PRTCs obtain their reference time in turn from one of the Global Navigation Satellite Systems (GNSS). The accuracy of this reference time is paramount to the operation of time synchronization throughout time-based networks. The International Telecommunication Union (ITU) has documented recommendation G.8272 that the time output of the PRTC should be accurate to within 100 ns or better when verified against an applicable primary time standard, e.g. Universal Coordinated Time (UTC). The time error of a PRTC output is notoriously difficult to measure due to the relativity of time, i.e. a reference is always required. The only way to provide an independent UTC reference in the field is to use another GNSS receiver, but even this will have accuracy uncertainties in calibrating the PRTC. A description of issues faced for GNSS-based PRTCs is provided, citing current inadequate test methods. This paper reports on an accepted solution by the ITU, integrating a GNSS simulator and a timing monitor instrument to verify the timing performance of the PRTC to better than 100 ns. The simulator generated RF signals precisely synchronized to the 1 pulse-per-second (1PPS) timing output, critical to ensure maximum 100 ns calibration is possible. Through the monitoring instrument, the output time from the device under test (DUT) and reference 1PPS time were compared and the time error calculated. A number of typical GNSS scenarios were performed, each over realistic time periods verifying the solution for a commercial PRTC.
Published in:	Proceedings of the 47th Annual Precise Time and Time Interval Systems and Applications Meeting January 25 - 28, 2016 Hyatt Regency Monterey Monterey, California
Pages:	218 - 222
Cite this article:	Chan, Richard, Vemuru, Ajay, Frost, Tim, "Calibrating Primary Reference Time Clocks for ITU G.8272 with GNSS Simulators," Proceedings of the 47th Annual Precise Time and Time Interval Systems and Applications Meeting, Monterey, California, January 2016, pp. 218-222. https://doi.org/10.33012/2016.13167
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