Fine Time Aiding and Pseudo-Synchronisation of GSM Networks

T. Pratt, R. Faragher, and P. Duffett-Smith

Abstract:	The integration of CDMA and GPS within a single mobile terminal, in support of the FCC E911 initiative, has greatly benefited from the GPS-based synchronisation of the CDMA network. Similar synchronisation is not available in GSM and W-CDMA networks which are, by design, asynchronous in operation, and this makes GPS integration into GSM and W-CDMA mobile terminals more complex and costly. Duffett-Smith & Tarlow [1] have described a new technology for pseudosynchronising a GSM system which can provide a time reference to an embedded GPS receiver with an accuracy better than 2 ƒÝs. This paper examines the scientific basis for GSM pseudosynchronisation, i.e. for using GSM base station clock signals as receptacles for GPS (universal) time in the mobile terminal (GSM handset). There are several key elements required to support this capability, including (a) measuring the position of the mobile terminal using the base station signals with a technique known as Matrix [3 - 6] and (b) receiving BTS signals of sufficiently-good quality from the GSM network from which the GPS time (or frequency) may be derived. In order to support Fine Time Aiding for GPS, we report the first measurements of the time stability of received GSM modulation signals through Allan standard deviation graphs for two independent networks at 900 and 1800 MHz over a range of distances from base stations. These support a systems engineering approach in which the GSM base stations are able to provide stable time references, after calibration, which is usable for many hours. A degradation of the calibration performance with increasing distance from a base station is noted in the measurements, and this is tentatively ascribed to delayspreading in the propagation channel for GSM signals primarily due to multi-path.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	167 - 173
Cite this article:	Pratt, T., Faragher, R., Duffett-Smith, P., "Fine Time Aiding and Pseudo-Synchronisation of GSM Networks," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 167-173.
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