Fine Time Aiding in Unsynchronised Cellular Systems: The Benefits for GPS Integration in Mobile Phones

P. Duffett-Smith and T. Pratt

Abstract:	GPS will play a central role in the provision of location based services using mobile cellular telephones. Many commercially-available handsets (terminals) already include integrated GPS components, mostly for use on CDMA and CDMA 2000 networks in which the transmitted signals are synchronised to GPS time. This synchronisation brings advantage in that it is relatively straightforward to provide the GPS receiver in the mobile terminal with assistance in the form of GPS time accurate to within approximately 10 ƒÝs. By a somewhat complex route, the time aiding extends the usability of the GPS service into buildings and other shielded environments. However, on GSM and W-CDMA networks, which dominate globally, the provision of time aiding is more difficult because the network transmitters are asynchronous by design. There is no concept of 'network time', and the transmitted signals have no pre-determined relationship with each other or with GPS time. Duffett-Smith et al. [1,2] have proposed and demonstrated a method of providing fine time aiding (FTA) in GSM and W-CDMA networks, called Enhanced GPS (E-GPS), which uses the Matrix method [3-6] of locating a mobile terminal from the network signals received by the terminal. In E-GPS, the network signals are used as a ¡¥receptacle¡¦ for GPS time after calibration. In ref. [1], they reported the first measurements made on GSM networks showing that FTA accuracies of 1 ƒÝsec or better were sustainable even over several hours. This conclusion is further supported in a companion paper to this one [7] which demonstrates that, in some circumstances, FTA at the sub 2 ƒÝs level is possible over elapsed time periods exceeding one day. In this paper, we examine and quantify the benefits which FTA brings in terms of the reduction in the complexity of a GPS implementation. We show how FTA combined with a precise knowledge of the local oscillator offset render it unnecessary to use massive parallel correlator hardware designs. On the contrary, we show that excellent in-building performance may be achieved using as few as two complex correlators per GPS satellite channel. These benefits come mainly from three factors: (a) a reduction in the code phase search window by a factor of more than 500; (b) frequency stabilisation of the local oscillator using the network signals; and (c) the use of a stronger statistical test in the signal detection algorithm for a given false-alarm rate. The changes in the statistical tests requirements in (c) provide an effective gain in receiver sensitivity of up to 6 dB whatever number of correlators is used. The further benefits of FTA can be ¡¥traded off¡¦ between cost saving (simpler GPS implementations), time saving (time to first acquisition), and sensitivity increase (penetration into inbuilding environments etc.). FTA at 2 ƒÝs accuracy provides the simplest and lowest-cost GPS receiver with an overall performance gain approaching 23 dB, giving it a performance in speed and sensitivity similar to that achieved using much more expensive parallel correlator designs.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	159 - 166
Cite this article:	Duffett-Smith, P., Pratt, T., "Fine Time Aiding in Unsynchronised Cellular Systems: The Benefits for GPS Integration in Mobile Phones," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 159-166.
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