Analysis of GPS Time-Transfer Accuracy in GSM and UMTS Networks and Possibilities to Improve Sensitivity

Jari Syrjärinne and Tuomas Kinnari

Abstract:	This paper presents a method for accurate GPS time transfer over GSM and UMTS cellular networks and gives estimates of its expected accuracy in both networks. Accurate GPS time assistance can play a major role in improving the sensitivity of GPS. In order to improve the receiver's sensitivity, the assisting network should provide time assistance with submillisecond accuracy and position assistance within at least a few tens of kilometers. With the assistance data, the receiver is capable of predicting data bit edges or even C/A code phases, and extending coherent signal integration up to 20 ms or even more. However, GSM and UMTS networks are asynchronous networks, unlike US CDMA. This means that they are not suitable for accurate time reference as such, nor suitable for transferring time assistance without considerable and unpredictable delays in transmission. If GSM or UMTS networks were used for time transfer, the expected time accuracy would be in the order of a few seconds due to unpredictable delays in network signaling layers. Therefore, new network elements, new signaling, and even modifications to cellular phones are needed to deliver dedicated time assistance through the network, over the air interface, down to the GPS receiver integrated with a mobile terminal. According to the analysis presented in the paper, the best possible time-transfer accuracy in GSM networks is 3 µs (1-sigma), and in UMTS networks roughly 2 µs (1- sigma), starting from a dedicated timing network element and ending up at the integrated GPS receiver. Seemingly, in both networks the presented GPS time- transfer method is accurate enough to not only improve acquisition sensitivity, but also to attempt direct reacquisition of the satellite signals. In the best-case scenario, it is possible to predict the expected signal code phases within a few chips.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	184 - 191
Cite this article:	Syrjärinne, Jari, Kinnari, Tuomas, "Analysis of GPS Time-Transfer Accuracy in GSM and UMTS Networks and Possibilities to Improve Sensitivity," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 184-191.
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