03.713GPPFunctional descriptionLocation Services (LCS)Release 1999Stage 2TS
The basic idea is to establish a GPS reference network (or a wide-area differential GPS network) whose receivers have clear views of the sky and can operate continuously. This reference network is also connected with the GSM network. At the request of an MS- or network-based application, the assistance data from the reference network is transmitted to the MS to increase performance of the GPS sensor. For classification, when the position is calculated at the network, we call it mobile-assisted solution. When the position is calculated at the handset, we call it mobile-based solution. If implemented properly, the assisted-GPS method should be able to:
1) Reduce the sensor start-up time;
2) Increase the sensor sensitivity; and
3) Consume less handset power than conventional GPS does.
Additional assisted data, such as differential GPS corrections, approximate handset location or cell base station location, and others can be transmitted to improve the location accuracy and decrease acquisition time.
If the GPS receiver does not know its approximate location, it will not be able to determine the visible satellites or estimate the range and Doppler frequency of these satellites. It has to search the entire code phase and frequency spaces to locate the visible satellites. For the code phase space, it spans from 0 to 1023 chips. For the frequency space, it spans from –4kHz to +4kHz. The relative movements between the satellites and receiver make the search even more time-consuming. Therefore, the time-to-first-fix (TTFF) is one important parameter to evaluate the quality of a receiver. For standalone GPS, this time could be more than 10 minutes. Clearly, this is unacceptable for certain applications such as E911. By transmitting assistance data over the GSM network, we can reduce the TTFF of a receiver to a few seconds. It significantly reduces the search window of the code phase and frequency spaces, hence, the start-up time. Furthermore, because of the availability of the satellite navigation message transmitted via the cellular network, it can also assist the receiver when the satellite signals are too weak to demodulate useful information. It reduces the handset power dissipation by going to the idle mode whenever there is no need for location services.
(BS stands for Base Station and SMLC stands for Serving Mobile Location Center)
Figure D.3: Assisted-GPS positioning system