Location Techniques for Cellular Phones
by Jaakko Lähteenmäki
One of the driving forces for location functions of mobile phones is the requirement by the FCC (Federal Communications Commission) in the USA: by October 2001 cellular systems shall be able to determine the location of a mobile phone in case of an emergency call. Mobile phone manufacturers and operators are now putting great effort into developing techniques to fulfil the FCCs requirement, and to facilitate implementation of other location-based services. A study on complementary location techniques optimized for urban and indoor environments was initiated at VTT Information Technology.
In addition to emergency applications, there is a wide range of other potential location applications including, for example, location-sensitive mobile Internet applications, fleet management and operators own network planning activities.
Currently, there are two main approaches for mobile phone location: a GPS (Global Positioning System) receiver module can be integrated into the handset or the transmitted signals of the cellular system can be used for location. Both approaches are based on computing the handset coordinates with the help of geometric distances retrieved from signal delay measurements. In open areas, these methods work well, but in built-up areas the signals from satellites or base stations do not reach the mobile phone directly, which leads to seriously degraded accuracy. Indoors, it is normally even not possible to receive a sufficient number of GPS satellites or base stations for location determination.
New Location Technique
VTT Information Technology has initiated a study on complementary location techniques optimized for urban and indoor environments. The new technique, depicted in the figure below, involves a location server with a large database including the measured or predicted field-strength values of several base stations in the area of interest. The location of the mobile phone can be computed by comparing the signals received by the phone to the signal values stored in the database.
The new location technique enables various signal characteristics to be utilized. In addition to field strength, for example, the predicted and measured delays and even impulse responses between the base station and mobile phone can be stored in a signal database and used as information for the location algorithm. Furthermore, if a GPS receiver is integrated into the phone, the GPS signal information may also be used to enhance the results in cases when satellite signals can be received. It is the objective of the first phase of the study to find out by computer simulation which signal characteristics should eventually be used and to determine the expected accuracy of the method.
Implementation of the new location technique involves some software modifications of the handset in order to enable the retrieval of received signal characteristics. Sending of the signal characteristics to the location server may be done by standard communication channels, eg the SMS (Short Message Service) of the GSM. A more efficient service can be achieved when the GPRS (General Packet Radio Service) becomes available. The third-generation systems will use wideband signals, which will increase the accuracy of location. In the development and implementation of the location method, the existing competence of VTT in radiowave propagation prediction and planning tools is being utilized.
Jaakko Lähteenmäki - VTT Information Technology
Tel: +358 9 456 6547