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Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region. The evolution to location-dependent services and applications in wireless systems continues to require the development of more accurate and reliable mobile positioning technologies. The major challenge to accurate location estimation is in creating techniques that yield acceptable performance when the direct path from the transmitter to the receiver is intermittently blocked. This is the Non-Line-Of-Sight (NLOS) problem, and it is known to be a major source of error since it systematically causes mobile to appear farther away from the base station (BS) than it actually is, thereby increasing the positioning error.

NEED FOR MOBILE TRACKING
Recent demands from new applications require positioning capabilities of mobile telephones or other devices. The ability to obtain the geo-location of the Mobile Telephone (MT) in the cellular system allows the network operators to facilitate new services to the mobile users. The most immediate motivation for the cellular system to provide MT position is enhanced in accident emergency services. The positioning of the mobile user could provide services like

¢ Emergency service for subscriber safety.
¢ Location sensitive billing.
¢ Cellular Fraud detection.
¢ Intelligent transport system services.
¢ Efficient and effective network performance and management.

Location Tracking Curve Method
The method proposed by us for tracking the location of a mobile telephone using curves connecting the points where circles intersect one another, the circles radii being the distances between BSs and the mobile telephone. The steps involved are:

a. Each base station nearer to a mobile telephone receives a predetermined signal from the mobile telephone and calculates the distance between the mobile telephone and the base station and the variances of time arrival of the signal at the base station;
b. A circle is drawn to have a radius being the distance and the coordinates of the base station being the center of the circle;
c. A pair of the first and the second base stations is selected among the base stations. A several location tracking curves connecting two intersection points between the selected circles corresponding to the first and the second base stations are drawn. One of the location tracking curves is selected using the variances of the first and the second base stations;
d. The steps c. and d. are repeated for the other pairs of the base stations;
e. The intersection points are obtained among the location tracking curves selected in step d. and e. and,
f. The location of the mobile telephone is determined using the coordinates of the intersection points obtained in step e.

The several location tracking curves are parts of circles with centers near to the base station with smaller variances between the first and the second base stations. The circles formed by the location tracking curves have the centers on a line connecting the coordinates of the first and the second base stations. The larger variances between the variances of the first and the second base stations are compared to the variances of the several location tracking curves, and one of the location tracking curves is selected according to the comparison result. The location coordinates of the mobile telephone are determined by averaging the coordinates of the intersection points obtained in step (f).

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