Cellular Positioning

Abstract

Location related products are the next major class of value added services that mobile network operators can offer their customers. Not only will operators be able to offer entirely new services to customers, but they will also be able to offer improvements on current services such as location-based prepaid or information services. The deployment of location based services is being spurred by several factors:

Competition

The need to find new revenue enhancing and differentiating value added services has been increasing and will continue to increase over time. Regulation The Federal Communications Commission (FCC) of the USA adopted a ruling in June 1996 (Docket no. 94-102) that requires all mobile network operators to provide location information on all calls to "911", the emergency services.
The FCC mandated that by 1 st October 2001, all wireless 911 calls must be pinpointed within125 meters, 67% of the time. On December 24 1998, the FCC amended its ruling to allow terminal based solutions as well as network based ones (CC Docket No. 94-102, Waivers for Handset-Based Approaches). There are a number of regulations that location based services must comply with, not least of all to protect the privacy of the user.
Mobile Streams believes that it is essential to comply with all such regulations fully. However, such regulations are only the starting point for such services- there are possibilities for a high degree of innovation in this new market that should not be overlooked.

Technology

There have been continuous improvements in handset, network and positioning technologies. For example, in 1999, Benefon, a Finnish GSM and NMT terminal vendor launched the ESC! GSM/ GPS mapping phone.

Needs Of Cellular Positioning

There are a number of reasons why it is useful to be able to pinpoint the position of a mobile telephone, some of which are described below. Location-Sensitive billing Different tariff can be provided depending upon the position of the cell phone. This allows the operator without a copper cable based PSTN to offer competitive rates for calls from home or office. Increased subscriber safety. A significant number of emergency calls like US.911 are coming from cell phones, and in most of the cases the caller can not provide the accurate information about their position. As a real life example let us take the following incident.
In February 1997 a person became stranded along a highway during a winter blizzard (Associated press 1997).She used her cellular phone to call for help but could not provide her location due to white-out conditions. To identify the callers approximate position authorities asked her to tell them when she could hear the search plane flying above. From the time of her first call forty hours elapsed before a ground rescue team reached her. An automatic positioning system would have allowed rescuers to reach her far sooner.

Positioning Techniques

There are a variety of ways in which position can be derived from the measurement of signals and these can be applied to any cellular system including GSM. The important measurements are the Time of Arrival (TOA), the Time Difference of Arrival (TDOA), the Angle of Arrival (AOA) and Carrier phase. All these measurement put the object to be positioned on a particular locus. Multiple measurements give multiple loci and the point of their intersection gives the position. If the density of the base stations is such that more measurements can be done than required then a least square approach can be used.
If the measurements are too few in number the loci will intersect at more than one point result in ambiguous position estimate. In the following discussion we assume that the mobile station and base station are lying in the same plane. This is approximately true for most networks unless the geography include hilly topology or high rise buildings.

Time of Arrival (TOA)

In a remote positioning system this involves the measurement of the propagation time of a signal from the mobile phone to a base station. Each measurement fixes the position of the mobile on a circle. With two stations there will be two circle and they can intersect in a maximum of two points. This gives rise to an ambiguity and it is resolved by including a priory information of the trajectory of the mobile phone or making a propagation time measurement to a third base station.
The TOA measurement requires exact time synchronization between the base stations and the receiver should have an accurate clock, so that the receiver knows the exact time of transmission and an exact TOA measurement have made by the receiver.