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
Competition :
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.
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.
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.
            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.
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.