Global Positioning System Reference
In-Depth Information
When a user makes a call a local base station is allocated a free channel that
will have a frequency and may have a time slot or code depending on the standard.
As the mobile transceiver moves the call or session is handed over to an adjacent
cell and a new channel is allocated seamlessly. When no calls are being made, the
central system still keeps a record of the local users by making regular silent calls
to check on the status. This is necessary so that outgoing calls can be routed
efficiently. If there were no location registers all users would need to be contacted
from every base station, everywhere, which would waste valuable resources. The
location register is therefore consulted by the system before contact is attempted
to find the nearest base stations to the user.
There are two parts to location registration. First, a home location register
keeps account of all details of each user. Second, a visitor location register keeps
account of those mobiles visiting an area. Details are swapped between registers
as mobiles roam. We can see, therefore, that a coarse Whereness system is in
operation keeping the positions for every active unit at all times.
Some location-based services (LBS) are based on location register
information (or Cell ID) and there is potential for many more. As cells get smaller,
in particular in dense urban areas, the accuracy improves but accuracy is poor in
very rural areas where the cells may be tens of kilometers apart.
There is, however, a complementary situation between cellular positioning
and GPS. In dense urban areas GPS is likely to be poor in performance due to
high-rise buildings, but cellular coverage is very good and cells closely spaced. In
contrast, GPS works very well in open country where cell base stations are sparse.
Although Cell ID is crude it comes at no extra cost other than the
requirements to make the embedded information available to interested parties.
With some systems (e.g., GSM) it is possible to use other, more accurate system
information. GSM is a time division multiplexed system where each physical
radio channel is shared between up to eight users who are allocated timeslots on
the carrier. To ensure that information from each mobile user is transmitted at
exactly the correct time, a “timing advance,” or TA, control system is used. When
a mobile first contacts a base station, it has no information concerning its range
and the time its signals will take (at light speed) to reach the base station. It
therefore only transmits an access burst in the center of a time slot and leaves a
substantial timing guard bands unmodulated. The base station detects approximate
range of the mobile from the time offset between the incoming access burst and its
own timing multiplexing. It then sends a correction factor to the mobile so that all
further time slots are aligned and no resources wasted on unnecessary guard
bands. As a call progresses the timing advance is constantly updated and
corrected.
It can be seen, therefore, that approximate range information is available and
some attempts have been made to use this information for positioning. For
example, if the network forces a mobile to register with three different and
geographically separate base stations then triangulation can be used to fix
position. Further improvements may be made by the enhanced cell global identity
