Global Positioning System Reference
In-Depth Information
(ECGI) technique, which includes signal strength measurements of the mobile's
signals and the enhanced observed time difference and uplink time difference of
arrival (EOTD and U-TDOA) techniques, which compares the arrival time of
signals from base stations to mobiles. In U-TDOA, the positioning is centralized
within the network whereas EOTD uses localized autonomous positioning within
the mobile unit. The trade-off of these extra positioning enhancements is the
complexity and investments required. Overall a typical system would at best be
giving positional accuracies of around 50m [11, 12].
Angle of arrival (AoA) measurements can be taken by using highly
directional antennae at base stations using phased array beam-forming techniques
(developed from military and satellite systems). In the first cellular systems,
simple sector antennae were used, for example, to illuminate three sectors, each
with 120°. The bases were placed not in the center of cells but at the corners of a
nominal honeycomb-shaped coverage pattern. These arrangements were fixed, but
with narrow spot beams that can be made to move (by automatic signal phasing)
and track targets, AoA information becomes more useful.
The biggest problem with all the cellular techniques for Whereness is that
they are not standardized. Some standards have emerged but they are optional.
Although an operator may offer an excellent and useful location-based service
(LBS), it would be unavailable to users who roam onto competitive networks and
possibly for visitors, for whom the equipment may lack vital software or
configuration. An example would be a network of location monitoring units
(LMUs) providing the means to measure signal timings at dedicated locations.
Access to the measurements would clearly be restricted to those who are
customers of the operator who has made the investment. Given the fragmented
approach to cellular standards in many global regions, it seems unlikely that a
unified positioning standard will emerge.
The most useful approach would be to treat all and any system positioning
services as inputs to a more general Whereness service with convergence
functions for each network (see Figure 3.2).
6.2.9
Hotspots and WiFi Positioning
Hotspots are the colloquial name for Internet wireless access points that are now
very common in most built environments and provide static broadband
connections with typical ranges up to 100m. The positioning techniques discussed
in this chapter are all applicable to this relatively new form of communications
and some interesting new approaches are emerging, but the most useful
opportunity is the ability to gain an accurate position indoors.
Most current hotspots conform to the WiFi standard (802.11g/b) operating on
the unlicensed industrial, scientific, and medical (ISM) band. The technology is
developing rapidly and various enhancements are expected including the opening
of other bands, some of which will be licensed to specific operators. A major
