Global Positioning System Reference
In-Depth Information
development is the increase in power leading to systems such as WiMax, which
operate at extended ranges (typically 1km).
Generally it is assumed that access base stations are fixed in position and the
user transceivers are portable but used while not in motion. WiFi and similar
protocols used by hotspots generally do not have the extra data protocols needed
to eliminate errors when truly mobile (in contrast with true cellular radio, which
contains significant data redundancy and protocol overheads).
Simple proximity is the most basic approach to positioning (similar to the
Cell ID approach) and a number of products are available that use this technique
[13, 14]. More advanced systems use fingerprinting and statistics. Measurements
are taken of the signals from all hotspots in range (and typically may be 10 or
more) and from past records the likelihood of position is calculated [15]. Intel's
Place Lab pioneered the technology of WiFi positioning and a suite of software is
available for experimental use that uses advanced techniques (e.g., the particle
filter). In order to collect a large number of mapped hotspots, it reuses the
wardriving records that have been collected by amateurs (see Section 2.3.7).
Lateration based on timing is also possible but is not widely available. Signal
angles can be measured and this approach is gaining momentum since the advent
of MIMO technology. Multiple input multiple output (MIMO) devices use
advanced antennae to improve communications performance.
The main drawback of the statistical approach of measuring signals a priori is
the effort it takes both to collect the initial survey and to update it should the local
environment change. For example, if an open plan office was mapped and then a
row of metal cupboards moved, the disturbance in the signal strength field would
be significant. It would be ideal to have an indoor system that worked rather like
GPS, using highly accurate time of flight pulse timing. Advances in
semiconductor devices have made this approach a reality quite recently in the
form of ultrawideband systems (UWB).
6.2.10
Ultrawideband Positioning
Until this technology was developed the only other approach to accurate indoor
positioning used ultrasonics (details of which are in Chapter 7). The term UWB is
used mainly to describe a new class of low-range radio communications where the
bandwidth is very high (typically 500MHz or 20% of the carrier frequency) [16].
The first UWB systems are the ones of interest to positioning since they use
nanosecond pulses of radio energy, which are ideal for accurate timing
applications such as positioning and imaging. More recently the term has included
systems with more conventional modulation that are not especially useful for
positioning.
The idea behind nanopulse UWB is radical and disruptive and is causing the
radio regulations to be modified to take into account its special characteristics. In
essence, it is a very simple idea that concerns the replacement of the normally
complex modulation and demodulation system of a digital wireless with a simple
