Home' Position : Position Dec Jan 2015 Contents RSSI is a potential solution. It works
relatively well in clear air. Then the RSSI
value does indeed drop off in a simple
way as a function of distance. A network
of three base stations would enable one to
derive a position by tri-lateration.
Sources of base stations are emerging.
Kontakt Beacons (www kontakt.io) and
Estimote (www estimote.com), both
from Poland, are simple, low-cost devices
driven by an API intended for easy
All the major manufacturers of smart
phones already support BLE. Indeed,
smart phones are emerging as a key ena-
bler of the technology. It should come as
no surprise that handset manufacturers
have begun building beacon units that can
be placed in a fixed location and commu-
nicate with phones as they move past.
Solving the positioning
However, from a location point of view,
these systems are relatively unsophisti-
cated. They depend, more or less, on the
presence or absence of a signal. The really
interesting question is whether one can
build a system to generate a proper indoor
location system in which the location of
a smart phone or a Bluetooth tag can be
identified in x and y.
For the last several years, electronics
and communications engineers have
been wrestling with the problem, and
the news is not good. For a start, the
RSSI is defined by a simple number,
usually between 0 and 15. This is
perfectly adequate for management of the
communications channel, but it imposes
an upper limit on the accuracy with which
one can measure an individual distance.
To a first approximation, if BLE's range
is taken at its design specification -- 100
metres -- and there really are only 16 steps
in the RSSI value, then the maximum
theoretical limit on accuracy is 6.25
metres per step. In practice, the steps
are larger at greater distance and smaller
closer in; there may be more resolution
in the RSSI and the maximum range may
not be 100 metres. Even so, we should not
expect great accuracy.
Even worse, the relationship between
distance and RSSI can be challenged
directly by interference. In practice, free
air, radio-quiet locations are few and far
between, and certainly not found in a
neighbourhood near you. In the presence
of radio noise, the reported RSSI number
will decrease. This is no small matter in the
ISM band, when ad hoc interference from
other devices is the rule rather than the ex-
ception. The problem is made worse when
reflections from walls, furnishing and
other obstructions are taken into account.
The bottom line: the signal strength varies
in a non-linear manner with distance.
What to do? Things are not as bad
as they sound. Another part of the
Bluetooth lexicon, the Link Quality
(LQ) signal can be pressed into service.
As its name suggests, LQ is used to
test the quality of the communications
link. Developers sometimes like to
use this number to test whether the
communications channel can support
their particular application. It can also
be used as a useful proxy for the extent
to which the RSSI signal is degraded.
There is also research to show that
BLE's Received Power Level is actually
a better proxy for distance than any of
the others. (See a paper from the IEEE A
Comprehensive Study of Bluetooth Signal
Parameters for Localisation by Houssain
and Wee-Seng for more on this topic).
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