Figure 1. An example of a cell-based
positioning concept in outdoor areas of
the city of Vienna in conjunction with the
trilateration concept for indoor areas
Alternative location methods for
absolute positioning in areas where no
GNSS position determination is possible
due to obstruction of the satellite signals
are needed in mobile positioning. Active
RFID (Radio Frequency Identification) can
be used also for position determination,
although the system was not only
developed for positioning and tracking but
mainly for identification of objects. Using
RFID in positioning, different approaches
can be distinguished, i.e., cell-based
positioning if the RFID tags are installed at
active landmarks (i.e., known locations) in
the surroundings, trilateration if ranges to
the RFID tags are deducted from received
signal strength (RSS in RFID terms)
values and location fingerprinting where
the measured signal power levels are used
directly to obtain a position fix. Using Cell
of Origin (CoO) the achievable positioning
accuracy depends on the size of the cell
and is therefore usually several metres up
to 10’s of metres using long range RFID
equipment. Higher positioning accuracies
can be obtained using trilateration
and fingerprinting. In this paper the
use of trilateration is investigated.
Background of active RFID
and positioning concepts
Radio Frequency Identification (RFID)
is an automatic identification method.
A RFID system consists of a tag, a
reader and an antenna. The tag is a
transponder that can be attached to or
incorporated into a product, animal, or
person for the purpose of identification
using radiowaves. The reader (i.e., a
transceiver) is able to read the stored
information of the tag in close proximity.
RFID tags contain antennas to enable
them to receive and respond to radiofrequency
queries from an RFID
transceiver. There are various typesof tags; i.e., passive, active and semipassive
tags. Passive RFID tags do not
have their own power supply and the
read range is less than for active tags,
i.e., in the range of about a few mm up to
several meters. Active RFID tags, on the
other hand, must have a power source,
and may have longer ranges and larger
memories than passive tags. Many active
tags have practical ranges of tens of
meters, and a battery life of up to several
years. Another advantage of the active
tags compared to the passive tags are that
they have larger memories and the ability
to store additional information (apart
from the tags’ ID) sent by transceiver.
For these reasons, the applications
described in this paper make use of active
RFID tags with a frequency range of
865.6-867.6 MHz. Further information
about the underlying technology can
be found in e.g. Finkenzeller (2002). To employ RFID for positioning and
tracking of objects, one strategy is to
install RFID readers at certain waypoints
(e.g. entrances of buildings, storage
rooms, shops, etc.) to detect an object
when passing by. For that purpose an
RFID tag is attached to or incoporated
in the object. This concept is employed
for example in theft protection of goods
in shops and in warehouse management
and logistics. A second approach for
using RFID in positioning would be to
install RFID tags at known locations (e.g.
at active landmarks) especially in areas
without GPS visibility (e.g. in tunnels,
under bridges, indoor environments,
etc.) and have a reader and antenna
installed in the mobile device carried by
the user. When the user passes by the
tag the RFID reader retrieves its ID and
other information (e.g. the location).
In the case of cell-based positioning,
i.e., Cell of Origin (CoO), the maximumrange of the RFID tag defines a cell of
circular shape in which a data exchange
between the tag and the reader is possible.
Using active RFID tags the positioning
accuracy therefore ranges between a
few meters up to tens of meters. In our
approach the maximum range of the
signal can then be set at around 20 m.
Higher positioning accuracies can be
obtained using trilateration if the ranges
to several tags are determined and are
used for intersection. For 3-D positioning
range measurements to at least three
tags are necessary. The ranges from the
antenna of the reader to the antenna of
the tag is deduced from the conversion
of signal power levels into distances.
Signal strength to distance
conversion for RFID range
deduction in trilateration
To transform the measured signal strength
from the RFID tag into a range between
the tag and the reader a conversion model
has to be employed. This conversion
can be performed using a radio wave
propagation model. Such a model is
an empirical mathematical formulation
for the characterization of radio wave
propagation as a function of frequency,
distance and other conditions. Such models
typically predict the path loss along a
link or the effective coverage area of a
transmitter. For indoor environments one
usable model is the ITU (International
Telecommunication Union) Indoor
Location Model (Wikipedia, 2008) that
estimates the path loss inside a room or
a closed area inside a building delimited
by walls of any form. It assumes a
logarithmic relationship between the
measured RSSI and the range from the
transmitter. Mathematically the ITU-R
model (Ranvier, 2004) can be described by
where
sT is the total signal strength in [dBm],
fc is the carrier frequency in [MHz],
n is the signal strength exponent,
d is the range between the RFID tag and
the RFID reader in [m] and
s f (n f )
is the floor penetration factor
of the signal strength which depends on
the number of floors between the RFID
tag and RFID reader in the building.
In the case of RFID the used parameters
might be different to those in equation (1).
In order to find out the suitable parameters
for a RFID system, a new simplified
equation using 3 fixed parameters as
given in equation (2) can be employed:
where
0 a and 1 a are coefficients found
during calibration using measurements
on a known baseline.
Then the parameter
is an
unknown coefficient that includes the
fixed carrier frequency and the number
of floors in the building and is
the range power loss coefficient. These
unknown paramters can be determined
using a calibration on a known baseline
inside the building. Then the distance
d between the RFID tag and the RFID
reader can be obtained from equation (3):
with the coefficients
For further improvement of the accuracy
of the logarithmic approximation, the
exponent in equation (3) can be extended
by a polynomial function of order p as
described in the following equation:
where
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