Active RFID trilateration for indoor positioning

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

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