This invention relates to a method to determine a field strength by a reader for telemetry units such as active or passive tags according to the preamble of claim 1.
The present invention covers the field of portable RFID-devices as active tags or passive tags. A typical RFID-System is disclosed in WO 1996/028941 A1 [1], especially of active tags. The present invention covers also the implementation on passive tags, especially on UHF tags where very long distance of communication can be achieved, and thus, determining the distance from tag to reader may be a very useful information.
Many applications involving the usage of telemetry require having information of distance to the reader from which it is in sight in order to provide some means for localization. In the radio-frequency domain, the distance information is an image of the amplitude of the electrical field received. The path loss, or attenuation needs to be calculated differently in open space or in confined environment. The involved principle is depicted in
A=(−27.6)+20·Log f+20·Log d
where
At f=2.4 GHz, the above mentioned formula becomes to
A=40+20·Log d
When the RF signal passes through solid objects, some of the signal power is absorbed. The most convenient way to express this is by adding an allowed loss to the Free Space loss.
Attenuation can vary greatly depending upon the structure of the object the signal is passing through. Metal or concrete in the barrier greatly increases the attenuation. Walls account for 10 to 15 dB depending upon the construction. Interior walls are on the low end and exterior walls create more loss. Floors of buildings account typically for an extra 12 to 27 dB of loss.
Considering a transmission at 2.4 GHz, the attenuation in building typically becomes
A=40+10·n·Log d+(Structure Loss)
where n is a factor to model the effect of scattering.
The scattering effect comes when the radiation is forced to deviate from its direct trajectory due to non-uniformities of the medium through which it passes. RF scattering involves some change in the energy of the radiation. The n factor is very difficult to predict and may vary substantially in the real life due to the physical modification of the area in which the radio transmission takes place such as movement of people, movement of objects.
In a different embodiment, the distance calculator is based on the measure of the phase shift related to the frequency shift, as described in patent WO2005/091013 [4].
In the case of confined areas the multipath of the travelling wave is much more difficult to predict. Several models are available, typically like the Rayleigh model. The purpose of this invention is to provide an image of the distance by knowing the value of the amplitude of the field received on the receiving section. This information is often abbreviated by the term RSSI, for Received Signal Strength Indication. The state of the art to provide the RSSI on the receiver of the reader is typically a diode detector connected on the analogue intermediate frequency signal. In some cases the chipset used to make the receiver of the reader doesn't provide either the RSSI or the analogue intermediate frequency. Therefore, building a field detector implies to use extra hardware on the RF section of the receiver in order to get this information. Building an RSSI on the RF section of the receiver is a difficult operation that may alter the performance of the overall system.
The object of the present invention is to overcome the problems presented by the above cited devices to provide a method to determine a field strength by a reader without having extra hardware on its radio section.
This object is solved by a method with the steps specified in claim 1 and a tag specified with the features given in claim 7.
Further advantageous embodiments are given in dependant claims.
One of the main applications and use of this method is for the distance detection/distance deduction/distance measuring of tags, mainly active tags. Passive tags can also benefit from this invention.
Another application, immediately derived, is the localization of tags. The purpose of this invention is to localize mobile tags inside a confined area.
The invention will be now described in a preferred embodiment with reference to the accompanying drawings wherein:
The telemetry unit, i.e. the transmission section of a tag, is sending a message with different amplitudes in its frame. This transmission with different amplitudes is called a step transmission or transmission in form of a step function. An example of this transmission is depicted in
The receiving unit—part of a reader 20—receives the step signal—sometimes also called chirp signal—of the telemetry unit—part of a tag 10—and decodes its digital portion. The digital portion of the step 3 contains the value A1, A2, A3, A4 of the amplitude. The minimum value of the amplitude decoded provides an indication of the distance to which the telemetry unit is transmitting the RF signal. In other word: The reader decodes the first understandable step 3 of the sequence to determine the amplitude of the power required by the telemetry unit to be in range of the reader, and therefore can deduct a distance. If the reader is placed at a distance such as the attenuation of the path loss only allows to receive the sequence . . . , . . . , [A3,D], [A4,D] the reader detects the information that amplitude of transmission A1 and A2 sent by the tag are not enough to cover the path loss brought by the distance and/or scattering effect and multipath loss. Therefore the distance from reader to tag is estimated to be at a reduced distance as compared to if the reader was able to decode the full step function sequence from A1 to A4.
The realization on tag will be described for an active and a passive tag:
For an active tag—see FIG. 3—the telemetry unit is fitted with a power amplifier 2. The gain of the power amplifier 2 is determined by the feed-back loop of the counter-reaction. The value of this counter-reaction can be digitally determined by the combination of the FET transistor Q1, Q2, Q3, Q4 which is switched ON or OFF; this is shown in a simplified representation according to
For a passive tag the amplitude of the transmission is based on the amount of RF energy reflected by the antenna 4, details see
An embodiment of a circuitry carrying out a step transmission according to the invention is depicted in
The ultimate goal is to have a position indication of the tag, at least on a floorplan represented by coordinates x,y. This information is provided by setting an array of telemetry receivers (readers) on the surface to monitor. Each tag has a unique digital identification number that can also be called a signature.
For simplicity of understanding, only the 2D representation is given here. The application for localization in volume is the same concept, with a 3D array of readers. The position detection in building is strongly affected by the multipath route of the wave. This multipath effect is highly unpredictable, even if some models tend to describe its behaviour, like the Rayleigh model.
The method for determining the x,y-position of the tag is based, at the first order, on the amplitude of power received by each of the three readers. The x,y-coordinate is located at the geometrical center of the three readers A, B, C. This method is described in the literature as the Nearest Neighbor or K-Nearest Neighbor KNN. The reference of the center of all three readers is measured with one tag placed physically at a known place, preferably the center. The instantaneous reception power is measured with a permanent reference tag and is used as a reference for the measurement of the moving tag. Unlike the teachings in the application WO 2004/073343 [2] which describes a probabilistic model to model the terminal's wireless environment, the presence of a permanent low power tag such as disclosed in the European patent application 07002606.7 allows to have a permanent sampling of the characteristics of the wireless environment. This feature is highly important because the characteristics of the wireless environment is tremendously versatile, depending on the presence of materials or people or any items. So in industrial environment it is believed that only a permanent sampling of the wireless environment is adequate.
The situation is depicted in
α·{right arrow over (Ptag)}A+β·{right arrow over (Ptag)}B+γ·{right arrow over (Ptag)}C=−(α′·{right arrow over (Pref)}A+β′·{right arrow over (Pref)}B+γ′·{right arrow over (Pref)}C)
In order to have a better precision in the position, especially in logistics environments where the tag Ptag is standing still during a long period, the distance measurement can be repeated several times and integrated over the time. Samples of positions can therefore be counted from (1, . . . , j). The plot of all the instantaneous samples of power measurement gives an elipsoïde of uncertainty, which can be averaged to provide the user with only one position indication:
Another way to reduce the uncertainty of measurement is to have more than three readers simultaneously in sight of the same reference tag. The area of the elipsoïde of uncertainty is reduced together with the quantity of readers able to capture the power indication of the tag.
A method to reduce the uncertainty of the position is to confirm the x,y-position calculated with three readers, by adjacent readers. This confirmation by adjacent readers is a very good help to reduce the uncertainty brought by the multipath fading in case the quantity of position samples (1, . . . , j) is very small.
ISM Industrial, Scientific and Medical; ISM band
Number | Date | Country | Kind |
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07005789.8 | Mar 2007 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP07/09352 | 3/21/2007 | WO | 00 | 7/24/2009 |