Intermodulation mitigation technique in an RFID system

Abstract

When multiple readers for RF transponders have to be placed in close proximity, such as in adjacent lanes of a highway toll barrier, they can be set to operate at different frequencies. When signals from two adjacent ones of the readers interfere, the resulting signal includes interference terms whose frequencies equal the sum of the reader frequencies and the difference between the reader frequencies. To remove such interference terms while passing the desired terms, a tag includes a low-pass or other frequency-selective filter.

Claims

1. A system comprising: a transponder; a first reader for supplying a first interrogation signal to the transponder at a first frequency and receiving a response signal from the transponder at the first frequency; a second reader for supplying a second interrogation signal to the. transponder at a second frequency and receiving a response signal from the transponder at the second frequency, the transponder mitigating interference effects of the second interrogation signal, wherein said first and second readers have intermodulation products that create interference; an antenna at the transponder for receiving the first and second interrogation signals from said first and second readers; a detector at the transponder and in electrical communication with the antenna, for detecting and outputting a baseband analog signal representing the response signal received by the antenna; a frequency-selective filter at the transponder and in electrical communication with the detector to receive the baseband analog signal output by the detector, for performing frequency-selective filtering of the baseband analog signal to output an analog signal in which the interference is reduced; and a signal processing circuit at the transponder and in electrical communication with the frequency-selective filter to receive the analog signal with the reduced interference, and configured to perform signal processing on the analog signal with the reduced interference to extract information from the analog signal with the reduced interference.

2. The system of claim 1, wherein the frequency-selective filter comprises a low-pass filter.

3. The system of claim 1, wherein the system uses frequency separation and time domain multiplexing to allow said first and second readers to operate closely together.

4. A system comprising: a plurality of readers at a plurality of locations in the system for supplying interrogation signals at a downlink data rate, each of the plurality of readers operating at a different frequency, wherein a frequency difference between the different frequencies is larger than the downlink data rate and wherein intermodulation products from at least two of said plurality of readers create interference; a plurality of transponders, each of the plurality of transponders comprising: an antenna for receiving the interrogation signals from the plurality of readers; a detector, in electrical communication with the antenna, for detecting and outputting a baseband analog signal representing the interrogation signal received by the antenna; a frequency-selective filter, in electrical communication with the detector to receive the baseband analog signal output by the detector, for performing frequency-selective filtering of the baseband analog signal to output an analog signal in which the interference is reduced; and a signal processing circuit at each of the plurality of transponders and in electrical communication with the frequency-selective filter to receive the analog signal with the reduced interference, and configured to perform signal processing on the analog signal with the reduced interference to extract information from the analog signal with the reduced interference.

5. The system of claim 4, wherein the frequency-selective filter comprises a low-pass filter.

6. The system of claim 4, wherein the system uses frequency separation and time domain multiplexing to allow the plurality of readers to operate closely together.

7. The of claim 6, wherein the plurality of readers are located in adjacent traffic lanes.

8. The system of claim 4, wherein the plurality of locations are locations in a highway toll barrier.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which:

(2) FIG. 1 is an aerial view of a portion of a toll facility in which a vehicle approaches a toll barrier;

(3) FIG. 2 is a side view of the vehicle and one of the reader antennas of FIG. 1;

(4) FIG. 3 is a schematic diagram of a tag according to the preferred embodiment;

(5) FIG. 4 is a graph of a desired signal to be detected by the tag of FIGS. 1 and 2;

(6) FIG. 5 is a graph of an interfering signal associated with the desired signal described in FIG. 4;

(7) FIG. 6 is a graph of a detected output with signals added together with no low-pass filter;

(8) FIG. 7 is a graph of a detected output with signals added together after filtering with a low-pass filter; and

(9) FIG. 8 is a circuit diagram of a low-pass filter that may be used in the tag of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(10) A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements throughout.

(11) FIG. 3 is a schematic diagram of a tag 104 according to the preferred embodiment. The tag 104 includes the following components connected in series: an antenna 300, detection circuitry (e.g., diode detector) 302, a low-pass filter 306, a limiter 308 and a baseband decoder and associated digital processing circuits 310.

(12) The tag 104 functions in the following manner. The antenna 300 receives an amplitude modulated RF signal from a reader, with interference from an adjacent reader. The detection circuitry 302 detects that signal and outputs a baseband analog signal with a high level of interference. The low-pass filter 306 low-pass filters the signal to reduce the level of interference, in a manner to be explained below. The baseband analog signal with the reduced level of interference is applied to the limiter 308, which applies a digital signal to the baseband decoder and digital processing circuits 310.

(13) The theory of operation of the tag 104 with the low-pass filter 306 will now be explain The theory of operation of the tag 104 with the low-pass filter 306 will now be explained with reference to FIGS. 4-7. FIG. 4 shows a desired signal; FIG. 5, an interfering signal; FIG. 6, the detected output, resulting from the desired and interfering signals, without the filter; and FIG. 7, the detected output with the filter.

(14) Let:

(15) V.sub.d(t) be the envelope of the desired on-off keyed RF signal;

(16) V.sub.i(t) be the envelope of the interfering on-off keyed RF signal;

(17) .sub.d be the frequency of the desired RF signal (microwave frequency);

(18) .sub.i be the frequency of the interfering signal;

(19) V.sub.o be the output of the diode detector; and

(20) V.sub.in be the input to the diode detector.

(21) Then the desired signal, shown in FIG. 4, is given by
V.sub.d(t)*cos(.sub.dt),

(22) while the interfering signal, shown in FIG. 5, is given by
V.sub.i(t)*cos(.sub.it).

(23) The signal received at the detector, shown in FIG. 6, is the sum of the desired and interfering signals, namely,
V.sub.in=V.sub.d(t)*cos(.sub.dt)+V.sub.i(t)*cos(.sub.it).

(24) The detection process is often modeled as a second-order process, such that
V.sub.o=V.sub.in.sup.2.

(25) Substituting from the above,
V.sub.o=(V.sub.d(t)*cos(.sub.dt)+V.sub.i(t)*cos(.sub.it)).sup.2
=V.sub.d(t).sup.2*cos.sup.2(.sub.dt)+(t).sup.2*cos.sup.2(.sub.it)+2*V.sub.i(t)*cos(.sub.dt)*V.sub.d(t)*cos(.sub.it).

(26) The first two terms in the equation above represent the output of the diode for the desired and interfering signals as if they were received separately, while the third term represents the results of the interference between those two signals. The effects of the interfering signal (the second term) cannot be removed by the tag. However, the effects of interference in the third term can be reduced as shown below.

(27) Using the following well known identity from trigonometry:
cos A cos B=0.5(cos(A+B)+cos(AB)),

(28) the third term can be rewritten as
V.sub.i(t)*V.sub.d(t)*cos((.sub.d+.sub.i)t)+V.sub.i(t)*V.sub.d(t)*cos((.sub.d.sub.i)t).

(29) The first term is approximately twice the frequency of the desired signal, while the second component is at the difference frequency of the two components. The second component can be filtered out (removed) by attaching a low-pass filter 306 to the output of the diode detector and setting the cutoff frequency of that filter well below the difference frequency. The difference frequency will be much less than the sum of the two frequencies; thus, the filter will also remove the first term. The resulting filtered signal is shown in FIG. 7.

(30) Various design options for the low-pass filter 306 will now be described. The filter implementation is directly related to the frequency difference between the desired and interfering signals. When that frequency difference is large compared to the data rate (for example, 10 times) a simple RC low-pass filter can be used because the filter will not remove any of the frequency components of the desired signal.

(31) An example of such a simple RC filter is shown in FIG. 8. As shown, a resistor 802 is connected between the input and output of the filter, while a capacitor 804 is connected between the output and a ground. Those skilled in the art who have reviewed the present disclosure will be able to select values of the resistance R of the resistor 802 and the capacitance C of the capacitor 804 in accordance with the overall circuitry and the difference in frequencies between adjacent ones of the readers. The filter of FIG. 8 is desirable because it has only two components and can easily be implemented in an ASIC in the tag.

(32) When the frequency difference is comparable to the data rate, the low-pass filter implementation will require many components to achieve the same level of performance because the filter must roll off quickly to allow the desired signal to get through and yet attenuate the difference frequency. Various designs for low-pass filters are known in the art and can be used in the present invention.

(33) While a preferred embodiment of the present invention has been set forth in detail above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, the use of the present invention in a toll collection system is illustrative rather than limiting. Also, depending on the values of .sub.d+.sub.i and .sub.d.sub.i, it may be necessary or desirable to use a high-pass filter instead of, or in addition to, the low-pass filter to remove the terms related to the interference effects without also filtering out the desired signal. Therefore, the present invention should be construed as limited only by the appended claims.