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.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to radio frequency communication systems and more particularly to a method and apparatus to mitigate intermodulation effects for adjacent readers in RFID systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Fields of endeavor ranging from manufacturing to highway toll collection present a continuing challenge to monitor the movements of objects. There is thus a continuing goal to interrogate the location of objects in an inexpensive and streamlined manner. 
         [0003]    One example of an identification system for monitoring the locations of objects uses an RF (radio-frequency) transponder device (commonly known simply as a “tag”) affixed to an object to be monitored, in which a controller or interrogator unit transmits an interrogation signal to the device. The tag receives the interrogation signal and then generates and transmits a responsive signal. 
         [0004]    An example will be explained with reference to  FIGS. 1 and 2 . In the “E-ZPass” sm  system for toll collection, in use from Massachusetts to Virginia, a vehicle  102  has a tag  104  mounted on its windshield. As the vehicle  102  approaches a toll barrier, an antenna  106  mounted in the toll barrier interrogates the tag  104 . The identification received from the tag  104  allows the authority maintaining the toll facility to deduct the amount of the toll from a prepaid account associated with the tag  104 . 
         [0005]    Currently in the art, RFID systems use frequency separation and time domain multiplexing in combination to allow multiple readers to operate closely together within the bandwidth limitations imposed by radio regulatory authorities. In transportation and other applications, there is a compelling need for readers to operate in close proximity. In the example of a toll collection system, as seen in  FIG. 1 , readers in many lanes of traffic  108  must be operated side by side to read tags  104  present in each lane  108  simultaneously. 
         [0006]    Many technical obstacles limit system performance when readers are closely spaced. One of the major problems or technical obstacles is downlink interference at the tag. That occurs when a tag  104  receives the downlink signals DLS from two or more antennas  106 , such that the downlink signals DLS interfere with one another. In systems using on-off keying, such interference results in waveform distortion after detection. 
       SUMMARY OF THE INVENTION 
       [0007]    Thus, an unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies associated with waveform distortion after detection of the downlink signals from adjacent readers. 
         [0008]    It is therefore an object of the present invention to provide an apparatus and method for mitigating intermodulation effects for adjacent readers in on-off keying RFID systems. 
         [0009]    To achieve the above and other objects, a frequency-selective filter such as a low-pass filter is added at the output of the diode detector of the antenna. The raw signal from the detection of the interfering downlink signals includes components based on the frequency of each of the interfering signals, the difference in the frequencies, and the sum of the frequencies. Thus, given a difference in frequencies between adjacent ones of the antennas, an appropriate frequency-selective filter can be used to filter out undesired components of the signal. 
         [0010]    During the design of the toll barrier or other facility at which the readers will be used, the frequencies are determined, so that the difference in frequencies between any two adjacent ones of the readers is known. That difference in frequencies is used to select the appropriate frequency-selective filter. Alternatively, if a particular frequency-selective filter is desired to be used, the design stage can include selection of the frequency difference accordingly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which: 
           [0012]      FIG. 1  is an aerial view of a portion of a toll facility in which a vehicle approaches a toll barrier; 
           [0013]      FIG. 2  is a side view of the vehicle and one of the antennas of  FIG. 1 ; 
           [0014]      FIG. 3  is a schematic diagram of a tag reader according to the preferred embodiment; 
           [0015]      FIG. 4  is a graph of a desired signal to be detected by the antenna of  FIGS. 1 and 2 ; 
           [0016]      FIG. 5  is a graph of an interfering signal associated with the desired signal described in  FIG. 4 ; 
           [0017]      FIG. 6  is a graph of a detected output with signals added together with no low-pass filter; 
           [0018]      FIG. 7  is a graph of a detected output with signals added together after filtering with a low-pass filter; and 
           [0019]      FIG. 8  is a circuit diagram of a low-pass filter that may be used in the tag reader of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    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. 
         [0021]      FIG. 3  is a schematic diagram of a tag reader  300  according to the preferred embodiment. The tag reader  300  includes the following components connected in series: an antenna  106 , 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 . 
         [0022]    The tag reader  300  functions in the following manner. The antenna receives an on-off-keying RF signal from a tag, with interference from an adjacent reader. The detection circuitry  304  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 . 
         [0023]    The theory of operation of the reader  300  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. 
         [0024]    Let: 
         [0025]    V d (t) be the envelope of the desired on-off keyed RF signal; 
         [0026]    V i (t) be the envelope of the interfering on-off keyed RF signal; 
         [0027]    ω d  be the frequency of the desired RF signal (microwave frequency); 
         [0028]    ω i  be the frequency of the interfering signal; 
         [0029]    V o  be the output of the diode detector; and 
         [0030]    V in  be the input to the diode detector. 
         [0031]    Then the desired signal, shown in  FIG. 4 , is given by 
         [0000]      V d (t)*cos(ω d t), 
         [0032]    while the interfering signal, shown in  FIG. 5 , is given by 
         [0000]      V i (t)*cos(ω i t). 
         [0033]    The signal received at the detector, shown in  FIG. 6 , is the sum of the desired and interfering signals, namely, 
         [0000]        V   in   =V   d ( t )*cos(ω d   t )+ V   i ( t )*cos(ω i   t ). 
         [0034]    The detection process is often modeled as a second-order process, such that 
         [0000]      V o =V in   2 . 
         [0035]    Substituting from the above, 
         [0000]    
       
         
           
             
               
                 
                   
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         [0036]    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. However, the effects of interference in the third term can be reduced as shown below. 
         [0037]    Using the following well known identity from trigonometry: 
         [0000]      cos  A  cos  B= 0.5(cos( A+B )+cos( A−B )), 
         [0038]    the third term can be rewritten as 
         [0000]        V   i ( t )* V   d ( t )*cos((ω d +ω i ) t )+ V   i ( t )* V   d ( t )*cos((ω d −ω i ) t ). 
         [0039]    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 . 
         [0040]    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. 
         [0041]    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  806 . 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. 
         [0042]    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. 
         [0043]    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 ω d +ω i  and ω d −ω 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.