# Patent

## Title
System and method for estimating range to an RFID tag

## Abstract

The a system for measuring distance between an RFID reader and tag, including an adaptive linear combiner, which is a tapped delay line with controllable weights on each tap, and outputs that are summed and subtracted from a reference to produce an error signal. After a sufficient number of cycles, the weight distribution indicates the delay of the received signal with respect to the reference, and by extension determines the distance between the tag and receiver.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This utility application is a continuation under 35 U.S.C. §120 of non-provisional patent application Ser. No. 14/734,655, filed on Jun. 9, 2015 and of non-provisional patent application Ser. No. 13/753,338, filed on Jan. 23, 2013 now U.S. Pat. No. 9,081,088, both of which also claim the benefit under 35 U.S.C. §119(e) of provisional application Ser. No. 61/594,906 filed on Feb. 3, 2012, entitled “Minimal Infrastructure System and Method for Determining Lane.” The entire disclosures of these applications are included herein by reference.


FIELD OF THE INVENTION
The invention relates generally to the field of vehicle tracking and tolling and particularly to determination of lane of travel of a vehicle.
BACKGROUND
Estimation of the distance to a tag in an RFID system is a useful feature. It would provide for an improved method cross-lane discrimination in RFID tolling stations, while also providing a platform to estimate vehicle speeds in the same type of application. While there are many techniques developed in prior art for range estimation, this approach uses digital components in the receiver. Patents in the general field include U.S. Pat. Nos. 6,600,443; 6,476,756 and 7,030,361.
Past methods include angle of arrival at multiple receivers, received signal strength indicator (RSSI), and variation of phase in a received signal. Each of these has their strengths but the present method particularly applies to a system with discrete (i.e. digitally sampled) signal inputs.
DESCRIPTION
The disclosed method is an adaptive linear combiner, which is a tapped delay line with controllable weights on each tap, and outputs that are summed and subtracted from a reference to produce an error signal. While there are multiple possible embodiments to control the weights based on the error signal, a least mean square (LMS) is described here. After a sufficient number of cycles, the weight distribution indicates the delay of the received signal with respect to the reference, and by extension determines the distance between the tag and receiver.
The invention composes an accurate method of identifying the physical location of a vehicle as it passes through an RFID-based tolling zone. One problem of RFID-based toll systems is occasional reads from lanes displaced laterally from the nominal lane beneath an antenna; these are termed cross-lane reads. The cross lane reads are typically handled in the computer that controls the readers by relatively complicated methods. This method, because of its embodiment in the reader, immediately discards cross lane reads because of their large range to the target compared to the range for cars in the proper lane. Another useful aspect of the invention is another method of measuring vehicle speed. While speed can be measured in other ways, this approach allows for range and speed to be measured simultaneously.
In an embodiment of the invention, there is a system in an RFID backscatter tag reader for determining distance from an antenna to a backscatter RFID tag. The system includes: a transmitter for transmitting a transmitted modulated signal to the tag; a receiver for receiving a delayed modulated backscattered signal from the tag; and a linear adaptive combiner which is a tapped delay line with controllable weights on each tap, and outputs that are summed and subtracted from a reference to produce an error signal. Modulation in the backscattered signal is input to the adaptive linear combiner, and the transmitted modulated signal is the reference to the combiner and weights for the taps of the linear adaptive combiner are approximately zero for all of the taps except for the tap corresponding to the delay of the backscattered signal and wherein the distance is determined from the delay. In a further embodiment, the linear adaptive combiner has a sample rate and the system operates at a plurality of sample rates for each distance determination and an optimum distance is determined from the plurality of results.
In a further embodiment, there is a method for determining distance between an RFID backscatter tag and a first antenna. The method includes the steps of: transmitting a modulated signal to the tag through the first antenna; receiving through the first antenna a modulated backscattered signal from the tag; inputting the modulated backscattered signal to an adaptive linear combiner having taps, the taps having weights; applying the modulated signal to the adaptive linear combiner as a reference signal; determining a first signal delay time by which of the taps has the lowest weight; and calculating distance between the tag and the first antenna from the first signal delay time. In a further embodiment of the method a difference in height between the first antenna and the tag is estimated. In a further embodiment of the method, all of the steps are performed with a second antenna located at the same height as the first antenna and a second adaptive linear combiner to produce a second signal delay time, wherein the distance between the first and second antennas is known and wherein the first and second signal delay times are compared to determine a difference in height between the antennas and the tag.



DESCRIPTION OF THE DRAWINGS
FIG. 1 is block diagram of an embodiment of a system for distance measurement;
FIG. 2 shows a reference signal (upper graph) and backscattered signal containing a small amount of noise and lagging by 15 samples or 15 nsec. (lower graph)
FIG. 3 shoes a calculated final weight distribution after 1000 cycles (upper graph) and an error signal for the input in FIG. 2. The weights are initially zero.
FIG. 4 is a diagram of a toll plaza antenna and a vehicle having an RFID tag.
FIG. 5 is a diagram showing dimensions for a multiple antenna configuration.



DETAILED SPECIFICATION
FIG. 1 is a block diagram of the system under consideration. The index k indicates the dimension of time. FIG. 1 shows an adaptive linear combiner, transversal filter format. The weights are updated at each time step as follows:



  
    
      
        
          W
          _
        
        
          0
          ,
          
            k
            +
            1
          
        
      
      =
      
        
          
            W
            _
          
          
            0
            ,
            k
          
        
        +
        
          
            μɛ
            k
          
          ⁢
          
            
              X
              _
            
            k
          
        
      
    
    ,
    
      

    
    ⁢
    
      
        
          W
          _
        
        
          1
          ,
          
            k
            +
            1
          
        
      
      =
      
        
          
            W
            _
          
          
            1
            ,
            k
          
        
        +
        
          
            μɛ
            k
          
          ⁢
          
            
              X
              _
            
            k
          
        
      
    
    ,
    
      

    
    ⁢
    
      
    
    ⁢
    
      ⋮
      ⁢
      
        
      
      ⁢
      ⋮
    
  




  
    
      
        W
        _
      
      
        N
        ,
        
          k
          +
          1
        
      
    
    =
    
      
        
          W
          _
        
        
          N
          ,
          k
        
      
      +
      
        
          μɛ
          k
        
        ⁢
        
          
            
              X
              _
            
            k
          
          .
        
      
    
  



This is the LMS algorithm. The parameter μ is a scalar gain coefficient. Each weight is adjusted at every time step according to the previous weight and previous input data, system error, and gain coefficient.
In an embodiment, the reader's transmitted CW energy is modulated during the uplink with a known pattern that will be recovered in the backscattered signal. The modulation in the backscattered signal becomes the input Xk to the adaptive linear combiner, while the transmitted signal is the reference. The proper pattern of weights is a value of approximately zero for all but the one corresponding to the delay of the backscattered signal.
The sample rate of the A/D converter is an important quantity affecting the resolution of the proposed technique. The filter operates on samples that are one time step apart, so the final distribution of weights has the same granularity. This implies that the base resolution of the system is no better than the inverse of the sample rate. For a sample rate of 700 MHz, for example, the sample time is 1.42 nsec. Using a free space signal propagation speed of 1 foot/nsec, this corresponds to a physical delay of 1.42 feet. This delay corresponds to the (round trip) total range between the tag and receiver. The height of the receiver antenna requires a translation of this range to the horizontal range, which is the parameter of interest. Deviations of a foot in the total range can imply several feet in the horizontal range.
Basic Adaptive Filter
Reference and backscattered signals are shown in FIG. 2. The sampling rate is 1 GHz. FIG. 2 shows a reference signal in the upper graph and in the lower graph a backscattered signal, containing a small amount of noise, and lagging by 15 samples or 15 nsec.
FIG. 3 shows calculated final weight distribution, after 1000 cycles, and error signal for the input in FIG. 2. The weights are initially zero.
All weights are approximately zero except for the one representing a delay of 15 samples. Note that when the true delay is not equal to an integral number of clock cycles, the resulting weight distribution is still primarily concentrated on a single integer, depending on interference levels. The algorithm produces a delay that effectively rounds the true delay to the nearest N seconds, where N is the time between samples.
Complications from Reader Antenna Height
The height of a typical reader antenna in a toll plaza (typically 18 feet) adds a complication to the problem, summarized in FIG. 4 and Table 1. FIG. 4 shows typical scenario dimensions as tabulated in Table 1, with range to tags for Δh=13 feet.









TABLE 1





Horizontal 
Distance 
Distance to 


distance
to tag 
tag in adjacent


to tag (x, feet)
(R, feet)
lane (feet)

















0
13
17.69


1
13.04
17.72


2
13.15
17.80


3
13.34
17.94


4
13.60
18.14


5
13.92
18.38


6
14.32
18.68


7
14.76
19.03


8
15.26
19.42


9
15.81
19.85


10
16.40
20.32


11
17.03
20.83


12
17.69
21.38









Therefore, a resolving the range to approximately a tenth of a foot is required to determine horizontal positions of a tag to within a foot. This is an order of magnitude less than the 1.42 feet limit on the range imposed by the sampling rate. The tags that are one lane away (12 feet) have ranges that are only about 4 feet greater for the same horizontal distance.
The adaptive filter can only resolve distances that are one ‘sample’ apart. However, utilizing different sampling rates will produce different range estimates. In general, changing the sampling rate in real time, producing one range estimate per sample rate, and choosing the maximum range estimate can theoretically achieve the required resolution.
For example, if Δh is 13 feet, and actual true horizontal distance is 10 feet. The table below summarizes the estimated horizontal distances for 10 different clock rates. The noise and interference are negligible. Table 2 shows estimated distances for Δh=13 feet and x=10 feet, as a function of sample time. Note that the total range is 2*Rest, and is equal to an integer multiple of the sample time.












TABLE 2







Sample time
Clock rate 
Rest 
Xest 



(nsec)
(MHz)
(feet)
(feet)





















1.428
700
15.7
8.8



1.449
690
15.94
9.2



1.47
680
16.17
9.6



1.492
670
15.66
8.7



1.513
660
15.89
9.1



1.535
651
16.11
9.5



1.556
643
16.34
9.9



1.577
634
15.77
8.9



1.599
625
15.99
9.3



1.62
617
16.2
9.7










The best and worst results in the forth column are highlighted. The spread between best and worst results, highlighted in Table 2, is about 1 foot. However, as the horizontal distance is reduced, the spread increases, as seen in the next example with the same parameters as the previous example, except the horizontal distance is 4.1 feet.












TABLE 3







Sample time
Clock rate
Rest 
Xest 



(nsec)
(MHz)
(feet)
(feet)





















1.428
700
13.56
3.9



1.449
690
13.04
1.0



1.47
680
13.23
2.5



1.492
670
13.43
3.4



1.513
660
13.62
4.1



1.535
651
13.04
1.0



1.556
643
13.22
2.4



1.577
634
13.41
3.3



1.599
625
13.59
4.0



1.62
617
12.96
(imaginary!)










Not only is the spread between best and worst estimates much larger, but the estimate is actually imaginary for one clock rate. This is because the horizontal distance is computed by Xest=√{square root over (R2est−Δh2,)} and the estimated value for Rest is smaller than the value of Δh. The example underscores the fact that at short horizontal distances, a single estimate (i.e. one clock period) can be off by several feet or not even be a real number.
In general, this technique would require several clock cycle changes during one uplink period, producing several estimates for the range. The maximum value among the estimates is the closest to the true range.
Complications of Variable Tag Height
Additional complication comes from variation in the tag height. In order to estimate the horizontal range (Xest), the value of Δh must be assumed. If the assumed Δh is too high, the value for Xest becomes imaginary at short ranges. If it is too low, the value for Xest suffers from low accuracy at short ranges. This is demonstrated in Table 4, which shows. Estimated horizontal distances with an assumed Δh of 13′ where actual Δh values vary per column.












TABLE 4







X 
Xest (feet) for
Xest (feet) for
Xest (feet) for



(feet)
true Δh = 12′
true Δh = 13′
true Δh = 14′





















10
8.6
9.9
11.2



9
7.5
9.0
10.4



8
6.2
8.0
9.5



7
4.8
6.9
8.7



6
3.3
6.0
7.8



5
0
5.0
7.2



4
j3.0
4.0
6.5



3
j4.1
2.9
6.0



2
j4.6
2.0
5.5



1
j5
0.8
5.3










If the assumed Δh is near the true Δh, the error is small. The error is as high as 5′ for the other cases, and imaginary ranges result if the assumed Δh is too high.
There are at least 2 ways to deal with this problem. The simplest and least accurate is to adjust the tag height assumption in real time when an imaginary range is computed. Another method is to add a second antenna and receiver, using the known separation between the antennas to determine a better assumption for Δh and therefore a more accurate horizontal distance estimate.
Method 1: Adjustment of Assumed Height
The simplest but least accurate approach is to begin the estimate process with an assumption for Δh that represents an average value. Then it is reduced as needed, when the resulting horizontal range estimate becomes imaginary. Consider the second column in Table 4, where the assumed Δh is 13′ and the true Δh is 12′. The error at a 10′ range is 1.4′ and increases as the tag moves closer to the antenna, reaching 5′ when the tag is 5′ away. At closer distances, the estimated range is imaginary with an assumed Δh of 13′, but if the assumed Δh is reduced to 12′, the error will be less than 1′. There may be more sophisticated methods of adjusting the assumed Δh that result in increased accuracy.
Altering the assumed Δh does not require re-running the adaptive filter through an additional cycle. The filter specifically computes the range R, not the horizontal range. The imaginary number results from the single calculation Xest=√{square root over (R2est−Δh2)}, and altering the assumed Δh and re-calculating can be done without re-estimating X.
Method 2: Multiple Antennas
For a given assumed height Δh, the estimated horizontal ranges are:

X1est=√{square root over (R2est−Δh2)} and X2est=√{square root over (R2est−Δh2)}.

and the difference between the two,

X1est−X2est=√{square root over (R1est−Δh)}−√{square root over (R2est−Δh)},

can be compared to the known quantity X1−X2.
If the estimated difference is greater/less than the true difference, the assumed Δh must be reduced/increased. This is repeated until estimated and true values of X1−X2 are within an acceptable tolerance. This method requires an additional antenna and processing bank. The potential for imaginary ranges is still present, and would require reducing the assumed value of Δh, as in the case of a single antenna.

## Claims

1. A system for determining distance from a plurality of antennas to a backscatter RFID tag comprising:
a transmitter for transmitting a transmitted modulated signal to the tag via first and second antennas,
a first receiver for receiving a first modulated backscattered signal having a first delay from the tag via said first antenna;
a second receiver for receiving a second modulated backscattered signal having a second delay from the tag via said second antenna;
a first linear adaptive combiner which is a delay line having first delay line taps with first controllable weights on each first delay line tap, and outputs that are summed and subtracted from a reference to produce a first error signal; and
a second linear adaptive combiner which is a delay line having second delay line taps with second controllable weights on each second delay line tap, and outputs that are summed and subtracted from said reference to produce a second error signal;
wherein modulation in the backscattered signal received by said first antenna is input to said first adaptive linear combiner, and said transmitted modulated signal is said reference to said first adaptive linear combiner;
wherein modulation in the backscattered signal received by said second antenna is input to said second adaptive linear combiner, and said transmitted modulated signal is said reference to said second adaptive linear combiner;
wherein the first and second controllable weights for said taps of said first and second linear adaptive combiners are approximately zero for all of said taps except for respective first and second combiner taps corresponding to the first and second delays of the backscattered signal; and
wherein the distance from the RFID tag to each of said antennas is determined from said first and second delays.

2. The system of claim 1, wherein said first and second antennas are mounted in a toll plaza and the tag in a vehicle.

3. The system of claim 1, wherein said first and second linear adaptive combiners operate at a plurality of sample rates and said distance is determined from a plurality of results obtained at said plurality of sample rates.

4. The system of claim 3, wherein said distance is determined by selecting a maximum distance from said plurality of results.

5. The system of claim 1, wherein said first controllable weights are adjusted based on a least means square fit of said first error signal and said second controllable weights are adjusted based on a least means square fit of said second error signal.

6. A method for determining distance from a plurality of antennas to a backscatter RFID tag comprising:
transmitting a modulated signal to the tag via first and second antennas,
receiving a first modulated backscattered signal having a first delay from the tag via said first antenna;
receiving a second modulated backscattered signal having a second delay from the tag via said second antenna;
inputting modulation in the backscattered signal received by said first antenna to a first linear adaptive combiner which is a delay line having first taps with first controllable weights on each tap;
inputting modulation in the backscattered signal received by said second antenna to a second linear adaptive combiner which is a delay line having second taps with second controllable weights on each tap;
inputting said transmitted modulated signal as a reference to said first linear adaptive combiner and second linear adaptive combiner;
summing and subtracting outputs of said first adaptive linear combiner from said reference to produce a first error signal;
summing and subtracting outputs of said second linear adaptive combiner from said reference to produce a second error signal;
wherein weights for said taps of said first and second linear adaptive combiners are approximately zero for all of said taps except for respective first and second combiner taps corresponding to the first and second delays of the backscattered signal and
determining the distance from the RFID tag to each of said antennas based on said first and second delays.

7. The method of claim 6, wherein said first and second antennas are mounted in a toll plaza and the tag in a vehicle.

8. The method of claim 6, further comprising operating said first and second linear adaptive combiners at a plurality of sample rates and said distance is determined from a plurality of results obtained at said plurality of sample rates.

9. The method of claim 8, wherein said distance is determined by selecting a maximum distance from said plurality of results.

10. The method of claim 6, further comprising adjusting said first controllable weights are based on a least means square fit of said first error signal and adjusting said second controllable weights adjusted based on a least means square fit of said second error signal.