Rfid reader systems detecting pilot tone

Abstract

RFID tags are commanded to generate a pilot tone in their backscatter. When the backscattered pilot tone is received in the reader, the pilot tone is used to estimate the tag period/frequency. Then, the estimate is used to seed and lock a symbol timing recovery loop, which provides a detected signal to one or more correlators for detecting the tag preamble. A delayed version of the received tag signal is compared against a baseline signal threshold established from the received signal to detect the pilot tone.

1 . A method for a Radio Frequency Identification (RFID) reader system, comprising: 
 Receiving from an RFID tag a backscattered signal;    estimating a frequency of a pilot tone in the backscattered signal;    recovering a timing of symbols in the backscattered signal using the estimated frequency; and    demodulating the backscattered signal based on the symbol timing to recover data.    
   
   
       2 . The method of  claim 1 , further comprising: 
 detecting the pilot tone.    
   
   
       3 . The method of  claim 2 , in which 
 the signal is backscattered from the RFID tag responsive to a command transmitted by the reader, and    in which the pilot tone is detected only if the command belongs in a subset of commands.    
   
   
       4 . The method of  claim 2 , in which 
 the pilot tone is detected by:    generating a baseline version of the backscattered signal;    generating a version of the backscattered signal that is delayed with respect to the backscattered signal; and    establishing detection when the delayed version reaches a detection relationship with the baseline version.    
   
   
       5 . The method of  claim 4 , in which 
 the detection relationship is that the delayed version exceed the baseline version.    
   
   
       6 . The method of  claim 4 , in which 
 the baseline version of the tag signal is generated as a preset portion of a magnitude of the tag signal.    
   
   
       7 . The method of  claim 6 , in which 
 the preset portion is substantially equal to 50%.    
   
   
       8 . The method of  claim 6 , in which 
 the baseline version has a waveform that rises up to a cusp, and then continues at substantially the same value, and    the delayed version is delayed enough so that detection is established after the cusp.    
   
   
       9 . The method of  claim 4 , in which 
 detection is established only if both the delayed version and the baseline version exceed a detection threshold.    
   
   
       10 . The method of  claim 9 , in which 
 the detection threshold is defined in terms of detected ambient noise.    
   
   
       11 . The method of  claim 9 , in which 
 the detection threshold is updated.    
   
   
       12 . The method of  claim 9 , in which 
 the backscattered signal is received responsive to a command signal transmitted by the RFID reader system,    the command signal causes the tag to be silent during a silent period, and    the detection threshold is defined from a power measurement made during the silent period.    
   
   
       13 . The method of  claim 1 , in which 
 the frequency is estimated by employing a Discrete Fourier Transform (DFT) of the received pilot tone.    
   
   
       14 . The method of  claim 1 , in which 
 the frequency is estimated by employing a Fast Fourier Transform (FFT) of the received pilot tone.    
   
   
       15 . The method of  claim 1 , in which 
 the frequency is estimated by correlating the backscattered tag signal with an expected version of the backscattered signal.    
   
   
       16 . The method of  claim 1 , further comprising: 
 identifying in the backscattered signal a preamble using the symbol timing, and    in which the backscattered signal is demodulated based on a timing of the preamble.    
   
   
       17 . The method of  claim 16 , in which 
 the preamble is identified using a single correlator.    
   
   
       18 . The method of  claim 16 , further comprising: 
 determining from the preamble timing a Time of Arrival (TOA) for a remainder of the backscattered signal; and    in which the data is recovered from the remainder.    
   
   
       19 . The method of  claim 18 , further comprising: 
 delaying the remainder of the backscattered signal the preamble by a delay time; and    adjusting the delay time to compensate for an amount of time taken to estimate the frequency.    
   
   
       20 . A Radio Frequency Identification (RFID) reader system component operable to receive a backscattered RFID tag signal that includes a pilot tone, the component comprising: 
 a frequency estimator operable to estimate a frequency of the pilot tone and to generate from it a frequency estimation signal;    a buffer/delay operable to generate a delayed version of the backscattered signal;    a symbol timing recovery loop controlled according to the frequency estimation signal, the symbol timing recovery loop operable to output, responsive to the delayed version of the backscattered signal, a TS signal that has recovered a timing of symbols in the backscattered signal; and    a decoder for decoding the TS signal to recover data.    
   
   
       21 . The component of  claim 20 , further comprising: 
 a detector for detecting the pilot tone.    
   
   
       22 . The component of  claim 20 , in which 
 the frequency estimator further detects the pilot tone.    
   
   
       23 . The component of  claim 20 , in which 
 the frequency is estimated by the frequency estimator by employing a Discrete Fourier Transform (DFT) of the received pilot tone.    
   
   
       24 . The component of  claim 20 , in which 
 the frequency is estimated by the frequency estimator by employing a Fast Fourier Transform (FFT) of the received pilot tone.    
   
   
       25 . The component of  claim 20 , in which 
 the frequency is estimated by the frequency estimator by correlating the backscattered tag signal with an expected version of the backscattered signal.    
   
   
       26 . The component of  claim 20 , in which a frequency estimate signal is generated according to a frequency of the backscattered signal, and 
 a frequency of the symbol timing recovery loop is locked responsive to the frequency estimate signal.    
   
   
       27 . The component of  claim 20 , in which 
 a symbol phase estimate signal is generated according to a symbol phase of the backscattered signal, and    a phase of the symbol timing recovery loop is controlled according to the symbol phase estimate signal.    
   
   
       28 . The component of  claim 20 , comprising: 
 at least one correlator operable to correlate the TS signal with an expected preamble signal for generating a correlation signal, and    in which the TS signal is demodulated only if the correlation signal exceeds a threshold.    
   
   
       29 . The component of  claim 28 , in which 
 only one correlator is used by the reader for correlating the TS signal.    
   
   
       30 . The component of  claim 20 , in which 
 the buffer/delay is implemented by a variable delay.    
   
   
       31 . The component of  claim 20 , in which 
 the buffer/delay is implemented by a variable rate buffer.    
   
   
       32 . The component of  claim 20 , in which 
 the delayed version of the tag signal is controlled to impose a nonzero delay time suitable for compensating for a delay from the frequency estimator.    
   
   
       33 . The component of  claim 20 , in which 
 the symbol timing recovery loop comprises: 
 a digital rate converter circuit arranged to: 
 obtain sample values of a tag response signal waveform at sample time points;  
 reconstruct signal values at target time points from the sample values using the frequency estimate signal; and  
 output the reconstructed signal values as the TS signal;  
 
 a timing error detector coupled to the digital rate converter;  
 a loop filter coupled to the phase detector; and  
 a numerically controlled oscillator (NCO) coupled to the loop filter and to the digital rate converter such that the timing error detector, the loop filter, and the NCO form a feedback loop.  
   
   
   
       34 . The component of  claim 33 , in which 
 the symbol timing recovery loop further comprises: 
 a matched filter coupled to an input of the digital rate converter.  
   
   
   
       35 . The component of  claim 33 , in which 
 the symbol timing recovery loop further comprises: 
 a matched filter coupled to between the digital rate converter and the timing error detector.  
   
   
   
       36 . The component of  claim 33 , in which 
 the digital rate converter includes: 
 an interpolating filter arranged to receive the sample time points, interpolate, and generate offset sample points;  
 a decimator coupled to the interpolating filter arranged to decimate a portion of the sample points such that offset sample points corresponding to the target time points are provided by the digital rate converter; and  
 a timing processor arranged to provide the interpolating filter and the decimator with a first and second control signals based on an output of the NCO.  
   
   
   
       37 . A Radio Frequency Identification (RFID) reader system component operable to receive a backscattered RFID tag signal that includes a pilot tone, the component comprising: 
 means for receiving from an RFID tag a backscattered signal;    means for detecting a pilot tone from the backscattered signal;    means for estimating a frequency of the pilot tone in the backscattered signal;    means for recovering a timing of symbols in the backscattered signal using the estimated frequency; and    means for demodulating the backscattered signal based on the symbol timing to recover data.    
   
   
       38 . The component of  claim 37 , in which 
 the means for detecting the pilot tone comprises:    means for generating a baseline version of the backscattered signal;    means for generating a version of the backscattered signal that is delayed with respect to the backscattered signal; and    means for establishing detection when the delayed version reaches a detection relationship with the baseline version.    
   
   
       39 . The component of  claim 37 , further comprising: 
 means for identifying in the backscattered signal a preamble using the symbol timing, in which the backscattered signal is demodulated based on a timing of the preamble.    
   
   
       40 . The component of  claim 37 , further comprising: 
 means for determining from the preamble timing a Time Of Arrival (TOA) for a remainder of the backscattered signal, in which the data is recovered from the remainder.    
   
   
       41 . The component of  claim 40 , further comprising: 
 means for determining from the preamble timing a Time Of Arrival (TOA) for a remainder of the backscattered signal, in which the data is recovered from the remainder.    
   
   
       42 . The component of  claim 41 , further comprising: 
 means for delaying the remainder of the backscattered signal the preamble by a delay time; and    means for adjusting the delay time to compensate for an amount of time taken to estimate the frequency.