Abstract:
A method and system for estimating position of a moving RFID tag is provided. The method includes transmitting at least one interrogator signal; receiving, at an interrogator antenna, a reflected signal from the RFID tag; determining phase changes of the reflected signal with respect to the phase of the at least one interrogator signal; weighting the phase changes based on instant power corresponding to the phase changes; producing a phase trajectory for the reflected signal based on the weighted phase changes; and estimating position of the RFID tag relative to the interrogator antenna based on the peak of the phase trajectory for the reflected signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application based on U.S. patent application Ser. No. 12/134,710 filed on Jun. 6, 2008, which issued as U.S. Pat. No. 8,149,093 on Apr. 3, 2012. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The following relates to RFID systems and, more particularly, to a method and apparatus for micro-localization of UHF passive RFID tags moving along a known, unidirectional path. 
       BACKGROUND OF THE INVENTION 
       [0003]    Radio Frequency Identification (RFID) systems use RFID tags to identify and/or track objects or living things. Typically, the tags are affixed to respective objects and when these tags are excited, they produce or reflect a magnetic or electric field at some frequency. The reflected field is modulated with an identifying code to identify the particular tag, and/or other useful information. 
         [0004]    An RFID tag may either be active or passive. Whereas active tags have a self-contained power supply and signal source, a passive tag receives an exciting signal at an exciting frequency from a transmitting antenna of an interrogator or reader positioned. Typically, the transmitting antenna is positioned at a portal. The exciting signal causes the RFID tag to transmit a signal, which is received by a receiving antenna adjacent to the transmitting antenna. The receiving antenna receives the modulated signal (magnetic or electromagnetic) produced by the excited tag and consequently the tag and the object to which it is attached can be identified. 
         [0005]    Interest in adopting RFID technology for use in automation systems and requiring minimal manual involvement is increasing rapidly. RFID systems are capable of providing real-time object visibility enabling continuous identification and location of all items and thereby providing real-time data management instead of simple snapshots. 
         [0006]    While the use of RFID tags is well known, most current RFID systems do not have the ability to locate fast moving tags (two meters per second i.e. 2 m/s or higher) with the accuracy required in many applications. Complexities are attributable to various factors including that the horizontal and vertical dimensions of the detection volume in which the RFID tags are to be read may contain several tags producing several signals, as well as noise, reflections and polarization losses. 
         [0007]    Prior approaches for addressing such complexities include confining the RF waves to a small volume using RF reflecting and absorbent materials, and/or controlling the angular extent of the interrogation zone (and thus the tag transmission zone) by using a two-element antenna to transmit a data signal with a directional sum pattern and a scrambled signal with a complementary difference pattern. Other approaches include the use of techniques relating to Doppler shift and triangulation. 
         [0008]    While various techniques for localization of RFID tags are known, improvements are of course desirable. 
         [0009]    It is an object of an aspect of the following to provide a method and system for wireless communications that addresses at least one of the above complexities. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect there is provided a method of estimating position of a moving RFID tag, comprising transmitting at least one interrogator signal; receiving, at an interrogator antenna, a reflected signal from the RFID tag; determining phase changes of the reflected signal with respect to the phase of the at least one interrogator signal; weighting the phase changes based on instant power corresponding to the phase changes; producing a phase trajectory for the reflected signal based on the weighted phase changes; and estimating position of the RFID tag relative to the interrogator antenna based on the peak of the phase trajectory for the reflected signal. 
         [0011]    According to another aspect there is provided a system for estimating position of a moving RFID tag, comprising a transmitter for transmitting at least one interrogator signal; a receiver in communication with at least one interrogator antenna receiving a reflected signal from the RFID tag; a phase detector determining phase changes of the reflected signal with respect to the phase of the at least one interrogator signal; a power detector determining instant power of the reflected signal corresponding to the phase changes; a phase peak estimator weighting the phase changes based on instant power and producing a phase trajectory for the reflected signal based on the weighted phase changes; and a position estimator estimating position of the RFID tag relative to the at least one interrogator antenna based on the peak of the phase trajectory. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Embodiments will now be described more fully with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  shows a block diagram of an illustrative Radio Frequency Identification (RFID) system; 
           [0014]      FIG. 2  shows a block diagram of an Interrogation unit used in the RFID system of  FIG. 1 , equipped with a Localization Processor; 
           [0015]      FIG. 3  shows a block diagram of an RFID tag used in the RFID system of  FIG. 1 ; 
           [0016]      FIG. 4  shows a block diagram of the Localization Processor used in the Interrogator of  FIG. 2 ; 
           [0017]      FIG. 5  shows the relative position between a moving RFID tag and the Interrogator antenna during phase and power measurements; 
           [0018]      FIG. 6  is a graph showing the averaged phase trajectory of demodulated signal received by an RFID tag moving under an Interrogator antenna; 
           [0019]      FIG. 7  is a graph showing the average trajectory of the gradient of the phase of the signal received from an RFID tag moving under an Interrogator antenna; and 
           [0020]      FIG. 8  is a graph showing the averaged power trajectory of the received signal from an RFID tag moving under an Interrogator antenna. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    Referring to  FIG. 1 , shown therein is a block diagram of an RFID system using passive technology (modulated backscattering). An Application Processor  101  communicates over Local Area Network  102  to a plurality of Interrogators  103 - 104 . The Interrogators may then each communicate with one or more of the Tags  105 - 107 . In reference with  FIG. 2 , the Interrogator  103  receives commands and information from an Application Processor  101 . A Processor  200  formats an Interrogator-Tag message ( 200   a ) based on the command and information received from the application Processor  101  to be sent to the Tag. The information signal ( 200   a ) may include information specific to Tag such as which Tag is to respond (each Tag may have a programmed identification number), instructions for the Tag&#39;s processor to execute or other information to be used and/or stored by the Tag&#39;s processor. With reference to  FIG. 2 , Local Oscillator  201  synthesizes a carrier wave (CW) signal, the Modulator  202  modulates the CW using Information Signal  200   a  and the Power Amplifier  203  applies the signal to an Antenna Switch/Combiner  204 . The Antenna Switch/Combiner  204  applies the amplified modulated signal to one or several transmit/receive antennae  205 - 206 . 
         [0022]    In the Tag  105  (see  FIG. 3 ), the antenna  301  receives the modulated signal. This signal is demodulated directly to baseband, using the Detector/Modulator  302 . The Information Signal  200   a  is then amplified by Amplifier  303  and bit synchronization is recovered in Clock Recovery circuit  304 . The resulting information detected using the recovered clock is sent to a tag Processor  305 . The processor  305  generates an Information Signal  305   a  based on the particular program executed by processor  305 . Signal  305   a  is eventually communicated to be sent from the Tag  105  back to the Interrogator (e.g.  103 ). Information Signal  305   a  is sent to a Modulator Control circuit  306  which uses the Information Signal  305   a  to modulate a subcarrier frequency generated by the Subcarrier generator  307  to produce signal  306   a.  The Modulated Subcarrier  306   a  is used by the Detector/Modulator  302  to modulate the CW received from Tag  105  to produce a backscattered (i.e. reflected) signal. A Battery  308  or other power supply provides power to the circuitry of Tag  105 . Power may also be received, for example, by using inductive coupling or microwaves. 
         [0023]    Returning to  FIG. 2 , the Interrogator  103  receives the modulated and reflected signal with the Antennae  205 - 206 , amplifies the signal with a Low Noise Amplifier  207  and demodulates the signal using a Quadrature Mixer  208 . Using the same Local Oscillator  201  as used in the transmit chain means the demodulation to baseband is done using Homodyne detection; this has advantages in that the received signal has the same reference as the Local Oscillator signal and it greatly reduces phase noise in the receiver. The Mixer  208  then sends the Quadrature Demodulated Signal  208   a  to a Filter/Amplifier  209  and a location processor  211 . The filtered and amplified signal—typically an Information Signal  209   a  carried on a subcarrier—is them demodulated from the subcarrier in the Demodulator  210  which then sends the Information Signal  210   a  to a Processor  200  to determine the content of the message. 
         [0024]    Using the above techniques, as an example an inexpensive, short-range, bi-directional digital radio communications channel can be implemented. 
         [0025]    We discuss now how a Modulated Backscattering system is used to determine the relative position between a Tag and an Interrogator antenna, as an example. For this example, assume that the Tag is moving in a constant direction and at a constant velocity under an Interrogator antenna during the period of time the measurement will be taken. Returning to  FIG. 2 , the Quadrature Signal  208   a  at the output of the Quadrature Mixer  208  is also applied to the Localization Processor  211 . The Localization Processor also receives Position Information Signals from an Optical Sensor  212  and/or Mechanical Sensor  213 , or any other position sensors. The Localization Processor  211  sends commands to the Processor  200  to specify which Tag is to respond, transmit power, antenna selection, and Information Signals such as Tag position estimate. The block diagram of the Localization Processor  211  is shown in  FIG. 4 . The Quadrature Signal  208   a  is filtered to remove data modulation and preserve only amplitude and phase changes caused by the Tag moving and then amplified by the Filter/Amplifier  401 ; the Filter/Amplifier  401  may or may not have the same characteristics as the Interrogator main Filter/Amplifier  209 . The filtered and amplified signal  401   a  is applied to a Phase Detector  402 . The Phase Detector  402  measures the phase difference between the transmitted signal (Local Oscillator) and the received signal. The phase difference is represented as: 
         [0000]      φ=atan( q/i )
 
         [0000]    where: q is the quadrature-phase component of the demodulated signal; and 
         [0026]    i is the in-phase component of the demodulated signal. 
         [0027]      FIG. 6  is a graph of the averaged phase trajectory (solid line) of demodulated signal received by an RFID tag moving under an Interrogator antenna. Raw data phase trajectory with multi path influence is shown as broken curve. Referring to  FIGS. 5 and 6 , as a Tag  501  is moving along direction x approaching an Interrogator antenna  502 , the received signal phase increases reaching a peak  601  at the antenna passing point, then it decreases when the tag is moving away from the antenna. 
         [0028]      FIG. 7  shows the average trajectory of the gradient of the phase (solid line) of the signal received from a tag moving under an Interrogator antenna. Raw data phase gradient trajectory with multi path influence is shown as broken curve.  FIG. 8  shows the averaged power trajectory (solid line) of the received signal from a tag moving under an Interrogator antenna. Raw data power trajectory with multi path influence is shown as broken curve. 
         [0029]    The mean phase spatial gradient is represented as: 
         [0000]      φ′= dφ/dx  
 
         [0000]    where: dφ is the phase differential; and 
         [0030]    dx is the differential displacement. 
         [0031]    As can be seen in  FIG. 7 , dφ crosses the zero line when the Tag passes by the antenna. By accurately detecting the zero crossing of the mean phase gradient, one can determine the moment a Tag passes a known position. 
         [0032]    For a single signal propagation path, the mean spatial gradient of the phase of the signal equals the mean Doppler, fd. In a practical situation, reflecting structures present in the vicinity of reading point cause a rich multipath radio propagation environment. In multipath channels, the mean phase spatial gradient is commonly denoted ‘random-FM’. The mean Doppler and the mean phase gradient are not always identical in multipath environments. However, this has no practical impact on the detection of the zero crossing point as only relative behavior of phase gradient before and after antenna passing point is needed for the identification of the zero crossing and consequently the antenna passing point. 
         [0033]    The multipath effect and measurement noise makes it difficult to detect the peak of the phase trajectory directly from measurements. The multipath propagation causes random phase jumps/steps (for the phase gradient this appears as random-FM transients/‘spikes’). Furthermore, different antennae connected to the same Interrogator may show a different peak position and different overlaid phase jumps. 
         [0034]    The phase gradient zero-crossing detection is performed by a Phase Gradient Null Estimator  403  as follows. First, the phase trajectories are found from the raw data received. Obvious outliers (jumps) are then detected, and mean powers around these jumps are measured using a Power Detector  404 . The measurements are weighted according to a relationship between instant power and magnitude of phase gradient transient. More particularly, instant power monitoring is used more precisely to identify outliers in phase and phase gradient. Following this, signal smoothing is performed. Finally, the measurements are averaged and a new phase peak estimate is extracted. Higher order phase derivatives can also be used to refine the passing point estimation. For example the 2 nd  order derivative of the phase (the phase curvature) can be used to identify a turn tangent occurring at the passing point. Furthermore, in more sophisticated implementations, the Phase Peak Estimator  403  can be a Kalman filter followed by a linear regression of the phase gradient to find the phase gradient trajectory zero crossing that also identifies the antenna passing point. 
         [0035]    The Phase Peak Information Signal  405   a  is applied to a Position Estimator  406  along with additional Position Information from Optical and Mechanical sensors  407 . Other auxiliary dimension, range or position information may be used and be retrieved from typical sensor systems and sources found in RFID and parcel applications, such as X-ray imaging, weight scale; acoustic/ultra-sound ranging and imaging, visual video and imaging, other radio radar. Finally, the Tag Position Information  211   a  is passed to the Interrogator Processor  200 , along with other Tag information such as Tag identification number. 
         [0036]    To narrow the Tag activation zone, the Interrogator antennae can be tilted to steer a null  702  in front of the reading gate (see  FIG. 5 ). 
         [0037]    Multiple antenna Interrogators can be used to compensate for random phase variation accompanying the envelope abrupt change caused by multipath. Combining phase information acquired by each antenna, one can smooth the phase gradient and compensate for correlated effects such as those caused by equipment imperfections. Speed sensors can be used to take into account tag speed variations. 
         [0038]    The method and system may be embodied in a software application including computer executable instructions executed by a processing unit such as a personal computer or other computing system environment. The software application may run as a stand-alone tool or may be incorporated into other available applications to provide enhanced functionality to those applications. The software application may comprise program modules including routines, programs, object components, data structures etc. and be embodied as computer readable program code stored on a computer readable medium. The computer readable medium is any data storage device that can store data, which can thereafter be read by a computer system. Examples of computer readable media include for example read-only memory, random-access memory, CD-ROMs, magnetic tape and optical data storage devices. The computer readable program code can also be distributed over a network including coupled computer systems so that the computer readable program code is stored and executed in a distributed fashion. 
         [0039]    Although embodiments have been described, those of skill in the art will appreciate that variations and modifications may be made without departing from the spirit and scope of the invention defined by the appended claims.