Abstract:
Passive radio frequency identification tags comprising a simple antenna and a circuit with a unique frequency-dependent response, in place of an RF-powered integrated circuit with control logic, is disclosed. The unique frequency-dependence of the circuit, which may simply be a feature of the antenna, conveys the tags identification. Multiple frequency-dependent circuit RFID tags can be simultaneously identified and located in 3-D space through use of antenna arrays and sampling at multiple frequencies in order to provide spatial resolution.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/645,273 filed Jan. 20, 2005, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to radio frequency identification tags, and more particularly to passive radio frequency identification tags and system thereof.  
       BACKGROUND OF THE INVENTION  
       [0003]     Radio Frequency Identification (“RFID”) is a generic term for technologies that use radio waves to automatically identify individual items. Objects can be identified using RFID by storing a serial number that identifies the object on a chip that is attached to an antenna. The chip and the antenna together are called an RFID tag. An RFID reader sends out electromagnetic waves that are received by the antenna on the RFID tag. Passive RFID tags draw power from this electromagnetic field to power the chip. Active tags use their own batteries to power the chip. There is also a version of a passive tag that does contain a battery, and is referred to as an “active/passive” tag. This tag has some of the attributes of a true active tag, but communicates in the same manner as a passive tag.  
         [0004]     RFID tags can also be distinguished by their memory type. Read/write memory, can be read as well as written into such that its data can be dynamically altered. Read only (typically “chipless”) type of tag memory is factory programmed and cannot be altered after the manufacturing process. According, the data of the RFID tag is static.  
         [0005]     A typical passive RFID tag is shown by  FIG. 1 , and generally indicated by symbol  10 . The RFID tag comprises an antenna circuit  20  and an integrated circuit  30 . The integrated circuit  30  may include an RF (or AC) rectifier  40  that converts RF (or AC) voltage to DC voltage, a modulation circuit  50  that is used to transmit stored data to a tag reader, a demodulation circuit  60  that is used to receive data from the tag reader, a memory circuit  70  that stores information, a logic circuit  80  that controls overall function of the device, etc. The antenna circuit  20  for a typical RFID tag may be formed by a parallel resonant LC circuit, where L is inductance and C is capacitance.  
         [0006]     When radio waves from a tag reader are encountered by the passive RFID tag  10 , the antenna circuit  20  within the tag forms a magnetic field. The integrated circuit  30  draws power from the antenna, via the rectifier  40 , energizing the circuits in the tag. The integrated circuit  30  uses that energy to transmit response codes by modulating the impedance the antenna circuit  20  presents to the interrogating field, thereby modulating the signal reflected back to the a tag reader antenna. Typical tag readers obtain a tag&#39;s ID information by requesting one bit at a time. The integrated circuit  30  on the tag processes signals from the tag reader to learn what bit is requested, and then sets backscatter modulation in order to respond with the requested bit. The integrated circuit  30  also provides control logic that allows tag readers to illuminate and communicate with multiple tags within its transmitted field. Without control logic, all illuminated tags would respond to queries from the tag reader simultaneously and cause interference with one another. Proper control logic and communication algorithms/protocols solve this multiple-tag collision problem. Unfortunately, the power requirements of the integrated circuit  30  largely drives the read range performance of passive RFID tags. Reading range is defined as the communication operating distance between the reader and tag. The reading range of a typical RFID tag may be less than ten meters, which is a significant limiting factor of passive RFID applications. Accordingly, improvements in passive RFID technologies are still needed.  
       SUMMARY OF THE INVENTION  
       [0007]     It is against the above background that present invention in one embodiment provides a passive radio frequency identification tag comprising a simple antenna and a circuit with a unique frequency-dependent response, in place of an RF-powered integrated circuit with control logic. Eliminating the power requirements of an integrated circuit greatly increases the effective reading range of the passive tag according to the present invention. In addition, the unique frequency-dependence of the circuit, which may simply be a feature of the antenna, conveys the tags identification, and may comprise linear elements and nonlinear elements to provide a nonlinear response. As a nonlinear response is easier to separate from a cluttered environment of mainly linear responses, multiple RFID tags according to the present invention can be simultaneously identified and located in 3-D space through the use of antenna arrays and sampling at multiple frequencies in order to provide spatial resolution. Accordingly, it is to be appreciated that the present invention in another embodiment provides an improved passive RFID tag system that permits multiple frequency-dependent circuit RFID tags to be simultaneously identified and located in a 3-D space, and also allow environmental clutter (e.g., nearby metal objects) to be spatially separated and segmented.  
         [0008]     These and other features and advantages of the invention will be more fully understood for the following detailed description of the invention taking together with the accompanying drawings. It is noted that the scope of the claims is defined by the recitations therein and not by the specific discussion of features and advantages set forth in the present description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The following description of the embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which:  
         [0010]      FIG. 1  is a prior art passive RFID tag that is powered by a rectifier/voltage converter which siphons off a significant portion of the RF energy captured by the antenna circuit of the tag;  
         [0011]      FIG. 2  is a frequency-dependent circuit passive RFID tag according to the present invention that does not depend on IC control logic to solve the collision problem; and  
         [0012]      FIG. 3  is a simple link budget analysis for the present invention. 
     
    
       [0013]     Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements, and with conventional parts removed, to help to improve understanding of embodiments of the present invention.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0014]     A plurality of simplified frequency-dependent circuit passive radio frequency identification (pRFID) tags in according to one illustrative embodiment of the present invention is shown generally by symbol  200  in  FIG. 2 . Each of the PFRID tags  200  comprises an antenna circuit  210  and each have a unique frequency-dependent circuit  220 . In one embodiment, the antenna circuit  210  and frequency-dependent circuit  220  are implemented on a substrate  230 . In one embodiment, the substrate  230  is formed from silicon, a fibrous material, such as paper or cloth, plastic, or other inexpensive material, and combinations thereof. The antenna circuit  210  is formed as a pattern, such as for example, of two spaced apart deposits of conductive material as shown in  FIG. 2 . The conductive materials include metals, carbon ink, carbon filled polymer, or other suitable conductive materials. The conductive pattern may be printed, painted, or otherwise disposed on the substrate  230 . In one embodiment, the antenna circuit  210  includes a receiving antenna array  210 A and a transmitting antenna array  210 B.  
         [0015]     It is to be appreciated that many methods for antenna and frequency-dependent circuit design are presently available. In one embodiment, the antenna pattern itself functions as the frequency-dependent circuit  220 . In another embodiment, the frequency-dependent circuit  220  may be similarly formed of conductive materials or separately prepared, such as by using a standard manufacturing process, and electrically coupled to the antenna circuit  210 . Therefore, it is to be appreciated that the present invention is not specific to any particular antenna design and/or circuit design. Rather, it is the combination of these elements that is innovative, and in practice, the antenna design and/or circuit design will vary significantly depending the application of interest.  
         [0016]     The frequency-dependent circuit  220  of each of the tags  200  is designed to have a unique response for each frequency within its specified spectrum that is used to provided identification and other data. The term “response” means, in the context of the present invention, that each of the tags  200  provides intelligible information when subjected to an appropriate interrogation signal  240 , such as from a RFID tag reader, which is indicated generally by symbol  250  in  FIG. 2 . Together, tags  200  and tag reader  250  provide one illustrative embodiment of a RFID system according to the present invention, which is illustrated generally by symbol  260 .  
         [0017]     When one of the tags  200  is illuminated by a particular frequency of RF energy, i.e. interrogation signal  240 , it responds at that frequency with a signal  270  having a unique phase and/or amplitude. This frequency dependent response, i.e. signal  270 , is received by an antenna system  280  of the RFID tag reader  250 .  
         [0018]     As shown in  FIG. 2 , the tag reader  250  includes the antenna system  280 , a radio frequency transceiver  290 , a digital signal processor (DSP)  300 , a control processor  310 . It should be noted the RFID tag reader  250  shown in  FIG. 2  can include other components, such as a display or user control  320 , and a communication device  330  for communicating data over a wired and/or wireless network (e.g. Ethernet, Power Over Ethernet, Bluetooth, infra-red, RF wireless LAN, etc.), such as to a network computer and/or other tag readers. In addition, the RFID tag reader  250  can include other components not shown, such as an internal and/or external power source which supplies the necessary power to the reader for proper operation such that the RFID tags  200  are located and identified within a specified space.  
         [0019]     In one illustrated embodiment, the antenna system  280  of the tag reader  250  is coupled to the transceiver  290 , which in turn is coupled to the DSP  300 , and the DSP is coupled to the control processor  310 . In one embodiment, the control processor  310  may be coupled to the DSP  300 , the transceiver  290 , and antenna system  280  for control and mode switching. It is to be appreciated that the transceiver  290  may also be in one embodiment separated into individual transmitter and receiver devices.  
         [0020]     The antenna system  280  may be one or a plurality of conventional antenna array structures separated by known distances. In one embodiment, the tag reader  250  transmits and receives RF electromagnetic radiation signals  240 ,  270  utilizing the antenna system  280 , as shown in  FIG. 2 . In one embodiment, one of the antennas  280  serves as the transmitting antenna and the other remaining antennas serve as the receiving antennas to provide angular signal separation. Such angular signal separation is analyzed by the processor  310  to provide location of the tags  200  in a space. In addition, the given responses by the tags  200  at multiple frequencies permits the control processor  310  to decode the responses and relate each of the given locations of the tags  200  to a unique tag ID.  
         [0021]     The control processor  310  operates to synchronize the components of the tag reader  250  to ensure effective operation. In one embodiment, the control processor  310  is networked to communicate and be RF phase synchronous with a plurality of tag readers  250  such that information on separate perspectives of a common scanned area can be analyzed as an aggregate image. It is contemplated that one or more tag readers  250  in the plurality can transmit the RF signals  240  simultaneously or non-simultaneously. It is also contemplated, as well, that one or more tag readers  250  can receive the reflected signals  270  from the RFID tags  200  simultaneously or non-simultaneously. These methods of cooperative signal processing provide the basis for a plurality of image perspectives and thereby can create an image of the scanned area showing the locations of the tags  200  from different perspectives.  
         [0022]     In one embodiment, the frequency-dependent circuit  220  provides a linear radio frequency response, and in another embodiment a linear and a non-linear radio frequency response. As used herein, a “linear radio frequency response” means that there is a linear relationship between stimulus and response where an increase in stimulus will result in a corresponding increase in response, and vice versa. A “non-linear radio frequency response” means that there is not a linear relationship between stimulus and response where an increase in stimulus will necessarily result in a corresponding increase in response, and vice versa. Since the ability to provide a circuit with a frequency-dependent response or a linear/nonlinear response is well within the skill of those in the related art, no further details about the same are provided. In one embodiment, the responses  270  of the tags  200  are statistically orthogonal to each other. In still another embodiments, in order to maintain signal strength, the responses  270  of the tags  200  vary in phase as a function of frequency.  
         [0023]     In the latter of the above embodiments, when the tags  200  are illuminated by a particular frequency of RF energy, each of the tags  200  will return a unique linear/nonlinear response. Each unique linear/nonlinear response is observed by the RFID reader  250 , and given responses at multiple excitations frequencies, a series of measurements can be decoded and related to a particular tag ID. However, unlike the typical pRFID tag design shown by  FIG. 1 , in the above illustrative embodiments of  FIG. 2 , there is no control logic to prevent all of the illuminated tags from replying simultaneously. For this reason, the use of multiple reader antennas  280  or tag readers  250  provide the geometric diversity necessary to spatially resolve the responses  270  from the plurality of tags  200 .  
         [0024]     Spatially resolving the tags  200  allows the unique identifications of each to be determined, and also provides 3-D position information for each tag. Resolving tags in 3-D also allows in one embodiment for the processor  310  to computationally spatially separate and segment environmental clutter (e.g., nearby metal objects) from the detection space. In addition, eliminating the power-consuming IC frees up considerable energy to be backscattered to the tag reader  250 .  
         [0025]     An analysis, shown in  FIG. 3 , suggests that an order of magnitude improvement in range performance in the disclosed embodiments of the present invention is provided. In particular,  FIG. 3  is a simple link budget analysis which indicates that tags  200  according to the present invention offer operating ranges up to 100 m, which is significantly in excess of the advertised ranges (&lt;10 m) of prior art passive RFID tags.  
         [0026]     The analysis presented hereinafter illustrates the primary functionality of a pRFID system  260  according to the present invention. As mentioned above, the pRFID system  260  comprises a plurality of the frequency-dependent circuit pRFID tags  200  and at least one tag reader  250 . Each of the tags  200  has a unique spectral response over a range of operating/excitation frequencies, such that response of the k th  tag to the p th  frequency is H k (f p ). One may choose to exploit all or only a subset of these frequencies (e.g., P of them) at any given instance. In another embodiment, in addition to each tag having a unique spectral response, all the tags  200  may also have an identical response at a given frequency or frequencies in order to provide a known reference.  
         [0027]     The tag reader  250  includes a multiple (N) of antennas elements  340  in each antenna system  280  that are geometrically separated. The precise configuration of antenna elements  340  will vary based on the application. Techniques for designing and fabricating antennas are also well-known. Through monostatic, bistatic or multistatic operation, each antenna system  280  will interrogate each illuminated tag at every frequency of interest. If multiple (M) tags  200  lie within the illuminated region, their recorded returns will consist of a superposition of tag responses. Thus, the spectral response observed by the q th  antenna at the p th  frequency will be according to Equation (1).  
                 S   q     ⁡     (     f   p     )       =       ∑     k   =   1     M     ⁢       A   qk     ⁢       H   k     ⁡     (     f   p     )       ⁢     exp   ⁡     (       -   j     ⁢       2   ⁢           ⁢   Π   ⁢           ⁢     f   p       c     ⁢     R   qk       )                   (   1   )             
 
 where A qk  is an arbitrary gain/phase related to the physical channel between the tag and reader. The round-trip range (monostatic or bistatic) from the transmitter to the tag to the receiver is R qk , is thus a function of the known transmitter position t q , the known receiver position r q , and the unknown tag position x k . In monostatic operation, the transmitter and receiver are collocated. The measurements represented by Equation (1) will be repeated for all antennas (for all q) and for all frequencies (for all p). 
 
         [0028]     One may then spatially resolve the M tags and obtain their identification codes through appropriate signal processing. In addition, significant work has already been done in 3-D RF imaging and extraction of RF phenomenology. Many algorithms exist for spatially resolving unique signatures such as those returned by the suggested pRFID tags. The goal is to estimate the tag positions (x k ,y k ,z k ) and/or their frequency-dependent responses Ĥ k (ƒ p ). This may be done parametrically. As mentioned earlier, many algorithms already exist to solve this problem. Many more algorithms are under development under different auspices. And, the specific algorithm that should be applied will vary on a case-by-case basis.  
         [0029]     Given estimates for tag positions and frequency-dependent responses, the estimated responses Ĥ k (ƒ p ) may be related back to tag identification numbers that are stored in an electronic database. The system may then supply the operator with a report that conveys the 3-D position and/or identification information associated with every tag that lies within the processed region.  
         [0030]     It is to be appreciated that the present invention is in one embodiment is an IC-less passive RFID tag, thereby providing a two way communication system with extreme power limitations on one end. One of the benefits is such a system permits the anti-collision algorithm to be implemented. In an alternate implementation, the PRFID tag embodiments of  FIG. 2  could include an additional integrated circuit (IC)  350  to allow writing operations at short range. During a write operation, the IC would be RF-powered and would alter the frequency-dependent circuit or the linear/nonlinear circuit as directed by the tag reader  250 . Read operations could be conducted at longer ranges using the general process described above.  
         [0031]     The present invention is suitable for personnel locating/tracking/monitoring and a variety of material handling functions that require product ID codes such as UPC symbols. These include product tracking, sorting systems, conveyor systems, baggage handlers, etc. Military applications include locating/tracking/monitoring of friendly forces, tracking of weapons and munitions, cooperative targeting, and the like are also envision applications of the present invention.  
         [0032]     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that is have the full scope permitted by the language of the following claims.