Patent Publication Number: US-2009219144-A1

Title: Method of Utilizing Parametric Reflective Technology

Description:
STATEMENT OF RELATED APPLICATIONS 
     The present application is a divisional application of co-pending U.S. patent application Ser. No. 11/158,624, which claims priority based on U.S. Provisional Patent Application Ser. No. 60/581,384, filed on Jun. 22, 2004, in the name of inventor Michael Gregory Pettus, entitled “Millimeter Wave RFID System Using Parametric Reflective Encoding,” commonly owned herewith. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directly generally to the field of radio frequency identification (RFID) interrogators and data tags as well as encoding and decoding methods. 
     BACKGROUND OF THE INVENTION 
     There are many existing technologies in current development and deployment that implement the desired function of identifying articles, objects, vehicles and personnel. Bar codes and magnetic strips are traditionally familiar as short range devices. More recently, techniques for increasing the read reliability are being used in the general area of radio frequency identification or RFID. 
     RFID technology utilizes a tag transponder, which is placed on an object, and a reader, also referred to herein as an interrogator, to read and identify the tag. RFID technologies are broadly categorized using “active” tags with the longest range, and “passive tags” with a much shorter range (typically less than 20 feet). The industry categorizes active tags as having a local power source (such as a battery) so that the active tag sends a signal to be read by the interrogator. The industry categorizes passive tags as those whose power is derived from the reader, whereby the passive tag re-transmits or transponds information upon receiving the signal from the reader. 
     In both of these categories of tags, there is an electronic circuit that is typically in the form of an integrated circuit or silicon chip, whereby the circuit stores and communicates identification data to the reader. In addition to the chip, the tag includes some form of antenna that is electrically connected to the chip. Active tags incorporate an antenna which communicates with the reader from the tag&#39;s own power source. For passive tags, the antenna acts as a transducer to convert radio frequency (RF) energy originating from the reader to electrical power, whereby the chip becomes energized and performs the communication function with the reader. 
     Considering that active and passive tags have electronic circuitry in the form of a chip, the manufacturing costs for each tag is significant. Not only is there a cost associated with the chip itself, but there are also numerous processing steps required in order to place the chip onto the tag. In addition, existing tags require a method of mechanically and electrically connecting the antenna to the chip, which adds to manufacturing costs. 
     It should also be noted that active and passive RFID tag technologies are fundamentally based on an interrogate-and-then-communicate sequence of operations. Therefore there is an amount of time for the interrogator to read the tag which is dependent on the RF bandwidth and the data rate of the communications channel between the interrogator and the tag. If more than one tag is within range of the interrogator, multiple interfering transmissions can result from the interrogator attempting to read a single tag. Also of note, in the types of RFID systems thus described, there are no straightforward methods to accurately locate and track a tag. The technologies described above provide only a method of identification. 
     What is needed is a chipless RFID system and method that would provide greater range between the interrogator and the tags, lower manufacturing costs of the tags, and less aggregate read time for multiple tags in proximity to each other. What is also needed is a system which accurately locates and tracks a tag. 
     OVERVIEW 
     A system and method for encoding and decoding information by use of radio frequency antennas includes one or more interrogator devices and RFID data tags. The RFID data tags include a plurality of antenna elements which are formed on a substrate or directly on an object. The antenna elements are oriented and have dimensions to provide polarization and phase information representing the encoded information on the RFID tag. The interrogator device scans an area and uses radar imaging technology to create an image of a scanned area. The device receives re-radiated RF signals from the antenna elements on the data tags, with the data tags represented on the image. The re-radiated RF signals preferably include polarization and phase information of each antenna element, whereby the information is utilized using radar signal imaging algorithms to decode the information on the RF data tag. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. 
         FIG. 1A  illustrates a radio frequency identification (RFID) system for use in a baggage identification setting in accordance with one example embodiment of the invention. 
         FIG. 1B  illustrates a luggage label having the RFID tag of one embodiment of the present invention. 
         FIG. 1C  illustrates a handheld RFID interrogator in accordance with one embodiment of the present invention. 
         FIG. 1D  illustrates a personal identification card having the RFID tag thereon in accordance with one embodiment of the present invention. 
         FIG. 1E  illustrates a container having multiple RFID tags thereon in accordance with one embodiment of the present invention. 
         FIG. 2  illustrates a schematic of the RFID interrogator in accordance with one embodiment of the present invention. 
         FIG. 3A  illustrates a schematic of an antenna configuration on a substrate of an RFID tag in accordance with one embodiment of the present invention. 
         FIG. 3B  illustrates a side view of the RFID tag in accordance with one embodiment of the present invention. 
         FIG. 3C  schematic of another RFID tag antenna configuration on a substrate in accordance with one embodiment of the present invention. 
         FIG. 4A  illustrates a schematic of another RFID tag antenna configuration on a substrate in accordance with one embodiment of the present invention. 
         FIG. 4B  illustrates a schematic of another RFID tag antenna configuration on a substrate in accordance with one embodiment of the present invention. 
         FIG. 5  illustrates a block diagram of the decoding process in accordance with one embodiment of the present invention. 
         FIG. 6  illustrates an example table of a data coding scheme for the RFID tag in accordance with one embodiment. 
         FIG. 7  illustrates a flow diagram of one example encoding process in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are described herein in the context of an RFID system utilizing parametric reflective technology. Those of ordinary skill in the art will realize that the following detailed description of the present invention is illustrative only and is not intended to be in any way limiting. Other embodiments of the present invention will readily suggest themselves to such skilled persons having the benefit of this disclosure. Reference will now be made in detail to implementations of the present invention as illustrated in the accompanying drawings. The same reference indicators will be used throughout the drawings and the following detailed description to refer to the same or like parts. 
     The present invention described herein preferably utilizes a very low cost RFID tag construction that does not require semiconductor or chip technologies. The present invention preferably can read the RFID tags at distances up to 100 meters as well as read and identify thousands of RFID tags per second. In addition, the present invention can preferably provide an accurate two and/or three dimensional location bearing of an RFID tag. If the RFID tag is located on a moving object, velocity and trajectory information can be computed by the interrogator of the present invention. The present invention preferably utilizes frequencies in the millimeter wave region, which allows detection of RFID tags behind foliage and non-metallic building materials. However, it is contemplated that the present invention utilizes frequencies in other ranges, and is not limited to the millimeter wave region. 
     The present invention can reduce operational expense, improve efficiencies and provide features to industry, government, homeland security, military, healthcare, education, transportation and consumers. The present invention can be used in a wide range of applications including, but not limited to: inventory identification; asset management tracking and shipping container location; vehicular access control (e.g. toll ways); moving vehicle identification; healthcare identification and tracking of patients, drugs, equipment and personnel identification, tracking and monitoring of personnel and equipment for security purposes; identification of luggage and packages at airports; systems for locating lost objects (e.g. keys, files, golf balls, clothing articles). 
       FIG. 1A  illustrates an RFID system in accordance with one example embodiment of the present invention. As shown in  FIG. 1A , the RFID system  10  includes one or more interrogators  100 , also referred to as readers, as well as one or more RFID tags  200 . The system shown in  FIG. 1A  depicts a conveyer belt  97  normally found in a baggage claim area in an airport, whereby various suitcases  98  and packages  99  are delivered on the conveyer belt  97 . As shown in  FIG. 1A , the packages  99  include one or more RFID tags  200  printed or affixed thereon, whereas the suitcases  98  each include an attachment  96  which includes the RFID tag  200  thereon.  FIGS. 1B ,  1 D and  1 E illustrate examples of the RFID tags  200  on the suitcases and packages. 
     The interrogators  100  are shown in  FIG. 1A  as stand alone units, whereby the interrogators locate, identify and optionally track each tag  200  as the items  98 ,  99  move on the conveyer belt  97 . In another embodiment, the interrogator is in the form of a handheld unit  101 , as shown in  FIG. 1C . It should be noted that the RFID system of the present invention is operable in a multitude of applications and settings, some of which are discussed below, and is in no way limiting to the examples shown and described herein. 
     The present system  10  preferably utilizes mathematical focus algorithms in the area of radar imaging to decode and identify the RFID tags. The type of mathematical focus algorithms that are used by the system  10  depends on the application. For example, as shown in  FIG. 1 , the present system  10  can use inverse synthetic aperture radar (ISAR) algorithms to identify a moving tag  200  (in synthetic aperture radar terminology (SAR), “moving target”) on a conveyor  97  which has a plurality of luggage articles  98 ,  99 , whereby the luggage articles are physically moving in a translational direction relative to one or more interrogators  100 . It should be noted that other mathematical focus algorithms can be used in the application shown in  FIG. 1A  and is not limited to SAR. In one embodiment, the interrogators  100  are fixed and disposed at various locations along the conveyor  97 . The interrogators  100  are preferably positioned to be orthogonal and parallel relative to the direction of motion of conveyor  97 . 
       FIG. 2  illustrates a schematic of the RFID interrogator in accordance with one embodiment of the present invention. It should be noted that the components in the interrogator  100  shown in  FIG. 2  are preferred, and the interrogator  100  can include other components not shown. As shown in  FIG. 2 , the interrogator  100  preferably includes an antenna structure  102 , a polarization and phase transmitting control block  110 , a radio frequency transmitter  112 , a digital signal processor (DSP)  114 , a control processor  116 , a radio frequency receiver  118  and a polarization and phase receiving control block  120 . Although not shown, the interrogator includes an internal and/or external power source which supplies the necessary power to the locate and identify the RFID tags  200  within a specified distance. Preferably, the antenna structure  102  of the interrogator  100  is coupled to the transmitting and receiving control blocks  110  and  120 . The transmitting control block  110  is preferably coupled to the radio frequency transmitter  112 , both of which are preferably coupled to the control processor  116 . The receiving control block  120  is preferably coupled to the radio frequency receiver  118 , both of which are coupled to the control processor  116 , as well. The radio frequency transmitter  112  and receiver  118  are preferably coupled to the DSP  114 , whereby the DSP  114  is coupled to the control processor  116 . 
     The control processor  116  preferably synchronizes the components of the interrogator  100  to ensure effective operation of the interrogator  100 . In one embodiment, the control processor  116  is coupled to a wired or wireless network via hard-wire or wireless communication techniques (e.g. Ethernet [such as Power Over Ethernet, POE], Bluetooth, infra-red, RF wireless LAN). In one embodiment, the interrogator  100  includes an integrated display or user control  124 , such as in a handheld unit ( FIG. 1C ), whereby the display  124  is coupled to the control processor  116 . In one embodiment, the interrogator  100  is coupled to an external display or user control  124 , such as on a network computer. 
     In one embodiment, a plurality of interrogators  100  are networked to communicate and be RF phase synchronous with one another 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 interrogators  100  in the plurality can transmit the RF signals simultaneously or non-simultaneously. It is also contemplated, as well, that one or more interrogators  100  can receive the reflected signals 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. 
     In the preferred embodiment, the interrogation device  100  transmits and receives RF electromagnetic radiation signals utilizing the antenna structure  102 , as shown in  FIG. 2 . In particular, the antenna structure preferably includes a transmitting antenna array  102 A and a receiving antenna array  102 B which are shown in separate portions of the antenna structure  102 . In the embodiment shown in  FIG. 2 , the transmitting antenna array  102 A is located in the top section of the structure  102  whereas the receiving antenna array  102 B is located in the bottom section. It is contemplated that the individual transmitting and receiving antennas  102 A,  102 B can be alternately arranged from top to bottom of the antenna structure  102  face. In another embodiment, one set of antennas serves the transmitting and receiving functions. In the embodiment shown in  FIG. 2 , the antenna structure  102  is shown as having a substantially flat, planar surface (two dimensional). In another embodiment, the antenna structure  102  has a non-planar configuration (e.g. conical, bulbed, angled) whereby the individual antennas in the array are positioned in three dimensions. 
     In general, the interrogator  100  utilizes one or more radio detection and ranging (RADAR) technologies to identify tags over a scanned area. The transmitting array  102 A of the interrogator  100  transmits electromagnetic radiation to a large area at a desired frequency. Preferably, the frequency ranges between 30 GHz and 300 GHz, depending on the application in which the system  10  is used. However, other frequencies outside this range are contemplated. The electromagnetic radiation is received at the RFID tags  200  in the scanned area, whereby the antenna structures of the RFID tags  200  resonate at the desired frequency and re-radiate the electromagnetic signals back toward the interrogator  100 . The interrogator  100  samples and stores the received signals from the RFID tag(s), as well as reflected electromagnetic radiation from all objects in the scanned area, and builds a signal phase history in a memory system. Through mathematical coherent phase analysis, the interrogator  100  preferably processes the phase history and polarization samples using general Synthetic Aperture Radar (SAR) signal processing algorithms, although other processing algorithms are contemplated. The interrogator  100  is then able to generate images of the scanned area from the phase history samples and associated polarization data to identify the RFID tags  200  in the area. In other words, the interrogator  100  is able to “view” the scanned area using RADAR technology and “see” the RFID tags and distinguish the tags  200  from other objects and RFID tags  200  by the orientations and dimensions of the antenna structures thereon. 
     The RFID system  10  of the present invention utilizes polarization and phase information, preferably in the millimeter wave range, to detect and identify the tags  200 . As shown in  FIG. 2 , the antenna structure  102  of the interrogator  100  includes several vertically positioned antennas  106  as well several horizontally positioned antennas  108  in the transmitting and receiving sections  102 A,  102 B. The individual antennas  106 ,  108  independently transmit and receive signals utilizing different RF polarization schemes. For example, the antenna structure  102  can transmit RF signals with vertical polarization (V) as well as horizontal polarization (H). This is preferably performed by energizing the vertically oriented antennas  106  for vertically polarized signals, V and energizing the horizontally oriented antennas  108  for horizontally polarized signals, H. 
     In regards to the antenna structure  102  in  FIG. 2 , the interrogator  100  can transmit signals utilizing one polarization scheme while the antennas in the receiving section  102 B can be controlled to receive signals of another polarization scheme. For example only, the interrogator  100  can transmit vertically polarized signals V and simultaneously receive signals which are horizontally polarized, H. 
     In addition, the interrogator  100  can control both the vertical and horizontal antennas to transmit and receive additional polarization parameters. The interrogator  100  can energize the vertically oriented antennas  106  and simultaneously energize the horizontally oriented antennas  108  with an appropriate phase difference to generate left-hand circular (LC) or right-hand circular (RC) polarized signals. Any combination of separate transmit and receive polarizations of vertical, horizontal, left-hand circular and right-hand circular (V, H, LC, RC) are preferably implemented in the present system  10 . This allows the present system  10  to make use of polarization differences as well as diversity so that the interrogator  100  can store received signals which are created with different transmit and receive polarization parameters, as will be discussed in more detail below. 
     The interrogator  100  generates and receives radio frequency signals in a range of frequencies that are compatible with the frequency range of the resonant antenna elements on the RFID tags. The frequency of the transmitting signal is modulated in time by a modulation signal generated by the DSP processor  114 . The DSP processor provides the modulation signal to the radio frequency transmitter  112 , whereby the radio frequency transmitter  112  preferably creates a frequency modulated continuous wave (FMCW) signal. As known in the art, the distance (z axis) between the interrogator  100  and the RFID tags  200  can be determined using the FMCW signal. In particular, the sweep rate and sweep bandwidth of the transmitter array antenna  102 A is measured and a beat frequency is created in the baseband by converting the signals received at the receiver array antenna  102 B and measuring the content of the beat frequency. 
     As shown in  FIG. 2 , the radio frequency transmitter  112  sends the FMCW signal to the transmit polarization and phase control block  110 , whereby the block  110  conditions the FMCW signal to a desired polarization and/or phase. The polarization and phase block  110  outputs the conditioned polarized and phased signal to the transmitting array antennas  102 A. The signal is then converted into electromagnetic radiation by the transmitting array antennas  102 A. Based on the conditioned signal, the transmitting array antenna  102 A can transmit vertical (V), horizontal (H), right-hand circular (RC) or left-hand circular (LC) polarized electromagnetic radiation along a beam scanning pattern. 
     The transmitting array antennas  102 A have the ability to directionally radiate electromagnetic radiation indicated by one or more patterns  126  in both the radial, translational, horizontal (x) and/or vertical (y) directions. The interrogator  100  preferably utilizes a mathematical focus method used in radar imagery to form a synthetic aperture in conjunction with beam scanning methods to interrogate large physical areas for RFID tags  200 . Beam scanning methods known in the art can be utilized to scan physical areas. In one embodiment, the interrogator  100  incorporates mechanical movement of the antenna(s) to radially scan and/or translate the beam to scan an area. In another embodiment, the interrogator  100  incorporates phased array beam forming and control to scan an area. In one embodiment, the transmit array antenna  102 A continuously transmits the electromagnetic radiation signals simultaneously while the receive array antenna  102 B continuously receives the re-radiated electromagnetic radiation. 
     In another embodiment, the transmit array antenna  102 A transmits the electromagnetic radiation in pulses while the receive array antenna  102 B receives reflected electromagnetic radiation between pulses from the transmit array  102 A. The interrogator can utilize an accurate electronic timing clock signal to measure the amount of time it takes for the signal to be transmitted and received back from the RFID tag(s)  200 . The distance to the tag(s)  100  is able to be calculated knowing the propagation speed of the transmitted pulse multiplied by the measured time difference of the transmitted pulse and the received pulse. The calculated distance (z axis) used in conjunction with the spatially scanned data (x, y) can be used to provide three dimensional positioning information for the tags  200 . It should be noted that the above is just an example embodiment, and the present invention can utilize any other appropriate methods to accurately transmit and receive the RF signals. 
     In one embodiment, the operational distance range between the interrogator  100  and various RFID tags  200  can be up to several hundred feet, depending on the environment and nature of any obstacles. In applications in which the electromagnetic radiation is transmitted and received at millimeter wave frequencies, various materials in the scanned area may absorb the RF energy. For example, in applications using the present system, it may be desired to interrogate tags  200  through paper and cardboard packaging materials, whereby the range between the interrogator  100  and the paper/cardboard packaging materials will be decreased to accommodate the attenuation. Considering that the present system  10  can be used in any conceivable RFID application, the present system  10  can be configured to determine the size of the desired beam pattern, the power of the RF transmission, and the types of focusing methods used, depending on the application. 
     The power used by the interrogator  100  depends on the distance range between the interrogator  100  and the RFID tag(s)  200  as well as frequency range(s) in which the system  10  operates as well as other factors. In one embodiment, the interrogator  100  is powered with 10 mW of transmit power and has an antenna gain of 30 dBi for each antenna array  102 A and  102 B, whereby the interrogator  100 , after digital signal processing, develops a signal level margin per RFID tag antenna element of 10 dB. Larger RFID tags  200  will reflect signals over longer distance ranges. This, however, is only one example, and the present interrogator and/or RFID tag(s) are not limited to these values. The power utilized by the present interrogator depends on the specific application and whether the interrogator is a licensed or unlicensed device with reference to spectrum regulations or whether the interrogator is utilized in military applications. The transmit power level can be between a few microwatts to 100 mW. Other power values are contemplated and are in no way limited to the values provided herein. 
     The interrogator  100  preferably receives reflected electromagnetic radiation at the receiving array antenna  102 B independently of the transmit array antenna  102 A. The electromagnetic radiation indicated by pattern  128  received by the interrogator  100  is reflected from at least one RFID tag  200 . As shown in  FIG. 2 , upon the antenna array  102 B receiving the reflected electromagnetic radiation, the signal is converted into a radio frequency signal by the antenna array  102 B. The received radio frequency signal is passed to the receive polarization and phase control block  120 . The polarization and phase control block  120  interfaces with the receive antenna array  102 B to create the desired polarization and phase response of the received signal and outputs the signal to the radio frequency receiver  118 , as shown in  FIG. 2 . The radio frequency receiver  118  converts the received signal to baseband signals, whereby the baseband signals are provided to the DSP processor  114 . The DSP processor  114  then preferably processes the received baseband signals using radar image signal processing algorithms to analyze the baseband signals. Such algorithms are derived from, but not limited to, algorithmic calculations utilized in synthetic aperture radar (SAR), inverse synthetic aperture radar (ISAR), interferometry SAR (InSAR), poliametric SAR (POLSAR), poliametric interferometry SAR (POLINSAR) and algorithms used in joint time frequency analysis (JTFA) applications. It should be noted that other radar signal imaging data processing methods are contemplated for use in the present system. Dependent on the applications in which the present system  10  is used, applying one or more of these types of image processing methods forms unique images of the scanned area. In one embodiment, an individual antenna on an RFID tag may be represented as a single pixel, whereby an optical view of the RFID tag(s)  200 , along with its representative information, is provided on a display screen to identify and track the particular tag(s)  200 . To better understand how the data is analyzed, a discussion of the antenna elements of the RFID tag  200  will first be discussed. 
       FIGS. 3-4  illustrate different embodiments of the RFID tag  200  of the present invention.  FIGS. 3A and 3B  illustrate one embodiment of the RFID tag  200  in accordance with the present invention. As shown in  FIG. 3A , the tag  200  includes a substrate layer  202  having one or more conductive antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  thereon. For sake of brevity, the antenna elements shown in relation to  FIG. 3A  are generally referred to as having reference numeral  204 , whereas the antenna elements will be described with their individual reference numerals of  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  where needed. It should be noted that the relative sizes, number and positions of the antenna elements  204  are illustrated in  FIG. 3A  as a non-limiting example, and are not limited thereto. It should also be noted that the antenna elements  204  are greatly exaggerated and not illustrated to scale, either individually, relative to each other, or relative to the substrate layer  202 . Although the present description discusses several different tag configurations, any of the tag configurations are applicable even if only one tag reference numeral is discussed. 
     In one embodiment, the substrate layer  202  is disposed on or is integral with an optional conductive ground plane  203 . The ground plane  203  can increase the radiation efficiency as well as allow greater control of the antenna element resonance, amplitude response, phase response and polarization response reflection parameters. However, operation without the use of a ground plane  203  will provide adequate response in many applications, including but not limited to, directly printing the antenna elements on the packaging containers  99  ( FIG. 1E ) or directly embedding the antenna structures into a manufactured product. 
     The antenna elements  204  are shown in  FIG. 3A  as having various length and width dimensions, whereby the width dimensions are preferably measured at the ends of the antenna dipoles. The length dimensions are preferably on the order of ½ wavelength, λ/2. The width dimensions of the antenna elements are preferably on the order of λ/10. As an example, at a frequency of 60 GHz, the dimension of a λ/2 antenna element is approximately 2.50 mm in free space. It is preferred that the antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  are positioned at least λ/2 apart from one another. It is contemplated, however, that the antenna elements can be separated less than λ/2 apart from one another. 
     The substrate layer material  202  provides an effect of decreasing the physical size of the wavelength which is associated with surface conductive elements according to equation (1). 
       λ g =λ/√ε r   (1) 
     As shown in equation 1, λ g  is physical wavelength (guide length), λ is the free space wavelength and ε r  is the relative permittivity or dielectric constant of the substrate layer  202  material. For example, if the material of the substrate layer  202  has a dielectric constant of 2.0, the physical wavelength (λ g ) of a conductive element on the surface at a frequency of 60 GHz would be 3.54 mm, and the λ g /2 element  206  would be 1.77 mm along the length dimension of the antenna element, according to equation ( 1 ). The thickness dimension of the substrate layer  202  is preferably on the order of λ/10 to λ/50, although other dimensions are contemplated. 
     The antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  of the tag  200  respond to incident electromagnetic radiation transmitted from the interrogator  100 . In particular, the antenna elements  204  will resonate and convert the incident electromagnetic radiation into electrical signals if the frequency of the incident electromagnetic radiation corresponds with the wavelength characteristic of that antenna element  204 . Upon being energized, the electrical signal produced by the antenna element  204  will flow through the conductive structure of the antenna element  204  as well as any terminating transmission line or electrical impedance which is coupled thereto. 
     Assuming the antenna element  204  is terminated directly into a low electrical impedance, as compared to the termination impedance of the antenna element itself, the antenna element  204  will immediately convert the electrical signal into electromagnetic radiation which is then re-radiated, also referred to as reflected, from the antenna element  204  to be received by the interrogator  100 . However, the reflected radiation has parameters and characteristics such as amplitude, phase and polarization which are used by the interrogator  100  to identify each particular antenna element  204 , the aggregate of which allows the interrogator  100  to identify the RFID tag  200  as well as any information related to the tag  200 . These parameters are controlled by the physical characteristics of the antenna element  204  that produces the reflection. 
     As shown in the embodiment in  FIG. 3A , the antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  have various dimensional lengths and rotational orientations. The dimensional lengths and orientations of the antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  control their particular phase and polarization responses, respectively, as well as allow the antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  to provide the information needed by the interrogator  100  to identify the antenna elements. Antenna element  204  in the upper left corner of the tag  200  in  FIG. 3A  preferably serves as a reference antenna element, whereby the orientations and thus polarization characteristics of the antenna elements  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  are determined relative to the reference antenna element  204 . This allows the overall rotational orientation of the tag  200  (and thus the object to which the tag  200  is affixed) to be irrelevant in reading the antenna elements. In other words, the interrogator  100  will be able to effectively identify a tag  200  by virtue of its antenna elements  204 ,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  218  irrespective of whether the tag  200  is right-side-up or upside down. It should be noted that the reference antenna element  204  can be located anywhere on the tag  200 , or object to which the tag  200  is affixed to, and is not limited to the upper left corner. It should be noted that there can be more than one reference antenna element per tag  200  without departing from the scope of the present invention. 
     As shown in  FIG. 3A , the antenna element  204  is shown having an orientation of 0° and being vertically polarized (V). The adjacent antenna element  206  in  FIG. 3A  is shown having an orientation of 90° and being horizontally polarized (H). The antenna elements  208  and  210  are shown at 45° and 135° (θ) and are polarized at their respective angles. Similarly, antenna elements  212  and  214  are shown to be vertically polarized (V), whereas antenna element  216  is shown to be horizontally polarized (H). Antenna element  218  is shown to be polarized at 45°. It should be noted that the particular rotational orientations shown and described are for example purposes and should not be limited to only those angles described. It is apparent that any angle, and thus polarization characteristic, is contemplated within the scope of the present invention. 
     For example purposes only, the antenna structures  204 ,  206 ,  208  and  210  have the same length dimension and thus re-radiate the RF signal to be in at the same phase. The antenna elements  212 ,  214 ,  216  and  218  in  FIG. 3A  are shown to have different length dimensions. Although antenna elements  212  and  214  are vertically polarized (0°) like the antenna element  204 , elements  212  and  214  have different length dimensions from each other as well as element  206  such that elements  212  and  214  have different phase response characteristics. Antenna elements  212  and  214  thus respond and reflect different phases of electromagnetic radiation from one another as well as antenna element  204 . Antenna element  216  is horizontally polarized at 90°, like antenna element  206 , however, antenna element  216  will respond and reflect a different phase than element  206  due to the difference in the length dimension. The same theory applies to antenna element  218 . 
       FIG. 3C  illustrates another embodiment of the RFID tag  300  according to the present invention. As shown in  FIG. 3C , the antenna elements  304 ,  306 ,  308 ,  310 ,  312 ,  314 ,  316 ,  318  are shown to be a strip of rectangular or square conductive material. The antenna elements are configured to be on the order of λ g /2 along both, the length and width dimensions, whereby the width dimension is such that the rectangular antenna elements respond to the resonant frequency. The antenna elements are also preferably positioned at least one λ/2 apart from one another. As with the RFID tag  200  in  FIG. 3A , the physical dimensions and orientations of the antenna elements vary in order to control individual antenna element phase and polarization reflective parameters. 
       FIG. 4A  illustrates another embodiment of an RFID tag  400  according to the present invention. As shown in  FIG. 4A , the antenna elements  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418  have the same length and width dimensions. In another embodiment, one or more of the antenna elements  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418  have length and width dimensions different from one another. Although eight antenna elements  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418  are shown on the substrate  402  in  FIG. 4A , it should be noted that any number of antenna elements, including only one, is contemplated. For sake of brevity, the antenna elements shown in relation to  FIG. 4A  are generally referred to as having reference numeral  404 , whereas the antenna elements will be described with their individual reference numerals of  404 ,  406 ,  408 ,  410 ,  412 ,  414 ,  416 ,  418  when needed. 
     Each antenna element shown in  FIG. 4A  includes a transmission line element extending therefrom. In particular, antenna element  404  includes transmission line element  420  whereas antenna element  406  includes transmission line element  422 , and so on. As shown in  FIG. 4A , the transmission line elements  420 ,  422 ,  424 ,  426 ,  428 ,  430 ,  432  and  434  vary in length and position with respect to one another, whereby the length and position of the transmission line elements control the phase and polarization parameters at which the antenna elements respond to the incident electromagnetic radiation. The transmission line element for each antenna element  404  in  FIG. 4A  includes a first transmission line  420 A and a second transmission line  420 B, whereby the first transmission line  420 A extends out perpendicularly from the antenna element  404 . The second transmission line  420 B extends from the first transmission line  420 A at a right angle and is parallel to the side of the antenna element  404 . 
     The position and overall length of the transmission line element  420  controls the antenna element&#39;s polarization and phase response when receiving the incident electromagnetic radiation. In  FIG. 4A , the orientation of the transmission line element  420  is 0° and is referenced as being vertically polarized (V). The orientation of the transmission line element  422  is 90° and is referenced as being horizontally polarized (H). For antenna elements  408  and  410 , the orientation of the transmission line elements  424  and  426  are 135° and 45°, respectively. 
     As stated, the overall length of the transmission line element  420  also controls the response of the phase parameter when receiving the incident electromagnetic radiation. As shown in  FIG. 4A , the transmission line element  420  has a greater length dimension than the transmission line element  428 , but has a smaller length dimension than the transmission line element  430 . The difference in the overall length dimension causes the antenna elements  404 ,  412  and  414  to reflect the incident electromagnetic radiation at a different phase. The other antenna elements  406 ,  408 ,  410 ,  416  and  418  have various length dimensions and orientations of the transmission line elements that affect the phase and polarization reflective parameters respectively. 
       FIG. 4B  illustrates another embodiment of the RFID tag  500  which preferably includes antenna elements  506 ,  508 ,  510 ,  512 ,  514 ,  516 ,  518  and  520 . As shown in  FIG. 4B , the antenna elements each include one or more transmission line elements whereby the transmission line elements vary in length, position and number with respect to one another. Antenna elements  508 ,  510 ,  512 ,  516 ,  518  and  520  are similar to the antenna elements discussed in  FIG. 4A  and will not be discussed again. However, antenna elements  504  and  512  each include two sets of transmission line elements  520  and  522 , respectively. 
     The antenna elements  504  and  512  shown in  FIG. 4B  are circularly polarized, whereas the remaining shown antenna elements are linearly polarized. For example, in antenna element  504 , the transmission line element  520  is oriented and shown to be greater in the length dimension than the transmission line element  522 . The orientation and greater length dimension preferably causes the transmission line element  520  to have a phase delay that is π/2 radians greater than the phase delay of the other transmission line element  522 . This difference in phase delay generates a quadrature phase condition on the adjacent sides of the antenna element  504  which creates a circular polarization parameter (e.g. LC, RC). In particular, the direction of the circular polarization (either right-hand or left-hand) is determined by which side of the square patch antenna is either leading or lagging in quadrature (π/2 radians). For example, the transmission line element  524  of the antenna element  512  is shown in  FIG. 4B  to have a greater length dimension that the transmission line element  526 . This difference in the length dimension between the transmission line elements  524  and  526  creates a quadrature phase condition on the adjacent sides of the antenna element  512  that is opposite in the circular polarization direction as compared to that of the antenna element  504 . These double tuned antenna elements  504 ,  512  allow the elements  504 ,  512  to provide additional information in the form of circular polarization of the electromagnetic radiation, as discussed below. 
     For one or more of the above discussed antenna structures, the structures are preferably made of conductive ink which can be printed on the substrate or other surface. The antenna structures, and thus the RFID tags, can be produced very inexpensively from a laser, ink jet or commercial printer. The antenna structures can also be printed using other conventional methods of making RFID tags. In another embodiment, the antenna structures can be etched, deposited or applied using any other appropriate method. 
       FIG. 5  illustrates a flow diagram of one example method in which the DSP processor  114  of the interrogator  100  processes and analyzes the received signal to decode and identify and retrieve information from the RFID tag  200 . As discussed above in relation to  FIG. 2 , the transmit polarization and phase control block  110 , in conjunction with transmitting array antenna  102 A and DSP processor  114 , create and transmit incident electromagnetic radiation that has polarization parameters sequenced in time. The polarization of the electromagnetic radiation can be vertical V, horizontal H, right-handed circular RC or left-handed circular LC. Once the incident electromagnetic radiation resonates the antenna structures of the RFID tag(s) ( FIGS. 3-4 ), the tag  200  reflects the radiation which is then received at the receiving array antenna  102 B of the interrogator  100 . The receive polarization and phase control block  120  ( FIG. 2 ), in conjunction and the DSP processor  114 , creates predetermined antenna configurations which are more sensitive to certain reflected electromagnetic radiations. The predetermined polarization configurations are sequenced and stored such that interrogator  100  is able to compare the polarization diversity of the received reflected signals to that of the stored configurations to aid in identifying and decoding the tag  200 . 
     Ordinary backscatter reflection from objects other than RFID tags will be received by the interrogator  100  and will have random polarization and phase compared to the antenna elements of on the tag(s). Thus, antennas with known phase and polarization parameters are printed into the coded pattern at known relative locations to establish an a priori reference return signal. This technique provides an effective method to lock onto the tag  200  and create a phase and polarization decoding reference for the interrogator. 
     By controlling how the polarized radiation is transmitted and received, the polarization configuration associated with the transmitted and received radiation controls how the sampled received signal information is stored in a time-indexed manner. In one embodiment, such received signal information is sequenced in time and indexed in memory. As shown in  FIG. 5 , the baseband signal is received from the radio frequency receiver block  118  ( FIG. 2 ). In one embodiment, the baseband signal is received in analog form, whereby the analog signal is converted into digital form (step  602 ). The converted digital signal is preferably made up of time sequences of transmit-receive polarization combinations of V t -V r , H t -H r , H t -V r , LC t -LC r , RC t -RC r , LC t -RC r , V t -LC r , V t -RC r , H t -LC r  and H t -RC r  at given times. For example, the polarization combination V t -LC r  represents data received when vertically polarized radiation was transmitted and left-circular polarized radiation was received for a particular time. Each polarization combination set of time-indexed polarization samples is preferably stored in one of separate memory locations known as polarization memory image panes, shown as  604 A- 604 J. It should be noted that the present invention is not limited to the polarization combinations shown and described herein and any number of individual or combination of polarization radiation that is transmitted and received is contemplated. 
     Radar image signal processing is applied to each memory pane in step  606 , preferably by performing Fourier Transform algorithms on the received data. Fourier transformed data is symbolized by two data locations in memory for each sample, whereby magnitude and phase information of each sample is able to be computed from the data. By computing the magnitude and phase information for each sample, the polarization and phase information of each antenna element in any number of RFID tags  200  in the scanned area can be determined. 
     As shown in  FIG. 5 , once the data is processed using Fourier Transform algorithms, synthetic aperture radar calculations are performed on the data to calculate compressed elevation data (y) samples  608 . If needed, phase correction calculations are then performed on the elevation data  608 , as shown in  610 . The corrected phase data  612  is then preferably processed again using Fourier Transform algorithms to calculate compressed azimuth data (x) samples  616 . From the elevation  608  and azimuth  616  data samples, a preliminary digital image  618  of the scanned area can be formed, whereby the multiple memory panes of data  620 A- 620 J are then used to decode the tags  200  and/or output a visual image  624  of both the tags  200  and the entire scanned area containing the tags  200 . 
     By creating multiple memory panes of data  620 A- 620 J, the present system is able to correlate and recognize tag pattern data. For example, if the transmitted array antenna  102 A is vertically polarized V and the received array antenna  102 B is also vertically polarized V at a given time, the data samples taken at that period are stored in a unique memory location, indexed by time, designated as V t -V r . If the transmitter is H and the receiver is V, a separate memory pane location is used and is distinguished as H t -V r , etc. Through valid mathematical permutations, the set of memory panes necessary to analyze the different combinations of transmit and receive polarizations comes out to be the set of V t -V r , H t -H r , H t -V r , LC t -LC r , RC t -RC r , LC t -RC r , V t -LC r , V t -RC r , H t -LC r  and H t -RC r . By mathematically correlating between these image panes, recognition of the tag&#39;s polarization and phase can be deduced. Inter memory pane mathematical correlation is then applied to derive image detail that emphasizes polarization content within the image. Unique images are then able to be formed for each set of time-indexed polarization combinations. 
     Accurate SAR imaging of the objects, walls or buildings around the tag(s)  200  can also be useful in processing the location of the tag(s)  200  within the context of the objects represented in scanned area. In one embodiment, the interrogators can be used in conjunction with a GPS system and graphical mapping software to locate and track the position of the RFID tag relative to geographic coordinates. It is contemplated that the RFID tag  200  as well as one or more interrogator&#39;s location can be indicated on a mapped graphical display for user recognition of the target&#39;s location. In one embodiment, this is performed by the system  10  mapping the interrogator&#39;s position by scanning the boundary area, such as the walls of a room. Once the boundary perimeter is established, the location of the interrogator within the perimeter can be determined and the bearings of the tags  200  can be determined as well. It should be noted that the locations of the interrogator  100  as well as the tags  200  can be determined in a stationary setting, such as a room, or while the interrogator and/or the tags  200  are moving. 
     As shown in  FIG. 5 , mathematical focus calculations preferably utilizing radar image signal processing are performed on the recognized individual tag structures  200  to deduce the frequency and phase response of one or more given tag antenna structures  200 . In deducing the polarization, frequency and phase response, the information on the tag  200  can be decoded  622 . For example purposes only,  FIG. 6  illustrates a table showing an example RFID tag antenna structure code decoding scheme. It should be noted that the table shown in  FIG. 6  only takes into account the polarization and phase characteristics of the antenna structures to decode the structures. For example purposes, the table in  FIG. 6  will be explained in relation to the RFID tag discussed in  FIG. 3A . However, it should be noted that the table can be used with any other antenna configuration and is not limited to that shown in  FIG. 3A . 
     As shown in  FIG. 6 , the table provides a four-level polarization code and a four-level phase code. The table is thus set up such that, together, there can be 16 different combinations or states represented from the polarization and phase information received from the RFID tag  200 . In binary data terms, the bit size of the code is defined by log 2  16=4. Therefore, the table in  FIG. 6  provides all possible combinations of a 4-bit code in terms of polarization and phase. 
     As stated above, in one embodiment, the interrogator  100  transmits RF signals as a modulated frequency carrier (e.g. FMCW) that is swept over a large frequency range many times per second. Considering that the tag antenna structures can be tuned to re-radiate different wavelengths of signals by changing the geometry of the structures, the represented polarization and phase combinations in the sample table in  FIG. 6  can be repeated at different, separated frequencies. This, in turn, allows for more bits to be encoded, and thus decoded, for a given antenna structure. The degree of frequency separation will depend on the antenna structure, electrical conductivity, and the substrate dielectric losses which both affect the resonance sharpness (i.e. Q factor). For example, if four frequency “bins” were used to analyze separate 4-bit polarization and phase data in each bin, a total of 4×16 or 64 total combinations would be feasible. Since log 2  64=6, a 6-bit code would thus result for each antenna structure utilizing a 4-bin frequency separation. 
     In one embodiment, the present system can accommodate for higher density codes in applications which desire higher quantities of coding of bits per antenna structure, thereby resulting in more encoded data per tag area. As a non limiting example of the number of data bits that can be coded into a given tag area, one square inch of tag can accommodate approximately 25 individual λ/2 microstrip patch antennas spaced at one wavelength apart (λ), or 5 mm at a frequency of 60 GHz. Using a 6-bit coding schema, the total number of bits encoded into a square inch of area would be 6×25=150 bits. Using the same coding schema at 92 GHz (λ=3.2 mm), approximately 60 individual patch antennas fit within one square inch. Thus, 360 bits (6×60) can be encoded. 
       FIG. 7  illustrates one example method in how the antenna elements can be encoded. As shown in  FIG. 7 , whatever information that is to be coded is initially converted from alphanumeric characters into a data coded format (e.g. ASCII) (step  700 ). The data characters are then preferably converted into appropriate phase and polarization values which can be standard or proprietary (step  702 ). Following, the phase and polarization values for each character is then preferably converted into the appropriate antenna structure (step  704 ). This entire process can be performed by a software program which then sends the information to a printer ( 706 ) to produce the antenna structure configuration. It is contemplated that reference antenna elements will be placed in the printed antenna configuration, as discussed above. In one embodiment, error correction coding methods (e.g. parity coding, turbo coding) are applied to the data which is to be eventually coded into the antenna elements. In one embodiment, the antenna elements are encoded and decoded utilizing encryption/decryption methods for security. Such methods include, but are not limited to, use of hash algorithms, digital signatures such as MD2, MD4, MD5 and/or SHA algorithms. Hash algorithms can also be used in the context of the present invention to improve signal randomness for detection enhancement. 
     While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims.