Abstract:
Due to design constraints and installation variability, RFID interrogator antennas do not always function optimally across the entire channel on which they are intended to operate due to diminished antenna bandwidth. Techniques are described for selecting a sub-band of frequencies within the channel on which a particular RFID interrogator can be operated to enhance operating efficiency. These techniques include a VSWR measurement technique and a read/no read technique are disclosed for identifying a useful sub-band of frequencies. The operation of a reader/interrogator is then limited to an identified sub-band so that an RFID interrogator/tag system can be operated efficiently.

Description:
BACKGROUND 
       [0001]    The invention relates in general to the arrangement and use of radio frequency identification (RFID) tags. In particular, the invention relates to the operation of a reader/interrogator of an RFID tag system. More specifically, the invention address the problem of a reader/interrogator not being fully effective over its entire designated frequency band due to antenna design compromises and installation constraints. It provides for a selection of a sub-band of frequencies within a designated frequency band on which an RFID reader/interrogator will operate. 
         [0002]    Radio frequency identification (RFID) tags are electronic devices that may be affixed to items whose presence is to be detected and/or monitored. RFID tags are classified based on standards defined by national and international standards bodies (e.g., EPCGlobal and ISO). Standard tag classes include Class 0, Class 1, and Class 1 Generation 2 (referred to herein as “Gen 2”). The presence of an RFID tag, and therefore the presence of the item to which the tag is affixed, may be checked and monitored wirelessly by an “RFID reader”, also known as a “reader-interrogator”, “interrogator”, or simply “reader.” Readers typically have one or more antennas for transmitting radio frequency signals to RFID tags and receiving responses from them. An RFID tag within range of a reader-transmitted signal responds with a signal including a unique identifier. 
         [0003]    With the maturation of RFID technology, efficient communication between tags and readers has become a key enabler in supply chain management, especially in manufacturing, shipping, and retail industries, as well as in building security installations, healthcare facilities, libraries, airports, warehouses etc. Many processes, as well as the status of many items, may be readily monitored via RFID tags. 
         [0004]    RFID systems generally operate using a frequency hop technique, thus, they transmit and receive signals on various frequencies within a communications channel in some predetermined or random sequence. In effect, they transmit bursts of frequencies in sequence at various center frequencies. 
         [0005]    Due to design constraints and construction variability, an RFID reader/interrogator and RFID tags do not always operate optimally and efficiently over an entire communication channel on which they are intended to operate. One contributor to this problem is the antenna of the reader/interrogator. In an “ideal” world a reader/interrogator antenna, such as, for example, a dipole antenna, would be constructed so as to be “full length”, i.e. its physical length is made to be ½ wavelength at an intended operating frequency. This operating frequency may be the center of a band of frequencies constituting a communication channel. 
         [0006]    A typical full length antenna has a pass band characteristic that permits it to operate reasonably efficiently over its entire intended communication channel. However, due to size constraints required by particular installations, the antenna of a reader/interrogator can not always be made to be full size. Design constraints may require that the antenna be shorter than ideal in order to fit within a certain size reader/interrogator or to fit the reader/interrogator within a small space allowed by a particular installation. A shorter than ideal antenna must be tuned to the correct center frequency using reactive elements. 
         [0007]    Also, in the “ideal” world, an antenna would be installed in “free space” in such a manner that its characteristics are not affected by the dielectric properties of objects nearby. However, in the real world, particular installations require that the antenna be situated in a manner that its characteristics are indeed affected by nearby objects, such as mounting structures, etc. 
         [0008]    Also, mechanical and electrical tolerances may accumulate during the manufacturing process which may result in an antenna frequency which is biased towards the upper or lower side of the communications channel. 
         [0009]    Due to these and other design compromises, an antenna of a reader/interrogator may perform with a less than ideal characteristic. The antenna may not function optimally across the entire communication channel on which it is intended to operate. 
         [0010]    For example, when a dipole antenna is constructed so that the physical dimension of its radiating element is less than ½ wavelength, it must be loaded with reactive elements in order to cause it to resonate near a center of an intended communication channel. The use of such reactive loading causes a normal antenna pass band characteristic to become more sharp, i.e. the roll off from its center frequency is more steep, and the operating bandwidth narrows more than it does for a full length antenna. Given this sharper roll off characteristic and narrower operating bandwidth, an interrogator antenna may have insufficient gain at certain frequencies to allow for reliable reception of signals and efficient response to signals. The incidence of “no read” responses from RFID tags interrogated may be too high to allow for efficient operation of the interrogator. 
         [0011]    In addition to antenna design constraints described above, there may be other design compromises and normal construction tolerances that contribute to an RFID reader/interrogator and tag system not performing optimally over its entire intended operating channel. 
         [0012]    What is needed, then, is an RFID reader/interrogator that can adapt its operation to compensate for an antenna that does not operate efficiently over an entire frequency band on which it is intended to operate. 
       SUMMARY OF THE INVENTION 
       [0013]    This section is for the purpose of summarizing some aspects of the inventions described more fully in other sections of this patent document. It briefly introduces some preferred embodiments. Simplifications or omissions may be made to avoid obscuring the purpose of the section. Such simplifications or omissions are not intended to limit the scope of the claimed inventions. 
         [0014]    To a degree, the frequencies on which an interrogator/RFID tag system is to operate can be selected in advance. A particular RFID interrogator can be programmed to use the selected frequencies. Thus, if the actual center frequency and bandwidth of an interrogator antenna can become known before the interrogator is installed and put into service, it can be programmed to frequency hop within a sub-band of “good” frequencies within an intended communication channel consistent with the actual characteristics of its antenna. 
         [0015]    The invention relates to an RFID interrogator which can be adapted to operate only within a sub-band of frequencies within an operating channel when it is not possible for it to operate efficiently over an entire channel on which they are intended to operate, such as, for example, because of antenna design constraints, installation constraints, or if another device is operating continuously within some portion of the channel and needs to be avoided. etc. 
         [0016]    To adapt, an interrogator, can analyze interrogation results from the entire channel to identify a sub-band of frequencies within a communication channel on which it operates most efficiently and then limits its operation to only a sub-band of frequencies at which efficient operation can be carried out. The reader/interrogator is programmed such that it operates on such an identified sub-band of frequencies rather than using all frequencies within the communication channel. This allows for its more efficient use in an actual installation by allowing a higher percentage of “reads” of RFID tags responsive to its transmitted interrogation signals. 
         [0017]    The invention described in this patent document relates in general to selecting an optimal sub-band of frequencies to which operation of an RFID interrogator should be limited within a designated communication channel. This limitation of frequencies can be accomplished by limiting the operation of a reader/interrogator to transmit interrogation signals only within the identified sub-band of frequencies. The use of a particular sub-band of frequencies allows the interrogator to operate at high efficiency. 
         [0018]    Techniques are described herein for optimizing the operation an already constructed RFID tag interrogator by selecting a sub-band of frequencies within an intended communication channel on which it can be operated efficiently. 
         [0019]    Normally, a reader/interrogator transmits interrogation signals. These signals are transmitted according to a frequency hopping scheme. After an RFID interrogator has been determined to operate efficiently within a sub-band of a communication channel, its operation is limited to transmitting interrogation signals only within an identified sub-band. 
         [0020]    One technique for identifying a sub-band of frequencies for a particular RFID interrogator measures the Voltage Standing Wave Ratio (VSWR) of its antenna across its entire intended communication channel. A sub-band of optimal frequencies is identified by determining a sub-band having acceptable VSWR measurements. 
         [0021]    A second technique for identifying a sub-band of frequencies for a particular RFID interrogator measures and tabulates responses to interrogation signals on various frequencies within the intended communication channel. A tabulation of “read” and “no read” responses indicates what frequencies are effective for interrogating the tag. A sub-band of optimal frequencies is determined based on a count of “reads” and “no read” responses. 
         [0022]    Using either technique, the information characterizing an already manufactured RFID interrogtor can be used to select a sub-band of frequencies on which the interrogator will interrogate RFID tags in actual use. By limiting interrogation to those frequencies that are effective, the interrogator can be operated with greater efficiency than it could be if the entire spectrum of the communication channel were used. Such optimization of the interrogator results in an RFID system that operates with enhanced efficiency. 
         [0023]    The invention can be implemented in numerous ways, including methods, systems, devices, and computer readable medium. Several embodiments of the invention are described below, but they are not the only ways to practice the invention described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0024]    The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
           [0025]    In the drawings, like reference numbers indicate identical or functionally similar elements. 
           [0026]    Additionally, references numbers which are the same, but vary by virtue of an appended letter of the alphabet (for example,  412 ,  412 R,  412 P,  412 S) or an appended letter and number (for example,  412 ,  412 S 1 ,  412 S 2 ) indicate elements which may be substantially the same or similar, but represent variations or modifications of the basic element. In some cases, the reference number without the appended letter or without the appended letter and number (for example,  412 ) may indicate a generic form of the element, while reference numbers with an appended letter or an appended letter and number (for example,  412 S,  412 S 1 ,  412 S 2 ,  412 P) may indicate a more particular or modified form of the element. 
           [0027]    Additionally, the leftmost digit(s) of a reference number identifies the drawing in which the reference number first appears. For example, an element labeled  412  typically indicates that the element first appeared in  FIG. 4 . 
           [0028]      FIG. 1  shows an environment where RFID readers (interrogators) communicate with an exemplary population of RFID tags. 
           [0029]      FIG. 2  is a block diagram of receiver and transmitter portions of an RFID reader. 
           [0030]      FIG. 3  is a block diagram of an exemplary radio frequency identification (RFID) tag. 
           [0031]      FIG. 4  is a schematic diagram of an RFID interrogator  400  having a classic design dipole antenna  402  and a frequency response associated therewith. 
           [0032]      FIG. 5  is a schematic diagram of a full size dipole antenna and its associated frequency response. 
           [0033]      FIG. 6  is a schematic diagrams of a reactive loaded dipole antenna and its associated frequency response. 
           [0034]      FIG. 7  shows a frequency response illustrating the “VSWR” technique according to the invention. 
           [0035]      FIG. 8  is a flowchart illustrating the VSWR technique for identifying an appropriate sub-band of frequencies for operation by an RFID interrogator according to the invention. 
           [0036]      FIG. 9  is a flowchart illustrating the “read/no-read” technique for identifying an appropriate sub-band of frequencies for operation by an RFID interrogator according to the invention. 
           [0037]      FIG. 10  is a graphical representation indicating how the read/no read technique is used to identify a sub-band of frequencies in which the RFID interrogator will be operated. 
           [0038]      FIG. 11  is a schematic diagram explaining how to operate an RFID tag system based on the principles of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention, including structures, systems, and methods, may be practiced without these specific details. The description and representation herein are the common means used by those experienced or skilled in the art to most effectively convey the substance of their work to others skilled in the art. In other instances, well-known methods, procedures, components, and circuitry have not been described in detail to avoid unnecessarily obscuring aspects of the invention. 
         [0040]    References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
         [0041]    The terms “reader” and “interrogator” are used interchangeably. They both refer to the device used to send an interrogation signal to an RFID tag and read any signal transmitted from or backscattered from an RFID tag. 
         [0042]    Exemplary Operating Environment 
         [0043]    Before describing embodiments of the invention in detail, it is helpful to describe an example RFID communications environment in which the inventions may be implemented.  FIG. 1  illustrates an environment  100  where RFID tag readers  104  (readers  104   a  and  104   b  shown in  FIG. 1 ) communicate with an exemplary population  120  of RFID tags  102 . As shown in  FIG. 1 , the population  120  of tags includes seven tags  102   a - 102   g . A population  120  may include any number of tags  102 . 
         [0044]    Environment  100  includes any number of one or more readers  104 . For example, environment  100  includes a first reader  104   a  and a second reader  104   b . Readers  104   a  and/or  104   b  may be requested by an external application to address the population of tags  120 . Alternatively, reader  104   a  and/or reader  104   b  may have internal logic that initiates communication, or may have a trigger mechanism that an operator of a reader  104  uses to initiate communication. Readers  104   a  and  104   b  may also communicate with each other in a reader network (see  FIG. 2 ). 
         [0045]    As shown in  FIG. 1 , reader  104   a  “reads” tags  120  by transmitting an interrogation signal  110   a  to the population of tags  120 . Interrogation signals may have signal carrier frequencies or may comprise a plurality of signals transmitted in a frequency hopping arrangement. Readers  104   a  and  104   b  typically operate in one or more of the frequency bands allotted for this type of RF communication. For example, the Federal Communication Commission (FCC) defined frequency bands of 902-928 MHz and 2400-2483.5 MHz for certain RFID applications. 
         [0046]    Tag population  120  may include tags  102  of various types, such as, for example, various classes of tags as enumerated above. Thus, in response to interrogation signals, the various tags  102  may transmit one or more response signals  112  to an interrogating reader  104 . Some of the tags, for example, may respond by alternatively reflecting and absorbing portions of signal  104  according to a time-based pattern or frequency. This technique for alternatively absorbing and reflecting signal  104  is referred to herein as backscatter modulation. Typically, such backscatter modulation may include one or more alpha-numeric characters that uniquely identify a particular tag. Readers  104   a  and  104   b  receive and obtain data from response signals  112 , such as an identification number of the responding tag  102 . In the embodiments described herein, a reader may be capable of communicating with tags  102  according to various suitable communication protocols, including Class 0, Class 1, EPC Gen 2, other binary traversal protocols and slotted aloha protocols, and any other protocols mentioned elsewhere herein, and future communication protocols. Additionally, tag population  120  may include one or more tags having the packed object format described herein and/or one or more tags not using the packed object format (e.g., standard ISO tags). 
         [0047]      FIG. 2  shows a block diagram of an example RFID reader  104 . Reader  104  includes one or more antennas  202 , a receiver and transmitter portion  220  (also referred to as transceiver  220 ), a baseband processor  212 , and a network interface  216 . These components of reader  104  may include software, hardware, and/or firmware, or any combination thereof, for performing their functions. 
         [0048]    Baseband processor  212  and network interface  216  are optionally present in reader  104 . Baseband processor  212  may be present in reader  104 , or may be located remote from reader  104 . For example, in an embodiment, network interface  216  may be present in reader  104 , to communicate between transceiver portion  220  and a remote server that includes baseband processor  212 . When baseband processor  212  is present in reader  104 , network interface  216  may be optionally present to communicate between baseband processor  212  and a remote server. In another embodiment, network interface  216  is not present in reader  104 . 
         [0049]    In an embodiment, reader  104  includes network interface  216  to interface reader  104  with a communications network  218 . As shown in  FIG. 2 , baseband processor  212  and network interface  216  communicate with each other via a communication link  222 . Network interface  216  is used to provide an interrogation request  210  to transceiver portion  220  (optionally through baseband processor  212 ), which may be received from a remote server coupled to communications network  218 . Baseband processor  212  optionally processes the data of interrogation request  210  prior to being sent to transceiver portion  220 . Transceiver  220  transmits the interrogation request via antenna  202 . 
         [0050]    Reader  104  has at least one antenna  202  for communicating with tags  102  and/or other readers  104 . Antenna(s)  202  may be any type of reader antenna known to persons skilled in the relevant art(s), including for example and without limitation, a vertical, dipole, monopole, loop, Yagi-Uda, slot, and patch antenna type. 
         [0051]    Transceiver  220  receives a tag response via antenna  202 . Transceiver  220  outputs a decoded data signal  214  generated from the tag response. Network interface  216  is used to transmit decoded data signal  214  received from transceiver portion  220  (optionally through baseband processor  212 ) to a remote server coupled to communications network  218 . Baseband processor  212  optionally processes the data of decoded data signal  214  prior to being sent over communications network  218 . 
         [0052]    In embodiments, network interface  216  enables a wired and/or wireless connection with communications network  218 . For example, network interface  216  may enable a wireless local area network (WLAN) link (including a IEEE 802.11 WLAN standard link), a BLUETOOTH link, and/or other types of wireless communication links. Communications network  218  may be a local area network (LAN), a wide area network (WAN) (e.g., the Internet), and/or a personal area network (PAN). 
         [0053]    In embodiments, a variety of mechanisms may be used to initiate an interrogation request by reader  104 . For example, an interrogation request may be initiated by a remote computer system/server that communicates with reader  104  over communications network  218 . Alternatively, reader  104  may include a finger-trigger mechanism, a keyboard, a graphical user interface (GUI), and/or a voice activated mechanism with which a user of reader  104  may interact to initiate an interrogation by reader  104 . An autonomous mode may be used where the reader interrogates based on a repeating timed duty cycle. 
         [0054]    In the example of  FIG. 2 , transceiver portion  220  includes a RF front-end  204 , a demodulator/decoder  206 , and a modulator/encoder  208 . These components of transceiver  220  may include software, hardware, and/or firmware, or any combination thereof, for performing their functions. Example description of these components is provided as follows. 
         [0055]    Modulator/encoder  208  receives interrogation request  210 , and is coupled to an input of RF front-end  204 . Modulator/encoder  208  encodes interrogation request  210  into a signal format, such as, for example, one of pulse-interval encoding (PIE), FMO, or Miller encoding formats, modulates the encoded signal, and outputs the modulated encoded interrogation signal to RF front-end  204 . 
         [0056]    RF front-end  204  may include one or more antenna matching elements, amplifiers, filters, an echo-cancellation unit, a down-converter, and/or an up-converter. RF front-end  204  receives a modulated encoded interrogation signal from modulator/encoder  208 , up-converts (if necessary) the interrogation signal, and transmits the interrogation signal to antenna  202  to be radiated. Furthermore, RF front-end  204  receives a tag response signal through antenna  202  and down-converts (if necessary) the response signal to a frequency range amenable to further signal processing. 
         [0057]    Demodulator/decoder  206  is coupled to an output of RF front-end  204 , receiving a modulated tag response signal from RF front-end  204 . In an EPC Gen 2 protocol environment, for example, the received modulated tag response signal may have been modulated according to amplitude shift keying (ASK) or phase shift keying (PSK) modulation techniques. Demodulator/decoder  206  demodulates the tag response signal. For example, the tag response signal may include backscattered data formatted according to FMO or Miller encoding formats in an EPC Gen 2 embodiment. Demodulator/decoder  206  outputs decoded data signal  214 . 
         [0058]    The configuration of transceiver  220  shown in  FIG. 2  is provided for purposes of illustration, and is not intended to be limiting. Transceiver  220  may be configured in numerous ways to modulate, transmit, receive, and demodulate RFID communication signals, as would be known to persons skilled in the relevant art(s). 
         [0059]    The invention described herein is applicable to any type of RFID tag, with suitable additional features, as described in further detail below in conjunction with  FIG. 4  and beyond.  FIG. 3  is a schematic block diagram of an example radio frequency identification (RFID) tag  102  as already known to those practiced in the art. Tag  102  includes a substrate  302 , an antenna  304 , and an integrated circuit (IC)  306 . Antenna  304  is formed on a surface of substrate  302 . Antenna  304  may include any number of one, two, or more separate antennas of any suitable antenna type, including for example dipole, loop, slot, and patch. IC  306  includes one or more integrated circuit chips/dies, and can include other electronic circuitry. IC  306  is attached to substrate  302 , and is coupled to antenna  304 . IC  306  may be attached to substrate  302  in a recessed and/or non-recessed location. 
         [0060]    IC  306  controls operation of tag  102 , and transmits signals to, and receives signals from RFID readers using antenna  304 . In the example of  FIG. 3 , IC  306  includes a memory  308 , a control logic  310 , a charge pump  312 , a demodulator  314 , and a modulator  316 . Inputs of charge pump  312 , and demodulator  314 , and an output of modulator  316  are coupled to antenna  304  by antenna signal  328 . 
         [0061]    Demodulator  314  demodulates a radio frequency communication signal (e.g., interrogation signal  110 ) on antenna signal  328  received from a reader by antenna  304 . Control logic  310  receives demodulated data of the radio frequency communication signal from demodulator  314  on an input signal  322 . Control logic  310  controls the operation of RFID tag  102 , based on internal logic, the information received from demodulator  314 , and the contents of memory  308 . For example, control logic  310  accesses memory  308  via a bus  320  to determine whether tag  102  is to transmit a logical “1” or a logical “0” (of identification number  318 ) in response to a reader interrogation. Control logic  310  outputs data to be transmitted to a reader (e.g., response signal  112 ) onto an output signal  324 . Control logic  310  may include software, firmware, and/or hardware, or any combination thereof. For example, control logic  310  may include digital circuitry, such as logic gates, and may be configured as a state machine in an embodiment. 
         [0062]    Modulator  316  is coupled to antenna  304  by antenna signal  328 , and receives output signal  324  from control logic  310 . Modulator  316  modulates data of output signal  324  (e.g., one or more bits of identification number  318 ) onto a radio frequency signal (e.g., a carrier signal transmitted by reader  104 ) received via antenna  304 . The modulated radio frequency signal is response signal  112  (see  FIG. 1 ), which is received by reader  104 . In one example embodiment, modulator  316  includes a switch, such as a single pole, single throw (SPST) switch. The switch is configured in such a manner as to change the return loss of antenna  304 . The return loss may be changed in any of a variety of ways. For example, the RF voltage at antenna  304  when the switch is in an “on” state may be set lower than the RF voltage at antenna  304  when the switch is in an “off” state by a predetermined percentage (e.g., 30 percent). This may be accomplished by any of a variety of methods known to persons skilled in the relevant art(s). 
         [0063]    Charge pump  312  (or other type of power generation module) is coupled to antenna  304  by antenna signal  328 . Charge pump  312  receives a radio frequency communication signal (e.g., a carrier signal transmitted by reader  104 ) from antenna  304 , and generates a direct current (DC) voltage level that is output on tag power signal  326 . Tag power signal  326  powers circuits of IC die  306 , including control logic  320 . 
         [0064]    Charge pump  312  rectifies a portion of the power of the radio frequency communication signal of antenna signal  328  to create a voltage power. Charge pump  312  increases the voltage level of the rectified power to a level sufficient to power circuits of IC die  306 . Charge pump  312  may also include a regulator to stabilize the voltage of tag power signal  326 . Charge pump  312  may be configured in any suitable way known to persons skilled in the relevant art(s). For description of an example charge pump applicable to tag  102 , refer to U.S. Pat. No. 6,734,797, titled “Identification tag Utilizing Charge Pumps for Voltage Supply Generation and Data Recovery,” which is incorporated by reference herein in its entirety. Alternative circuits for generating power in a tag, as would be known to persons skilled in the relevant art(s), may be present. Further description of charge pump  312  is provided below. 
         [0065]    It will be recognized by persons skilled in the relevant art(s) that tag  102  may include any number of modulators, demodulators, charge pumps, and antennas. Tag  102  may additionally include further elements, including an impedance matching network and/or other circuitry. Furthermore, although tag  102  is shown in  FIG. 3  as a passive tag, tag  102  may alternatively be an active tag (e.g., powered by a battery, not shown). 
         [0066]    Memory  308  is typically a non-volatile memory, but can alternatively be a volatile memory, such as a DRAM. Memory  308  stores data, including an identification number  318 . In a Gen-2 tag, tag memory  308  may be logically separated into four memory banks. 
         [0067]    Overview of Sub-Band Identification for RFID Interrogator 
         [0068]    The following sections of this specification, along with  FIGS. 4 through 11 , describe exemplary embodiments that incorporate the features of the inventions. The embodiment(s) described, and references in the specification to “exemplary embodiment”, “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described may include a particular procedure, operation, step, feature, structure, or characteristic, but every embodiment may not necessarily include the particular procedure, operation, step, feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular procedure, operation, step, feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such procedure, operation, step, feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
         [0069]    While specific methods and configurations are described, it should be understood that this is done for illustration purposes only. A person skilled in the art will recognize that other configurations and procedures may be used without departing from the spirit and scope of the invention. 
         [0070]    In particular, RFID reader and system embodiments are described wherein within a particular frequency band channel of operation, a sub-band of frequencies is selected on which the RFID reader can optimally operate. 
         [0071]      FIG. 4  schematically shows an RFID interrogator  400  having an integrated dipole antenna  402 . Dipole antenna  402  is a classic dipole design having a physical length equal to one-half wavelength at the center frequency of a channel defined by a band of frequencies in which RFID interrogator  400  is intended to operate. The graph in  FIG. 4  demonstrates characteristics of antenna  402  in a plot  406  indicating amplitude of radiated power of signals emitted by RFID interrogator  400  via antenna  402 . As shown in plot  406 , there is a frequency  408  at which antenna  402  is perfectly resonant. For frequencies greater than the resonant frequency  408  and for frequencies less than the resonant frequency  408 , the amplitude of radiated power is less than it is at the resonant frequency  408 . For purposes of discussion the full channel is divided into parts designated R 01 , R 02 , R 03 , R 04 , R 05 , and R 06 . 
         [0072]      FIGS. 5 and 6  demonstrate how the characteristic of an antenna changes when the antenna is shortened from its ideal ½ wavelength physical length.  FIG. 5  schematically shows an antenna characteristic  502  corresponding to a dipole antenna  504  that has a physical length  506  equal to ½ wavelength. In order to integrate an antenna into a smaller space than can accommodate a ½ wavelength antenna, the antenna can be made to have a physical length  508  that is less than ½ wavelength at the center of its intended operating channel, as shown in  FIG. 6 . In order to maintain a resonant frequency  510  at the center of its channel, a short antenna  512  must be loaded with inductive elements such as inductors  514  and  516  and capacitive elements such as capacitors  518  and  520 . However, as shown in plot  510 , the response curve of antenna  512  becomes much narrower than the response curve  502  of a full size antenna such as antenna  504 . Because of this narrower response curve, and because of other tolerances in building an RFID reader antenna, an RFID reader may not operate as desired over a full range of frequencies for a communication channel on which it is intended to operate. 
       VSWR Technique for Sub-Band Identification 
       [0073]      FIG. 7  is a frequency response illustrating the “reflected power” concept of the invention. One solution to the problem of having a less than ideal antenna response is to program an RFID interrogator to operate only on frequencies that are within a portion of an antenna response curve that permits a sufficient signal to be transmitted and received. RFID interrogators generally operate in a frequency hopping mode. Rather than use all frequencies available within a particular communication channel, the interrogator can be operated on only those frequencies that allow for good transmission of interrogation signals and reception of backscatter signals based on an actual response curve of an actual antenna integrated into the RFID interrogator. 
         [0074]    There are various ways to identify a sub-band of frequencies of a communication channel on which a particular interrogator should be operated in order to properly receive and transmit signals. One such technique is the measure the VSWR of the interrogator antenna over its entire intended communication channel frequency range and to then limit frequencies of actual use to those falling within a “sweet spot” of low VSWR. In  FIG. 7 , a full response curve  702  is shown with a sweet spot range from a first frequency  704  to a second frequency  706 . The range of frequencies from frequency  704  to frequency  706  defines a sub-band  708  constituting a sweet spot for an antenna installed in a particular RFID interrogator. 
         [0075]      FIG. 8  is a flowchart showing the process of identifying an appropriate sub-band of frequencies for operation by an RFID interrogator being matched to an integrated antenna using VSWR techniques according to the invention. Beginning at step  804 , a predetermined pseudo random frequency hopping sequence is begun. At step  806 , the first of the identified frequencies is used to test the antenna. A standard RFID interrogation is performed while the antenna and it&#39;s reflected power is measured at step  808 . The level of reflected power is recorded at step  810 . At step  812 , it is determined whether there are additional frequencies at which measurements are to take place. If there are additional frequencies, Then in step  811  the next frequency in the hop sequence is selected and control returns to step  806 . The process at step  806 ,  808 ,  810  and  811  continues until all frequencies within the gross frequency band have been tested and reflected power recorded. Once all frequencies have been tested, and there are no other frequencies to test, control passes to step  814  whereat a sub-band of frequencies is identified. Once the sub-band of frequencies has been identified, the RFID interrogator can be programmed at  816  to only used the identified frequencies for actual operation. Control ends at step  818 . Once a sub-band of frequencies has been identified, an interrogator can be programmed to send interrogation signals only on those frequencies within the identified sub-band. 
         [0076]    “Read/No Read” Technique for Sub-Band Identification 
         [0077]      FIG. 9  is a flowchart showing an alternative process of identifying an appropriate sub-band of frequencies for operation by an RFID tag being matched to an integrated antenna using a “read or no read” technique rather than measuring antenna VSWR. Once an RFID interrogator has been built the installed antenna&#39;s response is tested by transmitting actual interrogating signals throughout the entire channel on which it is intended to operate. Actual “reads” are measured to determine the response of test RFID tags. This technique is advantageous in that no VSWR measurements need to be made. The process is begun at step  902 . At step  904  a database of all possible channel numbers is initialized to zero. At step  906  the system awaits a trigger from any controlling process or user. When a trigger is received, the system advances to step  908  where the system selects the next frequency to operate on from a predetermined list of pseudorandomly generated channel numbers. The system then advances to step  910  where the actual RFID reads occur. The system will repeatedly loop through steps  910  and  912  until all RFID tags within range are interrogated. Once it is determined that no more unread tags remain in the interrogation space, the system advances to step  914  where a test is made to determine if the currently selected channel has been tested N times. N is an integer which represents the number of times each frequency must be tested before a channel efficiency comparison can be made accurately. The higher the number N, the greater the integration factor of the test, and the more that factors that are external to the system are averaged out of the measurement. This needs to be done to remove such factors as RF multipath, interference from other interrogators or other RF devices, tag distribution variances, and environmental variables are also minimize in the measurement. If it is determined that N has been satisfied for the current channel, the system will return to step  906  to repeat the above sequence, otherwise the system advances to step  916 . In step  916  any tag reads from the latest round of interrogations are aggregated with any reads from prior rounds of interrogations that have been stored in the current channel database. The results are used to overwrite the prior values in the current channel memory location. The current N value for the current channel is also incremented in the database. The system then advances to step  918 . In step  918 , a test is done to determine if all N values for all channals have reached their terminal values. If not, the system returns to step  906  to await a new trigger command, otherwise the system continues to step  920 . In step  920  a histogram is made from the database of channel reads to determine the optimal sub band for the interrogator to operate on. The system then continues to step  922  where the interrogator is programmed to operate only on the optimal sub band determined in step  920 . The process then terminates and returns to normal RFID operation using the new optimal sub band of channels. 
         [0078]      FIG. 10  is a graphical representation explaining step  920  in  FIG. 9 . The plot indicates how the cumulated read/no read results are used to help identify a sub-band of frequencies in which the RFID tag will be operated. As shown in  FIG. 10 , frequencies were selected for tests within a gross band of frequencies. A threshold can be established to help make a decision as to the appropriate number of reads for a given number of attempts are acceptable. 
         [0079]      FIG. 11  is a schematic diagram explaining how to operate an RFID tag system based on the principles of the invention. As in the system shown in  FIG. 1 , an interrogator (reader)  104   a  transmits interrogation signals  110   a  to RFID tags  102 . Reader  104   a  is not effective over its entire designated frequency band of operation because of design constraints for its antenna and or installation constraints. After identifying a sub-band of frequencies  708  on which the interrogator operates effectively, interrogation signals  110   a  are limited to those frequencies within the identified sub-band. 
         [0080]    Persons skilled in the relevant arts will recognize that the elements, methods, techniques, and principles of the inventions may be applied, with suitable modifications, to other kinds of radio frequency reporting systems which may employ mechanically modifiable elements. 
       CONCLUSION 
       [0081]    The above examples of a system and method for operating an RFID interrogator are exemplary only. Persons skilled in the relevant arts will recognize that a variety of alternatives may exist in terms of materials, relations of structural and operational elements, and methods of employing or applying the same. Such variations fall within the scope and spirit of the invention which is not limited by the particular examples described above. 
         [0082]    While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.