Abstract:
In one embodiment, an RFID reader and active tag (RAT) includes: a first beam forming means for interrogating a plurality of RFID tags using at least a first set of two antennas coupled to a first fixed phase feed network, the beam forming means being configured to adjust gains in the first fixed phase feed network to scan with respect to the plurality of RFID tags; and a second beam forming means for uploading RFID data from the interrogated plurality of RFID tags to an external access point using at least a second set of two antennas coupled to a second fixed phase feed network, the beam forming means being configured to adjust gains in the second fixed phase feed network to direct its RF beam at the external access point.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Ser. No. 10/860,526, filed Jun. 3, 2004, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to RFID applications, and more particularly to an RFID reader configured to wirelessly communicate with an access point.  
       BACKGROUND  
       [0003]     Radio Frequency Identification (RFID) systems represent the next step in automatic identification techniques started by the familiar bar code schemes.  
         [0004]     Unlike bar codes that can smear or be obscured by dirt, RFID tags are environmentally resilient. Whereas bar code systems require relatively close proximity and line-of-sight (LOS) contact between a scanner and the bar code being identified, RFID techniques do not require LOS contact and may be read at relatively large distances. This is a critical distinction because bar code systems often need manual intervention to ensure proximity and LOS contact between a bar code label and the bar code scanner. In sharp contrast, RFID systems eliminate the need for manual alignment between an RFID tag and an RFID reader or interrogator so as to enable readability of concealed RFID tags, thereby keeping labor costs at a minimum. Moreover, RFID tags may be written to one-time programmable (OTP) or write-many fashions whereas once a bar code label has been printed further modifications are impossible. These advantages of RFID systems have resulted in the rapid growth of this technology despite the higher costs of RFID tags as compared to a printed bar code label.  
         [0005]     The non-LOS nature of RFID systems is both a strength and a weakness, however, because one cannot be sure which RFID tags are being interrogated by a given reader. In addition, RFID tag antennas are inherently directional and thus the spatial orientation of the interrogating RF beam can be crucial in determining whether an interrogated RFID tag can receive enough energy to properly respond. This directionality is exacerbated in mobile applications such as interrogation of items on an assembly line. Moreover, it is customary in warehousing and shipping for goods to be palletized. Each item on a pallet may have its RFID tag antenna oriented differently, thus requiring different RF beam interrogation directions for optimal response. As a result, conventional RFID readers are often inefficient while being relatively expensive.  
         [0006]     Accordingly, there is a need in the art for improved low-cost RFID readers.  
       SUMMARY  
       [0007]     In accordance with one aspect of the invention, an RFID reader and active tag includes: a first plurality of antennas; a first fixed phase variable gain beam forming interface coupled to the first plurality of antennas; a wireless interface configured to communicate through the first fixed phase variable gain beam forming interface with an access point; a second plurality of antennas; a second fixed phase variable gain beam forming interface coupled to the second plurality of antennas; and an RFID interface configured to interrogate RFID tags through the second fixed phase variable gain beam forming interface.  
         [0008]     In accordance with another aspect of the invention, a method includes the acts of: beam forming to scan through a plurality of items to interrogate a corresponding plurality of RFID tags so as to obtain RFID data; storing the RFID data in a memory; and uploading the stored RFID data to an external access point.  
         [0009]     In accordance with another aspect of the invention, an RFID reader and active tag (RAT is provided that includes: a first beam forming means for interrogating a plurality of RFID tags using at least a first set of two antennas coupled to a first fixed phase feed network, the beam forming means being configured to adjust gains in the first fixed phase feed network to scan with respect to the plurality of RFID tags; and a second beam forming means for uploading RFID data from the interrogated plurality of RFID tags to an external access point using at least a second set of two antennas coupled to a second fixed phase feed network, the beam forming means being configured to adjust gains in the second fixed phase feed network to direct its RF beam at the external access point.  
         [0010]     The invention will be more fully understood upon consideration of the following detailed description, taken together with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a block diagram of an antenna array having a fixed-phase feed network configured to provide beam steering of received signals through gain adjustments according to one embodiment of the invention.  
         [0012]      FIG. 2  illustrates the beam-steering angles achieved by the antenna array of  FIG. 1  for a variety of gain settings.  
         [0013]      FIG. 3  is a block diagram of an antenna array having a fixed-phase feed network configured to provide beam steering of transmitted signals through gain adjustments according to one embodiment of the invention.  
         [0014]      FIG. 4  is a block diagram of an RFID reader and active tag (RAT) in accordance with an embodiment of the invention.  
         [0015]      FIG. 5  illustrates the RAT of  FIG. 4  in an exemplary industrial environment in accordance with an embodiment of the invention.  
         [0016]      FIG. 6   a  is a perspective view of a monopole RFID antenna in accordance with an embodiment of the invention.  
         [0017]      FIG. 6   b  is a cross-sectional view of the monopole RFID antenna of  FIG. 6   a.   
     
    
     DETAILED DESCRIPTION  
       [0018]     An RFID reader is provided that incorporates the beam forming techniques disclosed in U.S. Ser. No. 10/860,526 to enable the interrogation of multiple RFID tags such as those found on palletized or containerized goods. Because the RFID reader will use the efficient yet inexpensive-to-implement beam forming techniques of U.S. Ser. No. 10/860,526, the directionality problems encountered with reading RFID tags of varying orientations using a single RFID beam are alleviated. These same beam forming techniques may be applied to a wireless interface the RFID reader includes to wirelessly communicate with an external access point using a suitable wireless protocol such as IEEE 802.11. In that sense, the RFID reader also acts as an active RFID tag with respect to the access point. Because the RFID reader also acts as an active RFID tag in that it may be interrogated by a remote AP to provide RFID data it has obtained, it will be denoted as an RFID reader active tag (RAT) in the following discussions.  
         [0019]     Advantageously, the beam forming techniques disclosed in U.S. Ser. No. 10/860,526 may be conveniently integrated with conventional wireless interfaces in the RAT such as an 802.11 interface as well as conventional RFID interfaces.  
         [0020]     This integration is convenient because an 802.11 interface transmits and receives on a single RF channel in a half-duplex mode of operation. The same is true for an RFID interface (but at a different operating frequency). Because the beam forming technique disclosed in U.S. Ser. No. 10/860,526 is performed in the RF domain, this beam forming is non-intrusive and thus transparent to these signal RF channel interfaces. The single RF channel beam forming technique may be further described with respect to  FIG. 1 . A beam forming antenna array  100  including antennas  110  and  120  receives and transmits with respect to a fixed-phase feed network  105 . The lengths of each channel within the fixed-phase feed network may be equal if antennas  110  and  120  are configured to transmit and receive substantially orthogonal to each other. If they are aligned, however, as shown in  FIG. 1  such that their directivities are parallel, the fixed phase network should be configured so as to introduce a substantially ninety degree phase shift between antennas  110  and  120 . For example, a received signal from antenna  110  will couple through network  105  to be received at a beamforming circuit  115  leading in phase ninety degrees with respect to a received signal from antenna  120 . Examples of such a fixed-phase feed network may be seen in PCMCIA cards, wherein one antenna is maintained 90 degrees out of phase with another antenna to provide polarization diversity. However, rather than implement a complicated MEMs-type steering of antenna elements  110  and  120  as would be conventional in the prior art, variable gain provided by variable-gain amplifiers  125  and  130  electronically provides beam steering capability. Amplifiers  125  and  130  provide gain-adjusted output signals  126  and  131 , respectively, to a summing circuit  140 . Summing circuit  140  provides the vector sum of the gain-adjusted output signals from amplifiers  125  and  130  as output signal  150 . Variable-gain amplifiers  125  and  130  may take any suitable form. For example, amplifiers  125  and  130  may be implemented as Gilbert cells. A conventional Gilbert cell amplifier is constructed with six bipolar or MOS transistors (not illustrated) arranged as a cross-coupled differential amplifier. Regardless of the particular implementation for variable-gain amplifiers  125  and  130 , a controller  160  varies the relative gain relationship between the variable gain amplifiers to provide a desired phase relationship in the output signal  150 . This phase relationship directly applies to the beam steering angle achieved. For example, should controller  160  command variable-gain amplifiers  125  and  130  to provide gains such that their outputs  126  and  131  have the same amplitudes, the resulting phase relationship between signals  126  and  131  is as shown in  FIG. 2 . Such a relationship corresponds to a beam-steering angle φl of 45 degrees. However, by adjusting the relative gains amplifiers  125  and  130 , alternative beam-steering angles may be achieved. For example, by configuring amplifier  130  to invert its output and reducing the reducing the relative gain provided by amplifier  125 , a beam-steering angle φ 2  of approximately −195 degrees may be achieved. In this fashion, a full 360 degrees of beam steering may be achieved through appropriate gain and inversion adjustments. It will be appreciated that orthogonality (either in phase or antenna beam direction) is optimal for beam steering. However, other relationships may be used, at the cost of reduced beam steering capability. For example, feed network  105  could be constructed such that antenna  110  is fed 45 degrees (rather than 90 degrees) out of phase with respect to the antenna  120 .  
         [0021]     The fixed-phase feed network with variable gain steering approach discussed with respect to signal reception in  FIG. 1  may also be used for beam steering for transmission as well. For example, a full 360 degrees of beam steering may be achieved for transmitted signals. As seen in  FIG. 3 , antennas  110  are now oriented in space such that their RF antenna beam directivities are orthogonal to each other. In such an embodiment, a fixed phase feed network  305  is configured such that antennas  110  and  120  are fed in phase with each other. A pair of variable gain amplifiers  305  and  310  receive an identical RF feed from either an IF or baseband processing stage (not illustrated) and adjust the gains of output signals  306  and  311 , respectively, in response to gain commands from controller  160 . Fixed-phase feed network  105  transmits signals  311  and  306  such that they arrive in phase at antennas  110  and  120 , respectively. Depending upon the relative gains and whether amplifiers  305  and  310  are inverting, a full 360 degrees of beam steering may be achieved as discussed with respect to  FIG. 1 .  
         [0022]     It will be appreciated that the gain-based beam-steering described with respect to  FIGS. 1 and 3  may be applied to an array having an arbitrary number of antennas. Regardless of the number of antennas, the beam forming is transparent to the IF or baseband circuitry because it is performed in the RF domin, rather than in the IF or baseband domains. This beam forming may be applied in an exemplary embodiment of a RAT  400  as seen in  FIG. 4 . RAT  400  includes an RFID interface  405  configured to interrogate RFID tags as known in the art. Thus, RFID interface  405  generates an appropriate RF signal  406  for transmission through an antenna to the RFID tags that are to be interrogated. RFID interface  405  is also configured as known in the art to receive the resulting transmissions from the interrogated RFID tags as an RF signal  407 , which interface  405  demodulates to determine the encoded information in the interrogated RFID tags. In a conventional RFID reader, RF signal  406  would be transmitted and RF signal  407  received without any beam forming being performed. However, a fixed phase, variable gain beam forming interface circuit  410  receives RF signal  406  and drives a plurality of RFID antennas  420  as discussed above. Thus, RFID antennas  420  may be arranged to radiate in parallel such that a fixed phase network  425  coupling interface  410  and antennas  420  would introduce a phase difference. Alternatively, RFID antennas  420  may be oriented orthogonally in space as illustrated in  FIG. 4  such that fixed phase network  425  would not introduce a phase difference. Variable gain amplifiers (not illustrated) within beam forming interface  410  control the gain in each channel as discussed with respect to  FIGS. 1 and 3 . It will be appreciated that phase differences or spatial arrangements of less than  90  degrees may utilized as discussed above. A logic engine  430  implemented in, for example, a field programmable gate array (FPGA) controls RFID interface  405  and beam forming interface  410 . Thus logic engine  430  may perform the functions of controller  160  discussed with respect to  FIGS. 1 and 3 . RFID interface may operate at any appropriate RFID frequency such as 13.56 MHz, 433 MHz, 868 MHz, or 915 MHz (the latter three frequencies being typically referred to as UHF bands).  
         [0023]     RFID interface  405  may store the resulting RFID data from the interrogated tags in a memory such as flash memory  440 . In turn, an AP (not illustrated) interrogates RAT  400  to provide this RFID data. Thus, a wireless interface such as an 802.11 interface  450  retrieves the RFID data from memory  440  and modulates an RF signal  460  accordingly. A fixed phase, variable gain beam forming interface circuit  470  receives RF signal  460  and drives a plurality of 802.11 antennas  480  using a fixed phase feed network  485 . Logic engine  430  controls beam forming interface circuit  470  to provide the desired beam forming angle to transmit to the AP. In addition, the beam forming would also apply to a received RF signal  465  from the AP. As discussed with respect to antennas  420 , antennas  480  may be arranged to transmit and receive orthogonally to each other or in parallel. As illustrated, antennas  480  are arranged in parallel and thus fixed phase feed network  485  introduces a phase difference Φ such as ninety degrees.  
         [0024]     An exemplary usage of RAT  400  is illustrated in  FIG. 5 . RAT  400  is attached to a container or pallet  500  that includes a plurality of items each having their own RFID tag  505 . As shown by the emanations from tags  505 , each tag has its preferred direction of interrogation that may be different from other tags in container/pallet  500 . RAT  400  scans through a plurality of interrogation directions to interrogate RFID tags  505 . This type of scanning may be thorough, such as a full 360 degree scan as discussed with respect to  FIG. 2 . Alternatively, a subset of directions may be scanned. For example, in the X-Y plane, a beam at 0 degrees and 90 degrees may be used to interrogate the tags. Similarly, in the X-Z plane a beam at 0 and 90 degrees may also be used. Having interrogated the tags, the resulting RFID data may be uploaded by RAT  400  to an AP  510  through a beam  520  having an orientation determined by beam forming interface  470  of  FIG. 4 . Because the RFID scan is internal to the container, beam forming interface  410  may also be denoted as an internal beam forming interface. In contrast, AP  510  is typically somewhat remote from RAT  400  such that beam forming interface  470  may be denoted as an external beam forming interface.  
         [0025]     RAT  400  may be removably connected to container/pallet  500  using, for example, Velcro or other types of temporary adhesives. The 802.11 antennas may be provided on an internal card to RAT  400  such as a PCMCIA card. However, RFID antennas are typically lower frequency and thus larger than those used for 802.11 communication. For example, 802.11 communication is often performed at 2.4 GHz whereas RFID interrogation may be performed at just 900 MHz. Thus, it is convenient to implement RFID antennas  420  externally to RAT  400  and also  1   0  removably connected to container/pallet  500 . Having affixed the RFID antennas and RAT  400  to container/pallet  500 , a user would then couple RFID antennas  420  to RAT  400  to complete the configuration.  
         [0026]     It will be appreciated that any suitable antenna topology such as, for example, monopole, patch, dipole, or patch may be used to implement RFID antennas  420  and 802.11 antennas  480 . A convenient topology for RFID antennas  420  is a monopole such as a monopole  600  illustrated in  FIG. 6   a . As seen in cross-sectional view in  FIG. 6   b , monopole  600  may comprise a metal rod  630  surrounded by an expensive insulator such as plastic foam  620 . Because pallet/container  500  to which monopole  600  will be attached typically has a rectangular shape, plastic foam  620  may have an angular cross-section such that monopole  600  may be affixed to an angular edge of pallet/container  500 . An inner surface of the angular cross-section may include an adhesive layer such as Velcro that enables monopole antenna  600  to be removably affixed to pallet/container  500 . To keep the radiation from monopole antenna  600  directed within the contents of pallet/container  500 , an outer surface of insulating layer  620  may be covered with a reflecting metallic shield such as aluminum foil shield  650 . Shield  650  may be further covered with a protective layer such as a plastic layer  640 .  
         [0027]     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. The appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention.