Patent Publication Number: US-2011050421-A1

Title: Systems, methods and apparatus for determining direction of motion of a radio frequency identification (rfid) tag

Description:
TECHNICAL FIELD 
     Embodiments of the subject matter described herein relate generally to radio-frequency identification (RFID) technologies. More particularly, embodiments of the subject matter relate to RFID systems, methods, and apparatus for determining the direction of motion of an RFID tag. 
     BACKGROUND 
     Radio frequency identification (RFID) systems have achieved wide popularity in a number of applications, as they provide a cost-effective way to track the location of a large number of items in real time. Most RFID systems include two primary components: an RFID reader (also known as an interrogator or RFID reader device); and one or more RFID tags (also known as RFID transponders). The RFID reader generates or emits a radio-frequency (RF) interrogation signal (sometimes also called a polling signal). The RFID tag is a miniature device that is capable of responding to the RF interrogation signal by generating an RF response signal that is transmitted back to the RFID reader over an RF channel. The RF response signal is modulated in a manner that conveys identification data (i.e., a tag identifier (ID)) for the responding RFID tag back to the RFID reader. In large-scale applications, such as warehouses, retail spaces, and the like, many types of RFID tags may exist in the environment (or “site”). Likewise, multiple types of readers, such as RFID readers, active tag readers, 802.11 tag readers, Zigbee tag readers, etc., are typically used throughout the space, and may be linked by network controller or wireless switches and the like. 
     RFID systems are used in a number of different applications such as object tracking, security, inventory control/tracking in retail stores, warehouses, shipping centers, etc. For instance, in one inventory tracking application, some retails stores have begun using the RFID technology to track the location of items/inventory/articles/merchandise present in the store. In such applications, each item has an RFID tag attached to it so that the item can be tracked as it moves about an inventory space. 
     RFID “portals” can be implemented at different points (e.g., an entrance/exit to the inventory space) to automatically track whether or not RFID tags (and hence the items they are attached to) have passed through the portal. In essence, an RFID portal is a RFID reader located at a known position such as a boundary between an entry/exit point. To determine whether or not a particular RFID tag has entered or exited the portal, knowing the direction of travel of an RFID tag is of interest. 
       FIG. 1  is a block diagram of an RFID tag  102  tracking system  100 . The system  100  includes a fixed RFID reader  104  at a portal  103 . The portal  103  is located at a boundary between an inventory space  110  and an external space  120 . The RFID reader  104  is fixed at a known location or position. The particular known position can be determined by technologies and methods such as GPS location determination, dead-reckoning, manual input or any other technique, and specified using a Cartesian or other coordinate systems. This allows the location of the RFID reader  104  to be established with respect to the detection path  105 . The RFID tag  102  can be moved in a first direction  130  of motion and a second direction  140  of motion along a detection path  105  between the inventory space  110  and the external space  120 . The first direction  130  of motion is into the portal  103 , out of the external space  120  and into the inventory space  110 . The second direction  140  of motion is out of the portal  103 , that is, out of the inventory space  110  and into the external space  120 . The fixed RFID reader  104  is designed to read an RFID tag  102  as it passes through the portal  103 . This is of importance, for example, in an inventory tracking system when the RFID tag  102  is coupled an item that is moving through the portal  103  since it may be desirable to determine whether the item is exiting or entering through the portal  103 . 
     While it is desirable to know whether the RFID tag  102  has passed through the portal  103 , it is more desirable to know in what direction the RFID tag  102  was moving as it passed through the portal  103  so that an inventory or monitoring system (not shown) can keep track of whether the item has exited or entered through the portal  103 . This is particularly important in applications such as inventory control, etc., since it allows the relative location of an item (i.e., as being within an inventory space or as having left the inventory space) to be automatically tracked. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. 
         FIG. 1  illustrates a Radio Frequency Identification (RFID) tracking system; 
         FIG. 2  illustrates a block diagram of an RFID reader and a nearby RFID tag that can be used in accordance with some embodiments of the present disclosure; 
         FIG. 3  illustrates a RFID tracking system in accordance with some embodiments of the present disclosure; 
         FIG. 4  is a graph that illustrates expected RSSI curves of the RFID tag response signal at the RFID reader as a function of horizontal distance (d h ) of the RFID tag from an origin point along the detection path when the antenna is tilted; 
         FIG. 5  is a flowchart illustrating a method for determining direction of motion of an RFID tag in accordance with some other embodiments of the present disclosure; and 
         FIG. 6  is a graph that illustrates measured RSSI of response signals transmitted from a first RFID tag at the RFID reader as a function of time when the antenna is tilted at a tilt angle of 60°. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the invention or the application and uses of such embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, or the following detailed description. 
     Some embodiments of the present disclosure relate generally to determining direction of motion of an RFID tag. The many alternative embodiments of the invention may be described herein in terms of functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions. For example, an embodiment of the invention may employ various integrated circuit components, e.g., memory elements, digital signal processing elements, logic elements, look-up tables, or the like, which may carry out a variety of functions under the control of one or more microprocessors or other control devices. In addition, those skilled in the art will appreciate that embodiments of the present invention may be practiced in conjunction with any number of data transmission protocols and that the system described herein is merely one example embodiment of the invention. 
     For the sake of brevity, conventional techniques related to radio-frequency identification (RFID) data transmission, RFID system architectures, computing device architectures, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent example functional relationships between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in a practical embodiment. 
     The following description may refer to elements or nodes or features being “connected” or “coupled” together. As used herein, unless expressly stated otherwise, “connected” means that one element/node/feature/device directly communicates with another element/node/feature/device. Likewise, unless expressly stated otherwise, “coupled” means that one element/node/feature/device directly or indirectly communicates with another element/node/feature/device. For example, although the schematic shown in  FIG. 2 , described below, depicts one example arrangement of an RFID reader, additional intervening elements, devices, features, or components may be present in an embodiment of the invention. 
     Overview 
     Referring again to  FIG. 1 , although techniques have been developed for determining what direction the RFID tag  102  was moving in as it passed through the portal  103 , these techniques generally require the use of two RFID readers each having its own antenna, or an RFID reader with two spaced apart antennas at the portal  103 . For example, according to one technique the sequence of read events from the two antennas is used to determine the direction of motion. 
     Accordingly, it is desirable to provide improved methods, systems and apparatus for determining which direction an RFID tag is moving in as it passes by an RFID reader. It is also desirable to provide improved RFID systems and methods for determining relative location(s) of item(s) with respect to an entry/exit point of an inventory space. It would also be desirable if such RFID systems are easy to deploy, maintain and operate. To reduce the cost of such RFID tracking systems and simplify installation of such RFID tracking systems, it would be desirable to provide improved techniques that can reduce the number of RFID readers required and/or reduce the complexity of the RFID reader by using a single reader with a single antenna. 
     According to one embodiment, a method is provided for determining direction of motion of an RFID tag. The method can be performed by an RFID reader located at a portal. In accordance with one exemplary embodiment of this method, direction of motion of an RFID tag can be determined as it moves along a detection path. The RFID reader includes an antenna (e.g., a directional antenna) that is tilted at a tilt angle with respect to the detection path. The tilt angle with respect to the detection path is the angle between the antenna and a direction parallel to the detection path, and is greater than 0 degrees and less than 180 degrees. The direction of motion that the RFID tag is moving in is determined with respect to the RFID reader along the detection path. 
     Response signals from the RFID tag are received at the antenna at different times, and an RSSI sample of each response signal is measured. Based on the RSSI samples, an RSSI/time data point is generated for each of the RSSI samples. Each RSSI/time data point defines a measured RSSI value for a particular RSSI sample versus a time that particular RSSI sample was measured. Based on the plurality of RSSI/time data points, the direction of motion of the RFID tag can be determined. 
     In accordance with one embodiment, the RSSI/time data point that has a maximum measured RSSI value is defined as a maximum RSSI/time data point, and the time at which the maximum RSSI/time data point was measured is defined as the maximum time point. Thereafter, a first group of the plurality of RSSI/time data points that were measured at times prior to when the maximum time point was measured are determined, and a second group of the plurality of RSSI/time data points that were measured at times occurring after the time when the maximum time point was measured are determined. 
     A first linear regression is then computed to generate a first line having a first slope. The first linear regression is computed based on the first group of the plurality of RSSI/time data points that were measured at times prior to when the maximum time point was measured. A second linear regression is also computed to generate a second line having a second slope. The second linear regression is computed based on the second group of the plurality of RSSI/time data points that were measured at times occurring after the time when the maximum time point was measured. It is then determined which one of the first slope and the second slope has a greater magnitude. When the second slope has the greater magnitude, it is determined that the RFID tag is moving in a first direction of motion (e.g., into the portal), and when the first slope has the greater magnitude, it is determined that the RFID tag is moving in a second direction of motion (e.g., out of the portal). 
     The disclosed embodiments allow for the direction of motion of an RFID tag to be determined via a RFID reader with a single antenna. This not only reduces cost and complexity of such systems, it simplifies customer installation since only a single reader having a single antenna can be used to determine the direction of motion of an RFID tag. 
     Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings. Prior to describing some embodiments with reference to  FIGS. 3-14 , an example of an RFID reader and nearby RFID tag will then be described with reference to  FIG. 2 . 
     Exemplary RFID Reader 
       FIG. 2  illustrates a block diagram of an RFID reader  204  and nearby RFID tag  225  that can be used in accordance with some embodiments of the present disclosure. The RFID reader  204  can be implemented with an-off-the-shelf RFID reader  204 , or other computer or computing device that runs one or more suitably configured software applications. In the following description of  FIG. 2 , the RFID reader  204  is configured to communicate with an exemplary RFID tag  225 . 
     The functionality of the RFID reader  204  is explained with respect to various modules depicted in the block diagram. It is to be understood that the various modules are shown to facilitate better understanding of the RFID reader  204 , and that the modules included in the RFID reader  204  are not meant to be a limitation on embodiments of the present disclosure. Depending on the implementation, the RFID reader  204  may be a fixed device or a handheld portable device. For instance, in embodiments described above with respect to  FIG. 1D  above, the RFID readers  104  are fixed, whereas in other embodiments (e.g.,  FIG. 7 ) the RFID reader is nomadic and can move about the space or environment  110 . The following description of the RFID reader  204  has been explained with reference to components shown in  FIG. 2 . The RFID reader  204  is depicted in a simplified manner, and a practical embodiment can include many additional features and components. 
     Modules included in one implementation of the RFID reader  204  can generally include network interfaces  211  (that can include a wired network interface such as an Ethernet interface, and/or wireless interfaces, such as a WLAN interface), one or more other antennas  210 , a housing  212 , a display element  213  that is visible from the outside of the housing  212 , input devices  214  that are accessible from the outside of the housing  212 , an RFID electronics module  215  contained within the housing  212 , an RFID antenna  216  (which can be, but is not necessarily, contained within the housing  212 ) and a power module  221  (e.g., a AC power source or a DC power source such as a rechargeable battery). The input devices  214  can include a keypad, a touch panel, a keyboard attached to a PC communicating with the RFID reader  204  or other input/output elements such as imaging devices (e.g. cameras including a digital camera, a video camera, etc.) that can be used to take a real time image (e.g., video image or picture) of an area covered by the imaging device of the RFID reader. 
     The display  213  and input device  214  function as input/output elements for the operator of the RFID reader  204 . As will be described below, various software and hardware produce an image or graphical user interface (GUI) on the display  213  indicative of the position of the RIFD reader or readers, the RFID beacon tags  101 , and RFID item tags  102  with respect to the RFID reader  104  or readers within environment  110 . In various embodiments that will be described below, a coverage map (hereinafter also referred to as a map) can be displayed as a GUI on the display  213  (e.g., screen) of a RFID reader. The coverage map that is displayed on the display  213  of the RFID reader can display the entire space or environment  100  or any portion of the entire space or environment  100 . In each of the embodiments described below, the coverage map can indicate read range information for one or more of the RFID readers that appear on the coverage map. 
     The display  213  and input device  214  can be coupled to the RFID electronics module  215  as necessary to support input/output functions in a conventional manner. 
     The RFID electronics module  215  represents the hardware components, logical components, and software functionality of the RFID reader  204 . In practical embodiments, the RFID electronics module  215  can be physically realized as an integrated component, board, card, or package mounted within the housing  212 . As depicted in  FIG. 2 , the electronics module  215  can be coupled to one or more RFID antennas  216 , for example, via RF cables and RF connector assemblies. In one embodiment, multiple RFID antennas  216  are included. These RFID antennas  216  can include dual-polarized RFID antenna and circularly polarized RFID antenna. The RFID reader  204  can switch between the antennas to create different radiation patterns. 
     The RFID electronics module  215  may generally include a number of sub-modules, features, and components configured to support the functions described herein. For example, the electronics module  215  may include an RFID reader communication sub-module  217 , at least one processor  219 , memory  220 , an RFID power controller sub-module  222  and a location determination and map generation sub-module  223 . In a practical embodiment, the various sub-modules and functions need not be distinct physical or distinct functional elements. In other words, these (and other) functional modules of the RFID reader  204  may be realized as combined processing logic, a single application program, or the like. 
     The RFID electronics sub-module  215  also includes an RFID communication sub-module  217  designed to support RFID functions of the RFID reader  204  and to communicate with the RFID tags via RFID antenna(s)  216 . The RFID communication module  217  can include an RFID reader transceiver that includes a transmitter and a receiver with conventional circuitry to enable digital or analog transmissions over a wireless communication channel. The transceiver enables the RFID reader  204  to communicate with the RFID beacon tags  101 ,  102  via antenna(s)  216 . 
     For example, the RFID reader transceiver generates RFID interrogation signals and receives reflected RFID response signals generated by RFID tags in response to the interrogation signals. In the example embodiment described herein, the RFID communication sub-module  217  is designed to operate in the UHF frequency band designated for RFID systems. Alternate embodiments may instead utilize the High Frequency band or the Low Frequency band designated for RFID systems. The operation of RFID readers and RFID transceivers are generally known and, therefore, will not be described in detail herein. Notably, in this example embodiment, the RFID communication sub-module  217  is operable at various transmit power levels, as controlled by the RFID power controller  222  sub-module. The RFID power controller sub-module  222  can adjust the power of transmission of interrogation signals transmitted by the RFID antenna(s)  216 . The transmit power level or radio signal strength of the interrogation signals can be adjusted so that the interrogation signals can travel varying distances from the RFID reader  204 . For example, the operator of an RFID reader can adjust the transmit power level or radio signal strength to cover the area of interest, thus avoiding the interrogation or polling of items placed on other shelves or racks, which are of no interest in the current polling. In one non-limiting, exemplary embodiment, the RFID reader  204  provides a linear coverage for 10 feet of the space at a particular transmit power level, which translates into a circular coverage for 5 feet of the space at the particular transmit power level. The RFID power controller sub-module  222  can be embodied separately, or integrated with one or more other sub-modules. 
     The processor  219  can be any general purpose microprocessor, controller, or microcontroller that is suitably configured to control the operation of the RFID reader  204 . In practice, the processor  219  executes one or more software applications that provide the desired functionality for the RFID reader  204 , including the operating features described in more detail below. The memory  220  may be realized as any processor-readable medium, including an electronic circuit, a semiconductor memory device, a ROM, a flash memory, an erasable ROM, a floppy diskette, a CD-ROM, an optical disk, a hard disk, an organic memory element, or the like. As an example, the memory  220  is capable of storing RFID data captured by the RFID reader  204 . 
     The power module  221  provides operating power to the RFID reader  204 . In one embodiment, the power module  221  includes a battery that supplies power to the RFID reader  204 . In some implementations, the battery is rechargeable via ambient lighting so that each RFID reader can be trickle charged. Power status of the RFID readers is communicated back to the central monitoring server  106  via the wireless link or a wired communication link, and low power conditions can set off alert signals for servicing. The power module  221  can also indirectly supply operating power to the RFID tags  225 , if the RFID tags  225  are passive tags. Passive tags do not have a battery of their own, and therefore derive power from RF signals transmitted by the RFID readers. When a passive tag encounters radio waves from a reader, a coiled antenna within the RFID tag forms a field. The RFID tag draws power from it, energizing the circuits in the RFID tag. 
     The tag motion directionality module  223  is a processor or equivalently a software module running on a processor that is designed to measure RSSI samples of the response signals received from the RFID tag  225  at different times. The value of the RSSI samples changes as the RFID tag  225  moves along the tag detection path  105  towards the RFID reader  204 . The tag motion directionality module  223  generates a plurality of RSSI/time data points for each of the RSSI samples. Each RSSI/time data point defines a measured RSSI value for a particular RSSI sample versus a time that particular RSSI sample was measured. Based on plurality of RSSI/time data points, the tag motion directionality module  223  can then determine a direction of motion  130 ,  140  of the RFID tag  225  with respect to the RFID reader  204  as the RFID tag  225  passes the RFID reader  204  (e.g., as it moves through the portal  103 ). Depending on the direction  130 ,  140  that the RFID tag  225  is moving in, the RSSI values will have a different characteristic or signature. For instance, the RSSI values will slowly rise and abruptly disappear when moving in the first direction  130 . The signal has the opposite sequence when traveling in the other second direction  140 . As such, the tag motion directionality module  223  can determine whether the RFID tag  225  is moving in a first direction  130  of motion into the portal  103 , or a second direction  140  of motion out of the portal  103 . Different embodiments of processing performed by the tag motion directionality module  223  will be described below with reference to  FIG. 5 . 
     A RFID reader, such as the one described above, preferably is capable of functioning in one or more alternate modes, including the RFID reader mode. The primary functions of the RFID reader need not be limited to data capture and RFID tag interrogation. Rather, the RFID reader can be capable of multi-tasking and multi-functioning. Some functions, such as a bar-code scanner and alternate manual input interfaces, can also be present. In some embodiments, the RFID reader  204  can be a single device, while in others, multiple devices can combine various features to accomplish the functions listed above, and others desired for or necessary to the embodiment. A RFID reader, such as the one described above, is preferably used as in conjunction with the systems and methods described below. 
     The exemplary RFID tag  225  illustrated in  FIG. 2  includes an integrated circuit  227 , and includes an antenna  226 . The RFID antenna  226  can receive RF signals such as an interrogation signal  224  and transmit RF signals, such as response signals  228 . The integrated circuit  227  represents one or more modules cooperating to store and process information including demodulating RF interrogation signals and for modulating RF response signals, and other functions. 
     Examples of RFID tags include, but are not limited to, active tags, passive tags, semi-active tags, WiFi tags, 801.11 tags, and the like RFID tags. Note that the term “RFID” is not meant to limit the invention to any particular type of tag. That is, the term “tag” refers, in general, to any RF element that can be communicated with and has an ID (or “ID signal”) that can be read by another component. In general, RFID tags may be classified as either an active tag, a passive tag, a semi-active tag or a semi-passive tag. Active tags are devices that incorporate some form of power source (e.g., batteries, capacitors, or the like) and are typically always “on,” while passive tags are tags that are exclusively energized via an RF energy source received from a nearby antenna. Semi-active tags are tags with their own power source, but which are in a standby or inactive mode until they receive a signal from an external RFID reader, whereupon they “wake up” and operate for a time just as though they were active tags. A semi-passive tag is a tag with a battery source that is used to extend the range beyond that of a passive tag, but still user passive backscatter to communicate with the reader. While active tags are more powerful, and exhibit a greater range than passive tags, they also have a shorter lifetime and are more expensive. Such tags are well known in the art, and need not be described in detail herein. For example, one implementation of the RFID item tags is disclosed, for example, in U.S. patent application Ser. No. 12/185867, attorney docket number SBL08079, entitled “Method of Configuring RFID Reader” filed Aug. 5, 2008 and assigned to the assignee of the present invention, its contents being incorporated by reference in its entirety herein. 
     Each antenna  226  within RFID reader  204  has an associated RF read range (or “coverage area”), which depends upon, among other things, the gain of the respective antenna or strength of the transmit signal of the respective antenna. The read range corresponds to the coverage area around the antenna  216  in which a tag  225  may be read by that antenna, and may be defined by a variety of shapes, depending upon the nature of the antenna. 
     The exemplary RFID tag  225  can be positioned within transmission range or read range of the RFID reader  204 . When the RFID tag  225  receives the interrogation signal  224  with its RFID antenna  226 , the integrated circuit  227  can perform one or more operations in response, including demodulating the interrogation signal  224  (to know when and with what to respond) and modulating the interrogation signal  224  using “backscatter modulation” (e.g., modulating the reflection coefficient of its antenna with the information to respond with), and transmitting the modulated interrogation signal  224  from the RFID antenna  226  as a response signal  228 . 
     The RFID reader  204  can receive the response signal  228 , and extract useful information from it including, but is not limited to, the identity of the RFID tag  225  (i.e., a tag identifier). 
     Although not illustrated in  FIG. 2 , the RFID reader can communicate information with an access point or port, a wireless switch, and a monitoring server, such as that described, for example, in U.S. patent application Ser. No. 12/369,838, filed Feb. 12, 2009, entitled “Displaying Radio Frequency Identification (RFID) Read Range Of An RFID Reader Based On Feedback From Fixed RFID Beacon Tags,” and assigned to the assignee of the present invention, which is incorporated herein by reference in its entirety. 
     Various embodiments of the present disclosure will now be described with respect to  FIGS. 3-14 . 
     RFID Tracking System 
       FIG. 3  illustrates a Radio Frequency Identification (RFID) tracking system  300  in accordance with some embodiments of the present disclosure. The system  300  is similar to that illustrated in  FIG. 1 . As in  FIG. 1 , the system  300  includes an RFID tag  102 . However, in the disclosed embodiments, a single, fixed RFID reader  104  is used at the portal  103  to determine the relative direction of motion of the RFID tag  102 . This RFID reader  104  is “fixed” at known location/position/coordinate, and utilizes only a single antenna to determine direction in which an RFID tag  102  is moving. 
     The RFID tag  102  is not at fixed locations/positions/coordinates and can be moved around and taken into or out of the space  110 . The RFID tag  102  can move within the inventory space  110  and the external space  120 . Although the RFID tag  102  can move along the detection path  105 , it is also true that it can move anywhere within the inventory space  110  and the external space  120 . Moreover, it can move, then stop, move again, etc. In other words, its movement pattern is not necessarily linear (along the detection path  105 ) and is not necessarily continuous. However, in some cases, the RFID tag  102  can move on a path that can be in a first direction  130  of motion and a second direction  140  of motion along a detection path  105  at any particular time. In this example, like that in  FIG. 1 , the first direction  130  of motion is into the portal  103 , and the second direction  140  of motion is out of the portal  103 . The detection path  105  extends along between an inventory space  110  and a second space  120 , which in some implementations can be external to the inventory space  110 , and in other implementations can be a different portion of section of the inventory space  110 . As used herein, the inventory space  110  is a controlled space where items having RFID tags can be stored at least temporarily. The space  110  can be located within a building or other site (alternatively referred to as an “environment”). Note that while a single two-dimensional space  110  is illustrated in  FIG. 3 , the invention is not so limited. That is, space  110  may be any two-dimensional or three-dimensional space within or without a building and other structure. Example environments include, for example, single-story buildings, multi-story buildings, school campuses, commercial buildings, retail spaces, warehouses, and the like structures. 
     The fixed RFID reader  104  can be placed or located at an entry/exit point boundary  108  between the first space  110  and the second space  120  to define a portal  103  located a first distance  150  from the detection path  105 . The entry/exit point boundary  108  is aligned with a center plane of the RFID reader  104 . In one implementation, the portal  103  can be defined, for example, an entrance to a building or other structure. The fixed RFID reader  104  can interrogate the RFID tag  102  when it is within the read range of the reader  104 . In response, the tag  102  transmits response signals, which include relevant tag data including identification information for each RFID tag. The identification data for each RFID tag  102  is stored at the RFID reader  104  (and at a monitoring server) so that the RFID reader  104  knows which RFID tag  102  transmitted the response signal. When the RFID tag  102  is attached to an item, the RFID tag  102  can include information pertaining to details regarding that item (e.g., item type, price, size, quality, and the like). 
     As the RFID tag  102  moves along towards the RFID reader  104 , it can follow the detection path  105 . The detection path  105  extends in the x-direction, and hence the direction perpendicular to the detection path  105  can be defined as the y-direction. The angle θ is the angle between the RFID tag  102  and the direction perpendicular to the detection path  105  at any point in time as the RFID tag  102  moves along the detection path  105 . A tag distance (d tag ) is defined as the distance between the tag  102  and the RFID reader  104  at any particular time as the RFID tag  102  moves along the detection path  105 . 
     The RFID reader  104  transmits RF interrogation signals on a regular basis, and when the RFID tag  102  is within the read range of the RFID reader  104 , it will receive the interrogations signals. In response, the RFID tag  102  transmits RF response signals that can be received by the RFID reader  104 . 
     Upon receiving the RF response signals, the RFID reader  104  can measure a receive signal strength (RSS) of each response signal received from the RFID tag  102 . In particular, in accordance with the disclosed embodiments, when the RFID reader  104  receives the response signals, it can measure an RSSI value associated with each response signal and record it along with a time stamp which indicates when it was received. 
     In general, the closer the RFID tag  102  is to the RFID reader  104 , the greater the RSS measurement will be and vice-versa. As the RFID tag  102  moves along the detection path  105 , in many cases it will eventually pass through the portal  103  at the entry/exit point boundary  108  and hence past the RFID reader  104 . The receive signal strength of the response signal from the RFID tag  102  will be at a maximum at the entry/exit point boundary  108 . 
     The RFID reader  104  includes a transmitter, a receiver, a processor and a RFID antenna  170 . The antenna  170  can be directional RFID antenna  170  (sometimes also referred to as a beam antenna) is an antenna which radiates greater power in one or more directions allowing for a greater concentration of radiation in a certain direction, increased performance on transmit and receive, and reduced interference from unwanted sources. In accordance with the disclosed embodiments, the directional RFID antenna  170  is tilted at a “tilt angle.” The tilt angle (φ) is the angle between the detection path  105  (x-axis) and the antenna  170  of the RFID reader  104 . In other words, the antenna  170  is tilted at an angle (φ) with respect to the direction parallel to the detection path  105  (which is defined as the y-direction above). As used herein, the term “tilt angle” refers an orientation of the antenna  170  of the RFID reader  104  at an angle (φ) greater than 0° with respect to the detection path  105  (x-axis) but not perpendicular to the detection path  105  (x-axis). The antenna  170  of the RFID reader  104  is tilted an angle with respect to the detection path so that the antenna  170  points at an angle that is either into the portal or out of the portal. As will be described below, a single directional antenna that is tilted in one direction with respect to the detection path  105  provides enough asymmetry so that the reader  104  can determine whether the RFID tag  102  is moving in a direction that is into or out of the portal  103 . As such, the direction of motion of the RFID tag  102  can be determined using only one RFID reader  104  that has only a single antenna. 
     The received signal strength indicator (RSSI) of a response signal received at the RFID reader  104 , which is equal to the received signal power (P reader ) at the RFID reader  104  in dBm, can be expressed as shown in equation (1) as follows: 
     
       
         
           
             
               
                 
                   RSSI 
                   = 
                   
                     
                       P 
                       reader 
                     
                     = 
                     
                       
                         P 
                         tag 
                       
                       - 
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                         20 
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                           d 
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     where c is the speed of light, f is the transmit frequency of the interrogation signal, d tag  is the tag distance, θ is the angle between the RFID tag  102  and the direction perpendicular to the detection path  105 , and φ is the tilt angle of the antenna. 
     The power received by the RFID tag  102  (P tag ) can be expressed as shown in equation (2) as follows: 
     
       
         
           
             
               
                 
                   
                     P 
                     tag 
                   
                   = 
                   
                     30 
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                         ) 
                       
                     
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                         tag 
                       
                     
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                     Equation 
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     When the RFID reader&#39;s antenna  170  is tilted at a tilt angle (φ), the RSSI measured at the RFID reader  104  will vary in a predictable manner that depends on a horizontal distance (d h ) that the RFID tag  102  is located at from an origin point along the detection path  105 . This origin point is defined at the location where the entry/exit point boundary  108  (center plane of the RFID reader  104 ) intersects the detection path  105 . The first distance  150  between the RFID reader  104  and the origin point (O) of the detection path  105  is known. The origin point (O) is the point along the detection path  105  that crosses the plane of the portal. 
       FIG. 4  is a graph that illustrates expected RSSI curves of the RFID tag response signal at the RFID reader as a function of horizontal distance (d h ) of the RFID tag from an origin point along the detection path  105  when the antenna  170  is tilted. The expected RSSI curves are computed in dBm based on equations (1) and (2) above. The horizontal distance (d h ) in meters. In this example, the tilt angle (φ) of the antenna  170  is 30°.  FIG. 4  illustrates the expected RSSI versus distance curves at two different frequencies 902 MHz and 928 MHz. 
       FIG. 4  illustrates that the slopes around the maximum RSSI will be effected when the antenna  170  is tilted at an angle with respect to the detection path. In other words, when the antenna  170  is tilted and the RFID tag  102  is moves from left to right in  FIG. 3 , the expected RSSI should first have a sharp upward slope, then hit a maximum, and finally have a shallow downward slope. By contrast, when the antenna is tilted and the RFID tag  102  is moving from right to left in  FIG. 3 , then the expected RSSI should first have a shallow upward slope, then hit a maximum, and finally have a sharp downward slope. This will be explained in greater detail below with actual experimental results. 
     Referring again to  FIG. 3 , upon receiving response signals from the RFID tag  102 , the processor of the RFID reader  104  is designed to measure RSSI samples of the response signals received from the RFID tag  102  at different times. The value of the RSSI samples changes as the RFID tag  102  moves along the tag detection path  105  towards the RFID reader  104 . The RFID reader  104  generates a plurality of RSSI/time data points for each of the RSSI samples. Each RSSI/time data point defines a measured RSSI value for a particular RSSI sample versus a time that particular RSSI sample was measured. 
     Based on plurality of RSSI/time data points, the RFID reader can then determine a direction of motion  130 ,  140  of the RFID tag  102  with respect to the RFID reader  104  as the RFID tag  102  passes the RFID reader  104  (e.g., as it moves through the portal  103 ). Depending on the direction  130 ,  140  that the RFID tag  102  is moving in, the RSSI values will have a different characteristic or signature. For instance, the RSSI values will slowly rise and abruptly disappear when moving in the first direction  130 . The signal has the opposite sequence when traveling in the other second direction  140 . As such, the RFID reader  104  can determine whether the RFID tag  102  is moving in a first direction  130  of motion into the portal  103 , or a second direction  140  of motion out of the portal  103 . Different embodiments of processing performed by the processor will be described below with reference to  FIG. 5 . 
       FIG. 5  is a flowchart illustrating a method  500  for determining direction of motion of an RFID tag in accordance with some other embodiments of the present disclosure. In one implementation, the method  500  can be performed by a processor at the RFID reader  104 . In other implementations, the method  500  can be performed a network computer that is communicatively coupled to the RFID reader  104 , such as a monitoring server (not illustrated in  FIG. 3 , but incorporated by reference above). It is noted that steps  555  and  565  are optional and need not be performed in all implementations of the method  500 . 
     The method  500  begins at step  505  when the RFID reader  104  receives a response signal from the RFID tag  102 , at which point the RFID reader  104  creates a record for that RFID tag  102 . The antenna  170  of the RFID reader  104  is tilted an angle with respect to the detection path so that the antenna  170  points at an angle that is either into the portal or out of the portal. At step  510 , the processor begins tracking RSSIs from the RFID tag  102  with respect to time, and measures RSSI samples of the response signals received from the RFID tag  102  at different times. 
     At step  520 , the processor generates a plurality of RSSI/time data points for each of the RSSI samples. Each RSSI/time data point defines a measured RSSI value for a particular RSSI sample versus a time that particular RSSI sample was measured. In general, at least some of the RSSI samples correspond to response signals transmitted by the RFID tag  102  as the RFID tag  102  moves along the detection path  105  towards the RFID reader  104 . 
     Steps  530  through  590  describe further processing performed by the processor to determine a direction of motion of the RFID tag  102  with respect to the RFID reader  104  based on the plurality of RSSI/time data points. 
     As described above, the RFID tag  102  can be moving in the first direction  130  of motion into the portal  103  or in the second direction  140  of motion out of the portal  103 . Either way, when the RFID tag  102  is approaching the RFID reader  104 , a series of RSSI values will be measured. As the RFID tag  102  moves through the portal  103  and passes the RFID reader  104 , the RSSI sample taken when the RFID tag  102  is closest to the RFID reader  104  will have a maximum value. As the RFID tag  102  moves away from the RFID reader  104 , the maximum value will be followed by a series of RSSI samples having lower values. 
     At step  530 , the processor determines the one of the plurality of RSSI/time data points that has a maximum measured RSSI value. At step  540 , the processor defines the one of the plurality of RSSI/time data points that has the maximum measured RSSI value as the maximum RSSI/time data point, and defines the time at which the maximum RSSI/time data point was measured as a maximum time point (i.e., the time at which the RSSI/time data point having the maximum measured RSSI value was measured). At step  550 , the processor determines a first group of the plurality of RSSI/time data points that were measured at times prior to when the maximum time point was measured, and determines a second group of the plurality of RSSI/time data points that were measured at times occurring after the time when the maximum time point was measured. 
     Step  555  is optional. If it is not performed, then the method  500  can proceed to step  560  following step  550 . In implementations in which optional step  555  is performed, the processor determines whether a first number of RSSI/time data points in the first group of the plurality of RSSI/time data points is greater than a threshold number, and whether a second number of RSSI/time data points in the second group of the plurality of RSSI/time data points is greater than the threshold number. This check can be performed to ensure that an adequate number of data points are being used to make any subsequent decisions. The threshold numbers used for each comparison can be the same or different depending on the specific implementation. 
     If either the first number or the second number is less than the threshold number, then the method  500  loops back to step  510  so that additional RSSI samples of the signal received from the RFID tag  102  can be measured at different times, and additional RSSI/time data points for each of the RSSI samples can be recorded to improve the overall data set being used in subsequent determinations. If both the first number of RSSI/time data points in the first group and the second number of RSSI/time data points in the second group are greater than (or equal to) the threshold number, then the method  500  proceeds to step  560 . 
     At step  560 , the processor computes a first linear regression based on the first group to generate a first line having a first slope (i.e., a linear regression in the data before the maximum), and computes a second linear regression based on the second group to generate a second line having a second slope (i.e., a linear regression in the date after the maximum). In accordance with the disclosed embodiments, any known linear regression technique can be utilized to compute the first linear regression (of the first group of the plurality of RSSI/time data points that were measured at times prior to when the maximum time point was measured) and the second linear regression (of the second group of the plurality of RSSI/time data points that were measured at times occurring after the time when the maximum time point was measured). 
     As will be understood by those skilled in the art, a linear regression refers to any approach to modeling the relationship between one or more variables denoted y and one or more variables denoted X, such that the model depends linearly on the unknown parameters to be estimated from the data. In many cases, linear regression refers to a model in which the conditional mean of y given the value of X is an affine function of X. Less commonly, linear regression can refer to a model in which the median, or some other quantile of the conditional distribution of y given X is expressed as a linear function of X. Like all forms of regression analysis, linear regression focuses on the conditional probability distribution of y given X, rather than on the joint probability distribution of y and X, which is the domain of multivariate analysis. Linear regression models are often fit using the least squares approach, but may also be fit in other ways, such as by minimizing the “lack of fit” in some other norm, or by minimizing a penalized version of the least squares loss function as in ridge regression. 
     Step  565  is optional. If it is not performed, then the method  500  can proceed to step  570  following step  560 . In implementations in which optional step  565  is performed, the processor determines whether a magnitude of a difference between the first slope and the second slope is greater than or equal to a difference threshold. 
     If the magnitude of the difference between the first slope and the second slope is less than the difference threshold, then the method  500  would be deemed indeterminate at  567 . Other methods (not described herein) may be used to determine the direction of travel in this case. 
     If the magnitude of the difference between the first slope and the second slope is greater than or equal to the difference threshold, then the method  500  proceeds to step  570 , where the processor determines which of the first slope to the second slope has a greater magnitude. When the first slope has the greater magnitude, the method proceeds to step  580 , where the processor determines that RFID tag  102  is moving in the second direction  140  of motion that is the opposite direction that the antenna  170  is pointing in (i.e., in this case out of the portal  103  from left to right in  FIG. 3 ). When the second slope has the greater magnitude, the method  500  proceeds to step  590 , where the processor determines that the RFID tag  102  is moving in the first direction of motion is moving in the first direction  130  of motion that is the same direction that the antenna  170  is pointing in (i.e., in this case into the portal  103  from right to left in  FIG. 3 ). Once the direction of motion is determined at step  580 ,  590 , the result can be stored (e.g., at a monitoring server) with a time indication that indicates when the direction of motion was determined, and/or displayed to a user to show them in what direction that tag/item is moving. The monitoring server can use this information to perform inventory control and/or tracking using any techniques known in the art. For instance, when the RFID tag can not be immediately located, the user can determine if it has left a controlled area and went into an external space, or it is still within the inventory space and needs to be searched for further. 
       FIG. 6  is a graph that illustrates measured power of response signals transmitted from a first RFID tag at the RFID reader  104  in dB as a function of time (in seconds) when the antenna  170  is tilted at a tilt angle of 60°.  FIG. 6  was experimentally determined using a “first RFID tag” that was moving in the second direction  140  of motion or out of the portal  103  (i.e., from left to right in  FIG. 3 ). Each small circle on the graph represents a measured power sample of a response signal received from the RFID tag  102  at a particular time, or “power/time data point” that defines a measured power value for a particular sample versus a time that particular sample was measured. The actual RSSI values are offset from the measured power values. In this particular example, occurs at a “maximum time point” of 1.214 seconds. Each line on the graph represents a linear regression of data points before or after the maximum time point. In particular, the first line represents a first linear regression in the data before the maximum time point that is computed based on a first group of the data points that were measured at times prior to when the maximum time point was measured. Likewise, the second line represents a second linear regression in the data after the maximum time point that is computed based on a second group of the data points that were measured at times occurring after the time when the maximum time point was measured. In this example, the magnitude of the slope of the first line (1.2976) is greater than the magnitude of the slope (0.77698) of the second line. When the first slope has the greater magnitude, the processor determines that RFID tag  102  is moving in the second direction  140  of motion or out of the portal  103  (i.e., from left to right in  FIG. 3 ). As such, in this example, the correct decision was made regarding the direction of motion of the RFID tag  102 . 
     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application.