Abstract:
A system and method for locating radio-frequency identification tags within a predetermined area. The method can incorporate sub-threshold superposition response mapping techniques, alone, or in combination with other methods for locating radio-frequency identification tags such as but not limited to time differential on arrival (TDOA), frequency domain phase difference on arrival (FD-PDOA), and radio signal strength indication (RSSI). The system can include a plurality of antennas dispersed in a predefined area; one or more radio-frequency identification tags; a radio-frequency transceiver in communication with said antennas; a phase modulator coupled to the ra-dio-frequency transceiver; and a system controller in communication with said transceiver and said phase modulator. Calibration techniques can be employed to map con-structive interference zones for improved accuracy.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/067,736, entitled “System and Methods for RFID Tag Locating Using Constructive Interference,” filed Oct. 23, 2014. The disclosure of U.S. Provisional Application No. 62/067,736 is herein incorporated by reference in its entirety and for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates generally to the wireless communications field, and more specifically to new and useful systems and methods for using constructive interference such as use in radio frequency identification tag (RFID) tag locating. 
       BACKGROUND 
       [0003]    Being able to identify and track objects as they move throughout buildings or other indoor areas with high precision is important for a wide variety of applications. Systems designed to track objects in this way, often called real-time locating systems (RTLS), find use in manufacturing, warehousing, retail inventory management, and medicine, to name a few areas. Unfortunately, current methods of tag locating used by RTLS are frequently associated with cost and/or usability issues. Thus, there is the need in the wireless communications field to create systems and methods for RFID tag locating. This invention provides such new and useful systems and methods. 
       SUMMARY 
       [0004]    The present disclosure relates to a system for radio-frequency identification tag locating and associated methods. 
         [0005]    In accordance with an aspect disclosed herein, there is set forth a method for locating a radio-frequency identification tag, comprising analyzing first and second response signals received via a plurality of antennas from the radio-frequency identification tag and determining a position of the radio-frequency identification tag based upon said analyzing. In some embodiments, the determining can be accomplished through triangulation. 
         [0006]    In some embodiments of the method, the analyzing further comprises transmitting an initial activation signal from one of the plurality of antennas dispersed in a predefined area; and receiving a first response signal from the radio-frequency identification tag via the plurality of antennas. 
         [0007]    In some embodiments of the method, said transmitting comprises transmitting via the selected antenna selected from among the plurality of antennas. 
         [0008]    In some embodiments, said transmitting comprises transmitting via the selected antenna selected from among the plurality of antennas being dispersed in a predefined geographic area. 
         [0009]    In some embodiments of the method, said analyzing comprises altering a transmission signal property of the initial activation signal, transmitting a secondary activation signal with the altered property; and receiving a second response signal from the radio frequency identification tag via the plurality of the antennas. 
         [0010]    In some embodiments of the method, said receiving of the second response signal is conducted via the antennas. 
         [0011]    In some embodiments, said transmitting the secondary activation signal comprises broadcasting a plurality of sub-threshold radio-frequency identification signals to create a constructive interference pattern within the predefined area. 
         [0012]    In some embodiments, said transmitting comprises transmitting the plurality of sub-threshold radio-frequency identification signals via separate antennas. 
         [0013]    In some embodiments of the method, said altering of the transmission signal property of the initial activation signal includes adjusting a power level of the secondary activation signal such that only a selected radio-frequency identification tag located in an area of constructive interference will transmit the second response signal. The method further comprises altering at least one of antenna power and a phase of the secondary activation signal to produce a second, but partially overlapping, area of constructive interference. 
         [0014]    In some embodiments of the method, said receiving includes: receiving a radio signal strength indication from a plurality of radio-frequency identification tag; and identifying only a selected tag located in a first area of constructive interference due to a strength of the received radio signal strength indication. 
         [0015]    In some embodiments of the method, said receiving includes: calculating a read probability for a selected tag to respond when queried; and using the calculated read probability to locate the selected tag. 
         [0016]    In some embodiments, the method further comprises mapping a response pattern for the predefined area that compensates for environmental and structural interference. 
         [0017]    In some embodiments, said mapping includes traversing the predefined area with a robot with an attached radio-frequency identification tag, transmitting a plurality of calibration signals via the plurality of antennas at respective power levels and phases, and comparing a predetermined location of the robot with a triangulated position of the attached radio-frequency identification tag. 
         [0018]    In some embodiments, the mapping further comprises comparing the predetermined location of the robot to a predicted constructive interference zone, comparing constructive interference data to predicted interference data, and adjusting at least one of signal power or phase of the secondary activation signal until the constructive interference data matches the predicted interference data. 
         [0019]    In some embodiments, the method further comprises disposing a calibration radio-frequency identification tag on a person; determining a location of the person as the person traverses the predetermined area; comparing the location of the person to a predicted constructive interference zone; comparing constructive interference data to predicted interference data; and adjusting at least one of signal power or phase of the secondary activation signal until the constructive interference data matches the predicted interference data. 
         [0020]    In some embodiments, the location of the person is determined using a camera. 
         [0021]    In some embodiments, the camera can be any one of an RGB camera, a monochrome visible light camera, a 3-dimensional camera, an infrared camera, and an ultra violet camera. 
         [0022]    In some embodiments, the method further comprises determining a volume occupied by a person and an associated object. 
         [0023]    In some embodiments, the volume occupied by a person and an associated object informs a targeted constructive interference pattern. 
         [0024]    In some embodiments, the camera can identify the presence of the radio-frequency identification tag or an object with a predefined volume. 
         [0025]    In some embodiments, the method disclosed is employed in combination with at least one of other tag locating techniques. In some embodiments, the other techniques can include at least one of time difference of arrival, frequency domain phase difference on arrival, received signal strength indication measurement, and read probability measurement. 
         [0026]    In some embodiments, the method includes receiving environmental data including at least one of air humidity, air temperature, an environmental noise level, and a signal indicating a presence of people or objects in the preselected area, and adjusting signal power or phase of the secondary activation signal based on the environmental data to generate a constructive interference pattern. Sensors can be used to determine the background environmental radiation level. 
         [0027]    In some embodiments, the method further comprises converting the second response signal from the radio-frequency identification tag from an analog signal into a digital signal in order to identify a radio-frequency identification tag number. 
         [0028]    In some embodiments of the method, said altering includes changing one or more of an antenna radio pattern, an antenna orientation, a signal transmission power level, a frequency of the activation signal, a phase of the activation signal, and a beam-width of the activation signal to modify a constructive interference patterns. 
         [0029]    In some embodiments, the method further comprises using historical radio-frequency identification tag location data to further refine the constructive interference pattern. 
         [0030]    In some embodiments, the method further comprises calculating a velocity of a moving radio-frequency identification tag; predicting a new location of the moving radio-frequency identification tag based on the velocity of the moving radio-frequency identification tag; and altering the phase of the secondary activation signal based on the new location of the radio-frequency identification tag. 
         [0031]    In accordance with an aspect disclosed herein, there is set forth a system for locating a radio-frequency identification tag, comprising a plurality of antennas dispersed in a predefined area; one or more radio-frequency identification tags dispersed within the predefined area; a radio-frequency transceiver in communication with said antennas; a phase modulator electrically coupled to the radio-frequency transceiver; and a system controller in communication with said transceiver and said phase modulator. 
         [0032]    In some embodiments, the system controller enables sub-threshold superposition response mapping to calculate the location of the radio-frequency tags within the predefined area. 
         [0033]    In some embodiments, the plurality of antennas can comprise any one of the following antenna types including a patch antenna, a reflected antenna, a wire antenna, a bow-tie antenna, an aperture antenna, a loop-inductor antenna, and a fractal antenna. 
         [0034]    In some embodiments, the plurality of antennas can comprise more than one type of antenna. In some embodiments, the plurality of antennas are connected directly to the radio-frequency transceiver. In some embodiments of the system, the plurality of antennas are connected to the radio-frequency transceiver through one or more antenna splitters. 
         [0035]    In some embodiments of the system, the plurality of antennas are capable of both transmission and reception of signals from the radio-frequency identification tags. In some other embodiments, the plurality of antennas are capable only of transmission or reception of signals. 
         [0036]    In some embodiments of the system, the radio-frequency transceiver is capable of transmitting and receiving signals in a 900 megahertz frequency band. 
         [0037]    In some embodiments, the radio frequency transceiver is capable of modulating a power level of a transmission signal. 
         [0038]    In some embodiments, the system controller can calculate a location of a radio-frequency identification tag from radio-frequency identification response data. 
         [0039]    In some embodiments, the system controller can store maps of constructive interference patterns of a predetermined location in a storage device. 
         [0040]    In accordance with an aspect disclosed herein there is set forth a method for mapping a response pattern for a predefined area compensating for environmental and structural interference, comprising traversing the predefined area with a robot with an attached radio-frequency tag; transmitting a plurality of calibration signals at respective power levels and phases via a plurality of antennas disposed in the predefined area; and comparing a predetermined location of the robot with a triangulated position of the attached radio-frequency identification tag. 
         [0041]    In some embodiments, the method further comprises comparing the predetermined location of the robot to a predicted constructive interference zone; comparing constructive interference data to predicted interference data; and adjusting at least one of signal power or phase of the secondary activation signal until the constructive interference data matches the predicted interference data. 
         [0042]    In some embodiments, the method further comprises disposing a calibration radio-frequency identification tag on a person; determining a location of the person as the person traverses the predetermined location; comparing the location of the person to a predicted constructive interference zone; comparing constructive interference data to predicted interference data; and adjusting at least one of signal power or phase of the secondary activation signal until the constructive interference data matches the predicted interference data. 
         [0043]    In some embodiments, the method further comprises saving constructive interference data to a database. 
         [0044]    In accordance with an aspect disclosed herein, there is set forth a computer implemented method suitable for implementation on a processor comprising analyzing first and second response signals via a plurality of antennas from the radio-frequency identification tag; and triangulating a position of the radio-frequency identification tag based upon said analyzing, wherein said analyzing and triangulating are performed by a processor. 
         [0045]    In some methods, said analyzing further comprises transmitting an initial activation signal from one of a plurality of antennas dispersed in a predefined area; receiving a first response signal from the radio-frequency identification tag via the plurality of the antennas. 
         [0046]    In some embodiments of the method, said analyzing further includes altering a transmission signal property of the initial activation signal; transmitting a secondary activation signal with the altered property; and receiving a secondary response signal from the radio frequency identification tag via the plurality of the antennas. 
         [0047]    In some embodiments of the method, said transmitting comprises transmitting the plurality of sub-threshold radio-frequency identification signals to create a constructive interference pattern within the predefined area. 
         [0048]    In some embodiments, said transmitting comprises transmitting the plurality of sub-threshold radio-frequency identification signals via separate antennas. 
         [0049]    In some embodiments, said altering includes adjusting a power level of the secondary activation signal such that only the selected radio-frequency identification tag located in an area of constructive interference will transmit the second response signal; and altering at least one of antenna power and a phase of secondary activation signal to produce a second, but partially overlapping, area of constructive interference. 
         [0050]    In some embodiments, the method further comprises mapping a response pattern for the predefined area that compensates for environmental and structural interference. 
         [0051]    In some embodiments, said mapping includes traversing the predefined area with a robot with an attached radio-frequency tag; transmitting a plurality of calibration signals via the plurality of antennas at respective power levels and phases; and comparing a predetermined location of the robot with a triangulated position of the attached radio-frequency identification tag. 
         [0052]    In some embodiments, the method further comprises comparing the predetermined location of the robot to a predicted constructive interference zone; comparing constructive interference data to predicted interference data; and adjusting at least one of signal power or phase until the constructive interference data matches the predicted interference data. 
         [0053]    In some embodiments, the method further comprises disposing a calibration radio-frequency identification tag on a person; determining a location of the person as the person traverses the predetermined area; comparing the location of the person to a predicted constructive interference zone; comparing constructive interference data to predicted interference data; and adjusting at least one of signal power or phase of the secondary activation signal until the constructive interference data matches the predicted interference data. 
         [0054]    In some embodiments of the method, the location of the person is achieved using a camera. 
         [0055]    In some embodiments of the methods, the camera can be any one of an RGB camera, a monochrome visible light camera, a 3-dimensional camera, an infrared camera, and an ultra violet camera. 
         [0056]    One embodiment of the method, further comprises determining a volume occupied by a person and an associated object. In some embodiments, the volume occupied by a person and an associated object informs a targeted constructive interference pattern. In some embodiments of the method, the camera can identify the presence of the radio-frequency identification tag or an object with a predefined volume. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0057]      FIG. 1  is a diagram view of a system of a preferred embodiment. 
           [0058]      FIG. 2  is a diagram view of a prior-art RSSI locating technique. 
           [0059]      FIG. 3A  is an example view of constructive interference patterns generated by a system of a preferred embodiment. 
           [0060]      FIG. 3B  is another example view of constructive interference pattern generated by a system of the preferred embodiment. 
           [0061]      FIG. 4A  is an example view of constructive interference patterns generated by a system of a preferred embodiment. 
           [0062]      FIG. 4B  is another example view of constructive interference patterns generated by a system of the preferred embodiment. 
           [0063]      FIG. 5  is a diagram view of a system of a preferred embodiment. 
           [0064]      FIG. 6  is an example view of constructive interference patterns generated by a system of a preferred embodiment. 
           [0065]      FIG. 7  is a chart view of a method of a preferred embodiment. 
       
    
    
       [0066]    It should be noted that the figures are not drawn to scale and that elements of similar structures or functions are generally represented by like reference numerals for illustrative purposes throughout the figures. It also should be noted that the figures are only intended to facilitate the description of the preferred embodiments. The figures do not illustrate every aspect of the described embodiments and do not limit the scope of the present disclosure. 
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0067]    The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use this invention. 
       1. RFID Tag Locating System 
       [0068]    As shown in  FIG. 1 , a radio-frequency identification (RFID) tag locating system  100  includes a plurality of antennas  110 , an RFID transceiver  120 , a phase modulator  130 , and a system controller  140 . The system  100  may additionally include one or more reference RFID tags  150 . 
         [0069]    The system  100  functions to locate RFID tags within a three-dimensional volume of interest (or a two-dimensional plane of interest). The system  100  preferably determines tag location across time in order to track changes in tag location and/or tag movement. The system  100  is preferably designed and used to locate ultra-high frequency (UHF) passive RFID tags, but may additionally or alternatively be designed and used to locate passive RFID tags operating on any frequency spectrum. Additionally or alternatively, the system  100  may also be used with active RFID tags or any other suitable devices capable of responding selectively based on received RF signal power. 
         [0070]    Traditional RFID tag locating systems use one of several methods for tag location, including time difference of arrival (TDOA), phase difference of arrival (PDOA), and received signal strength indication (RSSI) measurement. All three of these methods can locate tags using trilateration. 
         [0071]    In the case of TDOA, a signal is sent to an RFID tag from one of three antennas. The tag receives the signal and transmits a signal in response. The response signal is then received at all three of the antennas at different times. The time between original signal transmission and reception of the response signal at each antenna can be used to determine the distance from the tag to each antenna, which can then be used to locate the RFID tag (relative to the antennas) using trilateration. The TDOA method is not typically used for UHF RFID tags simply because typical time differences are very small (and bandwidth available is narrow). 
         [0072]    There are several types of PDOA, including frequency domain PDOA (FD-PDOA). In FD-PDOA, a signal is sent to a tag from one of three antennas at a first frequency; the tag responds with a first response signal. Then the same antenna sends a signal at a second frequency (preferably close to the first frequency), and the tag responds with a second response signal. The phase difference between the first response signal and the second response signal (as measured at the first antenna) can give a distance from the tag to the first antenna. This process can be repeated for the other two antennas, producing three distances, which can be used to locate the tag using trilateration. 
         [0073]    In the case of RSSI measurement, as shown in  FIG. 2 , a signal is sent to an RFID tag from one (or more) of three antennas. The tag receives the signal and transmits a signal in response. The response signal is then received at all three of the antennas, each recording a different received signal strength (e.g., RSSI). The RSSI is used to estimate distance from each antenna, which can then be used to locate the tag relative to the antennas using trilateration. Since RSSI does not typically correspond well to distance, this method may suffer from accuracy issues. 
         [0074]    Another method for locating RFID tags is known as read probability measurement, described in U.S. Provisional Patent Application No. 61/928,303, which is incorporated in its entirety by this reference. To briefly summarize, read probability measurement takes advantage of RFID tag power-on thresholds (that is, the minimum amount of power a passive RFID tag must receive in order to transmit a readable response signal). The antennas modulate transmission power and record whether the tag responds or not at each transmission power. A number of these transmissions are used together to calculate a read probability (the probability that a tag will be read versus transmission power). By comparing this to an estimate or analysis of how transmission signal power changes with distance (and potentially direction) for each transmission power, a distance from each antenna can be determined, and trilateration can be performed. 
         [0075]    The system  100  preferably locates RFID tags using a method henceforth referred to as sub-threshold superposition response mapping (STSRM). This technique may be used independently of the method of RFID tag locating described previously, but may additionally or alternatively be used in conjunction with those methods. 
         [0076]    As with previously described methods, STSRM (described in more detail in the description of the method  200 ) also involves the use of multiple antennas; however, the locating process is very different from previously described methods. In STSRM, multiple antennas transmit a signal at the same time. The signals transmitted by the antennas interfere with each other, creating areas of constructive interference and areas of destructive interference. Based on the relative location of antennas, signal properties of the signal emitted by each antenna (e.g., phase, polarization, beam width, etc.), and the environment (e.g., obstacles in between antennas and tags), the interference pattern generated by signals can be predicted. The power of each antenna can be adjusted such that only areas of strong constructive interference (preferably a sparse pattern) have enough power to activate RFID tags; in other words, the individual signals are sub-activation-threshold in areas of interest. If an RFID tag is activated, it must then lie in one of these areas of constructive interference. After an RFID tag is located within a constructive interference area, the constructive interference pattern is then changed (by altering antenna power and phase) to produce a different, but partially overlapping, set of constructive interference points. This process of altering patterns may proceed until the RFID tag has been confined to a single location. 
         [0077]    In other embodiments, instead of using power level for threshold for activation of tags inside the target zone, received signal strength indication (RSSI) measurement can be used. Outside the constructive interference zone, there will be a marked decrease in RSSI. The varied RSSI signal levels can produce a steep gradient for tags located inside and outside the zone. This difference in RSSI can be used to eliminate tags located outside the target zone. For tags that are located on the boundary of a constructive interference zone, other techniques can be utilized to determine whether the tag is located inside or outside the zone. Examples of the constructive interference patterns produced during STSRM are as shown in  FIGS. 3A and 3B .  FIGS. 3A and 3B  include field contour plots, where field strength above a threshold is displayed as black and field strength below the threshold is displayed as white. In these examples, the plot threshold is chosen to be an RFID tag threshold; that is, RFID tags will only have enough power to respond to transmitted signals if they are in black areas. The examples shown in  FIG. 3A  and  FIG. 3B  are identical except for transmission power; P2&lt;P1. Because P2 is lower, fewer points in the area of interest are super-threshold (and thus potential locating resolution is higher). More examples are as shown in  FIGS. 4A and 4B ; in these examples, the transmission power is the same between  FIGS. 4A and 4B , but the relative phase of antennas is different. Note that all of these examples assume a uniform transmission media (e.g., air) and no reflections; these factors are often important in determining real-world constructive interference patterns. 
         [0078]    The locating process as described above discusses localization using only STSRM, but the technique may additionally or alternatively be used with other techniques to narrow the search field (i.e., how many patterns must be tested) or for other purposes. For example, read probability measurement may place an RFID tag within a 1×1 meter area with 95% accuracy, and STSRM could be used to further narrow down location within this area. 
         [0079]    The nature of constructive interference is that with multiple signals traveling in different directions, effects can be highly localized (on order of signal wavelength). Further, altering of phase can displace interference peaks by magnitudes substantially smaller than wavelength, meaning that STSRM is capable of achieving very high accuracy in locating RFID tags. 
         [0080]    The system  100  preferably enables the use of STSRM techniques to locate RFID tags; additionally or alternatively, the system.  100  may enable the use of other tag locating techniques in combination with or complementary to STSRM techniques. 
         [0081]    The antennas  110  function enable the system  100  to transmit signals to RFID tags and receive signals from the RFID tags. The antennas  110  convert conducted electric power into RF waves and/or vice versa, enabling the transmission and/or reception of RF communication. The antennas  110  are preferably made out of a conductive material (e.g. metal). The antennas  110  may additionally or alternatively include dielectric materials to modify the properties of the antennas  110  or to provide mechanical support. 
         [0082]    The antennas  110  may be of a variety of antenna types; for example, patch antennas (including rectangular and planar inverted F), reflector antennas, wire antennas (including dipole antennas), bow-tie antennas, aperture antennas, loop-inductor antennas, and fractal antennas. The plurality of antennas  110  can additionally include one or more type of antennas, and the types of antennas can include any suitable variations. 
         [0083]    The antenna  110  structure may be static or dynamic (e.g. a wire antenna that includes multiple sections that may be electrically connected or isolated depending on the state of the antenna). 
         [0084]    Antennas  110  may have isotropic or anisotropic radiation patterns (i.e., the antennas may be directional). If antennas  110  are directional, their radiation pattern may be dynamically alterable; for example, an antenna  110  substantially emitting radiation in one direction may be rotated so as to change the direction of radiation. 
         [0085]    The plurality of antennas  110  are preferably connected directly to RFID transceivers  120  with conductive wires, but may additionally or alternatively be connected to transceivers through any suitable method. The antennas  110  may be connected directly to RFID transceivers  120 , or may be connected RFID transceivers  120  through one or more antenna splitters. 
         [0086]    The system  100  preferably includes at least three antennas  110 , so as to be able to perform trilateration, but the system may additionally include any suitable number of antennas. In one implementation of the system  100 , the system  100  includes a rectangular grid of antennas  110 . Other embodiments can selectively assign antennas to various roles. In one embodiment a fixed number of antennas can be tasked with targeting a particular zone, while other antennas can be assigned to reducing secondary effects interference from other power zones which can occur some distance away from the targeted zone. 
         [0087]    The antennas  110  of the system  100  are preferably used both for transmission of signals to and reception of signals from RFID tags, but may additionally or alternatively antennas may be used only for transmission or only for reception. 
         [0088]    Antennas  110  are preferably located as to provide coverage for a particular indoor area. For example, antennas  110  might be oriented in a rectangle on the ceiling of a store in order to locate RFID tags contained within the rectangle. In this particular implementation, of the two solutions produced by trilateration, only one would be valid (the assumption being that no RFID tags are present above the ceiling). 
         [0089]    The RFID transceiver  120  functions to produce signals for transmission by the antennas  110 , as well as to analyze signals received by the antennas  110  from RFID tags. In one embodiment, the RFID transceiver preferably includes an RF transmitter capable of sending signals in the 900 MHz band and an RF receiver capable of receiving signals in the 900 MHz band, but may additionally or alternatively be any suitable transceiver capable of communicating with RFID tags. The 900 MHz band supports 902-928 MHz in North America. Alternatively the transmitter can operate in the 800 MHz band. The 800 MHz band supports 865-968 MHz in Europe. Alternatively, the transceiver can operate in the industrial, scientific and medical (ISM) radio band from 2.4-2.485 (Bluetooth Band), 2.4 gigahertz (12 cm) UHF and 5 gigahertz (6 cm) SHF ISM radio bands, 3.1-10 GHz (microwave band), and other UHF RFID tag emitter bands in use or later developed. 
         [0090]    The RFID transceiver  120  is preferably coupled directly to the antennas  110 , but may additionally be coupled to the antennas  110  through an antenna splitter or through any other components. 
         [0091]    The RFID transceiver  120  is preferably controlled by the system controller  140 , but may additionally or alternatively be controlled by any other component of the system  100 . The RFID transceiver  120  is preferably capable of modulating power to the antennas  110 , additionally or alternatively, power modulation may be accomplished by a device external to the RFID transceiver  120  (e.g., an active splitter). 
         [0092]    The phase modulator  130  functions to change the phase of the signal output by one or more antennas  110 . Changing the phase of any one of the antennas  110  has the effect of changing the far-field interference pattern (and thus the areas that RFID tags may be activated in). The phase modulator  130  is preferably part of the RFID transceiver  120 , but may additionally or alternatively be a component independent of the RFID transceiver  120 . 
         [0093]    If the phase modulator  130  is part of the RFID transceiver  120  and each antenna  110  (or antenna array) is connected to the RFID transceiver  120  individually (as shown in  FIG. 1 ), the phase modulator  130  preferably changes phase simply by modifying the digital signal intended for a particular antenna. For example, the carrier wave of an RF signal transmitted by an antenna  110  might have the form of cos [ωt+φ], where φ represents an alterable phase shift. The phase modulator  130  may simply adjust the value of φ to provide the signal with a particular phase. 
         [0094]    If the phase modulator  130  is part of or after an antenna splitter, as shown in  FIG. 5 , or otherwise operates on the analog signals intended for the antennas  110  (as opposed to the previous example, where the phase modulator  130  operates in the digital domain), the phase modulator  130  may consist of variable delay circuits connected to the antennas  110 . Additionally or alternatively, the phase modulator  130  may comprise any digital or analog circuit or component capable of altering the phase of the transmitted signals of one or more antennas  110 . 
         [0095]    The system controller  140  functions to control the output of the RFID transceiver  120  and the phase modulator  130 , as well as to process the signals received by the RFID transceiver  120 . The system controller  140  includes a microprocessor; the system controller  140  may be integrated with the RFID transceiver  120  and phase modulator  130 , but may additionally or alternatively be separate of one or both of the RFID transceiver and phase modulator  130 . 
         [0096]    The system controller  140  enables the system  100  to transform RFID response data into a location for an RFID tag. The system controller  140  preferably accomplishes this transformation by using a mapping of constructive interference patterns to physical locations to estimate the coordinates at which signal power rises above some activation threshold. This process is described in more detail in the sections on the method  200 . 
         [0097]    The system controller  140  preferably includes a processor and storage for the above-mentioned maps, but may additionally or alternatively store map data and configuration data in any suitable location (e.g., cloud-based servers). 
         [0098]    The system controller  140  preferably performs this transformation using stored maps. The system controller  140  may additionally or alternatively generate maps in real-time. These maps preferably allow the system controller to determine super-threshold areas of constructive interference based on transmission variables; for example, the location of antennas  110 , the angle of orientation of antennas  110 , the radiation pattern of antennas  110 , the phase, frequency, polarization, and power of signals transmitted by the RFID transceiver  120  (via the antennas  110 ), or any other applicable data. The maps may additionally or alternatively vary based on environmental variables, for example, the number of people within the area of interest. 
         [0099]    Constructive interference patterns may be strongly dependent on environment. For example, a change in positioning of shelves in a store might cause larges changes in the constructive interference pattern generated given a certain set of transmission parameters. For this reason, it may be helpful for the system controller  140  to have a calibration reference; for example, data defining how constructive interference patterns for a particular area. 
         [0100]    The calibration references may be static; for instance, the calibration references may be formed by a robot with an RFID tag traversing an area; the robot maps out the area while the system  100  outputs one or more constructive interference patterns. The system  100  outputs constructive interference patterns by transmitting a signal from one or more antennas  110  at particular transmission powers and phases. The robot map may be, through time synchronization, matched up to points of RFID tag activation, this data set is then compared to constructive interference data predicted by the system controller  140 . The system controller  140  may then adjust transmission variables (e.g., by adjusting transmission variable inputs to a prediction engine until prediction matches reality, or by adjusting actual transmission variable inputs until the robot output matches predictions). Static calibration processes may be performed in real-time with data gathering (e.g., as the robot moves around) or at a later time (using previously corrected data). 
         [0101]    Similarly, a robot may be used to map read probabilities for various locations within an area. For example, a robot may be used to map out an area while the system  100  outputs signals from one antenna (or serially from multiple antennas). The robot map, through time synchronization, may be matched up to points of RFID tag activation; this data may then be used to calculate read probabilities as a function of position in the area. As in the previous process, the system controller  140  may adjust transmission variables to match predictions to reality or vice versa. Read probability calibration processes may be performed in real-time with data gathering (e.g., as the robot moves around) or at a later time (using previously corrected data). 
         [0102]    The calibration references may additionally or alternatively be dynamic. In one example, the system  100  includes RFID reference tags  150  placed in known locations. These may be used to calibrate or recalibrate the system controller  140  mapping at any time. This allows the system  100  to be recalibrated easily when environmental factors (e.g., positioning of RF-signal-affecting objects, etc.) change. The system  100  preferably calibrates with references by predicting patterns that would activate particular reference tags, testing those patterns, and refining the patterns based on response or non-response. 
         [0103]    Calibration may additionally or alternatively be performed with the aid of non-STSRM techniques. For example, persons in a particular area may carry RFID tags that identify them. If the position of the persons can be located with precision (e.g., by a camera, or by another method, such as detecting wireless transmissions from their cellphone), the RFID tags they carry could be used to calibrate the system  100 . Cameras or other locating methods may additionally or alternatively be used at any point in order to refine or calibrate STSRM location data or the system  100 . 
         [0104]    For example, a camera (e.g., RGB camera, monochrome visible light camera, 3D camera, depth camera, infrared camera, or an ultra violet sensor etc.) could be used to recognize a person (either generically as a person, or as a particular person, using face recognition software, gait analysis, or another suitable technique). The camera may additionally or alternatively be used to calculate the volume occupied by the person and associated objects (e.g., a shopping cart). The location and volume occupied by the person and/or associated objects could be used to inform a particular constructive interference pattern; for example, to query RFID tags of objects contained within the person&#39;s bag or shopping cart. This could be used to determine particular items a person is carrying. Additionally or alternatively, location information, recognition data, visual data, or any other suitable camera data may be used in combination with STSRM data in any suitable manner in order to provide further information about the presence of RFID tags (or other objects) within a particular volume. 
         [0105]    The system controller  140  may additionally or alternatively use the antennas  110  to perform calibration; for example, the system controller  140  may transmit a signal at a first antenna  110  and receive it at a second antenna  110 . Because the relative locations of the antennas  110  are preferably known, the signal can be used to determine delay or phase shift due to environmental factors in the signal path. This information can be used to refine constructive interference pattern maps. 
         [0106]    In addition to controlling the calibration process, the system controller  140  preferably controls the transmissions used for RFID tag location. The system controller  140  preferably adjusts phase and transmission power to locate RFID tags in a small number of iterations (e.g., by optimizing for a minimum number of iterations given rough knowledge about the position of a tag). For example, the system controller  140  may know from a previous search that a tag is located in a particular area. If analysis of historical data suggests that the tag is likely to be in the same area, the system controller  140  may attempt to isolate the search to this area before trying other areas. The system controller  140  storage may analyze historical data related to tag location in a number of ways. Historical data preferably includes historical environmental data, historical absolute location data (e.g., the tag&#39;s location in coordinate space), historical relative location data (e.g., the tag&#39;s location relative to other tags or other references), behavioral data (e.g., the tag is likely to be in the middle of the area during the afternoon, but near the left edge during the evening), or any other suitable data. 
         [0107]    The system controller  140  preferably alters phase and transmission power of antennas  110  by controlling RF transceivers  120  and phase modulators  130 , but may additionally or alternatively alter antenna phase and transmission power in any suitable manner. 
         [0108]    The system controller  140  may locate RFID tags using only the STSRM method, but may additionally or alternatively locate RFID tags using a combination of methods; for instance, RSSI may be used to roughly locate RFID tags, and then STSRM may be used to locate RFID tags with higher resolution. If the system  100  performs multiple methods of tag locating, all methods are preferably directed by the system controller  140 , but the system controller  140  may alternatively direct only a subset of locating methods. 
         [0109]    The reference RFID tags  150  function to provide a calibration reference to the system  100 . The reference RFID tags are preferably substantially similar to the RFID tags located by the system  100 , but may additionally or alternatively any suitable type of RFID tag. The RFID tags preferably have a known tag identifier (i.e., the signal transmitted by the tag when interrogated) and a known position. Thus, when reference RFID tags  150  transmit, the system controller  140  can infer that the activation signal was above-threshold at the location of transmitting reference RFID tags. 
         [0110]    Reference RFID tags are preferably associated with a location that is static relative to the antennas  110 , but may additionally or alternatively be associated with a location in a different coordinate space. For example, reference RFID tags may be located with GPS coordinates, or with some particular object (e.g., a moveable cart may contain a reference RFID tag so that positions may be determined relative to that cart). 
         [0111]    Sub-threshold superposition response mapping (STSRM) techniques are not limited to the use of UHF radio frequency radiation. STRM techniques can be applied using ultrasound radiation. Ultrasound devices operate with frequencies from 20 kHz up to several gigahertz. Sound vibration can form constructive interference patterns similar to ultrahigh frequency radiation and STSRM techniques can be applied for the selective transmission of sound waves. 
         [0112]    In addition to using constructive interference mapping techniques for the locating of RFID tags, these methods could be used for other targeted transmission and receipt of energy. Such applications include, the targeted transmission of radiation resulting in constructive interference zones for the targeted transmission of energy for specific areas. This could be used for selected areas for transmitting radiation for wireless, remote recharging of portable electronic devices. In this way, concentration of the radiation to selective areas would reduce the harmful effects of radiation on humans with isometric radiation. 
         [0113]    Other uses included the selected targeting of areas for concentrated bandwidth distribution. In this way some areas would have higher bandwidth capabilities in these constructive interference zones than outside the constructive interference zones. In zones outside the target zone, the data rate would be significantly reduced. 
         [0114]    As shown in  FIG. 7 , a method  200  for sub-threshold superposition response mapping (STSRM) preferably includes transmitting a plurality of sub-threshold RFID activation signals from separate antennas S 210 , receiving a response signal from an RFID tag S 220 , altering transmission signal properties S 230 , receiving an additional response signal from the RFID tag S 240 , and calculating the RFID tag position S 250 . The method  200  may additionally include calibrating interference mapping S 260 . 
         [0115]    The method  200  functions to locate RFID tags within a specific volume (bounded by antenna range). The method  200  preferably results in a more accurate location estimate than from typical methods (e.g., TDOA, PDOA, etc.). 
         [0116]    Step S 210  includes transmitting a plurality of sub-threshold RFID activation signals from separate antennas. Step S 210  functions to create a constructive interference pattern within an area defined by the transmitting antenna range. The constructive interference pattern is a function of antenna and signal properties including antenna radiation pattern, antenna orientation, antenna type, transmission power, frequency, phase, beam-width, and other factors. 
         [0117]    The locations of the antennas are preferably known relative to each other. Antennas may additionally or alternatively be referenced to any coordinate frame of reference. 
         [0118]    The transmission power and relative phase of activation signals are preferably set based on an estimated constructive interference pattern, but may additionally or alternatively be based on any suitable instructions or data. The transmission power and relative phase of activation signals are preferably set such that only a small subset of the area covered by antenna range results in super-threshold signal power; that is, most of the area covered by antenna range does not have enough constructive interference to, activate an RFID tag. 
         [0119]    The particular power and phase settings chosen for each signal are preferably informed by historical data; that is, the interference pattern generated by Step S 210  is preferably intended to activate tags in a particular subset of in-range area where the tags are assumed to be. Additionally or alternatively, the power and phase settings chosen by Step S 210  may result from explicit settings (e.g., the first activation signals always have a relative phase of zero and a transmission power of 100 dBm), other data (e.g., data from other locating methods), or any other suitable instructions. 
         [0120]    Step S 210  may additionally or alternatively include receiving environmental data (e.g., humidity, presence of people or objects, temperature, environmental RF noise, etc.) or previous mapping information (e.g., a mapping of particular transmission settings to a constructive interference pattern). This data may be used to inform the transmission settings in order to more accurately generate particular constructive interference patterns. Previous mapping information or other calibration information preferably results from Step S 260 , but may additionally or alternatively come from any suitable source. 
         [0121]    Step S 220  includes receiving a response signal from an RFID tag. Step S 220  functions to provide data that can be used to generate information about the RFID tag&#39;s location. Based on the transmission settings of Step S 210  and the predicted mapping of signal strength (taking into account constructive interference), the location of the RFID tag may be confined to a set of points (or small areas) of constructive interference. Note that Step  210  may need to be iterated multiple times at different transmission settings before receiving a response signal from a particular RFID tag. 
         [0122]    Step S 220  preferably includes receiving an analog signal over one or more antennas; these antennas are preferably the same antennas used to transmit signal in Step S 210 , but may additionally or alternatively be any suitable antennas. This analog signal is preferably converted to a digital signal and analyzed to provide the locating system with the RFID tag ID. Additionally or alternatively, if the tag identifier is not important to a particular application, the signal may not be converted (e.g., an application that only cares about locating any tag, not a specific tag). 
         [0123]    Step S 230  includes altering transmission signal properties. Step S 230  functions to change the constructive interference pattern used to enable RFID tag responses. Step S 230  may occur after Step S 210  (if a desired tag is not located) or after Step S 220  (to refine the location of a particular tag). 
         [0124]    Step S 230  preferably includes altering one or more of antenna radiation pattern, antenna orientation, signal transmission power, frequency, phase, and beam-width in order to alter constructive interference patterns. 
         [0125]    The alterations made by Step S 230  preferably are informed by existing data or estimates pertaining to an RFID tag&#39;s location; additionally or alternatively, alterations may be made according to a static instruction set or in any other suitable manner. For example, if analysis of data from Step S 220  identifies an RFID tag as occupying a location in the first quadrant of a square area (i.e., x&gt;0 and y&gt;0) or in the third quadrant (x&lt;0, y&lt;0), and historical data suggests that the RFID tag is much more likely to be in the first quadrant, the alterations made by Step S 230  may produce an interference pattern more likely to provide location information on a tag located in the first quadrant. 
         [0126]    As a specific example of data pertaining to RFID tag location, the alterations made by Step S 230  are preferably informed by the results of previous alterations. For example, as shown in  FIG. 6 , a first pattern may be generated by Step S 210 , resulting in tag detection in Step S 220 . Step S 230  alters the transmission signal to produce a second pattern, which results in no detection. Assuming that the tag did not move significantly between the generation of pattern 1 and pattern 2, the tag must be located in the area found by subtracting pattern 2 from pattern 1. In this example, Step S 230  might then be run again, with the third pattern calculated to give more information about where the tag might be located within the area defined by the removal of pattern 2 area from pattern 1. While this example includes a detection and a non-detection, the same principles apply to two detections in a row. For example, if an RFID tag were detected in both pattern 1 and pattern 2, the RFID tag would be located within the intersection of pattern 1 and pattern 2 (again assuming no substantial movement between responses). 
         [0127]    Preferably, tags read by the method  200  do not move significantly while being located; but if it is expected that tags will move significantly while being located, the method  200  may include detecting tag velocity and adjusting locating techniques appropriately (e.g., predicting where a tag will be based on previously measured velocity and attempting to locate it at the predicted location). Tag velocity may be detected in any number of ways, including by the steps previously mentioned. Altering antenna phase only slightly has an effect of essentially shifting the constructive interference pattern without substantially altering it; by shifting constructive interference patterns slightly tag velocity can be determined even if tag location is not definitely known. For example, if the method  200  confines tag location to a first set of points defined by a constructive interference pattern, generates a shifted pattern and detects the tag in a second set of points defined by a second constructive interference pattern, the average velocity of the tag between the generation of those two patterns falls into a bound set of solutions. By performing additional pattern generations and/or by including some assumptions (e.g., maximum velocity the tag can move at, direction of velocity, etc.) the tag velocity can be determined. 
         [0128]    Step S 240  includes receiving an additional response signal from the RFID tag. Step S 240  is preferably substantially similar to Step S 220 . The results of the second response signal are preferably used in determining RFID tag position; the results may additionally be used to direct Step S 230  (e.g., by identifying an area of interest to search in). 
         [0129]    Steps S 230  and Steps  240  are preferably iterated until RFID tag location has been suitably confined. In some cases, Steps S 230  and S 240  may be iterated a set number of times; for instance, there may be a set of constructive interference patterns that can, to a desired resolution, locate any tag within an area of interest (regardless of tag location within the area of interest) and Steps S 230  and S 240  may be iterated until this set has been completed. Additionally or alternatively, Steps S 230  and Steps S 240  are iterated along with an intermediate iteration of Step S 250 ; for example, after each iteration of Step S 230  and Step S 240 , Step S 250  uses the results to further confine tag location and to direct parameters of the next iteration of Step S 230 , the iteration cycle continuing until Step S 250  has suitably determined tag location (e.g., by reducing possible tag location area to an area below some threshold area). 
         [0130]    Step S 250  includes calculating the RFID tag position. Step S 250  functions to determine or estimate where RFID tags are located based on responses to particular interference patterns. Step S 250  is preferably iterated along with steps S 230  and S 240 , but may additionally or alternatively be performed only after several iterations of Steps S 230  and S 240  or at any suitable time. 
         [0131]    Step S 250  preferably calculates RFID tag position by correlating RFID tag response or non-response to locations defined by constructive interference patterns. Step S 250  preferably produces RFID tag position data from RFID tag response data and transmission parameter sets (e.g., whether a tag responded or not for a particular transmission parameter set) by generating a transmission power field estimate (or other distribution correlated to RFID response rates) based on the transmission parameter set. 
         [0132]    The mapping between transmission parameter sets and transmission power fields is preferably set by Step S 260 , but may additionally or alternatively be set in any suitable manner. As described in Step S 260 , the mapping may vary solely on transmission power and phase (i.e., all other transmission parameters, including antenna location, and environmental variables are considered static) or the mapping may vary based on additional variables. For example, the mapping algorithm might also vary based on the number of people known to be in a particular area (changing the permittivity of the area, and thus the interference pattern) or based on antenna direction, if antenna direction is variable. 
         [0133]    Step S 250  may additionally or alternatively include calculating RFID tag position based on a combination of multiple locating methods (e.g., by locating an RFID tag to a particular area using a read probability method and then locating the tag within that area using STSRM). 
         [0134]    Step S 260  includes calibrating interference mapping. Step S 260  functions to increase the accuracy of the mapping between antenna fields (specifically, super-threshold and sub-threshold areas) and location (relative to antennas or otherwise). 
         [0135]    Step S 260  preferably calibrates interference mapping by generating calibration references, which are then used to predict antenna fields (or a related metric, such as areas of super-threshold power). Calibration references may be pre-generated; for instance, calibration references may be formed by a robot with an RFID tag traversing an area; the robot maps out the area while one or more constructive interference patterns are generated by antennas. The robot map may be, through time synchronization, matched up to points of RFID tag activation, this data set is then compared to predicted data. Transmission parameters may then be adjusted (e.g., by adjusting transmission variable inputs to a prediction engine until prediction matches reality, or by adjusting actual transmission variable inputs until the robot output matches predictions). Pre-generated calibration references may be calculated in real-time with data gathering (e.g., as the robot moves around) or at a later time. 
         [0136]    The calibration references may additionally or alternatively be generated in real-time (during operation of the method  200 ). In one example, RFID reference tags are placed in known locations. These may be used to generate calibration references at any time. This allows for easy recalibration when environmental factors (e.g., positioning of RF-signal-affecting objects, etc.) change. 
         [0137]    These calibration references are preferably measured for a range of transmission parameters. In particular, calibration references preferably contain enough information to accurately calculate antenna fields for significantly varying transmission parameters (e.g., transmission phase and power from each antenna). In some cases, this may not mean actually measuring fields for wide ranges of all parameters. For instance, if the phase delay of signals is independent of transmission power for a particular environment, and the antenna transmission characteristics are well-known, an accurate calibration may not require many data points at different transmission powers. 
         [0138]    Predicted fields are preferably generated by a modeling of the constructive interference fields based on calibration data collected as part of the method  200 ; additionally, the modeling may also be based on additional data. In one example, antenna fields for a particular set of transmission parameters not exactly sampled as part of calibration data may be predicted by interpolating calibration data. In another example, calibration data is used to model the permittivity (vs. coordinates) of a volume of interest, which are then used to predict antenna fields at any transmission power and phase. 
         [0139]    Calibration may additionally or alternatively be performed with the aid of non-STSRM techniques. For example, persons in a particular area may carry RFID tags that identify them. If the position of the persons can be located with precision (e.g., by a camera, or by another method, such as detecting wireless transmissions from their cellphone), the RFID tags they carry could be used to generate calibration data. Cameras or other locating methods may additionally or alternatively be used at any point in the method  200  in order to refine or calibrate STSRM location data. 
         [0140]    Step S 260  may additionally include calibrating read probabilities by generating read probability references. Read probability references may be pre-generated; for instance, read probability references may be formed by a robot with an RFID tag traversing an area; the robot maps out the area while signals are output from one antenna (or serially from multiple antennas). The robot map, through time synchronization, may be matched up to points of RFID tag activation; this data may then be used to calculate read probabilities as a function of position in the area. As in the previous process, the transmission variables may be adjusted to match predictions to reality or vice versa. Read probability calibration processes may be performed in real-time with data gathering (e.g., as the robot moves around) or at a later time (using previously corrected data). 
         [0141]    The description of the method  200  above provides examples directed to locating particular tags or of tags in a sparse environment; that is, scenarios where some amount of search is required to find a tag. A person skilled in the art will recognize that the method  200  is also applicable to systems where a large number of tags are located in some area, and the locations of many or all of those tags are of interest. In examples applying to such a situation, constructive interference patterns may not be generated to find a particular tag, but rather to provide the locations of all tags within a certain area; the strategy to locate all tags in an area (e.g., what patterns are generated and in what order) may be significantly different than a strategy to locate a single tag. 
         [0142]    The method  200  is preferably performed by the system  100  but may additionally or alternatively be performed by any suitable system. 
         [0143]    The methods of the preferred embodiment and variations thereof can be embodied and/or implemented at least in part as a machine configured to receive a computer-readable medium storing computer-readable instructions. The instructions are preferably executed by computer-executable components preferably integrated with an RFID tag locating system. The computer-readable medium can be stored on any suitable computer-readable media such as RAMs, ROMs, flash memory, EEPROMs, optical devices (CD or DVD), hard drives, floppy drives, or any suitable device. The computer-executable component is preferably a general or application specific processor, but any suitable dedicated hardware or hardware/firmware combination device can alternatively or additionally execute the instructions. 
         [0144]    The disclosed embodiments are susceptible to various modifications and alternative forms, and specific examples thereof have been shown by way of example in the drawings and herein described in detail. It should be understood, however, that the disclosed embodiments are not meant to be limited to the particular forms or methods disclosed, but to the contrary, the disclosed embodiments are to cover all modifications, equivalents, and alternatives.