Selection of the Best Sensor / Measurement Method for RFID Tag Location

Abstract

Radio frequency identification (RFID) systems use readers to query and locate passive RFID tags in stores, warehouses, and other environments. Signals from the reader power the tags, which modulate and backscatter signals toward the reader. A reader, or an appliance connected to a reader, can estimate the tag's location based on the angle of arrival (AOA) of the backscattered signal. In some situations, AOA measurements from different readers may yield different location estimates for the same tag. If these location estimates are sufficiently close to each other (e.g., within a reasonable margin of error or radius of inaccuracy), they can be averaged to improve accuracy. Otherwise, the appliance can measure the variance or another measure of variance for each reader's location estimate and then select the reader with the lowest variance as the preferred or best sensor for locating that tag, thereby improving accuracy and reducing processing time.

Description

Technical Field

[0001] Cross - reference to Related Applications This application claims the benefit of priority of U.S. Patent Application No. 63 / 490,834, filed on March 17, 2023, under 35 U.S.C. § 119(e), the entire disclosure of which is hereby incorporated by reference in its entirety for all purposes.

Background Art

[0002] Radio Frequency Identification (RFID) tags, or simply tags, can be attached to objects and are low - cost devices that provide expectations for the automatic tracking, positioning, sales check - out, and inventory counting of objects, among other commercial, industrial, and medical applications. There are passive, semi - active, and active RFID tags that can be wirelessly interrogated by an RFID tag reader (also called a reader or sensor) and can radiate a wireless RF response back to the reader. Each response can include information stored on the RFID tag, such as a tag identification number, an Electronic Product Code (EPC), or other unique alphanumeric sequence or identifier. Other information may also be included in the response.

[0003] Passive RFID tags do not have batteries and are therefore typically less expensive than semi-active and active RFID tags. Passive RFID tags are powered by an unmodulated continuous-wave RF signal from an RFID tag reader. This continuous-wave RF signal powers the passive RFID tag's circuitry and precedes queries or commands from the RFID tag reader in the form of modulated RF signals. The passive RFID tag receives and demodulates the modulated RF signal and responds to the RFID tag reader by modulating and backscattering a portion of the modulated RF signal. This modulated backscattered RF signal is the passive RFID tag's response to the query or command and is detected by the RFID tag reader. The response from a passive RFID tag is typically several orders of magnitude weaker than the RF signal from the RFID tag reader. This makes it more difficult to detect the response over longer ranges, limiting the range of passive RFID tags.

[0004] Each cycle in which a continuous-wave (CW) RF signal and a query or command are sent to a tag and a response from the tag is received is called a hop. The CW RF signal and query or command for each hop can be on different carrier frequencies; that is, the sensor can hop between carrier frequencies within a specific frequency band when querying different tags. Many tags can respond during a single hop, but because the FCC regulates the maximum duration of a hop, if the tag population is too large to read within a single hop, the sensor may periodically repeat hops until all tags in range have been read. The sensor may periodically continue querying tags in range to monitor the inventory of items to which the tags are attached.

[0005] Sensors can be used to estimate the location of a tag in three dimensions using one of several techniques. For example, each sensor may detect the amplitude or power of the tag's response, in addition to a unique modulation (e.g., EPC) that identifies which tag is responding to the query. In other words, each sensor can record a Received Signal Strength Indicator (RSSI) for each tag within range. If the sensor has an antenna array that can sense the angle of arrival (AOA) of the tag's response, the sensor can record the AOA in addition to, or instead of, the RSSI. A computer connected to the sensor can use the measured RSSI and / or AOA to estimate the tag's location. [Overview of the Initiative]

[0006] In practice, due to attenuation, dispersion, multipath propagation, manual errors in sensor placement, and other factors, a sensor or sensor cluster may identify multiple possible locations or places for a given RFID tag. In some cases, the location estimates are within an acceptable margin of error and can be averaged to improve location accuracy. However, in other cases, the location estimates are so far apart that they are considered different locations. In these cases, it may not be possible to determine which location estimate is correct without some additional information about the tag, sensor(s), or environment.

[0007] Fortunately, RFID systems can derive this additional information from the response itself, as well as other prior knowledge, to select the preferred or best sensor or sensor set for estimating the location of each RFID tag in the environment. The RFID system can select a different sensor or sensor set for each RFID tag purely on the indices derived from the detected response.

[0008] Selecting the best sensor or sensor set for estimating the location of each RFID tag in an environment has several advantages. Firstly, it increases the likelihood of accurately estimating the location of the RFID tags. Secondly, it reduces the time and processing power used to estimate the location of RFID tags, for example, by reducing the number of sensors queryed for a given RFID tag and / or by reducing the number of sensor measurements processed to determine the location of each RFID tag.

[0009] The “best sensor” technology disclosed herein can be implemented as a method for locating an RFID tag using first, second, and / or more RFID tag readers. The first and second RFID tag readers each detect first and second responses from a given RFID tag. The first and second RFID tag readers, or an appliance connected to the first and second RFID tag readers, each determine first and second sets of possible locations for the RFID tag based on the first and second responses. The appliance also determines a measure of variance for the first and second sets of possible locations, selects either the first or second set of possible locations based at least partially on the measure of variance, and then estimates the location of the RFID tag based on the selected set of possible locations, for example, by averaging the selected set of possible locations.

[0010] The appliance (or first RFID tag reader) can determine a first set of possible locations by determining the respective angle of arrival (AOA) at the first RFID tag reader for a first response and estimating a first set of possible locations for the RFID tag based on each AOA. To do this, the appliance or first RFID tag reader can filter out false or suspicious AOAs, for example, based on the elevation angle and / or distance between the RFID tag and the first RFID tag reader.

[0011] The measure of variance may include variance, standard deviation, or the hypotenuse of a triangle defined by the maximum and minimum x and y coordinates of the possible locations within a first set of possible locations. The appliance may select a first or second set of possible locations with a lower measure of variance to estimate the location of the RFID tag.

[0012] Another method for locating an RFID tag involves each of several RFID tag readers detecting responses from each of several RFID tags. Each RFID tag reader detects responses from each RFID tag. For each RFID tag / RFID tag reader combination, the RFID tag reader or an appliance connected to the RFID tag reader estimates a set of location estimates for that RFID tag based on the responses detected from that RFID tag by the RFID tag reader. The appliance determines a measure of variance for each of the set of location estimates for that RFID tag and, based on the measure of variance, selects one of the RFID tag readers as the preferred RFID tag reader for that RFID tag. The appliance then estimates the location of that RFID tag based only on the responses detected by the preferred RFID tag reader for that RFID tag.

[0013] If the appliance detects a change in the variance scale of the set of preferred RFID tag readers for a particular RFID tag, it can respond to that change by updating which RFID tag reader is the preferred RFID tag reader for that RFID tag. For example, if an RFID tag reader has a lower variance scale among its position estimates for that RFID tag, a different RFID tag reader among several RFID tag readers can be selected as the preferred RFID tag reader. Alternatively, for example, if the same RFID tag reader still has the lowest variance scale among its position estimates for that RFID tag, that same RFID tag reader can be selected as the preferred RFID tag reader.

[0014] Another method for locating an RFID tag includes detecting responses from the RFID tag in first and second RFID tag readers, respectively; determining first and second sets of possible locations for the RFID tag based on the first and second responses; performing a correlation process on the possible locations in the first and second sets of possible locations; and estimating the location of the RFID tag based on the correlation.

[0015] All combinations of the aforementioned concepts, and any additional concepts discussed in more detail below (provided that such concepts are not contradictory), are considered to be part of the subject matter of the invention disclosed herein. In particular, all combinations of the subject matter described in the claims, which appear at the end of this disclosure, are considered to be part of the subject matter of the invention disclosed herein. Terms used expressly herein, which may also appear in any disclosure incorporated by reference, should be given meanings that are most consistent with the specific concepts disclosed herein. Brief explanation of the drawing

[0016] Those skilled in the art will understand that the drawings are for illustrative purposes only and are not intended to limit the scope of the subject matter of the invention as described herein. The drawings are not necessarily to a fixed proportion, and in some examples, various aspects of the subject matter of the invention disclosed herein may be exaggerated or enlarged in the drawings to facilitate the understanding of different features. In the drawings, similar reference letters generally mean similar features (e.g., functionally similar and / or structurally similar components). [Brief explanation of the drawing]

[0017] [Figure 1A] This shows an environment such as a store or warehouse with several ceiling-mounted RFID tag readers and RFID tags. [Figure 1B] Figure 1A shows an RFID tag reader suitable for use in an RFID environment. [Figure 1C]Figure 1A shows an appliance suitable for controlling RFID tag readers in an RFID environment. [Figure 2A] This describes the process for estimating the location of an RFID tag using the response from that RFID tag detected by a single RFID tag reader. [Figure 2B] This describes the process for estimating the location of an RFID tag using responses from that RFID tag detected by several RFID tag readers. [Figure 3] This shows a machine representation of the state of an RFID tag location system that selects the best location estimate(s) for an RFID tag from among location estimates derived from tag responses detected by multiple RFID tag readers. [Figure 4] This document describes a process for estimating the location of different RFID tags from responses collected by different RFID tag readers within an RFID tag location system. [Figure 5] This document describes a process for estimating the location of different RFID tags from responses collected by different RFID tag readers within an RFID tag location system. [Figure 6] This is a polar coordinate plot showing the estimated position of a single RFID tag reader relative to a single RFID tag, with the RFID tag reader as the origin. [Figure 7] This is a plot of Cartesian coordinates of modal position estimates for a single RFID tag from two RFID tag readers, ab and cd. [Figure 8] This is a plot of Cartesian coordinates of position estimates for different RFID tags 1-5, including the modal position estimate for RFID tag 1. [Modes for carrying out the invention]

[0018] FIG. 1A shows an environment 10 having an RFID tag location determination system comprising a high density population of passive RFID tags 101a-101k (collectively, passive RFID tags 101, or simply RFID tags or tags), several RFID tag readers 150a-150g (collectively, RFID tag readers 150), also called leaders or sensors, an appliance 140, also called a central controller or inquiry controller, and optional cameras 130a-130c (collectively, cameras 130). The RFID tag readers 150 communicate with RFID tags 101 attached to objects (not shown) within the environment 10 and track their locations. These objects may be items for sale such as clothing, equipment or fixtures such as tables, shelves, walls, or doors, or persons such as employees, customers, or other visitors. The RFID environment 10 may be, for example, in a retail store or a warehouse, although other settings are possible.

[0019] The environment 10 may have fixtures 120a-120c (collectively, fixtures 120) that block, attenuate, and / or scatter RF signals transmitted by the RFID tag readers 150 and the passive RFID tags 101. These fixtures 120 may include shelves, racks, cabinets, etc. that may be used to hold objects to which at least some of the RFID tags 101 are attached. (Also, there may be RFID tags 101, for example, reference RFID tags 101 whose locations are known and that can be used to determine the locations of RFID tags 101 at unknown locations, on at least some of these fixtures 120.)

[0020] For example, some equipment 120 may include a metal shelf that holds one or more items for sale (not shown in FIG. 1A) tagged with RFID tags 101. The equipment 120 can be arranged in rows in some configurations, and aisles separate the rows to enable access to all objects tagged with RFID tags 101. The RFID environment 10 can be surrounded by walls 110 and 112, a ceiling, and a floor, all of which can reflect or scatter RF signals from RFID tag readers 150 and / or RFID tags 101.

[0021] The RFID tag readers 150 are preferably installed within the RFID environment 10 such that each RFID tag 101 within the environment can communicate with at least one of the RFID tag readers 150. In a typical installation, the RFID tag readers 150 are mounted to the ceiling or suspended from the ceiling. If the ceiling is a suspended ceiling or a double ceiling, the RFID tag readers 150 can be suspended from the ceiling panels, mounted to the ceiling panels, or disposed between the ceiling panels and the structural ceiling, and can be done, for example, according to the configuration described in International Application No. PCT / US2022 / 081761, filed on December 16, 2022, "Antenna Arrays and Signal Processing for RFID Tag Readers", the entire content of which is incorporated herein by reference for all purposes. Additionally, or alternatively, one or more of the RFID tag readers 150 can be attached to the walls 110 or 112 or to fixed equipment 120.

[0022] There may be one or more cameras 130a-130c (collectively, camera 130) installed within the environment 10 to capture images of at least a portion of the environment 10, as well as of people and objects within the environment 10. The cameras 130 can be communicatively connected to an appliance 140 that can receive and process images from the cameras 130 and data from an RFID tag reader 150. If necessary, the appliance 140 can use the images from the cameras 130 and data from the RFID tag reader 150 to locate RFID tags 101 and associate them with people and / or objects within the environment 10. The appliance 140 can also determine a route from a person's location to a specific RFID tag 101 / object, for example, if a person is looking for an object or if the object should be moved from its current location. The appliance 140 can recognize people from images or from RFID tags 101, smartphones, or other wireless devices carried by people, and can trigger sales or other inventory changes of objects based on the movement of people with objects.

[0023] The RFID tag readers 150 may communicate with each other and / or with the appliance 140 via a wireless or wired (e.g., Ethernet) connection. The appliance 140 may be a dedicated device having a processor, memory, power supply, and network interface(s), or it may be a appropriately programmed computer, laptop, or smartphone adapted to communicate with the RFID tag readers 150 and issue commands that the RFID tag readers 150 can recognize. The appliance 140 can also receive signals from the RFID tag readers 150. For example, the appliance 140 can instruct the RFID tag readers 150 to inventory all RFID tags 101 (and the items to which they are attached) in the environment 10, or to determine the location of one or more RFID tags 101 (and the items to which they are attached) in the environment 10. The appliance 140 can also instruct the RFID tag readers 150 to query the RFID tags 101 according to a schedule.

[0024] While one RFID tag reader 150 is querying an RFID tag 101, one or more other RFID tag readers 150 within range can await a response from the RFID tag. In other words, each RFID tag reader 150 can sequentially transmit a signal to the tag, and all RFID tag readers 150 await each response from the tag. In this configuration, the tag reader 150 that transmits the query or command is called the interrogator, and the other tag readers 150 are called listeners. The tag readers 150 can switch roles / hops alternately as interrogator and listener. For further details regarding scheduling, interrogator, and listener modes, see International Patent Application PCT / US2022 / 026198, entitled "RFID Tag Readers Switchable between Interrogator and Listener Modes" (which is incorporated herein by reference in its entirety for all purposes). The RFID tag reader 150 can transmit raw or processed data representing the response of the RFID tag to the appliance 140, which uses this data to identify and / or locate the RFID tag 101 and / or the object to which it is attached.

[0025] Each RFID tag reader 150 is equipped with an antenna array, such as a four-element square antenna array, which transmits signals to the RFID tag 101 and receives responses from the RFID tag 101. These signals and responses may experience attenuation, interference, scattering, and / or other effects as they propagate along the path or communication channel between the RFID tag reader 150 and the RFID tag 101. These effects may change over time, for example, as people and objects move within the environment 10. If the attenuation of the communication channel between a particular RFID reader 150 and a particular RFID tag 101 is too high, or if equipment 120, people, or objects block the path between them, the RFID tag reader 150 may be unable to communicate with its RFID tag 101.

[0026] In some cases, several RFID tag readers 150 may be able to detect responses from the same RFID tag 101. For example, in the environment of Figure 1A, all RFID tag readers 150a to 150c may be able to detect responses from RFID tag 101b via the line-of-sight (LOS) path indicated by the solid double-headed arrow in Figure 1A. If each RFID tag reader 150a to 150c measures the RSSI and / or angle of arrival (AOA) of the responses along the LOS path, the location of the tag can be estimated using those RSSIs and / or AOAs.

[0027] Ideally, the response from RFID tag 101a detected by RFID tag readers 150a-150c should indicate or correspond to the same location of RFID tag 101b. However, in practice, differences in the communication channels between RFID tag 101b and RFID tag readers 150a-150c, or differences between the RFID tag readers 150a-150c themselves, may cause the response from RFID tag 101b detected by RFID tag readers 150a-150c to appear to come from different locations. If all of these estimated locations are centered on the actual location of RFID tag 101b, they can be averaged to yield a more accurate estimated location. However, if one of the estimated locations deviates significantly from both the actual location and the other estimated locations, it can distort the average and degrade the accuracy of the RFID tag location estimate provided by the RFID tag location system.

[0028] For example, one or more of the communication channels between the reader 150 and the tag 101 may be multipath communication channels, and the signal travels along a non-line-of-sight (NLOS) path in addition to the LOS path, or instead. The response propagating along the NLOS path may be reflected or scattered by walls 112, equipment 120, and other objects within the environment 10. For example, the dashed double arrow in Figure 1A shows the NLOS path between the RFID tag reader 150a and the RFID tag 101b with reflection or scattering from equipment 120a. The AOA of the response received by the RFID tag reader 150a along this NLOS path estimates a false location estimate 101b' for the RFID tag 101b. As a result, the RFID tag reader 150a may return an incorrect location estimate (e.g., the average of the true / correct location estimate 101b and the false location estimate 101b'), or, if the location estimates are from different attempts to read the tag 101, it may return both the correct or true location estimate for RFID tag 101b and the false location estimate 101b' for RFID tag 101b. Without more information, it may be impossible to determine which of the location estimates is correct (or more likely to be correct), and averaging the estimates does not improve accuracy.

[0029] In this example, the false location estimate 101b' is caused by the multipath communication channel between the RFID tag reader 150a and the RFID tag 101b. Other effects, including the detection of scattered signals including query signals, aliasing in the sensor, and self-interference in the sensor, can also generate false location estimates for each communication channel. Without additional information, it may be impossible to distinguish the true location estimate from the false location estimate.

[0030] Fortunately, noise or variance in the location estimates, measurements from other RFID tag readers 150, location estimates from tags with similar EPCs, and / or prior knowledge about the tags, tag readers, and / or environment 10 can be used to determine which location estimate is more likely to be correct. This additional information can be used to select the preferred or best sensor 150 or sensor set 150 for estimating the location of each RFID tag 101 in the environment. For example, in Figure 1A, AOA measurements from RFID tag readers 150b and 150c produce a location estimate that matches one of the two location estimates from RFID tag reader 150a (i.e., the true location of tag 101b). Appliance 140 can use this match to discard one or both of the location estimates generated from the measurements from sensor 150a and designate sensors 150b and 150c as the best or preferred sensors for estimating the location of tag 101b.

[0031] Two position estimates from leader 150a form a modal group, which correspond to the same tag as indicated by the EPC coded in the response, but with very different azimuth angles. More generally, a modal cluster is a cluster of position estimates from one sensor for a given tag (EPC), where the elevation angles may be similar but the azimuth angles differ significantly (e.g., 120-180 degrees apart). Averaging AOA measurements or position estimates across different modal clusters usually does not produce a realistic estimate (e.g., the average may be below the sensor). In practice, modal behavior tends to be relatively rare.

[0032] While not bound by any particular theory, modal clusters can be attributed to grid lobes within an antenna array. As described below, a leader can locate a tag using a channel estimate that characterizes the communication channel between the leader and the tag at a given carrier frequency. The leader, or an interrogator controller connected to the leader, uses the channel estimate and the corresponding carrier frequency to determine the AOA measurement by selecting a peak value from the array manifold. In the case of multimodal (e.g., bimodal), there are two (or more) peaks of similar size in the array manifold. Selecting one peak for another can result in very different elevation and azimuth values ​​for the AOA measurement of that tag / leader pair. RFID Tag Reader Architecture

[0033] Figure 1B illustrates the reader 150 in more detail, including components that can be enabled or disabled when the reader 150 is in interrogator mode or listener mode. The reader 150 comprises an RF antenna array and front end 156, a processor 152, an RF calibration and tuning block 154, a hop generator 160, and a hop receiver 170. The RF antenna array and front end 156 may include one or more antenna elements (e.g., arranged in a multi-element antenna array) for transmitting an RFID query signal 151, a tag response 153, and optionally receiving RFID query signals from other readers, amplifiers, filters, and / or other analog RF components. The processor 152 may be implemented as a microcontroller, an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other suitable device, and controls the operation of the reader 150, including steering the reader's antenna array, if desired. The processor stores information in memory (not shown), retrieves information from memory, and communicates with the appliance 140 via a network connection (not shown), such as an Ethernet connection. If the leader 150 is configured to operate in interrogator and listener modes, the processor 152 switches the leader 150 between interrogator mode and listener mode, the hop generator 160 is disabled or turned off in listener mode and enabled or turned on in interrogator mode, and the hop receiver 170 is enabled or turned on in both modes. The RF calibration and tuning block 154 performs RF calibration and tuning functions.

[0034] The hop generator 160 generates a query signal 151 that the reader 150 transmits to the RFID tag 101. The hop generator 160 can also optionally generate commands or communication signals intended for other readers 150, for example, on a dedicated reader communication channel or in a specific preamble or payload. This comprises a digital command generator 162 that generates digital queries, commands, and / or other information transmitted by the query signal 151, and RF electronics 164 for converting the digital signals from the command generator 162 into analog signals suitable for transmission by the antenna array of the front end 156. The RF electronics 164 may include a digital-to-analog converter (DAC) that converts the digital signals into baseband analog signals, a mixer and local oscillator for mixing the baseband analog signals up to an intermediate frequency for broadcast, and filters and / or pulse shapers for removing sidebands and / or spurs.

[0035] The hop receiver 170 includes a receiver front end 172 connected to a command demodulator 174 and a tag response demodulator 176. Generally, the receiver front end 172 digitizes, downconverts, and estimates the phase of the RF signal detected by the antenna. There are various ways to configure the receiver front end 172, and in this embodiment, it receives analog common-mode and quadrature (I / Q) signals at higher frequencies (e.g., 40 MHz) and converts them to digital I / Q samples at baseband (e.g., 5 MHz).

[0036] In interrogator mode, the front end 172 also cancels any self-interference caused by the query signal 151, for example, due to leakage within the receiver. Fortunately, since the crosstalk correlates with the query signal 151, the receiver front end 172 can generally cancel crosstalk between different antenna elements and the circuits connected to those antenna elements. As described in International Patent Application PCT / US2022 / 081761, filed on 16 December 2022 (which is incorporated herein by reference in its entirety for all purposes), this crosstalk can be further reduced or suppressed by further separating the antenna elements from each other.

[0037] When leader 150 is in listener mode, it does not transmit query signals and does not perform self-interference cancellation. In listener mode, leader 150 detects channels on which other leaders 150 transmit query signals 151 and estimates the frequencies of those other query signals 151.

[0038] The command demodulator 174 is activated when leader 150 is in listener mode and demodulates commands from other leaders to replay query signals at a command bitrate (e.g., 40kbps to 160kbps). The command demodulator 174 uses the command payload to determine what a leader in interrogator mode is asking about tag 130 (e.g., modulation, preamble type, expected response type, etc.). For example, leader 150 in interrogator mode might request tag 130 to send the first 64 bits of its EPC using Miller-2 modulation at a backscatter link frequency (BLF) of 320kHz with a standard preamble. Leader 150 in listener mode uses this information to decode the tag response 153. The command demodulator 174 is deactivated when leader 120 is in interrogator mode.

[0039] The tag response demodulator 176 is activated in both interrogator mode and listener mode to demodulate baseband tag response I / Q samples to create a tag response signal at the tag response bitrate. Appliance architecture

[0040] Figure 1C shows the appliance 140, which is shown in more detail in Figure 3B. The controller appliance 140 may comprise one or more processors, non-volatile memory, and other logical devices, implemented as an integrated circuit and powered by appropriate power supplies and other housekeeping electronics. These processors and logical devices may comprise individual components that perform individual functions, and / or more general-purpose components that are programmed to perform various functions, either by themselves or in cooperation with other components of the controller appliance 140. For example, the controller appliance 140 may have communication interfaces, indicated as Ethernet connections Eth0 and Eth1, connected to a reader 150, a POS system, and / or other devices, and may comprise a central processor unit (CPU) 142 that runs an operating system (e.g., Alpine Linux OS) that manages the hardware and software resources of the controller appliance. The non-volatile memory of the controller appliance can store the operating system, other firmware and software, and tag state information.

[0041] Figure 1C illustrates appliance 140 as a block diagram, where each block represents a different function or subfunction performed by appliance 140. To monitor and update tag status, controller appliance 320 comprises or implements an in-store message router 180, an RFID query controller (RFID-IC) 182, a location status manager 184, a tag status manager 186, and a retail backend application programming interface (API) 188. The in-store message router 180 queues and routes messages exchanged between reader 150 and RFID-IC 182 via Ethernet connections Eth0 and Eth1. RFID-IC 182 employs a segmented media access controller (MAC) design with a lower MAC layer implemented in reader 150 and a higher MAC layer implemented in RFID-IC 182 to process messages exchanged with reader 150. The lower MAC layer determines timestamps and parameters estimated from the backscatter response of the RFID tag, which are useful for determining the location of the tag. The upper MAC layer schedules hop transmissions and the general purpose of each hop. The lower MAC layer performs more time-critical functions, such as actually scheduling when to send commands and how to respond to replies within a hop. The positioning layer, including the RFID-IC and / or reader 150, calculates the position of the RFID tag in a 3D coordinate system (e.g., Cartesian coordinates with the origin at a known location in a store or room) from the data coming from the MAC and PHY layers. Messages from reader 150 may also include data to be read from the RFID tag, including the RFID tag's EPC and other metadata.

[0042] The location status manager 184 and the tag status manager 182 track the location and status of RFID tags, respectively. The location status manager 184 receives the estimated location of the RFID tag from the RFID-IC 182 (e.g., within a Cartesian coordinate frame with the origin at a corner of the store) and determines where the RFID tag is located within the RFID environment (e.g., a room and a zone). Rooms and zones may be extracted from a 3D model of the store or space. In a retail RFID environment, rooms and zones may include pick-up areas, merchandise warehouses, sales floors, and changing rooms, with sales floors further divided into entrance / exit zones and checkout zones. Based on the location changes detected by the reader 150, the location status manager 184 updates the location of each RFID tag in the inventory database 190, which may be hosted locally or off-site (e.g., in the cloud).

[0043] The tag status manager 186 manages the status of tags, including their location and availability. Several possible available states exist, including, but not limited to, (1) available, (2) old (optional), (3) ignored, (4) missing, and / or (5) sold. Other states may also exist. The tag status manager 186 transitions the RFID tags 101 between these states based on the tag's response (or lack thereof) to queries from the reader 150 containing information about the tag's location, and the status of the tags stored in the inventory database 190. For further details on tag status and stateful inventory management, see International Application PCT / US2023 / 061645, filed on 31 January 2023, which is incorporated herein by reference in its entirety.

[0044] The tag state manager 186 updates the tag state stored in the inventory database 190 and forwards both the tag state and estimated tag location to the retail backend light API 188, which implements backend functions for inventory, restocking, and product lookup. The retail backend light API 188 can implement these functions via a web application gateway 194 that implements a Hypertext Transfer Protocol (HTTP) proxy and redirects Representational State Transfer (REST) ​​requests to the appropriate backend server (not shown). The web application gateway 194 can also provide user authentication and provides static files used by the browser to render web pages.

[0045] Figure 1C also shows some optional components of appliance 140, including a raw tag server 192, a spatial server 193, a Trivial File Transfer Protocol (TFTP) server 195, a multicast Domain Name Service (mDNS) server 196, a Network Time Protocol (NTP) server 197, a Secure Shell (SSH) server 198, and a Secure Sockets Layer (SSL) certificate store 199. The spatial server 193 handles firmware lifecycle management and configuration of the sensor 150 and a Power-over-Ethernet (PoE++) switch (not shown), which connect appliance 140 to the sensor 150. The raw tag server 192 retrieves tag metadata for legacy APIs, such as those used by API clients for system debugging. Upon booting, sensor 150 downloads executable images from the TFTP server 195. The mDNS server 196 allows appliance 140 to expose itself using the mDNS and DNS-SD protocols, for example, during debugging. The NTP server 197 connects to and synchronizes with a remote (e.g., internet-based) NTP server, providing NTP services to the sensor 150. The SSH server 198 is also used for debugging. Additionally, the SSL certificate store 199 authenticates the appliance 140 by hosting the server certificate used by the sensor 150 and the web server certificate used by the REST client. Estimation of RFID tag location

[0046] Figures 2A and 2B illustrate how the location of an RFID tag (e.g., RFID tag 101a in Figure 1A) is estimated using responses from that RFID tag detected by multiple RFID tag readers (e.g., RFID tag readers 150a-150b). Location estimation can be performed by each tag reader, by the appliance, or by both the tag readers and the appliance. The appliance typically has higher processing power than the embedded device (tag reader), but by offloading processing to the tag readers, it is easier to scale the system (e.g., by reducing the processing per tag reader performed by the appliance, the appliance can handle more tag readers), and the appliance can be effectively utilized for other tasks. Generally, the appliance performs a process that uses data acquired by multiple tag readers (e.g., triangulation) because individual tag readers may not have data from other tag readers.

[0047] Figure 2A illustrates a process 200 for estimating the location of an RFID tag using the response from that RFID tag detected by a single RFID tag reader (e.g., RFID tag reader 150a). The antenna array of the RFID tag reader may be characterized by an array manifold 202, which is a hypersurface describing the response of the antenna array to incident radio waves, i.e., the response from the RFID tag in this case. The array manifold 202 is correlated (204) with a complex channel estimate of the communication channel between the RFID tag and the RFID tag reader to form an estimate of the AOA of the response at the RFID tag reader.

[0048] AOA estimates can include estimates of both azimuth and elevation relative to the sensor. For an RFID tag reader mounted on the ceiling and facing downwards, an elevation of 180 degrees corresponds to boresight, i.e., directly downwards from the ceiling to the floor. An elevation of 90 degrees is at the same height as the ceiling. Since it is unlikely that an RFID tag is close to the ceiling, AOA estimates with elevations below a threshold (e.g., 110 degrees) can be filtered or removed as unrealistic using a digital sieve type filter or other suitable filter (206). Once the AOA estimates have been filtered, they can be used to estimate the position of the RFID tag in Cartesian coordinates, assuming that the RFID is at a specific height from the floor or at a specific vertical distance below the RFID tag reader (i.e., a fixed position in z, such as 1 meter above the floor) (208). When an RFID tag reader receives many responses from RFID tags, it can estimate the Area of ​​Arrow (AOA) and the location of each response, and then calculate an average location estimate using, for example, a finite impulse response (FIR) filter with N taps selected based on the desired filter performance (212). The average location estimate of a single RFID tag reader is called the IntraSensorMean.

[0049] The system also calculates a measure of the variance or spread of the distribution of position estimates derived from the AOA measurements of the RFID tag reader (210). The variance of the position estimates may be caused by noise in the responses detected by the sensor. This can be characterized by the variance or standard deviation of the position estimates. Another measure of variance is the hypotenuse of the triangle defined by the distance between the largest and smallest x and y coordinates.

number

[0050] The system may weight the location estimates based on their age or a decay coefficient such as an exponential decay coefficient or a step function when selecting the maximum and minimum x and y coordinates. It may also discard location estimates after a specific period (e.g., 1 hour, 6 hours, 12 hours, 1 day, or more) or as more location estimates are accumulated (e.g., 10, 25, 50, 100, or more). For example, a reader and / or interrogator controller may discard location estimates at a rate proportional to the tag reading rate, which may vary depending on the tag population. The reader and / or interrogator controller may also average location estimates taken over several consecutive periods (e.g., the last 10 location estimates) or a fixed period (e.g., the last 5, 10, 15, 30, or 60 minutes). Similarly, as older position estimates are attenuated or discarded and replaced with newer position estimates, the mean and hypotenuse or other measures of variance can be recalculated periodically. Attenuating or discarding x and y position estimates over time prevents inaccurate initial readings from limiting or distorting the hypotenuse.

[0051] Figure 2B illustrates a method for generating a preferred location estimate for the same RFID tag using location estimates from multiple RFID tag readers 150. Each RFID tag reader 150 within range detects multiple responses from the RFID tag and calculates multiple location estimates for the tag at a fixed height (z) (e.g., one location estimate for each detected response), an in-sensor average location estimate at the fixed height, and an in-sensor hypotenuse. For example, an RFID tag reader 150 can calculate an in-sensor average from two or more location estimates (e.g., 10, 100, or 1000 location estimates). If only one location estimate is available, the sensor 150 can report its location estimate, even though its noise characteristics may be worse than the average location estimate. The RFID tag reader 150 supplies these averages and hypotenuses, along with the RFID tag's EPC (Sensor_EPC_ID) and the number of responses or counts detected from the RFID tag, to the appliance 140. (EPC is included in each response and uniquely identifies the RFID tag that sent the response and the item to which the RFID tag is attached.) As described below, appliance 140 stores this information in its memory (252) and uses it to select a preferred or adopted position for the RFID tag (254).

[0052] Appliance 140 also receives the current or most recent location estimate(s) for each RFID tag reader for an RFID tag. If multiple location estimates are received from a given RFID tag reader for a given RFID tag, Appliance 140 can select the preferred or best location estimate(s) or calculate the mean, median, and / or mode of the location estimate(s) for further reporting and / or processing. Appliance 140 also selects the maximum and minimum x and y values ​​from the current location estimates from all sensors and uses these values ​​to calculate the inter-sensor hyphen(260) by equation (1).

[0053] Appliance 140 can select the adoption location of an RFID tag by using another measure of the hypotenuse or noise or variance of the current location estimate from RFID tag reader 150, for example, by discarding current location estimates that are too far from the acceptable or expected location range for the RFID tag (e.g., more than 2 meters or 1 or 2 standard deviations away). Inter-sensor hypotenuse is a measure of discrepancy and / or noise between sensors, while intra-sensor hypotenuse is a measure of noise for a particular sensor. Generally, intra-sensor hypotenuse can be used to prefer one sensor over another (the smaller the intra-sensor hypotenuse for a given sensor, the lower the noise in the measurements for that sensor). A small inter-sensor hypotenuse indicates that the measurements of the sensors are close to each other and can be averaged or otherwise combined for better performance. For example, the appliance may receive measurements for the same tag from two, three, four, five, or more sensors, each of which calculates its own intra-sensor hypotenuse or other measure of variance. The appliance selects the sensor(s) with the smallest internal hypotenuse or other measure of variance as the best or preferred sensor(s). For example, if the appliance is forced to re-select the best sensor(s) due to noise in the preferred position estimate, it updates its selection based on the recalculated internal hypotenuse or other measure of variance and re-selects the sensor(s) with a smaller internal hypotenuse or other measure of variance, even if that sensor was previously selected as the best or preferred sensor(s).

[0054] Appliance 140 can also use intra-sensor and inter-sensor hypotenuses or similar noise indicators to determine whether the RFID tag reader's location estimate for a particular RFID tag should be ignored. Table 1 (below) shows possible qualitative combinations of intra-sensor and inter-sensor hypotenuses from two or more RFID tag readers. Small intra-sensor and inter-sensor hypotenuses indicate that the RFID tag reader is making consistent location estimates that can be averaged or otherwise combined to reduce noise and / or increase the reliability of the estimated location. If the intra-sensor hypotenuse is small and the inter-sensor hypotenuse is large, the measurements from each RFID tag reader are consistent with other measurements from that RFID tag reader but not with measurements from other RFID tag readers. This may indicate that the measurement from one RFID tag reader is distorted or biased and should be discarded or ignored. This may also mean that the RFID tag reader is reading different tags that have the same EPC, in which case the measurements may be accurate but cannot be matched or combined. The probability of two tags having the same EPC is extremely low. The tag registration process is supposed to prevent duplicate EPCs, but the possibility cannot be completely eliminated. Generally, when the inter-sensor hypotenuse is large, measurements from noisier sensors (e.g., sensors with larger inter-sensor hypotenuses) should be discarded. [Table 1]

[0055] Large intra-sensor hypotenuses and small inter-sensor hypotenuses may indicate that the measurement environment is highly noisy and that averaging position estimates from all RFID tag readers may yield more accurate position estimates for the RFID tag. If necessary, position estimates from different sensors can be weighted, for example, based on the confidence level of the measurements, before being combined or averaged. Large intra-sensor and inter-sensor hypotenuses may indicate that the tag is moving frequently or rapidly, and / or that the environment is highly noisy, for example, due to other objects in the environment causing time-varying multipath effects.

[0056] If the hypotenuse between sensors is too large (for example, exceeding a predetermined threshold based on achievable measurement uncertainty, which can be tens to hundreds of centimeters), it indicates a lack of agreement among RFID tag readers regarding the estimated location of the RFID tag. In this case, the central controller may select a location estimate from one RFID tag reader or one set of RFID tag readers for reporting and further processing. In other words, the central controller selects a preferred or best set of RFID tag readers to measure the location of that RFID tag. This calculates and reports the estimated location of the RFID tag using measurements from the preferred RFID tag reader or set of RFID tag readers, and ignores or discards location estimates from other RFID tag readers(s). Selecting the best RFID tag reader or set of RFID tag readers for an RFID tag improves the accuracy of the system estimate of the RFID tag's location. It also increases the speed at which the system estimates the RFID tag's location by eliminating the need to obtain, wait for, and process measurements or location estimates from non-preferred, i.e., less likely, RFID tag readers that produce accurate results.

[0057] Generally, a central controller selects an RFID tag reader(s) that has the lowest intra-sensor variance (e.g., hypotenuse, standard deviation, or variance) and sufficient position estimates to be the best RFID tag reader(s) for a given RFID tag. For example, a central controller may select an RFID tag reader as the preferred RFID tag reader for a given RFID tag if it has an intra-sensor hypotenuse below a predetermined threshold (e.g., δ based on achievable spatial resolution, which can be tens to hundreds of centimeters) and at least N measurements (where N is a positive integer, e.g., 2, 5, 10, 25, 50, or 100).

[0058] The central controller can select a preferred RFID tag reader or set of RFID tag readers for each RFID tag. An RFID tag reader may be a preferred RFID tag reader (one of several) for a single RFID tag or set of RFID tags, but not for other RFID tags. Depending on the nature of the communication channels in the environment, the preferred RFID tag reader for a given set of RFID tags may be the RFID tag reader closest to that set of RFID tags, or an RFID tag reader with a line-of-sight path to that set of RFID tags. For example, based on the location estimate and the sensor's hypotenuse, the central controller may select the RFID tag reader with the shortest unobstructed path to a given RFID tag as the preferred RFID tag reader for that RFID tag.

[0059] The central controller may update the preferred RFID tag reader(s) for a given RFID tag if the variance of the location estimates for that RFID tag exceeds an acceptable limit or threshold. For example, the central controller may determine an updated hypotenuse each time it receives a new response from that RFID tag or a location estimate for that RFID tag. If the hypotenuse is within the acceptable limit, the central controller simply reports the location estimate and continues to process tag responses and location estimates (only) from the preferred RFID tag reader(s). (Even if the central controller reports location estimates only from the preferred RFID tag reader(s), it may continue to receive and store responses, location estimates, and other data for that RFID tag acquired by other sensors until that data becomes outdated.) If the hypotenuse exceeds the acceptable limit, the central controller may repeat the process of selecting the preferred sensor(s). In some cases, such as with a temporary violation of the acceptable variance / hypotenuse limit, the central controller may select the same preferred sensor(s) that it previously selected. State machine representation of an RFID tag location system

[0060] Figure 3 shows a state machine representation 300 for an RFID tag location system or interrogator controller / appliance that selects a preferred or best sensor for locating RFID tags. The appliance stores location estimates for each RFID tag, determines which sensor is best, and reports location estimates only from that sensor until it is determined that the sensor is no longer suitable. The state machine representation 300 can be for each sensor or for the appliance, as long as the appliance can delete older data / measurements. States include an initialization state 302, a standby or hold state 304, a data storage state 308, a data processing state 312, and a data deletion state 318. The system moves or cycles between states in response to events such as receiving a response from an RFID tag. Starting in the initialization state 302 until initialization is complete, at which point it transitions to the standby or hold state 304 and remains in that state until it receives a response from an RFID tag. Detection of a response indicates that the hop is complete (306) and causes the RFID tag reader to transition to the data storage state 308. In data storage state 308, the RFID tag reader instructs itself to update a data structure containing the location estimate, the sensor that read the tag, and the time the tag was read, based on the received response, and the data structure is stored in memory. Once the response data is stored in the data structure (310), the RFID tag reader enters data processing state 312, where it estimates the location of the RFID tag and calculates the in-sensor hypotenuse or other variance measure based on the detected response. Once processing is complete (314), the RFID tag reader enters data deletion state 316. In this data deletion state 316, the RFID tag reader removes old, outdated, obsolete, and / or invalid data from the data structure to prevent this data from distorting future location estimates. Once deletion is complete (318), the RFID tag reader returns to standby state 304 until it receives a new tag response at the end of the next hop.

[0061] The RFID tag reader can cycle through the states of the state machine representation 300 hop by hop, periodically (for example, every few seconds or every few hops), or in response to other events. When cycling through states periodically or in response to other events, the RFID tag reader stores data in the standby state 306 until the period elapses or a trigger event occurs, and then transitions to the data storage state 308. Implementing "Best Sensors / Best Measurements"

[0062] Figure 4 illustrates the process for locating an RFID tag using the best sensor / best measurement in an RFID sensing system. This RFID sensing system may perform this process separately for each RFID tag it is trying to locate (400). The process begins by determining whether a “best sensor” exists for the RFID tag to be located (402). If the RFID sensing system appliance has already selected a “best sensor” for the tag, and that sensor has recent measurements for that tag (404), and the position error of those recent measurements is less than a threshold distance (e.g., <2 meters) (406), the appliance does not change which sensor is the “best sensor” for that tag (408). Instead, it obtains a moving average of the position estimates of the best sensor for that tag (410), updates the tag's state (including the tag's position and associated positioning error) (412), and reports the position update to the higher layer(s) of the RFID sensing system (430).

[0063] If there is no “best sensor” for the RFID tag being located (402), if the sensor does not have recent measurements for the tag (404), or if the position error of the sensor's recent measurements is greater than a threshold distance (e.g., >2 meters) (406), the appliance selects the best (new) sensor for the tag. This is done by determining whether any sensor has multiple measurements for the tag being located (414). In that case, the appliance calculates the normalized position error for the measurements from each sensor (416), and then selects the sensor with the fewest measurements below the corresponding normalized position error as the best sensor (418). For example, the appliance may select the sensor with the lowest normalized position error, where at least 25% of its position estimates are below this error, as the best sensor. Otherwise, the appliance selects the sensor with the most recent measurements for the tag as the best sensor (424). The appliance calculates a moving average of the best sensor location estimates (420), updates the tag status (including the new best sensor), and reports the location update to the higher layer(s) of the RFID sensing system (430).

[0064] Figure 5 shows one implementation of process 500 for selecting the best sensor (here, sensor 1 to sensor 5) for each of several RFID tags (tags 1 to 4). This process 500 can be performed by an appliance, for example, as steps 416 and 418 of the tag location process in Figure 4. The RFID tag reader transmits information about itself, including its location, RFID tag data including elevation and azimuth data (AOA data) derived from the detected response, and information about the RFID tag, such as the tag's EPC (501). The appliance can hash the EPC for more efficient memory storage and retrieval (502), and then sort or index this information based on the relevant EPCs (epc1 to epc4) (503), each of which corresponds to a different RFID tag (e.g., epc1 for tag 1, epc2 for tag 2, etc.). (The appliance can retrieve tag and sensor data using hash tables based on the tag EPC and sensor ID, respectively.) The sensor and appliance can update the AOA data for each tag hop by hop.

[0065] The appliance calculates a position estimate for at least one, and possibly all, of the RFID tags for each RFID tag reader (sensor) that detects a response from its RFID tag and transfers the RFID tag's AOA data to the appliance. In Figure 5, the appliance estimates the positions of tag 1 / epc1 (504a) and tag 4 / epc4 (504b). The central controller hashes the sensor IDs for more efficient storage and retrieval (505a and 505b), sorts the RFID tag data (particularly the AOA data) for each sensor (sensor_aoa_data, sensor_id) (506a, 506b), and estimates the position of each tag / sensor pair (sensor_to_tag_position_data) from the corresponding AOA data (508a, 508b). In Figure 5, the appliance calculates a position estimate for tag 1 (sensor_position_data) from AOA data acquired by sensors 1, 2, and 3, and a position estimate for tag 4 from AOA data acquired by sensors 4 and 5. For example, the appliance can calculate one position estimate for each AOA measurement value from each sensor. Here, since the appliance has three AOA measurements of tag 1 from sensor 1, three AOA measurements of tag 1 from sensor 2, and two AOA measurements of tag 1 from sensor 3, it calculates three position estimates for tag 1 / sensor 1, three position estimates for tag 1 / sensor 2, and two position estimates for tag 1 / sensor 3. In another embodiment, a given RFID tag reader (e.g., sensor 2) may provide AOA data for multiple RFID tags (e.g., tag 1 and tag 2), in which case the central controller calculates a position estimate for each RFID tag measured by the RFID tag reader.

[0066] The appliance stores recent location estimates for each RFID tag (510a, 510b). This is to prevent these measurements from becoming obsolete or biased due to RFID tag movement, environmental changes, and / or noise reduction through averaging of AOA measurements, by deleting old and / or invalid location estimates. Using the stored location estimates, the appliance calculates intra-sensor and inter-sensor hypotenuse, location variance, or other measures of variance, noise, and / or (in)stability for the estimated location of each RFID tag (512a, 512b) as described above.

[0067] The appliance selects the best sensor for each tag based on the positional variance of the sensors that have a positional estimate for that tag (514a, 514b). Generally, the sensor with the lowest positional variance (e.g., the lowest hypotenuse or standard deviation) for a given tag is selected as the best sensor for that tag. In some cases, the appliance may select the best sensor based on multiple measures, such as positional variance or variability, the number of (recent) measurements, a priori information about the tag's location or RFID environment, and / or information derived from a camera or other sensor. If multiple sensors meet the best sensor selection criteria for a given tag, the appliance may select those sensors as the best sensors for that tag and average their positional estimates. Once the appliance has selected the sensors that provide the most likely accurate positional estimate ("best" or "preferred" sensor(s)) for a given tag, the appliance aggregates the positional estimates for that tag for each best sensor (516a, 516b). In Figure 5, the appliance selects sensor 2 as the best sensor for tag 1 and sensor 4 as the best sensor for tag 2. The appliance calculates a moving average or cumulative average of position estimates from the best sensors (518a, 518b), where the position estimates can be weighted by age, with newer position estimates being given a higher weight than older ones. The appliance can calculate this moving average over a predetermined or selectable time window or number of measurements (e.g., the last 30 seconds, 1 minute, 5 minutes, 10 minutes, 30 minutes, or 1 hour, or the last 10, 25, 50, 100, or 250 measurements). If the best sensor changes during this time window, the appliance can continue the moving average with the position estimates from the new best sensor or reset the moving average.If the appliance selects a set of sensors as the best sensors for a given tag, it can aggregate and average (and optionally weight them based on the hypotenuses within and / or between the corresponding sensors) the location estimates from those sensors to further improve the accuracy of the reported location estimate and report the aggregated or averaged location estimate. The appliance reports the moving average as the final location estimate to the user or other systems that use the location estimate, such as an inventory management system that tracks items to which RFID tags are attached (520a, 520b). Use the "best sensor" to distinguish between true and false position estimates.

[0068] Figures 6-8 illustrate different scenarios in which one or more RFID tag readers generate multiple location estimates for a single RFID tag, and how the “best sensor” generates the location estimate most likely to be correct. If the location estimates are close enough to each other, for example, within a radius equal to the spatial resolution of the RFID tag system, the appliance can average the location estimates to generate a more accurate estimate of the tag’s actual location. However, if the location estimates are significantly different, for example, several times a given error threshold, the appliance can determine which location estimate is more likely to be correct and discard the others.

[0069] Figure 6 is a polar coordinate plot showing the estimated position of a single RFID tag reader (sensor) relative to a single RFID tag, with the RFID tag reader (sensor) as the origin. Generally, the AOA measurement(s) of an RFID tag reader for a given RFID tag changes over time due to changes in the communication channel between the RFID tag reader and the RFID tag, or changes in the position of the RFID tag. Changes in the communication channel can be caused by the movement of people or other items in the environment (e.g., a customer or salesperson moving other items on a rack or shelf that holds an item with an RFID tag attached). Changes in the communication channel cause the AOA measurement, and therefore the estimated position, to change over time. If the hypotenuse within the sensor is small, the resulting estimated positions may be close enough to each other to form clusters or groups.

[0070] Typically, the location estimates from an RFID tag reader for a single tag form a single modal group. Within a modal group, the location estimates are based on tag responses encoding the same EPC, but detected along different azimuth and / or elevation angles. Averaging the location estimates within a single modal group can reduce the impact of noise on the AOA measurement. In some cases, including the example shown in Figure 6, the location estimates may form two or more separate, widely separated modal groups. In these cases, it may not be possible to determine whether one modal group is more likely to be correct than the other modal group(s). If one modal group is closer to the true location of the tag, it may not be possible to identify or estimate which modal group represents the true location of the RFID tag without further information. This additional information may include the RFID tag's EPC, the EPCs of other RFID tags in the environment, the layout of shelves, racks, and / or other equipment in the environment, and / or RFID tag location estimates from other sensors. For example, if the EPC of an RFID tag indicates that the tag is attached to a men's sweater, the RFID tag reader or appliance can use a location estimate of a group or cluster that places the RFID tag near or on equipment that holds or should hold other men's clothing, or near other RFID tags attached to men's sweaters.

[0071] Figure 6 also shows a single location estimate marked as “potential movement.” This location estimate represents a change in the channel estimate that the central controller determined to be due to the movement of the RFID tag, since the location estimate is sufficiently far from both modal groups for the RFID tag. In other words, the change in AOA of the tag's response is so large that the RFID tag is considered to have moved, at least temporarily. For example, a person may have taken the RFID tag and associated items from a clothing rack and then put them back on the clothing rack. When an RFID tag reader queries an RFID tag as it moves along a trajectory or through a series of locations, the RFID tag reader should generate a location estimate that follows that trajectory.

[0072] Figure 7 is a plot in Cartesian coordinates of modal clusters of single RFID tag position estimates derived from AOA measurements by two RFID tag readers (sensors ab and cd). In this embodiment, each RFID tag reader generates two modal clusters enclosed by solid lines, sensor ab generates two clusters in the upper right quadrant of the plot, and sensor cd generates clusters in the upper right and upper left quadrants. The central controller aggregates these sensor clusters into superclusters, each enclosed by dashed lines (clusters 1 and 2 in the upper right quadrant, and cluster 3 in the upper left quadrant). The clusters may be grouped based on predetermined or dynamic error thresholds, such as uncertainty or resolution of the position estimates, using density-based spatial clustering (DBSCAN) for noisy applications or another suitable clustering technique.

[0073] In Figure 7, when each RFID tag sensor reports multiple modal groups of RFID tags, the central controller may not be able to select the “best sensor” or correctly determine the location of the RFID tag. At best, the central controller may only estimate the RFID tag's location to be within the upper right quadrant, possibly within or near cluster 1, with uncertainty based on the distribution of location estimates within cluster 1 and / or the distribution across the cluster. For example, the central controller may select a matching modal group with a bimodal distribution as the most likely tag location (e.g., within cluster 1). It may also average the supporting measurements or select one of the matching modal groups as the most likely tag location.

[0074] If the central controller receives a single modal cluster of location estimates that falls within cluster 1 from another RFID tag reader, it may discard or ignore the location estimates from clusters 2 and 3. In other words, the central controller may select cluster 1 as the "best set" of sensor measurements. Alternatively, the central controller may determine that sensors ab and cd are not suitable "best sensors" and discard or ignore all of their measurements, choosing sensors that generate only a single cluster of location estimates. Similarly, if sensor ab reports only the modal group of cluster 1, the central controller may select cluster 1 as the "best set" of sensor measurements, or select sensor ab as the "best sensor."

[0075] Unfortunately, averaging location estimates from different modal groups tends to produce inaccurate or skewed location estimates. As a result, if an RFID tag reader generates many modal groups for a single RFID tag, the central controller may discard or ignore all location estimates from that RFID tag reader for that RFID tag. In other words, the central controller may not select an RFID tag reader as the "best sensor" for an RFID tag if that RFID tag reader generates location estimates that form multiple modal groups. Even if the central controller does not use the RFID tag reader's location estimates to estimate the true location of the RFID tag, it can still use them to select the "best sensor" by correlating the modal groups with modal or non-modal groups from other RFID tag readers.

[0076] Figure 8 is a plot of location estimates for five different RFID tags (labels 1-5). Shading at each location indicates the sensor that generated the location estimate. The first sensor generated two location estimates for tag 1, and four different sensors generated location estimates for tags 2-5. In this case, the central controller selects the location estimate that is most likely to be correct for tag 1 by correlating other independently acquired information about tag 1 with similar information about tags 2-5.

[0077] For example, a central controller can select a preferred or best position estimate based on the channel estimates for each tag / sensor pair, provided that the channel estimates generally do not change much over short distances. Each channel estimate represents the attenuation, noise, interference, distortion, dispersion, etc., introduced into the signal as it propagates from the sensor to the RFID tag and back. Some of these effects are due to propagation through free space, while others are due to components within the channel, such as filters, amplifiers, analog-to-digital converters (ADCs), and antennas. If the channel is a filter, the channel estimate can be thought of as the channel's transfer function.

[0078] The central controller can select the "best" or most likely correct location estimate for tag 1 by comparing the channel estimates of the first sensor and tag 1 with the channel estimates of other tag / sensor pairs. For example, if the channel estimate is closest to the channel estimates of tags 4 and 5, and the first sensor is close to the sensor that measured the location estimates of tags 4 and 5, the central controller will select the location estimate of tag 1 that is closest to the location estimates of tags 4 and 5.

[0079] The appliance can also use other information, including the tag's EPC, the Universal Product Code (UPC) of the item associated with or attached to the tag, the RSSI, or the distribution, to select the "best" location estimate for tag 1. For example, the appliance can also select the location estimate for tag 1 that is closest to the location estimates of tags that have similar EPCs or are associated with similar UPCs. Similarly, if an inventory system or database indicates that tags with a particular EPC or items with a particular UPC (e.g., men's underwear) are expected to be close to each other or near a particular location in the environment (e.g., on equipment for men's underwear), the appliance may select a location estimate for tag 1 based on this information regarding the EPC and / or UPC. The appliance can also use the number or number of location estimates in a cluster, with a higher number of measurements indicating a more likely correct cluster. These are just some of the other pieces of information that the sensor can use to determine multiple location estimates for the same tag. conclusion

[0080] While various embodiments of the invention have been described and illustrated herein, those skilled in the art will readily conceive of various other means and / or structures for carrying out the functions described herein and / or obtaining one or more of the results and / or advantages, and each of such variations and / or modifications will be considered to be within the scope of the embodiments of the invention described herein. More generally, those skilled in the art will readily understand that all parameters, dimensions, materials and configurations described herein are illustrative, and that actual parameters, dimensions, materials and / or configurations will depend on the specific use or of a few specific uses in which the teachings of the invention are used. Those skilled in the art will be able to recognize or confirm many equivalents of the specific embodiments of the invention described herein simply by using ordinary experimentation. Thus, it will be understood that the embodiments described herein are presented only as examples, and that embodiments of the invention may be practiced in ways other than those specifically described and claimed, within the scope of the appended claims and their equivalents. Embodiments of the invention of this disclosure cover each individual feature, system, article, material, kit and / or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and / or methods is included within the scope of the inventions of this disclosure, provided that such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent.

[0081] Furthermore, various inventive concepts may be embodied in one or more methods, and embodiments thereof have been provided. The actions performed as part of a method may be ordered in any preferred manner. As a result, embodiments may be constructed in which the actions are performed in a different order than those exemplified, and this may include performing some of the actions simultaneously, even when they are shown as actions that follow the exemplary embodiments.

[0082] All definitions defined and used herein should be understood to govern dictionary definitions, definitions incorporated by reference in documents, and / or the common meanings of the defined terms.

[0083] As used herein and in the claims, the indefinite articles "a" and "an" should be understood to mean "at least one" unless explicitly stated otherwise.

[0084] As used herein and in the claims, the phrase “and / or” should be understood to mean “either or both” of the combined components, that is, components that are conjunctive in some cases and disjunctive in others. Multiple components listed in “and / or” should be interpreted as “one or more” of the same type, i.e., coordinating components. Other components may optionally exist in addition to those specifically identified by the “and / or” clause, whether related or not to the components specifically identified. Thus, as an unrestrictive example, a reference to “A and / or B” when used in conjunction with unrestrictive language such as “including” may refer to A only in one embodiment (optionally including components other than B), B only in another embodiment (optionally including components other than A), and both A and B in yet another embodiment (optionally including other components), and so on.

[0085] As used herein and in the claims, “or” should be understood to have the same meaning as “and / or” as defined above. For example, when separating items in a list, “or” or “and / or” should be interpreted as inclusive, i.e., including at least one of the number of components or the list, but also including two or more, and optionally including additional items not on the list. Only terms that explicitly indicate the opposite, such as “one of,” “exactly one of,” or, when used in the claims, “consisting of,” refer to including exactly one component of the number of components or the list. In general, as used herein, the term “or” should be interpreted only when preceded by an exclusive term such as “either,” “one of,” “only one of,” or “exactly one of,” to indicate an exclusive choice (i.e., “one or the other, but not both”). “Consisting of” should have the usual meaning as used in the field of patent law when used in the claims.

[0086] As used herein and in the claims, the phrase “at least one” with respect to a list of one or more components should be understood to mean at least one component selected from any one or more components in the list of components, but not necessarily including at least one of each component specifically enumerated in the list of components or all components, nor excluding any combination of components in the list of components. This definition also allows for the presence of components other than those specifically identified in the list of components to which the phrase “at least one” refers, regardless of whether they are related to the specifically identified components. Therefore, in non-limiting embodiments, "at least one of A and B" (or equivalently "at least one of A or B," or equivalently "at least one of A and / or B") may mean, in one embodiment, that B is absent (and optionally includes components other than B), that at least one, optionally one or more A is included; in another embodiment, that A is absent (and optionally includes components other than A), that at least one, optionally one or more B is included; and in yet another embodiment, that at least one, optionally one or more A is included, and at least one, optionally one or more B is included (and optionally other components).

[0087] In the claims and the above specification, all transitional phrases, such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and similar phrases, are understood to be unrestrictive, meaning they include but are not limited to them. Only the transitional phrases “consisting of” and “consisting essentially of” are considered closed or semi-closed transitional phrases, respectively, as provided in Section 2111.03 of the U.S. Patent and Trademark Office's Patent Examination Procedure Manual.

Claims

1. A method for locating a radio frequency identification (RFID) tag, The first RFID tag reader among multiple RFID tag readers detects the first response from the RFID tag, The second RFID tag reader among the multiple RFID tag readers detects the second response from the RFID tag, Based on the first response, a first set of possible locations for the RFID tag is determined, Based on the second response, a second set of possible locations for the RFID tag is determined, Determining a measure of the variance of the first set of possible positions and a measure of the variance of the second set of possible positions, Selecting one of the first set of possible positions or the second set of possible positions as more accurate than the other of the first set of possible positions, based at least in part on the measure of variance of the first set of possible positions and the measure of variance of the first set of possible positions, The estimated location of the RFID tag is determined based on either the first set of possible locations or the second set of possible locations. Methods that include...

2. Determining a first set of the aforementioned possible positions is Determining the respective arrival angles of the first response at the first RFID tag reader, Based on the respective angles of arrival, estimate a first set of possible positions for the RFID tag. The method according to claim 1, including the method described in claim 1.

3. Determining each of the aforementioned angles of arrival is The method according to claim 2, comprising filtering the respective angles of arrival based on the angle of elevation and / or the distance between the RFID tag and the first RFID tag reader.

4. The method according to claim 1, wherein determining a measure of the variance of a first set of possible positions includes calculating at least one of the variance, standard deviation, or hypotenuse of a triangle defined by the maximum x and y coordinates and the minimum x and y coordinates among the possible positions in the first set of possible positions.

5. The method according to claim 1, wherein selecting one of the first set of possible positions or the second set of possible positions as being more accurate than the other of the first set of possible positions or the second set of possible positions includes selecting the one of the first set of possible positions or the second set of possible positions having a lower measure of variance.

6. The method according to claim 1, wherein determining the estimated position of the RFID tag includes averaging the possible positions within one of a first set of possible positions or a second set of possible positions.

7. A system for locating radio frequency identification (RFID) tags, A first RFID tag reader for detecting a first response from the RFID tag, A second RFID tag reader for detecting a second response from the RFID tag, An appliance operably connected to the first RFID tag reader and the second RFID tag reader, Based on the first response, a first set of possible locations for the RFID tag is determined. Based on the second response, a second set of possible locations for the RFID tag is determined. Determine the measure of variance for the first set of possible positions and the measure of variance for the second set of possible positions. Based at least in part on the measure of variance of the first set of possible positions and the measure of variance of the first set of possible positions, one of the first set of possible positions or the second set of possible positions is selected as being more accurate than the other of the first set of possible positions or the second set of possible positions. An appliance for determining the estimated location of the RFID tag based on either the first set of possible locations or the second set of possible locations. A system equipped with these features.

8. The aforementioned appliance By determining the respective arrival angles of the first response at the first RFID tag reader, and The system according to claim 7, configured to determine the first set of possible positions of the RFID tag by estimating the first set of possible positions of the RFID tag based on each of the angles of arrival.

9. The aforementioned appliance The system according to claim 8, configured to determine each of the angles of arrival by filtering each of the angles of arrival based on the elevation angle and / or the distance between the RFID tag and the first RFID tag reader.

10. The system according to claim 7, wherein the appliance is configured to determine a measure of the variance of the first set of possible locations by calculating at least one of the variance, standard deviation, or hypotenuse of a triangle defined by the maximum x and y coordinates and the minimum x and y coordinates among the possible locations in the first set of possible locations.

11. The system according to claim 7, wherein the appliance is configured to select one of the first set of possible positions or the second set of possible positions having a lower measure of variance as being more accurate than the other of the first set of possible positions or the second set of possible positions.

12. The system according to claim 7, wherein the appliance is configured to determine the estimated location of the RFID tag by averaging one of the possible locations from a first set of possible locations or a second set of possible locations.

13. A method for locating the location of a radio frequency identification (RFID) tag using an RFID tag reader, Each RFID tag reader detects a response from each RFID tag, For each RFID tag, For each RFID tag reader, a set of estimated location values ​​for the RFID tag is determined based on the response detected from the RFID tag by the RFID tag reader. To determine the scale of variance for each of the aforementioned set of position estimates, Based on the aforementioned variance measure, one of the RFID tag readers is selected as the preferred RFID tag reader for that RFID tag. The location of the RFID tag is estimated based solely on the response detected by a preferred RFID tag reader for that RFID tag. Methods that include...

14. The method according to claim 13, wherein determining the measure of each of the variances includes calculating at least one of the variances, standard deviations, or hypotenuses of triangles defined by the maximum and minimum x and y coordinates among the possible positions of each set of position estimates.

15. The method according to claim 13, wherein selecting one of the RFID tag readers as the preferred RFID tag reader includes selecting the RFID tag reader having the lowest measure of variance for that RFID tag.

16. The method according to claim 13, wherein selecting one of the RFID tag readers as the preferred RFID tag reader includes selecting a first RFID tag reader as the preferred RFID tag reader for a first RFID tag, and selecting a second RFID tag reader other than the first RFID tag reader as the preferred RFID tag reader for a second RFID tag.

17. The method according to claim 16, further comprising selecting one of the RFID tag readers as the preferred RFID tag reader, the first RFID tag reader as the preferred RFID tag reader for the third RFID tag.

18. For at least one of the RFID tags, Based on the response from the RFID tag detected by the preferred RFID tag reader, a change in the scale of the variance of the set of position estimates for the RFID tag is detected. In response to the change in the measure of the aforementioned variance, the selection of the preferred RFID tag reader for that RFID tag is updated. The method according to claim 13, further comprising:

19. The method according to claim 18, wherein updating the selection of preferred RFID tag readers for the RFID tag includes selecting one of the RFID tag readers as the preferred RFID tag reader.

20. The method according to claim 18, wherein updating the selection of preferred RFID tag readers for the RFID tag includes selecting the same RFID tag reader as the preferred RFID tag reader.

21. An appliance for a radio frequency identification (RFID) system, An interface for receiving RFID tag measurements from an RFID tag reader, A processor operably connected to the aforementioned interface, wherein for each RFID tag, For each RFID tag reader, a set of estimated location values ​​for that RFID tag is determined based on the measurement values ​​of that RFID tag by that RFID tag reader. Determine the scale of variance for each of the aforementioned set of position estimates. Based on the aforementioned variance measure, the first RFID tag reader among the RFID tag readers is selected as the preferred RFID tag reader for that RFID tag. A processor and An appliance equipped with these features.

22. The appliance according to claim 21, wherein the processor is configured to determine a measure of each variance by calculating at least one of the following: the variance, the standard deviation, or the hypotenuse of a triangle defined by the maximum x and y coordinates and the minimum x and y coordinates, between the possible positions of each set of position estimates.

23. The appliance according to claim 21, wherein the processor is configured to select the RFID tag reader having the lowest measure of variance as the preferred RFID tag reader for its RFID tag.

24. The appliance according to claim 21, wherein the processor is configured to select a second RFID tag reader other than the first RFID tag reader as the preferred RFID tag reader for the second RFID tag.

25. The appliance according to claim 24, wherein the processor is configured to select the first RFID tag reader as the preferred RFID tag reader for the third RFID tag.

26. The processor, for at least one of the RFID tags, Based on the response from the RFID tag detected by the preferred RFID tag reader, a change in the scale of the variance of the set of location estimates for the RFID tag is detected, and The appliance according to claim 21, further configured to update the selection of a preferred RFID tag reader for the RFID tag in response to a change in the scale of the aforementioned variance.

27. The appliance according to claim 26, wherein the processor is configured to update the selection by selecting one of the different RFID tag readers as the preferred RFID tag reader.

28. The appliance according to claim 26, wherein the processor is configured to update the selection by selecting the same RFID tag reader as the preferred RFID tag reader.

29. A method for locating a radio frequency identification (RFID) tag, Each of the multiple RFID tag readers detects a set of responses from the RFID tag, For each set of responses, determine the estimated location of the RFID tag. Determining the distribution of each of the sets of estimated positions of the RFID tags, From the aforementioned plurality of RFID tag readers, the RFID tag reader having the lowest variance is selected as the preferred RFID tag reader for the RFID tag. Based on the response from the RFID tag detected by the preferred RFID tag reader, the final estimated position of the RFID tag is determined. To report the final estimated location of the RFID tag. Methods that include...

30. A system for locating radio frequency identification (RFID) tags, Multiple RFID tag readers, each configured to detect a set of responses from the RFID tag, An appliance operably connected to the plurality of RFID tag readers, wherein (i) determines each set of estimated locations of the RFID tags based on the set of responses, (ii) determines the variance of each set of estimated locations of the RFID tags, (iii) selects the RFID tag reader with the lowest variance from the plurality of RFID tag readers as the preferred RFID tag reader for the RFID tags, and (iv) determines the final estimated location of the RFID tags based on the response from the RFID tags detected by the preferred RFID tag reader. A system equipped with these features.

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