Segmented inventory rounds for RFID tags

Abstract

When a radio-frequency identification (RFID) tag reader transmits a query, each RFID tag within range responds by transmitting a reply and switching the state of its inventoried flag for a persistence time depending on the session of query. Addressing a query to tags with inventoried flags in a particular state provokes replies from only tags with inventoried flags in that state. These inventoried flags can be used to silence tags, potentially accelerating queries of large groups tags. If a reader transmits a low-power query, that low-power query switches the state of the inventoried flags of the tags within range. If the reader transmits a higher-power query in the same session while the inventoried flags of those tags are still in the switched state, then those tags won't respond to the higher-power query. This enables tailoring the modulation rate and duration of higher-power query to tags that are farther from the sensor.

Description

Segmented Inventory Rounds for RFID TagsCROSS-REFERENCE TO RELATED APPLICATION S)

[0001] This application claims the priority benefit, under 35 U.S.C. 119(e), of U.S. Application No. 63 / 591,688, filed October 19, 2023, which is incorporated herein by reference in its entirety for all purposes.BACKGROUND

[0002] Radio-frequency identification (RFID) tags, or tags, are low-cost devices that can be attached to objects and offer the promise of automated tracking, locating, sales check-out, and inventory of the objects among other commercial and medical applications. There are passive, semi -active, and active types of RFID tags that can be wirelessly interrogated by an RFID tag reader, also called a reader, interrogator, or sensor, and emit wireless radio-frequency (RF) replies to the reader. Each reply can include information stored in the RFID tag, such as an electronic product code (EPC), tag identification number, or other alpha-numeric sequence. Other information may be included with the reply. Each EPC is unique and so can be used to identify the tag that sent a particular reply.

[0003] Passive RFID tags have no battery and are therefore typically less expensive than semiactive and active RFID tags. A passive RFID tag is powered by an unmodulated, continuous- wave RF signal from the RFID tag reader. This continuous-wave RF signal powers up the passive RFID tag’s circuitry and precedes a query or command from the RFID tag reader in the form of a modulated RF signal. The powered-up, 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 reply and is at the same carrier frequency as the RF signal from the RFID tag reader. The replies from passive RFID tags are typically many orders of magnitude weaker (lower amplitude) than the RF signals from the RFID tag readers and are detected by the RFID tag readers.

[0004] Each cycle of transmitting a continuous-wave RF signal and a query command or other signal from the sensor to the tag and receiving the tag’s reply at the sensor occurs at a different carrier frequency and is called a hop. A single sensor can repeat hops periodically at different carrier frequencies — i.e., it can hop among carrier frequencies — until it has read all of the tagswithin range, with each tag responding during a different time slot in a given hop or during a different hop. For ultrahigh frequency (UHF) passive RFID tags, the carrier frequencies are typically within bands of 865-868 MHz (Europe) or 902-928 MHz (North America).

[0005] The EPC™ Radio-Frequency Identity Protocols Generation-2 UHF RFID Standard for Air Interface Protocol for Communications at 860 MHz - 960 MHz defines physical and logical requirements for wireless communications between sensors and tags at 860-960 MHz. Under this protocol, called the EPC GEN 2 protocol for short, a sensor manages a group of tags using select, inventory, and access operations to choose a tag population, identify individual tags, and communicate with identified tags, respectively. For instance, a sensor can inventory tags by transmitting a Query command to a group of tags, each of which responds to the Query command. Transmitting a Query command starts an inventory round, which lasts until it is terminated by either a subsequent Query command (which also starts a new inventory round), a Select command, or a Challenge command.

[0006] The EPC GEN 2 protocol supports four sessions, Sessions 0-3, for inventorying tags. Each session is an inventory process for a sensor and an associated set of tags. Each tag maintains an independent inventoried flag for each session simultaneously (i.e., each tag maintains four separate inventoried flags). Each inventoried flag can be in one of two states, called states A and B. The state of a tag’s inventoried flag indicates whether the tag may respond to a sensor in a given session and enables the tag to track its inventoried status separately for each session.

[0007] A sensor begins an inventory round by selecting a session and transmitting a query command, or Query, to the nearby tags. The sensor and nearby tags operate within that session for the duration of the inventory round. The Query includes a Target field that selects whether tags with inventoried flags in state A or state B participate in the inventory round. By default, each tag begins the inventory round with its inventoried flag in state A. Each tag’s inventoried flag switches state (e.g., from state A to state B) for the session specified in the Query when the tag responds to the Query. Per the EPC GEN 2 protocol, each tag remains in its new state (e.g., state B) for a predetermined period called the persistence time, which is measured from the last rising edge of the last bit of the sensor transmission that caused the inventoried flag to change state. The tag does not respond to queries in the same session for tags in its original state (in this example, state A) from any sensor until the persistence time has elapsed, at which point it returns to its default state (usually state A), or it is commanded to return to its originalstate. The duration of the persistence time depends on the session and on whether or not the tag remains energized by a signal from the sensor after switching states:

[0008] In Session 0, a tag resets its inventoried flag to state A as soon as it is no longer being energized by the sensor, e.g., because the sensor has stopped transmitting or the tag is no longer within range of the sensor. In Session 1, a tag stays in state B for at least 0.5 seconds and up to 5 seconds after it has been switched from state A, whether or not it remains energized. In Sessions 2 and 3, a tag stays in the B state indefinitely so long as it remains energized and for at least 2 seconds if is not energized.

[0009] In Session 1, if the persistence time expires before the inventory round is over, then the tag will stay in state B unless the sensor commands it to change state or until the inventory round ends. If a tag in Session 1, 2, or 3 is in state B at the end of an inventory round, it will remain in state B until its persistence time expires, even after being de-energized. In every session, a sensor can also switch the state of a tag’s inventoried flag before the persistence time has elapsed, e.g., by transmitting a Select command that instructs the tag to toggle its inventoried flag.

[0010] By default, each tag begins the inventory round with its inventoried flag in state A and so will attempt to respond to the Query that initiates the inventory round. If two or more tags reply simultaneously, their replies may collide with each other at the sensor; that is, the replies from different tags may reach the sensor at the same time or may overlap in time with each other. Complete overlaps are called complete collisions, and partial overlaps are called partial collisions. A sensor cannot reliably detect colliding responses and so may not acknowledge responses that collide. This means that the tags whose responses collide must be interrogated again, increasing the total interrogation time. These additional interrogations also increase the total energy consumed by the sensor to interrogate a given set of tags.

[0011] Collisions can be prevented using an anti-collision protocol, such as the EPC Q scheme given in the EPC GEN 2 protocol. In the EPC Q scheme, the period for replying to a Query command is called a frame and is divided into 2eslots, where Q is an integer from 0 to 15 (inclusive) called the slot-count parameter, which is based on the estimated tag population. The sensor announces the slot-count parameter, Q, as part of the Query command. Each tag that receives the Query picks a random integer in the range 0 to 2e- 1 and loads this integer into its 15-bit slot counter. A tag that picks a 0 respond immediately; the other tags transition to an arbitrate state and wait fortheir slot counters to be decremented by subsequent commands from the sensor. At the end of the first slot, the sensor commands the tags to decrement their counters, and any tag whose slot counter has just reached 0 transmits its reply to the Query. These steps repeat until the hop terminates, the sensor has run through all 2eslots, or the sensor transmits a new Query or a QueryRep command. Generally, Q is large enough so that there is more than one slot in the frame for each tag to reduce the probability that two tags will pick the same slot. If a collision does occur, however, the corresponding tags can transmit replies during a subsequent frame.

[0012] In some cases, the number of tags exceeds the average number of tag reads per hop. In these cases, more often than not, it is not possible to read all the tags during the same hop. Unfortunately, the hop duration is usually limited by regulations (in the United States, Federal Communications Commission (FCC) regulations limit the hop duration to 400 milliseconds), so increasing the hop duration to accommodate more tags is not always possible. Instead, the extra tags are read during subsequent hops. Without the ability to exploit tag state memory (i.e., persistent inventoried flags), a tag that responds during the first hop in a series of hops would contend for slots in subsequent hops, potentially increasing the time it takes to inventory the tags.SUMMARY

[0013] It can be challenging to read large numbers of tags (e.g., 100 tags, 150 tags, 200 tags, 250 tags, or more) quickly with a single sensor, in part because of collisions and in part because the modulation rate of the Query (and of the tag replies) is limited by the path loss between the sensor and the tags. Because a Query causes every tag within range to reply and because the modulation rate scales with the energy per bit to noise power spectral density (Eb / N0), or signal-to-noise ration per bit, of the replies, the modulation rate should be low enough that the sensor can detect and demodulate the reply with the lowest Eb / No(i.e., the reply from the tagwith the highest path loss to the sensor). In a conventional inventory round, then, tags with low path loss typically operate at lower modulation rates than necessary, increasing the total time it takes to complete the inventory round.

[0014] Dividing an inventory round into segments, each of which addresses tags with a different range of path losses, makes it possible to interrogate tags with lower path losses at higher modulation rates. Such an inventorying method can be carried out at as follows on a first RFID tag with a first path loss to an RFID tag reader and a second RFID tag with a second path loss to the RFID tag reader greater than the first path loss. The first and second RFID tags each have an inventoried flag for a first session that is set to a first state and is switchable between the first state and a second state. An RFID tag reader transmits a first query in the first session at a first power level addressed to RFID tags having respective inventoried flags set to the first state. This first query energizes the first RFID tag and causes the first RFID tag to (i) transmit a reply to the first query and (ii) change its inventoried flag for the first session from the first state to the second state for a persistence time. The RFID tag reader detects the reply to the first query from the first RFID tag and transmits, while the inventoried flag for the first session of the first RFID tag is in the second state, a second query in the first session at a second power level greater than the first power level. This second query is addressed to (only) RFID tags having respective inventoried flags for the first session set to the first state. The second query energizes the first and second RFID tags and causes the second RFID tag to (i) transmit a reply to the second query and (ii) change its inventoried flag for the first session from the first state to the second state for the persistence time. The RFID tag reader detects the reply to the second query from the second RFID tag.

[0015] In this method, the first and second RFID tags can conform to the EPC™ Radio- Frequency Identity Protocols Generation-2 UHF RFID Specification for Air Interface Protocol for Communications at 860 MHz - 960 MHz, in which cases the first session can be Session 0, 1, 2, or 3 as defined in the EPC™ Radio-Frequency Identity Protocols Generation-2 UHF RFID Specification for Air Interface Protocol for Communications at 860 MHz - 960 MHz.

[0016] The first power level may be sufficient to energize the first RFID tag but not the second RFID tag.

[0017] The second query can cause the inventoried flag of the first RFID tag for the first session to remain in the second state.

[0018] Transmitting the second query may comprise transmitting the second query without deenergizing the first RFID tag. The RFID tag reader can transmit the second query at a lower modulation rate than the first query, over a longer period than the first query, at a different carrier frequency than the first query, and / or at a power level 2 dB to 3 dB higher than the power level of the first query. The RFID tag reader can also allow the first RFID tag to become de-energized for less than the persistence time between transmitting the first query and transmitting the second query.

[0019] If desired, the RFID tag reader can transmit a third query at a third power level greater than the second power level while the inventoried flags of the first RFID tag and the second RFID tag for the first session are in the second state. This third query is addressed to (only) RFID tags having respective inventoried flags for the first session set to the first state. The third query energizes the first the second RFID tags as well as a third RFID tag with a third path loss to the RFID tag reader that is greater than the second path loss. The third query causes the third RFID tag to (i) transmit a reply to the third query and (ii) change its inventoried flag for the first session from the first state to the second state for the persistence time. The RFID tag reader detects the reply to the third query from the third RFID tag. The third query can further cause the inventoried flags for the first session of the first RFID tag and the second RFID tag to remain in the second state.

[0020] Alternatively, an RFID tag reader can transmit a series of queries at increasing power levels (e.g., increasing at a rate of 2 dB to 3 dB per query to a maximum power level of 36 dBm) addressed to RFID tags with respective inventoried flags for the first session set to the first state. Each query in the series of queries energizes at least a subset of the RFID tags. This causes the energized RFID tags with respective inventoried flags for the first session set to the first state to (i) transmit respective replies to that query and (ii) change their respective inventoried flags for the first session from the first state to the second state for a persistence time longer than a total duration of the series of queries. The RFID tag reader detects the replies to each query in the series of queries .

[0021] The RFID tag reader can transmit the series of queries without gaps in transmission between successive queries or with gaps in transmission shorter than the persistence time between successive queries. The RFID tag reader can transmit different queries in the series of queries at different modulation rates, over periods of different durations, and / or at different carrier frequencies.

[0022] All combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. The terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein.BRIEF DESCRIPTIONS OF THE DRAWINGS

[0023] The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the inventive subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar and / or structurally similar components).

[0024] FIG. 1A shows a sensor reading tags at different ranges (path losses) in different segments of a segmented inventory round.

[0025] FIG. IB illustrates a timing diagram for a segmented inventory round.

[0026] FIG. 1C is a flow diagram for a segmented inventory round.

[0027] FIG. 2 depicts an RFID tag monitoring system that can inventory and locate RFID tags in an environment, such as a store or warehouse, using segmented inventory rounds.

[0028] FIG. 3 illustrates an RFID tag reader suitable for use in the RFID tag monitoring system of FIG. 2.

[0029] FIG. 4 illustrates an appliance or interrogator controller suitable for controlling RFID tag readers in the RFID tag monitoring system of FIG. 2.DETAILED DESCRIPTION

[0030] Dividing an inventory round into segments as in FIGS. 1 A-1C may reduce the amount of time it takes to inventory tags 101 within range of a sensor 120. In a segmented inventory round, the tags 101 are grouped into subsets by the loss or attenuation in their communicationschannels to the sensor 120. If the tags 101 are all oriented in the same way with respect to the sensor 120 and there are no obstructions between the sensor 120 and the tags 101, then the amount of loss or attenuation should scale with the range or distance between the sensor 120 and tag 101 as shown in FIG. 1A. Typically, the one-way path loss or attenuation is about 37 dB for a range of 2 meters. If the sensor transmits at +36 dBm and the signal experiences another 10 dB of loss at the tag due to imperfect absorption and reflection, the sensor should detect a reply from a tag at a range of about 2 meters at a level of about -48 dBm (= +36 dBm - 37 dB - 10 dB - 37 dB). In practice, obstructions, scattering, multipath effects, and tag orientation affect the loss in the communications channels between the tags 101 and the sensor 101, so a tag 101 that is close to the sensor 120 may have higher path loss than a tag 101 that is farther from the sensor 120.

[0031] Each inventory round is divided into several segments 11 or sub-rounds, each of which targets a different subset of tags 101. For example, an inventory round can be divided into two, three, four, five, or more segments (e.g., three or four segments), with the sensor’s transmission power incremented by 2 dB or 3 dB for successive segments to a maximum transmission power for the last segment. In the United States, the maximum transmission power is limited by the FCC to 36 dBm. Every 3 dB increase in transmission power increases the coverage area of the transmission by a factor of 2.

[0032] The segments 11 can be at the same carrier frequency or at different carrier frequencies, depending on the number of segments, the duration(s) of the segments, and the session. In the United States, the FCC limits the maximum hop duration to 400 ms, which sets a practical limit on the number of segments for a given carrier frequency and a given minimum segment duration. For instance, a single inventory round of 400 ms can be divided into four segments of 100 ms, each at the same carrier frequency and at a different power level (e.g., 27 dBm for the first segment, 30 dBm for the second segment, 33 dBm for the third segment, and 36 dBm for the fourth segment). Alternatively, the same 400 ms inventory round can be divided into four segments at different carrier frequencies and power levels. Longer inventory rounds (e.g., of 500 ms) can be divided into segments at different carrier frequencies to avoid violating the FCC upper limit of 400 ms per hop.

[0033] The segments can all be the same duration (e.g., 40 ms, 50 ms, 60 ms, 70 ms, 80 ms, 90 ms, or 100 ms) or different durations. If the transmission rate goes down for subsequent segments, then the segment duration may go up for subsequent segments to provide enough time for the sensor and tag(s) to transmit and receive at the lower modulation rates. Forinstance, if the modulation rate goes down by half for every subsequent segment (e.g., 640 kbps for the first segment, 320 kbps for the second segment, 160 kbps for the third segment, 80 kbps for the fourth segment, and so on), then the duration may double for every subsequent segment (e.g., 25 ms for the first segment, 50 ms for the second segment, 100 ms for the third segment, and 200 ms for the fourth segment) to account for the reduction in modulation rate. The modulation rate for a given segment may be selected to achieve an energy per bit to noise power spectral density ratio (Eb / No) of 10 dB, 13 dB, or another suitable target level.

[0034] The duration of each segment may also vary based on the expected or estimated tag population being interrogated in that segment. For example, if at the lowest power level (first segment), the sensor is expected to inventory 50 tags and to inventory 100 tags at the next power level (second segment), then the second segment may be twice as long as the first segment. Alternatively, consider that every 3 dB increase in signal power doubles the coverage area of the query broadcast by the sensor. If signal power increases by 3 dB with every segment, the first segment covers an area A, the second segment covers an area 2A (including the area A covered in the first segment), the third segment covers an area 4A (including the area 2A covered in the first and second segments), and so on. If the tags are distributed uniformly at a density of N / A across the entire area covered by the sensor, then the Query broadcast in the first segment should provoke replies from N tags; the Query broadcast in second segment should provoke replies from N tags (= N / A * (2A - A)); the Query broadcast in third segment should provoke replies from 2N tags (= N / A * (4A - 2A)); and so on. This means that the first and second segments should have the same duration, the third segment should have a duration twice as long as the durations of the first and second segments, and so on.

[0035] The segments can be contiguous — that is, without gaps in transmission between them — or non-contiguous — that is, separated by gaps (e.g., of 1-10 ms) in transmission — depending on the persistence behavior of the session for the inventory round. In Sessions 0, 2, and 3, the persistence time is indefinite for energized tags, suggesting that once a tag’s inventoried flag for Session 0, 2, or 3 is switched from one state to another state in an inventory round, then it will stay in that state until the tag is no longer energized. This suggests that a sensor could continue an inventory round indefinitely in Session 0, 2, or 3 without the tag switching state by hopping among frequencies so long as there are no gaps or breaks in transmission between hops. In Session 2 or 3, it may even be possible to continue an inventory round without the tag switching state with gaps less than the persistence time between hops. In Session 1, the persistence time is at least 500 ms (and up to 5 seconds) whether or not the tag remainsenergized, meaning that there can be gaps or breaks less than 500 ms between segments. In addition, the tag’s inventoried flag for Session 1 will stay in a switched state until the inventory round ends, meaning that a segmented inventory round in Session 1 could last longer than the maximum persistence time of 5 seconds.

[0036] For the purposes of illustration, FIGS. 1A and IB illustrates four subsets of tags 101-1 through 101-4 at ranges of < r through <from sensor 120 assuming that range is the only determinant of path loss / attenuation (i.e., there are no obstructions between the sensor 120 and the tags 101). The sensor 120 interrogates the subset of tags 101-1 through 101-4 in segments 11-1 through 11-4, respectively. In other examples, there may be more or fewer subsets of tags and segments / sub -rounds, tags at the same range may be in different subsets because they experience different amounts of attenuation, and / or tags at different ranges may be in the same subset because they experience roughly equal amounts of attenuation.

[0037] Optionally, before the first segment, the sensor 120 can transmit a high-power signal to all of the tags 101 within range that switches the tags’ inventoried flags to desired starting state, e.g., state A. During the first segment 11-1, the sensor 120 transmits a first Query 111-1 at a power level that is high enough to activate the subset of tags 101-1 with the lowest path loss to the sensor 120 and too low to activate tags 101 with higher pass loss (in this example, tags 101- 2, 3, and 4 at a range > r from the sensor 120). The tags 101-1 in this subset transmit replies 121-1 to the first Query command 111-1 and switch their inventoried flags, e.g., from state A to state B. (For simplicity, FIG. IB shows only one reply 121-1.) In Session 0 and optionally in Session 1, 2, or 3, the sensor 120 continues to transmit a continuous-wave (CW) signal to keep the tags 101-1 energized and in state B even after they have replied to the first Query 111- 1. In Session 1, 2, or 3, there can be a gap in transmission shorter than the persistence time between the first segment 11-1 and the second segment 11-2 without the inventoried flags of the energized tags 101-1 reverting to their default state (state A).

[0038] The second segment 11-2 follows the first segment 11-1, with the sensor 120 continuing to transmit the CW signal to keep tags 101-1 energized and their inventoried flags in state B. While keeping tags 101-1 energized in Session 0, 2, or 3 (or before the tags’ persistence time elapses in Session 1, 2, or 3), the sensor transmits a second Query command 111-2 at a power level that is high enough to activate the subset of tags 101-2 with the second-lowest path loss to the sensor 120 and too low to activate tags 101-3 and 101-4 with higher path loss. If the increased power level is insufficient to offset the maximum path loss of tags 101-2 read in the second segment 11-2, the second Query 111-2 can be transmitted at a lower modulation ratethan the first Query 111-1 to compensate for the higher path loss and potentially lower Eb / N0of the replies from tags 101-2. In addition, the second Query 111-2 can extend over a longer period than the first Query 111-1. The second Query 111-2 is in the same session as the first Query 111-1 and specifies that tags 101 with inventoried flags in state A should respond. Tags 101-2 in the second subset backscatter replies 121-2 to the second Query 111-2 and switch their inventoried flags from state A to state B. Tags 101-1 do not respond because their inventoried flags are already in state B. The sensor 120 either continues to transmit a CW signal or transmits with gaps shorter than the persistence time so that tags 101-1 and 101-2 remain energized and in state B through the third segment 11-3 and fourth segment 11-4.

[0039] The third segment 11-3 and fourth segment 11-4 of the inventory round proceed in similar fashion. For each subsequent segment 11, the sensor 120 increases the power level and optionally decreases the modulation rate of the Queries 111-3, 111-4 to offset the increased path loss to the remaining tags 101-3, 101-4. Because the previously inventoried tags 101-1, 101-2 remain energized, they stay in state B and do not respond to the subsequent Queries 111- 3, 111-4, which are directed to tags 101 in the same session in state A. In the third segment 11- 3, only tags 101-3 transmit replies 121-3; similarly, in the fourth segment 11-4, only those tags 101-4 transmit replies 121-4. The tags 101 that have already responded to a previous Query in this inventory round do not respond so long as they remain energized.

[0040] Once all of the segments 11 have been completed and all the tags 101 have responded, the inventory round ends. The sensor 120 stops energizing the tags 101, causing the tags 101 to revert to their default state for the session (here, state A) after the persistence time for the session elapses. Optionally, the sensor 120 or an appliance (not shown) coupled to the sensor 120 can locate the tags 101 based on the replies 121 as the sensor 120 receives the replies 121 and / or after the inventor round ends, e.g., using triangulation, trilateration, channel estimates, or any other suitable location method.

[0041] FIG. 1C gives an overview of a process for conducting a segmented inventory round. To start, the sensor 120 can optionally initialize the inventoried flags in a given session to state A (or state B) for all of the tags 101 within range (131). Then the sensor 120 transmits a Query 111 in that session at a first power level (133) to (only) those tags in the desired state (in this example, state A). The sensor 120 detects replies 121 from tags 101 within range while keeping the tags 101 within range energized in Session 0, 2, or 3 or before the tags’ persistence time elapses in Session 1 , 2, or 3 ; at the same time, the tags 101 within range switch their inventoried flags from state A to state B (or vice versa if initialized in state B) (135). An appliance coupledto the sensor 120 optionally locates the tags either when the sensor 120 detects the replies 121 or after the inventory round has been completed (141). If there are more segments in the segmented inventory round, then the sensor increments the power level of the Query (137) and optionally decrements the modulation rate of the Query (139) for subsequent segments, depending on the lowest expected Eb / N0and / or highest expected path loss.

[0042] In each subsequent segment, the tags that have already been read have inventoried flags in state B, they do not respond to the query. Put differently, switching a tag’s inventoried flag for the session of the query to state B inhibits or prevents the tag from responding to subsequent queries in the same session for as long as the tag’s inventoried flag remains in state B (i.e., for the persistence time, which can be indefinite in Sessions 0, 2, and 3 for tags that remain energized).

[0043] If desired, a sensor can use commands other than queries to toggle tags into a desired state and hence silence them during queries of tags in the other state. For example, a sensor can toggle the inventoried flags of tags with low path losses into state B for a given session by broadcasting a Select command at relatively low power. This effectively silences these tags in the session for the duration of the persistence time. Then the sensor can broadcast a higher- power query in the same session to tags in state A. This triggers responses from the tags in state A for that session that receive the higher-power query. The tags in state B for that session do not respond. This effectively allows the sensor to selectively inventory tags at a given range of path losses.

[0044] If a given subset of tags 101 includes more tags than can be read during a single segment, then the sensor 120 may transmit two or more Queries in a row at the same power level until all the tags 101 in that subset have been read. The sensor 120 can also adapt the slotcount parameter Q for each segment and / or the segment duration based on the real-time estimated tag population. For example, if the tags are distributed uniformly and the coverage area doubles with every segment as explained above, then the sensor can increase the slot-count parameter based on the expected number of tags to be read in that segment.

[0045] As mentioned above, when querying all of the tags in a single unsegmented hop, the query should be at a modulation rate slow enough for the tag with the highest path loss to produce a detectable reply. In a segmented hop, on the other hand, the sensor can vary the modulation rate of the Queries based on the (expected) Eb / Noof the tag replies. For example, the sensor may transmit Queries at higher modulation rates to tags with lower expected pathloss (e.g., closer tags) and at lower modulation rates to tags with high expected path loss (e.g., tags that are farther away). In other words, the sensor may decrease the modulation rate of the Query command for each subsequent segment to account for the increased path loss and lower expected Eb / N0of the detected tag replies. This enables the sensor to inventory closer tags faster and reduces the overall time it takes to inventory all of the tags with range of the sensor.RFID Tag Location System for Segmented Inventory Rounds

[0046] FIG. 2 depicts an RFID tag location system 100 that locates passive RFID tags 101a- 101k (collectively, RFID tags 101), or tags, in an environment 10, such as a retail store or warehouse, using segmented inventory rounds or segmented hops. The RFID tag location system 100 includes RFID tag readers 120a-120g (collectively, RFID tag readers 120), also called readers or sensors, that transmit interrogation signals to the tags 101 and detect replies from the tags 101; an interrogator controller or appliance 140 that is coupled to the sensors 120 (e.g., via wireless or wired connections); and optional cameras 130a-130c (collectively, cameras 130) that can be used to confirm the tags’ locations and / or to track motion of people and / or objects in the environment 10. The sensors 120 can also estimate the locations of the tag 101 based on the angles-of-arrival and / or channel estimates of the tags’ replies, e.g., as disclosed in International Application No. PCT / US2024 / 020357, entitled “Channel Estimation for Locating RFID Tags” and filed on March 18, 2024, which is incorporated herein by reference in its entirety for all purposes.

[0047] The sensors 120 report the EPCs of the tags, whether or not they have moved, and / or other information, such as angles-of-arrival and quality metrics, to the appliance 140. The appliance 140 uses this information to estimate the locations of the tags 101. (The appliance 140 can also be configured to determine channel estimates and use them to locate tags 101.) The RFID tags 101 are attached to objects (not shown) in the environment 10. These objects may be items for sale, such as articles of clothing; fixtures or furnishings 93a-93c (collectively, fixtures 93), such as tables, shelves, walls, or doors; or people, such as employees, customers, or other visitors. The environment 10 can be bounded by walls 91, a ceiling, and a floor, any of which can reflect or scatter RF signals from the RFID tag readers 120 and / or RFID tags 101.

[0048] The RFID tag readers 120 are preferably installed in the RFID environment 10 such that every RFID tag 101 in the environment can communicate with at least one of the RFID tag readers 120. In a typical installation, the RFID tag readers 120 are mounted to or suspended from the ceiling. If the ceiling is a drop ceiling or secondary ceiling, the RFID tag readers 120can be hung from the ceiling panels, mounted to the ceiling panels, or placed between the ceiling panels and the structural ceiling as disclosed in International Application No. PCT / US2022 / 081761, entitled “Antenna Arrays and Signal Processing for RFID Tag Readers” and filed on December 16, 2022. In addition, or instead, one or more of the RFID tag readers 120 can be mounted to the walls 91 or fixed furnishings 93.

[0049] The cameras 130 capture images of at least portions of the environment 10 and of people and things within the environment 10. The cameras 130 are communicatively coupled to the appliance 140, which receives and processes images from the cameras 130 and tag reply data from the RFID tag readers 120. If desired, the appliance 140 can use the images from the cameras 130 and the data from the RFID tag readers 120 to trigger tag readings by the readers 120 and / or to locate the RFID tags 101 and associate them with people and / or objects in the environment. The appliance 140 can also determine a route from a person’s location to a particular RFID tag 101 / object, for example, if the person is looking for the object or the object should be moved from its current location. The appliance 140 can recognize the person from the image or from an RFID tag 101, smartphone, or other wireless device carried by the person and can trigger a sale of the object or other inventory change based on the person’s movements with the object.

[0050] The RFID tag readers 120 may communicate with each other and / or with the appliance 140 via wireless or wired (e.g., Ethernet) connections. The appliance 140 may be a specialized computing device or a suitably programmed computer, laptop, or smartphone adapted to communicate with the RFID tag readers 120 and issue commands recognizable to the RFID tag readers 120. The appliance 140 can also receive signals from the RFID tag readers 120. For example, the appliance 140 can command the RFID tag readers 120 to inventory all RFID tags 101 (and attached items) in the environment 10 or to determine the location of one or more RFID tags 101 (and attached item(s)) in the environment 10. The appliance 140 can also command the RFID tag readers 120 to query the RFID tags 101 using particular segment settings, including the number of segments and the power levels, modulation frequencies, slot count parameters, and / or durations of the segments. The RFID tag readers 120 can send raw or processed data representing the RFID tags’ replies to the segmented hops to the appliance 140, which uses this data to identify and / or locate the RFID tags 101 and / or attached objects.

[0051] Each RFID tag reader 120 includes an antenna array, such as a four-element square antenna array, that transmits signals to the RFID tags 101 and receives replies from the RFID tags 101. The RFID tag readers 120 switch or hop the signals among different frequencychannels (carrier frequencies), e.g., within bands of 865-868 MHz (Europe) or 902-928 MHz (North America). They can also use their antenna arrays to steer the transmitted signals and / or receptivity patterns to different angles of arrival (AOAs). The RFID tag readers 120 detect replies from the RFID tags 101 with their antenna arrays too.

[0052] These signals and replies can experience attenuation (fading), interference, scattering, and / or other effects as they propagate between the RFID tag readers 120 and the RFID tags 101. This distortion can vary with carrier frequency and tends not to vary with time, neglecting variations due to movement of people (and objects carried by people) about the environment 10. If the fading between a particular RFID reader 120 and a particular RFID tag 101 is too high, or if a furnishing 93, person, or object blocks the pathway between them, then that RFID tag 101 may not detect signals from that RFID tag reader 120 and / or that RFID tag reader 120 may not detect replies from that RFID tag 101.RFID Tag Reader Architecture

[0053] FIG. 3 illustrates a reader 120 suitable for performing a segmented inventory round in greater detail. Unlike conventional RFID tag readers, this reader 120 can optionally be switched between interrogator mode, in which it transmits queries and receives tag replies, and listener mode, in which it does not transmit queries but receives tag replies and queries from other readers. Several of the reader’s components can be enabled or disabled if the reader 120 is in interrogator mode or listener mode. For more on interrogator and listener mode, see, e.g., U.S. Pre-Grant Publication No. 2024 / 0193381 Al, entitled “RFID Tag Readers Switchable Between Interrogator and Listener Modes” and filed on October 20, 2023, which is incorporated herein by reference in its entirety for all purposes.

[0054] The reader 120 includes an RF antenna array and front end 356, a processor 352, an RF calibration and tuning block 354, a hop generator 360, and a hop receiver 370. The RF antenna array and front end 356 may include one or more antenna elements (e.g., arranged in a multielement antenna array), amplifiers, filters, and / or other analog RF components for transmitting RFID interrogation signals 351 and receiving tag replies 353 and, optionally, RFID interrogation signals from other readers. The processor 352 may be implemented in a microcontroller, application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or other suitable device and controls the operation of the reader 120, including, if desired, steering of the reader’s antenna array. It stores information in and retrieves information from a memory (not shown), which may store lookup tables (LUTs), and communicates withthe appliance 140 via a network connection (not shown), such as an Ethernet connection. If the reader 120 is configured to operate in interrogator and listener modes, the processor 352 switches the reader 120 between interrogator and listener modes, with the hop generator 360 being disabled or off in listener mode and enabled or on in interrogator mode and the hop receiver 370 being enabled or on in both modes. The RF calibration and tuning block 354 performs RF calibration and tuning functions.

[0055] The hop generator 360 generates the interrogation signals 111 that the reader 120 transmits to the RFID tags 101. The hop generator 360 is configured to generate interrogation signals 111 at different (increasing) power levels (e.g., in 2 or 3 dB increments) and optionally different (decreasing) modulation rates (e.g., 640 kbps, 320 kbps, 160 kbps, 80 kbps, and so on) for segmented inventory rounds as described above. The hop generator 160 can optionally also generate commands or communications signals intended for other readers 120, e.g., on a dedicated reader communications channel or with particular preambles or payloads. It includes a digital command generator 362, which generates the digital queries, commands, and / or other information conveyed by the interrogation signals 111, and RF electronics 364 for turning the digital signals from the command generator 362 into analog signals suitable for transmission by the antenna array in the front end 356. The RF electronics 364 may include a digital-to- analog converter (DAC) that converts the digital signal into a baseband analog signal, a mixer and local oscillator to mix the baseband analog signal up to an intermediate frequency for broadcast, and filters and / or pulse shapers to remove sidebands and / or spurs.

[0056] The hop receiver 370 includes a receiver front end 372 coupled to a command demodulator 374 and a tag reply demodulator 376. Generally, the receiver front end 372 digitizes, downconverts, and estimates the phases of the RF signals detected by the antenna(s). There are a variety of ways to configure the receiver front end 372; in this example, it receives analog in-phase and quadrature (VQ) signals at higher frequency (e.g., 40 MHz) and converts them into digital VQ samples at baseband (e.g., 5 MHz).

[0057] In interrogator mode, the front end 372 also cancels any self-interference caused by the interrogation signals 111, for example, due to leakage within the receiver. Fortunately, the receiver front end 372 can generally cancel crosstalk between different antenna elements and the circuits coupled to those antenna elements because the crosstalk is correlated with the interrogation signal 111. This crosstalk can be further reduced or suppressed by spacing the antenna elements farther apart from each other as explained in International Application No. PCT / US2022 / 081761, entitled “Antenna Arrays and Signal Processing for RFID Tag Readers”and filed on December 16, 2022, which is incorporated herein by reference in its entirety for all purposes.

[0058] When the reader 120 is in listener mode, it does not transmit an interrogation signal, nor does it perform self-interference cancellation. In listener mode, the reader 120 detects the channels on which the other readers 120 transmit interrogation signals 111 and estimates the frequencies of those other interrogation signals 111.

[0059] The command demodulator 374 is enabled when the reader 120 is in listener mode and demodulates commands from other readers to reproduce the interrogator’s signals at the command bit rate (e.g., 40 kbps to 160 kbps). The command bit rate can vary with the segment, e.g., it can be lower for segments with higher path losses and / or longer durations. The command demodulator 374 uses the command payload to determine what the reader in interrogator mode is asking of the tag 101 (e.g., modulation, preamble type, expected reply type, etc.). For example, the reader 120 in interrogator mode may ask the tag 101 to send the first 64 bits of its EPC using Miller-2 modulation at 320 kHz backscatter link frequency (BLF) with the standard preamble. The readers 120 in listener mode use that information to decode the tag reply 121. The command demodulator 374 is disabled when the reader 120 is in interrogator mode.

[0060] The tag reply demodulator 376 is enabled in both interrogator and listener modes and demodulates the baseband tag reply EQ samples to produce tag reply signals at the tag reply bit rate.Interrogator Controller / Appliance Architecture

[0061] FIG. 4 shows the interrogator controller / appliance 140 in greater detail. The appliance 140 can include one or more processors, non-volatile memories, and other logic devices implemented as integrated circuits and powered by appropriate power supplies and other housekeeping electronics. These processors and logic devices may include discrete components that perform discrete functions and / or more general -purpose components that are programmed to perform a variety of functions, either by themselves or in concert with other components of the appliance 140. For instance, the appliance 140 may include a central processor unit (CPU) 442 running an operating system (e.g., Alpine Linux OS) that manages the appliance’s hardware and software resources, including communications interfaces, shown as Ethernet connections EthO and Ethl, connected to the readers 120, the POS system, and / or other devices. The appliance’s non-volatile memory can store the operating system and other firmware and software as well as tag state information.

[0062] FIG. 4 illustrates the appliance 140 as a block diagram, where each block represents a different function or sub-function performed by the appliance 140. To monitor and update tag states, the appliance 140 includes or implements an in-store message router 480, RFID interrogator controller (RFID-IC) 482, location state manager 484, tag state manager 486, and retail backend application programming interface (API) 488. The in-store message router 480 queues and routes messages exchanged between the readers 120 and RFID-IC 482 via the Ethernet connections EthO and Ethl. The RFID-IC 482 employs a split media access controller (MAC) to handle messages exchanged with the readers 120, with a lower MAC layer implemented in the readers 120 and an upper MAC layer implemented in the RFID-IC 482. The lower MAC layer determines a timestamp and parameters, estimated from the RFID tag’s backscattered response, useful for determining the tag’s position. The upper MAC layer schedules hop transmissions, the number of segments for each hop, and the general purpose of each hop. The lower MAC layer executes more time-critical functions, such as actually scheduling when to transmit commands and how to react to replies within a hop. A positioning layer comprising the RFID-IC and / or the reader(s) 120 calculates the RFID tag’s position in a 3D coordinate system (e.g., Cartesian coordinates with an origin at a known location in the store or room) from data coming from the MAC and PHY layers. The messages from the reader 120 may also include data read from the RFID tag, including the RFID tag’s EPC and other metadata.

[0063] The location state manager 484 and tag state manager 482 track the RFID tag’ s location and state, respectively. The location state manager 484 receives the RFID tag’s estimated location from the RFID-IC 482 (e.g., in a Cartesian coordinate frame with the origin at one corner of the store) and determines where (e.g., the room and zone) in the RFID environment in which the RFID tag is located. The rooms and zones may be extracted from a 3D model of the store or space. In a retail RFID environment, the rooms and zones can include a receiving area, stockroom, sales floor, and changing room, with the sales floor further divided into an entrance / exit zone and a checkout zone. The location state manager 484 updates each RFID tag’s location in an inventory database 490, which may be hosted locally or off site (e.g., in the cloud), based on changes in location detected by the reader(s) 120.

[0064] The tag state manager 486 manages the tag’s state, including its location and availability. There are several possible availability states, including but not limited to: (1) available; (2) stale (optional); (3) ignored; (4) missing; and / or (5) sold. There may be other states as well. The tag state manager 486 transitions the RFID tags 101 among these statesbased on the tags’ responses (or lack of responses) to queries from the readers 120, including information about the tags’ locations, and on the tag states stored in the inventory database 490. For more on tag states and stateful inventory management, please see International Application No. PCT / US2023 / 061645, entitled “Stateful Inventory for Monitoring RFID Tags” and filed on January 31, 2023, which is incorporated herein by reference in its entirety.

[0065] The tag state manager 486 updates the tag states stored in the inventory database 490 and forwards both the tag state and tag location estimate to the retail backend lite API 488, which implements the backend functions for inventory, restocking, and product lookup. The retail backend lite API 488 can implement these functions via a web app gateway 494, which implements a Hypertext Transfer Protocol (HTTP) proxy, redirecting Representational State Transfer (REST) requests to the appropriate backend server (not shown). The web app gateway 494 can also provide user authentication and authorization and serves the static files used by browsers to render web pages.

[0066] FIG. 4 also shows several optional components of the appliance 140, including a raw tag server 492, space server 493, Trivial File Transfer Protocol (TFTP) server 495, multicast Domain Name Service (mDNS) server 496, Network Time Protocol (NTP) server 497, Secure Shell (SSH) server 498, and Secure Sockets Layer (SSL) certificate store 499. The space server 493 handles firmware lifecycle management and configuration of the sensors 120 and Power- over-Ethernet (PoE++) switches (not shown) that connect the appliance 140 to the sensors 120. The raw tag server 492 retrieves tag metadata for legacy APIs, such as those used by API clients for system debugging. When they boot, the sensors 120 download executable images from the TFTP Server 495. The mDNS server 496 enables the appliance 140 to advertise itself using the mDNS and DNS-SD protocols, e.g., during debugging. The NTP server 497 connects and synchronizes with a remote (e.g., Internet-based) NTP server and provides NTP service to the sensors 120. The SSH server 498 is also used for debugging. And the SSL certificate store 499 hosts the server certificates used by the sensors 120 and the web-server certificate used by the REST clients to authenticate the appliance 140.Conclusion

[0067] While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and / or structures for performing the function and / or obtaining the results and / or one or more of the advantages described herein, and each of such variations and / or modifications is deemed to be within thescope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and / or configurations will depend upon the specific application or applications for which the inventive teachings is / are used. Those skilled in the art will recognize or be able to ascertain, using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to 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, if such features, systems, articles, materials, kits, and / or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure.

[0068] Also, various inventive concepts may be embodied as one or more methods, of which an example has been provided. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments.

[0069] All definitions, as defined and used herein, should be understood to control over dictionary definitions, definitions in documents incorporated by reference, and / or ordinary meanings of the defined terms.

[0070] The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

[0071] The phrase “and / or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the components so conjoined, i.e., components that are conjunctively present in some cases and disjunctively present in other cases. Multiple components listed with “and / or” should be construed in the same fashion, i.e., “one or more” of the components so conjoined. Other components may optionally be present other than the components specifically identified by the “and / or” clause, whether related or unrelated to those components specifically identified. Thus, as a non-limiting example, a reference to “A and / or B”, when used in conjunction with open-ended language such as “comprising” can refer, inone embodiment, to A only (optionally including components other than B); in another embodiment, to B only (optionally including components other than A); in yet another embodiment, to both A and B (optionally including other components); etc.

[0072] As used herein in the specification 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” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of components, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of’ or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one component of a number or list of components. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

[0073] As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more components, should be understood to mean at least one component selected from any one or more of the components in the list of components, but not necessarily including at least one of each and every component specifically listed within the list of components and not excluding any combinations of components in the list of components. This definition also allows that components may optionally be present other than the components specifically identified within the list of components to which the phrase “at least one” refers, whether related or unrelated to those components specifically identified. Thus, as a non-limiting example, “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”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including components other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including components other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other components); etc.

[0074] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” “composed of,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of’ and “consisting essentially of’ shallbe closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.

Claims

CLAIMS1. A method of inventorying a first radio-frequency identification (RFID) tag with a first path loss to an RFID tag reader and a second RFID tag with a second path loss to the RFID tag reader greater than the first path loss, each of the first RFID tag and the second RFID tag having an inventoried flag for a first session set to a first state and switchable between the first state and a second state, the method comprising: transmitting, by an RFID tag reader, a first query in the first session at a first power level addressed to RFID tags having respective inventoried flags set to the first state, the first query energizing the first RFID tag and causing the first RFID tag to (i) transmit a reply to the first query and (ii) change the inventoried flag of the first RFID tag for the first session from the first state to the second state for a persistence time; detecting, by the RFID tag reader, the reply to the first query from the first RFID tag; transmitting, by the RFID tag reader while the inventoried flag for the first session of the first RFID tag is in the second state, a second query in the first session at a second power level greater than the first power level and addressed to RFID tags having respective inventoried flags for the first session set to the first state, the second query energizing the first RFID tag and the second RFID tag and causing the second RFID tag to (i) transmit a reply to the second query and (ii) change the inventoried flag of the second RFID tag for the first session from the first state to the second state for the persistence time; and detecting, by the RFID tag reader, the reply to the second query from the second RFID tag.

2. The method of claim 1, wherein the first RFID tag and the second RFID tag conform to the EPC™ Radio-Frequency Identity Protocols Generation-2 UHF RFID Specification for Air Interface Protocol for Communications at 860 MHz - 960 MHz.

3. The method of claim 1, wherein the first power level is sufficient to energize the first RFID tag but not the second RFID tag.

4. The method of claim 1, wherein the second query further causes the inventoried flag of the first RFID tag for the first session to remain in the second state.

5. The method of claim 1, wherein transmitting the second query comprises transmitting the second query without de-energizing the first RFID tag.

6. The method of claim 1, wherein transmitting the second query comprises transmitting the second query at a lower modulation rate than the first query.

7. The method of claim 1, wherein transmitting the second query comprises transmitting the second query over a longer period than the first query.

8. The method of claim 1, wherein transmitting the second query comprises transmitting the second query at a different carrier frequency than the first query.

9. The method of claim 1, wherein the second power level is 2 dB to 3 dB higher than the first power level.

10. The method of claim 1, further comprising: allowing the first RFID tag to become de-energized for less than the persistence time between transmitting the first query and transmitting the second query.

11. The method of claim 1, further comprising: transmitting, by the RFID tag reader, while the inventoried flags of the first RFID tag and the second RFID tag for the first session are in the second state, a third query at a third power level greater than the second power level and addressed to RFID tags having respective inventoried flags for the first session set to the first state, the third query energizing the first RFID tag, the second RFID tag, and a third RFID tag with a third path loss to the RFID tag reader greater than the second path loss and causing the third RFID tag to (i) transmit a reply to the third query and (ii) change an inventoried flag of the third RFID tag for the first session from the first state to the second state for the persistence time; and detecting, by the RFID tag reader, the reply to the third query from the third RFID tag.

12. The method of claim 11, wherein the third query further causes the inventoried flags for the first session of the first RFID tag and the second RFID tag to remain in the second state.

13. The RFID tag reader configured to inventory the first RFID tag and the second RFID tag according to the method of claim 1.

14. A method of inventorying a radio-frequency identification (RFID) tags with different path losses to an RFID tag reader, each of the RFID tags having an inventoried flag for a firstsession set to a first state and switchable between the first state and a second state, the method comprising: transmitting, by an RFID tag reader, a series of queries at increasing power levels addressed to RFID tags with respective inventoried flags for the first session set to the first state, each query in the series of queries energizing at least a subset of the RFID tags and causing the energized RFID tags with inventoried flags for the first session set to the first state to (i) transmit respective replies to that query and (ii) change their respective inventoried flags for the first session from the first state to the second state for a persistence time longer than a total duration of the series of queries; and detecting, by the RFID tag reader, the respective replies to each query in the series of queries .

15. The method of claim 14, wherein the RFID tags conform to the EPC™ Radio- Frequency Identity Protocols Generation-2 UHF RFID Specification for Air Interface Protocol for Communications at 860 MHz - 960 MHz.

16. The method of claim 14, wherein the increasing power levels increase at a rate of 2 dB to 3 dB per query to a maximum power level of 36 dBm.

17. The method of claim 14, wherein transmitting the series of queries comprises transmitting the series of queries without gaps in transmission between successive queries.

18. The method of claim 14, wherein transmitting the series of queries comprises transmitting the series of queries with gaps in transmission shorter than the persistence time between successive queries.

19. The method of claim 14, wherein transmitting the series of queries comprises transmitting different queries at different modulation rates.

20. The method of claim 14, wherein transmitting the series of queries comprises transmitting different queries over different periods.

21. The method of claim 14, wherein transmitting the series of queries comprises transmitting different queries at different carrier frequencies.

22. The RFID tag reader configured to inventory the RFID tags according to the method of claim 14.

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