Frequency channel selection for interrogating RFID tags
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2026-02-05
- Publication Date
- 2026-08-13
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Figure US2026014052_13082026_PF_FP_ABST
Abstract
Description
Attorney Docket No. RADR-032W001Frequency Channel Selection for Interrogating 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 / 754,325, filed on February 5, 2025, 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] A passive RFID tag has no battery and is therefore typically less expensive than a semiactive or active RFID tag. 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 selectively reflecting or backscattering a portion of the RF signal from the RFID tag reader. 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 tagsAttorney Docket No. RADR-032W001within 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), which are divided into frequency channels, each of which is centered on a different carrier frequency with a bandwidth selected to accommodate the signal modulation.
[0005] In the United States, Federal Communications Commission (FCC) regulations require frequency-hopping systems operating in the 902-928 MHz band to hop among frequency channels that are selected at the system hopping rate from a pseudo randomly ordered list of frequency channels. These regulations also require each transmitter to use each channel frequency equally. They also limit the average occupancy time per channel frequency to no more than 400 milliseconds within a 10- or 20-second period, depending on the 20 dB bandwidth of the frequency channel. For more details, please see 47 C.F.R. § 15.247, which is incorporated herein by reference in its entirety.SUMMARY
[0006] The frequency hopping behavior required by FCC regulations effectively prohibits coordinating transmissions by different RFID tag readers. This almost inevitably leads to collisions or interference among transmissions to or from readers within range of each other: nearby readers may transmit on the same frequency channel simultaneously, or a reader may start transmitting on a frequency channel already being used by another reader. Even if nearby readers aren’t broadcasting on the same frequency channel, their transmissions and the corresponding tag replies can still interfere with each other if they are in adjacent or nearby frequency channels. Interference can degrade the fidelity with which the reader(s) and / or tag(s) receive signals, leading the reader(s) to repeat transmissions and increasing the time it takes to interrogate the nearby tag(s).
[0007] The present technology uses listen before talk (LBT), also called listen before transmit or clear channel assessment (CCA), to prevent interference and other problems associated with uncoordinated pseudo-random frequency hopping by RFID tag readers. One example of this technology includes a method of communicating with a radio-frequency identification (RFID) tag. An interrogator controller or an RFID tag reader selects a backscatter link frequency (BLF) for the RFID tag to use when replying to an RFID signal from the RFID tag reader. The interrogator controller or RFID tag reader also selects, based on the BLF, a number N of frequency channels for communicating with the RFID tag, where the number N is an integerAttorney Docket No. RADR-032W001greater than 0 (e.g., an odd integer between 0 and 10). The RFID tag reader selects, at random, a first frequency channel from among a plurality of frequency channels and, starting with the first frequency channel, makes measurements of power levels in each of at least N adjacent frequency channels in the plurality of frequency channels. The RFID tag reader identifies, based on the measurements of power levels, a block of N adjacent frequency channels in the plurality of frequency channels with power levels below a threshold power level (e.g., less than or equal to -60 dBm), which can be based on the BLF and the type of modulation used by the RFID when replying to the RFID tag reader. The RFID tag reader transmits a first signal to the RFID tag in a middle frequency channel of the block of N adjacent frequency channels.
[0008] In another LBT method, an RFID tag reader selects a first frequency channel from among a plurality of frequency channels. Starting with the first frequency channel, the RFID tag reader measures power in each of at least a subset of the plurality of frequency channels. The RFID tag reader identifies at least three adjacent frequency channels in the plurality of frequency channels with power below a threshold power level (e.g., less than or equal to -60 dBm), which can be based on the BLF and the type of modulation used by the RFID when replying to the RFID tag reader. The RFID tag reader transmits a first signal to the RFID tag in a middle frequency channel of the adjacent frequency channels.
[0009] The RFID tag reader can randomly choose one of the plurality of frequency channels as the first frequency channel. It can measure the power of an odd number (e.g., three, five, seven, nine, eleven, thirteen, or more) of adjacent frequency channels. It can also measure the power in each of fifty frequency channels. In some cases, the RFID tag reader can measure the power in each frequency channel by detecting the power in each frequency channel for at least 4 ps per frequency channel. In other cases, the RFID tag reader detects the power in each frequency channel for less than 4 ps per frequency channel.
[0010] In another LBT method, a first RFID tag reader measures power in each of a plurality of frequency channels. The first RFID tag reader identifies, from among the plurality of frequency channels, a block of consecutive frequency channels comprising a target frequency channel having a power greater than a first threshold power level surrounded by exclusion frequency channels having respective power levels below a second threshold power level lower (e.g., less than or equal to about -60 dBm) than the first threshold power level. (The second threshold power level can be based on the BLF of a reply from the RFID tag.) The first RFID tag reader detects a signal transmitted by a second RFID tag reader to the RFID tag on the target frequency channel and / or a reply transmitted by the RFID tag to the second RFID tagAttorney Docket No. RADR-032W001reader on the target frequency channel. The number of frequency channels in the block of consecutive frequency channels may depend on the BLF or data rate of the reply, which may be set or prescribed by the signal from the second RFID tag reader.
[0011] The first RFID tag reader can measure the power of three, five, seven, nine, or more adjacent frequency channels. It can also measure the power in each of fifty frequency channels. Measuring the power in each frequency channel can include detecting power in each frequency channel for at least 4 ps per frequency channel.
[0012] An example RFID tag reader configured perform LBT analysis may include an antenna and a receiver front. In operation, the antenna transmits an interrogation signal to an RFID tag on a target frequency channel in a band of frequency channels. This interrogation signal specifies a BLF and a modulation type, for the RFID to modulate a reply to the interrogation signal. The antenna also receives the reply to the interrogation signal from the RFID tag on the target frequency channel at the BLF. The receiver front end identifies a block of N adjacent frequency channels in a band of frequency channels having power levels less than a threshold power level and selects one frequency channel in the block of N adjacent frequency channels as the target frequency channel, where A is based on the BLF (e.g., N may be an integer between 0 and 10).
[0013] The RFID tag reader can be switchable between (i) an interrogator mode in which the RFID tag reader transmits the interrogation signal to an RFID tag and receives the reply to the interrogation signal from the RFID tag and (ii) a listener mode in which the RFID tag reader receives a reply to an interrogation signal from another RFID tag reader from the RFID tag. In listener mode, the receiver front end is configured to identify a first frequency channel having a power level above a threshold power level adjacent to frequency channels having respective power levels below the threshold power level and the antenna is configured to receive, on the first frequency channel, a reply from the RFID tag to the interrogation signal from the other RFID tag reader.
[0014] 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 alsoAttorney Docket No. RADR-032W001may 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
[0015] 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).
[0016] FIG. 1A illustrates an inventive listen-before-talk (LBT) process for selecting a frequency channel for interrogating one or more RFID tags.
[0017] FIG. IB illustrates the spectrum of a tag reply modulated at a backscatter link frequency (BLF) greater than the spacing between adjacent frequency channels.
[0018] FIG. 1C illustrates the spectrum of a tag reply modulated at a BLF less than the spacing between adjacent frequency channels but with side lobes that extend into adjacent frequency channels.
[0019] FIG. ID illustrates the spectrum of a tag reply modulated at a BLF low enough that the first and second side lobes fit within a single (target) frequency channel.
[0020] FIG. IE illustrates tag replies with BLFs and frequency channels chosen so that the maxima of each reply’s side lobes are aligned with the nulls between the other reply’s side lobes.
[0021] FIG. 2A illustrates RFID tag readers that use an inventive LBT process for hopping among frequency channels used to interrogate RFID tags.
[0022] FIG. 2B illustrates an RFID tag reader in interrogator mode and several RFID tag readers in listener mode interrogating an RFID tag according to an inventive LBT process.
[0023] FIG. 2C illustrates the hop timing used by an RFID tag reader following an inventive LBT process.Attorney Docket No. RADR-032W001
[0024] FIG. 3 is a block diagram of an RFID tag reader configured to hop among frequency channels using an inventive LBT process.
[0025] FIG. 4 is a block diagram of the receiver front end in the RFID tag reader of FIG. 3.
[0026] FIG. 5 A is a block diagram of the channel selector in the receiver front end of FIG. 4.
[0027] FIG. 5B shows a mask indicating the frequency channels for the RFID tag reader to scan in interrogator and listener modes.
[0028] FIG. 6A illustrates a hardware implementation of the channel detection and selection logic in the channel selector of FIG. 5 A.
[0029] FIG. 6B shows a mask identifying the block of frequency channels selected by the channel detection and selection logic for a sensor in interrogator mode to use for its next hop.
[0030] FIG. 7 is a state diagram for an RFID tag reader in listener mode executing an inventive LBT process.DETAILED DESCRIPTION
[0031] Listen-before-talk (LBT) reduces the risk of interference when interrogating RFID tags in a shared radio-frequency (RF) spectrum environment in compliance with FCC regulations. Interference can lead to corruption or distortion of signals transmitted by readers and the replies from tags to those signals, potentially forcing repeated attempts to the tags and increasing the time it takes to interrogate the tags successfully. Interference can also impair synchronization of the readers and of run-time reader calibration.
[0032] In LBT, an RFID tag reader, also called a reader, interrogator, or sensor, senses which frequency channels are unoccupied before picking a frequency channel for its next transmission or hop. LBT can be implemented in a variety of ways. For example, instead of transmitting during a given hop, a reader can measure the (average or instantaneous) power or power spectral density (PSD) on some or all of the frequency channels in the RFID band, then transmit on an unoccupied frequency channel. In the United States, the RFID band extends over a band from 902-928 MHz and includes 50 channels, each 500 kHz wide. A reader can measure the power or PSDs of the frequency channels at a rate of 4.125 ps per frequency channel, or 206.25 ps, for example, to measure the power levels on all 50 channels, then select a frequency channel to use for its next hop from among the frequency channels whose power levels are below a predetermined threshold. Alternatively, a reader can measure the frequency channels one at aAttorney Docket No. RADR-032W001time and begin transmitting as soon as it has identified a set of unoccupied frequency channels. This can be faster than measuring the power levels of all 50 frequency channels because the total measurement time may be lower: if the first five frequency channels are unoccupied, then the reader can start transmitting immediately instead of scanning the remaining 45 frequency channels. This also reduces the risk of another reader transmitting on those five free frequency channels while the reader scans the remaining 45 frequency channels. This leads to potentially lower latency between measuring (“listening”) and transmitting (“talking”), especially if implemented in a field-programmable gate array (FPGA) or other hardware.
[0033] Whether the reader measures the frequency channels one-by-one or all together, the reader generally attempts to identify a target channel for its next transmission. This target channel may be in the middle of a block of several unoccupied frequency channels in a row. The reader transmits on the central frequency channel in the block — the target channel — with the other frequency channels in the block acting as exclusion channels to reduce or mitigate interference with the tag replies, which are at the same carrier frequency as the reader’s transmission but may have bandwidths exceeding the 500 kHz channel spacing as explained in greater detail below.
[0034] FIG. 1 A shows how a reader identifies a block of frequency channels at or before the beginning of a hop in an inventive LBT process. The reader picks a starting frequency channel, at random, for beginning the scan. In this example, the frequency channels have a center-to-center frequency channel spacing equal to their widths; in other examples, there may be guard bands between adjacent frequency channels, causing the spacings to be greater than the widths. The reader measures the (average) power in each frequency channel and compares the measured power to a clear channel threshold power level (e.g., -60 dBm). If the measured power is below the clear channel threshold power level, then the reader determines that the frequency channel is unoccupied and suitable for use as either a target or exclusion channel. Once the reader has identified a block of adjacent frequency channels with measured power levels below the clear channel threshold power level, it picks the central frequency channel as the target channel and a suitable number of adjacent frequency channels as the exclusion channels. In FIG. 1A, there are four exclusion channels — two on each side of the target frequency channel.
[0035] The bandwidth of the tag replies, and hence the number of exclusion channels and the number of frequency channels in the block, depends on the replies’ data rate, which depends on the type of modulation and the backscatter link frequency (BLF). The data rate for FM0Attorney Docket No. RADR-032W001modulation is the BLF, which can be up to 640 kHz, or greater than the channel spacing. The data rates for Miller-2, Miller-4, and Miller-8 modulation are one-half, one-quarter, and one-eighth of the BLF, respectively. At a BLF greater than the channel spacing, the sensor uses enough exclusion channels on either side of the target frequency channel (e.g., two exclusion channels total for a BLF of 640 kHz) as shown in FIG. IB to ensure that signals from other sensors will not drown out or interfere with the much weaker replies.
[0036] The number of exclusion channels and the number of frequency channels in the block may also depend on the type of modulation used by the tag to respond to the reader. Some types of modulation are more robust to interference than other types of modulation and therefore may work satisfactorily with fewer exclusion channels. For example, Miller-2 modulation is more robust to interference than FMO modulation; Miller-4 modulation is more robust than Miller-2 modulation; and Miller-8 modulation is more robust than Miller-4 modulation. FIGS. IB and 1C show how exclusion channels reduce or avoid interference even when the BLF is less than the channel spacing. Each reply is modulated with square-wave modulation at the BLF and has a sinc-like spectrum with side lobes that contain an appreciable amount (e.g., about 10% total) of the power in the reply. Even if the BLF is lower than the 500 kHz channel spacing, as in FIG. 1C, these side lobes can extend into adjacent frequency channels. Selecting enough exclusion channels to accommodate the first harmonic or the first and third harmonics effectively improves the sensor’s sensitivity and extends the range at which the sensor can read a given tag.
[0037] Conversely, if the BLF is low enough that the first and second side lobes fit within the target channel as shown in FIG. ID, then the reader may not use any exclusion channels. Likewise, the reader may use fewer exclusion channels or even no exclusion channels, even for BLFs close to or greater than the channel spacing, if it expects to receive a reply at a high enough signal -to-noise ratio (SNR) (e.g., > 10 dB). If the SNR is high enough, then the extra power in the side lobes does not affect the measurement significantly and so can be neglected in favor of denser channel usage for concurrent or temporally overlapping hops.
[0038] For example, a tag reply modulated at a BLF of 640 kHz has first and third harmonics that fall within about 2.5 MHz of the tag reply’s carrier frequency (center frequency of the target channel). For a channel spacing of 500 kHz, the sensor should use four exclusion channels to avoid interference with the first and third harmonics. A tag reply modulated at a BLF of 320 kHz has a first harmonic that extends to 640 kHz, a third harmonic that extends to 1.28 MHz, and so on, so the sensor can use two exclusion channels on either side as in FIG.Attorney Docket No. RADR-032W001IB - provided that the low-pass filter (LPF) bandwidth is reduced to 1 MHz. Theoretically, a tag reply with a 160 kHz BLF should need a single exclusion channel, and a tag reply with an 80 kHz BLF should have a spectrum that first within the 500 kHz target channel as in FIG. ID.
[0039] If desired, the BLF can be selected to position the side lobe peaks and nulls between side lobes at or near the centers of successive frequency channels as shown in FIG. IE. In this example, the BLF is equal to twice the frequency channel spacing (e.g., 1 MHz). As a result, the nulls and peaks of each reply’s side lobes are aligned in an alternating fashion with the center frequencies of successive frequency channels. This makes it possible to choose exclusion channels in a way that allows the target channels for concurrent hops to be closer together. In FIG. IE, for example, a first sensor uses frequency channel no. 6 as the target channel and frequency channels 2, 4, 8, and 10 as exclusion channels, while a second sensor uses frequency channel no. 9 as the target channel and frequency channel nos. 5, 7, 11, and 13 as exclusion channels. Thus, in this example, the target and exclusion channels are not directly adjoining but instead are interleaved with the target and exclusion channels used for other hops. This allows higher BLFs and more efficient, more complete use of the available frequency channels.Listen-Before-Talk (LBT) for Interrogating RFID Tags
[0040] FIGS. 2A and 2B illustrate a system 100 that uses an interrogator controller 110, also called an appliance, and several RFID tag readers 120a-120d (collectively, sensors or readers 120) to interrogate and optionally locate RFID tags 130, or simply tags, with an inventive LBT process. Like conventional RFID tag readers, the readers 120 hop among frequency channels for transmitting signals to the tags 130. Unlike conventional RFID tag readers, the readers 120 measure the power levels or power spectral densities (PSDs) of several frequency channels before transmitting and select which frequency channel to use (i.e., to transmit on) based on the measurements according to the LBT process. This reduces the risk of multiple readers 120 transmitting on the same frequency channel at the same time, which in turn reduces both interference between signals and the time it takes to interrogate the tags 130. It also reduces the risk of a reader 120 transmitting within the band of a tag’s reply to another reader 120.
[0041] In addition, the readers 120 can be switched between an interrogator mode in which they transmit and receive and a listener or receive-only mode in which they receive but do not transmit. With LBT, one or more readers 120 can be in interrogator mode at the same time, with each transmitting on a different frequency channel, while the other readers 120 withinAttorney Docket No. RADR-032W001range are in listener mode. Each reader 120 that is in interrogator mode interrogates a tag 130. All of the readers 120 within range of tag, including the readers 120 in listener mode, receive and attempt to decode the tag’s reply. For N readers 120, this means making up to N measurements of the tag’s reply simultaneously even though only one reader 120 may be transmitting an interrogation message at a time. This TV-fold increase in the number of simultaneous measurements can be used to increase the speed (e.g., by a factor of TV), fidelity (e.g., by a factor of TV through incoherent averaging), or speed and fidelity of the RFID tag location performed by the system 100.
[0042] The readers 120 may make more simultaneous measurements in a round-robin fashion, with each reader 120 serving as the interrogator in turn while the other readers 120 act as listeners, further increasing measurement speed and / or fidelity. Because the listeners are not powering the tags, and hence do not suffer from self-interference, etc., they can detect tag responses at longer ranges, making it possible to make measurements from distances / locations that are simply not possible with conventional systems. Their measurements can be more susceptible to interference from other transmitters, however, so they use a version of LBT to identify which frequency channel the reader 120 in interrogator mode is (likely to be) using. For more on interrogator and listener modes, please see U.S. Pre-Grant Publication No.2024 / 0193381 Al, entitled “RFID Tag Readers Switchable Between Interrogator and Listener Modes,” which is incorporated herein by reference in its entirety for all purposes.
[0043] The readers 120 are connected to the interrogator controller 110 via respective Ethernet connections 112 or other suitable (usually wired) connections as shown in FIG. 2A. The Ethernet connections 112 can provide electrical power from the interrogator controller 110 to the readers 120 and may connect the readers 120 to each other as well. The interrogator controller 110 has a clock synchronized to network time and may use that clock to synchronize the readers 120, e.g., via the Ethernet connections 112 or using over-the-air transmissions from readers 120 in interrogator mode to readers 120 in listener mode. The readers 120 should be synchronized well enough that when different readers 120 time-stamp the received replies from the same tag 130 to the same interrogation signal 121 sent at the same time, the interrogator controller 110 can group and process the detected replies together. The synchronization should also be good enough to prevent excessive time between hops (e.g., allowing a minimum interhop spacing of 1 millisecond or less). If it is impractical or impossible to synchronize the readers 120, then the readers 120 may simply detect the broadcast commands 121 and report them to the interrogator controller 110 along with the replies 130. The readers 120 can alsoAttorney Docket No. RADR-032W001communicate with each other wirelessly (e.g., over the same RF channel used for communicating with the tags 130) using reader-specific commands instead of via the local area network provided by the interrogator controller 110 and Ethernet connections 112.
[0044] The interrogator controller 110 includes a processor that generates a schedule 113 for interrogating the RFID tags 130. The schedule 113 lists the time(s) at which each reader 120 should be in interrogator mode and in listener mode. The schedule 113 specifies when each hop should start and end. The hops are windows during which readers 120 in listener mode should expect to receive interrogation signals 121 from other readers 120 and tag replies prompted by those interrogation signals 121. The schedule 113 can also include the BLF to be used by the tags 130 when replying during each hop and, optionally, the number of exclusion channels for the readers 120 to use for each hop. The readers 120 can also determine the number of exclusion channels for each hop based on the BLF. The interrogator controller 110 stores the schedule 113 in a local memory coupled to the processor and transmits the schedule 113 to the RFID tag readers 120 via the Ethernet connections 112. (Alternatively, each reader 120 can schedule its hops independently and asynchronously instead of following a schedule set by the interrogator controller.)
[0045] Although the interrogator controller 110 can command readers 120 in interrogator mode when to start their hops (broadcast signals) to the tags 130, FCC regulations prohibit the interrogator controller 110 from specifying the frequency channels for these transmissions in advance. Instead, each reader 120 picks the frequency channel for transmission pseudo-randomly at the beginning of each broadcast or hop. Each reader 120 does this by sensing the power or PSD on that frequency channel and possibly on adjacent frequency channels, depending on the BLF for the hop. A processor in the reader 120 compares these measurements to a predetermined threshold, e.g., using a mask as described below, to identify a block of frequency channels that is not being used by other transmitters, including other readers 120. The reader 120 can compare each measurement to a single predetermined threshold or it can use different predetermined thresholds for different frequency channels. These different predetermined thresholds may depend on the overlap between the frequency channel and the spectrum of the tag reply. For example, a frequency channel that falls into the null between side lobes may be more tolerant to interference and hence subject to a higher threshold than a frequency channel that falls on the peak of the side lobe.
[0046] Each block of frequency channels includes an odd number of frequency channels, with a transmission channel at the center and optionally one or more adjacent channels (e.g., one,Attorney Docket No. RADR-032W001two, three, four, or more channels) on each side of the transmission channel. If the BLF is low enough (e.g., < 80 kHz), then no exclusion channels may be necessary — that is, the mask may include only the transmission channel. Once the reader 120 has identified a suitable block of frequency channels, it begins transmitting on the central frequency channel in that block, with the other frequency channels acting as exclusion channels. The exclusion channels prevent interference from affecting the tag’s reply, which is on the same frequency channel as the reader’s transmission, but may be modulated at a different (e.g., lower) rate and have sidelobes that extend into the exclusion channels.
[0047] The reader 120 can identify a suitable block of frequency channels by selecting a starting frequency channel, optionally at random, and measuring its power level or PSD. If the starting frequency channel’s power level or PSD is below the threshold, then the reader 120 can measure the power levels or PSDs of the adjacent frequency channels (i.e., the next higher and / or lower frequency channels). If the reader 120 encounters a frequency channel whose power level or PSD is above the predetermined threshold while scanning a block of frequency channels, it can either continue scanning or jump, possibly at random, to another frequency channel and begin the scanning process again. In any case, the reader 120 continues to measure power levels or PSDs of frequency channels and compare them to the threshold until it has identified a predetermined number (e.g., three, five, seven, nine, eleven, or any other odd number greater than two) of consecutive frequency channels with power levels or PSDs below the predetermined threshold power level or PSD.
[0048] Alternatively, a reader 120 in interrogator mode can measure the power levels or PSDs of every frequency channel in the frequency band (e.g., from 902-928 MHz) and identify one or more blocks of frequency channels with power levels or PSDs below the predetermined threshold power level or PSD. If the reader 120 identifies one suitable block of frequency channels, then it can transmit on the central frequency channel in that block. If the reader 120 identifies more than one suitable block of frequency channels, then it can pick a block (and central frequency channel) for the next hop at random from among the suitable blocks of frequency channels. For instance, if the reader 120 is looking for a block of at least nine unoccupied frequency channels and identifies ten consecutive frequency channels with power levels or PSDs below the predetermined threshold, it may pick either the fifth or sixth frequency channel at random in that ten-channel block as the transmission channel for the next hop.
[0049] FIG. 2C illustrates the receiver and transmitter timing for a hop executed by a reader 120 that listens before talking. During a clear channel assessment period at the beginning ofAttorney Docket No. RADR-032W001the hop, the reader 120 identifies which, if any, frequency channels are clear (unoccupied) by measuring PSDs and comparing them to a predetermined threshold as described above. The duration of the clear channel assessment period depends on the number of frequency channels being scanned. Listening to a single channel takes approximately 4 ps, corresponding to a maximum total listening time of approximately 200 ps for all 50 frequency channels. (Other implementations of readers may take more or less time to listen to the frequency channels.) This clear channel assessment continues until the reader has identified a block of clear frequency channels or the clear channel assessment period has elapsed. If the hop is scheduled to last 100 ms, then the clear channel assessment could expire at any time up to 100 ms, e.g., after 70 ms, 80 ms, or 90 ms, to ensure that the tags have enough time to respond to the reader’ s transmission. Shorter clear channel assessment periods (e.g., 1 ms, 5 ms, 10 ms, or 25 ms) are also possible. If the reader identifies a suitable block of clear frequency channels during the clear channel assessment period, the reader selects a suitable frequency channel and begins transmitting. If not, the reader can abort the hop.
[0050] The readers 120 can transmit and listen (i.e., measure PSDs) in a staggered or coordinated fashion to prevent or avoid more than one reader 120 broadcasting on the same frequency channel at the same time. For instance, if there are 50 readers 120 within range of each other, each reader can be assigned a different time slot for starting transmission. If each hop takes 100 ms, including a clear channel assessment period at the beginning of each hop of less than 10 ms, then the readers 120 can start their hops at staggered intervals, with the first reader 120 starting at time t = 0 ms, 110 ms, 210 ms, and so on; the second reader 120 starting at t = 10 ms, 110 ms, 210 ms, and so on; the third reader 120 starting at t = 20 ms, 120 ms, 220 ms, and so on.
[0051] Alternatively, if the readers 120 can each scan 50 frequency channels in a scan period of about 200 ps, they can be assigned transmit start slots spaced apart by a period equal to the scan period times the number of readers 120, possibly plus an additional margin to account for processing, etc. If there are 20 readers 120 numbered j = 0 to 19, each reader 120 can be assigned a transmit start slot of 200 / ps within a 4 ms repeating interval, sensor j transmitting at [0.000, 0.004, 0.008, 0.012, ..., 0.996] + 0.0002 / seconds. Each reader 120 could also operate with a slightly different hop duration to reduce the probability that two readers 120 complete transmission at the same time and start scanning again at the start time.
[0052] The windows or periods between hops may be greater than the clear channel assessment period to provide a margin for timing offsets and processing. If desired, the interrogatorAttorney Docket No. RADR-032W001controller 110 can schedule these listening windows or periods for the readers 120 and even re-schedule or command them dynamically, e.g., in response to user requests for interrogation of certain tags 130 or certain areas or by certain readers 120. The window or period between staggered hops can be adapted based on the number of readers 120 operating simultaneously in interrogator mode within range of each other. For a small number of readers (e.g., a number less than or equal to about 10% of the number of frequency channels — 5 or fewer for 50 frequency channels), staggering or offsetting hops by 1 ms may be more than sufficient. For a larger number of readers, the stagger or offset can be larger, e.g., 5 ms or 10 ms, to account for the extra time each reader 120 may have to wait for a suitable frequency channel to become free.
[0053] Once a reader 120 in interrogator mode has selected the target frequency channel for its next hop, it transmits an interrogation signal 121, such as a query or select command, on that frequency channel to one or more tags 130 within range. The tags 130 receive the interrogation signal 121 via a wireless, multipath channel 122 through the store, warehouse, factory, or other environment in which the system 100 is deployed. At least one of the tags 130 responds to the interrogation signal 121 with a tag reply 131 that arrives at the reader 120 in interrogation mode within a predefined time window after the interrogation signal 121. Nearby readers 120 in listener mode detect the interrogation signal 121 and tag reply 131 over the same wireless, multipath channel 122. The readers 120 provide information in and data derived from the tag reply 131 to the interrogator controller 110, which can use the information and data to update a database with the tag’s state and optionally locate the tag 130 within the environment.
[0054] Readers 120 in listener mode can also scan or listen at the start of each hop. Instead of listening forblocks of unoccupied frequency channels, readers 120 in listener mode listen for an occupied frequency channel (i.e., a frequency channel whose power level or PSD exceeds an occupied channel threshold power level or PSD (e.g., -60 dBm), which may be greater than the threshold power level or PSD used to identify unoccupied frequency channels) bordered on each side by at least a predetermined number of (e.g., one, two, three, four, or more) unoccupied frequency channels (exclusion channels). Once a reader 120 in listener mode has identified such an occupied frequency channel, it begins listening to both the interrogation signal(s) 121 and tag replies 131 on that frequency channel for the duration of the hop.
[0055] Like readers 120 in interrogator mode, readers 120 in listener mode can scan or listen to frequency channels according to a schedule dictated to the readers 120 by the interrogator controller 110. For instance, if it takes a clear channel assessment period (e.g., of approximatelyAttorney Docket No. RADR-032W0014-200 ps) at the beginning of each hop for the reader(s) 120 in interrogator mode to select a frequency channel and begin transmitting, the reader(s) 120 in listener mode may not begin scanning for occupied frequency channels until after the end of the clear channel assessment period. In other words, at the beginning of each hop period, a reader 120 in interrogator mode selects and begins transmitting on a frequency channel surrounded by unoccupied exclusion channels. During the beginning of that transmission, when the reader 120 in interrogator mode is broadcasting an unmodulated continuous-wave (cw) signal at the carrier frequency associated with the selected frequency channel, the readers 120 in listener mode listen for that cw signal surrounded by exclusion channels, then select and listen for subsequent modulated signals on the corresponding frequency channel.RFID Tag Reader Architecture
[0056] FIG. 3 illustrates the reader 120 in greater detail, including components that can be enabled if the reader 120 is in interrogator mode and disabled if the reader 120 is in listener mode. The reader 120 includes an RF antenna and front end 210, a processor 212, an RF calibration and tuning block 214, a hop generator 220, and a hop receiver 230. The RF antenna and front end 210 may include one or more antenna elements, amplifiers, filters, and / or other analog RF components for transmitting RFID interrogation signals 121, receiving tag replies 131 and RFID interrogation signals 121 from other readers, and listening (scanning for occupied and unoccupied frequency channels at the beginning of each hop). The processor 212 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. It stores information in and retrieves information from a memory (not shown) and communicates with the system controller via a network connection (not shown), such as an Ethernet connection. The processor 212 can be programmed to scan for and select frequency channels for each hop as described above and switches the reader 120 between interrogator and listener modes, with the hop generator 220 being disabled or off in interrogator mode and enabled or on in interrogator mode and the hop receiver 230 being enabled or on in both modes. The RF calibration and tuning block 214 performs RF calibration and tuning functions.
[0057] The hop generator 220 generates the interrogation signals 121 that the reader 120 transmits to the RFID tags 130 and nearby readers 120 in listener mode (FIGS. 2 A and 2B). It may 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 222, which generates the digital queries, commands,Attorney Docket No. RADR-032W001and / or other information conveyed by the interrogation signals 121, and RF electronics 224 for turning the digital signals from the command generator 222 into analog signals suitable for transmission by the antenna 210. The RF electronics 224 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 the carrier frequency, and filters and / or pulse shapers to remove sidebands and / or spurs.
[0058] The hop receiver 230 includes a receiver front end 232 coupled to a command demodulator 234 and a tag reply demodulator 236. Generally, the receiver front end 232 digitizes, down-converts, and estimates the phase of the RF signals detected by the antenna(s). When the reader 120 is in interrogator mode, the receiver front end 232 measures the power and / or PSD in each frequency channel as part of the LBT process described above. When the reader 120 is in listener mode, the receiver front end 232 detects the frequency channels on which the other readers 120 transmit interrogation signals 121. There are a variety of ways to configure the receiver front end 232; in this example, it receives analog in-phase and quadrature (I / Q) signals at 40 MHz and converts them into digital I / Q samples at baseband (5 MHz) as explained in greater detail below. In other examples, the receiver front end could include a low intermediate frequency heterodyne receiver or other suitable receiver.
[0059] The command demodulator 234 is enabled when the reader 120 is in listener mode and demodulates the baseband command I / Q samples to produce interrogator signals 231 at the command bit rate (e.g., 40 kbps to 160 kbps). The command demodulator 234 uses the command payload to determine what the reader 120 in interrogator mode is asking of the tag 130 (e.g., modulation, preamble type, expected reply type, etc.). For example, the reader 120 in interrogator mode may ask the tag 130 to send the first 64 bits of its electronic product code (EPC) using Miller-2 modulation at a 320 kHz BLF with the standard preamble. The readers 120 in listener mode use that information to decode the tag reply 131. The command demodulator 234 is disabled when the reader 120 is in interrogator mode. The tag reply demodulator 236 is enabled in both interrogator and listener modes and demodulates the baseband tag reply I / Q samples to produce tag reply signals 233 at the tag reply bit rate.
[0060] FIG. 4 shows one of many possible implementations of the receiver front end 232. This implementation includes an analog-to-digital converter (ADC) interface 310 that converts analog I / Q signals (e.g., at 40 MHz) on one or more channels into digital I / Q samples at (e.g., at 40 MS / s). The ADC interface 310 also generates an end-of-hop (EOH) signal when either the hop duration times out at the end of the hop or the clear channel assessment period endsAttorney Docket No. RADR-032W001before the reader 120 has detected suitable frequency channels for the hop. The EOH signal indicates that the reader 120 in listener mode can stop processing the current hop, and aggregate the hop statistics (e.g., power levels, time stamps of various events, etc.). The EOH signals that the receiver should prepare for the next hop or other next action, such as sleep or become idle.
[0061] A downconverter 320 coupled to the output of the ADC 310 mixes the digital I / Q samples down to baseband (e.g., 5 MS / s) and provides the samples to both a channel detector and selector 322 and to a decimator or decimation filter 330. When the reader 120 is in interrogator mode, the channel detector and selector 322 measures the power / PSD in each frequency channel and picks which channel to use for the hop. When the reader 120 is in listener mode, the channel detector and selector 322 detects and selects the channel of the detected interrogation signal 121 as described above. Once the channel detector and selector 322 has selected the frequency channel for the hop, the decimation filter 330 down-samples the digital samples for processing by the command demodulator 234 and tag reply demodulator 236. Depending on the BLF, an optional switchable or programmable filter 340 coupled to the output of the decimation filter 330 can be toggled on to further suppress undesired signals passed by the decimation filter 330 at the possible cost of removing tag reply side lobes. In other words, the receiver identifies the 500 kHz frequency channel for the hop, receives and converts signals on that frequency channel to baseband, and applies a baseband filter. It also down-samples the 40 MS / s digital data stream to 5 MS / s to reduce the digital signal processing load.
[0062] In operation, the channel selector 322 controls the ADC 310 and the downconverter 320 with channel detect and channel select messages. The channel detect message has a dual meaning: in interrogator mode, it indicates whether the reader 120 has detected a center frequency channel and enough adjacent frequency channels free of transmissions and interference; in listener mode, it indicates detection of an active (occupied) frequency channel with clear adjacent frequency channels. If the reader 120 does not detect enough clear frequency channels (interrogator mode) or an active frequency channel (listener mode) within the clear channel assessment period, the channel detect message stops the ADC 310 and downconverter 320 from generating and processing more ADC samples.
[0063] The channel select message indicates which frequency channel the downconverter 320 should convert to baseband. The channel selector 322 selects the value of the channel select message based on the bit-position of the binary ones in the channel mask. The downconverterAttorney Docket No. RADR-032W001320 down-converts that frequency channel to baseband — the channel selector 322 will measure power at DC to determine if there is a signal on that frequency channel. This is effectively a discrete Fourier transform (DFT) where the front end 232 steps through the frequency channels sequentially, downconverting each frequency channel to DC and measuring the DC power on that DC signal.
[0064] FIG. 5A illustrates the channel detector and selector 322 implemented as a correlator 402, power meter 404, channel selection logic 406, and channel detection logic 408. (Other implementations are also possible.) The channel detection logic 408 can be implemented in software executed by the reader’s processor 212 or in hardware, e.g., in a field-programmable gate array (FPGA) that is the processor 212 or is included in the processor 212.
[0065] The correlator 402 measures the power in each of m streams across the 40 MHz band that the sensors use to transmit and receive RFID signals, where each stream is from one of the reader’s m antenna elements (e.g., m = 4). The correlator 402 sums blocks of samples in each stream (e.g., blocks of 80 samples to produce a 500 kHz output from a 40 MHz input) and provides the resulting m power measurements — one for each antenna element stream — to the power meter 404. The power meter 404 averages the measurements for each frequency channel across the m antenna element streams over a predetermined period (e.g., the dwell time of a scanning filter) for each of N integration periods (e.g., N = 1) to yield the average power for each frequency channel. Increasing N increases the amount of averaging, reducing noise, but increases the total measurement time. The power meter 404 provides the average power measurements to the channel detection logic 408.
[0066] The channel detection logic 408 also receives logical inputs from the channel selection logic 406. These logical inputs include a channel select input, a channel skip input, and an available channel input. The channel select input is a channel selection from the set of channels identified by the channel select mask and indicates which frequency channels to scan when searching for target and exclusion channels. The channel skip input indicates which frequency channels in the mask to skip. (In Brazil, for example, the available RFID frequency channels span 902-907.5 MHz and 915-928 MHz, so frequency channels from 908-914.5 MHz should be skipped.) This enables the reader to reset its search for sequential frequency channels, if appropriate. And the available channel input may be a mask that indicates which frequency channels the sensor is allowed to use for transmitting signals, i.e., as target frequency channels.Attorney Docket No. RADR-032W001
[0067] FIG. 5B shows an example channel scan mask. It is a bitmask that indicates which channels the sensor should scan for power. This bitmask is for an FPGA, which uses 32-bit words, so it is split into two masks, one for frequency channels below 915 MHz and one for frequency channels above 915 MHz. The channel scan mask is a superset of the available channel masks. For example, if the available channel mask indicates that the sensor is allowed to transmit only on frequency channels 7, 8, 9, and 10 and the sensor is using two exclusion channels on either side of the center channel, then the available channel mask should include frequency channels 5, 6, 7, 8, 9, 10, 11, and 12, which encompasses all sets of five consecutive frequency channels with the allowed target (center) channels.
[0068] Each time the channel detection logic 408 begins a scan, it can shift the available channel mask to a randomly selected starting frequency channel index either generated by the reader 120 or supplied to the reader 120 by the interrogator controller 110. During the scan, the channel detection logic 408 compares the average detected power or PSD for a block of consecutive frequency channels in the channel scan mask to a detection threshold, which depends on whether the reader is in interrogator or listener mode. In interrogator mode, the reader looks for (unoccupied) frequency channels with power levels below a first threshold (e.g., -65 dBM), and in listener mode, the reader looks for (occupied) frequency channels with power levels above a second threshold (e.g., -60 dBm), which may be the same as or higher than the first threshold. The channel selection logic 408 selects the frequency channel(s) to use for the hop based on these comparisons.
[0069] FIG. 6A illustrates an example implementation of the channel detection logic 408 in logic blocks of an FPGA. Average channel power or PSD measurements flow into a first shift register 502 and then to a first comparator or comparison logic block 504 that compares each measurement to a channel detection threshold, which may depend on whether the reader is in interrogator or listener mode, and outputs a zero (0) or one (1) indicating whether the power / PSD is above or below the detection threshold. This output is coupled to one input of an XOR gate 510 whose other input receives a logical input, called inter enable, indicating whether interrogator mode is enabled. The XOR gate 510 ensures that the reader tests to see if the target channel to be above the channel det thresh in listener mode and below the channel det thresh in interrogator mode. A second comparator or comparison logic block 506 compares each measurement to the interference threshold and provides a zero (0) or one (1) indicating whether the power / PSD is above or below an interference detection threshold for declaring whether exclusion channels are free of interference to a second shift register 508,Attorney Docket No. RADR-032W001which stores one bit for the target frequency channel and one bit for each exclusion channel being used for the hop. The channel skip input inserts a one (1) into the second shift register 508 for every channel to be skipped. An AND gate 512 combines the values stored in the second shift register 508 with the available channel mask. A three-input AND gate 520 implements a logical conjunction of the outputs from the XOR gate 510, AND gate 512, and the available channel mask, which is stored in a third shift register 514. If all three inputs to the AND gate 520 are high, then the AND gate 520 produces a high output indicating detection of a suitable frequency channel for the hop. A fourth shift register 516 indicates which channel is the center or target channel in the current set of consecutive frequency channels and outputs the channel to use for the hop. If the sensor is testing frequency channels 1-5, then it tests channel 5 last and indicates that the sensor can use channel 3.
[0070] FIG. 7 illustrates a process 650 for channel monitoring, detection, and selection by a channel detector and selector 322 in a reader 120 in listener mode. When a hop starts (652), the power detector 404 scans the RFID band for a frequency channel with a PSD greater than the detection threshold PSD (654). When the channel detector and selector 322 scans for the channel with the maximum power in a “scan-for-max” mode, it tests every channel (e.g., all 50 channels). When it finds the channel with the highest or maximum power, it transitions to an ARMED state (656) and then rescans the other channels to make sure that the current channel is truly the channel with the maximum power. After scanning the other channels without finding a higher power, it selects the channel with the maximum power, entering the SELECTED state (658). (If more than one channel has the maximum power, then the channel detector and selector 322 can pick the first or lowest-frequency channel.) The channel detector and selector 322 can also simply pick the first channel that exceeds the detection threshold PSD as the channel with the maximum power (i.e., as the selected channel). The detection threshold can be based on historical measurements of commands from other sensors. For example, if the reader 120 measured a power level of 1000 (arbitrary units) from another reader during a previous hop, the threshold may be set to 800 for that other reader. It is also possible to perform runtime calibrations in which each reader 120 measures the ambient noise when no readers 120 are transmitting and using the ambient noise floor (possibly with a bias, e.g., of 10 dB) as a detection threshold.Conclusion
[0071] 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 forAttorney Docket No. RADR-032W001performing 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 the scope 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.
[0072] 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.
[0073] 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.
[0074] 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.”
[0075] 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 thoseAttorney Docket No. RADR-032W001components 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, in one 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.
[0076] 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.
[0077] 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.
[0078] In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,”Attorney Docket No. RADR-032W001“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’ shall be 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
Attorney Docket No. RADR-032W001CLAIMS1. A method of communicating with a radio-frequency identification (RFID) tag, the method comprising:selecting a backscatter link frequency (BLF) for the RFID tag to use when replying to an RFID signal from an RFID tag reader;selecting, based on the BLF, a number A of frequency channels for communicating with the RFID tag, where the number N is an integer greater than 0;selecting, at random, a first frequency channel from among a plurality of frequency channels;starting with the first frequency channel, making measurements of power levels in each of at least A adjacent frequency channels in the plurality of frequency channels;identifying, based on the measurements of power levels, a block of A adjacent frequency channels in the plurality of frequency channels with power levels below a threshold power level; andtransmitting, by an RFID tag reader, a RFID signal to the RFID tag in a middle frequency channel of the block of A adjacent frequency channels.
2. The method of claim 1, wherein A is an odd integer between 0 and 10.
3. The method of claim 1, wherein the threshold power level is based on the BLF and a type of modulation for the RFID tag to use when replying to the RFID signal from the RFID tag reader.
4. The method of claim 1, wherein the threshold power level is less than or equal to -60 dBm.
5. A radio-frequency identification (RFID) tag reader comprising:an antenna to transmit an interrogation signal to an RFID tag on a target frequency channel in a band of frequency channels, the interrogation signal specifying a backscatter link frequency (BLF) for the RFID to modulate a reply to the interrogation signal, and to receive the reply to the interrogation signal from the RFID tag on the target frequency channel at the BLF; anda receiver front end, operably coupled to the antenna, to identify a block of A adjacent frequency channels in a band of frequency channels having power levels less than a thresholdAttorney Docket No. RADR-032W001power level and to select one frequency channel in the block of N adjacent frequency channels as the target frequency channel, where Ais based on the BLF.
6. The RFID tag reader of claim 5, wherein N is an integer between 0 and 10.
7. The RFID tag reader of claim 5, wherein the RFID tag reader is switchable between (i) an interrogator mode in which the RFID tag reader transmits the interrogation signal to an RFID tag and receives the reply to the interrogation signal from the RFID tag and (ii) a listener mode receives a reply to an interrogation signal from another RFID tag reader from the RFID tag.
8. The RFID tag reader of claim 7, wherein, in listener mode, the receiver front end is configured to identify a first frequency channel having a power level above the threshold power level adjacent to frequency channels having respective power levels below the threshold power level and the antenna is configured to receive, on the first frequency channel, a reply from the RFID tag to the interrogation signal from the other RFID tag reader.
9. A method of communicating with a radio-frequency identification (RFID) tag, the method comprising:selecting, by an RFID tag reader, a first frequency channel from among a plurality of frequency channels;starting with the first frequency channel, measuring, by the RFID tag reader, power in each of at least a subset of the plurality of frequency channels;identifying, by the RFID tag reader, at least three adjacent frequency channels in the plurality of frequency channels with power below a threshold power level; and transmitting, by the RFID tag reader, a first signal to the RFID tag in a middle frequency channel of the at least three adjacent frequency channels.
10. The method of claim 9, wherein selecting the first frequency channel comprises randomly choosing one of the plurality of frequency channels as the first frequency channel.
11. The method of claim 9, wherein measuring the power in each of the at least the subset of the plurality of frequency channels comprises measuring the power of at least nine adjacent frequency channels.Attorney Docket No. RADR-032W00112. The method of claim 9, wherein measuring the power in each of the at least the subset of the plurality of frequency channels comprises measuring the power in each of fifty frequency channels.
13. The method of claim 9, wherein measuring the power in each of the at least the subset of the plurality of frequency channels comprises detecting power in each of the at least the subset of the plurality of frequency channels for at least 4 ps per frequency channel.
14. The method of claim 9, wherein the threshold power level is less than or equal to about -60 dBm.
15. The method of claim 9, wherein the threshold power level is based on a backscatter link frequency of a reply from the RFID tag.
16. A method of communicating with a radio-frequency identification (RFID) tag, the method comprising:measuring, by a first RFID tag reader, power in each of a plurality of frequency channels;identifying, by the first RFID tag reader from among the plurality of frequency channels, a block of consecutive frequency channels comprising a target frequency channel having a power greater than a first threshold power level surrounded by exclusion frequency channels having respective power levels below a second threshold power level lower than the first threshold power level; anddetecting, by the first RFID tag reader, a signal transmitted by a second RFID tag reader to the RFID tag on the target frequency channel and / or a reply transmitted by the RFID tag to the second RFID tag reader on the target frequency channel.
17. The method of claim 16, wherein measuring the power in each of the plurality of frequency channels comprises measuring the power of at least nine adjacent frequency channels.
18. The method of claim 16, wherein measuring the power in each of the plurality of frequency channels comprises measuring the power in each of fifty frequency channels.Attorney Docket No. RADR-032W00119. The method of claim 16, wherein measuring the power in each of the plurality of frequency channels comprises detecting power in each of the plurality of frequency channels for at least 4 ps per frequency channel.
20. The method of claim 16, wherein the second threshold power level is less than or equal to about -60 dBm.
21. The method of claim 16, wherein the second threshold power level is based on a backscatter link frequency of a reply from the RFID tag.