RFID frequency management using precision time protocol

Abstract

Certain aspects and features of the present disclosure relate to a method and / or a system that reduces interference in the operation of multiple radio frequency identification (RFID) readers in proximity to each other. The method uses network time resources available to all RFID readers in a group of readers to schedule frequency channel usage. Centralized control signaling is not required. Readers run a channel hop schedule based on system clocks that are universally synchronized using network time resources, and readers can determine how the other readers are sequencing in frequency by independently accessing or determining information about the other RFID readers in the group.

Description

Attorney Docket No.: 102832-1520032RFID FREQUENCY MANAGEMENT USING PRECISION TIME PROTOCOLCross-Reference to Related Application

[0001] This application claims priority to U.S. Provisional Patent Application 63 / 712,736 filed October 28, 2024, the entire disclosure of which is incorporated herein by reference.Technical Field

[0002] The present disclosure relates generally to radio frequency identification (RFID) systems. More particularly, though not exclusively, the present disclosure relates to systems that involve multiple RFID readers operating in proximity within regulatory regions where multiple carrier frequencies can be used.Background

[0003] RFID systems include, at least, an RFID reader and an antenna. The antenna is used by the RFID reader to transmit interrogation signals to RFID tags and receive responses from the RFID tags. Passive UHF RFID readers can read low-cost battery -free passive RFID tags at many meters distance without line of sight between the RFID reader and the RFID tag. Passive RFID readers transmit with enough power to energize the tags, so that nearby readers can cause interference with an RFID reader. As with most RF systems, maintaining physical and / or frequency distance between readers can reduce interference, as the receive filtering of the RFID reader in question will typically provide adjacent channel rejection.Summary

[0004] Aspects and features of this disclosure include a method of using precision time protocol (PTP) to manage frequencies used over time by RFID readers in a group of readers to minimize interference within the group. The method involves accessing a PTP signal at an RFID reader among a group of RFID readers, and accessing a channel hop sequence, a specified channel dwell time, and information regarding other RFID readers among the group of RFID readers. The method also involves determining an offset within the channel hop sequence based on the information regarding the other RFID readers. ThePage 1 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol method further involves synchronously transmitting an RFID interrogation signal using the PTP signal and the offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence.

[0005] Aspects and features of this disclosure also include a method of configuring an RFID reader to use PTP to manage frequencies used over time to minimize interference within the group. The method involves determining a number of available RFID frequency channels for a target location of an RFID reader. The method also involves selecting, from candidate channel increments, and based on the number of available RFID frequency channels, a channel increment for the RFID reader and producing a channel hop sequence based at least in part on the number of available RFID channels. The method further involves using a specified channel dwell time for the RFID reader and storing the specified channel dwell time and the channel hop sequence in the RFID reader to configure the RFID reader for synchronous RFID interrogation using a precision time protocol (PTP) signal.

[0006] Aspects and features of this disclosure may also include a non-transitory storage medium with instructions to perform the operations of one or more of the methods described. Aspects and features may also include RFID readers and / or processing systems to perform the operations of one or more of the methods described.Brief Description of the Drawings

[0007] FIG. 1 is a system block diagram showing an example of provisioning RFID readers according to some aspects of the present disclosure.

[0008] FIG. 2A and FIG. 2B illustrate two different views of a frequency vs. time plot showing examples of frequency curves for two RFID readers. FIG. 2A is a view showing the frequency channels being used in a free-running fashion, prior to beginning transmission according to a channel hop sequence, and FIG. 2B shows the frequency channels being used with aspects of the present disclosure once the channel hop sequence is engaged.

[0009] FIG. 3 is a diagram illustrating how to arrange readers in two-dimensional space to minimize frequency interference and allow frequency channel reuse according to example embodiments.Page 2 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0010] FIG. 4 is a system block diagram showing an RFID system in which readers according so some aspects of the present disclosure use RF frequency channels to interrogate for RFID tags when system time is synchronized using a network precision time protocol according to example embodiments.

[0011] FIG. 5 is a flowchart of a method to determine a channel increment for constructing a pseudorandom channel hop sequence according to example embodiments.

[0012] FIG. 6 is a flowchart of a method of configuring an RFID reader for synchronous RFID interrogation using a precision time protocol (PTP) signal to avoid interference among the plurality of RFID readers according to example embodiments. The example method of FIG. 6 may make use of the example method of FIG. 5.

[0013] FIG. 7 is a flowchart of an example method of initializing the synchronous transmission of an RFID interrogation signal using a PTP signal and an offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence, according to example embodiments.

[0014] FIG. 8 is an example flowchart of a method to autonomously determine the best offset in a pseudorandom hop sequence to use for transmitting an RFID interrogation signal using a PTP signal where the RFID reader has been configured with reader information according to example embodiments.

[0015] FIG. 9 is an example flowchart of a method to autonomously determine the best offset in a pseudorandom hop sequence to use for transmitting an RFID interrogation signal using a PTP signal where the RFID reader discovered the needed reader information after deployment, according to example embodiments.Detailed Description

[0016] Passive UHF RFID readers can read low-cost battery -free passive tags at many meters distance without line of sight. The readers transmit enough power to energize the tags, so multiple readers in proximity to each other can interfere with one another’s radio receivers. Use of different frequencies for reader operation can mitigate this problem, as the receive filtering on the RFID reader will typically have more rejection the farther adjacent readers are in frequency from a given reader’s own local oscillator. Unlike active radio protocols where many kinds of devices have transmitters, there is no concern of readers having to synchronize channels with the passive tags themselves, sincePage 3 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol the tags have no coherent receiver ability and instead use AM envelope decoding for commands and backscatter modulation to send information to the readers. Thus, tags respond to any reader on any carrier frequency within the frequency range of the tag.

[0017] When two reader signals are received at the same time, a tag can be unable to decode commands from either reader because of the beat frequency in the tag’s envelope detection. This phenomenon of multiple readers causing a tag to be unable to respond is known as “tag confusion” and is a more significant problem if the two readers are close in frequency. Thus, keeping a good frequency separation between nearby readers as they singulate tags improves tag function.

[0018] RFID readers often use industrial, scientific, and medical (ISM) frequency bands throughout the world for unlicensed operation, meaning no explicit license is needed for the frequency usage of the RFID reader if the reader adheres to the frequency use rules of the ISM band. The use of ISM bands may be predicated on laws or administrative regulations wherein operators of RFID equipment cannot allow the equipment to transmit on a fixed frequency channel for longer than some arbitrary time. There are limits on the duration of channel usage before the RFID reader is required to stop transmitting on a currently used channel and switch to a new channel. Further, some laws and / or regulations pertaining to ISM equipment, including RFID readers, may prohibit explicit, electronic signaling to provide frequency coordination among RFID devices unless the entire multi-reader system is tested and qualified together. Such system qualification can be quite costly and burdensome, as the configuration of RFID readers needed at a location may not be known until close to deployment, and may change as readers are replaced, updated, etc. Thus, manual monitoring and correction at some interval may be required.

[0019] Certain aspects and features of the present disclosure relate to a method and / or a system that reduces interference in the operation of multiple RFID readers in proximity to each other. Certain aspects and features relate to a process that uses network time resources available to readers in a group to schedule frequency channel usage. Centralized control signaling is not required. Readers run a channel hop schedule based on system clocks that are universally synchronized using network time resources, and readers can determine how the other readers are sequencing in frequency by independently accessing or determining information about the other RFID readers in the group.Page 4 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0020] As examples, this disclosure generally uses UTC time, meaning midnight on January 1, 1970, is the time origin, t=0, also known as the epoch. When using a network time protocol such as network time protocol (NTP), simple network time protocol (SNPT), or PTP, the devices have a common definition of time. For purposes of this disclosure all of the above standardized network time protocols, servers, etc. as well as any others, are referred to using the moniker “precision time protocol” (PTP). This disclosure assumes continuity from the right when considering channel frequency as a function of time. This means that at frequency channel switch times, the channel frequency is equal to the new channel frequency starting at that switch time.

[0021] An RFID reader can be configured for use in a group of RFID readers prior to deployment by determining a number of available RFID frequency channels for a target location of an RFID reader. A “target location” for purposes of this disclosure is meant to refer to a location where a specific regulatory framework applies to RFID systems, usually a country. A channel increment for the RFID reader can be selected and a channel hop sequence can be produced and stored, based at least in part on the number of available RFID channels and the channel increment. A channel dwell time for the RFID reader can also be specified and stored. If the number and identity of the readers in the group are known, additional information may be stored for access by the reader at initialization or boot up, for example, a channel step size and hope sequence based on channel and step factors determined through factorization calculations.

[0022] Once deployed, a reader according to aspects of this disclosure can use precision time protocol (PTP) to manage frequencies used over time, taking into account frequency characteristics that can be derived for RFID readers in the group of readers in order to minimize interference within the group. A reader in some examples accesses a channel hop sequence, a specified channel dwell time, and information regarding other RFID readers. The reader can determine an offset within the channel hop sequence based on information regarding the other RFID readers and synchronously transmit an RFID interrogation signal using the PTP signal and the offset; the reader’s transmission varies in frequency according to the channel hop sequence.

[0023] Information about the other readers used to manage transmissions based on the PTP signal can be configured prior to deployment if a specified group of readers is identified to be deployed and used together. Otherwise, a reader can determine thePage 5 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol information needed by listening to other readers that have already been initialized. For example, the reader can cycle through the channel hop sequence in listening mode and determine signal strength statistics for the available RFID frequency channels. An appropriate offset can be calculated by the reader itself using these signal strength statistics.

[0024] The RFID readers according to embodiments herein transmit as necessary while maintaining a minimum channel spacing relative to transmissions from nearby readers. In some examples, RFID readers manage frequency channels using just three parameters: (1) a specified channel hop sequence, (2) a specified channel dwell time, and (3) an offset into the hop sequence computed relative to a specified time, for example time t = 0.

[0025] These illustrative examples are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts. The following paragraphs describe various additional features and examples with reference to the drawings in which like numerals indicate like elements, and directional descriptions are used to describe the illustrative aspects but, like the illustrative aspects, should not be used to limit the present disclosure.

[0026] FIG. 1 is a system block diagram showing an example of a system 100 in which multiple RFID readers, for example, reader 1000 and reader 1003, are configured to operate according to the example described herein. The readers have antenna ports which are connectable to RFID antenna 2000 and RFID antenna 2002. The RFID readers may have additional antenna ports. Once operational, the readers will transmit radio signals to RFID tags 3000 using one of the attached antennas. The tags will communicate back to the readers by modulating information in their radar cross section, thereby creating a modulated backscatter signal which the readers demodulate.

[0027] Reader 1000 includes a digital signal processor (DSP) 1002, memory 1004, a radio 1008 connectable to the antennas, and a network interface 1010. Any of various types of network interfaces can be used. In some examples, the readers use Ethernet. The memory serves as a non-transitory computer readable medium to store software 1006 (microcode, firmware, instructions, or the like) that is associated with the DSP and is executed on the DSP to operate RFID reader 1000.Page 6 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0028] Reader 1003 includes a digital signal processor (DSP) 1012, memory 1014, a radio 1018 connectable to the antennas, and a network interface 1020. Any of various types of network interfaces can be used. Memory 1014 serves as a non-transitory computer readable medium to store software 1016 (microcode, code, firmware, instructions, or the like) that is associated with the DSP and is executed on the DSP to operate RFID reader 1003.

[0029] FIG. 1 shows the readers’ network connections attached to a network interface 4000, for example, an Ethernet switch, router, or network interface modem with integrated Ethernet switch. In this example, the network interface connects these readers to the enterprise network 104, which in turn connects to reader configuration functionality including processor 101 and reader configuration software 102. This reader configuration functionality, as an example, includes code or computer program instructions to configure the readers as described herein. The code or instructions may reside on a server, workstation, tablet, notebook computer, or any other computing device (not shown).

[0030] FIG. 1 also shows a reader-to-reader interference path 2100 wherein the transmission from a reader will be received by another reader when the two readers are operating. In this example, such interference is undesirable. This reader-on-reader interference can be referred to herein as “adjacent reader interference.” There would be some pathloss associated with interference path 2100. The interference pathloss is symmetric for monostatic antennas, though it is frequency dependent. Thus, on a given channel, the adjacent reader interference pathloss from one reader to another is substantially the same in both directions.

[0031] Still referring to FIG. 1, the reader radios 1008 and 1018 incorporate receive filters to reduce interference but these filters are most effective when the adjacent reader’s signal is a significant frequency distance away. This disclosure describes techniques, which maximize the frequency separation between readers by making use of a PTP signal from PTP server 3500. These techniques improve overall system performance and may allow lower complexity and less expensive receive filters, providing for the construction of less expensive RFID readers.

[0032] FIG. 2A shows an example time domain plot 200 of the frequency channel usage in a 9-channel frequency hop spread spectrum system (FHSS) for RFID. It can be assumed for purposes of this disclosure that the system is operating in a 50-channel FHSSPage 7 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol regulatory region for RFID with a maximum 400 millisecond use time on average for each channel within a 20 second period. There are regions throughout the world using other numbers of channels but 400 milliseconds is typical of the maximum use time in the United States and many other regions. Europe, India, and other regions can be referred to as European Telecommunication Standards Institute (“ETSI”) regions. In ETSI regions there are often fewer channels with no requirement to use all of the channels. The maximum use time in ETSI is typically 4 seconds, and when that limit is met, the RFID reader either needs to switch to another channel or turn off for at least 100 milliseconds.

[0033] FIG. 2A is an example FHSS with 1 second maximum use time and nine channels. FIG. 2A shows the channel verses time plots for two readers, plot 220 and plot 230. The readers both use the same sequence and timing but are free running with arbitrary start time and unsynchronized system clocks. In this sample from an ensemble of many possible waveforms, the two readers are sometimes close in frequency, and other times farther, but this state is random even though the frequency distance can have a profound impact on RFID system performance.

[0034] FIG. 2B shows an example time domain plot 300 of the frequency channel usage in the same hypothetical 9-channel FHSS system as in FIG. 2, except that the two readers, using example embodiments described herein, have synchronized clocks and are following the same channel hop sequence while being offset from one another in the channel hop sequence. In this case, channel usage 320 of one reader is consistently separated in frequency from channel usage 330 of the other reader.

[0035] FIG. 3 is a diagram showing an example of the tiling of two-dimensional space using hexagons 400. Using a hexagon to model the coverage of a reader, a tiling pattern involving four distinct frequencies vs. time usages can provide good spatial isolation between any two readers using the same frequency vs. time function. In this example then, A / =4, where AT is the number of readers in a group of readers. However, smaller or larger numbers of readers may be used together while employing the embodiments described herein to maintain frequency separation between readers. The use of a pre-planned group of readers with assigned reader number will be discussed in more detail below with respect to FIG. 6.

[0036] FIG. 4 is a system block diagram 400 showing an example of the incorporation of network PTP -based control into RFID readers 1000 and 1002, once thePage 8 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol readers have been configured as shown in FIG. 1 and have been initialized during or just after deployment. A PTP subsystem implemented at least in part by computer program code within memory 1004 and memory 1014 ensures that the system time within readers 1000 and 1002 is tightly synchronized with each other and with that of other readers in the group.

[0037] Continuing with FIG. 4, the memory 1004 serves as a non-transitory computer readable medium to store software (microcode, firmware, instructions, or the like) that is associated with the DSP and is executed on the DSP to operate the RFID reader 1000. In this example, software in memory 1004 of reader 1000 includes instructions 1013 that provide PTP control to operate the reader to synchronize time over the network via some precision time protocol such as IEEE-1588. The software can also use the network time to schedule radio frequency channel usage according to a frequency channel hop table representing a channel hop sequence according to some embodiments. The software can also optionally use a listen function to adjust the channel usage. From the synchronized time, the reader 1000 also runs computer program code, which provides a frequency hop timer algorithm to determine the current hop channel to be used.

[0038] Staying with FIG. 4, the memory 1014 serves as a non-transitory computer readable medium to store software (microcode, firmware, instructions, or the like) that is associated with the DSP and is executed on the DSP to operate the RFID reader 1003. In this example, software in memory 1014 of reader 1003 includes instructions 1023 that provide PTP control to operate the reader to synchronize time over the network via some precision time protocol such as that defined by IEEE standard 1588. The software can also use the network time to schedule radio frequency channel usage according to a frequency channel hop table representing a channel hop sequence according to some aspects of this disclosure. The software can also optionally use a listen function to adjust the channel usage. From the synchronized time, the reader 1003 also runs computer program code 1024, which provides a frequency hop timer algorithm to determine the current hop channel to be used. Network interface 4010 connects the readers to an enterprise network, WAN, or cloud computing system 5010. This connectivity can provide for updating the readers, receiving RFID even information, or other remote functions.Page 9 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0039] FIG. 5 is a flowchart illustrating an example method 1300 for determining a hop sequence channel increment to be used to construct a pseudorandom hop sequence according to example embodiments. In this example, method 1300 is executed by processor 101 executing reader configuration software 102 in FIG. 1, prior to deployment or installation of the reader. The process starts at block 1305, where the number of available channels N is determined. In this example, the number is fixed by region and may be determined based on input provided when configuring a reader for deployment, or by look up in a database, as examples. At block 1310 the processor factors number N into primes, referred to herein as prime channel factors. Block 1315 is a loop block by which a processor checks channel increments from 1 to TV- 1. At block 1320 the channel increment loop variable n is factored by the processor into primes, referred to herein as prime increment factors.

[0040] Continuing with FIG. 5, at block 1325, the processor determines any commonality between n and A, that is, if n and N share any common factors, and if so, the loop continues back to block 1315. If there are no common factors between n and A, then at block 1330, the magnitude difference abs(A / 2 - ri) is calculated. The magnitude difference from A / 2 is minimized so as to find the best channel increment. At block 1335, the processor determines if the current increment n yields a smaller magnitude difference than previous, tried increments, and if so, saves n as the best current increment at block 1340. Otherwise, the process continues iterating at block 1315 until all potential channel increments are checked, then the determined best increment is saved at block 1345.

[0041] FIG. 6 is a flowchart of an example method 600 for configuring the RFID reader for synchronous RFID interrogation using a precision time protocol (PTP) signal to avoid interference among the plurality of RFID readers according to example embodiments. The example method of FIG. 6 may make use of the example method of FIG. 5. The operations of this method are executed by a processor 101 performing reader configuration using software 102, shown in FIG. 1.

[0042] At block 602, the processor determines a number of available RFID frequency channels for a target location of an RFID reader, as described with respect to block 1305 in FIG. 5. At block 604, the processor selects, from a plurality of candidate channel increments, and based on the number of available RFID frequency channels, a channel increment for the RFID reader. Block 604 may be executed as described withPage 10 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol respect to blocks 1310 through 1345 of method 1300 shown in FIG. 5. At block 606, the processor produces a channel hop sequence based at least in part on the number of available RFID channels and the channel increment.

[0043] Continuing with FIG. 6, at block 608, the processor accesses a specified channel dwell time for the RFID reader and optionally accesses information about the readers in a predetermined group of readers that are to be deployed to a specific location. This information may be input by a user or stored in a database that is accessible to the reader being configured. The information about the other readers in a preconfigured group will be used to select an offset into the channel hop sequence for synchronous transmission of interrogation signals, ultimately leaving each reader in the group with a different offset. As an example, for vehicle tolling, a group of readers may be preconfigured for a specific tolling arch or plaza. As another example, readers may be configured in groups with a specified number of readers per group, as shown in and discussed above with respect to FIG. 3. The number of readers, the assigned reader numbers, and other information is known, and a reader can use this information at initialization (e.g., boot up) to determine an offset. This option will be described below with respect to FIG. 8.

[0044] Information about other readers may not be known prior to deployment in some circumstances, for example, in a retail or warehouse environment where readers are put into service as expansion is needed. In this instance, the reader gathers information about other readers as needed after installation. This option will be described in more detail below with respect to FIG. 9. In either case, referring again to FIG. 6, the processor configures the reader using the channel hop sequence, channel dwell time, and information about other readers, if available, at block 610. Method 600 configures the RFID reader for synchronous RFID interrogation using a precision time protocol (PTP) signal to avoid interference among the plurality of RFID readers.

[0045] FIG. 7 is a flowchart of an example method 700 of initializing the synchronous transmission of an RFID interrogation signal using a PTP signal and an offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence, according to example embodiments. Method 700 in this example is executed by a processor running computer program instructions. For example, a processor such as DSP 1002 and / or DSP 1012 can execute the instructions which are stored in non-Page 11 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol transitory memory 1004 or 1014, respectively. These instructions include but are not limited to those for PTP control as discussed above with respect to FIG. 4.

[0046] At block 702 of FIG. 7, the processor accesses a PTP signal at an RFID reader in a group of RFID readers. At block 704, the processor accesses the channel hop sequence, the specified channel dwell time, and information regarding other RFID readers in the group. The channel hop sequence and specified channel dwell time have been stored in the reader prior to deployment, as already discussed. Some information regarding the other readers may have been stored when the reader was configured and this information can be used to initialize the reader, as will be described below with respect to FIG. 8. This may be the case, as an example, when the reader is deployed in a preconfigured group. Alternatively, information, not necessarily the same information, about the other readers in the group may be accessed by the processor autonomously making measurements of the RF environment and storing the results, as will be discussed below with respect to FIG. 9. This may be the case, as an example, when the reader is deployed in a configuration that is not known in advance.

[0047] Continuing with FIG. 7, at block 706, the RFID reader determines an offset within the channel hop sequence based on the information regarding the other RFID readers. At block 708, the processor begins synchronously transmitting an RFID interrogation signal using the PTP signal and the offset. The RFID interrogation signal varies in frequency according to the channel hop sequence. The exact transmission time may be corrected slightly when and if needed as indicated by the PTP signal. Since all readers in the group operate in a different slot in the channel hop sequence at any given time, and all are synchronized in time without direct communication, both interference among readers and tag confusion are minimized.

[0048] FIG. 8 is a flowchart illustrating an example method 1400 of computing the current frequency channel hop sequence index. Method 1400 makes use of information that is stored at the time of configuration for deployment. This information includes the channel hop sequence and the available RFID frequency channels, as well as the current time based on the PTP signal. This information also includes information about the readers in the installation group, for example, the total number of readers and the reader number among the numbers assigned to the readers. At block 1405, the processor in the reader accesses the stored or otherwise available parameters:Page 12 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol- 1, the current network time- Z, the reader’s configured, specified channel dwell time. The channel dwell time is the amount of time the reader stays on a frequency channel before advancing to the next channel in the channel hop sequence. The dwell time T cannot exceed the maximum use time of the regulatory region being used.- M, the number of readers. This number can represent the number of distinct readers used in some tiling of space as in FIG. 4. More generally, this can be the number of readers used in a one-dimensional tiling, as when readers are arranged on an RFID vehicle tolling gantry, or even a two-dimensional tiling as shown in FIG. 4, or any other arrangement.- m, the integer reader number assigned with respect to the other readers in the installation.- N, the number of available frequency channels in the regulatory region.

[0049] Continuing with FIG. 8, at block 1410 the current dwell slot number is calculated by the reader, where ceil() is the ceiling function. A ceiling function rounds the input number up to the next decimal place or integer. At block 1415 the processor in the reader calculates a modulo AT reader index number k. While the assigned reader number m could be distinct, the resulting reader index number k is constrained to 0<=k<Mby the modulo calculation. At block 1420 the target channel separation is computed. In this example, this value is a real number and not constrained to be an integer. The target channel separation is the best-case channel separation given AT distinct reader index numbers and N available regulatory channels. At block 1425 the time 0 initial channel is computed using a rounding function, round(). At block 1430, the processor computes the current channel for reader m by determining the channel at time t=0 (the epoch) and rotating through the available channels to the current dwell slot number for modulo N channels. This current channel represents an offset into the channel hop sequence for the readers in the group of readers.

[0050] As an example, consider a reader arrangement with AZ=7 and N=50. The resulting sequence is shown in Table 1, below:Page 13 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time ProtocolPage 14 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time ProtocolEach reader sequences through the same pseudorandom channel hop sequence. The channel hop sequence is constructed in part using method 1300 shown in and discussed with respect to FIG. 5, and the offsets of the readers are computed as in method 1400 of FIG 8. The minimum channel separation is seven channels across the whole sequence.

[0051] FIG. 9 shows a flowchart for an example method 1500 for an RFID reader such as reader 1000 or reader 1003 in FIG. 5 to autonomously select the best frequency channel within the hop sequence for use when starting to transmit. This is a listen-before- talk operation to sense carrier levels from nearby RFID readers which are using compatible PTP-controlled dwell times (7) and channel hop sequences. Method 1500 can be used in deployments where complete information about the readers in a group of readers is not known in advance. The method starts at block 1505, when the processor in an RFID reader is used to estimate the optimum position in the channel hop sequence by listening to adjacent readers. Thus, the channel hop sequence and the specified channel dwell time T as inputs at block 1505 are known because they were stored in the reader when the reader was being configured for deployment in a group of readers.

[0052] At block 1510 the RFID reader tunes its radio’s frequency synthesizer to a mid-band frequency channel in order to access adjacent channels both above and below its own channel. The various other readers in the group may be transmitting both below and above the frequency where the RFID reader performing the method is initially transmitting. Using a band edge channel would be less useful because the channel distance metrics would be biased as compared to a mid-band channel. Using the mid-band channel allows the RFID reader selecting its offset determine signal strength statistics of both adjacent channel and co-channel transmissions from other readers. If the reader’s radio is AC coupled then the radio could be tuned to be slightly off channel so that cochannel signal strength can be estimated more readily. At block 1515 the processor in the reader computes the next frequency switch time given the current time t and channel dwell time T. The reader waits for the next frequency switch time before proceeding.Page 15 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0053] Continuing with FIG. 9, at block 1520, the process begins a loop and continues looping through the frequency channels in the channel hop sequence. Thus, this loop as shown in blocks 1520 and 1525 lasts for N*T seconds. For each dwell period, the processor at block 1525 samples the receiver signal strength for co-channel and adjacent channel signals on any antennas to which it is connected. Because nearby readers will also be switching between antennas while the frequency dwell persists, it is useful for the listening reader to sample all of its active antennas as often as it can within the dwell time to gather signal strength statistics for the available RFID frequency channel. More samples will typically lead to a more reliable estimation for the optimum hop table offset. At block 1530, when signal strength statistics for all channels in the hop sequence have been heard, the signal strength statistics are sorted by the processor according to some optimality criteria such as a minimum weighted average of the sampled co-channel and adjacent channel receive signal strength values. These signal strength statistics provide the information regarding other readers in the group that is used to determine an offset into the hop sequence. The hop table (channel hop sequence) offset, which provides the optimum listening mode signal strength statistics serves as the offset into the channel hop sequence. At block 1535 the RFID reader tunes its radio frequency synthesizer to the selected channel and begins reading tags.

[0054] To implement the functions of the devices described herein, a general- purpose processor such as a DSP, microcontroller, embedded controller, processor, or microprocessor can be used and firmware, software, or microcode can be stored in a tangible or non-transitory storage medium that is associated with the processor. Such a storage medium may be a memory integrated into the processor or maybe a memory chip that is addressed by the processor to perform control functions. Such firmware, software or microcode is executable by the processor and when executed, causes the processor to perform its control functions. Such firmware or software could also be stored in or on a tangible or non-transitory medium such as an optical disk or traditional removable or fixed magnetic medium such as a disk drive used to load the firmware or software into an RFID device or other RFID device. An RFID device in this context can refer to a reader, a mux, a server or other computer used to configure RFID readers, some other device, or any combination of the foregoing.Page 16 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol

[0055] The reader described as an example in this disclosure may also be implemented using a discrete component RFID reader design, such as one based on physically separate chips that provide DACs, ADCs, mixers, amplifiers, couplers, and the like. A processing function for a reader, as well as additional functional bocks or circuits in the reader can be implemented on a field programmable gate array (FPGA), or on an application specific integrated circuit (ASIC). A reader may also be implemented as a system-on-a-chip (SoC), wherein many of the subsystems are integrated together on a chip. Sometimes multichip SoC solutions can be used to ease manufacturability given the variations in process which may be required based on frequency, power, and the like.

[0056] The foregoing description of the examples, including illustrated examples, has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the subject matter to the precise forms disclosed. Numerous modifications, combinations, adaptations, uses, and installations thereof can be apparent to those skilled in the art without departing from the scope of this disclosure. The illustrative examples described above are given to introduce the reader to the general subject matter discussed here and are not intended to limit the scope of the disclosed concepts.Page 17 of 24US2008 32083532 1

Claims

Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time ProtocolClaimsWhat is claimed is:

1. A method comprising: accessing, by a radio frequency identification (RFID) reader among a plurality of RFID readers, a precision time protocol (PTP) signal; accessing, by the RFID reader, a channel hop sequence, a specified channel dwell time, and information regarding other RFID readers among the plurality of RFID readers; determining, by the RFID reader, an offset within the channel hop sequence based on the information regarding the other RFID readers; and synchronously transmitting an RFID interrogation signal using the PTP signal and the offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence.

2. The method of claim 1, wherein the information comprises signal strength statistics, the method further comprising: cycling through the channel hop sequence using the RFID reader in listening mode; determining, based on the cycling, the signal strength statistics for each of a plurality of available RFID frequency channels; and calculating the offset using the signal strength statistics.

3. The method of claim 2, further comprising: tuning the RFID reader in listening mode to a mid-band frequency to access adjacent channels above and below an initial listening channel; and cycling through the channel hop sequence beginning at the mid-band frequency to determine the signal strength statistics.

4. The method of claim 2 or claim 3, wherein the signal strength statistics comprise a minimum weighted average of sampled co-channel and adjacent channel receive strength values.Page 18 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol5. The method of claim 1, wherein the information comprises a number of readers in the plurality of RFID readers and an assigned reader number for the RFID reader, wherein the method further comprises: computing a current dwell slot number for the RFID reader; computing a synchronous start time for the number of readers using the PTP signal; and determining, using the current dwell slot number and the synchronous start time, a current channel for the RFID reader, the current channel defining the offset within the channel hop sequence.

6. The method of claim 5, further comprising computing the current dwell slot number using a ceiling function of a current time from the PTP signal and the specified channel dwell time.

7. The method of claim 5 or claim 6, further comprising: computing a target channel separation for the plurality of RFID readers based on the number of readers and available RFID frequency channels; and computing the synchronous start time based at least in part on the target channel separation.

8. A radio frequency identification (RFID) reader comprising: a processor; and a non-transitory storage medium associated with the processor, the non-transitory storage medium including instructions executable by the processor to cause the processor to: access a precision time protocol (PTP) signal at a radio frequency identification (RFID) reader among a plurality of RFID readers; access a channel hop sequence, a specified channel dwell time, and information regarding other RFID readers among the plurality of RFID readers; determine an offset within the channel hop sequence based on the information regarding the other RFID readers; andPage 19 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol synchronously transmit an RFID interrogation signal using the PTP signal and the offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence.

9. The RFID reader of claim 8, wherein the information comprises signal strength statistics, and wherein the instructions are executable to cause the processor to: tune the RFID reader in listening mode to a mid-band frequency to access adjacent channels above and below an initial listening channel; cycle through the channel hop sequence beginning at the mid-band frequency to determine the signal strength statistics. determine, based on the cycling, the signal strength statistics for each of a plurality of available RFID frequency channels; and calculate the offset using the signal strength statistics.

10. The RFID reader of claim 9, wherein the signal strength statistics comprise a minimum weighted average of sampled co-channel and adjacent channel receive strength values.

11. The RFID reader according to any of claims 8 through 10, wherein the information comprises a number of readers in the plurality of RFID readers and an assigned reader number for the RFID reader, and wherein the instructions are executable to cause the processor to: compute a current dwell slot number for the RFID reader; compute a synchronous start time for the number of readers using the PTP signal; and determine, using the current dwell slot number and the synchronous start time, a current channel for the RFID reader, the current channel defining the offset within the channel hop sequence.

12. The RFID reader of claim 11, wherein the instructions are executable to cause the processor to compute the current dwell slot number using a ceiling function of a current time from the PTP signal and the specified channel dwell time.Page 20 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol13. The RFID reader of claim 11 or claim 12, wherein the instructions are executable to cause the processor to: compute a target channel separation for the plurality of RFID readers based on the number of readers and available RFID frequency channels; and compute the synchronous start time based at least in part on the target channel separation.

14. A system comprising the plurality of RFID readers including the RFID reader according to any of claims 8 through 13, wherein each of the plurality of RFID readers uses a different offset.

15. A non-transitory computer-readable medium storing executable instructions, which when executed by a processor, cause the processor to perform operations comprising: accessing a precision time protocol (PTP) signal at a radio frequency identification (RFID) reader among a plurality of RFID readers; accessing a channel hop sequence, a specified channel dwell time, and information regarding other RFID readers among the plurality of RFID readers; determining an offset within the channel hop sequence based on the information regarding the other RFID readers; and synchronously transmitting an RFID interrogation signal using the PTP signal and the offset, wherein the RFID interrogation signal varies in frequency according to the channel hop sequence.

16. The non-transitory computer-readable medium of claim 15, wherein the information comprises signal strength statistics, and wherein the operations further comprise: cycling through the channel hop sequence using the RFID reader in listening mode; determining, based on the cycling, the signal strength statistics for each of a plurality of available RFID frequency channels; andPage 21 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol calculating the offset using the signal strength statistics.

17. The non-transitory computer-readable medium of claim 16, wherein the operations further comprise: tuning the RFID reader in listening mode to a mid-band frequency to access adjacent channels above and below an initial listening channel; and cycling through the channel hop sequence beginning at the mid-band frequency to determine the signal strength statistics.

18. The non-transitory computer-readable medium of claim 17 or claim 16, wherein the signal strength statistics comprise a minimum weighted average of sampled co-channel and adjacent channel receive strength values.

19. The non-transitory computer-readable medium according to any of claims 15 through 18, wherein the information comprises a number of readers in the plurality of RFID readers and an assigned reader number for the RFID reader, and wherein the operations further comprise: computing a current dwell slot number for the RFID reader; computing a synchronous start time for the number of readers using the PTP signal; and determining, using the current dwell slot number and the synchronous start time, a current channel for the RFID reader, the current channel defining the offset within the channel hop sequence.

20. The non-transitory computer-readable medium of claim 19, wherein the operations further comprise: computing the current dwell slot number using a ceiling function of a current time from the PTP signal and the specified channel dwell time; computing a target channel separation for the plurality of RFID readers based on the number of readers and available RFID frequency channels; and computing the synchronous start time based at least in part on the target channel separation.Page 22 of 24US2008 32083532 1Attorney Docket No.: 102832-1520032RFID Frequency Management using Precision Time Protocol21. A method compri sing : determining a number of available RFID frequency channels for a target location of an RFID reader; selecting, from a plurality of candidate channel increments, and based on the number of available RFID frequency channels, a channel increment for the RFID reader; producing a channel hop sequence based at least in part on the number of available RFID channels and the channel increment; accessing a specified channel dwell time for the RFID reader; and configuring, based on the specified channel dwell time and the channel hop sequence, the RFID reader for synchronous RFID interrogation using a precision time protocol (PTP) signal to avoid interference among a plurality of RFID readers.

22. The method of claim 21, further comprising: factor the number of available RFID frequency channels into prime channel factors; for each of a plurality of candidate channel increments, factor the candidate channel increment into prime increment factors; and determine a channel increment for the channel hop sequence based at least in part on commonality between the prime channel factors and the prime increment factors.

23. A non-transitory computer-readable medium storing executable instructions, which when executed by a processor, cause the processor to perform the method according to claim 21 or claim 22.

24. A system comprising: a processor; and a non-transitory computer-readable medium accessible by the processor and storing executable instructions, which when executed by the processor, cause the processor to perform the method according to claim 21 or claim 22.Page 23 of 24US2008 32083532 1

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