Array of very light readers for active RFID and location applications

Abstract

An example method includes estimating whether a collision occurred between a plurality of wide band response signals, the wide band response signals being responsive to a first interrogation signal transmitted by a first reader; when the collision is estimated to have occurred and the first reader received a first response from a particular tag, transmitting, by the first reader, a second interrogation signal, the second interrogation signal initiating a communication session between the reader and the particular tag; and transmitting, by the first reader, an acknowledgement packet to cause the tag to refrain from responding to a command during the communication session; and a second reader to refrain from interrogating the particular tag during the communication session.

Claims

1. A method comprising: estimating whether a collision occurred between a plurality of wide band response signals at a first reader, the wide band response signals being responsive to a first interrogation signal transmitted by the first reader; when the collision between the plurality of wide band response signals at the first reader is estimated to have occurred and the first reader received a first response from a first one of a plurality of tags: transmitting, by the first reader, a second interrogation signal, the second interrogation signal initiating a communication session between the first reader and the first tag; and transmitting, by the first reader, an acknowledgement packet to cause: the first tag to refrain from responding to a command during the communication session; and a second reader to refrain from interrogating the first tag during the communication session.

2. A method as defined in claim 1, further comprising limiting the first reader to receive replies having a combined reader transmission and tag reply time-of-flight within a predetermined amount of time.

3. A method as defined in claim 1, further comprising determining a round trip delay time of individual ones of the plurality of wide band response signals.

4. A method as defined in claim 1, further comprising estimating a quantity of the plurality of wide band response signals.

5. A system comprising: a tag configured to receive a first interrogation signal and to transmit a first wide band response signal in response to the first interrogation signal; a first reader to transmit the first interrogation signal and to receive a plurality of wide band response signals including the first wide band response signal from the tag, the first reader configured to, in response to an estimation that a collision occurred between the plurality of response signals at the first reader: transmit a second interrogation signal to initiate a communication session between the first reader and the tag; and transmit an acknowledgment packet to cause the tag to refrain from responding to a command during the communication session; and a second reader configured to, in response to receiving the acknowledgement packet, refrain from interrogating the tag during the communication session.

6. A system as defined in claim 5, wherein the tag is a wide-band active RFID tag comprising: a Very-Large-Scale Integration (VLSI) chip integrating an ultra wide band (UWB) front end with a digital transceiver; a UWB antenna; a power supply; and a clock source.

7. A system as defined in claim 5, further comprising a central network controller to dynamically determine overlapping coverage between the first and second readers by analyzing responses of the tag and other tags.

8. A system as defined in claim 5, wherein the first reader is configured to determine a round trip delay time of individual ones of the plurality of wide band response signals.

9. A system as defined in claim 5, wherein the first reader is to estimate a quantity of the plurality of wide band response signals.

10. A first reader, comprising: communication hardware to: transmit a first interrogation signal; and receive a plurality of response signals in response to the first interrogation signal, the plurality of response signals including a first response signal from a tag; and a processor to: when a collision is estimated to have occurred between the plurality of response signals: provide a second interrogation signal to be transmitted by the communication hardware, the second interrogation signal to initiate a communication session between the first reader and the tag; and provide an acknowledgement packet to be transmitted by the communication hardware, the acknowledgement packet to prohibit: the tag from responding to a command during the communication session; and a second reader from interrogating the tag during the communication session.

11. A first reader as defined in claim 10, further comprising: a controller configured to implement a Media Access Control (MAC) protocol; and a communication interface subsystem configured to operate with the controller, wherein the communication interface subsystem is to provide connection to a network through a channel separate from the tag.

12. A first reader as defined in claim 11, wherein the communication interface subsystem providing connection to the network includes communication via one or more of WiFi, Wimax, UWB, power lines, Bluetooth, or Ethernet.

13. A first reader as defined in claim 10, further comprising a tag module having at least one VLSI chip integrating a UWB front end and digital transceiver.

14. A first reader as defined in claim 10, wherein the processor is to determine a round trip delay time for individual ones of the plurality of response signals.

15. A first reader as defined in claim 10, wherein the processor is to estimate a quantity of the plurality of wide band response signals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

(2) FIG. 1 is a schematic block illustration of a preferred embodiment of a VLR constructed and operational according to the teachings of the present invention;

(3) FIG. 2 is a schematic block illustration of a first preferred embodiment of a modular VLR constructed and operational according to the teachings of the present invention;

(4) FIG. 3 is a schematic block illustration of a second preferred embodiment of a modular VLR constructed and operational according to the teachings of the present invention; and

(5) FIG. 4 is a schematic block illustration of an exemplary embodiment of a system network including Tags, VLR's, Readers, a Server and communication infrastructure constructed and operational according to the teachings of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) The present invention is a cost effective system that uses an array of Very Light Readers (VLR's) in which each VLR is based on an augmented RFID Tag circuit.

(7) The principles and operation of system that uses an array of VLR's according to the present invention may be better understood with reference to the drawings and the accompanying description.

(8) By way of introduction, the terms Very Light Reader, Very Light Readers, VLR (singular) and VLR's (plural) are used herein to refer to an augmented RFID Tag circuit as described herein and such terms may be used interchangeably.

(9) The VLR's of the present invention provide an alternative to the costly RF Readers of the prior art. The system of the present invention uses a relatively large quantity of VLR's per a given site, providing higher coverage and with a low installation and maintenance cost. In order to reduce the size and cost in comparison to known Readers, the functionality of a VLR is limited.

(10) The VLR's of the present invention are based on an augmented Tag circuit. That is, a Tag circuit operationally connected to additional components. The VLR's may be installed, for example, one in every room of the facility to be covered.

(11) Due to the limited functionality of the VLR's, several methods are suggested in the context of the present invention that improve overall network utilization. Embodiments of the present invention include cooperation between the VLR's that assist in increasing the effective throughput in the Tag to Reader communication, such as, but not limited to, algorithms to improve congestion control of a multiplicity of Readers/VLR's and a method to distribute the load between Readers/VLR's.

(12) Connection of VLR'S to the network can be done using various methods and devices currently known in the art. The VLR'S coordination requirements, as the result of the algorithms and methods described herein, are low such that no very reliable links, nor low latency, are required.

(13) An exemplary, non-limiting list of convenient low cost backbone connections includes wireless (e.g. Wifi, Wimax, Cellular, or UWB by non-limiting example), cable, fiber optic, or connection over power lines. It is to be understood that in order to function even with a non reliable link back to the central server the VLR's need to exhibit smart autonomous behavior as will be described herein.

(14) A Very Light Reader of the present invention uses UWB for communication between VLR's and Tags, and other ways of communication for back-haul communication. A preferred embodiment of a VLR device 2 of the present invention is composed of the following components illustrated in FIG. 1.

(15) At least one VLSI chip 4 integrating the UWB front end, high speed digital transceiver, and possibly an internal controller (CPU).

(16) A UWB Antenna 6.

(17) A power supply, preferably a battery 8.

(18) A controller 10 configured to implement the Media Access Control (MAC) protocol of the system, possibly assisted by the VLSI chip controller.

(19) A Communication Interface subsystem 12 that provides connection to the system network 20. Such a subsystem 12 can use, by non-limiting example, Wifi, Wimax, UWB, power lines, Bluetooth, Ethernet, or any other communication protocol or media.

(20) A clock source 14.

(21) It will be appreciated that generally an Active RFID Tag can process only one packet at a time, if each transmission takes typically 10 msec, a VLR based on such a Tag may probably handle around 20-40 packets (to/from Tags) per second. A Tag as described in the patent application PCT/IL2005/000506 Fast Synchronization of RF Tags to two of the present inventors, can receive multiple packets at the same time, and therefore could be used to benefit as the front end of a VLR of the present invention, and would enable handling of several tens of Tag messages per second.

(22) It will also be appreciated that by removing the controller 10 and the Communication Interface 12 the remaining portion of a VLR can be configured to operated as a RFID Tag (which may be attached to an object), using a low cost battery 8, possibly using a lower cost clock source 14, and typically operating the device in an alternating active and sleep modes.

(23) The power supply of the VLR 2 must be capable of operating the device always in an active state for the specified life time of the VLR 2 and provide power to the VLSI chip 4 enabling the CPU software to create control channels whereby the Master controller 10 uses the CPU as a slave to send and receive UWB packets. In other words, the CPU implements the layer 2 connection, and the controller 10 performs the network control, routing and congestion control. Note that such a configuration requires that the CPU in the VLSI chip 4 has some means for communicating messages to the controller 10, preferably with a simple communication protocol. This feature is not available in prior art Tags.

(24) As illustrated in FIGS. 2 and 3, the VLR of the present invention may be configured as a modular device having a base module and a plurality of Tag modules, in which the various operational components are deployed in one or the other of the base module and the Tag module. It will be understood that the addition of Tag modules increases the reception capacity of the modular VLR to which the Tags are added.

(25) A first preferred embodiment of a modular VLR 100, as illustrated in FIG. 2, is configured with a base module 102 that includes the controller 10 and the Communication Interface 12. The VLSI chip 4, UWB antenna 6, clock 14 and battery 8 are deployed in a Tag module 104 that may be interconnected to the base module 102 so as to be used as part of the VLR 100. It will be understood as discussed above, the Tag Module 104 may also be used as a standalone Tag. The base module 102, although illustrated here with three Tag modules 104 attached, may be configured to accept substantially any number of Tag modules depending on size and other hardware requirements of any particular end-use application.

(26) A second preferred embodiment of a modular VLR 150, as illustrated in FIG. 3, is configured with a base module 152 that includes the controller 10, the Communication Interface 12, UWB antenna 6, clock 14 and battery 8. The VLSI chip 4 is deployed in the Tag module 154 that may be interconnected to the base module 152 so as to be used as part of the VLR 150. In this embodiment, all of the Tags are powered by battery 8 and share the use of UWB antenna 6. Here too, although base module 152 is illustrated with three Tag modules 154 attached, base module 152 may be configured to accept substantially any number of Tag modules depending on size and other hardware requirements of any particular end-use application.

(27) A preferred embodiment of an overall system 200, as illustrated in FIG. 4, can be described as a layer 3 network, in which the central server 40 and the Readers 50 and VLR's 2 are connected using layer 2 connections such as, but not limited to power line, Ethernet and wireless connection. The VLR's 2 and the Tags 30 communicate in another layer 2 connection (wireless) with the VLR's 2 provide the routing.

(28) It is also possible to use the standard IP protocol for communicating with the VLR's or even with the Tags. If in order to save the cost of a processor and memory and decrease the overhead over the air, the Tags used do not implement the IP protocol, the VLR's provide the protocol conversion for back-end communication, thereby providing seamless use of IP in the RFID network. The server of the system can include a table which presents an updated status of the network such as, but not limited to, noting which Tags are in the proximity of each VLR. Such a map is very useful for performing an Asset Management application.

(29) It is noted, that for some systems it is advantageous to use the UWB Reader to Reader communications backbone instead or in conjunction with the other means of communications. In this way the coverage is improved, and system cost is reduced. In cases where there is low bandwidth use of the system, such a backbone is advantageous. In this mode of operation the VLR's form a mesh network. When is a server present to collect information or command the network, the server communicates with one or more of the Readers/VLR's using conventional means these Readers/VLR's relay the information using the UWB mesh network formed using the Reader/VLR to Reader/VLR UWB communications together with routing capabilities configured in the relay Readers/VLR's.

(30) One of the tasks commonly performed in RFID systems is determining which Tags are located in the vicinity of a specific Reader/VLR. For example, the Reader/VLR may be located near a gate and it is desired to know which Tags pass through the gate. Generally, the Reader/VLR transmits broadcast signals in its vicinity and the Tags receiving the broadcast signal respond to the broadcast signal. However, it is necessary to prevent the occurrence of collisions between the response signals of different Tags. Various methods have been suggested for preventing collisions, or in general terms, performing congestion control.

(31) One method for determining which Tags are located near the VLR is to take advantage of the implementation of the UWB link. As described in U.S. Provisional Patent Application 60/609,240 (to some of the present authors), UWB communications can employ interleaved impulse radio to allow multiple transmissions simultaneously. For example, a very short transmission pulse sequence of 100 nsec is employed for a single symbol transmission, while employing, as a non-limiting example, a relatively low symbol rate of 50 Ksps (time between symbols of 20 sec). This allows up to 200 simultaneous transmissions.

(32) In normal operation the Readers or Tags will randomly choose any of the abovementioned 200 possible transmission phases, while the receivers will listen or wait for answers in all of them as well. However, if Tags reply to a Reader transmission in a phase which is very close to the phase in which it was received, the reply will reach Reader in a phase influenced by the physical location of the Tags in relation to the Reader. The farther the Tag is from the Reader, the longer the time difference between the Reader's transmission and reception of the Tag's reply. Therefore, by limiting its reception to specific phases, the Reader can limit its receptions to Tags that are located within a predetermined distance range from the Reader. This provides a method, based on the combined time-of-flight for a transmission and reply, for determining which Tags are in the vicinity of the Reader, while ignoring replies from Tags that are outside of the predetermined range yet still answered that Reader's transition. That is to say, limiting a reader to receive replies from tags during predetermined time intervals so as to limit reception to replies whose combined reader transmission and tag reply time-of-flight falls within a predetermined length of time, such that replies are received only from tags that are located within a predetermined distance of the reader.

(33) This is also advantageous for reducing the number of reply receptions for a Reader whose reception capacity is limited, such as the limited reception capabilities of the VLR's of the present invention as discussed below.

(34) Considering the cost of providing high throughput per a single prior art Reader, one of the underlying principles of the present invention is that VLR's have a relatively low throughput. Unlike a standard Reader that can handle hundreds or even thousands of Tag messages per second, in a preferred embodiment, the VLR's are equipped to handle up to several tens of Tags per second. This limitation may pose a congestion problem in a network encompassing all the Tagged devices.

(35) In order to resolve the congestion issue, an embodiment of the present invention includes a system with a multitude of unsynchronized VLR's which have a distributed congestion resolving mechanism or payload sharing mechanism.

(36) For our discussion herein, we will concentrate on RFID systems that employ Ultra Wide Band (UWB) transmissions both for information transfer between Tags and Readers, as well as for locating a Tag. Furthermore, we assume that the system uses some kind of a multiple access, where a number of Tags can simultaneously reply to a Reader's query.

(37) Each VLR uses its ability to transmit and receive UWB signals to measure the time of flight and thus its distance from a Tag, as known in the prior art of UWB Readers. This information from all VLR's in the system is sufficient to enable the system to determine, with a high reliability, the closest VLR to each Tag. Once this is established, further queries to Tags can use this information and lower response times. Prior art low-cost Readers, which do not make use of UWB techniques, mainly use the signal strength (sometimes donated RSSI in the literature) to know which Reader is the closest to any given Tag. In other prior art systems, instead of Readers there are just beacon transmitters and the Tag has the responsibility to measure the signals from various beacons and decide its own location. These systems are at a disadvantage since the Tag needs to be smart. However, this method can be used to advantage in the system of the present invention, equipped with VLR's, in that the Tags do not need to be smart.

(38) It is to be noted that prior art congestion control mechanisms apply to one Reader transmitting at the time, and do not apply correctly to the problem of multiple Readers transmitting concurrently.

(39) The multi Reader network as described herein, where each VLR actually has a low inherent multi-access capability, calls for innovative way for congestion control.

(40) In addition to the VLR's, it is assumed that a wide-band Tag can process one or more packets at a time but not more than N packets simultaneously.

(41) Thus, according to the present invention, there is provided a method and a corresponding system for communicating between multiple Very Light Readers and a plurality of RF Tags, the method including the steps of: 1. Readers/VLR's transmit wide band interrogation signals (either unicast or multicast). 2. Tags wait to receive the wide band interrogation signals. 3. Tags receive the wide band interrogation signals which are possibly overlapping and if possible (according to whether the number of concurrent packet is not exceeded) transmit in response a wide band response signal. 4. A Reader/VLR receives the wide band response signal which may include other overlapping wide band response signals from the Tags, and (a) the Reader/VLR attempts to distinguish between the multiple wide band response signals from the Tags, and when distinguishable, detects the wide band response signals, transmitted by the Tags, (b) the Reader/VLR estimates the number of wide band response signals to the wide band interrogation signal, (c) the Reader/VLR analyzes the distinguishable wide band response signals in order to determine their round trip delay.

(42) 5. Each Reader/VLR estimates whether a collision between a plurality of the wide band response signals occurred and when the Reader/VLR estimates that the collision occurred, the Reader/VLR transmits at least one additional wide band interrogation signal. This additional wide band interrogation signal initiates a new communications session between the Reader/VLR and the RF Tags in order to reduce the collision probability between the wide band response signals transmitted by the Tags.

(43) 6. Each Reader/VLR that received a valid Tag response from a responding Tag transmits an acknowledgment packet toward the responding Tag in order that, (a) the responding Tag will not answer again to that broadcast session in case the broadcast command has to be issued again (due to the estimation that some collisions occurred during the previous session), (b) each Reader/VLR that receives the acknowledge packet that was transmitted from another Reader/VLR to a specific Tag and having the same session ID will know that the Tag already received the command and the Reader/VLR will not try again to reach that Tag, thus lowering congestion directed toward the Tags from multiple Reader/VLR transmissions. This Reader/VLR may still send his own packet to that Tag in order to measure the distance of that Tag from it.

(44) Each transmission request of a Reader/VLR, which can be a new command pending, an acknowledgment, or retransmission of a previous command, is conditioned on the network situation at the time. A Reader/VLR should not transmit when the Tags in its proximity are already occupied with other transmissions, nor should the Reader/VLR transmit when other Readers/VLR's are receiving many responses from Tags. It is a purpose of the present invention to describe a novel method for avoiding overcrowding the Tags and the Readers/VLR's. This is a modification of the well-known ALOHA protocol.

(45) Each Tag in its response packet will include a field specifying the number of active receivers. Reader/VLR A sends a packet and listens to the Tag responses. If a Tag does not respond, it may be either because that Tag did not hear that packet, or because that Tag is occupied with other packets from other Readers/VLR's. Reader/VLR A listens to the Tag responses to other Readers/VLR's transmissions and determines if the field specifying the number of occupied receivers indicates that the number of occupied receivers has reached a predetermined limit or other threshold. If the threshold has been reached, Reader/VLR A enters into a backoff state. It is advantageous to use an exponential backoff like in a standard ALOHA protocol.

(46) A Reader/VLR is in addition listening to all traffic of other Readers/VLR's. When a Readers/VLR's determines that its receivers were occupied above a threshold number of packets the Reader/VLR should enter a backoff state. In order to further protect the Readers/VLR's from overloading due to the requests of other Readers/VLR's, there is the same parameter of number of occupied receivers, also attached to any packet that a Reader/VLR is sending, so other Readers/VLR's can backoff their requests or transmitted packets. It is advantageous also in this case, to use an exponential backoff.

(47) In some preferred implementation of the system of the present invention, a Tag does not enter into backoff state when answering a Reader/VLR. This simplifies the operation of a Reader/VLR. In other embodiments the Tags refrain from answering a Reader/VLR if other the packets from Readers/VLR's contain a full occupancy in their special field of number of occupied receivers.

(48) Another method for reducing the congestion is by making sure that Readers/VLR's with overlapping coverage do not transmit at the same time. Overlapping of Readers/VLR's converge may be intentional when, for location purposes, each Tag is seen by multiple Readers/VLR's, and also in cases where accurate location is not required, due to installation and logistics requirements.

(49) Even in the case where only one Reader/VLR is transmitting, the fact that there is overlap in coverage can be used to increase the throughput of the system. In such a case, each Reader transmission includes some Reader specific information such as, but not limited to, a Reader ID. When a Reader/VLR receives a reply to a broadcast transmission, it will forward this reply to the server, even if this reply is to a transmission that was not originated from that specific Reader/VLR. The Reader/VLR also sends a message to the originating Reader/VLR indicating that it has received a reply from a Tag. The originating receiver will then send an acknowledgement to the Tag, indicating that its reply was received. This in effect creates a distributed Reader/VLR, which has multiple receivers. This way of coordinated networking increases the effective capacity of the VLR's.

(50) Moreover, the fact that a reply from a specific Tag, was received by Reader/VLR B, despite the fact that the Tag was replying to a transmission from Reader/VLR A, allows the network controller (server) to determine that Readers/VLR's A and B have overlapping coverage. That is to say, by analyzing which Readers/VLR's in the network forward each particular Tag response to the central network controller, the central network controller is able to determine an extent of coverage overlap between at least some of the Readers in the network. This information is useful to manage the network and determine which Readers/VLR's can transmit simultaneously.

(51) It will be appreciated that the above descriptions are intended only to serve as examples and that many other embodiments are possible within the spirit and the scope of the present invention.