TECHNICAL FIELD

The present invention relates to radio frequency identification (RFID), and in particular, to location of RFID tags.

BACKGROUND OF THE INVENTION

A conventional RFID system typically includes readers, also known as interrogators, and tags, also known as transponders. There are short range, near field systems as well as longer range, so-called ultra high frequency (UHF), systems. There are generally two type of standardized UHF RFID systems, active RFID systems, e.g., that conform to the ISO/IEC 18000-7, standard and passive RFID systems, e.g., that conform to the International Organization for Standardization and the International Electrotechnical Commission (ISO/IEC) 18000-6 family of standards. An active RFID tag has a battery to power the circuitry such that the tag can operate further away from the reader. A passive RFID tag receives both information and operating energy from an RF signal transmitted by a reader.

The most widely available passive RFID tags include ISO/IEC18000-6 Type C or EPC Radio-Frequency Identity Protocols Generation-2 UHF RFID (EPCglobal® Gen2) tags. A reader may obtain information from a passive tag by transmitting a continuous wave (CW) RF signal to the tag and having the tag respond by modulating the reflection coefficient of the tag's antenna, thereby backscattering the CW RF signal to create an information that is received by the reader.

Passive tags can operate at low frequency, high frequency, and ultra-high frequency. Low-frequency systems generally operate at, for example, 124 kHz, 125 kHz or 135 kHz. High-frequency systems use, for example, 13.56 MHz while ultra-high frequency systems use, for example, a band anywhere from 860 MHz to 960 MHz. Some systems also use, for example, 2.45 GHz or other areas of the radio spectrum.

There is also a hybrid type of tag that employs a battery and so need not get all of its operating energy from the reader's RF waveform. Such a tag is known as a battery assisted passive (BAP) RFID tag or a semi-passive tag. A BAP RFID tag, such as an ISO/IEC18000-6 Type D tag, transmits information to a reader using backscattering in a manner similar to that employed by a passive tag. An ISO/IEC 18000-6 type D compliant BAP tag, also known as tag-only-talk-after-listen (TOTAL) tag, only needs a continuous wave (CW) RF signal to activate and sustain communications since such tags only transmit messages with pre-programmed parameters.

An RFID reader, e.g., an ISO/IEC 18000-6 type C or EPCglobal Gen2 compliant RFID reader, usually starts an interrogation session by sending a CW RF signal to activate compatible RFID tags in its coverage range. The reader sends interrogation messages to the tags by modulating on the CW RF signal, which acts as a carrier, coded interrogation messages.

The communication between the RFID reader and each of the RFID tags is usually established using a random-access procedure such that a one-on-one connection between the RFID reader and one of the RFID tags is established. Such a procedure, upon completion, allows the data of each tag to be read even though there are other similar tags in the same general area, and so each tag is said to be singulated for the interrogation session. More generally, singulation is a method by which an RFID reader identifies and communicates with a specific tag. This is necessary because if multiple tags were to respond simultaneously to an interrogation message, such responses would interfere with each other, rendering it impossible for the reader to determine the contents of any of the responses. In a typical practical application, it is possible that hundreds of tags might be within the range of the reader. However, when all the tags cooperate with the tag reader and follow the same anti-collision protocol, also called a singulation protocol, then the reader can read data from each and every tag without interference from the other tags. Singulation has to be performed for each session for each tag. As each tag is singulated, the RFID tag may be accessed and updated for the interrogation session.

Each tag has a unique tag identification (UTID), e.g., a number, which may be referred to as electronic product code (EPC), stored in its memory and which distinguishes the tag from all other possible tags, and in particular, from amongst the various tags that are in range to communicate with the reader. The UTID is typically associated with a specific instance of a product that the tag is affiliated with. The UTID of an RFID tag may include a 96-bit unique item identification (UII) or an electronic product code (EPC) for EPCglobal Gen2 tag.

Passive RFID tags are typically very small and may be placed on a variety of items including, for example, equipment, products, or even people. Identification of such items may be made by a reader via their tags. Accordingly, RFID systems may be used to track inventory in a warehouse or the number of products on a store's shelves, to name just a few example applications.

Currently, standard passive RFID readers are not suitable for tracking locations and movements of RFID tags. This is because, in part, each RFID tag can only interact with a single RFID reader at any given time. A standard passive RFID reader utilizes some components of its transmit path along with parameters that are employed by its transmit path as part of, and in the operation of, its receive path. For example, the reader's receive path relies on the parameters of a tag coding scheme for an RFID interrogation session. The tag coding scheme parameters are sent to the RFID tags by the reader's transmit path and are used by the reader's receive path to correctly decode signals received in response from the RFID tags. Also, a carrier signal from a frequency synthesis circuit used by the reader for transmitting the CW RF signal is applied directly to the backscattering receive path of the reader to downconvert the tag response signals since the tag backscatters the CW RF signal to send back its responses.

These parameters are especially important for frequency hopping spread spectrum (FHSS) operation in which the carrier frequency is changed pseudo-randomly during a communication session to mitigate interference, e.g., as required by federal regulatory agencies. For this reason, prior art RFID arrangements that attempt to perform location of an RFID tag, e.g., a UHF RFID tag, by employing triangulation or multilateration (MLAT) techniques, have to use multiple readers or receiving antennas sequentially, i.e., at different times, to simulate the simultaneous measurements needed for such algorithms. This limitation significantly restricts the practical effectiveness of such arrangements in tracking moving tags since it usually takes a few seconds to accomplish each round of inventory sessions when there are multiple tags.

U.S. Pat. No. 8,294,554 discloses the use of a single reader that transmits and multiple readers that receive in parallel for use in determining a location for a tag. However, disadvantageously, such prior art arrangement does not appear to be suitable for use with FHSS operation.

As noted, an RFID reader usually transmits a strong CW RF signal so as to allow the tag to extract therefrom energy in order to power its internal circuitry and to create by backscattering an information signal that can be received by the reader. In some systems using digital modulation in the 902-928 MHz in the United States, e.g., an ISO/IEC18000-6 Type C or EPC Radio-Frequency Identity Protocols Generation-2 UHF (EPCglobal Gen2) RFID system, the reader may transmit up to 1 watt or 30 dBm from its antenna port. The receive sensitivity for the receive path of the RFID reader is usually limited by the interference from the leakage from the transmit path. Unfortunately, such leakage occurs even with sophisticated RF isolation and interference cancelation circuitry. Such high output RF power combined with spurious emission control restriction also requires sophisticated and large size electronic elements as well as a reliable power source which often increase the size and cost of passive RFID readers that are used for practical RFID tag tracking types of applications, thereby limiting their deployment. Furthermore, a reader usually has a large antenna so as to cover a large area in which there can be a large number of tags. The large size of the antenna usually limits location accuracy.

SUMMARY OF THE INVENTION

We have recognized that the challenges with passive RFID tag location systems that employ frequency hopping spread spectrum (FHSS) operation can be mitigated, in accordance with the principles of the invention, by an RFID detector that is suitable for use in a passive RFID tag system that employs frequency hopping spread spectrum (FHSS) operation, the RFID detector obtaining an indication of at least one characteristic of a CW RF signal employing a hopped-to carrier frequency that is being transmitted from an RFID tag reader, e.g., for use in activating the RFID tag to be located, the indication of the characteristic being obtained based on a signal that is received from a source other than the RFID detector. In accordance with further principles of the invention, a location, e.g., of the tag, of a group of tags, of the RFID detector, or of another RFID detector, may be determined based on the obtained indication of the at least one characteristic of the CW RF signal.

In accordance with an aspect of the invention, the RFID detector may obtain the indication of the characteristic of the CW RF signal based on the RFID detector's receipt of the CW RF signal transmitted from the tag reader. In accordance with an aspect of the invention, the RFID detector may obtain the indication of the CW RF signal characteristic by receiving such indication from an external source. In one embodiment of the invention, the indication of the characteristic of the CW RF signal is received from the external source in digital form, e.g., over a communication network.

In one embodiment of the invention, a characteristic of the CW RF signal that is indicated may be a frequency, e.g., a carrier frequency, thereof. In one embodiment of the invention a characteristic of the CW RF signal that is indicated may be a phase thereof.

In accordance with an aspect of the invention, the RFID detector extracts the RF signal that was sent by a tag using backscattering, i.e., the tag signal, from the total signal that is received at its antenna based on the obtained indication of the at least one characteristic of the CW RF signal. In this regard, it should be appreciated that the antenna of the RFID detector simultaneously receives the CW signal from the RFID tag reader and the backscattered signal from the RFID tag. In one embodiment of the invention, this is achieved by employing a regenerated carrier signal whose nature is based on the obtained indication.

In accordance with an aspect of the invention, toward the goal of determining a location, e.g., for at least the RFID tag, the RFID detector may use the obtained indication of the characteristic of the CW RF signal to determine at least one position-related parameter for the RFID tag. In an embodiment of the invention, the position-related parameter for the RFID tag may be based on the extracted backscattered signal from the RFID tag.

In one embodiment of the invention the RFID detector uses the obtained indication of the characteristic of the CW RF signal to measure the received signal strength indicator (RSSI) for the backscattered signal from the RFID tag. In an embodiment of the invention, the RSSI may be employed as the position-related parameter. In an embodiment of the invention the position-related parameter may be the RF phase rotation for the backscattered signal which is determined by the RFID detector based on the obtained indication of the characteristic of the CW RF signal. In yet another embodiment of the invention the position-related parameter may be a time stamp of an RF signal received from a singulated and identified tag, e.g., a time stamp indicating the arrival time of the backscattered signal. In yet a further embodiment of the invention, the position parameter is receipt of a valid signal from the RFID tag at the RFID detector.

In various embodiments of the invention, the RFID tag may be located using the at least one determined position parameter from one RFID detector either independently or in combination. Furthermore, determining the location of the RFID tag may be based on at least one position parameter from one RFID detector and information from at least one other device. Such information may be supplied by another RFID detector, e.g., one or more position parameters supplied therefrom. Such information may be information supplied by the RFID tag reader. Thus, for example, the obtained RSSI from the RFID detector may be combined with timing information from the RFID tag reader and an RSSI obtained by another RFID detector to determine the position of the tag.

In an embodiment of the invention, order to ascertain the location of an RFID tag, a particular backscattering from the tag should be a message from the RFID tag that is identifiable so that the device ascertaining the location can be aware that the versions of the message received at, respectively, an RFID detector and at least one other device, which may be, for example, another RFID detector or the RFID tag reader, correspond to the message from the RFID tag. This identification may be achieved, in one embodiment of the invention, based on the contents of the message, e.g., by determining that the message received at the RFID detector and the at least one other device have the same content. In another embodiment of the invention, the identification may be achieved based on a time of the message, e.g., a time of receipt and/or a time of transmission or the time of causing the transmission of the message by the RFID reader. Such time may be coordinated by the RFID reader or by a server.

More than one obtained characteristic may be employed to improve the accuracy of the process and the determined location.

In one embodiment, the RSSI measurements obtained at various RFID detectors in response to a tag message are normalized, which may enable the location of the RFID tag to be determined more accurately.

In accordance with another aspect of the invention, two or more objects, each of which has thereon a respective RFID tag, may be associated together by detecting certain attributes of their respective tags, such as their being within a predefined area at a particular time or being determined to be moving together.

