RFID tag and communication protocol for long range tag communications and power efficiency

An RFID tag has a frequency agile RF transceiver that transmits data from the RFID tag to a reader and a direct sequence spread spectrum RF receiver that receives communications from the reader. Messages between the reader and the RFID tag are divided into frames, and each frame contains a frame header transmitted by the reader and at least one time slot containing data transmitted by the RFID tag. The frame header contains a hop sequence and a frequency in a hop sequence to be used by the RFID tag in transmitting data to the reader.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a tag that can be suitably attached to an article and that operates in accordance with a communication protocol promoting long range communications and power efficiency.

BACKGROUND OF THE INVENTION

Various labels have been attached to articles so that the articles can be distinguished one from the other. For example, bar code labels are attached to articles of grocery and are scanned at a check-out counter in order to automatically identify the articles and to register the price of the articles as they are purchased.

Bar code labels have also been used in inventory control and monitoring. Accordingly, these bar codes may be scanned in order to track articles as they move into, through, and out of a storage area. It is also known to read the bar codes attached to articles in order to access various computer records regarding the articles.

Bar code labels, however, have several drawbacks. For example, computer stored records that are accessed when a bar code is read do not move with the corresponding article. Therefore, if the article to which the bar code label is attached is remote from the computer, the records concerning that article cannot be immediately accessed if necessary.

Moreover, bar code labels cannot be read remotely. Thus, if it is desired to take an inventory of articles currently in the storage area, personnel must physically scan each label on each article one at a time in order to determine which articles are presently in the storage area. Such scanning requires the physical presence of the personnel at the location of the articles and is extremely time consuming. Additionally, because bar code labels cannot be read remotely, they cannot be used as security devices that can be detected if the articles to which they are attached are improperly removed from a secured area.

Instead of bar coded labels, it is known to attach radio frequency identification (RFID) tags to the articles to be monitored. The RFID tags can be read, as can bar code labels. However, unlike bar code labels, reading RFID tags does not require the physical presence of personnel because the RFID tags can instead be read remotely. Thus, inventory can be taken more quickly because personnel are not required to walk around a storage area or other area in order to read the RFID tags. Moreover, because RFID tags can be read remotely, they can be used as security devices. Thus, if someone attempts to surreptitiously remove an article to which an RFID tag is attached from a secured area, a remote reader can sense the tag and provide an appropriate alarm.

RFID tags can be read one at a time or in groups. When multiple RFID tags in a group are read at the same time, the information transmitted by the multiple tags frequently collide. Accordingly, spread spectrum techniques, such as either direct sequence spread spectrum (DSSS) or frequency hopping, in the communications between the reader and the tags have been suggested in order to reduce the impact of such collisions. It is also known to interrogate a tag using either a direct sequence spread spectrum (DSSS) signal or a frequency hopping signal.

In one embodiment, the present invention combines a direct sequence spread spectrum RF receiver and a frequency hopping transmitter in an RFID tag in order to overcome deficiencies of prior art tags. In another embodiment, the present invention relies on a communications protocol that supports the use of frequency hopping communications between the RFID tag and a reader.

The combination of direct sequence spread spectrum for signal reception and frequency hopping for signal transmission relative to a tag has not been previously suggested. Both of these modulation techniques circumvent jamming or interference by other signals. Also, the use of a direct sequence spread spectrum RF receiver in a tag permits the tag to properly synchronize to, and decode, the signal received from the reader in a shorter period of time than if the reader transmits the signal using frequency hopping. At the same time, this arrangement permits the tag to be interrogated by the reader over long distances while conserving power.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an RFID tag comprises a frequency agile RF transmitter and a direct sequence spread spectrum RF receiver. The frequency agile RF transmitter transmits data to a reader, and the direct sequence spread spectrum RF receiver receives communications from the reader.

In accordance with another aspect of the present invention, an electrical signal transmitted between a reader and an RFID tag contains a message divided into frames. Each frame contains a frame header transmitted by the reader and at least one time slot containing data transmitted by the RFID tag, and the frame header contains a frequency in a hop sequence to be used by the RFID tag in transmitting the data.

In accordance with still another aspect of the present invention, a method of transmitting information contained in a plurality of frames between a reader and a tag comprises the following: transmitting a frame header in each of the frames from the reader to the tag, wherein the frame header contains a frequency in a hop sequence; and, transmitting data in a time slot of at least one of the frames from the tag to the reader, wherein the data is transmitted at the frequency.

In accordance with yet another aspect of the present invention, a method of transmitting information contained in a single frame between a plurality of tags and a reader comprises the following: receiving a frame header at each of the tags, wherein the frame header is contained in the frame, wherein the frame also contains a plurality of time slots, and wherein the frame header contains a frequency in a hop sequence and the number of time slots in the frame; receiving a time slot header in each of the time slots at each of the tags, wherein each time slot header contains a number of the corresponding time slot; each of the tags non-deterministically selecting a corresponding time slot; and, each of the tags transmitting data in the time slot that is selected by the corresponding tag.

DETAILED DESCRIPTION

As illustrated inFIG. 1, a tagging system10includes a long range reader12, a short range reader14, and an RFID tag16. The long range reader12includes an antenna18, and the RFID tag16similarly includes an antenna20. The antennas18and20establish a long range RF link between the long range reader12and the RFID tag16so that the long range reader12can remotely read the identification stored in a memory of the RFID tag16. The range of the long range reader12can be as high as several hundred feet. For example, the long range reader12can have an expected range of approximately 500 feet.

A secure link22between the short range reader14and the RFID tag16permits the short range reader14to read information from the RFID tag16in a more secure manner. That is, it may not be desirable for the long range reader12to read certain information stored in the RFID tag16because long range RF communications can be intercepted by a strategically placed surreptitious reader similar to the long range reader12. Accordingly, the secure link22increases the difficulty in illicitly acquiring the more sensitive information that may be stored on the RFID tag16.

The secure link22is shown inFIG. 1as a hard wire link between the short range reader14and the RFID tag16. Accordingly, the more sensitive information stored on the RFID tag16can be read by establishing a physical interconnection between the short range reader14and the RFID tag16. Alternatively, the secure link22may be a limited range magnetic link such as those provided by contact-free smart cards. As a still further alternative, the secure link22may be a very limited range RF link. Other alternatives will occur to those skilled in the art. The expected maximum range of the short range reader14over the secure link22, for example, may be less than two feet, and is expected, in typical usage, to be between six inches and eighteen inches.

An embodiment of the RFID tag16is shown in additional detail inFIG. 2. The RFID tag16includes a frequency agile (frequency hopping) RF transmitter30and a direct sequence spread spectrum RF receiver32. The frequency agile RF transmitter30and the direct sequence spread spectrum RF receiver32are coupled between the antenna20and a microprocessor34. Accordingly, the frequency agile RF transmitter30of the RFID tag16implements frequency hopping in transmitting information to the long range reader12, and the direct sequence spread spectrum RF receiver32of the RFID tag16implements direct sequence spread spectrum synchronization and decoding in receiving communications from the long range reader12.

The RFID tag16also includes an interface36between the microprocessor34and the short range reader14. Accordingly, the RFID tag16can transmit and receive communications to and from the short range reader14. In the case where the secure link22is a hardwire link, the interface36may simply be a plug that is connectible to a corresponding plug of the short range reader14. In the case where the secure link22is an RF link, the interface36may be an RF transceiver of any known type provided that this RF transmitter preferably has a much shorter range than the frequency agile RF transmitter30and the direct sequence spread spectrum RF receiver32. In the case where the secure link22is a magnetic link, the interface36may simply be a magnetic emitter/sensor capable of magnetically interfacing with the short range reader14.

The RFID tag16further comprises a memory38coupled to the microprocessor34. The memory38stores the ID of the RFID tag16that can be read by the long range reader12through the antennas18and20, the frequency agile RF transmitter30, the direct sequence spread spectrum RF receiver32, and the microprocessor34. The memory38also stores information supplied to it by the short range reader14through the secure link22, the interface36, and the microprocessor34. The memory38can additionally store information supplied by the long range reader12.

This information can include, for example, the inventory history of the article to which the RFID tag16is attached. Accordingly, the date that the article entered inventory, the date that the article left inventory, the length of time that the article has been in inventory, any movement into and out of inventory, and similar information may be stored in the memory38.

The information stored in the memory38may also include shipping manifests that indicate when and to whom the article is to be shipped. Moreover, in the case where individual articles with differing destinations are shipped in the same container, an RFID tag attached to the container, hereafter called a container tag, can be attached to the container. This container tag may be arranged to store the identity and destination of each article in the container. As articles are removed from the container, the information stored in the container tag can be updated to indicate which articles have been removed, the location at which the articles were removed, and the identity of the personnel who removed the articles.

The information stored in the memory38may further include maintenance, repair, and date of service records showing the maintenance and/or repair history of the corresponding article.

Other information related to the article may likewise be stored in the memory38. For example, the integrity of the information stored in the memory38can be assured by keeping a record of the modifications to the stored information and of the identity of the personnel making the modifications. As another example, records related to the production of the article may be stored in the memory of the tag.

Accordingly, any information about the article may be stored with the article instead of in a remote computer system or on paper.

Because the records are carried by the RFID tag16attached to a corresponding article, the RFID tag16eliminates the need to maintain paper or computer records of the life history of an article, the RFID tag16eliminates the problem of lost or misplaced records, and the RFID tag16improves operational efficiency by eliminating the requirement to retrieve records prior to accessing and/or operating on the article.

The RFID tag16may include a battery (not shown) that is coupled so that it supplies power to the frequency agile RF transmitter30, the direct sequence spread spectrum RF receiver32, the microprocessor34, the interface36(if necessary), and the memory38. Moreover, a plurality of sensors (also not shown) may be coupled to the microprocessor34. These sensors may include, for example, a temperature sensor, a humidity sensor, and other sensors such as a pressure sensor, a proximity sensor, an electromagnetic sensor, an optical sensor, a mechanical sensor, a chemical sensor, and/or the like. The microprocessor34stores the information from the sensors in the memory38, and this information may be read from the memory38by the short range reader14or by the long range reader12.

The microprocessor34may be arranged to further sense the voltage level of the battery. Accordingly, the microprocessor34stores this voltage level in the memory38, and this stored voltage level may be read from the memory38by the short range reader14or by the long range reader12. Thus, if the voltage level of the battery as read by either the short range reader14or the long range reader12indicates that the battery needs charging or replacement, suitable remedial action may be taken.

Because of the frequency agile RF transmitter30and the direct sequence spread spectrum RF receiver32, the RFID tag16is capable of relatively long range activation while providing a low power method for command-response activation by the long range reader12. This long range activation allows the RFID tag16to be placed at distances remote from the long range reader12for purposes of interrogating the RFID tag16for its unique tag number and possibly other information.

The frequency agile RF transmitter30and the direct sequence spread spectrum RF receiver32allow the tagging system10to operate in the FCC defined Industrial Scientific and Medical (ISM) bands at maximum legal power. Both frequency hopping as used by the frequency agile RF transmitter30and direct sequence spread spectrum communications as used by the direct sequence spread spectrum RF receiver32circumvent jamming by narrow-band signals using different methods of spreading the signal over a large bandwidth. The direct sequence spread spectrum RF receiver32can receive signals from the long range reader12within milliseconds of activation. By contrast, a frequency agile receiver must search a long frequency hopping sequence in order to receive signals from the long range reader12. The time required to make this search is typically longer than the time required to detect a direct spread spectrum sequence because the direct spread spectrum signal is either on a fixed frequency or on one of only a few frequencies.

An embodiment of the long range reader12is shown in additional detail inFIG. 3. The long range reader12includes a direct sequence spread spectrum RF transmitter50and a frequency agile RF receiver52coupled between the antenna18and a microprocessor54. The frequency agile RF receiver52of the long range reader12implements frequency hopping in receiving information from the frequency agile RF transmitter30of the RFID tag16. Moreover, the direct sequence spread spectrum transmitter52of the long range reader12implements direct sequence spread spectrum transmission in transmitting communications to the direct sequence spread spectrum RF receiver32of the RFID tag16.

The long range reader12further comprises a memory56coupled to the microprocessor34. The memory56stores the information that the long range reader12receives from the RFID tag16. The memory56also stores the software that supports the communication protocol as described herein.

This communication protocol governs the message format that is used between the long range reader12and the RFID tag16. According to this protocol, a message is comprised of a plurality of frames as shown inFIG. 4. Each frame is preferably no longer than the length of time the frequency agile RF transmitter30is allowed to dwell at any given frequency.

Each of the frames shown inFIG. 4has the construction shown inFIG. 5. Accordingly, each frame has a frame header and a number of time slots TS0–TSN. The frame header contains information about the long range reader12that is reading the RFID tag16. As shown inFIG. 6, the frame header contains (i) the state of the long range reader12, (ii) the hop sequence currently being used by the long range reader12to receive messages from the RFID tag16, (iii) and the current position (i.e., frequency) of the long range reader12in this hop sequence. The frame header can also contain such other information that is useful in the tagging system10. For example, the frame header may also contain the number (N+1) of time slots in the corresponding frame.

The long range reader12may have several reader states including, for example, an active communication state and a beacon state. In the active communication state, the long range reader12commands responses from one or more selected tags such as the RFID tag16. In the beacon state, the tags, such as the RFID tag16, self-initiate the transmission of messages to the long range reader12.

The hop sequence and/or the current position in the hop sequence as contained in the frame header are/is useful to tags that have limited signal processing capability. Such tags, for example, may have no capability themselves to determine the frequency (i.e., the current position in the hop sequence) onto which they should transmit their responses.

Moreover, each time slot may also include a time slot header and data as shown inFIG. 7, and each time slot header, as shown inFIG. 8, may contain the hop sequence and the current position in the hop sequence of the long range reader12. The time slot header may also contain the relative position, such as a time slot number (0,1, . . . , or N), of the corresponding time slot in the frame. This relative position information may be used by the RFID tag16to establish a relative timing interval into which the RFID tag16can transmit data. By transmitting the hop sequence and the current position in the hop sequence at the beginning of each time slot, the RFID tag16is aided in its rapid acquisition of the current hop sequence and frequency. Because the RFID tag16can acquire, from the time slot header in each time slot, sufficient information about the frequency and timing of the long range reader16, the RFID tag16may power down until such time that it expects the complete header information to be transmitted by the long range reader12. Therefore, the RFID tag16is able to substantially reduce the amount of power that it uses to determine the frequency and timing to be used by its frequency agile RF transmitter30in transmitting information in the data portion of the time slot.

As indicated above, the long range reader12transmits all headers, whether frame headers or time slot headers. The RFID tag16transits only in the data portion of the time slots. The RFID tag16may implement a non-deterministic method of selecting a time slot for the transmission of data. By using a non-deterministic method of selecting a time slot, the possibility of a plurality of tags transmitting data into the same time slot is minimized. For purposes of illustration, such a non-deterministic method of selecting a time slot could be embodied by a pseudo-random number generator that pseudo-randomly generates a number of a time slot into which its corresponding tag transmits its data. This implementation results in a communications protocol similar to, but not identical to, the Aloha protocol, a standard communications protocol.

The long range reader12can communicate directly with a specific tag or a group of specific tags. When the long range reader12is communicating directly with a specific tag or a group of specific tags, the long range reader12may suspend the transmission of time slot headers. This suspension indicates to all other tags that their communications are to be suspended. Also, all data may be transmitted between the long range reader12and the RFID tag16in packets having packet numbers so that both the long range reader12and the RFID tag16can detect missing or duplicate data. Moreover, acknowledgements can be used to signify a successful transmission between the long range reader12and the RFID tag16. A failure to receive an acknowledgement can cause re-transmission of the information. Once a transaction between the long range reader12and a specific tag or group of tags is complete, the long range reader12resumes transmitting the headers.

As shown inFIG. 9, when the RFID tag16detects a received message at the direct sequence spread spectrum RF receiver32as indicated by a block100, the RFID tag16parses the header information as indicated by a block102and, as indicated by a block104, stores the reader state, the hop sequence, the current position in the hop sequence, and any other information that is contained in the header.

As shown inFIG. 10, when it is time for the RFID tag16to transmit data as indicated by a block106, the RFID tag16selects the time slot in which it is to transmit the data as indicated by a block108, the RFID tag16selects the frequency at in which it is to transmit the data as indicated by a block110, and the RFID tag16causes the frequency agile RF transmitter30to transmit the data in the selected time slot using the selected frequency as indicated by a block112.

The time at which the RFID tag16is to transmit data (the block106) depends on the state of the long range reader12. If the reader state as contained in the header and stored by the RFID tag16(block104) indicates that the long range reader12is in the beacon mode, the RFID tag16self-originates the transmission of data. In this state, the RFID tag16, for example, may be arranged to transmit data periodically based on a timer. When the timer indicates that it is time to transmit, the RFID tag16pseudo-randomly selects a time slot as indicated by the block108, selects a transmission frequency as indicated by the block110, and transmits as indicated by the block112.

If the long range reader12is in an active communication state, the RFID tag16determines that it is time to transmit when it receives an interrogation message from the long range reader12. When the RFID tag16receives an interrogation message, the RFID tag16pseudo-randomly selects a time slot as indicated by the block108, selects a transmission frequency as indicated by the block110, and transmits as indicated by the block112.

Other reader states for the long range reader12are also possible.

As indicated above, the time slot in which the RFID tag16transmits data may be selected based on the pseudo-randomly generated number. The frequency at which the RFID tag16transmits data is selected based on the current position in the hop sequence as received (block100) and stored (block104) by the RFID tag16(block100).

Certain modifications of the present invention have been disclosed above. Other modifications will occur to those practicing in the art of the present invention. For example, the functions of the long range reader12as described above have been confined to reading information from the RFID tag16. However, the long range reader12can also be arranged to write information to the RFID tag16.

Also, as described above, the long range reader12is arranged to read the tag ID of the RFID tag16, and the short range reader14is arranged to read other information from the RFID tag16. However, the long range reader12may be arranged instead to read any combination of tag ID and other information from the RFID tag16, and the short range reader14may be similarly arranged to read any combination of the tag ID and other information from the RFID tag16.

Moreover, although the RFID tag16is shown as a microprocessor based tag inFIG. 2, the RFID tag16may instead comprise one or more digital circuit elements, and/or a programmable logic array, and/or a dedicated integrated circuit, etc.

Furthermore, the long range reader12as described above has a range of several hundred feet and could have an expected range of approximately 500 feet. However, this range could be longer or shorter depending on the application and/or other factors. Similarly, the range given above for the short range reader14could be other than as described above.

Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.