Abstract:
Apparatus and a method for enhancing the interference resistance of so-called RFID systems, wherein identification messages delivered from an identification device are longer than the time for which an anticipated interference or disturbance is calculated to continue, and wherein the messages contain redundancy, such as a checksum. The messages are delivered from the identification device repetitively, and are redundancy checked. In the event of any discrepancy in the redundancy check, such as when the checksum is checked against the data content of the message and is found in disagreement, successive messages are checked bit for bit against each other. Any deviations between otherwise previously rejected messages are assumed to be due to external interference and are thus successively substituted until the redundancy check gives an accepted result, such as through the medium of a checksum check according to the so-called CRC method.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
   This application claims the benefit of 371 of PCT/SE00/02143 filed on Nov. 1, 2000 and foreign application 9909 077-9 file in Sweden on Nov. 18, 1999. 
   BACKGROUND OF THE INVENTION 
   Automatic identification systems that include identification devices (also called data carriers, escort memories, cards, or ID tags) that deliver information from the identification devices by coding, modulation and reflection of an incident microwave signal in the form of an information containing sideband, without supplying further energy to the identification device, have been known in the art since the middle of the 1970s. Such RFID systems (Radio Frequency Identification systems) are related to “backscatter technology” and are particularly common in the microwave range, e.g. at 915 MHz, 2.45 GHz and 5.8 MHz. 2.45 GHz is also a frequency band that can be used for other radiation sources, such as RFDC links, for instance (Radio Frequency Data Communication links). This means that interference problems originating from RFDC systems can occur. 
   The present invention provides an RFID system that is robust with respect to interference from, among others, these RFDC systems, but also with respect to interference from other systems which give rise to interferences that are short and unsynchronized in relation to the identification messages. 
   As distinct from RFID, RFDC operates with microwave transmitters at both ends of the transmission link. RFDC systems often utilize so called spread spectrum technology, where the RFDC messages are transmitted at different microwave frequencies in accordance with a controlled pattern in order to counteract fading effects and interferences/disturbances, from other systems, and where the frequency hops rapidly between different values, so-called frequency hopping. 
   RFID systems can also operate in accordance with spread spectrum technology, and the invention can thus also be applied to minimize interferences from such systems, and also to suppress interferences from any other system which gives rise to interferences that are shorter than the identification messages of the RFID system. 
   One problem with present day identification systems is that they are highly sensitive to interferences from sources that deliver signals at the receiver frequency of the RFID systems, for instance in the 2.45 GHz band, which is freely available for many applications. Examples of interference sources include so-called Bluetooth links, systems according to the 802.11 standard, RFID systems that include so called downlinks (i.e. systems that send short microwave pulses to the identification device in order to activate and/or to send data to the identification device), RFID systems that include battery-free identification devices, which are energized from the microwave transmitter of the reader via a microwave signal that is modulated in some way or another, radar stations, microwave ovens, personal detectors in alarm systems, automatic door openers, video transmission links, etc. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an identification device and/or identification reader or write/read unit functioning in the microwave range. The invention is adapted to suppress interferences from primarily communications and/or identification systems that operate in accordance with a so-called spread spectrum technique in the form of frequency hopping, or so-called direct sequency. It also is adapted to suppress interferences from any other system that transmits microwave signals at fixed or hopping frequencies, and where the signal has been modulated. A microwave signal delivered from the reader is received by the identification device, modulated with an information sideband on one side or on both sides thereof and the information sideband(s) is/are reflected back to the reader without supplying said signal with further energy. The data content of the identification device includes redundancy, for instance by including a data checksum in the data message of said identification device. The message in the identification device is repeated unchanged at least two times in sequence, and the identification device message has a longer time duration than the anticipated interference, so that only a part of said message can be disturbed. In the event of the checksum/redundancy control indicating that the message has been disturbed when making a comparison between at least two mutually sequential messages, the reader establishes which data bits deviate between the messages, and, after intermediate storage of the messages in the reader, substitutes these deviating bits between the messages by trial and error until the checksum/redundancy control gives an accepted result and therewith registers the message as being a correct message. Alternatively, when all possible combinations have been tried and a correct result has still not been obtained, the test is continued with all possible combinations of the data content of subsequent messages. 
   The invention is further characterized in that the write/read unit may be of a kind that transmits pulsated microwave signals. The write/read unit in one particular embodiment varies its frequency between different values, so-called spread spectrum technology, by frequency hopping. In a further embodiment said unit carries out the redundancy check by checksum calculation in accordance with the CRC method. Only messages that fail the redundancy test are tested by substitution of disturbed parts in their identification messages. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to an exemplifying embodiment thereof, shown in the accompanying drawings, in which: 
       FIG. 1  illustrates an RFID system of the so-called backscatter type; 
       FIG. 2  illustrates an RFID system in an interfering field; 
       FIG. 3  illustrates two consecutive identification messages; 
       FIG. 4  illustrates interferences from a spread-spectrum RFDC system; 
       FIG. 5  illustrates how interfered messages have been reconstructed; and 
       FIG. 6  illustrates a technical solution for this reconstruction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  shows a read unit  1  and an identification device  2  that are interconnected via a microwave signal  3 . The read unit includes a microwave oscillator  4  which irradiates the identification device antenna  14  via the antenna  5 . The electronic unit  7  of the identification device  2  receives, encodes, modulates and reflects the signal from the read unit with information according to the data present in the electronic unit of the identification device, and therewith creates so-called identification messages  8  for the read unit. 
   The data content of these messages may be pre-programmed in the identification device or may be programmable, for instance, via microwaves or via a contact device not shown, or in some other way. 
   The identification messages  8  are delivered to a receiver antenna  6  in the read unit. The receiver antenna, however, need not necessarily be separate from the transmitter antenna  5 , but is so shown in the Figure solely for the sake of simplicity. 
   The microwave signal received from the identification device by the antenna  6  is transposed to a baseband in a mixer  9  in the read unit  1 , by mixing the signal with part of the signal sent by the antenna  5 , so as to re-create the identification messages  8  and pass said messages to the computer part  10  of the read unit. 
   Identification device data and other information can therewith be made available via the communications channel  13 , for instance by serial communication in accordance with some typical method. 
   An important function of the system described hitherto is its redundancy check of the identification messages. For instance, if the identification device is located close to its range limit, its signal to the antenna  6  will only marginally exceed the noise level of the system and certain bits in the message will be erroneous, i.e. the recreated message in  8   b  will not fully coincide with the message delivered by the identification device in  8   a.    
   It would be very unfortunate if such errors were reported to superordinate systems. For instance, an automatically identified train travelling at high speed could unintentionally be switched into a railway siding, or an automatically but erroneously identified parking customer could unintentionally cause billing for someone else, etc. This type of error is called a substitution error and is avoided by said redundancy check. In this document, only one redundancy check method, the checksum method, will be mentioned, even though several other methods can be applied to the same end. 
   Checksum calculations are based on the assumption that the message  8  does not only contain the data  11  to be transmitted, but that corresponding information has also been coded into a checksum  12 , and the checksum has been calculated on the basis of said data in accordance with some formula, for instance CRC16 or CRC32, and therewith has been programmed into the identification device in order to include the message. 
   The computer part  10  of the read unit is now able to differentiate between correct messages and erroneous messages with the aid of said formula, by comparing the content of the data part  11  with the checksum part  12 . The messages in which checksum and data do not agree are rejected by the computer part  10 , so that these messages cannot be reported further to the superordinate system via the communications channel  13 . So-called substitution error has been eliminated in this way, i.e. such errors where erroneous messages are reported as correct messages even though they are in fact erroneous, for instance by having been distorted by noise. This is known technology, described here to clarify the invention. 
     FIG. 2  illustrates an RFID system of the aforedescribed type in the presence of interferences, where reference numeral  21  identifies an RFDC transmitter which communicates with one or more other transmitters (not shown) via microwaves. Reference numeral  21 , however, may identify each other type of apparatus that transmits microwaves which may cause disturbances in the communication between identification device  2  and read unit  1 —also including other RFID systems. 
   However, for the sake of clarity it is assumed that the illustrated transmitter  21  is an RFDC transmitter according to the so-called Bluetooth standard at 2.45 GHz, more specifically communicating in the frequency band of 2400–2480 MHz permitted in accordance with CEPT and other authorities, said band also being assumed as standard for data communication in accordance with IEEE 802.11 and also that the RFID system functions at the frequency band of 2446–2454 MHz accepted by CEPT for identification systems, i.e. the so-called A VI band. 
   The signal  3   a  from the identification device will now have competition from the signal  5  from the RFDC transmitter. If the identification device is located at its range limit, e.g. 10 meters from the read unit, it can be shown theoretically that interference from the signal  5  can be significant even if the RFDC transmitter  21  should be located at a relatively long distance from the read unit, e.g. a distance of 100 meters. The invention is intended to make the RFID system resistant to these interferences. 
     FIG. 3  shows two successive RFID messages  31  and  32  with identical data fields  33  and checksum fields  34 . The messages are, however, partially disturbed, more specifically in the intervals  35  and  36 . For instance, interval  35  may represent 10 disturbed bits of a total of 100 bits in the data field  33 . 
     FIG. 4  shows how disturbances, or interferences, according to  FIG. 3  can occur. 
   It is assumed in this embodiment that the RFID system operates in the RFID band  41 , a band, which, for instance, may have a width of 8 MHz at 2450 MHz. The identification messages  31  and  32  are transmitted at the frequency in this band to which the read unit is set, or more specifically close to said frequency on either side or on both sides thereof in the form of a sideband or sidebands that contain identification message data. Naturally, the frequency of the RFID transmitter may hop between different frequencies in the RFID band from time to time, so-called spread spectrum technique. However, since this does not affect the principle of the invention, it is assumed for the sake of simplicity that the illustrated RFID system operates at a fixed frequency. 
   It is also assumed that the receiver of the read unit is optimized to receive the aforesaid information sideband in the best possible way, i.e. the receiver band is only as wide as is required for the messages to be transmitted in an optimal manner. If the bandwidth, for instance, should be 100 kHz, the data transmission rate of the message should be of the same order of magnitude. 
   It is assumed that the illustrated RFDC transmission of  FIG. 4  operates in accordance with the spread spectrum technique with hopping frequency over a band  44  that includes the RFID band, where the frequency, for instance, hops between 80 different channels over a bandwidth of 80 MHz. The RFDC link continues to transmit data at a relatively high data transmission rate, for instance a rate at which each individual channel takes up a frequency space of 1 MHz in width, wherewith the RFDC transmitter transmits constantly but at hopping frequencies according to the pattern  45 ,  46 ,  47 ,  48 , and so on. 
   The interference events that occur, and that the present invention protects against, have been shown in  FIG. 4 , where the frequency of the RFDC frequency  47  is shown to collide with the RFID message  31 . 
   Without the aid of the present invention, the earlier described redundancy check would have rejected the message  31  as a substitution error, and successive messages would also have been rejected due to non-agreement of their checksum calculations. The RFID link thus becomes totally blocked by the RFDC link. 
     FIG. 5  illustrates a testing procedure according to the described invention, and  FIG. 6  illustrates a device for carrying out the procedure. 
   The messages  31  and  32  illustrated in  FIG. 3  and originally containing identical data  33  and checksum  34 , but where a number of bits  35  and  36  have been disturbed in accordance with  FIG. 4 , result in rejection of both messages. 
   A processor  61  that in addition to carrying out standard redundancy checks in accordance with the aforedescribed also checks for deviations between mutually successive messages in order to establish possible differences between the messages. The incoming unaccepted messages are placed in a memory bank  62  for successive processing in the processor  61 . 
   If, for instance, the disturbed messages  31  and  32  are now compared with each other bit for bit, the processor will discover two uncertain areas  51  and  52  in which deviations occur. Nevertheless, the processor draws the conclusion that the two messages are identical, because both checksums  34  are identical. An interference or disturbance event has thus probably occurred. 
   The processor  61  then tests first the event  55 , by replacing the deviating area or region  52  in the message  31  with the alternative bit pattern  36 , but when carrying out a redundancy check will interpret the message as being erroneous, because the thus changed message  55  now includes errors both at  53  and  54 . 
   The processor will then test event  56 , by replacing the deviating area or region  51  with corresponding data in message  32 , and will therewith find that the thus changed message  56  agrees with the redundancy check. The message is therewith accepted. The computer part  10  will thus only receive messages when interferences, or disturbances, from different sources, for instance from Bluetooth-type RFDC systems, are filtered out. 
   In accordance with the present invention, the processor  61  may be given one of a number of different forms, e.g. the form of a gate matrix, and need not contain software. A hardware solution is to be preferred for dealing with simple interference events, as the solution can be carried out very rapidly. 
   In order to handle complicated interference events, the processor  61  may consist of a signal processor that has powerful mathematical functions, as this facilitates the use of advanced calculation algorithms for interference suppression. It may for example happen that the message contains disturbances at several positions, where a larger number of messages are interfered with in succession, and where disturbances also in the checksum need to be dealt with, and so on. 
   The illustrated embodiment thus constitutes only a limited description of how interferences, or disturbances, can be filtered out. Moreover, there has been illustrated with the aid of a simple example a technique which filters out those interferences, or disturbances, that can be expected when an RFID system is operated in a frequency range where several different applications are in good agreement with each other, e.g. in the so-called ISM band (Industrial, Scientific and Medical), 2.45 GHz. 
   Furthermore, there has been shown in the illustrated example solely an RFID system in which a non-modulated microwave signal is transmitted from the read unit, which may be the case when the identification device contains a constantly oscillating circuit for clocking its internal logic and its modulation circuits connected to the identification device antenna. 
   In another embodiment, the read unit may also be used to activate the identification device, for instance by transmitting pulsed microwave signals that are detected by circuits in the identification device, and therewith, for instance, starting an oscillator incorporated in the identification device for forward clocking of data to the modulator circuits connected to the identification device antenna. 
   In yet another embodiment, the read unit may transmit pulsed microwave signals for transferring data to the identification device for storage in a memory incorporated in the identification device and/or for controlling the function of said device. Consequently, the read unit is at times also referred to as a write/read unit.