AU2006269814A1 - Radio frequency identification (RFID) tags and techniques - Google Patents

Description

WO 2007/006085 PCT/AU2006/000964 1 RADIO FREQUENCY IDENTIFICATION (RFID) TAGS AND TECHNIQUES TECHNICAL FIELD 5 The present invention relates to radio frequency identification (RFID) tags and techniques relating thereto. BACKGROUND 10 Radio frequency identification (RFID) tags are commonly used in electronic article surveillance (EAS) systems for detecting the shoplifting of products from stores. EAS systems commonly include a RFID reader which, in turn, includes a transmitter gate for transmitting a probe signal to a tag, and a receiver gate for receiving a response signal from a tag. When a tag passes 15 between the gates, the tag receives the probe signal and then transmits a response to the transmitter gate which generates an alarm. The distance between the gates is typically of the order of a couple of metres and the operating frequencies for EAS systems generally range from 2 to 15 MHz. 20 It is desirable that the tags are small in size so that they can be effectively hidden in the products which they protect. Conventional RFID tags generally have a very limited range of operation owing to the relatively small amount of power derived from the probe signal they receive. The applicant perceives a need for a RFID tag and associated system having an alternative design to 25 those commonly used at present. The present invention relates to such an RFID tag and system, and various techniques associated with the tag and system. SUMMARY OF THE INVENTION 30 According to a first aspect of the present invention, there is provided a radio frequency identification (lFID) network for locating a RFID tag, the network including: WO 2007/006085 PCT/AU2006/000964 2 one or more nodes each including a RF transponder for relaying a signal whether from another node or the RFID tag, the relayed signal including information from which the location of the RFID tag can be determined; and a locator for receiving the relayed signal from the nodes and for 5 determining the location of the RFID tag using the information. The network may include a plurality of the nodes with each node being configured to determine whether to relay the signal. The determination of whether to relay the signal may be made using a count of the number of 10 nodes through which the signal has already been relayed, the count being included in the signal. Alternatively, the determination of whether to relay the signal can be made using the time elapsed since the creation of the signal, a time-stamp being included in the signal at creation of the signal. 15 The RFID network may further include a detector for detecting whether the RFID tag has been removed from a zone using the determined location of the RFID tag. The RFID network may further include an alarm generator which, upon detection of the removal of the RFID tag from the zone, transmits an alarm signal to the first one of the nodes to relay the signal. 20 Each node may further include an alarm which is activated upon the node receiving an RF alarm signal. The locator may be configured to generate an alarm signal when the relayed signal is not received within a predetermined time period. 25 The information may include an identifier corresponding to a node which reads the RFID tag. The information can be included with the signal by either the RFID tag or the node which reads the RFEL tag. When determining the location of the RFID tag, the locator may correspond the identifier with a 30 stored identifier associated with the location. Each transponder receives the signal and relays the signal at the same UHF frequency. The network may include the RFID tag. Each node may be WO 2007/006085 PCT/AU2006/000964 3 configured to periodically transmit a probe signal. The RFID tag may be configured to create the signal to be relayed upon receipt of the probe signal. Each node may further include a RFID reader for reading the information from 5 the RFID tag. The reader may include a transmitter for transmitting a probe signal to the RFID tag and a receiver for receiving a response signal including the information from the REID tag. The reader can include a scheduler for periodically transmitting the probe signal. 10 According to a second aspect of the present invention, there is provided a node for a radio frequency identification (RFID) network, the node including: a RF transponder for relaying a signal whether from another node or a RFID tag. 15 According to a third aspect of the present invention, there is provided a power supply for a radio frequency identification (RFID) tag, the power supply including: a storage capacitor for storing charge; a first diode through which a received signal can be supplied to the 20 storage capacitor; and a second diode through which charge is supplied to the storage capacitor from a battery. The power supply may further include a voltage limiter for limiting the voltage 25 level of the received signal supplied to the storage capacitor. The voltage limiter may include a zener diode. According to a fourth aspect of the present ir-ntion, there is provided an alarm system for detecting the removal of an radio frequency identification 30 (RFID) tag from a zone; the system including: at least one reader for reading the tag located within the zone; a detector for detecting when the tag has been removed from the zone; and WO 2007/006085 PCT/AU2006/000964 4 an alarm for generating an alarm signal when the tag has been removed from the zone. Preferably, the alarm system further includes an audible siren which sounds 5 responsive to the generation of the alarm signal. Preferably, the tag is periodically read by the reader. According to a fifth aspect of the present invention, there is provided an alarm 10 system for detecting the removal of an radio frequency identification (RFID) tag from a first zone; the system including: a first set of readers for reading the tag located in the first zone; a second set of readers for reading the tag located in a second zone, the second zone being adjacent to the first zone; and 15 a detector for detecting when the tag has moved from the first to the second zones. In one embodiment, the first set of readers may be a singleton set including a sole reader. 20 Preferably, the system further includes an alarm for generating an alarm signal when the tag has been moved from the first to the second zones. The alarm signal may be generated by a reader in the second zone and transmitted to a reader in the first zone. The alarm system may further include 25 an audible alarm which sounds responsive to the generation of the alarm signal. The audible alarm may be located in the first zone. According to a sixth aspect of the present i...ntion, there is provided a method for calculating backoff in a radio frequency identification (RFID) tag, 30 the method including the steps of: dividing a dividend stored in the RFID tag by a prime number; and obtaining the remainder of the division to thereby calculate the backoff.

WO 2007/006085 PCT/AU2006/000964 5 According to a seventh aspect of the present invention, there is provided a radio frequency identification (RFID) tag programmed with software instructions for calculating the backoff. Alternatively, the backoff could be calculated using hardware (e.g. an ASIC). 5 According to a eighth aspect of the present invention, there is provided a method for calculating the back off period in a radio frequency identification (RFID) tag, the method including the steps of: dividing a dividend stored in the RFID tag by a prime number; and 10 obtaining the remainder of the division; multiplying the remainder by a time period to thereby calculate the backoff period. Preferably, the dividend is a unique to a particular RFID tag. Even more 15 preferably, the dividend is a serial number of the RFID tag. The serial number is a suitable dividend as each RFID tag in the network has a unique serial number. According to an ninth aspect of the present invention, there is provided a 20 checkout system for determining the price of one or more products to which respective radio frequency identification (RFID) tags are attached, the system including: a RFID reader for reading each RFID tag to thereby determine the identity of each product; and 25 a calculator for calculating the price of the products by corresponding the determined identity of each product with a corresponding price and summing the prices of the products. Preferably, the system includes an immobilise. .or immobilising each RFID tag. 30 According to a tenth aspect of the present invention, there is provided an alarm system including: a radio frequency identification (RFID) reader; and WO 2007/006085 PCT/AU2006/000964 6 a camera for capturing one or more images when the reader reads a RFID tag. According to an eleventh aspect of the present invention, there is provided a 5 method for locating a radio frequency identification (RFID) tag in a RFID network, the method including the steps of: relaying a RF signal from the RFID tag using one or more nodes each including a RF transponder, the relayed signal including information from which the location of the RFID tag can be determined; 10 receiving the relayed signal from the nodes; and determining the location of the RFID tag using the information included in the received signal. BRIEF DESCRIPTION OF THE DRAWINGS 15 Preferred features, embodiments and variations of the invention may be discerned from the following Detailed Description which provides sufficient information for those skilled in the art to perform the invention. The Detailed Description is not to be regarded as limiting the scope of the preceding 20 Summary of the Invention in any way. The Detailed Description will make reference to a number of drawings as follows: Figure 1 is a block diagram of a radio frequency identification (RFID) network in accordance with a first embodiment of the present invention. 25 Figure 2 is a plan view of a RFID network in accordance with the first embodiment installed within a shopping complex. Figure 3 is a functional block diagram of a node for a RFID network in 30 accordance with the first embodiment. Figure 4 is a time-domain diagram of a transmitted amplitude shift keying (ASK) signal waveform.

WO 2007/006085 PCT/AU2006/000964 7 Figure 5 is a schematic diagram of a RFID tag in accordance with the first embodiment. Figure 6 is a circuit diagram of a receiver and power supply for a RFID tad in 5 accordance with another embodiment. Figure 7 is a block diagram of a controller of the RFID tag in accordance with the first embodiment. 10 Figure 8 is a structural diagram of a response signal transmitted by a RFID tag in accordance with the first embodiment. Figure 9 is a structural block diagram of a RFID node in accordance with the first embodiment. 15 Figure 10a is a schematic diagram of an automatic checkout including a RFID network in accordance with the first embodiment. Figure 10b is a structural block diagram of the structure of a response signal 20 transmitted by a RFID tag when implementing the automatic checkout. Figure 10c is a schematic diagram of a display of the automatic checkout for displaying items being purchased. 25 Figure 11 is a block diagram of a silent alarm system in accordance with a second embodiment of the present invention. DETAILED DESCRIPTION OF PREFERRED EmdJODIMENTS 30 RFID network Figure 1 is a block diagram showing a conventional RFID system 4, and a RFID network 5 in accordance with a first embodiment of the present invention. The RFID network has been retrofitted so as to interface with the WO 2007/006085 PCT/AU2006/000964 8 RFID system 4. A detailed description of the operation of the RFID system 4 and network 5 is provided below. The RFID system 4 includes a base station 6 which interfaces to a pair of 5 security gates and one or more passive RFID tags 8. The base station 6 periodically transmits an 8.2MHz probe signal 10 which is received by and powers a passive RFID tag 8. The passive RFID tag 8 transmits a tag response signal 12a which is of relatively low power. Accordingly, the base station 6 only receives the response signal 12a when the passive RFID tag 8 10 is in close proximity (e.g. 1 metre) to the base station 6. The RFID network 5 includes one or more active RFID tags 13 each having a stored unique identifier (e.g. serial number), a RFID node 14 having a stored unique identifier which includes a RFID reader for reading the RFID tags 13, 15 and a central station 16 which is in RF communication with the RFID reader 16. Each tag 13 also receives the 8.2MHz probe signal 10 transmitted from the base station 6. Each tag 13 includes an internal battery which powers an internal UHF transmitter that transmits a tag response signal 12b over a relatively high range (e.g. 50 metres). In the present embodiment, the tag 20 response signal 12b is transmitted at 433MHz, however in alternative embodiments, other UHF frequencies including 315 MHz, 868 MHz and 915MHz, 2.4GHz and 5.8GHz may be used. The base station 6 is not adapted to receive the tag response signal 12b. 25 Instead, the RFID node 14 includes a transponder which receives the tag response signal 12b and relays a (boosted) corresponding node response signal 12c to the central station 16 for processing. The tag response signal 12b and node response signal 12c may not be .antical (e.g. in frequency or information content), however, collectively form the same response signal 12 30 which is relayed from the tag 13 to the central station 16. In fact, each response signal 12b, 12c can be any RF indicator (including, for example, a carrier frequency only) which forms part of the collective response signal 12 sent from the tag 13 to the central station 16. In the present embodiment, the response signals 12b, 12c are both transmitted at 433MHz and the node WO 2007/006085 PCT/AU2006/000964 9 response signal 12c is transmitted after the tag response signal 12b is received so as to prevent the interference of the two signals. In an alternative embodiment, the network 5 would operate according to a channel arrangement whereby concurrent transmission of response signals 12 by two 5 respective nodes 14 of the network 5 can be performed. The RFID node 14 includes sufficiently sensitive receiver circuitry to effectively detect the tag response signal 12b when an active RFID tag 13 is located within 50 metres of the RFID node 14. The node 14 is generally powered by a mains power supply. Accordingly, the node response signal 12c 10 is of higher power than the tag response signal and therefore the central station 16 and the RFID node 14 can be located further than 50 metres apart, and typically up to several hundred metres apart. The range of both response signals 12b, 12c are power limited to an extent so that a license to operate the network 5 in the UHF range is not required. In an alternative embodiment, the 15 node 14 may be powered by a low voltage supply or battery. Figure 2 is a block diagram showing a RFID network 5 installed within a shopping complex 18 in accordance with the first embodiment for the purpose of preventing the shoplifting of products from stores 20 of the complex 18. 20 Active RFID tags 13 each having a unique stored identifier are attached to respective products. In the present example, the shopping complex 18 includes four stores 20a-20d interconnected by a common hallway 22. Respective RFID nodes 14a-14d are located within each store 20, and a further pair of RFID nodes 14e, 14f and a central station 16 is located within 25 the hallway 2. Referring to Figure 3, each RFID node 14 functionally includes a RFID reader 30 for reading RFID tags 13; and a RF tra 1 *.ponder 32 for receiving a response signal 12 from either another transponder 32 or a RFID tag 13 and 30 re-transmitting (i.e. relaying) the response signal 12. The RFID reader 30 and RF transponder 32 may include a common transceiver. In Figure 2, response signal 12 is relayed via nodes 14a, 14e and 14f to central station 16., That is, RFID node 14a relays tag response signal 12a to node 14e in the form of node response signal 12b, RFID node 14e relays node response signal 12b to WO 2007/006085 PCT/AU2006/000964 10 node 14f in the form of node response signal 12c, and RFID node 14f relays node response signal 12c to central station 16 in the form of node response signal 12d. Similarly, signals can be relayed from the central station 16 to RFID tags 13 via one or more nodes 14. 5 A detailed description of the operation of the network 5 in shopping complex 18 is now described with reference to a single active RFID tag 13a located in store 20a. A person skilled in the art will appreciate that analogous operations can be concurrently undertaken by the network 5 in relation to a plurality of 10 tags 13. Subsequent to receiving a probe signal 10 from RFID node 14a, active RFID tag 13a transmits a response signal 12 to the central station 16 via RFID nodes 14a, 14e, and 14f respectively. The reader 30 of node 14a reads tag 15 13a (i.e. transmits probe signal 10 and receives and processes tag response signal 12a) and relays the response signal 12a to node 14e. The transponders 32 in node 14e and node 14f relay the response signal 12 to a RF receiver of central station 16. The response signal 12 includes a node counter parameter which is incremented by a node 14 each time response signal 12 is relayed 20 through a respective RFID node 14. Hence, the value of the node counter parameter for respective stages in the network 5 is as follows: "0" for tag response signal 12a, "1" for node response signal 12b, "2" for node response signal 12c and "3" for node response signal 12d. 25 In one embodiment, a determination arrangement of each node 14 compares the value of the node counter parameter received with a predetermined threshold and, if the value of the node counter parameter exceeds the predetermined threshold, the response signal 111 Ii not be relayed. If the value of the node counter parameter does not exceed the predetermined threshold, 30 the node counter parameter is incremented by the node 14 and the response signal 12 is re-transmitted. In this manner, the response signal will not be indefinitely relayed throughout the network 5.

WO 2007/006085 PCT/AU2006/000964 11 In an alternative embodiment, the tag 13a includes a timestamp with response signal 12a upon its creation. The determination arrangement of each node 14 which receives the response signal 12 checks the timestamp. If the time elapsed since the timestamp was created (and hence the response signal 12 5 was created) exceeds a predetermined threshold, the response signal 12 is not retransmitted. That is, each node will only relay the response signal 12 in the event that the time elapsed is less than or equal to the predetermined threshold. 10 Each node 14 stores a unique node identifier. The node 14a transmits its node identifier to the tag 13a along with probe signal 10. The tag 13a then re transmits this node identifier along with the response signal 12 which is received by the central station 16. Each RFID node 14 includes a scheduler which periodically transmits a probe signal 10 and the tag 13a periodically 15 sends the node identifier, corresponding to node 14a, to the central station 16. As the tag moves about the shopping complex from store 20a to store 20d, the node identifier sent with response signals 12 transmitted from the tag 13a change responsive to the RFID reader which presently reads the tag 13a. In this case, the node identifier would change between nodes 14a, 14e, 14f and 20 14d respectively. This node identifier for tag 13a is periodically updated in a location register which is managed by central station 16. The central station 16 also manages a lookup table which stores the general location of each respective node in 25 the shopping complex 18 (i.e. store location or hallway) and a corresponding node identifier. In this manner, a locator in the central station 16 can locate the general whereabouts of the RFID tag 13a within the shopping complex 18 at any time by corresponding the node identifie. ... the location register with a corresponding general location stored in the lookup table. 30 The tag 13a is fastened to a product for sale in store 20a. When a consumer purchases the product, the tag 13a may be removed from the product and retained in the store. In the event that the tag 13a is removed from the store 14a (i.e. zone), the node identifier stored in the location register will change WO 2007/006085 PCT/AU2006/000964 12 (to the node identifier of node 14e). Central station 16 includes a detector for detecting this change when the tag 13a has been removed from the store 14e. Accordingly, the central station includes an alarm which generates and transmits an alarm signal to node 14a via nodes 14f and 14e respectively. 5 Node 14a includes an audible siren which sounds responsive to the generation and reception of the alarm signal. If a response signal 12 is no longer periodically received by the central station 16 from the tag 13a, it can be deduced by the central station 16 that the tag 10 13a is no longer within the shopping complex 18. That is, if the tag 13a is not read by a node 14, the tag 13a will not transmit a response signal 12 and a response signal 12 will not be received by the central station 16. In practice, each store 20 would include a plurality of active RFID tags 13 15 attached to respective products and respective storekeepers could be alerted of any unauthorized removal of a product from the store 20. The RFID system 5 is quickly installed in a shopping complex 18 as there is no need for wired connections between adjacent nodes 14. In addition, the nodes 14 collectively form the RFID network 5 which can be readily expanded by including a further 20 node 14 within communication distance of an existing node 14 in the network 5. Each node 14 and RFID tag 13 includes a RF transmitter (as described in detail below) for transmitting a response signal 12 having an Amplitude Shift 25 Keying (ASK) waveform as shown in Figure 4. The response signal 12 includes a series of pulsed sinusoids with a logical "1" having a longer pulse duration than a logical "0". RFID Tag 30 Figure 5 schematically shows an active RFID tag 13 in accordance with the first embodiment. The RFID tag 13 includes a receiver block 50, an address detection block 64, a power supply block 70, a data storage block 68, a WO 2007/006085 PCT/AU2006/000964 13 controller block 66 and an UHF transmitter block 72. Each block is described in detail below. Receiver Block 5 The receiver block 50 includes a magnetic loop antenna 52 which is coupled in parallel to a tuning capacitor 54 to form a tuned circuit having a Q-factor greater than fifty. The tuned circuit is tuned to the frequency of the probe signal 10 which may be, for example, 8.2MHz or 13.56 MHz. A half wave 10 rectifier is coupled to the tuned circuit and is formed by the diode 56 and capacitor 58. The output of the rectifier is coupled to a comparator 60. In use, the received probe signal passes through the rectifier and comparator 60 to generate an activation signal which then passes to a wake-up circuit 62 which wakes up the sleeping controller 66. 15 Address Detection Block The output of comparator 60 is also coupled to the address detection block 64 which receives the activation signal. The address detection block 64 evaluates 20 whether an address of a node 14 or the base station 6 is contained in the activation signal. If the address of the base station 6 is detected, an identifier of the tag 13 is included in the transmitted response signal 12. If the address of a node 5 is detected, identifier of both the tag 13 and the node 5 are included in the transmitted response signal 12. 25 If a valid address or command is detected in the probe signal 10 by the address detection block 64, the command is passed on to the controller 66 for processing. The command may be, for eA.mple, a product identifier command for updating the product identifier of the tag 13. 30 Power Supply Block A power supply block 70 is provided for selectively providing power to the wake-up circuit 62, address detection block 64, and data storage block 68.

WO 2007/006085 PCT/AU2006/000964 14 The power supply block 70 includes and auxiliary battery for powering the wake-up circuit 62, address detection block 64, and data storage block 68 when the signal strength of the received probe signal 10 is insufficient to power these blocks. 5 A low power switch (operating in the nano-ampere range) is provided to selectively connect the battery to the aforementioned blocks 62, 64, 68 when power supply block 70 detects that the probe signal strength is below a predetermined threshold. If the power supply block 70 detects that the probe 10 signal strength is above the predetermined threshold, the switch connects rectified signal 74 to the blocks 62, 64, 68 instead of the battery. In an alternative embodiment, the circuit arrangement shown in Figure 6 may be used to prolong the life of the battery 76 (typically beyond 10 years). The 15 rectified signal 74 is clamped by a voltage limiter in the form of zener diode 78. Subsequently the signal passes through diode 80 and supplies power to storage capacitor 84. Storage capacitor 84 stores electrical charge and supplies power to the blocks 62, 64, 68 of the RFID tag 13 through power feed line 86. Charge is supplied from the battery to the storage capacitor 82 via 20 diode 82 and therefore, even in the absence of a rectified signal 74, power is still maintained on power feed line 86. In the foregoing arrangement, power can be supplied by the battery 76 and rectified signal 74 concurrently so that the battery 76 effectively only provides any difference between the load power required and the rectified signal power 74 supplied, thereby prolonging the life 25 of the battery 76. Data Storage Block The data storage block 68 is coupled to the controller 66 and includes an 30 EEPROM. The EEPROM stores various information which may include the tag identifier (i.e. serial number), a customer identifier, a product identifier, an article code, the price of the product and the store in which the item is located.

WO 2007/006085 PCT/AU2006/000964 15 Typically, the serial number of a tag 13 has a length of 32 bits which allows for 4294967295 combinations. The product identifier is effectively the name of the product and can be of up to 31 characters long. The customer identification number is 16 bits long. The additional information stored in the 5 EEPROM is encrypted and can only be decrypted by users having the appropriate decryption key. Controller 10 As shown in Figure 7, the controller 66 includes a backoff calculator 90 for calculating the backoff period in the event of a collision between transmitted response signals 12 from two or more tags 13, and a data coding block 98 for encoding data (i.e. information) transmitted with the response signal 12. These features are described in detail below. 15 In the event that a node 14 experiences a collision of two or more response signals 12 transmitted by respective tags 13, the backoff calculator 90 in each tag will wait for a respective "backoff" period before attempting to retransmit the response signal 12. A different backoff in each tag 13 minimises the 20 probability that the response signals 12 will collide again. The backoff calculator in each tag 13 calculates a backoff period using the same algorithm in each tag 13 as described below. Each tag 13 within range of a node 14 receives the probe signal 10 from that 25 node 14 at approximately the same instant. The probe signal 10 is received by the receiver block 50 and the controller proceeds to process the information contained in the probe signal 10. The backoff period is divided up into an integral number of slot times in which a tag 13 c... transmit a response signal 12. In the present embodiment, the slot time is 5 milliseconds and the nodes 30 14 transmit a probe signal 10 every 0.5 seconds. Therefore, up to 100 respective tags 13 can transmit a response signal 12 over a maximum backoff period of 0.5 seconds between successive probe signal 10 transmissions from a node 14.

WO 2007/006085 PCT/AU2006/000964 16 Each RFID tag 13 stores the same array of prime numbers in the data storage block 68. An example of an array of prime numbers stored in each tag 13 is as follows: Location 0 1 2 3 4 Prime Number 97 71 43 31 13 5 Each tag has a unique serial number stored in the data storage block 68. An example of serial numbers for five tags 13 designated A to E is shown in the following table: Tag A B C D E Serial No. 12345678 23456789 34567891 45678912 56789123 Backoff 3 55 1 60 85 [slots] Backoff 15 275 5 300 425 period [ms] 10 In the event that each of tags A to E are serviced by the same node 14 and their transmitted response signals 12 collide, the node 14 will send a collision signal advising each tag 13 that there has been a collision. The collision signal will include the slot time in which the collision occurred. In the event of a 15 collision, each tag 13 receives the collision signal and therefore the collision slot time. Each tag 13 which caused the collision (i.e. which transmitted a response signal 12 in the collision slot time) calculates a backoff period which the tag will wait before attempting to retransmit. 20 The backoff for each tag 13 is the remainder whicn results from dividing the serial number (i.e. dividend) by the first prime number in the prime number array (i.e. at location 0). This operation is known as a modulus operation. The backoff for each tag 13 is then multiplied by the slot time (i.e. 5ms) to yield a different backoff period for each tag A to E as shown in the foregoing table. 25 WO 2007/006085 PCT/AU2006/000964 17 On occasion, two or more of tags 13 may calculate the same backoff period (e.g. say tags F and G). In this event, the response signals 12 re-transmitted by tags F, G will once again collide and, accordingly, the node 14 once again sends a collision signal. Tags F,G then calculate another backoff and backoff 5 period as previously described, however, by using the second prime number in the prime number array (i.e. at location 1) instead of the first prime number. In the rare event that there is yet another collision, the third prime number in the array (i.e. at location 2) is then used and so on. The foregoing algorithm advantageously provides a random backoff calculation which quickly and 10 effectively minimises the probability that consecutive retransmissions for the same nodes will collide, even although the same backoff algorithm is used. As shown in Figure 7, the data coding block 98 prepares data for transmission by the UHF transmitter 72 'and includes a convolutional encoder 92, 15 manchester encoder 94, and frame assembler 96 which are connected together in series. Data is read from the EEPROM of the data storage block 68 by the convolutional encoder 92 which, in turn, performs forward error correction (FEC) on the data to correct any errors introduced into data received by the receiver 50. The size of data transmitted with the probe and 20 response signals 10,12 is increased by about 50% using a 2/3 coding scheme. The convolutionally encoded data then passes to the manchester encoder 94 which effectively breaks up any continuous strings of logical "Is" or "Os" in the 25 data to be transmitted (i.e. removes any net DC offset). The manchester encoded data passes to the frame assembler 96 which appends a synchronisation pattern to the be,...ning of the data so as to enable the receiver of the node 14 to synchronise the received response 30 signal 12. The node 14 performs complementary operations to those of the data coding block 98 in order to extract the transmitted data as described further below. UHF transmitter WO 2007/006085 PCT/AU2006/000964 18 The UHF transmitter 72 is initially activated one slot time prior to transmitting the response signal 12 to allow the transmitter to stabilise prior to transmission. In the present embodiment, the transmitter 72 is an ASK 5 transmitter, however, frequency shift keying (FSK) could be alternatively used. The data structure of the response signal 12 is shown in Figure 8. The response signal 12 includes a synchronisation pattern 102 of '1', '0', '1', '0', '1', '0', '1', '1' which is first sent to enable the receiver in the node 14 to 10 synchronise the response signal 12. The response signal 12 further includes the following parameters: the tag identifier 104, the node identifier 106 and the node counter (initialised) 108. Node 15 Figure 9 is a block diagram showing a node 14 in accordance with the first embodiment. The reader 30 and transponder 32 of the node 14 shown in Figure 3 are implemented using the hardware shown in Figure 9. 20 The node 14 includes a receiver 120, a FEC and manchester decoder 122, a data processor 124, a node protocol expansion block 126, and a UHF transmitter 128 which are serially connected together. The node 14 further includes a time reference block 130. The node further 25 includes a collision detector 132 and a collision prime index calculator 134. The collision detector 132 detects when there has been a collision between response signals 12 transmitted by two or more tags 13 in which case, the node 14 transmits a collision signal to the tags , which includes the slot time in which the collision occurred. The collision signal is received by the receiver 30 block 50 of any RFID tags 13 within range of the node 14. Automated Checkout Further to the first embodiment of the present invention shown in Figure 2, there is also provided an automatic checkout 140 as shown in Figure 1Oa. The WO 2007/006085 PCT/AU2006/000964 19 automatic checkout 140 includes a single node 14 that is mounted at a location on the ceiling 14 of the shopping complex 18 which is proximal to a checkout desk 146. The transmit power of the node 14 is sufficient to read tags 13 located within a zone immediately surrounding the checkout desk 146, 5 and insufficient so that tags 13 located in other zones (i.e. controlled by other nodes 14) of the shopping complex 18 are not read. The checkout desk 146 is located at the exit of the shopping complex 18. A trolley 148 can be used to transport products to which tags 13 are fastened past the checkout desk 146. Typically, the trolley 148 is transported directly beneath the node 14. The 10 node 14 interfaces to the central station 16 which, in turn, interfaces with one or more displays 142. Each display serves to display details relating to the items in the trolley 148. The structure of a response signal 12 for implementing the automatic 15 checkout 140 is shown in Figure 10b. The response signal 12 includes the following parameters: synchronisation information 102, a tag identifier (e.g. serial number) 104, a node identifier 106 and a node counter 108. The response signal 12 further includes parameters of a product identifier 103 which identifies the product to which the tag is attached, a customer identifier 20 105 which identifies the store from which the product was procured, and other information 107 which is typically encrypted. The other information 107 includes information peculiar to a particular user (i.e. store) which is stored in an encrypted format in the data storage block 68. Other information 107 may include, for example, store location and warehouse information. 25 In use, the shopping trolley 148 is positioned beneath the node 14 at the checkout desk 146. A checkout operator 150 (or the customer) presses a button to initiate the node 14 sending out a prol~ signal 10 which is received by each of the tags 13 located in the trolley 148. In the present embodiment, 30 each tag 13 transmits a first response signal 12 in accordance with a calculated backoff period to avoid the collision of initial response signals 12 which would otherwise occur. Each tag 13 transmits the response signal 12 which is relayed via the node 14 to the central station 16. The central station WO 2007/006085 PCT/AU2006/000964 20 16 extracts the customer identifier 105 and product identifier 103 from each response signal 12. The central station 16 includes a price database which, in turn, includes a list 5 of product identifiers of available products for sale, and a list of prices of the products which each correspond to a product identifier. The central station 16 calculates the total price of the products in the trolley 148 which each correspond to an item in the list of product identifiers. The product identifier 103, customer identifier 105 and price of each item are displayed on each 10 display 142 along with the total price as shown in Figure 10c. The operator 150 then accepts payment for the products and issues a receipt to the customer. Once the payment has been confirmed, the central station 16 sends a notice 15 to a server for each store 20 from which a purchased product was procured advising the store server of the sale. The storekeepers of each store can access the price database of the central station 16, and modify the prices of products available for sale at that store using the store server. Typically, the store servers are connected to a central station server of the central station 16 20 via a network (e.g. Ethernet). The central station 16 further includes an immobiliser for removing the tag identifier (e.g. serial number) of the tags 13 attached to purchased products from the list of tag identifiers of products for sale. In this manner, the tags 13 25 attached to the purchased products are effectively immobilized whereby, if a tag 13 is returned to the automatic checkout 140, the product to which the tag 13 is attached cannot be purchased again as the tag 13 cannot be corresponded to an item in the list. If a produc. .3 returned to the store, the serial number of the corresponding tag 13 can be added to the list to thereby 30 re-activate the tag 13. The automated checkout 140 has the benefit of greatly increasing the speed at which products can be scanned by an operator. According to a second embodiment of the present invention, there is provided a silent alarm system 162 shown in Figure 11. The alarm system 162 includes WO 2007/006085 PCT/AU2006/000964 21 a RFID reader 30 which is located proximal to an exit of a store, and a digital camera 160 for taking a photograph of the exit when a response signal 12 is received by the reader 30 from a tag 13. The RFID reader 30 periodically transmits a probe signal 10 about the exit. In the event that a shoplifter carries 5 a product bearing a tag 13 through the exit, the reader will read the tag 13 and instruct the camera 160 to take a photograph of the exit (and the shoplifter). The silent alarm system 162 stores the photograph of the shoplifter exiting the store along with the product identifier 103 of the response signal 12 in a 10 database. The date and time may also be stored in the database by the system 162, and may be either stored in an electronic file or as a caption on the stored photograph. The silent alarm system 162 transmits the foregoing information to a security guard station so that a security guard can then apprehend the shoplifter. 15 A person skilled in the art will appreciate that many embodiments and variations can be made without departing from the ambit of the present invention. 20 In the first embodiment, the node identifier parameter in the response signal 12d received by central station 16 was included in response signal 12 by a tag 13. In an alternative embodiment, the node identifier parameter may instead be included in the response signal by node 14a. 25 In the shoplifting prevention scheme presented in the first embodiment, the central station 16 detected when tag 13a was removed from a store 20a and generated an alarm signal accordingly. In an alternative embodiment, the removal of the product from store 20a could ins-ad be detected by node 14e which, in turn, generates the alarm signal and sends it to node 14a. That is, a 30 reader (i.e. detector) in the second node 14e detects when the tag enters the hallway 22 (i.e. from a first zone in store 20a to a second zone in hallway 22). The node 14e may further include an audible siren which sounds responsive to the generation of the alarm signal. In yet another embodiment, two or more nodes 14 may regulate a single zone.

WO 2007/006085 PCT/AU2006/000964 22 In the preferred embodiment of the automatic checkout 140 described, the tags 13 sent a response signal 12 to the central station 16 upon purchase of the products to which the tags 13 were attached. In turn, the central station 5 16 included an immobilizer which removed the tag identifier (e.g. serial number) of the tags 13 attached to purchased products from the list of tag identifiers of products for sale, thereby immobilizing the tags 13. In an alternative embodiment, the central station 16 (or node 14) includes an immobilizer which sends a de-activation command to the tags 13 upon 10 purchase of the products. Typically, a special code would need to be entered by the checkout operator 150 and verified by the central station 16, before the de-activation command can be sent. The tags 13 receiving the de-activation command may set a de-activation flag, in which case the tags 13 will not transmit a response signal 12 when the de-activation flag is set, thereby 15 immobilizing the tags 13. A tag 13 may be re-activated (i.e. when a product is returned) if required. The checkout operator 150 would instruct the central station 16 (or node 14) to send an activation command to the tag 13 which would reset the de-activation 20 flag. The tag 13 would therefore transmit a response signal 12 upon receipt of a probe signal 10, provided the de-activation flag is not set. The silent alarm system 162 of the third embodiment included a digital camera 160 for capturing single images as a shoplifter leaves a store. In another 25 embodiment, the digital camera can be substituted with a video camera that records multiple images whilst the reader 30 reads a RFID tag 13. In yet another embodiment, a video camera would be provided which continuously records multiple images and information (e.g. -te, time, product identifier) can be added to any images which are captured whilst the reader 30 reads a 30 RFID tag 13. In yet another embodiment, the silent alarm system may include a camera orientation means for orienting and/or focusing the camera prior to capturing images.

WO 2007/006085 PCT/AU2006/000964 23 In compliance with the statute, the invention has been described in language more or less specific to structural or methodical features. It is to be understood that the invention is not limited to specific features shown or described since the means herein described comprises preferred forms of 5 putting the invention into effect.

Claims (41)

1. A radio frequency identification (RFID) network for locating a RFID tag, the network including: one or more nodes each including a RF transponder for relaying a signal whether from another node or the RFID tag, the relayed signal including information from which the location of the RFID tag can be determined; and a locator for receiving the relayed signal from the nodes and for determining the location of the RFID tag using the information.

2. A RFID network as claimed in claim 1, wherein the network includes a plurality of the nodes with each node being configured to determine whether to relay the signal.

3. A RFID network as claimed in claim 2, wherein the determination of whether to relay the signal is made using a count of the number of nodes through which the signal has already been relayed, the count being included in the signal.

4. A RFID network as claimed in claim 2, wherein the determination of whether to relay the signal is made using the time elapsed since the creation of the signal, a time-stamp being included in the signal at creation of the signal.

5. A RFID network as claimed in claim 1, further including a detector for detecting whether the RFID tag has been removed from a zone using the determined location of the RFID tag.

6. A RFID network as claimed in claim 5, further including an alarm generator which, upon detection of the removal of the RFID tag from the zone, transmits an alarm signal to the first one of the nodes to relay the signal.

7. A RFID network as claimed in claim 1, wherein each node further includes an alarm which is activated upon the node receiving an RF alarm signal. WO 2007/006085 PCT/AU2006/000964 25

8. A RFID network as claimed in claim 1, wherein the locator is configured to generate an alarm signal when the relayed signal is not received within a predetermined time period.

9. A RFID network as claimed in claim 1, wherein the information includes an identifier corresponding to a node which reads the RFID tag.

10. A RFID network as claimed in claim 9, wherein the information is included with the signal by either the RFID tag or the node which reads the RFID tag.

11. A RFID network as claimed in claim 9 wherein, when determining the location of the RFID tag, the locator corresponds the identifier with a stored identifier associated with the location.

12. A RFID network as claimed in claim 1, wherein each transponder receives the signal and relays the signal at the same UHF frequency.

13. A RFID network as claimed in claim 1, wherein each node is configured to periodically transmit a probe signal.

14. A RFID network as claimed in claim 13, wherein the RFID tag is configured to create the signal to be relayed upon receipt of the probe signal.

15. A RFID network as claimed in claim 1, wherein each node further includes a RFID reader for reading the information from the RFID tag.

16. A RFID network as claimed in claim 15, k.arein the reader includes a transmitter for transmitting a probe signal to the RFID tag and a receiver for receiving a response signal including the information from the RFID tag.

17. A RFID network as claimed in claim 16, wherein the reader includes a scheduler for periodically transmitting the probe signal. WO 2007/006085 PCT/AU2006/000964 26

18. A node for a radio frequency identification (RFID) network, the node including: a RF transponder for relaying a signal whether from another node or a RFID tag.

19. A node as claimed in claim 18, wherein the relayed signal includes information from which the location of the RFID tag can be determined.

20. A node as claimed in claim 18, wherein the node is configured to determine whether to relay the signal.

21. A node as claimed in claim 20, wherein the determination of whether to relay the signal is made using a count of the number of nodes through which the signal has already been relayed, the count being included in the signal.

22. A node as claimed in claim 20, wherein the determination of whether to relay the signal is made using the time elapsed since the creation of the signal, a time-stamp being included in the signal at creation of the signal.

23. A node as claimed in claim 18, further including an alarm which is activated upon the node receiving an RF alarm signal.

24. A node as claimed in claim 18, wherein the transponder receives the signal and relays the signal at the same UHF frequency.

25. A node as claimed in claim 18, further including a RFID reader for reading the information from the RFID tag.

26. A node as claimed in claim 25, wherein the reader includes a transmitter for transmitting a probe signal to a RFID tag and a receiver for receiving a response signal from the RFID tag.

27. A node as claimed in claim 26, wherein the reader includes a scheduler for periodically transmitting the probe signal. WO 2007/006085 PCT/AU2006/000964 27

28. A method for locating a radio frequency identification (RFID) tag in a RFID network, the method including the steps of: relaying a RF signal from the RFID tag using one or more nodes each including a RF transponder, the relayed signal including information from which the location of the RFID tag can be determined; receiving the relayed signal from the nodes; and determining the location of the RFID tag using the information included in the received signal.

29. A method as claimed in claim 28 wherein, during the step of relaying, each node determines whether to relay the signal.

30. A method as claimed in claim 28, further including the step of detecting whether the RFID tag has been removed from a zone using the determined location of the RFID tag.

31. A method as claimed in claim 30, further including the step of transmitting an alarm signal to the first one of the nodes to relay the signal upon detection of the removal of the REID tag from the zone.

32. A method as claimed in claim 28, further including the step of generating an alarm when the relayed signal is not received within a predetermined time period.

33. A method as claimed in claim 28 wherein, prior to the step of transmitting, the method further includes the step of transmitting a probe signal to the RFID tag from one of the nodes.

34. An alarm system for detecting the removal of a radio frequency identification (RFID) tag from a zone; the system including: at least one reader for reading the tag located within the zone; a detector for detecting when the tag has been removed from the zone; and WO 2007/006085 PCT/AU2006/000964 28 an alarm for generating an alarm signal when the tag has been removed from the zone.

35. An alarm system for detecting the removal of an radio frequency identification (RFID) tag from a first zone; the system including: a first set of readers for reading the tag located in the first zone; a second set of readers for reading the tag located in a second zone, the second zone being adjacent to the first zone; and a detector for detecting when the tag has moved from the first to the second zones.

36. A checkout system for determining the price of one or more products to which respective radio frequency identification (RFID) tags are attached, the system including: a RFID reader for reading each RFID tag to thereby determine the identity of each product; and a calculator for calculating the price of the products by corresponding the determined identity of each product with a corresponding price and summing the prices of the products.

37. An alarm system including: a radio frequency identification (RFID) reader; and a camera for capturing one or more images when the reader reads a RFID tag.

38. A power supply for a radio frequency identification (RFID) tag, the power supply including: a storage capacitor for storing charge; a first diode through which a received signal can be supplied to the storage capacitor; and a second diode through which charge is supplied to the storage capacitor from a battery. WO 2007/006085 PCT/AU2006/000964 29

39. A method for calculating backoff in a radio frequency identification (RFID) tag, the method including the steps of: dividing a dividend stored in the RFID tag by a prime number; and obtaining the remainder of the division to thereby calculate the backoff.

40. A method for calculating the back off period in a radio frequency identification (RFID) tag, the method including the steps of: dividing a dividend stored in the RFID tag by a prime number; and obtaining the remainder of the division; multiplying the remainder by a time period to thereby calculate the backoff period.

41. A radio frequency identification (RFID) tag programmed with software instructions for calculating the backoff in accordance with a method as claimed in any one of claim 39 or claim 40.