GB2353436A

GB2353436A - Tag interrogation system

Info

Publication number: GB2353436A
Application number: GB9916551A
Authority: GB
Inventors: Simon Meldrum
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1999-07-14
Filing date: 1999-07-14
Publication date: 2001-02-21
Anticipated expiration: 2019-07-14
Also published as: GB9916551D0; GB2353436B

Abstract

In a method for determining which of a plurality of identification devices having identification numbers within a predefined range are present within the transmission range of an interrogation device, an interrogation signal is broadcast from the interrogation device defining a range of identification numbers, a response signal is transmitted from each identification device which receives the interrogation signal and which has an identification number within the range defined by the interrogation signal and, depending upon the defined range of identification numbers and whether or not a response signal is received, a further range of interrogation signals is broadcast from the interrogation device defining a smaller range of the identification numbers. The process is repeated until all the tag identities are obtained.

Description

2353436 TAG INTERROGATION SYSTEM This invention relates to a system f or

determining which of a plurality of identification devices, each having a respective identification, are present within the transmission range of an interrogation device. Examples of such identification devices include tags which are attached to respective items so that the position of each item can be monitored.

Systems for determining which identification devices are present are widely known. In all of these systems, the identify of the devices present is determined on the basis of signals transmitted between an interrogating apparatus and the identification devices. It is therefore necessary to employ a scheme to avoid conflicts between the signals received from the identification devices by the interrogator.

In one such scheme, the interrogator sends a signal addressed to each identification device which could possibly be present in turn. Af ter each transmission, the interrogator waits f or a predetermined reply period to see if a reply signal is received confirming the presence of the particular identification device, before sending a signal addressed to the next identification 2 device.

In another scheme, each identification device is arranged to send a signal to the interrogator at a different frequency in response to a request signal addressed to all of the identification devices. Such a system is described, for example, in EPA- 0565583.

In a further scheme, each identification device is 10 arranged to send a signal to the interrogator in a different time slot in response to a request signal addressed to all of the identification devices.

US 5,640,151 describes a communication system for identifying, locating, trading and communication for other purposes with a large number of tags. To investigate the identification numbers of all of the tags located within a communication region around the interrogation device, firstly a signal is broadcast for the attention of all tags instructing the tags to transmit their identification numbers within a specified time window. The tags respond by transmitting their respective identification numbers to the interrogation device within the specified time window. If the signals conveying the identification numbers of two or more tags overlap, the interrogation device cannot identify those identification numbers. In this case, the interrogation 3 device sends signals addressed to each tag whose identification number has been successfully identified, instructing it not to respond to subsequent interrogation signals broadcast to all tags. The interrogator subsequently broadcasts a second interrogation signal specifying a generally smaller time window, and those of the tags whose identification numbers were not originally identified by the interrogator respond. The process is repeated until all the tags within transmission range of the interrogation device have been identified.

The methods described above suffer from a number of problems. In particular, they often result in systems which are complex and expensive, or which are not time- efficient in identifying the devices present. For example, in the system described in US 5640151, the tags are required to carry out a number of complex processing operations, which increases the complexity and cost of manufacture of each tag. This is a particular problem in systems that employ a very large number of tags.

The present invention has been made with the above problems in mind.

According to the invention, there is provided a signal processing apparatus or method for determining which of a plurality of identification devices having 4 identification numbers within a predefined range are present within the transmission range of an interrogation device, in which an interrogation signal is broadcast from the interrogation device defining a range of identification numbers, a response signal is transmitted from each identification device which receives the interrogation signal and which has. an identification number within the range defined by the interrogation signal and, depending upon the defined range of identification numbers and whether or not a response signal is received, a further range of interrogation signals- is broadcast from the interrogation device defining a smaller range of the identification numbers. The only operations that must be performed by the identification devices are to receive an interrogation signal, determine whether the device identification falls within the range of identification specified in the interrogation signal and send a response signal if it is determined that the device identification does fall within the specified range of identifications. Thus, the complexity of each tag can be significantly reduced leading to a reduction in cost.

An embodiment having the feature that the response 25 signals transmitted from the identification devices are identical (rather than the signal from each device specifying the identity of the device) has the advantage that the cost and complexity of the identification device can be further reduced.

Alternatively, an embodiment having the feature that each identification device transmits a response signal incorporating its device identification to the interrogation device has the advantage that the number of interrogation signals required to identify all the identification devices within transmission range of the interrogation device can be reduced.

The invention also provides a signal processing apparatus or method for determining which of a plurality of identification devices having identifications within a predefined numerical range are present within the transmission range of an interrogation device, in which an interrogation signal is broadcast from the interrogation device defining a range of identifications, a response signal is transmitted from each identification device which receives the interrogation signal and which has an identification within the range defined by the interrogation signal and these operations are repeated while storing data defining the identifications remaining to be checked, the stored data being updated depending upon whether or not a response signal is received for a defined range of identification numbers. By updating the stored data after each interrogation signal has been sent 6 and any response signals have been analysed, the number of interrogation signals required to identify all the identification devices within the transmission range of the interrogation device can be reduced.

The invention also provides an interrogation device or method for determining which of - a plurality of identification devices having device identifications within a predefined numerical range are present within a transmission range, in which interrogation signals defining a range of identifications are generated and broadcast and response signals are received from identification devices which receive the interrogation signal and which have an identification within the defined range of identifications, wherein data defining the range of identifications remaining to be checked is stored and updated in response to whether or not any response signals are received for a defined range of identifications.

The invention further provides an identification device comprisIng means for storing a device identification, means for receiving an interrogation signal defining a range of identifications, means for determining whether or not the device identification is within the specified range of identification and means for transmitting a response signal when said determining means determines 7 that the device identification is within the specified range of identifications.

An embodiment having the features that the interrogation signal has first and second parts, the first part specifying an identification made up of a number of elements and the second part indicating which of the elements of the specified identification are significant, and that the determining means determines whether or not the significant elements of the specified identification are the same as the corresponding elements of the device identification stored in the storing means enables a range of identifications to be transmitted using only two numbers which can easily be processed within the identification device to determine whether the device identification falls within the specified range. Additionally, in a preferred embodiment, the determining means comprises an exclusive-OR gate for performing a bi- twise exclusive-OR operation on said device identification and said first part and an AND gate for performing a bitwise AND operation on the result of the exclusive-OR operation and said second part, thus providing a simple and inexpensive way of determining whether the device identification falls within the specified range of identification.

Exemplary embodiments of the present invention will now 8 be described with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram illustrating a system 5 incorporating an interrogator and a plurality of tags; Figure 2 is a block diagram showing the components of an interrogator in the first embodiment; Figure 3 is a block diagram showing the components of tag in the first embodiment; Figures 4A and 4B show the processing steps used in the first embodiment for an interrogator to investigate the identity of one or more tags; Figure 5 shows the processing steps used in the first embodiment at steps SS in Figure 4A to set the initial parameters for a probe-type interrogation by the interrogator; Figure 6 shows the processing steps used in the first embodiment at step S7 in Figure 4A and steps S75 in Figure 8 to probe whether any tags are present within range of the interrogator having identification numbers within a specified numerical range; 9 Figures 7A to 7L are schematic diagrams f or an example interrogation which comprises a plurality of probe interrogations, each diagram indicating the range of tag identification numbers probed in a respective probe interrogation, and also showing parameters used in the probe interrogation.

Figure 8 shows the processing steps used in the f irst embodiment to perform a poll-type interrogation of a single tag.

First Embodiment With reference to Figure 1, a tag system includes a number of tags 1, each having a respective identification number. Typically, each tag 1 is attached to an article (such as a component in a warehouse, a food package in a supermarket, or an item of luggage etc) whose presence (or absence) is to be detected. An interrogator 3 is used to investigate which tags are present within the transmission range of the interrogator. More particularly, interrogator 3 is arranged to transmit signals defining one or more tag identification numbers, each tag is arranged to transmit a response if it has an identification number corresponding to the identification, or within the identification range, transmitted by the interrogator 3, and the interrogator 3 is arranged to process any received signals to determine which tags are present. In this embodiment, the signal transmitted by a tag 1 in response to a signal f rom the interrogator 3 contains no further information than the fact that the tag has an identification number corresponding to the identification, or within the identification range, transmitted by.the interrogator 3 (that is, the signal transmitted by a tag does not specify any part of the tag's identification number).

In this embodiment, interrogator 3 is operable to interrogate the tags 1 either to determine whether a specific, single tag is present, or to determine the identification of all tags which are present. This is done using two types of interrogation operation. In the first type of interrogation operation, known as a "poll", interrogator 3 transmits signals defining a single tag identification number, such that the tag with this identification number will respond if it is present. In the second type of interrogation operation, known as a "probe", interrogator 3 transmits signals specifying a range of identification numbers, and those of the tags 1 having an identification number which falls within the specified range respond by sending a signal back to the interrogator 3. On the basis of the received signals interrogator 3 performs further probe operations and poll operations to determine the identification of all tags present in an efficient manner. More particularly, as will be explained in detail below, by specifying a range of identification numbers, if no response is received f rom. the tags 1 then the interrogator 3 has found out in one probe operation that no tags 1 having identification numbers within that range are present and no further probe operations need be carried out. for that range of identif ication numbers. If a response is received from the tags 1, the interrogator 3 sends out an interrogation signal specifying a smaller range of identification numbers lying wholly within the range previously probed. These operations continue until the identification numbers of all of the tags are known.

As shown schematically in Figure 2, an interrogator 3 (indicated by broken lines) includes a processor 5 which is connected to a read only memory (ROM) 7 f or storing procedures used during operation of the interrogator 3, a random access memory (RAM) 9 to provide working space for running the procedure stored in the ROM 7 and a clock 11 for providing a clock signal.

Also connected to the processor 5 is an input device 13, such as a keypad, which enables an operator of the interrogator 3 to initiate an interrogation procedure, and a display 15 for displaying the results of an interrogation. The processor 5 is f urther connected to 12 a modulator 17 which modulates a radio-frequency (RF) wave in accordance with a signal generated by the processor 5 so that the signal can be transmitted via a transmission aerial 19 to the tags 1. A demodulator 21 is also connected to the processor 5 which is used to convert a signal received f rom, a tag I by a reception aerial 23 into a suitable form for input to the processor 5.

The interrogator 3 can be powered via the mains or battery.

Referring to Figure 3, a tag 1 (indicated by broken lines) includes a processor 31 which is connected to a ROM 33 for storing procedures used during operation of the tag I and a RAM 35 to provide working space for running a procedure stored in the ROM 33.

A reception aerial 37 is connected to the processor 31 via a demodulator 39 which demodulates the RFsignal received from interrogator 3 to provide an input signal to processor 31. The reception aerial 37 is also connected to a power supply unit 41 and a clock unit 43.

The power supply unit 41 converts a portion of the power of the RF wave carrying the RF signal into a power supply for the tag 1 and the clock unit 43 develops a clock signal for the tag 1 from the RF wave, these operations 13 being preferred in a conventional manner, for example as described in US 4857893. Although in Figure 3 only the connections with the processor 31 of the power supply unit 41 and clock unit 43 are shown, the power supply unit 41 and clock unit 43 provide power and clock signals respectively for all components of the tag 1 for which they are necessary.

A logic circuit 45 (generally indicated by dotted lines) comprises an ID memory 47, two shift registers, namely Register A 49 and Register B 51, an exclusive-OR (XOR) gate 53 and an AND gate 55. The ID memory 47 is a ROM which stores the identification number of the tag 1. The logic circuit 45 is used to determine whether the identification number stored in the ID memory 47 corresponds to identification, or is within the identification range, transmitted by the interrogator 3 as will be described hereinafter.

An output of the processor 31 is connected to a signal generator 57 for generating a signal to be transmitted to interrogator 3 via a transmission aerial 59.

For exemplary purposes, in this embodiment the identification number of each tag is a four-bit binary number. There are therefore sixteen possible identification numbers ranging from 0000 to 1111.

14 Figures 4A and 4B illustrate the procedure carried out when an interrogator 3 performs a tag interrogation.

The interrogation procedure is initiated, in step S1, by a user via the input device 13. The interrogator 3 then checks, in step S3, whether the user has indicated that a probe operation, to identify the identification numbers of all tags present, or a poll operation, to investigate whether a tag having a particular identification number is present, is to be carried out. The interrogation procedure moves to step SS if a probe operation is to be carried out.

The interrogation procedure to identify all tags present uses an algorithm which reduces the range of identification numbers being probed by a factor of two each time a response is received from the tags 1 until the range specified in the probe operation corresponds to a single identification number (this is, a poll operation). However, each time that the range of identification numbers is reduced by a factor of two, probe parameters are stored for investigating those identification numbers which have been removed from the range probed so that they may be probed at a later time.

In order to store the probe parameters for investigating the identification numbers which have been taken out of the range probed, a last-in first-out (LIFO) memory called a stack is used. In this embodiment, this stack memory f orms a part of the RAM 9 of the interrogator 3. The stack has the feature that numbers are output f rom.

the stack in the reverse order to the order in which they were input to the stack; in other words the later a number was added to the stack, the earlier it is removed from the stack.

The interrogator 3 specifies the range of identification numbers being probed by transmitting two four-bit numbers. one of the four-bit numbers, called the "Match",, is compared bit-by-bit by each receiving tag with its identification number. The other of the four- bit numbers, called the "Mask", indicates bit-by-bit which bits of the identification number must be identical to corresponding bits of the Match for the tag to respond to the probe operation. More particularly, the corresponding bit of the Mask is set to one if identity is required and zero if identity is not required. In other words, if, for those bits of the Mask which are one, the corresponding bits of the tag identification number and the Match are identical, the tag sends a signal to the interrogator 3 indicating that the identification number of the tag falls within the range being probed.

16 In this embodiment, four values are used for the Mask, that is 1000, 1100, 1110 and 1111. When the Mask is 1000, then the first bit of the identification number must be identical to the f irst bit of the Match for the identification number to fall within the range of the probe operation. Similarly, when the Mask is 1100, 1110 and 1111 the first two, three and four bits respectively of the identification number must be identical to the Match for the identification number to fall within the range of the probe operation.

Referring again to Figure 4A, in step S5 of the interrogation procedure, initial parameters for the probe operation are set. The procedure carried out in step S5 is shown in Figure 5 in which, at step S31, the Mask is set to be 1000 and, in step S33, the Match is set to be 0000. With these values of Mask and Match, the range of identification numbers to be probed is from 0000 to 0111.

As the range of identification numbers between 1000 and 1111 is not being probed in the initial operation, the parameters for the Mask and Match required to probe that range, namely a Mask of 1000 and a Match of 1000, are input to the stack in steps S35 and S37 respectively, for subsequent use as will be explained below.

Referring again to Figure 4A, in step S7 the tags 1 are 17 interrogated. The steps carried out in the procedure for interrogating the tags 1 are illustrated in Figure 6 in which the broken line separates those steps carried out by the interrogator 3 and those steps carried out by each 5 tag 1.

First, in step S41, the interrogator 3 generates a SyncO pulse which is detected by the tag in step S43 and informs the tag that data defining the Mask is about to be transmitted. The interrogator 3 then transmits, in step S45, the Mask which is detected by the tag 1 and stored bit-by-bit in Register A 49 in step S47.

A Syncl pulse, which informs the tag 1 that data defining the Match is about to be transmitted, is then generated, in step S49, by the interrogator 3 and detected, in step S51, bythe tag 1. Subsequently, in step S53 the Match is transmitted by the interrogator 3 and, in step S55, is detected by the tag 1 and stored bit-by-bit in Register B 51.

The interrogator 3 then generates a Sync2 pulse, in step S57, which tells the tag 1 to carry out a comparison of the identification number of the tag 1 stored in the ID memory 47, the Mask and the Match. When the Sync2 pulse is detected, in step S59, by the tag 1, the processor 31 controls the ID memory 47 and the register 18 B 51 to input their contents bit-by-bit to the exclusiveOR gate 53. The output of the exclusive-OR gate 53 for each bit is input to one of the inputs of the AND gate 55 where it is compared with the corresponding bit of the Match and the output of the AND gate 55 is input to the processor 31. Thus the tag 1 carries out the following logic operation:

((X^B)&A) where X is the identification number of the tag 1, B is the Match stored in Register B, A is the Mask stored in Register A and - and & represent bit-wise XOR and bitwise AND operations respectively.

If the processor 31 detects that the result of the logic operation is 0000 then a pulse is generated by the tag, in step S63, indicating that the identification number of the tag 1 f alls within the range being probed by the interrogator 3. Af ter the pulse has been generated, or if the result of the logic operation is not 0000, the interrogation procedure ends, as far as the tag 1 is concerned, until a new SyncO pulse has been detected.

Returning to Figure 4A, in step S9, the interrogator 3 determines whether any of the tags have responded. if there is a response from a tag 1, then it is determined, 19 in step S11, whether the Mask used in the previous interrogation to produce the tag response equals 1111, in other words whether just one identification number is being probed in a poll operation.

If the Mask is not equal to 1111, then the range of identification numbers being probed is reduced while using the same Match. This is achieved, in step S13, by calculating a variable New-Mask which will be the Mask for the next probe operation. New---Mask is calculated by carrying out a bit-wise OR operation on the current Mask and the current Mask rightshifted by one place. This has the effect of converting the most significant bit of the current Mask which is equal to zero to be equal to one with the other bits being identical to those of the current Mask. When a probe operation is carried out using New-Mask as the Mask and the current Match, the range of identification numbers being probed is halved.

As the range of identification numbers has been reduced, values of Mask and Match, which will be referred to as Deferred---Mask and Deferred--Match respectively, for probing the range of identification numbers being removed are stored so that the deferred range can be investigated in a later probe operation. Def erred---Mask is equal to New-Mask and therefore, in step S15, New-Mask is input into the stack. Deferred_Match. is obtained by switching the value of the bit of Match which corresponds to the least significant bit which is equal to 11111 in New-Match. With the initial probe parameters used in this embodiment this will always involve switching a zero to be a one.

More particularly, Deferred---Match is calculated using the equation:

Deferred---Match = Matchl(New-MaskAMask) where represents a bitwise OR operation and A represents a bit-wise XOR operation. Deferred---match is input into the stack in step S17. Then, Mask is set to be equal to New-Mask in step S19 and the interrogation procedure returns to step S7 in which the tags 1 are is again interrogated, as previously described.

on the other hand, if it is determined in step S11 that the Mask is equal to 1111, then a tag 1 is present whose identification number is identical to the current value of Match. The current value of Match is then stored in step S21 and it is subsequently determined in step S23 whether the stack, which stores details of the probe operations which have yet to be performed, is empty. If the stack is empty then the interrogation procedure ends in step S27, the identification numbers of all the tags present having been identified and stored for subsequent display on the display 15.

21 On the other hand, if it is determined in step S23 that the stack is not empty, then new values of Match and Mask, in that order, are obtained from the stack (as noted above, the stack being a last-in first-out data structure) in step S25 and the interrogation procedure returns to step S7 in which the tags 1 are again interrogated, as previously described..

Returning to step S59, if it is determined in this 10 step that no response has been received after an interrogation of the tags, then the interrogation procedure proceeds to step S23 in which it is determined whether the stack is empty and continues as described above.

In order to illustrate how the above interrogation procedure works, a specific example will be considered in which two tags are present, one having an identification number 0000 and one having an identification number 0111.

To aid the illustration, reference will be made to Figures 7A to 7L, each of which shows f or the given probe operation the values of Mask and Match, the values stored in the stack, and also the range of identification numbers being investigated. The range of identification numbers is illustrated using a "tree,, diagram which includes all sixteen possible tag identification numbers with the two identification numbers corresponding to the 22 tags present being underlined and the range of identification numbers probed in the operation being emphasised.

It will be seen as the probe operations are described that when a tag is present having an identification number within the probed range, the probe range being represented by a branch of the tree diagram, the branch is split into two sub-branches until no further splitting is possible, at which point only one identification number is being probed and thus the identification number of the responding tag is known. During the probing operations, whenever a range of identification numbers is probed and no response is returned from the tags 1, then the entire probe range, which forms a branch of the tree, can be discarded. As described above, for the first probe operation the Mask is set to be 1000, the Match is set to be 0000, and 1000 is input into the stack and then 1000 is again input into the stack (step SS).

First probe operation As the Mask is 1000 and the Match is 0000, in the f irst probe operation those identification numbers whose most significant bit equals 0, that is the identification 23 numbers 0000 to 0111 are investigated (step S7). This is shown in Figure 7A.

As the identification numbers of both the tags assumed to be present in this example have f irst digits equal to 0, both tags respond by issuing a signal. Having determined that a signal has been received from the tags 1 (step S9) and that the Mask is not equal to 1111 (step S11), the interrogator 3 calculates a New-Mask which equals 1100 (step S13). Then the interrogator 3 inputs New_Mask, which is identical to Deferred---Mask, into the stack (step S15), calculates Deferred--- Match to be 0100, and inputs Deferred---Match into the stack (step S17). The interrogator 3 then sets the Mask to be equal to New-Mask (step S19), leaves the Matchunchanged, and proceeds to the next probe operation.

Second probe operation With reference to Figure 7B, the second probe operation investigates the identification numbers 0000 to 0011, which is half of the numbers investigated in the first probe operation. In response, the tag 0000 responds.

Subsequently, in the same manner as described above, as the Mask is not 1111 a New_Mask of 1110 is calculated, a Deferred---Mask of 1110 is input to the stack, a Deferred---Match of 0010 is input to the stack, and Mask is 24 then set to equal New-Mask. The interrogation procedure then proceeds to the third probe operation.

Third probe operation 5 With reference to Figure 7C, the third probe operation investigates the identification numbers 0000 to 0001 and the tag 0000 responds. Subsequently, in the same manner as described above, as the Mask is not 1111, a New-Mask of 1111 is calculated, a Deferred---Mask of 1111 is input to the stack, a Deferred---Match of 0001 is input to the stack, and Mask is then set to equal New-Mask. The interrogation procedure then proceeds to the fourth probe operation.

Fourth probe operation With reference to Figure 7D, the fourth probe operation is effectively a poll operation, and investigates just the identification number 0000. The tag whose identification number is 0000 responds. As the Mask is 1111, the value of Match, that is 0000, is stored (step S21) as the identification number of a tag whose presence has been detected. Subsequently, the interrogator 3 then determines that the stack is not empty (step S23), and therefore sets the Match to be equal to the first number out of the stack, that is 0001, and the Mask to be equal to the next number out of the stack, that is 1111 (step S25). The investigation procedure then performs -the fifth probe operation.

Fifth probe operation With reference to Figure 7E, the fifth probe operation investigates just the identification number 0001. As no tag is present with thab identification number, the interrogator 3 determines that the stack is not empty (step S23) and sets the Match to be equal to the f irst number out of bhe stack, that is 0010, and the Mask to be equal to the next number out of the stack, that is 1110 (step S25).

Sixth probe operation With reference to Figure 7F, the sixth probe operation investigates the identification numbers 0010 and 0011.

As no tag is present with one of these identification numbers, the interrogator 3 determines that the stack is nob empty (step S23) and sets the Match to be equal to the first number out of the stack, that is 0100, and the Mask to be equal to the next number out of the stack, that is 1100 (step S25).

26 Seventh probe operation With reference to Figure 7G, the seventh probe operation investigates the identification numbers 0100 to 0111 and the tag 0111 responds. Subsequently, in the same manner as described above, as the Mask is not 1111 a New-Mask of 1110 is calculated, a Deferred---Mask of 1110 is input to the stack, a Deferred---Match of 0110 is input to the stack, and Mask is then set to equal New-Mask.

Eiqhth probe operation With reference to Figure 7H, the eighth probe operation investigates the identification numbers 0100 and 0101.

As no tag is present with one of these identification numbers, the interrogator 3 determines that the stack is not empty (step S23) and sets the Match to be equal to the first number out of the stack, that is 0110, and the Mask to be equal to the next number out of the stack, that is 1110 (step S25).

Ninth probe operation With reference to Figure 71, the ninth probe operation investigates the identification numbers 0110 and 0111 and the tag 0111 responds. Subsequently, in the same manner as described above. as the Mask is not 1111, a New-Mask 27 of 1111 is calculated, a Deferred---Mask of 1111 is input to the stack, a Deferred---Match of 0111 is input to the stack, and Mask is then set to equal New-Mask.

Tenth probe operation With reference to Figure 7J, the tenth probe operation is a poll operation which investigates just the identification number 0110. As no tag is present with that identification number, the interrogator 3 determines that the stack is not empty (step S23) and sets the Match to be equal to the first number out of the stack, that is 0111, and the Mask to be equal to the next number out of the stack, that is 1111 (step S25).

Eleventh probe operation With reference to Figure 7K, the eleventh probe operation is a poll operation which investigates just the identification number 0111 and the tag whose identification number is 0111 responds. As the Mask is 1111, the value of Match, that is 0111, is stored (step S21) as the identification number of the identified tag. Subsequently, the interrogator 3 determines that the stack is not empty (step S23), and therefore sets the Match to be equal to the f irst number out of the stack, that is 1000, and the Mask to be equal to the next number 28 out of the stack, that is 1000 (step S25).

Twelfth probe operation With reference to Figure 7L, the twelfth probe operation investigates the identification numbers 1000 and 1111. As no tag is present with an identification number within this range of numbers, the interrogator 3 determines that the stack is empty (step S23) and ends the interrogation procedure (step S27).

The exemplary interrogation procedure described above thus identified that only tags having identification numbers 0000 and 0111 were present. Further, only twelve probe operations were required whereas if each identification number was polled individually sixteen probe operations would have been needed.

The reason why the number of probe operations was reduced is that if there was no response when a range of identification numbers was probed, then the entirety of that range could be discarded. In certain situations, the identification procedure described above would require more probe operations than would be needed if each identification number were polled individually, for example if tags with all the identification numbers in the range 0000 to 1111 were present. However, the 29 identification procedure described above has been designed for the case where the number of possible identification numbers is many orders of magnitude greater than sixteen and only a small percentage of them 5 are likely to be present in the selection interrogated. In such a situation, as described above, the number of probe operations required to identify all the tags 1 present can be significantly reduced by probing a range of identification numbers and if no response is received from the tags 1 discarding the entirety of that range. Reducing the required number of probe operations has the advantage of reducing the amount of time taken to identify the identification numbers of all the tags 1 present.

Sometimes a user will simply want to know whether or not a tag with a particular identification number is present. In this case the user indicates that a poll operation is to be carried out. When the interrogator 3 detects, in step S3, (Figure 4A) that a poll operation is to be carried out, the poll operation is, carried out as shown in Figure 8.

Referring to Figure 8, the interrogator 3 waits for the 25 user to input via the input device 13 the identification number to be polled and, in step S71, sets Match to be equal to the input identification number. The interrogator then, in step S73, sets the Mask equal to 1111 and, in step S75, interrogates the tags 1 in the manner described previously with respect to Figure 6, and checks whether or not a response is returned from the tags in step S77.

if a response is returned from the tags, the interrogator 3 indicates, in step S79, that the target tag is present via the display 15 and the interrogation procedure ends in step S83. If no response is returned from the tags, the interrogator 3 indicates, in step S81, that the target tag is not present via the display 15 and the interrogation procedure ends in step S83.

The skilled person will recognise that the probe operation described above can be implemented by the following C source code in which interrogate (Mask, Match) performs the interrogate tags sub-routine (step S7 of Figure 4A.) and returns TRUE if any tags respond and FALSE if no tags respond.

void enumerate(unsigned Mask, unsigned Match) if ( interrogate(Mask,Match) if (Mask & 1) 31 printf("identified tag %d\n", Match); else 5 unsigned NewMask; NewMask (Mask (Mask>>1)); enumerate NewMask, Match); enumerate NewMask, Match NewMask Mask) 10 void main(void) enumerate 1 << ( N - 1 0 enumerate( 1<<(N-1 1 <<(N- 1 where N is the number of bits in the identification number.

The system described above has been designed so that the tags 1 can be manufactured cheaply. in particular, the logic operations to be carried out by a tag require only 32 a small amount of processing power and the tag does not have to send any coded data back to the interrogator 3.

Second Embodiment 5 In a second embodiment of the invention, the number of probe operations required to identify.a selection of tags is reduced at the expense of making the tags more complex and expensive to manufacture.

The components of the second embodiment are the same as those described above in the first embodiment, except that the signal generator 57 of each tag 1 is modified to transmit the identification number of that tag 1 along with a cyclic redundancy check (CRC) code. The CRC code is used by the interrogator 3 to verify that the associated identification number has been correctly received. Thus, when the identification number of a tag 1 falls within the range of identification numbers specified in a signal from an interrogator 3, the tag 1 transmits a signal having encoded therein the identification number of the tag back to the interrogator 3. If more than one tag transmits to the interrogator 3 in a single probe operation, then the interrogator 3 will receive a garbled signal and will not be able to unambiguously identify the identification numbers of the responding tags. In this case, the 33 interrogator 3 will carry out a further probe operation specifying a smaller range of identification numbers. if only one tag 1 responds, then the interrogator 3 will be able to unambiguously identify the responding tag 1, including using the CRC code to check that the signal from the responding tag 1 has been correctly received, and will know that no further probe operations need be carried out for the specified range of identification numbers.

The interrogation procedure carried out in the second embodiment differs from that of the first embodiment as illustrated in and described with reference to Figures 4A and 4B in that step S11 is changed from checking whether the mask equals 1111 to checking whether a tag has been unambiguously identified. The remainder of the interrogation procedure of the second embodiment is substantially identical to that of the first embodiment.

A number of modifications can be made to the abovedescribed embodiments without departing from the concept of the invention.

In the second embodiment a cyclic redundancy check (CRC) code is used to allow the interrogator 3 to check that the identification number of a responding tag 1 has been correctly received. Alternatively, a parity bit could be 34 employed.

The skilled person will appreciate that the tags described could be implemented as a single application- specific integrated circuit (ASIC) incorporating an antenna.

In the previously described embodiments, a display 15 is used to display the results of the interrogation procedure. However, the results could also be subjected to further processing operations. For example, a possible use of the invention is in the monitoring of the movement of luggage in an airport once the luggage has been checked in. At the check-in desk, a tag 1 is attached to each item of luggage. With interrogators 3 being located in all the regions of the airport, it is possible to find the location of a single piece of luggage, using a polling operation, or to check the identification numbers of all of the pieces of luggage located in a particular region, for example a pre-loading area in which luggage is placed immediately prior to being loaded onto an aeroplane. It will be appreciated that in such a system, a plurality of interrogators 3 are used, all of which being controlled by and responding to a central control processor.

Claims

1 A signal processing method for determining which of a plurality of identification devices having device identifications within a predefined numerical range are present within the transmission range of an interrogation device, the method comprising the steps of:

(a) broadcasting an interrogation signal from the interrogation device defining a range of the identifications for receipt by identification devices in the transmission range and storing data defining the identifications remaining to be checked; (b) transmitting a response signal from each identification device which receives the interrogation signal and which has an identification within the range defined by the interrogation signal for receipt by the interrogation device; and (c) repeating steps (a) and (b) in accordance with predefined rules to determine the identification of each identification device within the transmission range; wherein the predefined rules comprise:

upon receipt of at least one response signal by the interrogation device from which the identification of a responding identification device cannot be determined, steps (a) and (b) are repeated specifying a smaller range of the identifications in the interrogation signal; upon receipt of a response signal f rom which the 36 identification of an identification device can be determined but data is stored defining remaining identifications steps (a) and (b) are repeated specifying at least some of the remaining identifications in the interrogation signal; and if no response signal is received by the interrogation device but data is -stored defining remaining identifications then steps (a) and (b) are repeated specifying at least some of the remaining identifications in the interrogation signal.

2. A method according to claim 1, wherein the interrogation signal and the response signals are transmitted at radio frequencies.

3. A method according to either claim 1 or claim 2, wherein the interrogation signal has at least two parts, one of the parts specifying an identification made up of a number of elements and another of the parts indicating which of the elements of the specified identification are significant, and in said transmitting step each identification device which receives the interrogation signal and for which the significant elements of the specified identification are identical to the corresponding elements of the device identification transmits a response signal.

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4. A method according to any preceding claim, wherein the response signals transmitted in said transmitting step from each identification device which receives the interrogation signal and which has an identification within the range defined by the interrogation signal are identical.

5. A method according to any of claims 1 to 3, wherein in said transmitting step each identification device which receives the interrogation signal and which has an identification within the range defined by the interrogation signal transmits a response signal incorporating the device identification to the interrogation device.

6. A method according to any preceding claim, wherein each device identification comprises a binary number.

7. A signal processing system comprising an interrogation device and a plurality of identification devices, each identification device having a device identification within a predefined numerical range, wherein: (a) said interrogation device comprises: 25 means for generating an interrogation signal def ining a range of identif ications; means for broadcasting the interrogation signal for 38 receipt by identification devices within the transmission range of the interrogation device; means for receiving response signals from each identification device which receives the interrogation signal and has an identification which falls within the range of identifications defined in the interrogation signal; and means for storing data defining identifications remaining to be checked; (b) each of the plurality of identification devices comprises:

means for storing a device identification; means for receiving an interrogation signal broadcast by said interrogation device; means for determining whether the device identification falls within the range of identifications defined in the interrogation signal; and means for transmitting a response signal in response to the interrogation signal when the determining means determines that the device identification falls within the range of identification defined in interrogation signal, (c) the generating means of the interrogation device is arranged to generate a plurality of interrogation signals to determine the identification of each identification device within the transmission range of the interrogation device in accordance with predefined rules including:

39 upon receipt of at least one response signal by the interrogation device from which the identification of a responding identification device cannot be determined, broadcasting a further identification signal specifying asmaller range of the identifications and updating the data defining the identifications remains to be checked; upon a receipt of a response signal from which the identification of an identification device can be determined but data is stored defining remaining identifications, broadcasting a further interrogation signal specifying at least some of the remaining identifications and updating the data defining the identifications remains to be checked; and if no response signal is received by the interrogation device but data is stored defining remaining identifications, broadcasting a further interrogation signal specifying at least some of the remaining identifications and updating the data defining the identifications remains to be checked.

8. A system according to claim 7, wherein the broadcasting means for broadcasting the interrogation signal and the transmission means for transmitting a response signal both comprise radio frequency transmitters.

9. A system according to either claim 7 or claim 8, wherein the generation means is arranged to generate an interrogation signal having at least two parts, one of the parts specifying an identification made up of a number of elements and another of the parts indicating which of the elements of the specified identification are significant, and the determining means of each of the identification devices is arranged to determine whether or not the significant elements of the identification specified in the interrogation signal are identical to the corresponding elements of the device identification.

10. A system according to any of claims 7 to 9, wherein the means for transmitting a response signal of each of the plurality of identification devices are arranged to transmit the same signal.

11. A system according to any of claims 7 to 9, wherein for each of the plurality of identification devices the means for transmitting a response signal is arranged to transmit a response signal incorporating the device identification.

12. A system according to any of claims 7 to 11, wherein each of the device identifications comprises a binary number.

13. A method of operation of an interrogation device for 41 determining which of a plurality of identification devices having device identifications within a predefined numerical range are present within the transmission range of the interrogation device, the method comprising the 5 steps of:

(a) broadcasting an interrogation signal from the interrogation device defining a -range of the identifications for receipt by identification devices in the transmission range and storing data defining identifications remaining to be checked; (b) receiving response signals from each identification device which receives the interrogation signal and which has an identification within the range defined by the interrogation signal; and (c) repeating steps (a) and (b) in accordance with predefined rules to determine the identification of each identification device within the transmission range; wherein the predefined rules comprise:

upon receipt of at least one response signal from which the identification of a responding identification device cannot be determined, broadcasting a further identification signal specifying a smaller range of the identifications and updating the data definingthe identifications remains to be checked; upon a receipt of a response signal from which the identification of an identification device can be determined but data is stored defining remaining 42 identifications, broadcasting a furtherinterrogation signal specifying at least some of the remaining identifications and updating the data defining the identifications remains to be checked; and if no response signal is received but data is stored defining remaining identifications, broadcasting a further interrogation signal specifying at least some of the remaining identifications and updating the data defining the identifications remains to be checked.

14. A method according to claim 13, wherein in said broadcasting step an interrogation signal is broadcast at radio frequencies.

is

15. A method according to either claim 13 or claim 14, wherein the interrogation signal has a match part specifying an identification made up of a number of elements and a mask part indicating which of the elements of the specified identification are significant.

16. A method according to claim 15, wherein the step of storing data defining identifications remaining to be checked comprises storing pairs of match and mask parts defining identification numbers remaining to be checked. 25

17. A method according to any of claims 13 to 16, wherein upon receipt of at least one response signal from 43 which the identification of a responding identification device cannot be identified, the range of identifications defined in the further identification signal is halved.

18. A method according to any of claims 13 to 17, wherein each of the device identifications is a binary number.

19. An interrogation device for determining which of a plurality of identification devices having device identifications within a predefined numerical range are present within the transmission range of the interrogation device, said interrogation device comprising:

means for generating an interrogation signal defining a range of identifications; means for broadcasting the interrogation signal for receipt by identif Ication devices within the transmission range of the interrogation device; means for storing data defining the range of identifications remaining to be checked; and means for receiving response signals from identification devices which received the interrogation signal and which have an identification within the defined range of identifications; wherein said generating means is arranged to generate a plurality of interrogation signals to 44 determine the identification of each identification device within the transmission range of the interrogation device in accordance with predefined rules including: upon receipt of at least one response signal from which the identification of a responding identification device cannot be determined, broadcasting a further identification signal specifying a smaller range of the identifications and updating the data defining the identifications remains to be checked; upon a receipt of a response signal from which the identification of an identification device can be determined but data is stored defining remaining identifications, broadcasting a further interrogation signal specifying at least some of the remaining identifications and updating the data definingthe identifications remains to be checked; and if no response signal is received but data is stored defining remaining identifications, broadcasting a further interrogation signal specifying at least some of the remaining identifications and updating the data defining the identifications remains to be checked.

20. A device according to claim 19, wherein the broadcasting means is arranged to broadcast the interrogation signal at radio frequencies.

21. A device according to either claim 19 or claim 20, wherein the generation means is arranged to generate an interrogation signal having a match part specifying an identification made up of a number of elements and a mask part indicating which of the elements of the specified 5 identification are significant.

22. A device according to claim 21, wherein said storing means is arranged to store pairs of match and mask parts defining the identifications remaining to be checked.

23. A device according to any of claims 19 to 22, wherein upon receipt of at least one response signal from which the identification of a responding identification device cannot be identified, said generating means is arranged to halve the range of identifications broadcast in the further identification signal.

24. A device according to any of claims 19 to 23, wherein each of the device identifications comprises a binary number.

25. An identification device comprising: means for storing a device identification; means for receiving an interrogation signal having first and second parts, the first part specifying an identification made up of a number of elements and the second part indicating which of the elements of the 46 specified identification are significant; means for determining whether or not the significant elements of the specified identification are the same as the corresponding elements of the device identification stored in the storing means; and means for transmitting a response signal when said determining means determines that the significant elements of the specified identification are the same as the corresponding elements of the device identification stored in the storing means.

26. A device according to claim 25, wherein said generating means is arranged to generate a response signal at radio frequencies.

27. A device according to either claim 25 or claim 26, wherein the response signal sends no information about the device identification of the identification device.

28. A device according to either claim 25 or claim 26, wherein the response signal incorporates the device identification of the identification device.

29. A device according to any of claims 25 to 28, wherein the storing means and determining means form part of an application specific integrated circuit.

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30. A device according to any of claims 25 to 29, wherein said device identification and said first and second parts are binary numbers.

31. A device according to claim 30, wherein said determining means comprises:

an exclusive-OR gate for performing a bitwise exclusive-OR operation on said device identification and said first part; and an AND gate for performing a bitwise AND operation on the result from the exclusive-OR gate and said second part.

32. A method of operation of an identification device having a device identification, comprising:

receiving an interrogation signal from an interrogation device having at least two parts, one of the parts specifying an identification made up of a number of elements and another of the parts indicating which of the elements of the specified identification are significant; determining whether or not the significant elements of the specified identification comprise the corresponding elements of the device identification; and generating a response signal when it is determined that the significant elements of the specified identification comprise the corresponding elements of the 48 device identification.

33. A method according to claim 32, wherein in said generating step a response signal is generated at radio frequencies.

34. A method according to either claim 32 or claim 33, wherein in said generating step a response signal is generated which contains no information concerning the device identification of the identification device.

35. A method according to either claim 32 or claim 33, wherein in said generating step a response signal is generated which incorporates the device identification of the identification device.

36. A method according to any of claims 32 to 35, wherein said device identification and said first and second parts are binary numbers. 20

37. A method according to claim 36, wherein said determining step comprises: performing a bitwise exclusive-OR operation on said device identification and said first part; and 25 performing a bitwise AND operation on the result of the exclusive-OR operation and said second part, the number resulting from the AND operation being zero if 49 the device identification and the significant elements of the first part are the same.