GB2568915A

GB2568915A - Reshaping interrogation range

Info

Publication number: GB2568915A
Application number: GB1719961.3A
Authority: GB
Inventors: Moran Humberto
Original assignee: Individual
Current assignee: Individual
Priority date: 2017-11-30
Filing date: 2017-11-30
Publication date: 2019-06-05
Also published as: GB201719961D0

Abstract

A system comprises an interrogator 1, transponder tags 61-3, and a jammer 7, where an interrogator signal 41 is modified by a jamming signal 47 in the intersection between the interrogator range 5 and the jamming range 8. Within the jamming range tags cannot understand, read, or reply to the distorted interrogator signals. The tags may be RFID tags, and the interrogator may be an RFID reader. The frequency of the jamming signal may be changed to distort the encoded data of a command package or the encoded preamble or postamble of a command package of the interrogator signal, where the frequency of the jamming signal may be changed to transform low or high encoding pulses into high or low encoding pulses or suppress the high or low encoding pulses from the interrogator signal. The invention enables detection of products once they are removed from a shelf or display in a shop. The interrogator signals are jammed while the tag is in the display zone and once the item is removed from the display (and the jamming region) the tag is able to respond to interrogator signals.

Description

Reshaping interrogation range

Background

Portable transponders (hereafter referred to simply as “transponders” or “tags”) such as radio frequency identification (RFID) transponders, usually comprise one or more semiconductor chips having logic and/or data handling capabilities, attached to one or more interface devices, such as an antenna. A transponder can communicate with the external devices such as interrogators and, via such interrogators, with supporting infrastructure, for example software applications.

Transponders communicate with interrogators (also known as “readers” or “base stations”) typically via radio waves. The interrogation range varies from few millimetres to several meters depending on the type of transponder and interrogator, frequency, media, antenna, interference and other factors. Interrogators can, in turn, be connected to a network of other interrogators and computers running appropriate supporting or application software. A transponder system (hereafter “transponder system”) includes at least one interrogator and one transponder.

The interrogation field of an interrogator (hereafter “interrogation range”) is the 3D region in space within which the interrogator can read transponders. In transponder systems based on electromagnetic waves the interrogation range of an interrogator is determined by the radiation pattern of its antennae, as well known by the skilled in the art.

Transponders may be passive, which means that they are energised by the interrogation signal of the interrogator, for example through electric or electromagnetic induction, or active, which means that they are energised by an internal power source, for example a battery. Normally, passive transponders can only operate within the interrogation range of an interrogator. Passive transponders are described in US 3 713 148 A.

Commonly, transponders are used to identify or locate the objects to which they are physically attached (hereinafter referred to as “tagged objects”). Typically, a tagged object, through its transponder, identifies itself by replying with one or more identities from a global numbering scheme upon request from an interrogator.

The use of transponders is becoming widespread. For example, low-cost transponders are used to identify pallets, cases and units of fast moving consumer goods (FMCGs). Transponder systems are also employed to track assets in a variety of fields such as manufacturing, logistics and distribution, amusement, rental and leasing, and are used in factories to manage conveyor belts, in airports to track baggage, and in retail to track products. Leading manufacturers, distributors and retailers are promoting the use of RFID transponders to replace barcode-based product identification procedures and so improve stock visibility and automation.

Transponder systems, in particular RFID systems, are commonly used in applications that are friendly to the environment. At home, RFID tags can be used by ambient intelligence applications to make energy savings, for example by selecting the most efficient program for a washing machine load depending on electronically-tagged garments in the load. In industry, apart from bringing significant energy-saving improvements from more controlled operations, RFID transponders can help to improve the management of supply chains of perishable goods and so reduce the amount thrown away as waste. RFID can support applications for the recycling and re-use of packaging, for example in the automatic separation of empty containers. Other environment-friendly applications include the electronic tagging of protected species or trees to prevent illegal hunting or logging respectively. Our invention enables the use of smartphones within the retail environment (e.g. by detecting products picked by a shopper) and therefore opens up the possibility of learning more about the environmental characteristics of a specific product, for example use of energy and material and recycling instructions.

Communication between transponders and interrogators takes place using standard frequencies, protocols and numbering schemas. The purpose of such standards is to specify: (a) a set of valid

Page 2 commands and parameters to be transmitted by an interrogator and (b) a set of possible responses and actions to those commands by transponders. Over recent years, a variety of standard-defining groups have emerged, including International Organization for Standardization (ISO), International Electrotechnical Commission (IEC), ASTM International, DASH7 Alliance, and EPCglobal. Examples of standard wireless protocols for transponder systems are ISO 14443, ISO 15693, ISO/IEC 18000 Parts 2, 3, 4, 6, 6C and 7, ISO 18185 and EPC (TM) Gen2.

Within these standards interrogators mainly communicate with transponders by modulating their digital signal using amplitude or frequency modulation. As well-known by the skilled in the art, the former involves changing the strength (amplitude) of the signal according to the data transmitted whilst the latter involves changing the frequency of the signal according to the data transmitted. Digital radio transmitters such as interrogators encode data using as series of “high” and “low” pulses. These are calculated according to a number of encoding schemas, for example NRZ (nonreturn-to zero), Manchester, return-to-zero, bipolar etc.

An important purpose of these standards is to coordinate communications between interrogators and transponders, particularly when there is more than one transponder in range of an interrogator. Normally, all in-range transponders simultaneously listen to an interrogator, which can issue commands aimed at multiple transponders (hereinafter referred to as “collective commands”), or commands aimed at individual transponders (hereinafter referred to as “individual commands” or “access commands”). In most transponder systems, only the interrogator can detect or “hear” transponder responses. Usually, the interrogator cannot individually address transponders that have not been identified, for example, freshly-energised transponders. Such a lack of segregation and coordination results in an undesired behaviour known as “transponder collision” (hereinafter referred to as “collision”), whereby two or more transponders reply simultaneously to an interrogator command. Collision reduces communication speed and reliability.

Protocols in transponder system standards usually include advanced anti-collision mechanisms involving the use of purposely delayed responses, for example, skipping a number of further interrogator commands according to a randomly-generated counter. Reference is made to, for example, CN 101359361 A, US 2008 180220 A, CN 101256617 A, US 2004 140884 A, WO 02 41650A, TW 399190 B and KR 2010 0011711A. Mainstream RFID standards such as the ISO/IEC 18000-6C or EPC Gen2 also incorporate advanced anti-collision mechanisms. A number of standards define commands for the segregation of transponder sub-populations, and reference is made to ISO/IEC 18000-6C.

Collisions affect the performance of the inventorying of tagged objects, particularly in applications that:

-Comprise a large population of tagged objects;

-Involve frequent, random or unpredictable movement of tagged; and

-Require prompt detection of moving tagged.

One such applications is the tracking of products picked by shoppers in the retail environment. In such applications the underlying challenge is to promptly detect the arrival or departure of a tagged object from the range of an interrogator in the network.

The present invention addresses this challenge by preventing the detection of transponders on tagged objects that, due to their location relative to the interrogation range of one or more interrogators, are not within the interest of such interrogators. This is based on the insight that the range of interrogators can be reshaped through “jamming devices”. This reshaping reduces collisions and so improves the inventorying time of transponders that arrive or depart from the range of interrogators (moving tagged objects). An example of such tagged objects are retail products that do not need inventorying because they are resting on a shelf, but the detection of the movement of which - e.g. when a shopper picks them - gives place to valuable business applications.

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Note that there is prior art addressing the jamming of RFID signals, notably US 2015/0002273. This prior art describes methods and systems of blocking transponder signals by means of jamming transponders. Our invention differs from this prior art in which, in our invention, the jamming signal affects the signal transmitted by the interrogator (hereafter “interrogator signal”) instead of the transponder response, and in which the location of the jammers is relative to (and aims at) interrogators and not to transponders. This non-obvious alternative approach responds to the difference in purpose between this prior art and our invention: usually, the jamming of transponder replies responds to privacy or security considerations, for example to protect transponders on someone’s belongings. In the prior art transponders are jammed according to their proximity to the jamming device(s) and regardless of their relative location to interrogators. The purpose of our invention is to improve detection time of moving tagged objects by jamming the interrogator signal at localised areas, reshaping the interrogation range, and ultimately avoiding the detection of transponders located in areas that are out of the interest of interrogators. This is achieved by jamming the interrogator signal instead of the transponder replies as in the prior art.

Statement of invention

The present invention relates to inventorying transponders particularly, but not exclusively, radio frequency identification (RFID) transponders.

It is well-known by the skilled in the art that the combination of two electromagnetic signals of different frequencies (Fl, F2) gives place to an electromagnetic signal F3 of frequency that is the average of Fl and F2 [(Fl + F2) / 2] and that produces an amplitude-modulated sine wave of frequency ABS(Fi - F2) where ABS is “absolute value”. This is because when the two signals are in phase their energies combine and when they are out of phase they cancel out.

This invention describes a method that allows the reshaping of the interrogation range of an interrogator. A jamming device transmits a signal (hereafter “jamming signal”) similar to that of the interrogator yet different in frequency and radiation pattern (and optionally power). In the intersection of the interrogator range and the jamming range (defined below) the jamming signal combines with the interrogator signal. Because of its altered amplitude and/or frequency the combined signal does not comply with the standards so it is not understandable by transponders located within the radiation pattern of the antenna of the jamming device (hereinafter “jamming range”). Consequently, the transponders located within the jamming range will not reply to and will not be read by the interrogator. Effectively, the use of a jamming device reshapes the interrogation range of an interrogator because it subtracts the jammed region from it.

Advantages

The purpose of this invention is to enable applications that rely on the fast detection of tag arrivals or departures in specific places, for example the aisle of a retail store. Due to the low level of power required to jam the signal of an interrogator, jamming devices can be battery-operated and so lowcost and easy to install. This is particularly advantageous in the retail environment, where jamming devices can be used to occult product items resting on a shelf, display or rail whilst enabling the fast detection of product items taken by shoppers.

In addition, smart jamming devices can be activated or de-activated to dynamically change the range of an interrogator and so support a number of applications. For example, while an isle is empty of shoppers (e.g. detected through a motion sensor) the jamming devices may be deactivated to allow the nearby interrogators perform a full inventory of the contents of a shelf, display or rail. The method for enabling or disabling jamming devices is not part of this invention.

Drawings

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which:

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Figure 1 is a schematic diagram of a Transponder System that includes a Jamming Device;

Figure 2 is a schematic diagram of tagged objects;

Figure 3a is a schematic representation of the data of an interrogation signal to be modulated by amplitude.

Figure 3b is a schematic representation of the modulation by amplitude of the data of an interrogation signal.

Figure 3c is a schematic representation of a jamming signal.

Figure 3d is a schematic representation of an interrogation signal modulated by amplitude that has been combined with the jamming signal.

Figure 4a is a schematic representation of the data of an interrogation signal to be modulated by frequency.

Figure 4b is a schematic representation of the modulation by frequency of the data of an interrogation signal.

Figure 4c is a schematic representation of a jamming signal.

Figure 4d is a schematic representation of an interrogation signal modulated by frequency that has been combined with the jamming signal.

Detailed description

According to a first aspect of the present invention there is provided a method comprising a jamming device transmitting a jamming signal that distorts the signal transmitted by said interrogator, said distortion taking place within the intersection between the jamming range of said jamming device and said interrogation range of said interrogator.

Referring to Figure 1, the main embodiment of the invention comprises an interrogator 1 connected to antenna 2i by cable 31. Some embodiments may incorporate antenna 2i to the circuit of the interrogator 1 and need no cable 31, which is therefore optional. Antenna 2i generates electromagnetic wave 41 and has interrogation range 5, which is the region in space where transponders 6 can be read. The shape and size of interrogation range 5 is mainly determined by the radiation pattern of antenna 2i and the transmission power of interrogator 1, but may be altered (or reshaped) by the electromagnetic properties of objects near antenna 2i, for example large metallic objects (not shown). Jamming Device 7 is connected to its own antenna 2₇ through cable 3₇, also optional when antenna 2₇ is integrated with the circuit of Jamming Device 7. Antenna 2₇ generates electromagnetic wave 4₇ and has jamming range 8, which is the region of space where transponders 6 receive a jamming signal sufficiently strong to distort electromagnetic wave 4i and therefore prevent transponders 62 and 63 from understanding it and therefore be read by interrogator 1. Since transponder 61 is outside jamming range 8 it can still be read by interrogator 1. The shape and size of jamming range 8 is mainly determined by the radiation pattern of antenna 2₇ and the transmission power of jamming device 7, but may be altered (or reshaped) by the electromagnetic properties of objects near antenna 2₇, for example large metallic objects (not shown). Without loss of generality, interrogation range 5 and jamming range 8 can be 3-dimensional spaces or regions of various shapes and sizes and not two 2-dimensional circles as in the illustration.

Figure 2 illustrates first, second and third tagged objects 91, 92, 9₃ having one or more transponders 61, 62, 6_3a, 6₃b physically attached to them. In this example, first and second objects are tagged with single respective tags 61, 6₂ and the third object 9₃ is tagged with two tags 6_3a, 6₃b. A tag 61, 62, 6_3a, 6₃b may be permanently attached to an object, for example by being welded, glued or built-into the object, or temporarily attached to an object, for example using a removable or breakable bond 10.

Figure 3a illustrates by example encoded digital data prior to modulation (in this case using amplitude modulation or AM). The “high” and “low” pulses of the encoding of digital data are represented by iii and n₂ respectively. The encoding schema in the communication protocol establishes the pulse length 12, difference in amplitude 13 and time 14 between the “highs” iii and the “lows” ii₂ for the different pieces of data sent; and an optional “ramp-up” and/or “ramp-down”

Page 5 slope 15. Usually, the pulse length 12 and/or difference in time 14 are contingent on the data transferred. Note that the encoded data is not shown to proportion and is only shown partially (only two “high” pulses and two “low” pulses). In reality, a data stream is usually encoded through several “high” and “low” pulses of varying duration and/or separation, not just two as illustrated.

Figure 3b illustrates by example the modulated signal 41 of the encoded digital data in Figure 3a generated by interrogator 1 (Figure 1). The “high” and “low” pulses of the encoding of digital data can be seen as waves of equal frequency but different amplitude as shown by Hiam and 1I2AM respectively. Note that in Figure 3b the wave frequency and amplitude are not represented to scale or proportion and are for illustration purposes only.

Figure 3c illustrates by example a jamming signal 4₇ generated by a Jamming Device 7 (Figure 1). Note that the frequency of the jamming signal 4₇ is different from the frequency of the modulated signal 41 (Figures 1 and 3b). In this example the frequency of the jamming signal 4₇ is 1.6% lower than that of the modulation frequency 41 (Figure 3b) of interrogator 1 (Figure 1), but as detailed below the optimal set of jamming frequencies depends on the frequency (or range of frequencies) of interrogator 1 (Figure 1) and the distance 14 between encoding pulses. Note that in Figure 3c the wave frequency and amplitude are not represented to scale or proportion and are for illustration purposes only.

Figure 3d illustrates by example jammed signal 4i₊₇, which is the result of the combination of interrogation signal 41 (Figures 1 and 3b) and jamming signal 4₇ (Figures 1 and 3c) that takes place within the intersection of interrogation range 5 (Figure 1) and jamming range 8 (Figure 1). Note that the modulated and jammed “high” pulse Hiam+₇ is of amplitude not very different from that of modulated and jammed “low” pulse i12am+₇. Tags 62 and 63 (Figure 1), located within the intersection of interrogation range 5 (Figure 1) and jamming range 8 (Figure 1), will not be able to understand or decipher the data or command sent by interrogator 1 (Figure 1) because within this intersection the signal is jammed. In this Figure 3d during the transmission of the “high” pulse modulated in Hiam (Figure 3b) jamming signal 4₇ (Figure 3c) is on inverse phase relative to the interrogation signal 41 (Figure 3b) and so subtracts from and cancels part of it. Without loss of generality, a properly calculated jamming signal (see below) can also add to the “low” pulse of an interrogation signal and so transform it into a “high” pulse - jamming works both ways.

Figure 4a illustrates by example encoded digital data prior to modulation (in this case using frequency modulation or FM). The “high” and “low” pulses of the encoding of digital data are represented by Hi and n₂ respectively. The encoding schema in the communication protocol establishes the pulse length 12, difference in amplitude 13 and time 14 between the “highs” Hi and the “lows” ii₂ for the different pieces of data sent; and an optional “ramp-up” and/or “ramp-down” slope (not illustrated). Usually, the pulse length 12 and/or difference in time 14 are contingent on the data transferred. Note that the encoded data is not shown to proportion and is only shown partially (only two “high” pulses and two “low” pulses). In reality, a data stream is usually encoded through several “high” and “low” pulses of varying duration and/or separation, not just two as illustrated.

Figure 4b illustrates by example the modulated signal 41 of the encoded digital data in Figure 4a generated by interrogator 1 (Figure 1) using frequency modulation. The “high” and “low” pulses of the encoding of digital data can be seen as waves of equal amplitude but different frequency as shown by Hifm and h₂fm respectively. Note that in Figure 4b the wave frequency and amplitude are not represented to scale or proportion and are for illustration purposes only.

Figure 4c illustrates by example a jamming signal 4₇ generated by a Jamming Device 7 (see Figure 1). In this example the frequency of the jamming signal4₇ is 2.3% lower than that of the modulation frequency of the “low” pulses sent by interrogator 1 (Figure 1) shown in Figure 4b, but as detailed below the optimal set of jamming frequencies depends on the range of frequencies of interrogator 1 (Figure 1) and the distance between encoding pulses. Note that in Figure 4c the wave frequency and amplitude are not represented to scale or proportion and are for illustration purposes only.

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Figure 4d illustrates by example jammed signal 4i₊₇, which is the result of the combination of interrogation signal 4i (Figures 1 and 4b) and jamming signal 4₇ (Figures 1 and 4c) that takes place within the intersection of interrogation range 5 (Figure 1) and jamming range 8 (Figure 1). Note that the modulated and jammed “low” pulse i12fm+₇ is very weak and of amplitude significantly low compared with that of modulated “low” pulse h₂fm (Figure 4b). Tags 62 and 63 (Figure 1), located within the intersection of interrogation range 5 (Figure 1) and jamming range 8 (Figure 1), will not be able to understand or decipher the data or command sent by interrogator 1 (Figure 1) because within this intersection the signal for “low” pulse H2fm+₇ is too weak. Similarly, the other pulses of the modulated jammed signal 4₇ have been affected by the jamming signal and are distorted and therefore difficult to demodulate. In this Figure 4d during the transmission of the “low” pulse modulated in 11₂fm (Figure 4b) jamming signal 4₇ (Figure 4c) is on inverse phase relative to the interrogation signal 41 (Figure 4b) of “low” modulated pulse ii₂fm (Figure 4b) and so subtracts from and cancels most of it. Without loss of generality, a properly calculated jamming signal (see below) can also cancel the “high” pulse of an interrogation signal - jamming works both ways.

Calculation of the jamming frequencies

The calculation of the optimal set of jamming frequencies is dependent on the frequency or range of frequencies of interrogators and on the characteristics of the interrogator-transponder communication protocol, in particular to its encoding method and specifications. In this sense jamming can take place at four different levels:

1. Preamble or postamble: in most transponder systems commands sent to transponders by interrogators use a preamble and/or a postamble. A preamble signals the start of a command “unit” or “package and consists of a specific series of low and high pulses sent to transponders prior to sending commands and data. A postamble signals the end of a command “unit” or “package” and consists of a specific series of low and high pulses sent to transponders after commands and data. For example, in ISO/IEC 18000-6C a “Frame Sync” is sent at the beginning of all commands except for the “query” command (which has its own preamble). A “Frame Sync” in ISO/IEC 18000-6C consists of a low-pulse (delimiter), one low and one high short pulses (data-zero bit - see below) and a long high pulse followed by a short low pulse (Reader->Tag Calibration or RTCal). The duration of the pulses is established by a variable called Tari, which is the time-reference interval for interrogator-to-transponder signalling, and in ISO/IEC 18000-6C ranges from 6.25ps to 25ps. The duration of RTCal ranges from 2.5 Tari to 3.0 Tari. Any jamming signal transforming a “high” pulse into a “low” pulse or a “low” pulse into a “high” pulse (amplitude modulation), or suppressing either a “high” or a “low” pulse (frequency modulation) within the preamble of a command will prevent transponders from recognising the preamble and therefore to ignore the command.

2. Command: in most transponder systems a section of the command “unit” or “package” establishes the nature of the command. A command section is usually a data section with a specific binary value that identifies the command. Examples of commands in ISO/IEC 18000-6C are Query, Select, ACK, NACK, Request-random-number and Read. Any jamming signal transforming a “high” pulse into a “low” pulse or a “low” pulse into a “high” pulse (amplitude modulation), or suppressing either a “high” or a “low” pulse (frequency modulation) within the command section may prevent transponders from recognising the command and therefore to ignore it. However, care must be taken not to transform a command into another command and therefore tamper with the normal operation of the Transponder System.

3. Data: similarly to the above, in most transponder systems a section of the command “unit” or “package” includes data to be sent to transponders. Examples of data in ISO/IEC 180006C are handles, random numbers and selection patterns. Any jamming signal transforming a “high” pulse into a “low” pulse or a “low” pulse into a “high” pulse (amplitude modulation), or suppressing either a “high” or a “low” pulse (frequency modulation) within the data section of a command may prevent transponders from recognising the command and therefore to ignore it. However, when changing data care must be taken to prevent

Page 7 tampering with the normal operation of the Transponder System.

4. Redundancy check: in most transponder systems some command “units” of “packages” include data for redundancy check, usually the CRC (Cyclical Redundancy Check). For example, ISO/IEC 18000-6C uses CRC5 in the Query commands and CRC16 in transponder “access” commands (commands used to communicate with a specific transponder). Any jamming signal transforming a “high” pulse into a “low” pulse or a “low” pulse into a “high” pulse (amplitude modulation), or suppressing either a “high” or a “low” pulse (frequency modulation) within the redundancy section of a command may prevent transponders from recognising the command and therefore to ignore it. This can also be used in combination with some of the above jamming strategies as the redundancy check will fail if the data or command sections of the transponder command has been altered.

From the above it is clear that the two main ways to jam an interrogator signal are: (a) jamming the encoded data (which includes data in the command, data and redundancy check sections); or (b) jamming the preamble and/or postamble. The former depends on the encoding in the protocol used by the transponder system, and the latter depends on the specific preamble and/or postamble characteristics.

Some data encodings are easier to jam than others. For example, ISO/IEC 18000-6C uses amplitude modulation and Pulse Interval Encoding (PIE) for the transmission of commands and data from interrogators to transponders in binary format, and this encoding is particularly easy to jam. In PIE, “zero” data bits are represented by one high and one low short pulses and “one” data bits are represented by one long high and one short low pulse. The duration of a data bit zero is Tari as defined above, and the duration of a one bit varies between 1.5 Tari or 2 Tari. The jamming of the PIE encoding may be achieved by choosing a jamming frequency that goes in and out of phase with the interrogator frequency at a time more or less equivalent to Tari plus or minus a slight shift “S” so different data bits are jammed differently and so ensure that a command is not entirely out of phase when low pulses are transmitting (in which case the jamming would be ineffective). Referring again to Figure 3a, we can for example assume that the high and low pulses correspond to two consecutive zero data bits. The jamming signal renders the high pulse of the second data bit low and therefore makes the signal unintelligible to transponders. The jamming of NRZ can be achieved setting the jamming frequency so it goes in and out of phase every bit, every other bit and so on; plus a slight shift to make sure that different data bits are jammed differently. For this reason, in ISO/IEC 18000-6C a possible frequency Fj of a jamming signal can be calculated as Fj = Fi+i/Tari, with Fi being the transmission frequency of the interrogator. This jamming frequency Fi would cause the jamming signal to get in and out of phase during the transmission of every zero data bit. Other values are possible, for example a multiplier or multiple of Tari N and/or changing the jamming frequency by a small percentage to achieve a small phase shift per each data bit. Assuming isometric data and length of the “one” sequence as 2 Tari, a possible optimal value for the jamming frequency would be Fj = Fi+N/Tari with N=1.5. The value for the slight shift S is a percentage (up or down the frequency of the interrogator) that allows the certain jamming of at least one data bit within a number of data bits. For example, a 10% frequency shift by each encoded data bit will ensure that at least one out of 10 data bits is jammed. This is equivalent to: Fj = Fi+N*o.oi/Tari. In general: Fj = Fi+N*S/Tari. Protocols with a large number of data bits per command may be effectively jammed with a lower value of S, whilst protocols with a few data bits per command may require a higher value of S.

The jamming of the preamble or postamble gives more flexibility and it is more reliable because it does not risk changing the data or command code for another potentially intelligible data or command code. For example, calculating the jamming frequency so it “breaks” the RTCal high pulse in ISO/IEC 18000-6C in two parts would safely cause transponders to ignore the command. Since the high pulse for RTCal in this standard is quite long relative to the high pulses of the data bits, the range of possible jamming frequencies is also broader. The high pulse for RTCal ranges from 2 Tari to 2.5 Tari, depending on the duration of the low pulse (called PW, usually of duration Tari/2). This means that any jamming frequency Fj ranging from Fi + 0.5 / Tari to Fi + 2.5 Tari would make the preamble unintelligible to transponders. If the jamming target is the preamble or postamble the slight shift S is optional.

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Without loss of generality, other embodiments may be created as a variation or extension of the main embodiment described, for example adding interrogators and/or jamming devices to the transponder system. The interrogation range of one or more interrogators may be simultaneously 5 reshaped by one or more jamming devices. “Smart jamming devices” may detect the frequency and/or protocol of an interrogator and dynamically calculate the optimal jamming frequency by applying the aforementioned logic and maths. Jamming devices may be commanded to stop/start jamming to support the selective inventory of regions within interrogation range. Transponders can be selected or setup to work on different protocols, encoding, frequencies etc. to selectively 10 make them resilient to one or more specific jamming signals and so still be able to detect them within a jammed region. Different antennae for the jamming devices may be used to obtain jamming ranges (and therefore reshaped interrogation ranges) that suit the application needs.

Claims

1 A method of reshaping the interrogation range of an interrogator in a transponder system, the method comprising:

A jamming device transmitting a jamming signal that distorts the signal transmitted by said interrogator, said distortion taking place within the intersection between the jamming range of said jamming device and said interrogation range of said interrogator.

2 A method according to claim 1, wherein the method further comprises: Said interrogator modulating its signal using amplitude modulation.

3 A method according to claim 1, wherein the method further comprises: Said interrogator modulating its signal using frequency modulation.

4 A method according to any preceding claim, wherein the method further comprises: Choosing the frequency of said jamming device so said distortion affects the encoded data of a command package transmitted by said interrogator.

5 A method according to any one of claims 1 to 3, wherein the method further comprises: Choosing the frequency of said jamming device so said distortion affects the encoded preamble and/or postamble of a command package transmitted by said interrogator.

6 A method according to any preceding claim, wherein the method further comprises: Choosing the frequency of said jamming signal so said distortion transforms low encoding pulses in the signal transmitted by said interrogator into high encoding pulses.

7 A method according to any one of claims 1 to 5, wherein the method further comprises: Choosing the frequency of said jamming signal so said distortion transforms high encoding pulses in the signal transmitted by said interrogator into low encoding pulses.

8 A method according to any one of claims 1 to 5, wherein the method further comprises: Choosing the frequency of said jamming signal so said distortion suppresses or partially suppresses low and/or high encoding pulses in the signal transmitted by said interrogator.

9 A method according to any preceding claim, wherein the method further comprises:

Using more than one jamming device to reshape the interrogation range of said interrogator.

10 A method according to any preceding claim, wherein the method further comprises: Said jamming device reshaping the interrogation ranges of one or more interrogators.

11 Apparatus for reshaping the interrogation range of an interrogator in a transponder system, the apparatus comprising at least:

said interrogator; and a jamming device;

wherein the apparatus is configured to perform a method according to any one of claims 1 to 10;

12 Apparatus for reshaping the interrogation range of an interrogator in a transponder system, the apparatus comprising at least:

said interrogator; and a jamming device;

wherein the apparatus is configured so:

said jamming device transmitting a jamming signal that distorts the signal transmitted by said interrogator, said distortion taking place within the intersection between the jamming range of said jamming device and said interrogation range of said interrogator.