WO2010007540A1

WO2010007540A1 - Rfid transmission protocol and method of operating a transponder

Info

Publication number: WO2010007540A1
Application number: PCT/IB2009/051980
Authority: WO
Inventors: William Hendrik Hofmeyr; Hendrik Lodewyk Van Eeden
Original assignee: Ipico South Africa (Pty) Limited
Priority date: 2008-07-17
Filing date: 2009-05-13
Publication date: 2010-01-21

Abstract

A transponder (20) for a radio frequency identification system (10) comprises a detector (26) sensitive to a transmission by another source. A controller (22) of the transponder is responsive to the detector for delaying a response signal by the transponder, until after the transmission by the other source.

Description

RFID SYSTEM PROTOCOL AND METHOD OF OPERATING A TRANSPONDER

INTRODUCTION AND BACKGROUND

This invention relates to wireless radio frequency identification (RFID) systems and more particularly to a transponder forming part of such a system and a communications protocol for use with such as system. The invention also relates to a method of operating a transponder of an RFID system.

RFID systems are known in which a plurality of RFID transponders are activated by a power-up or interrogation signal from an RFID reader. The transponders then transmit respective response signals, usually comprising identification data, to a receiver, which typically forms part of an RFID reader or interrogator.

In general, if the response signals of two or more transponders overlap in time, the response signals clash and are lost, since the receiver cannot always distinguish the individual signals. There exist anti- collision protocols aimed at reducing or minimizing such clashes. According to the protocols, the transmissions of the transponders are organised in time, so that there exists a real probability that one transponder transmits its response signal at a time when no other transponder is transmitting, thereby enabling the interrogator to receive the response signal successfully. Another object of the protocols is to ensure that the transmissions of all the transponders in a transponder population to be read, are received successfully within a reasonable (as short as possible) time.

In one known class of RFID systems, the transponders initiate the anti- collision protocol by automatically transmitting a respective response signal, when the transponder is exposed to an energizing beam generated by the reader. These protocols are known as "Transponder Talks First" (TTF) protocols.

TTF identification systems are known in which the RFID reader transmits a mute signal, to mute a transponder after having read or identified the transponder. The purpose is to reduce the population of transponders still to be read and thereby accelerating the identification process. TTF identification systems are also known in which, when one transponder has started to transmit its response signal, the RFID reader transmits a signal to mute all other transponders. The purpose in this case is to prevent the other transponders from interfering with the currently transmitting transponder and in so doing, accelerate the identification process. A special case of TTF protocols is also known in which the reader does not send any commands to the transponders - these protocols may be referred to as "Transponder Talks Only" protocols, see for example USA 6, 154, 136. TTO transponders normally do not require any form of signal detector, since no signal or command is being sent by the reader, to control the anti-collision process. The main advantage of TTO protocols is that the lack of reader modulation results in less interference and the ability to deploy multiple TTO readers in close proximity and in the same frequency band. However, a possible disadvantage is that TTO protocols are perhaps slower than TTF or "Reader Talks First" (RTF) protocols, because there is no signalling from the reader to the transponders that can control or accelerate the anti-collision process.

OBJECT OF THE INVENTION

Accordingly, it is an object of the present invention to provide a transponder, a protocol, an RFID system and a method of operating a transponder with which the applicant believes the aforementioned disadvantages may at least be alleviated.

SUMMARY OF THE INVENTION

According to the invention there is provided a transponder for use in a population of transponders in a radio frequency identification system, the transponder comprising a detector sensitive to a transmission by another source of transmission; and a controller responsive to the detector for delaying transmission by the transponder of a response signal, until after the transmission by the other source.

The other source may be another transponder in the population of transponders.

The transponder is preferably configured automatically to transmit a response signal after having been exposed to an energizing field and does not require interrogation by the reader, before transmitting the response signal. Hence, the transponder may be suitable for use with a

"Transponder Talks First" (TTF) protocol, preferably a "Transponder Talks Only" (TTO) protocol.

The transponder may be configured to schedule transmissions of the response signal at pseudo-random inter-transmission intervals, the response signal comprising a data packet having a packet period and comprising a plurality of bits, each having a bit period.

The detector may be configured to be sensitive to the transmission by the other source during a first period before, preferably immediately before, each of the scheduled transmissions, and the controller may be responsive, in the event of a transmission by the other source, to delay the scheduled transmission by a second time period.

The detector may be an amplitude modulation detector configured to detect transmissions comprising a modulating signal having a bit rate of about 256kbits/s or 128kbits/s.

The first time period may be equal to the bit period or a plurality of bit periods (for example two or three), alternatively in the order of the packet period.

The second time period may be equal to the bit period or a plurality of bit periods (for example two or three), alternatively in the order of the packet period.

The transponder may be a dual frequency channel transponder utilizing a first frequency channel to receive the energizing signal and a second frequency channel to transmit the response signal and the detector may be sensitive to signals in the second channel. The first and second frequencies may be different. The invention also extends to a protocol for a radio frequency identification system comprising a reader and a plurality of transponders constituting a transponder population to be read by the reader, wherein there is no modulation by the reader of a reader signal, the transponders being configured to re-transmit a response signal at pseudo random inter-transmission intervals, at least some of the transponders being sensitive to transmissions by another source, and to delay a scheduled transmission of the response signal, until after the transmission by the other source.

The other source may be another transponder in the population, but may also be another source in the system or an external source.

Also included within the scope of the present invention is a radio frequency identification system comprising a transponder as herein defined and/or described.

Yet further included within the scope of the present invention is a radio frequency identification system utilizing the protocol as herein defined and/or described. The invention also includes within its scope a method of operating a transponder of a radio frequency identification system comprising a reader and a plurality of transponders constituting a transponder population, the method comprising the steps of causing the transponder: while being exposed to an energizing field, automatically to re-transmit a response signal at pseudo random inter- transmission intervals; during a first time period before a scheduled transmission of the response signal, to sense whether another source is transmitting; and if a transmission by another source is detected during the first time period, to delay the scheduled transmission by a second time period.

BRIEF DESCRIPTION OF THE ACCOMPANYING DIAGRAMS The invention will now further be described, by way of example only, with reference to the accompanying diagrams wherein: figure 1 is a basic block diagram of a radio frequency identification system and comprising a plurality of transponders; figure 2 is a basic block diagram of the transponder according to the invention; figure 3 is a flow diagram of a method according to the invention of operating a transponder; figure 4 is a first time diagram illustrating operation of the transponder and the system; figure 5 is a second time diagram illustrating operation of the transponder and the system; and figure 6 is a basic block diagram of another embodiment of the transponder according to the invention.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

A wireless radio frequency identification (RFID) system is generally designated by the reference numeral 10 in figure 1 .

The system 10 comprises a reader 1 2 and a population of transponders 1 4.1 to 1 4.n. In use, the reader generates an energizing signal 1 6, which is broadcast, to be received by the transponders of the population to be read. Where the transponders are passive transponders, the transponders derive energy from the signal 1 6 and store the derived energy in respective local charge storage devices (not shown) to power local circuits of the transponder. However, this invention is not limited to passive transponders, but may be used with active and semi-active or battery assisted transponders as well. In this example embodiment, the transponders 14.1 to 14.n are "Transponder Talks Only" (TTO) transponders and are configured automatically to respond by repeatedly retransmitting a response signal comprising a data packet, preferably comprising respective identification data (ID) associated with the transponder. Each transponder comprises means

(not shown) for generating pseudo-random inter-transmission intervals for separating any two consecutive response signals and a controller (not shown) for the generation means, to cause an average value of the inter-transmission intervals over a period of time to vary, preferably to increase. Such a system and associated transponders are disclosed in US 6, 1 54, 136, the contents of which are incorporated herein by this reference.

Referring to figure 2, in a presently preferred embodiment, a chip 20 of a passive UHF RFID transponder 14.1 to 14.n comprises a rectifier capable of rectifying incident RF energy in the UHF band followed by a regulator for supplying a known and steady supply voltage to the rest of the circuit, an oscillator for generating a 512 kHz clock to the logic, a Power-on-Reset to the logic, and a logic block 22 implementing a protocol engine comprising various digital circuits, state machines and a memory arrangement 24, e.g. EEPROM. When exposed to the energizing field 16, the UHF RFID transponder transmits (backscatters) by means of modulator 25 a response signal comprising a unique factory programmed ID code stored in the memory 24 at pseudo-random intervals. In one example embodiment, the ID code comprises 64 bits of information preceded by an 1 1 -bit preamble, for a total packet of 75 bits. The data is transmitted at a rate of approximately 256 kbits/s. The bit period is approximately 3.9 μ s, and the total time to transmit one packet of data is approximately 3.9 x 75 = 293μ s. The interval between response signal transmissions varies pseudo randomly in a range between 0 and

65 ms.

The chip 20 also comprises an AM detector 26 capable of detecting transmissions by another source, including similar transponders 14.1 to 14.n in the same reader beam 1 6, i.e. data transmissions at approximately 256 kbit/s. The detector is also capable of detecting noise sources in the same band that could potentially interfere with the reception by the reader of the response signals comprising the ID packets. Such noise sources could in practice be relatively wide band, implying that the detector 26 must be capable of detecting wide band interference, e.g. 10 kHz to 900 kHz. The detector should at least be sufficiently sensitive to detect backscatter modulation by neighboring transponders, typically at less than 0 dBm to 80 dBm.

Referring to figure 3, and as shown at 30, during the intervals between successive transmissions by any one of the transponders (say transponder 14.1 ) of response signals, that transponder 14.1 is, by virtue of the detector 26, sensitive to ongoing transmissions by any of the other transponders 14.2 to 14.n in the population that may be exposed to the energizing beam, or to noise that could cause interference. As shown at 32, if any such transmission or noise is detected in a first period T1 immediately preceding a scheduled transmission by transponder 14.1 of a response signal comprising the ID packet, the scheduled response signal is delayed by a second time T2 as shown at 34, so as to avoid a possible collision between the scheduled response signal and the ongoing transmission or noise.

This process can be repeated, i.e. the scheduled response signal can be delayed by additional T2 periods, until no transmission or noise is detected in a period T1 immediately preceding the scheduled transmission.

Referring now to figure 4, in one example embodiment, during first period T1 before a scheduled ID packet transmission 40 by transponder 14.1 (indicated at TR14. V), there is an on-going transmission of an ID packet 42 by transponder 14.2. A local controller of transponder 14.1 causes the scheduled transmission 40 to be delayed by a second time period 12, which is of the order of one or two or three bit periods, i.e. between 3.9//S and 1 1 .7μ s. The delayed and actual transmission of transponder 14.1 is shown at 44, and it is clear that interference by and/or on transmission 42 is avoided. As is clear from figure 3, transponder 14.1 remains so sensitive to on-going transmissions, until immediately before the actual transmission 44.

In another example embodiment shown in figure 5, T2 is of the order of a complete packet length, i.e. about 300// s. It will be appreciated that T1 may then in effect also be at least as long as T2.

Referring to figure 6, in a further embodiment, a passive dual frequency RFID transponder 50 comprises first and second coil antennas. The first coil 52 is resonant at a low frequency (LF), typically 125 kHz - 135 kHz. The second coil 54 is resonant at a high frequency (HF), typically 6.8 MHz or 1 3.56 MHz. Such a transponder is more fully described in ZA2003/9262. The RFID reader 12 (shown in figure 1 ) transmits an energizing beam 16 at the low frequency. This signal is picked up by the LF coil and rectified by rectifier 53 to provide power to the chip 56. The chip 56 comprises a regulator for supplying a known and steady supply voltage to the rest of the circuit, an oscillator for generating a 256 kHz clock to the logic, a Power-on- Reset to the logic, and a logic block implementing a protocol engine comprising various digital circuits, state machines and memory 58, e.g. EEPROM.

While being exposed to the energizing signal 1 6, the dual frequency RFID transponder 50 transmits a response signal comprising a unique factory programmed ID code stored in the memory 58 at pseudorandom intervals to the RFID reader 10. This is done by pulsing the HF coil 54 so that it rings at its resonant frequency. The ID code comprises 64 bits of information preceded by an 1 1 - bit preamble, for a total of 75 bits. The data is transmitted at a rate of approximately

128 kbits/s. The bit period is approximately 7.8 μs, and the total time to transmit one ID code or packet of data is approximately 7.8 x 75 = 586//S. The interval between response signal transmissions varies pseudo randomly between 0 and 130 ms.

When a dual frequency transponder of the kind described hereinbefore transmits a response signal to the reader, the HF coils of similar transponders in the transponder population ring in sympathy, and so pick up the transmitted pulses. An AM detector 60 connected to the HF coil is capable of detecting transmissions of other similar transponders in the same reader beam, i.e. data transmissions at approximately 128 kbit/s. The detector is also capable of detecting noise sources in the same band that could potentially interfere with the response signals comprising the ID packets. Such noise sources could in practice be quite wide band, implying that the detector must be capable of detecting wide band interference, e.g. 10 kHz to 450 kHz.

During the interval between successive response signals, the transponder 50, by virtue of detector 60 is sensitive to ongoing transmissions from other similar transponders that are also exposed to the reader energizing beam, or to noise that could cause interference.

If any such transmission or noise is detected in a period T1 immediately preceding a scheduled transmission by transponder 50 of a response signal, the scheduled response signal is delayed by a time T2, so as to avoid a possible collision between the scheduled response signal and the ongoing transmission or noise. In one embodiment T1 and T2 is of the order of one or two bit periods, i.e. between 7.8μs and 15.6//S. In another embodiment, T1 and T2 are of the order of a complete packet length, i.e. about 586 μs. T1 may also be of the order of one or two bit periods, i.e. between 7.8μs and 15.6//S, while T2 is of the order of a complete packet length, i.e. about 586 μs.

Claims

1 . A transponder for a radio frequency identification system, the transponder comprising a detector sensitive to a transmission by another source; and a controller responsive to the detector for delaying a response signal by the transponder, until after the transmission by the other source.

2. A transponder as claimed in claim 1 wherein the other source is another transponder in a population of transponders.

3. A transponder as claimed in claim 1 or claim 2, which is configured automatically to transmit the response signal after having been exposed to an energizing field.

4. A transponder as claimed in claim 3 wherein the transponder is suitable for use in at least one of a "Transponder Talks First" (TTF) protocol and a "Transponder Talks Only" (TTO) protocol.

5. A transponder as claimed in any one of claims 1 to 4 wherein the transponder is configured to schedule transmissions of the response signal at pseudo-random inter-transmission intervals, the response signal comprising a data packet having a packet period and comprising a plurality of bits having a bit period.

6. A transponder as claimed in claim 5 wherein the detector is configured to be sensitive to transmission by the other source during a first period immediately before each of the scheduled transmissions and wherein the controller is responsive, in the event of a transmission by the other source, to delay the scheduled transmission by a second time period.

7. A transponder as claimed in any one of claims 1 to 6 wherein the detector comprises an amplitude modulation detector configured to detect transmissions comprising a modulating signal having a bit rate of about 256kbits/s or 128kbits/s.

8. A transponder as claimed in claim 6 or claim 7 wherein the first time period is equal to one of: the bit period, a plurality of bit periods and the packet period.

9. A transponder as claimed in any one of claims 6 to wherein the second time period is equal to one of: the bit period, a plurality bit periods and the packet period.

10. A transponder as claimed in any one of claims 3 to 9 comprising first and second frequency channels, the transponder being configured to receive the energizing signal in the first channel, to transmit the response signal in the second channel and wherein the detector is sensitive to signals in the second channel.

1 1 . A radio frequency identification system comprising a reader and at least one transponder as claimed in any one of claims 1 to 10.

1 2. A radio frequency identification system comprising a reader and a plurality of transponders constituting a transponder population to be read by the reader, wherein there is no modulation by the reader of a reader signal, each transponder being configured to schedule transmissions of a respective response signal at pseudo random inter-transmission intervals, at least some of the transponders being sensitive to a transmission by another source during a first period before each of the scheduled transmissions and configured to delay the scheduled transmission until after the transmission by the other source.

13. A method of operating a transponder of a radio frequency identification system comprising a reader and a plurality of transponders constituting a transponder population, the method comprising the steps of causing the transponder: - while being exposed to an energizing field, automatically to re-transmit a response signal at pseudo random inter- transmission intervals; during a first period before a scheduled transmission of the response signal, to sense whether another source is transmitting; and if a transmission by another source is detected during the first period, to delay the scheduled transmission by a second time period.