AU1451692A - Identification apparatus and method - Google Patents

Description

IDENTIFICATION APPARATUS AND METHOD RELATED APPLICATION

Applicant(s) have disclosed a related invention in PCT Application No. PCT/AU88/00469 or its equivalent in Australia, Australian Patent Application No. 26093/88. The disclosure of this related application is herein , incorporated by reference. HELP QF INVENTION

The present invention relates to the field of identification of and/or communication between a number of remote objects or devices and an interrogator or receiver simultaneously. In particular, the present invention deals with communication and/or identification of one or more remote devices, such as transponders. The present invention also relates to a modification of the invention disclosed in the above related patent application. BACKGROUND ART The related application disclosed a system adapted to identify one or more remote transponders simultaneously.

The system disclosed utilized a number of remote transponders, each of which transmit a data or information signal at a carrier frequency selected randomly from a group of possible carrier frequencies. At each transmission a newly selected carrier frequency is used.

Furthermore, applicants are aware of "frequency-hopping" utilized in radar systems, where the remote transponder and interrogator are synchronised in their communication, and the data transmission occurs at a number of different carrier frequencies (frequency-hopping), the hopping between or selection of frequencies occurring in accordance with a predetermined sequence to which both transponder and interrogator are synchronised. This type of system is not adapted to communicate with a number of remote transponders simultaneously in a reliable manner. SUMMARY OF INVENTION

The present invention relates to a device in which communication is provided between the device and a receiver or interrogator by use of a single carrier frequency in conjunction with a selected modulation, such as a Direct-Sequence Spread Spectrum modulation technique. The present invention provides a device comprising: transmitter means adapted to transmit an information, code or identification signal; selection means adapted to select a modulation for transmission of said signal from a set of predetermined modulations; wherein the signal is transmitted using the selected modulation.

The present invention also provides a communication system comprising: at least one remote device each adapted to transmit a signal at a modulation selected from a set of predetermined modulations; and a receiver adapted to receive transmissions from at least one remote device.

The present invention preferably includes generating means for generation a set of predetermined modulations.

Preferably, transmission of each signal at the newly selected modulation occurs sequentially or cyclically. The device may also include means for extracting timing information or powering energy from an impinging electro-magnetic field.

Preferably, control means is provided to select a modulation. The control means may also reselect the same or another modulation for the next transmission. The selection or reselection may occur in accordance with a probability weighting, pseudo¬ random binary sequence or may be random. The present invention also provides a system for simultaneously identifying a first and second device, each device comprising code storage means, modulation means and an inductive receiver/transmitter means, the system comprising: magnetic field generator/radiator means for generating a magnetic field from which said first and second devices are adapted to extract power using said inductive means; each of said first and second devices, when so powered, respectively providing at least one unique code from the code storage means to the modulation means, said modulation means being adapted to provide at least one modulated code to the inductive means for transmission to a device identifying receiver; each device adapted to modulate said at least one code at at least one modulation randomly selected from a predetermined finite set of modulations, while each device remains powered; each device being embodied in a single (IC) chip.

The present invention further provides a method of communication between an interrogator and at least two remote receiving devices, said method comprising the steps of: radiating an interrogation signal from said interrogator to each remote device; generating a reply signal at a modulation selected from a predetermined set of modulations in each remote device; transmitting said reply signal from each remote device to the interrogator; and serially generating and transmitting the reply signal at a newly selected modulation from said predetermined set of modulations.

The interrogator of the communication system may also be adapted to receive a number of transmissions from any number of remote devices simultaneously, and demodulate the received transmissions to recover a signal from each remote device. This is made possible by having an interrogator adapted to receive a number of different modulations simultaneously. Preferably, the interrogator comprises a number of modulation receiving channels, the number of channels corresponding to the number of modulations available for selection for transmission of data by a remote device. The signal may be an identification, code and/or information signal. The signal is preferably RF. DESCRIPTION OF A PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, wherein:

Figure 1 shows a prior art DSSS transmitter; Figure 2 shows a prior art DSSS receiver; Figure 3 shows a device incorporating a transmitter according to the present invention;

Figure 4 shows a preferred form of PN generator; and Figure 5 shows a receiver according to the present invention. The preferred embodiment to be described operates on principles similar to those described in the aforementioned related application.

Although the specification refers to the term "device", any communication apparatus is also encompassed, including transponders and transmitter/receiver systems. The term "device" should therefore be given a broad interpretation the "device" may be passive or active. The term "modulation" is also intended to be construed broadly and encompass many forms of communication mediums, including a PN sequence. The term "interrogator" should also be interrupted broadly to include at least devices adapted to receive transmissions from and/or communicate with one or more remote devices.

The present invention in one form represents a modification or enhancement of the applicants related invention, whereby a single carrier frequency is used, in conjunction with a "Direct-Sequent Spread Spectrum" modulation technique. A transmitter for a Direct-Sequence Spread Spectrum (DSSS) system includes a source of pseudo-random data (usually binary), such data being generated at a rate considerably in excess of the system communication speed. Because of its similarity to natural noise, this data is commonly termed "Pseudo-Noise" (PN). This PN data is then mixed with the "real" data, so greatly increasing the requisite bandwidth, and transmitted.

The receiver or interrogator of the present invention must include, on each channel means to generate a Pn-sequence identical to that used by a remote device. That is, the interrogator includes a separate channel for each possible Pn-sequence which can be selected by a remote device. The Pn-sequence of each channel needs to be accurately synchronised with that of the transmitting remote device. Means for achieving such synchronisation are known (see, inter alia, Sklar, "Digitals Communications: Fundamentals & Applications", Prentice-Hall, 1988). To be effective, synchronisation needs to be very precise (within a fraction of the period of the clock driving the PN generator). When this occurs, the desired signal is greatly increased in amplitude, while other, un-synchronized signals are greatly attenuated. Of course, should the receiver's PN generator produce a PN sequenced different to that at the transmitter, synchronisation will be impossible. It is this selective amplification property which provides the basis for the present invention. A correlation operation is performed between the received signal and this local PN source, leaving the required data or signal. Unlike the prior art where a number of different carrier frequencies can be used, as shown in Figures 1 and 2 the present invention contemplates the use of a single carrier frequency for use by all remote devices, upon which a data or information signal is modulated in accordance with a particular type of modulation (AM, FM. etc) or in accordance with a particular pn-sequence.

Such a system is relatively highly resistant to interference (whether intentional or not), in that any received signal which does not correspond exactly to the local PN signal, will be converted into wideband noise, which may readily be filtered out. DESCRIPTION OF THE INVENTION

The invention provides a device and an object-identification system, characterised by the ability to identify (read signals from) several different objects simultaneously. Prior art systems employ a plurality of carrier radio frequencies, was described in the applicants related invention. The related invention provided an identifying device ("transponder", or "tag") to be attached to the object to be identified, said tag being adapted to transmit an identifying message at one of a plurality of radio frequencies. In use, the message is repeatedly transmitted, with the same or a new radio frequency being selected at each transmission. The receiving equipment provides a plurality of channels, each sensitive to one of the plurality of frequencies, and can receive message on all frequencies simultaneously. Separation of messages transmitted by a plurality of tags is provided by limiting the bandwidth of the receiver channels, such that each receives messages on one radio frequency only. The system is characterised by the ability to identify a plurality of tags at once, such plurality being randomly selected from an extremely large (quasi-infinite) total population of possible tags.

The present invention may be regarded as an improvement over or a modification of the related invention. In the present invention, a single radio frequency is used, and the tags are prevented from co-interfering by employing a plurality of PN sequences and/or a plurality of types of modulation and likewise a plurality of corresponding receivers, each such receiver adapted to synchronise to only one of the said plurality of PN sequences or types of modulation.

The transmitting portion of a device according to the present invention is shown conceptually in Fig. 3. Comparison with the prior art Fig. 1 will disclose the novelty, which resides in the provision of a plurality of PN or modulation sources, and a selection means (which said means operates identically to that of the cited prior invention) to choose one of the PN or modulation sources. The tag selects a PN or type of modulation source, transmits the message, selects a new (or the same) PN or type of modulation source, and repeats.

Rather than employing a plurality of separate PN generators, it will be advantageous to employ a single PN generator, adapted to receive as inputs, certain quantities ("co-efficients") which determine the generated sequence. An exemplary embodiment of such a circuit is shown in Fig. 4. The device comprises a shift register A, whose input is fed from the Exclusive-OR (XOR) function of two or more taps on the said shift register. Adroitly chosen taps will yield a plurality of long sequences, having good PN qualities. The exemplary embodiment employs two taps, implemented by the multiplexers B and C, which select two bits from the said register, and present them to the XOR gate D. The output of this gate is presented to the serial data input of register A.

By changing the selection inputs to the multiplexers B and C, a plurality of PN sequences may be produced. In the exemplary embodiment, a plurality of such selections are pre- determined, and stored in the Read-Only-Memory (ROM) E. When a value is presented to the address inputs of E, its outputs direct multiplexers B and C to select taps on the register A, and a PN sequence is generated. In the complete tag, a "selection means" is provided to select from the plurality of possible PN sequences.

A preferred receiving means are shown in Fig. 5. Each of the several PN or modulation generators generates a different member of the plurality of PN sequences or modulations available in the device. SUITABLE PN SEQUENCES

Suitable PN sequences can be characterised by 3 qualities:

1 . Their auto-correlation function will approximate to a "spike" or mathematical delta function. That is to say, if two copies of the same PN sequence are correlated together, a significant output will occur only when the two copies are in exact alignment. Any mutual misalignment will result in an essentially zero output.

2. The cross-correlation function between any two (different) PN sequences will be essentially flat, i.e. there will be minimal output for any possible relative alignment of the two sequences.

3. The PN sequence shall comprise approximately equal numbers of ONE and ZERO bits.

The present system utilises the interference-resistant quality of a DSSS channel to identify or communicate with specific devices such as transponders.

According to the invention, each device is adapted to hold a plurality of distinct PN sequences as a set of predetermined modulations. Alternatively, the device may hold a table of suitable coefficients, which may be passed to a digital logic circuit, such circuit being adapted to (when the said coefficients are presented as inputs) generate the required PN sequences or modulations.

A random or predetermined choice is made amongst these modulations and the randomly chosen modulation is used to transmit a signal which is to form the communication.

Before the signal is transmitted again, a new choice (from the available PN sequences) is made on a random or predetermined basis. Each bit of the message data is mixed with the chosen PN sequence, and transmitted, in practice this may imply that what is transmitted is either a selected modulation or in the case of a stored sequence, its complement, depending on whether signal is a logic signal and the data bit is a zero or one. The transmission bandwidth has been increased in proportion to the number of bits in the PN sequence. In an alternative system, in which only alternate bits of the PN sequence, rather than all bits, are reversed to suit the data polarity. Other schemes are also possible.

The complete message is transmitted preferably using a single modulation PN sequence. The PN sequence may extend over the entire message, or a shorter PN sequence may be used repeatedly for each message bit. When the entire message has been sent, a new sequence is then selected, from a group of available sequences, and the message is retransmitted. In the present system, the radio carrier frequency remains substantially constant.

Conveniently, the receiver or interrogator may operate on the homodyne principle as described on pages 14-15 and Figure 9 of the related application. In the case of a plurality of devices transmitting simultaneously, an interrogator will yield as output a wideband data stream, comprising the sum of messages received from all devices within range. Such messages will have been encoded using the plurality of PN sequences as previously described. This stream is passed simultaneously to a plurality of matched filters, each adapted to match one of the original, stored PN sequences. These filters may be of the class of "Transversal Filters", wherein the filter coefficients take the values +1 and -1 , following the bit-sequence of the original PN pattern. The auto-correlation property of the PN sequences (as above-described) implies that when a data pattern matching the original sequence is presented to such a filter, a sharp positive voltage spike appears at the output, as the received data passes (momentarily) into alignment with the filter coefficients. Similarly the complement of the given sequence will develop a sharp negative pulse. Non-matched data will merely generate low-level noise, which may be filtered out by a simple amplitude "window", or other circuitry.

The pulses so obtained represent one and zero bits of the original data.

The above-described cross-correlation property of the PN sequences enables messages encoded with a given PN sequence to be recovered by the appropriate matched filter, and no other. This property serves to separate out the various transmissions from one or more devices.

In a most preferred form, these transversal filters may be implemented by converting the receiver output to a sampled digital representation, and employing one of the many "digital signal processor" VLSI chips now available, to perform the filtering function in the digital domain. Many such chips are sufficiently powerful to simultaneously filter a plurality of channels (i.e. using different PN sequences).

An interrogator as described above is suitable for a PN sequence, however, the interrogator may be suitably constructed or adapted to receive and demodulate any type of modulation, depending on the form of modulation selected to transmit the signal from the remote device(s) to the interrogator, as would be understood by those of the art. In yet another alternative, it is possible to combine the transponder and system similar to that disclosed in the related application and the present invention, thus providing a plurality of remote devices, each modulated by a plurality of modulations or PN sequences and/or frequencies. This will require a plurality of receivers in the interrogator (one for each carrier frequency), each being adapted to demodulate a number of received transmissions at a selected modulation. Such a system could simultaneously identify very large numbers of transponders, in that it provides a system utilising a plurality P1 of radio frequencies, with a second plurality P2 of PN sequences employed upon each of the said frequencies, so obtaining a larger plurality (up to P1^*P2) of simultaneously useable channels. While such a system would require a more elaborate receiver means (having up to P1^*P2 receiving sections) than either of the description above, it would permit rapid, simultaneous identification of very large numbers of objections.

It may also be possible to utilise various forms of modulation (AM, FM, PWM and others as is known in the art). By selecting one modulation (say AM) for the first transmission, then selecting another modulation say (FM) for the second modulation, etc. Selection may be carried out in a random or predetermined sequence.

Claims (28)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 . A device comprising: transmitter means adapted to transmit an information, code or identification signal; selection means adapted to select a modulation for transmission of said signal from a set of predetermined modulations; wherein the signal is transmitted using the selected modulation.

2. A device as claimed in Claim 1 , wherein selection of said modulation is random.

3. A device as claimed in Claim 1 or 2, wherein the signal is successively or repetitively transmitted at each transmission at a newly selected modulation from the set of modulations.

4. A device as claimed in Claim 1 , 2 or 3, wherein each modulation comprised PN sequence selected from a group of possible Pn-sequences.

5. A device as claimed in Claim 1 , wherein the modulation is one of DSSS, AM, FM or PWM.

6. A communication system comprising: at least one remote device each adapted to transmit a signal at a modulation selected from a set of predetermined modulations; and an interrogator adapted to receive transmissions from at least one remote device.

7. A system as claimed in Claim 6 wherein the interrogator includes a number of receiver channels adapted to receive transmissions from the remote devices, the number of channels corresponding to the number of modulations available for use by each remote device in the set of predetermined modulations.

8. A communication system as claimed in Claim 6, wherein the receiver is further adapted to receive a number of transmissions simultaneously and subsequently demodulate the received transmissions.

9. A device as claimed in Claim 1 , further comprising: generating means for generating said set of predetermined modulations.

1 0. A device as claimed in Claim 1 , wherein said transmitter means includes means for transmitting each signal at said selected modulation sequentially or cyclically.

1 1 . A device as claimed in Claim 1 , wherein said transmitter means includes means for transmitting said number of information signals in response to an interrogation signal.

12. A device as claimed in Claim 1 , wherein said transmitter means includes means for transmitting one of said information signals as a unique identification signal of the transponder.

13. An identification system having a device as claimed in Claim 1.

14. A transponder comprising: transmitter means adapted to transmit at least one signal from the transponder to an external receiver; said transmitter means including means to transmit said at least one signal at a newly selected modulation selected from a predetermined set of modulations.

15. A device as claimed in Claim 1 , including means for extracting said timing information from a surrounding electromagnetic field.

1 6. A device as claimed in Claim 1 , including means for extracting powering energy also from said electromagnetic field.

1 7. A device as claimed in Claim 1 , further comprising: control means for selecting at least one selected modulation from the plurality of predetermined modulations, said selected modulation(s) being used to modulate said signal.

18. A device as claimed in Claim 1 , wherein the control means includes means for re- selecting the same at least one selected frequency or another at least one selected frequency from the plurality of predetermined frequencies for retransmission of said signal.

1 9. A device as claimed in Claim 18, including means for continuing to reselect frequency(s) and retransmit said signal while the transponder is powered.

20. A device as claimed in Claim 1 , including means whereby the selection or the reselection is performed in accordance with a predetermined weighted probability.

21 . A transponder as claimed in Claim 17, wherein the control means comprise means for using a pseudo-random binary sequence (PRBS) logic circuit to randomly select or reselect said selected frequency(s).

22. A system of device identification comprising:

at least two identifiable devices spaced apart, and a device identifier, wherein each identifiable device includes transmitter means adapted to transmit a signal to the identifier, and each transmitter means being adapted to transmit said signal at a newly selected modulation selected from a predetermined set of modulations.

23. A system for simultaneously identifying a first and second device, each device comprising code storage means, modulation means and an inductive receiver/transmitter means, the system comprising: magnetic field generator/radiator means for generating a magnetic field from which said first and second devices are adapted to extract power using said inductive means; each of said first and second devices, when so powered, respectively providing at least one unique code from the code storage means to the modulation means, said modulation means being adapted to provide at least one modulated code to the inductive means for transmission to a device identifying receiver; each device adapted to modulate said at least one code at at least one modulation randomly selected from a predetermined finite set of modulations, while each device remains powered; each device being embodied in a single (IC) chip.

24. A communication system comprising:

a first and a second device, said devices being adapted to continuously transmit respective first and second data signals while each device is powered, the first and second data signals having a first and a second respective modulations selected from respective first and second predetermined sets of possible modulations; said first and second devices including respective first and second selector means for selecting the first and the second respective modulation from said respective sets of modulations; receiver means for receiving the first and second data signals simultaneously; and demodulator means for demodulating the first and second data signals to obtain a first and second respective device data signal.

25. A method of communication between an interrogator and at least two remote receiving devices, said method comprising the steps of: radiating an interrogation signal from said interrogator to each remote device; generating a reply signal at a modulation selected from a predetermined set of modulations in each remote device; transmitting said reply signal from each remote device to the interrogator; and serially generating and transmitting the reply signal at a newly selected modulation from said predetermined set of modulations.

26. A method as claimed in Claim 25, wherein each remote device transmits the reply signal simultaneously.

27. A method as claimed in Claim 25, wherein the reply signal includes a code unique to each remote device.

28. A method of identifying at least two remote devices, comprising the steps of:

providing an interrogator for radiating an interrogation signal to said remote devices,

upon receiving said interrogation signal, each remote device transmitting a unique identification signal, said identification signal being transmitted at a newly selected modulation selected from a predetermined set of modulations.