WO2008118019A2

WO2008118019A2 - Transponder system

Info

Publication number: WO2008118019A2
Application number: PCT/NL2008/050173
Authority: WO
Inventors: Franciscus Hedrikus Elferink; Erik Bert Busking
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 2007-03-27
Filing date: 2008-03-26
Publication date: 2008-10-02
Also published as: WO2008118019A3

Abstract

Transponder system comprising an FMCW radar transceiver (1) arranged to transmit an FMCW signal (2) and to receive the backscattered reflections (3). The transponder (4) comprises an antenna (5) connected to a switch (6), arranged to modulate the terminating impedance of the antenna, thus modulating the backscattered signal. The modulating switch is controlled by digital control means (7, 10) arranged to be controlled by voice (9) or data produced by a human user, thus using the transponder as a communication device for digital voice or data from the user to the transceiver, which communication cannot be intercepted by any external system. The FMCW transceiver can measure the distance and direction of the transponder. The system is very attractive for military etc. operations. The reflected signal may be frequency shifted and amplified by a booster module (20,21).

Description

Transponder system

DESCRIPTION

The invention concerns a transponder system comprising an RF transceiver which is arranged to transmit an RF signal and to receive backscattered reflections of the transmitted signal from a passive transponder which comprises means which are arranged to modulate the backscattered signal.

Modulated backscattering is known e.g. from US2006/0132301, disclosing a base unit which transmits a wireless signal to a passive transponder, which passive transponder modulates the impedance of its antenna, thereby altering reflections of the wireless signal off its antenna. The base unit detects the changes in the reflected signal which is used to distinguish data bits.

Besides, US6147605 discloses a radio frequency identification tag including a first and second antenna element and a radio frequency identification circuit which includes a load modulation circuit, optimized for radio frequency identification applications.

Finally, US6084503 discloses a radio-interrogated surface- wave technology sensor, in which the sensitive element is an impedance which is electrically connected as termination to a surface-wave structure of the sensor.

The known transponder systems thus comprise an RF transceiver which is arranged to transmit an RF signal and to receive backscattered reflections of the transmitted RF signal. A (local) transponder comprises an antenna which is connected to a switch or modulator which is arranged to switch (modulate) the antenna's (terminating) impedance between at least two values, the switch being controlled by digital control means, e.g. keying/modulating the transponder's identification (ID) code or system status information upon reception of the RF signal from the RF transceiver, and thus (modulated) backscattering the transponder's ID code ("ID interrogation") .

One aim of the present invention is to "transmit" (rather "to reflect") not (only) fixed data, like a transponder bound ID code or status information but (besides) "variable" signals like voice, produced by a human being and entered into the transponder via a microphone, thus using such modulated backscattering type transponder as a (one-way) communication device, viz. for the transmission of digital voice or data from the side of the transponder to the side of the RF transceiver, wherein, however, the source/location of those voice or data cannot be located by external means, due to the passive nature of the (modulating) transponder. This aim makes the transponder very suitable for e.g. military or similar "secret" operations.

A second aim of the present invention is to enable a reliable and precise location of a (e.g. voice/data backscattering) transponder, viz. by using radar, which is, due to its nature (viz. based on reflection time measurement and processing rather than field strength measurement and processing) , excellently suitable for locating objects like e.g. transponders. This aim still strengthens the system's suitability for military etc. operations. Another aim is to use "Frequency Modulated Continuous Wave" (FMCW) radar for the RF transceiver, as FMCW is - rather than e.g. Continuous Wave (CW) or Doppler (based) radar, which is solely able to detect displacement speeds (and thus often used for speed measurements of e.g. road traffic) - capable to measure both the distance and the direction (angle, altitude, inclination) of (in this case) the (voice/data backscattering) transponder w.r.t. the location of the transceiver (base station) . In FMCW the RF transceiver' s transmitter emits a RF FM chirp signal and receives the backscattered echo from the transponder. The signal propagation delay given by the distance from the transmitter to the transponder (and back) causes a beat signal in the transceiver's receiver. By means of advanced signal processing (e.g. Fourier Transform) the beat frequency is converted to an accurate distance. The FMCW principle has been illustrated in Fig. 10.

Summarizing, the invention aims to provide a system which is able to communicate voice and/or data from the side of the transponder towards a base station (mobile or fixed) , wherein the (passive) source of the data/voice cannot be located by any external means except the system's (FMCW) radar based base station/transceiver, but wherein the position of that source can be determined very accurately by that FMCW radar based base station. These aims make the system according to the invention very attractive for military etc. operations, undercover operations etc. For example the (voice/data modulated) transponder could be part of the communication equipment of a front soldier, commando or a secret agent. The communication towards the side of the transponder (i.e. towards the soldier, commando, secret agent) could be performed in a conventional way, viz. by providing the soldier etc. with a normal small radio receiver as, after all, the position of such (well shielded) military radio receiver cannot be detected by external means (of the enemy) . Thus, two-way communication can be achieved without the chance to betray the communicator' s position by any external detection/monitoring system.

To reach the aims, according to the invention, in a transponder system comprising an RF transceiver which is arranged to transmit at least one RF signal and to receive backscattered reflections of said at least one transmitted RF signal; the transponder system, moreover, comprising at least one transponder comprising at least one antenna or antenna array connected to a switch or switch array respectively, arranged to switch (or modulate) the terminating impedance of each relevant antenna between at least two values, thus modulating the backscattered signal, and which modulating switch or switch array is controlled by digital control means, it is preferred that - the control means are arranged to be controlled by voice or data produced by a human user, thus using the transponder as a communication device for the transmission of digital voice or data from the side of the user to the side of the RF transceiver; - the RF transceiver is a FMCW radar based transceiver, arranged to measure the distance and the direction of said at least one transponder to the location of the transceiver (base station) .

In order to be suitable for e.g. military etc. operations which require the transponder to have minimal dimensions without damaging its operation effectiveness, it is preferred that the transponder' s antenna has the form of a micro strip or printed antenna, e.g. a micro strip patch antenna or patch antenna, which is very advantageous in view of their efficiency, small (esp. flat) dimensions and their insensitivity for effects of parasitic capacitance, such as the so-called hand (or body) effect.

It is noted that the (impedance modulating) switch or modulator may have any form of an (electronic) switch) , phase shifter, quadrature modulator, (e.g. PLL based) frequency modulator etc., all of them being able to modulate the reflected/backscattered signal.

Finally, two additional improvements will be addressed now.

To improve the reflective capacity some form of corner reflector or retro reflector antenna may be used. Such reflector can be defined as any instrument used to cause reflected radiation to return along paths parallel to those of their corresponding incident rays; one type, the corner reflector, is an efficient radar target (source: www.answers.com/topic/retroreflector). To that end the transponder system according to the invention comprises at least one antenna array including an array of mainly coplanar antenna surfaces, each single antenna of it including at least two of such coplanar antenna surfaces located around a symmetry axis which is common for all single antennas of the array.

To be able to detect e.g. the mutual geometrical position of at least two transponders, in the transponder system according to the invention, the RF transceiver may be arranged to transmit at least two RF signals and to receive reflections of said at least two transmitted RF signals, where the transponder system, moreover, comprises at least two separate transponders, their antenna (s) or antenna array (s) being tuned to different frequencies selected from said at least two RF signals. In this way the relative position of the at least two transponders can be detected. This option may be used to detect the mutual position of at least two transponders within one body, thus enabling to detect that body's altitude and/or inclination, or within separate bodies, enabling the mutual position (altitude and/or inclination) of the bodies w.r.t. each other.

As an alternative for enabling the system to detect e.g. the mutual geometrical position of at least two transponders, the at least two transponders, each comprising digital control means, are arranged for keying the relevant switch (or switch array respectively) with a carrier which is modulated by the voice and/or data. To discriminate between the different (at least two) transponders, the carrier frequencies of different transponders preferably have mutually different values. The transceiver, moreover, comprises means which are arranged to discriminate between reflections from the different transponders based on their different carrier frequencies.

Below the invention will be discussed more in detail referring to an exemplary embodiment.

Exemplary Embodiment

Fig. 1 shows schematically an exemplary embodiment of a transponder system according to the invention; Fig. 2 shows the structure of an antenna which preferably may be used, in plan view, Fig. 3 in bottom view; Fig. 4 shows the circuitry of the embodiment of the transponder part of Fig. 1 more in detail, Fig. 5 including means for setting and/or programming the transponder part; Fig. 6 shows the radiation pattern of the exemplary antenna structure;

Fig. 7 shows a planar corner reflector configuration; Fig. 8 shows a system using two RF frequencies for e.g. altitude and/or inclination detection. Fig. 9 shows an alternative system for e.g. altitude and/or inclination detection, using discriminating frequencies of carrier waves modulated by voice and/or data; Fig. 10 gives a general illustration of FMCW radar; Fig. 11 shows a transponder including a reflection booster module .

The system shown in Fig. 1 comprises an FMCW transceiver 1 which is arranged to transmit an FMCW signal 2 and to receive reflections 3 of the transmitted FMCW signal 2. The transponder system further comprises an FMWC transponder 4 which comprises an antenna 5 which is tuned to the FMWC signal's frequency. The antenna 5 is connected to a switch 6 which is arranged to switch the antenna's impedance between at least two values; the switch is controlled by digital control means 7, which may be a microcontroller, which is fed by a battery 8.

The control module 7 is controlled by digital data from a voice module, comprising a microphone 9 and an A/D converter 10.

The circuitry of Fig. 1 is thus arranged to control the reflectivity of the dipole 5 by means of the switch 6, which is controlled - via de A/D converter 10 and the microcontroller 7 - by any signal picked up by the microphone 9, the (analogue) microphone signal being converted, by the A/D converter 10, to a digital signal, which the microcontroller 7 uses to key the switch 6 with a carrier wave which is modulated by voice (or data) and thus to key the impedance of the dipole 5 with said carrier. In that way, when the dipole 5 receives a signal 2 from transceiver 1, the extent in which the signal 2 will be reflected to the transceiver 1 is thus modulated by the digitized microphone signal, causing that de transceiver 1 is able, by measuring the reflected (backscattered) signal strength to "read out" the digital reflections modulated by the microphone input. Subsequently the voice (or data) signal can be obtained by demodulating the reflected signal, e.g. using the carrier wave frequency.

In this way the transponder 4 can act as a remote (secret) microphone, which can only be detected by means of a (directional) radar signal. Discovery of such a (passive modulated) transponder is substantially more difficult than a secret microphone system including a transmitter which broadcasts a modulated RF carrier.

By using different carrier frequencies the signals from different transponders may be discriminated. Alternatively, different RF (radar) frequencies may be used which, however, will require transceivers using multiple RF frequencies. Both alternatives will additionally be discussed hereinafter.

The current consumption of the microcontroller, the AD- converter and the switch is that low that e.g. a lithium button battery cell may be sufficient for uninterrupted use during several years. To increase the battery's life time the transponder may - under control of the microcontroller - be activated only for restricted periods. For the main part of the time the transponder may be then in a so-called sleep mode .

Preferably, but not necessarily, the antenna is a micro strip or printed antenna, more preferably a micro strip patch antenna or patch antenna, the structure of which is shown separately in Figs. 2 and 3.

In telecommunication, there are several types of micro strip antennas (also known as printed antennas) the most common of which is the micro strip patch antenna or patch antenna. A patch antenna is a narrowband, wide-beam antenna fabricated e.g. by etching the antenna element pattern in metal trace bonded to an insulating substrate. Because such antennas have a very low profile, are mechanically rugged and can be conformable, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile radio communications devices. Micro strip antennas are also relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. They are usually employed at UHF and higher frequencies because the size of the antenna is directly tied to the wavelength at the resonant frequency. An advantage inherent to patch antennas is the ability to have polarization diversity. Patch antennas can easily be designed to have Vertical, Horizontal, Right Hand Circular (RHCP) or Left Hand Circular (LHCP) Polarizations with a single antenna feed point. This unique property allows patch antennas to be used in many types of communications links that may have varied requirements. Fig. 2 shows a plan view of configuration of the patch antenna as used in the present embodiment. The surfaces 11 and 12 of the patch antenna are conductive layers supported by an intermediate carrier 13 of an electrically insulating material. The patch antenna has a square shape, one side having a length of λ/4. The location of the feed point 14 is determinative for the impedance and the polarization of the antenna.

Fig. 3 shows the bottom view of the antenna, showing its ground surface 12 up.

Fig. 4 shows the configuration of the transponder more in detail, viz. a λ/4 line 14 connected with the switch. The combination of a make-and-break type switch 6 and the λ/4 line performs better than using an on-off type switch as shown in Fig. 1.

Fig. 5 shows that the processor 7 may be set up and/or programmed by using an inductive coupling comprising two coils 15 and 16 to be able to control the processor 7 from an external programmer 17.

Fig. 6 shows a sensitivity curve of the patch antenna. The antenna's sensitivity is high and has an opening angle of about 60°. From the rear side the sensitivity is about zero, thus enabling the transponder e.g. to be used in the vicinity of a (human) body without being hindered by any "hand effect", which would give signal attenuation.

Referring now to Fig. 7, to improve the reflective capacity an (e.g. planar) corner reflector or retro reflector antenna may be used which enables any reflected radiation to return along paths parallel to those of their corresponding incident rays. To that end the transponder system shown in Fig. 7 comprises an antenna array including an array of mainly co- planar antenna surfaces 5, e.g. provided on a carrier 13, each single antenna of it including at least two of such coplanar antenna surfaces 5 located around a symmetry axis 18 which is common for all single antennas of the array. Each single antenna (formed by two antenna surfaces 5) of the antenna array is connected to a switch (6) which is part of a switch array, arranged to switch the terminating impedance of each antenna between at least two values; the switches 6 of the switch array are controlled by the digital control means 7.

To be able to detect the mutual geometrical position of at least two transponders, in the transponder system according to the invention, Fig. 8 illustrates that the FMCW transceiver 1 may be arranged to transmit e.g. two FMCW signals 2a, 2b and to receive reflections 3a, 3b of those two transmitted FMCW signals. The transponder system, moreover, comprises (e.g.) at least two separate transponders 4a, 4b. Their antenna (s) or antenna array (s) (see e.g. Fig. 7) are tuned to different frequencies selected from said at least two FMCW signals, e.g. transponder 4a to the frequency of the signals 2a and 3a, transponder 4b to the frequency of signals 2b and 3b. In this way the relative position of the separate transponders 4a, 4b can be detected. This option may be used to detect the mutual position of at least two transponders within one body 19, thus enabling to detect that body's altitude and/or inclination, or within separate bodies, enabling the mutual position (altitude and/or inclination) of the bodies w.r.t. each other which will be obvious for any person skilled in the art. Fig. 9 shows an alternative and preferred system for e.g. altitude and/or inclination detection, using different frequencies of carrier waves which are modulated by voice and/or data. In Fig. 9 two transponders 4a, 4b comprise digital modules 7a, 7b arranged for keying the relevant switches or switch array (not shown here) with carriers modulated by voice and/or data. The carrier frequencies of both transponders have different values, indicated by fl and f2. The transceiver 1 comprises means (not shown) which are arranged to discriminate between reflections from the different transponders (4a, 4b) based on their different carrier frequencies f1 and f2.

Fig. 10 gives a general illustration of FMCW radar. During one frequency sweep the radar response is sampled which results in a time series of n samples. Before a fast Fourier transform (FFT) can be taken from the time series a window should be used to eliminate the effect of start and end discontinuities. Each bin from this range FFT represents the amplitude of the reflectivity of the environment on a certain distance. Each sweep will produce one range FFT. When we consider the sweep number as the new discrete time it is possible to define a new n dimensional time series (the nth range bin amplitude as function of the sweep number) . The transponder information can be recovered (data recovery) from this new time series. This results in an ID code for a given range bin.

Finally, Fig. 11 shows an optional addition to the circuitry of the transponder, enabling it to boost its reflection power. The circuitry of Fig. 11 is similar to Fig. 4, except the addition of a booster module which is formed by an RF amplifier 20 and a frequency shifter 21. The amplifier is able to amplify ("boost") the signal received by the antenna and to supply the amplified signal back to the same antenna. To be able to use the same antenna, viz. for reception of the signal (from the FMCW transceiver 1) and for transmitting (reflecting) the amplified signal without feedback instabilities, it is preferred to distinguish the transmitted signal from the received signal, e.g. by shifting its (basic) carrier frequency from f to (f+fojffset) ) • Preferably the frequency offset is thus that the frequencies f and

(f+ fo(ffset) ) belong to different frequency bands. In this optional case the (basic, i.e. not FM modulated) transmission frequency of the transceiver 1 is f and its (basic) reception frequency is (f+fo(ffset) ) • ∑ⁿ this way the principle of impedance keying (viz. by digitized speech or data) remains unchanged, while booster module 20,21 improves the transponder's gain and thus its usability. The booster module can be enabled - via processor 7 - solely when voice is coming in from the user's microphone, thus providing that the transponder is quite silent when not actively in use.

Claims

1. Transponder system comprising an RF transceiver (1) which is arranged to transmit at least one RF signal (2) and to receive backscattered reflections (3) of said at least one transmitted RF signal; the transponder system, moreover, comprising at least one transponder (4) comprising at least one antenna (5) or antenna array connected to a switch (6) or switch array respectively, arranged to switch the terminating impedance of each relevant antenna between at least two values, thus modulating the backscattered signal, and which modulating switch or switch array is controlled by digital control circuit (7, 10), wherein the control circuit is arranged to be controlled by voice (9) or data produced by a human user, thus using the transponder as a communication device for the transmission of digital voice or data from the side of the user to the side of the RF transceiver; the RF transceiver is a FMCW radar based transceiver, arranged to measure the distance and the direction of said at least one transponder to the location of the transceiver (base station) .

2. Transponder system according to claim 1, wherein at least one antenna (5) or antenna array comprises at least one micro strip antenna or printed antenna.

3. Transponder system according to claim 2, wherein a,t least one antenna (5) or antenna array comprises at least one micro strip patch antenna or patch antenna.

4. Transponder system according to claim 2 or 3, wherein at least one antenna (5) or antenna array comprises an insulated carrier (13) having, for each single antenna, a first conductive layer (11) on one side and, either common or not common for some or all relevant antennas, a second conductive layer (12) on the other side.

5. Transponder system according to claim 1, comprising at least one antenna array including an array of mainly co- planar antenna surfaces, each single antenna of it including at least two of such coplanar antenna surfaces (5) located around a symmetry axis (18) which is common for all single antennas of the array.

6. Transponder system according to claim 1, the RF transceiver (1) being arranged to transmit at least two RF signals (2a, 2b) and to receive reflections (3a, 3b) of said at least two transmitted RF signals, the transponder system, moreover, comprising at least two transponders (4a, 4b) each comprising at least one antenna (5) or antenna array, tuned to a separate frequency of said at least two RF signals.

7. Transponder for a system according to claim 1, the digital control circuit (7, 10) being arranged for keying the switch (6) or switch array respectively with a carrier modulated by voice and/or data produced by a human user, thus using the transponder as a communication device for the transmission of digital voice or data from the side of the user to the side of the RF transceiver.

8. Transponder according to claim 7, comprising a booster module (20, 21) arranged to amplify the signal received by the antenna and to supply the amplified signal back to the same antenna.

9. Transponder according to claim 8, the booster module comprising means for shifting the received frequency over an offset value and for reflecting the amplified and shifted signal .