WO2012131337A1 - Procédé et appareil de communication rf - Google Patents

Procédé et appareil de communication rf Download PDF

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Publication number
WO2012131337A1
WO2012131337A1 PCT/GB2012/050649 GB2012050649W WO2012131337A1 WO 2012131337 A1 WO2012131337 A1 WO 2012131337A1 GB 2012050649 W GB2012050649 W GB 2012050649W WO 2012131337 A1 WO2012131337 A1 WO 2012131337A1
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WO
WIPO (PCT)
Prior art keywords
signal
frequency
shifted
carrier frequency
interrogation
Prior art date
Application number
PCT/GB2012/050649
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English (en)
Inventor
Alan David Hart
Adam Armitage
Amanda Kay SINDEN
Duncan Christopher Frederick Levett
Original Assignee
Mbda Uk Limited
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Publication date
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Publication of WO2012131337A1 publication Critical patent/WO2012131337A1/fr

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/06—Receivers
    • H04B1/16—Circuits
    • H04B1/30—Circuits for homodyne or synchrodyne receivers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S13/00—Systems using the reflection or reradiation of radio waves, e.g. radar systems; Analogous systems using reflection or reradiation of waves whose nature or wavelength is irrelevant or unspecified
    • G01S13/74—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems
    • G01S13/75—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems using transponders powered from received waves, e.g. using passive transponders, or using passive reflectors
    • G01S13/751—Systems using reradiation of radio waves, e.g. secondary radar systems; Analogous systems using transponders powered from received waves, e.g. using passive transponders, or using passive reflectors wherein the responder or reflector radiates a coded signal
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/38—Transceivers, i.e. devices in which transmitter and receiver form a structural unit and in which at least one part is used for functions of transmitting and receiving
    • H04B1/40—Circuits
    • H04B1/50—Circuits using different frequencies for the two directions of communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/59—Responders; Transponders

Definitions

  • This invention relates to the field of radio-frequency (RF) communications, and in particular to a communications method and apparatus that provides a low risk of interception by third parties.
  • RF radio-frequency
  • the power budget is typically symmetrical; i.e. both devices radiate approximately equal amounts of RF power.
  • a passive communications system one of the two devices radiates much more RF power than the other: in the extreme case, the interrogating device radiates all of the power, and the interrogated (deployed) device contributes no additional RF power.
  • the signal returned from a transponder may be at much lower amplitude than the clutter signals (for example from the ground and/or nearby structures). It would be advantageous to provide a communications method and apparatus in which one or more of the aforementioned disadvantages is eliminated or at least reduced.
  • a first aspect of the invention provides a method of communicating with a previously deployed communication system, the method comprising:
  • (1 ) receiving, in response to an interrogation signal that has been transmitted to the deployed system at a first carrier frequency, a response signal from the deployed system, the response signal comprising (i) the interrogation signal shifted by the deployed system to a second, different, carrier frequency, the second carrier frequency being separated from the first carrier frequency by a shift frequency, the shifted interrogation signal carrying a message signal from the deployed system, and (ii) unwanted signals, at the first carrier frequency, resulting from reflections of the interrogation signal;
  • the method enables communication with a deployed system that does not radiate any power additional to that received from the interrogation signal; the method is thus particularly suitable for covert communications applications.
  • the deployed communication system may be a transponder.
  • the transponder may be configured to provide no overall gain (although it may be that the transponder provides amplification to the received interrogation signal to compensate for any reduction in signal power resulting from, for example, losses within electronics in the transponder).
  • the method may further include the step of transmitting the interrogation signal to the deployed system.
  • the transmission of the interrogation signal to the deployed system may have been carried out by non- cooperating radio sources, for example sources of public television or radio broadcasts.
  • the extraction of the message signal from the filtered downshifted signal may include the steps of mixing the filtered downshifted signal with a signal at an intermediate frequency to produce an upshifted signal at the intermediate frequency and demodulating the message signal from the upshifted signal.
  • the downshifted signal may consist of an in-phase component signal and a quadrature component signal.
  • the filtering of the downshifted signal to remove the interrogation signal and to produce the filtered downshifted signal may then be carried out in parallel on the in-phase component signal and the quadrature component signal.
  • the response signal is mixed with a signal substantially at the second carrier frequency. It may be that, in the downshifted signal, the shifted interrogation signal has been shifted substantially to 0 Hz. It may be that the sideband is not shifted to exactly 0 Hz, as it may be undesirable for the downshifted signal to include a significant DC (i.e. 0 Hz) component. It may be that, in the downshifted signal, the unwanted signals have been shifted substantially to the shift frequency.
  • the message signal will have a bandwidth: it may be that the shifted interrogation signal is shifted to a frequency larger than or approximately equal to half the bandwidth of the message signal; in that case, the DC component of the downshifted signal will be small. It may be that the downshifted signal includes no or substantially no DC component.
  • the interrogation signal may be an electromagnetic signal.
  • the interrogation signal may be a RF signal.
  • the carrier frequency may be a conventional communications band (e.g. the S-band or C-band). Use of a conventional communications band (and especially of frequencies that are well- utilised, e.g. commercial mobile-phone carrier frequencies or WiFi frequencies) would be advantageous for covert operation, as it could reduce the risk of the covert communications being detected.
  • the carrier frequency may be in a higher band for point-to-point communications (e.g. the X-band or Ku band). For a very compact system, it may be that even higher frequencies are used (e.g. the K-band or the Ka band or higher EHF signals), so that antennas can be kept small.
  • the shift frequency is less than 10 "5 octaves, or even less than 10 "6 octaves relative to the carrier frequency of the interrogation signal. It may be that, in the response signal, the shift frequency is less than twice the bandwidth of the message signal. It may be that, in the response signal, the shift frequency is more than 5 times, preferably more than 10 times, the bandwidth of the message signal. It may be that the shift frequency is between 2 and 10 times the bandwidth of the message signal. It may be that the shift frequency is between 2 and 5 times, the bandwidth of the message signal.
  • the frequency separation between the shifted unwanted signals and the nearest edge of the bandwidth of the message signal (on the further shifted interrogation signal) is more than 0.1 octave, preferably more than 1 octave, or still more preferably more than 10 octaves relative to the bandwidth of the message signal.
  • the greatly increased separation in the downshifted signal between the message signal and the unwanted signals compared with that separation in the response signal means that it is much easier to filter the unwanted signals from the shifted interrogation signal containing the message signal.
  • the message signal may be encoded on the interrogation signal by the deployed system before the frequency shift is applied.
  • the message signal may be encoded on the shifted interrogation signal by the deployed system.
  • the message signal may be encoded by the deployed system on the interrogation signal or on the shifted interrogation signal by for example FM, AM, PSK, FSK, ASK, OFDM or any other suitable encoding method.
  • the message signal may be analogue.
  • the message signal may be digital.
  • a second aspect of the invention provides a receiver for receiving, in response to an interrogation signal that has been transmitted to a deployed system at a first carrier frequency, a response signal from the deployed system, the response signal comprising (i) the interrogation signal shifted by the deployed system to a second, different, carrier frequency, the second carrier frequency being separated from the first carrier frequency by a shift frequency, the shifted interrogation signal carrying a message signal from the deployed system, and (ii) unwanted signals, at the first carrier frequency, resulting from reflections of the interrogation signal, the receiver including:
  • a mixer configured to mix the response signal with a signal at a frequency separated from the second carrier frequency by no more than the cut-off frequency of the low-pass filter minus half the bandwidth of the message signal to produce a downshifted signal comprising the shifted interrogation signal further shifted to a frequency lower than the cut-off frequency and the unwanted signals shifted to a frequency higher than the cut-off frequency
  • the low-pass filter being arranged to remove the unwanted signal from the downshifted signal and thereby to produce a filtered downshifted signal; the receiver further comprising a decoder configured to extract the message signal from the filtered downshifted signal.
  • the communications device may further include at least one antenna.
  • the mixer may be a quadrature mixer for generating an in-phase signal component and a quadrature signal component displaced in phase relative to the in-phase signal component.
  • the low-pass filter may comprise a first filter for low-pass filtering the in-phase signal component and a second filter, in parallel with the first filter, for low-pass filtering the quadrature component.
  • the decoder may comprise a demodulator.
  • a third aspect of the invention provides a communications device including a receiver according to the second aspect of the invention and further including a transmitter for transmitting the interrogation signal to the deployed system.
  • the transmitter may include a source of the interrogation signal.
  • the device may include an antenna shared by the transmitter and the receiver.
  • the device may include a circulator linking the transmitter and the receiver to the antenna.
  • a fourth aspect of the invention provides a method of communicating from a deployed system, the method comprising the deployed system:
  • a fifth aspect of the invention provides a deployable communication system comprising:
  • a mixer configured to form a response signal in the form of the interrogation signal shifted by the deployable system to a second, different, carrier frequency, the second carrier frequency being separated from the first carrier frequency by a shift frequency, the shifted interrogation signal carrying a message signal from the deployable system;
  • the deployable system is a transponder. It may be that the transponder is configured to provide no overall gain between the received interrogation signal and the transmitted response signal.
  • Figure 1 is (a) a schematic diagram showing an example apparatus according to the invention and indicating the frequency of various signals at different stages in the apparatus, and (b) a more detailed schematic diagram of an example implementation of part of the apparatus shown in (a);
  • Figure 2 is a system block diagram of a transponder from an example embodiment of the invention.
  • Figure 3 is a system block diagram of a transceiver according to an example embodiment of the invention.
  • Figure 4 is an example implementation of an IQ frequency up-mixer from the transceiver of Fig. 3;
  • Figure 5 is a schematic representation of signal magnitude as a function of frequency for (a) a signal leaving the transponder of Fig. 2 and (b) the signal after mixing to shift a sideband to 0 kHz, in the transceiver of Fig. 3.
  • data is transferred from a deployed communication system, in the form of a transponder, to an interrogating transceiver system, in such a manner that the deployed system modifies and retransmits EM radiation which impinges upon it.
  • the interrogating system operates as an RF illumination source, specifically a line-of-sight illuminator directed at the deployed system.
  • the deployed system applies a frequency shift and message encoding to the interrogating signal which is incident upon it and then retransmits the signal.
  • frequency modulation is tailored to generate a single side band (SSB) separated 100 kHz from the carrier frequency which contains the encoded data.
  • the receiver of the interrogating system collects and processes returned signals to extract data encoded by the deployed system.
  • the returned signal typically comprises the encoded data and significant levels of undesired signals from clutter.
  • the example system thus provides a data link that comprises a compact low-power covert deployed subsystem, capable of operating in remote regions, and an interrogating subsystem which interprets signals returned to it from the deployed subsystem. Both analogue and digital communications are possible using this data link.
  • the receiver sub-system extracts the 100kHz sideband from returned carrier signals. As the carrier reflections can dominate the sideband by as much as 100dB, a highly selective technique is employed to reject the unwanted carrier.
  • the 100kHz sideband contains the required data in frequency modulation (FM) form, which can then be decoded using standard techniques.
  • FM frequency modulation
  • the example system (Fig. 1 (a)) comprises a transmitter 5, a receiver 10 and a transponder 40.
  • the transmitter 5 comprises a source 20 of an interrogation signal at a frequency ⁇ cameri, in this example 14 GHz, which is fed to a transmission antenna 30, which transmits the interrogation signal to the transponder 40.
  • the transponder 40 comprises a receiving antenna 50, which receives the interrogation signal from the transmitter 5 and feeds it to a signal processing block 60.
  • the interrogation signal is frequency-shifted and then encoded with a message signal to form a response signal.
  • the frequency shift is to a frequency i C amer2 separated from the carrier frequency icameri by a sideband-carrier separation frequency i S hm-
  • the response signal is fed to a transmission antenna 70, which transmits the response signal to the receiver 10.
  • the response signal includes the original carrier icameri, as well as the shifted carrier icame , due to reflections from objects in the environment.
  • the receiver comprises a receiving antenna 80, which receives the response signal from the transponder 40.
  • the response signal is fed to a mixer 90, where it is mixed with a signal from a local oscillator 100.
  • the signal from the local oscillator 100 is at the second carrier frequency icame , and so the mixing results in a downshifted signal, with the component of the signal previously at learned shifted substantially to 0 kHz, and the component of the signal previously at icameri shifted substantially to the transponder shift frequency i S hitt (see Fig. 5(b)).
  • the downshifted signal is fed to a low-pass filter 1 10, which removes the downshifted carrier signal (i.e.
  • the filtered downshifted signal is fed to a decoder 120 which extracts the message signal originally applied by the transponder 40.
  • the message signal is passed to the receiver output 130.
  • Fig. 1 (b) shows an example implementation of the decoder, comprising a local oscillator 124, a mixer 122 and a demodulator 126.
  • the mixer 122 mixes the filtered downshifted signal with a signal at an intermediate frequency f; to produce an upshifted signal at the intermediate frequency f,.
  • the upshifted signal is fed to the demodulator 126, which demodulates the message signal from the upshifted signal.
  • the message signal is then passed to the receiver output 130.
  • the deployed transponder system is designed to impart message signal on to the illuminating RF signal based on an input signal.
  • the system block diagram of Fig. 2 shows the transponder 40 in more detail.
  • the frequency shift and encoding of the message signal are achieved simultaneously by using a mixer to provide the frequency shift whilst encoding the message as a frequency modulation.
  • the interrogating signal is received by the receiving antenna 50 and fed to the a mixer 150, where it is frequency modulated with the message signal by mixing it with a signal from a voltage-controlled oscillator 160, which is fed the message signal from a signal input 170.
  • the mixer 150 is chosen to be an active mixer, in order to accommodate the large frequency difference between the interrogating signal and the message signal.
  • the resulting FM response signal is amplified in an amplifier 180 and then fed to the transmitting antenna 70.
  • Fig. 3 shows the receiver 10 and transmitter 5 in more detail.
  • the role of the interrogating system (the transceiver comprising transmitter 5 and receiver 10) is to extract and decode the FM signal that has been imparted by the deployed transponder 40. As mentioned above, that task is complicated by the fact that, in some environments, an extremely high carrier- to-FM-sideband signal ratio may be present.
  • the 14 GHz carrier frequency 210 i.e. the signal resulting from environmental reflections
  • the 20kHz wide FM sideband 220 i.e. the shifted carrier signal transmitted by the transponder
  • That separation (0.000003 octaves) is very small compared with the carrier frequency, and consequently a filter having an extremely high Quality Factor' would be required to isolate one from the other.
  • the FM sideband is shifted to baseband.
  • a quadrature mixer allows the 20kHz FM band to be represented as a ⁇ 10kHz complex signal.
  • the shifted carrier 210' which is now located at 100kHz, is 3.3 octaves higher in frequency than the edge of the shifted FM sideband 220', as depicted in Fig. 5 (b). Separation of these two signals is then possible using a high-quality low-pass filter.
  • the 14GHz CW carrier signal is generated, amplified and radiated towards the deployed transponder 40.
  • the response signal is received by the antenna 80 and then amplified by an amplifier 84.
  • the amplified signal is passed through a complex mixer 90 to generate both in- phase and quadrature signals.
  • the mixer 90 is excited by a local oscillator 100 slightly offset from the carrier frequency to allow the FM sideband to be mixed to baseband.
  • a twin filter chain 1 10a, 1 10b is utilised to reject the carrier from both the I and Q output of the mixer 90.
  • the filter output is amplified by amplifiers 105a, 105b and DC offset compensation is applied to prevent saturation of the following stages.
  • a second mixer 122 is employed to translate the baseband signal to an industry standard 455kHz centre frequency, that allows the demodulation process to be completed by a commercial off-the-shelf FM demodulator unit 126.
  • the transmit oscillator 22 In the transmitter 5, the transmit oscillator 22 generates the 14GHz CW interrogation signal which will be used to illuminate the deployed transponder 40.
  • the phase-noise characteristics of the transmit oscillator 22 are important as they govern how much of the carrier frequency spills over into adjacent parts of the frequency spectrum: of particular concern is how much of the carrier leaks into the information-bearing band situated at 90kHz to 1 10kHz from the carrier.
  • This example embodiment employs an Agilent E8257D as the transmit oscillator 22, which has extremely stable phase characteristics and exhibits - 1 13dBc/Hz at 100kHz separation from the carrier.
  • the interrogation signal generated by the transmit oscillator 22 is fed to a transmit amplifier 24, which is provided to increase the available output power.
  • the amplifier is a Miteq amplifier model number MPN4- 02001800-23P.
  • the interrogation signal then passes to a RF isolator 26, which is incorporated into the design to prevent reflected power damaging either the transmit amplifier 24 or oscillator 22.
  • the RF isolator 26 is a Ditom model number D3I7018.
  • the interrogation signal then passes to the transmit antenna 30, which in this example is a lens horn antenna model number CW820-GA, manufactured by Flann Microwave.
  • the receive antenna 80 is identical to the transmit antenna 30, offering the same antenna gain and beam pattern.
  • the two antennas 30, 80 are mounted side by side such that the transmit and receive beam pointing vectors are parallel.
  • the received response signal passes through an RF limiter 82, which is included to reduce the possibility of damage to subsequent amplifying components, which is otherwise a risk, for example, if the clutter signal is higher than expected or the receive and transmit antennas 30, 80 are aligned such that the direct cross talk becomes significant.
  • the limiter 82 is Miteq's part number MPL02018-2-33.
  • the operation of the limiter 82 is such that, in the event of signals greater than the limiting threshold entering the device, the attenuation of the limiter 82 increases. (The increase in attenuation affects all signals regardless of their magnitude; thus unfortunately the limiter 82 in this example cannot be used to decrease the dynamic range between the desired and undesired signals.)
  • the response signal next passes into a receive amplifier sub-system 84, which increases the magnitude of both the desired and undesired signal prior to entry into the quadrature mixer 90.
  • the amplifier 84 is a Miteq amplifier part number AFS-02001800-24-1 OP-4.
  • the RF quadrature mixer 90 is employed to down-convert the RF response signal to baseband for further processing.
  • the RF quadrature mixer 90 is a Miteq component part number IR0218LC1 Q.
  • the quadrature mixer 90 mixes with the response signal a signal from a local oscillator 100.
  • the local oscillator is an Agilent E8257D, set to a frequency such that the desired sideband is mixed down to DC; in this example, the local oscillator frequency is 14GHZ + 100kHz.
  • a small additional frequency offset is employed such that the 100kHz quiescent frequency does not down convert exactly to DC which would be blocked by the AC coupling of some stages in this example embodiment of the invention.
  • the downshifted signal is next fed into audio frequency sub-systems 105-126, which require additional power, which in this example is supplied by a commercial off-the-shelf power supply unit.
  • Two pairs of audio-band amplifiers 105a,b, 1 15a,b are employed (one of each pair 105a, 1 15a for the l-component of the downshifted signal, the other of each pair 105b, 1 15b for the Q-component of the downshifted signal).
  • the first pair 105a,b magnify the downshifted signal output from the mixer 90, prior to the low-pass filtering.
  • a second pair of audio-band amplifier 1 15a,b are employed to magnify the low-pass-filtered output prior to up-mixing.
  • the amplifiers 105a,b, 1 15a,b exhibit a low-pass response, where the gain rolls off at frequencies above 10kHz with a response of -3dB/octave or better.
  • each amplifier pair 105a,b, 1 15a,b are matched such that their gain profiles are within 5% of each other.
  • the audio amplifiers 105a,b, 1 15a,b are AC-coupled with a time constant of at least 1 second.
  • the low-pass filters 1 10a,b, between the amplifiers have a passband from 0-10kHz and have a sufficiently high stop-band attenuation rate that signals at 100kHz are attenuated by at least 100dB.
  • the filter is a D100L8L-10kHz, an 8-pole Bessel low pass filter, which exhibits a stop band attenuation rate of 48dB/octave.
  • the I and Q channels of the filtered downshifted signal are frequency up-converted to produce an up-shifted signal in the band of a commercial off-the-shelf FM demodulator.
  • Both the I and Q channels are processed simultaneously and their phases with respect to each other preserved: as shown in Fig. 4, that is achieved using a pair of audio mixers 125a, 125b and a 455kHz oscillator 124 which is capable of generating two quadrature local oscillator signals.
  • a differential amplifier 127 is used to perform image rejection.
  • the 445kHz quadrature oscillator 124 is a square wave oscillator with two identical frequency outputs, one of which is phase shifted by 90°; such an arrangement can be readily achieved using commercial off-the shelf hardware.
  • the two mixers 125a, 125b are double-balanced mixers to reduce the input and local oscillator breakthrough to the amplifier stage 122.
  • a differential amplifier 122 with a bandwidth of approximately 1 MHz is utilised to perform the image rejection.
  • a commercial off-the-shelf FM demodulator IC 126 regenerates the original time-varying message voltage signal.
  • the demodulator 126 is a Japan Radio Company component NJM2590 in an SSOP14 package.
  • one option is to replace the receive antenna 80 with an RF circulator, and thereby to use the transmit antenna 30 simultaneously as a receive antenna. That would simplify the antenna configuration and beam pointing requirements, but is likely to significantly increase carrier breakthrough.
  • the receive antenna 50 and the transmit antenna 70 of the transponder 40 could be replaced with an arrangement including just the transmit antenna 70, with an RF circulator coupling signals received at that transmit antenna 70 to the signal processing block 60.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Radar Systems Or Details Thereof (AREA)
  • Near-Field Transmission Systems (AREA)

Abstract

L'invention concerne un procédé de communication avec un système de communication déployé, par exemple un transpondeur (40), dans lequel un signal de réponse est reçu du transpondeur (40) en réponse à un signal d'interrogation qui a été émis vers le transpondeur (40) à une première fréquence porteuse (210). Le signal de réponse comprend : (i) le signal d'interrogation décalé par le transpondeur vers une seconde fréquence porteuse (220) différente, la seconde fréquence porteuse (220) étant séparée de la première fréquence porteuse par une fréquence de décalage; (ii) un signal de message du transpondeur (40), porté sur le signal d'interrogation décalé (220); et (iii) des signaux non souhaités, à la première fréquence porteuse (210), qui résultent de réflexions du signal d'interrogation. Un filtre passe-bas (110) possède une fréquence de coupure inférieure à la fréquence de décalage. Le signal de réponse est mélangé avec un signal à une fréquence séparée de la seconde fréquence porteuse par au plus la fréquence de coupure du filtre passe-bas (110) moins la moitié de la bande passante du signal de réponse, pour donner un signal abaissé comprenant le signal d'interrogation décalé, à nouveau décalé vers une fréquence inférieure à la fréquence de coupure, et les signaux non souhaités décalés vers une fréquence (210') supérieure à la fréquence de coupure. Le filtre passe-bas (110) est utilisé pour éliminer les signaux non souhaités dans le signal abaissé et pour produire ainsi un signal abaissé et filtré. Le signal de message est extrait du signal abaissé et filtré.
PCT/GB2012/050649 2011-03-25 2012-03-23 Procédé et appareil de communication rf WO2012131337A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB1105032.5A GB2489416A (en) 2011-03-25 2011-03-25 A transponder shifts a received interrogation signal in frequency and uses the shifted signal as a carrier signal for data transmissions to a reader
GB1105032.5 2011-03-25

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WILLIAMSON F R ET AL: "A CODED RADAR REFLECTOR FOR REMOTE IDENTIFICATION OF PERSONNEL AND VEHICLES", PROCEEDINGS OF THE NATIONAL RADAR CONFERENCE. LYNNFIELD, APR. 20 - 22, 1993; [PROCEEDINGS OF THE NATIONAL RADAR CONFERENCE], NEW YORK, IEEE, US, vol. -, 20 April 1993 (1993-04-20), pages 186 - 191, XP002065550 *

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