GB2489416A

GB2489416A - A transponder shifts a received interrogation signal in frequency and uses the shifted signal as a carrier signal for data transmissions to a reader

Info

Publication number: GB2489416A
Application number: GB1105032.5A
Authority: GB
Inventors: Alan David Hart; Adam Armitage; Amanda Kay Sinden; Duncan Christopher Fredrick Levett
Original assignee: MBDA UK Ltd
Current assignee: MBDA UK Ltd
Priority date: 2011-03-25
Filing date: 2011-03-25
Publication date: 2012-10-03
Also published as: WO2012131337A1; GB201105032D0

Abstract

In method of communicating with a deployed communication system, for example a transponder (40), a response signal is received from the transponder (40) in response to an interrogation signal that has been transmitted to the transponder (40) at a first carrier frequency, fcarrier1 (210 in Fig. 5a). The response signal comprises the interrogation signal shifted by the transponder to a second, different, carrier frequency, fcarrier2 (220, Fig. 5a). The second carrier frequency (220) is separated from the first carrier frequency by a shift frequency, fshift. A message signal from the transponder (40) is carried on the shifted interrogation signal (220). The response signal also contains unwanted signals, at the first carrier frequency (210), caused by reflections of the interrogation signal. A low-pass filter (110) has a cut-off frequency lower than the shift frequency. The response signal is mixed 90 with an oscillator signal 100 at or close to the second carrier frequency to produce a downshifted signal f0 comprising the shifted interrogation signal further shifted to baseband, or at least to a frequency lower than the cut-off frequency, and the unwanted signals shifted to a frequency fshift (210', Fig. 5b) or at least to a frequency higher than the cut-off frequency. The message signal is extracted from the filtered downshifted signal.

Description

AN RF COMMuNIcATION METHOD AND APPARATUS

FIELD OF THE INVENTION

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.

BACKGROUND ART

In a conventional two-way RF communications system between two devices, the power budget is typically symmetrical; i.e. both devices radiate approximately equal amounts of RF power. In 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. There are potential applications for a passive or very low power data link that require the link to operate in a high-clutter environment, i.e. an environment in which reflections from objects in the environment are large. In such cases, 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.

DISCLOSURE OF THE INVENTION

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; (2) providing a low-pass filter having a cut-off frequency lower than the shift frequency; (3) mixing 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; (4) using the low-pass filter to remove the unwanted signals from the downshifted signal and thereby to produce a filtered downshifted signal; and (5) extracting the message signal from the filtered downshifted signal.

Advantageously, 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. Alternatively, 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.

It may be that 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.

It may be that, in the response signal, the shift frequency is less than I 0 octaves, or even less than io° 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 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.

It may be that, in the downshifted signal, the frequency separation between the shifted unwanted signals and the nearest edge of the bandwidth of the message signal (on the turther shifted interrogation signal) is more than 0.1 octave, preferably more than I 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.

It may be that the transmission of the interrogation and the receiving of the response signal are carried out using a common antenna.

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: (1) a low-pass filter having a cut-off frequency lower than the shift frequency, and (2) 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: (1) receiving an interrogation signal that has been transmitted at a first carrier frequency; (2) forming a response signal in the form of 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 (3) transmitting the response signal to a receiver as claimed in claim 1 0.

A fifth aspect of the invention provides a deployable communication system comprising: (1) an antenna for receiving an interrogation signal that has been transmitted at a first carrier frequency; (2) 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; and (3) an antenna for transmitting the response signal to a receiver.

It may be that 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.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the first aspect of the invention may incorporate any of the features described with reference to the receiver of the second aspect of the invention and vice versa.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: 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 10 frequency up-mixer from the transceiver of Fig. 3; and 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.

For convenience and economy, the same reference numerals are used in different figures to label identical or similar elements of the apparatus shown.

DETAILED DESCRIPTION

In an example communication system according to an embodiment of the invention, 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. In this example embodiment, as described further below, frequency modulation is tailored to generate a sngie side band (555) 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 1 00kHz sideband contains the required data in frequency modulation (FM) form, which can then be decoded using standard techniques.

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 fcarrieri, in this example 14 0Hz, 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. in the 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 fcarrfer2 separated from the carrier frequency fcaeri by a sideband-carrier separation frequency fshlft. The response signal is fed to a transmission antenna 70, which transmits the response signal to the receiver 10. As shown in Fig. 5(a), the response signal includes the original carrier fcarrieri, as well as the shifted carrier fcarr/er2, 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 fcarrfer2, and so the mixing results in a downshifted signal, with the component of the signal previously at fcarrjer2 shifted substantially to 0 kHz, and the component of the signal previously at fcarrjerj shifted substantially to the transponder shift frequency fshfft (see Fig. 5(b)). The downshifted signal is fed to a low-pass filter 110, which removes the downshifted carrier signal (i.e. the component of the signal now at the transponder shift frequency fshjft) to produce a filtered downshifted signal at f0, i.e. substantially 0 kHz. 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 f1 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.

In this example embodiment, 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.

The system block diagram of 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.

In the example system of Fig. 3, the 14 GHz carrier frequency 210 (i.e. the signal resulting from environmental reflections) is separated from the 20kHz wide FM sideband 220 (i.e. the shifted carrier signal transmitted by the transponder) by approximately 1 00kHz, as shown in Figure 5(a). 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.

Using a mixer technique, 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.

In the transmitter 5, the 140Hz OW 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 11 Oa, 11 Ob is utilised to reject the carrier from both the I and 0 output of the mixer 90. The filter output is amplified by amplifiers 105a, 1 05b 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 blocks of the system of Fig. 3 will now be described in more detail.

In the transmitter 5, the transmit oscillator 22 generates the 140Hz OW 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 110kHz from the carrier. This example embodiment employs an Agilent E82570 as the transmit oscillator 22, which has extremely stable phase characteristics and exhibits - 11 3d Bc/Hz at 1 00kHz 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.

In this example, the amplifier is a Miteq amplifier model number MPN4- 1 800-23 F. 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. In this example, the AF isolator 26 is a Ditom model number D3l7018.

The interrogation signal then passes to the transmit antenna 30, which in this example is a lens horn antenna model number 0W820-GA, manufactured by Flann Microwave.

Turning to the receiver 10, in this example, 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. In this example embodiment, the limiter 82 is Miteq's part number MPLO2O18-2-33. 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. In this example, the amplifier 84 is a Miteq amplifier part number AFS-02001 800-24-1 OF-4.

The RF quadrature mixer 90 is employed to down-convert the RF response signal to baseband for further processing. In this example, the RF quadrature mixer 90 is a Miteq component part number IRO218LC1Q. The quadrature mixer 90 mixes with the response signal a signal from a local oscillator 100. In this example, 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 + 1 00kHz. In practice, 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 1 05a,b, 11 5a,b are employed (one of each pair 1 05a, 11 5a for the I-component of the downshifted signal, the other of each pair 105b, 115b for the 0-component of the downshifted signal). The first pair 1 05a,b magnify the downshifted signal output from the mixer 90, prior to the low-pass filtering. A second pair of audio-band amplifier 115a,b are employed to magnify the low-pass-filtered output prior to up-mixing. The amplifiers 105a,b, 115a,b exhibit a low-pass response, where the gain rolls off at frequencies above 10kHz with a response of -3dB/octave or better. The gain of each amplifier pair 1 05a,b, 11 5a,b are matched such that their gain profiles are within 5% of each other. The audio amplifiers 105a,b, 115a,b are AC-coupled with a time constant of at least 1 second.

The low-pass filters 11 Oa,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. In this example, the filter is a Dl 00L8L-1 0kHz, an 8-pole Bessel low pass filter, which exhibits a stop band attenuation rate of 48dB/octave.

Next, the I and 0 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 0 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.

Finally, a commercial off-the-shelf FM demodulator IC 126 regenerates the original time-varying message voltage signal. In this example, the demodulator 126 is a Japan Radio Company component NJM2590 in an SSOP14 package.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein.

For example, 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.

Similarly, 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.

The specific embodiment described above employs a single mixer 150 to achieve both the frequency shift and the FM encoding of the message signal, in alternative embodiments of the invention the encoding of the message is achieved (by other conventional encoding methods, e.g. AM, P3K, FSK, ASK or OFDM) either before or after the frequency shifting by the mixer.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may be absent in other embodiments.

Claims

CLAIMS1. A method of communicating with a 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; (2) providing a low-pass filter having a cut-off frequency lower than the shift frequency; (3) mixing 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; (4) using the low-pass filter to remove the unwanted signals from the downshifted signal and thereby to produce a filtered downshifted signal; and (5) extracting the message signal from the filtered downshifted signal.
2. A method as claimed in claim 1, in which the deployed communication system is a transponder
3. A method as claimed in claim I or claim 2, further including the step of transmitting the interrogation signal to the deployed system.
4. A method as claimed in claim 1 or claim 2, in which the transmission of the interrogation signal to the deployed system has been carried out by non-cooperating radio sources.
5. A method as claimed in any preceding claim, in which the extraction of the message signal from the filtered downshifted signal includes 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.
6. A method as claimed in any preceding claim, in which the response signal is mixed with a signal substantially at the second carrier frequency.
7. A method as claimed in any preceding claim, in which, in the downshifted signal, the shifted interrogation signal has been shifted substantially to 0 Hz.
8. A method as claimed in any preceding claim, in which, in the downshifted signal, the unwanted signals have been shifted substantially to the shift frequency.
9. A method as claimed in any of claims 1 to 5, in which, in the downshifted signal, the shifted interrogation signal has been shifted to a frequency larger than or approximately equal to half the bandwidth of the message signal.
10. A method as claimed in any preceding claim, in which the shift frequency is between 2 and 10 times the bandwidth of the message signal.
11. 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: (1) a low-pass filter having a cut-off frequency lower than the shift frequency, and (2) 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.
12. A communications device including a receiver as claimed in claim 11, and further comprising a transmitter for transmitting the interrogation signal to the deployed system.
13. A communications device as claimed in claim 12, including an antenna shared by the transmitter and the receiver.
14. A method of communicating from a deployed system, the method comprising the deployed system: (1) receiving an interrogation signal that has been transmitted at a first carrier frequency; (2) forming a response signal in the form of 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 (3) transmitting the response signal to a receiver as claimed in claim 11.
15. A deployable communication system comprising: (1) an antenna for receiving an interrogation signal that has been transmitted at a first carrier frequency; (2) 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; and (3) an antenna for transmitting the response signal to a receiver.
16. A deployable communication system as claimed in claim 15, said deployable system being a transponder.
17. A deployable communication system as claimed in claim 16, configured to provide no overall gain between the received interrogation signal and the transmitted response signal.