Patent Description:
Throughout this specification, the following definitions are employed:.

The need to transmit signals (e.g., RF, acoustic or seismic signals) back towards a transmitting source (such as a wireless or RF device/system, a satellite device/system, an acoustic device/system, a seismic device/system and the like) in a precise manner arises in many communication systems and applications. When knowing where exactly the transmitting source is located, it can be assured that the transmitted signal reaches said source. This issue can be especially important, when communicating with distant sources, such as satellites, and/or when the accuracy in transmitting the signal in a precise direction is important.

The problem of transmitting RF signals back towards the transmitting source has been recognized in the prior art, and various systems have been developed to provide a solution, disclosing various time-reversal and phase-conjugation techniques. For example, a prior art method called "heterodyning" or "heterodyne mixing" is a phase conjugation method, according to which a signal received at each antenna is multiplied by means of a local oscillator, having a frequency which is two times of the frequency of the received signal; the signal is then filtered via low pass filter (LPF). The method may handle both far as well as near-field targets, and can be implemented in various antenna array geometries. Heterodyne mixing is a narrowband method that may handle only signals of known frequencies; otherwise, it requires frequency estimation of the received signals. Furthermore, the heterodyne mixing method cannot handle simultaneous signals.

In addition, there are several prior art systems, such as phased arrays. For using such arrays, determining an angular direction of the transmitting source is required, as well as also calculating the phase differences of the signal to be transmitted towards the source from each antenna of said phased array. It should be noted that calculating the signal phase differences requires relatively complex processing, and also the usage of a phase shifter is required. In addition, the accuracy in calculating the phase differences of the signal depends on how accurately an angular direction of the source is determined. Often, prior to being received by means of antennas, a RF signal is reflected from several accidental reflectors, such as tower blocks, balconies and the like. Thus, measurements of the source angular direction depend on such accidental factors, which in turn lead to receiving incorrect results. Further, for improving the measurements when operating at relatively high frequencies, e.g., above <NUM> (GigaHertz), the antennas have to be positioned relatively close to one another, which may cause juxtaposition of antennas within said phased array (especially, when receiving RF signals), and can also cause antennas to overheat during RF signals transmission. Moreover, this can result in disturbances in calculating phase differences of the signal to be transmitted from each antenna. Usually, the antennas within the phased array have to be positioned on a straight plane in order to be able to further determine and calculate the required angular direction of the signal to be transmitted by means of each antenna (in order to get the greatest overall ERP (Effective Radiated Power) in the direction to which the RF signal is transmitted).

For example, <CIT> relates to a mobile radio base station capable of communicating with a large number of mobile stations by implementing space diversity and time-division retransmission techniques in a digital communication system. The digital base station contains a plurality of antenna elements and a plurality of retransmission branches associated in a one-to-one relationship. When the base station transmits a signal back to the mobile station, each retransmission branch adds the conjugate of its associated random phase to the signal to be transmitted, allowing the environment to "undo" the effect of the conjugate random phase so that the signals transmitted by the plurality of antenna elements will arrive coherently at the mobile station. According to <CIT>, the retro-directivity is achieved by analog implementation of the phase conjugation for transmitting narrowband signals with known frequencies, each narrowband signal at the same time.

Further, <CIT> discloses a system and method for automatically generating a return beam in the direction of a received beam. The system includes a phased array antenna for receiving a radio frequency signal having a first wavefront from a first direction. In response to this signal, the second signal is provided having a second wavefront. The second signal is a phase conjugate of the first signal and is transmitted in a reverse direction relative to the direction of the first signal.

Also, according to the prior art, a corner reflector can be used for reflecting electromagnetic waves back towards the transmitting source. The RF wave hits the surface of the corner reflector, and due to its unique structure, the wave is reflected back towards the source. The corner reflector can be used for communication applications, where signals have to be immediately reflected back to the source. For example, by placing such a reflector on the Moon, this can help measure the Moon's orbit in a more accurate manner. According to <CIT>, a detailed mapping of the magnetosphere is made possible by deploying hundreds of attitude-impervious micro-satellites, in the form of small corner reflectors with piezoelectric mirror surfaces, from a single mother satellite at spacings of as little as <NUM> in equatorial and elliptical orbits. The micro-satellites carry magneto-sensors whose output is transmitted to a ground station by modulating the reflection of a laser beam transmitted to the micro-satellite by the ground station. <CIT> relates to transmitting narrowband signals of a known frequency, each narrowband signal at the same time.

In addition, the prior art teaches about a Van Atta reflector array (<CIT>) that is an array, in which elements are interconnected to reradiate received energy back in the direction of arrival. The Van Atta reflector array requires ULA (Uniform Linear Array) geometry and can handle signals received from far sources to ensure planar wavefronts.

The International Patent Application <CIT> discloses to retro-transmit signals received at a base station back to a mobile terminal, the retro-transmission enabled by either applying phase conjugation or applying a time reversal of samples.

There is a continues need in the prior art to provide a digital retro-directive method and system configured to enable transmitting a signal from a plurality of antennas towards a transmitting source in a substantially simultaneous (synchronous) manner and in a substantially precise angular direction, without the need to measure such direction.

Also, there is a need to provide a retro-directive method and system that enables simultaneous transmitting of signals from each antenna back towards a transmitting source without calculating signals phase differences, and further enables transmitting signals having unknown frequencies. In addition, there is a need o provide a retro-directive method and system, wherein there is no need in positioning antennas on a straight plane for enabling said simultaneous transmitting.

Moreover, there is a need to provide a retro-directive method being applicable for both narrow and wideband communication systems, further enabling simultaneous multisignal communication with multiple far-distanced and/or near-field transmitting sources.

The present invention relates to a digital retro-directive method and system configured to receive signals (such as RF signals, electromagnetic signals, acoustic, seismic signals, or photonic signals, etc.) from one or more transmitting sources (such as a wireless or RF device/system, a satellite device/system, an acoustic device/system, a seismic device/system, a photonic device/system, and the like), and transmit signals back towards said transmitting sources from a plurality of antennas (e.g., antenna elements) in a substantially synchronous manner and in a substantially precise angular direction, without the need to measure such direction.

In order to understand the invention and to see how it may be carried out in practice, preferred examples will now be described, by way of non-limiting examples only, with reference to the accompanying drawings, in which:.

In other instances, well-known methods, systems, procedures, techniques, units/components and the like have not been described in detail so as not to obscure the present invention.

It is emphasized, that many of the given embodiments and examples do not fall under the scope of the invention, however, are useful to understand the subject-matter of the claimed invention.

<FIG> is a schematic illustration of a digital retro-directive system <NUM> for receiving signals from one or more transmitting sources, such as a source <NUM> and transmitting signals back towards said source <NUM> from a plurality of antennas in a substantially simultaneous (synchronous) manner and in a substantially precise angular direction, according to an embodiment. According to this embodiment , system <NUM> is a multi-channel system, comprising a plurality of antennas (such as two or more antennas <NUM>', <NUM>" and <NUM>"', for example), wherein each antenna is connected to its corresponding communication unit <NUM>. Communication unit <NUM> comprises a receiver <NUM> for receiving an incoming signal (such as an RF signal, an electromagnetic signal, an acoustic signal, a seismic signal, a photonic signal, etc.); an A/D (Analog-to-Digital) unit <NUM> for sampling the received signal, giving rise to signal samples; a digital memory unit, such as a DRFM (Digital Radio Frequency Memory) unit <NUM> for storing the signal samples in its memory to be reconstructed later; a digital retro-processing unit <NUM> for reconstructing a signal from signal samples stored within DRFM unit <NUM>, said samples used as carriers for data to be transmitted, giving rise to a reconstructed digital signal; an D/A (Digital-to-Analog) unit <NUM> for converting said reconstructed digital signal to a reconstructed analog signal; and a transmitter <NUM> for transmitting the reconstructed analog signal back towards source <NUM>. Hereinafter, for simplicity, the signals are considered to be RF signals; however, it should be noted that the signals can be of any type, such as electromagnetic (EM) signals, acoustic signals, seismic signals, photonic signals, etc..

According to an embodiment, the RF signal transmitted from transmitting source <NUM> (such as, a wireless or RF device/system (e.g., a cellular system, a Wi-Fi system, a radar system, etc.), a satellite device/system, an acoustic device/system, a seismic device/system, and the like) is received by means of antennas <NUM>', <NUM>" and <NUM>‴. Then, the received signal is sampled by means of corresponding unit, such as A/D unit <NUM>. It should be noted that samples are performed substantially simultaneously by means of A/D units <NUM> of said antennas <NUM>', <NUM>" and <NUM>‴. For example, for each time period of a double bandwidth length of the received RF signal (according to the Nyquist-Shannon sampling theorem), one sample is made by means of each A/D unit <NUM>. Each sample contains information regarding the amplitude and phase of the signal at a specific time instant (e.g., t=<NUM> [nsec] (nanoseconds), <NUM> [nsec], etc.).

For example, it is supposed that the received signal ϕR,n(t) is defined by ϕR,n(t)= AR · cos(wt + φR,n), wherein AR is an amplitude, w is a radian frequency, t is a time instant, φR,n is a phase, n is an antenna serial number (e.g., <NUM>, <NUM>,. ), and R symbolizes that the signal is received. Then, for example, at t=<NUM> and t=<NUM> [nsec], signal ϕR,n(t) received by means of antenna <NUM>' (n=<NUM>) is sampled, and the following samples are obtained: <MAT>.

According to an embodiment of the present invention, the signal samples (a set of signal samples) ϕR,n(<NUM>) and ϕR,n(<NUM>) are stored in the sampling order, on a time scale, within corresponding DRFM units <NUM>, and then such samples can be used as carriers for data to be transmitted back towards transmitting source <NUM>. Said samples contain information regarding amplitude AR and reception phase φR,n of the signal received from said source <NUM> at each antenna <NUM>', <NUM>" and <NUM>‴. The incoming signal can be sampled, for example, during <NUM> [µsec](microseconds), while each sample can be taken each <NUM> [nsec]. It should be noted that since all samples are performed substantially simultaneously by means of A/D units <NUM> of said antennas <NUM>', <NUM>" and <NUM>‴, then the set of signal samples, stored within each DRFM unit <NUM>, is substantially identical.

According to an embodiment, the signal samples are reordered in an inverse sampling order by means of processing unit <NUM>, such that the first sample becomes the last one, for a predefined period of time (e.g., for <NUM> microsecond), resulting in the time-reversal of the signal. Then, a new RF (digital) signal is reconstructed by using such inverted samples as data carriers, and then it is converted to an analog signal by means of each corresponding unit, such as D/A (Digital/Analog) unit <NUM> of each antenna <NUM>', <NUM>" and <NUM>‴. After this, it is transmitted back towards source <NUM>. As a result, , the signal is transmitted towards source <NUM> without measuring an angular direction of said source <NUM>, and without calculating the phase differences of the signal to be transmitted from each antenna within a predefined set (array) of antennas, such as antennas <NUM>', <NUM>" and <NUM>‴ (it should be noted that according to the prior art, the signal phase differences for each antenna have to be calculated for achieving the optimal effective radiation pattern (ERP) in a desired angular direction). For example, it is supposed that the first sample of the incoming signal received by antenna <NUM>' is R<NUM>, the second sample is R<NUM>, the third sample is R<NUM> and the fourth sample is R<NUM>. Thus, by inverting an order of the samples on the time scale, such that the first sample is R<NUM>, the second is R<NUM>, the third is R<NUM> and the fourth sample is R<NUM>, and then using said samples as data carriers of the signal to be transmitted, enables transmitting said signal back towards the source in a substantially precise angular direction. Similarly, the samples of antenna n are reordered such that the first sample is Rn4, the second is Rn3, the third is Rn2 and the fourth sample is Rn1. Further, according to an embodiment, there is no need in positioning (locating) antennas <NUM>', <NUM>" and <NUM>‴ on a straight plane because there is no need in determining phase differences of the outgoing signal to be transmitted back towards transmitting source <NUM> from each antenna. It should be noted that since the set of signal samples, stored within each DRFM unit <NUM>, is substantially identical (because all samples are performed substantially simultaneously by means of A/D units <NUM> of said antennas <NUM>', <NUM>" and <NUM>‴), then performing D/A conversion by means of D/A units <NUM> of said antennas <NUM>', <NUM>" and <NUM>'" is also done substantially simultaneously at each antenna. As a result, reconstructed signals are transmitted back towards source <NUM> substantially simultaneously (synchronously) from each antenna (e.g., from antennas <NUM>', <NUM>" and <NUM>‴) within a set of antennas, and then, in turn, said reconstructed signals are coherently received at transmitting source <NUM>. It should be noted that according to an embodiment, each antenna <NUM>', <NUM>", and <NUM>‴ is an antenna element within a set of mutually synchronized antenna elements, such as shown on <FIG>. Also, according to another embodiment of the present invention, each antenna <NUM>', <NUM>", and/or <NUM>‴ comprises two or more mutually synchronized antenna elements, wherein each antenna element can have a communication unit, such as unit <NUM>.

Further, it should be noted that according to an embodiment, by inverting the order of signal samples, the modulation of the transmitted (outgoing) signal ϕT,n(t) (compared to the modulation of the received (incoming) signal ϕR,n(t)) is also inverted. For example, if the modulation of the received signal is linear, then the modulation of the transmitted signal is also linear, but inverted on a time scale. According to an embodiment of the present invention, transmitting source <NUM> is configured to transmit incoming RF signals and receive back outgoing RF signals, reconstructed from samples of said incoming RF signals, wherein for reconstructing the outgoing RF signals the samples are organized in an inverted sampling order, on a time scale. Transmitting source <NUM> receives said outgoing RF signals from at least one antenna, demodulates them and processes the demodulated data. According to another embodiment, for demodulating said outgoing RF signals, transmitting source <NUM> is configured to determine that these RF signals have inverted modulation. For this, transmitting source <NUM> can be provided with corresponding software and/or hardware processing means.

It is supposed, for example, that the incoming RF signal, received by means of antenna <NUM>', is defined by ϕR,n(t) = A · cos(wt + φR,n), wherein A is an amplitude of said signal, w is a radian frequency, t is a time parameter, φR,n is a phase, n is an antenna serial number (e.g., <NUM>, <NUM>,. ), and the letter R symbolizes that the signal is received. The requirement for getting a constructive interference in the angular direction, from which the signal is received, is: ∀n, φR,n + φT,n = const, wherein const is any constant (e.g., <NUM>, <NUM>, -<NUM>, etc.) substantially identical for all antenna elements, and φT,n is the phase of the signal to be transmitted from antenna n. This requirement is equivalent to ∀n,m φT,n - φT,m = -(φR,n - φR,m), and therefore, it ensures that all signals arrive to source <NUM> with the same phase. Thus, the transmitted (output) RF signal may be represented as: <MAT> wherein φT,n is a phase of the transmitted signal, T' relates to a predefined period of time (e.g., <NUM> microsecond), T symbolizes that the signal is transmitted, and const is selected as: -ωT' (w is a radian frequency). From the above it follows that if the signal is transmitted from its end to the beginning (on a time scale, by inverting an order of signal samples, used as data carriers), a constructive interference in a direction from which the signal was received is achieved. As a result, each reconstructed RF signal is transmitted back towards source <NUM> from each corresponding antenna <NUM>', <NUM>" and <NUM>‴ in a substantially precise angular direction.

It should be noted that according to an embodiment, each reconstructed RF signal is transmitted back towards source <NUM> by means of the same antenna (e.g., antenna <NUM>', <NUM>" or <NUM>‴), through which the original corresponding signal from source <NUM> was received. Further, it should be noted that according to an embodiment of the present invention, the reconstructed RF signals are transmitted back towards source <NUM> in a substantially simultaneous (synchronous) manner from each antenna within a set of antennas (e.g., the set of antennas comprising two or more antennas, such as antennas <NUM>', <NUM>" and <NUM>‴).

According to another embodiment the multi-pass communication with two or more transmitting sources <NUM> is enabled, without the need in determining phases of the signals to be transmitted back towards said sources from a plurality of antennas <NUM>', <NUM>" and/or <NUM>‴ (without the need in determining angular directions of said two or more transmitting sources), and without the need in positioning the plurality antennas on a straight plane. In addition, said two or more transmitting sources <NUM> can be either far-distanced and/or located in proximity (near-field) to said plurality of antennas <NUM>', <NUM>" and/or <NUM>‴. It should be noted that each source can transmit/receive a different RF signal. Further, it should be noted that according to an embodiment of the present invention, antennas <NUM>', <NUM>" and/or <NUM>‴ (their corresponding communication units <NUM>) are not synchronized with transmitting source <NUM>, i.e., are asynchronous to said source <NUM> either at the time and/or frequency domain.

According to an embodiment, system <NUM> can be used in various applications, such as medical, military, space applications and the like. For example, by placing a set (plurality) of antennas around a human body, positioning a transmitting source (transmitter) within a cancerous growth inside the body, then receiving a signal from said transmitter, and after this transmitting a signal back to said source from the plurality of antennas in a substantially synchronous manner, can destroy such a cancerous growth at its exact location, and not accidentally damage other organs within the body.

According to another embodiment after the incoming signal is sampled by means of corresponding communication unit <NUM> of each antenna (by means of its corresponding A/D unit <NUM>), the phase sign of the signal received at each antenna is inverted by means of digital retro-processing unit <NUM>, resulting in obtaining phase conjugation of the signal. It should be noted that phase conjugation corresponds to selecting const (∀n, φR,n + φT,n =const) to be equal "<NUM>", and therefore ∀n, φT,n = -φR,n and, as result, the retro-directive transmission is enabled. This can be achieved, for example, by digitally multiplying each signal sample by a function/signal, such as cosine(2ωt), and then digitally low-pass filtering the result of the above multiplication. Also, it should be noted that signal sampling is performed substantially synchronously by means of A/D units <NUM> of each antenna <NUM>', <NUM>" and <NUM>‴. The phase conjugated samples are stored within DRFM unit <NUM> of each antenna, and the amplitude of each signal sample is related to a particular time instant (e.g., t=<NUM> [nsec], <NUM> [nsec], etc.). Since all samples are obtained substantially simultaneously, the set of signal samples, stored within each DRFM unit <NUM>, is substantially identical (up to a time shift). It should be noted that according to an embodiment the signal samples can be used later as carriers for transmitting data back towards source <NUM>.

It is supposed, for example, that received signal ϕR,n(t) is defined by ϕR,n(t) = AR · cos(wt + φR,n) and transmitted signal ϕT,n(t) is defined by ϕT,n(t) = AT ·cos(wt + φT,n), wherein AR and AT are amplitudes, w is a radian frequency, t is a time parameter, φR,n and φT,n are corresponding phases, n is an antenna serial number (e.g., <NUM>, <NUM>,. ), the letter R symbolizes the signal that is received, and the letter T symbolizes the signal that is transmitted. The requirement for getting a constructive interference in a direction from which the signal was received is: ∀n, φR,n + φT,n =const, wherein const is any constant (e.g., <NUM>, <NUM>, -<NUM>, etc.) substantially identical for all antenna elements. Thus, for example, for const=<NUM>, signal ϕT,n(t) to be transmitted can be such as: ϕT,n(t)= AT cos(ωt + φT,n)= AT cos(ωt + (const -φR,n))= AT cos(ωt - φR,n). Signal ϕT,n(t) is transmitted towards source <NUM> without measuring an angular direction of said source <NUM>, and without calculating the phase differences of the signal to be transmitted from each antenna (it should be noted that according to the prior art, the signal phase differences for each antenna have to be calculated in order to achieve an effective radiation pattern in a desired angular direction). As a result, there is also no need in positioning antennas <NUM>', <NUM>" and <NUM>‴ on a straight plane because there is no need in determining the above signal phase differences.

According to an embodiment, the phase conjugation used for transmitting signals back towards transmitting source <NUM> may be applied to each frequency component of a wideband signal: <MAT> <MAT> wherein ϕR,n(t) is a received signal, ϕT,n(t) is a transmitted signal, AR,l and AT,l are amplitudes, wi is a radian frequency, t is a time parameter, <MAT> and are <MAT> are phases of corresponding received and transmitted signals, n is an antenna serial number (e.g., <NUM>, <NUM>,. ), and i is an index varied from <NUM> to L. This may be achieved by performing spectral decomposition of said wideband signal and performing phase conjugation of each component, such that: <MAT> wherein FFT is a Fast Fourier Transform, spectrally decompositing the signal to its frequency components; IFFT is an Inverse FFT; conj relates to obtaining a complex conjugate of a result of the FFT; and N is the number of samples to be retro-transmitted, i.e., N is the FFT length.

It should be noted that the above wideband phase conjugation is mathematically equivalent to the time reversal (of the cyclic continuation of the signal to be conjugated); this arises due to phase anti-symmetry of the Fast Fourier Transform. In addition, it should be noted that according to another embodiment of the present invention, other spectral decomposition analyses or other transforms can be implemented instead of the Fast Fourier Transform and Inverse Fast Fourier Transform.

According to an embodiment, for a narrowband signal, the phase shift Δφn in Δφn = φT,n - φR,n = const - <NUM>φR,n (φR,n + φT,n =const ) is equivalent to the time-reversal (through the term -<NUM>φR,n is responsible for the retro effect) and an arbitrary time delay <MAT>. It should be noted that const is any constant (e.g., <NUM>, <NUM>, -<NUM>, etc.); φT,n is the phase of the signal to be transmitted; φR,n is the phase of the received signal; and w is a radian frequency (w= <NUM>π · f (f is a frequency).

According to another embodiment, this may be applied to each frequency component of a wideband signal: <MAT> <MAT> wherein <MAT> and hence ∀i, <MAT>. α is a constant; and i is an index. The definition of the constants Ci ensures that the time delays are identical for all frequency components from i=<NUM> to i=L. As a result, the constructive interference of all frequency components may be obtained by assigning any parameters α∈<IMG> (real number).

It should be noted that according to an embodiment, by using the above presented time reversal and phase conjugation methods, the constructive interference at transmitting source <NUM> is independent of the bandwidth and modulation of the transmitted signal. Also, an assumption that transmitting source <NUM> is far-distanced to allow linear phase front, is not required. In addition, since the above equations (of ϕR,n(t) and ϕT,n(t)) for a wideband signal may represent a group of narrowband transmitting sources as well, the requirement for <MAT> ensures the retro-effect (either phase conjugation or time reversal) independent of the number of transmitting sources and their locations.

Also, it should be noted that according to an embodiment, a constructive interference may be achieved by assigning any real value to parameter α , wherein α∈<IMG>. For example, Ci =<NUM> with α=<NUM> (for performing phase conjugation), or Ci = -ωl ·T with α=-T (for performing time reversal).

Further, it should be noted that according to an embodiment, the location of transmitting source <NUM>, and/or antennas <NUM>', <NUM>", <NUM>‴, etc., and/or any component/unit of communication unit <NUM> (such as receiver <NUM> and/or transmitter <NUM>) can be either static or mobile: for example, they can be airborne, sailed by a ship or carried by a vehicle, person, and the like. Also, it should be noted that if said transmitting source <NUM> and/or antennas <NUM>', <NUM>", <NUM>‴ are dynamically relocated, then an appropriate compensation for such relocation can be applied.

It should be noted that according to an embodiment, the reconstructed digital signal is transmitted back towards transmitting source <NUM> by means of, for example, two or more antennas <NUM>', <NUM>", <NUM>‴ with a delay (e.g., a second, a minute, an hour, and the like).

In addition, it should be noted that according to another embodiment , digital retro-processing unit <NUM> is incorporated within transmitter <NUM>.

Further, it should be noted transmitting source <NUM> may also receive signals transmitted towards it, and process the received signals accordingly.

<FIG> are schematic illustrations of digital retro-processing unit <NUM>, according to an embodiment of the present invention. Digital retro-processing unit <NUM> of <FIG> comprises a time-reversal unit <NUM>, which enables recording T seconds of samples of received signal ϕR,n(t), and then enables reordering the recorded samples in a reversed order, equivalent to assigning α=-T in equation Ci = α · wl , presented above. By recording T seconds of samples of received signal <MAT>, the following signal is obtained at each antenna (e.g., by ignoring the communication channel noise): <MAT> wherein A is an amplitude of the signal; w is a radian frequency; t is a time parameter; <MAT> is a phase of the ith spectral component of received signal <MAT>; n is an antenna serial number (e.g., <NUM>, <NUM>,. ); i is a signal index; and the letter R symbolizes that the signal is received.

According to an embodiment, the constructive interference is achieved by reordering the recorded signal samples from the end to beginning, such that the signal to be transmitted is: <MAT>.

From the above it follows that if the signal is transmitted from its end to the beginning (on a time scale, by inverting an order of signal samples, used as data carriers), a constructive interference in a direction, from which the signal was received, is achieved. As a result, each reconstructed RF signal is transmitted back towards source <NUM> (<FIG>) from each corresponding antenna <NUM>', <NUM>" and <NUM>‴ (<FIG>) in a substantially precise angular direction.

It should be noted that according to this embodiment, by inverting the order of signal samples, the modulation of the transmitted (outgoing) signal ϕT,n(t) (compared to the modulation of the received (incoming) signal ϕR,n(t)) is also inverted. For example, if the modulation of the received signal is linear, then the modulation of the transmitted signal is linear but inverted in time (time reversal). For example, if the received signal is characterized by having frequencies raising from the lower to higher frequency, then inverting an order of the signal samples results in obtaining a signal, in which frequencies are changed from the higher to lower frequency. According to an embodiment, transmitting source <NUM> is configured to transmit incoming RF signals and receive back outgoing RF signals, reconstructed from samples of said incoming RF signals, wherein for reconstructing the outgoing RF signals the samples are organized in an inverted sampling order, on a time scale. Transmitting source <NUM> receives said outgoing RF signals from at least one antenna, demodulates them and processes the demodulated data. According to another embodiment, for demodulating said outgoing RF signals, transmitting source <NUM> is configured to determine that these RF signals have inverted modulation. For this, transmitting source <NUM> can be provided with corresponding software and/or hardware processing means.

According to another embodiment, digital retro-processing unit <NUM> further comprises modulation unit <NUM>, as shown on <FIG> for modulating the signal with the inverted order of its samples. For this, time dependency t is added to the constant β. This leads to modulating the signal from transmitting source <NUM>, using the received signal as data carrier. Modulated transmitted signal ϕT,n(t) can be as follows: <MAT> wherein β(t) represents the amplitude modulation (AM).

According to still another embodiment, digital retro-processing unit <NUM> comprises phase conjugation unit <NUM> and, optionally, modulation unit <NUM>, as presented on <FIG>. According to this embodiment, after the incoming signal is sampled by means of corresponding communication unit <NUM> (<FIG>) of each antenna, a sign of the phase of each received signal is inverted by means of processing unit <NUM>, resulting in obtaining phase conjugation of the signal. This can be done, for example, by digitally multiplying each sample by a function/signal, such as cosine(2wt) , and then digitally low-pass filtering the result. Then, the phase conjugated samples (having inverted phase signs) are stored within DRFM unit <NUM> (<FIG>) of each antenna. It should be noted that the amplitude of each signal sample relates to a particular time instant (e.g., t=<NUM> [nsec], <NUM> [nsec], etc.). The signal samples can be used later as carriers for transmitting data back towards source <NUM> (<FIG>).

It should be noted that for the case of phase conjugation (for example, for narrowband signals), constant α=<NUM>, and in turn constant C=<NUM> ( <MAT> and C = α · w , wherein <MAT> is a phase of the received signal; <MAT> is a phase of the transmitted signal; n is an antenna serial number (e.g., <NUM>, <NUM>,. ); the letter R symbolizes that the signal is received, and the letter T symbolizes that the signal is transmitted). According to an embodiment of the present invention, the phase conjugated signal to be transmitted is modulated by means of modulation unit <NUM>. For this, time dependency t is added to the constant α. This leads to digitally modulating the signal from transmitting source <NUM>, using the received signal as data carrier: <MAT> wherein <MAT> is a phase of the transmitted signal; and <MAT> is a phase of the received signal. As a result, modulated transmitted signal ϕT,n(t) can be as follows: <MAT> wherein β(t) represents the amplitude modulation.

It should be noted that the phase conjugated signal to be transmitted can be modulated by its amplitude and phase by means of using parameters β(t) and α(t), respectively.

According to a further embodiment, digital retro-processing unit <NUM> comprises FFT (Fast Fourier Transform) unit <NUM>, phase conjugation unit <NUM>, IFFT (Inverse FFT) unit <NUM> and, optionally, modulation unit <NUM>, as presented on <FIG>. This embodiment can be used for receiving/transmitting wideband signals ϕR,n(t) and ϕT,n(t), wherein <MAT> is a phase of the transmitted signal and <MAT> is a phase of the received signal, and <MAT> (α=<NUM>, and in turn Ci=<NUM>, wherein i is a signal index).

Thus, the overall retro-transmitted wideband signal is given by: <MAT> wherein FFT is a Fast Fourier Transform, spectrally decompositing the signal to its frequency components; IFFT is an Inverse FFT; conj relates to obtaining a complex conjugate of a result of the FFT; and N is the number of samples to be retro-transmitted. It should be noted that phases <MAT> are obtained by means of the Fourier analysis, the phase conjugation ensures that <MAT>, and the inverse transform (IFFT) yields the phase conjugated signal. Further, the phase conjugated signal to be transmitted is modulated by means of modulation unit <NUM>. For this, time dependency t is added to constant α( Ci = α · wi). This leads to digitally modulating the signal from transmitting source <NUM>, using the received signal as data carrier: <MAT> wherein <MAT> is a phase of the transmitted signal; and <MAT> is a phase of the received signal. As a result, a modulated wideband transmitted signal ϕT,n(t) can be as follows: <MAT> wherein β(t) is the amplitude modulation of transmitted signal; and L is the number of simultaneously received narrowband signals. It should be noted that the phase conjugated signal to be transmitted can be modulated by its amplitude or phase by means of using parameters β(t) and α(t). Then, the modulated wideband signal ϕT,n(t) may be retro-transmitted, when required. This can be achieved by the following operation on the recorded (stored) samples: <MAT> wherein <MAT> (fs is a sampling frequency and N the FFT length).

<FIG> is a schematic illustration of a digital retro-directive system <NUM> for receiving signals from one or more transmitting sources, such as a source <NUM> and transmitting signals back towards said source <NUM> in a substantially simultaneous manner and in a substantially precise angular direction, according to another embodiment of the present invention. According to this embodiment, a corner reflector <NUM>' is provided, being connected to a plurality of antennas (such as two or more antenna elements <NUM>', <NUM>", <NUM>‴, for example) by means of its intersecting substantially flat surfaces <NUM>, <NUM> and/or <NUM>.

According to an embodiment each antenna <NUM>', <NUM>" and <NUM>" can be positioned on (connected to) the one or more intersecting flat surfaces (e.g., flat surface <NUM>) of the comer reflector, and the antennas can be distanced one from another at a distance of, for example, a half wavelength <MAT> of the highest radio frequency to be used for the signal transmission. Further, each antenna is connected to its corresponding communication units <NUM> (<FIG>) that comprises a receiver <NUM> (<FIG>) for receiving an incoming RF signal; an A/D (Analog-to-Digital) unit <NUM> (<FIG>) for sampling the received RF signal; a digital memory unit, such as a DRFM (Digital Radio Frequency Memory) unit <NUM> (<FIG>) for storing the signal samples in its memory to be reconstructed later; a digital retro-processing unit <NUM> (<FIG>) for reconstructing a RF signal from signal samples stored within DRFM unit <NUM> (<FIG>), said samples used as carriers for data to be transmitted, giving rise to a reconstructed digital RF signal; a D/A (Digital-to-Analog) unit <NUM> for converting said reconstructed digital RF signal to a reconstructed analog RF signal; and a transmitter <NUM> (<FIG>) for transmitting the reconstructed analog RF signal back towards source <NUM>.

It is supposed that the incoming RF signal from transmitting source <NUM> hits corner reflector <NUM>', and then it is detected by means of the set of antennas attached to the surface (such as antennas <NUM>',<NUM>" and <NUM>‴). The RF signal is sampled by means of a corresponding unit, such as A/D unit <NUM> of each antenna, and then the signal samples are stored within corresponding DRFM units <NUM> of each antenna, for later usage. It should be noted that since all samples (resulting in a set of samples) are performed substantially simultaneously (synchronously) by means of A/D units <NUM> of said antennas, and then the set of signal samples, stored within each DRFM unit <NUM>, is substantially identical. After this, upon the need, RF signals are reconstructed from the stored signal samples (by means of processing unit <NUM> of each antenna). Each reconstructed RF digital signal is converted back to an analog signal by means of a corresponding unit, such as Digital-to-Analog (D/A) unit <NUM>. Then, the analog signal is transmitted by means of transmitter <NUM> of each corresponding antenna within set of antennas, and when the reconstructed RF signal hits the corner reflector, it is reflected back towards source <NUM>. It should be noted that since the set of signal samples, stored within each DRFM unit <NUM>, is substantially identical (because all samples are performed substantially simultaneously by means of A/D units <NUM> of said antennas <NUM>', <NUM>" and <NUM>‴), then performing D/A conversion by means of D/A units <NUM> of said antennas <NUM>', <NUM>" and <NUM>‴ is also done substantially simultaneously at each antenna. As a result, the signal after the D/A conversion is transmitted back towards source <NUM> substantially simultaneously from each antenna.

It should be noted that according to an embodiment, due to the geometry of the corner reflector, the round signal communication paths from transmitting source <NUM> to each antenna <NUM>', <NUM>" and <NUM>‴, and vice versa, are equal. Thus, to achieve the constructive interference at transmitting source <NUM>, the shift of phase <MAT> of the signal to be transmitted, relative to phase <MAT> of the received signal, is given by: <MAT>.

This means that phases of signals, received at each antenna, substantially do not require manipulations for enabling their retro-directivity back towards transmitting source <NUM>. Constant C defines an arbitrary phase delay, applied substantially identically to all antennas within the set.

It should be noted that according to an embodiment, antenna elements <NUM>', <NUM>", <NUM>‴ are mutually synchronized for enabling transmitting the reconstructed RF signal back towards transmitting source <NUM> in a substantially precise angular direction. In addition, it should be further noted that according to this embodiment, modulation of the transmitted signal ϕT,n(t) remains the same as the modulation of the received signal ϕR,n(t). According to an embodiment, signal ϕT,n(t) is transmitted towards source <NUM> (by being reflected from corresponding corner reflector) without measuring an angular direction of said source <NUM>, and without calculating the phase differences of the signal to be transmitted from each antenna <NUM>', <NUM>" and <NUM>‴ (that is required, according to the prior art, for getting the greatest overall ERP in the direction, to which the RF signal is transmitted). As a result, there is also no need in positioning antennas within each set of antennas <NUM>', <NUM>" and <NUM>‴ on a straight plane because there is no need in determining the above signal phase differences.

<FIG> is a schematic illustration of digital retro-processing unit <NUM> to be provided for each antenna <NUM>', <NUM>" of <NUM>'" (<FIG>) located on corner reflector <NUM>', according to another embodiment of the present invention. According to an embodiment of the present invention, the signal to be transmitted is modulated by means of modulation unit <NUM>. For this, time dependency t is added to constant α ( Ci = α · wl and <MAT> ). This leads to digitally modulating the signal from transmitting source <NUM> (<FIG>), using the received signal as data carrier: <MAT> wherein <MAT> is a phase of the transmitted signal; and <MAT> is a phase of the received signal. As a result, a modulated transmitted wideband signal ϕT,n(t) can be as follows: <MAT> wherein β(t) is the amplitude modulation of transmitted signal; and L is a number of simultaneously received narrowband signals. It should be noted that the phase conjugated signal to be transmitted can be modulated by its amplitude or phase by means of using parameters β(t) and α(t). Then, the modulated wideband signal ϕT,n(t) may be retro-transmitted, when required. The modulation can be implemented as follows: <MAT> wherein <MAT> (fs is a sampling frequency and N the FFT length).

<FIG> is a flow chart of a retro-directive method for transmitting signals back towards transmitting source <NUM> (<FIG>) in a substantially synchronous manner and in a substantially precise angular direction, according to an embodiment of the present invention. At step <NUM>, the incoming RF signal is received by means of antennas <NUM>', <NUM>" and <NUM>‴ (<FIG>). Then, at step <NUM>, the signal is sampled by means of A/D unit <NUM> (<FIG>) provided within communication unit <NUM> (<FIG>) of each antenna. At step <NUM>, each signal sample is stored within corresponding DRFM units <NUM> (<FIG>) of each antenna. After this, at step <NUM>, an order of the signal samples is inverted, such that for a predefined period of time (e.g., <NUM> microsecond), the first sample becomes the last one. At step <NUM>, a new signal is reconstructed by using said inverted signal samples as carriers for data to be transmitted, giving rise to a reconstructed digital RF signal. Then, at step <NUM>, said reconstructed digital RF signal is converted by means of D/A unit <NUM> (<FIG>) into a reconstructed analog RF signal, which is transmitted back towards source <NUM> by means of transmitter <NUM>, at step <NUM>. For example, it is supposed that the first sample of a signal received by antenna <NUM>' is R<NUM>, the second is R<NUM>, the third is R<NUM> and the fourth sample is R<NUM>. Thus, by reordering said samples in an inverse order, such that the first sample is R<NUM>, the second is R<NUM>, the third is R<NUM> and the fourth sample is R<NUM>, and then using said samples as carriers for the data to be transmitted, enables transmitting said signal back towards source <NUM> in a substantially precise angular direction. Further, according to an embodiment of the present invention, there is no need in positioning antennas <NUM>', <NUM>" and <NUM>‴ on a straight plane since there is no need in determining phase differences of a signal to be transmitted from each antenna.

<FIG> is a flow chart of a retro-directive method for transmitting signals back towards transmitting source <NUM> (<FIG>) in a substantially synchronous manner and in a substantially precise angular direction, according to another embodiment of the present invention. At step <NUM>, the incoming RF signal is received by means of each antenna <NUM>', <NUM>" and <NUM>‴ (<FIG>). Then, at step <NUM>, the signal is sampled by means of A/D unit <NUM> (<FIG>) provided within communication unit <NUM> (<FIG>) of each antenna. After this, at step <NUM>, the amplitude of each signal sample is stored within corresponding DRFM unit <NUM> (<FIG>) of each antenna. It should be noted that the amplitude of each sample relates to a particular time instant (e.g., t=<NUM> [nsec], <NUM> [nsec], etc.). At step <NUM>, a sign of the initial phase of each signal is inverted (for example, by means of multiplying each signal by a function/signal, such as cosine(2wt) for narrowband signal, or by using FFT for wideband signals. This results in obtaining phase conjugated signals. At step <NUM>, a new signal is reconstructed by using said samples that have been phase conjugated as carriers of data to be transmitted, giving rise to a reconstructed digital RF signal. Then, at step <NUM>, said reconstructed digital RF signal is converted by means of D/A unit <NUM> (<FIG>) into a reconstructed analog RF signal, which is transmitted back towards source <NUM> by means of transmitter <NUM> (<FIG>), at step <NUM>.

It should be noted that according to an embodiment of the present invention, each reconstructed RF signal is transmitted back towards source <NUM> by means of the same antenna (e.g., , antenna <NUM>', <NUM>" or <NUM>"'), through which the original corresponding signal from source <NUM> was received.

<FIG> is a flow chart of a retro-directive method for transmitting signals back towards source <NUM> (<FIG>) in a substantially synchronous manner and in a substantially precise angular direction, according to still another embodiment of the present invention. According to this embodiment, a set of antennas, such as antennas <NUM>', <NUM>" and <NUM>'" (<FIG>), is connected to corner reflector <NUM>' (<FIG>). According to an embodiment of the present invention, the antennas can be positioned on one or more intersecting flat surfaces of the corner reflector, and the antennas can be distanced one from another at the distance of, for example, a half wavelength of the highest radio frequency to be used for the signal transmission. Further, each antenna is connected to its corresponding communication units <NUM> (<FIG>).

At step <NUM>, the RF signal from transmitting source <NUM> hits a corner reflector <NUM>', and then it is received by a set of antennas <NUM>', <NUM>", <NUM>"'. Then, at step <NUM>, the signal is sampled by means of A/D unit <NUM> (<FIG>) provided within communication unit <NUM> of each antenna within said set. After this, at step <NUM>, each signal sample is stored within corresponding DRFM units <NUM> (<FIG>) of each antenna. At step <NUM>, a new RF signal is reconstructed from said data samples, which are used as carriers for the data to be transmitted, giving rise to a reconstructed digital RF signal. Then, at step <NUM>, said reconstructed digital RF signal is converted by means of D/A unit <NUM> (<FIG>) into a reconstructed analog RF signal, which is (at step <NUM>) transmitted by means of transmitter <NUM> (<FIG>) of each antenna within set of antennas, and when the reconstructed RF signal hits corner reflector <NUM>', it is reflected back towards source <NUM>. When reflected by the corner reflector, the transmitted RF signal goes in substantially precise angular direction back towards source <NUM>. In addition, it should be noted that according to this embodiment of the present invention, modulation of the transmitted signal ϕT,n(t) remains the same as the modulation of the received signal ϕR,n(t).

<FIG> and <FIG> are sample illustrations of (random) locations of a plurality of antennas <NUM>, <NUM>, and <FIG> and <FIG> are constructive interference maps <NUM>, <NUM> of signals transmitted from said plurality of antennas, respectively, according to an embodiment. Illustrations <NUM> and <NUM> present a plurality of randomly positioned antennas, wherein each antenna is represented by a circle. Illustration <NUM> presents a constructive interference map of signals transmitted from said plurality of antennas <NUM> back towards transmitting source <NUM> (<FIG>), which is represented by point <NUM>' (or <NUM>") on the graph. It should be noted that at point <NUM>' (or <NUM>"), the greatest ERP (Effective Radiated Power) from the plurality of antennas <NUM> is received (the greater the ERP, the darker its grayscale representation on interference maps <NUM> and <NUM>). According to an embodiment, the signals are transmitted towards source <NUM>' (or <NUM>") from the plurality of antennas <NUM> without measuring an angular direction of said source and without calculating the phase differences of the signal transmitted from each antenna. As a result, there is also no need in positioning the plurality of antennas <NUM> on a straight plane because there is no need in determining the above signal phase differences.

Further, illustration <NUM> is a constructive interference map of signals transmitted from a plurality of antennas <NUM> back towards two transmitting sources, the first source represented by points <NUM>' (or <NUM>") and the second source represented by curve <NUM>. This illustration <NUM> relates to communication with two or more transmitting sources. Similarly to illustration <NUM>, there is no need in measuring an angular direction of said sources, and there is no need in calculating the phase differences of the RF signal to be transmitted from each antenna. Further, it is not necessary to position a plurality of antennas <NUM> on a straight plane.

Claim 1:
A system for retro-transmitting signals, substantially simultaneously towards two or more transmitting sources (<NUM>); wherein the two or more transmitting sources need not be synchronized with the system, and each source transmits a different narrow-band RF signal, the system comprises:
two or more antenna elements (<NUM>) and two or more respective communication units (<NUM>), each communication unit of each respective antenna element comprises: a receiver (<NUM>) configured to receive, via its respective antenna element, a wideband signal signal comprising the different narrow-band RF signals of the two or more transmitting sources;
and an analog-to-digital unit (<NUM>) configured to sample the received signal; a digital memory unit, a processing unit, a digital-to-analog unit, a transmitting unit;
the digital memory unit (<NUM>) configured to store samples of the received wideband signal received during a period of time by the respective antenna element (<NUM>), from the two or more transmitting sources (<NUM>), and sampled by the analog-to-digital unit (<NUM>), whereby the samples from each antenna element (<NUM>) are stored with an order corresponding to their given sampling order;
the processing unit (<NUM>) configured and operable for reconstructing a signal including:
deciding about at least one sub-group of signal samples that are sampled within said period of time; inverting the sampling orders of said samples of the signal within said at least one sub-group; reconstructing said signal by using said signal samples, which have the inverted sampling order, as data carriers, giving rise to a reconstructed digital signal; so as to reconstruct retro-transmission wideband signals that are transmitted back to said two or more transmitting sources when transmitted synchronously via respective antenna elements; and
the digital-to-analog units (<NUM>: connected to respective transmitter units (<NUM>) of said respective antenna elements and adapted for converting the respective reconstructed digital signals into respective analog transmission signals, substantially simultaneously, such that the analog transmission signals are transmitted in synchronization with one another, thereby enabling said system to retro-transmit the retro-transmission wideband signals towards said two or more transmitting sources simultaneously and thereby enabling coherent receipt of a respective reconstructed signal at each transmitting source.