Patent Publication Number: US-2022236403-A1

Title: Ofdm radar sensor system having an actively retransmitting repeater

Description:
FIELD 
     The present invention relates to an OFDM radar sensor system. 
     BACKGROUND INFORMATION 
     Digital modulation methods utilizing a plurality of carrier frequencies are known as OFDM (orthogonal frequency division multiplexing) methods. Use of OFDM methods for radar systems is being increasingly investigated. In an OFDM method, a frequency band is split up into a plurality of orthogonal sub-bands of respective subcarriers (FDM, frequency division multiplexing), and OFDM symbols are transmitted sequentially, one after another. The transmitted signal of an OFDM symbol is made up of subcarrier signals, which are modulated in accordance with a modulation scheme of the symbol, are orthogonal to each other, and are transmitted simultaneously within the OFDM symbol period. To that end, the subcarrier frequencies are selected in such a manner, that in the frequency spectrum, the maximum of a subcarrier lies on a zero crossing of the other subcarriers. 
     In the case of the received signal, a distance of a radar object may be estimated in light of the travel time of the OFDM symbols, while a speed estimate may be made in light of a phase characteristic over a sequence of OFDM symbols; the phase characteristic resulting from the Doppler effect. A plurality of radar objects generate a sum of delayed and Doppler-shifted echoes of the transmitted OFDM signal. Using a cycle header (prefix) in front of the symbol period, superposed radar echoes having different travel times may be separated from radar echoes of a subsequent OFDM-Symbol. 
     “Design of Low-Power Active Tags for Operation with 77-81 GHz FMCW Radar,” by M. S. Dadash, J. Hasch, P. Chevalier, A. Cathelin, N. Cahoon and S. P. Voinigescu, IEEE Transactions on Microwave Theory and Techniques, vol. 65, no. 12, pp. 5377-5388, December 2017, describes an active transponder (“active tag”), which modulates the phase of a received radar signal, using a square-wave signal, and emits it again. The active tag may be detected by the radar sensor in light of the reflected signal and may, in this manner, indicate a target object provided with the active tag. 
     SUMMARY 
     An object of the present invention is to provide a new OFDM radar sensor system, which includes a plurality of transmitting and receiving units, and in which monostatic and bistatic radar target responses of the transmitting and receiving units may be evaluated. 
     This object of the present invention may be achieved by an OFDM radar sensor system in accordance with an example embodiment having a plurality of transmitting and receiving units; one of the transmitting and receiving units being an OFDM radar sensor; and another of the transmitting and receiving units being a repeater, which is configured to modulate a signal generated and transmitted by the OFDM radar sensor into a signal orthogonal to the signal received by the repeater, and to emit the modulated signal; and the OFDM radar sensor being configured to separate a portion of a signal received by the OFDM radar sensor, which portion corresponds to the modulated signal, from a monostatic portion of the signal received by the OFDM radar sensor. 
     Consequently, the repeater is configured to modulate the signal, which is received by the repeater and is a signal generated and transmitted by the OFDM radar sensor, and to emit the modulated signal; the modulated signal being orthogonal to the signal received by the repeater. In other words, the repeater is configured to generate a modulated signal from a received signal by modulating the received signal, and to emit the modulated signal. 
     The signal generated and transmitted by the OFDM radar sensor and received by the repeater is subsequently referred to as the signal received by the repeater, as well. 
     The repeater may also be referred to as a transceiver. 
     In this case, a monostatic portion is understood to be a portion of the received signal, which is received as a radar echo of the transmitted signal transmitted by the OFDM radar sensor on a transmitting and receiving path, without interposition of the repeater. Monostatic signal portions, on whose transmitting and receiving path the repeater is not traversed, are distinguished from bistatic signal portions, which, starting from the OFDM radar sensor, have run through a transmitting and receiving path including the repeater and have consequently been transmitted, finally, by the repeater. Due to the orthogonality, the monostatic and bistatic signal portions of the signal received by the OFDM radar sensor do not interfere with each other. 
     The orthogonality relates to the OFDM encoding of the transmitted signal. Here, in particular, an orthogonal signal is understood to be a signal orthogonal with respect to the OFDM encoding of the transmitted signal and/or with respect to the signal received by the repeater. In particular, two signals, such as the signal received by the repeater and the modulated signal, are orthogonal to each other, if OFDM subcarriers contained, that is, occupied, in one of the two signals are each orthogonal to the respective OFDM subcarriers contained, that is, occupied, in the other signal. The orthogonality may be present, for example, due to use of different frequency bands in place of the frequency bands of the transmitted signal and/or radar echo, or due to the use of a different frequency band. 
     Since the bistatic signal portions received are orthogonal to the monostatic signal portions, they may be reliably distinguished from the monostatic signal portions at the OFDM radar sensor. Since the repeater also emits the modulated signal in an active manner, marked attenuation of the retransmitted signal, as would occur, for instance, in the case of a passive signal reflector, may be prevented. In particular, a signal strength reduced by running through the transmitting and receiving path twice may be compensated for completely or at least partially. 
     The repeater corresponds to a “virtual OFDM radar sensor,” whose associated radar echoes are evaluated, however, at the actual OFDM radar sensor. Both the evaluation and the modulation into the HF frequency band and the demodulation into the baseband take place only at a main transmitting and receiving unit, the OFDM radar sensor. Monostatic signal portions received simultaneously at the OFDM radar sensor and bistatic signal portions corresponding to the modulated signal may be evaluated separately due to their orthogonality. The transmitting and receiving units are allowed to cooperate with each other in the distributed OFDM radar sensor system; the transmitting and receiving units being both autonomous and including an OFDM radar sensor, as well as at least one repeater. Since the repeaters carry out only one modulation, for example, a simple frequency shift of the received and retransmitted signal, all of the signals received by the OFDM radar sensor return to the radar frequency of the local oscillator of the OFDM radar sensor, which means that accurate evaluation of the radar echoes of the transmitting and receiving units is rendered possible. In particular, the received radar echoes coming from the different transmitting and receiving units originate from, in each instance, the same OFDM symbol during a given OFDM symbol period, which means that the amplitudes and phase shifts of the radar echoes at the OFDM radar sensor are coherent and may be determined centrally in a precise manner. The orthogonal signals allow the signal source (OFDM radar sensor or repeater) to be assigned in an unequivocal manner during the evaluation in the OFDM radar sensor. Thus, a very wide virtual aperture of the OFDM radar sensor system may be generated as a function of the relative configuration of one or more repeaters with respect to the OFDM radar sensor. The effect of phase jitter may be minimized by the use of a local oscillator only in the OFDM radar sensor. Therefore, improved position-finding of radar targets is rendered possible. In particular, an evaluation from two different sensor positions is enabled, namely, from the actual OFDM radar sensor and from a virtual sensor position corresponding to the position of the repeater. This may be advantageous, in particular, for evaluating targets in the short range below 50 or 100 meters. 
     In particular, the repeater may be configured to modulate a radar echo, which is received by the repeater and is of the transmitted signal transmitted by the OFDM radar sensor, into, in particular, a signal orthogonal to the radar echo, and to emit the modulated signal. In other words, the repeater may be configured to generate a modulated signal from a radar echo, which is received by a radar target and is of a transmitted signal transmitted by the OFDM radar sensor, by modulating the received radar echo, and to emit the modulated signal. In particular, the modulated signal may be retransmitted on the same transmitting and receiving path. 
     Preferred refinements of the present invention are disclosed herein. 
     The signal emitted by the repeater preferably includes the signal received by the repeater, shifted in frequency by a predefined frequency shift. In other words, the signal received by the repeater and shifted in frequency by a predefined frequency shift, using modulation, is contained in the signal emitted by the repeater. Consequently, the modulated signal generated by the repeater includes frequency components, which are shifted in frequency by a predefined frequency shift with respect to corresponding frequency components of the signal received by the repeater. In this context, it is particularly advantageous that in the specific repeater, orthogonal signals, which enable unequivocal assignment of the signal source (OFDM radar sensor or repeater) during the evaluation in the OFDM radar sensor, may be generated, using simple circuit engineering devices. In particular, an HF oscillator is not necessary in the repeater, and a repeater constructed relatively simply may be used. This is particularly advantageous with regard to the robustness and the manufacturing costs of the system. 
     In accordance with an example embodiment of the present invention, the repeater is preferably configured to modulate the signal generated and transmitted by the OFDM radar sensor and received by the repeater into the signal orthogonal to the signal received by the repeater, using a shift in frequency by a predefined frequency shift. Thus, the orthogonality is produced through shifting by a frequency shift or frequency offset. Therefore, the object is achieved by an OFDM radar sensor system having a plurality of transmitting and receiving units; one of the transmitting and receiving units being an OFDM radar sensor; and another of the transmitting and receiving units being a repeater, which is configured to modulate a signal generated and transmitted by the OFDM radar sensor and received by the repeater, by shifting the frequency by a predefined frequency shift, into a signal orthogonal to the signal received by the repeater, and to emit the modulated signal; and the OFDM radar sensor being configured to separate a portion of a signal received by the OFDM radar sensor, which portion corresponds to the modulated signal, from a monostatic portion of the signal received by the OFDM radar sensor. 
     A particular advantage of modulating by shifting a frequency by a predefined frequency shift, is that the evaluation at the actual OFDM radar sensor for the received signal portions corresponding to the modulated signal may take place in the same manner as the evaluation of the monostatic, received signal portions. Therefore, the expenditure for signal processing in the OFDM radar sensor system may be advantageously minimized in spite of the provision of a “virtual radar sensor” at the location of the repeater. In this context, the separation of the monostatic and bistatic signal portions is still allowed by the orthogonal modulation. 
     The shift in frequency by a predefined frequency shift is preferably carried out by shifting the phase of an I/Q signal; the phase shift being varied in accordance with a harmonic oscillation, at a frequency corresponding to the predefined frequency shift. In this manner, conversion of the complex frequency of the I/Q signal may be carried out, in which no unwanted, second sideband is formed. However, other types of modulation, such as phase modulation or amplitude modulation, are also possible. 
     The repeater preferably has a modulator for shifting the frequency of a signal received by the repeater by a predefined frequency spacing; the modulator including: an I/Q splitter, which is configured to provide I/Q signal components, which are 90° out of phase from each other, based on a reference radar frequency, from a signal received by the repeater; multipliers, which are configured to multiply the specific I/Q signal components by respective I/Q modulation signal components of a modulation signal in a manner retaining the algebraic sign; the modulation signal having a frequency, which corresponds to the predefined frequency spacing; and an output, at which output signal components of the multipliers are combined. The modulation signal preferably corresponds to a harmonic oscillation. The signal orthogonal to the signal received by the repeater is provided at the output. The modulator allows a real shift in frequency by a frequency spacing corresponding to a modulation frequency, to be carried out without generating unwanted harmonics of the modulation frequency in the process; a construction of the repeater that is simple with regard to circuit engineering still being rendered possible. Thus, in particular, no HF oscillator is necessary in the repeater, and no HF oscillator signal has to be supplied. It is also advantageous that a system may be implemented, in which no synchronization signals have to be transmitted over signal connecting lines between the transmitting and receiving units. It is also advantageous that, for example, the I/Q splitter may be constructed as a passive network, such as an RLC network or, in particular, an LC network. Thus, in spite of a relatively simple design with regard to circuit engineering, the modulator described allows an OFDM symbol to be generated from an OFDM symbol contained in the signal received by the repeater; the former OFDM symbol being orthogonal to the latter OFDM symbol and then being emitted by the repeater. It also advantageous that with the aid of the repeater, a highly robust system may be provided. In addition, the possible, separate evaluation of the monostatic and bistatic portions of the received signal at the OFDM radar sensor yields a high performance of the system. 
     The reference radar frequency may be, for example, a reference carrier frequency or main carrier frequency of the OFDM symbols of the radar sensor and may correspond to the frequency of a local HF oscillator of the OFDM radar sensor. 
     The combination is preferably a linear combination and may be, in particular, addition or subtraction. The output may be, for example, a sum output, at which the output signal components of the multipliers are summed. Depending on the algebraic sign of the I/Q modulation signal components, a differential output may also be used, at which the output signal components of the multipliers are subtracted. The output signal components are combined while retaining the phases. 
     In one preferred specific embodiment of the present invention, the predefined frequency shift is a frequency shift, in which an OFDM subcarrier contained in the signal received by the repeater is orthogonal to a corresponding OFDM subcarrier in the modulated signal, which is shifted by the frequency shift. This is the case, when the condition, that frequency shift Δf 0  be equal to an integral multiple of the reciprocal of symbol period T, is satisfied: Δf 0 =k/T, where k is a whole number not equal to zero. In particular, frequency shift Δf 0  may be an integral multiple of the subcarrier spacing Δf of the OFDM frequency scheme: Δf 0 =kΔf. Alternatively, or at the same time, frequency shift Δf 0  may be greater than a bandwidth of the transmitted signal transmitted by the OFDM radar sensor or of the signal received by the repeater. The modulated signal is preferably shifted by the above-mentioned frequency shift with respect to the signal received by the repeater. 
     The signal generated and transmitted by the OFDM radar sensor preferably includes unoccupied OFDM subcarriers in the frequency spectrum; from occupied OFDM subcarriers in the signal received by the repeater, the repeater being configured to generate OFDM subcarriers, which are shifted in frequency and lie in frequency ranges, which correspond to frequency ranges of unassigned OFDM subcarriers in the signal received by the repeater. 
     Using an increased, e.g., doubled, carrier spacing of the occupied subcarriers, the synthetically generated subcarriers of the signal emitted by the repeater may be transmitted in the gaps between the occupied subcarriers. For example, in the modulated signal, subcarriers are occupied, which are interleaved (in the frequency space or, more precisely, in the OFDM carrier spectrum) with occupied subcarriers in the signal generated and transmitted by the OFDM radar sensor and received by the repeater. This means that in the modulated signal, subcarriers are occupied, which lie between the occupied subcarriers of the signal received by the repeater. 
     In one preferred specific embodiment of the present invention, the transmitted signal transmitted by the OFDM radar sensor occupies only every nth subcarrier of the OFDM frequency scheme; n being a natural number greater than 1; and with respect to the signal received by the repeater, the modulated signal being shifted by a frequency shift Δf 0 , which corresponds to (m+pn) times a subcarrier spacing Δf of the OFDM frequency scheme; m being a natural number less than n, and p being a whole number. This means that the frequency spacing between two occupied subcarriers of an OFDM symbol is nΔf, and that the frequency shift generated in the repeater is Δf 0 =(m+pn)Δf. 
     Preferably, p=0. That is, the frequency shift is m times the subcarrier spacing, where m is a natural number less than n. By interleaving the subcarriers used by the OFDM radar sensor for the transmitted signal and the subcarriers used by the repeater for the modulated signal, in such a manner, the frequency range of the transmitted signal may overlap the frequency range of the modulated signal to a maximum degree, which means that the signal transmission characteristics of the respective transmitting and receiving paths of the monostatic and bistatic, received signal portions are as similar as possible. Thus, in one example, only every second subcarrier may be occupied for the transmitted signal transmitted by the OFDM radar sensor (n=2), and the repeater may shift (modulate) the received signal by a subcarrier spacing and retransmit it (Δf 0 =+/−Δf). 
     In an expanded system, which includes a plurality of repeaters of the type described, in the case of n&gt;2, orthogonally modulated signals of (n-1) repeaters may be interleaved with the monostatic signal portions and may still be separated at the OFDM radar sensor. 
     The portion of the received signal corresponding to the modulated signal is preferably separated from a monostatic portion of the received signal, by separately evaluating frequency ranges of the received signal. This allows separate evaluation for corresponding parts of a frequency spectrum of the received signal to be undertaken in a simple manner for a specific, transmitted OFDM symbol. In the frequency spectrum, the signal portions may be separated in a particularly simple manner. 
     The OFDM radar sensor is preferably configured to detect OFDM symbols, which correspond to monostatic radar echoes, in one or more first frequency ranges of the received signal, and to detect radar echoes of OFDM symbols, which correspond to bistatic radar echoes of the signal modulated by the repeater, in one or more other, second frequency ranges of the received signal. Consequently, in light of the frequency ranges, OFDM symbols are recognized as a modulated signal coming from the repeater and are distinguished from monostatic signals. 
     When the repeater modulates the signal transmitted by the OFDM radar sensor and received by the repeater into the signal orthogonal to the signal received by the repeater, by shifting the frequency by a predefined frequency shift, the one or more second frequency ranges correspond to the one or more first frequency ranges of the received signal shifted by the frequency shift. Subcarrier frequencies within first frequency ranges are evaluated as OFDM symbols of a monostatic radar response, subcarrier frequencies within other, second frequency ranges are evaluated as frequency-shifted OFDM symbols of a bistatic radar response. 
     The one or more first frequency ranges preferably include frequencies, which correspond to the occupied OFDM subcarriers of the transmitted signal. The one or more second frequency ranges preferably include frequencies, which are orthogonal to the occupied OFDM subcarriers of the transmitted signal. 
     For example, the OFDM radar sensor may be configured to separate first frequency ranges of the received signal, which include frequencies that correspond to the occupied OFDM subcarriers of the transmitted signal, from other, second frequency ranges of the received signal, which include frequencies that are orthogonal to the occupied OFDM subcarriers of the transmitted signal. Consequently, the first frequency ranges are separated from the second frequency ranges in the frequency spectrum of the signal received by the OFDM radar sensor, and respective, present frequency bands of OFDM subcarriers may be evaluated separately. 
     In each instance, for example, radar echoes of OFDM symbols of the transmitted signal and/or radar echoes of the modulated signal of the OFDM symbols present in the repeater may be determined separately in the separate frequency ranges. That is, radar echoes of OFDM symbols of the transmitted signal included in the first frequency ranges may be detected, and radar echoes, which are present in the second frequency ranges and are of OFDM symbols contained in the modulated signal of the repeater, may be detected. 
     In an evaluation of the portion of the received signal corresponding to the modulated signal, the OFDM radar sensor is preferably configured to take into account a double signal propagation time and a double Doppler shift, which result from the transmitting and receiving path twice covered between the OFDM radar sensor and the repeater, for estimating distance and relative radial velocity of a radar target. Depending on the installation positions of the two components, the transmitting and receiving path between the OFDM radar sensor and the repeater is approximately twice as long as the travel time and/or distance in the monostatic radar echoes for the same radar object. For example, the OFDM radar sensor may be configured to assign a radar echo detected in the frequency spectrum of the monostatic signal portion at a first frequency position, to a radar echo, which is detected in the frequency spectrum of the portion of the received signal corresponding to the modulated signal, at a corresponding, second frequency position; the first frequency position corresponding to the single Doppler shift of a radar echo, and the second frequency position corresponding to the double Doppler shift of a radar echo of the same radar target. Accordingly, the OFDM radar sensor may be configured to associate a delay of a radar echo detected in the monostatic portion of the received signal, which delay corresponds to the single signal propagation time of the radar echo between the sensor and the radar target, to a delay of a radar echo detected in the monostatic portion of the received signal, for the same radar target, which delay corresponds to two run-throughs of a transmitting and receiving path between the sensor, radar target, and repeater. 
     Therefore, to estimate the distance and speed of a radar object for the bistatic radar echoes, it is taken into account that the transmitting and receiving path between the OFDM radar sensor and the repeater is run through twice. 
     The OFDM radar sensor system is preferably an OFDM radar sensor system for a motor vehicle. The plurality of transmitting and receiving units are preferably transmitting and receiving units for placement at separate positions on a motor vehicle. The object is further achieved by a motor vehicle including the OFDM radar sensor system. 
     Below, exemplary embodiments are explained in greater detail with reference to the figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic sketch of a motor vehicle, including an OFDM radar sensor system having an OFDM radar sensor and a repeater, in accordance with an example embodiment of the present invention. 
         FIG. 2  shows a schematic representation of the repeater, in accordance with an example embodiment of the present invention. 
         FIG. 3  shows partial spectra of OFDM symbols. 
         FIG. 4  shows a schematic illustration of a signal characteristic of an OFDM symbol. 
         FIG. 5  shows schematic representations of radar echoes of OFDM symbols. 
         FIG. 6  shows a schematic layout of the OFDM radar sensor, in accordance with an example embodiment of the present invention. 
         FIG. 7  shows a schematic basic circuit diagram of a modulator of the repeater, in accordance with an example embodiment of the present invention. 
         FIG. 8  shows a schematic representation of a further example of a repeater, in accordance with the present invention. 
         FIG. 9  shows schematic representations of radar echoes of OFDM symbols according to a further example. 
         FIG. 10  shows partial spectra of OFDM symbols according to a further example. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     The OFDM radar sensor system shown in  FIG. 1  is installed in a motor vehicle  10  and includes transmitting and receiving units in the form of an OFDM radar sensor  12  and an active repeater  14 , which are installed in motor vehicle  10  at a lateral distance B from each other, for example, at the vehicle front end. OFDM radar sensor  12  generates and emits a transmitted signal  16 , which is reflected or scattered by a radar target  18  and is received by OFDM radar sensor  12  as a radar echo  20 . Reflected, transmitted signal  16  is also received by repeater  14  as a radar echo  22 . 
     Repeater  14  amplifies the signal generated and transmitted by OFDM radar sensor  12  and received by repeater  14  as a radar echo  22  and modulates it into a signal  24 , which is orthogonal to radar echo  22  and is transmitted by repeater  14 . The signal  24  transmitted by the repeater is reflected anew by radar target  18  and is received by OFDM radar sensor  12  as a modulated radar echo  26 . Thus, the received signal of OFDM radar sensor  12  includes a monostatic portion, which contains the direct radar echo  20  of radar target  18 , and a bistatic portion, which contains modulated radar echo  26  and therefore corresponds to modulated signal  24 . 
     OFDM radar sensor  12  may be, for example, an angle-resolving OFDM radar sensor, by which the angle φ 1  at which signal  20  is received by radar target  18 , may be estimated. Repeater  14  may be, for example, a transceiver, whose transmitting and/or receiving antenna(e) have a relatively wide field of view in the elevation direction and in the azimuthal direction. The visual range of repeater  14  may correspond to, for example, a visual range of OFDM radar sensor  12  for a given distance range. As illustrated in  FIG. 1 , this allows the signal  22  reflected to repeater  14  to be retransmitted to OFDM radar sensor  12  on the same transmitting and receiving path while being reflected again at radar target  18 . Angle φ 2 , at which radar echo  22  is received by the repeater, may differ from angle φ 1 , which means that non-central radar targets  18  also deliver monostatic and bistatic radar echoes  20 ,  26 . 
       FIG. 2  schematically shows the repeater  14  having a receiving antenna  28  and a transmitting antenna  30 . A direction of a portion of transmitted signal  24  is shown in  FIG. 2 ; coming from this direction, signal  24  being reflected again at radar target  18  and received by OFDM radar sensor  12 . 
     Repeater  14  includes at least one amplifier, in the example, a receiving amplifier  32  and a transmitting amplifier  34 . In addition, repeater  14  includes a modulator  36  for shifting the frequency of the signal received and re-emitted in modulated form. Modulator  36  subjects received signal  22  to a shift in frequency by a predefined frequency shift Δf 0 , in accordance with a shift in a phase of a complex frequency of signal  22 ; the phase varying according to a harmonic oscillation. The phase shift is controlled via amplitudes I, Q of I/Q signal portions, as explained below, using the example of  FIG. 7 . 
       FIG. 3  schematically shows a portion of a spectrum of an OFDM symbol of transmitted signal  16 . Signal amplitude A is represented versus frequency f. In an OFDM signal having an OFDM symbol of symbol period T, subcarriers, whose minimum frequency spacing Δf satisfies the orthogonality condition T=1/Δf, are available for the OFDM modulation. In the case of a subcarrier frequency spacing of Δf, the numbers of the periods of the oscillations of the subcarriers within symbol period T differ by exactly one period or a multiple of it, which means that the subcarriers are orthogonal to each other. 
     In transmitted signal  16  of OFDM radar sensor  12 , only every nth subcarrier is occupied in an OFDM symbol. In the example shown in  FIG. 3 , n=2. For the purpose of simplified representation,  FIG. 3  shows only two occupied subcarriers of the OFDM symbol at frequencies f 1  and f 2 . 
     Neglecting a Doppler shift, the frequency spectrum of signal  16  shown in  FIG. 3  corresponds to the frequency spectrum of the radar echo  22  received by repeater  14 . Received signal  22  usually includes a superposition of time-delayed and possibly Doppler-shifted radar echoes, of which only a partial spectrum of a single radar echo  22  is shown in  FIG. 3 . 
     Modulator  36  effects a shift in frequency of signal  22  by a frequency shift Δf 0 , which corresponds to the minimum subcarrier spacing Δf. In  FIG. 3 , the frequency spectrum of the amplified signal  24  retransmitted by repeater  14  is shown schematically with the same amplitude, using a dotted line. Due to the shift in frequency, signals  22 ,  24  are orthogonal to each other. The occupied subcarriers of the modulated radar echo are situated in the gaps of the OFDM symbols received as radar echo  22 , as illustrated in  FIG. 3 . 
       FIG. 4  schematically represents a signal characteristic of an OFDM symbol of transmitted signal  16  over time t. The individual, occupied subcarriers of the signal are modulated according to an OFDM modulation scheme; for example, each occupied subcarrier being modulated, using a complex amplitude. 
       FIG. 5  schematically illustrates a monostatic radar echo  20  of an OFDM symbol, which is contained in the received signal of OFDM radar sensor  12  and includes a plurality of first frequency ranges  38 , in which respective subcarriers are situated; as well as an OFDM symbol in second frequency ranges  40 , which is contained in bistatic radar echo  26 . The monostatic signal portions  20  received by OFDM radar sensor  12  lie in first frequency ranges  38  and usually contain a superposition of time-delayed and possibly Doppler-shifted radar echoes. In contrast, the bistatically received radar echoes  26  additionally have the shift in frequency by the frequency spacing Δf 0  and have, furthermore, a delay and possibly a Doppler shift, which corresponds to two run-throughs of the transmitting and receiving path via radar target  18 . Due to the different frequency ranges  38 ,  40 , the monostatic and the bistatic signal portions may be processed separately. 
       FIG. 6  schematically shows a basic circuit diagram of OFDM radar sensor  12 , including a transmitting branch  42  and a receiving branch  44 . For each OFDM symbol step, a modulation symbol s, which includes subsymbols for the individual subcarriers, is converted to an OFDM symbol x in the time domain with the aid of inverse Fourier transformation. In this context, OFDM symbol x includes, as is conventional, the actual OFDM symbol of symbol length T, as well as a header (cyclic prefix), which is a copy of an end section of the OFDM symbol. OFDM symbol x is converted by a digital-to-analog converter to an analog signal, with the aid of which an I/Q modulator  45  modulates transmitting frequency f 0  of a local oscillator LO, in order to generate transmitted signal  16 . 
     In receiving branch  44 , the received signal, which contains signal portions  20  and  26 , is demodulated in an I/Q demodulator  46 , using the radar frequency of local oscillator LO, and digitized by an analog-to-digital converter, and a Fourier transformation is carried out with the aid of FFT. In the Fourier transformation, the subcarriers contained in received signal  20 ,  26  are mapped onto separate frequency positions in the frequency spectrum. 
     First frequency ranges  38  of the frequency spectrum and second frequency ranges  40  of the frequency spectrum are then processed further in separate processing branches. For the first frequency ranges  38 , which correspond to the monostatic radar echoes, a complex spectral division of the received signal by transmitted OFDM signal s is carried out. This may be referred to as normalizing of the received signal portion. The processing is carried out for the consecutive OFDM symbols s of an OFDM radar measurement. Thus, a sum of complex exponents generated by the travel time and the Doppler shift is obtained in a two-dimensional spectrum E 1  according to the subcarriers and the sequence of OFDM symbols s. 
     In contrast, the signal portions of second frequency ranges  40  corresponding to the modulated signal of repeater  14  are additionally subjected to demodulation in the form of a shift in frequency by frequency spacing Δf 0 , by which repeater  14  modulated the transmitted signal. The further processing, using complex spectral division by the sequence of OFDM symbols s then corresponds to the processing of the monostatic signals, and a two-dimensional spectrum E 2  is obtained. 
     Respective detection devices  47  evaluate the 2-D spectra E 1 , E 2  obtained in the two separate processing branches for frequency ranges  38  and  40  and detect radar objects from peaks in spectra E 1 , E 2 . An evaluation device  48  evaluates the detected radar objects. In this context, radar objects, which are detected in light of signals from second frequency ranges  40 , at frequency positions, which correspond to the double Doppler shift of the radar echoes, are correlated to radar objects, which are detected in light of signals from first frequency ranges  38 , at frequency positions, which correspond to a corresponding, single Doppler shift. In a similar manner, objects, which are detected in light of signals from second frequency ranges that exhibit a double travel time, are assigned to corresponding objects, which are detected in light of signals from first frequency ranges  38  having a single travel time. 
       FIG. 7  schematically shows a basic circuit diagram of modulator  36 . An input  49  of modulator  36  is connected to an I/Q splitter  50 , which splits up the input signal of modulator  36  into an in-phase signal and a quadrature signal. In other words, I/Q splitter  50  provides the input signal with a phase shift of 0° in a first, in  FIG. 7 , upper, signal branch, and provides the input signal with a phase shift of 90° in the second, in  FIG. 7 , lower, signal branch. I/Q splitter  50  is constructed in a conventional manner, using a passive LC network. The upper and the lower signal branch include multipliers  52  and  54 , respectively, which are illustrated symbolically in  FIG. 7  by an amplifier having adjustable amplitude, as well as by a symbol, which represents the possible sign change of the signal. Multipliers  52 ,  54  receive I/Q components of a modulation signal as further input variables. The modulation signal is a harmonic oscillation having a frequency corresponding to frequency shift Δf 0 , e.g., I=sin(2πtΔf 0 ) and Q=cos(2πtΔf 0 ). The outputs of multipliers  52 ,  54  are summed in-phase at output  56  of modulator  36 , using a summing element  58 . Consequently, a phase shift of the input signal is carried out by modulator  36 ; the complex phase-shift vector rotating at the frequency Δf 0 . 
     The exemplary embodiments described are given as examples for illustrating the present invention and may be modified. 
     Thus, for example, a repeater  14 ′ shown in  FIG. 8  may be used in place of the repeater  14  shown in  FIG. 2 . Repeater  14 ′ differs from the example of  FIG. 2 , in that a joint transmitting/receiving antenna  60  is provided, which is connected to modulator  36  via a directional coupler  42  and input amplifier  32  and/or output amplifier  34 . Directional coupler  62  includes three input/output terminals. Directional coupler  62  couples the signal received by antenna  60  into modulator  36  via amplifier  32 . The output signal of modulator  36  and/or of amplifier  34  is coupled in in the direction of antenna  60  for transmitting. 
       FIG. 9  shows a representation corresponding to  FIG. 5 , in accordance with a further exemplary embodiment. In this example, the frequency shift Δf 0  effected by modulator  36  of repeater  14  corresponds to the occupied bandwidth of an OFDM symbol or is greater than it. In the OFDM symbols of the transmitted signal of OFDM radar sensor  12 , directly consecutive subcarriers, which are present together in a first frequency range  38 , are occupied in this example. The bistatically received radar echoes are contained in a second frequency range  40 , which is shifted by frequency shift Δf 0  with respect to first frequency range  38 . When q consecutive subcarriers are used for an OFDM symbol, then, in this example, the frequency spacing is qΔf, where Δf is the spacing between two consecutive subcarriers. 
     The above-described examples of  FIG. 2  and  FIG. 5  may be generalized in a corresponding manner to an OFDM radar sensor system having a plurality of active repeaters  14 . In a representation corresponding to  FIG. 3 ,  FIG. 10  shows a portion of an OFDM symbol of a further example of a transmitted signal  16 , where only every third subcarrier is occupied. Then, using a frequency shift  2 Δf 0 , a further repeater  14  may generate a modulated signal, which is orthogonal to the signal of first repeater  14  shifted by Δf 0 . OFDM radar sensor  12  may then distinguish the radar echoes coming from the different repeaters  14  from each other and from the monostatic radar echoes, in light of their position in corresponding frequency ranges, and in each instance, process them separately in three processing branches.