OFDM radar sensor system with active repeater

Abstract

An OFDM radar sensor system having a plurality of transmitting and receiving units (12, 14), wherein one of the transmitting and receiving units is an OFDM radar sensor (12), wherein another of the transmitting and receiving units is a repeater (14), wherein the repeater is configured to modulate a signal (22) generated and transmitted by the OFDM radar sensor (12) and received by the repeater (14) into a signal orthogonal to the signal (22) received by the repeater (14) and to emit the modulated signal (24), wherein the OFDM radar sensor (12) is configured to separate a component (26) of a signal (20, 26) received by the OFDM radar sensor (12) corresponding to the modulated signal (24) from a monostatic component (20) of the signal (20, 26) received by the OFDM radar sensor (12).

Description

Technical Field

[0001] The invention relates to an OFDM radar sensor system. Background Art

[0002] Digital modulation methods with multiple carrier frequencies are called OFDM (orthogonal frequency division multiplexing). OFDM methods are increasingly being studied for use in radar systems. In the OFDM method, the frequency band is divided into corresponding subcarriers ( OFDM symbols are transmitted sequentially one after another. The transmitted signal of an OFDM symbol is composed of mutually orthogonal subcarrier signals (English: sub-carrier signals) modulated according to the modulation scheme of the symbol, which are transmitted simultaneously within the OFDM symbol period. For this purpose, the subcarrier frequencies are selected so that the maximum value of one subcarrier in the spectrum is located at the zero crossing point of the other subcarriers.

[0003] In the received signal, the distance of the radar object can be estimated based on the propagation time of the OFDM symbol, and the speed can be estimated based on the phase change process caused by the Doppler effect on the sequence of OFDM symbols. Multiple radar objects cause the sum of delayed and Doppler-shifted echoes of the transmitted OFDM signal. The cyclic prefix before the symbol period can separate overlapping radar echoes with different propagation times from radar echoes of subsequent OFDM symbols.

[0004] "Design of Low-Power Active Tags for Operation with 77-81GHz FMCW Radar" (MS Dadash, J. Hasch, P. Chevalier, A. Cathelin, N. Cahoon and SP Voinigescu, IEEE Transactions on Microwave Theory and Techniques, Vol. 65, No. 12, pp. 5377-5388, December 2017) describes an active transponder ("active tag") that phase modulates a received radar signal with the aid of a square wave signal and re-transmits it. The "active tag" can be identified by a radar sensor based on the echo signal, and can thereby identify a target object equipped with the "active tag". Summary of the invention

[0005] The object of the present invention is to provide a novel OFDM radar sensor system having a plurality of transmitting and receiving units, wherein the monostatic and bistatic radar target responses of the transmitting and receiving units can be evaluated.

[0006] According to the invention, this object is achieved by an OFDM radar sensor system having a plurality of transmitting and receiving units, wherein one of the transmitting and receiving units is an OFDM radar sensor, wherein another of the transmitting and receiving units is a repeater, which is configured to modulate a signal generated and transmitted by the OFDM radar sensor and received by the repeater into a signal orthogonal to the signal received by the repeater and to emit the modulated signal, wherein the OFDM radar sensor is configured to separate a component of the signal received by the OFDM radar sensor that corresponds to the modulated signal from a monostatic component of the signal received by the OFDM radar sensor.

[0007] Therefore, the repeater is configured to modulate a signal received by the repeater (the signal generated and transmitted by the OFDM radar sensor) and emit a modulated signal, wherein the modulated signal is orthogonal to the signal received by the repeater. In other words, the repeater is configured to generate a modulated signal from the received signal by modulating the received signal and emit the modulated signal.

[0008] The signal generated and transmitted by the OFDM radar sensor and received by the repeater is also referred to below as the signal received by the repeater.

[0009] A repeater may also be called a transceiver.

[0010] In this context, a monostatic component is understood to be a component of the received signal that is received as a radar echo of a transmit signal transmitted by an OFDM radar sensor on a transmit and receive path without an inserted repeater. Monostatic signal components that do not pass through repeaters on their transmit and receive paths are distinguished from bistatic signal components that pass through a transmit and receive path starting from the OFDM radar sensor and including repeaters and are therefore ultimately transmitted by the repeaters. Due to orthogonality, the monostatic and bistatic signal components of the signal received by the OFDM radar sensor do not interfere with each other.

[0011] The orthogonality relates to the OFDM coding of the transmitted signal. In particular, an orthogonal signal is understood here to be an orthogonal signal with respect to the OFDM coding of the transmitted signal or the signal received by the repeater. In particular, two signals are orthogonal to one another, for example a signal received by a repeater and a modulated signal, if the OFDM subcarriers contained in one of the two signals, i.e. occupied, are respectively orthogonal to the corresponding OFDM subcarriers contained in the other signal, i.e. occupied. The orthogonality can be given, for example, by using a different frequency band than the frequency band of the transmitted signal or the radar echo or by using another frequency range.

[0012] Since the received bistatic signal components are orthogonal to the monostatic signal components, the received bistatic signal components can be reliably distinguished from the monostatic signal components at the OFDM radar sensor. In addition, since the repeater actively emits the modulated signal, a strong attenuation of the forwarded signal, such as would occur, for example, in a passive signal reflector, can be avoided. In particular, the signal strength that is reduced due to the double passage through the transmission and reception path can be fully or at least partially compensated.

[0013] The repeater corresponds to a "virtual OFDM radar sensor", but its associated radar echo is analyzed at the real OFDM radar sensor. Not only the analysis but also the modulation into the radio frequency band and the demodulation into the baseband are performed only at the main transmitting and receiving unit (OFDM radar sensor). The monostatic signal component and the bistatic signal component corresponding to the modulated signal received simultaneously at the OFDM radar sensor can be analyzed separately due to their orthogonality. It is possible to cooperate with each other in a distributed OFDM radar sensor system, wherein the transmitting and receiving units are autonomous at the same time and include an OFDM radar sensor and at least one repeater. Since the repeater only performs modulation, for example a simple frequency shift of the received and re-transmitted signal, all signals received by the OFDM radar sensor are returned to the radar frequency of the local oscillator of the OFDM radar sensor, so that the radar echoes of the transmitting and receiving units can be accurately analyzed. In particular, the received radar echoes originating from different transmitting and receiving units are respectively returned to the same OFDM symbol during a given OFDM symbol period, so that the amplitude and phase shift of the radar echoes at the OFDM radar sensor are coherent and can be determined centrally and accurately. Orthogonal signals enable the signal source (OFDM radar sensor or repeater) to be uniquely and clearly assigned in the analysis process in the OFDM radar sensor. According to the relative arrangement of one or more repeaters and the OFDM radar sensor, a very wide virtual aperture of the OFDM radar sensor system can be generated. By using a local oscillator only in the OFDM radar sensor, the influence of phase noise can be minimized. Therefore, improved positioning of radar targets can be achieved. In particular, analysis can be achieved from two different sensor positions, namely from the real OFDM radar sensor and from the virtual sensor position corresponding to the repeater position. This may be advantageous in particular for analyzing targets in a close range of less than 50 or 100 meters.

[0014] The repeater can be configured to modulate the radar echo received by the repeater of the signal transmitted by the OFDM radar sensor, in particular to modulate it into a signal orthogonal to the radar echo, and emit the modulated signal. In other words, the repeater can be configured to generate a modulated signal from the radar echo received from the radar target of the transmission signal transmitted by the OFDM radar sensor by modulating the received radar echo, and emit the modulated signal. In particular, the modulated signal can be forwarded on the same transmission and reception path.

[0015] Preferred embodiments emerge from the dependent claims.

[0016] Preferably, the signal sent by the repeater contains a signal received by the repeater and frequency-shifted with a predetermined frequency offset. In other words, the signal received by the repeater and frequency-shifted with a predetermined frequency offset by modulation is contained in the signal sent by the repeater. Therefore, the modulated signal generated by the repeater has the following frequency components: relative to the corresponding frequency components of the signal received by the repeater, the frequency components are frequency-shifted with a predetermined frequency offset. It is particularly advantageous here that in the corresponding repeater, the generation of orthogonal signals is realized with a simple circuit technology device, and the orthogonal signal enables the unique and clear allocation of the signal source (OFDM radar sensor or repeater) in the analysis and processing in the OFDM radar sensor. In particular, no radio frequency oscillator is required in the repeater, and a relatively simple repeater can be used. This is particularly advantageous in terms of the robustness of the system and the manufacturing cost.

[0017] Preferably, the repeater is configured to modulate the signal generated and transmitted by the OFDM radar sensor and received by the repeater into a signal orthogonal to the signal received by the repeater by frequency shifting by a predetermined frequency offset. Thus, the orthogonality is established by a frequency shift or a frequency offset. Therefore, the task is solved by an OFDM radar sensor system having a plurality of transmitting and receiving units, wherein one of the transmitting and receiving units is an OFDM radar sensor, wherein another of the transmitting and receiving units is a repeater, wherein the repeater is configured to modulate the signal generated and transmitted by the OFDM radar sensor and received by the repeater into a signal orthogonal to the signal received by the repeater by frequency shifting by a predetermined frequency offset and emit the modulated signal, wherein the OFDM radar sensor is configured to separate a component of the signal received by the OFDM radar sensor corresponding to the modulated signal from a monobasic component of the signal received by the OFDM radar sensor.

[0018] Modulation by frequency shifting with a predetermined frequency offset has the particular advantage that the evaluation of the received signal components corresponding to the modulated signal can be performed at a real OFDM radar sensor in the same manner as the evaluation of the monostatic received signal components. In an advantageous manner, the signal processing effort in the OFDM radar sensor system can thus be minimized despite the provision of a "virtual radar sensor" at the location of the repeater. However, the separation of the monostatic and bistatic signal components can be achieved here by quadrature modulation.

[0019] The frequency shift with a predetermined frequency offset is preferably performed by a phase shift of the I / Q signal, wherein the phase shift changes as a function of the resonance at a frequency corresponding to the predetermined frequency offset. In this way, a conversion of the complex frequency of the I / Q signal can be performed without undesired second sidebands occurring. However, other types of modulation are also conceivable, such as phase modulation or amplitude modulation.

[0020] Preferably, the repeater has a modulator for frequency shifting the signal received by the repeater by a predetermined frequency spacing, wherein the modulator has: an I / Q splitter, which is configured to provide I / Q signal components from the signal received by the repeater, which are phase-shifted by 90° relative to one another with respect to a reference radar frequency; a multiplier, which is configured to sign-multiply the respective I / Q signal components with the respective I / Q modulation signal components of a modulation signal, wherein the modulation signal has a frequency corresponding to the predetermined frequency spacing; an output, at which the output signal components of the multiplier are combined. The modulation signal preferably corresponds to a resonance. A signal orthogonal to the signal received by the repeater is provided at the output. The modulator enables a true frequency shift with a frequency spacing corresponding to the modulation frequency, without generating undesired harmonics of the modulation frequency, wherein a simple circuit design of the repeater can still be achieved. In this way, in particular, no radio frequency oscillator is required in the repeater, and no radio frequency oscillator signal has to be provided. It is also advantageous that a system can be realized in which a synchronization signal does not have to be transmitted between the transmitting and receiving units via a signal connection line. It is also advantageous that, for example, the I / Q splitter can be constructed as a passive network, for example as an RLC network or in particular as an LC network. Therefore, the described modulator makes it possible to achieve: despite a relatively simple structure in circuit technology, an OFDM symbol orthogonal to the OFDM symbol contained in the signal received by the repeater is generated, which is then emitted by the repeater. It is further advantageous that a very robust system can be created with the aid of a repeater. In addition, a very high performance of the system is obtained by the possible separate analysis and processing of the monostatic and bistatic components of the received signal at the OFDM radar sensor.

[0021] For example, the reference radar frequency may be a reference or main carrier frequency of an OFDM symbol of the radar sensor and may correspond to the frequency of a local radio frequency oscillator of the OFDM radar sensor.

[0022] The combination is preferably a linear combination and can in particular be an addition or a subtraction. The output can be, for example, a sum output, at which the output signal components of the multipliers are added. Depending on the signs of the I / Q modulated signal components, a differential output can also be used, at which the output signal components of the multipliers are subtracted. The combination of the output signal components is performed while preserving the phase.

[0023] In a preferred embodiment, the predetermined frequency offset is a frequency offset in which the OFDM subcarriers contained in the signal received by the repeater are orthogonal to the corresponding OFDM subcarriers in the modulated signal shifted by the frequency offset. This is the case if the condition that the frequency offset Δf0 is equal to one or more times, an integer multiple of the inverse of the symbol period T is met: Δf0=k / T, where k is a non-zero integer. The frequency offset Δf0 can in particular be an integer multiple of the subcarrier spacing Δf of the OFDM frequency scheme: Δf0=kΔf. Alternatively or simultaneously, the frequency offset Δf0 can be greater than the bandwidth of the transmission signal transmitted by the OFDM radar sensor or the signal received by the repeater. The modulated signal is preferably shifted by the mentioned frequency offset relative to the signal received by the repeater.

[0024] Preferably, the signal generated and transmitted by the OFDM radar sensor has unoccupied OFDM subcarriers in the frequency spectrum, wherein the repeater is configured to generate frequency-shifted OFDM subcarriers from occupied OFDM subcarriers in the signal received by the repeater, wherein the frequency-shifted OFDM subcarriers are located in a frequency range corresponding to the frequency range of unoccupied OFDM subcarriers in the signal received by the repeater.

[0025] By increasing, for example doubling, the carrier spacing of the occupied subcarriers, synthetically generated subcarriers of the signal emitted by the repeater can be transmitted in the gaps between the occupied subcarriers. For example, in the modulated signal, subcarriers are occupied that are interlaced (in frequency space or more precisely in the OFDM carrier spectrum) with subcarriers occupied in the signal generated and transmitted by the OFDM radar sensor and received by the repeater. That is, in the modulated signal, subcarriers are occupied that are located between the occupied subcarriers of the signal received by the repeater.

[0026] In a preferred embodiment, the transmission signal transmitted by the OFDM radar sensor occupies only every nth subcarrier of the OFDM frequency scheme, where n is a natural number greater than 1, wherein the modulated signal is shifted relative to the signal received by the repeater by a frequency offset Δf0, which corresponds to (m+pn) times the subcarrier spacing Δf of the OFDM frequency scheme, where m is a natural number less than n and p is an integer. This means that the frequency spacing between two occupied subcarriers of an OFDM symbol is nΔf, and the frequency offset generated in the repeater is Δf0=(m+pn)Δf.

[0027] Preferably p=0. That is to say, the frequency offset is m times the subcarrier spacing, where m is a natural number less than n. By such an interleaving of the subcarriers used by the OFDM radar sensor for transmitting signals and the subcarriers used by the repeater for modulating signals, the frequency range of the transmitted signal can overlap the frequency range of the modulated signal to the maximum extent possible, so that the signal transmission characteristics of the corresponding transmission and reception paths of the received signal components of the monobase and the bibase are as similar as possible. Thus, in the example, only every second subcarrier can be laid out for the transmit signal transmitted by the OFDM radar sensor (n=2), and the repeater can shift (modulate) the received signal by the subcarrier spacing and transmit it again (Δf0=+ / -Δf).

[0028] In an extended system including multiple repeaters of the described type, the orthogonal modulated signals from (n-1) repeaters can be interleaved with the monostatic signal components in the case of n>2 and still be separated at the OFDM radar sensor.

[0029] Preferably, the components of the received signal corresponding to the modulation signal are separated from the monobasic components of the received signal by separate evaluation of the frequency range of the received signal. This makes it possible to carry out separate evaluation of the corresponding parts of the spectrum of the received signal for the corresponding transmitted OFDM symbols in a simple manner. In the spectrum, the signal components can be separated particularly easily.

[0030] Preferably, the OFDM radar sensor is configured to detect in one or more first frequency ranges of the received signal an OFDM symbol corresponding to a monostatic radar echo, and in one or more other, second frequency ranges of the received signal an OFDM symbol corresponding to a bistatic radar echo of a signal modulated by a repeater. Thus, based on the frequency range, the OFDM symbol is identified as a modulated signal originating from the repeater and distinguished from a monostatic signal.

[0031] When the repeater modulates the signal transmitted by the OFDM radar sensor and received by the repeater into a signal orthogonal to the signal received by the repeater by frequency shifting by a predetermined frequency offset, the one or more second frequency ranges correspond to the one or more first frequency ranges of the received signal shifted by the frequency offset. The subcarrier frequencies in the first frequency range are analyzed and processed as OFDM symbols of a monostatic radar response, and the subcarrier frequencies in the second, other frequency ranges are analyzed and processed as frequency-shifted OFDM symbols of a bistatic radar response.

[0032] The one or more first frequency ranges preferably include frequencies corresponding to occupied OFDM subcarriers of the transmitted signal.The one or more second frequency ranges preferably include frequencies orthogonal to the occupied OFDM subcarriers of the transmitted signal.

[0033] For example, an OFDM radar sensor can be configured to separate a first frequency range of a received signal, the first frequency range including frequencies corresponding to occupied OFDM subcarriers of a transmission signal, from a further, second frequency range of a received signal, the further, second frequency range including frequencies orthogonal to the occupied OFDM subcarriers of the transmission signal. Thus, in the frequency spectrum of the signal received by the OFDM radar sensor, the first frequency range is separated from the second frequency range, and the corresponding contained frequency bands of the OFDM subcarriers can be evaluated separately.

[0034] In separate frequency ranges, for example, radar echoes of OFDM symbols of the transmission signal or radar echoes of the modulated signal of OFDM symbols contained in the repeater can be determined separately. That is, radar echoes contained in the first frequency range of OFDM symbols of the transmission signal can be detected, and radar echoes contained in the second frequency range of OFDM symbols contained in the modulated signal of the repeater can be detected.

[0035] Preferably, the OFDM radar sensor is configured to take into account the twice signal propagation time and twice Doppler shift resulting from the two passes through the transmission and reception path between the OFDM radar sensor and the repeater when evaluating the component of the received signal corresponding to the modulated signal, for estimating the distance and relative radial velocity of the radar target. Depending on the installation position of the two components, the transmission and reception path between the OFDM radar sensor and the repeater is approximately twice as long as the propagation time or distance in the monostatic radar echo for the same radar object. For example, the OFDM radar sensor can be configured to assign a radar echo detected at a first frequency position in the spectrum of the monostatic signal component to a radar echo detected at a corresponding second frequency position in the spectrum of the component of the received signal corresponding to the modulated signal; wherein the first frequency position corresponds to one Doppler shift of the radar echo, and the second frequency position corresponds to twice the Doppler shift of the radar echo of the same radar target. Accordingly, the OFDM radar sensor can be configured to assign a delay detected in the monostatic component of the received signal of a radar echo, which delay corresponds to one signal propagation time of the radar echo between the sensor and the radar target, to a delay detected in the monostatic component of the received signal of the radar echo of the same radar target, which delay corresponds to two passes through the transmission and reception path between the sensor, the radar target and the repeater.

[0036] Therefore, for the estimation of the distance and velocity of the radar object, a bistatic radar echo is considered: twice passing through the transmit and receive path between the OFDM radar sensor and the repeater.

[0037] 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 being arranged at separate locations on the motor vehicle. The object is further achieved by a motor vehicle having an OFDM radar sensor system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The following further describes the embodiments based on the accompanying drawings. The accompanying drawings show:

[0039] Figure 1 A schematic sketch of a motor vehicle having an OFDM radar sensor system with an OFDM radar sensor and a repeater is shown;

[0040] Figure 2 A schematic diagram showing a repeater;

[0041] Figure 3 shows a portion of the spectrum of an OFDM symbol;

[0042] Figure 4A schematic illustration showing the signal change process of an OFDM symbol;

[0043] Figure 5 A schematic diagram showing a radar echo of an OFDM symbol;

[0044] Figure 6 shows the schematic structure of an OFDM radar sensor;

[0045] Figure 7 A basic circuit diagram showing the modulator of a repeater;

[0046] Figure 8 A schematic diagram showing another example of a repeater;

[0047] Fig. 9 a schematic diagram showing a radar echo according to another example OFDM symbol; and

[0048] Fig.10 A portion of the spectrum of an OFDM symbol according to another example is shown. DETAILED DESCRIPTION

[0049] exist Figure 1 The OFDM radar sensor system shown in FIG. 1 is installed in a motor vehicle 10 and comprises a transmitting and receiving unit in the form of an OFDM radar sensor 12 and an active repeater 14, which are installed in the motor vehicle 10, for example at the front of the vehicle, at a lateral spacing B from one another. The OFDM radar sensor 12 generates and emits a transmit signal 16, which is reflected or scattered by a radar target 18 and received by the OFDM radar sensor 12 as a radar echo 20. The reflected transmit signal 16 is also received by the repeater 14 as a radar echo 22.

[0050] The repeater 14 amplifies the signal generated and transmitted by the OFDM radar sensor 12 and received by the repeater 14 as a radar echo 22, and modulates the signal into a signal 24 orthogonal to the radar echo 22, which is transmitted by the repeater 14. The signal 24 transmitted by the repeater is reflected again by the radar target 18 and received by the OFDM radar sensor 12 as a modulated radar echo 26. Therefore, the received signal of the OFDM radar sensor 12 includes a monostatic component including the direct radar echo 20 of the radar target 18 and a bistatic component including the modulated radar echo 26 and thus corresponding to the modulated signal 24.

[0051] For example, OFDM radar sensor 12 may be an angle-resolved OFDM radar sensor, by means of which the angle at which signal 20 is received by radar target 18 may be estimated. For example, the repeater 14 may be a transceiver having one or more transmitting or receiving antennas with a relatively wide field of view in elevation and in azimuth. For example, the field of view of the repeater 14 may correspond to the field of view of the OFDM radar sensor 12 within a given distance range. Figure 1 As described in FIG. 1 , the signal 22 reflected to the repeater 14 can thus be forwarded to the OFDM radar sensor 12 on the same transmission and reception path after being reflected again at the radar target 18. The angle at which the radar echo 22 is received by the repeater Possible angle Different, and thus off-center, radar targets 18 also provide monostatic and bistatic radar returns 20 , 26 .

[0052] Figure 2 A repeater 14 is schematically shown with a receiving antenna 28 and a transmitting antenna 30. Figure 2 , the direction from which a portion of the transmitted signal 24 is indicated, from which direction the signal 24 is reflected again at the radar target 18 and is received by the OFDM radar sensor 12 .

[0053] The repeater 14 comprises at least one amplifier, in the example a receiving amplifier 32 and a transmitting amplifier 34. In addition, the repeater 014 comprises a modulator 36 for frequency shifting the received signal and re-transmitted in a modulated manner. The modulator 36 subjects the received signal 22 to a frequency shift of a predetermined frequency shift Δf0, corresponding to a shift in the phase of the complex frequency of the signal 22, wherein the phase changes depending on the resonance. The phase shift is controlled by the amplitudes I, Q of the I / Q signal components, as described below with Figure 7 As illustrated by the example.

[0054] Figure 3 A portion of the spectrum of an OFDM symbol of a transmission signal 16 is schematically shown. The signal amplitude A at frequency f is shown. In an OFDM signal with an OFDM symbol of symbol duration T, subcarriers are available for OFDM modulation whose minimum frequency spacing Δf satisfies the orthogonality condition T=1 / Δf. In the case of a subcarrier frequency spacing of Δf, within the symbol duration T, the number of oscillation periods of the subcarriers differs by exactly one period or a multiple thereof, so that the subcarriers are mutually orthogonal.

[0055] In the transmit signal 16 of the OFDM radar sensor 12, only every nth subcarrier is occupied in an OFDM symbol. Figure 3 In the example shown in , n = 2. For simplicity, Figure 3 Only two occupied subcarriers of an OFDM symbol at frequencies f1 and f2 are shown.

[0056] Signal 16 is Figure 3 The spectrum shown in corresponds to the spectrum of the radar echo 22 received by the repeater 14—ignoring the Doppler shift. The received signal 22 generally contains a superposition of time-delayed and possibly Doppler-shifted radar echoes, wherein Figure 3 Only part of the spectrum of a single radar echo 22 is shown in FIG. The modulator 36 causes the signal 22 to be frequency-shifted by a frequency offset Δf0, which corresponds to the minimum subcarrier spacing Δf. The spectrum of the signal 24 amplified and forwarded by the repeater 14 is shown in FIG. Figure 3 Schematically shown in dashed lines with the same amplitude. Due to the frequency shift, the signals 22, 24 are orthogonal to each other. The subcarriers occupied by the modulated radar echo are located in the gaps of the OFDM symbol received as radar echo 22, as shown in Figure 3 As described in .

[0057] Figure 4 The signal profile of an OFDM symbol of a transmission signal 16 over time t is schematically shown. In accordance with the OFDM modulation scheme, the individual occupied subcarriers of the signal are modulated, wherein, for example, each occupied subcarrier is modulated with a complex amplitude.

[0058] Figure 5 A monostatic radar echo 20 contained in the received signal of an OFDM radar sensor 12 is schematically illustrated, comprising a plurality of first frequency ranges 38, in which the corresponding subcarriers are located, and an OFDM symbol contained in a bistatic radar echo 26 in a second frequency range 40. The monostatic signal component 20 received by the OFDM radar sensor 12 is located in the first frequency range 38 and generally comprises a superposition of time-delayed and possibly Doppler-shifted radar echoes. In contrast, the bistatic received radar echo 26 additionally has a frequency shift of a frequency spacing Δf0 and also a delay and possibly a Doppler shift, which corresponds to two passes through the transmission and reception path via the radar target 18. Due to the different frequency ranges 38, 40, the monostatic and bistatic signal components can be processed separately.

[0059] Figure 6The basic circuit diagram of an OFDM radar sensor 12 with a transmission branch 42 and a reception branch 44 is schematically shown. For each OFDM symbol step, the modulation symbol s, which comprises the sub-symbols of the individual subcarriers, is converted into an OFDM symbol x in the time domain by means of an inverse Fourier transform. The OFDM symbol x comprises here in a manner known per se the actual OFDM symbol of symbol length T and a prefix (cyclic prefix), which is a copy of the end segment of the OFDM symbol. The OFDM symbol x is converted by a DA converter into an analog signal, with the aid of which an I / Q modulator 45 modulates the transmission frequency f0 of the local oscillator LO in order to generate the transmission signal 16.

[0060] In the receiving branch 44, the received signal containing the signal components 20 and 26 is demodulated in an I / Q demodulator 46 with the aid of the radar frequency of the local oscillator LO, digitized by an AD converter and Fourier transformed with the aid of an FFT. In the Fourier transformation, the subcarriers contained in the received signal 20, 26 are mapped to separate frequency positions in the frequency spectrum.

[0061] Then, the first frequency range 38 of the spectrum and the second frequency range 40 of the spectrum are further processed separately in separate processing branches. For the first frequency range 38 corresponding to the monostatic radar echo, the received signal is subjected to complex spectrum division by the transmitted OFDM signal s. This can be called normalization of the received signal components. This processing is performed for the OFDM symbols s that are consecutive to each other measured by the OFDM radar. Therefore, in the two-dimensional spectrum E1, the sum of the complex exponentials generated by the delay and Doppler shift is obtained, corresponding to the sequence of subcarriers and OFDM symbols s.

[0062] In contrast, the signal components of the second frequency range 40 corresponding to the modulated signal of the repeater 14 are additionally demodulated in the form of a frequency shift of the frequency spacing Δf0 with which the transmitted signal was modulated by the repeater 14. The further processing with the complex spectrum division by the sequence of OFDM symbols s then corresponds to the processing of a monostatic signal and results in a two-dimensional spectrum E2.

[0063] The corresponding detection device 47 analyzes the two-dimensional spectra E1, E2 obtained for the frequency ranges 38 and 40 in two separate processing branches and detects radar objects based on the peaks in the spectra E1, E2. The analysis device 48 analyzes the detected radar objects. Here, the radar objects detected based on the signal from the second frequency range 40 at the frequency position corresponding to twice the Doppler shift of the radar echo are linked to the radar objects detected based on the signal from the first frequency range 38 at the frequency position corresponding to the corresponding one-time Doppler shift. In a similar manner, the objects detected based on the signal from the second frequency range 40 with twice the propagation time are assigned to the corresponding objects detected based on the signal from the first frequency range 38 with once the propagation time.

[0064] Figure 7 The basic circuit diagram of the modulator 36 is schematically shown. The input terminal 49 of the modulator 36 is connected to the I / Q splitter 50, which splits the input signal of the modulator 36 into an in-phase signal and a quadrature signal. In other words, the I / Q splitter 50 Figure 7 An input signal with a 0° phase shift is provided in the first signal branch in the upper middle part and Figure 7 The input signal is provided with a 90° phase shift in the second signal branch in the lower middle part. The I / Q splitter 50 is constructed in a manner known per se from a passive LC network. The upper and lower signal branches each comprise a multiplier 52 or 54, which Figure 7 , which is symbolically illustrated by an amplifier with adjustable amplitude and a sign representing a possible sign change of the signal. The multipliers 52, 54 receive the I / Q components of the modulation signal as further input variables. The modulation signal involves a resonance with a frequency corresponding to the frequency shift Δf0, for example, I=sin (2πtΔf0) and Q=cos(2πtΔf0). The outputs of the multipliers 52, 54 are summed in a phase-corrected manner at the output 56 of the modulator 36 by means of a summing element 58. Thus, a phase shift of the input signal is performed by the modulator 36, wherein the complex phase shift vector rotates (rotates) with the frequency Δf0.

[0065] The described embodiments are presented by way of example for the purpose of illustrating the invention and may be modified.

[0066] Thus, for example, instead of Figure 2 The repeater 14 shown in FIG. Figure 8 The repeater 14' shown in FIG. Figure 2The example of FIG. 1 is different in that a common transmit / receive antenna 60 is provided, which is connected to the modulator 36 via a directional coupler 42 and an input amplifier 32 and / or an output amplifier 34. The directional coupler 62 includes three input / output connections. The directional coupler 62 couples the signal received by the antenna 60 into the modulator 36 via the amplifier 32. The output signal of the modulator 36 or the amplifier 34 is coupled in the direction of the antenna 60 for transmission.

[0067] Fig. 9 According to another embodiment, a Figure 5 . In this example, the frequency shift Δf0 caused by the modulator 36 of the repeater 14 corresponds to the occupied bandwidth of the OFDM symbol or is greater than this bandwidth. In the OFDM symbol of the transmission signal of the OFDM radar sensor 12, in this example, subcarriers that follow one another immediately are occupied, which are together contained in the first frequency range 38. The radar echo received by the bistatic signal is contained in a second frequency range 40, which is offset by a frequency shift Δf0 relative to the first frequency range 38. When q consecutive subcarriers are used for the OFDM symbol, the frequency spacing in this example is qΔf, where Δf is the spacing between two consecutive subcarriers.

[0068] Figure 2 and Figure 5 The above-described example of can be generalized in a corresponding manner to an OFDM radar sensor system having a plurality of active repeaters 14 . Fig.10 Corresponding to Figure 3 , a portion of an OFDM symbol of another example of a transmitted signal 16 is shown in the diagram, wherein only every third subcarrier is occupied. Then, another repeater 14 can generate a modulated signal by a frequency shift of 2Δf0, which is orthogonal to the signal of the first repeater 14 shifted by Δf0. Then, the OFDM radar sensor 12 can distinguish the radar echoes from different repeaters 14 from each other and from the monostatic radar echo based on the position of the radar echoes in the corresponding frequency range, and process the radar echoes separately in three processing branches respectively.

Claims

1. An OFDM radar sensor system having a plurality of transmitting and receiving units (12, 14), wherein: One of the transmitting and receiving units is an OFDM radar sensor (12), wherein the other of the transmitting and receiving units is a repeater (14), the repeater being configured to modulate a signal (22) generated and transmitted by the OFDM radar sensor (12) and received by the repeater (14) into a signal orthogonal to the signal (22) received by the repeater (14) and to emit a modulated signal (24), wherein the OFDM radar sensor (12) is configured to separate a component (26) of the signal (20, 26) received by the OFDM radar sensor (12) corresponding to the modulated signal (24) from a monobasic component (20) of the signal (20, 26) received by the OFDM radar sensor (12 ... component (26) of the signal (20, 26) received by the OFDM radar sensor (12) is separated from the monobasic component (20) of the signal (20, 26) received by the OFDM radar sensor (12), wherein the component (26) The invention relates to a radar object detected at a frequency position corresponding to twice the Doppler shift of a radar echo by a component (26) of a signal (20, 26) received by an OFDM radar sensor (12) corresponding to the modulated signal (24) and associated with a radar object detected at a frequency position corresponding to twice the Doppler shift of a radar echo based on a monostatic component (20) of the signal (20, 26) received by the OFDM radar sensor (12) at a frequency position corresponding to once the Doppler shift, and / or to a radar object detected based on a component (26) of a signal (20, 26) received by the OFDM radar sensor (12) corresponding to the modulated signal (24) and having twice the propagation time, to a radar object detected based on a monostatic component (20) of the signal (20, 26) received by the OFDM radar sensor (12) and having once the propagation time.

2. The OFDM radar sensor system according to claim 1, wherein the signal emitted by the repeater (14) contains a signal received by the repeater (14) which is frequency shifted by a predetermined frequency offset (Δf0).

3. The OFDM radar sensor system according to claim 2, wherein: The predetermined frequency offset (Δf0) is a frequency offset (Δf0) in which, in the case of the frequency offset, the OFDM subcarriers contained in the signal (22) received by the repeater are orthogonal to the corresponding OFDM subcarriers in the modulated signal (24) offset by the frequency offset (Δf0).

4. An OFDM radar sensor system according to any one of the preceding claims, in which the repeater (14) is configured to modulate the signal (22) generated and transmitted by the OFDM radar sensor (12) and received by the repeater (14) into a signal (24) orthogonal to the signal (22) received by the repeater (14) by frequency shifting it with a predetermined frequency offset (Δf0).

5. The OFDM radar sensor system according to any one of claims 2 to 4, wherein the frequency shift is performed by a phase shift of the I / Q signal with the predetermined frequency offset (Δf0), wherein: The phase shift varies according to the resonance at a frequency corresponding to the predetermined frequency shift (Δf0).

6. The OFDM radar sensor system according to claim 1 , wherein the repeater ( 14 ) has a modulator ( 36 ) for frequency shifting a signal ( 22 ) received by the repeater ( 14 ) by a predetermined frequency offset ( Δf0 ), wherein: The modulator (36) has: an I / Q splitter (50) arranged to provide, from the signal (22) received by the repeater (14), I / Q signal components, the I / Q signal components being phase-shifted relative to each other by 90° with respect to a reference radar frequency (f0), a multiplier (52, 54) configured to sign-multiply the respective I / Q signal component by the respective I / Q modulation signal component of the modulation signal, wherein the modulation signal has a frequency corresponding to the predetermined frequency offset (Δf0), and An output terminal (56) at which the output signal components of the multipliers (52, 54) are combined.

7. The OFDM radar sensor system according to claim 1 , wherein the signal ( 16 ) generated and transmitted by the OFDM radar sensor ( 12 ) has unoccupied OFDM subcarriers in the frequency spectrum, wherein: The repeater (14) is configured to generate frequency-shifted OFDM subcarriers from occupied OFDM subcarriers in a signal received by the repeater (14), wherein the frequency-shifted OFDM subcarriers are located in a frequency range that corresponds to a frequency range of unoccupied OFDM subcarriers in the signal received by the repeater (14).

8. The OFDM radar sensor system according to any one of the preceding claims, in which the transmission signal (16) transmitted by the OFDM radar sensor (12) occupies only every n-th subcarrier, wherein n is a natural number greater than 1, wherein the modulated signal (24) is shifted with respect to the signal (22) received by the repeater (14) by a frequency offset (Δf0), the frequency offset corresponding to (m+pn) times the subcarrier spacing (Δf), wherein m is a natural number less than n and p is an integer.

9. An OFDM radar sensor system according to any of the preceding claims, in which a component (26) of the signal (20, 26) received by the OFDM radar sensor (12) corresponding to the modulated signal (24) is separated from a monostatic component (20) of the signal (20, 26) received by the OFDM radar sensor (12) by separately evaluating the frequency range (38, 40) of the received signal (20, 26).

10. An OFDM radar sensor system according to any of the preceding claims, in which the OFDM radar sensor (12) is configured to detect OFDM symbols (s) corresponding to monostatic radar echoes in one or more first frequency ranges (38) of the signal (20, 26) received by the OFDM radar sensor (12), and to detect radar echoes corresponding to OFDM symbols of bistatic radar echoes (26) of the signal (24) modulated by the repeater (14) in one or more other, second frequency ranges (40) of the signal (20, 26) received by the OFDM radar sensor (12).

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