OFDM RADAR SENSOR SYSTEM HAVING AN ACTIVELY RETRANSMITTING REPEATER

Abstract

An OFDM radar sensor system having a plurality of transmitting and receiving units. One of the transmitting and receiving units is an OFDM radar sensor, and 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. The OFDM radar sensor is 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.

Claims

1-10. (canceled)

11. An OFDM radar sensor system, comprising: 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, into a signal orthogonal to the signal received by the repeater, and to emit the modulated signal, 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.

12. The OFDM radar sensor system as recited in claim 11, wherein the signal emitted by the repeater includes the signal received by the repeater, shifted in frequency by a predefined frequency shift.

13. The OFDM radar sensor system as recited in claim 12, wherein the predefined frequency shift is a frequency shift in which an OFDM subcarrier included 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.

14. The OFDM radar sensor system as recited in claim 11, wherein the repeater is configured to modulate the signal generated and transmitted by the OFDM radar sensor and received by the repeater, using a shift in frequency by a predefined frequency shift, into the signal orthogonal to the signal received by the repeater.

15. The OFDM radar sensor system as recited in claim 12, wherein the shift in frequency by the predefined frequency shift is carried out by shifting a phase of an I/Q signal, and the phase shift is varied in accordance with a harmonic oscillation, at a frequency corresponding to the predefined frequency shift.

16. The OFDM radar sensor system as recited in claim 11, wherein the repeater has a modulator for shifting a frequency of a signal received by the repeater by a predefined frequency spacing, the modulator including: an I/Q splitter configured to provide I/Q signal components which are 90° out of phase from each other with respect to a reference radar frequency, from the signal received by the repeater; multipliers configured to multiply the I/Q signal components by respective I/Q modulation signal components of a modulation signal while retaining an 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.

17. The OFDM radar sensor system as recited in claim 11, wherein the signal generated and emitted by the OFDM radar sensor includes unoccupied OFDM subcarriers in a frequency spectrum, and from occupied OFDM subcarriers in the signal received by the repeater, the repeater is configured to generate OFDM subcarriers, which are shifted in frequency and lie in frequency ranges that correspond to frequency ranges of unoccupied OFDM subcarriers in the signal received by the repeater.

18. The OFDM radar sensor system as recited in claim 11, wherein the transmitted signal transmitted by the OFDM radar sensor occupies only every nth subcarrier, n being a natural number greater than 1, and with respect to the signal received by the repeater, the modulated signal is shifted by a frequency shift, which corresponds to (m+pn) times a subcarrier spacing, m being a natural number less than n, and p is a whole number.

19. The OFDM radar sensor system as recited in claim 11, wherein a portion of the signal received by the OFDM radar sensor and corresponding to the modulated signal, is separated from a monostatic portion of the signal received by the OFDM radar sensor, by separately evaluating frequency ranges of the received signal.

20. The OFDM radar sensor system as recited in claim 11, wherein the OFDM radar sensor is configured to detect OFDM symbols, which correspond to monostatic radar echoes, in one or more first frequency ranges of the signal received by the OFDM radar sensor, 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 signal received by the OFDM radar sensor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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.

[0040] FIG. 2 shows a schematic representation of the repeater, in accordance with an example embodiment of the present invention.

[0041] FIG. 3 shows partial spectra of OFDM symbols.

[0042] FIG. 4 shows a schematic illustration of a signal characteristic of an OFDM symbol.

[0043] FIG. 5 shows schematic representations of radar echoes of OFDM symbols.

[0044] FIG. 6 shows a schematic layout of the OFDM radar sensor, in accordance with an example embodiment of the present invention.

[0045] FIG. 7 shows a schematic basic circuit diagram of a modulator of the repeater, in accordance with an example embodiment of the present invention.

[0046] FIG. 8 shows a schematic representation of a further example of a repeater, in accordance with the present invention.

[0047] FIG. 9 shows schematic representations of radar echoes of OFDM symbols according to a further example.

[0048] FIG. 10 shows partial spectra of OFDM symbols according to a further example.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0049] 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.

[0050] 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.

[0051] OFDM radar sensor 12 may be, for example, an angle-resolving OFDM radar sensor, by which the angle φ1 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 φ2, at which radar echo 22 is received by the repeater, may differ from angle φ1, which means that non-central radar targets 18 also deliver monostatic and bistatic radar echoes 20, 26.

[0052] 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.

[0053] 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 Δf0, 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.

[0054] 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.

[0055] 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 f1 and f2.

[0056] 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.

[0057] Modulator 36 effects a shift in frequency of signal 22 by a frequency shift Δf0, 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.

[0058] 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.

[0059] 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 Δf0 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.

[0060] 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 f0 of a local oscillator LO, in order to generate transmitted signal 16.

[0061] 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.

[0062] 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 E1 according to the subcarriers and the sequence of OFDM symbols s.

[0063] 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 Δf0, 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 E2 is obtained.

[0064] Respective detection devices 47 evaluate the 2-D spectra E1, E2 obtained in the two separate processing branches for frequency ranges 38 and 40 and detect radar objects from peaks in spectra E1, E2. 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.

[0065] 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 Δf0, e.g., I=sin(2πtΔf0) and Q=cos(2πtΔf0). 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 Δf0.

[0066] The exemplary embodiments described are given as examples for illustrating the present invention and may be modified.

[0067] 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.

[0068] FIG. 9 shows a representation corresponding to FIG. 5, in accordance with a further exemplary embodiment. In this example, the frequency shift Δf0 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 Δf0 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.

[0069] 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 2Δf0, a further repeater 14 may generate a modulated signal, which is orthogonal to the signal of first repeater 14 shifted by Δf0. 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.