Radar sensor with a digitally modulated transmission signal

Abstract

The invention relates to a radar sensor having a transmitting part (10) comprising a digital modulation stage (16, 18) for generating a transmission signal (c) by digitally modulating a base signal (a), and having a receiving part (12) comprising a digital demodulation stage (24) and a mixer (20) which mixes the received signal with the base signal and feeds the mixing result to the digital demodulation stage (24), characterized in that the transmitting part (10) has a base signal generator (14) for generating a frequency-modulated signal as the base signal (a).

Description

[0001] R. 413960

[0002] - 1 -

[0003] Description

[0004] title

[0005] Radar sensor with digitally modulated transmission signal

[0006] Description

[0007] The invention relates to a radar sensor with a transmitter part comprising a digital modulation stage for generating a transmit signal by digital modulation of a base signal, and with a receiver part comprising a digital demodulation stage and a mixer that mixes the received signal with the base signal and feeds the mixing result to the digital demodulation stage.

[0008] In particular, the invention relates to a radar sensor that serves to detect the traffic environment in a driver assistance system for motor vehicles or in an autonomously driving vehicle.

[0009] State of the art

[0010] Some well-known radar sensors use linear frequency chirps as their transmit signal. After transmission and reception, these chirps are reflected and, in the receiver section of the radar sensor, mixed "analogously" with the original transmit signal to create a baseband signal. The frequency of this baseband signal depends primarily on the distance to a detected target and therefore allows for distance measurement. In the case of relative movement of the target, the rapidly transmitted chirps exhibit relative phase shifts, which also enable the determination of the target's radial velocity. R. 413960

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[0012] In radar sensors of this type, the signals from nearby targets, which appear as strong signals in the receiver due to lower free-space path loss, can be attenuated by a high-pass filter. This reduces the difference in reception performance between near and far targets (dynamic range). As a result, the signal amplification in the receiver can be optimized for more distant targets, and the analog-to-digital converter (ADC), which digitizes the baseband signal, only needs to have a lower resolution.

[0013] Other well-known radar sensors operate with digitally modulated transmit signals. To generate the transmit signal, a local oscillator produces a base signal with a fixed frequency. A modulation signal is then superimposed on this base signal in a modulator. This modulation signal is generated using a digital-to-analog converter (DAC) based on digital signals. In the receiver, the received signal is mixed with the base signal to obtain a signal in an intermediate frequency range. This signal is then digitized by a fast analog-to-digital converter (ADC). The actual demodulation of the received signal then takes place using digital signal processing.

[0014] This digital modulation technique allows the generation of more complex modulation signals, such as PMCW (Phase Modulated Continuous Wave) or OFDM (Orthogonal Frequency Division Multiplex) waveforms, enabling flexible adaptation to different applications. For example, in an angle-resolving MIMO (Multiple Input Multiple Output) radar, the transmitted signals from several simultaneously transmitting antennas can be orthogonalized using OFDM, allowing the signals from the different antennas to be separated at the receiver. However, with known radar sensors using digital modulation, it is no longer possible to perform distance-dependent amplitude compression using high-pass filtering, as is possible with a chirp radar. Furthermore, the AD and DA converters must be designed with very wide bandwidth to process the modulation signals. R. 413960

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[0016] Disclosure of the invention

[0017] The object of the invention is to create a radar sensor that enables flexible adaptation to different tasks and situations, while allowing the use of DA and AD converters with lower bandwidth.

[0018] This problem is solved according to the invention by the fact that the transmitting part has a basic signal generator for generating a frequency-modulated signal as the basic signal.

[0019] The invention thus proposes the use of a "hybrid" modulation method, which achieves many of the advantages of complex digital modulation signals while simultaneously avoiding disadvantages compared to the existing chirp radar method or broadband purely digital modulation processing. The advantages of analog demodulation (also referred to as "correlation" in radar technology) of a chirp radar can thus be combined with the advantages of complex modulation signals, in order, for example, to simultaneously detect the near and far ranges with different radar parameters (e.g., different range resolution, velocity unambiguity, field of view, etc.).

[0020] In particular, the invention offers the following advantages:

[0021] By using a "basic chirp", distance-dependent amplitude compensation is possible using a high-pass filter or band-pass filter by using a chirp signal for modulation.

[0022] Complex modulation signals such as FDM (Frequency Division Multiplex), CDM (Code Division Multiplex), or DDM (Doppler Division Multiplex) can be used, and target location can be achieved using different radar parameters simultaneously. By superimposing the modulation signal onto an already frequency-modulated base chirp, the bandwidth of the combined signal (the transmitted signal) is spread, so that the D / A converter in the transmitting section only needs to have a small bandwidth. R. 413960

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[0024] Non-idealities (e.g., non-linearities) of analog components such as mixers, power amplifiers, and transmitter modulators can be compensated for by adjusting (pre-distortion) the modulation signal generated by the DA converter.

[0025] In the receiving section, mixing with the base chirp causes a compression of the bandwidth (reciprocal to the transmitting operation), so that the AD mixer only needs to have a smaller bandwidth.

[0026] It is also possible to fall back to the classic chirp operation by switching the modulator to a "blow-through" mode in which no additional digital modulation is applied.

[0027] The following section explains an exemplary embodiment in more detail with reference to the drawing. The drawing shows:

[0028] Fig. 1 A block diagram of a radar sensor according to the invention;

[0029] Fig. 2(a)-(e) Frequency / time diagrams of different signals in the radar sensor according to Fig. 1 ;

[0030] Fig. 3 (a)-(c) Diagrams analogous to Fig. 2(a)-(c) for another modulation example;

[0031] Fig. 4(a)-(c) Diagrams analogous to Fig. 2(a)-(c) for a further modulation example; and Fig. 5 a block diagram of a radar sensor according to the state of the art.

[0032] Technology.

[0033] To explain the operating principle of a radar sensor according to the invention, a prior art radar sensor that generates a digitally modulated transmission signal will first be described with reference to Fig. 5. The radar sensor has a

[0034] A transmitter section 10' and a receiver section 12', separated by a dashed line in Fig. 5. In the transmitter section 10', a signal generator 14' produces a high-frequency signal (base signal) with a fixed frequency of, for example, 77 GHz. R. 413960

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[0036] The DA converter 16' and modulator 18' together form a digital modulation stage for modulating the base signal. Digital signals DS are fed to the DA converter 16', which are then converted within the converter into an analog modulation signal. The frequency response of this signal is shown in a small frequency-time diagram next to the converter's symbol. In the example shown, the modulation signal is a multi-tone signal in which two signal components with different frequencies are superimposed. One signal component consists of relatively flat frequency ramps (chirps) that repeat periodically. The other frequency component is modulated in the form of steeper frequency ramps with a higher center frequency and passes through two ramps in each modulation cycle. In general, the modulation signal can consist of various components, e.g.Frequency chirps with different start times, offset frequencies, frequency slopes, or from completely different modulation forms such as OFDM and PMCW. In the modulator 18', the modulation signal is superimposed on the fixed frequency signal generated by the signal generator 14'. In this way, a modulated transmit signal is created, which is radiated via a transmitting antenna TX.

[0037] The signal reflected from the radar target is received in the receiver section 12' by a receiving antenna RX. The received signal is then mixed in a mixer 20' with the fixed-frequency signal from the signal generator 14'. This produces an analog intermediate frequency signal whose frequency response and composition are comparable to the modulation signal, but which encodes information about the distances and velocities of the detected radar targets. This intermediate frequency signal is digitized in an analog-to-digital converter 22', and the digital data is fed to a digital demodulation stage 24'. Further digital processing then takes place in a processor 26', where the distances and relative velocities of the detected radar targets are determined in a known manner.In practice, the radar sensor will usually have several transmit and receive channels, so that the tracking angles of the radar targets can also be determined in the processor 26°.

[0038] In the method described above, the A / D converter 22' must operate very quickly, as it has to sample the entire bandwidth of the receiving channel. R. 413960

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[0040] Figure 1 shows a block diagram of a radar sensor according to the invention. This radar sensor also has a transmitter 10 and a receiver 12. In the receiver, a base signal generator 14, e.g., a modulated LO source (e.g., classical VCO+PLL) or a DDS-based frequency generator, generates a frequency-modulated base signal (a), which is modulated, for example, like a transmit signal of a chirp radar. Similar to Figure 5, a DA converter 16 in a digital modulation stage generates a modulation signal (b), which, however, is now not superimposed on a fixed-frequency signal in a modulator 18, but on the already frequency-modulated base signal (a). Further frequency components are thus modulated onto the base signal. Ideally, these are implemented with a smaller signal amplitude or with corresponding pre-distortion, so that a downstream transmitter output stage can operate with high efficiency while avoiding the emergence of nonlinearities, known, for example, from...It can be operated from typical OFDM radars. The modulated signals, for example frequency-shifted chirps for FDM, phase-modulated chirps for DDM, or complex modulation signals, are designed so that they do not overlap in the time and frequency spectrum and therefore cannot interfere with each other.

[0041] The power of the additionally modulated component relative to the power of the base signal can be adjusted as needed. This can be used to create a

[0042] To generate a multi-tone signal that is virtually unaffected by the non-idealities of the output stage that typically become problematic when using multi-tone signals. The distribution of signal power can then be adjusted depending on the respective frequency components of the transmitted signal (e.g., long-range mode with more power and short-range mode with less power).

[0043] A transmit signal (c) generated by digital modulation is radiated by a transmitting antenna TX, and the radar echo is received by a receiving antenna RX and mixed with the base signal (a) in a mixer 20. This yields an intermediate frequency signal (d), which is filtered in a bandpass or highpass filter 28 and then digitized in an analog-to-digital converter 22. Since the received signal has been mixed with the frequency-modulated base signal (a) in the mixer 20, partial demodulation of the received signal has already taken place in the mixer. The final R. 413960

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[0045] Demodulation then takes place in a digital demodulation stage 24, followed by further processing in a processor 26.

[0046] Figure 2(a) shows the frequency response of the basic signal (a). In the example shown, this signal consists of a sequence of chirps 30 with the same duration, the same slope, and the same center frequency.

[0047] The frequency response of the modulation signal (b) is shown in Fig. 2(b). In this example, the modulation signal has a narrowband component 32 with a relatively low frequency, which is constant in this example, and a wider, higher-frequency component, which in this example consists of a sequence of chirps 34 with varying steepness. The two components can be optimized separately depending on the sensing task (e.g., a first signal for long-range operation and a second modulation signal for short-range operation). Further independent modulation signals can be added, which can then be correlated in the receiver in combination with the first and the nth transmit modulation signals.

[0048] Fig. 2(c) shows the transmitted signal (c) generated in the modulator 18. This signal contains chirps 36, which are created by modulating the chirps 30 with the component 32, as well as steeper and higher-frequency chirps 38, which are created by modulating the chirps 30 with the chirps 34 of the modulation signal.

[0049] Fig. 2(d) shows the frequency response of the intermediate frequency signal (d) at the output of mixer 20. As a result of partial demodulation, this signal has a frequency response comparable to that of the modulation signal (b). Accordingly, signal (d) has a component 40 with an approximately constant frequency and a component consisting of chirps 42 with varying steepness. Fig. 2(e) shows the frequency response BP of the bandpass filter 28. It can be seen that the chirps 42 pass through the bandpass filter practically unchanged, while component 40 is attenuated more strongly the lower its frequency. The frequency of signal (d) is a beat frequency equal to the frequency difference R. 413960

[0050] - 8 - between the base signal (a) and the received signal (c). Since this frequency difference is essentially proportional to the signal propagation time from the radar sensor to the target and back to the radar sensor, the frequency of the signal (d) is a measure of the distance to the radar target. Since the amplitude of the received signal also depends on this distance, the bandpass filter 28 causes the frequency component 40 to be attenuated more strongly the closer the target is and, consequently, the larger the amplitude of the received signal.

[0051] Comparing Figures 2(c) and 2(d), it can be seen that the partial demodulation of the received signal in mixer 20 significantly reduces the signal's bandwidth. This has the advantage that the analog-to-digital converter 22 also does not need to have a particularly high bandwidth.

[0052] The component 40 resulting from the chirp signal 36 can be evaluated in the conventional manner. The wider-bandwidth component 42, however, must first be digitally correlated in the demodulation stage 24. In the case of OFDM, this can be done, for example, using a matched filter; in the case of another chirp signal, by means of a digital mixing process. Since both signal components can be evaluated in parallel, it is possible to take the information from the other signal component into account during evaluation. For example, the Doppler evaluation of the chirp signal can be used to correct an OFDM signal by the respective Doppler shift and thus avoid inter-carrier interference before the matched filter operation decodes the subcarriers. When using more than one transmitter, a different signal can be modulated onto each transmitter, and optionally, the transmitted signal can be selected for downmixing in the receiver.

[0053] In the example shown in Fig. 2, the chirp sequences 34 in the modulation signal (b) repeat periodically with a repetition time T equal to the repetition time of the chirps 30 in the base signal. However, modulation patterns with different repetition times are also conceivable. Fig. 3 shows an example where the modulation signal (b) contains chirps 44 with a repetition time R. 413960

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[0055] T m only half the repetition time T of the chirps 30 is In this case the modulated transmit signal (c) contains chirp sequences 46, 48 with the repetition time T.

[0056] Fig. 4, on the other hand, shows an example where the modulation signal (b) contains 50 chirps, which have a slightly longer repetition time T. m have such that the repetition time T of the chirps 30 is not an integer multiple of T mTo nevertheless obtain chirps 46, 48 with the repetition time T in the modulated transmitted signal (c), the digital control of the DA converter 16 must be changed. At the time when the second chirp 50 begins in Fig. 4(b), chirp 30 has not yet completed its first cycle. Rather, chirp 30 only ends at a time t. s Since the frequency of the base signal (a) drops abruptly at this point, the frequency of the corresponding chirp 48 in the transmitted signal (c) would also decrease abruptly. To compensate for this, the frequency of the second chirp 50 in the modulation signal in Fig. 4(b) is adjusted at time T s increased dramatically.

[0057] By adjusting the modulation bandwidth and repetition rate, the signal components can be conditioned for different tasks. In a system with more than one transmitter, multiple transmitters can transmit a phase-coherent signal component (either a local oscillator chirp or a broadband signal) in such a way that, through the positioning and relative phase of the signals, analog beamforming is achieved at the transmitter side. This, together with the design of the bandwidth and repetition rate, can be used, for example, for a long-range application.

Claims

R. 413960 - 10 - Claims 1. Radar sensor with a transmitter (10) comprising a digital modulation stage (16, 18) for generating a transmit signal (c) by digital modulation of a base signal (a), and with a receiver (12) comprising a digital demodulation stage (24) and a mixer (20) that mixes the received signal with the base signal and supplies the mixing result to the digital demodulation stage (24), characterized in that the transmitter (10) comprises a base signal generator (14) for generating a frequency-modulated signal as the base signal (a).

2. Radar sensor according to claim 1, wherein the basic signal (a) is a chirp signal 3. Radar sensor according to claim 1 or 2, wherein a modulation signal (d) generated in the digital modulation stage (16, 18), which is modulated up to the basic signal (a), has a narrowband component (32) and at least one broadband component (34) whose frequency is higher than that of the narrowband component (32).

4. Radar sensor according to claim 3, wherein the at least one broadband component (34) is an OFDM signal 5. Radar sensor according to claim 3 or 4, wherein the at least one broadband component (34) is a PMCW signal.

6. Radar sensor according to any one of claims 3 to 5, wherein the at least one broadband component is an FDM, CDM or DDM signal. R. 413960 - 11 - 7. Radar sensor according to one of claims 3 to 6, wherein the at least one broadband component (34) is a chirp signal 8. Radar sensor according to one of the preceding claims, wherein both the basic signal (a) and a modulation signal (b) generated in the digital modulation stage (16, 18) are cyclic signals.

9. Radar sensor according to claim 8, wherein the base signal (a) and the modulation signal (b) have the same cycle time (T).

10. Radar sensor according to claim 8, wherein the cycle time (T) of the base signal (a) is an integer multiple of the cycle time (T m ) of the modulation signal (b).

11. Radar sensor according to one of the preceding claims, wherein the receiver part (12) has a bandpass or highpass filter (28) for filtering the output signal of the mixer (20).

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