Synchronization method for at least two digital radar sensors and radar network

Abstract

The invention relates to a multiple-input multiple-output (MIMO) radar network and a method for synchronizing such a multiple-input multiple-output radar network. The method comprises synchronizing at least two radar sensors generated by digital signals having a broadband by transmitting, by one radar sensor, at least two multiplexed frequency modulated signals having a defined frequency offset and receiving, by the other radar sensor, the signals. Through the defined frequency offset, the frequency offset between the radar sensors can be determined or estimated, and the clock generators can be synchronized with each other. The invention also relates to a radar network.

Description

Field of the Invention

[0001] The present invention relates to a multiple-input multiple-output (MIMO) radar network and a method for synchronizing such a multiple-input multiple-output radar network. In particular, the present invention relates to a method for synchronizing a radar network that is used for environmental detection in a driver assistance system of a motor vehicle, and to such a radar network. Background Art

[0002] Generally, the following modulation method is used for a radar network that is a component of a driver assistance system in a motor vehicle: the modulation method frequency-modulates a signal in a chirp method. Modern radar networks are so-called multiple-input multiple-output (MIMO) radar networks that have a plurality of transmitters and a plurality of receivers in the form of radar sensors. Here, for example, each radar sensor can be designed to be both a transmitter and a receiver at the same time. Here, chirp signals are transmitted by means of a plurality of transmitters, and the reflected signals are received by means of a plurality of receivers. Here, voltage-controlled oscillators (VCOs) are used to generate the chirp signals, and the received signals are demodulated by analog mixing with the transmitted signal before analog-to-digital conversion, thereby greatly reducing the bandwidth.

[0003] Radar sensors with broadband digital signal generation (hereinafter simply referred to as digital radar sensors) are also used, in which the signal is directly generated by a digital-to-analog converter at a sampling rate of at least several hundred MHz, for example greater than 200 MHz, using a pre-given local oscillator (LO). Similarly, the reflected signal is mixed with the pre-given local oscillator, and is provided for analysis and processing by means of an analog-to-digital converter at a sampling rate of at least several hundred MHz, for example greater than 200 MHz, without prior analog demodulation. Thus, such a digital radar sensor can operate in a large number of modulation methods, which significantly increases the scope of use of the digital radar sensor compared to an analog radar sensor. The digital radar sensor is also capable of receiving the complete bandwidth or most of the bandwidth of the signal spectrum used in the radar network at a single point in time, and has very good time resolution. In a multiple-input multiple-output (MIMO) radar network with digital radar sensors, a commonly used modulation method for generating transmitted signals is the frequency-division multiplexing method (FDM), for example the orthogonal frequency-division multiplexing method (OFDM).

[0004] Each radar sensor has its own clock generator (Clock) for signal generation and processing. Since an increasing number of individual radar sensors are installed in motor vehicles with driver assistance systems, it may be necessary to synchronize these radar sensors or their clock generators, especially to avoid interference between the radar sensors, to enable coherent signal processing of signals from different sensors, and to operate the radar network as a cooperative radar. In addition, each clock generator is subject to drift in its clock frequency, such as due to temperature drift. Therefore, it may be necessary to synchronize the clock generators regularly. Summary of the Invention

[0005] Accordingly, the object of the present invention is to provide a synchronization method for clock synchronization of a plurality of digital radar sensors, each of which has its own clock generator. Another object of the present invention is to implement such a method in a simple and low-cost manner without additional hardware components for connecting the radar sensors. Another object of the present invention is to provide a radar network operating with such a synchronization method.

[0006] According to the present invention, these objects are solved by a synchronization method according to the present invention for at least two digital radar sensors and a radar network according to the present invention having at least a first and a second digital radar sensor. Advantageous configurations and developments of the present invention are described below.

[0007] The synchronization method according to the present invention for at least two digital radar sensors comprises the following steps:

[0008] At least two signals are transmitted by a first radar sensor, wherein the signals are derived from the same clock generator or synchronized clock generators of the transmitting radar sensor, wherein the signals are modulated by a frequency division multiplexing method and have the same frequency variation process during a common transmission, and have a defined frequency offset Δf from each other; the transmitted signals are received by at least one additional radar sensor; the complex frequency is converted to an intermediate frequency band suitable for digitization; the received signals are analog-to-digital converted to complex digital signals; the complex signals are conjugated to obtain complex conjugate signals; the complex signals are mixed with the complex conjugate signals to obtain signals containing the frequency offset Δf of the received signals as the main frequency component; the signals are mixed with a complex sine wave or a complex square wave signal, wherein a complex sine wave or a complex square wave signal having a defined frequency offset Δf as the target frequency is generated from the local clock generator of at least one additional receiving radar sensor; the obtained signals are filtered to obtain the main frequency component; all the clock generators of at least one additional receiving radar sensor are controlled by using the main frequency component through a control loop such that the clock generators of at least one additional receiving radar sensor are synchronized with the clock generator of the transmitting radar sensor; or, based on the main frequency component, the transmitted signals and / or the received signals are corrected according to the estimation of the frequency offset between the transmitting radar sensor and at least one additional receiving radar sensor.

[0009] By deriving the transmitted signals from the same clock generator or synchronized clock generators of the transmitting radar sensor, the frequency offset of the signals corresponds to a defined frequency offset. Generally, Δf << f is applicable for applications in driver assistance systems c , wherein, f c is the carrier frequency of the radar signal. In these cases, the frequency offset Δf is only insignificantly affected by the Doppler frequency shift generated due to the reflection of the signal at the target, because the influence of the Doppler frequency shift on different components of the frequency division multiplexing modulated signal is almost the same. The signal propagation time also does not affect the received frequency of the signal. Therefore, the frequency offset of the received signal basically corresponds to the defined frequency offset of the transmitted signal.

[0010] At least one additional receiving radar sensor (hereinafter the term "receiving radar sensor" refers to at least one additional receiving radar sensor, but is not limited thereto and may also include a plurality of additional receiving radar sensors) receives the entire bandwidth of the signals mixed to the complex baseband and is capable of analog-to-digital converting the entire baseband bandwidth. The radar sensor operating by using the method according to the present invention is preferably a radar sensor (digital radar sensor) with broadband digital signal generation. The conjugation of the complex signal is performed by reversing the sign of the imaginary part of the complex signal to obtain a complex conjugate signal.

[0011] The complex signal is mixed with the complex conjugate signal by a complex multiplier. The signal thus obtained contains the frequency offset Δf of the received signal as the difference frequency, i.e., the defined frequency offset of the transmitted signal, which constitutes the main frequency component of the signal.

[0012] Mixing the signal containing the frequency offset Δf of the received signal as the main frequency component with a complex sine wave or a complex square wave signal generated using the defined frequency offset Δf produces a signal with a main frequency component close to 0 Hz. This is achieved as follows: The complex sine wave is generated from the local clock generator signal of the receiving radar sensor by means of a numerically controlled oscillator (NCO), or the complex square wave signal is generated from the local clock generator signal of the receiving radar sensor by means of a square wave generator. Here, using a square wave generator instead of an NCO reduces the cost of the corresponding radar sensor.

[0013] Due to the difference in the clock frequencies of the clock generators of the transmitting radar sensor and the receiving radar sensor (this difference is caused by the respective drifts of their clock frequencies as described above), a frequency offset occurs between the transmitting radar sensor and the receiving radar sensor. In the receiving radar sensor, the defined frequency offset -Δf is used as the target frequency for generating the complex sine wave or the complex square wave signal. Since the complex sine wave or the complex square wave signal is generated using the local clock generator, which is not synchronized with the clock generator of the transmitting radar sensor, the actual frequency of the complex sine wave or the complex square wave signal deviates from the defined frequency offset Δf of the transmitting radar sensor corresponding to the frequency offset between the radars.

[0014] In other words, the defined frequency offset Δf of the transmitting radar sensor does not correspond to the actual frequency offset Δf of the receiving radar sensor. Rx Therefore, mixing the complex sine wave or the complex square wave signal with the frequency of the actual frequency offset -Δf with the received signal (which contains the frequency offset Δf of the received signal as the main frequency component) does not result in exactly 0 Hz. Rx This can be illustrated by an example: In the case where the frequency offset between the transmitting radar sensor and the receiving radar sensor is 1% and the defined frequency offset between the transmitted signals is 50 MHz, if the clock generator of the transmitting radar sensor is 1% faster than the clock generator of the receiving radar sensor, a deviation of +0.5 Hz will be obtained after mixing with the complex sine wave or the complex square wave signal. Correspondingly, if the clock generator of the transmitting radar sensor is 1% slower than the clock generator of the receiving radar sensor, a deviation of -0.5 Hz will be obtained.

[0015]

[0016] ​Here, the main frequency component of the mixed signal is preferably filtered by means of a low-pass filter, for example by means of a moving-average filter, and the deviation of the main frequency component of the mixed signal from 0 Hz is obtained from the deviation between the radar sensors.

[0017] The filtered main frequency component can now be used to control all clock generators of at least one additional receiving radar sensor via a control loop, or the filtered main frequency component can be used to correct the transmitted signal and / or the received signal based on an estimation of the frequency offset between the transmitting radar sensor and at least one additional receiving radar sensor. Here, the transmitted signal and / or the received signal of the receiving radar sensor is preferably corrected, but the transmitted signal and / or the received signal of the transmitting radar sensor can also be corrected. In this case, the method includes the step of exchanging data between the receiving radar sensor and the transmitting radar sensor. Here, the method is not limited to control or correction; here, these steps can be combined and carried out simultaneously or in any order. For example, the control system can synchronize all clock generators of at least one additional receiving radar sensor with the clock generator of the transmitting radar sensor, and can use the estimation of the frequency offset to correct the remaining technical deviations when generating the transmitted signal and / or the received signal.

[0018] Here, synchronization does not mean perfect synchronization, but approximate synchronization. Depending on the intended use of the radar network synchronized by this method, this can include deviations of a few Hz that are technically tolerable. For example, the remaining deviations may be due to the persistent temperature drift of the clock generators of the individual radar sensors.

[0019] Here, the control loop is preferably a phase-locked loop (PLL: Phase-Locked-Loop) and / or a frequency-locked loop (FLL: Frequency-Locked-Loop). According to an embodiment of the method, for example, it is possible to switch between the PLL and the FLL, or they can be combined or used simultaneously.

[0020] According to a preferred embodiment of the present invention, the synchronization method includes a stop signal that, if no suitable input signal exists, pauses the control of all clock generators via the control loop and / or pauses the correction of the transmitted signal and / or the received signal based on the estimation of the frequency offset.

[0021] Preferably, the stop signal is controlled by a threshold detector that receives as input signal the filtered signal filtered to obtain the main frequency component and pauses the control loop when the input signal is below the threshold. Preferably, the stop signal is also and / or alternatively controlled by interference detection of the radar sensor and pauses the control loop in the presence of an interference signal. For example, the stop signal is controlled by a threshold detector that receives the filtered main frequency component as the input signal. When the signal has a power above the defined threshold, loop control and / or correction of the signal is activated, otherwise loop control and / or correction of the signal is paused.

[0022] Here, the output signal of the threshold detector can also be used for a plausibility check in which it is checked whether the received signal is not interference. This, like controlling the stop signal via interference detection, increases the robustness of the method according to the invention.

[0023] According to another preferred embodiment of the invention, the control loop includes a programmable reference oscillator, and all clock generator signals of at least one additional receiving radar sensor originate from this reference oscillator.

[0024] Here, the programmable reference oscillator is arranged in the control loop and is preferably tracked. Preferably, the clock generator signal for complex frequency conversion and the clock generator signal for the carrier frequency of the receiving radar sensor originate from this reference oscillator, which can be the same frequency. Further preferably, the clock generator signal for the logic clock of the receiving radar sensor originates from this reference oscillator, which can also be, for example, a PLL or a clock generator frequency divider.

[0025] According to another preferred embodiment of the invention, the setting of the reference oscillator is performed by a digital-to-analog converter and the control voltage generated by the digital-to-analog converter or by configuration parameters. This depends on the type of reference oscillator used. For example, the output signal of the loop filter of the control loop is used as the input signal of the digital-to-analog converter.

[0026] According to another preferred embodiment of the invention, the control loop includes frequency generation with a direct digital synthesizer (DDS: Direct-Digital-Synthesis), and all clock generator signals of at least one additional receiving radar sensor originate from this DDS.

[0027] Preferably, at least the clock generator signal for complex frequency conversion and the clock generator signal for receiving the carrier frequency of the radar sensor are derived from the DDS, which may be the same frequency. Further preferably, the logic clock for receiving the radar sensor is set by means of the DDS, which may also be, for example, a PLL or a clock generator frequency divider. For example, the output signal of the loop filter of the control loop is used as the control word of the DDS, whereby the frequency generation of the DDS is set and further preferably tracked.

[0028] According to another preferred embodiment of the invention, when correcting the transmitted signal and / or the received signal, the baseband of the transmitted signal and / or the baseband of the received signal are mixed with a complex sine wave, wherein a complex sine wave with an estimated frequency offset Δf is generated E as the complex sine wave of the target frequency of the clock generator.

[0029] Here, for example, the complex sine wave is generated by means of the NCO of the receiving radar sensor. Here, preferably, the complex sine wave is complex conjugated before being mixed with the transmitted signal.

[0030] According to another preferred embodiment of the invention, the estimation of the frequency offset is further used to correct the generation of the transmitted signal and / or the processing of the received signal.

[0031] The invention also includes a synchronization method according to one of the described embodiments, wherein each of at least two signals is transmitted by a respective transmitting antenna of a first radar sensor, and at least two transmitting antennas of the first radar sensor are arranged at a distance from each other; wherein at least one additional receiving radar sensor receives the transmitted signals using more than one receiving antenna, wherein the number and distance of the receiving antennas correspond to the number and distance of the transmitting antennas; wherein analog-to-digital conversion converts the received signals into complex digital signals of a first receiving antenna and complex signals of at least one additional receiving antenna; wherein the complex conjugation of the complex signals to obtain complex conjugate signals is performed by complex conjugating the complex signal of one of the receiving antennas to obtain a complex conjugate signal; and wherein the mixing of the complex signals with the complex conjugate signals to obtain a signal containing the frequency offset Δf of the received signal as the main frequency component is performed by mixing the complex conjugate signal with the complex signal of another receiving antenna.

[0032] When using more receiving antennas than transmitting antennas, correspondingly, the signals of two of the receiving antennas are mixed with each other, and the results of all comparison processes are mixed with each other to obtain an average value.

[0033] The invention further includes a radar network having at least first and second digital radar sensors. The method according to one of the above-described embodiments or extensions is implemented using this radar network. Here, each radar sensor preferably includes at least one transmitting antenna and at least one receiving antenna, which can be implemented as conventional radar antennas. Additionally, they can be implemented such that each transmitting antenna is also a receiving antenna at the same time. For example, each radar sensor includes at least two transmitting and / or receiving antennas, and the at least two transmitting and / or receiving antennas are arranged at a distance from each other. Here, a digital radar sensor refers to a radar sensor with broadband digital signal generation, in which signals are processed with a sampling rate of at least several hundred MHz, for example greater than 200 MHz, by means of a digital-to-analog converter and an analog-to-digital converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The preferred embodiments of the present invention will be described in more detail hereinafter with reference to the drawings. The drawings show:

[0035] Figure 1 A block diagram showing a synchronization method according to a first embodiment of the present invention;

[0036] FIG. 2 shows the signals in each step of the synchronization method according to a first embodiment of the present invention;

[0037] Figure 3 A block diagram showing a synchronization method according to a second embodiment of the present invention;

[0038] Figure 4 A block diagram showing a synchronization method according to a third embodiment of the present invention;

[0039] Figure 5 Shows the correction of the frequency deviation between the transmitting radar sensor and the receiving radar sensor according to the third embodiment;

[0040] Figure 6 Shows two radar sensors according to a fourth preferred embodiment of the method;

[0041] Figure 7 A block diagram showing a synchronization method according to a fourth embodiment of the present invention;

[0042] FIG. 8 shows the signals in each step of the synchronization method according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0043] Figure 1FIG. 10 shows a block diagram of a synchronization method according to a first embodiment of the present invention. Block diagram 10 shows the reception of the transmitted signal by at least one receiving radar sensor and subsequent signal processing. Here, the receiving radar sensor receives the signal via a radar antenna (Rx) 12. The received signal is amplified by an amplifier 14 and mixed into the complex baseband by an in-phase quadrature mixer (I&Q mixer) 16. In the I&Q mixing process, on the one hand, the received signal is mixed with the carrier frequency generated by the local oscillator in a manner that the phase position remains unchanged and I data 18 is generated. At the same time, the signal is mixed with a carrier frequency phase-shifted by 90° and Q data 20 is generated. This converts the complex frequency into an intermediate frequency band suitable for digitization. The resulting signal is converted into a digitized complex signal A at a sampling rate of at least several hundred MHz (e.g., greater than 200 MHz) by an analog-to-digital converter 22 (see Figure 2A ).

[0044] Figure 2A Two digitized signals f1 and f2 are shown here, which are transmitted and received simultaneously (i.e., together) and with the same ramp duration and the same ramp angle, and are offset from each other due to a frequency offset Δf.

[0045] Then, the complex signal A is subjected to complex conjugation by a conjugator 24, where the sign of the imaginary part is inverted, resulting in a complex conjugate signal B (see Figure 2B ).

[0046] Figure 2B The complex conjugate signal B is shown, where the partial signal -f1 corresponds to the non-conjugated partial signal f1, and the partial signal -f2 corresponds to the non-conjugated partial signal f2. The complex signal A is mixed with the complex conjugate signal B by a complex multiplier 26. The complex conjugation, the mixing using the complex multiplier, and subsequent steps can all be implemented and executed completely digitally by the implemented analog-to-digital converter 22. The signal C obtained by mixing (see Figure 2C ) contains the main frequency component HK. This represents the difference frequency of the received signal A and corresponds to the frequency offset Δf.

[0047] Then, the signal C is mixed with a complex sine wave (generated by an NCO 30) by a mixer 28. Here, the NCO generates a complex sine wave with a defined frequency offset -Δf as the target frequency. This frequency is provided to the NCO by the local clock generator.

[0048] Here, the target frequency of the receiving radar sensor and the defined frequency offset Δf thereby can deviate from the target frequency of the transmitting radar sensor and the defined frequency offset Δf. This is due to the drift of the respective clock generators of the radar sensors, and this drift causes the actual frequencies and thus also the actual frequency offsets to deviate from each other. Due to this method, only the deviation between the clock generator of the receiving radar sensor and the clock generator of the transmitting radar sensor is determined here, and the deviation of the actual frequency of the clock generator of the transmitting radar sensor from the theoretically determined defined frequency offset is independent of the synchronization method. If the frequency offset of the transmitting radar sensor is Δf and the actual frequency offset of the receiving radar sensor is Δf Rx , then Δf≠Δf Rx applies.

[0049] Therefore, the complex sine wave generated by the NCO does not precisely correspond in terms of its frequency to the frequency offset of the received signal that is signal C, and the received signal has a frequency offset Δf as the main component HK. By mixing the complex sine wave with signal C by mixer 28 to generate signal D (see Figure 2D ), this signal D has the main component HK, and the main component HK is close to 0 Hz but has a frequency offset relative to 0 Hz. This frequency offset corresponds to Δf Rx and is generated by the frequency offset between the transmitting and receiving radar sensors and the defined frequency offset Δf.

[0050] Then, the main component HK can be filtered using filter 32, preferably by a low-pass filter, for example, by a Moving-Average-Filter, to filter the main component HK. The resulting signal E (see Figure 2E ) only has the main component HK.

[0051] Signal E now serves as the input signal of a frequency control loop, and the frequency control loop includes a discriminator 34, a loop filter 36, and a settable reference oscillator 38. According to the embodiment of the discriminator, the control loop is a Phase-Locked-Loop (PLL) or a Frequency-Locked-Loop (FLL). Here, the control loop can also be a combination of PLL and FLL, where a switch can be made between these types. The precise implementation determines the speed of the control loop here, that is, the reaction time.

[0052] The output signal of the loop filter 36 is hereby used as the input signal of the adjustable reference oscillator 38 and thus as the control signal of the adjustable reference oscillator 38. The adjustable reference oscillator here can be a voltage-controlled oscillator (VCO), then the output signal of the loop filter 36 controls a digital-to-analog converter (not shown here), which generates a control voltage as the input signal of the reference oscillator 38. The adjustable reference oscillator 38 here can be a numerically controlled oscillator (NCO), then the output signal of the loop filter 36 is a control value or a configuration parameter. The loop filter 36 tracks the adjustable reference oscillator 38.

[0053] The synchronization method according to the invention hereby includes a threshold detector 40, which generates a stop signal for controlling the loop. The threshold detector 40 hereby receives the signal E as the input signal and activates the control loop when the power of the signal E is higher than the defined threshold. If there is no signal with the corresponding power, the threshold detector 40 pauses the control loop via the loop filter 36. Thus, in case of an error signal due to interference or loss of the transmitted synchronization signal, the control loop can be paused and false tracking of the reference oscillator 38 can be prevented.

[0054] Then a clock signal for the clock generators 42, 44 of the receiving radar sensor is derived from the adjustable reference oscillator 38, which clock signal generates, for example, the carrier frequency of the radar signal and the carrier frequency of the I&Q mixing process. Here, the clock generator 42 is, for example, a PLL for generating the carrier frequency of the radar signal and thus generating the carrier frequency of the I&Q mixing process. The clock generator 44 is, for example, the logic clock of the receiving radar sensor, which serves as the clock generator of the NCO 30. The logic clock here can be, for example, a PLL or a clock generator divider.

[0055] The reference oscillator 38 is set via the control loop such that the reference oscillator is synchronized with the clock generator of the transmitting radar sensor, from which the transmitted signal originates. Since the clock generators 42, 44 of the receiving radar sensor track the adjustable reference oscillator 38, they are also synchronized with the clock generator of the transmitting radar sensor. This enables the radar sensor to operate accurately and without interference and enables coherent signal processing of different radar sensors.

[0056] Figure 3 A block diagram 110 of a second embodiment of the invention is shown. In the following, regarding the parts that are the same as in the first embodiment, reference is made to the description of those parts and no further elaboration is made.

[0057] In a second embodiment, the control loop includes a direct digital synthesizer (DDS) 46 instead of a programmable reference oscillator. A clock generator signal for the receiving radar sensor is derived from the direct digital synthesizer, and the clock generator signal generates, for example, the carrier frequency of the radar signal and the carrier frequency for I&Q mixing processing. In addition, a clock generator signal for the logic clock 144 is derived from the direct digital synthesizer, and the logic clock can be, for example, a clock generator frequency divider. The logic clock 144 is, for example, the clock generator of the NCO 30 here.

[0058] Figure 4 FIG. 210 is a block diagram showing a third embodiment of the present invention. Hereinafter, for parts identical to those of the first embodiment, reference is made to the description of those parts and no further elaboration is provided.

[0059] In Figure 4 the output signal of the filter 32 is used to estimate the frequency offset between radars, that is, the deviation between the frequency offset of the receiving radar and the frequency offset of the transmitting radar. The current frequency offset is estimated by the discriminator 234 and filtered by the filter 236 over a defined time period. Here, for example, the estimated deviation Δf E is averaged.

[0060] As Figure 5 shown in E the output signal of the filter 236, that is, the estimated deviation Δf

[0061] for example, is then used as the target frequency for setting the NCO 230, where the internal clock generator 244 (for example, the logic clock of the receiving radar sensor) serves as the clock generator of the NCO 230. The NCO can be, for example, the NCO 30 here, but can also be a dedicated NCO. E The NCO 230 generates a complex sine wave with the target frequency Δf

[0062] which is then mixed into the respective baseband of the transmitting radar sensor or the receiving radar sensor by mixers 248 and 250. Here, before mixing into the digital baseband of the transmitting radar sensor, the complex sine wave is complex conjugated by the conjugator 252. Here, this embodiment is not limited to mixing into the basebands of both the transmitting radar sensor and the receiving radar sensor, and synchronization can also be achieved by correcting the signals of only one of the transmitting radar sensor and the receiving radar sensor.

[0063] The generation of the transmitted signal further includes the following process blocks here: performing digital-to-analog conversion by means of a digital-to-analog converter 222, performing I&Q mixing processing by means of an I&Q mixer 216, performing amplification by means of an amplifier 214, and transmitting the transmitted signal by means of a radar antenna (Tx) 212. The carrier frequency of the radar signal or the carrier frequency of the I&Q mixing processing is generated by a local clock generator 242 in the receiving radar sensor.

[0064] Figure 6 Two radar sensors of a fourth preferred embodiment of the method are shown. In this embodiment of the method, each of the two transmitted signals f1, f2 is transmitted by one of two antennas Tx1, Tx2 of a transmitting radar sensor 54. Antenna Tx1 transmits signal f1 here, and antenna Tx2 transmits signal f2 here. The antennas are arranged at a distance d from each other.

[0065] Signals f1, f2 are received by a receiving radar sensor 56 having antennas Rx1 and Rx2. Here, each antenna Rx1, Rx2 receives two signals, that is, Rx1 receives signal f 1-Rx1 and f 2-Rx1 , Rx2 receives signal f 1-Rx2 and f 2-Rx2 . Due to the reflection of signals f1, f2 by the radar sensor 56, there is an angle-related phase difference between the respective received signals in view of the distance d between the transmitting antennas Tx1, Tx2. In order to eliminate the phase difference for the synchronization method, the transmitted signals are received by antennas Rx1 and Rx2, which also have a distance d from each other, and the synchronization method is implemented as shown in Figure 7 .

[0066] In Figure 7 , signals f1 and f2 are received by antennas 12, 312 (Rx1, Rx2) respectively. Here, the method does not limit that antenna 12 corresponds to antenna Rx1 and antenna 312 corresponds to antenna Rx2. The received signals are amplified by amplifiers 14, 314 respectively and mixed into the complex baseband by means of I&Q mixers 16, 316 respectively. By modulating according to the in-phase quadrature method, the corresponding signals are mixed in a manner that the phase positions remain unchanged and I data 18, 318 are generated. At the same time, the corresponding signals are mixed with a carrier frequency shifted by 90° and Q data 20, 320 are generated. These signals are respectively converted into digitized complex signals A2 or A3 at a sampling rate of at least several hundred MHz (for example, greater than 200 MHz) by means of analog-to-digital converters 22, 322 (see Figure 8A ).

[0067] Figure 8A Two digitized signals f 1-Rx1 , f 2-Rx1 or f1-Rx2 , f 2-Rx2 .

[0068] Then, the complex conjugate of the complex signal A3 is performed by the conjugator 324, where the sign of the imaginary part is inverted to obtain the complex conjugate signal B2. Figure 8B The complex conjugate signal B2 is shown, where the partial signal -f 1-Rx2 corresponds to the non-conjugate partial signal f1, and the partial signal -f 2-Rx2 corresponds to the non-conjugate partial signal f2. The complex signal A2 is mixed with the complex conjugate signal B2 by the complex multiplier 26. The complex conjugation, the mixing using the complex multiplier, and the subsequent steps can all be implemented and executed completely digitally through the implemented analog-to-digital conversions 22, 322. The signal C2 obtained by mixing (see Figure 8C ) contains the main frequency component f 1-Rx1 -f 2-Rx2 . This represents the difference frequency of the transmitted signal and corresponds to the frequency offset Δf.

[0069] Here, the subsequent steps of the fourth embodiment of the method can be performed according to any one of the first to third embodiments of the method, so they are not shown here.

[0070] The method according to the invention according to any one of the embodiments can also be performed here using a plurality of radar sensors in a radar network. Here, one radar sensor transmits the described synchronization signal, and each of the plurality of receiving radar sensors performs the method according to the invention. Here, the adaptation of the transmitted signal can be performed in a suitable manner, for example, using the average value of the estimated deviations of all receiving radar sensors as the correction value of the transmitted signal.

[0071] The disclosed embodiments are not limited to their respective features, but can be combined with each other in any way allowed by the technology.

Claims

1. A method for synchronizing at least two digital radar sensors, the method comprising the following steps: At least two signals are sent by the first radar sensor, wherein the signal originates from the same clock generator as the transmitting radar sensor or a synchronized clock generator, wherein the signals are modulated in a frequency division multiplexing method and have the same frequency variation process during common transmission and have a defined frequency offset Δf between each other; receiving the transmitted signal by at least one further radar sensor; The complex frequency is converted into an intermediate frequency band suitable for digitization; Converting the received signal into a complex digital signal (A); Conjugating the complex signal to obtain a complex conjugate signal (B); Mixing the complex signal (A) with the complex conjugate signal (B) to obtain a signal (C) containing the frequency offset Δf of the received signal as a main frequency component; Mixing the signal (C) with a complex sine wave or a complex square wave signal, wherein the complex sine wave or complex square wave signal having a defined frequency offset Δf as a target frequency is generated from a local clock generator of the at least one further receiving radar sensor; Filter the obtained signal (D) to obtain the main frequency component (E); using the main frequency component to control all clock generators of the at least one further receiving radar sensor by means of a control loop so that the clock generators of the at least one further receiving radar sensor are synchronized with the clock generator of the transmitting radar sensor; or Based on the main frequency component, a transmit signal and / or a receive signal is corrected according to an estimation of a frequency offset between the transmit radar sensor and the at least one further receive radar sensor.

2. The synchronization method according to claim 1, in, The synchronization method comprises a stop signal which suspends the control of all clock generators by a control loop or suspends the correction of the transmission signal and / or the reception signal based on an estimate of the frequency offset if no suitable input signal is present.

3. The synchronization method according to claim 1 or 2, in, The control loop includes a settable reference oscillator, and Therein, all clock generator signals of the at least one further receiving radar sensor originate from the reference oscillator.

4. The synchronization method according to claim 3, in, The setting of the reference oscillator is performed by means of a digital-to-analog converter and a control voltage generated by the digital-to-analog converter or by means of configuration parameters.

5. The synchronization method according to claim 1, in, The control loop includes a frequency generation with a direct digital synthesizer (DDS), and Therein, all clock generator signals of the at least one further receiving radar sensor originate from the direct digital synthesizer.

6. The synchronization method according to claim 1, in, When correcting the transmission signal and / or the reception signal, the baseband of the transmission signal and / or the baseband of the reception signal is mixed with a complex sine wave, Therein, a complex sine wave having an estimated frequency offset Δf is generated as a target frequency of a clock generator.

7. The synchronization method according to claim 6, in, The estimate of the frequency offset is further used to correct the generation of the transmit signal and / or the processing of the received signal.

8. A synchronization method according to any one of the preceding claims, in, Each of the at least two signals is transmitted by a corresponding transmitting antenna of the first radar sensor, and the at least two transmitting antennas of the first radar sensor are arranged at a distance (d) from each other; wherein the at least one further receiving radar sensor receives the transmitted signal using more than one receiving antenna, the number and distance (d) of the receiving antennas corresponding to the number and distance (d) of the transmitting antennas; wherein the analog-to-digital conversion converts the received signal into a complex digital signal (A2) of a first receiving antenna and a complex signal (A3) of at least one other receiving antenna; wherein conjugating the complex signal to obtain the complex conjugate signal is performed by conjugating the complex signal (A3) of one of the receiving antennas to obtain the complex conjugate signal (B2); and The mixing of the complex signal with the complex conjugate signal to obtain a signal containing the frequency offset Δf of the received signal as a main frequency component is performed by mixing the complex conjugate signal (B2) with the complex signal (A2) of another receiving antenna.

9. The synchronization method according to any one of the preceding claims, in, The complex sine wave is generated by a numerically controlled oscillator, or The complex square wave signal is generated by a square wave generator. 10 . A radar network having at least a first and a second digital radar sensor, with which the method according to claim 1 is carried out.

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