Synchronization procedure for digital radar sensors

Abstract

The invention relates to a multiple-input-multiple-output (MIMO) radar network and a method for synchronizing such a network. The method comprises the synchronization of at least two radar sensors with broadband digital signal generation by transmitting at least two multiplexed frequency-modulated signals with a defined frequency offset by one radar sensor and receiving the signals by another radar sensor. Using 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.

Description

The invention relates to a multiple-input multiple-output (MIMO) radar network and a method for synchronizing such a radar network. In particular, the invention relates to a method for synchronizing radar networks which are used in driver assistance systems of motor vehicles for detecting the environment, and to such radar networks.Prior ArtFor radar networks as a component of driver assistance systems in motor vehicles, modulation methods are usually used which frequency modulate the signals in a chirp method. Modern radar networks are so-called multiple-input-multiple-output (MIMO) radar networks which have a plurality of transmitters and a plurality of receivers in the form of radar sensors. In this case, for example, each radar sensor can be designed to be transmitter and receiver at the same time. In this case, the chirp signals (chirps) are transmitted by means of a plurality of transmitters and the reflected signals are received by means of a plurality of receivers. The chirps are generated with voltage-controlled oscillators (VCOs) and received signals are demodulated by analog mixing with the transmission signal before the analog-to-digital conversion and are thus greatly reduced in the bandwidth.Radar sensors with broadband digital signal generation (referred to below as digital radar sensors for short) are also used, in which the signals are generated directly by digital-to-analog converters with sample rates of at least several 100 MHz, for example of more than 200 MHz, using a predefined local oscillator (LO). Reflected signals are likewise mixed with a predefined local oscillator and provided for evaluation by means of an analog-to-digital converter with sample rates of at least several 100 MHz, for example of more than 200 MHz, without analog demodulation having been carried out beforehand. As a result, such digital radar sensors can operate in a large number of modulation methods, which significantly increases their spectrum of use compared to analog radar sensors. They are also capable of receiving the full bandwidth or large portions of the bandwidth of the signal spectrum used in a radar network at a time and have very good temporal resolution. In multiple-input multiple-output (MIMO) radar networks with digital radar sensors, a frequently used modulation method for generating the transmission signals is frequency-division multiplexing (FDM), for example as orthogonal frequency-division multiplexing (OFDM).The individual radar sensors each have their own clock generators (clocks) for signal generation and processing. Since an increasing number of individual radar sensors are installed in motor vehicles with driver assistance systems, synchronization of these radar sensors or their clocks may be necessary, in particular in order to avoid interference between the radar sensors and in order to enable coherent signal processing of the signals of the different sensors and to operate the radar network as a cooperative radar. In addition, the respective clock generators are subject to a drift in their clock frequency, for example a temperature drift. Thus, periodic synchronization of the clocks may be required.Object of the InventionIt is therefore an object of the invention to provide a synchronization method for the clock synchronization of a plurality of digital radar sensors, each of which has its own clock generator. A further object of the invention is to enable such a method in a simple and cost-effective manner without additionally requiring hardware components for connecting the radar sensors. Another object of the invention is to provide a radar network for operation with such a synchronization method.Disclosure of the InventionThese objects are achieved according to the invention by the method specified in claim 1 and by the radar network specified in claim 10. Advantageous embodiments and further developments of the invention are evident from the dependent claims.A synchronization method according to the present invention for at least two digital radar sensors includes the following steps:transmitting at least two signals by a first radar sensor, the signals being derived from the same or synchronous clock generators of the transmitting radar sensor, the signals being modulated in a frequency multiplex method and having the same frequency profile during the joint transmission and having a defined frequency offset Δf with respect to one another; receiving the transmitted signals by at least one further radar sensor; complex frequency conversion into an intermediate frequency band suitable for the digitalization; analog-digital conversion of the received signals into a complex digital signal; conjugation of the complex signal to obtain a complex conjugate signal; mixing of the complex signal with the complex conjugate signal to obtain a signal which contains the frequency offset Δf of the received signals as a main frequency component; Mixing of the signal with a complex sine or a complex square-wave signal, wherein the complex sine or the complex square-wave signal with the defined frequency offset Δf is generated as a setpoint frequency from a local clock generator of the at least one further receiving radar sensor; filtering of the obtained signal to obtain the main frequency component; regulating all clock generators of the at least one further receiving radar sensor using the main frequency component by means of a control loop, such that the clock generators of the at least one further receiving radar sensor are synchronous with the clock generators of the transmitting radar sensor; or correcting the transmission signals and / or the reception signals on the basis of an estimate of the frequency offset between the transmitting radar sensor and the at least one further receiving radar sensor on the basis of the main frequency component.By deriving the transmitted signals from the same or synchronous clock generators of the transmitting radar sensor, the frequency offset of the signals corresponds to the defined frequency offset. Normally, for applications in driver assistance systems, Δf" f c where f c is the carrier frequency of the radar signal. In these cases, the frequency offset Δf is influenced only insignificantly by the Doppler shift generated by the reflection of the signals on a target, since the Doppler shift has an almost identical effect on the different components of the frequency multiplex-modulated signal. The signal transit time also does not influence the received frequencies of the signal. Therefore, the frequency offset of the received signals corresponds substantially to the defined frequency offset of the transmitted signals.The at least one further receiving radar sensor (in the following the term "the receiving radar sensor" means at least one further receiving radar sensor, but is not limited thereto and can also comprise a plurality of further receiving radar sensors) receives the complete bandwidth of the signals, which is mixed into the complex baseband and can analog-digital convert the complete baseband bandwidth. The radar sensors operated using the method according to the present invention are preferably radar sensors having broadband digital signal generation (digital radar sensors). Conjugation of the complex signal to obtain the complex conjugate signal is performed by inverting the sign of the imaginary part of the complex signal.The complex signal is mixed with the complex conjugate signal by a complex multiplier. The signal thus obtained contains as the differential frequency the frequency offset Δf of the received signals, i.e. the defined frequency offset of the transmitted signals, which forms the main frequency component of the signal.Mixing the signal including the frequency offset Δf of the received signals as a main frequency component with a complex sine or a complex square wave signal generated using the defined frequency offset Δf yields a signal having a main frequency component near 0 Hz. This results from the fact that the complex sine is generated from a 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 a local clock generator signal of the receiving radar sensor by means of a square-wave generator. The use of a square wave generator instead of an NCO reduces the costs of a corresponding radar sensor.The differences in the clock frequencies of the clocks of the transmitting radar sensor and the clocks of the receiving radar sensor-which, as described, arise as a result of the respective drift of their clock frequencies-result in a frequency offset between the transmitting radar sensor and the receiving radar sensor. The defined frequency offset -Δf is used in the receiving radar sensor as a setpoint frequency for generating the complex sine wave or the complex square wave signal. Since the complex sine or the complex square wave signal is generated using a local clock generator and this is not synchronous with the clock generators of the transmitting radar sensor, the actual frequency of the complex sine or the complex square wave signal corresponding to the frequency offset between the radars deviates from the defined frequency offset Δf of the transmitting radar sensor.In other words, the defined frequency offset Δf of the transmitting radar sensor does not correspond to the actual frequency offset Δf Rx of the receiving radar sensor. Therefore, mixing a complex sine or complex square wave signal having the actual frequency offset -Δf Rx as a frequency with the received signal including the frequency offset Δf of the received signals as a main frequency component does not give exactly 0 Hz.This can be made clear by way of an example: in the case of a frequency offset of 1% between the transmitting radar sensor and the receiving radar sensor and a defined frequency offset of 50 MHz between the transmitted signals, a deviation of +0.5 Hz results after the mixing with the complex sine or the complex square-wave signal if the clock generators of the transmitting radar sensor are 1% faster than the clock generators of the receiving radar sensor. Accordingly, a deviation of -0.5 Hz results if the clocks of the transmitting radar sensor are 1 % slower than the clocks of the receiving radar sensor.The main frequency component of the mixed signal, the deviation from 0 Hz of which results from the deviation between the radar sensors, is preferably filtered by means of a low-pass filter, for example by means of a moving average filter (moving average filter).The filtered main frequency component can now either be used to control all clocks of the at least one further receiving radar sensor by a control loop, or it can be used to correct the transmission signals and / or the reception signals on the basis of an estimate of the frequency offset between the transmitting radar sensor and the at least one further receiving radar sensor. In this case, the transmission signals and / or the reception signals of the receiving radar sensor are preferably corrected, but the transmission signals and / or the reception signals of the transmitting radar sensor can also be corrected. In such a case, the method includes a step of data exchange between the receiving and the transmitting radar sensor. The method is not limited to either regulation or correction, and the steps can be combined and carried out simultaneously or in any order. For example, a control system can synchronize all clocks of the at least one further receiving radar sensor with the clocks of the transmitting radar sensor, and existing residual deviations that arise from the technical state can be corrected when generating the transmission signals and / or the reception signals using the estimation of the frequency offset.In this context, synchronous means not exactly synchronous but approximately synchronous. Depending on the intended use of a radar network synchronized with the method, this can comprise a technically tolerable deviation of a few Hz. Still existing deviations can result, for example, from a still existing temperature drift of the clock generators of the individual radar sensors.The control loop is preferably a phase-locked loop (PLL) and / or a frequency-locked loop (FLL). Depending on the embodiment of the method, it is possible, for example, to switch between a PLL and an FLL or these can be combined or used simultaneously.According to a preferred embodiment of the invention, the synchronization method comprises a hold signal which pauses the regulation of all clock generators by a control loop and / or the correction of the transmission signals and / or the reception signals on the basis of an estimate of the frequency offset if no suitable input signal is present.Preferably, the hold signal is controlled by a threshold detector which receives the signal filtered by the filtering to obtain the main frequency component as an input signal and pauses the control loop at an input signal below a threshold value. Preferably, the stop signal is also and / or alternatively controlled by an interference detection of the radar sensor and the control loop is paused in the case of an interference signal. For example, the hold signal is controlled by a threshold detector which receives the filtered main frequency component as an input signal. In the case of a signal having a power above a defined threshold value, the control loop and / or the correction of the signals is activated and otherwise paused.The output signal of the threshold value detector can also be used for a plausibility check, in which a check is made as to whether the received signals are not interferences. This, like the control of the stop signal via interference detection, increases the robustness of the method according to the invention.According to a further preferred embodiment of the invention, the control loop comprises an adjustable reference oscillator and all clock generator signals of the at least one further receiving radar sensor are derived from the reference oscillator.In this case, the adjustable reference oscillator is set and preferably tracked in the control loop. The clock generator signals for the complex frequency conversion and for the carrier frequency of the receiving radar sensor are preferably derived from the reference oscillator; this can be the same frequency. The clock generator signal for the logic clock of the receiving radar sensor is furthermore preferably derived from the reference oscillator; this can also be a PLL or a clock generator divider, for example.According to a further preferred embodiment of the invention, the adjustment of the reference oscillator is carried out by a digital-to-analog converter and a control voltage generated thereby or by a configuration parameter. This depends on the type of reference oscillator used. For example, the output signal of a loop filter of the control loop is used as an input signal for the digital-to-analog converter.According to a further preferred embodiment of the invention, the control loop comprises the frequency generation with a direct digital synthesis (DDS), and all clock generator signals of the at least one further receiving radar sensor are derived from the DDS.Preferably, at least the clock generator signals for the complex frequency conversion and for the carrier frequency of the receiving radar sensor are derived from the DDS; this can be the same frequency. The logical clock of the receiving radar sensor is also preferably set by means of the DDS; this can also be a PLL or a clock generator divider, for example. For example, the output signal of a loop filter of the control loop is used as a control word for the DDS, whereby the frequency generation of the DDS is adjusted and more preferably tracked.According to a further preferred embodiment of the invention, during the correction of the transmission signals and / or of the reception signals, the baseband of the transmission signals and / or of the reception signals is mixed with a complex sine, wherein the complex sine is generated with the estimated frequency offset Δf E as the setpoint frequency of a clock generator.The complex sine is generated, for example, by means of an NCO of the receiving radar sensor. Preferably, the complex sine is complex-conjugated with the transmission signals before mixing.According to a further preferred embodiment of the invention, the generation of transmission signals and / or the processing of received signals is further corrected using the estimation of the frequency offset.The invention further comprises a synchronization method according to one of the described embodiments, wherein each of the at least two signals is transmitted by a respective transmission antenna of the first radar sensor and the at least two transmission antennas of the first radar sensor are arranged at a distance from one another; wherein the at least one further receiving radar sensor receives the transmitted signals with more than one reception antenna, wherein the number and the distance of the reception antennas correspond to the number and the distance of the transmission antennas; wherein the analog-to-digital conversion converts the received signals into a complex digital signal of a first reception antenna and a complex signal of at least one further reception antenna; wherein the conjugation of the complex signal to obtain a complex conjugate signal is carried out by conjugating the complex signal of one of the reception antennas to obtain the complex conjugate signal; and wherein the mixing of the complex signal with the complex conjugate signal to obtain a signal containing the frequency offset Δf of the received signals as a main frequency component is performed by mixing the complex conjugate signal with the complex signal of another receiving antenna.When using more reception antennas than transmission antennas, the signals from two of the reception antennas are respectively mixed with each other, and the result of the method of all the comparisons is mixed with each other to obtain an average value.The invention further comprises a radar network having at least a first and a second digital radar sensor. The method according to one of the above-described embodiments or refinements is carried out with the radar network.Each radar sensor preferably comprises at least one transmitting antenna and at least one receiving antenna, which can be embodied as usual radar antennas and which can furthermore be embodied such that each transmitting antenna is simultaneously a receiving antenna. For example, each radar sensor includes at least two transmitting and / or receiving antennas, which are situated at a distance from one another. Digital radar sensors refer to radar sensors with broadband digital signal generation, in which the signals are processed by means of digital-to-analog converters and analog-to-digital converters with sample rates of at least several 100 Mhz, for example of more than 200 Mhz.Preferred embodiments of the invention are explained in more detail below with reference to the figures. The following are shown: FIG. 1 is a block diagram of the synchronizing method according to a first embodiment of the present invention; FIG. 2 shows signals in various steps of the synchronizing method according to the first embodiment of the present invention; FIG. 3 is a block diagram of the synchronization method according to a second embodiment of the present invention; FIG. 4 is a block diagram showing the synchronizing method according to a third embodiment of the present invention; FIG. 5 shows the correction of the frequency deviation between the transmitting radar sensor and the receiving radar sensor according to the third embodiment; FIG. 6 shows two radar sensors of a fourth preferred specific embodiment of the method; FIG. 7 is a block diagram showing the synchronizing method according to the fourth embodiment of the present invention; FIG. 8 shows signals in various steps of the synchronizing method according to the fourth embodiment of the present invention.FIG. 1 shows a block diagram 10 of the synchronization method according to the invention according to a first embodiment of the invention. Block diagram 10 represents the reception of the transmitted signals by at least one receiving radar sensor and the subsequent signal processing. The receiving radar sensor receives the signal through 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 (I&Q) mixer 16. In the I&Q mixing method, the received signal is mixed in unchanged phase with the carrier frequency generated by the local oscillator and yields the I data 18. At the same time, the signal is mixed with a carrier frequency phase shifted by 90° and yields the Q data 20. The signals obtained are converted into a digitized complex signal A (see FIG. 2A ) by means of an analog-to-digital converter 22 with sample rates of at least several 100 MHz, for example of more than 200 MHz.FIG. 2A shows the two digitized signals f 1 and f 2, which were transmitted and received simultaneously, i.e. jointly, and with the same ramp duration and the same ramp angle and are offset with respect to one another by the frequency offset Δf.The complex signal A is then complex-conjugated by a conjugator 24, inverting the sign of the imaginary part and obtaining the complex-conjugated signal B (see Fig. 2B).FIG. 2B shows the complex conjugate signal B, wherein the subsignal - f 1 corresponds to the unconjugated subsignal f 1 and the subsignal - f 1 corresponds to the unconjugated subsignal f 1. The complex signal A is mixed with the complex conjugate signal B by a complex multiplier 26. Both the complex conjugation and the mixing with a complex multiplier and the subsequent steps can be implemented and carried out completely digitally by the effected analog-to-digital conversion 22. The signal C obtained by the mixing (see FIG. 2C ) contains a main frequency component HK. This represents the differential frequency of the received signals A and corresponds to the frequency offset Δf.Subsequently, the signal C is mixed by a mixer 28 with a complex sine-generated by an NCO 30. The NCO generates the complex sine with the defined frequency offset -Δf as the desired frequency. This frequency is provided to the NCO from a local clock.In this case, the setpoint frequency and thus the defined frequency offset Δf of the receiving radar sensor may deviate from the setpoint frequency and the defined frequency offset Δf of the transmitting radar sensor. This is due to the drift of the respective clock generators of the radar sensors, which can cause the actual frequencies and thus also the actual frequency offsets to deviate from one another. Due to the method, only the deviation of the clocks of the receiving radar sensor from the clocks of the transmitting radar sensor will be determined, the deviation of the actual frequency of the clocks of the transmitting radar sensor from the theoretically determined defined frequency offset is not relevant for 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 Δf ≠Δf Rx. holds.Thus, the frequency of the complex sine generated by the NCO does not correspond exactly to the frequency offset of the received signals as signal C with the frequency offset Δf as the main component HK. Mixing the complex sine with the signal C by the mixer 28 results in a signal D (see FIG. 2D ) having a main component HK that is close to zero Hz but has a frequency offset to zero Hz. This frequency offset corresponds to Δf Rx and results from the frequency offset between the transmitting and the receiving radar sensor and the defined frequency offset Δf.The main component HK can then be filtered with a filter 32, preferably with a low-pass filter, for example with a moving average filter (moving average filter). The signal E obtained (see FIG. 2E ) has only the main component HK.The signal E is now used as input to a frequency locked loop comprising a discriminator 34 and a loop filter 36 and an adjustable reference oscillator 38. Depending on the embodiment of the discriminator 34, the control loop is a phase-locked loop (PLL) or frequency-locked loop (FLL). The control loop can also be a combination of PLL and FLL, it being possible to switch between the types. The exact implementation determines the speed of the control loop, i.e. the reaction time.The output signal of the loop filter 36 serves as an input signal and thus as a control signal of the adjustable reference oscillator 38; this 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 an input signal for the reference oscillator 38. The adjustable reference oscillator 38 may be a numerically controlled oscillator (NCO), then the output signal of the loop filter 36 is a control value or configuration parameter. The loop filter 36 tracks the adjustable reference oscillator 38.The synchronization method according to the invention comprises a threshold value detector 40, which generates a holding signal for the control loop. The threshold value detector 40 receives the signal E as an input signal and activates the control loop if the power of the signal E is above a defined threshold value. If no signal with a corresponding power is present, the threshold value detector 40 pauses the control loop via the loop filter 36.The clock signals for the clocks 42, 44 of the receiving radar sensor are then derived from the adjustable reference oscillator 38, which generate, for example, the carrier frequency of the radar signal and of the I&Q mixing method. In this case, the clock generator 42 is, for example, a PLL for generating the carrier frequency of the radar signal and thus the carrier frequency of the I&Q mixing method. And the clock generator 44 is, for example, the logic clock of the receiving radar sensor, which functions as a clock generator for the NCO 30. The logic clock can be, for example, a PLL or a clock generator divider.The control loop sets the reference oscillator 38 in such a way that it is synchronous with the clock generators of the transmitting radar sensor from which the transmitted signals are derived. Because the clocks 42, 44 of the receiving radar sensor are tracked to the adjustable reference oscillator 38, they are likewise synchronized with the clocks of the transmitting radar sensor. This enables precise and interference-free operation of the radar sensors and coherent signal processing of the different radar sensors.Figure 3 shows a block diagram 110 of a second embodiment of the invention. Hereinafter, as for the parts identical to the first embodiment, the description thereof will be referred to and repetition will be omitted.In the second embodiment, the control loop includes a direct digital synthesis (DDS) 46 instead of a variable reference oscillator. The clock generator signals of the receiving radar sensor are derived from the latter, wherein these generate, for example, the carrier frequency of the radar signal and of the I&Q mixing method. Furthermore, the clock signal for the logic clock 144 is derived from it; this can be a clock divider, for example. The logic clock 144 is, for example, a clock generator for the NCO 30.Figure 4 shows a block diagram 210 of a third embodiment of the invention. Hereinafter, as for the parts identical to the first embodiment, the description thereof will be referred to and repetition will be omitted.In FIG. 4, the output signal of the filter 32 is used to estimate the frequency offset between the radars, i.e. the deviation of the frequency offset of the receiving radar from the frequency offset of the transmitting radar. The current frequency offset is estimated by a discriminator 234 and filtered by a filter 236 over a defined period of time. Here, for example, the estimated deviation Δf E is averaged.As shown in FIG. 5, the output signal of the filter 236, i.e., for example, the estimated deviation Δf E, is subsequently used as a setpoint frequency for setting an NCO 230, wherein an internal clock generator 244-for example, the logic clock of the receiving radar sensor-functions as a clock generator for the NCO 230. The NCO may be, for example, the NCO 30, but may also be a dedicated NCO. The NCO 230 generates a complex sine with the setpoint frequency Δf E, which is then mixed with the mixers 248 and 250 into the respective baseband of the transmitting radar sensor or of the receiving radar sensor. In this case, the complex sine is complex-conjugated by means of a conjugator 252 before mixing into the digital baseband of the transmitting radar sensor. The specific embodiment is not limited to being mixed into both the baseband of the transmitting and the receiving radar sensor; the synchronization may also be carried out only by correcting the signals of one of the two.In this case, it is also possible to correct existing frequency deviations between the transmitting radar sensor and the receiving radar sensor during the generation of transmission signals and / or the processing of received signals using the estimation of the frequency offset.The generation of transmission signals further comprises the method blocks of digital-to-analog conversion by means of a digital-to-analog converter 222, the execution of the I&Q mixing process by means of an I&Q mixer 216, the amplification by means of an amplifier 214 and the transmission of the transmission signals by means of a radar antenna (Tx) 212. The carrier frequency of the radar signal or of the I&Q mixing method is generated in the receiving radar sensor by a local clock generator 242.FIG. 6 shows two radar sensors of a fourth preferred specific embodiment of the method. In this specific embodiment of the method, each of the two transmission signals f 1, f 2 is transmitted by one of two antennas Tx 1, Tx 2 of transmitting radar sensor 54. The antenna Tx 1 transmits the signal f 1 and the antenna Tx 2 transmits the signal f 2. The antennas are arranged at a distance d.The signals f 1, f 2 are received by the receiving radar sensor 56 with the antennas Rx 1 and Rx 2, wherein each antenna Rx 1, Rx 2 receives both signals, i.e. Rx 1 receives the signals f 1-Rx1 and f 2-Rx1 and Rx 2 receives the signals f 1-Rx2 and f 2-Rx2. Because radar sensor 56 receives the reflection of signals f 1, f 2 there is an angle-dependent phase difference between the respective received signals due to distance d of transmitting antennas Tx 1, Tx 2. To eliminate these for the synchronization method, the transmitted signals are received by the antennas Rx1 and Rx2 which are also at the distance d from each other, and the synchronization method is carried out as shown in FIG. 7.In FIG. 7, signals f 1 and f 2 are received by antennas 12, 312 (Rx1, Rx2), respectively. Here, the method is not set to the fact that the antenna 12 corresponds to the antenna Rx 1 and the antenna 312 corresponds to the antenna Rx 2. The received signals are each amplified by an amplifier 14, 314 and are each mixed into the complex baseband by means of an I&Q mixer 16, 316. By the modulation according to the in-phase & quadrature method, the corresponding signal is mixed with the unchanged phase position and yields the I data 18, 318. At the same time, the respective signal is mixed with a carrier frequency phase-shifted by 90° and yields the Q data 20, 320. These are converted by means of an analog-to-digital converter 22, 322 with sample rates of at least several 100 MHz, for example of more than 200 MHz, in each case one digitized complex signal A2 or A3 (see FIG. 8A ).FIG. 8A shows the two digitized signals f 1-Rx1, f 2-Rx1 and f 1-Rx2, f 2-Rx2.The complex signal A3 is then complex-conjugated by a conjugator 324, inverting the sign of the imaginary part and obtaining the complex-conjugated signal B2. FIG. 8B shows the complex conjugate signal B2 wherein the subsignal -f 1-Rx2 corresponds to the unconjugated subsignal f 1 and the subsignal -f 2-Rx2 corresponds to the unconjugated subsignal f 2. The complex signal A2 is mixed with the complex conjugate signal B2 by the complex multiplier 26. Both the complex conjugation and the mixing with a complex multiplier and the subsequent steps can be implemented and carried out completely digitally by the effected analog-to-digital conversion 22, 322. The signal C 2 obtained by the mixing (see FIG. 8C ) includes a main frequency component f 1-Rx1- f 2-Rx2. This represents the differential frequency of the transmitted signals and corresponds to the frequency offset Δf.In this case, the following steps of the fourth embodiment of the method can be carried out according to any of the first to third embodiments of the method and are therefore not illustrated in any more detail here.The method according to the invention according to any of the described embodiments can also be carried out with a plurality of radar sensors in a radar network. In this case, a radar sensor transmits the described synchronization signals and each of the plurality of receiving radar sensors executes the method according to the present invention. The adaptation of the transmission signals can be carried out in a suitable manner, for example, an average value of the estimated deviation of all receiving radar sensors is used as the correction value of the transmission signals.The disclosed embodiments are not limited to their respective features but can be combined with one another in any desired manner, insofar as technically possible.

Claims

Synchronization method for at least two digital radar sensors, comprising the steps: transmitting at least two signals by a first radar sensor, wherein the signals are derived from the same or synchronous clock generators of the transmitting radar sensor, wherein the signals are modulated in a frequency multiplex method and have the same frequency profile during the joint transmission and have a defined frequency offset Δf with respect to one another; receiving the transmitted signals by at least one further radar sensor; complex frequency conversion into an intermediate frequency band suitable for the digitalization; analog-to-digital conversion of the received signals into a complex digital signal (A); conjugation of 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) which contains the frequency offset Δf of the received signals as a main frequency component; mixing the signal (C) with a complex sine or a complex square signal, wherein the complex sine or the complex square signal with the defined frequency offset Δf is generated as a setpoint frequency from a local clock generator of the at least one further receiving radar sensor; filtering the obtained signal (D) to obtain the main frequency component (E); regulating all clock generators of the at least one further receiving radar sensor using the main frequency component by means of a control loop, such that the clock generators of the at least one further receiving radar sensor are synchronous with the clock generators of the transmitting radar sensor; or correction of the transmission signals and / or of the reception signals on the basis of an estimate of the frequency offset between the transmitting radar sensor and the at least one further receiving radar sensor on the basis of the main frequency component.Synchronization method according to Claim 1, wherein the synchronization method comprises a hold signal which pauses the regulation of all the clocks by a control loop or the correction of the transmission signals and / or of the reception signals on the basis of an estimate of the frequency offset if no suitable input signal is present.Synchronization method according to Claim 1 or 2, wherein the control loop comprises an adjustable reference oscillator and wherein all clock generator signals of the at least one further receiving radar sensor are derived from the reference oscillator.The synchronization method according to claim 3, wherein the adjustment of the reference oscillator is performed by a digital-to-analog converter and a control voltage generated thereby or by a configuration parameter.Synchronization method according to Claim 1, wherein the control loop comprises the frequency generation with a direct digital synthesis (DDS), and wherein all clock generator signals of the at least one further receiving radar sensor are derived from the direct digital synthesis.Synchronization method according to Claim 1, wherein, during the correction of the transmission signals and / or of the reception signals, the baseband of the transmission signals and / or of the reception signals is mixed with a complex sine, wherein the complex sine is generated with the estimated frequency offset Δf as the setpoint frequency of a clock generator.The synchronization method according to claim 6, wherein further the generation of transmission signals and / or the processing of received signals is corrected using the estimation of the frequency offset.Synchronization method according to one of the preceding claims, wherein each of the at least two signals is transmitted by a respective transmission antenna of the first radar sensor and the at least two transmission antennas of the first radar sensor are arranged at a distance (d) from one another; wherein the at least one further receiving radar sensor receives the transmitted signals with more than one reception antenna, wherein the number and the distance (d) of the reception antennas correspond to the number and the distance (d) of the transmission antennas; wherein the analog-to-digital conversion converts the received signals into a complex digital signal (A2) of a first reception antenna and a complex signal (A3) of at least one further reception antenna; wherein the conjugation of the complex signal to obtain a complex conjugate signal is carried out by the conjugation of the complex signal (A3) of one of the reception antennas to obtain the complex conjugate signal (B2); and wherein the mixing of the complex signal with the complex conjugate signal to obtain a signal containing the frequency offset Δf of the received signals as a main frequency component is performed by mixing the complex conjugate signal (B2) with the complex signal (A2) of another receiving antenna.The synchronization method according to any of the preceding claims, wherein the complex sine is generated by a numerically controlled oscillator or wherein the complex square wave signal is generated by a square wave generator.Radar network having at least one first and one second digital radar sensor, with which the method according to one of Claims 1 to 9 is carried out.

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