RADAR SENSOR SYSTEM

Description

[0001] The invention relates to a radar sensor system. State of the art

[0002] Vehicles with advanced driver assistance or automated driving functions are increasingly being equipped with radar sensors. The goal of using more radar sensors is to improve the performance of automated or semi-automated driving functions compared to using single sensors. Current solutions in this area consist of radar sensors that perform extensive internal data processing of the received radar waves. This allows the radar sensors to provide object- or location-level data for further analysis by the vehicle. While this reduces the amount of data transmitted to the vehicle, it requires the radar sensors to have higher processing power and larger memory capacities.

[0003] However, the parking functions in particular suffer from the properties of wave propagation, whereby, for example, walls reflect strongly and thus less reflective objects, such as posts, are masked.

[0004] Bistastatic analyses in connection with ultrasound are known. These involve evaluating the time-of-flight differences between acoustic and electrical signals.

[0005] DE 10 2014 212 280 A1 discloses a method for determining the relative velocity of a radar target in which an FMCW radar measurement is performed with a transmitted signal whose modulation pattern comprises temporally interleaved sequences (JSFMCW modulation method).

[0006] DE 10 2014 212 284 A1 discloses a MIMO-FMCW radar sensor and a MIMO time-division multiplexing method for locating a radar target, in which an FMCW radar measurement is performed with a transmit signal whose modulation pattern for different transmit switching states, which differ in the selection of the antenna elements used for transmitting, comprises temporally interleaved sequences of ramps.

[0007] Disclosure document US 2018 / 0113206 A1 discloses a radar system.

[0008] The patent application DE 10 2015 218 542 A1 discloses a radar sensor. Disclosure of the invention

[0009] The object underlying the invention can be seen as providing an improved radar sensor system that supports improved object classification.

[0010] This problem is solved by means of the subject matter of the independent claim. Advantageous embodiments of the invention are the subject of dependent dependent claims.

[0011] According to a first aspect, the problem is solved with a radar sensor system having the features of claim 1, comprising: at least two radar sensors, each with at least one transmitter and at least one receiver, wherein the detection ranges of the two radar sensors overlap at least partially, wherein the two radar sensors are spaced apart in a defined manner, wherein the transmission signals of the two radar sensors can be synchronized in such a way that radiation reflected from an object by one radar sensor, which was emitted by the other radar sensor, can be evaluated by means of an evaluation device.

[0012] In this way, the proposed radar sensor system enables improved object classification. The system is particularly useful for short-range applications, such as vehicle parking systems. It utilizes the principle that objects are illuminated from one direction, while signals are received from another direction, evaluated, and classified.

[0013] According to a second aspect, which is not part of the invention, the problem is solved by a method for manufacturing a radar sensor system comprising the steps: Providing at least two radar sensors, wherein the detection ranges of the two radar sensors overlap at least partially, wherein the two sensors are arranged at a defined distance apart, wherein the transmission signals of the two radar sensors can be synchronized in such a way that radiation reflected from an object by one radar sensor, which was emitted by the other radar sensor, can be evaluated by means of an evaluation device.

[0014] According to a third aspect, which is not part of the invention, the problem is solved by a method for operating a radar sensor system. wherein a frequency offset (Δf) between the two radar sensors can be determined and compensated by having the transmitters of the radar sensors transmit alternately, while simultaneously the receivers of the non-transmitting radar sensors receive, wherein the transmitting signals of the two radar sensors are synchronized in such a way that radiation reflected from an object by one radar sensor, which was emitted by the other radar sensor, is evaluated by means of an evaluation device.

[0015] A preferred embodiment of the radar sensor system, which is not part of the invention, is characterized in that the at least two radar sensors transmit essentially at the same time, transmit essentially at the same frequency, and have essentially the same modulation parameters. This ensures good synchronization of the transmitted signals, thereby enabling accurate bistatic evaluation.

[0016] Another preferred embodiment of the radar sensor system is characterized in that FMCW ramps with essentially identical modulation parameters can be transmitted by means of the at least two radar sensors. This also supports good synchronization of the radar sensor transmission signals.

[0017] The radar sensor system according to the invention is characterized in that a frequency offset between the two radar sensors can be determined and compensated by having the transmitters of the radar sensors transmit alternately, while the receivers of the non-transmitting radar sensors simultaneously receive. Advantageously, the frequency offset between the two radar sensors can be corrected in this way by having the radar sensors alternately function as transmitters and receivers.

[0018] The radar sensor system according to the invention is characterized in that the frequency offset between the two radar sensors can be determined and compensated by measuring two baseband frequencies for two unknowns when measuring one target object, and by measuring 2N baseband frequencies for N+1 unknowns when measuring N target objects. Advantageously, the frequency offset between the radar sensors can be determined more accurately in this way as the number of targets increases.

[0019] Another preferred embodiment of the radar sensor system is characterized by the fact that a frequency offset between the two radar sensors can be determined and compensated for in order to synchronize the FMCW ramps. This supports further optimization of the synchronization of the radar sensor transmission signals.

[0020] Another preferred embodiment of the radar sensor system is characterized by the fact that temporally interleaved FMCW ramps can be transmitted using the two radar sensors. This provides a good application for the JSFMCW modulation method.

[0021] The radar sensor system according to the invention is characterized in that a bistastatic and a monostatic radar cross-section of the object can be determined by means of the evaluation device. Depending on the angle of incidence and reflection, good object detection and classification can thus be carried out.

[0022] Another preferred embodiment of the radar sensor system is characterized in that the signal power of the transmitted signals from the radar sensors can be defined and taken into account for synchronizing these signals. In this way, the synchronization of the radar sensor signals can be advantageously optimized.

[0023] In the following, preferred embodiments of the invention are explained in more detail with reference to highly simplified schematic representations.

[0024] This shows: Fig. 1 is a schematic representation of a proposed radar sensor system; and Fig. 2 is a schematic representation of a method for manufacturing a radar sensor system, wherein the method for manufacturing a radar sensor system is not part of the invention.

[0025] In the following, "bistatic evaluation" means that signals from a system consisting of a sender and a receiver are evaluated, whereby the sender and receiver are not located in the same place, but are spaced apart from each other.

[0026] Fig. 1 Figure 1 shows a schematic representation of a proposed radar sensor system 100. A first radar sensor 10 and a second radar sensor 20, spaced apart from the first radar sensor 10, are visible. These are functionally connected to an evaluation unit 30. Thus, a system of radar sensors 10 and 20 is proposed, wherein each radar sensor 10 or 20 can evaluate radar waves emitted by the other radar sensor 10 or 20 and reflected by an object 200. Fig. 1 It is indicated that a transmitted signal emitted by the second radar sensor 20 is reflected by the object 200, with the reflected signal being received by the first radar sensor 10. The number two radar sensors 10, 20 is merely exemplary; the radar sensor system 100 can also include more than two radar sensors 10, 20.

[0027] The aforementioned evaluation of the radiation emitted by another radar sensor enables the determination of both the monostatic and the bisstatic radar cross-section (RCS) of object 200. This RCS represents a measure of the reflectivity of object 200 and thus allows for improved detection and classification of objects, particularly for parking applications in a motor vehicle. Due to the proximity of object 200, a large angular difference to radar sensors 10 and 20 can be achieved.

[0028] This is made possible by performing a bistatic evaluation, which is advantageously possible with the proposed approach without significant additional costs.

[0029] To enable bistatic evaluation, two or more radar sensors 10, 20 with transmitters and receivers are necessary, whereby the detection ranges must overlap at least partially. Furthermore, the radar sensors 10, 20 of the radar sensor system 100 must be synchronized to a certain degree so that a receiver can meaningfully evaluate the waves from another "external transmitter".

[0030] According to the invention, the synchronization is essentially based on synchronizing the reference oscillators (not shown) of the radar sensors 10, 20. Each radar sensor 10, 20 has a quartz oscillator, and the deviation from a reference frequency is regularly determined via a communication interface (Ethernet according to the invention, not shown) using timestamps. Ethernet has the advantage that there is a corresponding standard, PTP (Precision Time Protocol), which, with hardware support, enables time synchronization in the range of 10 ns.

[0031] In this way, according to the invention, it is possible to synchronize the reception of one radar sensor 10, 20 with the transmission of the other radar sensor 10, 20, so that a transmitted signal from the other radar sensor 10, 20 is also received. For example, when using FMCW ramps as transmit signals with a slope of 5 MHz / µs, a frequency offset of 50 kHz results, which is well within the baseband of typically about 5 MHz. This frequency offset represents a frequency error that results in a range error, which is why it is desirable to keep the frequency error as small as possible.

[0032] By synchronizing the transmission signals of the two radar sensors 10 and 20, it is possible to synchronize the center frequency and the slope of the locally generated oscillator frequencies. Achieving an accuracy of 1 ppm results in a frequency offset of approximately 76 kHz. Overall, the frequency offset remains well within the baseband bandwidth, thus fulfilling the primary synchronization requirement. This ensures that the error is small enough to prevent the bistatic signals from leaving the evaluation range. The aim is to minimize the synchronization error to optimize distance estimation.

[0033] The resulting frequency offset must be compensated for, as this can distort the distance estimation. For the baseband frequency f bb, the following applies: f bb = f TX t − τ − T TX − RX − f RX t with : τSignal propagation time T TX-RX Time offset between transmitter and receiver (positive if the transmitter starts before the receiver) τ=dTX,RXc d TX,RX Total path from transmitter to receiver c Propagation speed fTXt=fs,TX+s⋅tfRXt=fs,RX+s⋅t sRamp slope of the FMCW ramps fs Start frequency tTime

[0034] Substituting this into the equation yields: f bb = f TX t − τ − T TX − RX − f RX t = f s , TX + s ⋅ t − τ − T TX − RX − f s , RX − s ⋅ t = f s , TX − f s , RX − s ⋅ τ + T TX − RX = − s c ⋅ d TX , RX + f s , TX − f s , RX − s ⋅ T TX − RX = − s c ⋅ d TX , RX + Δ f Δf Frequency offset fs,TX - fs,RX Difference in starting frequencies

[0035] The received baseband frequency also depends on the offset of the oscillator frequencies and the timing error.

[0036] Equations (1) to (4) thus specify that the baseband frequency f bb depends on the ramp signals and the propagation delays and that the frequency offset Δf must also be taken into account.

[0037] This error, in the form of the frequency offset Δf, is corrected by alternating the transmitter and receiver, i.e., by assigning the transmitter alternately to the first radar sensor 10 and the second radar sensor 20, thus allowing the two radar sensors 10 and 20 to act as transmitters alternately. This switching of transmitter and receiver can be effectively combined with the aforementioned, well-known JSFMCW modulation method, in which, within a time-division multiplexing sequence, not only transmitters from the system's own radar sensor but also transmitters from external radar sensors occupy a ramp.

[0038] In a scenario of a radar sensor system 100 with two radar sensors 10, 20, the above general equation (4) must be modified as follows: f bb , 12 = − s c ⋅ d TX , RX + f s , 1 − f s , 2 − s ⋅ T 1 − 2 = − s c ⋅ d TX , RX + Δ f f bb , 21 = − s c ⋅ d TX , RX + f s , 2 − f s , 1 − s ⋅ T 2 − 1 = − s c ⋅ d TX , RX − Δ f f bb,12 Baseband frequency: Sensor 1 transmits, Sensor 2 receives f bb2,1 Baseband frequency: Sensor 2 transmits, Sensor 1 receives fs,1 Start frequency Sensor 1 fs,2 Start frequency Sensor 2 T 1-2 Time offset between Sensor 1 and Sensor 2, where Sensor 1 transmits where, due to symmetry, the following applies: fs,1−fs,2=−fs,2−fs,1T1−2=−T2−1 T 2-1 3 Time offset between sensor 2 and sensor 1, where sensor 2 sends

[0039] In the bistatic evaluation, all baseband frequencies are thus shifted by the offset, the sign of which is reversed when wave propagation is reversed, i.e., when transmitter and receiver are swapped.

[0040] For a target object of 200, two baseband frequencies are measured for two unknowns; for N targets, this increases to 2N baseband frequencies for N + 1 unknowns. This means that the true frequency and the frequency offset or error are measured. With more target objects, the number of baseband frequencies increases accordingly, but advantageously, the number of unknowns does not increase proportionally.

[0041] This mathematical problem is fundamentally solvable, although due to the shift in the solution, a correlation or least-squares approach is preferred.

[0042] To optimize the computational effort required to solve the mathematical problem, it is conceivable to include the signal power of the transmitted signals and, for example, to use only signals with high or similar power in order to determine the offset at least roughly.

[0043] The synchronization measures mentioned above ensure that a receiver can receive signals from another sender, thus enabling a bistatic evaluation.

[0044] Alternating the operation of the radar sensors at short intervals is advantageous because it compensates for synchronization errors in the form of frequency offsets, which can also change. This is possible, for example, if one radar sensor heats up slightly differently than another and / or if the radar sensors exhibit different drifts. Furthermore, this method allows for the consideration of changing driving situations, ensuring that the measurements from the radar sensors are relevant to each other during bista analysis.

[0045] The proposed bistastatic analysis makes it possible to identify objects that would otherwise be undetectable. Furthermore, the proposed bistastatic analysis can improve the classification of the identified objects.

[0046] The proposed approach is limited by phase noise, which is uncorrelated with two independent oscillators of the two sensors and can therefore cause high noise levels. For bistatic evaluation, practical application is therefore primarily intended for short-range applications (e.g., parking applications) where no low-phase-noise technology is available.

[0047] An improvement is achieved if a reference frequency is also transmitted via a communication interface (or separately via a vehicle bus due to EMC shielding), so that the local oscillators and their PLLs use a coherent reference, ensuring that the phase noise within the PLL loop bandwidth is at least somewhat correlated, which can also suppress the phase noise in the bista-static evaluation for a defined frequency range, at least slightly.

[0048] The radar sensor system 200 can be configured as an FMCW radar, e.g. an FMCW chirp sequence radar, but can also be operated with other modulation types.

[0049] Advantageously, the proposed radar sensor system 100 can be implemented not only with full radar sensors, but also with radar sensor heads, with a central control unit for processing the signals from the radar sensor heads.

[0050] The proposed concept could involve, for example, five radar sensors on a vehicle, positioned at the front left, front right, and front center, as well as at the rear left and rear right. The three front radar sensors would have partially overlapping fields of view. If necessary, the radar sensors mentioned could be supplemented with additional satellite sensors to further optimize the proposed method.

[0051] Advantageously, the proposed method for operating a radar sensor system 100 can be designed as software that runs on the radar sensor system 100. This facilitates easy adaptability of the method.

[0052] Fig. 2 shows a basic sequence of a method for manufacturing a radar sensor system 100, wherein the method for manufacturing a radar sensor system 100 is not part of the invention.

[0053] In step 300, at least two radar sensors 10, 20 are provided, wherein the detection ranges of the two radar sensors 10, 20 overlap at least partially, wherein the two radar sensors 10, 20 are arranged at a defined distance apart, wherein the transmitted signals of the two radar sensors 10, 20 can be synchronized in such a way that radiation reflected from an object 200 by one radar sensor 10, 20, which was emitted by the other radar sensor 10, 20, can be evaluated by means of an evaluation device 30.

[0054] In summary, the proposed radar sensor system enables bistatic evaluation with minimal effort required for synchronizing the oscillators in the sensors. This makes bistatic evaluation cost-effective. For example, the proposed bistatic evaluation mode of the radar sensor system can be activated for specific applications.

Claims

1. Radar sensor system (100), comprising: - at least two radar sensors (10, 20) that each have at least one transmitter and at least one receiver, detection areas of the two radar sensors (10, 20) at least partially overlapping, the two radar sensors (10, 20) being a defined distance apart, transmission signals of the two radar sensors (10, 20) being able to be synchronized in such a way that an evaluation device (30) can be used to evaluate radiation of one radar sensor (10, 20) that has been reflected by an object (200) and was emitted by the other radar sensor (10, 20), the synchronization being based substantially on a synchronization of the reference oscillators of the radar sensors (10, 20), - each radar sensor (10, 20) having a crystal oscillator, an Ethernet communication interface being used to regularly determine by way of timestamps the deviation from a reference frequency so that it is possible to synchronize the timing of the reception of one radar sensor (10, 20) with that of the transmission of the other radar sensor (10, 20) using the Precision Time Protocol so that a transmission signal of the other radar sensor (10, 20) is also received, - the evaluation device (30) being able to be used to determine a bistatic and a monostatic radar cross-section of the object (200), - characterized in that a frequency offset (Δf) between the two radar sensors (10, 20) is determined and compensated for by virtue of the transmitters of the radar sensors (10, 20) alternately transmitting, with the receivers of the non-transmitting radar sensors (10, 20) simultaneously receiving, and - the frequency offset (Δf) between the two radar sensors (10, 20) is determined and compensated for by measuring two baseband frequencies for two unknowns when there is one target object (200) and by measuring 2N baseband frequencies for N+1 unknowns when there are N target objects.

2. Radar sensor system (100) according to Claim 1, characterized in that the at least two radar sensors (10, 20) are used to transmit FMCW ramps that have substantially identical modulation parameters.

3. Radar sensor system (100) according to Claim 1 or 2, characterized in that the transmission signals of the radar sensors (10, 20) are synchronized by factoring in signal powers of the transmission signals and using only signals with high or similar power to at least roughly determine an offset, the offset being the shift in the baseband frequencies of the respective two transmitting radar sensors in the bistatic evaluation, the arithmetic sign of which offset reverses for inverse wave propagation when the transmitter and the receiver are interchanged.