Radar sensor network and method for determining the relative velocity of a radar target

Abstract

The invention relates to a radar sensor network (100) and a method for determining the relative velocity (v) of a radar target (18), in which phase-modulated transmission signals (30-1; 30-2; 30-3) are transmitted by means of a plurality of transmission antenna elements (14) of each of a plurality of radar sensors (10), wherein a ramp-like frequency-modulated transmission signal (30-1; 30-2; 30-3) of a respective radar sensor has temporally nested sequences (1a; 1b; 1c; 1d;...) of ramps (40) which follow on from one another in a time-offset manner with a predefined time interval (Tr2r) within the respective sequence (1a; 1b; 1c; 1d;..), wherein a phase of the transmission signal (30-1; 30-2; 30-3) is phase-modulated with a code for each transmission antenna element (14) of a radar sensor (10), and wherein a measurement cycle of the plurality of radar sensors (10) comprises sections (42) which follow on from one another temporally and which are each allocated to one of the radar sensors (10) and comprise one ramp (40) or a plurality of ramps (40) of the transmission signal (30-1; 30-2; 30-3) exclusively of the radar sensor (10) allocated to the section; an estimated value for the relative velocity (v) of the radar target (18) is determined from the received response signals.

Description

[0001] Description

[0002] title

[0003] Radar sensor network and method for determining the relative velocity of a radar target

[0004] The invention relates to a radar sensor network and a method for determining the relative speed of a radar target. The radar sensor network can, in particular, be a radar sensor network for motor vehicles. The radar sensors of the radar sensor network can, in particular, be configured for an FMCW (frequency modulated continuous wave) measurement method with linear frequency ramps and an evaluation of the received signals using discrete Fourier transformation, in particular an FFT (Fast Fourier Transformation). Background

[0005] Radar sensors are used to implement comfort functions such as Adaptive Cruise Control and safety functions such as emergency braking. The main advantage of such sensors is that they directly measure physical quantities and do not require an interpretation of

[0006] Images, such as those from a video camera. Radar sensors transmit high-frequency radar beams via an antenna structure and receive the beams reflected from objects. The detected objects can be stationary or moving. Using the received radar beams, the distance and direction (angle) to the object can be calculated. Furthermore, the relative speed of an object to the radar sensor can be calculated. Typical radar sensors operate in a frequency range between 76 and 81 GHz.

[0007] The so-called chirp sequence method is a well-known technique for radar modulation. This involves transmitting many fast frequency ramps, known as chirps. To increase accuracy and angular separation, multiple transmitting antennas are often used to create an enlarged virtual aperture using the MIMO (multiple input multiple output) principle. The transmitted signals must be separated upon reception. A simple method for this is time-division multiplexing, in which the time interval between chirps with the same transmitting antenna is increased by a factor of N = the number of transmitting antennas. As a result, the unambiguous measurement range for the relative velocity is also reduced by a factor of N.

[0008] State of the art

[0009] US 2019 / 0353770 A1 and DE 10 2017 200 317 A1 describe a radar sensor and a method for determining the relative speed of a radar target. The speed of objects is estimated using a radar sensor with multiple transmitting antennas. Several nested sequences of frequency ramps are transmitted using multiple transmitting antennas. Individual phase coding using a harmonic code is performed for each transmitting antenna. To estimate the object's speed, ambiguities are resolved using code multiplexing.

[0010] Disclosure of the invention

[0011] In cooperative radar sensor networks, a distinction is made between systems with coherent distribution of the RF oscillator signal and those without. Systems with distribution of the RF oscillator signal exhibit significantly better phase noise performance, but the distribution of the oscillator signal is associated with considerable complexity, so a solution without distribution of the oscillator signal is advantageous for practical implementation.

[0012] In a cooperative radar sensor network without RF oscillator signal distribution, response signal analysis may involve evaluating response signals received by one radar sensor against transmitted signals from another radar sensor in the radar network. This may involve calibration or compensation between non-coherent radar sensors. The analysis may be performed, for example, according to techniques described in the two documents mentioned below:

[0013] M. Göttinger, F. Kirsch, P. Gulden and M. Vossiek, "Coherent Full-Duplex Double-Sided Two-Way Ranging and Velocity Measurement Between Separate Incoherent Radio Units," IEEE Transactions on Microwave Theory and Techniques, vol. 67, no. 5, pp. 2045-2061 , May 2019, doi:

[0014] 10.1109 / TMTT.2019.2902553;

[0015] A. Dürr, B. Schweizer, J. Bechter and C. Waldschmidt, "Phase Noise Mitigation for Multistatic FMCW Radar Sensor Networks Using Carrier Transmission," IEEE Microwave and Wireless Components Letters, vol. 28, no. 12, pp. 1143- 1145, Dec. 2018, doi: 10.1109 / LMWC.2018.2878983.

[0016] If the code division multiplexing method described in US 2019 / 0353770 A1 and DE 10 2017 200 317 A1 were used to multiplex multiple sensors in a cooperative sensor network without a common oscillator signal, the uncorrelated phase noise of the various local oscillators would typically cause a significant increase in the noise level in the received signals of all sensors, severely limiting the sensitivity and dynamic range of the radar sensor network. The monostatic signals (transmission and reception on the same radar sensor with their own oscillator signal) would also exhibit this degradation.

[0017] The object of the invention is therefore to provide a modulation method for a radar sensor network which enables a clear speed measurement in the individual sensor and at the same time realizes a good phase noise behavior in the monostatic signal response.

[0018] The object is achieved according to the invention by a method for determining a relative speed of a radar target, comprising the steps:

[0019] Emitting phase-modulated transmission signals by means of a plurality of transmitting antenna elements of each of a plurality of radar sensors, wherein a ramp-shaped frequency-modulated transmission signal from a respective one of the radar sensors is generated, which has a plurality of temporally interleaved sequences of ramps, wherein the ramps follow one another within the respective sequence with a predetermined time interval, wherein a phase of the generated ramp-shaped frequency-modulated transmission signal for each transmitting antenna element of a radar sensor is phase-modulated with a code, and wherein a measuring cycle of the plurality of radar sensors comprises temporally successive sections, each of which is assigned to one of the radar sensors and comprises one or more ramps of the transmission signal exclusively of the radar sensor assigned to the section, receiving response signals to the emitted phase-modulated transmission signals;

[0020] Calculating a two-dimensional spectrum for each sequence of the transmitted signal of a respective radar sensor by a two-dimensional Fourier transform of baseband signals of the received response signals, wherein the transformation is performed in a first dimension per ramp and in a second dimension via a ramp index of the sequence of ramps;

[0021] Determining values ​​for relative velocities of a radar target that are periodic with a predetermined velocity period based on a peak in one of the calculated two-dimensional spectra;

[0022] Determining an estimate of the relative velocity of the radar target based on matches in phase relationship between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for several of the determined values ​​of relative velocities.

[0023] The ramp sequences are also referred to as ramp sequences below. Unless otherwise stated, the term "sensor" refers to a radar sensor. The ramps are also referred to as chirps.

[0024] In this way, a clear speed measurement is possible based on the phase-modulated transmission signals transmitted by the transmitting antenna elements of a respective radar sensor and the response signals received (for example, in particular by means of at least one receiving antenna element of the same radar sensor), without this speed measurement being impaired by simultaneously transmitted transmission signals from another of the radar sensors. This is because at each transmission time, only one of the radar sensors transmits the ramp-shaped frequency-modulated transmission signal.

[0025] The core of the invention is thus a modulation method that operates each individual sensor using a modulation method, in particular a JSFMCW modulation method (Joint Sampling FMCW), with code division multiplexing (e.g., DDM, Doppler Division Multiplexing), and operates the various individual sensors in time division multiplexing. Since only one sensor transmits radar beams at a time, the phase noise behavior of the individual sensor is determined only by the phase noise of the local oscillator or the transmission signal processing of the individual sensor, thus allowing high sensitivity and signal dynamics in the monostatic response (i.e., the same radar sensor transmits and receives). By using the JSFMCW method with DDM, high sensitivity is achieved in the individual sensor (all transmitters of a radar sensor are active simultaneously) and, at the same time, unambiguous speed measurement is enabled without the existence of bistatic paths (i.e.,, paths between different radar sensors) is assumed.

[0026] The method makes it possible to extend the MIMO principle to a network of multiple cooperative radar sensors, thus further increasing performance. The fundamental challenges of a clear measurement range in relative velocity and transmitter multiplexing remain manageable.

[0027] The terms "monostatic" and "bistatic" are used herein, on the one hand, in relation to an antenna system of a radar sensor. In a bistatic antenna system, the transmitting antenna elements are different from the receiving antenna elements, and in a monostatic antenna concept of a radar sensor, the same antenna elements are used for transmitting and receiving. On the other hand, the terms "monostatic" and "bistatic" are used herein in relation to a received radar signal from one of the multiple radar sensors. The term "monostatic" is used to mean that a received radar signal is based on a transmitted signal transmitted by the same radar sensor, and the term "bistatic" is used to mean that a received radar signal is based on a transmitted signal transmitted by another of the multiple radar sensors.The transmit and receive antenna elements of radar sensors can each have a similar design. For example, they can each consist of a patch antenna array.

[0028] Receiving response signals to the transmitted phase-modulated transmission signals can comprise: wherein at least response signals are received by at least one receiving antenna element of the same radar sensor for the phase-modulated transmission signal transmitted by the transmitting antenna elements of a respective radar sensor. That is, at least monostatic response signals are received.

[0029] A measurement cycle of the multiple radar sensors comprises temporally successive sections, each of which is assigned to one of the radar sensors and comprises one or more ramps of the transmission signal of the radar sensor assigned to the section. In particular, the sections can each comprise one or more ramps of the transmission signal exclusively of the radar sensor assigned to the section. Thus, at any given time, the transmitted, phase-modulated, ramp-shaped frequency-modulated transmission signal only passes through a maximum of one of the multiple radar sensors.

[0030] Ramp.

[0031] Thus, the respective radar sensors transmit with a time offset according to a time-division multiplexing method. In other words, the individual ramps of the sequences of a respective radar sensor do not overlap in time with the ramps of the sequences of the other radar sensors, or they are arranged in a time-division multiplexing method of the radar sensors so that they do not overlap in time with the ramps of the sequences of the other radar sensors. In particular, while a ramp of the transmission signal of a respective radar sensor is traversing, only the respective radar sensor transmits. Thus, at a respective time, a transmission signal from only one of the respective radar sensors (i.e. transmission signals from the transmission antenna elements of only one of the respective radar sensors) is transmitted in a ramp-shaped, frequency-modulated manner. The phase-modulated transmission signals of the radar sensor are transmitted simultaneously by means of the multiple transmission antenna elements of the respective radar sensor.In a respective radar sensor, the transmission signal is thus transmitted simultaneously (synchronously) by several transmitting antenna elements, with the respective coding modulated by phase modulation. A different code is used for each simultaneously transmitting transmitting antenna element. The ramps of the ramp sequences of the transmission signals of the transmitting antenna elements of a respective one of the plurality of radar sensors are arranged in time gaps between the ramps of the ramp sequences of the transmission signals of the transmitting antenna elements of the respective other one of the plurality of radar sensors; they are arranged according to a time-division multiplexing scheme. In particular, the transmission of phase-modulated transmission signals can comprise: wherein the respective radar sensors transmit the phase-modulated, ramp-shaped frequency-modulated transmission signal according to a time-division multiplexing method of the plurality of radar sensors.The method may include the following step: Assigning a response signal to one of the multiple transmit antenna elements. This may be done, in particular, based on the code of the transmit signal of the transmit antenna element. To assign the response signals or the transmit signals to a transmit antenna element, noise-like or harmonic codes are possible, for example. Based on the assignment, an angle estimate of the detected radar target may be made.

[0032] In embodiments, the code is a harmonic code. In this context, "harmonic" means that the phase modulation describes a discrete, harmonic oscillation. A harmonic code is thus understood here as follows: A phase of the generated ramp-shaped, frequency-modulated transmission signal is phase-modulated for each transmitting antenna element of a radar sensor with a (complex) harmonic code, wherein the harmonic code describes a discrete, harmonic oscillation. Examples of harmonic codes include harmonic sequences such as 1 , 1 , 1 ,

[0033] 1 , -1 , 1 , -1 , or 1 , i, -1 , -i, 1 , ... or other harmonic frequencies are possible. Phase modulation using harmonic codes leads to additional ambiguities for the velocities to be estimated. These ambiguities can also be resolved.

[0034] The transmission signal of a radar sensor can, for example, be generated from an RF oscillator signal. The transmission signals of the plurality of radar sensors can be coherent or non-coherent with one another. In embodiments, the transmission signal and / or an RF oscillator signal of the respective radar sensor is generated by the respective radar sensor. For example, a reference signal can be generated and distributed to at least one or more of the plurality of radar sensors, wherein the at least one or more of the plurality of radar sensors generate the respective transmission signal based on the reference signal. The reference signal can, for example, be a clock signal or an RF oscillator signal. The latter enables coherent transmission signals from the radar sensors. In other embodiments, the transmission signals of the radar sensors are generated by an RF oscillator and distributed to at least one of the plurality of radar sensors.This enables coherent transmission signals from the radar sensors.

[0035] In particular, calculating a two-dimensional spectrum for each sequence of the transmitted signal of a respective radar sensor can comprise: calculating, for a respective receive channel of a respective radar sensor, a two-dimensional spectrum for each sequence of the transmitted signal of the respective radar sensor. This means that for each receive channel and / or each receive antenna element of a radar sensor, a two-dimensional spectrum is calculated for each sequence of the transmitted signal of the radar sensor. The received response signals thus comprise monostatically received radar signals. The respective receive channel is assigned to a respective receive antenna element.

[0036] In the two-dimensional Fourier transformation of baseband signals of the received response signals, the transformation is performed in a first dimension per ramp and in a second dimension via a ramp index of the ramp sequence. In other words, the transformation is performed ramp by ramp in a first dimension and in a second dimension via a ramp index that counts the ramps within the sequence.

[0037] Values ​​for relative velocities of a radar target are determined that are periodic with a predetermined velocity period based on a peak in one of the calculated two-dimensional spectra. In other words, the method comprises: determining values ​​for relative velocities of a radar target that are periodic with a predetermined velocity period starting from a peak in one of the calculated two-dimensional spectra.

[0038] The method comprises determining an estimated value for the relative speed of the radar target based on phase relationship matches between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for a plurality of the determined relative speed values. For example, the method may comprise identifying phase relationship matches between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for a plurality of the determined relative speed values, and selecting an estimated value for the relative speed of the radar target based on the identified phase relationship match.In particular, determining an estimate of the relative velocity of the radar target may comprise: identifying matches in phase relationships between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for a plurality of the determined values ​​of relative velocities; and selecting an estimate of the relative velocity of the radar target based on the identified match in the phase relationship.

[0039] In embodiments, the plurality of radar sensors each have a circuit board (printed circuit board), wherein the circuit boards of the respective radar sensors are separated from one another. In embodiments, the plurality of radar sensors are arranged at different positions on the vehicle, separated from one another. In particular, the plurality of radar sensors have overlapping or identical fields of view.

[0040] The ramp sequences of the individual sensors can have identical parameters (duration and frequency swing of the chirps, number of chirps, (possibly time-variable) center frequency, start times of the individual chirps), however, this is not a prerequisite for the method. In embodiments, within a sequence of ramps, the ramps have the same ramp gradient, the same difference of the ramp center frequencies, the same ramp center frequency, the same ramp duration and / or the same ramp frequency swing. In embodiments, ramps with the same ramp index in the ramp sequences of the radar sensors each have the same

[0041] Ramp gradient, the same ramp center frequency, the same ramp duration, the same ramp frequency swing, and / or the same time offset to a ramp with a ramp index one lower in the same ramp sequence. In embodiments, the transmission signals of the plurality of radar sensors have different time offsets between their ramp sequences (more precisely: between successive ramps, one of which belongs to a respective ramp sequence, or between the start times of their ramp sequences), i.e. at least one or more of the time offsets between the ramp sequences of a radar sensor differs / differs from each of the time offsets between the successive start times of the ramp sequences of at least one of the other radar sensors, or from each of the time offsets between the successive start times of the ramp sequences of each other of the radar sensors.

[0042] In embodiments, the transmission signals of the plurality of radar sensors have different time intervals with which the ramps follow one another with a time offset within the respective ramp sequence, and / or different ramp durations (durations of the chirps).

[0043] In embodiments, the ramp sequences of a respective radar sensor have equidistant time offsets from one another. In other words, ramps each having the same ramp index of the ramp sequences of a respective radar sensor have equidistant time offsets from one another. This means that their ramp start times are arranged equidistantly in time. In other embodiments, the ramp sequences of a respective radar sensor each have non-equidistant time offsets from one another in pairs. In other words, ramps each having the same ramp index of the ramp sequences of a respective radar sensor each have non-equidistant time offsets from one another in pairs. This means that their ramp start times are arranged non-equidistantly in time.

[0044] In embodiments, the transmission signal of the respective radar sensor comprises or forms a sequence of ramp groups arranged sequentially, wherein each ramp group comprises (or consists of) a ramp from each of the sequences of ramps of the transmission signal of the radar sensor. These can, in particular, be ramps each with the same ramp index. Thus, the ramp groups of a sequence of ramp groups follow one another chronologically. The sequence can be seamless and / or spaced.

[0045] In embodiments, a measurement cycle of the plurality of radar sensors comprises temporally successive blocks, each block comprising a ramp group from each of the radar sensors. In preferred embodiments, each block comprises exactly one ramp group from each of the radar sensors.

[0046] For example, the measurement cycle of the multiple radar sensors can be divided into blocks.

[0047] In particular, a measurement cycle of the plurality of radar sensors may comprise temporally successive blocks, wherein each block comprises (exactly) one ramp group of each of the radar sensors, wherein the ramps of one ramp group of the block are temporally interleaved with the ramps of each other of the ramp groups of the block.

[0048] In preferred embodiments, in each block, the ramps of a ramp group of the block are temporally interleaved with the ramps of each other of the ramp groups of the block. Such a radar modulation scheme is also referred to below as radar modulation scheme #1. Thus, each ramp group of a block is distributed across at least two sections assigned to the radar sensor of the ramp group. Thus, the ramps of a ramp group of a radar sensor are interleaved with ramps of a ramp group of each other of the radar sensors. More precisely, the ramps of a ramp group of the ramp group sequence of the transmission signal of a respective radar sensor are (temporally) interleaved with the ramps of a ramp group of the ramp group sequence of the transmission signal of each of the other radar sensors.In particular, between each two consecutive ramps of a respective sequence of ramps of a transmission signal of a radar sensor, at least one ramp of each of the sequences of ramps of the transmission signals of the other radar sensors can be arranged, ie, can be passed through.

[0049] In radar modulation scheme #1, the multiple radar sensors are operated in an interleaved time-division multiplexing method, which minimizes the time offset between the radar sensors. Several transmit antenna elements are simultaneously active using code division multiplexing, particularly Doppler division multiplexing (DDM).

[0050] The time offset of the ramp sequences of the individual sensors can be selected to be equidistant or non-equidistant, depending on the application and further signal processing. An equidistant time offset may simplify implementation and optimization. The precise design of the modulation scheme can be optimized for speed clarity by taking into account the properties of the individual ramp sequences. For example, it is conceivable to vary the time intervals from chirp to chirp within a sequence.

[0051] In embodiments, the method comprises: performing digital beamforming for a radar target via transmission channels assigned to the transmission antenna elements and / or reception channels assigned to reception antenna elements of at least two of the plurality of radar sensors, wherein phase differences between the radar sensors that depend on the relative speed of the radar target are taken into account when performing the digital beamforming. In particular, a respective phase difference between two of the radar sensors for a radar target can be determined based on at least one (in particular the determined) estimated value for the relative speed of the radar target and based on a time offset (within a block) between the ramp groups of the two radar sensors. The (digital) beamforming can also be referred to as (digital) beamforming. In particular, the digital beamforming can be performed via several (e.g. all) transmission channels and / or several (e.g.all) receive channels of the at least two radar sensors. Beamforming across multiple (e.g., all) transmission channels is referred to as transmit-side beamforming. Beamforming across multiple (e.g., all) receive channels is referred to as receive-side beamforming. This beamforming is also referred to as MIMO (multiple-input-multiple-output) beamforming. The phase difference corresponds to a phase change in the received radar signals due to a relative movement of the radar target during the time offset between the ramp groups of the different radar sensors. Taking the respective phase difference into account makes it possible to jointly and coherently evaluate two-dimensional spectra originating from different radar sensors.Performing digital beamforming can, in particular, comprise a phase-correct summation of the calculated two-dimensional spectra or of parts of the calculated two-dimensional spectra corresponding to the radar target. Digital beamforming can be performed for received monostatic radar signals (response signals) or for received monostatic and bistatic radar signals (response signals). When processing monostatic response signals from different radar sensors, the phase difference between the respective radar sensors is taken into account. When processing bistatic response signals received by one radar sensor in response to phase-modulated transmission signals emitted by another radar sensor, the phase difference between the respective radar sensors is taken into account.By means of the unambiguous speed measurement of the individual sensors, a speed-dependent phase correction (the phase difference) can be determined in order to compensate for the time offset between the bistatic and / or monostatic signals and thus enable a joint coherent processing of the signals from several sensors.

[0052] The unique speed measurement of the individual sensors also allows for partial processing on the individual sensors: For example, MIMO beamforming (transmit-side and receive-side beamforming) can already be performed on the individual sensors. The determination of the unique speed and the beamforming can occur before or after the detection of a radar target.

[0053] The described processing can be carried out completely on a central evaluation unit, e.g. a central control unit, or in partial aspects on the individual sensors, e.g. on evaluation units of the individual radar sensors.

[0054] In embodiments, the method comprises determining an angle estimate for the radar target. Determining the angle estimate for the radar target may, for example, comprise performing digital beamforming for the radar target.

[0055] In embodiments, performing the digital beamforming for a radar target comprises a first sub-step in which digital beamforming for the radar target is performed via transmission channels assigned to the transmission antenna elements and / or reception channels assigned to the reception antenna elements of a respective one of the at least two of the plurality of radar sensors, and a second sub-step in which the digital beamformings of the respective at least two radar sensors are combined, taking into account the phase differences between the radar sensors that depend on the relative speed of the radar target.

[0056] In embodiments, receiving response signals to the transmitted phase-modulated transmission signals comprises: wherein, for the phase-modulated transmission signal transmitted by the transmitting antenna elements of a respective radar sensor, at least response signals are received by means of at least one receiving antenna element of the same radar sensor and response signals are received by means of at least one receiving antenna element of another of the plurality of radar sensors. This means that monostatic and bistatic response signals are received. The evaluation of the response signals can thus comprise an evaluation of response signals received in one radar sensor to transmission signals from another of the radar sensors in the radar network.

[0057] If necessary (e.g., excessive phase noise in the bistatic response), the evaluation of the bistatic signals can alternatively be dispensed with and only the monostatic signals of the individual sensors can be evaluated. This enables a fallback position without having to change the modulation method. In one embodiment, the steps of calculating a two-dimensional spectrum, determining values ​​for relative speeds of a radar target, determining an estimated value for the relative speed of the radar target, and optionally performing digital beamforming and / or determining an angle estimate are carried out at least for monostatic response signals of a respective radar sensor, wherein the steps are optionally also carried out for bistatic response signals of a respective radar sensor.The method may comprise deciding on the implementation of the steps also for bistatic response signals depending on a (peak) signal-to-noise ratio of the bistatic response signals and / or depending on a noise level of the bistatic response signals.

[0058] In further preferred embodiments, a measuring cycle of the plurality of radar sensors comprises temporally successive sub-cycles, wherein each sub-cycle is assigned to one of the radar sensors and comprises the ramp groups of the assigned radar sensor. In particular, each sub-cycle can be assigned to one of the radar sensors and comprise the ramp groups exclusively of the assigned radar sensor. Such a radar modulation scheme is also referred to below as radar modulation scheme #2. In particular, for example, the sequences of the ramp groups of the radar sensors can follow one another in time. This means: the sequence of the ramp groups of a first of the radar sensors is followed by the sequence of the ramp groups of a next of the radar sensors. Thus, a sub-cycle assigned to a radar sensor corresponds to a section assigned to this radar sensor.

[0059] For example, a measurement cycle of the plurality of radar sensors (a cycle of the transmission signals of the plurality of radar sensors) can comprise respective sub-cycles in which only one of the radar sensors transmits, wherein the sub-cycles follow one another, and wherein in a respective sub-cycle only the temporally nested sequences of ramps of the respective radar sensor are run through.

[0060] In radar modulation scheme #2, the multiple radar sensors are operated sequentially in a time-division multiplexing process, with only one of the radar sensors transmitting in each sub-cycle. Unlike radar modulation scheme #1, radar modulation scheme #2 does not temporally interleave the sensors' transmissions. Instead, the individual sensors transmit sequentially, with all sensors preferably receiving continuously.

[0061] With sequential transmission of individual sensors, the accuracy of the speed measurement of the individual sensor may not be sufficient to enable coherent processing of the transmitted signals across all sensors. If the accuracy is sufficient, the evaluation can be performed as described above. Otherwise, separate evaluation of the "sub-cycles" is recommended, whereby the transmitted signals of the currently active sensor and the received signals (RX) of all sensors can be evaluated. The sequential transmission of the individual sensors results in an update rate of the detected targets that is increased by the number of sensors, since processing takes place in each "sub-cycle."

[0062] When evaluating the bistatic response signals, different virtual antenna arrangements with different ambiguity properties in the angle estimation are obtained due to the different activations of the transmitters (TX) in the “sub-cycles”.

[0063] As an alternative to separate processing of the “sub-cycles” and the resulting increased update rate, a non-coherent averaging of the results of the sub-cycles can also be advantageous. The non-coherent averaging can be applied to

[0064] spectral plane before target detection or by averaging the angular spectra after target detection.

[0065] In embodiments, the method comprises performing digital beamforming over transmit channels assigned to the transmit antenna elements and / or receive channels assigned to receive antenna elements of a respective one of the plurality of radar sensors for a radar target and for a respective sub-cycle. The digital beamforming can be performed for received monostatic radar signals or for received monostatic and bistatic radar signals. When processing bistatic response signals received by one radar sensor in response to phase-modulated transmit signals emitted by another radar sensor, different virtual antenna arrangements with different ambiguity properties in the angle estimation are obtained due to the different activations of the transmitters (TX) in the "sub-cycles." These ambiguities can be resolved.

[0066] In embodiments, the method comprises determining an angle estimate for the radar target. Determining the angle estimate for the radar target may, for example, comprise performing digital beamforming for the radar target.

[0067] Determining an angle estimate may include: determining a respective ambiguous angle estimate for the radar target for monostatic and bistatic received response signals of a respective radar sensor; and determining an unambiguous angle estimate for the radar target based on the respective ambiguous angle estimates for the radar target.

[0068] In embodiments, the digital beamforming is carried out via transmit channels assigned to the transmit antenna elements and / or receive channels assigned to the receive antenna elements by a respective one of at least two of the plurality of radar sensors for a radar target and for a respective respective sub-cycle, the method further comprising: combining the digital beamformings of the at least two radar sensors by non-coherently averaging the digital beamformings of the at least two radar sensors and respective sub-cycles or by averaging angular spectra of the radar target of the at least two radar sensors and respective sub-cycles obtained by the digital beamformings.

[0069] In embodiments, receiving response signals to the transmitted phase-modulated transmission signals comprises: wherein, for the phase-modulated transmission signal transmitted by the transmitting antenna elements of a respective radar sensor, at least response signals are received by means of at least one receiving antenna element of the same radar sensor and response signals are received by means of at least one receiving antenna element of another of the plurality of radar sensors. That is, monostatic and bistatic response signals are received.

[0070] According to one aspect, the object is achieved by a radar sensor network for determining a relative speed of a radar target, comprising a plurality of radar sensors and at least one evaluation unit, wherein each of the plurality of radar sensors comprises: a plurality of transmitting antenna elements; a signal generating device designed to generate a ramp-shaped frequency-modulated transmission signal and to provide it to the transmitting antenna elements, wherein the transmission signal has a plurality of temporally interleaved sequences of ramps, wherein the ramps follow one another within the respective sequence with a predetermined time interval (Tr2r), wherein a phase of the generated ramp-shaped frequency-modulated transmission signal is phase-modulated with a code for each transmitting antenna element of a radar sensor;and at least one receiving antenna element configured to receive a response signal to the transmitted phase-modulated transmission signals, wherein the evaluation unit is configured to determine values ​​for relative speeds of a radar target that are periodic with a predetermined speed period based on a peak in one of two-dimensional spectra calculated for each sequence of the transmission signal of a respective radar sensor; and to determine an estimated value for the relative speed of the radar target based on correspondences in phase relationships between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for several of the determined values ​​of relative speeds.

[0071] In preferred embodiments, each of the multiple radar sensors comprises multiple receiving antenna elements. The multiple receiving antenna elements of a radar sensor are preferably spatially separated. Thus, the radar sensor forms a so-called MIMO (multiple input multiple output) system with multiple transmit and receive channels. The radar sensor network is, in particular, a cooperative radar sensor network.

[0072] The evaluation unit can comprise a central evaluation unit and local evaluation units of the respective radar sensors. The local evaluation units can be part of the respective radar sensors. The evaluation unit can be designed in particular to carry out the described method. The radar sensor network can comprise a control unit. The control unit can comprise the evaluation unit. The control unit can be designed to carry out the described method. The control unit can comprise a central control unit and local control units of the respective radar sensors. The local control units can be part of the respective radar sensors. The local control units can each comprise the local evaluation unit. Exemplary embodiments are explained in more detail below with reference to drawings. They show:

[0073] Fig. 1 is a schematic representation of a cooperative radar sensor network with several radar sensors and a central control unit;

[0074] Fig. 2 is a schematic representation of a time-frequency diagram of transmission signals from several radar sensors according to one embodiment;

[0075] Fig. 3 is a schematic representation of a flowchart underlying a method according to an embodiment;

[0076] Fig. 4 is a schematic representation of a time-frequency diagram of transmission signals from several radar sensors according to another embodiment;

[0077] Fig. 5 is a schematic representation of virtual antenna arrangements according to another embodiment; and

[0078] Fig. 6 is a schematic representation of a flow chart according to a method according to another embodiment.

[0079] Fig. 1 shows a schematic representation of a radar sensor network 100 with several FMCW radar sensors 10-1 and 10-2 and a central control unit 20. In practice, the number of radar sensors 10 can be greater. Each of the radar sensors 10-1, 10-2 comprises a local control and evaluation unit 12. Furthermore, each of the sensor sensors 10-1, 10-2 comprises two transmitter antenna elements 14-1, 14-2 and two receiver antenna elements 16-1, 16-2. In practice, larger numbers of antenna elements are possible.

[0080] Radar sensors 10-1, 10-2 are installed on a same side of a motor vehicle, for example, from, and are designed to measure distances d, angles and relative speeds v of radar targets 18, for example, of vehicles driving ahead or of obstacles.

[0081] In the example shown, radar sensors 10-1, 10-2 each have a bistatic antenna system in which different antenna elements are used for transmitting and receiving. However, a monostatic antenna system can also be used, in which the respective radar sensor uses the same antenna elements for transmitting and receiving.

[0082] The respective control and evaluation unit 12 comprises a signal generation device 17, which generates a transmission signal and is designed to individually modulate the phase of the supplied transmission signal for each transmission antenna element 14-1, 14-2. In the example, harmonic codes are used for the phase modulation of the transmission signals, with a different code being used for each transmission antenna element 14-1, 14-2. The phase-modulated transmission signals are fed to the plurality of transmission antenna elements 14-1, 14-2 and transmitted by them. The transmitted radar signals reflected by an object 18 are received by the reception antenna elements 16-1, 16-2. The received signals are downconverted to baseband signals by the control and evaluation unit 12 and evaluated. The frequency of the transmitted signal can be modulated within a measurement cycle of a radar measurement with sequences of rising or falling ramps.Examples of ramp-shaped frequency-modulated transmission signals are explained in more detail below.

[0083] The antenna elements 14-1, 14-2, 16-1, 16-2 can be arranged in different positions in a direction in which the radar sensor 10 has angle resolution. This requires, in particular, a plurality of receiving antenna elements 16-1, 16-2 arranged at equal intervals on a straight line (ULA; Uniform Linear Array). The same applies to the transmitting antenna elements 14-1, 14-2, whereby the transmitting and receiving antenna elements 14-1, 14-2, 16-1, 16-2 do not necessarily have to be arranged on the same straight line. If the radar sensor 10 is to be used to measure azimuth angles of objects, the straight lines on which the antenna elements are arranged run horizontally. In a sensor for measuring elevation angles, however, the antenna elements would be arranged on vertical lines. A two-dimensional antenna array is also conceivable, allowing both azimuth and elevation angles to be measured.

[0084] 20

[0085] The transmitting antenna elements 14-1, 14-2 and the receiving antenna elements 16-1, 16-2 can each be constructed similarly and, for example, have identical viewing areas. The transmitting and receiving antenna elements 14-1, 14-2, 16-1, 16-2 can each consist of a patch antenna array, for example.

[0086] The central control and evaluation unit 20, in turn, controls the control and evaluation units 12 of the radar sensors 10-1, 10-2 and performs a cross-radar sensor portion of the evaluation of the received radar signals. Fig. 2 schematically shows a diagram for the frequency f of transmission signals 30-1, 30-2, 30-3 from three radar sensors 10 of a cooperative radar sensor network 100, which can basically correspond to the structure of Fig. 1, but includes three FMCW radar sensors 10. The transmission signals 30-1, 30-2, 30-3 of the three radar sensors 10 are represented by different line thicknesses.

[0087] In each radar sensor 10, the corresponding ramp-shaped, frequency-modulated transmission signal 30-1, 30-2, or 30-3 is generated. The respective transmission signal 30-1, 30-2, 30-3 comprises several ramp sequences for each radar sensor 10, four ramp sequences in the example shown. The ramp sequences of the transmission signal 30-1 of the first radar sensor 10 are designated 1a, 1b, 1c, 1d in Fig. 1. Accordingly, the ramp sequences of the transmission signal 30-2 of the second radar sensor 10 are designated 2a, 2b, 2c, 2d, and the ramp sequences of the transmission signal 30-3 of the third radar sensor 10 are designated 3a, 3b, 3c, 3d. The respective ramp sequences are represented by different line types.

[0088] In Fig. 2, only the first two ramps 40 of each ramp sequence are shown. Each illustrated ramp 40 corresponds to a corresponding section 42 of the respective transmission signal 30-1, 30-2, or 30-3. The transmission signal 30-1, 30-2, or 30-3, frequency-modulated according to ramp 40, is phase-modulated with a harmonic code for each transmission antenna element 14 of the respective radar sensor and transmitted by the transmission antenna element 14, as explained with reference to Fig. 1. For example, the harmonic code is identical for all ramp sequences 1a, 1b, 1c, 1d within a ramp group 44, and a next code setting is selected for each subsequent ramp group 44 (where each ramp group 44 includes a ramp 40 from each of the ramp sequences 1a, 1b, 1c, 1d of the radar sensor 10). Fig. 2 shows an example of a radar modulation scheme #1 for three radar sensors 10, each with several, e.g.four transmit antenna elements 14, which are active simultaneously using Doppler division multiplexing (DDM) with harmonic codes. The three sensors 10 are operated in an interleaved time-division multiplexing method in order to keep the time offset between the sensors 10 as small as possible. The ramp-shaped, frequency-modulated transmission signal 30-1, 30-2, or 30-3 of a respective one of the radar sensors 10 has several temporally interleaved ramp sequences. For example, the transmission signal 30-1 of the first radar sensor 10 has the four temporally interconnected ramp sequences 1a, 1b, 1c, and 1d. Within the respective ramp sequence, the ramps follow one another with a predetermined time interval Tr2r. A measuring cycle of the radar sensors 10, of which FIG.2, only the first two ramps of each ramp sequence are shown, comprises temporally successive sections 42, each of which is assigned to one of the radar sensors 10 and thus to the corresponding transmission signal 30-1, 30-2 or 30-3 of the respective radar sensor 10. Each section 42 comprises one ramp 40 or several ramps 40 of the transmission signal exclusively of the radar sensor 10 assigned to this section 42. Thus, the first two sections 42 marked in Fig. 2 comprise exclusively ramps 40 exclusively of the first radar sensor 10, wherein the ramps belong to its transmission signal 30-1.

[0089] For the respective transmission signal 30-1, 30-2, 30-3, the term ramp group refers to a group of those ramps of the ramp sequences of the transmission signal that have the same ramp index. For example, the transmission signal 30-1 of the first radar sensor 10 comprises ramp groups 44, each of which comprises a ramp 40 from the ramp sequences 1a, 1b, 1c, and 1d. In radar modulation scheme #1, the transmission signal 30-1, 30-2, 30-3 of the respective radar sensor 10 thus comprises a sequence 46 of ramp groups 44 arranged sequentially in time, each ramp group 44 comprising a ramp 40 from each of the ramp sequences 1a, 1b, 1c, 1d of the radar sensor 10. At the same time, the measurement cycle of the multiple radar sensors 10 is divided into temporally successive blocks 48. Each block 48 comprises a ramp group 44 from each of the radar sensors 10, i.e., a ramp group from each of the transmission signals 30-1, 30-2, 30-3 of the radar sensors 10.Blocks 48 follow with the time interval T. r 2r with a time delay. In each block 48, the ramps 40 of each ramp group 44 of the block 48 are temporally interleaved with the ramps 40 of each other of the ramp groups 44 of the block 48. For example, if one considers in Fig. 2 in the first block 48 that ramp group 44 which includes the ramps 40 with ramp index 1 of the first transmission signal 30-1, i.e. those ramps with ramp index 1 of the ramp sequences 1a, 1b, 1c and 1d, these ramps 40 are temporally interleaved with the ramps of the other ramp groups 44 of the block 48: The block 48 includes a second ramp group 44, which in each case includes the ramps with ramp index 1 of the ramp sequences 2a, 2b, 2c and 2d, and the block 48 includes a third ramp group 44, which in each case includes the ramps with ramp index 1 of the Ramp sequences 3a, 3b, 3c and 3d.

[0090] In the example shown, in a block 48, the ramps 40 of the first ramp group 44 have a time offset T12 to the ramps 40 of the second ramp group 44 and a time offset T13 to the ramps 40 of the third ramp group 44. The respective time offset T12, T13 is significantly smaller than the time interval Tr2r of the ramps within a respective ramp sequence. Pauses occur between some of the ramps of a respective ramp group 44. Furthermore, pauses occur between some consecutive ramps of the different transmission signals 30-1, 30-2, 30-3. The further evaluation of the received radar signals corresponds to the flow diagram explained below in Fig. 3 of a method for determining a relative speed of a radar target 18. In a step 60, the transmission and reception of the radar modulation scheme #1 takes place according to the above explanation.This includes transmitting the phase-modulated transmission signals 30-1, 30-2, 30-3 and receiving response signals to the transmitted phase-modulated transmission signals.

[0091] In a step 62, a 2D FFT of the individual ramp sequences 1a, 1b, 1c, 1d, or 2a, 2b, 2c, 2d, or 3a, 3b, 3c, 3d is carried out for each individual sensor. For each ramp sequence of the transmitted signal 30-1, 30-2, 30-3 of the respective radar sensor 10, a two-dimensional spectrum is calculated by a 2-dimensional Fourier transform, for example a 2D FFT, of baseband signals of the received response signals. The transformation is carried out in a first dimension for each ramp 40 and in a second dimension via the ramp index j of the respective ramp sequence. The sizes of the respective transformations, i.e. their respective numbers of bins (sampling points or support points), are preferably uniform for all spectra for the first dimension and for the second dimension.

[0092] Due to the relative speed v of the radar target 18 and the time offset Tab, Tac, Tad (Fig. 2) between the partial measurements corresponding to the individual ramp sequences, a phase difference occurs between the partial measurements. The phase difference between the partial measurements is referred to as

[0093] Phase difference between the complex amplitudes (spectral values) of a peak occurring at the same position in the two-dimensional spectra. However, due to the relatively large time offset Tab, Tac, Tad between the corresponding ramps 40 of the respective ramp sequences, the determination of the phase differences between the partial measurements does not allow any direct conclusions to be drawn about the relative velocity v. This is because the periodicity of the phases results in an ambiguity for the corresponding value of the relative velocity v for a single phase difference. In the example, the different sensors 10 have the same time offsets Tab, Tac, Tad. In a variant of the example, the time offsets Tab, Tac, Tad of the different sensors are selected differently. This then results in different ambiguities of the relative velocity v for the respective radar sensors, which simplifies the resolution of the ambiguities.

[0094] From the obtained two-dimensional complex spectra, a power spectrum is calculated by forming the square of the absolute value of the respective spectral values, and the power spectra are combined point by point by summation or averaging to form an integrated two-dimensional power spectrum.

[0095] The position of a peak corresponding to a radar target 18 in the power spectrum, which is subsequently specified as bin k, I, corresponds to the position of the peak in the individual spectra. From the first dimension, corresponding to bin k of the position of the peak, according to the FMCW equation k = 2 / c(dF

[0096] + fovT), a linear relationship between the relative velocity v and the distance d of the radar target is obtained. Where c is the speed of light, F is the ramp range, T is the ramp duration of a single ramp, and fo is the average transmission frequency. If the frequency difference between consecutive ramps in a sequence is zero, the peak position in the second dimension I contains only information about the relative velocity v of the radar target.

[0097] There is a linear relationship between the relative velocity v and the distance d. In the example shown, due to a relatively large time interval Tr2r, the information about the relative velocity of the radar target obtained from sampling the Doppler frequency is ambiguity, since the Doppler frequency resulting from the relative movement at velocity v is not clearly sampled due to the relatively large time intervals Tr2r of the respective ramp sequence. In addition to the linear relationship between the relative velocity v and the distance d, which results from the frequency bin k, periodic values ​​of the relative velocity v result from the frequency bin I, so that possible value pairs (v, d) of relative velocity and distance of the detected radar target 18 are obtained.

[0098] The ambiguity of the determined velocity v is now resolved, as explained below. To evaluate the measured phase difference, a control vector a(v) of an ideal measurement is calculated as a function of the relative velocity v. A measurement vector a mis defined accordingly, whereby instead of the expected, velocity-dependent complex values, the complex amplitudes (spectral values) at the position of the peak of the calculated two-dimensional spectra of the partial measurements are used as components of the vector. Based on the measurement vector and the control vector, a normalized likelihood function in the form of a relative velocity spectrum S(v) is defined. Maxima of the likelihood function correspond to the most probable values ​​of the parameter v. Taken on its own, the relative velocity spectrum S(v) can be ambiguous; a maximum at the maximum value 1 corresponds to an optimal match of the ideal phase shifts resulting for the respective relative velocity v with the measured phase shift according to the measurement vector.However, an evaluation of the function S(v) is only required at the points corresponding to the periodic values ​​of the relative velocity v obtained from the evaluations according to the position of the peak in the bins (k, I). A maximum agreement obtained here at a relative velocity v, where the function S(v) assumes the expected maximum value of 1, corresponds to the actual value of the relative velocity v.

[0099] The ambiguity resulting from the peak position can thus be resolved using the additional information from the phase relationship. Based on the linear relationship, an estimate for the distance d corresponding to the selected estimate for the relative velocity v is determined.

[0100] The time signals (baseband signals) corresponding to the different ramp sequences are first processed separately. The detection of a radar target occurs in the power spectrum obtained by non-coherent integration. Based on the detection and the complex amplitudes at the peak position, the ambiguity of the velocity v is then resolved.

[0101] The above statements refer to conventional radar analysis without code division multiplexing for the various transmit antenna elements 14-1, 14-2, etc. Furthermore, the phase coding of the transmit signals for the individual transmit antenna elements with a harmonic code leads to additional ambiguities that must also be resolved. In the case of code division multiplexing, a transmitter with a corresponding code assignment experiences a shift in the peaks in the second dimension of the spectrum. This results in NTX peaks in the spectrum for a number of NTX transmitters. This additional ambiguity must also be resolved.

[0102] Due to the harmonic phase modulation of the transmitted signals, a transmitter shift is equivalent to a bin shift in the Doppler dimension. This bin shift is equivalent to a changed estimate of the relevant relative velocities v. In a preferred embodiment with harmonic codes for NTX transmitters (10 NTX transmit antenna elements per radar sensor), the transmitter offset corresponds to a 1-bin offset of A = N_slow / NTX. The velocity offset is therefore continued equivalent to the subsampling in the Doppler or, in other words, the distances between the evaluated relative velocities are "filled" across the transmitters. To determine the relative velocity of the radar object, these additional ambiguities due to the code division multiplex must now also be evaluated.

[0103] The described method can also be extended to perform a radar measurement with multiple receive channels of a radar sensor. For each receive channel, a measurement vector a_m(n) is then obtained for the nth channel.

[0104] In Fig. 3, a step 64 comprises a step 64-1 of resolving the speed ambiguities and a step 64-2 of target detection. The resolution of the speed ambiguities can occur before or after target detection. The procedure is the same as for the code division multiplexing method in US 2019 / 0353770 A1 or DE 10 2017 200 317 A1 and as explained above.

[0105] According to variant 1 of the method of Fig. 3, step 64 is followed by step 66 for compensating the time offsets between the bistatic and monostatic signals. For this purpose, a phase difference between the respective radar sensors 10 is taken into account, which depends on the relative speed v of the radar target 18. The respective phase difference Acp-i2 between two radar sensors 10-1, 10-2 can be determined, for example, according to the following formula:

[0106]

[0107] Where c corresponds to the speed of light, fo to the center frequency of the chirp sequence modulation (ramp), T12 to the time offset between the ramp sequences of the two sensors (or between the monostatic and bistatic response received in one sensor), and v to the relative speed of the target determined in the individual sensor. If the target is far enough away (far field), then each of the two individual sensors measures a (virtually) identical speed. At close range, the two measured speeds of the sensors can differ. For the bistatic signal response, the average of the two speeds measured (by the respective radar sensors) must be considered.

[0108] The respective phase difference A(pi2) between two of the radar sensors 10 for a radar target 18 is thus determined based on the determined estimated value for the relative speed v of the radar target 18 and based on a respective time offset Ti2 (or T13) within a block 48 between the ramp groups 44 of the respective radar sensors 10.

[0109] The compensation of the time offsets is carried out, for example, by applying a phase correction to the complex amplitudes of the bistatic response signal, e.g. by multiplying exp(-1.0i A(pi2). The complex amplitudes of the monostatic response signals and the bistatic response signals then only contain the angular information, and the angle of the radar target can be determined using known methods for angle estimation (e.g. deterministic maximum likelihood estimator, DML). The complex amplitudes are compared, for example, with a measured antenna pattern, and the estimated angle is the angular position with the best agreement.

[0110] For angle estimation, in a step 68, digital beamforming for a radar target is performed via the transmission channels assigned to the transmitter antenna elements 14 and / or via the reception channels assigned to the reception antenna elements 16 of all radar sensors 10 or a subset of the radar sensors 10. Step 68 is thus a step of coherent MIMO beamforming via the transmission and reception channels of all sensors 10 or a subset.

[0111] According to variant 2 of the method of Fig. 3, the implementation of digital beamforming for a radar target is divided into two substeps. In a step 68-1, coherent MIMO beamforming is performed for each sensor 10. In this first substep, digital beamforming for the radar target is thus performed via the transmission channels assigned to the transmission antenna elements and / or via the reception channels assigned to the reception antenna elements of a respective radar sensor 10. At this point, after step 68-1, the monostatic results can be output and / or further processed in a step 69.

[0112] Subsequently, step 66 of compensating the time offsets between the bistatic and monostatic signals is carried out. After step 66, a step 68-2 follows in which a coherent combination of the partial beamformings of the individual sensors is carried out. The digital beamformings of the respective radar sensors 10 are combined, taking into account the phase differences between the radar sensors 10, which depend on the relative speed v of the radar target 18. Fig. 4 schematically shows a diagram for the frequency f of transmitted signals 30-1, 30-2, 30-3 from three radar sensors 10 of a cooperative radar sensor network 100, which can basically correspond to the structure of Fig. 1, but comprises three FMCW radar sensors 10. The transmitted signals 30-1, 30-2, 30-3 of the three radar sensors 10 are again represented by different line thicknesses.

[0113] In each radar sensor 10, the corresponding ramp-shaped, frequency-modulated transmission signal 30-1, 30-2, or 30-3 is generated. The respective transmission signal 30-1, 30-2, 30-3 comprises several ramp sequences for each radar sensor 10, in the example shown, four ramp sequences 1a, 1b, 1c, 1d or 2a, 2b, 2c, 2d or 3a, 3b, 3c, 3d. The respective ramp sequences are represented by different line types.

[0114] In Fig. 4, only the first and last ramps 40 of each ramp sequence are shown. The successive ramps 40, which belong to a same transmission signal 30-1, 30-2, 30-3, together form a section 42 of the respective transmission signal 30-1, 30-2, or 30-3. The transmission signal 30-1, 30-2, or 30-3, frequency-modulated according to the ramp 40, is phase-modulated with a harmonic code for each transmission antenna element 14 of the respective radar sensor and transmitted by the transmission antenna element 14.

[0115] Fig. 4 shows an example of a radar modulation scheme #2 for three radar sensors 10, each with several, e.g., four, transmit antenna elements 14, which are simultaneously active by means of a Doppler Division Multiplex (DDM) with harmonic codes. In this example, instead of the temporal interleaving of the transmission operation of the sensors of Fig. 2 and Fig. 3, a sequential transmission operation of the individual sensors takes place, with all sensors receiving continuously. The ramp-shaped, frequency-modulated transmission signal 30-1, 30-2, or 30-3 of a respective one of the radar sensors 10 has several temporally interleaved ramp sequences. For example, the transmission signal 30-1 of the first radar sensor 10 has the four temporally interconnected ramp sequences 1a, 1b, 1c, and 1d. Within the respective ramp sequence, the ramps follow each other at a predetermined time interval T r2r with a time offset from one another. A measurement cycle of the radar sensors 10, of which only the first ramp and the last ramp of each ramp sequence are shown in Fig. 4, comprises temporally successive sections 42, each of which is assigned to one of the radar sensors 10 and thus assigned to the corresponding transmission signal 30-1, 30-2 or 30-3 of the respective radar sensor 10. Each section 42 comprises all ramps 40 of the ramp sequences of the transmission signal exclusively of the radar sensor 10 assigned to this section 42. Thus, the first section 42, identified in Fig. 4, comprises exclusively ramps 40 exclusively of the first radar sensor 10, wherein the ramps belong to its transmission signal 30-1.

[0116] For the respective transmission signal 30-1, 30-2, 30-3, the term ramp group again refers to a group of those ramps of the ramp sequences of the transmission signal that have the same ramp index j. For example, the transmission signal 30-1 of the first radar sensor 10 comprises ramp groups 44, each of which comprises a ramp 40 from the ramp sequences 1a, 1b, 1c, and 1d. Thus, in radar modulation scheme #2, the transmission signal 30-1, 30-2, 30-3 of the respective radar sensor 10 also comprises a sequence 46 of ramp groups 44 arranged sequentially, each ramp group 44 comprising a ramp 40 from each of the ramp sequences 1a, 1b, 1c, 1d of the radar sensor 10. Pauses occur between some of the ramps of a respective ramp group 44.

[0117] At the same time, the measurement cycle of the multiple radar sensors 10 is divided into temporally successive subcycles 70-1, 70-2, 70-3. Each subcycle 70-1, 70-2, 70-3 is assigned to one of the radar sensors 10 and includes the ramp groups 44 exclusively of the assigned radar sensor 10. Subcycle 70-1 of the first radar sensor 10 includes the transmission signal of the first radar sensor 10. Accordingly, subcycles 70-2 and 70-3 each include the transmission signal of the corresponding second radar sensor 10 and third radar sensor 10, respectively.

[0118] The further evaluation is carried out according to the method of Fig. 6 explained below. The processing is initially carried out individually for each sub-cycle 70, with differences from the method of Fig. 3 being explained in particular below.

[0119] In a step 80, the radar modulation scheme #2 is transmitted and received. Step 80 corresponds to step 60 of Fig. 3, with the difference that the radar modulation scheme #2 is used, for example, according to Fig. 4.

[0120] For each subcycle 70-1, 70-2, 70-3, processing then takes place in a subsequent section 800 of the method as follows.

[0121] In a step 82, a 2D FFT of the individual ramp sequences is performed in each of the sensors. In this respect, step 82 corresponds to step 62 in Fig. 3. In the example, the different sensors 10 or transmission signals 30-1, 30-2, 30-3 have the same time intervals Tr2r, with which the ramps follow one another within the respective ramp sequence, and the same ramp durations T_fast. In variants of the example, the time intervals Tr2r and / or the ramp durations T_fast of the different sensors 10 or transmission signals 30-1, 30-2, 30-3 are selected differently. This results in different velocity ambiguities in the spectra.

[0122] In a step 84, the speed ambiguities are resolved before or after target detection. Step 84 corresponds to step 64 of Fig. 3 and can, in particular, include step 84-1 of resolving the speed ambiguities corresponding to step 64-1, as well as step 84-2 of target detection, corresponding to step 64-2.

[0123] With sequential transmission of the individual sensors 10, the accuracy of the speed measurement of the individual sensor is generally not sufficient to enable coherent processing of the transmission signals across all sensors. Instead, a separate evaluation of the so-called "subcycles" is performed, whereby the transmission signals of the currently active sensor and the reception signals (RX) of all sensors can be evaluated.

[0124] In a variant 1 of section 800, in a step 86, coherent MIMO beamforming is performed across all transmit and receive channels of the current subcycle. Thus, digital beamforming is performed via the transmit channels assigned to the transmit antenna elements 14 and / or via the receive channels assigned to the receive antenna elements 16 from the respective radar sensor 10 for a radar target 18 and for the respective subcycle 70. In particular, in step 86, an angle estimate for the radar target 18 is determined, wherein determining the angle estimate for the radar target comprises performing digital beamforming for the radar target. In a variant 2 of section 800, the execution of the digital beamforming for the sub-cycle is divided into a sub-step 86-1 of a coherent MIMO beamforming per sensor and a sub-step 86-2 of a coherent combination of the partial beamforming of the individual sensors.After step 86-1, the monostatic results can be output and / or further processed in a step 87. The combining of the partial beamformings in sub-step 86-2 can again be performed taking into account the time offsets between the signals from sensors 10, ie, it can include compensation for the time offsets.

[0125] A second section of the method may follow the first section 800.

[0126] According to a variant of the method, the results are output for each sub-cycle in a step 90. The sequential transmission operation of the individual sensors 10 results in an update rate of the detected targets that is increased by the number of sensors, since processing occurs in each "sub-cycle."

[0127] At the same time, due to the different activations of the transmitters (TX), i.e. transmitter antenna elements 14, in the sub-cycles 70, different virtual antenna arrangements with different ambiguity properties in the angle estimation are obtained. This is illustrated schematically in Fig. 5. In the upper part, Fig. 5 shows an arrangement of the actively transmitting transmit antenna element arrangement TX of a first radar sensor 10 and the three receiving receive antenna element arrays RX of the three radar sensors 10, corresponding to the sub-cycle 70-1. This results in a corresponding virtual antenna arrangement. In the lower part, Fig. 5 shows an arrangement of the actively transmitting transmit antenna element arrangement TX of the second radar sensor 10 and the three receiving receive antenna element arrays RX of the three radar sensors 10, corresponding to the sub-cycle 70-2. This results in a corresponding, different virtual antenna arrangement.

[0128] The sensors 10 can also have different array arrangements TX and / or RX, e.g. with different distances between the antenna elements.

[0129] According to an alternative variant or in addition to the processing with step 90, the method according to a variant may comprise steps 92 and 94, which are explained below.

[0130] In a step 92, a non-coherent averaging of the partial cycles 70 takes place. The non-coherent averaging of the partial cycles can take place at the spectral level before the detection of the targets, or it can take place by averaging the angular spectra after target detection. In the first case, the range Doppler spectra are averaged in terms of power in order to subsequently achieve improved detection performance. However, the complex amplitudes of the non-averaged spectra are subsequently required in order to be able to determine individual angle estimates / angular spectra for each of the partial cycles. In the second case, detection is first performed in the individual spectrum and then an angular spectrum is calculated for each partial cycle 70 (e.g., using a DML estimator). The angular spectra of the partial cycles can then be non-coherently averaged.

[0131] The method thus comprises, in step 92, combining the digital beamforms of the radar sensors by non-coherently averaging the digital beamforms of the radar sensors and the respective sub-cycles, or by averaging the angular spectra of the radar target of the radar sensors and the respective sub-cycles obtained by the digital beamforms. The overall results are output in a step 94. Thus, the non-coherent averaging can already be performed at the spectral level before the targets are detected, or by averaging the angular spectra after the targets are detected.

[0132] In the examples of Fig. 2 and Fig. 4, the ramp sequences of the individual transmission signals 30-1, 30-2, 30-3 each have identical parameters such as ramp duration T_fast, ramp pitch F_fast, ramp center frequency fo, and thus the identical frequency response. Fig. 2 shows the following parameters as examples: ramp duration T_fast, ramp pitch F_fast, ramp center frequency fo. The ramp center frequency here corresponds to the mean transmission frequency fo. In other examples, the ramp center frequency of the ramps can also increase or decrease during the transmission of a ramp sequence. For example, consecutive ramps within a ramp sequence can have the same difference in ramp center frequencies.For a time T_slow of a ramp sequence (corresponding to a duration of the measurement cycle) and a frequency swing F_slow of the ramp center frequencies during the entire ramp sequence, the ramp slope s_slow of the ramp center frequencies can be described by s_slow = F_slow / T_slow.

[0133] In particular, ramps 40 with the same ramp index j in the nested ramp sequences can each have the same ramp gradient F_fast / T_fast.

Claims

Patent claims 1. A method for determining a relative speed (v) of a radar target (18), comprising the steps of: transmitting phase-modulated transmission signals (30-1; 30-2; 30-3) by means of a plurality of transmit antenna elements (14) of each of a plurality of radar sensors (10), wherein a ramp-shaped frequency-modulated transmission signal (30-1; 30-2; 30-3) of a respective one of the radar sensors is generated, which has a plurality of temporally interleaved sequences (1a; 1b; 1c; 1d; ...) of ramps (40), wherein the ramps (40) within the respective sequence (1a; 1b; 1c; 1d; ...) follow one another with a predetermined time interval (Tr2r), wherein a phase of the generated ramp-shaped frequency-modulated transmission signal (30-1; 30-2; 30-3) is phase-modulated with a code for each transmission antenna element (14) of a radar sensor (10), and wherein a measuring cycle of the plurality of radar sensors (10) comprises temporally successive sections (42), which are each assigned to one of the radar sensors (10) and comprise one ramp (40) or several ramps (40) of the transmission signal (30-1; 30-2; 30-3) exclusively of the radar sensor (10) assigned to the section, receiving response signals to the transmitted phase-modulated transmission signals (30-1; 30-2; 30-3);. Calculating a two-dimensional spectrum for each sequence (1a; 1b; 1c; 1d; ... ) of the transmitted signal (30-1; 30-2; 30-3) of a respective radar sensor (10) by a two-dimensional Fourier transformation of baseband signals of the received response signals, wherein the transformation is carried out in a first dimension for each ramp and in a second dimension via a ramp index of the sequence of ramps; Determining values ​​for relative velocities (v) of a radar target (18) that are periodic with a predetermined velocity period based on a peak in one of the calculated two-dimensional spectra; Determining an estimated value for the relative speed (v) of the radar target (18) based on matches in phase relationship between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for several of the determined values ​​of relative speeds (v).

2. The method according to claim 1, wherein the transmission signal (30-1; 30-2; 30-3) of the respective radar sensor (10) comprises a sequence (46) of ramp groups (44) arranged one after the other in time, wherein each ramp group (44) comprises a ramp (40) from each of the sequences (1a; 1b; 1c; 1d; ... ) of the ramps of the transmission signal of the radar sensor (10), wherein a measuring cycle of the plurality of radar sensors (10) comprises blocks (48) which are successive in time, wherein each block (48) comprises a ramp group (44) from each of the radar sensors (10).

3. The method according to claim 2, wherein in each block (48), the ramps (40) of a ramp group (44) of the block (48) are temporally interleaved with the ramps (40) of each other of the ramp groups (44) of the block (48).

4. The method according to one of the preceding claims, wherein the transmission signals (30-1; 30-2; 30-3) of the plurality of radar sensors (10) have different time offsets (Tab; Tac; Tad) between their sequences (1a; 1b; 1c; 1d; ...) of the ramps (40).

5. The method according to one of the preceding claims, comprising: Carrying out a digital beamforming for a radar target (18) via transmission channels assigned to the transmission antenna elements (14) and / or reception channels assigned to reception antenna elements (16) of at least two of the plurality of radar sensors (10), wherein, when carrying out the digital beamforming, the relative speed (v) of the radar target (18) dependent phase differences between the radar sensors (10) are taken into account, wherein a respective phase difference between two of the radar sensors (10) for a radar target (18) is determined based on at least one estimated value for the relative speed (v) of the radar target (18) and based on a time offset (T12; ... ) (within a block) between the ramp groups (44) of the two radar sensors (10).

6. The method of claim 5, comprising: determining an angle estimate for the radar target (18), wherein determining the angle estimate for the radar target (18) comprises performing digital beamforming for the radar target (18).

7. The method according to claim 5 or 6, wherein carrying out the digital beamforming for a radar target (18) comprises a first sub-step (68-1) in which a digital beamforming for the radar target (18) is carried out via transmission channels assigned to the transmission antenna elements (14) and / or reception channels assigned to the reception antenna elements (16) of a respective one of the at least two of the plurality of radar sensors (10), and a second sub-step (68-2) in which the digital beamformings of the respective at least two radar sensors (10) are combined, taking into account the phase differences between the radar sensors (10) that are dependent on the relative speed (v) of the radar target (18).

8. The method according to claim 1, wherein the transmission signal (30-1; 30-2; 30-3) of the respective radar sensor (10) comprises a sequence (46) of ramp groups (44) arranged one after the other in time, wherein each ramp group (44) comprises a ramp (40) from each of the sequences (1a; 1b; 1c; 1d; ... ) of the ramps of the transmission signal of the radar sensor (10), wherein a measuring cycle of the plurality of radar sensors (10) comprises sub-cycles (70-1; 70-2; 70-3) following one after the other in time, wherein each sub-cycle (70-1; 70-2; 70-3) is assigned to one of the radar sensors (10) and comprises the ramp groups (44) exclusively of the assigned radar sensor (10).

9. The method according to claim 8, wherein the transmission signals (30-1; 30-2; 30-3) of the plurality of radar sensors (10) have different time intervals (Tr2r) with which the ramps (40) follow one another with a time offset within the respective sequence (1 a; 1 b; 1 c; 1 d; ... ) of the ramps (40), and / or different ramp durations (T_fast).

10. The method according to claim 8 or 9, comprising: Carrying out digital beamforming via transmission channels assigned to the transmission antenna elements (14) and / or reception channels assigned to reception antenna elements (16) from a respective one of the plurality of radar sensors (10) for a radar target (18) and for a respective sub-cycle (70-1; 70-2; 70-3).

11. The method of claim 10, comprising: determining an angle estimate for the radar target (18), wherein determining the angle estimate for the radar target (18) comprises performing digital beamforming for the radar target (18).

12. The method according to claim 10 or 11, wherein the digital beamforming is carried out via transmit channels assigned to the transmit antenna elements (14) and / or receive channels assigned to the receive antenna elements (16) by a respective one of at least two of the plurality of radar sensors (10) for a radar target (18) and for a respective respective sub-cycle (70-1; 70-2; 70-3), the method further comprising: Combining (92) the digital beamforming of the at least two radar sensors (10) by non-coherent averaging of the digital beamforming of the at least two radar sensors and respective sub-cycles (70-1; 70-2; 70-3) or by averaging angle spectra of the radar target (18) of the at least two radar sensors (10) and the respective partial cycles (70-1; 70-2; 70-3) obtained by the digital beamforming.

13. The method according to one of the preceding claims, wherein the receiving of response signals to the emitted phase-modulated transmission signals (30-1; 30-2; 30-3) comprises: wherein, in response to the phase-modulated transmission signal (30-1; 30-2; 30-3) transmitted by the transmitting antenna elements (14) of a respective radar sensor (10), at least response signals are received by means of at least one receiving antenna element (16) of the same radar sensor (10), and response signals are received by means of at least one receiving antenna element (16) of another of the plurality of radar sensors (10).

14. Radar sensor network (100) for determining a relative speed of a radar target, comprising a plurality of radar sensors (10) and at least one evaluation unit (20, 12), wherein each of the plurality of radar sensors (10) comprises: a plurality of transmitting antenna elements (14); a signal generating device (17) designed to generate a ramp-shaped frequency-modulated transmitting signal (30-1; 30-2; 30-3) and to provide it to the transmitting antenna elements, wherein the transmitting signal (30-1; 30-2; 30-3) comprises a plurality of temporally interleaved sequences (1a; 1 b; 1 c; 1 d; ... ) of ramps (40), wherein the ramps (40) within the respective sequence (1 a; 1 b; 1 c; 1 d; ... ) follow one another with a predetermined time interval (Tr2r), wherein a phase of the generated ramp-shaped frequency-modulated transmission signal (30-1 ; 30-2; 30-3) is phase-modulated with a code for each transmission antenna element (14) of a radar sensor (10); and at least one receiving antenna element (16) designed to receive a response signal to the transmitted phase-modulated transmission signals (30-1; 30-2; 30-3), wherein the evaluation unit (20, 12) is designed to determine values ​​for relative speeds (v) of a radar target (18) that are periodic with a predetermined speed period, based on a peak in one of two-dimensional spectra calculated for each sequence of the transmission signal (30-1; 30-2; 30-3) of a respective radar sensor (10); and to determine an estimated value for the relative speed (v) of the radar target (18) based on matches in the phase relationship between values ​​of the two-dimensional spectra at the same positions with expected phase relationships for several of the determined values ​​of relative speeds (v).

15. Radar sensor network according to claim 14, wherein each of the plurality of radar sensors (10) comprises a plurality of receiving antenna elements (16).

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