MIMO Radar System

Abstract

To provide a MIMO radar system including a simply constructed antenna array, in which relative velocities and location angles of radar objects can be determined in a short measuring time with high accuracy and a large uniqueness range.SOLUTION: A MIMO radar system includes: a transmitter array having multiple transmitting antennas situated at a distance from one another in an angle resolution direction; a receiver array having multiple receiving antennas situated at a distance from one another in the angle resolution direction; and a control and evaluation unit. The antenna distances in one of the transmitter and receiver arrays exceeds a Nyquist limit for unambiguous angle measurement, but the antenna distances in the combination of the transmitter and receiver arrays is below the Nyquist limit.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a MIMO radar system, in particular to a MIMO radar system for vehicles. [Background technology]

[0002] Radar systems are being used in vehicles to detect the traffic environment over a wider range and provide information on the distance, relative speed and direction angle of the vehicle or of located objects such as obstacles to one or more safety or comfort functions that relieve the driver from the burden of steering the vehicle or completely or partially replace the human driver. Here, multiple input-multiple output (MIMO) systems, in which multiple transmit and receive antennas are used, are increasingly being used.

[0003] WO 2018 / 076005 mentions different types of MIMO radar systems. The transmitters and / or receivers can be located at different positions. Virtual channels can be created using mutually orthogonal codes. Time Division Multiple Access (TDMA) and / or Frequency Division Multiple Access (FDMA) can be used.

[0004] From DE 10 2014 212 284 A1 a MIMO radar measurement method is known in which a transmission signal is frequency modulated in a ramp manner with a modulation model in which ramp trains are assigned to different transmission switching states, which differ in terms of the selection of antenna elements used for transmission, and which overlap each other in time. A transmission switching state is in turn assigned to a number of trains which overlap each other in time. Based on the peak positions of the two-dimensional spectrum of the signal obtained for the train, a value of the relative velocity of a radar target, which is periodic with a predefined velocity period, is determined. The phase relationship of the spectral values ​​of the spectrum of the train of transmission switching states is compared with the phase relationship expected for the respective periodic value of the relative velocity, and based on the result of the comparison an estimate of the relative velocity is selected.

[0005] US Patent Application Publication No. 2017 / 0160380 describes a MIMO radar system in which multiple transmit antennas transmit simultaneously, using Pseudo-Random Phase Modulation (PRPM) to randomly vary the phase of the signals routed to each transmit antenna to obtain orthogonality between simultaneously emitted and received signals.

[0006] The digital modulation method using multiple carrier frequencies is known as orthogonal frequency division multiplexing (OFDM). The use of OFDM in radar systems is being increasingly considered. In OFDM, the frequency band is divided into multiple orthogonal sub-bands or sub-carriers (FDM, frequency division multiplexing), and OFDM symbols are transmitted sequentially. The transmitted signal for an OFDM symbol is composed of mutually orthogonal sub-carrier signals modulated according to the symbol modulation pattern, and transmitted simultaneously within the OFDM symbol period.

[0007] The applicant has already proposed a MIMO radar system in which the repetition rate of the multiplexed array is below the Nyquist limit for an unambiguous Doppler measurement, so that the measurement of the relative velocity is ambiguous, albeit with high resolution. To resolve this ambiguity, the proposed method exploits the fact that demultiplexing methods, in which signals that can be uniquely assigned to the individual transmitting antennas of the transmitting array are reconstructed from the received signals, only give high quality results if the phase offset due to the Doppler effect is appropriately corrected. However, this requires knowledge of the relative velocity of the object in question. The ambiguity is resolved by testing various ambiguity hypotheses and selecting the one that provides the highest quality standard signal during demultiplexing. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2018 / 076005 [Patent Document 2] DE 102014212284 [Patent Document 3] US Patent Application Publication No. 2017 / 0160380 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a MIMO radar system having an easily configured antenna array that can determine the relative velocity and positioning angle of a radar target with high accuracy and a wide unambiguity range in a short measurement time. [Means for solving the problem]

[0010] This problem is solved according to the present invention by: a transmitting array having a plurality of transmitting antennas spaced apart from one another in an angularly resolved direction; a receiving array having a plurality of receiving antennas spaced apart from one another in an angularly resolved direction; a control evaluation device; This problem is solved by a MIMO radar system equipped with the antenna spacing of either the transmit array or the receive array is above the Nyquist limit for unambiguous angle measurement (spatial frequencies below the Nyquist limit), but the antenna spacing of the combined transmit and receive array is below this Nyquist limit (spatial frequencies above the Nyquist limit); The control evaluation device is - transmitting a transmission signal via the transmit array in accordance with a periodic multiplexing pattern in each of a number of repeated measurement cycles, the time sequence of the multiplexing pattern being selected to allow an unambiguous Doppler measurement; - transforming the signals received in a measurement cycle into a detection space of at least two dimensions, one dimension representing an estimate of the Doppler shift and the other dimension representing an estimate of the positioning angle of the object to be located, the estimate being ambiguous in at least one of these dimensions; - checking ambiguity hypotheses against the estimate, each test comprising a Doppler correction of the received signals based on a respective ambiguity hypothesis, providing a mapping of transmit antennas to combinations of Doppler corrected received signals, and determining a quality measure of the ambiguity hypothesis; - selecting the ambiguity hypothesis having the highest quality criterion for the unambiguous Doppler measurement; - performing another angle estimation based on the transmit array and the receive array, the angle estimation based on the transmit array using a mapping of the transmit antennas to a combination of Doppler corrected receive signals based on a selected ambiguity hypothesis; - fusing the results of the separate angle estimates into a unique angle measurement; The system is configured to:

[0011] The present invention extends the previously proposed method in that not only the Doppler measurements are undersampled, i.e. sampled below the Nyquist limit, but also the angle measurements. For this, either the aperture of the transmit array or the aperture of the receive array is selected to provide high angular separation, but with ambiguous results. In contrast, the aperture of the respective other array is selected to fill the gap between the individual antennas to the extent that unambiguous results are obtained, but with lower angular resolution. The two angle measurements are then fused to provide unambiguous measurements with high resolution, corresponding to a synthetic aperture generated with a relatively small total number of transmit and receive antennas. As a result, a common data compression is achieved in the dimensions "relative velocity" and "angle".

[0012] However, in this case, angle estimation based on a transmitting array requires the reconstruction of signals that can be uniquely assigned to the individual transmitting antennas, and therefore the correction of the phase advance due to the Doppler effect, thus eliminating both the ambiguity of the Doppler measurement on the one hand and the ambiguity of the angle measurement on the other hand, which allows an efficient and resource-saving signal evaluation.

[0013] Also disclosed herein is a method for estimating range, Doppler and angle in a MIMO radar system, the method comprising: - estimating a distance; - a step of ambiguous Doppler estimation in a first stage, - estimating an angle in a first stage using a receive array; - performing joint Doppler and angle estimation in a second stage using a transmit array to resolve ambiguities and complement the angle information; has.

[0014] Advantageous embodiments and further variants of the invention emerge from the dependent claims.

[0015] In one embodiment, the radar system is an FMCW or chirp sequence radar, in which the frequency of the transmitted signal is modulated according to a train of steep frequency ramps, so-called chirps, whose slope is so large that the Doppler effect on the ramps is negligible, and thus a pure propagation time measurement, i.e. a distance measurement, is performed. The relative velocity measurement is then performed by evaluating the ramp-to-ramp phase shift due to the Doppler effect.

[0016] According to the MIMO principle, appropriate multiplexing and demultiplexing methods are required to separate the signals transmitted from different transmit antennas from one another, in particular code division multiplexing and time division multiplexing methods.

[0017] In code division multiplexing, the signals transmitted simultaneously from the transmit antennas are coded with orthogonal or quasi-orthogonal codes set in a code matrix. The transmitted signal is then divided into a sequence of periodically repeating code blocks, each setting a different code distribution for a different transmit antenna. The repetition rate at which the code blocks are transmitted corresponds to the sampling rate for the Doppler measurement. If the inverse of this repetition rate, i.e. the period duration, exceeds a certain value (the Nyquist limit), the results of the Doppler measurement become ambiguous.

[0018] For example, if a single code block contains nCI code instances, each presetting a different code distribution for a transmit antenna, the phase of the signal received with a single code block forms a vector with nCI components. For a square code matrix, nCI is equal to the number of simultaneous transmit antennas of the transmit array. The received signal can then be decoded by multiplying the vector by the inverse of the code matrix. However, this is prerequisite that the orthogonality of the signals decoded upon reflection from the radar target is maintained or at least can be restored. If the relative velocity of the radar target is different from zero, the Doppler effect will result in some disruption of the orthogonality, resulting in the decoded signal for a given transmit antenna also containing signal components originating from other transmit antennas.

[0019] In order to obtain a measurement result with a high quality criterion, the received signal must therefore be corrected according to the Doppler effect, but this is only successful if, among the ambiguous values ​​obtained in the measurement of the Doppler shift, the value that exactly corresponds to the true relative velocity of the object is selected. In this way, the ambiguity can be resolved using a quality criterion. In the case of orthogonal codes, the quality of the angle estimation can for example be a criterion for the quality of the decoding.

[0020] In other embodiments, a time division multiplexing scheme can be used instead of a code multiplexing scheme, where only a single transmit antenna of the transmit array is active at any one time, and the different transmit antennas are switched according to a periodic pattern, where the order in which the individual antennas are activated is generally not the same as the order in which the antennas are spatially arranged in the transmit array. Thus, the relative motion of the radar target leads to characteristic phase shifts between the signals received from the different transmit antennas due to the time offsets at which the signals were transmitted, which are distinct from the phase shifts caused by the angle-dependent propagation time differences when the signals are emitted at a certain angle relative to the normal of the transmit array. Again, the phase error due to the relative motion can be corrected by selecting the correct hypothesis or the correct pair of hypotheses among various ambiguity hypotheses about the Doppler shift and / or the positioning angle of the object, and performing corrections based on these hypotheses.

[0021] Embodiments of the invention which operate with a combination of code division multiplexing and time division multiplexing are also contemplated.

[0022] Similarly, embodiments in which the code matrix is ​​not square are also contemplated. If the number of code instances is smaller than the number of transmit antennas, then under-determined equations are obtained upon decoding, which can be solved with plausible additional assumptions (e.g., regarding the number of simultaneously located radar targets). Conversely, if there are more code instances than transmit antennas, then over-determined equations are obtained. In that case, greater robustness of the ambiguity resolution is obtained, for example, against signal noise or other interference effects.

[0023] In one embodiment, the receive array has a large aperture that is not completely occluded, so that angle estimates based on the receive array are high resolution but ambiguous, while the transmit array has a small aperture that is completely occluded, thus allowing unambiguous angle measurements at lower resolution, however, in other embodiments the transmit array may have a large aperture and the receive array a smaller aperture.

[0024] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. [Brief description of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic diagram of the analog portion of a MIMO radar system with independent range and velocity determination. [Diagram 2] FIG. 2 is a diagram showing the frequency of an FMCW transmission signal and the modulation pattern of the transmission signal. [Diagram 3] FIG. 1 is a diagram of an antenna array of a radar system. [Figure 4] FIG. 2 is a diagram of the positioning angle range of the radar system. [Diagram 5] 1 is a block diagram of a digital signal evaluation device according to an embodiment of the present invention; [Figure 6] FIG. 11 is a block diagram of a digital signal evaluation device according to another embodiment of the present invention. [Figure 7] FIG. 11 is a block diagram of a digital signal evaluation device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] 1 to 4, an embodiment of a high-speed chirp MIMO radar system will be described as an example of an FMCW-MIMO radar system in which transmission signal encoding is performed by phase modulation. Fig. 1 shows a schematic simplified configuration of an analog section of the radar system.

[0027] The frequency modulation device 10 controls a high frequency oscillator 12 which generates a train of identical signals in the form of frequency ramps for a number of transmitting antennas 14. In each of a number of transmitting channels, a respective phase modulator 16 preceding an amplifier 18 modulates the phase of the signal according to a respective code 20 generated by a code generator 22. The phase modulated signal is emitted via one of the transmitting antennas 14. The transmitted signal reflected by an object 24 is received by a number of receiving antennas 26, where it is mixed by a mixer 28 with the non-phase modulated signal component from the high frequency oscillator 12 and brought to a low frequency range. Then, A / D conversion is performed by an A / D converter 30 in the usual way.

[0028] FIG. 2 shows a schematic diagram of the frequency transition of a transmission signal, and below that, the pattern of code blocks 32 by which the transmission signal is phase modulated is shown.

[0029] A "fast chirp" frequency modulation pattern is used having a train of relatively "fast" frequency gradients 34, so that range and velocity estimates can be made substantially independent of each other, for example using a two-dimensional Fourier transform. In particular, the Doppler shift in the gradients can be neglected.

[0030] According to Fig. 2, the codes 20 for each transmit antenna 14 are grouped together in a code block 32. The code block 32 associates with each signal for each transmit antenna a code value A, B, C, ... of the code 20. The respective code value defines the phase with which the phase modulator 16 modulates the signal. At each code instant, also called code instance I, i.e. at each position within the code 20, the code block 32 defines the code value for each of the transmit antennas. The number of codes 20 in the code block 32 corresponds to the number of simultaneously transmitting transmit antennas. In the sequence of code instances I, I=1, ..., m, the phase modulation goes through the code values ​​of the code for each transmit antenna. As shown in Fig. 2, the code block 32 is divided into a set of code instances of periodic duration T C2C The repetition rate 1 / T corresponds to C2C 2. The index C#=1...k in Fig. 2 counts the repetitions of the code blocks, while the index TX=1...n (n=m) numbers the transmit antennas. The codes 20 of the code block 32 are orthogonal to each other (preferably fully orthogonal, or alternatively quasi-orthogonal, i.e. small cross-correlation between the codes). Thus, the signals of the individual transmit antennas are coded by codes, and the transmitted signals are orthogonal to each other in order to allow signal separation in the receive channel.

[0031] 3, the transmitting antennas 14 form a transmitting array 36 and the receiving antennas 26 form a receiving array 38. In the example shown, both arrays are two-dimensional, allowing MIMO angle measurements in both azimuth and elevation.

[0032] In the receive array 38, the receive antennas 26 are equally spaced in the angular resolution direction y, e.g., azimuth. The distance between the individual receive antennas is so large that a large aperture and a correspondingly high angular resolution can be achieved with just a few antennas. However, the Nyquist uniqueness criterion is not met because the antenna-to-antenna distance is now greater than half the wavelength of the radar radiation.

[0033] In Fig. 4, the field of view of the radar sensor is shown to include angles from -θ to +θ with respect to an axis X perpendicular to the plane of the receiving array 38. The result of the angle measurement is unambiguous only if the positioning angle is within a significantly small interval from -θa to +θa. If larger positioning angles cannot be excluded, the measurement is not unambiguous since there are several angles for which the phase relationship between the signals at the receiving antennas is the same.

[0034] In the example shown in FIG. 3, the receiving antennas 26 are also arranged at equal intervals in the elevation angle direction (angle resolution direction z), and since the antenna intervals are large in this direction as well, non-unique undersampling is performed.

[0035] The transmitting antennas 14 of the transmit array 36 are unevenly spaced in azimuth, with the spacing selected to provide unambiguous angle measurements, but with a significantly smaller aperture than that of the receive array 38, resulting in poor angular resolution. Also, in elevation, the transmit array 36 is configured for unambiguous angle measurements with a small aperture.

[0036] 3 further shows a composite array 40 that is obtained when combining each receive antenna 26 with each transmit antenna 14 such that the propagation time differences of the signal from the transmit antenna to the object and from the object to the receive antenna are additive. Ultimately, it is the aperture of this virtual array 40 that determines the resolution of the radar sensor. However, the signal components emanating from the various transmit antennas 14 need to be separated from each other in the received signal so that the receive array can be disambiguated.

[0037] Also, in the example shown in Figure 3, for each y position of the receive antenna, all z positions are also occupied by the receive antenna, so the two angular resolved directions y and z in the receive array 38 are separated from each other. In contrast, the transmit array 36 is an example of a non-separated array where for some y positions (the two right positions in Figure 3), all z positions are not occupied. In general, a separated array makes data evaluation easier, and a non-separated array requires fewer antenna elements. The decision between separated and non-separated arrays can be made differently for the transmit and receive sides depending on their respective requirements.

[0038] Also, an equidistant arrangement of the antenna elements (in azimuth and / or elevation) facilitates the evaluation of the data, since it allows, for example, the use of a Fast Fourier Transform (FFT), whereas a non-equidistant arrangement of the antennas has the advantage that a unique angular range (FIG. 4) can be optimized for a given aperture, as here is the case for the transmitting antenna 14.

[0039] Overall, all combinations of equidistant and non-equidistant or separated and non-separated geometries are contemplated in the radar systems described herein. Similarly, embodiments are possible in which the transmit array is configured for ambiguous high resolution angle measurements while the receive array is configured for unambiguous angle measurements with lower angular resolution.

[0040] Next, an example of an evaluation device for evaluating a received signal obtained using the antenna array shown in FIG. 3 and the multiplexing pattern shown in FIG. 2 will be described with reference to FIG.

[0041] The digital data provided by the A / D converter 30 are sampled over each complete measurement cycle. The total number of (complex) signal values ​​recorded within a measurement cycle is given by the product of the number of receive antennas 26 of the receive array 38, nRX, the number of code instances I, nCI, the number of repetitions of the code block 32 within a measurement cycle, ns, and the number of scan points on a single frequency ramp 34, nf. In a processing stage 42, the data sampled over a measurement cycle are subjected to a four-dimensional Fourier transform (4D-FFT). This results in a four-dimensional spectrum with the dimensions "Azimuth 1", "Elevation 1", "Doppler 1", and "Distance". The "Azimuth 1" dimension indicates the distribution of complex amplitudes over the azimuth positioning angle range based on the data of the receive antennas 26 arranged in the same row in the azimuth direction. Correspondingly, the "Elevation 1" dimension indicates the distribution over the elevation angle range based on the data of the receive antennas 26 arranged in the same row in the elevation direction. The "Doppler 1" dimension is determined by the code block repetition rate 1 / T C2C 3 shows the Doppler spectrum obtained by a "slow" scan on each frequency gradient 34. It should be noted that the results in the above three dimensions are ambiguous due to the respective undersampling. The "distance" dimension shows the distance spectrum based on a "fast" scan on each frequency gradient 34. In this dimension the results are unambiguous. For each transmitted gradient a separate spectrum is obtained.

[0042] The 4D spectrum is integrated non-coherently (addition of the absolute values ​​of the complex amplitudes). This results in an amplitude distribution in a 4D detection space 44. To each point of this 4D space is assigned a specific value of the amplitude sum, in which each located object is represented in the form of a peak (local maximum) at a specific distance, a specific Doppler shift, a specific azimuth angle and a specific elevation angle, but each of the last three values ​​is ambiguous, so that only one of several hypotheses regarding the relative velocity and only one of several hypotheses regarding the azimuth and elevation angles can be associated with the object. Then, in this detection space 44, the 4D coordinates of the detected peaks are searched to represent each detection result. For each of these points there is (before the non-coherent integration) an nCI complex amplitude, which forms a vector with nCI components and is further evaluated to resolve the remaining ambiguities.

[0043] In addition to this, various triplets of ambiguity hypotheses are checked in a checking step 46. Each ambiguity hypothesis contains a value of the Doppler shift that is still taken into account in view of the ambiguous value "Doppler1", one possible value of the azimuth angle that is taken into account based on the ambiguous value "Azimuth1", and one of the elevation angles represented by the ambiguous value "Elevation1". The sum of all these triplets thus covers all possible combinations of Doppler shift and angle. In particular, each triplet contains an assumption value for the Doppler shift and thus for the relative velocity. This value can be used to correct the phase that occurs in the signal vector depending on the relative velocity. This restores the orthogonality of the codes within the code instance so that, if the hypothesis for the relative velocity holds, a correct decoding is possible, resulting in a decoded signal vector whose components respectively indicate the phase of the signal emitted by one of the transmitting antennas 14. Then, based on the components of this vector belonging to the transmitting antennas 14 arranged in the azimuth direction, an unambiguous (but low-resolution) value for the azimuth angle can be determined, and correspondingly, an unambiguous value for the elevation angle can be determined based on the components belonging to the transmitting antennas arranged in the elevation direction.

[0044] If the assumptions about the relative velocity are wrong, the decoding will not be completely successful, and the quality of the azimuth and elevation angle estimates will also be degraded.

[0045] Based on these criteria, ambiguity resolution can be performed in a second detection stage 48. For this purpose, the phase-corrected and decoded signal vectors obtained in the testing of the ambiguity hypotheses are summed, for example, into a coherently added three-dimensional spectrum. This spectrum has the dimensions "Doppler2", "Azimuth2", and "Elevation2", and the sharpest (and highest) peaks in this spectrum represent the true unambiguous values ​​for the relative velocity, azimuth and elevation of the objects. Similarly, multiple targets can be resolved in this three-dimensional space.

[0046] The results obtained in the first detection stage 14, "Doppler1", "Azimuth1" and "Elevation1", are high-resolution but ambiguous, whereas the results obtained in the second detection stage 48 are low-resolution but unambiguous. These results can be combined by selecting, from among the high-resolution values ​​of "Doppler1", the value that best matches "Doppler2", as the final and unambiguous value for the relative velocity. The azimuth and elevation values ​​are fused in a similar way.

[0047] Since each localization object may have a different relative velocity, for each detection result obtained in the detection space 44, a separate inspection step 46 and a second detection step 48 are performed.

[0048] The radar system structure shown in Figure 1 also allows for an alternative mode of operation in which the transmit signals are transmitted in a time division multiplexed manner rather than a code division multiplexed manner. In that case, the code generator 22 drives the individual amplifiers 18 so that only a single transmit antenna is active at any one time, and switches between the transmit antennas in a specific sequence. These switches then occur over a period of duration T in Figure 2. C2C14. The transmitted signals are repeated periodically, always in the same order, with period durations corresponding to 14. No coding of the transmitted signals is necessary, since they are already separated from one another by being transmitted offset in time. Nevertheless, a phase correction of the received signals is also necessary in time division multiplexing in order to compensate for the time offset between the signals transmitted alternately from the transmitting antennas 14. Similar to the phase correction for orthogonality recovery in code division multiplexing, the phase correction in this case is also based on an ambiguity hypothesis for the relative velocities.

[0049] Likewise, a combined code division multiplexing and time division multiplexing operating mode is also conceivable, in which the transmit antennas 14 are divided into groups each transmitting simultaneously and whose signals are coded with a corresponding smaller code matrix.

[0050] There are also various options for the type of signal evaluation shown in FIG.

[0051] FIG. 6 shows an example in which the digital data sampled over a measurement cycle first undergoes only a two-dimensional Fourier transformation in the dimensions “Doppler1” and “Distance” in the processing stage 42. Thereby, for each detected object, the first detection stage provides only a value for the distance and different hypotheses for the value of “Doppler1”. The complex amplitudes for the detected objects are provided to a first angle estimation stage 50, where an angle estimation is performed based on the phase of the values ​​obtained for the different receiving antennas 26. In the example shown, for the sake of clarity, it is assumed that the radar sensor has an angle resolution only in azimuth. It can of course be extended to an angle estimation of the elevation angle.

[0052] In a check stage 46, the ambiguity hypotheses are checked by phase correction for each assumed relative velocity and decoding of the phase-corrected signal vector. In this way, an unambiguous value for the relative velocity "Doppler 2" is obtained, as well as a set of signals TX associated with simultaneously active transmit antennas 14. These signals are then used in a second angle estimation stage 52 to perform an angle estimation based on the transmit array 36, which results in an unambiguous value for the azimuth angle "Azimuth 2". In this way, in a second detection stage 48, a three-dimensional spectrum in the dimensions "Doppler 2", "Azimuth 1" and "Azimuth 2" is obtained for each detection result of the first detection stage. The values ​​"Doppler 1" and "Doppler 2" as well as the values ​​"Azimuth 1" and "Azimuth 2" are then fused in the manner already described.

[0053] Another variant of the evaluation method is shown in Fig. 7. In this method, in the processing stage 42, a Fourier transformation is performed in three dimensions in the dimensions "Doppler 1", "Distance", "Azimuth 1" so that in the detection space 44, an unambiguous distance value and ambiguous values ​​"Doppler 1", "Distance", "Azimuth 1" are obtained. Then, in the examination stage 46, a phase correction and a decoding are performed for each detection result and each Doppler hypothesis. Then, in the decoded signals, in the angle estimation stage 52, an angle estimation is performed on the basis of the transmission array 36 so that an unambiguous value "Azimuth 2" is obtained for the azimuth angle. Then, in the second detection stage 48, a peak in the two-dimensional spectrum in the dimensions "Doppler 2" and "Azimuth 2" is searched for for each detection result, after which the detection results obtained in the detection stages 44 and 48 are merged in the manner already described.

[0054] Also, in a radar system where, unlike FIG. 3, the transmit array has a large, unobstructed aperture and the receive array has a smaller aperture, the roles of the transmit and receive arrays can be reversed and the evaluation method described above can be applied in a similar manner.

Claims

1. a transmitting array (36) having a number of transmitting antennas (14) arranged at a distance from one another in an angularly resolved direction (y, z); a receiving array (38) having a number of receiving antennas (26) arranged at a distance from one another in said angular resolution direction; a control evaluation device, A MIMO radar system comprising: the antenna spacing at either the transmit array or the receive array is above the Nyquist limit for unambiguous angle measurement, but the antenna spacing at the combined transmit array and receive array is below this Nyquist limit; The control evaluation device, - transmitting a transmission signal via said transmit array in each of a number of recurring measurement cycles according to a periodic multiplexing pattern, the time sequence of said multiplexing pattern being selected so as to allow an unambiguous Doppler measurement; - transforming the signals received during a measurement cycle into an at least two-dimensional detection space (44), one dimension of which represents an estimate of the Doppler shift ("Doppler 1") and the other dimension of which represents estimates of the positioning angles ("Azimuth 1"), ("Elevation 1") of the object to be located, said estimates being ambiguous in at least one of these dimensions; - checking ambiguity hypotheses against said estimate, each test comprising a Doppler correction of said received signals based on a respective ambiguity hypothesis, providing a mapping of said transmit antennas (14) to a combination of Doppler corrected received signals, and determining a quality measure of said ambiguity hypothesis; - selecting the ambiguity hypothesis with the highest quality criterion for the unambiguous Doppler measurement ("Doppler2"); - performing another angle estimation based on the transmit array (36) and the receive array (38), the angle estimation based on the transmit array (36) using a mapping of the transmit antennas to the combination of the Doppler corrected receive signals based on the selected ambiguity hypothesis; - fusing the results of said further angle estimates into an unambiguous angle measurement; A radar system configured for:

2. 2. The radar system of claim 1, wherein the multiplexing pattern includes code division multiplexing for at least some of the transmit antennas (14).

3. 3. The radar system of claim 1, wherein the multiplexing pattern includes time division multiplexing for at least some of the transmit antennas (14).

4. 4. A radar system according to claim 1, wherein the transmission signal (20) comprises a train of frequency ramps (34) whose slopes are configured for distance measurement according to the FMCW principle, and the Doppler measurement is based on the relative phase of signals obtained in a periodic repetition of the multiplexing pattern.

5. 5. The radar system of claim 1, wherein the transmit array is configured for unambiguous angle measurements and the receive array is configured for high-resolution ambiguous angle measurements.

6. 5. The radar system of claim 1, wherein the receive array is configured for unambiguous angle measurements and the transmit array is configured for high-resolution ambiguous angle measurements.

7. 7. A radar system according to claim 1, wherein the receiving antennas (26) and / or the transmitting antennas (14) are arranged equidistant in the angularly resolved directions (y,z).

8. 8. The radar system according to claim 1, wherein in the at least two-dimensional detection space (44), both the estimate for the Doppler shift ("Doppler 1") and the estimate for the positioning angle ("Azimuth 1") are ambiguous, and the testing of the ambiguity hypotheses comprises testing different combinations of hypotheses for the Doppler shift and hypotheses for the positioning angle.

9. 9. The radar system according to claim 1, wherein an angle estimation (50) is performed based on the receiving array (38) separately from the testing of the ambiguity hypotheses.

10. 10. The radar system of claim 9, wherein the receiving antennas (26) are equidistantly positioned in an angularly resolved direction (y,z) and the angle estimation is performed by a Fast Fourier Transform (FFT) based on the receiving array (38).

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