Coherent multistatic radar systems, particularly for use in vehicles

Abstract

The invention relates to a radar system (3) which can be used in particular in a vehicle. The radar system (3) has a radar sensor (5) for transmitting and receiving a first radar signal (25) and an evaluation device (9) for processing the radar signals (27, 33) received by the radar sensor (5). The radar system (3) is characterized in that it also has at least one active radar tag (7) which is configured to amplify and modulate the received radar signal (29) and retransmit it as a second radar signal (31), and the evaluation device (9) is configured to determine information about the object (23) based on a component (27) of the first radar signal (25) reflected at the object (23) and received by the radar sensor (5) and a component (33) of the second radar signal (31) reflected at the object (23) and received by the radar sensor (5).

Description

Technical Field

[0001] The invention relates to a radar system which can be used in particular in a vehicle. Background Art

[0002] Radar systems are used to detect distant objects. The aim is to be able to determine at least the position of an object with the help of a radar system, but also possibly information about its speed of movement and / or direction of movement. To this end, radar systems use radar sensors that transmit radar signals in the form of electromagnetic waves and detect the echoes of these radar signals, i.e. the components of the radar signal that are reflected from an object.

[0003] In particular, radar systems are used in modern vehicles to automatically detect objects such as other vehicles or obstacles, for example in order to support the vehicle driver by assistance systems or even to autonomously control the vehicle based on the information obtained in this way. Summary of the invention

[0004] Against this background, the method proposed herein introduces a radar system and a vehicle equipped with such a radar system according to the independent claims. Advantageous further developments and improvements of the method proposed herein are derived from the description and are described in the dependent claims.

[0005] Advantages of the present invention

[0006] Embodiments of the present invention can advantageously use a radar system of relatively simple design to improve the accuracy of measuring the position, speed and / or direction of movement of an object.Such a more accurate radar system can be advantageously used in a vehicle.

[0007] According to a first aspect of the invention, a radar system is described, which can be used in particular in a vehicle. The radar system has a radar sensor for transmitting and receiving a first radar signal and an evaluation device for processing the radar signal received by the radar sensor. The radar system is characterized in that it has a so-called active radar tag, which is configured to amplify and modulate the received radar signal and retransmit it as a second radar signal. The radar system is further characterized in that the evaluation device is configured to determine information about the object based on a component of the first radar signal reflected at the object and received by the radar sensor and a component of the second radar signal reflected at the object and received by the radar sensor.

[0008] A second aspect of the invention relates to a vehicle having a radar system according to an embodiment of the first aspect of the invention.

[0009] Ideas related to embodiments of the present invention may be considered to be based, inter alia, on the concepts and discoveries described below.

[0010] In order to more accurately determine the position and motion of an object, i.e. to improve the angular resolution of the radar system, it is recommended to establish a network of spatially distributed radar sensors. Each radar sensor used in this network should have a mature radar sensor, including a signal generation unit, a transmitter and a receiver. Each radar sensor is controlled by an RF analog signal, which is shared between the participating radar sensors through a physical connection.

[0011] However, interconnecting spatially separated radar sensors to increase the angular resolution is currently only done in the field of radio astronomy. Interferometry is used for this purpose. The coherent evaluation of spatially separated radar signals can only be performed using highly specific techniques, mostly under laboratory conditions.

[0012] There are other methods for the collaborative evaluation of multiple spatially separated radar sensors. However, they do not produce any coherence.

[0013] For the coherent evaluation of multiple radar signals, it is usually necessary to distribute the high-frequency transmission signal to multiple separately placed radar sensors, for example via rigid hollow conductors. Alternatively, a common low-frequency reference signal can be provided to the radar sensors, which can be used to generate the coherent transmission signal. Both methods require highly specialized radar hardware and a very large amount of mechanical effort. Therefore, these methods have not yet been used outside of idealized laboratory settings.

[0014] When jointly evaluating the signals from multiple radar sensors without coherence, the signal-to-noise ratio (SN ratio) usually deteriorates significantly, making operation beyond a few meters almost impossible. The lack of signal coherence also usually makes phase evaluation, which is required for angle and velocity estimation, impossible, for example.

[0015] The method proposed in this paper proposes a radar system in which a spatially distributed network of radar sensors is virtually constructed, whose proposed network maintains signal and noise coherence. This makes it possible to measure not only the distance to an object based on the phase information that still exists, but also the speed and angle of movement of the target, i.e. the speed and angle of a partial area of ​​an object or an extended object. The spatial distribution of the radar sensors can lead to a greater angular resolution. This allows direct estimation of motion parameters, in particular of extended objects such as vehicles, based on different viewing angles of the object caused by the spatial separation of the virtual sensor positions.

[0016] In summary, with the embodiments of the radar system presented herein, a network is established which, in addition to at least one radar sensor, also has at least one so-called active radar tag. A radar tag is sometimes also referred to as an active tag or a repeater tag. A radar tag is a system that can receive radar signals, amplify them and then retransmit them in modulated form. In particular, a radar tag can receive, amplify and modulate HF radar signals reflected from an observed object and then retransmit them without destroying the noise coherence, in particular the phase noise coherence. As a result, the signal and noise coherence is preserved, so that the phase information of the observed object can also be evaluated and thus remain available for, for example, angle and velocity estimation.

[0017] Therefore, the radar tag of the proposed radar system cannot generate radar signals independently. However, it can receive, modulate, actively amplify and then retransmit the incoming radar signals.

[0018] The radar sensor of the radar system is capable of receiving radar signals across a relatively wide frequency spectrum range, so that not only a first radar signal generated and transmitted by the radar sensor itself, but also a second radar signal modulated and returned by the radar tag can be received by the radar sensor.

[0019] The evaluation device of the radar system proposed in this article is able to determine information about the observed object based on the components of the first radar signal reflected by the object and received by the radar sensor. In addition, the evaluation device is also able to determine information about the object based on the components of the second radar signal reflected by the object and also received by the radar sensor. Since the second radar signal represents a modulated image of the first radar signal, on the one hand, the phase information contained in the first radar signal is largely coherently contained in them. On the other hand, due to the implemented modulation, the second radar signal differs slightly from the first radar signal, but is sufficient for the radar sensor or the evaluation device, respectively, to distinguish between the two radar signals.

[0020] The use of radar tags makes it possible to build a distributed radar sensor network, and since it is virtual, there is no need to establish physical connections between radar sensors. The radar tag receives the first radar signal sent by the radar sensor and reflected by the object towards the radar tag, amplifies and modulates it, and then retransmits it to the radar sensor acting as the transmitting sensor node. This provides additional phase information of the observed object based on the position of the radar tag, which can be used for angle and velocity estimation, for example.

[0021] By using one or more radar tags together with at least one radar sensor, additional virtual sensor positions can thus be generated. The radar tag can thus be regarded as an additional sensor, whose received second radar signals can be coherently evaluated at the physical radar sensor or by an evaluation device connected to the radar sensor. Very large apertures of the virtual sensor positions can be created by phase coherence. This allows very high angular resolutions to be achieved. The obtained phase coherence also allows the evaluation of velocity information of the observed object. The signal-to-noise ratio of the target is also largely unaffected by the retained noise coherence.

[0022] Embodiments of the radar system described herein generally do not require specially developed radar hardware. By adding one or more active radar tags and adjusting the information collection in the evaluation device, a general radar system may be able to determine position and motion information about an observed object with high accuracy.

[0023] The radar tag itself can have a relatively simple design, consisting of only a few components. In particular, the radar tag does not necessarily require digital circuitry. The radar tag only requires power and a suitable modulated signal, which places little demands on the mechanical or electrical design of the network generated by the radar system described in this article.

[0024] Several different active radar tags have been described previously that could be used in the radar system described in this article. One possible active radar tag was described by Sarkas et al. in the 2009 IEEE Radio Frequency Integrated Circuit Symposium (DOI: 10.1109 / RFIC.2009.5135576: “W-band 65-nm CMOS and SiGe BiCMOS transmitter and receiver with lumped IQ phase shifters”). Another possible active radar tag was described by Dadash et al. in “Design of Low-Power Active Tags for Operation with 77-81-GHz FMCW Radar”, published in the IEEE Transactions on Microwave Theory and Techniques, Vol. 65, No. 12, December 2017 (DOI: 10.1109 / TMTT.2017.2769079).

[0025] According to one embodiment, the active radar tag is configured to retransmit a received radar signal as a second radar signal after shifting and amplifying it in frequency through frequency variation.

[0026] In other words, an active radar tag can simulate a received radar signal by changing the frequency of the received radar signal by a small amount, i.e. performing a small frequency shift, before retransmitting the modulated radar signal as a second radar signal. On the one hand, such frequency modulation is technically easy to implement in a radar tag. On the other hand, the second radar signal, which is frequency shifted due to the frequency change, can be easily received by the radar sensor and distinguished from the first radar signal or the echo of the first radar signal due to the frequency difference.

[0027] In particular, the frequency change can be between 0.5 kHz and 2 MHz, preferably between 2 kHz and 500 kHz, more preferably between 10 kHz and 100 kHz. On the one hand, such a frequency change has only a small effect on the phase information in the radar signal, which is usually transmitted at frequencies in the range of many gigahertz. On the other hand, a second radar signal with such a frequency change can be distinguished from the unchanged first radar signal by technically simple means.

[0028] According to one embodiment, the active radar tag is configured to amplify the received radar signal by at least 10 dB, preferably at least 20 dB, and then retransmit it as the second radar signal.

[0029] In other words, the radar tag should be able to amplify radar signals that arrive at an object as a first radar signal from a radar sensor and then continue to the radar tag as an echo, thereby losing considerable signal strength, by at least up to 10 dB or more, in order to then amplify them for retransmission as a second radar signal. The ability to retransmit the second radar signal with sufficiently large amplification ensures that the second radar signal, which again loses signal strength on the way back to the radar sensor, eventually still arrives at the radar sensor with sufficient signal strength to be reliably detected there. Typically, radar tags do not allow gains greater than 40 dB.

[0030] "At least up to 10 dB" can be understood to mean that the radar tag is generally capable of amplifying the received signal by at least 10 dB, but, depending on the specific circumstances and signal strength of the received signal, this amplification potential does not need to be fully utilized in all cases; on the contrary, lower amplification can also be achieved when relatively strong signals are received.

[0031] According to one embodiment, the radar sensor is a MIMO radar sensor having a plurality of transmit antennas and a plurality of receive antennas.

[0032] In a MIMO (Multiple Input Multiple Output) radar sensor, each transmit antenna is able to transmit any transmit signal independently of the other transmit antennas, which can be received by each receive antenna, then digitized and then subjected to general radar signal processing. From the array of N transmit antennas and the array of K receive antennas, a virtual array of KN elements with a greatly enlarged virtual aperture is calculated.

[0033] MIMO radar systems can be used to increase spatial resolution and significantly improve anti-interference capabilities. By improving the signal-to-noise ratio, the probability of target detection is also increased.

[0034] The radar sensor of the radar system described herein can preferably be a monostatic MIMO radar sensor, in which the antennas are concentrated in a very small space, similar to a point target in a traditional radar system. In contrast, for a bistatic or "distributed" MIMO radar sensor, the antennas are widely distributed in space, so that each antenna can observe the target from a different perspective. The monostatic MIMO radar system is similar in design to a sparse array of phased array antennas, in which each radiator has its own transceiver module and its own A / D converter. However, in a phased array antenna, each radiator only transmits a (possibly time-shifted) copy of the transmission signal, which has been generated in a central waveform generator. In a MIMO radar system, each radiator has its own waveform generator, and then each radiator uses a separate waveform. This separate waveform is also the basis for assigning the echo signal to its source. In order to process the radar signal more efficiently, each individual transmission signal ("adaptive waveform") can then be modified specifically to improve the signal-to-noise ratio (SNR) and signal-to-interference-plus-noise ratio (SINR) of each individual target for subsequent scanning purposes. If the individual waveforms are generated synchronously with each other in the transmitter, i.e. based on a synchronous clock from a common "mother generator", this is called a coherent MIMO system.

[0035] According to a more specific embodiment, a MIMO radar sensor may have fewer than four transmit antennas and fewer than five receive antennas.

[0036] Conventionally, attempts have been made to improve the resolution of radar systems by including more antennas in their MIMO radar sensors than the conventional three transmit antennas and four receive antennas. For example, MIMO radar sensors with six transmit antennas and eight receive antennas have been proposed to improve the resolution. However, this brings greater complexity and therefore greater manufacturing effort and higher costs.

[0037] Using the radar system proposed in this article, higher resolution can be achieved without increasing the complexity of the radar sensor used. In particular, a simple-to-design, low-cost MIMO radar sensor with up to three transmit antennas and up to four receive antennas can be used.

[0038] According to an embodiment, both the first radar signal and the second radar signal may be in a frequency range between 40 GHz and 120 GHz, preferably in a frequency range between 60 GHz and 100 GHz and more preferably in a frequency range between 70 GHz and 90 GHz.

[0039] In other words, the radar sensor may be configured to transmit and receive radar signals within the frequency range. Furthermore, the active radar tag may be configured to receive, amplify, modulate and retransmit radar signals within the frequency range. For example, radar tags designed to operate within the frequency range of 77 to 81 GHz have been proposed.

[0040] In particular, the radar signal used in the radar system may be a frequency modulated continuous wave (FMCW) radar signal.

[0041] According to an embodiment, the radar sensor and the active radar tag may be laterally spaced apart from each other by a distance between 1 cm and 10 m, preferably between 10 cm and 2 m, more preferably between 20 cm and 2 m.

[0042] In other words, the radar sensor and the one or more active radar tags can be arranged relative to each other in such a way that they are clearly spaced apart, for example ten to a thousand times further apart than the spacing between the multiple antennas within the radar sensor itself. The distance between the radar sensor and the active radar tag can be, for example, ten to a thousand times the wavelength of the radar signal used. On the one hand, such a lateral distance makes it possible to achieve a high spatial resolution with the proposed radar system. On the other hand, ambiguities in the interpretation of the received radar signals can still be analyzed without difficulty.

[0043] The radar sensor and the radar tag can be arranged on a common surface, such as the vehicle body, for example in the bumper or the front apron. However, they can also be arranged on different surfaces and / or at different heights. For example, the radar tag can be attached to the side of the vehicle body and the radar sensor to the front of the vehicle body, or vice versa.

[0044] According to one embodiment, the active radar tag may be configured to transmit a second radar signal in an opposite direction to the received radar signal.

[0045] In other words, the active radar tag can be configured so that the receiving antenna can receive the first radar signal from a certain angle range, and the transmitting antenna can send back the modulated and amplified second radar signal substantially in the direction in which the first radar signal was received. This means that the second radar signal is largely sent to the target from which the first radar signal is reflected to the active radar tag in the form of an echo.

[0046] With such an embodiment, first radar signals may be sent from a radar sensor to an object, wherein a portion of these first radar signals are reflected directly back to the radar sensor and another portion of the first radar signals are reflected toward an active radar tag. A second radar signal may then be sent from the active radar tag back to the object and reflected by the object as an echo to the radar sensor. The radar sensor may distinguish between the echo of the first radar signal and the echo of the second radar signal based on the modulation performed at the active radar tag.

[0047] In summary, the radar system is thus able to act as a virtual array network, where an object is irradiated by radar signals from different angles, and thus the radar sensor can receive echoes from different angles. Thus, objects can be detected not only along a monostatic path, but also along a bistatic path or, in the case of using multiple active radar tags, along a multistatic path. Due to the different angle-dependent radar signatures of an object, an increased resolution can be achieved.

[0048] According to an alternative embodiment, the active radar tag may be configured to transmit a second radar signal in a direction towards the radar sensor.

[0049] In other words, the active radar tag can send the second radar signal not primarily toward the object from which the echo of the first radar signal was received, but directly toward the radar sensor. This can shorten the path that the second radar signal must travel before reaching the radar sensor, thereby reducing the attenuation loss of the radar signal.

[0050] The embodiments of the radar system described herein can be used in particular in a vehicle according to an embodiment of the second aspect of the invention. On the one hand, the fact that the proposed radar system is able to achieve an increased resolution and thus can increase the probability of detecting distant objects, such as other vehicles or obstacles, can be advantageously exploited. On the other hand, the fact that the proposed radar system requires relatively few, simple and inexpensive components can be advantageously exploited.

[0051] It should be noted that some possible features and advantages of the present invention are described herein with reference to various embodiments. Those skilled in the art will recognize that these features can be appropriately combined, modified or interchanged to arrive at other embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] The embodiments of the present invention are described below in conjunction with the accompanying drawings, wherein the drawings and descriptions should not be interpreted as limiting the present invention.

[0053] Figure 1 A vehicle with a radar system according to an embodiment of the invention is shown viewed from above.

[0054] Figure 2The signal paths of the bistatic paths in a radar system according to an embodiment of the present invention are shown.

[0055] Figure 3 An example of an active radar tag for a radar system according to an embodiment of the present invention is shown.

[0056] Figure 4 The signal paths of a bistatic path in a radar system according to an alternative embodiment of the present invention are shown.

[0057] Figure 5 The signal paths of a bistatic path in a radar system according to another alternative embodiment of the present invention are shown.

[0058] The accompanying drawings are schematic and not drawn to scale. Identical reference numerals in the drawings denote identical or identically acting features. DETAILED DESCRIPTION

[0059] Figure 1 A vehicle 1 in the form of an automobile with a radar system 3 according to an embodiment of the invention is shown. The radar system 3 comprises a radar sensor 5, two active radar tags 7 and an evaluation device 9.

[0060] In the example shown, the radar sensor 5 is arranged in the center of the front of the vehicle 1, for example in the area of ​​a bumper or skirt on the body of the vehicle 1. Two active radar tags 7 are located at opposite sides of the same front of the vehicle 1. However, such an arrangement is only exemplary. In principle, the radar sensor 5 and the active radar tag 7 can be arranged at different positions on the body of the vehicle 1.

[0061] The radar sensor 5 is connected to a power supply (not shown) and is designed to send and receive a first radar signal. To this end, the radar sensor 5 has a signal generator, at least one transmitting antenna and at least one receiving antenna. Preferably, the radar sensor 5 is equipped as a MIMO radar sensor having multiple transmitting antennas and multiple receiving antennas.

[0062] In the example shown, the evaluation device 9 is arranged outside the radar sensor 5 and is connected thereto for the exchange of signals. Alternatively, the evaluation device 9 can also be integrated into the radar sensor 5 .

[0063] The active radar tag 7 is configured to receive, amplify, modulate and then retransmit the input radar signal as a second radar signal. To this end, each active radar tag 7 has at least one receiving antenna, an amplifier, a modulator and a transmitting antenna.

[0064] Figure 3An exemplary embodiment of an active radar tag 7 is shown. The radar tag 7 has a receiving antenna 11, a three-stage low noise amplifier (LNA) 13, a mixer 15, an external local oscillator 17, a variable gain amplifier (VGA) 19 and a transmitting antenna 21.

[0065] The active radar tag 7 is capable of amplifying and slightly changing the frequency of FMCW radar signals in a frequency band, for example, around 77 GHz.

[0066] Figure 2 The signal paths of radar signals transmitted and received by the radar system 3 are shown.

[0067] The radar sensor 5 sends a first radar signal 25 in the direction of the object 23. The first radar signal 25 is shown as a dashed line. At the object 23, a portion of this first radar signal 25 is reflected directly back to the radar sensor 5 and can be received there as an echo 27 of the first radar signal 25. The echo 27 of the first radar signal 25 is shown as a dashed line. Another portion 29 of the first radar signal 25 is reflected towards the active radar tag 7. This portion 29 of the first radar signal 25 is received, amplified and modulated by the active radar tag 7 before being sent back to the object 23 as a second radar signal 31. This second radar signal 31 is reflected again by the object 23 and can then be received by the radar sensor 5 as an echo 33.

[0068] Evaluation device 9 can then gather information about object 23 based on echo 27 corresponding to the component of first radar signal 25 reflected from object 23 and echo 33 corresponding to the component of second radar signal 31 reflected from object 23 .

[0069] In the example shown, for simplicity, the radar system 3 comprises only a single active radar tag 7, whereby a bistatic path is formed by the first and second radar signals 25, 31 and their echoes 27, 29, 33. However, the radar system 3 may also comprise a plurality of active radar tags 7, resulting in a multistatic path.

[0070] Figure 4 and Figure 5 The signal paths of radar signals transmitted and received back in an alternatively configured radar system 3 are shown.

[0071] In these cases as well, radar sensor 5 transmits first radar signal 25 in the direction of object 23 and at object 23 a portion of this first radar signal 25 is reflected directly back to radar sensor 5 and can be received there as echo 27 of first radar signal 25. Figure 4In the embodiment shown in , another part 29 of the first radar signal 25 is reflected towards the active radar tag 7. This part 29 of the first radar signal 25 is received, amplified and modulated by the active radar tag 7. It is then sent directly to the radar sensor 5 as a second radar signal 31 and is received by it. Alternatively, in Figure 5 In the embodiment shown in , a portion of the first radar signal 25 is sent directly toward the radar tag 7, amplified and modulated there, and then sent as a second radar signal 31 toward the object 23, from where the echo 33 is sent to the radar sensor 5 and received by it. These embodiments also result in a bistatic path, which can be significantly shorter than Figure 2 The path in the embodiment.

[0072] It should be noted that the inventors of the present invention intend to disclose further details of possible embodiments of the radar system described herein, including details of active radar tags that may be used therein, as well as details of possible signal generation and signal evaluation in the radar system, in a scientific article entitled “Coherent Multistatic MIMO Radar Networks based on Repeater Tags”, which is expected to be published in IEEE Transactions on Microwave Theory and Techniques.

[0073] Finally, it should be noted that terms such as "having", "including", etc. do not exclude other elements or steps, and terms such as "a", "an", or "the", etc. do not exclude a plurality. Reference signs in the claims should not be considered as limitations.

Claims

1. A radar system, in particular for a vehicle, comprising: a radar sensor for sending and receiving a first radar signal; an evaluation device for processing radar signals received by the radar sensor after reflection from an object separated from the radar system; Features The radar system also has at least one active radar tag, which is configured to retransmit the received radar signal as a second radar signal after amplification and modulation, wherein the active radar tag is configured to retransmit the received radar signal as the second radar signal after being shifted and amplified in frequency by frequency variation, The radar sensor and the active radar tag are laterally spaced apart from one another by a distance of between 1 cm and 10 m, and the evaluation device is configured to determine information about the object based on a component of the first radar signal received by the radar sensor and a component of the second radar signal received by the radar sensor.

2. The radar system of claim 1, wherein the frequency varies between 0.5 kHz and 2 MHz.

3. The radar system of claim 1, wherein the active radar tag is configured to amplify the received radar signal by at least 10 dB and then retransmit it as the second radar signal. 4 . The radar system according to claim 1 , wherein the radar sensor is a MIMO radar sensor having a plurality of transmitting antennas and a plurality of receiving antennas. 5 . The radar system of claim 4 , wherein the MIMO radar sensor has fewer than four transmit antennas and fewer than five receive antennas. 6 . The radar system of claim 1 , wherein both the first radar signal and the second radar signal are within a frequency range between 40 GHz and 120 GHz. 7 . The radar system of claim 1 , wherein the active radar tag is configured to transmit a second radar signal in an opposite direction to the received radar signal. 8 . The radar system of claim 1 , wherein the active radar tag is configured to transmit the second radar signal in a direction toward the radar sensor.

9. The radar system of claim 1, wherein the distance is between 10 cm and 2 m.

10. A vehicle having a radar system according to any one of the preceding claims.

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