Distributed radar system with efficient Doppler measurement

Abstract

The present invention relates to a central unit (Z) for a distributed radar system, which can be operated, for example, in a means of transportation. The central unit (Z) is configured to provide a first transmit signal (SC) in the form of a continuous sequence of chirp signals to a designated radar transmitter unit, and to provide second transmit signals (SI) alternately in the form of an intermittent sequence of chirp signals to the other radar transmitter units. The chirp signals of the first transmit signal (SC) and the chirp signals of the second transmit signals (SI) have the same signal waveform but different modulation times. The chirp signals of the first transmit signal (SC) are also transmitted during pauses between the chirp signals of the second transmit signals (SI). The first radar echo signal (RC) resulting from the first transmit signal (SC) is evaluated for a Doppler measurement.The second radar echo signals (RI) resulting from the second transmitted signals (SI) are used for detecting objects in the surrounding area. The present invention further relates to a distributed radar system with such a central unit (Z), as well as a transmitting module and a receiving module for a distributed radar system. The invention also relates to a method and a computer program for operating such a central unit (Z), as well as a means of transport comprising a distributed radar system with such a central unit (Z).

Description

The present invention relates to a central unit for a distributed radar system, which can be operated, for example, in a means of transportation. The present invention further relates to a distributed radar system with such a central unit. The invention also relates to a method and a computer program for operating such a central unit, as well as a means of transportation comprising a distributed radar system with such a central unit. For driver assistance systems and safety systems in fully automated driving, the safest possible environmental perception is essential. This is achieved by capturing the environment using sensors such as radar, lidar, and camera sensors integrated into the vehicle. Based on the acquired sensor data, an environmental model can then be created using a suitable machine learning model. Perception modules can be used for this purpose, enabling the recognition of learned objects in the environment and forwarding this information to a planning module. The planning module can then take the recognized objects into account for trajectory planning and safe vehicle control. Particularly important here is a holistic 360° 3D capture, which allows for the complete 360-degree recording of all static and dynamic objects and the creation of the highest possible resolution 3D models of the environment. This also applies to objects below the vehicle, such as...may have gotten there during a prolonged period of standstill. While lidar-based systems are capable of providing precise distance measurements and can also be used for classification, they are expensive and complex to design. Furthermore, lidar systems are susceptible to weather conditions such as rain, fog, or direct sunlight. Radar sensors, on the other hand, deliver reliable and fail-safe data in all weather conditions. Even poor visibility conditions like rain, fog, snow, dust, and darkness hardly affect their accuracy. However, their resolution is currently limited. For example, radar sensors used in series production in the automotive sector have a resolution of approximately 2°.This is insufficient, for example, to meet the requirements for levels 4 and 5 of automated driving with safe driving functions, as these require radar sensors to provide three-dimensional images with a high resolution in the range of 0.1° and below, with high insensitivity to interference from their environment. This cannot be achieved with conventional radar technology because the resolution of such systems is too low. Currently under development are so-called photonic radar systems, in which driver signals in the GHz range can be distributed to a large number of radar sensors using an optical carrier signal in the THz range. This allows for the co-integration of electronic and photonic components on a single semiconductor substrate, enabling extremely compact form factors for the individual radar sensors and, consequently, arrays with a large number of such radar sensors integrated into the vehicle. During the processing of the radar echo signals received by the radar sensors, the high-frequency radar signal information is downconverted in the individual sensor units. The received radio frequency (RF) signal, typically in the GHz range, is converted into a lower intermediate frequency (IF) signal.intermediate frequency (IF), typically in the sub-GHz range, is converted. German patent application DE 10 2017 221 257 A1 discloses a radar system in which signal transmission between a central unit and a radar transmitter or radar receiver is implemented optically. For this purpose, a radar driver signal is optically generated in the central unit and transmitted via at least one optical fiber to at least one radar receiver and / or at least one radar transmitter. In the radar transmitter, the radar driver signal is then converted into an electrical radar driver signal and used to drive a radar transmitter. A radar echo signal received by a radar receiver is mixed with the electrical radar driver signal in a mixer of the radar receiver. The mixed signal is then modulated onto the optical driver signal by means of a modulation unit, coupled into the optical fiber, and transmitted back to the central unit.The central unit receives the modulated optical signal and evaluates it using an evaluation unit. The result is then provided as radar information. DE 10 2021 128 147 A1 describes an antenna device for a motor vehicle for transmitting and / or receiving electromagnetic radiation, comprising at least one first antenna element designed using liquid crystal technology for transmitting and / or receiving electromagnetic radiation, and an electronic computing device designed to generate a control signal for the first antenna element. The antenna device includes at least one second antenna element designed using liquid crystal technology for transmitting and / or receiving electromagnetic radiation, wherein, depending on the control signal, the first antenna element and / or the second antenna element are activated for transmitting and / or receiving. Distributed radar systems, also known as "distributed" or bistatic MIMO (Multiple Input Multiple Output) systems, observe potential targets from different angles to minimize fluctuation losses. However, calculating the target coordinates before coherently summing the individual signals is significantly more complex and computationally intensive compared to monostatic radar systems due to the varying signal propagation times. The use of distributed transmitting and receiving antennas results in different signal propagation times between the transmitting antenna, the receiving antenna, and a potential target, which in turn lead to differing measurement distances. These differing measurement distances, in turn, result in a non-linear phase term, which prevents coherent integration over the dimension of the receiving channels in favor of a better signal-to-noise ratio. The compensation of such nonlinear phase terms for subsequent Doppler processing involves considerable computational effort and a massive utilization of the corresponding computing unit. One objective of the invention is to provide more efficient solutions for Doppler measurement using a distributed radar system. This problem is solved by the independent claims. Preferred embodiments of the invention are the subject of the dependent claims. According to a first aspect of the invention, a central unit for a distributed radar system, which in addition to the central unit comprises two or more radar transmitters and two or more radar receivers, is configured: - to provide a first transmission signal in the form of a continuous sequence of chirp signals for one of the two or more radar transmitters; - to provide second transmission signals alternately in the form of an intermittent sequence of chirp signals for the other of the two or more radar transmitters, wherein the chirp signals of the first transmission signal and the chirp signals of the second transmission signals have the same signal waveform but different modulation times, and wherein the chirp signals of the first transmission signal are sent in pauses between the chirp signals of the second transmission signals;- to obtain first radar echo signals from a first subset of the two or more radar receiving units, resulting from the first transmitted signal; - to obtain second radar echo signals from a second subset of the two or more radar receiving units, resulting from the second transmitted signals; and - to evaluate the first radar echo signals obtained from the first subset for a Doppler measurement. According to a further aspect of the invention, a method for operating a central unit for a distributed radar system, which in addition to the central unit comprises two or more radar transmitters and two or more radar receivers, is provided, wherein the central unit: - provides a first transmission signal in the form of a continuous sequence of chirp signals for one of the two or more radar transmitters; - provides second transmission signals alternately in the form of an intermittent sequence of chirp signals for the other of the two or more radar transmitters, wherein the chirp signals of the first transmission signal and the chirp signals of the second transmission signals have the same signal waveform but different modulation times, and wherein the chirp signals of the first transmission signal are sent in pauses between the chirp signals of the second transmission signals;- first radar echo signals are received from a first subset of the two or more radar receiving units, resulting from the first transmitted signal; - second radar echo signals are received from a second subset of the two or more radar receiving units, resulting from the second transmitted signals; and - the first radar echo signals received from the first subset are evaluated for a Doppler measurement. According to a further aspect of the invention, a computer program contains instructions which, when executed by a processor, cause the processor to perform the following steps for operating a central unit for a distributed radar system, which, in addition to the central unit, comprises two or more radar transmitters and two or more radar receivers: - providing a first transmission signal in the form of a continuous sequence of chirp signals for one of the two or more radar transmitters;- Alternating provision of second transmit signals, each in the form of an intermittent sequence of chirp signals, to the other of the two or more radar transmitting units, wherein the chirp signals of the first transmit signal and the chirp signals of the second transmit signals have the same signal waveform but different modulation times, and wherein the chirp signals of the first transmit signal are sent in pauses between the chirp signals of the second transmit signals; - Receiving first radar echo signals resulting from the first transmit signal from a first subset of the two or more radar receiving units; - Receiving second radar echo signals resulting from the second transmit signals from a second subset of the two or more radar receiving units; and - Evaluating the first radar echo signals received from the first subset for a Doppler measurement. The term "computer" is to be understood broadly. In particular, it also includes control units, embedded systems, and other processor-based data processing devices. Furthermore, the individual steps are not necessarily executed directly by the processor. It is equally possible that the processor controls or utilizes external components to perform individual steps. The computer program can, for example, be made available for electronic retrieval or be stored on a computer-readable storage medium. In the solution according to the invention, the distributed radar system includes a designated radar transmitter that emits a continuous sequence of chirp signals. At least one designated radar receiver is also provided, which receives the time-delayed echo signal of this chirp sequence, which is used for Doppler measurement. The at least one designated radar receiver is preferably arranged in close proximity to the designated radar transmitter. Second transmission signals, each in the form of an intermittent sequence of chirp signals, are alternately provided to the other radar transmitters, so that the other radar transmitters alternately emit chirp signals. The dead time of the intermittent sequence of chirp signals is used to emit a rapid chirp of the continuous sequence of chirp signals.The chirps of the first transmitted signal and the second transmitted signals have the same signal waveform, but different modulation times, i.e., up chirps or down chirps are repeated. The solution according to the invention allows the use of less powerful computing units, resulting in cost savings. Furthermore, it reduces algorithmic complexity, which helps prevent downstream implementation errors. Finally, the solution according to the invention contributes to meeting the ASIL-B requirements for safety-related sensor systems. According to one aspect of the invention, the distributed radar system is designed as a photonic radar system. The co-integration of electronic and photonic components on a single semiconductor substrate in a photonic radar system enables extremely compact form factors for the individual radar sensors. This allows arrays with a large number of such radar sensors to be integrated into a vehicle. According to one aspect of the invention, the first transmitted signal and the second transmitted signals have the same bandwidth. This has the advantage that the components of the distributed radar system can be optimized for this bandwidth. According to one aspect of the invention, the first and second transmitted signals are shaped by two signal generators. In this way, the characteristics of both transmitted signals can be individually controlled. Alternatively, the first and second transmitted signals are shaped by adjusting at least one parameter of a signal generator. This has the advantage that only a single signal generator is required. According to one aspect of the invention, the radar receivers of the first subset of the two or more radar receivers form a uniform linear array (ULA). This allows the integration gain due to the coherent received signal to be profitably exploited for an improvement in the signal-to-noise ratio. According to another aspect of the invention, a distributed radar system comprising a central unit according to the invention is provided. In the distributed radar system according to the invention, the radar transmitting units and the radar receiving units can each be integrated into a common unit. The distributed radar system according to the invention can, for example, be designed as a photonic radar system. The radar system, the central unit, and the method according to the invention can be used, in particular, in any means of transport, such as motor vehicles, ships, or aircraft, where radar-based environmental sensing is performed, for example, for driver assistance systems or for automatic or autonomous operation. The motor vehicles can be, in particular, passenger cars, commercial vehicles, trucks, or buses. However, application in radar-based environmental sensing in other technical fields is also possible. Further features of the present invention will become apparent from the following description and the appended claims in conjunction with the figures. Fig. 1 schematically shows a photonic radar system as an example of a distributed radar system; Fig. 2 schematically shows a central unit for a distributed radar system; Fig. 3 schematically shows a method for operating a central unit according to the invention for a distributed radar system; Fig. 4 shows an exemplary integration of the radar system on the vehicle surface of a passenger car; Fig. 5 shows a signal model based on two synchronous, successive linear frequency-modulated ramps; Fig. 6 shows a sequence based on the signal model from Fig. 5; Fig. 7 illustrates a transmit and receive sequence for the signal model from Fig. 5; Fig. 8 shows a first embodiment of a backend for a distributed radar system; Fig.Figure 9 shows a second embodiment of a backend for a distributed radar system; Figure 10 shows radar transmitter units of a frontend for a distributed radar system; Figure 11 shows a transmitter module for a distributed radar system; Figure 12 shows a receiver module for a distributed radar system; and Figure 13 shows radar receiver units of a frontend for a distributed radar system. To better understand the principles of the present invention, embodiments of the invention are explained in more detail below with reference to the figures. It is understood that the invention is not limited to these embodiments and that the described features can also be combined or modified without leaving the scope of protection of the invention as defined in the appended claims. Fig. 1 schematically shows a photonic radar system RS as an example of a distributed radar system. The radar system RS comprises a central unit Z, several radar transmitters S-1, S-2, S-3,..., Sn, and several radar receivers E-1, E-2, E-3,..., En. The central unit Z is referred to as the backend, and the radar transmitters and radar receivers E-1, E-2, E-3,..., En are referred to as the frontend. Although the radar transmitters S-1, S-2, S-3,..., Sn and the radar receivers E-1, E-2, E-3,..., En are shown here as separate units, they can also be implemented using combined units, each integrating both a transmitter and a receiver. The central unit Z is connected via one or more transmission media G to the radar transmitting units S-1, S-2, S-3,..., Sn and the radar receiving units E-1, E-2, E-3,..., En, wherein the transmission media G can be, in particular, one or more optical fibers. In the case of an integrated unit containing circuits for transmitting and receiving, the signal only needs to be sent to this common unit. The central processing unit Z generates a frequency-modulated continuous wave (FMCW) signal and processes and evaluates the signals generated by the radar receiver units E-1, E-2, E-3, ..., En. Instead of a frequency-modulated continuous wave signal, a signal with a different waveform can also be generated. This centralized processing and evaluation of the signals allows the individual radar sensors to be designed as small and cost-effectively as possible. In the central unit Z, a radar driver signal, which consists of a radar carrier signal with frequency fcarrier and a radar ramp signal with frequency frramper, is modulated onto an optical carrier signal. The frequency of the radar driver signal is preferably only a fraction of the carrier frequency required to drive the individual radar transmitters. For example, the signal to be transmitted can be modulated with 1 / 8 of the radar frequency. With a carrier frequency of 77 GHz, currently common for road vehicles, this results in a frequency of 9.625 GHz. The optical carrier signal modulated with the radar driver signal is then coupled into the transmission medium G. The optical carrier signal modulated by the radar driver signal is coupled out of the transmission medium G by each of the individual radar transmitter units S-1, S-2, S-3,..., Sn by means of a coupling unit (not shown), and the radar driver signal is separated in each case. If this radar driver signal is only a fraction of the radar frequency used, for example, 1 / 8 of the radar frequency, it is first amplified eightfold in each of the individual radar transmitter units S-1, S-2, S-3,..., Sn. The resulting signal then drives the respective radar transmitters in the radar transmitter units S-1, S-2, S-3,..., Sn. The individual radar transmitters include, in particular, radar antennas, each of which then transmits a radar signal. Additionally, the optical carrier signal is also transmitted to the individual radar receivers E-1, E-2, E-3,..., En and likewise coupled out of the transmission medium G by means of a coupling unit. Radar echo signals are received by the individual radar receivers E-1, E-2, E-3,..., En and modulated onto the optical carrier signal. The resulting signal is coupled into the transmission medium G by means of further coupling units and sent back to the central unit Z. The central unit Z receives the signals from the radar receivers E-1, E-2, E-3,..., En, evaluates them, and provides derived radar information. This radar information can then be further processed, for example, to create or update an environmental model. The radar transmitting units S-1, S-2, S-3,..., Sn and radar receiving units E-1, E-2, E-3,..., En can each be designed as separate electronically and photonically cointegrated chips (so-called "EPIC chips") or implemented on a single electronically or photonically integrated chip. Silicon photonics technology can be used for the cointegration of the electronic and photonic components, enabling the monolithic integration of photonic components, high-frequency electronics, and digital electronics on a single chip. A hybrid implementation using separate electronic (EIC) and photonic (PIC) chips is also possible. The integration of optical components into the chip can be achieved, for example, using so-called silicon-on-insulator (SOI) regions, while the integration of electronic components can be accomplished using bulk silicon regions. In SOI regions, a thin silicon layer is separated from the silicon substrate by an insulating layer, such as silicon dioxide. Since silicon is transparent at the near-infrared wavelengths common in optical communication technology, and the refractive indices of silicon and silicon dioxide differ significantly in this wavelength range, various optical components can be implemented using SOI structures. This allows for high signal quality with low parasitic interference, particularly at high data rates.The integration of the RF circuits for the radar antennas, including the frequency multiplier, with the photonic circuit can be implemented in a monolithic design without additional wire or flip-chip bonding. Furthermore, chips can be optically and electrically tested directly at the wafer level. Additionally, the scalability to large volumes in the highly integrated manufacturing of electronically and photonically integrated circuits enables a significant reduction in assembly costs and a more efficient cost structure. The basic principle behind measuring the Doppler velocity of a dynamic object using an FMCW signal model is based on the sequential transmission of several so-called chirps, i.e., frequency-modulated radar signals. This generates a measurement sequence that significantly increases the observation period of an object compared to a single measurement. A phase term relevant for Doppler velocity measurement propagates with each newly emitted measurement signal within the sequence, generating a signal waveform whose underlying frequency is proportional to the Doppler velocity. If an arbitrary sampling step "n" of the measurement chirp is set constant across all chirps within the sequence, the measurement signal sn of a measurement m can be described according to formula (1): Here, A denotes the amplitude of the mixed received signal, v the velocity of any object measured during the measurement period, TRPI the time interval between two successive chirps, α the ratio between the bandwidth and the modulation time of a chirp, d(p) the bistatic distance between the target and the pair of transmitting and receiving antennas, TS the sampling interval for acquiring the sample samples, c0 the speed of light, and R0 an arbitrary default distance to describe a radial distance. In the case of a monostatic radar, or a bistastatic radar where the spatial distance between transmitting and receiving antennas is negligible, d(p) = 2R0 applies. The outbound and return paths between the transmitting antenna, receiving antenna, and target are the same. With each additional measurement m within the measurement sequence, only the phase term exp, which describes the dependence on the Doppler velocity v, changes in formula (1). If the radar aperture includes additional receiving antennas for which the described property d(p) = 2R0 still holds, the individual measurement signals can be coherently added according to formula (1) to improve the signal-to-noise ratio. The final determination of the Doppler frequency is performed by frequency analysis in the form of a discrete Fourier transform. For bistatic radars, where transmitting and receiving antennas are spatially distributed, the previously made assumption d(p) = 2R0 is no longer valid. Instead, d(p) ≠ 2R0, since the radial distance between the transmitting antenna position pTx and a target p differs from the radial distance between the receiving antenna position pRx and the target p. Therefore, for d(p): This property of distributed radars has a significant impact on the calculation of the Doppler velocity, since d(p) within the phase terms exp and exp influences the measurement signal nonlinearly. A coherent integration over all receiving antennas, which is advantageous for the signal-to-noise ratio, is no longer feasible. Instead, such an approach can, in the worst case, cause destructive interference and cancel out essential signal components. To counteract this problem, all nonlinear phase components in the measurement signal must be compensated according to formula (1) before processing the Doppler when using a distributed radar.Since the direction angles inherent in formula (2) are unknown during the Doppler measurement process step, hypotheses about all targets within all angular and distance intervals within the radar's field of view must be formulated and applied to compensate for the nonlinear phase components. Starting with the input data set, this is multiplied by a hypothesis data set, accumulated over the dimension of the receiving antennas, and finally Fourier-transformed over the dimension of the received sequence. Each hypothesis data set assumes a target in the direction of an assumed solid angle, which occurs within each distance gate. The effort required to compensate for the nonlinear phase terms increases linearly according to the granularity by which the individual hypotheses differ from one another.With a conventional approach that merely subjects the transmission sequence of a receiving antenna of a receiving channel to a Fourier transformation, there is a risk of no longer being able to distinguish target information from noise. Figure 2 schematically illustrates an embodiment of a central unit according to the invention for a distributed radar system. The distributed radar system is again, by way of example, a photonic radar system. The thin lines in Figure 2 represent electrical connections, and the thick lines represent connections via optical waveguides. To provide the optical carrier signal, the central unit Z comprises a light source 11. This can be, in particular, a laser diode that emits continuous laser light in the near-infrared range. Preferably, the wavelength of the laser diode is in a range where optical losses and dispersion are as low as possible, for example, 850 nm, 1310 nm, or 1550 nm. However, an ASE (Amplified Spontaneous Emission) light source can also be used, which emits light generated by spontaneous emission and subsequently optically amplified by stimulated emission. The optical carrier signal generated by the light source 11 is fed to an electro-optic modulator 12, which modulates the electrical radar driver signal, consisting of the radar carrier signal and the radar ramp signal, onto the optical carrier signal. The radar ramp signal is generated by a ramp generator 13. For example, the electro-optic modulator 12 can be a Mach-Zehnder modulator, in which the optical carrier signal is first split into two waveguides where phase modulation is performed and then recombined. Depending on their relative phase, the partial signals superimpose to form the modulated output signal. Alternative modulation principles, such as those using directly modulated light sources, are also possible. The optical carrier signal modulated by the electro-optical modulator 12 is then fed to an optical processing unit 14, which ensures that a first transmission signal SC is provided for one of the radar transmitting units in the form of a continuous sequence of chirp signals and that second transmission signals SI are provided alternately for the other radar transmitting units, each in the form of an intermittent sequence of chirp signals. To receive and evaluate the signals received by the radar receivers, which are distance-dependent, time-delayed copies of the signals emitted by the radar transmitters, the central unit Z comprises an optical receiver 15 and an evaluation unit 16. First radar echo signals RC, resulting from the first transmitted signal SC, are received from a first subset of the radar receivers, and second radar echo signals RI, resulting from the second transmitted signals SI, are received from a second subset. The optical receiver 15 includes, for each modulated signal RC, RI of an associated radar receiver to be evaluated, a photodiode 17, a transimpedance amplifier 18, and an IQ mixer 19. The optical receiver 15 may also include filter units for selecting individual wavelength ranges, etc.Each of the modulated signals RC, RI is converted into an output current by the photodiode 17. The output current is then converted into a proportional voltage signal in the transimpedance amplifier 18 and subsequently demodulated in the IQ mixer 19. The demodulated signal is fed to the evaluation unit 16, which derives radar information from the signal. This radar information can then be output and further processed, for example, for environmental sensing. Additionally, the electrical and optical components of the central unit Z can be controlled via one or more optional control interfaces and control units 20. In particular, this includes switching the electrical and optical components on and off, parameterizing them, diagnosing them, etc. Furthermore, communication with the radar transmitters and receivers can also be established via the control interfaces and control units 20, and this communication can be carried out electrically, optically, or electrically and optically. For clarity, the control interfaces and control units 20 are shown in a single component in Fig. 2; however, in a given implementation, they can also be divided into several blocks. Fig. 3 schematically shows a method for operating a central unit according to the invention for a distributed radar system, which, in addition to the central unit, comprises two or more radar transmitters and two or more radar receivers. In this method, the central unit provides a first transmission signal in the form of a continuous sequence of chirp signals S1 for one of the two or more radar transmitters. For the other two or more radar transmitters, second transmission signals are provided alternately, each in the form of an intermittent sequence of chirp signals S2. The chirp signals of the first transmission signal and the chirp signals of the second transmission signals have the same signal waveform but different modulation times. Furthermore, the chirp signals of the first transmission signal are sent during pauses between the chirp signals of the second transmission signals.First radar echo signals S3, resulting from the first transmitted signal, are received from a first subset of the two or more radar receiving units. Second radar echo signals S4, resulting from the second transmitted signals, are received from a second subset of the two or more radar receiving units. The first radar echo signals received from the first subset are then evaluated for a Doppler measurement S5. Figure 4 schematically shows an exemplary large-area integration of a multitude of antenna chips A, each of which can comprise the described radar transmitting units and / or radar receiving units, on the vehicle surface of a means of transport F, here a passenger car. For example, the windshield, rear window, and bumper can be used for integration into the front and rear, and the vehicle floor, roof, and B-pillar for integration along the sides of the vehicle. The central unit of the photonic radar system is not shown in this illustration and can, for example, be implemented as a radar control unit located in the vehicle's interior or engine compartment. Likewise, the figure omits the representation of the transmission media between the central unit and the antenna chips. Further details of the invention will be explained below with reference to Figures 5, 6, 7, 8, 9, 10, 11, 12 to 13. In the described embodiments, the distributed radar system is a photonic radar system. To significantly reduce the computational effort required to compensate for the nonlinear phase term in sparsely distributed transmitting and receiving antennas, according to the invention, a fast chirp propagating in the same direction is emitted within the dead time of the chirp sequence used to generate a virtual aperture array. This emitted fast chirp preferably has the same bandwidth as the chirp used to generate the virtual aperture. Figure 5 shows a corresponding signal model based on two consecutive linear frequency-modulated ramps running concurrently. After the modulation period of the transmit signal for generating the virtual antenna array, the so-called MIMO up-chirp time TMu, the duty cycle (Tdde) follows. This duty cycle is used to emit a second transmit ramp of the same bandwidth, but with a steeper curve, within a so-called Doppler up-chirp time TDue. The basic principle described here is therefore to use an alternating sequence of up-chirps and down-chirps with different modulation times. Fig. 6 shows a sequence based on the signal model from Fig. 5. When considering a sequence of transmitted signals following the scheme shown in Fig. 5, it becomes clear that the pulse repetition interval (TPRI) encompasses the time intervals TMu and TDu. Analytical analysis of the measurement signal for Doppler calculation reveals that TPRI represents the sampling time at which this measurement signal is sampled. Consequently, this also influences the uniqueness range, taking into account the Nyquist criterion. The aim of the solution according to the invention is therefore to keep the gradients of both frequency-modulated transmission signals as short as possible in order to ensure a large uniqueness range: Further consideration of this requirement reveals that, when examining the resolving power using Doppler, this requirement has a positive effect on the separation, see formula (5). Shorter pulse repetition intervals lead to a finer resolving power: Fig. 7 illustrates a transmit and receive sequence for the signal model from Fig. 5 for two MIMO cycles MC1 and MC2. To effectively use the signal modulation described above to reduce the computational complexity of Doppler calculations through large apertures, the transmission control of the transmitting antennas and the receive control of the receiving antennas must be modified. This necessity becomes clear upon closer examination of equation (3). This equation is divided into two essential phase terms: a linear component and a nonlinear component. In particular, the nonlinear component influences the phase of the Doppler term due to the bista distance between the transmitting antenna, the receiving antenna, and the target. Depending on the transmitting and receiving antenna pair, this distance varies and induces a nonlinearity that affects the phase term of the Doppler signal. To limit this influence, or rather, to reduce it, the transmission control of the transmitting antennas must be modified.To completely eliminate this, a transmit / receive behavior is sought that follows that shown in Fig. 7. Here, the transmitted signal, generated within the MIMO up-chirp time TMu, is emitted by the transmitting antennas required for establishing the virtual aperture, represented here by Tx0-Tx9. As shown in Fig. 7, all receiving antennas, represented by Rx0, receive a sequence of echo signals that follows the transmission sequence emitted by the transmitting antennas Tx0-Tx9. This results in the nonlinearity described in equation (3) within the phase terms of the Doppler signal. To avoid this, the transmitted signal, generated within the time interval denoted by Doppler up-chirp time TDu, is emitted by a designated transmitting antenna TxDemit. The echo signal of this emitted transmission sequence is captured by a designated group of receiving antennas, here exemplified as RxD / RxULA. Ideally, this group is arranged as a uniform linear array (ULA), as this allows the properties of coherent integration over the receiving channels of the ULA structure to be exploited to improve the signal-to-noise ratio. The described procedure allows for the generation and reception of a continuous sequence of chirps without nonlinearities, resulting in continuous phase propagation within a frequency bin in the receive channel RxULA. Implementing the solution according to the invention requires only a manageable increase in effort in the antenna control, signal generation, and the amount of data acquired by the analog-to-digital converters. In particular, the data volume of the received signals acquired by the ULA structure doubles. Fig. 8 shows a first embodiment of a backend or central processing unit Z for a distributed radar system. Fig. 8a) shows the central processing unit Z and Fig. 8b) shows the provided chirp sequences SC and SI. The thin lines represent electrical connections, and the thick lines represent connections via optical waveguides. Starting with an electrically frequency-modulated transmission signal, this is modulated onto an optical carrier signal from a light source 11 by means of an electro-optical modulator 12. Different transmission sequences of the output signals SC and SI are provided via an optical control unit 23 and an optical switch 22. One of the output channels SC provides a continuous chirp sequence, while the remaining output channels SI switch between the individual chirps of the sequence. The dead time of the intermittent sequence of chirp signals of the transmission signal SI is used to emit a fast chirp from the continuous sequence of chirp signals of the transmission signal SC. Both transmission signals SC and SI are generated by the same signal generator using different rise and fall times. Optionally, an arrayed waveguide grating 24 can be provided to divide the optical carrier signal of the light source 11 into different carrier signals based on the wavelength. In addition, a feedback loop 21 can be provided, for example, for controlling the carrier-envelope offset (CEO) or the carrier-envelope phase (CEP). To receive and evaluate the signals RC, RI received by the radar receivers, the central unit Z comprises one photodiode 17 for each modulated signal RC, RI from an associated radar receiver. First radar echo signals RC, resulting from the first transmitted signal SC, are received from a first subset of the radar receivers, and second radar echo signals RI, resulting from the second transmitted signals SI, are received from a second subset. The output signals of the photodiodes 17 are digitized via a digital interface 25, e.g., an analog-to-digital converter, and fed to an evaluation unit 16, e.g., a PC or a graphics processing unit. Optionally, a signal processing unit 26 can be connected upstream, e.g., for a fast Fourier transform (FFT). Communication between the central unit Z and the radar transmitting and receiving units can take place via an electrical output channel Eo and an electrical input channel Ei. Fig. 9 shows a second embodiment of a backend or central processing unit Z for a distributed radar system. Fig. 9a) shows the central processing unit Z and Fig. 9b) shows the provided chirp sequences SC and SI. The thin lines again represent electrical connections, and the thick lines represent connections via optical waveguides. The backend largely corresponds to that shown in Fig. 8; however, two different signal generators are responsible for generating the transmission signals. Both signal generators are synchronized such that each emits during the dead time of the other. The output signals of the signal generators are modulated onto an optical carrier signal from a light source 11 by means of two electro-optic modulators 12. While one output signal is passed on to an optical switch 22 and generates an alternating sequence of transmissions for the output signals SI, the other output signal is fed directly to the output channel SC and provides a continuous transmission sequence. Fig. 10 shows radar transmitter units S-1, S-2, SK, SU of a front end for the back end from Fig. 8 or Fig. 9. The radar transmitter units S-1, S-2, SK, SU are shown as semiconductor structures. According to the virtual array to be configured, the continuously switching output channels are connected to the inputs of the transmitting antennas Tx1 to TxK via optical couplers 30. The back end output that generates a continuous sequence of chirps is connected to the input for the designated transmitting antenna TxU via an optical coupler 30. The signals for driving the transmitting antennas Tx1 to TxU are each amplified by an amplifier 31. In this embodiment, all components for signal transmission are integrated on the chip. Fig. 11 shows a transmitter module SM for a distributed radar system. The transmitter module SM is depicted as a semiconductor structure. Both the continuously switching output channels and the continuous sequence of chirps are transmitted to a control element 34 via optical couplers 30. This control element is supplied with a control signal from the backend. According to the virtual array to be configured, the continuously switching output channels are assigned by the control element 34 to the inputs of the transmitting antennas Tx1 to TxK. The continuous sequence of chirps, on the other hand, is assigned by the control element 34 to the input for the designated transmitting antenna TxU. The signals for driving the transmitting antennas Tx1 to TxU are each amplified by an amplifier 31. In this embodiment, all components for signal transmission are integrated on a single chip. The depicted transmitter module SM is compatible with both the backend from Fig. 8 and the backend from Fig. 11.9 can be combined. Fig. 12 shows a receiver module EM for a distributed radar system. The receiver module EM is depicted as a semiconductor structure in which all components and receiving antennas Rx1 to RxU are integrated on a single chip. A control signal, fed from the backend, controls the corresponding receiving interface via a control element 34 and determines which transmit signal is fed to a mixer 32 and which receiving antenna Rx1 to RxU is switched to be sensitive for signal reception. The corresponding received signal is mixed with the reference signal, i.e., the transmit signal, by means of the mixer 32. The intermediate signal thus generated is passed on to an electro-optical modulator 33, electro-optically converted, and forwarded to the backend via an optical coupler 30 and an optical fiber. The receiver module EM shown can be combined with both the backend from Fig. 8 and the backend from Fig. 9. Fig. 13 shows radar receiver units Ei, E-i+1, EL of a front end for the back end from Fig. 8 or Fig. 9. The radar receiver units Ei, E-i+1, EL are shown as semiconductor structures, with each component integrated as an independent module for signal reception within a single semiconductor structure. The corresponding output channels of the back end are connected to the inputs of the receiving antennas Rxi, Rxi+1, RxL via an optical coupler 30. The received signals from the receiving antennas Rxi, Rxi+1, RxL are each amplified by an amplifier 31. The received signal is then mixed with the reference signal, i.e., the transmitted signal, by means of a mixer 32. The intermediate signal thus generated is passed to an electro-optical modulator 33, electro-optically converted, and transmitted to the back end via an optical coupler 30 and an optical fiber.The hardware configuration of the overall system determines which transmission signal from the backend is connected to which radar receiver unit Ei, E-i+1, EL. In particular, the individual modules can be configured as a uniform linear array. Reference symbol list A Antenna chip E-1, E-2, E-3, En Radar receiver unit Ei Electrical input channel Eo Electrical output channel EM Receiver module F Means of transport G Transmission medium MC1, MC2 MIMO cycle RC First radar echo signal RI Second radar echo signal RS Distributed radar system Rxi Receiving antenna S-1, S-2, S-3,Sn Radar transmitting unit SC First transmit signal SI Second transmit signal SM Transmitting module Td Dead time TDu Doppler up-chirp time TMu MIMO up-chirp time TPRI Pulse repetition interval Txi Transmitting antenna Z Central processing unit 11 Light source 12 Electro-optical modulator 13 Ramp generator 14 Optical processing unit 15 Optical receiving unit 16 Evaluation unit 17 Photodiode 18 Transimpedance amplifier 19 IQ mixer 20 Control interface and control unit 21 Feedback loop 22 Optical switch 23 Optical control unit 24 Arrayed waveguide grating 25 Digital interface 26 Signal processing unit 30 Optical coupler 31 Amplifier 32 Mixer 33 Electro-optical modulator 34 Control element S1 Provide first transmit signal S2 Provide second transmit signals S3 Receive first radar echo signals S4 Receive second radar echo signals S5 Evaluate the first radar echo signals for Doppler measurement,

Claims

Central unit (Z) for a distributed radar system (RS), which, in addition to the central unit (Z), comprises two or more radar transmitting units (S-1, S-2, S-3, Sn) and two or more radar receiving units (E-1, E-2, E-3, En), wherein the central unit (Z) is configured: - to provide a first transmit signal (SC) in the form of a continuous sequence of chirp signals (S1) for one of the two or more radar transmitting units (S-1, S-2, S-3, Sn); - to provide second transmit signals (SI) alternately in the form of an intermittent sequence of chirp signals (S2) for the other of the two or more radar transmitting units (S-1, S-2, S-3, Sn), wherein the chirp signals of the first transmit signal (SC) and the chirp signals of the second transmit signals (SI) have the same signal waveform but different modulation times, and wherein the Chirp signals of the first transmission signal (SC) are sent during pauses between the chirp signals of the second transmission signals (SI);- to obtain first radar echo signals (RC) from a first subset of the two or more radar receiving units (E-1, E-2, E-3, En) (S3), resulting from the first transmitted signal (SC); - to obtain second radar echo signals (RI) from a second subset of the two or more radar receiving units (E-1, E-2, E-3, En) (S4), resulting from the second transmitted signals (SI); and - to evaluate the first radar echo signals (RC) obtained from the first subset for a Doppler measurement (S5).; Central unit (Z) according to claim 1, wherein the distributed radar system (RS) is configured as a photonic radar system. Central unit (Z) according to claim 1 or 2, wherein the first transmit signal (SC) and the second transmit signals (SI) have the same bandwidth. Central unit (Z) according to one of the preceding claims, wherein the first transmit signal (SC) and the second transmit signals (SI) are shaped by two signal generators or by adjusting at least one parameter of a signal generator. Central unit (Z) according to one of the preceding claims, wherein the radar receiving units (E-1, E-2, E-3, En) of the first subset of the two or more radar receiving units (E-1, E-2, E-3, En) form a uniform linear array. Method for operating a central unit (Z) for a distributed radar system (RS), which, in addition to the central unit (Z), comprises two or more radar transmitting units (S-1, S-2, S-3, Sn) and two or more radar receiving units (E-1, E-2, E-3, En), wherein the central unit (Z) provides: - a first transmit signal (SC) in the form of a continuous sequence of chirp signals (S1) for one of the two or more radar transmitting units (S-1, S-2, S-3, Sn);- Second transmit signals (SI) are provided alternately to the other of the two or more radar transmitting units (S-1, S-2, S-3, Sn) in the form of an intermittent sequence of chirp signals (S2), wherein the chirp signals of the first transmit signal (SC) and the chirp signals of the second transmit signals (SI) have the same signal waveform but different modulation times, and wherein the chirp signals of the first transmit signal (SC) are sent in pauses between the chirp signals of the second transmit signals (SI); - First radar echo signals (RC) resulting from the first transmit signal (SC) are received from a first subset of the two or more radar receiving units (E-1, E-2, E-3, En) (S3); - Second radar echo signals (RI) are received from a second subset of the two or more radar receiving units (E-1, E-2, E-3, En). (S4), which result from the second transmission signals (SI);and- the first radar echo signals (RC) received from the first subset are evaluated for a Doppler measurement (S5).; Computer program with instructions which, when executed by a processor, cause the processor to perform the steps of the method according to claim 6. Distributed radar system (RS) with a central unit (Z) according to one of claims 1 to 5. Means of transport with a distributed radar system (RS) according to claim 8.

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