DE102013220131B4 - Imaging system with orthogonal transmit signals - Google Patents

Abstract

Method for imaging an object, with the following steps:simultaneous transmission (402) of signals for object illumination in the microwave range or in the millimeter wave range by a plurality of transmitting antennas (104), each of the plurality of transmitting antennas (104) transmitting on its own transmission frequency and the transmission frequencies are selected such that the transmission signals transmitted thereon are mutually orthogonal;receiving (404) signals reflected from the object by a plurality of receiving antennas (106); anddetermining (406) an image of the object based on the signals received from the receiving antennas (106);wherein each of the transmitting antennas (104) emits signals sequentially in time according to its own frequency vector,andwherein the frequency vector comprises a plurality of transmission frequencies (f_100, f_101, f_102) defined,wherein each frequency vector comprises a sequence of transmission frequencies (f_100, f_101, f_102, f_200, f_201, f_202), andcorresponding transmission frequencies (f_100, f_200) of different frequency vectors have a mutual frequency offset Δf,wherein for a separation of the signals with frequency offset Δf the signals received from the receiving antennas are sampled with a number N_sample of samples with N_sample = 2 * N_carrier + 2, where N_carrier indicates the number of simultaneously active transmitting antennas.

Description

The present invention relates to a method and a system for illuminating and imaging objects using electromagnetic millimeter wave radiation.

Millimeter wave or microwave systems are increasingly being used in non-destructive material testing and for personal scanners. For example, millimeter waves are suitable for detecting metallic as well as non-metallic objects, for example ceramic objects. At the same time, air, materials such as those used for clothing, etc. are transparent to such radiation. Thus, millimeter or microwave waves are suitable, for example, for non-contact detection of objects hidden under clothing. Corresponding scanners can supplement or replace metal detectors, for example in the area of security checks at airports or security-relevant public facilities.

When such a scanner is operated, a microwave signal with a given bandwidth is generated and emitted by at least one antenna in the direction of a human body with objects that may be hidden under clothing. The microwave signals are reflected from the body and hidden objects. The reflections are received by at least one antenna. Transmitting and receiving antennas can be provided separately.

A scanner can also include a multiplicity of transmitting and/or receiving antennas. Such a scanner can be operated, for example, in a multistatic mode, in which a transmitting antenna emits a signal and reflections of this signal are received by a number or all of the receiving antennas according to amplitude and phase position.

Aperture synthesis methods can be used, e.g. digital beamforming (DBF) methods, in which the emitted and/or received radiation for object scanning can be focused without mechanical movement of the antennas or electromagnetic bundling by lenses or the like . Instead, focusing only takes place through targeted activation of the transmitting antennas and/or evaluation of the signals detected by the receiving antennas. In the case of the latter, a signal reflected from a given point in space in the direction of a number of antennas and received there is evaluated accordingly, for example using a software-based algorithm.

From the DE 10 2011 005 145 A1 a millimeter wave scanner is known which works according to a multistatic method. The scanner includes an antenna array made up of one or more antenna clusters. In this case, an antenna cluster comprises a plurality of transmitting antennas, which are arranged in a predetermined area, and a number of receiving antennas, which are arranged in another area. A larger aperture can be achieved by interconnecting several antenna clusters. In this way, measurements can be made in a larger spatial area without having to change the spatial position or orientation of an antenna cluster.

To image objects that move in relation to the scanner, the measurement times for exposure or illumination of the object should be short enough to minimize image blurring and thus obtain usable images. For example, in the area of personal scanners, measurement times of up to 15 milliseconds, preferably 10 milliseconds, are usually considered sufficiently short to obtain usable images.

In order to achieve such short measurement times, however, correspondingly powerful hardware components are required for a sufficiently high imaging quality (i.e. high measurement dynamics) and therefore a high integration density of the system for practical implementations. However, there are limits in terms of technical feasibility and/or system costs.

WO 2009/036507 A1 discloses a radar imaging system for capturing an image of an object when visually impaired.

YANG, Jungang [et al.]: Synthetic aperture radar imaging using stepped frequency waveform. In: IEEE Transactions on Geoscience and Remote Sensing, Vol. 50, 2012, no. 5, pp. 2026-2036. - ISSN 0196-2892 discloses a synthetic aperture radar imaging system using a stepped frequency waveform.

One object of the present invention is to propose a method or system for imaging an object in the microwave or millimeter wave range, which optimizes the competing requirements of short measurement times and a practically achievable high integration density at reasonable costs.

The object is achieved by the features of claim 1 for the method aspect and by the features of claim 9 for the system aspect.

According to the invention, a method for imaging an object is proposed, which comprises the following steps: Simultaneous transmission of signals for object illumination in the microwave range or in the millimeter wave range by a plurality of transmission antennas, each of the plurality of transmission antennas transmitting on its own transmission frequency and the transmission frequencies being selected in this way that the transmission signals sent thereon are orthogonal to one another; receiving signals reflected from the object by a plurality of receiving antennas; and determining an image of the object based on the signals received from the receiving antennas; wherein each of the transmitting antennas emits signals in chronological succession according to its own frequency vector, and wherein the frequency vector defines a plurality of transmission frequencies, each frequency vector comprising a sequence of transmission frequencies, and transmission frequencies of different frequency vectors which correspond to one another have a mutual frequency offset Δf, whereby for a separation of the signals with frequency offset Δf, the signals received by the receiving antennas are sampled with a number N_sample of samples with N_sample=2*N_carrier+2, where N_carrier indicates the number of simultaneously active transmitting antennas.

The system can be designed to illuminate and image an object such as a human being or an animal or inanimate object. Certain embodiments of the system have an intended use for detecting objects concealed on a human's body under clothing, which may include non-metallic objects such as ceramic weapons or body-molded objects made from plastic explosives.

The transmitting and/or receiving antennas can be horn antennas, for example. The electromagnetic radiation can include microwaves, for example, radiation in the wavelength range from 1 millimeter to 10 millimeters. The terms "millimeterwave" and "microwave" are sometimes used interchangeably in this disclosure. In some embodiments, a frequency of the transmitted signals is 70 gigahertz or more, for example in a range from 70 gigahertz to 81 gigahertz.

To illuminate an object, signals can be transmitted and/or received in the direction of an object, for example preferably in the direction of a specified object or measurement area. Additionally or alternatively, in certain embodiments, synthetic focusing can be performed at the transmitter end and/or at the receiver end. The imaging of the object can be determined, for example, based on synthetic focusing, as is known from digital beam forming (DBF, “Digital Beam Forming”) processes.

Transmission signals transmitted simultaneously by several transmission antennas should be orthogonal to one another; it can be, for example, OFDM (“Orthogonal Frequency Division Multiplex”) signals. This means that within a given bandwidth in the frequency range, there is a given frequency offset Δf between simultaneously transmitted signals, as is known for FDM (“Frequency Division Multiplex”) systems. In addition, the frequency offset Δf should be selected in a special way, namely in such a way that the frequencies of the simultaneously transmitted signals each differ by a whole multiple (in the sense of the natural numbers) of a fundamental frequency. If, for example, two signals with frequencies f_1 and f_2 are transmitted simultaneously and these signals (or the corresponding signals reflected by the object as received by the receiver) are to be orthogonal to one another, the following must apply to the frequencies: f_1 = f_LO + m * f, f_2 = f LO + n * f, where m, n are natural numbers, f is a fundamental frequency, e.g. a fraction of a sampling frequency, and f_LO is a conversion frequency (LO, "Local Oscillator") around which a receiver Some systems convert a received signal into a mostly lower frequency band, e.g. an intermediate frequency (IF). For a system with no mixing, e.g. no down-conversion to an IF signal at the receiver, in the above equations f_LO = 0 would be in units of Hertz or some other frequency measurement unit.

Some embodiments provide that each of the transmission antennas emits signals according to its own frequency vector. The frequency vector specifies a plurality of transmission frequencies, ie a sequence or series of transmission frequencies. A sequence or series of transmission signals is then transmitted in chronological succession with these frequencies. For example, a transmission signal with this frequency can be transmitted exactly once for each frequency provided according to the frequency vector, with switching to the next transmission frequency after a certain period of time, e.g. a measurement time interval, as specified by the frequency vector.

In certain embodiments, arrays of transmit antennas are provided. The groups differ in that each group is assigned its own frequency vector, ie there are at least as many different frequency vectors as there are groups of transmitting antennas. The frequency vectors of the different groups can be of the same length, or can be one under specify a different number of transmission frequencies. For example, only the transmitting antennas of one group or groups can transmit at a certain point in time, while the antennas of another group or groups are not transmitting at that point in time.

In certain embodiments, the frequency vectors differ at all frequency points, ie transmission frequencies, so that each transmission frequency is assigned to exactly one group. But this does not necessarily have to be the case. The only important thing is that several transmission signals are transmitted simultaneously without interference (ie orthogonally to one another). In some embodiments, the frequency vectors of the different groups differ from one another by only at least one transmission frequency or only a few transmission frequencies. If the transmission frequencies of two or more groups match, they do not necessarily have to be transmitted at the same time. If the transmission frequencies of two vectors match for one and the same measurement time, this is equivalent to a dynamic grouping in which one antenna or several or all transmission antennas are assigned to different groups during the measurement process.
Thus, in a system with several transmitting antennas (at a specific measurement time), each transmitting antenna can be assigned to exactly one group. However, it is also conceivable that some transmission antennas of a system are not assigned to any group, for example these antennas are not used in general or for a momentary measurement time. If an antenna is not assigned to a group with its frequency vectors and is still to be used for the measurement process, a frequency vector must be provided for this antenna, even if this has only a single frequency point or a smaller or larger number of frequency points than the other frequency vectors has frequency points.

At least one frequency vector can include a sequence of equidistant transmission frequencies, i.e. the transmission frequencies of the vector differ by a constant frequency spacing D_f or a multiple thereof. In one example, a frequency vector V_1 may include a total of 64 frequencies, where f_0 = 70 GHz (gigahertz), f_1 = f_0 + D_f, f_2 = f_0 + 2D_f, ... , f_63 = f_0 + 63D_f, and D_f = 158.73 MHz (megahertz).

If there are several transmitter groups in some embodiments, there are also several frequency vectors, with the frequency vectors having a mutual frequency offset Δf or a multiple thereof, so that transmission frequencies of different frequency vectors that correspond to one another have a mutual frequency offset Δf, ie approximately transmission frequencies at the same positions within them respective frequency vectors. For a frequency offset of Δf = 16.67 MHz, for example, the transmission frequencies for the example vector Vf_1 above would be: f_100 = 70 GHz, ..., f_163 = 80 GHz; for a vector Vf_2 shifted by 1Δf: f_200 = 70.01667 GHz, ..., f_263 = 80.01667 GHz; for a vector Vf_3 shifted by 2Δf: f_300 = 70.03333 GHz, ..., f_363 = 80.03333 GHz, etc.

As can be seen from the example above, the frequency vectors can cover a measurement bandwidth of 10 GHz or more, for example. As can also be seen, a fixed frequency offset Δf can be provided in particular. The frequency offset Δf can, for example, be selected in such a way that signals of different frequency vectors transmitted in parallel are not only orthogonal, but also lie within a reception bandwidth of the reception antennas or, more precisely, of reception channels of the downstream signal processing, so that simultaneously transmitted signals from a number of transmission antennas can be received simultaneously. for example, based on a reception bandwidth as it results from a sampling rate in the receiver.

Digital signal processing methods can be used to determine the image of the illuminated object. For example, the received signals can be subjected to a Discrete Fourier Transform (DFT). Alternatively, a digital downconversion can be performed and an averaging filter can be applied.

For an analog-to-digital conversion, an analog representation of a received signal has to be sampled. In order to be able to separate signals received in parallel with a frequency offset Δf, in certain embodiments the signals received by the receiving antennas are sampled with a number N_sample of samples, where N_sample=2*N_carrier+2, where N_carrier specifies the number of simultaneously active transmitting antennas.

In some of these embodiments, the frequency offset Δf is chosen such that Δf=f_sample/N_sample=f_sample/(2*N_carrier+2), where f_sample indicates a sampling frequency of the signals received by the receiving antennas. Such a frequency offset enables efficient signal separation.

In other embodiments, the signals received by the receiving antennas are sampled with a number N_sample of samples, where N_sample=2*N_carrier+1 (N_carrier again indicates the number of simultaneously active transmit antennas). In this case, the frequency offset can be chosen to be Δf=f_sample/(2*N_carrier) if f_sample again indicates a sampling frequency of the signals received by the receiving antennas.

In some embodiments, predetermined received signals are masked out to determine the image of the object. For example, specific reception times are masked in order to minimize antenna crosstalk, for example from one transmitting antenna to an adjacent receiving antenna.

According to the invention, a system for imaging an object is also proposed, which comprises: a plurality of transmitting antennas for the simultaneous transmission of signals for object illumination in the microwave range or in the millimeter wave range, each of the plurality of transmitting antennas being designed to transmit on its own transmission frequency and wherein the transmission frequencies are chosen such that the transmission signals transmitted thereon are orthogonal to one another; a plurality of receiving antennas for receiving signals reflected from the object; and a processing unit for determining an image of the object based on the signals received from the receiving antennas; wherein each of the transmitting antennas is used to emit signals in chronological succession according to its own frequency vector, and wherein the frequency vector defines a plurality of transmission frequencies, each frequency vector comprising a sequence of transmission frequencies, and transmission frequencies of different frequency vectors which correspond to one another have a mutual frequency offset Δf, the processing unit also serves for separating the signals with frequency offset Δf to sample the signals received from the receiving antennas with a number N_sample of samples with N_sample=2*N_carrier+2, where N_carrier indicates the number of simultaneously active transmitting antennas. The system can be a person scanner, for example.

The system may include a plurality of clusters each having a plurality of transmit antennas. All transmitting antennas in a cluster can be assigned to exactly one transmitter group with its own frequency vector. In some embodiments of the system, the clusters each include a plurality of receive antennas. The processing unit can be designed to determine an image of the object based on the signals of the receiving antennas, regardless of their cluster or group affiliation. For example, the simultaneously transmitted transmission signals of several transmitter groups can be received by all receiving antennas of the system.

The system can be designed for the simultaneous reading out of all reception channels of a cluster, several clusters, or all reception channels of the system.

The invention provides systems and methods for imaging an object in the microwave or millimeter wave range, which make it possible for a plurality of transmitting antennas to transmit simultaneously and for the reflected signals to be measured simultaneously. The processing of reflected signals, i.e. in particular the separation of parallel signals, can be carried out particularly efficiently when using orthogonal transmission signals. In this way, a gain in measurement time can be realized, which leads to a reduction in motion blur in the images. The hardware requirements remain essentially unchanged.

The limited reception bandwidth of realistic hardware concepts can be taken into account by arranging the transmission antennas into groups, with each group emitting transmission signals according to its own frequency vector. The frequency vectors of the groups can be offset in a suitable manner, e.g. slightly, so that received signals from different groups lie within the reception bandwidth of the reception channel. On the one hand, parallel reception and thus a gain in measurement time are possible. On the other hand, signal separation can also be implemented with a minimum of effort. In this way, the hardware requirements can be minimized.

• 1 ein Ausführungsbeispiel eines erfindungsgemäßen Abbildungssystems;
• 2 eine Detailansicht des Systems aus 1 mit einem Antennencluster;
• 3 in schematischer Form funktionale Komponenten des Systems aus 1 für eine Signalverarbeitung;
• 4 ein Flussdiagramm zur Veranschaulichung eines Ausführungsbeispiels des Betriebsverfahrens des Systems der 1 - 3;
• 5 eine schematische Darstellung eines Antennenclusters zur Veranschaulichung eines ersten Ausführungsbeispiels einer Sendergruppierung;
• 6A eine schematische Darstellung einer Senderapertur mit Antennenclustern zur Veranschaulichung eines zweiten Ausführungsbeispiels einer Sendergruppierung;
• 6B eine weitere Darstellung der Senderapertur der 6C zur Veranschaulichung eines dritten Ausführungsbeispiels einer Sendergruppierung;
• 7A ein Diagramm zur Veranschaulichung eines ersten Ausführungsbeispiels eines Frequenzrasters aus mehreren versetzten Frequenzvektoren;
• 7B ein Diagramm zur Veranschaulichung eines zweiten Ausführungsbeispiels eines Frequenzrasters;
• 8A ein Spektraldiagramm zur Veranschaulichung eines Zweitonsignals nach ZF-Umsetzung; und
• 8B ein Spektraldiagramm zur Veranschaulichung einer Signaltrennung bei dem Zweitonsignal der 8A.

Further aspects and advantages of the invention are described below by way of example with reference to the accompanying drawings. Here show:

• 1 an embodiment of an imaging system according to the invention;
• 2 a detailed view of the system 1 with an antenna cluster;
• 3 functional components of the system in schematic form 1 for signal processing;
• 4 a flowchart to illustrate an embodiment of the method of operation of the system of FIG 1 - 3 ;
• 5 a schematic representation of an antenna cluster to illustrate a first embodiment of a transmitter grouping;
• 6A a schematic representation of a transmitter aperture with antenna clusters to illustrate a second embodiment of a transmitter grouping;
• 6B another representation of the transmitter aperture of the 6C to illustrate a third embodiment of a transmitter grouping;
• 7A a diagram to illustrate a first embodiment of a frequency grid from a plurality of offset frequency vectors;
• 7B a diagram to illustrate a second embodiment of a frequency grid;
• 8A a spectral diagram to illustrate a two-tone signal after IF conversion; and
• 8B a spectral diagram to illustrate a signal separation in the two-tone signal 8A .

In 1 an exemplary embodiment of an imaging system 100 according to the invention is shown schematically. The system 100 here comprises a vertical frame 102 in which a multiplicity of analog front-end modules or antenna clusters 120, each with a multiplicity of transmitting antennas 104 and receiving antennas 106, are arranged. The transmitting antennas 104 are designed to radiate microwaves into an area 108 in front of the frame 102 and thus to illuminate an object located there, for example a person. The reception antennas 106 serve to receive radiation reflected from the object.

A base plate 112 with a reflector element 114 is located on the floor in front of the frame 102. Plate 112 and element 114 are arranged at an angle of 90° to the frame 102 with the antennas 104, 106 and delimit the illumination area 108 at the bottom. Microwaves transmitted from the transmission antennas 104 are reflected by the reflector element 114 in the direction of the object. In addition, radiation reflected by the object is reflected in the direction of the receiving antennas 106 . The element 114 can comprise a metallic reflecting plate, for example, on which a person to be illuminated stands in an area denoted by the symbol 116 .

The system 100 implements a planar, thinned out ("sparse") aperture with the 32 modules 120; an aperture size can be about 2 square meters. The operating frequencies or transmission frequencies of the emitted microwave or millimeter waves can be in the range from 70 GHz to over 80 GHz. Although a thinned-out aperture is realized by the antenna cluster 120, the 1 system 100 shown have a total of about 3000 transmitting antennas or transmitting channels and about 3000 receiving antennas or receiving channels. The organization of the transmit/receive antennas in the clusters 120 can at the same time represent an optimized implementation with regard to an integration density of the hardware (HW), system footprint, etc.

A processing unit 110 serves to control the antennas 104 , 106 and to determine an image of the object based on signals which represent the radiation received by the receiving antennas 106 . In order to achieve a suitable lateral resolution, the processing unit 110 is designed for a transmitter and receiver-side synthetic focusing; in particular, the receiving antennas can be focused virtually, for example, on specific points on the object or in the object region 108 using DBF (digital beamforming). A desired axial resolution (i.e. in the direction of propagation of the electromagnetic wave) is achieved by suitable SFCW (“Stepped Frequency Continuous Wave”) methods.

The antennas 104, 106 can be operated in a multistatic mode according to the invention. In particular, in such a mode, multiple transmit antennas 104 transmit simultaneously, with the reflected signals being received by a plurality of receive antennas 106 .

In 2 the structure of a front-end module or antenna cluster 120 is sketched schematically from the direction of the irradiation area 108 . The cluster 120 includes a plurality of transmit antennas 104 and a plurality of receive antennas 106. In a practical implementation, an antenna cluster can have e.g. 96 transmit antennas and 96 receive antennas, but in 2 drawn fewer antennas, which is sufficient to explain aspects essential to the invention.

At the in 2 In the configuration shown, the transmit antennas 104 are stacked in a vertical pattern 122 separate from the receive antennas 106 in a horizontal pattern 124 . Instead of in vertical 122 or horizontal 124 rows, transmitting or receiving antennas can alternatively or additionally also be arranged in other, also overlapping patterns, for example alternating in a common pattern. However, for reasons of clarity, only the in 2 configuration shown.

3 FIG. 12 shows some functional components of the processing unit 110 in the form of a schematic block diagram 1 . Also indicated only schematically is an antenna array 302 with a plurality of front-end modules 120 and transmitting/receiving antennas 104/106 integrated therein. The processing unit 110 includes a digital backend 304 and a component 306 for image reconstruction.

An operating method of the system 100 and in particular of the processing unit 110 is described below with reference to the flow chart of FIG 4 described. The method relates generally to a technique 400 for illuminating and imaging an object in the microwave/millimeter wave range.

In step 402, the processing unit 110, for example the backend component 304, controls a plurality of transmitting antennas 104 in such a way that they simultaneously (simultaneously, at the same time) emit millimeter wave radiation or signals for object illumination. In this case, each of the controlled transmission antennas 104 transmits at an individual transmission frequency that differs from the transmission frequency or frequencies of the other antennas transmitting in parallel. The transmission frequencies of the antennas transmitting in parallel are selected in such a way that the transmission signals transmitted thereon are orthogonal to one another. This is explained in detail below.

For the parallel activation of the antennas 104, the backend component 304 sends a corresponding control signal to a signal generator SigGen 308, which sends high-frequency signals or RF (“Radio Frequency”) signals to the antenna array 302 via a corresponding plurality of amplifiers 310. In the example of 3 two RF signals 311 are applied to the array 302 to generate a two-tone signal. At the array 302, the transmission signals are sent to two transmission antennas 104, which are thereby excited in parallel to emit microwaves. The antennas 104 can be located on one module 120 or on different modules 120 . The details of how signal generators and amplifiers for controlling antennas work are known to those skilled in the art.

In step 404 signals reflected from the object are received by the receiving antennas 106 . Corresponding to the simultaneously transmitted, mutually orthogonal transmission signals, the reflections include simultaneously received, mutually orthogonal reception signals (the different signal propagation paths or times are neglected here). A conversion to an intermediate frequency IF can already take place at the antenna array 302, for example each module 120 can contain its own component for IF conversion. The IF reception signals of all antenna modules 120 or all reception antennas 106 can then be read out in parallel and transmitted 312 to the backend 304. Alternatively, an IF conversion can also only take place in the backend.

The backend component 304 receives 312 the parallel multi-tone measurement data on the intermediate frequency IF and performs analog-to-digital conversion (ADC, “Analog Digital Conversion”) in a subcomponent 314 . For example, a digital conversion for all channels of an antenna cluster or all clusters can be carried out in parallel by appropriate hardware. Alternatively, an ADC can also already be carried out on the antenna array 302 .

In a sub-component 316 in the backend, signal separation SigSep (“signal separation”) is carried out based on the orthogonal properties of the received signals. This is also discussed in detail below. As indicated by the arrows 318 , the separated signals are then fed separately to the image acquisition ImgRec (“image reconstruction”) in component 306 . As an alternative or in addition, a uniform data record can also be transferred to component 306 .

In step 406, based on the transmitted data set 318, the component 306 determines an image of the object that is in the illumination area 108 ( 1 ) of the system 100 is located. A calibration using calibration data 320 may be required to determine the image, for example based on a free space measurement carried out beforehand, ie a measurement of the illumination area 108 without an object and/or without a reflector element 114. In this way, for example, antenna crosstalk can be corrected. In step 408 the method ends; for example, the object image may be stored and/or displayed on a console connected to the system 100.

In order to achieve the desired lateral and axial resolution for the image reconstruction, each receiving antenna 106 can be measured against each transmitting transmitting antenna 104 at all frequency points, ie at all transmission frequencies used. Because a number of transmitting antennas 104 are transmitting in parallel and corresponding signals are received and processed, a measurement time gain can be achieved compared to a strictly sequential activation of the transmitting antennas, in which case only exactly one of the transmitting antennas is transmitting. According to the invention, this can be based on the parallel, ie simultaneous transmission of orthogonal transmission signals, without requiring a significantly more complex hardware configuration at the receiving antennas or the antenna clusters, in the backend, or during image reconstruction.

An exemplary image reconstruction can use a specialized two-tone or general multi-tone algorithm. Alternatively, partial images can also be created from the separate signal components of the various transmitter groups using a single-tone reconstruction algorithm. If necessary, the partial images are to be calibrated separately. However, the measurement data for the free space measurement and/or reflector measurement for the calibration can be carried out with all transmitter groups and orthogonal signals transmitted simultaneously, i.e. no separate measurement data recordings are required for the calibration of the partial images. If necessary, the raw data can also be calibrated based on a single, complete data set, i.e. the data set that includes all measurement data of all transmitter groups or frequency points of the different frequency vectors. The measurement data must only be separated at the point in the process where there is no longer a two-tone reconstruction algorithm, for example when the calibrated or corrected measurement data is transferred to the partial reconstruction of the partial images.

The calibrated partial images are then to be added coherently to obtain an overall image. The frequency offset between the transmitter groups can be chosen so small that the image properties change only insignificantly compared to conventional imaging methods with only one frequency vector. The frequency offset is discussed in detail below.

In the image reconstruction in the component 306 according to 3 predefined reception signals can be masked out in order to minimize antenna crosstalk. The corresponding measured values can relate to specific combinations of transmitting/receiving antennas, for example geometrically adjacent pairs of antennas. Masking of the corresponding measured values, for example during image reconstruction, only has an insignificant effect on the imaging quality.

5 illustrates a simple example of channel grouping. A single antenna cluster 120 is shown schematically, a plurality of, for example, a total of 96 transmitting antennas 104 and 96 receiving antennas 106 being merely indicated by solid lines 502, 504, 506, 508. The transmitting antennas 104 are arranged in two vertical rows 502, 504 and the receiving antennas 106 in two horizontal rows 506, 508. The rows could also be arranged in other ways, e.g. X-shaped, or could be arranged in a circle, semi-circle, etc.

In a conventional, strictly sequential measurement process, for a given measurement frequency, the transmit antennas 104 would be activated one after the other, e.g. starting with the transmit antenna in row 502 at the bottom, through the other transmit antennas in the row 502 up, continuing with the transmit antenna in series 504 at the bottom via the further transmission antennas in the row 504 to the last transmission antenna in the row 504 at the top. Meanwhile, all of the receive antennas of rows 506, 508 would be active regardless of which of the transmit antennas is currently transmitting.

According to the schematic representation in 5 another activation pattern is pursued according to the invention. Two of the transmitting antennas 104 are always activated at the same time, for example one in the row 502 and one in the row 504. For example, the bottom antennas of each row can be activated in parallel first, then continuously up to the top antenna in each of the rows 502, 504 as indicated by arrows 510, 512.

The antennas in row 502 are excited to emit signals with a frequency f_100, for example, and the antennas in row 504 to emit signals with a frequency f_200. The frequencies f_100 and f_200 are selected in such a way that the corresponding transmission signals are orthogonal to one another. At any transmission time, only one antenna in row 502 is active on frequency f_100 and one antenna in row 504 on frequency f_200, ie it applies here as well as to other exemplary embodiments that all signals transmitted at the same time are orthogonal to one another, and therefore through suitable signal processing based thereon can be efficiently separated. Concrete examples for the realization of the orthogonality condition are discussed further below in connection with the digital signal separation of the corresponding reflected signals.

The simultaneous transmission and parallel reception of measurement signals makes it possible to save measurement time without resulting in losses in the image reconstruction, for example with regard to an axial and/or lateral image resolution. After the measured values have been recorded, a switch is made simultaneously to the next transmitter antenna in each group.

All or some of the receiving antennas 106 can be permanently active for taking measurements; in particular, the activation of the receiving antennas 106 is independent of the control mechanism for the transmitting antennas 104. Possible deviations from the permanent activation of the Receiving antennas, for example due to DBF mechanisms, the masking of individual measured values, etc. are otherwise ignored in the context of this application. In certain exemplary embodiments, DBF mechanisms and other methods for image reconstruction, including possible masking of data points, are only used during the post-processing of the measurement data, so that the receiving antennas are active at all times during a reception period for recording measurement data.

In order to achieve a suitable axial resolution, each transmission antenna transmits at a plurality of different transmission frequencies one after the other. A corresponding frequency vector Vf_1 for the transmitting antennas of the row 502 can specify 64 frequencies f_100-f_163, for example. A frequency vector Vf_2 for the transmission antennas in the row 504 can specify, for example, 64 frequencies f_200 - f_263, with all 128 frequencies f_100, f-101, ..., f_163, f_200, f_201, ... f_263 being able to differ from one another, but not mandatory. All that is necessary is that in the pairings of the frequencies transmitted at the same time, e.g. (f_100, f_200), ... , (f_163, f_263), the transmission frequencies differ in such a way that the frequencies of each of these pairings are orthogonal to one another.

In a conventional, strictly sequential measurement method, each of the 96 transmission antennas of the cluster 120, for example, would transmit at 64 measurement frequencies, ie there would be 6144 transmission or reception times to achieve 6144 measurement results. With a transmitter grouping as in 5 illustrated, one of 48 array 502 antennas and one of 48 array 504 antennas each transmit in parallel. This method is repeated for all 64 positions of the frequency vectors Vf_1, Vf 2 introduced above as an example for the rows 502 and 504, so that a number of 6144 measurement results can again be obtained from 3072 measurement times.

This consideration already results in a measurement time gain of a factor of 2 that can be achieved through parallel measurements. However, the prerequisite for this is that the reflected parallel signals can also be received, ie can be separated from the downstream signal processing. The signal separation is discussed in detail below, in particular with regard to the requirement that the complexity of the signal processing should not be significantly higher than in a strictly sequential measurement process. Regarding the here based on 5 In the example described, it should be mentioned that a measurement time gain of a factor of 2 cannot be fully achieved in practical implementations due to the requirements for signal separation, but a factor of 1.6, for example, can be achieved.

When grouping channels according to 5 two opposing antennas are activated in each case, but other activation patterns are also possible. Thus, the antennas in rows 502 and 504 could be activated from top to bottom instead of from bottom to top (arrows 510, 512), or also in opposite directions. In addition, the transmitter groupings need not include geometrically adjacent transmit antennas, but the groups can overlap geometrically. Thus, a first group of transmitters may include about every other transmitter antenna in both row 502 and row 504, and a second group of transmitters the remaining antennas.

More than two transmitter groups can also be formed. Thus, a first group of transmitters could include the transmitter antennas of the lower half of row 502, a second group of transmitters the transmitter antennas of the upper half of row 502, a third group of transmitters the transmitter antennas of the lower half of row 504, and a fourth group of transmitters the transmitter antennas of the upper half of row 504. At each transmission time, one antenna of each of the four groups would transmit, with each of the four transmission signals transmitted at the same time being orthogonal to the three other signals in a specific exemplary embodiment. As discussed in detail elsewhere, the measurement speed increases with an increasing number of transmitter groups due to gains in measurement time.

A system includes a plurality of antenna clusters, such as the system 100 of FIG 1 , then the transmitter groups and activation patterns can be extended over all antenna clusters. For example, the simultaneous transmission of orthogonal transmission signals with frequencies f_100 and f_200, as in 5 illustrated, be performed sequentially for all clusters of a system. Based on the frequency vectors discussed above as an example, it is conceivable that measurements over all frequency pairs (f_100, f_200), first the frequency pairing (f_100, f_200) could be measured across all antenna clusters, then the frequency pairing (f_101, f_201), etc.

6A shows schematically a system 600 with an aperture implemented by 16 antenna clusters 602 . Each of the clusters 602 can be equipped with transmitting and receiving antennas, for example, as is the case above for the transmitting cluster 120 of FIG 1 , 2 or. 5 was portrayed. Alternatively can The clusters 602 could also only include transmit antennas, while the receive antennas would be implemented differently.

All of the transmit antennas of the system 600 are assigned to one of two groups 604,606. The group 604 contains the transmission antennas of the upper 8 clusters 602, the group 606 contains the transmission antennas of the lower 8 clusters 602. Thus here, unlike the one based on FIG 5 described example, all transmission antennas of any cluster 602 are always assigned to exactly one group. The receiving antennas of the 16 clusters 602, if any, are not assigned to any group.

During a measurement process, one of the transmitting antennas in group 604 and one of the transmitting antennas in group 606 is activated at the same time. The order of activation in each group is illustrated by arrows 608,610. In relation to each of the groups 604, 606, the transmitting antennas of the respective lower left cluster 612 or 614 are activated one after the other, followed by the transmitting antennas of the other clusters 602 belonging to the respective group in a counter-clockwise order as indicated by the arrows 608 , 610 indicated. At a certain point in time, for example, cluster 616 of group 604 and, in parallel, cluster 618 of group 606 could be activated, as indicated by clusters 616, 618 provided with a dashed box.

Within a cluster 602, the transmission antennas can be activated according to a scheme as indicated by the arrows 620 and 622 for the clusters 612 and 614, respectively.

Each of the two groups 604, 606 is assigned its own frequency vector Vf_1, Vf_2. Correspondingly, simultaneous transmission signals are each transmitted at transmission frequencies f_100 and f_200 in such a way that the transmission signal of group 604 is orthogonal to that of group 606 in each case. Examples of the spectral occupancy by the transmitter groups 604, 606 are described further below.

Examples of parallel activation patterns of groups 604 and 606 are described below. A measurement process can thus begin with the transmission antennas of the group 604 emitting a transmission signal of a frequency f_100 one after the other. Simultaneously, the transmitting antennas of the group 606 send out a transmission signal of a frequency f_200 one after the other. The transmitting antenna located at the bottom left in the cluster 612 or 614 can transmit first, while the transmitting antenna located at the top right in the cluster 624 or 626 transmits last. The run can then be repeated with the other frequency pairings (f_101, f201), ....

Because the transmitter groups 604 and 606 each group geometrically related or adjacently arranged clusters or transmitting antennas, the image reconstruction results in an upper (group 604) or lower (group 606) partial image with correspondingly better or brighter illumination in the object area. However, if the partial images are processed together in a suitable manner, or at least added in a suitable (coherent) manner during image processing, such effects should not be visible in an overall image.

6B 6 again shows the system 600 with the 16 antenna clusters 602 as described above, but a different transmitter grouping is carried out here, in which the clusters and transmitting antennas of the system 600 are assigned to one of four groups 652, 654, 656, 658. The order of activation of the clusters of each group is indicated by arrows 660,662,664,666. At a given time, for example, a group 652 cluster 668 transmit antenna, a group 654 cluster 670 transmit antenna, a group 656 cluster 672 transmit antenna, and a group 658 cluster 674 transmit antenna could be activated in parallel, as indicated by the dashed lines Boxes of these clusters are indicated.

Each of the four groups 652, 654, 656, 658 is assigned its own frequency vector. Transmission frequencies f_100, f_200, f_300 and f_400 are specified at corresponding positions within the four vectors in such a way that the transmission signals of the four groups that are transmitted at the same time are orthogonal to one another. Four measuring points are then recorded in parallel at each measuring time. Examples of the spectral occupancy of the signals from transmitter groups 652 - 658 are described below.

The transmitter groups 652-658 also group geometrically adjacent clusters or transmitting antennas. However, it is not absolutely necessary for the groups to define geometric partial apertures; the groups can also penetrate or overlap one another. In any case, ultimately several measurement sequences (corresponding to the number of groups) are realized simultaneously on a total aperture. An advantage of the joint grouping of geometrically connected clusters or antennas can be that they can be assigned symmetrical activation patterns, as in the examples in FIG 6A and 6B indicated by arrows 608, 610, 660-666. Because closely adjacent transmit antennas of the different groups are not activated at the same time, the occurrence of transmit/receive receiver pairs that are susceptible to antenna crosstalk.

In general, a large number of different activation patterns are conceivable both within and between the antenna clusters of a group.

In the exemplary embodiments described here, the groups defined on a total aperture are of the same size, i.e. the groups each include the same number of clusters or transmitting antennas. Such a grouping is advantageous for an efficient measurement process, but not absolutely necessary. If the groups are of different sizes, at certain times, for example, only a one-tone signal is received instead of a two-tone signal, or only a three-, two-, or one-tone signal instead of a four-tone signal; however, a gain in measuring time can also be achieved with these configurations.

7A FIG. 12 illustrates an example of a spectral occupancy or distribution of the measurement points of two transmitter groups, ie the frequency vectors of these groups, for example of the two groups 604 and 606 6A . A transmitter group has exactly one frequency vector, vice versa, a frequency vector is always assigned to a specific transmitter group.

The frequency vector Vf_1 of the group 604 defines a sequence of 64 transmission frequencies f_100, f_101, f_102, etc. The transmission frequencies are arranged uniformly, i.e. equidistantly, in a frequency range between 70 GHz and 80 GHz; thus each successive transmission frequency has a distance D_f of about 160 MHz. Such a frequency vector enables an optimized axial resolution (SFCW), but frequency vectors do not necessarily have to define equidistant frequency points; advantages according to the invention can also be realized in the case of non-equidistant or only partially equidistant frequency points. If the frequency vector covers a measurement bandwidth of around 10 gigahertz, as here, axial resolutions in the range of around 10 millimeters can be achieved.

The frequency vector Vf_2 of group 606 also defines a sequence of 64 transmit frequencies f_200, f_201, f_202, etc. As shown in FIG 7A As can be seen, the transmission frequencies are also arranged equidistantly from one another in this example with a respective spectral spacing of D_f.

The frequency vectors Vf_1 and Vf_2 are slightly offset from each other with a frequency offset Δf. The frequency offset Δf is selected in such a way that the transmission frequencies of the two transmitter groups result in orthogonal transmission signals, i.e. the transmission signals from the transmission antennas in group 604 with the transmission frequency f_100 are orthogonal to the transmission signals from the transmission antennas in group 606 with the transmission frequency f_200.

The frequency offset Δf can be selected to be smaller than the spectral distance D_f of the measurement points of a frequency vector. The reason for this is discussed below. In the present example, the frequency offset Δf can be in a range between 10 MHz and 50 MHz, for example. A frequency grid results which roughly corresponds to the pattern of the transmission frequencies, as specified by a single frequency vector, with two transmission frequencies of the groups lying close together at accumulation points 702, 704, 706 in the spectrum. If transmission signals with the frequencies of the accumulation points are sent out at the same time, two-tone signals with the respective frequency pairs (f_100, f_200), (f_101, f_201), etc. result during the measurement process.

The orthogonality condition does not necessarily have to be fulfilled between the transmission frequencies that occupy the corresponding position in the two vectors Vf_1 or Vf_2, i.e. it does not necessarily have to be the transmission frequencies of the accumulation points from which the orthogonal transmission signals result; it is only important that simultaneously transmitted transmitter signals of the different groups are orthogonal to each other. However, in the examples, for purposes of discussion, it is assumed that the signals transmitted at the same time are each transmitted at the frequencies of one of the burst points.

7B shows another example of a spectral occupancy by a plurality of frequency vectors suitably offset from one another. Four frequency vectors are defined, e.g. for transmitter groups 652-658 6B . The frequency vector Vf_1 of the group 652 defines a sequence of 64 transmission frequencies f_100, f_101, f_102, etc. The transmission frequencies can in turn be arranged equidistantly in a frequency range between 70 GHz and 80 GHz with a spectral spacing D_f of approximately 160 MHz. The frequency vector Vf_1 can, for example, be defined identically to the vector Vf_1 from 7A .

The frequency vectors Vf_2, Vf_3 and Vf_4 of the groups 655, 656, 658 also each define a sequence of 64 transmission frequencies f_200, f_201,..., or f_300,..., or f_400,.... The transmission frequencies of the vectors Vf_2, Vf_3 and Vf 4 can be derived from the transmission frequencies of vector Vf_1 by a respective frequency offset of Δf, 2Δf, and 3Δf, respectively. The value of Δf can be used in the examples of 7A and 7B same or ver be divorced If the assignments of the 7A and 7B However, if they are implemented with one and the same system with, for example, a fixed sampling rate, Δf will be different.
In general, the transmission frequencies of N frequency vectors can be generated from the transmission frequencies of an output vector by a frequency offset of n*Δf for n=0 (original vector), ..., N-1.

However, it is not absolutely necessary for more than two frequency vectors for adjacent vectors to be offset from one another by one and the same fixed amount. For example, one of the four frequency vectors in 7B fall away. The only important thing here is that the orthogonality condition for simultaneously transmitted transmission signals is met.

in the in 7B The spectrum shown again results in a frequency grid which roughly corresponds to the pattern of the transmission frequencies, as specified by a single frequency vector, but with a plurality of transmission frequencies at accumulation points 752, 754, 756 in the spectrum. These accumulation points in the spectrum arise when the values of Δf are noticeably smaller than the value of D_f; more precisely, if 2Δf in the case of 7A , or 4Δf in the case of the 7B is smaller than D_f. If the respective frequency vectors are measured in the same way for a plurality of transmitter groups, the simultaneously transmitted transmission signals of the different transmitter groups have the frequencies at the spectral location of the accumulation points, i.e. the orthogonality condition for the transmitter signals must be fulfilled for the transmission frequencies that make up an accumulation point.

In addition to the orthogonality condition, the spectral width B_f of the accumulation points, which in the example of 7A a value of 2Δf, in the example the 7B has a value of 4Δf, can be selected such that B_f is less than or equal to a reception bandwidth of the reception antennas or the downstream signal processing, ie the reception channels. Only in this case is it possible to receive the transmission signals that are sent out at the same time, and thus to save measurement time.

By covering the entire measurement bandwidth with a frequency grid with accumulation points, which are created by frequency vectors that are slightly offset from one another, the number of measurement points in the spectrum increases according to the number of transmitter groups provided. However, the total number of measured values obtained remains constant because each transmitting antenna only measures its own frequency vector. The difference to a conventional measurement process is that at a given spectral point not only a single measurement frequency is measured, but rather several measurement frequencies that are slightly offset from each other; i.e. around Δf in the example of 7A or by a maximum of 3Δf in the case of 7B .

A gain in measurement time can be realized because the measurement frequencies are offset from one another in a suitable manner, namely in such a way that the resulting transmission signals are orthogonal and can therefore be efficiently separated from one another by suitable signal processing and can therefore be transmitted simultaneously. In addition, a frequency offset between the vectors, such as that in Figs 7A or 7B offset Δf illustrated by way of example in each case, can be selected in such a way that it enables parallel reception of the orthogonal signals with hardware that can be implemented in practice, as will be discussed below.

Signal processing for signal separation can take place in the digital part of the receiver as in 3 illustrated by the backend component 304. In this case, after intermediate frequency conversion, the analog received signal is digitized in the ADC component 314 with a sampling frequency f_sample or f_s (“sampling frequency”). Signal separation of the digitized signal is then performed in component 316. Methods of discrete Fourier transformation (DFT), for example, can be used for signal separation; these are particularly suitable for the efficient recording of orthogonal signals. Methods for this are known from the field of using OFDM methods for data transmission methods and corresponding communication systems and will not be discussed further here.
It should only be mentioned that OFDM methods for signal separation do not require steep filter functions that require long measurement times and/or complex implementations, which would conflict with the goal of the shortest possible measurement time and/or a desirably high integration density.

As an alternative to the DFT method, a digital down-conversion with downstream filtering can also be carried out, for example, in a filter bank in a series of parallel channels. An averaging filter can be used here as a filter.

For efficient measurement data acquisition, the shortest possible sampling time or a filter of the lowest possible order should be used. The number N_sample of the required sampling values ("samples") increases with the number N_carrier of the Signals or carriers to be separated. The following applies to certain exemplary embodiments: $$\text{N\_sample}=2<figure-callout\; id="2"\; label="*\; N\_carrier\; +"\; filenames="DE102013220131B4\_0009.png"\; state="\{\{state\}\}">*</figure-callout>\text{N\_carrier}+2.$$

4 sampling values are required for the sampling of single-tone signals, but 6 or 10 sampling values are required for two-tone or four-tone signals, i.e. the required measuring times are correspondingly longer. The gain in measurement time through the parallel transmission and measurement of orthogonal signals is thus canceled out again by the longer reception time required for the separation of multi-tone signals, but only partially. In a typical implementation for two-tone signals, the receiving antennas only have to be active two sampling periods longer than for single-tone signals, which corresponds to an additional sampling time of around 20 nanoseconds at a sampling frequency of f_sample=100 MHz.

If the intermediate frequency bandwidth specified with the sampling rate f_sample of the ADC component 314 is to be fully utilized, the following applies to the frequency offset already introduced above: $$\Delta \text{f}=\text{f\_sample}/\text{N\_sample}.$$

For example, the relationship between samples and the number of carrier signals according to Eq. (1) and a sampling rate of f_sample = 100 MHz for the ADC component 314 is assumed. For the two-tone operation of transmitter groups 604 and 606 off 6A an approximate offset or frequency offset Δf = 16.67 MHz results. Assuming a constant offset, and related to the spectral occupancy as in 7A shown, in which the lowest transmitter frequency of the first transmitter group 604 is at f_100=70.00000 GHz, so the lowest transmitter frequency of the second transmitter group 606 should be at f_200=70.01667 GHz, so that f_100 and f_200 can be received both orthogonally to one another and in parallel.

With an equidistant occupancy of the spectrum between 70 GHz and 80 GHz, consecutive transmission frequencies of a frequency vector (Vf_1 or Vf_2) have a distance D_f = 158.73 MHz. The next frequency pair at the accumulation point 704 consequently includes the transmission frequencies (f_101 = 70.15873 GHz, f_201 = 70.17540 GHz), etc.

For the four-tone operation of the 6B and 7B the value of D_f can also be chosen as above, but the frequencies at the accumulation points 752, 754, 756 are only offset from one another by Δf=10 MHz. In this way, the corresponding transmitter signals can also be received in parallel and at the same time orthogonally, ie efficiently separated from one another.

8A Figure 12 is a frequency diagram illustrating an IF spectrum of an orthogonal two-tone signal sampled at frequency f_sample (f_s). Reference numerals 802, 804, 806 and 808 denote carrier signals, while numeral 810 denotes a DC component of the ADC component 314. FIG. According to the above Eq. (1) apart by Δf = f_sample / 6.

This spectrum can, for example, be digitally downmixed. 8B illustrates the result, ie the baseband spectrum, where corresponding carriers are given the same reference numbers as in FIG 8A are designated. Due to the mixing process, an additional spectral line 812 appears at f_sample / 2.

For illustration is in 8B Furthermore, a transfer function 814 of a digital filter is drawn in, namely a sliding 6-tap averaging filter. The number of taps corresponds to the number N_Sample of samples discussed above. A 10-tap filter would be required to filter four-tone signals.

In the spectral representation of 8B the transfer function of the 6-tap filter has zeros at k * f_sample / 6. If the carrier signals are placed at the spectral distance of these zeros from one another, one carrier 816 can be recorded after digital frequency conversion to baseband, while all others have the zeros of the transfer function coincide, i.e. be filtered out. In the example of 8B the carrier signal 806 becomes off 8A recorded, the carrier signal 808 is suppressed. A corresponding baseband transformation must be carried out to record the signal 808 .

In certain embodiments of the invention, the transmission antennas of a microwave aperture are divided into transmitter groups, which emit transmission signals at frequencies that are slightly offset from one another. This means that an object does not have to be illuminated in a strictly sequential order by the transmitter antennas, but that several antennas can illuminate the object in parallel, one antenna from each transmission group.

If orthogonal signals are emitted by a suitable choice of the frequency offset between the transmitter groups, a signal separation of the signals reflected by the object is possible in an efficient manner. The frequency offset can optionally be selected in such a way that none are increased or there are hardly any increased demands on the bandwidth of the receiving channels. In this way it can be achieved that the complexity of the hardware for the signal processing increases only insignificantly, which also applies to the outlay and the costs of a manageable integration density.

In the signal processing at the receiving end, a certain DFT or a certain filtering in the receiving channel can favor one of the signals while the other parallel signals are suppressed. If a corresponding frequency offset of the parallel transmission signals is selected for this, the orthogonality condition can be realized in this way.

The gain in measurement time compared to a strictly sequential measurement results in a first approximation from the number of transmitter groups, i.e. the signals transmitted in parallel. However, longer reception times or larger filter lengths are necessary for signal separation. Measurement time gains of about a factor of 1.6 could be obtained for two-tone signals without noticeable changes in the image quality of still objects. This means that the image quality of moving objects is improved due to the shorter measurement time and the correspondingly reduced motion blur. For signals with more than two tones, the gain in measurement time increases further; a profit of up to a factor of 4, for example, is conceivable.

Theoretical explanations of technical effects brought about by the invention are not intended to limit the scope of the invention. The invention is also not limited to the exemplary embodiments described here and the aspects highlighted therein; on the contrary, within the scope indicated by the appended claims, a large number of modifications are possible, which are within the scope of actions of a person skilled in the art. In particular, certain combinations of features described separately above are evident to the person skilled in the art as expedient or advantageous.

Claims (12)

Method of imaging an object, comprising the following steps: Simultaneous transmission (402) of signals for object illumination in the microwave range or in the millimeter wave range by a plurality of transmitting antennas (104), each of the plurality of transmitting antennas (104) transmitting on its own transmission frequency and the transmission frequencies being selected such that the transmission signals transmitted thereon correspond to one another are orthogonal; receiving (404) signals reflected from the object by a plurality of receiving antennas (106); and determining (406) an image of the object based on the signals received from the receiving antennas (106); wherein each of the transmitting antennas (104) emits signals one after the other according to its own frequency vector, and wherein the frequency vector defines a plurality of transmission frequencies (f_100, f_101, f_102), each frequency vector comprising a sequence of transmit frequencies (f_100, f_101, f_102, f_200, f_201, f_202), and Corresponding transmission frequencies (f_100, f_200) of different frequency vectors have a mutual frequency offset Δf, where for a separation of the signals with a frequency offset Δf, the signals received by the receiving antennas are sampled with a number N_sample of samples with N_sample=2*N_carrier+2, where N_carrier indicates the number of simultaneously active transmitting antennas. procedure after claim 1 , wherein the transmission antennas are assigned to different groups (604, 606) and each group (604, 606) of transmission antennas is assigned its own frequency vector. procedure after claim 1 or 2 , where the frequency offset Δf is selected such that signals of different frequency vectors lie within a receiving bandwidth (B_f). Method according to one of the preceding claims, wherein the frequency offset Δf is chosen such that Δf = f_sample / N_sample applies, where f_sample indicates a sampling frequency of the signals received by the receiving antennas. Method according to one of the preceding claims, wherein digital signal processing of the received signals comprises a digital Fourier transformation, or a digital downconversion and application of an averaging filter. Method according to one of the preceding claims, wherein a frequency of the transmitted signals is 70 gigahertz or more and the frequency vectors cover a measurement bandwidth of 10 gigahertz or more. A method according to any one of the preceding claims, wherein the imaging of the object is determined based on synthetic focusing. Method according to one of the preceding claims, in which predetermined received signals are emitted to determine the image of the object blinded to minimize antenna crosstalk. System for imaging an object, comprising: a plurality of transmitting antennas (104) for the simultaneous transmission of signals for object illumination in the microwave range or in the millimeter wave range, each of the plurality of transmitting antennas (104) serving to transmit on its own transmission frequency and the transmission frequencies being selected such that those transmitted thereon transmission signals are mutually orthogonal; a plurality of receiving antennas (106) for receiving signals reflected from the object; and a processing unit (110) for determining an image of the object based on the signals received from the receiving antennas (106); wherein each of the transmitting antennas (104) is used to emit signals in chronological succession according to its own frequency vector, and wherein the frequency vector defines a plurality of transmission frequencies (f_100, f_101, f_102), each frequency vector comprising a sequence of transmit frequencies (f_100, f_101, f_102, f_200, f_201, f_202), and Corresponding transmission frequencies (f_100, f_200) of different frequency vectors have a mutual frequency offset Δf, the processing unit (110) also serving to sample the signals received by the receiving antennas with a number N_sample of sample values for a separation of the signals with a frequency offset Δf N_sample = 2 * N_carrier + 2, where N_carrier indicates the number of simultaneously active transmit antennas. system after claim 9 , comprising a plurality of antenna clusters (120, 602) each having a plurality of transmitting antennas (104), wherein all transmitting antennas of an antenna cluster (120, 602) are assigned to exactly one transmitter group (604, 606) with its own frequency vector. system after claim 10 , wherein the antenna cluster (120) each comprise a plurality of receiving antennas (106), and the processing unit (110) is designed to determine an image of the object based on the signals of the receiving antennas (106) regardless of their cluster membership. system after claim 10 , wherein the system (100) is designed for the simultaneous reading out and/or digitizing of measurement data of all reception channels of at least one antenna cluster (120).

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