DE102013220131A1 - Imaging system with orthogonal transmit signals - Google Patents

Abstract

The present invention relates to a method and system for illuminating and imaging objects using millimeter waves. According to a method aspect of the invention, multiple transmit antennas simultaneously transmit signals, each antenna transmitting at its own transmit frequency and the transmit signals orthogonal to each other. Reflected signals from the object (806, 808) are received by a plurality of receive antennas and based thereon an image of the object is determined.

Description

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

Millimeter-wave and microwave systems are increasingly used in non-destructive testing and for personal scanners. Millimeter waves are suitable, for example, for the detection of 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, for example, are suitable for non-contact detection under the clothing of hidden objects. Corresponding scanners can complement or replace metal detectors, such as airport security or security-related public facilities.

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

A scanner may also include a plurality of transmitting and / or receiving antennas. Such a scanner can, for example, be operated in a multistatic mode in which a transmitting antenna transmits a signal and reflections of this signal are received in amplitude and in phase position from a plurality or all of the receiving antennas.

Methods of aperture synthesis may be used, for example, digital beamforming (DBF) techniques which include focusing the emitted and / or received radiation for object scanning without mechanical movement of the antennas or electromagnetic focusing by lenses or the like can take place. Instead, focusing takes place only by targeted control of the transmitting antennas and / or evaluation of the signals detected by the receiving antennas. In the case of the latter, for example, by means of a software-based algorithm, a signal reflected from a given point in space in the direction of a plurality of antennas and received there is correspondingly evaluated.

From the DE 10 2011 005 145 A1 a millimeter-wave scanner is known which operates according to a multistatic method. The scanner includes an antenna assembly constructed of one or more antenna clusters. An antenna cluster in this case comprises a plurality of transmitting antennas, which are arranged in a predetermined area, and a number of receiving antennas, which are arranged in a different area. By interconnecting multiple antenna clusters, a larger aperture can be achieved. In this way it is possible to measure in a larger spatial area without having to change the spatial position or orientation of an antenna cluster.

To image objects moving in relation to the scanner, the exposure time for illuminating or illuminating the subject should be sufficiently short to minimize image blur and obtain usable images. Thus, for example, in the field of personal scanners usually measuring times of up to 15 milliseconds, preferably 10 milliseconds, are considered to be sufficiently short to obtain usable images.

To achieve such short measurement times, however, sufficiently high imaging quality (i.e., high dynamic range) requires correspondingly high-performance hardware components, and therefore, for practical implementations, also a high integration density of the system. However, there are limits as far as technical feasibility and / or system costs are concerned.

An 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 practically achievable high integration density at a reasonable cost.

The object is achieved for the method aspect by the features of claim 1 and for the system aspect by the features of claim 12.

According to the invention, a method for imaging an object is proposed, which comprises the following steps: Simultaneous emission of signals for object illumination in the microwave or millimeter-wave range by a plurality of transmit antennas, each of the plurality of transmit antennas transmitting on its own transmission frequency and the transmission frequencies are selected in that the transmission signals transmitted thereon are mutually orthogonal; Receiving from the object reflected signals by a plurality of receive antennas; and determining an image of the object based on the signals received by the receiving antennas.

The system may be designed to illuminate and depict an object, such as a human or even an animal or inanimate object. Certain embodiments of the system have the purpose of detecting objects hidden on a person's body under their clothing which may include non-metallic objects such as ceramic weapons or sculpted plastic explosives objects.

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

For object illumination, signals can be transmitted and / or received in the direction of an object, for example, preferably in the direction of a predetermined object or measuring area. Additionally or alternatively, in certain embodiments, a transmitter-side and / or receiver-side synthetic focusing can be performed. The image of the object may be determined based on, for example, synthetic focusing, as known from digital beamforming (DBF) techniques.

Transmit signals emitted simultaneously by multiple transmit antennas should be orthogonal to each other; it may, for example, be OFDM ("Orthogonal Frequency Division Multiplex") signals. By this it is first understood that within a given bandwidth in the frequency range between simultaneously transmitted signals a given frequency offset Δf is present, as known for FDM ("Frequency Division Multiplex") systems. The frequency offset Δf should moreover be chosen in a special way, namely in such a way that the frequencies of the signals transmitted simultaneously 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 sent out simultaneously, and if these signals (or the corresponding signals reflected by the object as received by the receiver) are 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, for example a fraction of a sampling frequency, and f_LO is a local oscillator (LO), around which a receiver in some Systems, a received signal in a mostly lower frequency band, for example. An intermediate frequency (IF), converts. For a system without mixing, for example, without down-mixing to an IF signal in the receiver, in the above equations, f_LO = 0 would be in units of Hertz or some other unit of frequency measurement.

Some embodiments provide that each of the transmit 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. With these frequencies, a sequence or series of transmission signals is then transmitted in chronological succession. For example. For example, for each frequency provided according to the frequency vector, a transmission signal with this frequency can be emitted just once, after a certain period of time, e.g. a measurement time interval, as specified by the frequency vector to the next transmission frequency is incremented.

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

In certain embodiments, the frequency vectors differ in all frequency points, ie transmission frequencies, so that each transmission frequency is only assigned to exactly one group. But this does not necessarily have to be the case. It is only important that multiple transmit signals are transmitted simultaneously without interference (ie orthogonal to one another). In some embodiments, the frequency vectors of the different groups differ from each other 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 coincide for one and the same measurement time, this is equivalent to a dynamic grouping in which one or more or all of the transmit antennas are assigned to different groups during the measurement process. Thus, in a system with multiple transmit antennas (at a given time of measurement), each transmit antenna may be assigned to exactly one group. It However, it is also conceivable that some transmit antennas of a system are not assigned to a group, for example, these antennas are not used generally or for a current measurement time. If an antenna is not assigned to a group with its frequency vectors and is nevertheless to be used for the measuring process, a frequency vector must be provided for this antenna, even if it has, for example, only a single frequency point or a smaller or larger number than the other frequency vectors Having frequency points.

At least one frequency vector may comprise a sequence of equidistant transmission frequencies, i. 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 comprise 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 (Mega Hertz).

In some embodiments, if there are multiple transmitter groups, there are also a plurality of frequency vectors, the frequency vectors may have a mutual frequency offset Δf or a multiple thereof, such that corresponding transmit frequencies of different frequency vectors have a mutual frequency offset Δf, i. about transmission frequencies at equal positions within their respective frequency vectors. For a frequency offset of, for example, Δf = 16.67 MHz, the transmission frequencies for the above example vector Vf_1 would be: f_100 = 70 GHz,..., F_163 = 80 GHz; for a vector Vf_2 offset by 1Δf: f_200 = 70.01667 GHz, ..., f_263 = 80.01667 GHz; for a vector Vf_3 offset by 2Δf: f_300 = 70.03333 GHz, ..., f_363 = 80.03333 GHz, etc.

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

When determining the image of the illuminated object, methods of digital signal processing can be used. For example. For example, the received signals may be subjected to discrete Fourier transform (DFT). Alternatively, for example, digital down-conversion may be performed and an averaging filter applied.

For an analog-to-digital conversion, an analog representation of a received signal is 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 receive antennas are sampled with a number N_sample of samples, where N_sample = 2 * N_carrier + 2, where N_carrier indicates the number of simultaneously active transmit 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 receive 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 Δf = f_sample / (2 · N_carrier) can be selected when f_sample again indicates a sampling frequency of the received signals from the receiving antennas.

In some embodiments, predetermined receive signals are masked to determine the image of the object. For example. For example, certain receive times are masked to minimize antenna crosstalk from, for example, a transmit antenna to an adjacent receive antenna.

According to the invention, a system for imaging an object is proposed, which comprises: a plurality of transmission antennas for simultaneously emitting signals for object illumination in the microwave range or in the millimeter-wave range, wherein each of the plurality of transmission antennas is adapted to transmit at its own transmission frequency and wherein the Transmission frequencies are selected so that the transmitted signals are orthogonal to each other; 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 by the receiving antennas. The system may, for example, be a personal scanner.

The system may include a plurality of clusters each having a plurality of transmit antennas. All transmit antennas of 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 may be configured to determine an image of the object based on the signals of the receiving antennas, regardless of their cluster or group membership. For example. For example, the simultaneously transmitted transmission signals of several transmitter groups can be received by all receiving antennas of the system.

The system may be configured to simultaneously read all of the receive channels of a cluster, multiple clusters, or all receive channels of the system.

The invention provides systems and methods for imaging an object in the micrometre or millimeter-wave range, which allow multiple transmit antennas to transmit simultaneously and simultaneously measure the reflected signals. The processing of reflected signals, i. 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, resulting in a reduction of the motion blur in the images. The hardware requirements remain essentially unchanged.

The limited reception bandwidth of realistic hardware designs can be accommodated by arranging the transmission antennas in groups, each group emitting transmission signals according to its own frequency vector. The frequency vectors of the groups may be suitably detected e.g. are slightly offset from each other so that received signals of different groups are within the receiving bandwidth of the receiving channel. Thus, on the one hand, a parallel reception and thus measuring time gain possible. On the other hand, a signal separation can be realized with minimal effort. In this way, the hardware requirements can be minimized.

Further aspects and advantages of the invention will be described below by way of example with reference to the accompanying drawings. Hereby 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 in schematic form functional components of the system 1 for signal processing;

4 a flowchart illustrating an embodiment of the operating method of the system 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 for illustrating a second embodiment of a transmitter grouping;

6B another illustration of the transmitter aperture of

6C to illustrate a third embodiment of a transmitter grouping;

7A a diagram illustrating a first embodiment of a frequency grid of a plurality of offset frequency vectors;

7B a diagram illustrating a second embodiment of a frequency grid;

8A a spectral diagram illustrating a two-tone signal after ZF conversion; and

8B a spectral diagram for illustrating a signal separation in the two-tone signal of 8A ,

In 1 is an embodiment of an imaging system according to the invention 100 shown schematically. The system 100 here includes a vertical frame 102 , in which a variety of analog front-end modules or antenna clusters 120 each with a plurality of transmitting antennas 104 as well as receiving antennas 106 are arranged. The transmitting antennas 104 are designed to microwave in one area 108 in front of the frame 102 to illuminate, and to illuminate such an object there, for example, a person. The receiving antennas 106 serve to receive reflected radiation from the object.

On the floor in front of the frame 102 there is a bottom plate 112 with a reflector element 114 , plate 112 and element 114 are at an angle of 90 ° to the frame 102 with the antennas 104 . 106 arranged and limit here the illumination area 108 downward. From the transmitting antennas 104 be sent out microwaves through the reflector element 114 reflected in the direction of the object. In addition, radiation reflected from the object is directed toward the receiving antennas 106 reflected. The element 114 may include, for example, a metallic reflective plate upon which a person to be illuminated passes through the symbol 116 designated area represents.

The system 100 realized with the 32 modules 120 a planar, sparse aperture; An aperture size can be about 2 square meters. The operating frequencies or the transmission frequencies of the emitted micro- or. Millimeter waves can range from 70 GHz to over 80 GHz. Although it is through the antenna cluster 120 a thinned aperture realized, nevertheless, the in 1 shown system 100 have a total of about 3000 transmitting antennas or transmission channels and about 3000 receiving antennas or receiving channels. The organization of the transmit / receive antennas in the clusters 120 can simultaneously represent an optimized realization 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 determining an image of the object based on signals from those of the receiving antennas 106 represent received radiation. To achieve a suitable lateral resolution is the processing unit 110 formed to a transmitter and receiver synthetic focussing, in particular, the receiving antennas by means of DBF ("Digital Beamforming") virtually eg. On certain points on the object or in the object area 108 be focused. A desired axial resolution (ie in the direction of propagation direction 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 are transmitting 104 at the same time, the reflected signals from a plurality of the receiving antennas 106 be received.

In 2 schematically the structure of a front end module or antenna cluster 120 from the direction of the irradiation area 108 outlined. The cluster 120 includes a plurality of transmitting antennas 104 and a plurality of receiving antennas 106 , In a practical implementation, an antenna cluster may have, for example, 96 transmit antennas and 96 receive antennas, however, in 2 fewer antennas drawn, which is sufficient for explaining aspects essential to the invention.

At the in 2 configuration shown are the transmitting antennas 104 in a vertical pattern 122 one above the other separately to the receiving antennas 106 in a horizontal pattern 124 arranged. Instead of vertical 122 or horizontal 124 Rows can alternatively or additionally be arranged in other, also overlapping patterns, for example alternately in a common pattern. However, for reasons of clarity, below only in the 2 shown configuration.

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

A method of operation of the system 100 and in particular the processing unit 110 will be described below with reference to the flowchart of 4 described. The method generally relates to a technique 400 for illumination and imaging of an object in the micro / millimeter wave range.

In step 402 controls the processing unit 110 , for example the backend component 304 , several transmitting antennas 104 such that they emit simultaneous (simultaneous, simultaneous) millimeter wave radiation or signals for object illumination. In this case, each of the controlled transmission antennas transmits 104 at an individual transmission frequency different from the transmission frequency or transmission frequencies of the other parallel transmitting antennas. The transmission frequencies of the antennas transmitting in parallel are chosen so that the transmitted signals transmitted to them are mutually orthogonal. This will be explained in detail below.

For parallel control of the antennas 104 returns the backend component 304 a corresponding control signal to a signal generator SigGen 308 , the radio frequency (RF) signals via a corresponding plurality of amplifiers 310 to the antenna array 302 gives. In the example of 3 become two RF signals 311 for generating a two-tone signal to the array 302 given. At the array 302 the transmit signals are sent to two transmit antennas 104 given thereby stimulated parallel to the emission of microwaves. The antennas 104 can be on a module 120 or on different modules 120 are located. The details of the operation of signal generators and amplifiers for driving antennas are known in the art.

In step 404 be reflected by the object signals through the receiving antennas 106 receive. In accordance with the simultaneously emitted mutually orthogonal transmission signals, the reflections comprise simultaneously received, mutually orthogonal received signals (the different signal propagation paths or times are neglected here). A conversion to an intermediate frequency ZF can already be done on the antenna array 302 For example, it can be any module 120 contain a separate component for ZF conversion. The IF receive signals of all antenna modules 120 or all receiving antennas 106 can then be read in parallel and sent to the backend 304 be transmitted 312 , Alternatively, an IF implementation can also be done in the backend.

The backend component 304 receives 312 the parallel multi-tone measurement data on the intermediate frequency ZF and performs an analog-to-digital conversion (ADC, "Analog Digital Conversion") in a subcomponent 314 by. For example, a digital conversion for all channels of an antenna cluster or all clusters can be done in parallel by corresponding HW. Alternatively, an ADC may already be on the antenna array 302 be performed.

In a subcomponent 316 In the backend, a signal separation SigSep ("signal separation") is performed based on the orthogonal properties of the received signals. These too will be discussed in detail below. As by the arrows 318 the separated signals are then indicated separately in the image acquisition ImgRec ("image reconstruction") component 306 fed. Alternatively or additionally, a uniform data record can also be sent to the component 306 be handed over.

In step 406 determines the component 306 based on the submitted record 318 an illustration of the object that is in the illumination area 108 ( 1 ) of the system 100 located. For the determination of a calibration can be based on calibration data 320 be necessary, for example, based on a previously carried out free space measurement, ie a measurement of the illumination area 108 without object and / or without reflector element 114 , As a result, for example, an antenna crosstalk can be corrected. In step 408 the procedure ends; For example, the object image may be stored and / or on one with the system 100 connected console.

In order to achieve the desired lateral and axial resolution for the image reconstruction, each receiving antenna 106 against every sending antenna 104 be measured at all frequency points, ie at all transmission frequencies used. Because several transmit antennas 104 send in parallel and receive and process corresponding signals, a measurement time can be realized compared to a strictly sequential activation of the transmit antennas, in which only one of the transmit antennas sends. According to the invention, this can be done based on the parallel, ie simultaneous emission of orthogonal transmit signals, without requiring a substantially more complex hardware configuration at the receive antennas or the antenna clusters, in the backend, or during the image reconstruction compared to a sequential illumination.

An exemplary image reconstruction may resort to a specialized two-tone or multi-tone algorithm. Alternatively, sub-images can also be created using the single-tone reconstruction algorithm from the separate signal components of the various transmitter groups. The partial images may need to be calibrated separately. However, the measurement data acquisition for clearance measurement and / or reflector measurement for calibration can be performed with all transmitter groups and simultaneously transmitted orthogonal signals, i. no separate measurement data recordings are required for the calibration of the partial images. Possibly. Also, the calibration of the raw data may be based on a single, complete data set, i. the data set, which includes all measured data of all transmitter groups or frequency points of the different frequency vectors. A separation of the measurement data must first take place at the point in the process at which no two-tone reconstruction algorithm is no longer present, for example during the transfer of the calibrated or corrected measurement data to the partial reconstruction of the partial images.

The calibrated fields should then be added coherently to obtain an overall picture. The frequency offset between the transmitter groups can be chosen so small that the image properties change only insignificantly in comparison to conventional imaging methods with only one frequency vector. The frequency offset will be discussed in detail below.

During image reconstruction in the component 306 according to 3 predetermined received signals can be hidden 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. A masking of the corresponding measured values, for example during image reconstruction, has only a negligible effect on the imaging quality.

5 illustrates a simple example of a transmitter grouping. Schematically represented is a single antenna cluster 120 , wherein a plurality of total, for example, 96 transmitting antennas 104 and 96 receiving antennas 106 with solid lines 502 . 504 . 506 . 508 is merely indicated. The transmitting antennas 104 are in two vertical rows 502 . 504 and the receiving antennas 106 in two horizontal rows 506 . 508 arranged. The rows could also be arranged differently, for example. X-shaped, or could be circular, semicircular, etc. arranged.

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

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

The antennas in the row 502 are excited for the transmission of signals, for example. With a frequency f_100, the antennas in the series 504 for the transmission of signals with a frequency f_200. The frequencies f_100 and f_200 are chosen so that the corresponding transmission signals are orthogonal to one another. At any time of transmission, there is always only one antenna in the row 502 on the frequency f_100 and one antenna of the series 504 is active on the frequency f_200, ie it applies here as well as for other embodiments, that all simultaneously transmitted signals are orthogonal to each other, and therefore can be efficiently separated by appropriately based thereon signal processing. Concrete examples of the realization of the orthogonality condition are discussed below in the context of the digital signal separation of the respective reflected signals.

The simultaneous transmission and the parallel reception of measurement signals makes it possible to realize a gain in measurement time without resulting in any loss of image reconstruction, for example in relation to an axial and / or lateral image resolution. After recording the measured values, the system switches simultaneously to the next transmitter antenna of each group.

All or some of the receiving antennas 106 can be permanently active for measured value recording; in particular, the activation of the receiving antennas 106 independent of the drive mechanism for the transmit 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 disregarded for the rest in the context of this application. In certain embodiments, DBF mechanisms as well as other methods of image reconstruction including possible masking of data points are only applied during the post-processing of the measurement data, so that the receive antennas are invariably active at any time during a receive period for measurement data acquisition.

To achieve suitable axial resolution, each transmit antenna transmits at a plurality of different transmission frequencies in succession. A corresponding frequency vector Vf_1 for the transmit antennas of the series 502 can for example specify 64 frequencies f_100-f_163. A frequency vector Vf_2 for the transmit antennas in the series 504 can, for example. 64 Specify frequencies f_200-f_263, where all 128 Frequencies f_100, f-101, ..., f_163, f_200, f_201, ... f_263 can differ from one another, but do not necessarily have to. All that is necessary is that in the pairings of simultaneously emitted frequencies, eg (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 eg 96 transmit antennas of the cluster would 120 send to 64 measuring frequencies, ie there would be 6144 send resp. Receiving times to obtain 6144 measurement results. For a station grouping like in 5 each one of 48 antennas of the series transmits 502 and one of 48 antennas in the series 504 parallel. This method is used for all 64 positions of the above introduced by way of example frequency vectors Vf_1, Vf_2 for the rows 502 respectively. 504 repeated, so that from 3072 measurement times in turn a number of 6144 measurement results can be obtained.

From this consideration results already obtained by parallel measurements measuring time gain by a factor of 2. However, the prerequisite for this is that the reflected parallel signals can also be received, that can be separated from the downstream signal processing. The signal separation will be discussed in detail below, particularly with respect to the requirement that the complexity of the signal processing should not be significantly higher than with one strictly sequential measurement. In terms of here by way of 5 described example may be that a measurement time gain of a factor of 2 in practical implementations due to the requirements for signal separation can not be fully achieved, but a factor of, for example, 1.6.

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

It is also possible to form more than two transmitter groups. So a first transmitter group could be the transmit antennas of the lower half of row 502 a second transmitter group comprises the transmit antennas of the upper half of row 502 , a third transmitter group the transmit antennas of the lower half of row 504 and a fourth transmitter group the transmit antennas of the upper half of the row 504 , At each transmit time, one antenna of the four groups would each transmit, and in a particular embodiment, each of four simultaneously transmitted transmit signals is orthogonal to the three other signals. As discussed in detail elsewhere, the measurement speed increases due to measurement time gains with an increasing number of transmitter groups.

A system includes a plurality of antenna clusters, such as the system 100 out 1 , then the transmitter groups and activation patterns can be extended over all the antenna clusters. For example, the simultaneous transmission of orthogonal transmission signals with frequencies f_100 and f_200, as in 5 illustrated sequentially for all clusters of a system. With reference to the frequency vectors discussed above by way of example, it is conceivable that measurements are first carried out with one cluster over all frequency pairs (f_100, f_200),..., (F_163, f_263), then with a next antenna cluster, etc. In another exemplary embodiment 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 schematically shows a system 600 with one through 16 antenna cluster 602 realized aperture. Each of the clusters 602 may for example be equipped with transmitting and receiving antennas as above for the transmission cluster 120 of the 1 . 2 respectively. 5 was described. Alternatively, the clusters could 602 include only transmit antennas while the receive antennas would be implemented differently.

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

During a measurement process, one of the transmit antennas of the group will be active at the same time 604 and one of the transmit antennas of the group 606 activated. The order of activation in each group is indicated by the arrows 608 . 610 illustrated. Related to each of the groups 604 . 606 The transmit antennas of the respective cluster on the lower left will be consecutive in time 612 respectively. 614 followed by the transmit antennas of the other clusters belonging to the respective group 602 in a counterclockwise order as indicated by the arrows 608 . 610 indicated. At some point, for example, the cluster could 616 the group 604 and in parallel the cluster 618 the group 606 be activated, such as by the dashed box provided with clusters 616 . 618 indicated.

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

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

The following are examples of parallel activation patterns of the groups 604 and 606 portrayed. Thus, a measurement process may begin with the transmit antennas of the group 604 one after the other emit a transmission signal of a frequency f_100. At the same time, the transmit antennas send the group 606 one after the other a transmission signal of a frequency f_200. It can be the first one in the lower left of the cluster 612 respectively. 614 Send antenna, while the top right in the cluster 624 respectively. 626 transmitter antenna last sends. The run can then with the further frequency pairings (f_101, f201), ... are repeated.

Because the transmitter groups 604 and 606 group geometrically contiguous or adjacently arranged clusters or transmit antennas, an upper (group 604 ) or lower (group 606 ) Partial image with a correspondingly better or brighter illumination in the object area. If the partial images are suitably processed together, or at least suitably (coherently) added during image processing, such effects should not be visible in an overall image.

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

Each of the four groups 652 . 654 . 656 . 658 is assigned a separate frequency vector. At corresponding positions within the four vectors, transmission frequencies f_100, f_200, f_300 and f_400 are respectively predetermined in such a way that the transmission signals of the four groups transmitted at the same time are mutually orthogonal. Then, four measuring points are recorded in parallel at each measuring time. Examples of the spectral occupancy by the signals of the transmitter groups 652 - 658 are described below.

Also the transmitter groups 652 - 658 each group geometrically adjacent clusters or transmit antennas. However, it is not absolutely necessary that the groups define geometric subapertures; the groups can also penetrate or overlap. In any case, several measurement sequences (corresponding to the number of groups) are ultimately realized simultaneously on a total aperture. An advantage in the joint grouping of geometrically connected clusters or antennas may be that they can be assigned symmetric activation patterns as in the examples of FIGS 6A and 6B through the arrows 608 . 610 . 660 - 666 indicated. Thus, because adjacent transmit antennas of the different groups are not activated simultaneously, the presence of transmit / receive pairs that are susceptible to antenna crosstalk can be minimized.

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

In the embodiments described here, the groups defined on a total aperture are the same size, i. the groups each comprise the same number of clusters or transmit antennas. For an efficient measuring process, such a grouping is advantageous, but not absolutely necessary. If the groups are of different sizes, at certain times e.g. only a single-tone signal instead of a two-tone signal, or only a three-tone, two-tone, or single-tone signal instead of a four-tone signal; However, it can be achieved in these configurations, a measuring time gain.

7A illustrates an example of a spectral occupancy or distribution of the measurement points of two transmitter groups, ie the frequency vectors of these groups, eg. The two groups 604 and 606 out 6A , A transmitter group has exactly one frequency vector, conversely 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 uniform in a frequency range between 70 GHz and 80 GHz, ie arranged equidistantly; Thus, each successive transmission frequencies have a distance D_f of about 160 MHz. Such a frequency vector enables optimized axial resolution (SFCW), but frequency vectors by no means necessarily have to define equidistant frequency points; Advantages of the invention can also be realized with non-equidistant or only partially equidistant frequency points. If the frequency vector covers a measuring bandwidth of approximately 10 gigahertz, axial resolutions in the range of approximately 10 millimeters can be achieved.

The frequency vector Vf_2 of the group 606 also defines a sequence of 64 transmission frequencies f_200, f_201, f_202, etc. As shown 7A can be seen, the transmission frequencies are arranged equidistant from each other with a respective spectral distance of D_f in this example.

The frequency vectors Vf_1 and Vf_2 are slightly offset from each other with a frequency offset Δf. The frequency offset .DELTA.f is chosen so that the transmission frequencies of the two transmitter groups lead to orthogonal transmission signals, so that, for example, the transmission signals of the Transmission antennas of the group 604 with the transmission frequency f_100 orthogonal to the transmission signals of the transmission antennas of the group 606 with the transmission frequency f_200.

The frequency offset Δf can be chosen to be smaller than the spectral distance D_f of the measuring points of a frequency vector. The reason for this is discussed below. In the present example, the frequency offset Δf may, for example, be in a range between 10 MHz and 50 MHz. The result is a frequency grid which roughly corresponds to the pattern of transmission frequencies, as dictated by a single frequency vector, but at cluster points 702 . 704 . 706 in the spectrum two transmission frequencies of the groups are close together. If transmit signals with the frequencies of the cluster points are transmitted 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 which occupy the corresponding position in the two vectors Vf_1 and Vf_2, i. it is not absolutely necessary to be the transmitter frequencies of the accumulation points from which the orthogonal transmitter signals emerge; is important only that simultaneously emitted 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 emitted at the frequencies of one of the clustering points.

7B shows a further example of a spectral occupancy by a plurality of suitably staggered frequency vectors. Four frequency vectors are defined, for example for the transmitter groups 652 - 658 out 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 in a frequency range between 70 GHz and 80 GHz equidistant with a spectral distance D_f of about 160 MHz. The frequency vector Vf_1 can be defined identically, for example, as the vector Vf_1 7A ,

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

However, it is not absolutely necessary that, in the case of more than two frequency vectors, neighboring vectors are offset from each other by one and the same fixed amount. For example, one of the four frequency vectors in 7B eliminated. It is also important here only 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 dictated 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 appreciably smaller than the value of D_f; more precisely, if 2Δf in the case of 7A , or 4Δf in the case of 7B is smaller than D_f. If the respective frequency vectors are measured in the same way in the case of a plurality of transmitter groups, the transmit signals of the different transmitter groups transmitted at the same time have the frequencies at the spectral location of the cluster points, ie the orthogonality condition for the transmitter signals must be satisfied for the transmitter frequencies which make up a cluster point.

In addition to the orthogonality condition, the spectral width B_f of the cluster points, which in the example of FIG 7A a value of 2Δf, in the example of 7B has a value of 4Δf, so that B_f is less than or equal to a receiving bandwidth of the receiving antennas or the downstream signal processing, ie the receiving channels. Only in this case, a simultaneous reception of the simultaneously transmitted transmission signals is possible and thus the realization of a measuring time gain.

By occupying the entire measurement bandwidth with a frequency grid with accumulation points, which arise by each slightly offset from each other frequency vectors, the number of measurement points in the spectrum increases according to the number of provided transmitter groups. However, overall, the 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 it is measured at a given spectral point not only with a single measurement frequency, but with multiple measurement frequencies, the slightly against each other are offset; ie about 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 suitably offset from each other, namely such that the resulting transmit signals are orthogonal, thus can be efficiently separated from each other by appropriate signal processing and therefore can be transmitted simultaneously. In addition, a frequency offset between the vectors, much like the one in the 7A or 7B each exemplified offset .DELTA.f be selected so that it allows a parallel reception of the orthogonal signals with practically realizable hardware cost, as discussed below.

A signal processing for signal separation can be done in the digital part of the receiver as in 3 based on the backend component 304 illustrated. Here, after intermediate frequency conversion, the analog received signal in the ADC component 314 digitized with a sampling frequency f_sample or f_s ("sampling frequency"). A signal separation of the digitized signal is then in the component 316 carried out. For example, discrete Fourier transform (DFT) techniques may 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 application of OFDM methods for data transmission methods and corresponding communication systems and will not be dealt with here. It should merely be mentioned that OFDM signal separation methods do not require steep filter functions which require long measurement times and / or complex implementations, which would be contrary to the goal of the shortest possible measurement time and / or a desirably high integration density.

As an alternative to DFT methods, it is also possible, for example, to carry out a digital downwards mixing with downstream filtering in a filter bank in a series of parallel channels. The filter used here can be an averaging filter.

For efficient measurement data recording, the shortest possible sampling time or a filter with the lowest possible order should be used. The number N_sample of the required samples increases with the number N_carrier of the signals or carriers to be separated. For certain embodiments: N_sample = 2 · N_carrier + 2. (1)

For sampling one-tone signals, 4 samples are required here, but for 2-tone and 4-tone signals, 6 and 10 samples, respectively, are required; The required measuring times are extended accordingly. The measurement time gain by parallel transmission and measurement of orthogonal signals is thus canceled by the longer for the separation of multi-tone signals receiving time, but only partially. Thus, in a typical implementation for two-tone signals, the receive antennas need only be active for two sampling periods longer than for single-tone signals, which corresponds to an additional sampling time of about 20 nanoseconds at a sampling frequency of f_sample = 100 MHz.

Let's use the sample rate f_sample of the ADC component 314 predetermined intermediate frequency bandwidth are fully utilized, then applies to the already introduced above frequency offset: Δf = f_sample / N_sample. (2)

By way of example, the relationship between samples and the number of carrier signals according to Eq. (1) and a sample rate of f_sample = 100 MHz for the ADC component 314 accepted. For the two-tone operation of the transmitter groups 604 and 606 out 6A results in an approximate offset or frequency offset Δf = 16.67 MHz. Assuming a constant offset, and related to the spectral occupancy as in 7A at about the lowest transmitter frequency of the first transmitter group 604 at f_100 = 70.00000 GHz, so should the lowest station frequency of the second station group 606 at f_200 = 70.01667 GHz, so that f_100 and f_200 are both orthogonal to each other and can be received in parallel.

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

For the four-tone operation of the 6B and 7B For example, the value of D_f can be selected as above, but the frequencies at the cluster points are 752 . 754 . 756 only offset by Δf = 10 MHz. In this way, the corresponding transmitter signals can be received in parallel and at the same time orthogonal, ie efficiently separated from each other here.

8A FIG. 12 is a frequency diagram illustrating an IF spectrum of an orthogonal two-tone signal sampled at the frequency f_sample (f_s). references 802 . 804 . 806 and 808 denote carrier signals while the numeral 810 a DC component of the ADC component 314 designated. The carrier signals are in accordance with the above equation. (1) apart by Δf = f_sample / 6.

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

By way of illustration is in 8B furthermore a transfer function 814 of a digital filter, a sliding 6-tap averaging filter. The number of taps corresponds to the number N_sample of samples discussed above. For the filtering of quarter-tone signals, a 10-tap filter would be required.

In the spectral representation of 8B has the transfer function of the 6-tap filter zero at k · f_sample / 6. If the carrier signals are placed in the spectral distance of these zeros to each other, after the digital frequency conversion to the baseband each one carrier 816 while all others coincide with the zeroes of the transfer function, that is, be filtered out. In the example of 8B becomes the carrier signal 806 out 8A recorded, the carrier signal 808 is suppressed. To record the signal 808 a corresponding baseband transformation has to be made.

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

If orthogonal signals are emitted by a suitable choice of the frequency offset between the transmitter groups, signal separation of the signals reflected by the object is possible in an efficient manner. The frequency offset can optionally be chosen so that no increased or hardly increased demands are made on the bandwidth of the receiving channels. In this way it can be achieved that the complexity of the hardware for the signal processing only slightly increased, which also applies to the cost and the cost of a manageable integration density.

In the reception-side signal processing, a certain DFT or a certain filtering in the reception channel may prefer one of the signals while the other parallel signals are being suppressed. If a corresponding frequency offset of the parallel transmission signals is selected for this purpose, the orthogonality condition can be realized in this way.

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

Theoretical explanations by the invention effected technical effects are not intended to limit the scope of the invention. Also, the invention is not limited to the embodiments described herein and the aspects highlighted therein; Rather, within the scope given by the appended claims a variety of modifications are possible, which are within the scope of expert action. In particular, the person skilled in certain combinations of features described separately above as appropriate or advantageous.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102011005145 A1 [0006]

Claims (15)

Method for mapping 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 transmit antennas ( 104 ), wherein each of the plurality of transmitting antennas ( 104 ) transmits on its own transmission frequency and the transmission frequencies are selected so that the transmission signals transmitted thereon are mutually orthogonal; Receive ( 404 ) from the object of reflected signals by a plurality of receiving antennas ( 106 ); and determining ( 406 ) an image of the object based on that of the receiving antennas ( 106 ) received signals. The method of claim 1, wherein each of the transmit antennas ( 104 ) transmits temporally successive signals according to its own frequency vector, and wherein the frequency vector defines a plurality of transmission frequencies (f_100, f_101, f_102). Method according to claim 2, wherein the transmission antennas are different groups ( 604 . 606 ) and each group ( 604 . 606 ) is assigned a separate frequency vector from transmit antennas.  The method of claim 2 or 3, wherein each frequency vector comprises a sequence of transmission 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 method of claim 4, wherein the frequency offset Δf is selected so that signals of different frequency vectors are within a reception bandwidth (B_f).  The method of claim 4 or 5, wherein for separating the frequency offset signals Δf the signals received by the receive 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 transmit antennas.  The method of claim 6, wherein the frequency offset Δf is chosen such that Δf = f_sample / N_sample, wherein f_sample indicates a sampling frequency of the signals received by the receiving antennas.  Method according to one of the preceding claims, wherein a digital signal processing of the received signals comprises a digital Fourier transformation, or a digital down-conversion and application of an averaging filter.  The method of any 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.  The method of any one of the preceding claims, wherein the mapping of the object is determined based on a synthetic focus.  Method according to one of the preceding claims, wherein for determining the image of the object predetermined reception signals are hidden in order to minimize antenna crosstalk. A system for imaging an object, comprising: a plurality of transmit antennas ( 104 ) for the simultaneous transmission of signals for object illumination in the microwave range or in the millimeter-wave range, wherein each of the plurality of transmit antennas ( 104 ) is used to transmit on its own transmission frequency and wherein the transmission frequencies are selected so that the transmission signals transmitted thereon 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 that of the receiving antennas ( 106 ) received signals. The system of claim 12, comprising a plurality of antenna clusters ( 120 . 602 ) each having a plurality of transmitting antennas ( 104 ), whereby all transmit antennas of an antenna cluster ( 120 . 602 ) exactly one transmitter group ( 604 . 606 ) are assigned with their own frequency vector. The system of claim 13, wherein the antenna clusters ( 120 ) each have a plurality of receiving antennas ( 106 ), and the processing unit ( 110 ) is formed, an image of the object based on the signals of the receiving antennas ( 106 ) regardless of their cluster affiliation. The system of claim 13, wherein the system ( 100 ) for the simultaneous reading and / or digitizing of measurement data of all reception channels of at least one antenna cluster ( 120 ) is trained.

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