Method and system for near field reconstruction in an indirect far field system

Abstract

The invention relates to a method and a system for near field reconstruction in an indirect far field system. The method comprises the steps of measuring a reference antenna (14) for orthogonal field components in a direct spherical near field system at a suitably geometry defined distance (R0), measuring the reference antenna (14) for orthogonal field components in an indirect far field system, and optimizing the orthogonal field components in the indirect far field system with respect to the orthogonal field components in the direct spherical near field system.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for near field reconstruction in an indirect far field system and a corresponding system, in particular for a Compact Antenna Test Range (CATR) implementing stable far field conditions in a compact test environment. BACKGROUND

[0002] Over The Air (OTA) measurements try to determine the performance of radio transmitters and receivers, e.g. mobile phones, in a test environment that closely mimics the conditions under which the device will be tested. For example, fifth generation millimeter wave (5G mmWave) devices operating above 24 GHz contain millimeter-sized antenna arrays or dipoles that become an integral part of the device module package. Therefore, the only way to characterize and test the performance of the antennas as part of the final product is by means of OTA measurements.

[0003] For example, US 8,750,354 B1 describes a method of scanning a phased array of digital beamforming in a near field by creating an open architecture method and the ability to perform measurements that replace analog reference points with digital reference points. SUMMARY

[0004] It is therefore an object of the present invention to provide a method and a system for near field reconstruction in an indirect far field system, in particular by combining an indirect far field method and a near field method in order to obtain the above mentioned advantages from both methods.

[0005] This object is achieved by the features of the independent claims. The dependent claims contain further improvements.

[0006] According to a first aspect of the present invention, a method for near field reconstruction in an indirect far field system is provided. The method comprises the steps of measuring a reference antenna for orthogonal field components in a direct spherical near field system at a suitably geometry defined distance, measuring the reference antenna for orthogonal field components in an indirect far field system, and optimizing the orthogonal field components in the indirect far field system with respect to the orthogonal field components in the direct spherical near field system. Thus, advantageously, a near field method and an indirect far field method are combined, thereby reconstructing a near field in an indirect far field system.

[0007] According to a first preferred implementation form of the first aspect of the present application, the method further comprises the step of applying a Fourier transform, thereby transforming the quadrature field components in the direct spherical near-field system at other distances. Advantageously, near-field measurements can be inferred using near- far algorithms and / or near-near algorithms, thereby significantly improving side lobe level accuracy.

[0008] According to a second preferred implementation form of the first aspect of the present application, the method further comprises the step of identifying an equivalent distance such that the quadrature field components measured in the indirect far-field system are closest to the quadrature field components in the transformed direct spherical near-field system. Advantageously, the equivalent distance at which the field is measured in the indirect far-field system is identified in a simplified manner in order to perform the near-field transformation.

[0009] According to a further preferred implementation form of the first aspect of the present application, the method further comprises the step of transforming the quadrature field components in amplitude and phase by utilizing the equivalent distance of the indirect far-field system. Advantageously, additional phase information enables the most accurate evaluation of the field pattern in amplitude, especially in the nulls and side lobes.

[0010] According to a further preferred implementation form of the first aspect of the present application, the method further comprises the step of calculating the near-field current of the reference antenna in the reactive near-field by forward and / or backward propagating the transformed amplitude and phase of the indirect far-field system. Advantageously, the near-field current obtained in the reactive near-field of the reference antenna allows identifying critical antenna design parameters, such as antenna design flaws, Specific Absorption Rate (SAR), etc.

[0011] According to a second aspect of the present application, a system for near-field reconstruction in an indirect far-field system is provided. The system comprises a measurement antenna, a measurement unit and a processing unit. In this context, the measurement unit is adapted to measure a reference antenna for quadrature field components in a direct spherical near-field system at appropriately geometry-defined distances. The measurement unit is further adapted to measure the reference antenna for quadrature field components in an indirect far-field system. Furthermore, the processing unit is adapted to optimize the quadrature field components in the indirect far-field system with respect to the quadrature field components in the direct spherical near-field system.

[0012] Hence, the reference antenna is characterized for each relevant field component (e.g. E θ 、 ) in a direct spherical near-field system at defined distances. An indirect far-field system is assumed, e.g. a CATR, in which the reference antenna is also characterized for a field component pattern. Hence, the near-field method and the indirect far-field method are advantageously combined, thereby reconstructing the near-field in the indirect far-field system.

[0013] According to a first preferred implementation form of the second aspect of the present application, the processing unit is further adapted to apply a Fourier transform to transform the orthogonal field components in the direct spherical near-field system at other distances. Thus, the field components measured at other distances can be transformed using a Near-Field to Far-Field (NF / FF) algorithm and / or a Near-Field to Near-Field (NF / NF) algorithm, e.g. E θ 、 The other distances are preferably not equal to the defined distance at which the reference antenna was previously characterized in the direct spherical near-field system. Advantageously, the NF / FF algorithm and / or the NF / NF algorithm can be used to extrapolate the near-field measurements, thereby significantly improving the side lobe level accuracy.

[0014] According to a second preferred implementation form of the second aspect of the present application, the processing unit is further adapted to identify an equivalent distance such that the orthogonal field components measured in the indirect far-field system are closest to the transformed orthogonal field components in the direct spherical near-field system. Advantageously, the equivalent distance at which the field is measured in the indirect far-field system is identified in a simplified manner in order to perform the near-field transformation.

[0015] According to a further preferred implementation form of the second aspect of the present application, the equivalent distance corresponds to an equivalent range length of the indirect far-field system. Advantageously, by knowing the equivalent range length of the indirect far-field system, additional power measurements, e.g. total radiated power, time-averaged power density, etc., can be easily performed.

[0016] According to a further preferred implementation form of the second aspect of the present application, the processing unit is further adapted to transform the orthogonal field components in amplitude and phase by utilizing the equivalent distance of the indirect far-field system. Advantageously, the additional phase information together with the amplitude enables the most accurate evaluation of the field pattern, especially in the nulls and side lobes.

[0017] According to a further preferred implementation form of the second aspect of the present application, the processing unit is further adapted to calculate the near-field current of the reference antenna in the reactive near-field by forward and / or backward propagating the transformed amplitude and phase of the indirect far-field system. Advantageously, the near-field current obtained in the reactive near-field of the reference antenna allows identifying critical antenna design parameters, e.g. antenna design flaws, Specific Absorption Rate (SAR), etc.

[0018] According to a further preferred implementation form of the second aspect of the present application, the system further comprises a turntable adapted to rotate the reference antenna relative to the measurement antenna along at least two axes. Advantageously, the field components, e.g. E θ 、

[0019] According to a further preferred implementation form of the second aspect of the present application, the system further comprises a reflector, preferably a shaped reflector. This reflector provides the means to convert the spherical phase front to and / or from the measurement antenna to a planar phase front close to the aperture. This approach advantageously allows to create a compact range of planar wave fields at a distance much shorter than required under the traditional far field criterion.

[0020] According to a further preferred implementation form of the second aspect of the present application, the system further comprises a positioning unit adapted to control the rotation of the turntable and the spatial position of the reflector. Advantageously, the measurement accuracy is further improved.

[0021] According to a further preferred implementation form of the second aspect of the present application, the system further comprises an anechoic chamber enclosing the reference antenna, the measurement antenna, the turntable and the reflector. Advantageously, the anechoic chamber's standing wave suppression capabilities greatly improve the OTA measurement reliability. BRIEF DESCRIPTION OF DRAWINGS

[0022] Example embodiments of the present application will now be further explained by way of example only, and without limitation, with reference to the accompanying drawings. In the drawings:

[0023] Figure 1 An antenna radiation area in free space is shown by way of example,

[0024] Figure 2a A block diagram of a far field simulation from a near field measurement is shown by way of example,

[0025] Figure 2b A block diagram of a far field measurement in a CATR environment is shown by way of example,

[0026] Figure 3 A flow chart of an exemplary implementation of the inventive method according to the first aspect of the present application is shown,

[0027] Figure 4 A first exemplary implementation of the system according to the second aspect of the present application is shown,

[0028] Figure 5 A second exemplary implementation of the system according to the second aspect of the present application is shown, and

[0029] Figure 6 An exemplary block diagram of a near field simulation from a far field measurement according to the second aspect of the present application is shown. DETAILED DESCRIPTION

[0030] Reference will now be made in detail to the embodiments of the application, examples of which are illustrated in the accompanying drawings. The following embodiments of the application are described in order to explain the present application to persons skilled in the art. Therefore, the following embodiments of the present application are not limited to the only ones disclosed below and many modifications are possible to those skilled in the art based on the technical concepts of the present application.

[0031] As Figure 1 shown, an antenna transmitting a Radio Frequency (RF) signal radiates ElectroMagnetic (EM) waves into free space, where the EM field characteristics change according to the distance from the radiating antenna. The antenna radiation area is generally divided into two areas: a near-field area and a far-field area.

[0032] The near-field area is the area next to the antenna, which is defined by the following distance relationship:

[0033]

[0034] wherein,

[0035] R NF is the near-field measurement distance from the antenna,

[0036] D is the maximum linear dimension of the antenna, and

[0037] λ is the wavelength of the EM wave.

[0038] However, in this area, the field is somewhat unpredictable and ambiguous due to the overlap and mixing of the reactive field of the antenna.

[0039] The far-field area starts from the far-field boundary, which is defined by the following distance relationship:

[0040]

[0041] In this area, the EM field is dominated by the radiated field, which is mutually orthogonal and orthogonal to the propagation direction (as the propagation direction of a plane wave). Generally, the OTA measurement is based on the far-field area measurement, because the antenna is often used to transmit signals at a long distance (considered to be within the far-field area). However, the measurement in the far-field area requires that the distance from the antenna must be much larger than the size of the antenna and the wavelength. Therefore, a direct far-field measurement setup requires a large test environment, and the Side Lobe Level (SLL) accuracy is severely limited due to the physical phenomena of propagation.

[0042] The near-field measurement enables the antenna probing within the radiated near-field area. For example, U.S. Patent No. 8,750,354B1 shows an open architecture design of a digital near-field test system for near-field testing phased array antennas, which allows the capability of testing with components using individual phased array antennas in combination with a near-field scanner probe.

[0043] One way to simulate far field conditions is to measure the antenna in the near field region and transform the near field electric field measurements to far field components. As shown in Figure 2a The near field to far field transform (NF / FF) test method calculates performance metrics defined for the far field by using a mathematical near field to far field transform 2. Thus, first the near field beam pattern 1 of the antenna radiation is measured. Next, these measurements are translated into far field metrics 3 using the mathematical near field to far field transform 2.

[0044] Although the above NF / FF transform test method achieves a higher SLL accuracy, the near field test setup is more complex than a direct far field measurement and is very limited in its application to OTA measurements, because the method requires phase information in addition to the amplitude, increasing the complexity of measuring modulated signals.

[0045] Another way to create stable far field conditions is the indirect far field method, which creates a far field environment inside a test chamber by means of e.g. a parabolic reflector, a Fresnel lens, etc. using a transformation. As shown in Figure 2b The CATR test environment utilizes a parabolic reflector 8 in order to linearize the radiated signal 7 sent to the Device Under Test (DUT) 4. In case the DUT 4 radiates a wave front 9 to the reflector 8, the reflector 8 subsequently makes the radiated spherical wave front parallel into the feed antenna 6. The distance between the DUT 4 and the feed antenna 6 is sufficient to make the outgoing spherical wave almost planar in front of the DUT 4 on its way from the DUT 4 to the feed antenna 6. The DUT 4 is usually mounted on a low scattering mount 5.

[0046] Although the indirect far field method is more complex than the direct far field method, the indirect far field method can be performed in a more compact environment and can advantageously achieve good far field conditions with a higher SLL accuracy. However, in contrast to the direct far field method, it is not possible to simulate near field conditions from the indirect far field measurement, since the far field distance is considered as the focal length, i.e. the distance between the feed antenna 6 and the reflector 8. In other words, the near field simulation requires the correct knowledge of the equivalent distance at which the measurement field is taken, which is not available from the indirect far field measurement.

[0047] Thus, the indirect far field measurement method, like the CATR, and the near field measurement method are thus mutually exclusive. The indirect far field method in fact allows to create good far field conditions in a compact environment and also allows to measure the typical quantities seen in OTA measurements with good accuracy by field amplitude measurements only. In contrast, the near field measurement method cannot cover all types of OTA measurements, because they require the measurement of the phase of the electric and / or magnetic field components.

[0048] However, if phase can be measured, near-field measurement methods can be highly advantageous. For example, using near-field to far-field transformation algorithms, measurements taken near the DUT can be accurately extrapolated to infinity and produce the most accurate far-field pattern assessments, especially at nulls and sidelobes. Furthermore, near-field to near-field transformation algorithms can be used, and the field measured on the surface surrounding the DUT can be backpropagated towards the DUT, allowing, for example, observation of near-field currents to identify antenna design flaws. Backpropagation is also beneficial for power density assessment, enabling evaluation of near-field human exposure in millimeter-wave units.

[0049] exist Figure 3 The diagram illustrates a flowchart of an exemplary embodiment of the method according to a first aspect of the invention. In a first step 100, a reference antenna is used to measure the orthogonal field components in a direct spherical near-field system at a geometrically defined distance. In a second step 101, a reference antenna is used to measure the orthogonal field components in an indirect far-field system. In a third step 102, the orthogonal field components in the indirect far-field system are optimized relative to the orthogonal field components in the direct spherical near-field system.

[0050] In addition, the method of the present invention may also include the following steps: applying Fourier transform to transform the orthogonal field components in the direct spherical near-field system at other distances.

[0051] The method may be more advantageous if it further includes the step of identifying equivalent distances such that the orthogonal field components measured in the indirect far-field system are closest to the orthogonal field components in the transformed direct spherical near-field system.

[0052] Furthermore, the method according to the first aspect of the invention may also include the step of transforming the orthogonal field components according to amplitude and phase by utilizing the equivalent distance of the indirect far-field system.

[0053] In addition, the method of the present invention may also include the following steps: calculating the near-field current of the reference antenna in the near-field by means of the transformed amplitude and phase of the forward and / or reverse propagation indirect far-field system.

[0054] exist Figure 4 The image shows a first exemplary embodiment of a system 10 according to a second aspect of the invention. The system 10 focuses on performing near-field measurements for two orthogonal field components E. θ , Characterizing reference antenna 14. Measurement unit 12 measures the antenna 11 at a properly defined distance (e.g., such as...). Figure 1The reference antenna 14 is measured at a distance R0. For post-processing the measurement data, the measurement unit (Meas) 12 is further connected to a processing unit (Proc) 13. The connection between the measurement unit 12 and the processing unit 13 does not necessarily have to be a wired connection, a wireless connection in the form of a Wireless Local Area Network (WLAN), a network-based interface, etc. are also feasible. In this context, the processing unit 13 is adapted to apply a Fourier transform, thereby transforming the orthogonal field components E θ 、 wherein

[0055] R≠R0

[0056] The processing unit 13 advantageously stores the measured and inferred field components for all distances R, R0.

[0057] The reference antenna 14 is typically driven by an antenna drive circuit 18, which is placed in close proximity to the reference antenna 14 in order to reduce insertion loss and signal attenuation. The reference antenna 14 and the antenna drive circuit 18 can be collectively referred to as a device under test (DUT) 15, such as a mobile phone, a radio set, a short-range device, etc. The DUT 15 is located on a turntable 16, which is rotatable along at least two axes by a positioning unit (POS) 17. The positioning unit 17 can be controlled externally, e.g. by control commands inputted and / or generated from the processing unit 13.

[0058] The system 10 further comprises an anechoic chamber 19, which encloses the measurement antenna 11, the reference antenna 14, the DUT 15 and the turntable 16.

[0059] A second exemplary embodiment of a system 20 according to the second aspect of the present application is shown in Figure 5 The system 20 differs from the system 10 of Figure 4 in that the system 20 comprises a shaped reflector 21 in order to perform far-field measurements of the reference antenna 14 for both orthogonal field components E θ 、 and the field pattern. The shaped reflector 21 is preferably a parabolic reflector, which is located within the anechoic chamber 19 and enables the indirect far-field method to efficiently measure the far-field pattern of the DUT 15. In this context, the positioning unit 17 is further adapted to control the spatial position of the shaped reflector 21. Here, the processing unit 13 also stores the far-field measurement data.

[0060] A third exemplary embodiment of a system 20 according to the second aspect of the present application is shown in Figure 6An exemplary block diagram of near-field simulation from far-field measurements according to a second aspect of the invention is shown. Since processing unit 13 has already obtained near-field and far-field measurement data, optimization of far-field measurement data 22 is performed relative to near-field measurement data 23 based on a specific distance (e.g., using the L2 norm). The optimization algorithm can be defined based on the following inverse Fourier transform:

[0061] f x,y =A∫∫E x,y e -jk.r dxdy

[0062] in,

[0063] A is the amplitude, and

[0064] e -jk.r Let be the phase, which is a function of the distance r.

[0065] Therefore, the distance r can be varied until the result matches a near-field measurement for a specific distance (any location between R and R0). Thus, the equivalent distance of the indirect far field can be identified, for example, as... Figure 1 The distance R shown eq .

[0066] Once the processing unit 13 identifies the equivalent distance R of the indirect far field eq Then the field components can be transformed according to amplitude and phase, and the field components can be propagated forward or backward to any other surface using the following near-field transformation algorithm:

[0067] f x,y =A∫∫E x,y e +jk.r dxdy

[0068] Of particular advantage is that the simplified near-field transformation algorithm is sufficient to calculate the near-field current of DUT 5 in the reactive near-field and can also calculate SAR values ​​to assess near-field human exposure in millimeter-wave units. In contrast, conventional near-field probes disrupt the field around DUT 15 and undesirably introduce measurement errors.

[0069] Embodiments of the present application can be realized by hardware, software, or any combination thereof. Various embodiments of the present application can be realized by one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), processors, controllers, micro-controllers, microprocessors, etc.

[0070] While various embodiments of the present application have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous modifications to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the application. Therefore, the breadth and scope of the present application should not be limited by any of the above described embodiments, but should be defined in accordance with the following claims and their equivalents.

[0071] While the present application has been illustrated and described in relation to one or more implementation, various equivalents changes and modifications have been suggested by the preceding description, and these equivalents changes and modifications are within the spirit and scope of the present application. In addition, while a particular feature of the present application can have been disclosed with respect to only one of multiple implementations, such feature can be combined with one or more other features of the other implementations as can be desired and advantageous for any given or particular application.

Claims

1. A method for near field reconstruction in an indirect far field system, the method comprising the steps of: measuring a reference antenna (14) for orthogonal field components in a direct spherical near field system at a well-defined distance (R0), measuring the reference antenna (14) for orthogonal field components in an indirect far field system, and optimizing the orthogonal field components in the indirect far field system with respect to the orthogonal field components in the direct spherical near field system, wherein the optimizing step comprising the steps of: applying a Fourier transform, thereby transforming the orthogonal field components in the direct spherical near field system at other distances, and identifying an equivalent distance (R eq ) such that the quadrature field components measured in the indirect far field system are closest to the quadrature field components in the transformed direct spherical near field system.

2. The method according to claim 1, wherein, The method further comprises the step of transforming the quadrature field components in amplitude and phase by utilizing the equivalent distance (R eq ) of the indirect far field system.

3. The method according to claim 2, wherein the method further comprising the step of calculating the near field current of the reference antenna (14) in a reaction near field by forward and / or backward propagating the transformed amplitudes and phases of the indirect far field system.

4. A system (10, 20) for near field reconstruction in an indirect far field system, the system comprising: a measurement antenna (11), a measurement unit (12), and a processing unit (13), wherein the measurement unit (12) is adapted to measure a reference antenna (14) for orthogonal field components in a direct spherical near field system at a well-defined distance (R0), wherein the measurement unit (12) is further adapted to measure the reference antenna (14) for orthogonal field components in an indirect far field system, and wherein the processing unit (13) is adapted to optimize the orthogonal field components in the indirect far field system with respect to the orthogonal field components in the direct spherical near field system, wherein, for the optimization, the processing unit (13) is further adapted to: apply a Fourier transform, thereby transforming the orthogonal field components in the direct spherical near field system at other distances, and identifying an equivalent distance (R eq ) such that the quadrature field components measured in the indirect far field system are closest to the quadrature field components in the transformed direct spherical near field system.

5. The system according to claim 4, wherein, The equivalent distance (R eq ) corresponds to an equivalent range length of the indirect far-field system.

6. The system according to claim 4, wherein The processing unit (13) is further adapted to transform the quadrature field components in amplitude and phase by using the equivalent distance (R eq ) of the indirect far field system.

7. The system according to claim 6, wherein the processing unit (13) is further adapted to calculate the near field current of the reference antenna (14) in a reaction near field by forward and / or backward propagating the transformed amplitudes and phases of the indirect far field system.

8. The system according to any one of claims 4 to 7, wherein, the system (10, 20) further comprising a turntable (16) adapted to rotate the reference antenna (14) with respect to the measurement antenna (11) along at least two axes, and / or wherein the system (10, 20) further comprises a reflector (21).

9. The system according to claim 8, wherein the system (10, 20) further comprising a positioning unit (17) adapted to control the rotation of the turntable (16) and the spatial position of the reflector (21).

10. The system according to claim 8, wherein the system (10, 20) further comprising an anechoic chamber (19) enclosing the reference antenna (14), the measurement antenna (11), the turntable (16) and the reflector (21).

11. The system of claim 8, wherein the reflector (21) is a shaped reflector.

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