Method for self-calibration of radar systems.

Abstract

The present invention relates to a method for self-calibration of a radar system (RS) comprising at least two antenna groups, each assigned with at least one transmit channel and at least one receive channel. First, the radar system (RS) measures (10) a number of targets. Then, for each antenna group, range and Doppler information processing (11) and target detection (12) are performed to obtain, for each target, a reflection list (L) with complex amplitudes. Then, for each channel, compensation (20) for amplitude differences is performed. Then, by two-dimensional linear regression (21), a regression plane is estimated that minimizes the mean-square distance of the phase measurements of the channels of the antenna group. For each channel, the difference between the measured phase value and the regression plane is calculated to obtain an intra-group phase correction value (K intra ) is obtained. Furthermore, the distance between two regression planes of different antenna groups is calculated modulo 2π (25) to obtain the inter-group phase correction value (K inter ) is obtained. Finally, the intra-group phase correction value (K intra ) and the intergroup phase correction value (K inter ) are used to compensate (26) the control vectors for each channel.

Description

[Technical Field]

[0001] The present invention relates to a method for self-calibration of a radar system comprising at least two antenna groups, to which at least one transmit channel and at least one receive channel are assigned. [Background technology]

[0002] Radar systems are used to measure the range, relative velocity, and azimuth and elevation angles of objects. Angle estimation typically involves a group of antennas, also known as an antenna array, which can be used for both transmission and reception. Digital beamforming captures the angle of incidence of reflected plane waves generated by targets at long range. For beamforming, the phase difference of the reflected waves is evaluated across multiple receive channels. The received signals of the antenna group can be processed by a transformation as if they were measured by a virtual receiver. The evaluation is traditionally performed using previously stored control vectors, which can be applied in various ways. For example, in a Bartlett beamformer, expected phase differences for various angles of incidence are stored in control vectors, which are then correlated. Alternatively, model-based estimation can also be performed based on the control vectors.

[0003] Conventionally, control vectors are measured over angles (one or more angular sections) for each individual sensor during a single end-of-line calibration and then stored in non-volatile memory, taking into account phase errors (phase offsets) that may occur due to various effects, such as manufacturing tolerances of the antenna or feed lines, or the interaction of electromagnetic waves with the radome, housing, or circuit board.

[0004] Antenna groups also allow the surrounding environment to be divided into angular cells that can be acquired separately from each other, allowing targets of the same range and radial velocity to be separated from each other by their angular position, where each radar beam of the radar system becomes a spatial filter.

[0005] Special functions in vehicle automation require better angle measurements in terms of both accuracy and resolution. To achieve this, antenna groups with large apertures are used. To control the corresponding transceivers, transceivers are typically grouped and cascaded, resulting in a transceiver hierarchy. To perform coherent measurements of the incidence angle of the reflected wave, all transceivers are preferably connected to a reference oscillator. For this purpose, high-frequency lines are typically used to distribute the reference oscillator signal to the transceivers. However, the larger the aperture, the longer the high-frequency lines become.

[0006] Currently, patch antennas are typically used as radar antennas in vehicles. They are attached to the surface of a circuit board and powered by a stripline. The size of the circuit board increases proportionally to the size of the opening. Therefore, larger circuit boards increase the likelihood of uneven temperature distribution on the board due to internal and / or external influences. Examples of internal influences include strong heating of various component groups. Examples of external influences include partial shielding of the circuit board and heating by adjacent component groups and / or air currents.

[0007] Factors such as uneven temperature distribution and component aging affect radar system measurements by changing the previously calibrated phase error. If angle estimation is subsequently performed using the initially stored control vector, the performance of the angle estimation will be degraded. In particular, the side lobes in the angle spectrum will increase, the main lobe will be weakened and broadened, and the position of the main lobe will change (beam pointing error). As a result, the dynamic range, accuracy, and resolution of the angle estimation will be reduced.

[0008] Self-calibration of radar systems is known from M. Harter et al., "Error analysis and self-calibration of a digital beamforming radar system," 2015 IEEE MTT-S International Conference on Microwaves for Intelligent Mobility (ICMIM), 2015, pp. 1-4. Here, it is assumed that the wavefront of the reflected wave at the antenna is flat. This assumption is only satisfactorily met when the target is in the far field. The far-field boundary is typically defined by Equation 1, which relates the aperture length D and wavelength λ.

[0009]

number

[0010] Assuming the target is in the far field, a one-dimensional linear regression is performed on the received phases of the antenna elements of the receiving antenna group, thereby estimating the most likely wavefront profile. The phase error to be corrected is then calculated from this. The estimated phase error from the target is applied to each target angle without restriction of generality. It is irrelevant whether the radar system has a radome or is located, for example, behind a bumper.

[0011] A further prerequisite for self-calibration according to the above document is that the antenna elements are equally spaced along one dimension. Here, the following arrangement is given: to calibrate the azimuth control vector, the antenna elements are arranged horizontally, and to calibrate the elevation control vector, the antenna elements are arranged vertically.

[0012] In general, self-calibration can be used for a variety of modulation methods. Today, typical transmission frequencies are 24 GHz or 77 GHz, and the maximum demonstrable bandwidth is less than 4 GHz, typically around 0.5 GHz.

[0013] Today's radar systems in the automotive field typically use FMCW modulation (Frequency Modulated Continuous Wave Radar) with a fast ramp (rapid chirp modulation), in which multiple linear frequency ramps of the same slope are sent out sequentially. Mixing the current transmitted signal with the received signal generates a low-frequency signal whose frequency (called the beat frequency) is proportional to the range. Systems are typically designed so that the beat frequency component caused by the Doppler frequency is negligibly small. The range information obtained from the beat frequency is nearly unique, and the Doppler shift can then be determined by observing the temporal evolution of the phase of the complex range signal over the entire ramp. Range and velocity determinations are performed independently of each other, typically using a two-dimensional Fourier transform. The angle estimation described above is performed downstream of the range and velocity estimations. [Prior art documents] [Non-patent literature]

[0014] [Non-Patent Document 1] M. Harter et al. “Error analysis and self-calibration of a digital beamforming radar system” 2015 IEEE MTT-S International Conference on Microwaves for Intelligent Mobility(ICMIM), 2015, pp.1-4 Summary of the Invention [Means for solving the problem]

[0015] A method for self-calibration of a radar system (RS) is proposed, which includes at least two antenna groups, each assigned with at least one transmit channel and at least one receive channel. The geometry of the antenna groups, i.e., the locations of the transmit and receive antennas, is known. Furthermore, the transceiver hierarchy of the transceivers used to form the antenna groups, whose elements are subject to similar phase errors, is known. These data are part of the radar system specification.

[0016] First, multiple targets are measured by the radar system. To do this, a group of transmitting antennas sends out electromagnetic waves in the direction of the targets. The electromagnetic waves reflected from the targets are then received by a group of receiving antennas and evaluated using digital beamforming. The measurement is performed using an angular resolution method, where the surrounding environment is divided into angular cells by the group of antennas.

[0017] For each antenna group, range and Doppler information is processed (also called range-Doppler processing). This processing is preferably performed by two-dimensional fast Fourier transformation, but can also be performed by other algorithms known per se. Target detection within the angular cells is then performed. For this purpose, the following steps are preferably performed: for each angular cell, the energy is calculated. Furthermore, a threshold for the estimated noise energy of such an angular cell is estimated. The calculated energy is compared with the noise energy threshold, so that the calculated energy in each cell is distinguished from the estimated noise energy. If several adjacent angular cells exceed the threshold for the estimated noise energy, a maximum value of the measured complex data is determined for each transmitter-receiver combination. The above steps result in a reflection list, in which for each target, the range, relative velocity, and complex amplitude measured on each virtual channel are stored.

[0018] First, for each channel, the amplitude difference is compensated for. To this end, the following steps are preferably performed: The average received power over all signal amplitudes is calculated, where Equation 2 is used in particular.

[0019]

number

[0020] where:

[0021]

number

[0022] represents the mean amplitude,

[0023]

number

[0024] represents the individual complex amplitudes measured, which are summed over m = 1 M transmitters and n = 1 N receivers. Then, for each channel, the deviation of the amplitude relative to the average received power is calculated. In particular, the deviation is calculated by the quotient according to Equation 3:

[0025]

number

[0026] This deviation is then used to compensate for amplitude errors for each channel. This step allows for calibration of the received power. Subsequently, an intra-group calibration is performed for each antenna group. For this, a two-dimensional linear regression is performed for each antenna group. Here, a regression plane is estimated in which the mean square distance of the phase measurements of the channels of the antenna group is minimized. As mentioned at the beginning, the two-dimensional regression uses the antenna group geometry data, which is known as part of the radar system specifications. Here, for each channel, the difference between the measured phase value and the regression plane is calculated. This results in an intra-group phase correction value for the respective antenna group. Now, this step is repeated for all antenna groups, thereby resulting in an intra-group phase correction value for all (virtual) channels.

[0027] If the phase error is not angle-dependent, for example, if the phase field is based on the aging of the radar system, measurements of all targets can be averaged for each channel. This results in an average gain. If the phase error is angle-dependent, for example, if the radar system is located behind a bumper, each angle can be calibrated with respect to a specific target. Preferably, intermediate angles where there are no specific targets can be interpolated. Here, the angles where targets exist must be close enough to each other so that the sampling theorem holds for the phase error. By performing multiple measurement cycles in sequence, targets can be captured within angle cells that were previously only interpolated or where phase error estimation was not possible.

[0028] Next, inter-group calibration is performed for at least two different antenna groups to determine the phase error between the different antenna groups. For each antenna group, a regression plane is calculated as described above. The distance between the two regression planes of the two different antenna groups is then calculated, modulo 2π, to obtain an inter-group phase correction value for the two antenna groups. Once the regression planes are compensated with the inter-group phase correction value, all of the regression planes of the antenna groups overlap with each other.

[0029] Finally, the control vectors for each channel are compensated using the intra-group and inter-group phase correction values, after which angle estimation can be performed according to known methods.

[0030] Due to the two-dimensional regression, the method can generally be applied to all geometries of antenna groups and is not restricted in terms of orientation or distance. The antenna groups do not necessarily have to be arranged along a horizontal or vertical line, but can be freely positioned. Furthermore, the antenna elements do not have to be positioned equidistant from each other, but can be located at any distance from each other. Furthermore, the phase error for each channel does not have to be evenly distributed, and different average phase errors are possible for each antenna group. The method can also be used for multiple-input multiple-output radar sensors (MIMO).

[0031] Exemplary embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description. [Brief explanation of the drawings]

[0032] [Figure 1] 1 is a schematic diagram of an antenna of a radar system in which the method according to the invention is used; [Figure 2] 1 is a flow diagram of an embodiment of a method according to the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0033] 1 shows an antenna of a radar system RS (not shown in detail) to which the method according to the invention is applied. The antenna comprises a number of antenna elements Rx1, Rx2, Rx3, Rx4, Tx1, Tx2, Tx3, Tx4, which emit radar signals as transmit antenna elements Tx1, Tx2, Tx3, and Tx4 and receive reflected radar signals as receive antenna elements Rx1, Rx2, Rx3, and Rx4. This means that it is a MIMO (multiple input multiple output) antenna. Furthermore, transceivers T1 and T2 are provided, to which the antenna elements Rx1, Rx2, Rx3, Rx4, Tx1, Tx2, Tx3, and Tx4 are connected. The first system-on-chip SoC1 (also referred to as system-on-chip, or SoC) comprises a first transceiver T1, two transmit antenna elements Tx1 and Tx2 connected to the first transceiver T1, and two receive antenna elements Rx1 and Rx2 connected to the first transceiver T1. The second system-on-chip SoC2 comprises a second transceiver T2, two transmit antenna elements Tx3 and Tx4 connected to the second transceiver T2, and two receive antenna elements Rx3 and Rx4 connected to the second transceiver T2. The transmit antenna elements Tx1 and Tx2, together with the receive antenna elements Rx1 and Rx2 of the first system-on-chip SoC1, form a first antenna group, and the transmit antenna elements Tx3 and Tx4, together with the receive antenna elements Rx3 and Rx4 of the second system-on-chip SoC2, form a second antenna group. The two systems-on-chips SoC1 and SoC2 are symmetrically configured and identically designed. The antenna group geometry G is known and stored in the radar system RS specifications (see Figure 2). Furthermore, the transceiver hierarchy of the transceivers T1 and T2 of the radar system RS is known.

[0034] FIG. 2 shows a flow chart of an embodiment of the method according to the invention. First, the radar system RS performs measurements 10 of multiple targets in the surrounding environment using the antenna elements Rx1, Rx2, Rx3, Rx4, Tx1, Tx2, Tx3, and Tx4. The measurements 10 are performed using an angular resolution method, where the surrounding environment is divided into angular cells by the antenna groups. Range and Doppler information processing 11 is then performed using a Fast Fourier Transform. Target detection 12 within the angular cells follows. For each angular cell, the energy is calculated, and a threshold for the noise energy is also calculated. The calculated energy is then differentiated in each cell against the threshold for the estimated noise energy. If several adjacent angular cells exceed the threshold for the estimated noise energy, a maximum value of the measured complex data is determined for each transmitter-receiver combination.

[0035] For self-calibration, the amplitude differences of the respective channels are compensated 20. To this end, the average received power over all signal amplitudes is calculated according to equation 2.

[0036]

number

[0037] where:

[0038]

number

[0039] represents the mean amplitude,

[0040]

number

[0041] represents the individual complex amplitudes measured, which are summed over m = 1 M transmitters and n = 1 N receivers. Then, for each channel, the deviation of the amplitude relative to the average received power is calculated according to Equation 3.

[0042]

number

[0043] The deviation is then used to compensate 20 for amplitude errors for each channel. For each antenna group of the systems-on-chips SoC1, SoC2, a two-dimensional linear regression 21 is performed. Here, in each case, a regression plane is estimated that minimizes the mean square distance of the phase measurements of the channels of the antenna group. In the two-dimensional regression, the geometry data G of the antenna group is used. Here, for each channel, the difference between the measured phase value and the regression plane is calculated. This results in an intra-group phase correction value for the respective antenna group. If the phase error is angle-dependent, each angle is calibrated with respect to a unique target 22. Intermediate angles for which there is no unique target are interpolated 23. If the phase error does not have angle dependence, it is averaged over the measurements of all targets for each channel 24. This is then repeated for all antenna groups, thereby resulting in an intra-group phase correction value K for all (virtual) channels. intra is obtained.

[0044] Furthermore, the distance between the two calculated regression planes of the two antenna groups of the first system-on-chip SoC1 and the second system-on-chip is calculated 25, where modulo 2π is applied, thereby obtaining an inter-group phase correction value K for the two antenna groups. inter is obtained.

[0045] Finally, the intra-group phase correction value K intra and the inter-group phase correction value K inter Compensation 26 of the control vector for each channel is performed using After self-calibration, further evaluations such as angle estimation 13 can be performed from the reflection list L.

Claims

1. 1. A method for self-calibration of a radar system (RS) comprising at least two antenna groups each having at least one transmit channel and at least one receive channel assigned thereto, the method comprising: measuring (10) a plurality of targets by said radar system (RS); - for each antenna group, processing (11) the range and Doppler information of said targets and detecting (12) said targets to obtain for each target a reflection list (L) with complex amplitudes; Compensating (20) for amplitude differences for each channel; - estimating, using a two-dimensional linear regression (21), the regression plane that minimizes the mean square distance of the phase measurements of the channels of the antenna group; For each channel, the difference between the measured phase value and the regression plane is calculated to obtain an intra-group phase correction value (K ｉｎｔｒａ ) and The distance between two regression planes of different antenna groups is calculated modulo 2π (25) to obtain the inter-group phase correction value (K ｉｎｔｅｒ ) and The intra-group phase correction value (K ｉｎｔｒａ ) and the inter-group phase correction value (K ｉｎｔｅｒ ) to compensate (26) the control vector for each channel; A method characterized by:

2. The intra-group phase correction value (K ｉｎｔｒａ 2. The method of claim 1, wherein when calculating θ, the angular dependence of the phase error is not given and is averaged (24) over measurements of all targets for each channel.

3. The intra-group phase correction value (K ｉｎｔｒａ 2. The method of claim 1, wherein when calculating θ, ...

4. 4. The method of claim 3, wherein intermediate angles for which there is no specific target are interpolated (23).

5. Method according to any one of claims 1 to 4, characterized in that the processing (11) of the range and Doppler information is performed by means of a Fast Fourier Transform.

6. The detection (12) of the target discriminating the energy at each angular cell against a threshold on the estimated noise energy; determining a local maximum value if multiple adjacent angular cells exceed the threshold; The method according to any one of claims 1 to 5, characterized in that it is carried out by

7. The compensation (20) of the amplitude difference calculating the average received power; calculating the deviation of the amplitude of each channel from the average received power; compensating the amplitude in response to the deviation; The method according to any one of claims 1 to 6, characterized in that it is carried out by

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