Patent Description:
Radar systems become even more common in various use cases for person and cargo transport. While radar systems have been implemented into planes for a long time, the implementation of such radar systems in automotive applications becomes more and more important.

For example, those radar systems are used in driver assistance systems, such as lane change assistants, brake assistants or adaptive cruise controls. The radar systems are also used in automatic driving systems that are configured to control the vehicle at least partially in an automatic manner, in particular autonomously.

In order to test these radar systems, the device under test, e.g. the car comprising the radar system, usually is placed in a test chamber that comprises a radar target simulation system. The radar target simulation system simulates a radar target having a certain velocity and generates a corresponding radar signal that can be used to test characteristics of the radar system.

Typically, the radar system has a certain baseband bandwidth, which causes deviations of the actual radar signal frequency from the carrier frequency of the radar signal. Modern automotive radar systems may have a carrier frequency of about <NUM> and a bandwidth of about <NUM>, and the bandwidth may cause sizable errors in the simulation in this case.

<CIT> discloses a device for simulating at least one echo signal of a signal. The device comprises a manipulation unit that may add a delay to the signal, shift the frequency of the signal and/or change the amplitude of the signal.

<CIT> discloses a radar target simulation device. An electronic circuit of the radar target simulation device has an adjustable multiplication element with which a frequency change signal can be set in order to simulate a speed of a radar target to the radar device.

<CIT> discloses a radio-frequency receiver with a built-in self-test circuitry. The radio-frequency receiver may be an automotive radar receiver.

"<NPL>, describes the calculation of a Doppler frequency shift for a FMCW radar.

The object of the invention therefore is to provide a radar target simulation system as well as a method for operating a radar target simulation system that provides a precise simulation of radar targets regardless of the carrier frequency and bandwidth.

According to the invention, the problem is solved by a radar target simulation system according to claim <NUM>.

Generally speaking, the radar target simulation system according to the invention is configured to correctly simulate the effect of the Doppler, which is found to have two conceptionally different contributions. Hence, the radar target simulation system takes both conceptionally different contributions into account in order to correctly simulate the Doppler effect.

The first contribution depends on the carrier frequency of a radar signal, which is constant over time and which causes a Doppler frequency shift that only depends on the velocity of the radar target that is to be simulated. The effect of the first contribution is handled by the frequency shifting unit that appropriately adapts the frequency of the input signal by a frequency shift.

The second contribution depends on the baseband frequency of the radar signal, which varies over time. The invention is based on the finding that the effect of the second contribution can be described by a rescaling of the input signal in time domain and/or in frequency domain. Therefore, the effect of the second contribution is handled by the resampling unit by a rescaling of the input signal in time domain and/or in frequency domain.

Thus, the radar target simulation system according to the invention is capable of correctly simulating a moving radar target irrespective of the carrier frequency and the baseband frequency of a radar system of a device under test to be tested. This is due to the fact that both contributions to the Doppler shift are correctly treated by the radar target simulation system according to the invention.

In fact, the resampling unit is configured to rescale the input signal in time domain and/or in frequency domain based on the baseband frequency of the input signal and the velocity of the moving radar target that is to be simulated. This ensures that the different contributions to the baseband frequency of the radar signal varying over time are taken into account.

The digital to analog converter may convert at least <NUM> GS/s (<NUM> · <NUM><NUM> samples per second), preferably at least <NUM> GS/s, for example <NUM> GS/s.

Likewise, the analog to digital converter may have a sampling rate of at least <NUM> GS/s, preferably at least <NUM> GS/s, for example <NUM> GS/s.

The analog to digital converter and the digital to analog converter have the same sampling frequency. In other words, the rescaling of the input signal in time domain and/or in frequency domain is completely done by the resampling unit, and not by a differing sampling rate of the analog to digital converter and the digital to analog converter.

According to one aspect of the present invention, the resampling unit is configured to contract the input signal in time domain and/or to expand the input signal in frequency domain if the radar target that is to be simulated is approaching, and/or characterized in that the resampling unit is configured to expand the input signal in time domain and/or to contract the input signal in frequency domain if the radar target that is to be simulated is departing. Thus, the baseband frequency contribution to the Doppler shift is correctly accounted for in both cases, i.e. in the cases that the radar target to be simulated is approaching or departing from the device under test.

According to another aspect of the present invention, the delay unit is configured to delay the input signal via a first-in-first-out technique. The delay unit may therefore also be called "FIFO". The delay unit stores a predetermined number of sample points that are part of the digitized input signal and forward these sample points to the resampling unit in the same order as received, namely in the correct order. Thus, the digitized input signal is forwarded to the resampling unit unaltered with respect to the order, but with the predetermined time delay. The intermediate time interval between the sample points may be maintained. Put differently, all sample points are delayed by the same amount of time.

The delay unit may be configured to store a predetermined number of samples, wherein the predetermined number of samples is adjustable. This accounts for the fact that due to the rescaling of the digitized input signal the time length of the digitized input signal increases or decreases. Accordingly, the number of sample points used to describe a certain portion of the digitized input signal has to be adapted as well.

According to one embodiment of the present invention, the delay unit is configured to adjust the number of samples based on the velocity of the moving radar target that is to be simulated. It was found that the time after which the number of samples has to be adjusted only depends on the velocity of the radar target relative to the device on the test and on the speed of light. Thus, the number of samples is correctly adjusted based on the velocity of the radar target.

In a further embodiment of the present invention, the resampling unit is configured to continuously rescale the input signal. Thus, the input signal is not rescaled in time domain and/or in frequency domain by a bigger factor at once but rather continuously from a starting scale to a target scale.

According to a further embodiment of the present disclosure, at least a second digital processing channel is provided, wherein the second digital processing channel is connected to the analog to digital converter. The second digital processing channel comprises a second delay unit, a second resampling unit and a second frequency shifting unit. The second delay unit is configured to receive the digitized input signal from the analog to digital converter and to forward the input signal to the second resampling unit and/or to the second frequency shifting unit with a predetermined time delay. The second frequency shifting unit is configured to adapt the frequency of the input signal based on the carrier frequency of the input signal and based on a velocity of a second moving radar target that is to be simulated. The second resampling unit is configured to rescale the input signal in time domain and/or in frequency domain based on at least one of the momentary frequency of the input signal, the baseband frequency of the input signal, the carrier frequency of the input signal and/or the velocity of the second moving radar target that is to be simulated.

Accordingly, a second radar target can be simulated in the second digital processing channel, wherein the first radar target and the second radar target may have velocities that are different from each other. Thus, at least two radar targets having different velocities can be simulated with the radar target simulation system according to the invention.

Further digital processing channels may be provided, wherein each of the processing channels may be used to simulate one radar target. Thus, several radar targets having different velocities can be simulated with the radar target simulation system according to the invention.

Of course, at least some of the velocities may also be pairwise equal.

A merging unit may be provided, wherein the merging unit is connected to the processing channels and is configured to superpose output signals of the processing channels. The merging unit combines the individual signals of the digital processing channels that each resemble a specific radar target. Thus, the merged signal contains information on all of the radar targets that are simulated in the individual digital processing channels. Put differently, the merged signal relates to a superposed signal encompassing the individual signals of each processing channel.

According to one aspect of the invention, the frequency shifting unit is established as a variable numerically controlled oscillator. The output of the numerically controlled oscillator, also called NOC, is a complex valued function of the form exp[<NUM>πi fNCO · t], wherein fNCO is the frequency of the numerically controlled oscillator. Thus, multiplying the digitized input signal with the output of the numerically controlled oscillator results in frequency shift of the digitized input signal.

Particularly, if only one digital processing channel is provided, the frequency shifting unit may also be established as an analog frequency shifter.

According to the invention, the problem is further solved by a radar test system comprising a device under test and the radar target simulation system as described above, wherein the device under test comprises a radar system. Regarding the advantages and properties of the test system, reference is made to the explanations given above with respect to the radar target simulation system, which also hold for the radar test system and vice versa.

The device under test may be established as an automotive radar system. For example, the automotive radar system is part of a driver assistance system such as an adaptive cruise control, an emergency brake system or a lane-keeping assistant. The automotive radar system may also be part of an automatic driving system that is configured to control the vehicle at least partially in an automatic manner, in particular autonomously.

Alternatively, the device under test may be any other kind of vehicle having a radar system. For example, the device under test may be a plane, a ship or a utility vehicle such as a truck.

According to the invention, the problem is further solved by a method for operating a radar target simulation system according to claim <NUM>.

Regarding the advantages and properties of method for operating the radar target simulation system, reference is made to the explanations given above with respect to the radar target simulation system, which also hold for the method and vice versa.

For instance, the input signal is generated based on a radar signal that was received previously.

According to one aspect of the present invention, at least two moving radar targets are simulated, wherein the steps of adapting the frequency and rescaling the frequency of the input signal are performed for each of the radar targets that are to be simulated. Therein, each of the digital processing channels of the radar target simulation system may be used to simulate one radar target having a certain velocity, wherein the velocities of the radar targets may be different from each other.

Thus, the invention provides a method for operating a radar target simulation system that enables a simultaneous simulation of several different radar targets that each may have a different velocity.

<FIG> schematically shows a block diagram of a radar test system <NUM> comprising a device under test <NUM> and a radar target simulation system <NUM>.

The device under test <NUM> may be a motor vehicle having an automotive radar system <NUM>. For example, the automotive radar system <NUM> is part of a driver assistance system such as an adaptive cruise control, an emergency brake system or a lane-keeping assistant. The automotive radar system <NUM> may also be part of an automatic driving system that is configured to control the vehicle at least partially automatic, in particular autonomous.

Alternatively, the device under test <NUM> may be any other kind of vehicle having a radar system <NUM>. For example, the device under test may be a plane, a ship or a utility vehicle such as a truck.

In the shown embodiment, the radar target simulation system <NUM> comprises a front end <NUM> having at least one antenna <NUM>, an analog to digital converter <NUM>, several digital processing channels <NUM>, a merging unit <NUM> and a digital to analog converter <NUM>.

In general, the radar target simulation system <NUM> may comprise only one or two digital processing channels <NUM>. In the following, the general case of m digital processing channels <NUM> is described, wherein m is a positive natural number.

Each of the digital processing channels <NUM> is connected to the analog to digital converter <NUM> downstream of the analog to digital converter <NUM>.

Therein and in the following, the terms "downstream" and "upstream" denote the direction of propagation of an electric signal, wherein the electric signal propagates from the upstream component to the downstream component. Thus, in the case above, the electrical signal propagates from the analog to digital converter <NUM> to the digital processing channels <NUM>.

Moreover, each of the digital processing channels <NUM> is connected to the merging unit <NUM> upstream of the merging unit <NUM>.

The digital processing channels <NUM> each comprise a delay unit <NUM>, a resampling unit <NUM> and a frequency shifting unit <NUM>.

In each digital processing channel <NUM>, the delay unit <NUM> is connected to the analog to digital converter <NUM> downstream of the analog to digital converter <NUM>. The resampling unit <NUM> is connected to the delay unit <NUM> downstream of the delay unit <NUM>. The frequency shifting unit <NUM> is connected to the resampling unit <NUM> downstream of the resampling unit <NUM> and the frequency shifting unit <NUM> is connected to the merging unit <NUM> upstream of the merging unit <NUM>.

Alternatively, the positions of the resampling unit <NUM> and of the frequency shifting unit may be interchanged such that the frequency shifting unit <NUM> is interconnected between the delay unit <NUM> and the resampling unit <NUM>.

The functionality of the individual components will be described in more detail below.

Generally speaking, the radar test system <NUM> is configured to test the radar system <NUM> of the device under test <NUM>.

For this purpose, the radar target simulation system <NUM> is configured to receive a radar signal from the radar system <NUM>. The radar target simulator system <NUM> then simulates at least one, in particular several moving targets each having a certain velocity with respect to the device under test <NUM>. Therein, each of the digital processing channels <NUM> is used to simulate one radar target having a certain velocity, wherein the velocities of the radar targets simulated may be different from each other.

Based on the simulation of the radar targets, the radar target simulation system <NUM> generates an appropriate simulated radar signal that has properties as if the radar signal generated by the device under test <NUM> were indeed reflected from the radar targets.

Then, the simulated radar signal may be evaluated on a computer and/or transmitted to the radar system <NUM> of the device under test <NUM> and can, e.g., be used to test characteristics of the radar system <NUM>.

Therein, the radar target simulation system <NUM> needs to correctly take into account the Doppler shift that is caused by the motion of the radar targets to be simulated relative to the device under test <NUM>.

More precisely, the Doppler shift ΔfDoppler is related to the momentary frequency fCW of the radar signal and to the magnitude of the relative velocity |vrel,i| of the i-th radar target, wherein i = <NUM>,<NUM>,. , m, by <MAT>.

Therein, c is the speed of light, wherein the "+" sign holds for a target approaching the device under test <NUM> and the "-" sign holds for a radar target departing from the device under test <NUM>.

In the following, the index "i" will be dropped in order to avoid a clutter of indices and in order to increase legibility. However, it is implicitly assumed that the radar targets to be simulated may have velocities that are different from each other.

Usually, automotive radar systems send out a radar signal having a baseband frequency f that is varying over time and that is modulated on a carrier having a carrier frequency f<NUM> being constant over time. The baseband frequency f typically is much smaller than the carrier frequency f<NUM>, i.e. f « f<NUM>. For example, f<NUM> ≥ <NUM> f.

Equation (E. <NUM>) can be rewritten to read <MAT>.

Accordingly, the Doppler frequency shift has two components. The first component, which will be denoted as the leading order contribution ΔfDoppler,LO in the following, is due to the first term on the right hand side of equation (E. <NUM>), i.e. <MAT>.

The second component, which will be denoted as the frequency dependent contribution ΔfDoppler(f) in the following, is due to the second term on the right hand side of equation (E. <NUM>), i.e. <MAT>.

As already mentioned above, the carrier frequency f<NUM> is usually much bigger than the baseband frequency f. However, neglecting the second term in equation (E. <NUM>) can cause non-negligible errors if high precision is aimed for and/or if the radar system <NUM> has a high bandwidth.

For example, there are automotive radar systems having a carrier frequency of f<NUM> = <NUM> and a bandwidth of about <NUM>. In this case, the first term in equation (E. <NUM>) is not negligible, as is shown in <FIG>.

<FIG> shows a plot of Doppler frequency error plotted against exact velocity and carrier frequency, wherein the Doppler frequency error is illustrated by means of level curves.

As can clearly be seen in <FIG>, the absolute value of the error rises with rising velocity of the radar target to be simulated. Moreover, the absolute error rises with rising deviation of the momentary radar frequency from the carrier frequency f<NUM> = <NUM> GHz.

Accordingly, in the excerpt of the radar frequency-velocity plane shown in <FIG>, the error is maximal for f = ± <NUM>GHz and vrel = ±<NUM> m/s. In this case, the error is bigger than <NUM>.

Analogously, <FIG> each show a diagram of a velocity error plotted against the exact velocity of the radar target to be simulated and the radar frequency. From <FIG> it is apparent that the absolute value of the velocity error rises with both rising absolute value of exact velocity and with rising absolute value of the deviation of the radar frequency from the carrier frequency f<NUM> = <NUM> GHz.

As can further be seen from <FIG>, the relative error increases with increasing deviation of the radar frequency from the carrier frequency f<NUM> = <NUM>. For example, for f = ±<NUM>GHz, the relative error is about <NUM>%.

Thus, in order to correctly simulate a moving radar target with high precision, the second term on the right hand side of equation (E. <NUM>) needs to be taken into account.

For this purpose, the radar test system <NUM>, more specifically the radar target simulation system <NUM> is configured to perform the method described in the following with reference to <FIG>.

First, a radar signal is generated by the device under test <NUM> or rather by the radar system <NUM> of the device under test <NUM> (step S1).

The radar signal is received by the front end <NUM> via the antenna <NUM>, wherein an input signal is generated by the front end <NUM> based on the received radar signal, which input signal is forwarded to the analog to digital converter <NUM> (step S2).

The input signal is then digitized by the analog to digital converter <NUM> (step S3). The input signal may be digitized with a sampling rate of at least <NUM> GS/s (<NUM> · <NUM><NUM> samples per second), preferably with a sampling rate of at least <NUM> GS/s, for example with a sampling rate of <NUM> GS/s.

Particularly, the sampling rate of the analog to digital converter <NUM> is constant over time.

The digitized input signal is then forwarded to each of the digital processing channels <NUM>, more precisely to each of the delay units <NUM> (step S4).

The delay units <NUM> receive the digitized input signal and forward the unaltered digitized input signal to the respective resampling unit <NUM> with a predetermined time delay (step S5).

The delay units <NUM> may each use a first-in-first-out technique (FIFO technique) to delay the digitized input signal. The delay units <NUM> may therefore also be called "FIFOs".

The delay units <NUM> store a predetermined number of sample points that are part of the digitized input signal and forward these sample points to the resampling units <NUM> in the same order as received. Thus, the digitized input signal is forwarded unaltered to the respective resampling unit <NUM>.

The digitized input signal is then rescaled in time domain and/or in frequency domain by the resampling unit <NUM> in order to account for the deviation of the radar signal frequency from the carrier frequency f<NUM>, i.e. to account for the baseband frequency f (step S6).

Therein, the resampling unit <NUM> may be established as a linear interpolator or as a polyphase finite impulse response filter.

Generally speaking, the frequency dependent Doppler shift of equation (E. <NUM>) is tantamount to a rescaling of the frequency axis by a factor of <MAT>, as can be seen from <MAT> wherein X(f) is a function describing the digitized input signal in frequency domain.

For the time domain function x(t) corresponding to the frequency domain function X(f), it is well-known that scaling the time axis with a factor α corresponds to a scaling of the frequency axis with a factor <NUM>/α. More precisely, it holds <MAT>.

Using the fact that due to the sampling of the digitized input signal the time t only takes discrete values t = n · T, wherein T = <NUM>/fS is the inverse of the sampling frequency fS, and further using the geometric series for small values of x, i.e. <NUM>/(<NUM> ± x) ≈ <NUM> ∓ x, one obtains the following equation: <MAT> and <MAT>.

In other words, scaling the frequency axis with a factor <MAT> corresponds to scaling the time axis with a factor <MAT>, as long as |vrel| « c, which is always the case in the use cases discussed above.

Accordingly, the resampling unit <NUM> rescales the frequency axis by a factor R± and/or the time axis by a factor R∓.

More precisely, each of the resampling units <NUM> is associated with one particular radar target that is to be simulated. Accordingly, each resampling unit <NUM> rescales the digitized input signal in time domain and/or in frequency domain with an appropriate scaling factor based on the velocity of the associated radar target.

Due to the rescaling of the time axis and/or of the frequency axis, the number of samples stored in the delay unit <NUM> needs to be adjusted (step S7), as is indicated by the dotted arrows in <FIG>.

In other words, the depth of the FIFO, i.e. the delay unit <NUM> is adjusted based on the respective velocity of the radar target that is to be simulated.

More precisely, the number of stored samples is increased if the respective radar target that is to be simulated departs from the device under test <NUM>. Analogously, the number of stored samples is reduced if the respective radar target approaches the device under test <NUM>.

Therein, the number of samples stored in the delay unit <NUM> is adjusted after Δn = c/(<NUM>|vrel|) samples. Thus, the number of samples after which the stores number of samples is adjusted is independent of the sampling frequency, but only depends on the velocity of the respective radar target that is to be simulated.

The combined effect of step S6 and step S7, i.e. the combined effect of continuously rescaling the digitized input signal in time domain and/or in frequency domain and adjusting the number of stores samples of Δn samples can be seen from the following equation: <MAT>.

As is apparent from the right hand side of equation (E. <NUM>), the input signal is contracted ("-" sign) or expanded ("+" sign) by one sample period time T after n = Δn = c/(<NUM>|vrel|) sample points.

As c/(<NUM>|vrel|) usually is a non-integer number, Δn may be rounded in order to determine the number of samples after which the number of samples stores in the delay unit <NUM> is increased or decreased.

In other words, one sample point is respectively removed from or added to the storage of the delay unit <NUM> in the appropriate moment when the input signal is contracted or expanded by one sample period time T.

The rescaled input signal is then forwarded to the frequency shifting unit <NUM>. Generally speaking, the frequency shifting unit <NUM> shifts the frequency of the rescaled input signal by a fixed amount, namely by ΔfDoppler,LO (step S8).

In principal, this can be achieved by any suitable method known from the state of the art.

Particularly, the frequency shifting unit <NUM> is established as a numerically controlled local oscillator having a frequency that is equal to the carrier frequency f<NUM> multiplied by <MAT>.

The output of the numerically controlled oscillator is a complex-valued signal <MAT>.

Multiplying the rescaled input signal with the output xNCO(t) of the numerically controlled oscillator results in a frequency shift of the rescaled input signal by <MAT>.

Thus, in steps S6 to S8, both contributions to the Doppler shift in equation (E. <NUM>) are correctly incorporated into the simulation of the radar targets.

It is emphasized that in each of the digital processing channels <NUM>, the appropriate velocity vrel,i for the i-th radar target is used.

The individual rescaled and frequency shifted input signals propagating in the digital processing channels are combined into a single signal by the merging unit <NUM>, thereby generating a merged input signal (step S9).

Accordingly, the merged input signal contains information on all m radar targets that are to be simulated, in particular information on the velocities and on the position of the radar targets relative to the device under test.

The merged input signal is converted into an analog radar transmission signal by the digital to analog converter <NUM> (step S10).

Therein, the digital to analog converter <NUM> converts at least <NUM> GS/s (<NUM> · <NUM><NUM> samples per second), preferably at least <NUM> GS/s, for example <NUM> GS/s.

The sampling rate of the analog to digital converter <NUM> and the conversion rate of the digital to analog converter <NUM> are equal to each other.

The radar transmission signal may then be forwarded to a radar antenna, in particular to the antenna <NUM>, via which the radar transmission signal is transmitted to the device under test <NUM>.

Summarizing, the radar test system <NUM> according to the disclosure provides an exact simulation of the Doppler frequency shift of one or several moving radar targets, wherein the individual radar targets may have different velocities.

Claim 1:
A radar target simulation system for simulating at least one moving radar target, comprising an analog to digital converter (<NUM>) and at least one digital processing channel (<NUM>), wherein the digital processing channel (<NUM>) is connected to the analog to digital converter (<NUM>),
wherein the digital processing channel (<NUM>) comprises a delay unit (<NUM>), a resampling unit (<NUM>) and a frequency shifting unit (<NUM>),
wherein the delay unit (<NUM>) is configured to receive a digitized input signal from the analog to digital converter (<NUM>) and to forward the input signal to the resampling unit (<NUM>) and/or to the frequency shifting unit (<NUM>) with a predetermined time delay,
wherein the frequency shifting unit (<NUM>) is configured to adapt a frequency of the input signal based on a carrier frequency of the input signal and based on a velocity of the moving radar target that is to be simulated,
wherein a digital to analog converter (<NUM>) is provided, wherein the digital to analog converter (<NUM>) is connected to the processing channel (<NUM>) downstream of the processing channel (<NUM>), wherein the analog to digital converter (<NUM>) and the digital to analog converter (<NUM>) have the same sampling frequency,
wherein the resampling unit (<NUM>) is configured to rescale the input signal in time domain and/or in frequency domain based on a baseband frequency of the input signal and the velocity of the moving radar target that is to be simulated, wherein a frequency axis is rescaled by a factor <MAT> according to <MAT> and/or wherein a time axis is rescaled by a factor <MAT> according to <MAT>,
wherein the frequency shifting unit (<NUM>) is configured to shift the frequency of the rescaled input signal by <MAT>, and
wherein f<NUM> is the carrier frequency, f is the baseband frequency, vrel is the velocity of the moving radar target that is to be simulated, and c is the speed of light.