Method for suppressing interference signals in a fast chirp fmcw radar sensor for motor vehicles

Abstract

The invention relates to a method for suppressing interference signals having a known interference signal frequency spectrum in a fast chirp FMCW radar sensor for motor vehicles, in which method a baseband signal formed by mixing a transmitted signal with a received signal is subjected to an at least two-dimensional discrete Fourier transform, one of the at least two dimensions representing a Doppler spectrum, i.e. a spectrum of Doppler frequencies determined by radial velocities of radar targets relative to the radar sensor, and in which method the starting times (t1, ..., tn) of chirps (28) in the transmitted signal are selected such that their distances differ from one another by non-linearly varying time intervals ΔTn, characterised in that the variation width Δ of the time intervals ΔTn is selected depending on the interference signal frequency spectrum such that at least one interference signal frequency fi in the Doppler spectrum is specifically suppressed.

Description

[0001]R.404087 February 15, 2023 Description Title Method for suppressing interference signals in a rapid chirp FMCW radar sensor for motor vehicles The invention relates to a method for suppressing interference signals with a known interference signal frequency spectrum in a rapid chirp FMCW radar sensor for motor vehicles, in which method a baseband signal formed by mixing a transmitted signal with a received signal is subjected to an at least two-dimensional discrete Fourier transformation, wherein one of the at least two dimensions represents a Doppler spectrum, that is to say a spectrum of Doppler frequencies determined by radial velocities of radar targets relative to the radar sensor, and in which the starting times of chirps in the transmitted signal are selected such that their distances differ from one another by non-linearly varying time intervals ^tn.State of the art In an FMCW (Frequency Modulated Continuous Wave) radar, the frequency of the transmitted radar signal is ramp-modulated. If the received radar echo is then mixed with part of the signal transmitted at the same time, the frequency difference between the two signals leads to a beat, producing a baseband signal with a frequency that corresponds to the frequency difference between the transmitted and received signals. R.404087 15.02.2023 Since this frequency difference is proportional to the signal propagation time and thus proportional to the distance of the located radar target, the frequency of the baseband signal contains information about the object's distance. In the case of moving radar targets, however, the frequency of the baseband signal also depends on the relative speed between the radar target and the radar sensor due to the Doppler effect.In a rapid-chirp FMCW radar sensor, very steep frequency ramps, so-called chirps, are transmitted in rapid succession. The steep ramp slope increases the sensor's distance sensitivity. However, the ramp slope has no influence on the frequency shift caused by the Doppler effect, so that with a sufficiently steep ramp slope, the Doppler effect can be neglected compared to the distance-dependent frequency shift. In the Fourier spectrum, each detected object then appears as a peak at a specific frequency, which indicates the object's distance to a good approximation. To obtain information about the relative speed, one exploits the fact that the radar echoes obtained from different frequency ramps exhibit a phase shift that depends on the relative speed.A second Fourier transformation, in which not the signal obtained on a single ramp is transformed, but rather the signals received for corresponding support points in the successive frequency ramps, produces a two-dimensional Fourier spectrum in which the first dimension indicates the distance of the object and the second dimension the Doppler frequency and thus the relative speed. DE 102009016480 B4 describes a radar sensor operating according to this principle, in which the time intervals between successive chirps are varied according to a random sequence. This variation of the time intervals, in conjunction with a special object tracking method, is intended to improve object plausibility and the resolution of ambiguities in determining relative speeds.As a side effect, however, it is also mentioned that the variation in the time intervals can lead to greater robustness of radar location against interference signals. Disclosure of the invention The object of the invention is to improve the suppression of these interference signals in scenarios in which interference signals with a known frequency or frequency distribution are introduced into the radar sensor. This object is achieved according to the invention in that, in a radar sensor of the type mentioned at the beginning, the variation width ^ of the time intervals ^tn is selected as a function of the interference signal frequency spectrum such that at least one interference signal frequency fi in the Doppler spectrum is specifically suppressed. The invention exploits the fact that the variation in the distances between the chirps leads to phase shifts in the baseband signal, which are then converted into frequency shifts in the Doppler spectrum during the Fourier transformation.In principle, this effect in the Doppler spectrum broadens and flattens not only those peaks caused by interference signals, but also those peaks caused by the wanted signal, which indicate the relative speed of real radar targets. For a signal with a given frequency, however, the flattening of the signal peaks depends not only on the variation range of the chirp spacing, but also on the frequency of the signal in question. If the frequency of an interference signal is known, the variation range can be adjusted so that those signal peaks caused by the interference signal are specifically suppressed, while the spectrum for wanted signals, which usually have a different frequency, is hardly affected, thus improving the detectability of real radar targets.The invention can be used, for example, to suppress interference signals caused by a clocked voltage regulator used, for example, to supply power to the radar sensor and / or other electronic components in the vehicle. The switching frequency of the voltage regulator leads to ripple in the supply voltage for the radar electronics. In the sensitive circuit components of the radar sensor's transmit and receive circuits, this can lead to the coupling of an interference frequency into the transmit or receive signal and ultimately impair the evaluation of the received signal. One known method for reducing such interference is to use passive filters (LC filters) to attenuate the amplitude of the ripple to such an extent that no interference can be detected in the spectrum of the received signal.However, due to the high sensitivity of the transmit and receive circuit components, the requirements for the filter effect are very high, making this type of interference suppression very complex. Another known approach to reducing the maximum interference power at the switching frequency and its higher harmonics is to vary the switching frequency over time (spread spectrum method) in order to distribute the power spectrally. In general, however, the options for varying the switching frequency of the voltage regulator are very limited due to the boundary conditions that must be observed, so this approach does not always lead to success. The method according to the invention has the advantage that the interference frequency does not have to be spread; instead, only the associated peak in the Doppler spectrum is spread. R.404087 15.02.2023 However, the method according to the invention can also be used to suppress other interference signals that typically arise in the vicinity of the radar sensor in a motor vehicle and whose frequencies are known. Advantageous embodiments and further developments of the invention are specified in the subclaims. In some known rapid chirp radar sensors, the chirps are grouped into packets separated from one another by certain time gaps. In this case, it is possible to vary the start times of the individual packets, while the intervals between the chirps within a packet can be uniform. The range of variation of the time intervals is then only limited by the minimum size of the time gaps and can thus be significantly greater than the time interval between two consecutive chirps within a packet.Of course, the distances between the chirps within a packet can also be varied, albeit with a smaller variation range. The two different variation ranges can then be used, for example, to suppress two different interference signals. The subject matter of the invention is also a radar sensor in which the method described above is implemented and in which the variation range of the time intervals is matched to the frequency of an internal interference signal source of the radar sensor. The subject matter of the invention is also a motor vehicle with a radar sensor in which the method described above is implemented and in which the variation range of the time intervals is matched to the frequency of an interference signal source present in the motor vehicle. R.404087 15.02.2023 Exemplary embodiments of the invention are explained in more detail below with reference to the drawings. They show: Fig.Fig. 1 is a block diagram of an FMCW radar sensor to which the invention is applicable; Fig. 2 is a compilation of diagrams to explain the operation of a rapid chirp FMCW radar sensor; Fig. 3 is an example of a modulation scheme for a radar sensor according to Fig. 2; Fig. 4 is a further example of a modulation scheme for a rapid chirp FMCW radar according to another embodiment; Fig. 5 is a simplified representation of details of a modulation scheme based on the principle illustrated in Fig. 4; Fig. 6 is another variant of the modulation scheme according to Fig. 4; and Fig. 7 is a sketch of a motor vehicle with a radar sensor and an interference signal source. A typical structure of a radar sensor to which the invention is applicable will be explained using a block diagram shown in Fig. 1. The R.404087 15.02.2023 radar sensor has a functional group 10 for generating a transmission signal 12 in the radar frequency range (76-81 GHz).This signal is transmitted to transmitting antennas 16 via parallel amplifier and phase-shifter circuits 14. Received signals, e.g., radar echoes received from various radar targets, are received via several parallel receiving antennas 18 and mixed with the transmitted signal 12 via mixers 20, thereby converting them into so-called baseband signals with lower frequencies. In the baseband range, the signals are amplified, filtered, and digitized by suitable circuits 22. Further processing of the baseband signals for detecting the radar targets and for calculating measured variables such as distances, relative speeds, and location angles of the radar targets takes place in a digital processing stage 24. The properties of the radar sensor with regard to measuring the distance and speed of objects are essentially determined by the signal shape or modulation of the transmitted signals.Current radar sensors are mostly based on variations of so-called rapid chirp or chirp sequence modulation, which is illustrated in Fig. 2. The frequency f of the transmitted signals 12 is shown there as a function of time t. A modulation sequence 26 for detecting the environment consists of a series of uniform FMCW ramps or chirps 28, which are transmitted at an equidistant time interval TR2R. The digital received signals in the baseband are transformed into a spectrum 30 for each transmit and receive channel using a two-dimensional Fourier transformation (Fast Fourier Transformation FFT). In a first R.404087 15.02.2023 dimension D1, Fourier transformations are performed for the individual chirps 28. The input variables are the complex amplitudes of the baseband signal at times corresponding to a sequence of support points 32 on the chirp 28.Each power peak 34, 36 in the one-dimensional spectrum in this dimension D1 corresponds to a radar target to which the radar sensor is at a certain distance proportional to the frequency of the peak. In a second dimension D2, Fourier transformations are performed over corresponding support points 32 in the successive chirps 28. A reflected signal thus leads to a power peak 34', 36' in the two-dimensional spectrum 30, the frequency position of which in the dimension D2 indicates the relative speed of the respective radar target. The spectrum 30 is accordingly divided into distance bins 38 in the first dimension D1 and into speed bins 40 in the second dimension D2. Within each distance bin 38, the distribution of the signal power across the speed bins 40 represents a Doppler spectrum.If the reflected signal has sufficient power, the power peaks 34', 36' assigned to the various radar targets can be detected and each assigned to a pair of distance bin and speed bin. The modulation parameters of the chirp sequence modulation assign a specific range of object distances to each distance bin 38 and a specific range of relative speeds to each speed bin 40. The coordinates (distance and speed) of a located radar target can thus be determined using the indices of the distance and speed bins. The location angles of the radar targets are determined in a known manner using the phase relationships between the signals received in the various receive channels. According to the invention, the method described above, in particular the modulation scheme used, is modified such that signals from known R.404087 15.02.2023 interference sources in the spectrum 30 can be suppressed more specifically. Instead of transmitting the chirps equidistantly in time, the starting times of the individual chirps 28 are each offset by a nonlinearly varying time interval ∆Tn. This correspondingly "jitters" or "distorts" the sampling times for Doppler processing of the radar sensor's received signals. As shown in Fig. 3, the starting times tn of the chirps 28 (for n > 1) are determined according to the following formula (1): tn = t1 + (n-1)T0 + ^Tn (1) The distance between the starting times of two consecutive chirps 28 is thus each composed of a fixed component T0 and a variable time interval ^Tn. The variable time interval ^Tn is always greater than 0 and smaller than a certain variation range ^. An upper bound for the variation range ^ ^results from the fact that the consecutive chirps 28 must not overlap. If T.r the ramp duration must therefore apply: ^ ^ ^ T0- T r(2) The time intervals ^Tn should vary irregularly, i.e., in particular, non-linearly, and can be generated, for example, using a random number generator. Optionally, a fixed sequence of time intervals ^Tn can be specified, which repeats after a certain (as large as possible) number of chirps 28. Preferably, the time intervals are evenly distributed over the interval [0, ^]. Fig. 4 shows another variant of a conventional chirp sequence method. In this case, several chirps 28, which follow one another at equal intervals, are combined to form packets P1, P2, ..., Pn, which are separated from one another by certain time gaps. The center frequency f1, f2, ..., fn of the chirps 28 differs from packet to packet and is varied linearly over the sequence of the packets.In the known methods of this type, the starting times ^1, ^2, …, ^n of the packets P1, P2, …, Pn are separated by constant time intervals T. p2p separated from each other. A possible adaptation of the invention to this version of the chirp sequence method is shown in Fig. 5. Here, only the initial sequences of three consecutive packets P n , P n+1 , P n+2 shown. The interval between the initial times ^ m , ^ m+1 or ^ m+1 ^ ^ ^ m+2 two consecutive packets consists of a fixed part T0 and a variable time interval ^T n+1 or ^T n+2 . The distances T R2Rbetween the start times of consecutive chirps 28 within each packet, however, are constant. The time intervals ^Tn+1, ^ ^ ^ ^ can also only vary within a certain range of variation ^, which in this case is limited by the fact that the consecutive packets must not overlap. The larger the time gaps between the individual packets Pn, ..., the more scope there is for varying the time intervals ^T n+1 ^ ^… . Fig.6 shows a variant in which the starting times ^ n , ^ n+1...the packets vary in the same way as in Fig. 5, but in which the start times of the chirps 28 are also varied within each packet. Within each packet, the intervals between the start times of the chirps are composed of a fixed portion T0 and a variable portion ^T2, ^T3 or ^TK, ^TK+1. The indices k and k+1 symbolize that the sequence in which the time intervals vary can be different for different packets. Alternatively, the same variation scheme can of course also be used for all packets. R.404087 15.02.2023 The effect of varying the time intervals ^Tn can be explained using the following signal model. For the phases ^ n of the received signals obtained from the nth chirp or the nth packet due to a reflection from a single object, the following relationship applies: ^ n = 4^ (f ^ / c) ∙ v ∙ t n + ^0= 4^ (f ^ / c) ∙ v ∙ (n ∙ T0+ ^Tn ) (3) In it is f ^ the center frequency of the chirp or packet, c the speed of light, v the relative speed of the object, the starting time of the nth chirp or packet, and ^0 the phase of the received signal that would be obtained for an object with the relative speed v = 0. If an interference signal with the frequency fi is superimposed on the received signal, the following relationship applies: ^n = 2^ ((2f^ / c) ∙ v + fi) ∙ tn + ^0 = 4^ (f ^ / c) ∙ v ∙ (n ∙ T0+ ^T n ) + 2^ ∙ f i ∙ (n ∙ T0+ ^T n ) + ^0(4) In this expression, (2f ^ / c) the Doppler frequency f d , which should be determined as accurately as possible in spectrum 30 in order to measure the relative velocity v. Therefore: ^ n = 2^f d ∙ (n ∙ T0+ ^T n ) + 2^ ∙ f i ∙ (n ∙ T0+ ^T n ) + ^0= 2^ f d ∙ n ∙ T0+ 2^ f d ∙ ^T n + 2^ f i ∙ n ∙ T0+ 2^ f i^T n + ^0(5) The variation of the time intervals ^Tn leads to a desired phase modulation of the interference signal due to the interference signal term 2^ fi ^Tn and to an undesired phase modulation of the wanted signal due to the wanted signal term 2^ fd ∙ ^Tn. Due to the Fourier transformation in the second dimension D2, these phase modulations cause a "smearing" of the power peak over a frequency range, which can extend over several speed bins, and a corresponding flattening of the peak. To suppress an interference signal with a high interference signal frequency fi, one can now vary the variation width ^ of the time intervals ^T n so that they are in the order of magnitude ^ = 1 / 2f iThe noise peak is then broadened and flattened as desired, so that it is no longer mistakenly interpreted as a reflection from an object. Furthermore, all odd harmonics of the noise frequency are suppressed because the chirp sequence consists of pairs of chirps whose phases are offset by 180° due to phase modulation. In contrast, the useful signal peak is smeared by a factor of 1 / f i<< 1 smaller, and side lobes of the useful signal peak are effectively suppressed regardless of the Doppler frequency of the object, so that the real radar target remains easily detectable. Fig. 7 schematically shows a motor vehicle 42 containing an interference signal source 44 with a known frequency spectrum. This motor vehicle also has a radar sensor 46 operating according to the principle described above. The variation width ^ in the radar sensor 46 is tuned to the dominant interference signal frequency fi of the interference signal source 44, so that its interference signals are effectively suppressed when evaluating the radar echoes.

Claims

R.404087 15.02.2023 Claims 1. Method for suppressing interference signals with a known interference signal frequency spectrum in a rapid chirp FMCW radar sensor (46) for motor vehicles (42), in which a baseband signal formed by mixing a transmitted signal with a received signal is subjected to an at least two-dimensional discrete Fourier transformation, wherein one (D2) of the at least two dimensions (D1, D2) represents a Doppler spectrum, that is to say a spectrum of Doppler frequencies determined by radial velocities of radar targets relative to the radar sensor, and in which the starting times of chirps (28) in the transmitted signal are selected such that their spacings are non-linearly varying time intervals ^T n differ from each other, characterized in that the variation range ^ of the time intervals ^t nis selected as a function of the interference signal frequency spectrum such that at least one interference signal frequency fi in the Doppler spectrum is specifically suppressed.

2. Method according to claim 1, wherein the transmitted signal consists of a sequence of packets (P1, ..., Pn), each comprising a plurality of chirps 28 and separated from one another by time gaps, and wherein the distances between the start times ^1, ..., ^n of the packets (P1, ... Pn)) differ from one another by non-linearly varying time intervals ^tn.

3. Method according to claim 1, wherein the chirps (28) within each packet have equidistant time intervals TR2R from one another.

4. Method according to claim 2 or 3, wherein the chirps (28) belonging to different packets (P1, ... Pn) differ from one another in their center frequency (f1, f2). R.404087 15.02.2023 5. Method according to one of the preceding claims, for suppressing interference signals with a dominant interference signal frequency fi, in which the variation width ^ of the time intervals ^T n between 1 / 4f i and 1 / f i 6. Method according to one of the preceding claims, in which the time intervals between the starting times (t1, ..., tn; ^1, ..., ^n) of the chirps (28) or the ramps (P1, ... Pn) are each additively composed of a fixed component (T0) and the varying time interval (^T 1, ^ ^ ^ ^ ^ ^T n ^ ^ ^where the consecutive time intervals ( ^T 1, ..., ^T n) form a random sequence or pseudorandom sequence and are evenly distributed over the interval [0, ^].

7. A radar sensor having an internal interference signal source and in which the method according to one of claims 1 to 5 for suppressing interference signal frequencies fi of the internal interference signal source is implemented.

8. A motor vehicle (42) having an internal interference signal source (44) and a radar sensor (46) in which the method according to one of claims 1 to 5 for suppressing interference signal frequencies of the interference signal source (44) is implemented.

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