Ddm radar sensor

Abstract

The invention relates to a radar sensor having a plurality of transmission antennas (Txi; 0 ≤ i ≤ 3), a signal generator (10) for generating a transmission signal in the form of repeating chirp sequences (12), and a phase modulator (14) which supplies the transmission signal to the transmission antennas with specific phase differences between the antennas. The radar sensor is characterized in that the phases (φ 0,0, φ 0,3) of the first chirp of each sequence differ from one another for at least two of the transmission antennas.

Description

[0001] R.413893

[0002] - 1 - Description

[0003] title

[0004] DDM radar sensor

[0005] Description

[0006] The invention relates to a radar sensor with multiple transmitting antennas, a signal generator for generating a transmit signal in the form of repeating chirp sequences, and a phase modulator that supplies the transmit signal to the transmitting antennas with specific phase differences from antenna to antenna.

[0007] In particular, the invention relates to a DDM (Doppler Division Multiplex) radar sensor, which is used in driver assistance systems of motor vehicles or in autonomous driving systems for detecting the traffic environment.

[0008] State of the art

[0009] The radar sensor's multiple transmitting antennas are spatially offset from one another, enabling angle-resolved detection of radar targets. Orthogonal signals are fed to the transmitting antennas, allowing multiple antennas to be active simultaneously while still enabling the signals transmitted by different antennas to be separated at the receiver. Distance information about the detected radar targets is obtained by mixing the received radar echo with a portion of the transmitted signal, resulting in a lower-frequency (beat) signal whose frequency corresponds to the frequency difference between the transmitted and received signals. Since the frequency of the transmitted signal increases ramp-wise during a chirp, this frequency difference is proportional to the signal's travel time and thus to the distance of the radar target (R.413893).

[0010] - 2 - dependent. If the radar target has a non-zero radial velocity relative to the radar sensor, an additional frequency shift occurs due to the Doppler effect. However, this does not significantly distort the distance measurement if the chirp ramp steep enough. The Doppler shift does, however, cause the received signals to exhibit a certain phase progression from chirp to chirp, which allows the radial velocity of the target to be measured.

[0011] The DDM modulation method ensures the orthogonality of the transmitted signals and enables a long radar sensor range and high resolution in the velocity dimension at comparatively moderate costs. The DDM phase modulator incorporates a phase shifter for each of the simultaneously active transmitting antennas. This shifter rotates the phase of the signal forwarded to the antenna such that the phase of the signal transmitted in the current chirp is rotated by a specific angle relative to the phase of the preceding chirp. This angle varies from antenna to antenna. This phase shift from chirp to chirp has the same effect on the received signal as the Doppler shift caused by the radar target's own motion. In this way, each transmitting antenna is characterized by a distinctive "virtual" Doppler shift.

[0012] For a radar sensor to receive operational approval, certain criteria regarding frequency and transmission power must be met. In particular, most countries impose limits on the so-called peak transmission power, i.e., the transmission power in the direction of radiation where the transmission beam reaches its main maximum. For the radar sensor's range, however, the average transmission power is more relevant, meaning the transmission power averaged over the entire angular range. The ratio between these transmission powers is called the PAPR (Peak to Average Power Ratio).

[0013] Disclosure of invention R.413893

[0014] - 3 -

[0015] The object of the invention is to create a radar sensor with reduced PAPR.

[0016] This problem is solved according to the invention by the fact that the phases of the first chip of each sequence are different from each other for at least two of the transmitting antennas.

[0017] The invention exploits the fact that, in phase-modulated radar sensors, only the phase relationships between chirps and / or antennas are relevant. For example, in a DDM radar, only the phase difference between chirps is crucial for the orthogonality of the transmitted signals, not the phase of the first chirp transmitted in each sequence. This so-called start-up phase therefore represents a degree of freedom that allows the radar sensor to be optimized with respect to the phase-to-phase ratio (PAPR). It has been shown that the PAPR of a radar sensor in which the transmitted signals of at least two antennas differ during the start-up phase is lower than the PAPR of a conventional radar sensor in which all antennas have the same start-up phase. By cleverly selecting different combinations of start-up phases, the radar sensor can therefore be optimized with respect to the PAPR. This enables a long range with reduced peak transmit power.A PAPR reduced in this way also has the advantage that, due to the reduced peak transmit power, the risk of a very strong radar echo causing signal distortions (e.g., nonlinearities or clipping) in individual hardware components of the radar sensor's receiver section is also reduced. Advantageous embodiments and further developments of the invention are described in the dependent claims.

[0018] In a DDM radar, the phase progression from chirp to chirp is typically 2T% (360%) for any integer n across all transmitting antennas. Consequently, in each antenna, the (n+1)th chirp will again have the initial phase. Similarly, the initial phases for the different antennas can be chosen such that the initial phase difference between any two antennas is 2K / s (360° / s) for any integer s. R.413893

[0019] - 4 -

[0020] If the radar sensor has any number of NTX transmitting antennas, the start-up phase for one (the first) of these antennas can be freely chosen. Only for the remaining (NTX - 1) antennas is there the restriction that at least one start-up phase must be different from the start-up phase for the first antenna. The start-up phase for the first antenna thus represents an additional degree of freedom that can be used for optimization.

[0021] For example, it has been shown that the accuracy of the phase shifters used in the phase modulator can be improved by appropriately selecting this initial start-up phase. If, for instance, a phase shifter is intended to increase the phase increment in 90° steps, it turns out that in practice the phase angle is often rotated by an angle that can deviate from the target value of 90° by up to 2°. It can happen that identical phase shifters, when generating a phase sequence of 0° - 90° - 180° - 270°-..., produce larger or more frequent errors than when generating, for example, a phase sequence of 45° - 135° - 225° - 315°-.... Therefore, to generate signals that are as perfectly orthogonal as possible and to minimize crosstalk between transmission channels, it can be advantageous to use a start-up phase that deviates from 0° for the first antenna, rather than a start-up phase of 0°. If the start-up phases for the remaining antennas are with regard to the

[0022] If PAPR is optimized, then the phase shifts of all other phase shifters are also set in such a way that the error frequency is reduced.

[0023] The following section explains an exemplary embodiment in more detail with reference to the drawing. The drawing shows:

[0024] Fig. 1 shows a block diagram of a transmitter section of a radar sensor according to the invention;

[0025] Fig. 2 shows a frequency / time diagram for a transmitted signal;

[0026] Fig. 3 shows a diagrammatic representation of phase progression in a prior art DDM radar; R.413893

[0027] - 5 -

[0028] Fig. 4 shows a representation analogous to Fig. 3 for a device according to the invention.

[0029] Radar sensor; Fig. 5 a comparison of antenna diagrams for the phase configurations according to Figures 3 and 4;

[0030] Fig. 6 shows the result of a simulation of peak power outputs for randomly selected combinations of start-up phases; and

[0031] Fig. 7 shows a diagrammatic representation of phase shifter errors as a function of the start phase.

[0032] Figure 1 schematically shows the transmitting section of a radar sensor, which has four transmitting antennas Txi (0 < i < 3). A signal generator 10 produces a transmit signal with the frequency profile shown in Figure 2. This signal consists of sequences of so-called chirps 12, repeatedly transmitted at regular time intervals, in which the frequency f increases linearly as a function of time t. This transmit signal is fed to each of the transmitting antennas Txi via a phase shifter 14, which rotates the phase of the signal by a specific angle for each chirp. If m (0 < m < 3) is

[0033] If index is the number that counts the chirps 12 in the sequence, and i is the index that counts the transmitting antennas, then the complex amplitude of the transmitted signal in chirp m in antenna m can, for example, have the following form:

[0034] In this, a is a real constant, j is the imaginary unit, co(t) is the (circular) frequency dependent on time t, and <p m i a phase angle. The phase shifters 14 can be controlled so that they adjust the phase angle. <p m i For example, adjust according to the following formula:

[0035] <Pm,i = 2it / N Tx ) * i * m + <p o i R.413893

[0036] - 6 -

[0037] This refers to <p o i the so-called start-up phase. This start-up phase has no influence on the orthogonality of the signals transmitted by the different antennas and is therefore set to zero for all transmitting antennas in conventional radar sensors.

[0038] In Fig. 3 the phase angles are shown. <p m i The signals for a conventional DDM radar with four antennas are shown. At antenna i = 0, (p O m The phase angle is zero for all four chirps. For antenna i = 1, the phase angle is zero for the first chirp and is then increased by 90° for each subsequent chirp. For antenna i = 2, the phase angle is increased by 180° from chirp to chirp, starting with the phase angle. <p 0 2 = 0. For antenna i = 3, the phase angle is increased by 270° from chirp to chirp, also starting with <p 0 3 = 0 for the first chirp. In the example shown in Fig. 3, the number (four) of the different phase angles (0°, 90°, 180° and 270°) is equal to the number of transmitting antennas. However, the invention is also applicable to radar sensors where the number of antennas is less than the number of phase angles, so that one or more phase channels remain unused.

[0039] Fig. 4 shows an example of a phase configuration according to the invention. As in Fig. 3, the phase angle for all antennas is increased by a fixed angle from chirp to chirp, in this example by 0° for antenna i = 0, by 90° for antenna i = 1, by 180° for antenna i = 2, and by 270° for antenna i = 3. However, a difference from Fig. 3 is that the start phase <p o i The angle is different from 0 for all four antennas. For antennas i = 0 to i = 2, it is 45°, while for antenna i = 3 it is 135°. This is a special case of the general principle of the invention, which distinguishes the start-up phase for at least one antenna (i = 3) from the start-up phases for the others.

[0040] The antennas differ. In the example shown, the start-up phase differs. <p 03For antenna i = 3, the start phases for the other antennas differ by 180°. In general, the start phases for each pair of transmitting antennas can differ by any desired angle. Thus, in a radar sensor according to the invention with four transmitting antennas, there are four free parameters, namely... <p 0 0 , <p Oil — <p OiO , <p0,2 ~ (Po,i und <p 0 3 — <p 0 2 , which can be varied to optimize the radar system. Generally, for a radar sensor with N there are Tx Transmitting antennas N Tx Free parameters. R.413893

[0041] - 7 -

[0042] Figure 5 compares the antenna diagrams for a conventional radar sensor with a start-up phase of 0° and a radar sensor according to the invention with optimized start-up phases. In both diagrams, curve 16 shows the angular distribution of the transmitted power as a function of the azimuth angle α (assuming that the entire power is radiated without interference from a single antenna). This curve 16 is also the effective antenna diagram / locating field after data processing.

[0043] Curves 18, 20, 22, and 24 each show the angular distribution for the chirps m = 0 to m = 3, which are transmitted simultaneously by the four transmitting antennas and therefore interfere with each other. In the left-hand diagram (start phase 0°), a symmetrical pattern results, and at an azimuth angle a = 0°, the peak power P, indicated by curve 16, is reached in chirp m = 0 (solid curve). In contrast, in the right-hand diagram, the start phases for the four transmitting antennas are shifted relative to each other, resulting in an asymmetrical angular distribution and only a peak power P* is achieved, which is significantly lower than P, by 2.8 dB in the example shown. The average power, averaged over all chirps and azimuth angles, is still given by the area under curve 16. This means that by choosing the start phases, a significant reduction in the PAPR (Peak to Average Ratio) was achieved.The invention thus makes it possible to comply with the prescribed upper limit for peak power while achieving a higher average power and therefore a greater range of the radar sensor.

[0044] For a given antenna arrangement, the optimal configuration of the start phases can be calculated using a mathematical model or determined through test series or simulations.

[0045] Figure 6 shows the result of a Monte Carlo optimization in which 100 different start-up phase configurations were randomly selected and the peak power P* was calculated. It can be seen that in some configurations, for example configuration no. 95, a reduction in peak power of up to 2.8 dB is achieved compared to the least favorable configurations.

[0046] This reduction in peak power is independent of the choice of start-up phase. <p 0 0for the antenna i = 0. This additional degree of freedom can therefore be used for an R.413893

[0047] - 8 -

[0048] Optimization can be used to account for certain non-idealities of the radar sensor's hardware components. For example, the phase shifters 14 typically each have two channels that determine the real and imaginary parts of the complex amplitude. If the phase shifter is intended to effect a phase shift of 90° from chirp to chirp, starting from a start phase of 0°, the change in the real and imaginary parts from chirp to chirp is maximal, and deviations between the target and actual values ​​in the channels can lead to the actual phase shift being not 90°, but, for example, 91.5°. Conversely, if only the phase sequence starts with a start phase of, for example, 45°, the changes in the two channels are smaller in magnitude, resulting in a smaller phase step error.

[0049] Figure 7 shows the magnitude of the phase step error for a typical phase shifter plotted against the start-up phase. It can be seen that the phase step error is minimal with a start-up phase of approximately 45°. For this reason, it is advantageous if, as in Figure 4, the start-up phase is... <p 0 0 For antenna i = 0, a value of approximately 45° is chosen. This reduction in phase step error leads to smaller deviations of the transmitted signals from perfect orthogonality and thus to less crosstalk between the different transmission channels, thereby improving the interpretability of the received signals.

Claims

R.413893 - 9 - Claims 1. Radar sensor with multiple transmitting antennas (Txi; 0 < i < 3), a signal generator (10) for generating a transmit signal in the form of repeating chirp sequences (12) and a phase modulator (14) which supplies the transmit signal to the transmitting antennas with specific phase differences from antenna to antenna, characterized in that the phases ( <p o ,o, ^0,3) of the first chirp of each sequence are different for at least two of the transmitting antennas.

2. Radar sensor according to claim 1, wherein the phase modulator (14) is a DDM phase modulator.

3. Radar sensor according to claim 1 or 2, wherein the phase progression from chirp to chirp for each transmitting antenna is 2n / n, where n is an integer constant specific to the respective antenna.

4. Radar sensor according to claim 3, wherein for each pair of transmitting antennas (Txi) the phases ( <p o i) the first chirps of each sequence differ by 2n / s, where s is an integer constant.

5. Radar sensor according to one of the preceding claims, wherein the phase ( <p i 0 ) of the first chirp of each sequence for all transmitting antennas (Txi ) is different from zero.

6. Radar sensor according to claim 5, wherein the phase of the first chirp of each sequence for all transmitting antennas (Txi) is between 40° and 50°.

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