CN113267751A - Anti-interference method for vehicle-mounted millimeter wave radar - Google Patents
Anti-interference method for vehicle-mounted millimeter wave radar Download PDFInfo
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Abstract
The invention relates to an anti-interference method for a vehicle-mounted millimeter wave radar, which comprises the step of integrally coding each transmitting chirp when transmitting signals, wherein the initial phase of different chirp is 0 or pi. The whole phase shift of 0 or pi to each chirp received signal is carried out according to the rule opposite to the emission during the receiving, the demodulation is carried out, and the subsequent processing is carried out after the demodulation, the peak power of the Doppler spectrum of the interference signal is obviously reduced, and when the power of the interference signal is not very large, the peak power is directly reduced to be below a noise threshold and cannot be detected; when the interference signal power is large, although the noise floor is exceeded, no obvious peak exists, and when CFAR detection is used, the CFAR threshold is not exceeded, and the target is judged to be a false target.
Description
Technical Field
The invention relates to the technical field of radar systems, in particular to an anti-interference method for a vehicle-mounted millimeter wave radar.
Background
The vehicle-mounted millimeter wave radar mainly works in the frequency bands of 24GHz and 77GHz and is used for achieving the functions of blind area monitoring, lane changing assisting, collision early warning, self-adaptive cruise, automatic parking and the like. With the popularization of millimeter wave radars, vehicles with millimeter wave radars mounted on roads are more and more, the vehicle-mounted millimeter wave radars generally adopt a continuous wave chirp sequence modulation technology, the radar transmits N chirp sequences, each sequence is called a chirp, as shown in fig. 1, a chirp signal is transmitted in one chirp, a time domain graph of the chirp signal is shown in fig. 2, and the frequency of the transmitted signal is linearly modulated and linearly increases along with time. Taking a specific waveform as an example, the time-frequency relationship is shown in fig. 3, and the frequency of the transmitted waveform is linearly increased from 77GHz to 77.2GHz, which takes 40us, to generate a chirp signal with a 200MHz bandwidth.
The received signal and the transmitted signal of the radar are mixed to obtain a difference frequency signal, the difference frequency signal is sampled and then the range-Doppler frequency spectrum of the target echo is obtained through two-dimensional FFT calculation, as shown in FIG. 4, however, the condition that the vehicle-mounted millimeter wave radar is interfered with radar signals of other vehicles frequently in the application process, so that the vehicle-mounted millimeter wave radar system cannot work normally, and therefore the vehicle-mounted millimeter wave radar has to take proper anti-interference measures.
The existing vehicle-mounted millimeter wave radar mainly adopts anti-interference measures of frequency hopping, new working frequency can be set for the radar at intervals, so that the probability that two vehicles running on the road work at the same frequency at the same moment can be reduced, the mutual interference condition can be reduced, and the radar is a very good anti-interference means.
However, as the working bandwidth of the vehicle-mounted millimeter wave radar is wider and wider (greater than 200MHz), the probability of overlapping of the frequencies of two adjacent millimeter wave radars still exists, and only one anti-interference measure of frequency hopping cannot meet the use requirement, so that a new anti-interference measure needs to be added, and the influence of mutual interference is further reduced.
Disclosure of Invention
The invention mainly aims to provide an anti-interference method for a vehicle-mounted millimeter wave radar, which is applied to a vehicle-mounted millimeter wave radar system and is used for weakening the influence of interference signals on the radar system.
An anti-interference method for a vehicle-mounted millimeter wave radar is characterized by comprising the following steps:
step 2, when the vehicle-mounted millimeter wave radar transmits signals, modulating the initial phase of a chirp sequence of the transmitted signals by using the pseudo-random two-phase code, and transmitting after changing the initial phase of each chirp sequence;
step 3, receiving a frequency modulation signal, and mixing the frequency modulation signal with the N chirp sequences in the original continuous wave transmitted in the step 1;
step 4, performing one-dimensional Fourier transform on the mixing signal, and then demodulating the signal subjected to one-dimensional Fourier transform according to the code element during transmission;
and 6, carrying out CFAR detection on the Doppler frequency.
Further, in step 4, the demodulation method includes demodulating chirp with symbol 0, multiplying the signal by 1, multiplying the signal by-1 with chirp with symbol 1.
Further, when modulating the N chirp sequences, each chirp adopts a different initial phase.
The invention has the beneficial effects that:
firstly, the method comprises the following steps: the peak power of the Doppler spectrum is obviously reduced, and when the power of an interference signal is not very large, the peak power is directly reduced to be below a noise threshold and cannot be detected;
secondly, because the Doppler spectrum of the interference signal is noisy, when the power of the interference signal is large, although the power exceeds the noise floor, no obvious peak exists, and when CFAR detection is used, the power does not pass through a CFAR threshold and is judged as a false target.
Drawings
FIG. 1 is a schematic diagram of continuous wave chirp sequence modulation;
fig. 2 is a time domain diagram of a transmit waveform within a chirp;
fig. 3 is a graph of time versus frequency for a transmit waveform in a chirp;
FIG. 4 is a flowchart of a conventional vehicle-mounted millimeter wave radar;
fig. 5 is a schematic diagram of a frequency difference structure of each chirp after frequency mixing;
FIG. 6 is a flowchart of the operation of the vehicle millimeter wave radar of the present invention;
FIG. 7 is a schematic diagram of modulating an initial phase of a set of transmit waveforms;
FIG. 8 is a waveform diagram of a transmitted signal with a symbol of 0;
FIG. 9 is a waveform diagram of a transmitted signal with symbol 1;
FIG. 10 is a Doppler spectrum with an interference signal to noise ratio of 25 dB;
FIG. 11 is a Doppler spectrum with an interference signal to noise ratio of 40 dB;
FIG. 12 is a diagram of an over-detection threshold for Doppler spectrum without using codes;
figure 13 is a diagram of a doppler spectrum using codes without over-detection thresholds.
Detailed Description
The embodiment of the invention provides an anti-interference method for a vehicle-mounted millimeter wave radar, and the theoretical basis and the method of the method are described in detail below, so that the invention can be better understood according to the embodiment.
Since the frequency difference between the mixed transmission signal and the received signal is proportional to the delay time, the distance is obtained by obtaining the delay time, and when the mixing is performed by using N chirp sequences, the frequency difference of each chirp is constant, as shown in fig. 5, and the frequency is proportional to the distance of the target. Therefore, the same frequency difference can be obtained by performing Fourier transform on the signals after mixing each chirp, the target distance can be obtained by obtaining the frequency, although the frequency difference of each chirp is the same, the initial phase corresponding to the frequency has a fixed phase difference between adjacent chirps due to the movement of an object, and the phase difference is in direct proportion to the Doppler frequency of the target, so the Doppler frequency can be obtained by arranging the phase differences of N chirps and performing Fourier transform.
According to the above theoretical basis, the method for resisting interference by the vehicle-mounted millimeter wave radar of the embodiment, as shown in fig. 6, includes:
step 2, when the vehicle-mounted millimeter wave radar transmits signals, modulating the initial phase of the chirp sequence of the transmitted signals by using the pseudo-random two-phase code, as shown in fig. 7, changing the initial phase of each chirp sequence, and then transmitting the signals, wherein when the code is 0, the initial phase is 0, as shown in fig. 8; when the code is 1, the initial phase is pi, and as shown in fig. 9, when modulating N chirp sequences, each chirp adopts a different initial phase;
step 3, receiving a frequency modulation signal, and mixing the frequency modulation signal with the N chirp sequences in the original continuous wave transmitted in the step 1;
step 4, performing one-dimensional Fourier transform on the mixing signal, then demodulating the signal subjected to one-dimensional Fourier transform according to the transmitted code element, wherein the demodulation mode is a chirp with a code element of 0, multiplying the signal by 1, and multiplying the signal by-1 with the code element of 1;
and 6, carrying out CFAR detection on the Doppler frequency.
In the embodiment, when the signals are transmitted, each transmitting chirp is integrally coded, the initial phase of different chirp is 0 or pi, when the signals are received, the phase of the signals received by each chirp is integrally shifted by 0 or pi according to a rule opposite to the transmitting rule, the signals are demodulated, the subsequent processing is carried out after the demodulation, and the subsequent processing flow is completely the same as that of the conventional millimeter wave vehicle-mounted radar system.
It should be emphasized that, in the present application, two-phase coding is added between the chrips, unlike most of the chirp modulation, the transmitted chirp signals are not changed, and when the initial phase of each chirp transmission signal is changed, the effect is that the phase coding is performed on the doppler frequency, the transmission signal in the chirp is not coded, that is, the frequency corresponding to the distance is not coded, the doppler frequency coding has no influence on the existing hardware architecture, and the two-phase coding is used to modulate the transmission initial phase and demodulate the received initial phase, so that the false target caused by the interference signal can be reduced.
This brings the immediate benefits of: after the method is used, the system can work normally when receiving normal target echo signals, and the method is not different from the traditional method; when the system receives the interference signal, the interference signal is subjected to noise in the Doppler frequency domain dimension, so that the Doppler frequency of the interference signal cannot have an obvious peak value, and when CFAR is used for detection, the interference signal cannot pass through a CFAR threshold and is judged as a false target, and the influence of the interference signal on the system is greatly reduced.
It is found through experiments that, taking a pulse doppler radar with 512 pulses as an example, when the signal-to-noise ratio of the interference signal is 25dB, the doppler spectrum of the interference signal is as shown in fig. 10. After the code is used, the Doppler spectrum peak is pressed down by 26dB and is lower than a noise floor, and the target detection cannot be influenced.
When the signal-to-noise ratio of the interference signal is 40dB, the doppler spectrum obtained by the processing is as shown in fig. 11, and it can be clearly seen that: the doppler spectrum of the unused codes has sharp peaks; the peak of the doppler spectrum using the code is still above the noise floor after a 26dB reduction, but its distribution exhibits "noising" characteristics, with no noticeable spikes.
In the subsequent CFAR detection of radar, different effects occur when coding is used and coding is not used. Without a coded system, the peak of the doppler spectrum of the interfering signal may exceed the CFAR threshold, resulting in reporting false targets, as shown in fig. 12.
By using the coded system, the Doppler peak of the interference signal cannot exceed the CFAR threshold, and the reported false target cannot be caused. As shown in fig. 13.
The effect of encoding systems with different numbers of chirp is shown in table 1.
TABLE 1 statistics of the number of different pulses
Now, taking a certain vehicle-mounted millimeter wave radar as an example to explain the measurement effect:
the working center frequency is 77GHz, the working bandwidth is 125MHz, the working system is a continuous wave frequency modulation sequence modulation system, the pulse number of one working cycle is 512, and the detected objects are mainly people and vehicles.
By practice, the transmitted signal is modulated and demodulated using an m-sequence to produce a 512-symbol bi-phase code.
The other vehicle-mounted millimeter wave radar is used as an interference source, the center frequency is 77.02GHz, the difference between the center frequency and the tested radar is 20MHz, and the working frequency ranges of the two radars have a 105MHz overlapping region. Two radars are oppositely arranged, work at the same time at a distance of 3 meters, and when no two-phase code is used, more than 20 false targets are used within 1 second, so that the use of the system is seriously influenced. After the two-phase coding is adopted, the number of false targets is greatly reduced, the number of false targets is less than 1 in 1 second, the system can normally work, and the anti-interference effect is very obvious.
The above-mentioned embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.
Claims (3)
1. An anti-interference method for a vehicle-mounted radar is characterized by comprising the following steps:
step 1, acquiring a pseudo-random two-phase code with a code element number N corresponding to the number N of chirp sequences contained in a period of transmission of a vehicle-mounted millimeter wave radar;
step 2, when the vehicle-mounted millimeter wave radar transmits signals, modulating the initial phase of a chirp sequence of the transmitted signals by using the pseudo-random two-phase code, and transmitting after changing the initial phase of each chirp sequence;
step 3, receiving a frequency modulation signal, and mixing the frequency modulation signal with the N chirp sequences in the original continuous wave transmitted in the step 1;
step 4, performing one-dimensional Fourier transform on the mixing signal, and then demodulating the signal subjected to one-dimensional Fourier transform according to the code element during transmission;
step 5, performing two-dimensional Fourier transform on the phase difference arrangement of the N chirp sequences;
and 6, carrying out CFAR detection on the Doppler frequency.
2. The method for resisting interference of the vehicle-mounted radar as recited in claim 1, wherein the demodulation method in step 4 comprises the steps of demodulating chirp with a code element of 0, multiplying a signal by 1, multiplying the signal by-1 with the chirp with the code element of 1.
3. The method for resisting interference of vehicular radar according to claim 1, wherein when modulating the N chirp sequences, each chirp has a different initial phase.
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Cited By (3)
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| CN116436743A (en) * | 2023-04-06 | 2023-07-14 | 南京厚华通信设备有限责任公司 | Waveform coding timing synchronization recovery method in LoRa modulation |
| CN116577738A (en) * | 2023-07-12 | 2023-08-11 | 南京隼眼电子科技有限公司 | Radar same-frequency anti-interference method and device, radar equipment and storage medium |
| CN117388836A (en) * | 2023-10-27 | 2024-01-12 | 深圳承泰科技有限公司 | Signal processing method of multiple wave-transmitting modes capable of increasing number of point clouds |
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