CN110208804A - Transmitted waveform design method and target acquisition algorithm suitable for automobile collision avoidance radar - Google Patents

Abstract

The invention discloses a transmission waveform design method and a target detection algorithm suitable for automobile anti-collision radars, comprising the following steps: determining that the system of the radar transmission signal is a multi-order linear frequency keying system; determining the transmission waveform according to a given index Parameters and indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and minimum signal detectable noise ratio in the frequency domain. The parameters required to determine a waveform are: FM bandwidth, coherent Processing interval, the number of LFSK signals, the number of steps of each LFSK signal, and the initial frequency difference between LFSK signals; process the echo data of the transmitted signal, and calculate the distance and speed of the target. The invention has a moderate amount of calculation, improves the detection accuracy on the basis of unambiguous distance measurement, and effectively reduces the detection blind area caused by the transmission and reception leakage.

Description

Transmission Waveform Design Method and Target Detection Algorithm Applicable to Automotive Anti-collision Radar

technical field

The invention belongs to the technical field of radar information, in particular to a transmission waveform design method and a target detection algorithm suitable for automobile anti-collision radars.

Background technique

In order to improve the safety performance of automobile driving and reduce the occurrence of traffic accidents, Advanced Driver Assistance System (ADAS) has become a research hotspot. The corresponding braking signal is given to the car, so that the car has the ability of collision warning or autonomous collision avoidance.

Millimeter wave radar has a long detection range and high accuracy, making it a reliable choice for ADAS sensors. The current vehicle-mounted millimeter-wave radars are mainly 24GHz and 77GHz, which are used for rearward assistance and forward detection of vehicles respectively. In terms of waveform selection, existing anti-collision radars generally use Linear Frequency Modulated Continuous Waveform (LFMCW), and some systems use Multiple Frequency ShiftKeying (Multiple Frequency ShiftKeying. Different detection performance and processing complexity.

Linear frequency modulation continuous wave radar uses the beat signal frequency corresponding to different sweep frequency bands to solve the coupling of distance and speed. It has a simple structure and low transmission power, and has advantages in detecting short-range targets. However, there are some deficiencies in this system in terms of excluding false targets and selecting correct pairs. The classic symmetrical triangular LFM-CW system cannot perform target matching, and it is generally not used when there are multiple targets; the LFM-CW system with variable FM slope needs to solve multiple equations when pairing targets, and the algorithm is more complicated, and when the number of targets increases , the amount of computation and memory usage will increase exponentially; the introduction of the linear frequency modulation continuous wave system of point frequency signals reduces the complexity of target pairing, but the accuracy of pairing also decreases; if moving target detection ( Moving Targets Detection.MTD) solves the distance and speed of the target. Although the step of target pairing is omitted, it requires high hardware requirements for the detection of high-speed targets, which increases the cost of the product.

The multiple frequency keying system does not need to perform target pairing. It uses the beat signal frequency and phase difference corresponding to each LFSK signal to solve the coupling of distance and speed, completely eliminating the interference of false targets, and the calculation amount is relatively small. Advantages when probing. However, the result of the detection largely depends on the estimation of the phase difference. Since the value range of the phase difference itself is small, the small error of the phase difference estimation is small when the SNR is not high or the coherent accumulation time is short. It will also bring about a more obvious decline in the accuracy of distance velocity measurement.

Contents of the invention

The purpose of the present invention is to provide a transmission waveform design method and a target detection algorithm suitable for automobile anti-collision radars, improve the accuracy of target detection under the premise of moderate calculation amount, and reduce the detection blind area caused by transmission and reception leakage.

The technical solution that realizes the object of the present invention is: a kind of transmission waveform design method that is applicable to automobile anti-collision radar, comprises the following steps:

Step 1, determine that the system of the radar transmitting signal is the MS-LFSK system, a cycle of MS-LFSK waveform is composed of an up-sweep frequency segment and a down-sweep frequency segment, and each frequency scan segment is synthesized by several LFSK signals;

Step 2. Determine the parameters of the transmission waveform according to the given indicators. The indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and minimum signal detectable noise ratio in the frequency domain. Determine The parameters required for a waveform of one cycle include: FM bandwidth, coherent processing interval, number of LFSK signals, number of steps for each LFSK signal, and initial frequency difference between LFSK signals.

A target detection algorithm suitable for automobile collision avoidance radar, comprising the following steps:

Step 1, determine that the system of the radar transmitting signal is the MS-LFSK system, a cycle of MS-LFSK waveform is composed of an up-sweep frequency segment and a down-sweep frequency segment, and each frequency scan segment is synthesized by several LFSK signals;

Step 2. Determine the parameters of the transmission waveform according to the given indicators. The indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and minimum signal detectable noise ratio in the frequency domain. Determine The parameters required for a waveform are: FM bandwidth, coherent processing interval, number of LFSK signals, number of steps for each LFSK signal, and initial frequency difference between LFSK signals;

Step 3, process the echo data of the transmitted signal to solve the distance and speed of the target: mix the received echo signal with the local oscillator to obtain the beat signal, sample it and correspond to the LFSK signal according to the echo data Rearrange it according to the situation, do FFT and periodogram accumulation on the rearranged data according to the sweep cycle, and perform OS-CFAR on the obtained two-dimensional spectrum, the obtained spectrum peak point corresponds to a target, and can be calculated according to the peak coordinates The frequency of the beat signal corresponding to the target is obtained, and then the phase difference of the peak point at the adjacent LFSK signal is obtained by calculating the phase-frequency response, and the frequency and phase difference of the beat signal are integrated, and after defuzzification processing, the target's distance and speed.

Compared with the prior art, the present invention has the remarkable advantages as follows: (1) the present invention combines the advantages of the linear frequency modulation continuous wave system and the multiple frequency keying system, and makes up for their deficiencies; (2) when detecting the target, Using the frequency and phase difference of the beat signal to solve the ambiguity of distance and speed, there is no interference from false targets, and no target pairing is required, so the amount of calculation is moderate; (3) The transmission waveform is composed of multiple groups of LFSK signals with different frequency intervals , On the basis of unambiguous distance, the accuracy of distance measurement and speed measurement is significantly improved, and the measurement is more accurate; (4) When detecting targets, the upper and lower scanning frequency bands are used to work together, effectively reducing the detection blind zone.

Description of drawings

FIG. 1 is an overall schematic diagram of an MS-LFSK transmission waveform.

Fig. 2(a) is a schematic diagram of MS-LFSK up-sweep frequency band, and Fig. 2(b) is a schematic diagram of MS-LFSK down-sweep frequency band.

Fig. 3 is the relationship diagram of the phase difference estimation error and the ranging error caused by it in the present invention, wherein (a) is a schematic diagram of the probability density of the phase difference estimation error, and (b) is a schematic diagram of the probability density of the ranging error.

Fig. 4 is a flow chart of signal processing of the above frequency sweep band as an example in the present invention.

Fig. 5 is a schematic diagram of distance deblurring in the present invention.

Fig. 6 is a schematic diagram of blind areas of the present invention, wherein (a) is a schematic diagram of detection blind areas in up and down frequency scanning bands, and (b) is a schematic diagram of blind areas in MS-LFSK up and down frequency scanning joint detection.

Fig. 7 is a comparison diagram of the detection accuracy of MS-LFSK and LFMCW and MFSK of the present invention as the input signal-to-noise ratio changes, wherein (a), (b), (c), and (d) are respectively the average value of distance measurement, the average value of speed measurement, Schematic diagram of the comparison between the distance measurement standard deviation and the speed measurement standard deviation.

Detailed ways

The present invention proposes a transmission waveform design method suitable for automobile anti-collision radar, and the target detection algorithm corresponding to the generated waveform, which mainly includes three steps: waveform structure design, waveform parameter design and signal processing design, wherein the first two steps are In the steps of the waveform design method, the third step is the process of using the designed launch waveform for target detection. The structural design of the MS-LFSK launch waveform is shown in Figure 1.

Step 1: Design the structure of the MS-LFSK transmission waveform: determine that the system of the radar transmission signal is the MS-LFSK system, and a cycle of MS-LFSK waveform is composed of an up-sweep frequency band and a down-sweep frequency band, and each sweep frequency band is composed of several LFSK signals synthesis;

The MS-LFSK waveform of one cycle of the present invention is composed of an up-sweep frequency segment and a down-sweep frequency segment, as shown in FIG. 1 . An up-sweep frequency segment and a down-sweep frequency segment together form a coherent processing cycle, and each frequency scan segment is formed by alternately arranging M LFSK signals.

The waveform of the up-sweep frequency band is shown in Figure 2(a), which is composed of M LFSK signals S _m,n : each frequency step signal, such as S _1,N , has N steps, and the The duration is T _Step , the time interval between two steps is (M-1)T _Step , and the frequency difference is f _Incr ; with respect to the M frequency step signals as a whole, the start time of each signal is T _Step lags behind the previous signal, and the starting frequency is also reduced compared to the previous signal That is:

is the frequency corresponding to the step S _m,n .

The waveform of the down-sweep frequency is a mirror-symmetrical form of the up-sweep frequency waveform, and the waveform is shown in Fig. 2(b).

Step 2: Design the waveform parameters of the MS-LFSK transmission signal: Determine the parameters of the transmission waveform according to the given indicators. The indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and The minimum detectable noise ratio in the frequency domain, the parameters required to determine a waveform are: FM bandwidth B, phase coherent period T _CPI , the number of LFSK signals M, the number of steps of each LFSK signal N and the starting point between LFSK signals initial frequency difference

Taking the up-sweep frequency of the transmitted signal as an example, the FM bandwidth is determined by the distance resolution ΔR, namely

where c is the speed of light;

When determining the coherent processing period T _CPI , in order to prevent the range gate from moving when doing FFT, it is required that the maximum displacement of the vehicle must not exceed the distance resolution ΔR within a frequency sweep period, that is,

V _max is the maximum detectable speed;

After obtaining the value of T _CPI , according to the formula The frequency sweep slope can also be obtained;

According to the sampling theorem, the sampling rate of each frequency keyed signal It should not be less than the difference between the maximum value and the minimum value of the beat signal frequency, that is

Where R _max is the maximum detection distance, |V _max | is the absolute value of the maximum detection velocity, f _d.max is the maximum value of the Doppler frequency, τ _max is the maximum value of the echo delay, λ is the wavelength of the transmitted signal . And in order to use FFT to improve the operation speed, on the basis of the above formula, the step number N of each LFSK signal should also satisfy:

log ₂ (N)∈Z (5)

Z is an integer;

Therefore, according to N, the frequency difference f _Incr of adjacent steps in an LFSK signal and the duration of each step T _Step can be obtained:

In the MS-LFSK system, the frequency and phase of the beat signals corresponding to different LFSK signals of the target are the same, and the ambiguity of distance and velocity can be resolved by using the frequency and phase difference at the target spectral line. The phase at the peak spectral lines corresponding to two adjacent LFSK signals and do difference, get phase difference

The distance R ₀ from the target satisfies:

f _b is the beat signal frequency at the target spectral line;

remember The estimated error of The smaller the distance is, the higher the accuracy is, and it follows that the expectation is 0 and the variance is normal distribution of , where Satisfy:

SNR is the signal-to-noise ratio in the time domain, (SNR·N) constitutes the signal-to-noise ratio in the frequency domain after the beat signal is matched and filtered, δ is the error between the real value and the measured value of the beat frequency, and δ satisfies |δ|≤0.5;

As shown in Figure 3(a), this is the probability density map of the phase difference estimation error of the target at the minimum detectable signal-to-noise ratio in the frequency domain and when δ=0.5, because the distribution of the phase difference estimation error and the ranging error is the same , so the probability density of the ranging error is similar to that of the phase difference, as shown in Figure 3(b).

Under the condition of the minimum detectable signal-to-noise ratio, choose an appropriate confidence interval (1-α) for interval estimation of normal distribution, and get:

From this, the maximum value of the phase difference estimation error under the condition of the minimum detectable signal-to-noise ratio can be estimated

according to and ranging accuracy σ _τ to calculate the minimum value of the unambiguous distance

When the unambiguous distance of the transmitted signal is not less than When , it corresponds to the calculated distance and can meet the requirements of ranging accuracy.

The upper limit of the unambiguous distance can theoretically select the maximum detection distance. Considering the influence of noise, it can be relaxed to:

In the present invention, the method of unambiguous distance recursion is used to improve detection accuracy, and the unambiguous distance needs to be bisected from start equally as which is

up to unambiguous distance less than

The difference from the initial frequency of each LFSK signal Satisfy the following relationship:

smaller, The larger it is, the less likely it is to produce ranging blur.

The number of bisections determines the number M of LFSK signals, which are:

From this we can get each unambiguous distance then Substituting the value of into formula (14), we can get

So far, the up-sweep waveform of the MS-LFSK signal can be determined according to the above parameters, and the down-sweep waveform is the mirror image of the up-sweep, so the transmit waveform parameters of the down-sweep can also be deduced.

Step 3: Design the signal processing method of MS-LFSK: process the echo data of the transmitted signal, and solve the distance and speed of the target: mix the received echo signal with the local oscillator to obtain the beat signal, and sample it And rearrange it according to the echo data corresponding to the LFSK signal, do FFT and periodogram accumulation for the rearranged data according to the frequency sweep cycle, and perform OS-CFAR on the obtained two-dimensional spectrum, and the peak point of the obtained spectrum is Corresponding to a target, the beat signal frequency corresponding to the target can be calculated according to the peak coordinates, and then the phase difference of the peak point at the adjacent LFSK signal is calculated from the phase-frequency response, and the beat signal frequency and phase difference are integrated, and then defuzzified After processing, the distance and speed of the target can be calculated.

In the up-sweep frequency band of MS-LFSK, the process of signal processing is shown in Fig. 4 . The specific process is as follows:

Sample the beat signal. Mix the received echo signal with the local oscillator to obtain the beat signal, and sample the beat signal at the end of each T _Step to obtain an array of sampling results

Data rearrangement. the array According to the value of M, it is rearranged into a two-dimensional array of M×N, that is, each row of the two-dimensional array corresponds to a beat result of a complete frequency step signal arranged in the order of transmission.

FFT with periodogram accumulation. Perform FFT on each row of the two-dimensional array separately to obtain M frequency spectra W _k [N], k=1,2,...,M. Take the modulus of W _k [N], do the modulus square and then add up the results of the M rows correspondingly to complete the accumulation of the periodogram, that is

Perform constant false alarm detection for spectral peaks. Since there may be many targets in the actual environment, OS-CFAR is adopted.

Find the frequency and phase corresponding to the spectral peak. Assuming that the detection unit where the spectral peak exceeding the threshold is located is p, then the corresponding beat frequency f _b (p) can be determined by the following formula:

The phase corresponding to the spectral peak for:

Then the phase difference between two adjacent spectral peaks is:

Solve for blur distance According to known parameters f _b (p) and Solve the fuzzy distance by the following formula

k=1,2,...,M-1

In the above formula, The value of The reduction of the distance gradually becomes more accurate, but it also becomes more and more blurred, so it is necessary to deblur the distance, as shown in Figure 5. Use R ₀ to represent the distance of the final detected target, first put Assign the value to R ₀ , and then calculate when k is from 2 to M-1 Find q ^k (p) satisfying:

and iterate the new result to R ₀ , ie

Iterate until k=M-1 to obtain the final measured target distance R ₀ .

The speed at which the target is solved. Substitute R ₀ into the following formula to get the speed V ₀ of the target:

So far, the calculation of the target distance and speed has been completed in the up-sweep frequency band, and the data processing method in the down-sweep frequency band is consistent with the up-sweep frequency band.

In addition, in the actual system, since the antenna of the continuous wave radar is separated from the transceiver and works all the time, the receiving antenna will also receive the transmitted signal while receiving the target echo signal, then the interference zero will be generated after mixing. frequency signal, which may overwhelm the target echo signal near zero frequency, forming a detection blind zone, as shown in Figure 6(a). It can be seen from the schematic diagram that most of the detection blind areas in the upper and lower scanning frequency bands do not overlap. Therefore, in MS-LFSK, using the upper and lower scanning frequency bands to jointly detect targets can detect targets that are far away from the original blind area in the upper scanning frequency band, and detect them in the lower scanning frequency band. Go to the approaching target in the original blind area to reduce the detection blind area, as shown in Figure 6(b).

The present invention will be described in detail below in conjunction with examples.

Example

In the present embodiment, the radar index of setting is as shown in table 1:

Table 1

The parameters of the MS-LFSK waveform designed according to the above indicators are shown in Table 2.

Table 2

In addition, in order to compare the performance of various transmission waveforms, we designed the waveform parameters of LFMCW and MFSK under the same index.

Taking LFMCW as an example, the transmit waveform parameters under this system are shown in Table 3:

table 3

The transmit waveform parameters under the MFSK system are shown in Table 4:

Table 4

Figure 7 shows the simulation results of matlab. Set a single target with a distance of 30m and a speed of 10m/s, set the frequency domain signal-to-noise ratio between 15dB and 40dB, and measure the distance and speed of the three waveforms every 1dB. Take 1000 Monte Carlo experiments and study the mean and standard deviation of the distances and velocities obtained from them.

In the figure, * indicates the detection effect of the frequency modulation slope LFMCW, + indicates the detection result of the MFSK signal, and o depicts the detection result under the MS-LFSK system.

It can be seen from Fig. 7(a) and Fig. 7(b) that when the SNR is high, all three waveforms can accurately measure distance and speed; and when the SNR in the frequency domain is lower than 20dB, the FM slope LFMCW The measurement mean value of MFSK has a large deviation, and the MFSK signal can no longer be measured correctly. On the contrary, the measurement result of the MS-LFSK waveform is still very accurate.

It can be seen from Fig. 7(c) and Fig. 7(d) that when the SNR is high, the accuracy of the distance measurement and speed measurement of the three waveforms is relatively high; and when the SNR in the frequency domain is lower than 19dB, the FM slope The accuracy of distance measurement and speed measurement of LFMCW has dropped significantly; when the signal-to-noise ratio in the frequency domain is lower than 24dB, the measurement accuracy of MFSK signals has not reached the standard. In the process of changing the SNR in the frequency domain, the measurement accuracy of the MS-LFSK waveform is always high.

To sum up the above, measured from the two aspects of detection accuracy and precision, when the signal-to-noise ratio in the frequency domain decreases, the detection performance of the FM slope LFMCW decreases significantly, and the performance of the MFSK system is more obvious in terms of performance degradation. On the contrary, in In the process of frequency domain SNR change, the detection effect of MS-LFSK waveform has been very good.

Claims (7)

1. a method for designing a launch waveform suitable for automobile anti-collision radar, is characterized in that, comprises the following steps: Step 1, determine that the system of the radar transmitting signal is the MS-LFSK system, a cycle of MS-LFSK waveform is composed of an up-sweep frequency segment and a down-sweep frequency segment, and each frequency scan segment is synthesized by several LFSK signals; Step 2. Determine the parameters of the transmission waveform according to the given indicators. The indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and minimum signal detectable noise ratio in the frequency domain. Determine The parameters required for a waveform of one cycle include: FM bandwidth, coherent processing interval, number of LFSK signals, number of steps for each LFSK signal, and initial frequency difference between LFSK signals. 2. according to claim 1, be applicable to the transmission waveform design method of automobile anti-collision radar, it is characterized in that, step 1 is specially: A cycle of MS-LFSK waveform consists of an up-sweep frequency band and a down-sweep frequency band, and an up-sweep frequency band and a down-sweep frequency band together form a coherent processing cycle T _CPI , and each sweep frequency band is formed by alternate arrangement of M LFSK signals ; The up-sweep frequency band is composed of M LFSK signals S _m,n : each frequency step signal has N steps, where the duration of each step is T _Step , and the time interval between two steps is ( M-1) T _Step , the frequency difference is f _Incr ; the start time of each signal is T _Step behind the previous signal, and the start frequency is also smaller than that of the previous signal That is: is the frequency corresponding to the step S _m,n ; The waveform of the frequency sweep down is the mirror symmetrical form of the frequency sweep up waveform. 3. according to claim 1, be applicable to the launch waveform of automobile anti-collision radar and corresponding target detection algorithm, it is characterized in that, the specific method of step 2 is: First, the transmit waveform parameters that need to be known are as follows: FM bandwidth B, coherent processing period T _CPI , number of LFSK signals M, step number N of each LFSK signal and initial frequency difference between each LFSK signal Taking the up-sweep frequency of the transmitted signal as an example, the FM bandwidth is determined by the distance resolution ΔR, namely where c is the speed of light; When determining the coherent processing period T _CPI , the maximum displacement of the vehicle must not exceed the distance resolution ΔR within a frequency sweep period, that is, V _max is the maximum detectable speed; After obtaining the value of T _CPI , according to the formula The frequency sweep slope can be obtained; According to the sampling theorem, the sampling rate of each frequency keyed signal It should not be less than the difference between the maximum value and the minimum value of the beat signal frequency, that is where R _max is the maximum detectable distance, |V _max | is the absolute value of the maximum detectable velocity, f _d.max is the maximum value of Doppler frequency, τ _max is the maximum value of echo delay, λ is the transmitted signal wavelength; on the basis of the above formula, the step number N of each LFSK signal should also satisfy: log ₂ (N)∈Z (5) Z is an integer; Therefore, according to N, the frequency difference f _Incr of adjacent steps in an LFSK signal and the duration of each step T _Step can be obtained: The phase at the peak spectral line corresponding to two adjacent LFSK signals and do difference, get phase difference The distance R ₀ from the target satisfies: f _b is the beat signal frequency corresponding to the target spectral line; The estimated error of It has an expectation of 0 and a variance of normal distribution of , where Satisfy: SNR is the signal-to-noise ratio in the time domain, (SNR·N) constitutes the signal-to-noise ratio in the frequency domain after the beat signal is matched and filtered, δ is the error between the real value of the beat frequency and the measured value, and δ satisfies |δ|≤0.5; Under the condition of the minimum detectable signal-to-noise ratio, choose the confidence interval (1-α) for the interval estimation of the normal distribution, and get: From this, the maximum value of the phase difference estimation error under the condition of the minimum detectable signal-to-noise ratio is estimated according to and ranging accuracy σ _τ to calculate the minimum value of the unambiguous distance Find an upper bound on the unambiguous distance Use the bisection method to divide the unambiguous distance from start equally as which is up to unambiguous distance less than The difference from the initial frequency of each LFSK signal Satisfy the following relationship: The number of bisections determines the number M of LFSK signals, which are: From this, each unambiguous distance can be obtained then Substituting the value of into formula (14), we get Determine the up-sweep waveform of the MS-LFSK signal according to the above parameters, and the down-sweep waveform is the mirror image of the up-sweep. 4. A target detection algorithm applicable to automobile collision avoidance radar, is characterized in that, comprises the following steps: Step 1, determine that the system of the radar transmitting signal is the MS-LFSK system, a cycle of MS-LFSK waveform is composed of an up-sweep frequency segment and a down-sweep frequency segment, and each frequency scan segment is synthesized by several LFSK signals; Step 2. Determine the parameters of the transmission waveform according to the given indicators. The indicators include: radar transmission frequency band, maximum detection distance, maximum detection speed, distance resolution, ranging accuracy, speed measurement accuracy and minimum signal detectable noise ratio in the frequency domain. Determine The parameters required for a cycle waveform are: FM bandwidth, coherent processing interval, number of LFSK signals, number of steps for each LFSK signal, and initial frequency difference between LFSK signals; Step 3, process the echo data of the transmitted signal to solve the distance and speed of the target: mix the received echo signal with the local oscillator to obtain the beat signal, sample it and correspond to the LFSK signal according to the echo data Rearrange it according to the situation, do FFT and periodogram accumulation on the rearranged data according to the sweep cycle, and perform OS-CFAR on the obtained two-dimensional spectrum, the obtained spectrum peak point corresponds to a target, and calculate the target according to the peak coordinates The corresponding beating signal frequency is calculated from the phase-frequency response to obtain the phase difference of the peak point at the adjacent LFSK signal, and the frequency and phase difference of the beating signal are integrated, and then the distance and speed of the target are obtained through defuzzification processing. 5. the target detection algorithm applicable to automobile collision avoidance radar according to claim 4, is characterized in that, step 1 is specially: A cycle of MS-LFSK waveform consists of an up-sweep frequency band and a down-sweep frequency band, and an up-sweep frequency band and a down-sweep frequency band together form a coherent processing cycle T _CPI , and each sweep frequency band is formed by alternate arrangement of M LFSK signals ; The up-sweep frequency band is composed of M LFSK signals S _m,n : each frequency step signal has N steps, where the duration of each step is T _Step , and the time interval between two steps is ( M-1) T _Step , the frequency difference is f _Incr ; as far as the M frequency step signals are concerned as a whole, the start time of each signal is T _Step behind the previous signal, and the start frequency is also higher than that of the previous signal. decrease That is: is the frequency corresponding to the step S _m,n ; The waveform of the frequency sweep down is the mirror symmetrical form of the frequency sweep up waveform. 6. the target detection algorithm applicable to automobile collision avoidance radar according to claim 4, characterized in that, step 2 is specifically: First, the transmit waveform parameters that need to be known are as follows: FM bandwidth B, coherent processing period T _CPI , number of LFSK signals M, step number N of each LFSK signal and initial frequency difference between each LFSK signal Taking the up-sweep frequency of the transmitted signal as an example, the FM bandwidth is determined by the distance resolution ΔR, namely where c is the speed of light; When determining the coherent processing period T _CPI , the maximum displacement of the vehicle must not exceed the distance resolution ΔR within a frequency sweep period, that is, V _max is the maximum detectable speed; After obtaining the value of T _CPI , according to the formula The frequency sweep slope can be obtained; According to the sampling theorem, the sampling rate of each frequency keyed signal It should not be less than the difference between the maximum value and the minimum value of the beat signal frequency, that is where R _max is the maximum detectable distance, |V _max | is the absolute value of the maximum detectable velocity, f _d.max is the maximum value of Doppler frequency, τ _max is the maximum value of echo delay, λ is the transmitted signal wavelength; on the basis of the above formula, the step number N of each LFSK signal should also satisfy: log ₂ (N)∈Z (5) Z is an integer; Therefore, according to N, the frequency difference f _Incr of adjacent steps in an LFSK signal and the duration of each step T _Step can be obtained: The phase at the peak spectral line corresponding to two adjacent LFSK signals and do difference, get phase difference The distance R ₀ from the target satisfies: f _b is the beat signal frequency corresponding to the target spectral line; The estimated error of It has an expectation of 0 and a variance of normal distribution of , where Satisfy: SNR is the signal-to-noise ratio in the time domain, (SNR·N) constitutes the signal-to-noise ratio in the frequency domain after the beat signal is matched and filtered, δ is the error between the real value of the beat frequency and the measured value, and δ satisfies |δ|≤0.5; Under the condition of the minimum detectable signal-to-noise ratio, choose the confidence interval (1-α) for the interval estimation of the normal distribution, and get: From this, the maximum value of the phase difference estimation error under the condition of the minimum detectable signal-to-noise ratio is estimated according to and ranging accuracy σ _τ to calculate the minimum value of the unambiguous distance Find an upper bound on the unambiguous distance Use the bisection method to divide the unambiguous distance from start equally as which is up to unambiguous distance less than The difference from the initial frequency of each LFSK signal Satisfy the following relationship: The number of bisections determines the number M of LFSK signals, which are: From this, each unambiguous distance can be obtained then The value of is substituted into formula (14), to get Determine the up-sweep waveform of the MS-LFSK signal according to the above parameters, and the down-sweep waveform is the mirror image of the up-sweep. 7. the target detection algorithm that is applicable to automobile anti-collision radar according to claim 4, is characterized in that, step 3 is specially: Sampling the beat signal; mixing the received echo signal with the local oscillator signal to obtain the beat signal, and sampling the beat signal at the end of each T _Step to obtain an array of sampling results data rearrangement; array According to the value of M, it is rearranged into a two-dimensional array of M×N, that is, each row of the two-dimensional array corresponds to the beat result of a complete frequency step signal arranged in the order of transmission; FFT and periodogram accumulation; perform FFT on each row of the two-dimensional array separately to obtain M spectrum W _k [N],k=1,2,...,M; take the modulus of W _k [N] and do the modulus After squaring, the results of the M rows are added correspondingly to complete the accumulation of the periodogram, that is Carry out constant false alarm detection of spectral peaks; Solve the frequency and phase corresponding to the spectral peak; assuming that the detection unit where the spectral peak exceeds the threshold is p, then the corresponding beat frequency f _b (p) can be determined by the following formula: The phase corresponding to this spectral peak for: The phase difference between two adjacent spectral peaks is: Solve for blur distance According to known parameters f _b (p) and Solve the fuzzy distance by the following formula Solve the unambiguous distance; use R ₀ to represent the distance of the final detected target, first put Assign the value of R 0 to R ₀ , and then calculate when k is from 2 to M-1 Find q ^k (p) satisfying: and iterate the new result to R ₀ , ie Iterate until k=M-1 to obtain the final measured target distance R ₀ . Find the speed of the target; Substitute R ₀ into the following formula to get the speed V ₀ of the target: So far, the calculation of the target distance and speed has been completed in the up-sweep frequency band, and the data processing method in the down-sweep frequency band is consistent with the up-sweep frequency band.