CN110609264B - Target echo Doppler frequency estimation method for pulse laser radar - Google Patents
Target echo Doppler frequency estimation method for pulse laser radar Download PDFInfo
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Abstract
The invention discloses a target echo Doppler frequency estimation method for a pulse laser radar, and relates to the technical field of laser radar signal processing. Aiming at the linear frequency modulation echo signal, when the signal-to-noise ratio is high, the Doppler frequency shift can be roughly estimated only according to the signal bandwidth and the frequency of the transmitted signal through Fourier transform, and the technical scheme of the invention can directly and accurately estimate the central frequency of the signal so as to obtain the Doppler frequency shift of the echo; when the signal-to-noise ratio is low, the signal frequency can not be observed by the Fourier transform method, and the Doppler frequency of the echo signal can still be estimated in a small error range by the technical scheme of the invention.
Description
Technical Field
The invention relates to the technical field of laser radar signal processing, in particular to an echo signal Doppler frequency offset estimation method of a linear frequency modulation pulse laser radar, which is particularly suitable for estimating the Doppler frequency offset of a single-pulse periodic target echo in a low signal-to-noise ratio environment.
Background
Lidar is a measurement method that is currently rapidly developed and widely used. The method has the advantages of short operation time, high measurement precision, small weather influence, safe operation, no limitation of terrain or region, and capability of simultaneously measuring the ground layer and the non-ground layer. Due to the constant velocity of the light, the time is related to the distance of the emitter, and thus spatial information is detected along the electron beam trajectory. The radial velocity of the target is detected by receiving a signal frequency offset from the target. It has very high angle resolution, distance resolution and strong anti-interference ability. Therefore, with the continuous development and popularization of the technology, the laser radar is widely developed in the military and civil fields.
The laser radar firstly transmits laser pulses through a laser transmitter, then a telescope receives reflected signals, a photoelectric detector is used for carrying out photoelectric conversion on the reflected signals, and the reflected signals are converted into digital signals after sampling to carry out signal processing so as to obtain target information.
Due to the complex working environment of the radar, more background noise can be mixed in the echo, and the signal-to-noise ratio of the laser radar echo signal is lower. Aiming at the signal characteristics of the laser radar, the current general signal processing method is incoherent accumulation, and the incoherent accumulation method is adopted to carry out frequency domain accumulation on the signal, so that the signal spectrum is averaged, and the signal spectrum identification degree is improved, but the method only has limited improvement on the signal-to-noise ratio, and still cannot effectively identify the signal spectrum line for echo signals with low signal-to-noise ratio; if the transmitted signal has full coherence, coherent accumulation can be directly performed, however, due to the limitation of the laser radar stimulated light emitter, random abrupt change of the phase is easily introduced, and pulse coherence is difficult to achieve. In addition, due to the characteristics of the chirp signal, the frequency spectrum of the echo signal occupies a certain bandwidth, and it is difficult to directly observe the center frequency of the frequency spectrum under the influence of noise, so that doppler information to a target cannot be obtained
Therefore, how to effectively detect the weak echo signal of the laser radar in real time under the condition of low signal-to-noise ratio and effectively estimate the echo Doppler frequency offset is a key problem in the application of the laser radar.
Disclosure of Invention
Aiming at the existing problems, the invention provides a frequency estimation method of frequency domain windowing convolution, which realizes Doppler frequency offset estimation of echo signals of a linear frequency modulation pulse laser radar and can effectively estimate Doppler information of a target under the condition of low signal-to-noise ratio only by one pulse period.
Assuming that there is a moving object, when the pulse of the reflected signal reaches the receiver, the signal frequency is shifted due to the existence of the doppler effect; the shift in the received signal frequency reflects the velocity information of the target; echo data in a pulse repetition period is selected, fourier transform is carried out, and the time domain is converted into the frequency domain. And performing Fourier transform on the transmitted signal, converting the transmitted signal from a time domain to a frequency domain, windowing the transmitted signal in the frequency domain by taking the bandwidth as a scale, and intercepting the effective range of the frequency spectrum. And convolving the frequency spectrum after windowing and the frequency spectrum of the echo signal in a frequency domain. And performing peak value search on the convolution result, wherein the position corresponding to the maximum value is the Doppler frequency of the echo signal. The transmitting signal and the echo signal are both chirp signals, the frequency spectrums of the transmitting signal and the echo signal can be approximate to two rectangular windows in the frequency domain, therefore, the rectangular windows are convoluted in the frequency domain, a spectrum peak can be formed at the position of the Doppler frequency, and the peak value search is carried out on the convolution result, so that the Doppler frequency of the echo signal can be obtained.
The technical scheme of the invention is a target echo Doppler frequency estimation method aiming at a pulse laser radar, wherein a transmitting signal and an echo signal of the pulse laser radar are both linear frequency modulation signals, and the method comprises the following steps:
step 1: after receiving an echo signal, a receiver firstly performs frequency selection filtering and amplification to filter noise and clutter out of a signal frequency band; then, frequency mixing and band-pass filtering amplification are carried out, and radio frequency signals are converted into intermediate frequency signals;
and 2, step: performing A/D conversion on the intermediate frequency signal in the step 1 to obtain a digital signal, performing digital down-conversion, and performing quadrature frequency mixing and low-pass filtering to obtain I, Q two paths of baseband signals;
and step 3: synthesizing the two paths of baseband signals in the step 2 into a complex signal according to the sampling rate f of the receiver s And the pulse repetition period PRI of the transmitted signal and the PRI synchronizing signal to select a segment of length f s * Taking a complex signal in a pulse repetition period of the PRI as an echo signal, and performing Fourier transform to convert the complex signal from a time domain to a frequency domain;
and 4, step 4: preprocessing an original baseband emission signal, performing Fourier transform on the baseband emission signal with the pulse width tau, converting the baseband emission signal from a time domain to a frequency domain to obtain a frequency spectrum signal, wherein the frequency spectrum range is [ -f [ ] s /2,f s /2]The bandwidth Bw is far larger than the signal bandwidth, the signal in the frequency spectrum signal bandwidth is intercepted by carrying out frequency domain windowing by taking the central frequency as the center and the bandwidth Bw as the scale, and the window length is the bandwidth Bw;
and 5: convolving the frequency spectrum in the transmission signal bandwidth intercepted in the step 4 with the echo signal frequency spectrum obtained in the step 1 in a frequency domain, and performing convolution according to the frequency axis-f in the convolution process s /2～f s /2 is changed into-f s /2～f s The coordinate axis of the convolution result is converted,/2 + Bw;
and 6: and searching the maximum value of the convolution result after the coordinate axis conversion on a frequency axis, wherein the coordinate of the frequency axis corresponding to the maximum value is the Doppler frequency shift of the echo signal plus the half bandwidth Bw/2, and the frequency coordinate corresponding to the echo signal minus the Bw/2 is the Doppler frequency shift of the target echo.
Further, the signals obtained by performing a/D conversion on the intermediate frequency signals in step 2 are:
x(t)＝s(t)+n(t)
wherein:
wherein f is 0 Carrier frequency of signal, f d Is Doppler shift, n (t) is white Gaussian noise;
in step 4, the original baseband emission signal is:
where μ is the slope of the frequency change.
The technical scheme of the invention is firstly proposed in the aspect of estimating the center frequency of a linear frequency modulation signal, aiming at a linear frequency modulation echo signal, when the signal-to-noise ratio is high, the Doppler frequency shift can be only roughly estimated according to the signal bandwidth and the frequency of a transmitted signal through Fourier transform, and the technical scheme of the invention can directly and accurately estimate the center frequency of the signal and further obtain the Doppler frequency shift of the echo; when the signal-to-noise ratio is low, the signal frequency can not be observed by the Fourier transform method, and the Doppler frequency of the echo signal can still be estimated in a small error range by the technical scheme of the invention.
Drawings
FIG. 1 is a flow chart of the algorithm of the present invention;
FIG. 2 is a spectrum diagram of a transmitted signal;
FIG. 3 is a graph of the spectrum of an echo signal at a signal-to-noise ratio of 0 dB;
FIG. 4 is a frequency domain windowed spectrum diagram of a transmitted signal
FIG. 5 shows the frequency estimation result in 0 dB;
FIG. 6 is a diagram of the echo signal spectrum at-5 dB;
FIG. 7 shows the result of frequency estimation in-5 dB;
FIG. 8 is a graph of the spectrum of the echo signal at-10 dB;
FIG. 9 shows the result of the frequency estimation in-10 dB.
Detailed Description
In fig. 1, the expression of the transmission signal is as follows
Wherein μ =3 x 10 13 Hz/s is the slope of the frequency change, and the bandwidth is 60MHz. Its spectrogram is shown in fig. 2.
Let the intermediate frequency after A/D sampling of the received signal be expressed as follows
x(t)＝s(t)+n(t) (2)
In the formula
Is a useful signal, wherein f 0 =100MHz as the carrier frequency of the signal, f d The doppler shift is =200MHz, the white gaussian noise is n (t), and the spectrum obtained by adding the white gaussian noise having a signal-to-noise ratio of 0dB is shown in fig. 3.
The frequency spectrum of the transmitted signal is windowed in the frequency domain with the scale of bandwidth, and the signal bandwidth range in fig. 2 is intercepted, and the result is shown in fig. 4.
The frequency spectrum after windowing and the frequency spectrum of the echo signal are convolved in the frequency domain, and the obtained result is shown in fig. 5. The frequency corresponding to the peak point is the Doppler frequency of the echo signal of 200MHz.
The invention will be further elucidated with reference to specific embodiments.
Example 1:
step one, the center frequency of a transmitting signal is taken as 100MHz, the pulse width is 2us, the bandwidth is 60MHz, the pulse repetition period is 10us, and the laser wavelength is 1500nm.
And step two, setting the sampling rate to be 1GHz, simulating the signal-to-noise ratio to be-5 dB, simulating the Doppler frequency shift of a target to be 100MHz, namely simulating the speed of the target to be 75m/s, and showing the spectrum of an echo signal as shown in figure 6.
And step three, adding a window with the bandwidth of 60MHz to the frequency spectrum of the transmitting signal in a frequency domain, and intercepting an effective frequency spectrum.
And step four, convolving the frequency spectrum after windowing and the frequency spectrum of the echo signal in a frequency domain, wherein the result is shown in fig. 7. The frequency measurement result is 100MHz and the frequency estimation error is 0Hz.
And fifthly, performing Monte Carlo simulation for 500 times, wherein the average Doppler frequency detection result is 99999200Hz, and the average measurement error is 800Hz.
Example 2:
step one, the center frequency of a transmitting signal is taken as 100MHz, the pulse width is 2us, the bandwidth is 60MHz, the pulse repetition period is 10us, and the laser wavelength is 1500nm.
And step two, setting the sampling rate to be 1GHz, simulating the signal-to-noise ratio to be-10 dB, simulating the Doppler frequency shift of a target to be 55MHz, namely simulating the speed of the target to be 41.25m/s, and enabling the spectrum of the echo signal to be as shown in figure 8, wherein the effective range of the spectrum cannot be observed at the moment.
And step three, adding a window with the bandwidth of 60MHz to the frequency spectrum of the transmitting signal in a frequency domain, and intercepting an effective frequency spectrum.
And step four, convolving the frequency spectrum after windowing and the frequency spectrum of the echo signal in a frequency domain, wherein the result is shown in fig. 9. The frequency measurement result is 100MHz and the frequency estimation error is 0Hz.
And fifthly, performing Monte Carlo simulation for 500 times, wherein the average Doppler frequency detection result is 55005600Hz, and the average measurement error is 5600Hz.
Claims (2)
1. A target echo Doppler frequency estimation method for a pulse laser radar is disclosed, wherein a transmitting signal and an echo signal of the pulse laser radar are both linear frequency modulation signals, and the method comprises the following steps:
step 1: after receiving an echo signal, a receiver firstly performs frequency selection filtering and amplification to filter noise and clutter out of a signal frequency band; then, frequency mixing and band-pass filtering amplification are carried out, and the radio frequency signal is changed into an intermediate frequency signal;
step 2: performing A/D conversion on the intermediate frequency signal in the step (1), converting the intermediate frequency signal into a digital signal, performing digital down-conversion, and performing quadrature frequency mixing and low-pass filtering to obtain I, Q two paths of baseband signals;
and step 3: synthesizing the two paths of baseband signals in the step 2 into a complex signal according to the sampling rate f of the receiver s And the pulse repetition period PRI of the transmitted signal and the PRI synchronizing signal to select a segment of length f s * Taking a complex signal in a pulse repetition period of the PRI as an echo signal, and then performing Fourier transform to convert the complex signal from a time domain to a frequency domain;
and 4, step 4: preprocessing an original baseband emission signal, performing Fourier transform on the baseband emission signal with the pulse width tau, converting the time domain into the frequency domain to obtain a frequency spectrum signal, wherein the frequency spectrum range is [ -f ] s /2,f s /2]The bandwidth Bw is far larger than the signal bandwidth, the signal in the frequency spectrum signal bandwidth is intercepted by carrying out frequency domain windowing by taking the central frequency as the center and the bandwidth Bw as the scale, and the window length is the bandwidth Bw;
and 5: convolving the frequency spectrum in the transmission signal bandwidth intercepted in the step 4 with the echo signal frequency spectrum obtained in the step 1 in a frequency domain according to a volumeFrequency axis in the product process is represented by-f s /2～f s /2 is changed into-f s /2～f s The coordinate axis of the convolution result is converted by/2 Bw;
and 6: and carrying out maximum value search on the convolution result after coordinate axis conversion on a frequency axis, wherein the coordinate of the frequency axis corresponding to the maximum value is the Doppler frequency shift of the echo signal plus half bandwidth Bw/2, and the frequency coordinate corresponding to the echo signal minus Bw/2 is the Doppler frequency shift of the target echo.
2. The method according to claim 1, wherein the signal obtained by performing a/D conversion on the intermediate frequency signal in step 2 is:
x(t)＝s(t)+n(t)
wherein:
wherein f is 0 Carrier frequency of signal, f d Is Doppler shift, n (t) is white Gaussian noise;
in step 4, the original baseband emission signal is:
where μ is the slope of the frequency change.
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