A non-coherent detection lidar signal form and detection method based on Barker code modulation

Abstract

The present invention discloses a signal form and detection method for Barker code modulation of an incoherent laser radar. The steps are: 1: modulating the incoherent laser radar pulses into "0" and "1" signals according to the Barker code rule; 2: transmitting the laser radar signal, reflecting it from a target, and then receiving and processing it by the radar to obtain a corresponding digital signal; 3: applying a certain amount of bias to the digital signal within a pulse repetition period, where the bias value is the average value of the digital signal obtained by receiving and processing the current background light; 4: matching filtering the biased digital signal using the Barker code modulated pulse signal; and 5: further processing the matched filtered signal to detect the target and estimate its parameters. In a low-cost incoherent laser radar system, the application of Barker code modulation to the transmitted signal can achieve a significant improvement in the signal-to-noise ratio through digital pulse compression technology, thereby improving the detection performance and ranging accuracy of the incoherent laser radar.

Description

Technical Field

[0001] The present invention relates to the technical field of laser radar signal processing, and in particular to a signal form and detection method for Barker code modulation of an incoherent laser radar. The method is suitable for detecting and ranging low-reflectivity targets and long-distance targets using an incoherent laser radar. The pulse compression technology is applied in an incoherent laser radar system to improve the detection performance of the laser radar to a certain extent. Background Art

[0002] Existing lidar systems primarily include coherent lidar and incoherent lidar (direct detection lidar). Coherent lidar offers numerous advantages, including superior detection performance and robust anti-interference capabilities, but its optical system is complex and expensive. Incoherent lidar, while relatively simple and inexpensive, has a shorter detection range and is susceptible to interference from weather conditions such as cloud, fog, rain, and snow.

[0003] At present, laser radar signal modulation mainly includes: laser intensity modulation, laser pulse position modulation, phase modulation (based on pseudo-random code), laser linear frequency modulation signal, etc.

[0004] Laser pulse position modulation signal can obtain better matched filtering results, but it requires a larger number of pulses to ensure the matched filtering effect. In this case, the signal's ability to detect moving targets decreases.

[0005] Phase modulation signals, laser linear frequency modulation signals, and frequency modulated continuous wave signals belong to the signal system of coherent detection lidar. Although they have excellent detection performance, they are relatively expensive and are currently less used. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to improve the radar range and resistance to severe weather interference in an incoherent lidar system. The purpose of the present invention is to provide a signal form and detection method for incoherent lidar Barker code modulation. In a low-cost incoherent lidar system, the Barker code modulation transmission signal is applied, and a significant signal-to-noise ratio improvement can be achieved through digital pulse compression technology, thereby improving the detection performance and ranging accuracy of the incoherent lidar.

[0007] In order to achieve the above-mentioned purpose, the present invention adopts the following technical measures:

[0008] A signal form and detection method for Barker code modulation of an incoherent laser radar, comprising the following steps:

[0009] Step 1: Modulate the incoherent lidar pulse into "0" and "1" according to the Barker code rule;

[0010] In this step, the incoherent laser radar pulse is modulated into "0" and "1" according to the Barker code rule, specifically including:

[0011] A. Select a Barker code. Currently discovered Barker codes include: [1 1], [-1 1], [1 1 -1], [1 1 -11], [1 1 1 -1], [1 1 1 -1 1], [1 1 1 -1 -1 1 -1], [1 1 1 -1 -1 -1 -1 1 -1], [1 1 1 1 1 -1 -1 -1 1 -1 1 -1].

[0012] Each of the above Barker codes has four groups of isomorphic codes, namely original code, inverse code, reverse sequence code, and reverse complement code.

[0013] example:

[0014] Original code [1 1 1 -1]

[0015] One's complement [-1 -1 -1 1]

[0016] Reverse code [-1 1 1 1]

[0017] Negative complement [1 -1 -1 -1]

[0018] B. Assume that the selected Barker code sequence is c k (“0” “1” modulation), its code length is n, a single code element is a rectangular wave, and its pulse width is t p .

[0019] Then the laser radar transmits a signal s t (t) is expressed as follows:

[0020]

[0021] Among them, A is the amplitude of the transmitted laser signal, n is the code length, c k is the Barker code sequence, k=1,2,…,n is the code element number, rect is the rectangular window function, t is the time, t p is the pulse width.

[0022] The beneficial effect of the above further scheme is: by selecting a suitable Barker code and modulating it on the laser radar transmit pulse, the laser radar transmit signal is reflected, received and processed by the target, thereby obtaining a better matched filtering effect, thereby improving the signal-to-noise ratio without improving the laser performance.

[0023] Step 2: After the laser radar signal is transmitted, it is reflected by the target and then received and processed by the radar to obtain a corresponding digital signal; further: in step 2, the laser radar target reflection signal is received and processed specifically as follows:

[0024] 1) The reflected light signal of the laser radar target is converted into an electrical signal through the radar optical system and the photoelectric detector. The target echo signal power Pr is:

[0025]

[0026] Among them, η sys is the laser radar system loss, which mainly includes the optical system loss and the electronic system loss; η ato =e -2μR is the atmospheric loss, R is the target distance, μ is the atmospheric attenuation coefficient; P t is the transmitted signal power; is the signal transmission gain, θ a is the azimuthal beam width of the laser beam, θ c is the pitch beam width of the laser beam; σ is the target scattering cross-sectional area corresponding to the resolution unit; is the effective receiving area of ​​the receiving telescope, D is the aperture of the receiving telescope; Ω is the backscattering solid angle.

[0027] In addition, the final photodetector output electrical signal also includes: background light noise electrical signal, system thermal noise electrical signal.

[0028] 2): Use analog-to-digital conversion equipment to convert the electrical signal output by the photodetector into a digital signal.

[0029] The above further solution has the beneficial effect that by digitizing the received signal, a computer can further process the signal and obtain target detection results and measurement data. In addition, the target echo signal power estimation can be used as auxiliary data for subsequent processing of the received signal.

[0030] Step 3: Apply a certain amount of bias to the digital signal within a pulse repetition period to reduce the overall signal level (which may be negative); the specific implementation is:

[0031] 1) The applied bias should be the current estimated value of the background light interference. Without emitting a laser signal, record the electrical signal output by the photodetector and the digital signal converted from the electrical signal. Take the statistical average of the measured background light interference intensity as the estimated value.

[0032] 2) The applied bias should be updated in real time according to the changes in background light intensity. When the target echo signal is obtained, the digital-to-analog converter outputs the digital signal (including the target echo signal, background light interference signal, system noise, etc.) minus the bias value;

[0033] The beneficial effect of the above further scheme is that the background light interference is converted and output into a DC or low-frequency signal after passing through filters, photoelectric detectors and other devices, which will affect the detection performance of the incoherent laser radar to the target. Considering that the background light intensity change rate is much lower than the laser pulse repetition frequency, that is, the background light interference intensity has a strong correlation within several laser pulse repetition cycles, the current background light interference intensity is estimated and can be used as an offset value within several laser pulse repetition cycles. The digital-to-analog conversion output digital signal (including target echo signal, background light interference signal, system noise, etc.) minus the offset value can reduce the impact of background light interference to a certain extent.

[0034] Step 4: Use the pulse signal modulated by the Barker code to perform matched filtering on the biased digital signal; the specific implementation is:

[0035] The digital signal s after bias processing r (t) Use the Barker code with the same modulation to perform matched filtering, and the filtering result is s f (t), expressed as:

[0036]

[0037] Where s f (t) is the filtering result, s r (t) is the digital signal output by the analog-to-digital converter, s t (t) is the laser pulse signal modulated by the reverse Barker code, and t is time.

[0038] The beneficial effect of the above further solution is: improving the radar range resolution and receiving signal-to-noise ratio through matched filtering processing.

[0039] Step 5: Further process the matched filtered signal to detect the target and estimate its parameters.

[0040] The specific ones include:

[0041] 1): The filtering result s f (t) Perform constant false alarm processing and set the decision threshold, s f The points in (t) that exceed the decision threshold are determined to be targets.

[0042] 2): For points that exceed the decision threshold, determine the distance unit they are in and use the radar ranging formula to estimate the target distance, specifically:

[0043] R=ct / 2

[0044] Where c is the speed of light, and t is the time interval between the moment a certain distance unit is located and the moment the laser pulse is emitted.

[0045] The beneficial effect of the above further solution is that: by utilizing the currently mature lidar target detection and estimation algorithm, the target can be detected more accurately and the precise target distance can be obtained.

[0046] The most critical step is step 1, which modulates the incoherent lidar transmission signal according to the Barker code. The modulation method is a combination of laser pulse position modulation and laser pulse width modulation. Its special signal modulation method enables the echo signal to obtain a certain degree of signal-to-noise ratio improvement after pulse compression processing.

[0047] At present, most laser radars that can use pulse compression technology to improve the signal-to-noise ratio are coherent laser radars (linear frequency modulation signals). Coherent laser radars are expensive and have complex systems, making them difficult to put into practical use on a large scale.

[0048] Laser pulse position modulation signals can achieve better matched filtering results, but they require a higher number of pulses to maintain the matched filtering effect. In this case, the signal's ability to detect moving targets is reduced. Barker codes, on the other hand, have a shorter symbol length, with a maximum single Barker code length of 13 bits. If a 13-bit Barker code is used to modulate the transmitted signal, its ability to detect moving targets is not affected.

[0049] Compared with the prior art, the present invention has the following advantages and effects:

[0050] The present invention discloses a signal form and detection method for Barker code modulation of an incoherent laser radar. In an incoherent laser radar system, the waveform of the emitted laser pulse is modulated according to the Barker code, and then the echo is subjected to pulse compression processing. This can further improve the signal-to-noise ratio of the incoherent laser radar without increasing the peak power of the laser emission, thereby enhancing the detection capability of low-reflectivity targets and long-distance targets, and improving the ability to resist weather interference such as clouds, fog, rain, and snow. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1 Schematic diagram of laser signal modulation and emission according to the present invention.

[0052] Figure 2 This is the response curve of a 1064nm filter.

[0053] Figure 3 Schematic diagram of the signal receiving and processing system of the present invention.

[0054] Figure 4 This is a MATLAB simulation diagram of the time domain digital signal output by the analog-to-digital converter of the present invention (SNR = 0 dB, using 13-bit Barker code).

[0055] Figure 5 This is a Matlab simulation diagram of the time domain digital signal after matched filtering of the present invention.

[0056] Figure 6 This is a decision threshold diagram of the time domain digital signal after matched filtering in the present invention.

[0057] Figure 7 This is a comparison chart of the detection performance between the present invention and the rectangular wave incoherent lidar (10,000 Monte Carlo experiments).

[0058] Figure 8 It is a flow chart of the present invention. DETAILED DESCRIPTION

[0059] Example 1:

[0060] A signal form and detection method for Barker code modulation of an incoherent laser radar, comprising the following steps:

[0061] 1. Modulate the incoherent lidar pulses into "0" and "1" according to the Barker code rule;

[0062] In this step, the incoherent laser radar pulse is modulated into "0" and "1" according to the Barker code rule, specifically including:

[0063] A. Select a Barker code. Currently discovered Barker codes include: [1 1], [-1 1], [1 1 -1], [1 1 -11], [1 1 1 -1], [1 1 1 -1 1], [1 1 1 -1 -1 1 -1], [1 1 1 -1 -1 -1 -1 1 -1], [1 1 1 1 1 -1 -1 -1 1 -1 1 -1].

[0064] Each of the above Barker codes has four groups of isomorphic codes, namely original code, inverse code, reverse sequence code, and reverse complement code.

[0065] example:

[0066] Original code [1 1 1 -1]

[0067] One's complement [-1 -1 -1 1]

[0068] Reverse code [-1 1 1 1]

[0069] Negative complement [1 -1 -1 -1]

[0070] B. Assume that the selected Barker code sequence is c k (“0” “1” modulation), its code length is n, a single code element is a rectangular wave, and its pulse width is t p .

[0071] Then the laser radar transmits a signal s t (t) is expressed as follows:

[0072]

[0073] Where A is the amplitude of the transmitted laser signal, c k is the Barker code sequence, t p is the pulse width, n is the code length, k=1,2,…,n represents the code element number, t is the time, and rect represents the rectangular function.

[0074] The laser emission signal is modulated using a Barker code. Specifically, when the current symbol is "1," the laser emits light; when the current symbol is "0," the laser does not emit light. Barker code modulation of the laser signal is achieved by controlling the laser's emission time and intervals.

[0075] Figure 1 This is a schematic diagram of laser signal modulation and transmission in this embodiment. Using encoding and control equipment, two lasers sharing a common optical path are simultaneously controlled, coordinating (alternating) their laser signal emission, controlling their emission timing and intervals, and ultimately modulating and transmitting the laser signals according to the Barker code principle. While meeting the functional specifications of the lidar, these steps further reduce laser performance requirements and, to a certain extent, lower costs.

[0076] 2. After the laser radar signal is transmitted, it is reflected by the target and then received and processed by the radar to obtain a corresponding digital signal. Further: In this step, the laser radar target reflection signal is received and processed specifically as follows:

[0077] Step 21: The target echo signal and the background light interference signal enter the laser radar receiving system through the filter, filtering out the background light interference signal outside the laser radar working band, greatly reducing the interference to the target detection. Figure 2 This is the response curve of a 1064nm filter. When the laser radar operating wavelength is 1064nm, the transmittance of the laser signal at this wavelength is about 93%, and the transmittance of light signals at other wavelengths is extremely low, which can effectively filter out most of the background light interference.

[0078] Step 22: The target echo signal is converted into an electrical signal by the photoelectric detector. The target echo signal power Pr is:

[0079]

[0080] Among them, η sys is the laser radar system loss, which mainly includes the optical system loss and the electronic system loss; η ato =e -2μR is the atmospheric loss, e is a natural number, R is the target distance, μ is the atmospheric attenuation coefficient; P t is the transmitted signal power; is the signal transmission gain, θ a is the azimuthal beam width of the laser beam, θ cis the pitch beam width of the laser beam; σ is the target scattering cross-sectional area corresponding to the resolution unit; is the effective receiving area of ​​the receiving telescope, D is the aperture of the receiving telescope; Ω is the backscattering solid angle.

[0081] Step 23: Use an analog-to-digital conversion device to convert the electrical signal output by the photodetector into a digital signal.

[0082] Figure 3 Schematic diagram of the lidar signal receiving and processing system. Figure 4 It is the digital signal obtained by modulating the transmitted signal with a 13-bit Barker code under the premise of a signal-to-noise ratio of 0dB.

[0083] 3. Apply a certain amount of bias to the digital signal within a pulse repetition period to reduce the overall signal level (which may be negative); the specific implementation is:

[0084] Step 31: The applied bias should be the current estimated value of the background light interference. Without emitting a laser signal, record the electrical signal output by the photodetector and the digital signal converted from the electrical signal, and perform a statistical average of the measured background light interference intensity to use as the estimated value.

[0085] Step 32: The applied bias should be updated in real time according to the change in background light intensity. When the target echo signal is obtained, the digital-to-analog converter outputs the digital signal (including the target echo signal, background light interference signal, system noise, etc.) minus the bias value.

[0086] Considering the received power similar to the radar range equation, the effect of the filter

[0087] 4. Using a pulse signal modulated by a Barker code to perform matched filtering on a biased digital signal; the specific implementation is:

[0088] The digital signal s after bias processing r (t) Use the Barker code with the same modulation to perform matched filtering, and the filtering result is s f (t), expressed as:

[0089]

[0090] s t The laser emission signal is modulated by Barker code.

[0091] Through matched filtering processing, the radar range resolution and receiving signal-to-noise ratio are improved.

[0092] Figure 5 For the time domain signal after matching filtering, a significant improvement in the signal-to-noise ratio can be observed.

[0093] 5. Further process the signal after matched filtering to detect the target and estimate its parameters.

[0094] The specific ones include:

[0095] Step 51: Obtain the filtering result s in step 4 f Variance of (t):

[0096]

[0097] Where K is s f The number of sampling points (signal length), s f (k) is the kth sampling point value. For s f Mean.

[0098] Step 52: Calculate the decision threshold based on the variance of the filtering result and the false alarm probability:

[0099]

[0100] Where T is the decision threshold, P fa is the false alarm probability, and σ is the signal variance.

[0101] The filtered result s f (t) Perform constant false alarm processing and set the decision threshold, s f The points in (t) that exceed the decision threshold are determined to be targets.

[0102] Figure 6 When the false alarm probability is 0.001, the decision threshold (red line) is calculated after the time domain signal is processed by matched filtering. At this time, the target can be effectively detected and there is no false alarm within this pulse repetition period.

[0103] Step 53: For the points exceeding the decision threshold, determine the distance unit in which they are located and estimate the target distance using the radar ranging formula, specifically:

[0104] R=ct / 2

[0105] Where c is the speed of light, and t is the time interval between the moment a certain distance unit is located and the moment the laser pulse is emitted.

[0106] Through the above specific technical measures, the detection performance of the incoherent laser radar with Barker code modulation was tested. 10,000 Monte Carlo experiments were designed and compared with the ordinary rectangular wave signal incoherent laser radar. The detection probability curve of the same target is shown as follows: Figure 7As shown, when the signal-to-noise ratio is low, in the range of approximately -10 to -15 dB, the detection probability of both lidars is low, below 10%. At this time, the detection probability of the rectangular wave signal incoherent lidar is slightly higher than that of the present invention. When the signal-to-noise ratio is higher than -10 dB, the detection performance of the present invention improves rapidly, and its detection probability is significantly higher than that of the rectangular wave signal incoherent lidar. When the signal-to-noise ratio is approximately 3 dB, the detection probability of the present invention is close to 100%, indicating better detection performance.

[0107] In summary, the present invention has relatively better detection performance when the signal-to-noise ratio is low (not extremely low). That is, when the target echo is weak, it can guarantee the detection probability to a certain extent. For example, when the target has low reflectivity to the operating wavelength light wave, or when the laser signal is affected by rain, fog, haze, smoke, and dust.

Claims

1. A detection method for non-coherent detection laser radar signal form based on Barker code modulation, characterized in that: The steps include: Step 1: Modulate the incoherent lidar pulses into "0" and "1" according to the Barker code rule; Step 2: After the laser radar signal is transmitted, it is reflected by the target and then received and processed by the radar to obtain the corresponding digital signal; Step 3: Apply a certain amount of bias to the digital signal within a pulse repetition period, where the bias value is the average value of the digital signal obtained by receiving and processing the current background light; Step 4: Use the pulse signal modulated by the Barker code to perform matched filtering on the biased digital signal; Step 5: Further process the matched filtered signal to detect the target and estimate its parameters; The step 1 of modulating the incoherent laser radar pulse according to the Barker code rule includes: Step 11: Select a Barker code. Currently discovered Barker codes include: [1 1], [-1 1], [1 1 -1], [1 1 -11], [1 1 1 -1], [1 1 1 -1 1], [1 1 1 -1 -1 1 -1], [1 1 1 -1 -1 -1 -1 1 -1], [1 1 1 1 1 -1 -1 -1 1 -1 1 -1]; Each of the above Barker codes has four groups of isomorphic codes, namely original code, inverse code, reverse sequence code, and reverse complement code: Original code [1 1 1 -1] One's complement [-1 -1 -1 1] Reverse code [-1 1 1 1] Inverse complement [1 -1 -1 -1]; Step 12: The selected Barker code sequence is c k , "0" "1" modulation, its code length is n, a single code element is a rectangular wave, its pulse width is t p ; Then the laser radar transmits a signal s t (t) is expressed as follows: Where: A is the amplitude of the transmitted laser signal, n is the code length, c k is the Barker code sequence, k=1,2,…,n is the code element number, rect is the rectangular window function, t is the time, t p is the pulse width.

2. The method for detecting the form of a non-coherent detection laser radar signal based on Barker code modulation according to claim 1, characterized in that: The step 2 of receiving and processing the target reflected signal to obtain the corresponding digital signal includes: Step 21: The laser radar target reflected light signal is converted into an electrical signal through the radar optical system and photoelectric detector. The target echo signal power Pr is: Where: η sys is the laser radar system loss, including optical system loss and electronic system loss; η ato =e -2μR is the atmospheric loss, R is the target distance, μ is the atmospheric attenuation coefficient; P t is the transmitted signal power; is the signal transmission gain, θ a is the azimuthal beam width of the laser beam, θ c is the pitch beam width of the laser beam; σ is the target scattering cross-sectional area corresponding to the resolution unit; is the effective receiving area of ​​the receiving telescope, D is the aperture of the receiving telescope; Ω is the backscattering solid angle; The output electrical signals of the photodetector include: background light noise electrical signals and system thermal noise electrical signals; Step 22: Use an analog-to-digital conversion device to convert the electrical signal output by the photodetector into a digital signal.

3. The method for detecting the form of a non-coherent detection laser radar signal based on Barker code modulation according to claim 1, characterized in that: The implementation of step 3 is: Step 31: The applied bias should be the current estimated value of the background light interference. Without emitting a laser signal, record the electrical signal output by the photodetector and the digital signal converted from the electrical signal, and perform a statistical average of the measured background light interference intensity to calculate the value. Step 32: The applied bias should be updated in real time according to the change of background light intensity. When the target echo signal is obtained, the digital-to-analog converter outputs a digital signal: including the target echo signal, the background light interference signal, and the system noise minus the bias value.

4. The method for detecting the form of a non-coherent detection laser radar signal based on Barker code modulation according to claim 1, characterized in that: The implementation of step 4 is as follows: Step 41: bias the digital signal s r (t) Use the Barker code with the same modulation to perform matched filtering, and the filtering result is s f (t), expressed as: Step 42: In the filtering result s f (t) The pulse compression result of the search target echo and its pulse compression time sidelobe.

5. The method for detecting the form of a non-coherent detection laser radar signal based on Barker code modulation according to claim 1, characterized in that: The step 5 includes: Step 51: The filtered result s f (t) Perform constant false alarm processing and set the decision threshold, s f The points in (t) that exceed the decision threshold are determined to be targets; Step 52: For the points exceeding the decision threshold, determine the distance unit where they are located and use the radar ranging formula to calculate the target distance, specifically: R=ct / 2 Where c is the speed of light, and t is the time interval between the moment a certain distance unit is located and the moment the laser pulse is emitted.

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