A method to improve the Doppler tolerance problem of NLFM signals

Abstract

The present invention relates to a method for improving the Doppler tolerance problem of NLFM signals, and belongs to the technical field of radar signal processing. The method for improving the Doppler tolerance problem of NLFM signals is a solution to the pulse compression performance and main lobe peak delay problems of NLFM signals with Doppler frequency shift. On the one hand, spectrum correction filtering is used to improve the signal pulse compression performance. The spectrum correction filter is designed by making the spectrum correction filter output an ideal window function, thereby significantly suppressing the sidelobe level of the pulse compression result of the NLFM signal with Doppler frequency shift. On the other hand, to address the waveform main lobe peak delay problem caused by Doppler frequency shift, the true peak position of the echo signal is calculated by taking the average of the main lobe peak time of the pulse compression waveform under positive and negative frequency modulation functions, thereby avoiding the influence of Doppler frequency shift on radar ranging.

Description

Technical Field

[0001] The present invention belongs to the field of radar signal processing technology, and specifically provides a method for improving the pulse compression performance of NLFM signals and the main lobe peak delay problem of the pulse compression waveform under Doppler frequency shift. The method is mainly used to reduce the sidelobe level of the pulse compression result of the NLFM signal with Doppler frequency shift, while solving the main lobe peak delay problem of the pulse compression waveform, thereby avoiding the influence of Doppler frequency shift on radar ranging. Background Art

[0002] Compared to LFM signals, NLFM signals directly transfer the role of the weighting network to the NLFM signal's transmission and reception systems during the design process. This eliminates the need for weighted filtering during pulse compression to achieve lower signal sidelobe levels and avoids the signal-to-noise ratio loss caused by weighting. Consequently, NLFM has been widely used in recent years. However, the primary drawback of NLFM signals is their poor Doppler performance. When a large Doppler shift exists in the echo signal, the mainlobe-to-sidelobe ratio of the pulse compression waveform increases significantly, creating asymmetry between the two sides of the mainlobe. Furthermore, the mainlobe peak time is delayed, and this phenomenon becomes increasingly pronounced with increasing Doppler shift.

[0003] To address the pulse compression performance issues of NLFM signals with Doppler shift, some have proposed designing nonlinear frequency modulation signal waveforms using polynomial fitting. The Doppler tolerance of the NLFM signals designed using this method has been analyzed. Experimental results show that, under certain conditions, the signal can achieve a wide Doppler shift range. When the Doppler shift is 10 kHz, the first sidelobe level of the pulse compression result is -29.14 dB, which in many cases does not meet practical requirements. Others have proposed improving the Doppler sensitivity and signal-to-noise ratio loss of the NLFM signal's main-sidelobe ratio by swapping the order of pulse compression and FFT, and performing Doppler shift compensation on the pulse compression coefficients in each frequency-domain filter bank. However, this method is computationally intensive.

[0004] To address the main lobe peak delay problem of the NLFM signal pulse compression waveform with Doppler shift, and considering that the pulse compression results of the LFM signal under Doppler shift also have this problem, the most commonly used method is to use the up-down frequency modulation Doppler compensation algorithm to eliminate the influence of Doppler shift on the range measurement of linear frequency modulation pulse compression radar, and calculate the true delay of the target echo by the average of the two delays of up-down frequency modulation and down-up frequency modulation. Summary of the Invention

[0005] Technical problems to be solved

[0006] In order to overcome the shortcomings of the prior art, the present invention proposes a method for improving the Doppler tolerance problem of NLFM signals.

[0007] Technical Solution

[0008] A method for improving the Doppler tolerance of NLFM signals is characterized by comprising two parts: improving the signal pulse compression performance by using spectrum correction filtering, and designing a spectrum correction filter by making the spectrum correction filter output an ideal window function, thereby significantly suppressing the sidelobe levels of the pulse compression results of the NLFM signal with Doppler frequency shift; and addressing the waveform main lobe peak delay caused by Doppler frequency shift, calculating the true peak position of the echo signal by averaging the main lobe peak time of the pulse compression waveform under positive and negative frequency modulation functions, thereby avoiding the influence of Doppler frequency shift on radar ranging.

[0009] A further technical solution of the present invention is as follows:

[0010] S1: The matched filter h(t) for the NLFM signal with Doppler shift is obtained as follows:

[0011] h(t)=s * (-t)

[0012]

[0013] Where s(t) is the Doppler-shifted NLFM signal, a(t) represents the amplitude function, represents the phase function.

[0014] Assume that the Doppler shift is f d ;

[0015] S2: Combining the following formula and the spectrum of the Hamming window function, we can get the spectrum correction filter h′(t):

[0016] h′(t)=h(t)*w(t)

[0017] S3: Perform a convolution operation on the NLFM signal s(t) under Doppler frequency shift and the spectrum correction filter h′(t) to obtain the pulse compression result of the NLFM signal after spectrum correction filtering:

[0018]

[0019] A further technical solution of the present invention is to calculate the main lobe peak moment of the echo pulse pressure result, that is, the true echo peak moment t max :

[0020]

[0021] Wherein, Δt1 is the time delay caused by the waveform s1(t) under positive frequency modulation, and Δt2 is the time delay caused by the waveform s2(t) under negative frequency modulation.

[0022] A computer system, characterized in that it includes: one or more processors, and a computer-readable storage medium for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors implement the above-mentioned method.

[0023] A computer-readable storage medium is characterized by storing computer-executable instructions, which are used to implement the above method when executed.

[0024] A computer program, characterized by comprising computer executable instructions, wherein the instructions are used to implement the above method when executed.

[0025] Beneficial effects

[0026] The present invention provides a method for improving the Doppler tolerance problem of NLFM signals, which solves the Doppler tolerance problem of NLFM signals.

[0027] (1) To address the pulse compression performance issues of NLFM signals with Doppler frequency shift, the present invention proposes a spectrum correction filtering method to suppress the signal sidelobe level. This method applies the principle of spectrum correction filtering to the pulse compression process of NLFM signals. By making the output signal spectrum close to the ideal window function, a spectrum correction filter is designed, thereby significantly reducing the sidelobe level of the pulse compression result and improving the pulse compression performance of the NLFM signal.

[0028] (2) Regarding the time delay problem of the main lobe peak moment of the pulse pressure waveform, the present invention proposes to calculate the true peak position of the echo signal by taking the average of the main lobe peak time of the pulse pressure waveform under the positive and negative frequency modulation functions of the NLFM signal, thereby solving the time delay problem of the pulse pressure waveform peak under Doppler frequency shift.

[0029] The method for improving the Doppler performance of NLFM signals proposed in this invention significantly reduces the sidelobe level of the NLFM signal pulse compression result under Doppler frequency shift. It also solves the problem of pulse compression waveform peak delay under Doppler frequency shift, thus preventing the impact of Doppler frequency shift on radar ranging. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0031] Figure 1 Schematic diagram of the spectrum characteristics of the NLFM signal used in the present invention;

[0032] Figure 2 Schematic diagram of the NLFM signal frequency domain matching pulse compression waveform used in the present invention;

[0033] Figure 3Schematic diagram of the effect of Doppler frequency shift on the matching pulse pressure waveform of NLFM signal used in the present invention;

[0034] Figure 4 Schematic diagram of the spectrum correction filtering principle used in the present invention;

[0035] Figure 5 Schematic diagram of the Hamming window spectrum waveform used in the present invention;

[0036] Figure 6 Schematic diagram of the NLFM signal spectrum correction filtered pulse compression waveform when there is no Doppler frequency shift used in the present invention;

[0037] Figure 7 Schematic diagram of pulse compression performance analysis of NLFM signal after time spectrum correction filtering using the present invention (Doppler frequency shift is ±30KHz);

[0038] Figure 8 Schematic diagram of pulse compression performance analysis of NLFM signal after time spectrum correction filtering using the present invention (Doppler frequency shift is ±50KHz);

[0039] Figure 9 1. It is a schematic diagram of the main lobe peak time of the NLFM signal pulse compression waveform when there is no Doppler frequency shift used in the present invention;

[0040] Figure 10 Schematic diagram of the main lobe peak delay problem of the NLFM signal pulse compression waveform under Doppler frequency shift when using the present invention (positive frequency modulation);

[0041] Figure 11 Schematic diagram of the main lobe peak delay problem of the NLFM signal pulse compression waveform under Doppler frequency shift when using the present invention (negative frequency modulation);

[0042] Figure 12 Flowchart of the present invention. DETAILED DESCRIPTION

[0043] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0044] The present invention provides a method for improving the Doppler tolerance problem of NLFM signals, such as Figure 12As shown. By making the output of the spectrum correction filter an ideal window function, the corresponding spectrum correction filter is designed, thereby improving the pulse compression performance of the NLFM signal with Doppler frequency shift and further suppressing the signal sidelobe level. Without changing the waveform parameters of the NLFM signal, different spectrum correction filters can be designed by changing the spectrum of the ideal window function, thereby reducing the signal sidelobe level and meeting actual needs. By taking the average of the main lobe peak time of the pulse compression result under the positive and negative frequency modulation functions as the true peak time of the echo signal, the time delay problem of the main lobe of the pulse compression waveform of the NLFM signal with Doppler frequency shift is solved, and the influence of the Doppler frequency shift on the radar ranging is avoided. The details are as follows:

[0045] (1) To address the problem of increased sidelobe levels in the pulse compression waveform caused by the Doppler tolerance of the NLFM signal, the present invention performs spectrum correction filtering on the NLFM signal with Doppler frequency shift. By making the output signal spectrum after spectrum correction filtering close to the ideal window function, the spectrum correction filter is designed, thereby better suppressing the sidelobe levels of the signal pulse compression waveform.

[0046] In the present invention, it is assumed that the expression of the NLFM signal is

[0047]

[0048] Where a(t) represents the amplitude function, represents the phase function. Assume that the Doppler shift is f d , then the NLFM signal with Doppler shift can be expressed as

[0049]

[0050] The corresponding expression of the matched filter for the NLFM signal with Doppler frequency shift is:

[0051] h(t)=s * (-t) (1-3)

[0052] According to the spectrum correction filtering principle, see Figure 4 , assuming that the output signal spectrum is an ideal window function W(f), the corresponding time domain expression is w(t), and the time domain expression of the spectrum correction filter is obtained as

[0053] h′(t)=h(t)*w(t) (1-4)

[0054] Then the pulse pressure output of the NLFM signal s(t) with Doppler frequency shift after spectrum correction filtering is:

[0055]

[0056] Take the bandwidth B as 10MHz, the pulse width T as 20us, and the sampling rate fs is 40MHz, and the Doppler shift f d Take ±30KHz and ±50KHz respectively, and choose Hamming window as the window function.

[0057] First, according to formula (1-3), the matched filter h(t) of the NLFM signal with Doppler frequency shift can be obtained. Then, combined with formula (1-4) and the spectrum of the Hamming window function, the spectrum correction filter h′(t) can be obtained. Finally, the NLFM signal s(t) under Doppler frequency shift and the spectrum correction filter h′(t) are convolved to obtain the pulse pressure result of the NLFM signal after spectrum correction filtering.

[0058] The method proposed by the present invention for improving the pulse compression performance of NLFM signals under Doppler frequency shift based on spectrum correction filtering can significantly suppress the signal sidelobe level. d = ±30KHz, the first sidelobe level of the pulse waveform is reduced from -20.23dB to -46.2dB; when f d =±50KHz, the first sidelobe level of the pulse compression waveform is reduced from -15.71dB to -43.07dB.

[0059] (2) Regarding the time delay problem of the main lobe peak moment of the pulse pressure waveform, the present invention proposes to calculate the true peak position of the echo signal by taking the average of the main lobe peak time of the pulse pressure waveform under the positive and negative frequency modulation functions of the NLFM signal, thereby solving the time delay problem of the pulse pressure waveform peak under Doppler frequency shift.

[0060] In the present invention, it is assumed that the NLFM signal expression of positive frequency modulation is The Doppler shift is f d , then the positive and negative NLFM signals with Doppler shift can be expressed as

[0061]

[0062] Let the time delay of waveform s1(t) under positive frequency modulation be Δt1, and the time delay of waveform s2(t) under negative frequency modulation be Δt2, then the peak time of the main lobe of the echo pulse pressure result is t max It can be expressed as

[0063]

[0064] Under the radar design parameters shown in Table 1, when there is no Doppler shift in the echo signal, the peak of the main lobe of the pulse compression waveform appears at -1.2us, see Figure 9 When there is a 40KHz Doppler frequency shift in the echo signal, the peak of the pulse pressure waveform under positive frequency modulation is at -1.325us, see Figure 10 ; The peak value of the pulse pressure waveform under negative frequency modulation is at -1.075us, see Figure 11 Combining formula (1-7), we can calculate that the actual echo peak time is -1.2us.

[0065] In order to enable those skilled in the art to better understand the present invention, the present invention is described in detail below with reference to specific embodiments.

[0066] (1) Improve the impact of Doppler frequency shift on NLFM signal pulse compression performance:

[0067] Step 1: According to the actual requirements for the sidelobe level of the NLFM signal pulse compression waveform with Doppler frequency shift, a suitable window function spectrum is selected to design a spectrum correction filter;

[0068] For example, the spectrum characteristics of NLFM signal are as follows: Figure 1 As shown, the matching pulse pressure waveform is as follows Figure 2 As shown, the sidelobe level is -36.19dB; when there is Doppler frequency shift, the sidelobe level of the pulse compression result increases rapidly, see Figure 3 According to the spectrum correction filtering principle, see Figure 4 , combined with formula (1-4), select the Hamming window function spectrum to design the spectrum correction filter, see Figure 5 .

[0069] Step 2: Combine equations (1-5) and perform pulse compression processing on the NLFM signal with Doppler frequency shift using the spectrum correction filter obtained in step 1;

[0070] For example, after spectrum correction processing is performed on the signal waveform selected in step 1, the sidelobe level of the NLFM signal pulse compression waveform can be reduced to -55.83dB, see Figure 6 When the Doppler frequency shift is ±30KHz, the sidelobe level is reduced from -20.23dB to -46.2dB; when the Doppler frequency shift is ±50KHz, the sidelobe level is reduced from -15.71dB to -43.07dB. For details, see Figure 7 and Figure 8 .

[0071] Step 3: Observe whether the first sidelobe level of the signal pulse compression result obtained in step 2 meets the actual requirements. If not, repeat the above steps and reselect the appropriate window function spectrum to design the spectrum correction filter until the first sidelobe level meets the actual requirements.

[0072] (2) Solve the delay problem caused by Doppler frequency shift on the main lobe peak of the NLFM signal pulse waveform:

[0073] Step 1: For the NLFM signal with Doppler frequency shift, use positive frequency modulation to obtain the signal pulse compression waveform, and record the peak time of the main lobe of the waveform Δt1;

[0074] Step 2: For the NLFM signal, negative frequency modulation is used to obtain a signal pulse waveform, and the peak moment of the main lobe of the waveform Δt2 is recorded;

[0075] Step 3: Calculate the true peak moment of the echo main lobe by combining equations (1-7).

[0076] For example, see Figure 9 When there is no Doppler shift in the echo signal, the main lobe peak appears at -1.2us. Under Doppler shift, the main lobe peak of the signal pulse pressure result appears at -1.325us when the frequency is positive; the peak of the pulse pressure waveform is at -1.075us when the frequency is negative. Figure 10 、 11 According to formula (1-7), it can be calculated that the true peak moment of the echo at this time is -1.2us.

[0077] Table 1 NLFM signal waveform parameter design

[0078] Time width T Bandwidth B <![CDATA[Sampling rate f s > Window function 20us 10MHz 40MHz hamming

[0079] The above description is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with this technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed in the present invention, and these modifications or replacements should all be included in the scope of protection of the present invention.

Claims

1. A method for improving the Doppler tolerance problem of NLFM signals, characterized in that The method consists of two parts: first, using spectrum correction filtering to improve the signal pulse compression performance. By making the spectrum correction filter output an ideal window function, a spectrum correction filter is designed, thereby significantly suppressing the sidelobe level of the pulse compression result of the NLFM signal with Doppler frequency shift; second, to address the waveform main lobe peak delay caused by Doppler frequency shift, the true peak position of the echo signal is calculated by taking the average of the main lobe peak time of the pulse compression waveform under positive and negative frequency modulation functions, thus avoiding the influence of Doppler frequency shift on radar ranging; The specific steps to suppress the sidelobe level are as follows: S1: The matched filter of the NLFM signal with Doppler shift is obtained according to the following formula : in, is the Doppler-shifted NLFM signal, represents the amplitude function, represents the phase function, assuming that the Doppler shift is ; S2: Combine the following formula and The spectrum of the window function can be obtained by spectrum correction filter : S3: NLFM signal under Doppler frequency shift Sum spectrum correction filter By performing convolution operation, we can obtain the pulse pressure result of the NLFM signal after spectrum correction filtering: Observe whether the first sidelobe level of the obtained signal pulse compression result meets the actual requirements; if not, repeat the above steps and reselect the appropriate window function spectrum to design the spectrum correction filter until the first sidelobe level meets the actual requirements; To address the waveform main lobe peak delay problem caused by Doppler frequency shift, the true peak position of the echo signal is calculated by averaging the main lobe peak time of the pulse pressure waveform under positive and negative frequency modulation functions, including the following steps: Step 1: For the NLFM signal with Doppler frequency shift, use positive frequency modulation to obtain the signal pulse waveform and record the peak moment of the main lobe of the waveform. ; Step 2: For the above NLFM signal, use negative frequency modulation to obtain the signal pulse waveform and record the peak moment of the main lobe of the waveform at this time. ; Step 3: Calculate the peak moment of the main lobe of the echo pulse pressure result, that is, the true echo peak moment : in, The waveform is under positive frequency modulation The delay generated, Negative frequency modulation waveform The resulting delay.

2. A computer system, characterized in that include: One or more processors, and a computer-readable storage medium for storing one or more programs, wherein when the one or more programs are executed by the one or more processors, the one or more processors are enabled to implement the method of claim 1.

3. A computer-readable storage medium, characterized in that Computer-executable instructions are stored, and when the instructions are executed, they are used to implement the method of claim 1.

4. A computer program product, characterized in that The invention comprises computer executable instructions, which are used to implement the method of claim 1 when the instructions are executed.

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