Disclosure of Invention
The invention aims to solve the technical problem of providing an LFM-PC signal and a fuzzy function optimization method thereof, which can improve the anti-interference performance of SAR and increase the Doppler tolerance of the signal.
In order to solve the above technical problem, the present invention provides an LFM-PC signal and a method for optimizing its fuzzy function, comprising the steps of:
(1) the LFM-PC signal is obtained by modulating an LFM signal by a phase coding signal, and the two signals are directly multiplied in a time domain; the ambiguity function χ of the LFM-PC signal is obtained according to the definition of the ambiguity functionu(τ, ξ) is
Where τ is time, ξ is Doppler frequency, u is
PC(t) is a phase encoded signal, K ═ B
L/T
pIs the chirp rate, B
L、T
pBandwidth and time width of the LFM signal, respectively; p is the code length and is the code length,
is the value of the n-th sub-pulse,
is a rectangular pulse p
k(t) and p
l(t) may be expressed as:
in the formula (I), the compound is shown in the specification,
t
bis the sub-pulse width, and T
p=Pt
b,
(2) Introducing vectors
And a subpulse cross-ambiguity function matrix H, where ψ is a vector of 1 XP dimension and the phase set of the phase encoded signal
One-to-one correspondence, each element in ψ has a value range of [0,2 π ].
The blur function can be expressed in the form of vector multiplication according to equations (1) and (3):
χu(τ,ξ)=|SHH(τ,ξ)S|2 (4)
(3) and (3) performing waveform optimization by using a sequence quadratic programming method, and increasing the Doppler tolerance of the LFM-PC signal.
Preferably, in the step (3), the waveform optimization by using the sequential quadratic programming method specifically includes the following steps: (31) defining a normalized fuzzy function;
in the formula, τξxi/K is the position of the main peak of the autocorrelation function at the Doppler frequency xi, | SHH(τξξ) S | is the dominant peak of the autocorrelation function at the Doppler frequency ξ, which for a given Doppler frequency ξ, | SHH(τξXi) S | is oneA constant;
(32) the above problem can be regarded as a constrained nonlinear programming problem, and the following objective function is established:
wherein, I
ΩFor the paravalvular region of pulse pressure, let's assume
0Is the width of the main lobe, then I
ΩIn the range of
(33) Introducing variables, t further converting equation (6) into an inequality constrained nonlinear programming problem:
in the formula, t is both an objective function and a variable, and the physical meaning of t is the upper bound of the peak value side lobe ratio of the normalized fuzzy function; (34) the nonlinear programming problem of the inequality constraint can be solved by adopting a sequential quadratic programming method, and the optimization problem can be solved by directly adopting an fmincon function in an MATLAB optimization tool.
Preferably, the feasibility of the optimization problem in step (34) that can be solved by the sequential quadratic programming method is demonstrated as follows: since the denominator of F (τ, ξ) is a constant for a given doppler frequency ξ, it is only necessary to consider the quadratic differentiability of the numerator to make γ (τ, ξ) ═ SHH (t, ξ) S, then
Wherein
As an hadamard product;
Equation (9) can be simplified as:
therefore, the objective function and the constraint in the formula (7) both satisfy quadratic continuous differentiability, and can be solved by adopting a sequential quadratic programming method.
The invention has the beneficial effects that: according to the invention, a group of orthogonal phase coding signals are used as radar emission signals, so that the anti-interference performance of the SAR is improved; compared with the LFM, the new signal has better anti-interference performance and has larger Doppler tolerance compared with the phase coding signal; and optimizing a fuzzy function by adopting a minimization peak sidelobe level criterion and applying a sequence quadratic programming method, thereby increasing the Doppler tolerance of the signal.
Detailed Description
As shown in fig. 1, an LFM-PC signal and its fuzzy function optimization method includes the following steps:
(1) the LFM-PC signal is obtained by modulating an LFM signal by a phase coding signal, and the two signals are directly multiplied in a time domain; the ambiguity function χ of the LFM-PC signal is obtained according to the definition of the ambiguity functionu(τ, ξ) is
Where τ is time, ξ is Doppler frequency, u is
PC(t) is a phase encoded signal, K ═ B
L/T
pIs the chirp rate, B
L、T
pBandwidth and time width of the LFM signal, respectively; p is the code length and is the code length,
is the value of the n-th sub-pulse,
is a rectangular pulse p
k(t) and p
l(t) may be expressed as:
in the formula (I), the compound is shown in the specification,
t
bis the sub-pulse width, and T
p=Pt
b,
(2) Introducing vectors
And a subpulse cross-ambiguity function matrix H, where ψ is a vector of 1 XP dimension and the phase set of the phase encoded signal
One-to-one correspondence, each element in ψ has a value range of [0,2 π ].
The blur function can be expressed in the form of vector multiplication according to equations (1) and (3):
χu(τ,ξ)=|SHH(τ,ξ)S|2 (4)
(3) and (3) performing waveform optimization by using a sequence quadratic programming method, and increasing the Doppler tolerance of the LFM-PC signal.
The simulation data of this embodiment is set as follows: the time width of the signal is 40 mus, the bandwidth of the LFM signal is 20MHz, and the modulation frequency is 5 multiplied by 1011Hz/s, a sampling frequency of 40MHz, a code length of the phase encoded signal of 160, and a symbol width t of the phase encoded signalb=Tp/P, assuming the Doppler range to be optimized is (-B)L/P,BL/P) according to a speed resolution of 0.5/TpAnd (6) sampling.
Referring to fig. 1, an LFM-PC signal and its fuzzy function optimization method includes the following steps:
step 1: the LFM-PC signal is obtained by modulating the LFM signal by the phase coding signal, and the two signals are directly multiplied in a time domain. The ambiguity function χ of the LFM-PC signal is obtained according to the definition of the ambiguity functionu(τ, ξ) is
Where τ is time, ξ is Doppler frequency, u is
PC(t) is a phase encoded signal with chirp rate K ═ B
L/T
p=5×10
11The bandwidth and the time width of the Hz/s and LFM signals are respectively B
L=20MHz、T
p40 μ s; the code length P is 160 which is,
is the value of the n-th sub-pulse,
is a rectangular pulse p
k(t) and p
l(t) may be expressed as:
in the formula (I), the compound is shown in the specification,
sub-pulse width t
b0.25. mu.s, and T
p=Pt
b=40μs,
Step 2: introducing vectors
And a subpulse cross-ambiguity function matrix H, where ψ is a vector of 1 XP dimension and the phase set of the phase encoded signal
One-to-one correspondence, each element in ψ has a value range of [0,2 π ].
The blur function can be expressed in the form of vector multiplication according to equations (1) and (3):
χu(τ,ξ)=|SHH(τ,ξ)S|2 (4)
and step 3: the method for optimizing the waveform by using the sequence quadratic programming method to increase the Doppler tolerance of the LFM-PC signal comprises the following specific steps:
step 3-1: defining a normalized blur function
In the formula, τξxi/K is the position of the main peak of the autocorrelation function at the Doppler frequency xi, | SHH(τξξ) S | is the dominant peak of the autocorrelation function at the doppler frequency ξ. For a given Doppler frequencyThe ratio xi, | SHH(τξξ) S | is a constant.
Step 3-2: the above problem can be regarded as a constrained nonlinear programming problem, and the following objective function is established
Wherein, I
ΩThe paravalvular region of pulse pressure. Let τ be
0Is the width of the main lobe, then I
ΩIn the range of
Step 3-3: introducing variables, t further converting equation (6) into an inequality constrained nonlinear programming problem:
where t is both the objective function and the variable, its physical meaning is the upper bound of the peak-to-side lobe ratio of the normalized blur function.
Step 3-4: the nonlinear programming problem of the inequality constraint can be solved by adopting a sequential quadratic programming method, and the optimization problem can be solved by directly adopting an fmincon function in an MATLAB optimization tool.
The feasibility that the optimization problem in the step 3-4 can be solved by adopting a sequence quadratic programming method is proved as follows: since the denominator of F (τ, ξ) is a constant for a given doppler frequency ξ, it is only necessary to consider the quadratic differentiability of the numerator to make γ (τ, ξ) ═ SHH (t, ξ) S, then
Wherein
As an hadamard product;
Equation (9) can be simplified as:
therefore, the objective function and the constraint in the formula (7) both satisfy quadratic continuous differentiability, and can be solved by adopting a sequential quadratic programming method.
FIG. 2(a) is an ambiguity plot of the LFM-PC signal before optimization; FIG. 2(b) is an ambiguity diagram of the LFM-PC signal after optimization; FIG. 2(c) shows the shape of the different Doppler frequency cuts | χ (τ, ξ) | before optimization; fig. 2(d) shows the shape of the different doppler frequency cuts χ (τ, ξ) after optimization.
While the invention has been shown and described with respect to the preferred embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.