pulsecompression_demo_fxj.m

对于两个目标的雷达脉冲信号压缩,通过MATLAB仿真实现

% function pulsecompression_demo
% 分析LFM信号在不同距离时的脉压性能
% 不考虑噪声因素
% 结论：时延不会改变信号的幅度频谱，因此无论目标的远近，匹配滤波输出的峰值都是一样的。
% （匹配滤波即相关，可放在频域中完成，由于不同时延的回波的频谱一样，都和匹配滤波器的频谱重合，因此都能完全脉压）
% 2007-05-29
% Copyright (c) fuxiongjun 2007-2009

close all;
clear all;

cj=sqrt(-1);

c=3e8;                   % Propagation speed
pi2=2*pi;

ntarget=2;

%  参数设置  
fc = 30e6;     % 载频 central frequency
wc=pi2*fc;
B = 2e6;      % chirp带宽
fs=80e6;       % 采样率 sample frequency
Ts =1/fs;
Tp = 5e-6;    % 脉宽,s
t0 = -Tp/2 : 1/fs : Tp/2-1/fs;        % 时间刻度
Prt=30e-6;                      % 脉冲重复周期
k = B/ Tp
A=1;            % LFM信号幅度


renta=c/fc;

Number=length(t0);                       % 一个LFM信号内的采样点数

freq=(fs/Number).*(0:Number-1);         % 频率刻度, Hz
 
Xc=2.e3;                       % 设定参考点与雷达的绝对距离（采样波门起始位置）Reference point lacation, the start of sampling gate
distance=[100 375];  % 设定ntarget个目标与参考点的距离，必须>0。Distance between the target i and reference point  (must >=0)
fn=1*ones(1,ntarget);          % scatterer strenth


XObj(1)=Xc+distance(1);          % Range distance of target 1 (m)
XObj(2)=Xc+distance(2);          % Range distance of target 2 (m)


Xc0=Xc;
Xend=Xc0+max(distance)+Tp/4;     % the farest distance among all the target, 多给出1/4个脉宽的接收距离（时间），作为裕量
 

Tstart=2*Xc/c;                  % Start time of sampling, absolute time雷达开始采样时的距离是本身距离的两倍
Tf=2*Xend/c+Tp;                 % End time of sampling , absolute time
 

% Measurement parameters
n=round((Tf-Tstart)/Ts);            % Number of time samples
t=Tstart+(0:n-1)*Ts;               % Time array for data acquisition, absolute time

td0=t-2*Xc/c;   % 匹配滤波器位置在Xc处

weightingwindow1=[ones(1,length(t0))  zeros(1,n-length(t0))];%窗
s0=exp(cj*2*pi*(fc.*td0+0.5*k.*td0.^2)).*weightingwindow1;    
% Baseband conversion
sb0=s0.*exp(-cj*wc*t);  

sb0tmp=sb0; 
sb0tmp=sb0tmp(1:length(t0)); 
sb0_matchfilter=conj(fliplr(sb0tmp));

s=zeros(1,n);              % Initialize echoed signal array
sb=zeros(1,n);   
fsb=zeros(1,n+length(t0)-1);  
fsmb=zeros(1,n+length(t0)-1);  
smb=zeros(1,n+length(t0)-1);  


for i=1:ntarget;
      td=t-2*XObj(i)/c;   
      LocateL=min(find(td>=0));
      modify_k=1; 

      backwave1=i*fn(i).*exp(cj*2*pi*(fc.*td+0.5*k.*td.^2));    
      
      weightingwindow2=[zeros(1,LocateL-modify_k)  ones(1,length(t0))  zeros(1,n-LocateL+modify_k-length(t0))];
      s=s+backwave1.*weightingwindow2;      
 end;
 	  sb=s.*exp(-cj*wc*t);     
    smb=conv(sb0_matchfilter,sb);    
      
      
    counter=5;
    figure(counter);
    xx=linspace(1,n+length(t0)-1,n+length(t0)-1);
    xt=(xx-1).*Ts-(length(t0)-1)*Ts;
    Rx=xt.*c/2;
    smbtmp=abs(smb);
    smbtmp_norm=smbtmp/max(smbtmp);  % 归一化
    smbtmp_norm_dB=20*log10(smbtmp_norm+realmin);
    index_lowerThan60dB=find(smbtmp_norm_dB<-60);  % if the dB value is lower than -60,than put it as -60
    smbtmp_norm_dB(index_lowerThan60dB)=-60;
    plot(Rx,smbtmp);
  	title('Matched Filtering of single pulse')  %  
    axis tight;
  	xlabel('Range /m')
    ylabel('dB');
    

% 测试实际产生的两个目标的回波的频谱与匹配滤波器频谱的关系 （已去除载频，所以频谱分布在低频）
for i=1:ntarget;
      td=t-2*XObj(i)/c;   
      LocateL=min(find(td>=0));
      modify_k=1;  
      backwave1(i,:)=fn(i).*exp(cj*2*pi*(fc.*td+0.5*k.*td.^2));    
      weightingwindow=[zeros(1,LocateL-modify_k)  ones(1,length(t0))  zeros(1,n-LocateL+modify_k-length(t0))];
      s(i,:)=backwave1(i,:).*weightingwindow;    
      sb(i,:)=s(i,:).*exp(-cj*wc*t); 
 end;
 	
freqs21=abs(fft(sb(1,:)));
freqs22=abs(fft(sb(2,:)));
freqmatch=abs(fft(sb0_matchfilter,n));

freqpadzero=(fs/n).*(0:n-1);         % 频率刻度, Hz
    
counter=counter+1;
figure(counter);
plot(freqpadzero,freqmatch);
hold on;
plot(freqpadzero,freqs21,'g');
hold on;
plot(freqpadzero,freqs22,'r');
hold off;
xlabel('Frequency /Hz');
legend('spectrum of no delay','spectrum of with delay 1','spectrum of with delay 2');