stftlong.m

雷达回波信号

% STFTLONG.M
%  STFTLONG
%  Function X = stftlong(x,w,a,b);
%  Description : STFTLONG(...) determines the STFT of a long signal x with
%                respect to a window w over a lattice with lattice constants
%                a (time) and b (frequ).
%  Input :       x=a vector representing the function (size n)
%                w=a vector representing the window function
%                a,b=two integers representing the lattice constants
%                    (they must be dividers of n)
%  Output :      X=a new matrix (size n*n when no lattice constants are
%                    given, size (n/a)*(n/b) otherwise)
%  Usage :       stftlong(x,w);  to have a stft with a=b=1
%                stftlong(x,w,a,b);
%  See also :    STFT
%
% ---------------------------------------------------------------
%
% COPYRIGHT : (c) NUHAG, Dept.Math., University of Vienna, AUSTRIA
%             http://nuhag.eu/
%             Permission is granted to modify and re-distribute this
%             code in any manner as long as this notice is preserved.
%             All standard disclaimers apply.
%
% STFTLONG
% Function X = stftlong(x,w,a,b);
% Description : STFTLONG(...) determines the STFT of a long signal x with
%               respect to a window w over a lattice with lattice constants
%               a (time) and b (frequ).
% Input :       x=a vector representing the function (size n)
%               w=a vector representing the window function
%               a,b=two integers representing the lattice constants
%                   (they must be dividers of n)
% Output :      X=a new matrix (size n*n when no lattice constants are
%                   given, size (n/a)*(n/b) otherwise)
% Usage :       stftlong(x,w);  to have a stft with a=b=1
%               stftlong(x,w,a,b);
% See also :    STFT


% uses symmetric extension of signal at boundaries



function X = stftlong(x,w,a,b)

if nargin < 3;  a = 1; b = 1; end;
l = length(x);
n = length(w);
% check if length of window divides length of signal
nl = l/n;
if rem(nl,1) ~= 0;
  nn = n*ceil(nl) - l;
end
na1 = floor((n-a)/2);
na2 = ceil((n-a)/2);
% simple zero padding for taking care of boundary effects
% maybe an antisymmetric extension should be carried out
xx = [x(na1:-1:1) x x(l:-1:l-na2+1)];
ll = length(xx);

coeff = zeros(1+(ll-n)/a,n/b);


for jj = 1:1+(ll-n)/a;
   x = xx((jj-1)*a+1:(jj-1)*a+n);
   y  = x.* w;
   y1 = perbas(y,b);
   v = fft(y1);
   coeff(jj,:) = v;
end;
[mm,nn] = size(coeff);
X = coeff.';
%X = rot(X,l/2+1);