function splace = sepsignalclose(N,spa,band)
%% generate the support of a signal with length N and sparsity spa while
%% the support must be separated at least by band and 
%% at least two indices are separated exactly by band
  region = 1:N;
  % select first two close objects
  new  = unidrnd(N,1,1);
  region = setdiff(region,(new-band):(new+band));
  new1 = new - band - 1;
  new2 = new + band + 1;
  if new2 > N
      splace = [new ; new1];
      region = setdiff(region,(new1-band):(new1+band));
  else
      splace = [new ; new2];
      region = setdiff(region,(new2-band):(new2+band));
  end
  while length(splace) < spa
       new = unidrnd(length(region),1,1);
       new = region(new);
       splace = [splace ; new];
       region = setdiff(region,(new-band):(new+band));
       region = intersect(region,1:N);
  end
