function splace = sepsignal(N,spa,band)
%% generate the support of a signal with length N and sparsity spa while
%% the support must be separted at least by band
  splace = [];
  region = 1:N;
  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
