function y = approximate(x,M)% approximate(x,M) - approximates noisy symbol vector x with valid symbols% in M%   x - noisy symbol vector%   M - matrix representing constellation mapping%M=[0 0 1;0 1 j;1 0 -1;1 1 -j];W=length(M(1,:));L=length(M(:,1));N = length(x);a = M(:,W);alpha=zeros(N,1);for i=1:N    mindist=100;    symnum=0;    for k=1:L        if abs(a(k)-x(i))<=abs(mindist)            mindist=abs(a(k)-x(i));            alpha(i)=a(k);        else        end    endend        y=alpha;