% 映射后的數字信息通過方差為var的高斯信道
function x_ch=g_ch(modulation_sig,var)
           w1=randn(length(modulation_sig),1);
           w1=w1-mean(w1)*ones(size(w1));
           w1=sqrt(var)*(w1/std(w1));
           
           w2=randn(length(modulation_sig),1);
           w2=w2-mean(w2)*ones(size(w2));
           w2=sqrt(var)*(w2/std(w2));
           
           x_ch(:,1)=modulation_sig(:,1)+w1;
           x_ch(:,2)=modulation_sig(:,2)+w2;