function  Pb=VBLAST_QR_N(SNR_dB)
%------------------------------------------------------------------------
%本程序是對兩發兩收情況下采用空時分層碼的性能分析
%星座映射采用的是BPSK映射
% SNR_dB 接收天線上的接收信噪比，在仿真的過程中把接收信號的功率歸一化為1
% Pb是誤比特率
%-------------------------------------------------------------------------
SNR=10^(SNR_dB/10);
Cons=sqrt(1/2)*[1 -1]; % BPSK星座
E=1; %接收端的信號功率
No=E/(SNR);%高斯白噪聲的功率譜密度
Frame=100000;
ErrorNum=0;
for i=1:1:Frame
    x1=Cons((rand(1)>=1/2)+1); % 隨機產生發送數據
    x2=Cons((rand(1)>=1/2)+1);
    X=[x1 x2 ]'; % 進行空時編碼
    h11=sqrt(1/2)*(randn(1)+j*randn(1));  % 產生信道 兩個發送天線到接收天線的信道
    h12=sqrt(1/2)*(randn(1)+j*randn(1));
    h21=sqrt(1/2)*(randn(1)+j*randn(1));
    h22=sqrt(1/2)*(randn(1)+j*randn(1));
    H=[h11 h12;h21 h22];
    N=sqrt(No/2)*(randn(2,1)+j*randn(2,1)); %接收端的噪聲
    Y=H*X+N;
    [U,R] = qr(H);
    y = U'*Y;
    decoder_output = zeros(2,1);
    decoder_output(2,:) = y(2,:)/(R(2,2)+eps);
   for t = 2 : -1 : 1
		interf = zeros(1);
		for f = (t+1):2
			interf = R(t,f)*decoder_output(f,:) + interf;
        end
		decoder_output(t,:) = (y(t,:) - interf)/(R(t,t)+eps);
    end
    for k=1:2
      if real(decoder_output(k,:))>=0
        decoder_output(k,:)=Cons(1);
      else decoder_output(k,:)=Cons(2);
      end
    end
    ErrorNum=ErrorNum+(x1~=decoder_output(1,:))+(x2~=decoder_output(2,:));
end
Pb=ErrorNum/(Frame*2);
end