%該程序用來完成對輸入信號進行4QAM解調，屬于《鏈路級仿真軟件設計》程序二的4QAM解調模塊
%
%編程日期：2006－3－7
function [llr]=deqam8(x,h,SNR_db)
%[llr]=deqam8(x)
%x是復向量，其中向量元素表示輸入到解調器中的信號
%h為信道估值向量，是復向量，其中向量元素表示對應采樣時刻的信道狀態。
%SNR_db為信道噪比。
%llr為x通過解調器后的輸出信號,是實向量，表示經過相關解調后的數據“似然比“信息。
%SNR_db為信道信噪比，信號功率為1，噪聲功率為n_power
%                             |   
%                             4
%                       3     |    2
%             －－－－7－－－－－－－－1－－－        
%                       8     |    5   
%                             6 
%                             |   
%
%星座圖表
constel_diagram=[1,sqrt(2)/2+sqrt(2)/2*j,-sqrt(2)/2+sqrt(2)/2*j,j,sqrt(2)/2-sqrt(2)/2*j,-j,-1,-sqrt(2)/2-sqrt(2)/2*j];%[1 2 4 3 7 8 6 5]
%SNR為信道信噪比，信號功率為1,轉化為線性值
SNR_linr=10^(SNR_db/10);
%得到信道估計值的平方
h_square=abs(h).^2;
%噪聲方差
%len為輸入信號的長度
len=length(x);
%存儲似然比信息
llr=zeros(1,3*len);
%計算信號到各個星座點的映射距離
temp=[abs(x-constel_diagram(1)),abs(x-constel_diagram(2)),abs(x-constel_diagram(3)),abs(x-constel_diagram(4)),abs(x-constel_diagram(5)),abs(x-constel_diagram(6)),...
    abs(x-constel_diagram(7)),abs(x-constel_diagram(8))].^2;
%下面是計算各個信息比特的似然比信息llr
for m=1:len
    temp2=temp(m:len:8*len);
    [y,z]=sort(temp2);
    
    switch z(1)
        case 1
        llr(3*(m-1)+1)=h_square(m)*(temp2(5)-temp2(1));
        llr(3*(m-1)+2)=h_square(m)*(temp2(3)-temp2(1));
        llr(3*(m-1)+3)=h_square(m)*(temp2(2)-temp2(1));
        %llr(3*(m-1))=h_square(m)*(temp2(3)-temp2(1));
        case 2
         llr(3*(m-1)+1)=h_square(m)*(temp2(6)-temp2(2));
         llr(3*(m-1)+2)=h_square(m)*(temp2(4)-temp2(2));
         llr(3*(m-1)+3)=h_square(m)*(temp2(2)-temp2(1));
         %llr(3*(m-1)+4)=h_square(m)*(temp2(4)-temp2(2));
        case 3
         llr(3*(m-1)+1)=h_square(m)*(temp2(7)-temp2(3));
         llr(3*(m-1)+2)=h_square(m)*(temp2(3)-temp2(1));
         llr(3*(m-1)+3)=h_square(m)*(temp2(4)-temp2(3));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(3)-temp2(1));
        case 4
         llr(3*(m-1)+1)=h_square(m)*(temp2(8)-temp2(4));
         llr(3*(m-1)+2)=h_square(m)*(temp2(4)-temp2(2));
         llr(3*(m-1)+3)=h_square(m)*(temp2(4)-temp2(3));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(4)-temp2(2));
        case 5
         llr(3*(m-1)+1)=h_square(m)*(temp2(5)-temp2(1));
         llr(3*(m-1)+2)=h_square(m)*(temp2(7)-temp2(5));
         llr(3*(m-1)+3)=h_square(m)*(temp2(6)-temp2(5));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(7)-temp2(5));
        case 6
         llr(3*(m-1)+1)=h_square(m)*(temp2(6)-temp2(2));
         llr(3*(m-1)+2)=h_square(m)*(temp2(8)-temp2(6));
         llr(3*(m-1)+3)=h_square(m)*(temp2(6)-temp2(5));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(8)-temp2(6));
        case 7
         llr(3*(m-1)+1)=h_square(m)*(temp2(7)-temp2(3));
         llr(3*(m-1)+2)=h_square(m)*(temp2(7)-temp2(5));
         llr(3*(m-1)+3)=h_square(m)*(temp2(8)-temp2(7));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(7)-temp2(5));
        otherwise
         llr(3*(m-1)+1)=h_square(m)*(temp2(8)-temp2(4));
         llr(3*(m-1)+2)=h_square(m)*(temp2(8)-temp2(6));
         llr(3*(m-1)+3)=h_square(m)*(temp2(8)-temp2(7));
         %llr(4*(m-1)+4)=h_square(m)*(temp2(8)-temp2(6));
     end
 end
 llr=llr/SNR_linr;