% 這個函數對qpsk信號進行解調；
% y是解調出的比特信息；
% 對于調制和解調來說，其載波頻率，采樣頻率，時間點n都應該是一致的
% 注意在將二進制信號轉化為4進制時有一個順序關系，比如0101序列
% 可以從左至右，也可以從右至左，
% 由于調制是按符號來調制的，解調時也要按相反的方向，先解調出符號
% 再轉化為比特流；

% function [y_demod_bit,y_demod_symbol]=...
%                              qpsk_demod_ZiBian(x,fc,fs,n)
function [y_demod_bit]=...
            qpsk_demod_ZiBian(x,fc,fs,symbol_t,theta)
% 注意把握好fc,fs,n之間的關系，是一個四進制碼元內的載波信號點
% 是整數個周期；
% x表示的是輸入的調制信號，是列向量，
% fc是載波頻率，fs是采樣頻率，
% n是每一個四進制碼元下擁有的載波信號采樣點數；

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if nargin<5
    error('the input arguments are not complete');
end;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if size(x,1)<=size(x,2)
    x=x';
else
    x=x;
end;   % 把x變換成列向量；
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 時間的設置要注意兩點：采樣點數和采樣間隔；
n=ceil(symbol_t*fs);
t=(0:n-1)/fs;
% 一個二進制碼元下的載波：
signal_q=sin(2*pi*fc*t);
signal_i=cos(2*pi*fc*t);
signal_mod=x;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% 下面是解調部分：
symbol_demod=[];
base_4=[0 1 2 3]';
mod_4=pskmod(base_4, 4 , theta);


for i=1:n:(size(signal_mod,1)-n+1)
    demod_i=signal_i*signal_mod(i:i+n-1)/(n/2);
    demod_q=signal_q*signal_mod(i:i+n-1)/(n/2);
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 注意進行相關檢測，
    % 進行相關檢測，選出最大的；
%     base_4=[0 1 2 3]';
%     mod_4=pskmod(base_4, 4 , theta);
    position=1; % 初始默認值；
    aa=0;
    for ii=1:size(mod_4,1)
        ww=[demod_i,demod_q]*[real(mod_4(ii)),imag(mod_4(ii))]';
        if ww>=aa
           position=ii;
           aa=ww;
        end;
    end;
    symbol_demod_temp=base_4(position);
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% 下面的是硬判決，根本不能用；      
%     if (demod_i>=0) & (demod_q>=0)
%         symbol_demod_temp=0;
%     elseif (demod_i<0) & (demod_q>=0)
%         symbol_demod_temp=1;
%     elseif (demod_i<0) & (demod_q<0)
%         symbol_demod_temp=2;
%     elseif (demod_i>=0) & (demod_q<0)
%         symbol_demod_temp=3;
%     end;
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   % 先解調出符號；
    symbol_demod=[symbol_demod,symbol_demod_temp]; 
end;

y_demod_symbol=symbol_demod;

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
if size(symbol_demod,1)<=size(symbol_demod,2)
    symbol_demod=symbol_demod';
else
    symbol_demod=symbol_demod;
end;   % 把symbol_demod變換成列向量；
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

    % 再由符號恢復出比特流；
    % symbol_demod是解調出的符號信息；
    bit_demod=zeros(2*size(symbol_demod,1),1);
    for ii=1:size(symbol_demod,1)
        if (symbol_demod(ii)==0)
           bit_demod(2*ii-1)=0;
           bit_demod(2*ii)=0;
        elseif (symbol_demod(ii)==1)
           bit_demod(2*ii-1)=1;
           bit_demod(2*ii)=0;
        elseif (symbol_demod(ii)==2)
           bit_demod(2*ii-1)=0;
           bit_demod(2*ii)=1; 
        elseif (symbol_demod(ii)==3)
           bit_demod(2*ii-1)=1;
           bit_demod(2*ii)=1;
        end;
    end;
    % y是解調出的比特信息；
    y_demod_bit=bit_demod;