% 這是一個調(diào)制的函數(shù)，理解它對各種調(diào)制方法起到促進(jìn)作用；
% 本程序的基帶信號到調(diào)制是很好的例子；

function mod_symbols = tx_modulate(bits_in, modulation)

full_len = length(bits_in);

% BPSK modulation

% findstr(modulation, 'BPSK')表示在modulation中找到BPSK；

if ~isempty(findstr(modulation, 'BPSK'))
   % Angle [pi/4 -3*pi/4] corresponds to 
   % Gray code vector [0 1], respectively.
   % generates BPSK symbols
   
   % 這僅是選擇相位，沒有考慮頻率問題；
   table=exp(j*[0 -pi]);  
   % Gray code mapping pattern for BPSK symbols
   
   % 把上面table的值翻轉(zhuǎn)了一下；
   table=table([1 0]+1); 
   
   % bits_in是輸入的基帶信號；
   inp=bits_in;
   % maps transmitted bits into BPSK symbols
   % inp+1的取值在2，1之間，完成映射，選擇相位；
   mod_symbols=table(inp+1);  
   
   % QPSK modulation
   % 下面是 QPSK 調(diào)制
elseif ~isempty(findstr(modulation, 'QPSK'))
   % Angle [pi/4 3*pi/4 -3*pi/4 -pi/4] corresponds to 
   % Gray code vector [00 10 11 01], respectively.
   % generates QPSK symbols
   % 有 4 種相位；
   table=exp(j*[-3/4*pi 3/4*pi 1/4*pi -1/4*pi]);  
   % Gray code mapping pattern for QPSK symbols
   table=table([0 1 3 2]+1); 
   inp=reshape(bits_in,2,full_len/2);
   % maps transmitted bits into QPSK symbols
   mod_symbols=table([2 1]*inp+1);  


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   % 16-QAM modulation
   % 下面是 16-QAM 調(diào)制
elseif ~isempty(findstr(modulation, '16QAM'))
   % generates 16QAM symbols
   m=1;
   % k是實(shí)軸，l是虛軸；
   for k=-3:2:3
      for l=-3:2:3
         table(m) = (k+j*l)/sqrt(10); % power normalization
         m=m+1;
      end;
   end;
    % Gray code mapping pattern for 8-PSK symbols
    % 有16種情況；
   table=table([0 1 3 2 4 5 7 6 12 13 15 14 8 9 11 10]+1);
   inp=reshape(bits_in,4,full_len/4);
   % maps transmitted bits into 16QAM symbols
   % 施以不同的權(quán)重；
   mod_symbols=table([8 4 2 1]*inp+1);  
   
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   % 64-QAM modulation
elseif ~isempty(findstr(modulation, '64QAM'))
   % generates 64QAM symbols
   m=1;
  % k是實(shí)軸，l是虛軸；
   for k=-7:2:7
      for l=-7:2:7
         table(m) = (k+j*l)/sqrt(42); % power normalization
         m=m+1;
      end;
   end;
   table=table([[ 0  1  3  2  7  6  4  5]...
         8+[ 0  1  3  2  7  6  4  5]... 
         24+[ 0  1  3  2  7  6  4  5]...
         16+[ 0  1  3  2  7  6  4  5]...
         56+[ 0  1  3  2  7  6  4  5]...
         48+[ 0  1  3  2  7  6  4  5]...
         32+[ 0  1  3  2  7  6  4  5]...
         40+[ 0  1  3  2  7  6  4  5]]+1);
   
   inp=reshape(bits_in,6,full_len/6);
   % maps transmitted bits into 64QAM symbol
   mod_symbols=table([32 16 8 4 2 1]*inp+1);  
else
   error('Unimplemented modulation');
end;