www.gusucode.com > matlab编程LDPC编码的BP译码算法的程序源码 > matlab编程LDPC编码的BP译码算法的程序源码/code/main.m

    
%					      %
%---------------------------------------------%
function [x_hat, success, k] = ldpc_decode(f0,f1,H)
% decoding of binary LDPC as in Elec. Letters by MacKay&Neal 13March1997
% For notations see the same reference.
% function [x_hat, success, k] = ldpc_decode(y,f0,f1,H)
% outputs the estimate x_hat of the ENCODED sequence for
% the received vector y with channel likelihoods of '0' and '1's
% in f0 and f1 and parity check matrix H. Success==1 signals
% successful decoding. Maximum number of iterations（重复迭代） is set to 100.
% k returns number of iterations until convergence（收敛）.
%
% Example:
% We assume G is systematic G=[A|I] and, obviously, mod(G*H',2)=0
%         sigma = 1;                          % AWGN noise deviation
%         x = (sign(randn(1,size(G,1)))+1)/2; % random bits
%         y = mod(x*G,2);                     % coding 
%         z = 2*y-1;                          % BPSK modulation
%         z=z + sigma*randn(1,size(G,2));     % AWGN transmission
%
%         f1=1./(1+exp(-2*z/sigma^2));        % likelihoods 
%         by兰朋朋 ./矩阵元素右除 A./B表示A(i,j)/B(i,j)
%         f0=1-f1;
%         [z_hat, success, k] = ldpc_decode(z,f0,f1,H);
%         x_hat = z_hat(size(G,2)+1-size(G,1):size(G,2));
%         x_hat = x_hat'; 

%   Copyright (c) 1999 by Igor Kozintsev igor@ifp.uiuc.edu
%   $Revision: 1.1 $  $Date: 1999/07/11 $
%   fixed high-SNR decoding

[m,n] = size(H);
if m>n, 
    H=H'; 
    [m,n] = size(H); 
end
if ~issparse(H) % make H sparse if it is not sparse yet，%by兰朋朋 函数的功能是如果该函数不是稀疏矩阵，就把它转化为稀疏矩阵
%by兰朋朋 在MATLAB中，issparse函数用于判断一个稀疏矩阵存储方式是否是sparse storage organization
%by兰朋朋 如果稀疏矩阵的存储方式是sparse storage organization，则返回逻辑1；否则返回逻辑0。
   [ii,jj,sH] = find(H);
%by兰朋朋 返回矩阵H中的非零元素所在的行ii和所在的列jj,和矩阵的非零元素sH
   H = sparse(ii,jj,sH,m,n);
%by兰朋朋 [i,j,s] = find(S);
%by兰朋朋 [m,n] = size(S);
%by兰朋朋 S = sparse(i,j,s,m,n);   
end

%by兰朋朋 BP译码算法
%initialization（没看懂）
[ii,jj] = find(H);             % subscript index to nonzero elements of H 
indx = sub2ind(size(H),ii,jj); % linear index to nonzero elements of H
%by兰朋朋 该函数用于把数组中元素下标转换为该元素在数组中对应的索引值
q0 = H * spdiags(f0(:),0,n,n);
%by兰朋朋 A=spdiags(B,d,m,n)，返回带状稀疏矩阵A。
%by兰朋朋 参数m,n为原带状矩阵的行数与列数。
%by兰朋朋 B为r×p阶矩阵，这里r=min(m,n)，p为原带状矩阵所有非零对角线的条数，矩阵B的第i列即为原带状矩阵的第i条非零对角线， d为长度为p的向量。
sq0 = full(q0(indx)); 
%by兰朋朋 在MATLAB中，该函数用于把一个稀疏矩阵（sparse matrix）转换成一个全矩阵（full matrix）。
sff0 = sq0;

q1 = H * spdiags(f1(:),0,n,n); 
sq1 = full(q1(indx));
sff1 = sq1;

%迭代过程
%iterations
k=0;
success = 0;
max_iter = 100;
while ((success == 0) & (k < max_iter)),
   k = k+1;
   
   %horizontal step，第一步
   sdq = sq0 - sq1; sdq(find(sdq==0)) = 1e-20; % if   f0 = f1 = .5
   %by兰朋朋 若先验概率一样的话，令sdq为一个很小很小的数
   dq = sparse(ii,jj,sdq,m,n);
   Pdq_v = full(real(exp(sum(spfun('log',dq),2)))); % this is ugly but works :)
   %将dq在对数域求和，即将dq相乘，构成数组 Pdq_v
   Pdq = spdiags(Pdq_v(:),0,m,m) * H;
   sPdq = full(Pdq(indx));
   sr0 = (1+sPdq./sdq)./2; sr0(find(abs(sr0) < 1e-20)) = 1e-20;
   sr1 = (1-sPdq./sdq)./2; sr1(find(abs(sr1) < 1e-20)) = 1e-20;
   r0 = sparse(ii,jj,sr0,m,n);
   r1 = sparse(ii,jj,sr1,m,n);
   
   %vertical step
   Pr0_v = full(real(exp(sum(spfun('log',r0),1))));
   Pr0 = H * spdiags(Pr0_v(:),0,n,n);
   sPr0 = full(Pr0(indx));
   Q0 = full(sum(sparse(ii,jj,sPr0.*sff0,m,n),1))';
   sq0 = sPr0.*sff0./sr0;
   
   Pr1_v = full(real(exp(sum(spfun('log',r1),1))));
   Pr1 = H * spdiags(Pr1_v(:),0,n,n);
   sPr1 = full(Pr1(indx)); 
   Q1 = full(sum(sparse(ii,jj,sPr1.*sff1,m,n),1))';
   sq1 = sPr1.*sff1./sr1;
   
   sqq = sq0+sq1;
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%自己加的
%    if sqq<=0
%        sqq=1e-20;
%    end
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
   sq0 = (sq0+10*eps)./(sqq+10*eps);
   sq1 = (sq1+10*eps)./(sqq+10*eps);
   
   %tentative decoding
   QQ = Q0+Q1;
   Q0 = Q0./QQ;
   Q1 = Q1./QQ;
   
   x_hat = (sign(Q1-Q0)+1)/2;
    if rem(H*x_hat,2) == 0, success = 1; end
end