ldpcencode.m

一些奇偶校验矩阵的构造-matlab 消除四环

% LDPC encoder.
%   Prasanna Sethuraman, March-2004

function x = ldpcEncode(u, H, B, L, U)  % matlab passes these arguments by reference.

%   u : input vector    L x 1
%   x : code word       N x 1

%   H : Parity Check matrix     LxN / (N-L)xN

if u==0
    [dummy,N] = size(H);
    x=zeros(1,N);
    return;
end;

[M,N] = size(H);
K = N-M;

% Generate Code word
% x = [c s]         s : 1 x K   and c : 1 x (N-K)
% H = [A B]         H : (N-K) x N
% Hx' =  Ac' + Bs'  A : (N-K) x (N-K)   and B : (N-K) x K
% c = inv(A)*Bs'    since Hx' is zero for a code vector x

% A = gf(full(H(:,1:N-K)),1);
% c = inv(A) * H(:,N-K+1:N)*u';      % No check is made to avoid the problem of non existence of inv(B)
% 
% x=[s c'];

% Method below is not used due to problems with matlab's Gaussian Elimination

%   Encoding:   H = [I P]       I : (N-L)x(N-L) and P: (N-L)xL
%               G = [P' I]      P': Lx(N-L)     and I: LxL
%               x = G'u         

%   Parity Chk: Hx = 0

% x = mod(G'*u,2); 


% Method described in Neal's documentation
% U = sparse(M,M); L = sparse(M,M);
% 
% F = H;
% for i=1:M
%     j=i;
%     while(H(j,j)==0 & j<=M)
%         j=j+1;
%     end;
%     F(:,[i j])=F(:,[j i]);  % Swap columns
%     F([i j],:)=F([j i],:);  % Swap rows
%     H(:,[i j])=H(:,[j i]);  % Swap columns
%     H([i j],:)=H([j i],:);  % Swap rows
%     U(1:i,i)=F(1:i,i);
%     L(i+1:M,i)=F(i+1:M,i);
% end;
% 
% B = H(:,M+1:N);

z = B*u;
y=lsolve(L,z);
c=usolve(U,y);
% c=Ainv*z;
% a nasty trick
str = struct(c);   % column vector
x = [u; double(str.x)];