```
function H = makeLdpc(M, N, method, noCycle, onePerCol)
% Create R = 1/2 low density parity check matrix
%
% M : Number of row
% N : Number of column
% method : Method for distributing non-zero element
% {0} Evencol : For each column, place 1s uniformly at random
% {1} Evenboth: For each column and row, place 1s uniformly at random
% noCyle : Length-4 cycle
% {0} Ignore (do nothing)
% {1} Eliminate
% onePerCol: Number of ones per column
%
% H : Low density parity check matrix
%
%
% Copyright Bagawan S. Nugroho, 2007
% http://bsnugroho.googlepages.com
% Number of ones per row (N/M ratio must be 2)
if N/M ~= 2
fprintf('Code rate must be 1/2\n');
end
onePerRow = (N/M)*onePerCol;
fprintf('Creating LDPC matrix...\n');
switch method
% Evencol
case {0}
% Distribute 1s uniformly at random within column
for i = 1:N
onesInCol(:, i) = randperm(M)';
end
% Create non zero elements (1s) index
r = reshape(onesInCol(1:onePerCol, :), N*onePerCol, 1);
tmp = repmat([1:N], onePerCol, 1);
c = reshape(tmp, N*onePerCol, 1);
% Create sparse matrix H
H = full(sparse(r, c, 1, M, N));
% Evenboth
case {1}
% Distribute 1s uniformly at random within column
for i = 1:N
onesInCol(:, i) = randperm(M)';
end
% Create non zero elements (1s) index
r = reshape(onesInCol(1:onePerCol, :), N*onePerCol, 1);
tmp = repmat([1:N], onePerCol, 1);
c = reshape(tmp, N*onePerCol, 1);
% Make the number of 1s between rows as uniform as possible
% Order row index
[r, ix] = sort(r);
% Order column index based on row index
for i = 1:N*onePerCol
cSort(i, :) = c(ix(i));
end
% Create new row index with uniform weight
tmp = repmat([1:M], onePerRow, 1);
r = reshape(tmp, N*onePerCol, 1);
% Create sparse matrix H
% Remove any duplicate non zero elements index using logical AND
S = and(sparse(r, cSort, 1, M, N), ones(M, N));
H = full(S);
end % switch
% Check rows that have no 1 or only have one 1
for i = 1:M
n = randperm(N);
% Add two 1s if row has no 1
if length(find(r == i)) == 0
H(i, n(1)) = 1;
H(i, n(2)) = 1;
% Add one 1 if row has only one 1
elseif length(find(r == i)) == 1
H(i, n(1)) = 1;
end
end % for i
% If desired, eliminate any length-4 cycle
if noCycle == 1
for i = 1:M
% Look for pair of row - column
for j = (i + 1):M
w = and(H(i, :), H(j, :));
c1 = find(w);
lc = length(c1);
if lc > 1
% If found, flip one 1 to 0 in the row with less number of 1s
if length(find(H(i, :))) < length(find(H(j, :)))
% Repeat the process until only one column left
for cc = 1:lc - 1
H(j, c1(cc)) = 0;
end
else
for cc = 1:lc - 1
H(i, c1(cc)) = 0;
end
end % if
end % if
end % for j
end % for i
end % if
fprintf('LDPC matrix is created.\n');```case {0}
% 为每列生成随机排列的行索引
for i = 1:N
onesInCol(:, i) = randperm(M)';
end
% 构造非零元素索引
r = reshape(onesInCol(1:onePerCol, :), N*onePerCol, 1);% 展开行索引
tmp = repmat([1:N], onePerCol, 1);% 生成列索引模板
c = reshape(tmp, N*onePerCol, 1); % 展开列索引
% 生成稀疏矩阵并转换为完整矩阵
H = full(sparse(r, c, 1, M, N));
为我详细讲解，每一步是如何实现的
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