title: converting ldpc parity matrix to alist format (for gnuradio ldpc encoder use)

LDPC（低密度奇偶校验码）是通信中常用的FEC（前向纠错）编码之一。

GnuRadio的LDPC编解码器接受的输入格式是alist格式的校验矩阵（通常记作H矩阵），本文提供了将H矩阵转换为alist格式的工具。

alist是一种通用的稀疏矩阵存储格式，具体定义见 XXXXXXXXXXXXXXXXXXXX/mackay/codes/XXXXXXXXml

可用以下matlab代码将矩阵x转换为alist，并保存到名字为filename的文件。

<code>function write_alist(x, filename)
    n = size(x,1);
    m = size(x,2);
    num_nlist = sum(x,1)';
    num_mlist = sum(x,2);
    biggest_num_m = max(num_mlist);
    biggest_num_n = max(num_nlist);
    for i = 1:n
        nlist{i} = find(x(i, :) > 0);
    end
    for i = 1:m
        mlist{i} = find(x(:, i) > 0)';
    end

    fd = fopen(filename, 'w');
    
    fprintf(fd, '%d %d\n', n, m);
    fprintf(fd, '%d %d\n', biggest_num_n, biggest_num_m);
    
    fprintf(fd, '%d ', num_nlist);
    fprintf(fd, '\n');
    fprintf(fd, '%d ', num_mlist);
    fprintf(fd, '\n');
    
    for i=1:length(nlist)
        fprintf(fd, '%d ', nlist{i});
        fprintf(fd, '\n');
    end
    for j=1:length(mlist)
        fprintf(fd, '%d ', mlist{j});
        fprintf(fd, '\n');
    end
    
    fclose(fd);
end</code>

调用示例：

<code>% convert sparse matrix to alist format
% <a href=/l?t=aHR0cDovL3d3dy5pbmZlcmVuY2Uub3JnLnVrL21hY2theS9jb2Rlcy9hbGlzdC5odG1s target="_blank">http://www.inference.org.uk/mackay/codes/alist.html</a>

clear all;
x= [0 0 1 0 0 0 0 1 0 1 0 0 1 0 0 0;
0 0 0 1 0 0 1 0 1 0 0 0 1 0 0 0;
0 1 0 0 1 0 1 0 0 1 0 0 0 0 0 0;
0 0 0 1 0 1 0 0 0 0 1 0 0 1 0 0;
0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 1;
1 0 0 0 0 1 0 0 1 1 0 0 0 0 0 0;
0 0 0 1 0 0 0 1 0 0 0 1 0 0 1 0;
0 1 0 0 0 1 0 0 0 0 0 1 0 0 0 1;
1 0 0 0 0 0 1 0 0 0 0 0 0 1 0 1;
0 0 1 0 1 0 0 0 0 0 0 1 0 1 0 0;
0 1 0 0 0 0 0 0 0 0 1 0 1 0 1 0;
1 0 0 0 1 0 0 1 0 0 1 0 0 0 0 0;];

write_alist(x, 'x.alist');</code>

输出结果与原文链接中的例子完全一致。

======TODO:补充CCSDS C2 LDPC 7/8的例子=====