enc.c

用于LDPC编码译码的仿真实现。包括随机生成校验矩阵、由校验矩阵产生生成矩阵、编码、加随机噪声、译码等内容。原作者是老外

/* ENC.C - Encoding procedures. */

/* Copyright (c) 2000, 2001 by Radford M. Neal 
 *
 * Permission is granted for anyone to copy, use, or modify this program 
 * for purposes of research or education, provided this copyright notice 
 * is retained, and note is made of any changes that have been made. 
 *
 * This program is distributed without any warranty, express or implied.
 * As this program was written for research purposes only, it has not been
 * tested to the degree that would be advisable in any important application.
 * All use of this program is entirely at the user's own risk.
 */

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#include "rand.h"
#include "alloc.h"
#include "mod2sparse.h"
#include "rcode.h"
#include "enc.h"


/* The procedures in this module obtain the generator matrix to use for
   encoding from the global variables declared in rcode.h */


/* ENCODE A BLOCK USING A SPARSE REPRESENTATION OF THE GENERATOR MATRIX. */
//使用生成矩阵的稀疏矩阵表示编码一个分组
void sparse_encode
( int *sblk,     //源比特(N-M)
  int *cblk      //编码比特N
)
{
  int i, j;

  mod2entry   *e;
  int *x, *y;

  x = chk_alloc (M, sizeof *x);
  y = chk_alloc (M, sizeof *y);

  /* Multiply the vector of source bits by the systematic columns of the 
     parity check matrix, giving x.  Also copy these bits to the coded block. */

  for (i = 0; i<M; i++) x[i] = 0;

  for (j = M; j<N; j++)
  { 
    cblk[cols[j]] = sblk[j-M];

    if (sblk[j-M]==1)
    { for (e = mod2sparse_first_in_col(H,cols[j]);
           !mod2sparse_at_end(e);
           e = mod2sparse_next_in_col(e))
      { x[mod2sparse_row(e)] ^= 1;
      }
    }
  }
 
  /* Solve Ly=x for y by forward substitution, then U(cblk)=y by backward
     substitution. */

  if (!mod2sparse_forward_sub(L,rows,x,y)
   || !mod2sparse_backward_sub(U,cols,y,cblk))
  { 
    abort(); /* Shouldn't occur, even if the parity check matrix has 
                redundant rows */
  }

  free(x);
  free(y);
}