enc.c

用C语言编写的LDPC译码程序

/* ENC.C - Encoding procedures. */

/* Copyright (c) 2000 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 "mod2dense.h"
#include "mod2convert.h"
#include "rcode.h"
#include "enc.h"


/* ENCODE A BLOCK USING A SPARSE REPRESENTATION OF THE GENERATOR MATRIX. */

void sparse_encode
( char *sblk,
  char *cblk
)
{
  int i, j;
  int b;

  mod2entry *e;
  char *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);
}


/* ENCODE A BLOCK USING DENSE REPRESENTATION OF GENERATOR MATRIX. */

void dense_encode
( char *sblk,
  char *cblk,
  mod2dense *u,
  mod2dense *v
)
{
  int j;

  /* Copy source bits to the systematic part of the coded block. */

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

  /* Multiply by Inv(A) X B to produce check bits. */

  for (j = M; j<N; j++)
  { mod2dense_set(u,j-M,0,sblk[j-M]); 
  }
  
  mod2dense_multiply(G,u,v);

  /* Copy check bits to the right places in the coded block. */

  for (j = 0; j<M; j++)
  { cblk[cols[j]] = mod2dense_get(v,j,0);
  }
}


/* ENCODE A BLOCK USING MIXED REPRESENTATION OF GENERATOR MATRIX. */

void mixed_encode
( char *sblk,
  char *cblk,
  mod2dense *u,
  mod2dense *v
)
{
  mod2entry *e;
  int j;

  /* Multiply the vector of source bits by the message bit columns of the 
     parity check matrix.  Also copy these bits to the coded block. */

  mod2dense_clear(u);

  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))
      { (void) mod2dense_flip(u,mod2sparse_row(e),0);
      }
    }
  }

  /* Multiply by Inv(A) to produce check bits. */

  mod2dense_multiply(G,u,v);

  /* Copy check bits to the right places in the coded block. */

  for (j = 0; j<M; j++)
  { cblk[cols[j]] = mod2dense_get(v,j,0);
  }
}