viterbi.c

一些纠错编码的c程序实现

/* Viterbi decoder for arbitrary convolutional code
 * viterbi27 and viterbi37 for the r=1/2 and r=1/3 K=7 codes are faster
 * Copyright 1999 Phil Karn, KA9Q
 * May be used under the terms of the GNU Public License
 */

/* Select code here */
#define	K 15			/* Constraint length */
#define N 6			/* Number of symbols per data bit */
#define Polys	Poly615		/* Select polynomials here */

/* Rate 1/2 codes */
unsigned int Poly23[] = { 0x7, 0x5 };		/* K=3 */
unsigned int Poly24[] = { 0xf, 0xb };		/* k=4 */
unsigned int Poly25[] = { 0x17, 0x19 };		/* k=5 */
unsigned int Poly25a[] = { 0x13, 0x1b };	/* k=5, used in GSM?  */
unsigned int Poly26[] = { 0x2f, 0x35 };		/* k=6  */
unsigned int Poly27[] = { 0x6d, 0x4f };		/* k=7; very popular with NASA and industry  */
unsigned int Poly28[] = { 0x9f, 0xe5 };		/* k=8  */
unsigned int Poly29[] = { 0x1af, 0x11d };	/* k=9; used in IS-95 CDMA  */
unsigned int Poly215[] = { 0x45dd, 0x69e3 };	/* k = 15  */

/* Rate 1/3 codes */
unsigned int Poly33[] = { 0x7, 0x7, 0x5 };	/* k = 3 */
unsigned int Poly34[] = { 0xf, 0xb, 0xd };	/* k = 4 */
unsigned int Poly35[] = { 0x1f, 0x1b, 0x15 };	/* k = 5 */
unsigned int Poly36[] = { 0x2f, 0x35, 0x39 };	/* k = 6 */
unsigned int Poly37[] = { 0x4f, 0x57, 0x6d };	/* k = 7; also popular with NASA and industry  */
unsigned int Poly38[] = { 0xef, 0x9b, 0xa9 };	/* k = 8  */
unsigned int Poly39[] = { 0x1ed, 0x19b, 0x127 }; /* k = 9; used in IS-95 CDMA  */

/* Rate 1/6 code */
unsigned int Poly615[] = { 042631, 047245, 073363, 056507, 077267, 064537 }; /* k=15 on Mars Pathfinder */


#if (K>=6)
#define	D	(1 << (K-6))
#else
#define	D	1
#endif

#include <memory.h>
#define NULL ((void *)0)

extern unsigned char Partab[];	/* Parity lookup table */

int Rate = N;

int Syms[1 << K];
int VDInit;

inline int
parity(int x)
{
  x ^= (x >> 16);
  x ^= (x >> 8);
  return Partab[x & 0xff];
}

/* Convolutionally encode data into binary symbols */
encode(
unsigned char *symbols,
unsigned char *data,
unsigned int nbytes,
unsigned int startstate,
unsigned int endstate)
{
  int i,j;
  unsigned int encstate = startstate;

  while(nbytes-- != 0){
    for(i=7;i>=0;i--){
      encstate = (encstate << 1) | ((*data >> i) & 1);
      for(j=0;j<N;j++)
	*symbols++ = parity(encstate & Polys[j]);
    }
    data++;
  }
  /* Flush out tail */
  for(i=K-2;i>=0;i--){
    encstate = (encstate << 1) | ((endstate >> i) & 1);
    for(j=0;j<N;j++)
      *symbols++ = parity(encstate & Polys[j]);
  }
  return 0;
}

/* Viterbi decoder */
int
viterbi(
unsigned long *metric,	/* Final path metric (returned value) */
unsigned char *data,	/* Decoded output data */
unsigned char *symbols,	/* Raw deinterleaved input symbols */
unsigned int nbits,	/* Number of output bits */
int mettab[2][256],	/* Metric table, [sent sym][rx symbol] */
unsigned int startstate,         /* Encoder starting state */
unsigned int endstate            /* Encoder ending state */
){
  int bitcnt = -(K-1);
  long m0,m1;
  int i,j,sym;
  int mets[1 << N];
  unsigned long paths[(nbits+K-1)*D],*pp,mask;
  long cmetric[1 << (K-1)],nmetric[1 << (K-1)];
  
  memset(paths,0,sizeof(paths));

  if(!VDInit){
    for(i=0;i<(1<<K);i++){
      sym = 0;
      for(j=0;j<N;j++)
	sym = (sym << 1) + parity(i & Polys[j]);
      Syms[i] = sym;
    }
    VDInit++;
  }

  startstate &= ~((1<<(K-1)) - 1);
  endstate &= ~((1<<(K-1)) - 1);

  /* Initialize starting metrics */
  for(i=0;i< 1<<(K-1);i++)
    cmetric[i] = -999999;
  cmetric[startstate] = 0;

  pp = paths;
  for(;;){ /* For each data bit */
    /* Read input symbols and compute branch metrics */
    for(i=0;i< 1<<N;i++){
      mets[i] = 0;
      for(j=0;j<N;j++){
	mets[i] += mettab[(i >> (N-j-1)) & 1][symbols[j]];
      }
    }
    symbols += N;
    /* Run the add-compare-select operations */
    mask = 1;
    for(i=0;i< 1 << (K-1);i+=2){
      int b1,b2;
      
      b1 = mets[Syms[i]];
      nmetric[i] = m0 = cmetric[i/2] + b1; 
      b2 = mets[Syms[i+1]];
      b1 -= b2;
      m1 = cmetric[(i/2) + (1<<(K-2))] + b2;
      if(m1 > m0){
	nmetric[i] = m1;
	*pp |= mask;
      }
      m0 -= b1;
      nmetric[i+1] = m0;
      m1 += b1;
      if(m1 > m0){
	nmetric[i+1] = m1;
	*pp |= mask << 1;
      }
      mask <<= 2;
      if(mask == 0){
	mask = 1;
	pp++;
      }
    }
    if(mask != 1){
      pp++;
    }
    if(++bitcnt == nbits){
      *metric = nmetric[endstate];
      break;
    }
    memcpy(cmetric,nmetric,sizeof(cmetric));
  }

  /* Chain back from terminal state to produce decoded data */
  if(data == NULL)
    return 0;/* Discard output */
  memset(data,0,(nbits+7)/8); /* round up in case nbits % 8 != 0 */

  for(i=nbits-1;i >= 0;i--){
    pp -= D;
    if(pp[endstate >> 5] & (1 << (endstate & 31))){
      endstate |= (1 << (K-1));
      data[i>>3] |= 0x80 >> (i&7);
    }
    endstate >>= 1;
  }
  return 0;
}