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/*
* Copyright 1995 Phil Karn, KA9Q
* Copyright 2008 Free Software Foundation, Inc.
*
* This file is part of GNU Radio
*
* GNU Radio is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3, or (at your option)
* any later version.
*
* GNU Radio is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GNU Radio; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
/*
* Viterbi decoder for K=7 rate=1/2 convolutional code
* Some modifications from original Karn code by Matt Ettus
*/
#include "viterbi.h"
/* The two generator polynomials for the NASA Standard K=7 code.
* Since these polynomials are known to be optimal for this constraint
* length there is not much point in changing them. But if you do, you
* will have to regenerate the BUTTERFLY macro calls in viterbi()
*/
#define POLYA 0x6d
#define POLYB 0x4f
/* The basic Viterbi decoder operation, called a "butterfly"
* operation because of the way it looks on a trellis diagram. Each
* butterfly involves an Add-Compare-Select (ACS) operation on the two nodes
* where the 0 and 1 paths from the current node merge at the next step of
* the trellis.
*
* The code polynomials are assumed to have 1's on both ends. Given a
* function encode_state() that returns the two symbols for a given
* encoder state in the low two bits, such a code will have the following
* identities for even 'n' < 64:
*
* encode_state(n) = encode_state(n+65)
* encode_state(n+1) = encode_state(n+64) = (3 ^ encode_state(n))
*
* Any convolutional code you would actually want to use will have
* these properties, so these assumptions aren't too limiting.
*
* Doing this as a macro lets the compiler evaluate at compile time the
* many expressions that depend on the loop index and encoder state and
* emit them as immediate arguments.
* This makes an enormous difference on register-starved machines such
* as the Intel x86 family where evaluating these expressions at runtime
* would spill over into memory.
*/
#define BUTTERFLY(i,sym) { \
int m0,m1;\
\
/* ACS for 0 branch */\
m0 = state[i].metric + mets[sym]; /* 2*i */\
m1 = state[i+32].metric + mets[3^sym]; /* 2*i + 64 */\
if(m0 > m1){\
next[2*i].metric = m0;\
next[2*i].path = state[i].path << 1;\
} else {\
next[2*i].metric = m1;\
next[2*i].path = (state[i+32].path << 1)|1;\
}\
/* ACS for 1 branch */\
m0 = state[i].metric + mets[3^sym]; /* 2*i + 1 */\
m1 = state[i+32].metric + mets[sym]; /* 2*i + 65 */\
if(m0 > m1){\
next[2*i+1].metric = m0;\
next[2*i+1].path = state[i].path << 1;\
} else {\
next[2*i+1].metric = m1;\
next[2*i+1].path = (state[i+32].path << 1)|1;\
}\
}
extern unsigned char Partab[]; /* Parity lookup table */
/* Convolutionally encode data into binary symbols */
unsigned char
encode(unsigned char *symbols,
unsigned char *data,
unsigned int nbytes,
unsigned char encstate)
{
int i;
while(nbytes-- != 0){
for(i=7;i>=0;i--){
encstate = (encstate << 1) | ((*data >> i) & 1);
*symbols++ = Partab[encstate & POLYA];
*symbols++ = Partab[encstate & POLYB];
}
data++;
}
return encstate;
}
/* 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 bitcnt = 0;
int mets[4];
long bestmetric;
int beststate,i;
struct viterbi_state state0[64],state1[64],*state,*next;
state = state0;
next = state1;
/* Initialize starting metrics to prefer 0 state */
state[0].metric = 0;
for(i=1;i<64;i++)
state[i].metric = -999999;
state[0].path = 0;
for(bitcnt = 0;bitcnt < nbits;bitcnt++){
/* Read input symbol pair and compute all possible branch
* metrics
*/
mets[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
mets[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
mets[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];
mets[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
symbols += 2;
/* These macro calls were generated by genbut.c */
BUTTERFLY(0,0);
BUTTERFLY(1,1);
BUTTERFLY(2,3);
BUTTERFLY(3,2);
BUTTERFLY(4,3);
BUTTERFLY(5,2);
BUTTERFLY(6,0);
BUTTERFLY(7,1);
BUTTERFLY(8,0);
BUTTERFLY(9,1);
BUTTERFLY(10,3);
BUTTERFLY(11,2);
BUTTERFLY(12,3);
BUTTERFLY(13,2);
BUTTERFLY(14,0);
BUTTERFLY(15,1);
BUTTERFLY(16,2);
BUTTERFLY(17,3);
BUTTERFLY(18,1);
BUTTERFLY(19,0);
BUTTERFLY(20,1);
BUTTERFLY(21,0);
BUTTERFLY(22,2);
BUTTERFLY(23,3);
BUTTERFLY(24,2);
BUTTERFLY(25,3);
BUTTERFLY(26,1);
BUTTERFLY(27,0);
BUTTERFLY(28,1);
BUTTERFLY(29,0);
BUTTERFLY(30,2);
BUTTERFLY(31,3);
/* Swap current and next states */
if(bitcnt & 1){
state = state0;
next = state1;
} else {
state = state1;
next = state0;
}
// ETTUS
//if(bitcnt > nbits-7){
/* In tail, poison non-zero nodes */
//for(i=1;i<64;i += 2)
// state[i].metric = -9999999;
//}
/* Produce output every 8 bits once path memory is full */
if((bitcnt % 8) == 5 && bitcnt > 32){
/* Find current best path */
bestmetric = state[0].metric;
beststate = 0;
for(i=1;i<64;i++){
if(state[i].metric > bestmetric){
bestmetric = state[i].metric;
beststate = i;
}
}
#ifdef notdef
printf("metrics[%d] = %d state = %lx\n",beststate,
state[beststate].metric,state[beststate].path);
#endif
*data++ = state[beststate].path >> 24;
}
}
/* Output remaining bits from 0 state */
// ETTUS Find best state instead
bestmetric = state[0].metric;
beststate = 0;
for(i=1;i<64;i++){
if(state[i].metric > bestmetric){
bestmetric = state[i].metric;
beststate = i;
}
}
if((i = bitcnt % 8) != 6)
state[beststate].path <<= 6-i;
*data++ = state[beststate].path >> 24;
*data++ = state[beststate].path >> 16;
*data++ = state[beststate].path >> 8;
*data = state[beststate].path;
//printf ("BS = %d\tBSM = %d\tM0 = %d\n",beststate,state[beststate].metric,state[0].metric);
*metric = state[beststate].metric;
return 0;
}
void
viterbi_chunks_init(struct viterbi_state* state) {
// Initialize starting metrics to prefer 0 state
int i;
state[0].metric = 0;
state[0].path = 0;
for(i=1;i<64;i++)
state[i].metric = -999999;
}
void
viterbi_butterfly8(unsigned char *symbols, int mettab[2][256], struct viterbi_state *state0, struct viterbi_state *state1)
{
unsigned int bitcnt;
int mets[4];
struct viterbi_state *state, *next;
state = state0;
next = state1;
// Operate on 16 symbols (8 bits) at a time
for(bitcnt = 0;bitcnt < 8;bitcnt++){
// Read input symbol pair and compute all possible branch metrics
mets[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
mets[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
mets[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];
mets[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
symbols += 2;
// These macro calls were generated by genbut.c
BUTTERFLY(0,0);BUTTERFLY(1,1);BUTTERFLY(2,3);BUTTERFLY(3,2);
BUTTERFLY(4,3);BUTTERFLY(5,2);BUTTERFLY(6,0);BUTTERFLY(7,1);
BUTTERFLY(8,0);BUTTERFLY(9,1);BUTTERFLY(10,3);BUTTERFLY(11,2);
BUTTERFLY(12,3);BUTTERFLY(13,2);BUTTERFLY(14,0);BUTTERFLY(15,1);
BUTTERFLY(16,2);BUTTERFLY(17,3);BUTTERFLY(18,1);BUTTERFLY(19,0);
BUTTERFLY(20,1);BUTTERFLY(21,0);BUTTERFLY(22,2);BUTTERFLY(23,3);
BUTTERFLY(24,2);BUTTERFLY(25,3);BUTTERFLY(26,1);BUTTERFLY(27,0);
BUTTERFLY(28,1);BUTTERFLY(29,0);BUTTERFLY(30,2);BUTTERFLY(31,3);
// Swap current and next states
if(bitcnt & 1){
state = state0;
next = state1;
} else {
state = state1;
next = state0;
}
}
}
void
viterbi_butterfly2(unsigned char *symbols, int mettab[2][256], struct viterbi_state *state0, struct viterbi_state *state1)
{
//unsigned int bitcnt;
int mets[4];
struct viterbi_state *state, *next;
state = state0;
next = state1;
// Operate on 4 symbols (2 bits) at a time
// Read input symbol pair and compute all possible branch metrics
mets[0] = mettab[0][symbols[0]] + mettab[0][symbols[1]];
mets[1] = mettab[0][symbols[0]] + mettab[1][symbols[1]];
mets[2] = mettab[1][symbols[0]] + mettab[0][symbols[1]];
mets[3] = mettab[1][symbols[0]] + mettab[1][symbols[1]];
// These macro calls were generated by genbut.c
BUTTERFLY(0,0);BUTTERFLY(1,1);BUTTERFLY(2,3);BUTTERFLY(3,2);
BUTTERFLY(4,3);BUTTERFLY(5,2);BUTTERFLY(6,0);BUTTERFLY(7,1);
BUTTERFLY(8,0);BUTTERFLY(9,1);BUTTERFLY(10,3);BUTTERFLY(11,2);
BUTTERFLY(12,3);BUTTERFLY(13,2);BUTTERFLY(14,0);BUTTERFLY(15,1);
BUTTERFLY(16,2);BUTTERFLY(17,3);BUTTERFLY(18,1);BUTTERFLY(19,0);
BUTTERFLY(20,1);BUTTERFLY(21,0);BUTTERFLY(22,2);BUTTERFLY(23,3);
BUTTERFLY(24,2);BUTTERFLY(25,3);BUTTERFLY(26,1);BUTTERFLY(27,0);
BUTTERFLY(28,1);BUTTERFLY(29,0);BUTTERFLY(30,2);BUTTERFLY(31,3);
state = state1;
next = state0;
// Read input symbol pair and compute all possible branch metrics
mets[0] = mettab[0][symbols[2]] + mettab[0][symbols[3]];
mets[1] = mettab[0][symbols[2]] + mettab[1][symbols[3]];
mets[2] = mettab[1][symbols[2]] + mettab[0][symbols[3]];
mets[3] = mettab[1][symbols[2]] + mettab[1][symbols[3]];
// These macro calls were generated by genbut.c
BUTTERFLY(0,0);BUTTERFLY(1,1);BUTTERFLY(2,3);BUTTERFLY(3,2);
BUTTERFLY(4,3);BUTTERFLY(5,2);BUTTERFLY(6,0);BUTTERFLY(7,1);
BUTTERFLY(8,0);BUTTERFLY(9,1);BUTTERFLY(10,3);BUTTERFLY(11,2);
BUTTERFLY(12,3);BUTTERFLY(13,2);BUTTERFLY(14,0);BUTTERFLY(15,1);
BUTTERFLY(16,2);BUTTERFLY(17,3);BUTTERFLY(18,1);BUTTERFLY(19,0);
BUTTERFLY(20,1);BUTTERFLY(21,0);BUTTERFLY(22,2);BUTTERFLY(23,3);
BUTTERFLY(24,2);BUTTERFLY(25,3);BUTTERFLY(26,1);BUTTERFLY(27,0);
BUTTERFLY(28,1);BUTTERFLY(29,0);BUTTERFLY(30,2);BUTTERFLY(31,3);
}
unsigned char
viterbi_get_output(struct viterbi_state *state, unsigned char *outbuf) {
// Produce output every 8 bits once path memory is full
// if((bitcnt % 8) == 5 && bitcnt > 32) {
// Find current best path
unsigned int i,beststate;
int bestmetric;
bestmetric = state[0].metric;
beststate = 0;
for(i=1;i<64;i++)
if(state[i].metric > bestmetric) {
bestmetric = state[i].metric;
beststate = i;
}
*outbuf = state[beststate].path >> 24;
return bestmetric;
}
//printf ("BS = %d\tBSM = %d\tM0 = %d\n",beststate,state[beststate].metric,state[0].metric);
// In tail, poison non-zero nodes
//if(bits_out > packet_size-7)
// for(i=1;i<64;i += 2)
// state[i].metric = -9999999;
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