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/* This file was taken from gsm-tvoid */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
//#include "burst_types.h"
#include "conv.h"
//#include "fire_crc.h"
/*
* Convolutional encoding and Viterbi decoding for the GSM CCH+TCH channel.
*/
/* The class 1 bits are encoded with the 1/2 rate convolutional code defined by
* the polynomials:
* G0 = 1 + D3+ D4
* G1 = 1 + D + D3+ D4
* The coded bits {c(0), c(1),..., c(455)} are then defined by:
* class 1: c(2k) = u(k) + u(k-3) + u(k-4)
* c(2k+1) = u(k) + u(k-1) + u(k-3) + u(k-4) for k = 0,1,...,188
* u(k) = 0 for k < 0
* class 2:c(378+k) = d(182+k) for k = 0,1,....,77
*/
/*
* Convolutional encoding:
*
* G_0 = 1 + x^3 + x^4
* G_1 = 1 + x + x^3 + x^4
*
* i.e.,
*
* c_{2k} = u_k + u_{k - 3} + u_{k - 4}
* c_{2k + 1} = u_k + u_{k - 1} + u_{k - 3} + u_{k - 4}
*/
#define K 5
#define MAX_ERROR(size) (2 * size + 1)
/*
* Given the current state and input bit, what are the output bits?
*
* encode[current_state][input_bit]
*/
static const unsigned int encode[1 << (K - 1)][2] = {
{0, 3}, {3, 0}, {3, 0}, {0, 3},
{0, 3}, {3, 0}, {3, 0}, {0, 3},
{1, 2}, {2, 1}, {2, 1}, {1, 2},
{1, 2}, {2, 1}, {2, 1}, {1, 2}
};
/*
* Given the current state and input bit, what is the next state?
*
* next_state[current_state][input_bit]
*/
static const unsigned int next_state[1 << (K - 1)][2] = {
{0, 8}, {0, 8}, {1, 9}, {1, 9},
{2, 10}, {2, 10}, {3, 11}, {3, 11},
{4, 12}, {4, 12}, {5, 13}, {5, 13},
{6, 14}, {6, 14}, {7, 15}, {7, 15}
};
/*
* Given the previous state and the current state, what input bit caused
* the transition? If it is impossible to transition between the two
* states, the value is 2.
*
* prev_next_state[previous_state][current_state]
*/
static const unsigned int prev_next_state[1 << (K - 1)][1 << (K - 1)] = {
{ 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2},
{ 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2, 2},
{ 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2},
{ 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2, 2},
{ 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2},
{ 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2, 2},
{ 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2},
{ 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2, 2},
{ 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2},
{ 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2, 2},
{ 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2},
{ 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2, 2},
{ 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2},
{ 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1, 2},
{ 2, 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1},
{ 2, 2, 2, 2, 2, 2, 2, 0, 2, 2, 2, 2, 2, 2, 2, 1}
};
static inline unsigned int hamming_distance2(unsigned int w) {
return (w & 1) + !!(w & 2);
}
/*
static void conv_encode(unsigned char *data, unsigned char *output,
unsigned int input_size) {
unsigned int i, state = 0, o;
// encode data
for(i = 0; i < input_size; i++) {
o = encode[state][data[i]];
state = next_state[state][data[i]];
*output++ = !!(o & 2);
*output++ = o & 1;
}
}
*/
int
conv_decode(unsigned char *output, unsigned char *data,
unsigned int input_size) {
int i, t;
unsigned int rdata, state, nstate, b, o, distance, accumulated_error,
min_state, min_error, cur_state;
unsigned int max_error = MAX_ERROR(input_size);
unsigned int ae[1 << (K - 1)];
unsigned int nae[1 << (K - 1)]; // next accumulated error
unsigned int state_history[1 << (K - 1)][CONV_MAX_INPUT_SIZE + 1];
// initialize accumulated error, assume starting state is 0
for(i = 0; i < (1 << (K - 1)); i++)
ae[i] = nae[i] = max_error;
ae[0] = 0;
// build trellis
for(t = 0; t < input_size; t++) {
// get received data symbol
rdata = (data[2 * t] << 1) | data[2 * t + 1];
// for each state
for(state = 0; state < (1 << (K - 1)); state++) {
// make sure this state is possible
if(ae[state] >= max_error)
continue;
// find all states we lead to
for(b = 0; b < 2; b++) {
// get next state given input bit b
nstate = next_state[state][b];
// find output for this transition
o = encode[state][b];
// calculate distance from received data
distance = hamming_distance2(rdata ^ o);
// choose surviving path
accumulated_error = ae[state] + distance;
if(accumulated_error < nae[nstate]) {
// save error for surviving state
nae[nstate] = accumulated_error;
// update state history
state_history[nstate][t + 1] = state;
}
}
}
// get accumulated error ready for next time slice
for(i = 0; i < (1 << (K - 1)); i++) {
ae[i] = nae[i];
nae[i] = max_error;
}
}
// the final state is the state with the fewest errors
min_state = (unsigned int)-1;
min_error = max_error;
for(i = 0; i < (1 << (K - 1)); i++) {
if(ae[i] < min_error) {
min_state = i;
min_error = ae[i];
}
}
// trace the path
cur_state = min_state;
for(t = input_size; t >= 1; t--) {
min_state = cur_state;
cur_state = state_history[cur_state][t]; // get previous
output[t - 1] = prev_next_state[cur_state][min_state];
}
// return the number of errors detected (hard-decision)
return min_error;
}
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