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libosmocore/src/conv.c

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/*! \file conv.c
* Generic convolutional encoding / decoding. */
/*
* Copyright (C) 2011 Sylvain Munaut <tnt@246tNt.com>
*
* All Rights Reserved
*
* SPDX-License-Identifier: GPL-2.0+
*
* This program 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 2 of the License, or
* (at your option) any later version.
*
* This program 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 this program; if not, write to the Free Software Foundation, Inc.,
* 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
*/
/*! \addtogroup conv
* @{
* Osmocom convolutional encoder and decoder.
*
* \file conv.c */
#include "config.h"
#ifdef HAVE_ALLOCA_H
#include <alloca.h>
#endif
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <osmocom/core/bits.h>
#include <osmocom/core/conv.h>
/* ------------------------------------------------------------------------ */
/* Common */
/* ------------------------------------------------------------------------ */
int
osmo_conv_get_input_length(const struct osmo_conv_code *code, int len)
{
return len <= 0 ? code->len : len;
}
int
osmo_conv_get_output_length(const struct osmo_conv_code *code, int len)
{
int pbits, in_len, out_len;
/* Input length */
in_len = osmo_conv_get_input_length(code, len);
/* Output length */
out_len = in_len * code->N;
if (code->term == CONV_TERM_FLUSH)
out_len += code->N * (code->K - 1);
/* Count punctured bits */
if (code->puncture) {
for (pbits=0; code->puncture[pbits] >= 0; pbits++);
out_len -= pbits;
}
return out_len;
}
/* ------------------------------------------------------------------------ */
/* Encoding */
/* ------------------------------------------------------------------------ */
/*! Initialize a convolutional encoder
* \param[in,out] encoder Encoder state to initialize
* \param[in] code Description of convolutional code
*/
void
osmo_conv_encode_init(struct osmo_conv_encoder *encoder,
const struct osmo_conv_code *code)
{
memset(encoder, 0x00, sizeof(struct osmo_conv_encoder));
encoder->code = code;
}
void
osmo_conv_encode_load_state(struct osmo_conv_encoder *encoder,
const ubit_t *input)
{
int i;
uint8_t state = 0;
for (i=0; i<(encoder->code->K-1); i++)
state = (state << 1) | input[i];
encoder->state = state;
}
static inline int
_conv_encode_do_output(struct osmo_conv_encoder *encoder,
uint8_t out, ubit_t *output)
{
const struct osmo_conv_code *code = encoder->code;
int o_idx = 0;
int j;
if (code->puncture) {
for (j=0; j<code->N; j++)
{
int bit_no = code->N - j - 1;
int r_idx = encoder->i_idx * code->N + j;
if (code->puncture[encoder->p_idx] == r_idx)
encoder->p_idx++;
else
output[o_idx++] = (out >> bit_no) & 1;
}
} else {
for (j=0; j<code->N; j++)
{
int bit_no = code->N - j - 1;
output[o_idx++] = (out >> bit_no) & 1;
}
}
return o_idx;
}
int
osmo_conv_encode_raw(struct osmo_conv_encoder *encoder,
const ubit_t *input, ubit_t *output, int n)
{
const struct osmo_conv_code *code = encoder->code;
uint8_t state;
int i;
int o_idx;
o_idx = 0;
state = encoder->state;
for (i=0; i<n; i++) {
int bit = input[i];
uint8_t out;
out = code->next_output[state][bit];
state = code->next_state[state][bit];
o_idx += _conv_encode_do_output(encoder, out, &output[o_idx]);
encoder->i_idx++;
}
encoder->state = state;
return o_idx;
}
int
osmo_conv_encode_flush(struct osmo_conv_encoder *encoder,
ubit_t *output)
{
const struct osmo_conv_code *code = encoder->code;
uint8_t state;
int n;
int i;
int o_idx;
n = code->K - 1;
o_idx = 0;
state = encoder->state;
for (i=0; i<n; i++) {
uint8_t out;
if (code->next_term_output) {
out = code->next_term_output[state];
state = code->next_term_state[state];
} else {
out = code->next_output[state][0];
state = code->next_state[state][0];
}
o_idx += _conv_encode_do_output(encoder, out, &output[o_idx]);
encoder->i_idx++;
}
encoder->state = state;
return o_idx;
}
/*! All-in-one convolutional encoding function
* \param[in] code description of convolutional code to be used
* \param[in] input array of unpacked bits (uncoded)
* \param[out] output array of unpacked bits (encoded)
* \return Number of produced output bits
*
* This is an all-in-one function, taking care of
* \ref osmo_conv_init, \ref osmo_conv_encode_load_state,
* \ref osmo_conv_encode_raw and \ref osmo_conv_encode_flush as needed.
*/
int
osmo_conv_encode(const struct osmo_conv_code *code,
const ubit_t *input, ubit_t *output)
{
struct osmo_conv_encoder encoder;
int l;
osmo_conv_encode_init(&encoder, code);
if (code->term == CONV_TERM_TAIL_BITING) {
int eidx = code->len - code->K + 1;
osmo_conv_encode_load_state(&encoder, &input[eidx]);
}
l = osmo_conv_encode_raw(&encoder, input, output, code->len);
if (code->term == CONV_TERM_FLUSH)
l += osmo_conv_encode_flush(&encoder, &output[l]);
return l;
}
/* ------------------------------------------------------------------------ */
/* Decoding (viterbi) */
/* ------------------------------------------------------------------------ */
#define MAX_AE 0x00ffffff
/* Forward declaration for accerlated decoding with certain codes */
int
osmo_conv_decode_acc(const struct osmo_conv_code *code,
const sbit_t *input, ubit_t *output);
void
osmo_conv_decode_init(struct osmo_conv_decoder *decoder,
const struct osmo_conv_code *code, int len, int start_state)
{
int n_states;
/* Init */
if (len <= 0)
len = code->len;
n_states = 1 << (code->K - 1);
memset(decoder, 0x00, sizeof(struct osmo_conv_decoder));
decoder->code = code;
decoder->n_states = n_states;
decoder->len = len;
/* Allocate arrays */
decoder->ae = malloc(sizeof(unsigned int) * n_states);
decoder->ae_next = malloc(sizeof(unsigned int) * n_states);
decoder->state_history = malloc(sizeof(uint8_t) * n_states * (len + decoder->code->K - 1));
/* Classic reset */
osmo_conv_decode_reset(decoder, start_state);
}
void
osmo_conv_decode_reset(struct osmo_conv_decoder *decoder, int start_state)
{
int i;
/* Reset indexes */
decoder->o_idx = 0;
decoder->p_idx = 0;
/* Initial error */
if (start_state < 0) {
/* All states possible */
memset(decoder->ae, 0x00, sizeof(unsigned int) * decoder->n_states);
} else {
/* Fixed start state */
for (i=0; i<decoder->n_states; i++) {
decoder->ae[i] = (i == start_state) ? 0 : MAX_AE;
}
}
}
void
osmo_conv_decode_rewind(struct osmo_conv_decoder *decoder)
{
int i;
unsigned int min_ae = MAX_AE;
/* Reset indexes */
decoder->o_idx = 0;
decoder->p_idx = 0;
/* Initial error normalize (remove constant) */
for (i=0; i<decoder->n_states; i++) {
if (decoder->ae[i] < min_ae)
min_ae = decoder->ae[i];
}
for (i=0; i<decoder->n_states; i++)
decoder->ae[i] -= min_ae;
}
void
osmo_conv_decode_deinit(struct osmo_conv_decoder *decoder)
{
free(decoder->ae);
free(decoder->ae_next);
free(decoder->state_history);
memset(decoder, 0x00, sizeof(struct osmo_conv_decoder));
}
int
osmo_conv_decode_scan(struct osmo_conv_decoder *decoder,
const sbit_t *input, int n)
{
const struct osmo_conv_code *code = decoder->code;
int i, s, b, j;
int n_states;
unsigned int *ae;
unsigned int *ae_next;
uint8_t *state_history;
sbit_t *in_sym;
int i_idx, p_idx;
/* Prepare */
n_states = decoder->n_states;
ae = decoder->ae;
ae_next = decoder->ae_next;
state_history = &decoder->state_history[n_states * decoder->o_idx];
in_sym = alloca(sizeof(sbit_t) * code->N);
i_idx = 0;
p_idx = decoder->p_idx;
/* Scan the treillis */
for (i=0; i<n; i++)
{
/* Reset next accumulated error */
for (s=0; s<n_states; s++) {
ae_next[s] = MAX_AE;
}
/* Get input */
if (code->puncture) {
/* Hard way ... */
for (j=0; j<code->N; j++) {
int idx = ((decoder->o_idx + i) * code->N) + j;
if (idx == code->puncture[p_idx]) {
in_sym[j] = 0; /* Undefined */
p_idx++;
} else {
in_sym[j] = input[i_idx];
i_idx++;
}
}
} else {
/* Easy, just copy N bits */
memcpy(in_sym, &input[i_idx], code->N);
i_idx += code->N;
}
/* Scan all state */
for (s=0; s<n_states; s++)
{
/* Scan possible input bits */
for (b=0; b<2; b++)
{
int nae, ov, e;
uint8_t m;
/* Next output and state */
uint8_t out = code->next_output[s][b];
uint8_t state = code->next_state[s][b];
/* New error for this path */
nae = ae[s]; /* start from last error */
m = 1 << (code->N - 1); /* mask for 'out' bit selection */
for (j=0; j<code->N; j++) {
int is = (int)in_sym[j];
if (is) {
ov = (out & m) ? -127 : 127; /* sbit_t value for it */
e = is - ov; /* raw error for this bit */
nae += (e * e) >> 9; /* acc the squared/scaled value */
}
m >>= 1; /* next mask bit */
}
/* Is it survivor ? */
if (ae_next[state] > nae) {
ae_next[state] = nae;
state_history[(n_states * i) + state] = s;
}
}
}
/* Copy accumulated error */
memcpy(ae, ae_next, sizeof(unsigned int) * n_states);
}
/* Update decoder state */
decoder->p_idx = p_idx;
decoder->o_idx += n;
return i_idx;
}
int
osmo_conv_decode_flush(struct osmo_conv_decoder *decoder,
const sbit_t *input)
{
const struct osmo_conv_code *code = decoder->code;
int i, s, j;
int n_states;
unsigned int *ae;
unsigned int *ae_next;
uint8_t *state_history;
sbit_t *in_sym;
int i_idx, p_idx;
/* Prepare */
n_states = decoder->n_states;
ae = decoder->ae;
ae_next = decoder->ae_next;
state_history = &decoder->state_history[n_states * decoder->o_idx];
in_sym = alloca(sizeof(sbit_t) * code->N);
i_idx = 0;
p_idx = decoder->p_idx;
/* Scan the treillis */
for (i=0; i<code->K-1; i++)
{
/* Reset next accumulated error */
for (s=0; s<n_states; s++) {
ae_next[s] = MAX_AE;
}
/* Get input */
if (code->puncture) {
/* Hard way ... */
for (j=0; j<code->N; j++) {
int idx = ((decoder->o_idx + i) * code->N) + j;
if (idx == code->puncture[p_idx]) {
in_sym[j] = 0; /* Undefined */
p_idx++;
} else {
in_sym[j] = input[i_idx];
i_idx++;
}
}
} else {
/* Easy, just copy N bits */
memcpy(in_sym, &input[i_idx], code->N);
i_idx += code->N;
}
/* Scan all state */
for (s=0; s<n_states; s++)
{
int nae, ov, e;
uint8_t m;
/* Next output and state */
uint8_t out;
uint8_t state;
if (code->next_term_output) {
out = code->next_term_output[s];
state = code->next_term_state[s];
} else {
out = code->next_output[s][0];
state = code->next_state[s][0];
}
/* New error for this path */
nae = ae[s]; /* start from last error */
m = 1 << (code->N - 1); /* mask for 'out' bit selection */
for (j=0; j<code->N; j++) {
int is = (int)in_sym[j];
if (is) {
ov = (out & m) ? -127 : 127; /* sbit_t value for it */
e = is - ov; /* raw error for this bit */
nae += (e * e) >> 9; /* acc the squared/scaled value */
}
m >>= 1; /* next mask bit */
}
/* Is it survivor ? */
if (ae_next[state] > nae) {
ae_next[state] = nae;
state_history[(n_states * i) + state] = s;
}
}
/* Copy accumulated error */
memcpy(ae, ae_next, sizeof(unsigned int) * n_states);
}
/* Update decoder state */
decoder->p_idx = p_idx;
decoder->o_idx += code->K - 1;
return i_idx;
}
int
osmo_conv_decode_get_output(struct osmo_conv_decoder *decoder,
ubit_t *output, int has_flush, int end_state)
{
const struct osmo_conv_code *code = decoder->code;
int min_ae;
uint8_t min_state, cur_state;
int i, s, n;
uint8_t *sh_ptr;
/* End state ? */
if (end_state < 0) {
/* Find state with least error */
min_ae = MAX_AE;
min_state = 0xff;
for (s=0; s<decoder->n_states; s++)
{
if (decoder->ae[s] < min_ae) {
min_ae = decoder->ae[s];
min_state = s;
}
}
if (min_state == 0xff)
return -1;
} else {
min_state = (uint8_t) end_state;
min_ae = decoder->ae[end_state];
}
/* Traceback */
cur_state = min_state;
n = decoder->o_idx;
sh_ptr = &decoder->state_history[decoder->n_states * (n-1)];
/* No output for the K-1 termination input bits */
if (has_flush) {
for (i=0; i<code->K-1; i++) {
cur_state = sh_ptr[cur_state];
sh_ptr -= decoder->n_states;
}
n -= code->K - 1;
}
/* Generate output backward */
for (i=n-1; i>=0; i--)
{
min_state = cur_state;
cur_state = sh_ptr[cur_state];
sh_ptr -= decoder->n_states;
if (code->next_state[cur_state][0] == min_state)
output[i] = 0;
else
output[i] = 1;
}
return min_ae;
}
/*! All-in-one convolutional decoding function
* \param[in] code description of convolutional code to be used
* \param[in] input array of soft bits (coded)
* \param[out] output array of unpacked bits (decoded)
*
* This is an all-in-one function, taking care of
* \ref osmo_conv_decode_init, \ref osmo_conv_decode_scan,
* \ref osmo_conv_decode_flush, \ref osmo_conv_decode_get_output and
* \ref osmo_conv_decode_deinit.
*/
int
osmo_conv_decode(const struct osmo_conv_code *code,
const sbit_t *input, ubit_t *output)
{
struct osmo_conv_decoder decoder;
int rv, l;
/* Use accelerated implementation for supported codes */
if ((code->N <= 4) && ((code->K == 5) || (code->K == 7)))
return osmo_conv_decode_acc(code, input, output);
osmo_conv_decode_init(&decoder, code, 0, 0);
if (code->term == CONV_TERM_TAIL_BITING) {
osmo_conv_decode_scan(&decoder, input, code->len);
osmo_conv_decode_rewind(&decoder);
}
l = osmo_conv_decode_scan(&decoder, input, code->len);
if (code->term == CONV_TERM_FLUSH)
osmo_conv_decode_flush(&decoder, &input[l]);
rv = osmo_conv_decode_get_output(&decoder, output,
code->term == CONV_TERM_FLUSH, /* has_flush */
code->term == CONV_TERM_FLUSH ? 0 : -1 /* end_state */
);
osmo_conv_decode_deinit(&decoder);
return rv;
}
/*! @} */
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