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/* GMR-1 SDR - DKABs bursts */
/* See GMR-1 05.004 (ETSI TS 101 376-5-4 V1.2.1) - Section 6.1
* GMR-1 05.002 (ETSI TS 101 376-5-2 V1.1.1) - Section 7.4.6 */
/* (C) 2011-2016 by Sylvain Munaut <tnt@246tNt.com>
* All Rights Reserved
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License as published by
* the Free Software Foundation; either version 3 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 Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
/*! \addtogroup dkab
* @{
*/
/*! \file sdr/dkab.c
* \brief Osmocom GMR-1 DKABs bursts implementation
*/
#include <complex.h>
#include <math.h>
#include <errno.h>
#include <stdint.h>
#include <stdint.h>
#include <stdio.h>
#include <osmocom/core/bits.h>
#include <osmocom/dsp/cxvec.h>
#include <osmocom/dsp/cxvec_math.h>
#include <osmocom/gmr1/sdr/defs.h>
#include <osmocom/gmr1/sdr/dkab.h>
/*! \brief Ratio between peak power and valley power for DKAB detection */
#define DKAB_PWR_RATIO_THRESHOLD 10.0f
/*! \brief Finds the precise TOA of a DKAB burts by looking for power spikes
* \param[in] burst Complex signal of the burst
* \param[in] sps Oversampling used in the input complex signal
* \param[in] p DKAB position
* \param[out] toa_p Pointer to TOA return variable
* \returns 0 for success, 1 if DKAB not found, -errno for fatal errors
*/
static int
_gmr1_dkab_find_toa(struct osmo_cxvec *burst, int sps, int p, float *toa_p)
{
struct osmo_cxvec *pwr = NULL;
int rv, w, i, ofs[2], d, mi;
float mp, toa;
float egy_peak, egy_valley;
int l_peak, l_valley, toa_i;
/* Window size */
w = burst->len - (GMR1_DKAB_SYMS * sps) + 1;
if (w <= 0)
return -EINVAL;
/* Energy vector */
pwr = osmo_cxvec_alloc(w);
if (!pwr)
return -ENOMEM;
ofs[0] = sps * (2 + p); /* First KAB position */
ofs[1] = sps * (2 + p + 59); /* Second KAB position */
d = sps * 5; /* Length of KAB */
pwr->data[0] = 0.0f;
for (i=0; i<d; i++) {
pwr->data[0] +=
osmo_normsqf(burst->data[ofs[0]+i]) +
osmo_normsqf(burst->data[ofs[1]+i]);
}
mi = 0; /* Max index */
mp = pwr->data[0]; /* Max pwr */
for (i=0; i<w-1; i++)
{
float np;
np = pwr->data[i]
- osmo_normsqf(burst->data[ofs[0]+i])
- osmo_normsqf(burst->data[ofs[1]+i])
+ osmo_normsqf(burst->data[ofs[0]+d+i])
+ osmo_normsqf(burst->data[ofs[1]+d+i]);
pwr->data[i+1] = np;
if (np > mp) {
mi = i+1;
mp = np;
}
}
pwr->len = w;
/* Weigh & center peak */
toa = (float)mi;
if ((mi > 0) && (mi < (w-1)))
toa += 0.5f * (-pwr->data[mi-1] + pwr->data[mi+1]) /
(-pwr->data[mi-1] + 2.0f * pwr->data[mi] - pwr->data[mi+1]);
toa += ((float)(sps-1)) / 2.0f;
*toa_p = toa;
toa_i = (int)roundf(toa);
/* Check the ratio between the peaks and valley to validate */
egy_peak = 0.0f;
l_peak = d * 2;
for (i=0; i<d; i++) {
egy_peak +=
osmo_normsqf(burst->data[toa_i+ofs[0]+i]) +
osmo_normsqf(burst->data[toa_i+ofs[1]+i]);
}
egy_peak /= l_peak;
egy_valley = 0.0f;
l_valley = ofs[1] - ofs[0] - d;
for (i=0; i<l_valley; i++)
egy_valley += osmo_normsqf(burst->data[toa_i+ofs[0]+d+i]);
egy_valley /= l_valley;
rv = ((egy_peak /egy_valley) > DKAB_PWR_RATIO_THRESHOLD) ? 0 : 1;
/* Done */
osmo_cxvec_free(pwr);
return rv;
}
/*! \brief Converts a burst into softbits given proper TOA
* \param[in] burst Complex signal of the burst
* \param[in] sps Oversampling used in the input complex signal
* \param[in] p DKAB position
* \param[in] toa The TOA to use to extract symbols
* \param[out] ebits Encoded soft bits return array
* \returns 0 for success. -errno for errors
*/
static int
_gmr1_dkab_soft_bits(struct osmo_cxvec *burst, int sps, int p, float toa,
sbit_t *ebits)
{
int i, toa_i, ofs[2], o;
float pd;
toa_i = (int)roundf(toa);
ofs[0] = toa_i + sps * (2 + p); /* First DKAB */
ofs[1] = toa_i + sps * (2 + p + 59); /* Second DKAB */
for (i=0; i<8; i++) {
o = ofs[i>>2] + sps * (i&3);
pd = cargf(burst->data[o] * conjf(burst->data[o+sps]));
ebits[i] = (sbit_t)roundf((0.5f - (fabsf(pd) / M_PIf)) * 254.0f);
}
return 0;
}
/*! \brief All-in-one finding and demodulation of DKAB bursts
* \param[in] burst_in Complex signal of the burst
* \param[in] sps Oversampling used in the input complex signal
* \param[in] freq_shift Frequency shift to pre-apply to burst_in (rad/sym)
* \param[in] p DKAB position
* \param[out] ebits Encoded soft bits return array
* \param[out] toa_p Pointer to TOA return variable
* \returns 0 for success, 1 if DKAB not found, -errno for fatal errors
*
* burst_in is expected to be longer than necessary. Any extra length will be
* used as 'search window' to find proper alignement. Good practice is to have
* a few samples too much in front and a few samples after the expected TOA.
*/
int
gmr1_dkab_demod(struct osmo_cxvec *burst_in, int sps, float freq_shift, int p,
sbit_t *ebits, float *toa_p)
{
struct osmo_cxvec *burst = NULL;
int rv;
/* Normalize the burst and counter rotate by pi/4 */
burst = osmo_cxvec_sig_normalize(burst_in, 1, (freq_shift - (M_PIf/4)) / sps, NULL);
if (!burst) {
rv = -ENOMEM;
goto err;
}
/* Find TOA */
rv = _gmr1_dkab_find_toa(burst, sps, p, toa_p);
if (rv)
goto err;
/* Demodulate into soft bits */
rv = _gmr1_dkab_soft_bits(burst, sps, p, *toa_p, ebits);
/* Done */
err:
osmo_cxvec_free(burst);
return rv;
}
/*! @} */
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