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/*
* gsm_ab.c
*
* GSM access burst SDR support
*
* Copyright (C) 2013 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/>.
*/
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <osmocom/core/bits.h>
#include <osmocom/dsp/cxvec.h>
#include <osmocom/dsp/cxvec_math.h>
#include "gmsk.h"
#include "gsm_ab.h"
const ubit_t gsm_ab_train[] = {
0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1,
1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0,
0, 0, 1, 1, 1, 1, 0, 0, 0,
};
int
gsm_ab_detect(const struct osmo_gmsk_state *gs,
const struct osmo_gmsk_trainseq *rach,
const struct osmo_cxvec *burst, float thresh,
float complex *peak, float *toa)
{
struct osmo_cxvec *cb = NULL;
int found = 0;
cb = osmo_cxvec_correlate(rach->seq, burst, 1, NULL);
if (!cb)
return -ENOMEM;
*toa = osmo_cxvec_peak_energy_find(cb, 5, PEAK_WEIGH_WIN_CENTER, peak);
if (*toa < 0.0 || *toa >= cb->len)
goto done;
*toa = *toa - rach->toa - 8*gs->sps;
*peak = *peak / rach->gain;
found = 1;
done:
osmo_cxvec_free(cb);
if (!found) {
*toa = 0.0f;
*peak = 1.0f;
}
return !found;
}
sbit_t *
gsm_ab_demodulate(struct osmo_gmsk_state *gs,
struct osmo_cxvec *burst, float complex channel, float toa)
{
sbit_t *rv;
int i;
/* Return vector */
rv = malloc(GSM_AB_BITS * sizeof(sbit_t));
if (!rv)
return NULL;
/* Demod */
osmo_cxvec_scale(burst, (1.0f / channel), burst);
osmo_cxvec_delay(burst, -toa, burst);
osmo_gmsk_rotate_rev(gs, burst);
/* Mapping / Slicing */
for (i=0; i<GSM_AB_BITS; i++) {
int v = (int)(-127.0f * crealf(burst->data[i*gs->sps]));
if (v > 127)
v = 127;
else if (v < -127)
v = -127;
rv[i] = v;
}
return rv;
}
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