sdr/dkab: Add support for Dual Keep Alive Bursts demodulation

The actual data out of the demodulation is not tested tough since
the upper layers can't handle that yet.

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

2 files changed, 191 insertions, 1 deletions

diff --git a/src/sdr/Makefile.am b/src/sdr/Makefile.am
index b2edef9..0d92d22 100644
--- a/src/sdr/Makefile.am
+++ b/src/sdr/Makefile.am
@@ -4,4 +4,4 @@ AM_LDFLAGS = $(LIBOSMOCORE_LIBS) $(LIBOSMOSDR_LIBS) $(FFTW3F_LIBS)
 
 noinst_LIBRARIES = libgmr1-sdr.a
 
-libgmr1_sdr_a_SOURCES = fcch.c nb.c pi4cxpsk.c
+libgmr1_sdr_a_SOURCES = dkab.c fcch.c nb.c pi4cxpsk.c
diff --git a/src/sdr/dkab.c b/src/sdr/dkab.c
new file mode 100644
index 0000000..265d254
--- /dev/null
+++ b/src/sdr/dkab.c
@@ -0,0 +1,190 @@
+/* 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 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/sdr/cxvec.h>
+#include <osmocom/sdr/cxvec_math.h>
+
+#include <osmocom/gmr1/sdr/defs.h>
+#include <osmocom/gmr1/sdr/dkab.h>
+
+
+/*! \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. -errno for errors
+ */
+static int
+_gmr1_dkab_find_toa(struct osmo_cxvec *burst, int sps, int p, float *toa_p)
+{
+	struct osmo_cxvec *pwr = NULL;
+	int w, i, ofs[2], d, mi;
+	float mp, toa;
+
+	/* 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;
+
+	/* Done */
+	osmo_cxvec_free(pwr);
+
+	return 0;
+}
+
+/*! \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 + 159);	/* Second DKAB */
+
+	for (i=0; i<8; i++) {
+		o = ofs[i>>2] + (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. -errno for 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;
+}
+
+/*! }@ */