1 files changed, 105 insertions, 0 deletions
diff --git a/src/host/layer23/src/transceiver/gsm_ab.c b/src/host/layer23/src/transceiver/gsm_ab.c
new file mode 100644
index 00000000..903a5c3f
--- /dev/null
+++ b/src/host/layer23/src/transceiver/gsm_ab.c
@@ -0,0 +1,105 @@
+/*
+ * 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;
+}