[handover] first functional handover implementation

With this commit, we can successfully hand over a channel from one cell to
another cell.  We implement asynchronous intra-BSC (but inter-BTS) handover.

Changes:
* introduce new DHO log category
* extend rsl_chan_activate_lchan() with argument for HO reference
* introduce actual minimal handover decision making in handover_decision.c
* various fixes to bsc_handover_start() in  handover_logic.c

1 files changed, 111 insertions, 0 deletions

diff --git a/openbsc/src/handover_decision.c b/openbsc/src/handover_decision.c
new file mode 100644
index 000000000..06eb86507
--- /dev/null
+++ b/openbsc/src/handover_decision.c
@@ -0,0 +1,111 @@
+/* Handover Decision making for Inter-BTS (Intra-BSC) Handover.  This
+ * only implements the handover algorithm/decision, but not execution
+ * of it */
+
+/* (C) 2009 by Harald Welte <laforge@gnumonks.org>
+ *
+ * All Rights Reserved
+ *
+ * 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.
+ *
+ */
+
+#include <stdlib.h>
+#include <errno.h>
+
+#include <openbsc/msgb.h>
+#include <openbsc/debug.h>
+#include <openbsc/gsm_data.h>
+#include <openbsc/meas_rep.h>
+#include <openbsc/signal.h>
+#include <openbsc/talloc.h>
+#include <openbsc/handover.h>
+
+static int handover_to_arfcn_bsic(struct gsm_lchan *lchan,
+				  u_int16_t arfcn, u_int8_t bsic)
+{
+	struct gsm_bts *new_bts;
+
+	/* resolve the gsm_bts structure for the best neighbor */
+	new_bts = gsm_bts_neighbor(lchan->ts->trx->bts, arfcn, bsic);
+	if (!new_bts) {
+		DEBUGP(DHO, "unable to determine neighbor BTS for ARFCN %u BSIC %u ?!?\n", arfcn, bsic);
+		return -EINVAL;
+	}
+
+	/* and actually try to handover to that cell */
+	return bsc_handover_start(lchan, new_bts);
+}
+
+#define RXLEV_HYST 3
+
+/* process an already parsed measurement report */
+static int process_meas_rep(struct gsm_meas_rep *mr)
+{
+	struct gsm_meas_rep_cell *mr_cell = NULL;
+	unsigned int best_better_db;
+	int i;
+
+	DEBUGP(DHO, "process meas res: ");
+
+	/* FIXME: implement actual averaging over multiple measurement
+	 * reports */
+
+	/* find the best cell in this report that is at least RXLEV_HYST
+	 * better than the current serving cell */
+	for (i = 0; i < mr->num_cell; i++) {
+		unsigned int better;
+		if (mr->cell[i].rxlev < mr->dl.full.rx_lev + RXLEV_HYST)
+			continue;
+
+		better = mr->cell[i].rxlev - mr->dl.full.rx_lev;
+		if (better > best_better_db) {
+			mr_cell = &mr->cell[i];
+			best_better_db = better;
+		}
+	}
+
+	if (mr_cell) {
+		DEBUGPC(DHO, "Cell on ARFCN %u is better, starting handover\n", mr_cell->arfcn);
+		return handover_to_arfcn_bsic(mr->lchan, mr_cell->arfcn,
+						mr_cell->bsic);
+	}
+
+	DEBUGPC(DHO, "No better cell\n");
+	return 0;
+}
+
+static int ho_dec_sig_cb(unsigned int subsys, unsigned int signal,
+			   void *handler_data, void *signal_data)
+{
+	struct gsm_meas_rep *mr;
+
+	if (subsys != SS_LCHAN)
+		return 0;
+
+	switch (signal) {
+	case S_LCHAN_MEAS_REP:
+		mr = signal_data;
+		process_meas_rep(mr);
+		break;
+	}
+
+	return 0;
+}
+
+void on_dso_load_ho_dec(void)
+{
+	register_signal_handler(SS_LCHAN, ho_dec_sig_cb, NULL);
+}