hodec2: add penalty-time low-rxqual-ho

When bad RxQual causes handover to a cell with weaker RxLev, then
handover oscillation *will* happen, as shown in test_rxqual.ho_vty.
Introduce a penalty timer for a cell where we had bad RxQual.

This delays handover back to the cell with stronger RxLev by the penalty
timeout; hopefully the interference is gone after the timeout.

Usually, we set new configuration elements so that osmo-bsc behaves the
same as before the config item was added. In this instance, this makes
no sense, because no-one ever wants handover oscillation from bad
RxQual, which is guaranteed to happen without the new penalty timer. Set
it to 60 seconds by default, same as other penalty timers.

Related: SYS#5911
Change-Id: I057b156604a104a26a7ce45d1c7adadbf452c932

4 files changed, 44 insertions, 1 deletions

diff --git a/include/osmocom/bsc/handover_cfg.h b/include/osmocom/bsc/handover_cfg.h
index 2bd26812e..7d68e6317 100644
--- a/include/osmocom/bsc/handover_cfg.h
+++ b/include/osmocom/bsc/handover_cfg.h
@@ -268,6 +268,13 @@ static inline int bool2i(bool arg)
 		"Time to suspend re-assignment after an lchan was re-assigned because of low RxQual\n" \
 		"Seconds\n") \
 	\
+	HO_CFG_ONE_MEMBER(int, hodec2_penalty_low_rxqual_ho, 60, \
+		"handover2 ", "penalty-time low-rxqual-ho", "<0-99999>", atoi, "%d", as_is, \
+		HO_CFG_STR_HANDOVER2 \
+		HO_CFG_STR_PENALTY_TIME \
+		"Time to suspend handover back to a cell after bad RxQual caused handover away from it\n" \
+		"Seconds\n") \
+	\
 	HO_CFG_ONE_MEMBER(int, hodec2_retries, 0, \
 		"handover2 ", "retries", "<0-9>", atoi, "%d", as_is, \
 		HO_CFG_STR_HANDOVER2 \
diff --git a/src/osmo-bsc/handover_decision_2.c b/src/osmo-bsc/handover_decision_2.c
index 14bc2d4b0..8627910e8 100644
--- a/src/osmo-bsc/handover_decision_2.c
+++ b/src/osmo-bsc/handover_decision_2.c
@@ -914,6 +914,19 @@ static int trigger_local_ho_or_as(struct ho_candidate *c, uint8_t requirements)
 				 full_rate ? "TCH/F" : "TCH/H",
 				 ho_reason_name(global_ho_reason));
 		handover_request(&req);
+
+		/* Apply penalty timer hodec2_penalty_low_rxqual_ho */
+		if (global_ho_reason == HO_REASON_INTERFERENCE
+		    || global_ho_reason == HO_REASON_BAD_QUALITY) {
+			struct gsm0808_cell_id bts_id;
+			struct gsm_subscriber_connection *conn = c->current.lchan->conn;
+			int timeout = ho_get_hodec2_penalty_low_rxqual_ho(c->current.bts->ho);
+			gsm_bts_cell_id(&bts_id, c->current.bts);
+			LOGPHOCAND(c, LOGL_DEBUG, "Applying penalty-time low-rxqual-ho %d s on bts %u (%s), reason: %s\n",
+				   timeout, c->current.bts->nr, gsm0808_cell_id_name_c(OTC_SELECT, &bts_id),
+				   ho_reason_name(global_ho_reason));
+			penalty_timers_add(conn, &conn->hodec2.penalty_timers, &bts_id, timeout);
+		}
 	}
 	return 0;
 }
diff --git a/tests/handover/test_rxqual.ho_vty b/tests/handover/test_rxqual.ho_vty
index f64de525d..6f86cf47f 100644
--- a/tests/handover/test_rxqual.ho_vty
+++ b/tests/handover/test_rxqual.ho_vty
@@ -10,6 +10,10 @@
 # See Performance Enhancements in a Frequency Hopping GSM Network (Nielsen Wigard 2002), Chapter
 # 2.1.1, "Interference" in the list of triggers on p.157.
 
+# first show undesired oscillation when penalty-time low-rxqual-ho is disabled
+network
+ handover2 penalty-time low-rxqual-ho 0
+
 create-n-bts 2
 set-ts-use trx 0 0 states * TCH/F - - - - - -
 meas-rep repeat 9 lchan 0 0 1 0 rxlev 40 rxqual 6 ta 0 neighbors 30
@@ -20,10 +24,26 @@ expect-ts-use trx 0 0 states * - - - - - - -
 expect-ts-use trx 1 0 states * TCH/F - - - - - -
 
 # Now the channel is on bts 1, which has lower rxlev than bts 0.
-# The result is an undesired ho oscillation
+# The result is an undesired ho oscillation, because the penalty timer is zero
 meas-rep lchan 1 0 1 0 rxlev 30 rxqual 0 ta 0 neighbors 40
 expect-ho from lchan 1 0 1 0 to lchan 0 0 1 0
+
+# Set a proper penalty timeout and report bad-rxqual again
+network
+ handover2 penalty-time low-rxqual-ho 10
 meas-rep repeat 10 lchan 0 0 1 0 rxlev 40 rxqual 6 ta 0 neighbors 30
 expect-ho from lchan 0 0 1 0 to lchan 1 0 1 0
+
+# This time the penalty timer prevents oscillation
+meas-rep repeat 10 lchan 1 0 1 0 rxlev 30 rxqual 0 ta 0 neighbors 40
+expect-no-chan
+
+# After the penalty timeout passes, we do go back to the cell with stronger rxlev
+wait 10
 meas-rep lchan 1 0 1 0 rxlev 30 rxqual 0 ta 0 neighbors 40
 expect-ho from lchan 1 0 1 0 to lchan 0 0 1 0
+# If the rxqual is still bad here after the penalty timeout, well, then we quickly snap back to the weaker cell, once
+meas-rep repeat 10 lchan 0 0 1 0 rxlev 40 rxqual 6 ta 0 neighbors 30
+expect-ho from lchan 0 0 1 0 to lchan 1 0 1 0
+meas-rep repeat 10 lchan 1 0 1 0 rxlev 30 rxqual 0 ta 0 neighbors 40
+expect-no-chan
diff --git a/tests/handover_cfg.vty b/tests/handover_cfg.vty
index eb3ec3ed7..fb3c73c49 100644
--- a/tests/handover_cfg.vty
+++ b/tests/handover_cfg.vty
@@ -190,6 +190,7 @@ OsmoBSC(config-net)# list
   handover2 penalty-time failed-ho (<0-99999>|default)
   handover2 penalty-time failed-assignment (<0-99999>|default)
   handover2 penalty-time low-rxqual-assignment (<0-99999>|default)
+  handover2 penalty-time low-rxqual-ho (<0-99999>|default)
   handover2 retries (<0-9>|default)
   handover2 congestion-check (disabled|<1-999>|now)
 ...
@@ -389,6 +390,7 @@ OsmoBSC(config-net)# handover2 penalty-time ?
   failed-ho              Time to suspend handover for a subscriber after a failed handover into this cell; see also 'handover2 retries'
   failed-assignment      Time to suspend handover for a subscriber after a failed re-assignment within this cell; see also 'handover2 retries'
   low-rxqual-assignment  Time to suspend re-assignment after an lchan was re-assigned because of low RxQual
+  low-rxqual-ho          Time to suspend handover back to a cell after bad RxQual caused handover away from it
 
 OsmoBSC(config-net)# handover2 penalty-time max-distance ?
   <0-99999>  Seconds
@@ -613,6 +615,7 @@ OsmoBSC(config-net-bts)# handover2 penalty-time ?
   failed-ho              Time to suspend handover for a subscriber after a failed handover into this cell; see also 'handover2 retries'
   failed-assignment      Time to suspend handover for a subscriber after a failed re-assignment within this cell; see also 'handover2 retries'
   low-rxqual-assignment  Time to suspend re-assignment after an lchan was re-assigned because of low RxQual
+  low-rxqual-ho          Time to suspend handover back to a cell after bad RxQual caused handover away from it
 
 OsmoBSC(config-net-bts)# handover2 penalty-time max-distance ?
   <0-99999>  Seconds