power_control.c: Limit speed of announced MS Power Level value changes

It's not a good idea to request big changes in MS Power based on
sporadic bad signal received, let's instead change announced MS power
levels more smoothly to avoid possible big signal strength fluctations,
similar to what is already done in osmo-bts-trx specific loop (loops.c).

Related: OS#1851
Change-Id: Iecc4ec7e21471ec853ad2d5659af4052aba5444c

3 files changed, 83 insertions, 9 deletions

diff --git a/src/common/power_control.c b/src/common/power_control.c
index d08ed5e5..d2a6c1dd 100644
--- a/src/common/power_control.c
+++ b/src/common/power_control.c
@@ -30,6 +30,11 @@
 #include <osmo-bts/measurement.h>
 #include <osmo-bts/bts_model.h>
 #include <osmo-bts/l1sap.h>
+#include <osmo-bts/power_control.h>
+
+/* how many dB do we raise/lower as maximum (1 ms power level = 2 dB) */
+#define MS_RAISE_MAX_DB 4
+#define MS_LOWER_MAX_DB 8
 
  /*! compute the new MS POWER LEVEL communicated to the MS and store it in lchan.
   *  \param lchan logical channel for which to compute (and in which to store) new power value.
@@ -51,10 +56,6 @@ int lchan_ms_pwr_ctrl(struct gsm_lchan *lchan,
 	if (lchan->ms_power_ctrl.fixed)
 		return 0;
 
-	/* How many dBs measured power should be increased (+) or decreased (-)
-	   to reach expected power. */
-	diff = bts->ul_power_target - rxLevel;
-
 	current_dbm = ms_pwr_dbm(band, ms_power);
 	if (current_dbm < 0) {
 		LOGPLCHAN(lchan, DLOOP, LOGL_NOTICE,
@@ -70,6 +71,18 @@ int lchan_ms_pwr_ctrl(struct gsm_lchan *lchan,
 		return 0;
 	}
 
+	/* How many dBs measured power should be increased (+) or decreased (-)
+	   to reach expected power. */
+	diff = bts->ul_power_target - rxLevel;
+
+	/* don't ever change more than MS_{LOWER,RAISE}_MAX_DBM during one loop
+	   iteration, i.e. reduce the speed at which the MS transmit power can
+	   change. A higher value means a lower level (and vice versa) */
+	if (diff > MS_RAISE_MAX_DB)
+		diff = MS_RAISE_MAX_DB;
+	else if (diff < -MS_LOWER_MAX_DB)
+		diff = -MS_LOWER_MAX_DB;
+
 	new_dbm = current_dbm + diff;
 
 	/* Make sure new_dbm is never negative. ms_pwr_ctl_lvl() can later on
diff --git a/tests/power/power_test.c b/tests/power/power_test.c
index 30fbb643..9b1a762b 100644
--- a/tests/power/power_test.c
+++ b/tests/power/power_test.c
@@ -58,22 +58,64 @@ static void test_power_loop(void)
 	lchan->state = LCHAN_S_NONE;
 	lchan->ms_power_ctrl.current = ms_pwr_ctl_lvl(GSM_BAND_1800, 0);
 	OSMO_ASSERT(lchan->ms_power_ctrl.current == 15);
+	lchan->ms_power_ctrl.max = ms_pwr_ctl_lvl(GSM_BAND_1800, 26);
+	OSMO_ASSERT(lchan->ms_power_ctrl.max == 2);
 
 	/* Simply clamping */
 	apply_power_test(lchan, -60, 0, 15);
 
 	/*
 	 * Now 15 dB too little and we should power it up. Could be a
-	 * power level of 7 or 8 for 15 dBm
+	 * power level of 7 or 8 for 15 dBm. However, since we limit peace at
+	 * which we change values, expect several steps of MS_RAISE_MAX_DB/2 levels:
 	 */
+	apply_power_test(lchan, -90, 1, 13);
+	apply_power_test(lchan, -90, 1, 11);
+	apply_power_test(lchan, -90, 1, 9);
 	apply_power_test(lchan, -90, 1, 7);
-
-	/* It should be clamped to level 0 and 30 dBm */
-	apply_power_test(lchan, -100, 1, 0);
+	apply_power_test(lchan, -90, 1, 5);
+
+	/* Check good RSSI value keeps it at same power level: */
+	apply_power_test(lchan, bts.ul_power_target, 0, 5);
+
+	apply_power_test(lchan, -90, 1, 3);
+	apply_power_test(lchan, -90, 1, 2); /* .max is pwr lvl 2 */
+	apply_power_test(lchan, -90, 0, 2); /* .max is pwr lvl 2 */
+
+	lchan->ms_power_ctrl.max = ms_pwr_ctl_lvl(GSM_BAND_1800, 30);
+	OSMO_ASSERT(lchan->ms_power_ctrl.max == 0);
+	apply_power_test(lchan, -90, 1, 0); /* .max is pwr lvl 0 */
+	apply_power_test(lchan, -90, 0, 0); /* .max is pwr lvl 0 */
+
+	lchan->ms_power_ctrl.max = ms_pwr_ctl_lvl(GSM_BAND_1800, 36);
+	OSMO_ASSERT(lchan->ms_power_ctrl.max == 29);
+	apply_power_test(lchan, -90, 1, 30);
+	apply_power_test(lchan, -90, 1, 29);
+	apply_power_test(lchan, -90, 0, 29);
+
+	/* Check good RSSI value keeps it at same power level: */
+	apply_power_test(lchan, bts.ul_power_target, 0, 29);
+
+	/* Now go down, steps are double size in this direction: */
+	apply_power_test(lchan, -45, 1, 1);
+	apply_power_test(lchan, -45, 1, 5);
+	apply_power_test(lchan, -45, 1, 9);
+
+	/* Go down only one level down and up: */
+	apply_power_test(lchan, bts.ul_power_target + 2, 1, 10);
+	apply_power_test(lchan, bts.ul_power_target - 2, 1, 9);
+
+	/* Check if BSC requesting a low max power is applied after loop calculation: */
+	lchan->ms_power_ctrl.max = ms_pwr_ctl_lvl(GSM_BAND_1800, 2);
+	OSMO_ASSERT(lchan->ms_power_ctrl.max == 14);
+	apply_power_test(lchan, bts.ul_power_target + 2, 1, 14);
+	/* Set back a more normal max: */
+	lchan->ms_power_ctrl.max = ms_pwr_ctl_lvl(GSM_BAND_1800, 30);
+	OSMO_ASSERT(lchan->ms_power_ctrl.max == 0);
 
 	/* Fix it and jump down */
 	lchan->ms_power_ctrl.fixed = true;
-	apply_power_test(lchan, -60, 0, 0);
+	apply_power_test(lchan, -60, 0, 14);
 
 	/* And leave it again */
 	lchan->ms_power_ctrl.fixed = false;
diff --git a/tests/power/power_test.ok b/tests/power/power_test.ok
index cf0a38b4..335a3d3b 100644
--- a/tests/power/power_test.ok
+++ b/tests/power/power_test.ok
@@ -1,7 +1,26 @@
 Testing power loop...
 power control [0]: MS current power 15
+power control [1]: MS current power 13
+power control [1]: MS current power 11
+power control [1]: MS current power 9
 power control [1]: MS current power 7
+power control [1]: MS current power 5
+power control [0]: MS current power 5
+power control [1]: MS current power 3
+power control [1]: MS current power 2
+power control [0]: MS current power 2
 power control [1]: MS current power 0
 power control [0]: MS current power 0
+power control [1]: MS current power 30
+power control [1]: MS current power 29
+power control [0]: MS current power 29
+power control [0]: MS current power 29
+power control [1]: MS current power 1
+power control [1]: MS current power 5
+power control [1]: MS current power 9
+power control [1]: MS current power 10
+power control [1]: MS current power 9
+power control [1]: MS current power 14
+power control [0]: MS current power 14
 power control [1]: MS current power 15
 Power loop test OK