power_control: implement BCCH carrier power reduction operation

The BCCH carrier (sometimes called C0) of a BTS shall maintain
discontinuous Downlink transmission at full power in order to
stay 'visible' to the mobile stations.  Because of that, early
versions of 3GPP TS 45.008 prohibited BS power reduction on C0.

However, starting from version 13.0.0 (2015-11) there is a feature
called 'BCCH carrier power reduction operation'.  This is a special
mode of operation, where the variation of RF level for some
timeslots is relaxed for the purpose of energy saving.

In BCCH carrier power reduction operation, for timeslots on the
C0 carrier, except timeslots carrying BCCH/CCCH, the output power
may be lower than the output power used for timeslots carrying
BCCH/CCCH.  In this case the maximum allowed difference in output
power actually transmitted by the BTS is 6 dB.

Introduce a VTY command to turn on and off the BCCH carrier power
reduction operation.  Also introduce a CTRL command.  On the
A-bis/RSL, abuse the BS POWER CONTROL message by setting
the Channel Number IE to 0x80 (RSL_CHAN_BCCH).

Currently, only osmo-bts-trx is supported.  A value greater than
zero makes it reduce the power on *inactive* timeslots of the
BCCH carrier.  Sending zero disables the BCCH power reduction
mode completely.

For more details, see 3GPP TS 45.008, section 7.1, and 3GPP TR 45.926.

Change-Id: I047fce33d4d3e4c569dd006ba17858467a2f4783
Related: SYS#4919

1 files changed, 33 insertions, 0 deletions

diff --git a/src/osmo-bsc/bts_osmobts.c b/src/osmo-bsc/bts_osmobts.c
index 1814ada3d..9813a265d 100644
--- a/src/osmo-bsc/bts_osmobts.c
+++ b/src/osmo-bsc/bts_osmobts.c
@@ -1,6 +1,7 @@
 /* Osmocom OsmoBTS specific code */
 
 /* (C) 2010-2012 by Harald Welte <laforge@gnumonks.org>
+ * (C) 2021 by sysmocom - s.m.f.c. GmbH <info@sysmocom.de>
  *
  * All Rights Reserved
  *
@@ -43,6 +44,35 @@ extern struct gsm_bts_model bts_model_nanobts;
 
 static struct gsm_bts_model model_osmobts;
 
+static int power_ctrl_set_c0_power_red(const struct gsm_bts *bts,
+				       const uint8_t red)
+{
+	struct abis_rsl_dchan_hdr *dh;
+	struct msgb *msg;
+
+	msg = rsl_msgb_alloc();
+	if (msg == NULL)
+		return -ENOMEM;
+
+	LOGP(DRSL, LOGL_NOTICE, "%sabling BCCH carrier power reduction "
+	     "operation mode for BTS%u (maximum %u dB)\n",
+	     red ? "En" : "Dis", bts->nr, red);
+
+	/* Abuse the standard BS POWER CONTROL message by specifying 'Common Channel'
+	 * in the Protocol Discriminator field and 'BCCH' in the Channel Number IE. */
+	dh = (struct abis_rsl_dchan_hdr *) msgb_put(msg, sizeof(*dh));
+	dh->c.msg_discr = ABIS_RSL_MDISC_COM_CHAN;
+	dh->c.msg_type = RSL_MT_BS_POWER_CONTROL;
+	dh->ie_chan = RSL_IE_CHAN_NR;
+	dh->chan_nr = RSL_CHAN_BCCH;
+
+	msgb_tv_put(msg, RSL_IE_BS_POWER, red / 2);
+
+	msg->dst = bts->c0->rsl_link_primary;
+
+	return abis_rsl_sendmsg(msg);
+}
+
 int bts_model_osmobts_init(void)
 {
 	model_osmobts = bts_model_nanobts;
@@ -52,6 +82,9 @@ int bts_model_osmobts_init(void)
 	/* Unlike nanoBTS, osmo-bts does support SI2bis and SI2ter fine */
 	model_osmobts.force_combined_si = false;
 
+	/* Power control API */
+	model_osmobts.power_ctrl_set_c0_power_red = &power_ctrl_set_c0_power_red;
+
 	model_osmobts.features.data = &model_osmobts._features_data[0];
 	model_osmobts.features.data_len =
 				sizeof(model_osmobts._features_data);