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, 81 insertions, 0 deletions

diff --git a/doc/manuals/chapters/power_control.adoc b/doc/manuals/chapters/power_control.adoc
index 4c1a275b8..bb87c850c 100644
--- a/doc/manuals/chapters/power_control.adoc
+++ b/doc/manuals/chapters/power_control.adoc
@@ -327,3 +327,84 @@ network
 // TODO: Document other power control parameters:
 //		OsmoBSC(config-net-bts)# ms max power <0-40>
 //		OsmoBSC(config-net-bts-trx)# max_power_red <0-100>
+
+=== BCCH carrier power reduction operation
+
+According to 3GPP TS 45.008, section 7.1, the BCCH carrier (sometimes called C0) of
+a BTS shall maintain continuous Downlink transmission at full power in order to
+stay "visible" to the mobile stations.  Because of that, early versions of this 3GPP
+document prohibited BS power reduction on C0.  However, a new feature was introduced
+in version 13.0.0 (2015-11) - "BCCH carrier power reduction operation".
+
+This is a special mode of operation, in which the variation of RF power level for
+some timeslots is relaxed for the purpose of energy saving.  In other words, the
+output power on some timeslots, except the timeslot(s) carrying BCCH/CCCH, can be
+lower than the full power.  In this case the maximum allowed difference is 6 dB.
+
+Of course, energy saving comes at a price and has impacts to the network KPI.  In
+particular, it does negatively affect cell reselection performance and does increase
+handover failure and call drop rates.  This is why BCCH carrier power reduction
+operation mode is not enabled by default.  More information on potential impact
+and the simulation results can be found in 3GPP TR 45.926.
+
+==== Supported BTS models
+
+At the time of writing this manual, the only BTS model that can be instructed to
+enter or leave the BCCH power reduction mode is osmo-bts-trx.  Support for other
+BTS vendors/models may be added in the future.
+
+TIP: If you're using OsmoBTS, make sure that it reports feature #021 "BCCH carrier
+power reduction mode" in the feature vector.  This can be checked by issuing
+`show bts` command in OsmoBSC's VTY interface.
+
+==== Managing BCCH carrier power reduction
+
+The BCCH carrier power reduction can be controlled via the CTRL and VTY interfaces.
+There is currently no logic in OsmoBSC for automatic activation and deactivation
+of this mode, so it's up to the network operator (or an external monitoring suite)
+when and depending on which factors to toggle it.  Setting a value greater than
+zero enables the BCCH power reduction mode;  setting zero disables it completely.
+
+.Example: Activating BCCH carrier power reduction via the VTY
+----
+OsmoBSC> enable
+OsmoBSC# bts 0 <1> c0-power-reduction ?
+  <0-6>  Power reduction value (in dB, even numbers only)
+OsmoBSC# bts 0 <1> c0-power-reduction 4 <2>
+----
+<1> BTS number for which to activate BCCH carrier power reduction
+<2> Maximum BCCH carrier power reduction (in 2 dB steps, 4 dB in this example)
+
+.Example: Activating BCCH carrier power reduction via the CTRL
+----
+$ osmo_ctrl.py \
+	--host 127.0.0.1 <1> -p 4249 \
+	--set "bts.0.c0-power-reduction" 4 <2>
+----
+<1> Remote address of the host running osmo-bsc (localhost in this example)
+<2> Maximum BCCH carrier power reduction (in 2 dB steps, 4 dB in this example)
+
+Once activated, it's possible to introspect the current maximum reduction value:
+
+.Example: Checking BCCH carrier power reduction state via the VTY
+----
+OsmoBSC> enable
+OsmoBSC# show bts 0 <1>
+BTS 0 is of osmo-bts type in band DCS1800, has CI 0 LAC 1, BSIC 63 (NCC=7, BCC=7) and 2 TRX
+  Description: (null)
+  ARFCNs: 751 753
+  BCCH carrier power reduction (maximum): 4 dB <2>
+  ...
+----
+<1> BTS number for which to show BCCH carrier power reduction state
+<2> Maximum BCCH carrier power reduction currently applied
+
+.Example: Checking BCCH carrier power reduction state via the CTRL
+----
+$ osmo_ctrl.py \
+	--host 127.0.0.1 <1> -p 4249 \
+	--get "bts.0.c0-power-reduction"
+Got message: b'GET_REPLY 3652121201381481804 bts.0.c0-power-reduction 4 <2>'
+----
+<1> Remote address of the host running osmo-bsc (localhost in this example)
+<2> Maximum BCCH carrier power reduction currently applied