osmo-bts-trx: implement BCCH carrier power reduction mode

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, in the recent 3GPP TS 45.008 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.

The power reduction operation can be controlled by the BSC by
sending BS POWER CONTROL on the A-bis/RSL with the Channel Number
IE set to 0x80 (RSL_CHAN_BCCH).  This makes osmo-bts reduce the
transmission power on inactive timeslots of the BCCH carrier.

This is a non-standard, Osmocom specific extension, so indicate
support of this feature to the BSC in the feature vector.  Also
add a VTY command to allow enabling/disabling the power reduction
locally.  Add some signalling notes to the A-bis/RSL manual.

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

Change-Id: I3dcee6e910ccc61c5c63c728db9ea04327e2fc98
Depends: I69283b3f35988fc7a1a1dcf1a1ad3b67f08ec716
Related: SYS#4919

1 files changed, 20 insertions, 0 deletions

diff --git a/doc/manuals/abis/rsl.adoc b/doc/manuals/abis/rsl.adoc
index 1205a447..b4c2609f 100644
--- a/doc/manuals/abis/rsl.adoc
+++ b/doc/manuals/abis/rsl.adoc
@@ -476,6 +476,26 @@ signaling for it.
 See <<OSMO_ETWS_CMD>> for the Osmocom implementation.
 
 
+=== 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 discontinuous 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
+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.
+
+Unfortunately, 3GPP did not specify in which way the BTS is instructed to activate
+and deactivate the BCCH carrier power reduction mode.  Osmocom had to invent their
+own non-standard approach: the BSC needs to send _BS POWER CONTROL_ message with
+the _Channel Number_ IE set to 0x80 (BCCH) and the _Message Discriminator_ set to
+0x06 (Common Channel Management messages).
+
+
 === Message Formats and Contents
 
 [[rsl_crcx_msg]]