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, 7 insertions, 0 deletions

diff --git a/include/osmocom/bsc/bts.h b/include/osmocom/bsc/bts.h
index 49691e023..65c3be511 100644
--- a/include/osmocom/bsc/bts.h
+++ b/include/osmocom/bsc/bts.h
@@ -236,6 +236,8 @@ struct gsm_bts_model {
 	int (*power_ctrl_enc_rsl_params)(struct msgb *msg, const struct gsm_power_ctrl_params *cp);
 	/* (Optional) function for sending default MS/BS Power Control paramaters */
 	int (*power_ctrl_send_def_params)(const struct gsm_bts_trx *trx);
+	/* (Optional) function for toggling BCCH carrier power reduction operation */
+	int (*power_ctrl_set_c0_power_red)(const struct gsm_bts *bts, const uint8_t red);
 
 	void (*config_write_bts)(struct vty *vty, struct gsm_bts *bts);
 	void (*config_write_trx)(struct vty *vty, struct gsm_bts_trx *trx);
@@ -543,6 +545,9 @@ struct gsm_bts {
 	struct gsm_power_ctrl_params ms_power_ctrl;
 	struct gsm_power_ctrl_params bs_power_ctrl;
 
+	/* Maximum BCCH carrier power reduction */
+	uint8_t c0_max_power_red_db;
+
 	/* Interference Measurement Parameters */
 	struct gsm_interf_meas_params interf_meas_params;
 
@@ -691,6 +696,8 @@ int bts_count_free_ts(struct gsm_bts *bts, enum gsm_phys_chan_config pchan);
 
 int gsm_bts_set_system_infos(struct gsm_bts *bts);
 
+int gsm_bts_set_c0_power_red(struct gsm_bts *bts, const uint8_t red);
+
 int gsm_bts_model_register(struct gsm_bts_model *model);
 struct gsm_bts_model *bts_model_find(enum gsm_bts_type type);