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doc/manuals: add documentation for Temporary ACCH Overpower 

Change-Id: Ie995dac6c7083a2b0b4d582f71f55b3736381edb
Related: SYS#5319
This commit is contained in:
Vadim Yanitskiy 2021-11-12 22:44:12 +03:00
parent 9a310f818f
commit a507875ae7
1 changed files with 58 additions and 0 deletions
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doc/manuals/chapters/power_control.adoc
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@ -565,3 +565,61 @@ 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
=== Temporary ACCH overpower
Temporary overpower (TOP) is a power control technique that allows to improve
SACCH/FACCH performance in case of bad C/I. The key idea of TOP is to
increment the BS transmit power by 2..4 dB only for FACCH/SACCH bursts, while
keeping all voice bursts at the lower (normal) level as determined by the
downlink power control loop. This allows to reduce call drop rate and
increase capacity in deployments with tight frequency reuse.
NOTE: It's not possible to increase the current BS power beyond the maximum
transmit power level supported by the PHY. Thus if the BTS is already
transmitting at full power, the overpower logic cannot increase it even
further. This is also why TOP must be employed *together with BS power
control*, either static or dynamic.
The main area of use for TOP is traffic channels employing the AMR (Adaptive
Multi Rate) codec, which is more robust to interference than the associated
signalling channels. While AMR provides sufficient speech quality even at
very low C/I levels, the associated signalling channels may be suffering from
channel coding errors. This imbalance can be compensated by employing TOP,
which can be efficiently combined with the ACCH repetition technique.
This feature requires no support on the mobile station side and can be used
with UEs implementing the most recent 3GPP relese features, as well as legacy
UEs. However, it needs to be implemented in the BTS. Given that TOP itself
is not specified in 3GPP specifications, osmo-bsc uses Osmocom specific
A-bis/RSL IEs in order to activate it. Therefore, only the recent osmo-bts
versions may be instructed to activate this feature. Make sure that feature
#023 "FACCH/SACCH Temporary overpower" is present in the feature vector.
This can be checked by issuing `show bts` command in OsmoBSC's VTY interface.
TOP is disabled by default. Below is a configuration example enabling it:
----
network
bts 0
overpower dl-acch 2 <1>
overpower rxqual 4 <2>
----
<1> Overpower of maximum 2 dB for both SACCH and FACCH.
<2> Enable TOP only if RxQual is worse than 4 (BER >= 1.6%).
It's also possible to enable TOP only for FACCH or SACCH selectively, or
or keep it enabled permanently regardless of the reported RxQual:
----
OsmoBSC(config-net-bts)# overpower ?
dl-acch Enable overpower for both SACCH and FACCH
dl-sacch Enable overpower for SACCH only
dl-facch Enable overpower for FACCH only
OsmoBSC(config-net-bts)# overpower rxqual 0?
0 BER >= 0% (always on)
----
These parameters are indicated to the BTS during a logical channel activation
or modifications procedures, so they can be changed at run-time.