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author | Philipp Maier <pmaier@sysmocom.de> | 2021-02-11 17:18:12 +0100 |
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committer | Philipp Maier <pmaier@sysmocom.de> | 2021-02-11 23:16:50 +0100 |
commit | ff5dcf7fd3b955cbf5670f55082d4650f2b5578f (patch) | |
tree | e5bb2d69fd1fd57dcec803381dc8a040aba11ca1 | |
parent | a2f21b3bc4e152eca428498011a57b6f0f1e3b9c (diff) |
bts.adoc: describe ACCH repetition settings
The manual currently does not mention ACCH repetition yet. Lets
add some info on how to set up ACCH repetition correctly.
Change-Id: I1e27ac955882497bbeefac0c830708dd18ad46b3
Related: SYS#5114
-rw-r--r-- | doc/manuals/chapters/bts.adoc | 69 |
1 files changed, 69 insertions, 0 deletions
diff --git a/doc/manuals/chapters/bts.adoc b/doc/manuals/chapters/bts.adoc index 6e7a308c7..e4167757b 100644 --- a/doc/manuals/chapters/bts.adoc +++ b/doc/manuals/chapters/bts.adoc @@ -511,6 +511,75 @@ bts 0 <5> The rotate subset size parameter will be increased one ACC slot at a time: len=0 -> len=1 -> len=2 -> len=3 <6> The time until the subset size is further increased will be calculated based on current channel load +=== Configuring FACCH/SACCH repetition + +osmo-bts supports repetition of FACCH, uplink SACCH and downlink SACCH as +described in _3GPP TS 44.006_ <<3gpp-ts-44.006>>. When the feature is enabled +it is applied dynamically, depending on the rf signal quality and MS +capabilities. FACCH/SACCH repetition (or ACCH repetition) repeats the channel +block transmission two times. This allows the transceiver to combine the symbols +from two separate transmissions, which increases the probability that even a +weak signal can be decoded. + +Enabling ACCH repetition is especially recommended when using the AMR speech +codec. AMR already provides a forward error correction that is superior to +the forward error correction used with FACCH or SACCH. ACCH repetition is a +good way to even out this imbalance. + +The VTY configuration allows to enable repetition for all three channel types +separately. For FACCH the operator has the option to restrict the repetition +to LAPDM command frames only. Alternatively it is also possible to allow all +LAPDM frame types for repetition. The following example shows a typical +configuration where ACCH repetition is fully enabled. + +.Example typical configuration of ACCH repetition parameters at VTY BTS node +---- +OsmoBSC(config-net-bts)# repeat dl-facch all +OsmoBSC(config-net-bts)# repeat ul-sacch +OsmoBSC(config-net-bts)# repeat dl-sacch +OsmoBSC(config-net-bts)# repeat rxqual 4 +---- + +It should be noted that unless the repetition is enabled explicitly, the +repetition is turned off by default. If no threshold (see <<acch_rep_thr>>) is +set, the default value 4 (BER >= 1.6%) will be used. The following example shows +a minimal configuration where the repetition is only activated for FACCH LAPDM +command frames. + +.Example minimal configuration of ACCH repetition parameters at VTY BTS node +---- +OsmoBSC(config-net-bts)# repeat dl-facch command +---- + +Since it is not worthwhile to apply any repetition when the signal conditions +are good enough to ensure a reliable transmission in one round, the operator +has the option to set a threshold based on RXQUAL/BER at which the repetition +is switched on. The threshold mechanism implements a hysteresis to prevent +bouncing between repetition on and repetition off. Only when the signal quality +is increased again by two rxqual levels, the repetition is turned off again. It +is even possible to permanently enable repetition, regardless of the signal +quality. + +[[acch_rep_thr]] +.ACCH repetition thresholds +[options="header",cols="20%,40%,40%"] +|=== +|rxqual |enable threshold |disable threshold +|0 |(repetition always on) |(repetition always on) +|1 |asciimath:[BER >= 0.2%] |asciimath:[BER = 0%] +|2 |asciimath:[BER >= 0.4%] |asciimath:[BER = 0%] +|3 |asciimath:[BER >= 0.8%] |asciimath:[BER <= 0.2%] +|4 |asciimath:[BER >= 1.6%] |asciimath:[BER <= 0.4%] +|5 |asciimath:[BER >= 3.2%] |asciimath:[BER <= 0.8%] +|6 |asciimath:[BER >= 6.4%] |asciimath:[BER <= 1.6%] +|7 |asciimath:[BER >= 12.8%] |asciimath:[BER <= 3.2%] +|=== + +NOTE: osmo-bsc only sets the ACCH repetition parameters via RSL. Whether ACCH +repetition can be used depends on the BTS model and osmo-bts version. To +find out if a BTS supports ACCH repetition (BTS_FEAT_ACCH_REP), the VTY +command `show bts` can be used. + ==== RACH Parameter Configuration The following parameters allow control over how the MS can access the random |