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Power Control Loop: Set P_CON_INTERVAL to 1 by default 

TS 45.008 section 4.7.1:
"""
Upon receipt of a command from an SACCH to change its power level on the corresponding uplink channel, the MS
shall change to the new level at a rate of one nominal 2 dB power control step every 60 ms (13 TDMA frames), i.e. a
range change of 15 steps should take about 900 ms. The change shall commence at the first TDMA frame belonging to
the next reporting period (as specified in subclause 8.4). The MS shall change the power one nominal 2 dB step at a
time, at a rate of one step every 60 ms following the initial change, irrespective of whether actual transmission takes
place or not.
"""

Since the reported MS_PWR in L1 SACCH Header is, according to specs, the
one used for the last block of the previous SACCH period, it becomes
clear the first SACCH block after a requested MS Power Level change by
the network may contain mismatches between the announced MS_PWR by the
MS and the measured Rxlev/RxQual. Hence, let's better use a
P_CON_INTERVAL of 1 which retriggers the MS Power Control Loop every second
SACCH block.

Related: SYS#5371
Change-Id: Iade5b597e0e56b07c6d78995fcec7c641e4e643f
This commit is contained in:
Pau Espin 2021-09-13 17:35:31 +02:00
parent 506aa70d77
commit 8367411772
4 changed files with 47 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
doc/manuals/chapters/power_control.adoc
View File

@ -150,20 +150,46 @@ Having requested a transmit power level, the MS/BS power control loop may option
be suspended for a certain number of SACCH multiframes defined by VTY parameter
`ctrl-interval`. Given that SACCH is relatively slow and transmission of a data block
takes 480 ms, suspension allows an observation of the effect of one power control
decision before initiating the next one.
decision before initiating the next one. This is mostly important due to the
fact that MS must change the transmit power at nominal steps of 2dB every 60ms
(TS 45.008 sec 4.7.1). As a result, if the network requests the MS to change its
transmit power by several MS Power Levels at a time, the MS will do so gradually
over the next measurement period. Hence, upon next received L1 SACCH block, the
_MS_PWR_ value announced by the MS will match only the one used to transmit the
last block, and so the related measured RxLevel/RxQual values will be
inaccurate. By skipping one or several SACCH blocks, the algorithm will always
use values which match correctly the announced _MS_PWR_ and the measured
RxLevel/RxQual (because the _MS_PWR_ will already have changed and hence will be
kept stable over that measurement period).
----
OsmoBSC(config-bs-power-ctrl)# ctrl-interval ?
<0-31> P_CON_INTERVAL, in units of 2 SACCH periods (0.96 seconds)
----
By default, the suspension interval is set to 0 for both MS/BS power control loops,
therefore the power control decision is taken every 480 ms (one SACCH block period).
Setting `ctrl-interval` to 1 increases the interval to 960 ms, so basically every
second Uplink SACCH block is skipped; value 2 corresponds to the interval of
1920 ms, so 3/4 received SACCH blocks are skipped.
The value to use for this parameter is closely related to that of VTY option
`step-size inc <2-6> red <2-4>`, which configures the maximum step (in dB) at
which the MS Power can be requested to changed when the MS Power Control Loop is
triggered. The higher the `step-size`, the more time it will need the MS to do
the necessary ramping of 2 dB steps, and hence the amount of time required for
the MS to settle on the requested MS Power Level for an entire SACCH block. Or
equally, the time the network can start using the full measurement period to
trigger the MS Power Control Loop again with reliable measurements
(`P_CON_INTERVAL`).
3GPP TS 45.008 briefly mentions this parameter in table A.1 (P_Con_INTERVAL).
By default, increment `step-size` is set to 4 dB and the decrement `step-size`
is set to 2 dB, hence the MS requiring `4 * 60 = 240` milliseconds. That's less
than 1 measurement period (480 ms), hence only the first measurement period
needs to be skipped. Therefore, the suspension interval is set to 1 for both
MS/BS power control loops, and so the power control decision is taken every 960
ms (every second SACCH block period).
Setting `ctrl-interval` to 0 increases the interval to 480 ms, so basically no
SACCH block is skipped and MS Power Control loop is triggered upon receival of
every UL SACCH block. Value 2 corresponds to the interval of 1920 ms, so 3/4
received SACCH blocks are skipped.
3GPP TS 45.008 briefly mentions this parameter in table A.1 (`P_Con_INTERVAL`).
A small time graph is depicted below for better understanding of the meaning of
values for this parameter, since it is not obvious at all.
@ -173,8 +199,8 @@ values for this parameter, since it is not obvious at all.
|<-->| - one SACCH multi-frame period
| |
|----|----|----|----|----|----|----|----|----> SACCH multi-frames
a) * * * * * * * * * P_CON_INTERVAL=0 (0.48 s, default)
b) * * * * * P_CON_INTERVAL=1 (0.96 s)
a) * * * * * * * * * P_CON_INTERVAL=0 (0.48 s)
b) * * * * * P_CON_INTERVAL=1 (0.96 s, default)
c) * * * P_CON_INTERVAL=2 (1.92 s)
d) * * P_CON_INTERVAL=3 (2.88 s)
e) * * P_CON_INTERVAL=4 (3.84 s)

5
src/osmo-bsc/bts_vty.c
View File

@ -2978,7 +2978,7 @@ DEFUN_USRATTR(cfg_power_ctrl_ctrl_interval,
X(BSC_VTY_ATTR_NEW_LCHAN),
"ctrl-interval <0-31>",
"Set power control interval (for dynamic mode)\n"
"P_CON_INTERVAL, in units of 2 SACCH periods (0.96 seconds)\n")
"P_CON_INTERVAL, in units of 2 SACCH periods (0.96 seconds)(default=1)\n")
{
struct gsm_power_ctrl_params *params = vty->index;
@ -3928,8 +3928,7 @@ static void config_write_power_ctrl(struct vty *vty, unsigned int indent,
if (cp->dir == GSM_PWR_CTRL_DIR_DL)
cfg_out(" bs-power dyn-max %u%s", cp->bs_power_max_db, VTY_NEWLINE);
if (cp->ctrl_interval > 0)
cfg_out(" ctrl-interval %u%s", cp->ctrl_interval, VTY_NEWLINE);
cfg_out(" ctrl-interval %u%s", cp->ctrl_interval, VTY_NEWLINE);
cfg_out(" step-size inc %u red %u%s",
cp->inc_step_size_db, cp->red_step_size_db,
VTY_NEWLINE);

2
src/osmo-bsc/gsm_data.c
View File

@ -1142,6 +1142,8 @@ const struct gsm_power_ctrl_params power_ctrl_params_def = {
.inc_step_size_db = 4, /* 2, 4, or 6 dB */
.red_step_size_db = 2, /* 2 or 4 dB */
.ctrl_interval = 1, /* Trigger loop every second SACCH block. TS 45.008 sec 4.7.1 */
/* RxLev measurement parameters */
.rxlev_meas = {
/* Thresholds for RxLev (see 3GPP TS 45.008, A.3.2.1) */

10
tests/power_ctrl.vty
View File

@ -7,6 +7,7 @@ OsmoBSC# show running-config
mode static
ms-power-control
mode dyn-bts
ctrl-interval 1
step-size inc 4 red 2
rxlev-thresh lower 32 upper 38
rxlev-thresh-comp lower 10 12 upper 19 20
@ -70,6 +71,7 @@ OsmoBSC(config-bs-power-ctrl)# show running-config
bs-power-control
mode dyn-bts
bs-power dyn-max 12
ctrl-interval 1
step-size inc 4 red 2
rxlev-thresh lower 32 upper 38
rxlev-thresh-comp lower 10 12 upper 19 20
@ -96,9 +98,11 @@ OsmoBSC(config-bs-power-ctrl)# show running-config
bs-power-control
...
ctrl-interval 31
... !ctrl-interval
...
ms-power-control
... !ctrl-interval
...
ctrl-interval 1
...
OsmoBSC(config-bs-power-ctrl)# exit
OsmoBSC(config-net-bts)# ms-power-control
@ -131,6 +135,7 @@ OsmoBSC(config-ms-power-ctrl)# show running-config
...
ms-power-control
mode dyn-bts
ctrl-interval 1
step-size inc 4 red 2
rxlev-thresh lower 32 upper 38
rxlev-thresh-comp lower 10 12 upper 19 20
@ -283,6 +288,7 @@ OsmoBSC(config-net-bts)# show running-config
mode static
ms-power-control
mode dyn-bts
ctrl-interval 1
step-size inc 4 red 2
rxlev-thresh lower 32 upper 38
rxlev-thresh-comp lower 10 12 upper 19 20