ERICSSON WCDMA RADIO ACCESS NETWORK
RETAINABILITY &
ACCESSIBILITY GUIDELINE
 Ericsson AB 2012
The contents of this product are subject to revision without notice due to continued progress in methodology,
design and manufacturing.
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Revision history
Rev
Date
Description
A
2012-06-28
First version
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Contents
1 Introduction ....................................................................... 4
1.1
1.2
1.3
1.4
Purpose and scope ...............................................................................4
Abbreviations ........................................................................................4
Concepts ...............................................................................................6
Conventions ..........................................................................................6
2 Priority order ...................................................................... 6
2.1 Stepwise approach................................................................................6
3 Improvements in general .................................................. 7
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Overview ...............................................................................................7
Increasing supervision timers – W12B .................................................8
Maximum downlink power for R99 RABs ...........................................12
Increased FACH1 power .....................................................................16
Improved RACH capacity ....................................................................17
UL SRB protection for speech only RABs ..........................................19
Minimum downlink power for R99 RABs ............................................20
4 Improvements for Multi-RAB .......................................... 21
4.1
4.2
4.3
4.4
Overview .............................................................................................21
Disable high-rate Multi-RABs ..............................................................21
Tuning of activation time .....................................................................22
Disable CQI repetition .........................................................................24
5 Improving radio environment ......................................... 25
5.1
5.2
5.3
5.4
5.5
Overview .............................................................................................25
Power offset ........................................................................................26
EUL optimization .................................................................................26
Initial SIR target ...................................................................................28
BLER target interactive .......................................................................29
6 References ....................................................................... 32
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1
Introduction
1.1
Purpose and scope
The descriptions and recommendations in this document are valid up to the
Ericsson WCDMA W12.1 RAN release.
The aim of the document is to give help with parameter tuning that improves
accessibility and retainability KPIs in the network. The suggested parameter
changes are based on live network experience from one or several live networks,
where improvements have been seen after parameter changes.
Although, previous trials show improvements, the proposed parameters should
always be evaluated in the network before deployment on a larger scale. It is
not certain that there will be improvements in all networks.
The network could be tuned in many different ways, for coverage, for capacity
etc. In this guideline it is assumed that the network traffic is dominated by
Smartphone users.
1.2
Abbreviations
A-DCH
Associated Dedicated Channel
ACK
Acknowledgement
CPICH
Common Pilot Channel
CS
Circuit Switched
CQI
Channel Quality Indicator
DCH
Dedicated Channel
DPDCH
Dedicated Physical Data Channel
DPCCH
Dedicated Physical Control Channel
EUL
Enhanced Uplink
FACH
Forward Access Channel
GGSN
Gateway GPRS Support Node
HSDPA
High Speed Downlink Packet Access
KPI
Key Performance Indicator
LA
Location Area
NACK
Not Acknowledge
PDU
Packet Data Units
PS
Packet Switched
R99
WCDMA Release 99 (all RABs except EUL and HSDPA)
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RAB
Radio Access Bearer
RA
Routing Area
RLC
Radio Link Control
RRC
Radio Resource Control
SDU
Service Data Unit
SF
Spreading Factor
SGSN
Serving GPRS Support Node
SIR
Signal to Interference Ratio
SRB
Signaling Access Bearer
TTI
Transmission Time Interval
UTRAN
Universal Terrestrial Radio Access Network
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1.3
Concepts
In this guideline the following concepts are used.
Dedicated mode: When the UE is on Cell_DCH, HS-DSCH or E-DCH.
HSDPA: When only HDSPA is used in the downlink channel.
HSPA: When HSDPA is used in the downlink and EUL is used in the uplink.
1.4
Conventions
In this document, parameters are presented in bold font type and counters in
courier font type, respectively.
dchRcLostT
Timer that is started when all radio links for
a connection are lost. At time-out, the radio
connection is considered lost.
pmNoRrcReqDeniedAdmDlPwr
Number of RRC Connection Requests denied
by admission control due to lack of DL
Power.
2
Priority order
2.1
Stepwise approach
It is suggested that a stepwise approach is used when optimizing accessibility and
retainability parameters, i.e. a parameter is changed and evaluated before moving
on to the next parameter/group of parameters. In some cases it is recommended
that a group of parameters is changed on the same time since there may be
dependencies between the parameters.
Table 1 indicates in which order the proposed changes should be done, starting
with the most important and then moving down in priority order.
The parameter changes are divided into three impact areas:
•
General improvements:
this group includes improvements that will have an impact on all RABs.
•
Multi-RAB improvements:
this group includes improvements on Multi-RAB performance.
•
Radio environment:
this group includes changes that improve radio environment, in one way
or another, for example by reducing interference in uplink or downlink.
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The improved radio environment has a positive impact on accessibility
and retainability.
Table 1
Proposed parameter changes, priority and improvement area.
Prio
Impact
Parameter change
Improvement
area
Section
Comments
1
General
Increasing supervision
timers – W12B
Retainability /
Accessibility
3.2
System software is
W12B release
1
General
Maximum downlink power
for R99 RABs
Retainability
3.3
2
General
Increased FACH1 power
Accessibility
3.4
2
Radio
Power offset
Retainability /
Accessibility
5.2
2
Radio
EUL optimization
Retainability /
Accessibility
5.3
2
Radio
Initial SIR target
Retainability /
Accessibility
5.4
2
Radio
BLER target interactive
Retainability /
Accessibility
5.5
3
General
Improved RACH capacity
Accessibility
3.5
3
Multi-RAB
Disable high-rate MultiRABs
Retainability
4.2
3
Multi-RAB
Tuning of activation time
Retainability
4.3
3
Multi-RAB
Disable CQI repetition
Retainability
4.4
3
General
UL SRB protection for
speech only RABs
Retainability
3.6
4
General
Minimum downlink power
for R99 RABs
Retainability
3.7
3
Improvements in general
3.1
Overview
This section describes retainability and accessibility improvements that have an
impact on both single- and Multi-RAB connections.
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3.2
Increasing supervision timers – W12B
3.2.1
Improvement area
Retainability and Accessibility
3.2.2
Overview
In W12B L1 and L3 timers are operator parameters. The only system constants
that remain are the L2 RLC parameters, which already have improved default
values in W12B, and therefore do not needed to be changed.
The parameters dchSynchReconfTime, dchNonSynchReconfTime and
rrcConnSetupTime are new in W12B. They replace system constants and hard
coded values.
These recommendations are valid when the feature Call Re-Establishment is OFF.
At the writing of this document recommendations on how to set parameters when
Call Re-Establishment is ON is not included. This will be added later when more
field trial experience has been collected from the feature.
3.2.3
Summary of changes
In Table 2 all proposed parameter changes are presented. More information about
the parameter changes can be found in the referenced sections.
Table 2
Proposed parameter changes W12B
Target
Improvement
Area
Parameter Name
Level
Default
Value
Modified
Value
Operator/
Ericsson
Retainability
T313
RNC
3s
7s
Operator
Retainability
dchRcLostT
RNC
5s
7s
Operator
Retainability
Accessibility
dchSynchReconfTime
RNC
01
10 s
Operator
Retainability
dchNonSynchReconfTi
me
RNC
01
10 s
Operator
Accessibility
rrcConnSetupTime
RNC
5s
9s
Operator
Section
1 The value 0 is used to select the same behavior as in pre-W12B, in which pre-W12B system constants and hard coded values are
used.
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3.2.4
UE/UTRAN Radio Link Failure
Background
Layer 1 timers for Radio Link Failure on UE and RNC side can be increased. That
way more time is allowed for the UE to be in poor radio before the radio
connection is improved again. This way the risk for dropped call is reduced.
The time for UE Radio Link Failure is calculated using parameter N313 and timer
T313. After receiving N313 (default: 100) consecutive "out of sync" indications
from layer 1, the UE starts timer T313. If T313 expires, the UE consider it as a
Radio Link Failure and goes to Idle. The total time for the UE to detect RL failure
is calculated as:
Total time UE = N313 * 10 ms + T313
UTRAN RL failure detection is controlled by the Radio Connection Supervision
(RCS) and Radio Link Set (RLS) Supervision functions. After receiving
nOutSyncInd (default: 10) consecutive frames, UTRAN starts timer rlFailureT
(default: 1s). If rlFailureT expires, the RLS function considers the connection as
out-of-sync and reports RL Failure to SRNC. When RL Failure is received on all
radio links in the connection, the SRNC starts timer dchRcLostT and when it
expires the connection is considered lost by RCS and the call is counted as
dropped. The total time for UTRAN to detect RL failure:
Total time RNC = nOutSyncInd * 10 ms + rlFailureT + dchRcLostT
Figure 1 shows how uplink and downlink RCS interacts (with default parameters).
When the UE looses sync in the downlink it will turn off its transmitter and only
monitor downlink until in sync again. At that stage it will be the UE only that can
recover the radio link. Since the UE is turning off its transmitter there will be no
in sync detected in the uplink. To recover the radio link the UE must get in sync
on the downlink, and then start to transmit on the uplink again.
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dchRcLostT
Uplink_RLS
(last RL)
System release
NOutsyncInd rlFailureT (1sec)
(10)
Out of sync Out of sync
5sec
No receiving in-sync indicator due to UE TX power off
starts
Uplink_RCS
Drop Call
Out of sync
UE goes to idle
Downlink_RCS
N313 (100)
On
T313 (3sec)
If ‘Out of synch’ detected over the last 160ms,
•‘Out of sync’ reporting is started (every frame)
•UE shall shut its transmitter off within 40 ms.
T313 timer expiry
(Dl Radio Link Failure)
UE Tx Pwr
Off
Figure 1 Interaction between uplink timer dchRLostT and downlink timer T313
(with default values).
It is important to align timers on uplink and downlink to detect RL. If for example
dchRcLostT is set to a short value it does not matter setting the timer T313 to a
long value since the call will be released when timer dchRcLostT expires. Also it
is important to set the timer dchRcLostT to longer value than T313, so that the
system does not release the call before the UE goes to idle (at expiry of T313)
Proposed changes
UE radio link failure
Change T313 from 3 to 7 (3 s  7 s)
Total time UE = N313 * 10ms + T313 = 100 * 0.010 + 7 = 8 s
RNC radio link failure
Change dchRcLostT from 50 to 70 (5 s  7 s)
Total time RNC = nOutSyncInd * 10ms + rlFailureT + dchRcLostT
= 10 * 0.010 + 1 + 7 = 8.1 s
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3.2.5
DCH to DCH synchronized configurations
Background
The timer dchSynchReconfTime is a L3 timer for supervision of all CELL_DCH
to CELL_DCH synchronized re-configurations including RAB Establishment,
RAB Release, Channel Switching and HS Serving Cell Change.
Proposed change
Change dchSynchReconfTime from 0 to 10 (use old pre-W12B timers with
default values  use timer dchSynchReconfTime with value 10 s)
3.2.6
DCH to DCH non-synchronized configurations
Background
The timer dchNonSynchReconfTime is a L3 timer for supervision of all
CELL_DCH to CELL_DCH non-synchronized re-configurations including Active
Set Update and RB Reconfiguration.
Proposed change
Change dchNonSynchReconfTime from 0 to 10 (use old pre-W12B timers
with default values  use timer dchNonSynchReconfTime with value 10 s)
3.2.7
RRC connection setup
Background
The timer rrcConnSetupTime is a L3 timer value for supervision for the RRC
Connection Setup procedure.
Proposed change
Change rrcConnSetupTime from 5 to 9 (5 s  9 s)
3.2.8
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS accessibility
•
CS & PS retainability
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•
UL RSSI
•
Mac-HS Throughput
Counter formulas can be found in [7] and [8].
3.3
Maximum downlink power for R99 RABs
3.3.1
Improvement area
Retainability
3.3.2
Background
DL power mapping curve
By using the parameters minimumRate, minPwrMax, interRate,
interPwrMax, maxRate and maxPwrMax the so called DL power mapping
curve is constructed. The parameters decide three points on the curve giving the
maximum transmission power (relative CPICH power) for corresponding rate [1].
Between these points the maximum transmission power is interpolated for rates
not given by the parameters minimumRate, interRate and maxRate, see Figure
2.
Relative radio link power
1590
7760
40690
maxPwrMax
48
interPwrMax
38
minPwrMax
0
minimumRate
interRate
Figure 2
maxRate
Maximum RL rate
Downlink power mapping curve with default values on parameters.
Improving speech retainability
It has been seen in various field trials that a very efficient way to improve Speech
retainability is to increase minPwrMax. This will increase maximum
transmission power for all radio links with a transmission rate up to
minimumRate (default 15.9 kbps). More power can be used if the radio link gets
poor and the call can be maintained longer, see Figure 3.
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Relative radio link power
1590
7760
40690
maxPwrMax
48
interPwrMax
38
minPwrMax
20
0
minimumRate
maxRate
interRate
Figure 3
dB).
Maximum RL rate
Downlink power mapping with increased minPwrMax (from 0 dB to 2
Improving speech retainability in smart phone intense cells
For cells with high HSDPA load (many HSDPA users) the A-DCH power can
constitute of a large part of the used downlink power. In these cases it may be
unwanted to further increase the A-DCH power by increasing minPwrMax, since
this will not only increase the possible power for Speech, but also for A-DCH
(radio link rate = 3.7 kbps) [1]. Instead it is suggested to move the minimumRate
point to 3.7 kbps and use the interRate point to increase maximum transmission
power for Speech. This way it is possible to use more power for Speech while ADCH is kept the same, see Figure 4.
Relative radio link power
370
40690
1590
maxPwrMax
48
interPwrMax
20
minPwrMax
0
ADCH power kept at 0 dB
minimumRate
interRate
maxRate
Maximum RL rate
Figure 4 The power control curve is changes by moving the point for
minimumRate and interRate. This way the maximum A-DCH power can be kept
the same.
If lower Speech AMR rates (than 12.2 kbps) are used it is possible to move the
inteRate point to a lower value than 15.9 kbps. The following Radio Link Rates
can then be used:
• 8450 kbps for Speech AMR 4.75 kbps
• 9600 kbps for Speech AMR 5.9 kbps
• 11650 kbps for Speech AMR 7.95 kbps
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Figure 5 shows an example where the power control curve has been adapted so
that the maximum power is increased for all AMR rates above 5.9 kbps (Radio
Link Rate = 9600 kbps).
Relative radio link power
40690
370 960
maxPwrMax
48
interPwrMax
20
interRate point moved from 1590 to 960
0
minPwrMax
minimumRate
interRate
maxRate
Maximum RL rate
Figure 5 The power control curve is adapted to also give retainability
improvements for Speech AMR 5.9 kbps (and all other higher Speech AMR rates.
As seen in Figure 4 the maximum transmission power for interactive R99 RABs is
also lowered by this change. This is good from a HSDPA performance view, since
this leads to that less non-HS power is consumed leaving more power for
HSDPA. To further improve HSDPA performance is possible to lower the
maximum transmission power for maxRate from 4.8 dB to 3 dB. This will lead to
even lower non-HS power consumption, see Figure 6. The drawback with this
solution is that DL coverage is reduced for R99 high rate bearers. However, this
may not be an issue in Smartphone dense networks with a high number of
HSDPA users. In these networks high R99 rates are not really wanted at all since
they are very expensive from a DL power point of view, it is much more efficient
to send data on HSDPA. For high capacity networks it is recommended to limit
the number of high rate R99, or even switched off them completely [3].
Relative radio link power
370
1590
40690
48
maxPwrMax
30
interPwrMax
R99 interactive power lowered for high rates
0
minPwrMax
minimumRate
interRate
20
maxRate
Maximum RL rate
Figure 6 The maximum power for R99 interactive can be further lowered to
improve HSDPA performance.
Another step in further improving the HSDPA performance is to lower the
possible transmission power for A-DCH. Since there can be a large number
HSDPA users in a cell, each one having one A-DCH consuming non-HS power, it
can lead to quite good improvements in available HSDPA power. Field trials have
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indicated that it is possible to lower maximum A-DCH power from 0 dB to 1.5 dB without any KPI degradation, see Figure 7.
Relative radio link power
350
40690
1590
48
maxPwrMax
30
interPwrMax
20
0
minPwrMax
ADCH power lowered 1.5dB
minimumRate
interRate
-15
maxRate
Maximum RL rate
Figure 7 A-DCH power consumption can be lowered by changing minPwrMax
from 0 dB to -1.5 dB.
3.3.3
Proposed change
The proposed changes are different depending on if the network/cells are HSDPA
capacity limited.
If the network/cell is not HSDPA capacity limited and it is important that high
rate R99 interactive bearers have the same coverage:
Change minPwrMax from 0 to 20 (0 dB  2 dB)
Trials indicate that it is possible to increase minPwrMax to 3 dB in case DL
power is not limiting.
If the network/cell is HSDPA capacity limited:
Do following changes:
a) minimumRate from 1590 to 370 (15.9 kbps  3.7 kbps)
b) minPwrMax from 0 to -15 (0 dB  -1.5 dB)
c) interRate from 7760 to 1590 (77.6 kbps  15.9 kbps)
d) interPwrMax from 38 to 20 (3.8 dB  2 dB)
e) maxPwrMax from 48 to 30 (4.8 dB  3 dB)
3.3.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
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•
CS & PS accessibility
•
CS & PS retainability
•
Mac-HS Throughput
Counter formulas can be found in [7] and [8].
3.4
Increased FACH1 power
3.4.1
Improvement area
Accessibility
3.4.2
Background
FACH1 is used to carry control information (Broadcast Control Channel (BCCH),
Common Control Channel (CCCH) or Dedicated Control Channel (DCCH)) to
the UEs in the cell [6].
It has been seen in field that both PS and CS accessibility can be improved by
increasing FACH1 power from default (1.8 dB relative CPICH). FACH1 power is
changed with the parameter maxFach1Power.
If the UE is in Idle Mode and trying to access the network, the UE sends the
RACH message: “RRC Connection Request” to the RNC to request for a
dedicated channel. The RNC checks available resources with admission control
and sends a “RRC Connection Setup” message on FACH (which carriers logical
control channel). This message gives information about the dedicated channel to
be setup, see Figure 8.
Figure 8
RRC Connection Request/Setup
By increasing the power of FACH1, the UEs will have higher probability to
receive RRC Connection Setup, as an answer to a RRC Connection Request,
especially in low coverage areas.
3.4.3
Proposed changes
Change maxFach1Power from 18 to 38 (1.8 dB  3.8 dB)
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It’s recommended to do this change in small steps, e.g. 1dB.
In some networks, maxFach1Power has been set to 5.3 dB relative CPICH power
with good accessibility results. However, if power is a limiting factor, e.g. high
capacity cells, the changes in maxFach1Power may result in negative
accessibility results. It’s recommended to change the parameter maxFach1Power
in smaller steps (e.g. 1 dB steps) and balance improved accessibility against
increased downlink power.
3.4.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS accessibility
•
CS & PS retainability
•
MAC-HS throughput
•
Non-HS power
•
Failures due to lack of DL power at RRC using
pmNoRrcReqDeniedAdmDlPwr and
pmNoFailedRabEstAttemptLackDlPwr
•
Overall failures at RRC, e.g. pmTotNoRrcConnectReqCs and
pmTotNoRrcConnectReqCsSucc.
•
Failures after admission measured with pmNoFailedAfterAdm.
It is expected that the number of RRC failures after admission is decreased
after increasing FACH1 power. With too low FACH1 power resources get
allocated, but FACH1 cannot complete the setup du to too low power. The
counter pmNoFailedAfterAdm is incremented when a function that has
been granted admission for a UE fails after being admitted due to a problem
in the RRC or RAB Setup procedure.
Counter formulas can be found in [7] and [8].
3.5
Improved RACH capacity
3.5.1
Improvement area
Accessibility
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3.5.2
Background
With the increase of new terminals, like Smartphones, the number of PS users will
increase in the network. Due to this, more capacity is needed in CELL_DCH and
CELL_FACH.
The parameter spreadingFactor changes the spreading factor and slot format on
the RACH channel. RACH capacity is improved by changing spreadingFactor
from default 64 to 32. This changes RACH slot format from 20 ms to 10 ms,
which reduces the risk for RACH collisions and therefore accessibility is
improved.
Value mapping:
•
SF32: TTI = 10 ms.
•
SF64: TTI = 20 ms.
Note that setting spreadingFactor = 32 may reduce RACH coverage. If this is
seen it is recommended to tune parameter powerOffsetPpm, which sets the
power level for the RACH message based on the received pre-amble power [4].
3.5.3
Proposed change
Change spreadingFactor for RACH from 64 to 32
3.5.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS accessibility
•
CS & PS retainability
•
Random access failure messages.
•
UL RSSI
•
MAC HS Cell throughput
•
Latency
Counter formulas can be found in [7] and [8].
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3.6
UL SRB protection for speech only RABs
3.6.1
Improvement area
Retainability
3.6.2
Background
Power control ensures that an agreed quality is kept on the connection in terms of
Block Error Rate (BLER). By adjusting BLER target it is possible to control
transmitted power on uplink and downlink.
The recommendation described here focus on UL BLER for speech only RABs,
and more specifically for UL SRB protection for speech only RABs. With the
proposed change, the UL BLER target is adjusted from 1% to 0.4% for SRB only
(UL BLER for speech transport channels will not be changed). The objective is to
get the UE to transmit more power on SRB and that way improves its protection.
The proposed change has shown some improvements in CS retainability in live
network trials. No noticeable change in UL RSSI has been seen in field since
more power will be transmitted on SRB only. No impact on the battery life is
expected.
The parameters are set per UeRc and Transport Channel (UeRcTrCh) and are
expressed as: 10*log(blerQualityTargetUl). For example setting the parameter
blerQualityTargetUl to -20 will result in a UL BLER target of 1% (-20 =
10*log(0.01)). Similarly setting blerQualityTargetUl to -24 will result in 0.4%
UL BLER
3.6.3
Proposed change
Change blerQualityTargetUl from -20 to -24 (BLER 1%  0.4%)
The proposed change can be done on following UeRc/UeRcTrCh:
3.6.4
•
UeRc = 2, UeRcTrCh = 1 (Speech 12.2 kbps)
•
UeRc = 33, UeRcTrCh = 1 (Speech 7.95 kbps)
•
UeRc = 34, UeRcTrCh = 1 (Speech 5.9 kbps)
•
UeRc = 35, UeRcTrCh = 1 (Speech 4.75 kbps)
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
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•
CS Call Success Rate
•
CS Retainability
•
UL RSSI
•
EUL throughput
•
Measured UL BLER
•
SIR and SIR Error
Counter formulas can be found in [7] and [8].
3.7
Minimum downlink power for R99 RABs
3.7.1
Improvement area
Retainability
3.7.2
Background
It is recommended to keep the minimum transmitted DL radio link power not
more than 25 dB from maximum transmission power [1]. If minimum DL
transmitted code power is set to a too low level, there is the risk (depending on the
mobile vendor) that UEs cause a sudden rush in DL power behavior. This can
result in that DL cell congestion is unnecessarily detected. The minimum
downlink power is decided by the parameter minPwrRl and is set relative
CPICH. Some live network tests have shown that this may have an impact on
retainability.
3.7.3
Proposed change
The default value for minPwrRl (-15 dB) assumes that CPICH is set to -10 dB
relative maximum downlink power (10% of total downlink power is used for
CPICH). If lower CPICH power is used, for example 5% of total downlink power
(-13 dB), it is recommended that the parameter minPwrRl is adjusted. The
following rule should be used when tuning minPwrRl:
minPwrRl = max DL Tx Power – primaryCpichPower – 25
Example:
The maximum DL TX power is 40 dBm.
a) If primaryCpichPower is set to 30 dBm (10% of total DL Tx power), then
minPwrRl should be set to: 40 – 30 – 25 = -15 dB
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b) If primaryCpichPower is set to 27 dBm (5% of total DL Tx power), then
minPwrRl should be set to: 40 – 27 – 25 = -12 dB
3.7.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate
•
CS & PS Retainability Rate
Counter formulas can be found in [8].
4
Improvements for Multi-RAB
4.1
Overview
This section describes retainability and accessibility improvements that have an
impact mainly on Multi-RAB connections.
4.2
Disable high-rate Multi-RABs
4.2.1
Improvement area
Retainability
4.2.2
Background
Field trials have shown that Multi-RAB drops constitute a large portion of the
total dropped Speech calls in a network. It has been seen that the drop call rate on
Multi-RAB can be several times larger than a single Speech RAB. The main
reasons for this are:
•
Muti-RABs with low SF on UL are less UL SRB protected, resulting in
higher call drop probability in poor radio conditions.
•
A high number of Multi-RAB states increase the probability of drop call
due to many radio bearer transitions.
The proposed change has shown good retainability improvement in field.
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4.2.3
Proposed change
Deactivate all Multi-RAB combinations with an uplink rate larger
than 64 kbps
4.2.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate
•
CS & PS Retainability Rate
•
MAC-HS throughput
Counter formulas can be found in [7] and [8].
4.3
Tuning of activation time
4.3.1
Improvement area
Retainability
4.3.2
Background
The activation time for a new radio link is decided by a Connection Frame
Number (CFN) and is calculated by the system. The CFN when the
reconfiguration should be done is calculated based on a number of factors, like
message length, RLC retransmissions etc. [2].
When switching between different RAB combinations and setting the activation
time (decided by CFN), it is possible to add an extra margin to the calculated
value by the parameter cfnOffsetMarginSrbDchDl when the SRB is on DCH
DL. The CFN offset (Δ CFN) is then calculated as:
Δ CFN = calculated value + cfnOffsetMarginSrbDchDl
If the CFN offset is not long enough the RBS will change configuration before the
UE and the UE will loose sync and there is a risk for dropped call. On the other
hand setting the CFN offset too long may also cause dropped calls.
Figure 9 shows how the parameter cfnOffsetMarginSrbDchDl is used when
reconfiguring radio link:
•
The RNC informs the RBS that the existing radio link needs to be
reconfigured by sending a Radio Link Reconfigurations Prepare message.
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•
The RBS responds with a Radio Link Reconfiguration Ready message, but
waits with changing the radio link.
•
When resources have been allocated in the RNC a Radio Link Commit
Message is sent to the RBS. This message contains the CFN at which the
radio link should be changed. Enough margin must be applied on the
current CFN so that there is time to send the Radio Bearer Setup message to
the UE and for the UE to change radio link configuration. The margin, Δ
CFN is decided by the calculated value + cfnOffsetMarginSrbDchDl.
•
The UE sends a Radio Bearer Setup Complete message to the RNC
UE
RBS
RNC
Reconf. Prepare
Reconf. Ready
Current CFN
Reconf. Commit
Δ CFN
Activation CFN
r
Radio Beare
Setup
Margin so that reconfiguration can be done in UE and RBS
at the same time
Radio Bearer
Set
Figure 9
4.3.3
up Complete
Reconfiguration of a radio link
Proposed change
Change cfnOffsetMarginSrbDchDl from 40 to 80 (40 frames  80 frames)
Field trials have shown good improvement on CS retainability by doing this
change. No other negative KPI impacts have been observed.
One frame equals 10 ms, meaning that the activation time is prolonged 400 ms by
this change.
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4.3.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate
•
CS & PS Retainability Rate
•
Latency for PS
Counter formulas can be found in [7] and [8].
4.4
Disable CQI repetition
4.4.1
Improvement area
Retainability
4.4.2
Background
The RBS can initiate updates of the CQI Repetition Factor, CQI Feedback Cycle
and ACK/NACK Repetition Factors, see [4]. The parameter updates are initiated
by the CQI reception performance. CQI Repetition Factor, CQI Feedback Cycle
and ACK/NACK Repetition Factors are:
•
Increased if the number of consecutive CQI errors > cqiErrors.
•
Reduced if no CQI errors for at least cqiErrorsAbsent consecutive CQI
reports.
The update of CQI Repetition Factor, CQI Feedback cycle and ACK/NACK
repetition factors are timer supervised physical channel reconfiguration
procedures (hardcoded Layer 3 timer). If the timer times out the call is dropped,
which is typical if the UE is in poor radio. This is a reason to disable CQI
repetition.
The RBS triggered updates can be turned off by setting the parameters cqiErrors
and cqiErrorsAbsent to 0.
Note that there is a dependency to the parameters initialCqiRepetitionFactor and
initialAcknackRepetitionFactor, which sets the initial number for CQI and
ACK/NACK repetitions. If CQI repetition is turned OFF, the number of
CQI/ACK/NACK messages will always be the same as set by parameters
initialCqiRepetitionFactor and initialAcknackRepetitionFactor (since the
RBS can not update the values). Normally there is no reason to use initial
repetition. Therefore it is recommended to use default 1 for
initialCqiRepetitionFactor and initialAcknackRepetitionFactor.
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4.4.3
Proposed changes
Change cqiErrors from 10 to 0
(10 consecutive CQI errors  RBS triggered updates turned off)
Change cqiErrorsAbsent from 10 to 0
(10 consecutive CQI reports with no errors  RBS triggered updates turned off)
Assure that initialCqiRepetitionFactor and initialAcknackRepetitionFactor
are set to 1 (default value)
Only UeRcs containing HS-DSCH are affected by the change
The reconfiguration procedure is only triggered if HS-DPCCH performance
becomes bad, meaning that the impact of the parameter change may differ
between cells depending on radio quality.
4.4.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate
•
CS & PS Retainability Rate
•
UL RSSI
•
MAC HS Cell throughput
•
EUL HARQ retransmission rate
•
Latency for PS
Counter formulas can be found in [7] and [8].
5
Improving radio environment
5.1
Overview
This section includes recommendations on how to improve the radio environment.
An improvement of the radio environment (lower interference in uplink and
downlink) is beneficial from a retainability and accessibility perspective.
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5.2
Power offset
5.2.1
Improvement area
Retainability and Accessibility
5.2.2
Background
The parameter pO3 sets the power offset between pilot bits and data bits in the
downlink [4]. It has been seen in previous field trials have shown very positive
effects on used A-DCH power in the downlink. The A-DCH power can be
substantial in cells with many HSDPA users [3]. The change improves HSDPA
performance since less non-HS power is used in the downlink. The reduced power
consumption also has a positive effect on downlink interference, i.e. downlink
interference is lowered.
5.2.3
Proposed change
Change pO3 to 0 (0 dB)
5.2.4
Observability
To evaluate the impact of the changes on parameter pO3, the following should be
monitored:
•
CS & PS accessibility, EUL accessibility
•
CS & PS retainability, EUL retainability
•
Non-HS power
•
HSDPA user- and cell throughput
Counter formulas can be found in [7] and [8].
5.3
EUL optimization
5.3.1
Improvement area
Retainability and Accessibility
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5.3.2
Reduce max SIR target
Background
The uplink SIR is limited by sirMax for EUL 10 ms users and by sirMaxTti2 for
EUL 2 ms users. It is recommended to tune in particular sirMaxTti2 since it has
been seen in field that the default value is too high and may cause instability
problems and high noise rise peaks. Tuned values will result in lower RSSI,
which in turn will have a positive effect on retainability for all RABs.
Proposed Change
Normal operation: change sirMaxTti2 from 173 to 120 (17.3 dB  12 dB)
High capacity: change sirMaxTti2 from 173 to 80 (17.3 dB  8 dB)
Ensure that sirMax is set to 100 (10 dB)
The 17.3 dB setting for sirMaxTti2 is too high even for single user peak rate
demos, where 14 dB is a more suitable value. For normal network operation
12 dB is a well balanced value (between capacity and peak rate). In high capacity
cells with many EUL users sirMaxTti2 can be lowered to 8 dB to ensure that
noise rise is kept well in bounds.
For EUL 10 ms it is recommended to use sirMax 10 dB, both with and without
EUL. For high capacity cells when UL 384 kbps and UL 128 kbps are disable it
may be possible to lower sirMax even further.
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS accessibility, EUL accessibility
•
CS & PS retainability, EUL retainability
•
UL RSSI
•
HSDPA & EUL cell throughput
Counter formulas can be found in [7] and [8].
5.3.3
Increase Transmission Target Error
Background
Uplink outer loop power control regulates the SIR target for EUL and is steered
by the Transmission Target Error, which sets the quality of the radio link in terms
of HARQ retransmissions [4].
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Transmission target error is controlled by the parameters
transmissionTargetError for EUL 10ms users and
transmissionTargetErrorTtti2 for EUL 2ms users.
It has been seen in field trials that it is beneficial from an uplink interference and
EUL throughput perspective to increase TTE in the cell if there are more than 4 –
5 EUL users in the cell. The only negative impact is lower EUL peak rate [3]. The
lower uplink interference has a positive effect on retainability.
Proposed change
Change transmissionTargetError from 10 to 100 (1%  10%)
Change transmissionTargetErrorTti2 from 20 to 50 (2%  5%)
For high capacity networks with many EUL 2ms users it is also possible to set
transmissionTargetErrorTti2 to 100 (10%).
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS accessibility, EUL accessibility
•
CS & PS retainability, EUL retainability
•
UL RSSI
•
HSDPA & EUL cell throughput
•
EUL HARQ retransmission rate
•
Latency for PS
Counter formulas can be found in [7] and [8].
5.4
Initial SIR target
5.4.1
Improvement area
Retainability and Accessibility
5.4.2
Background
It has been seen in field trials that initial SIR target has a significant impact on
uplink interference, especially in highly loaded cells [3]. Lowering uplink
interference has a positive effect on retainability and accessibility.
The initial SIR target is set with the parameters:
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5.4.3
•
ulInitSirTargetSrb for standalone SRB
•
ulInitSirTargetLow for RABs having SF ≥ 32
•
ulInitSirTargetHigh for RABs having SF = 16 or 8
•
ulInitSirTargetExtraHigh for RABs having SF ≤ 4
Proposed changes
Change ulInitSirTargetSrb to 30 (3 dB)
Change ulInitSirTargetLow to 30 (3 dB)
Change ulInitSirTargetHigh to 70 (7 dB)
Change ulInitSirTargetExtraHigh to 70 (7 dB)
This setting is recommended for high capacity networks.
5.4.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate
•
CS & PS Retainability Rate
•
UL RSSI
Counter formulas can be found in [7] and [8].
5.5
BLER target interactive
5.5.1
Improvement area
Retainability and Accessibility
5.5.2
Background
Power control ensures that an agreed quality is kept on the connection in terms of
Block Error Rate (BLER). By adjusting BLER target it is possible to control
transmitted power on uplink and downlink.
It has been seen in filed trials that it is possible to increase BLER target for
interactive RABs to reduce the transmitted power on uplink and downlink. This
has resulted in improved RSSI in uplink and lower power consumption in
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downlink. The result has been seen in field as improved accessibility in downlink
due to less power blocking at admission. The parameter change also improves
capacity in uplink and downlink [3].
BLER is set with the following parameters:
•
blerQualityTargetUl for UL BLER
•
blerQualityTargetDl for DL BLER
The parameters are set per UeRc and Transport Channel (UeRcTrCh) and are
expressed as: 10*log(blerQualityTarget). For example setting the parameter
blerQualityTargetUl to -20 will result in a UL BLER target of 1% (-20 =
10*log(0.01)). Similarly setting blerQualityTargetUl to -13 will result in 5% UL
BLER
5.5.3
Proposed change
For PS interactive RABs change:
blerQualityTargetUl and blerQualityTargetDl from 1% to 5%
The BLER targets should be adjusted on the uplink and downlink for single PS
DCH RABs (both SRB and DCH transport channels).
For HSDPA RABs field trials have shown that it is possible to change BLER
target for ADCH from 0.1% to 1%, and in some cases all the way to 5% with
positive effect on consumed downlink power (especially in cells with many
HSDPA users). No negative impact has been seen in KPIs. Here it is
recommended to do the change in steps (0.1%  1%  5%) while closely
monitoring HSDPA retainability.
For Multi-RABs the BLER target is adjusted on the PS DCH transport channel
only. It should not be changed for SRB in this case since it may have a negative
effect on Speech retainability.
The parameter changes are summarized in Table 3.
Table 3
Summary of changes on blerQualityTargetUl and
blerQualityTargetDl.
RAB
TrCh
blerQualityTargetUl
blerQualityTargetDl
PS DCH/DCH
SRB
1%  5%
1%  5%
PS DCH
1%  5%
1%  5%
SRB
1%  5%
0.1%  1%  5%
PS DCH
1%  5%
–
SRB
–
0.1%  1%  5%
PS DCH/HS
PS EUL/HS
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Multi-RABs Speech + PS DCH/DCH
Multi-RABs Speech + PS DCH/HS
PS DCH
–
–
SRB
–
–
PS DCH
1%  5%
1%  5%
SRB
–
–
PS DCH
1%  5%
–
In total there are more than 130 parameter changes involved, see Excel sheet
below for all details.
BLER target
interactive.xls
5.5.4
Observability
To evaluate the impact of the parameter changes the following KPIs should be
monitored:
•
CS & PS Call Success Rate, RRC & RAB power blocking
•
CS & PS Retainability Rate
•
UL RSSI
•
Latency for PS
Counter formulas can be found in [7] and [8].
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6
References
1. User Description, Capacity Management, 83/1553-HSD 101 02
2. User Description, Connection Handling, 4/1553-HSD 101 02
3. High Capacity RN parameter tuning, 3/100 56-HSD 101 02
4. User Description, Power Control, 80/1553-HSD 101 02
5. User Description, LA, RA and URA Planning, 2/100 56-HSD 101 02
6. Common Control Channel Guideline, 63/100 56-HSD 101 02/6
7. Guideline for HSPA Performance Indicators, 33/100 56-HSD 101 02/10
8. User Description, Radio Network KPI, 120/1553-HSD 101 02
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