Idle Mode Parameter Optimization

1 © Nokia Siemens Networks Presentation / Author / DateFor internal use

Slowing Down the Inter RAT Cell Reselections; Parameter Tuning & Monitoring GuideVersion 1.0

23.09.2007 Reunanen Jussi


Agenda

• Treselection

• Hysteresis between 3G->2G and 2G->3G cell reselections

• PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble

• PrxAlpha

• PrxTarget


Treselection – Introduction

Treselection: how long the reselection conditions must be fulfilled before reselection is triggered

• Impacts all cell reselections : Inter RAT, intra frequency and inter frequency

• The UE reselects the new cell, if the cell reselection criteria (R-criteria, see next slide) are fulfilled during a time interval Treselection


Treselection – Introduction

First ranking of all the cells based on CPICH RSCP (WCDMA) and RSSI (GSM)

Rs = CPICH RSCP + Qhyst1Rn= Rxlev(n) - Qoffset1



Rn (GSM) > Rs (WCDMA)And

Rxlev (GSM) >QrxlevMin

Rn (GSM) > Rs (WCDMA)And

Rxlev (GSM) >QrxlevMin

YesNo

Cell re-selection to GSM

Cell re-selection to GSM

Neighbour WCDMA or GSM cell calculation with offset

parameter

Serving WCDMA cell calculation, with

hysteresis parameter

UE starts GSM measurements if CPICH Ec/No < qQualMin + sSearchRAT

UE starts GSM measurements if CPICH Ec/No < qQualMin + sSearchRAT

SintraSearch

SinterSearch

SsearchRAT

CPICH EcNo

qQualMin

Second ranking only for WCDMA cells based on CPICH Ec/No

Rs = CPICH Ec/No + Qhyst2Rn=CPICH_Ec/No(n)-Qoffset2

Second ranking only for WCDMA cells based on CPICH Ec/No

Rs = CPICH Ec/No + Qhyst2Rn=CPICH_Ec/No(n)-Qoffset2 Cell re-selection to

WCDMA cell of highest R value

Cell re-selection to WCDMA cell of highest

R value


Treselection – Recommended Values

As this parameter impacts on all the cell reselections too long Treselection timer might cause problems in high mobility areas but too short timer causes too fast cell reselections and eventually causes also cell reselection ping pongRecommended value 1s should work in every conditions i.e. enough averaging to make sure that correct cell is selectedHowever careful testing is needed to check the performance of different areas• (Dense) Urban area, slow moving UEs with occasional need for fast and

accurate (to correct cell) reselections e.g. outdoor to indoor scenarios or city highways – in some cases cell by cell parameter tuning is performed to find most optimal value between 0s and 2s but typically 1s is optimal value when workload is considered as well

• Highways, fast moving UEs must reselect correct cell – typically 1s works the best (however occasionally also 0s might be needed in fast speed outdoor to indoor cell reselections e.g. tunnels)

• Rural areas, slow or fast moving UEs need very often reselect between different RATs and make proper cell reselections even when the coverage is poor – typically 1s works the best

• Location Area Borders, usually the coverage is fairly poor – typically 1s works the best but sometimes to reduce location area reselection ping pong 1s is used when going from LA1 to LA2 and 2s from LA2 to LA1


Treselection – KPIs and Analysis

RRC connection request amount for inter RAT cell reselection ratio to all RRC Connection request causes

• When Treselection is increased this KPI should decrease

RRC connection request amount for registrations ratio to all RRC Connection request causes

• When Treselection is increased this KPI should decrease

TP_ATTRRC_CONN_SM1001C0

_ATTSELL_RE_SELINTR_RAT_C M1001C42


ON_ATTSREGISTRATI M1001C46



At the same time the CSSR and especially the RRC Connection Setup and Access Complete must be monitored

• The KPIs below should remain the same despite the Treselection modification or even get better

• If the CSSRs or RRC Connection Setup and Access Complete decrease the Treselection value should be decreased

• RNC_94d: RRC Setup and Access Complete Ratio from network point of view

• RNC_565e: CSSR CS Voice (RAS05.1) or RNC_565d : CSSR CS Voice (RAS05)

• The RRC Connection Setup attempts should decrease– M1001C0 RRC_CONN_STP_ATT



• Following RRC Connection Setup failure causes should decrease as the amount of RRC Connection setup attempts is decreasing

– M1001C3 RRC_CONN_STP_FAIL_AC

– M1001C4 RRC_CONN_STP_FAIL_BTS

– M1001C5 RRC_CONN_STP_FAIL_TRANS

– M1001C530 RRC_CONN_STP_FAIL_IUB_AAL2

• Above indicate saved resources and more detailed analysis can be done based on counters below

– BTS CE : RAS05.1

– Iub : AAL2 Path Average Reserved Bandwidth % : RAS05

E_DLMAX_USED_C M5001C3 E_DLMAX_USED_C M5001C3

CE_UL MAX_USED_ M5001C4 CE_UL MAX_USED_ M5001C4

E_DLMIN_USED_C M5001C5 E_DLMIN_USED_C M5001C5CE_UL MIN_USED_ M5001C6 CE_UL MIN_USED_ M5001C6

E_DLAVG_USED_C M5001C7 E_DLAVG_USED_C M5001C7

E_ULAVG_USED_C M5001C8 E_ULAVG_USED_C M5001C8

RATEGUAR_CELL_AAL2_PATH_ M550C0

SNBR_SAMPLE M550C7ATEVED_CELL_R SUM_RESERM550C1

100%


Hysteresis Between 3G->2G and 2G->3G Cell Reselections – Introduction & Recommended Values

Parameters for cell reselections

• Qqualmin = -18dB Ssearch_RAT =2dB -> the 3G->2G cell reselection starts when Ec/No hits -16dB

• FDDQmin/GPRSfddQmin = -14dB (6) and QsearchP/Qsearch = always

The cell reselection paramters 3G -> 2G and 2G -> 3G provide only 2dB hysteresis which is not enough and can been noticed from the RNC statistics as high amount of INTR_RAT_CELL_RE_SEL_ATTS from all the RRC Connection Setup Attempts

• Recommendation is to adjust the GFDM/FDM from -14dB to -10dB (or even up to -8dB) to provide 6 to 8 dB hysteresis between 3G to 2G cell reselection and 2G to 3G cell reselection

On top of Treselection the above parameters will slow down further the 2G to 3G and 3G to 2G cell reselections


Hysteresis Between 3G->2G and 2G->3G Cell Reselections – Introduction & Recommended ValuesHowever careful testing is needed to check the performance of different areas

• UEs must reselect correct cell in case of 3G outdoor to certain 2G – in these cases adjacency based parameter Adjqoffset1 can be used to prioritize certain 2G neighbor

– Impacts the R-criteria as shown below






Hysteresis Between 3G->2G and 2G->3G Cell Reselections – KPIs and AnalysisRRC connection request amount for inter RAT cell reselection ratio to all RRC Connection request causes

• When hysteresis is increased this KPI should decrease

RRC connection request amount for registrations ratio to all RRC Connection request causes

• When hysteresis is increased this KPI should decrease


_ATTSELL_RE_SELINTR_RAT_C M1001C42


ON_ATTSREGISTRATI M1001C46


Hysteresis Between 3G->2G and 2G->3G Cell Reselections – KPIs and AnalysisAt the same time the CSSR and especially the RRC Connection Setup and Access Complete must be monitored

• The KPIs below should remain the same despite the hysteresis modification or even get better

• If the CSSRs or RRC Connection Setup and Access Complete decrease the hysteresis value should be decreased



• The RRC Connection Setup attempts should decrease– M1001C0 RRC_CONN_STP_ATT


Hysteresis Between 3G->2G and 2G->3G Cell Reselections – KPIs and Analysis• Following RRC Connection Setup failure causes should decrease as the

amount of RRC Connection setup attempts is decreasing– M1001C3 RRC_CONN_STP_FAIL_AC

– M1001C4 RRC_CONN_STP_FAIL_BTS

– M1001C5 RRC_CONN_STP_FAIL_TRANS

– M1001C530 RRC_CONN_STP_FAIL_IUB_AAL2

• Above indicate saved resources and more detailed analysis can be done based on counters below

– BTS CE : RAS05.1

– Iub : AAL2 Path Average Reserved Bandwidth % : RAS05

E_DLMAX_USED_C M5001C3 E_DLMAX_USED_C M5001C3

CE_UL MAX_USED_ M5001C4 CE_UL MAX_USED_ M5001C4

E_DLMIN_USED_C M5001C5 E_DLMIN_USED_C M5001C5CE_UL MIN_USED_ M5001C6 CE_UL MIN_USED_ M5001C6

E_DLAVG_USED_C M5001C7 E_DLAVG_USED_C M5001C7

E_ULAVG_USED_C M5001C8 E_ULAVG_USED_C M5001C8

RATEGUAR_CELL_AAL2_PATH_ M550C0

SNBR_SAMPLE M550C7ATEVED_CELL_R SUM_RESERM550C1

100%


CLUSTER JBK09

0%

20%

40%

60%

80%

100%

<50% 50-70% 70-85% 85-100%

85-100% 1 1

70-85% 3 7 1 1

50-70% 12 10 9 7 6 8

<50% 41 39 47 50 51 48

6/ 19/ 2007 6/ 20/ 2007 6/ 21/ 2007 7/ 10/ 2007 7/ 11/ 2007 7/ 12/ 2007

BEFORE AFTER

Cluster JBK09

Count of % INTR_RAT_CELL_RESEL

Comparison Date

Grouping1

Cluster JBK09 Percentage Distribution after implementation:

☺Decreasing black and red colour as high percentage of Inter Rat Cell Reselection

☺Increasing green colour as low percentage of Inter Rat Cell Reselection

Decreasing

☺Before 25% of Cells having >50% of all RRC setups for inter RAT cell reselection

☺After 12% of cells having >50% of all RRC setups for Inter RAT cell reselection

Hysteresis Between 3G->2G and 2G->3G Cell Reselections – KPI Analysis Example

Hysteresis from 2dB -> 6dB


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – Introduction • During drive testing it is usually noted that there are call setup failures where the

network does not seem to respond to RRC Connection Requests with RRC Connection Setup –message

• These are problems due to the spiky UL noise and due to that the power ramping is not aggressive enough to provide high enough Tx power for the terminal during open loop PC

MAJOR ROADS INNER & OUTER BLOCKED COUNT BREAKDOWN

No Immediate Assignment, 1

No Radio Bearer Setup Complete, 2

No Alerting or Connect, 1

N-300 and T-300 expiry, 4

No Setup, 1RRC Connection

Reject, 3


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – Introduction

The parameters affecting to open loop power control are, in brackets are the recommended values:

• PRACH_preamble_retrans (7)

• RACH_tx_Max (16)

• PowerOffsetLastPreamblePRACHmessage (2 dB)

• PowerRampStepPRACHpreamble (2dB)

DownlinkBS

L1 ACK / AICH

UplinkMS Preamble

1

Not detected

Message partPreamble2

PreamblePRACH_preamble_retrans

# PRACH preambles transmitted during one PRACH cycle without receiving AICH

response

UEtxPowerMaxPRACH

… … … …

RACH_tx_Max# preamble power ramping cycles that can be done before RACH transmission

failure is reported

PowerRampStepPRACHpreamble

PowerOffsetLastPreamblePRACHmessage

Ptx = CPICHtransmissionPower-RSCP(CPICH) +RSSI(BS) + PRACHRequiredReceivedCI (-20dB)

Ptx = CPICHtransmissionPower-RSCP(CPICH) +RSSI(BS) + PRACHRequiredReceivedCI (-20dB)


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – Recommended Values

• Typically the parameter PRACHRequiredReceivedCI is set to -18…-20dB instead of the default -25dB (typically -20dB is enough)

• Also the parameter PowerRampStepPRACHpreamble is adjusted in case the problem of missed MT calls or call setup failures due to missing RRC Connection Setup still persists (default 2dB usually works the best but in some networks also 3dB is used)

• Check also the values for RACH_tx_Max and PRACH_preamble_retrans– They should be set accordingly as:

– PRACH_preamble_retrans (7)

– RACH_tx_Max (16)

• Also the parameters should all be set in line as below:– PowerOffsetLastPreamblePRACHmessage (2 dB)

– PowerRampStepPRACHpreamble (2dB)

– PRACHRequiredReceivedCI (-20dB)

– PRACH_preamble_retrans (7)

– RACH_tx_Max (16)


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – Recommended Values

• If for example PowerRampStepPRACHpreamble is set to 3dB then other parameters should be adjusted with extra care to avoid unnecessary UL noise spikes

• Current defaults are tested to work properly without excessive increase in UL noise


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – KPIs and Analysis• The impact of this parameter has to be tested with drive testing (as in case the

PRACH preambles are not heard and therefore the RRC Connection Request is not heard by the BTS/RNC no RRC connection setup attempt counter is incremented)

• From the network statistics following should be monitored– RNC_177b Noise Floor of the System– RNC_101b Average Uplink load

▪ Related to above the M1001C3 RRC_CONN_STP_FAIL_AC should not increase (if do then load level reduction actions are needed, explained in PrxAlpha tuning section)

– Secondary KPIs to follow▪ If the CSSRs or RRC Connection Setup and Access Complete decrease the hysteresis

value should be decreased▪ RNC_94d: RRC Setup and Access Complete Ratio from network point of view▪ RNC_565e: CSSR CS Voice (RAS05.1) or RNC_565d : CSSR CS Voice (RAS05)▪ The RRC Connection Setup attempts should increase but the failures should not increase

(especially AC rejections which are related to UL noise increase / Average UL load)• M1001C0 RRC_CONN_STP_ATT• M1001C4 RRC_CONN_STP_FAIL_BTS • M1001C5 RRC_CONN_STP_FAIL_TRANS • M1001C530 RRC_CONN_STP_FAIL_IUB_AAL2


PRACHRequiredReceivedCI and PowerRampStepPRACHpreamble – KPI Analysis Example

Two values for PRACHRequiredReceivedCI tested (drive testing)

100%

0% 0% 0%

88%

2% 5% 6%

0%

20%

40%

60%

80%

100%

1 2 3 4

# RRC Connection Request Messages per call setup

%

PRACH req. C/I = -20dB PRACH req. C/I = -25dB

Clear improvement in number of needed RRC Connection Request messages per call. For –20dB 100% of established calls are setup with only 1 RRC Connection Request message

Clear improvement number of sent preambles per RRC Connection Request for –20dB case. For –20dB 50% of cases the needed number of preambles is <=4 where as for –25dB it is ~6.5

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1 2 3 4 5 6 7 8

PRACH req. C/I = -25dB PRACH req. C/I = -20dB

There should be significant improvement also for call setup delay

• -20dB & -25dB : UL interference conditions are at the same level (reported in SIB 7 for both cases)



Clear improvement in call setup delay for –20dB case. ~65% of the established calls are through with only 3.5 – 3.7s delay and the >5.5s delay “tail” disappears (in this case).

Two values for PRACHRequiredReceivedCI tested (drive testing)• -20dB

• -25dB

• UL interference conditions are at the same level (reported in SIB 7 for both cases)

96.2%

100.0%

94%

95%

96%

97%

98%

99%

100%

-25dB -20dB

Call Setup Success Rate

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

120.0%

<3

.5s

3.5

s -

3.7

s

3.7

s -

3.9

s

3.9

s -

4.1

s

4.1

s-4

.3s

4.3

s-4

.5s

4.5

s-4

.7s

4.7

s-4

.9s

4.9

s-5

.1s

5.1

s-5

.3s

5.3

s-5

.5s

>5

.5s

Call Setup Delay (seconds) RRC Conn. Req. to Alerting

PRACH req. C/I = -25 PRACH req. C/I = -20



AMR Voice Before 1st drive 2nd drive

Average Preamble Counts 2.40 2.01 1.617

Average RACH_TX_Pwr -9.11 -7.09 -6.581

average UL interference -102.42 -102.07 -101.53

Video Before 1st drive 2nd drive

Average preamble count 2.84 1.77 1.933

Average RACH_TX_Pwr -6.95 -6.45 -6.888

average UL interference -102.46 -102.15 -101.331. The number of resend for PRACH (Preamble Count) has reduced after the

parameters changing, that mean shorter time to build the radio link. 1st drive is after PRACHRequiredReceivedCI been changed (-25dB -> -20dB), while 2nd drive is after RACH_tx_max been changed (8 -> 16)

2. Meanwhile the RACH_TX_Pwr for both AMR and Video services is increased, which mean more power is consumed by the UE .

3. The slighlty increased UL interference can be tried to be reduced by taking the alpha trimmed mean filter into use to filter the Prx measurements in BTS



AMR (MOC) Before 1st drive 2nd drive

Average CS call setup time (MOC) 2.36s 2.44s 2.38s

Call Setup Success Rate 97.87% 99.30% 100.00%


PrxAlpha – Introduction

In case the indicators below have increased due to the open loop PC parameter modifications

• RNC_177b Noise Floor of the System• RNC_101b Average Uplink load

Or in generally if there are high amount of rejections due to following rasons• M1001C3 RRC_CONN_STP_FAIL_AC• M1001C80 RAB_STP_FAIL_CS_VOICE_AC• M1001C85 RAB_STP_FAIL_CS_CONV_AC

The BTS PrxTotal filtering parameters can be modified i.e. how the BTS averages the PrxTotal measurements before sending the results to the RNC in Radio Resource Indication messages



Frame mean of Prx_total is pre-calculated using arithmetic mean of the slot values (15 slots)

Averaged frames are sorted based on PrxMeasAveWindow frames

Filtered values are calculated by taking arithmetic frame mean values which are between ALPHA and (1-APLHA) defined by PrxAlpha

• N x lowest and highest measurements removed

• N x (1- 2) measurements remain

Av. Frame measurements

Prx• PrxMeasAveWindow = 10 & PrxAlpha = 0.2

• Frame averages are calculated for 10 frames

• 2 max. & 2 min. values excluded : 6 values remaining

• Final average Prx_total is calculated on the 6 values



RADIO RESOURCE INDICATION (RRI) messagePrx Total is reported to the RNC for LC, AC and PS usage, on every RRIindicationperiod (200 ms)

The BTS is measuring:Prx Total

BTS sends the LASTEST averaged measurement result to RNC every RRI period

RNC is calculating a sliding window averaged value over the values given by RRI messagesFrame

measurements

Prx

…

RRIxRRIx+1RRIx+2 RRIx+n

Sliding window average result on every RRI periodWinAcRabSetup

UI

m

jjntotalrx Totalx

mP

1

_ _Pr1

BTS uses ALPHA TRIMMED MEAN FILTERPrxAlphaPrxMeasAvWindow


PrxAlpha – Recommended Values & Example

PrxAlpha=0.1

PrxMeasAveWindow=10

RRIPeriod=200ms

PrxAlpha=0

PrxMeasAveWindow=10

RRIPeriod=200ms

Clearly some impact when changing the PrxAlpha -> suggests that the interference conditions are somewhat spiky and the PrxTotal values could be further reduced adjusting the PrxAlpha

The PrxAlpha parameter value should be adjusted in 0.1 step intervals and KPIs monitored for each parameter value


PrxAlpha – KPIs and Analysis

Following indicators should be monitored for each PrxAlpha value

• RNC_177b Noise Floor of the System

• RNC_101b Average Uplink load

• M1001C3 RRC_CONN_STP_FAIL_AC

• M1001C80 RAB_STP_FAIL_CS_VOICE_AC

• M1001C85 RAB_STP_FAIL_CS_CONV_AC


PrxTarget – Introduction

If the received Non-controllable power (RT) + the estimated power increase caused by the to be established RAB is above the PrxTarget, the RT RAB setup is rejectedIf the received Non-controllable power (RT) is above the PrxTarget limit then the RRC Connection Setup is rejected• However note that for the following RRC Connection Request

cause values the AC limits are not used (in UL nor in DL)– Emergency call– Registration– Detach– Originating High Priority Signaling– Terminating High Priority Signalling– Inter-RAT cell re-selection– Inter-RAT cell change order


PrxTarget – Recommended Values & Example

TOTAL NUMBER OF RRC CONN STP FAIL AC

0

1000

2000

3000

4000

5000

6000

6/1/

2007

6/2/

2007

6/3/

2007

6/4/

2007

6/5/

2007

6/6/

2007

6/7/

2007

6/8/

2007

6/9/

2007

6/10

/200

7

6/11

/200

7

6/12

/200

7

6/13

/200

7

6/14

/200

7

6/15

/200

7

6/16

/200

7

6/17

/200

7

6/18

/200

7

6/19

/200

7

6/20

/200

7

6/21

/200

7

6/22

/200

7

6/23

/200

7

6/26

/200

7

6/27

/200

7

6/28

/200

7

6/29

/200

7

6/30

/200

7

7/1/

2007

Total Moving Avg

WCELL Name (All) Batch 4th Batch

Sum of RRC_CONN_STP_FAIL_AC

Date

In case the PrxAlpha tuning does not reduce the AC rejections then the PrxTarget can be increased to reduce the AC rejections

Typically just 2dB increase provides significant reduction on the AC rejections

Starting 16 June 2007 PtxTarget modified 4dB -> 6dBStarting 16 June 2007 PtxTarget modified 4dB -> 6dB


PrxTarget – Recommended Values

First the following counters should be checked • RNC_177b Noise Floor of the System

– The above should be compared with the parameter value PrxNoise (WCEL)

– Max(RNC_177b, PrxNoise) = A


• M1000C0 AVE_PRXTOT_CLASS_0, M1000C1 PRXTOT_DENOM_0






PrxTarget – Recommended Values

First the following counters should be checked • The PrxTotal distribution should be calculated from above and the needed

PrxTarget distribution and average– Average needed PrxTarget; A – RNC101b

– Distribution can be calculated the same way A-AVE_PRXTOT_CLASS_X and the % of samples for each class PRXTOT_DENOM_X/(PRXTOT_DENOM_1+ PRXTOT_DENOM_2+ PRXTOT_DENOM_3+ PRXTOT_DENOM_4+ PRXTOT_DENOM_5)

• Then the PrxTarget parameter tuning should be performed in steps of 2dB towards the highest calculated PrxTotal value

• Max PrxTarget value should not exceed 10dB (in normal conditions without any special interferer)


PrxTarget – KPIs and Analysis

When the PrxTarget is modified following KPIs should be monitored• RNC_177b Noise Floor of the System


• M1001C3 RRC_CONN_STP_FAIL_AC

• M1001C80 RAB_STP_FAIL_CS_VOICE_AC

• M1001C85 RAB_STP_FAIL_CS_CONV_AC








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Idle Mode Parameter Optimization