TTP30024 - RF Parameter Optimization

8/12/2019 TTP30024 - RF Parameter Optimization

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Knowledge SharingKey RF Parameter Optimization

All Rights Reserved © Alcatel-Lucent 2006

Alex Anokye

RF Engineering Group, Murray Hill, NJ

Documents from NPE/TIS/CEC

Documents from Devesh Patel and Mattijs Visser

June 23, 2010




Structure of this talk

Primary goal of this talk is Knowledge sharing experience from LTE

Trials

Start with Some Key RF Performance Impacting Parameters With Real examples from Orange & BostonTrials. Lessons learned from a technology

trial help refine RF optimization procedure

Share Analysis of Handoff Failures Test performed in MH, New Jersey

Follow with Data Analysis of RRC Layer 3 message Flow with eDAT Helps RF Engineers how to troubleshoot RF issues

End with Open Issues and Proposed Resolutions from Phase 4 Trial Helps to understand product functionality as we migrate to FSA and Deployments




•RF Parameter Optimization




Introduction

Presentation discusses key LTE RF performance impacting parameters

Detailed description on 52 parameters

Ranges of these parameters are provided in the following excel sheet

Key in the sense likely to have the biggest impact on end-user performance and capacity

Many of these parameters are categorized as “fixed” parameters

“Fixed” means not typically customized from cell to cell or as part of RF optimization process

– Some exceptions such as settings optimized for a stadium cell / special events orspecific to certain hardware platform

These are algorithmic parameters - settings optimized based on simulations and lab studies

Some amount of fine tuning in field, usually as part of focused performance studies in acommercial system after a critical mass of traffic available




LTE Key Parameters

DownlinkdlSinrThresholdBetweenCLMimoTwoLayersAndOneLayer: 10.0 dB dlSinrThresholdBetweenOLMimoAndTxDiv: 10.0 dB dlSinrThresholdBetweenCLMimoAndTxDiv: 10.0 dB

Uplink

initialSIRtargetValueForPUSCHSemiStaticUsers: 4.0 dB initialSIRtargetValueForPUSCHnonSemiStaticUsers: 4.0 dB

HandoverfilterCoefficientRSRP: fc8 filterCoefficientRSRQ: fc8 reportAmount RSRP: r8 Reporting Interval RSRP: 240 ms hysteresis RSRP: 2 dB

eventA3Offset: 0 qRxLevMin: -120 dBmqRxLevMinIntraFreqNeighbor: -120 dBm




Switching point lies around CQI = 10-11

i.e. computed SINR ~ 15dB

if dlSinrThresholdBetweenOLMimoAndTxDiv = 15dB

dlSinrThresholdBetweenOLMimoAndTxDiv

• dlSinrThresholdBetweenOLMimoAndTxDiv = Signal to Noise ratio threshold for switching between

OL Mimo mode and TxDiv mode

• It does not have anything to do with the RS-SNR shown in LLDM or Agilent NiXT tool, but is

related to the SINR calculated as below

• If computed SINR > dlSinrThresholdBetweenOLMimoAndTxDiv + 1dB, then eNB will switch to DL OL

Mimo mode (as long as filtered value for the UE reported rank > 1.6).

• It switches back to TxDiv if SINR < dlSinrThresholdBetweenOLMimoAndTxDiv


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7

Threshold for MIMO switching

Background

At “higher SINR, lower Tx antenna correlation” regimes, MIMO allows for higher data

rates compared to Tx diversity

– Benefit from rank of 2 (i.e., 2 parallel data streams possible)

– Vice-versa for low SINR levels

Parameter Impact / Tradeoffs

Impacts DL MCS/throughput

– Higher switching threshold values will reduce DL data rate otherwiseachievable in the higher SINR regime

– Single stream used instead of 2 even if Tx ant correlation is low

– Lower values could allow MIMO too soon -> increases the likelihood thatHARQ Retrx/BLER will be higher than achievable with Tx diversity -> lowerMCS -> lower DL data rate/throughput at the UE




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9

initialSIRtargetValueForPUSCHnonSemiStaticUsers

Problem: max UL throughput in the field is lower than expected for BE

Suspicion: initialSIRtargetValueForPUSCHnonSemiStaticUsers parameter

When trying to reach the max theoretical UL throughput in the field in static tests,

it is essential to get close to the ideal condition as possible:

• The mobile should be located close to the site, within the main beam of

the antenna.

• The RSSI should be around -40dBm and the SNR around 25dB for

each of the 2 antennas.

• It is worth to play around separately with each of the antennas

until we reach the desired values on each one [this is also

recommended in DL throughput issues as well]

• The other elements that could impact on the throughput [UL and

DL] are the MTU settings in the drive-test computer (it is

recommended to use TCP Optimizer) and the FTP server

configuration.





initialSIRtargetValueForPUSCHnonSemiStaticUsers configures the initial UL SIR target for non semi-static users.

Closed-loop power control allows the eNodeB to send power control commands to the UE.

The UE will consequently increase or decrease its transmission power to reach a defined SIR target for the received UE

signal at the eNodeB.

UE Tx power increase toreach SIR target at eNodeB





The higher the parameter, the higher the transmission of the mobile on the PUSCH channel (for non semi-static users)

And consequently the higher the received signal quality at the eNodeB.

Higher MCS used on the UL => higher throughputs

As single UE test is performed in the field, this parameter can be changed from 10 to 18dB to experience max UL

throughput.

Up to 16Mbps were reached instead of the previous 12Mbps.

In multi-UE environment, too high value for this parameter may generate more UL interference and eventually lower

total throughput on the UL.

In case of multi-UE, it is recommended to use the default value of 10dB.

Note: If the total transmission on the mobile is already at its max, i.e. 23dBm, then the UL throughput is already limited

for another reason (most probably pathloss).

Conclusion: initialSIRtargetValueForPUSCHnonSemiStaticUsers value was changed from10 to 18




Attachment procedure

Problem: UE can not attach to the network, even if located close to the site (Cachan).

Suspicion: qRxLevMin parameter

3-sector siteinstalled on 2

poles

No attach possible

from this position

(RSRP = -102dBm)

30°

135°270°




Attachment procedure: qRxLevMin

qRxLevMin configures the minimum required Rx level in the cell for the mobile to be allowed to attach to the network.

Default DLU value: -50, i.e. -100dBm (2*IE).

From drive test data, most locations at cell edge, the signal is between -100dBm and -110dBm, with still reasonable good performance.

The value was changed to -55, i.e. -110dBm.

The distance range where the UE can attach was increased by a factor of 1.75

Note: -110dBm per Resource Element for the RS (Reference Signal) corresponds to -90dBm for the total RS power.

A slightly lower value of qRxLevMin is still possible, but it is not recommended to go lower as reasonable good quality when attaching to thenetwork is required.

Note: Please do the distinction between qRxLevMin and qRxLevMinIntraFreqNeighbour.

qRxLevMinIntraFreqNeighbour is set by default to -70, i.e. -140dBm, and is used in HO procedures to configure the minimum required Rx level inthe target cell for the mobile to be allowed to handover to that target cell.

Conclusion: qRxLevMin value was changed from -50to -55 (i.e. -110dBm).




RxLevMin & RxLevMinOffset (to favor a particular cell)




Handover optimization

Problem: a lot of ping pongs and call drops in the handover areas.

Suspicion: The following HO parameters were suspected to be the reason:

1- hysteresis

2- TimeToTrigger

3- filterCoefficientRSRP

The tuning steps were done on the parameters with the same order mentioned above




Handover optimization

Handover hysteresis: measurement difference between a source cell and a target cell to trigger a measurement report (from UE to eNodeB).

Time-to-trigger: time during which specific criteria for the event needs to be met in order to trigger a measurement report (from UE to eNodeB).

The trigger metric can be RSRP and/or RSRQ.

Handover between cells 54 and 55

-110.00

-105.00

-100.00

-95.00

-90.00

-85.00

-80.00

-75.00

-70.00

-65.00

-60.00

R S R P ( d B m )

RSRP (cell 55)

RSRP (cell 54)

RSRP Handover

hysteresis = 5 dB

timeToTrigger




Handover optimization: RSRP measurement report




Handover optimization: RSRQ measurement report




Handover Optimization: timeToTrigger

Parameter timeToTrigger configures the time-to-trigger for the HO.

The higher its value, the less ping pongs will be seen in the field.

Current default value was 20ms.

No performance enhancements were seen in the field.

Was discovered that changing this parameter will not have any effect unless the new

value is above 200ms

The reason being that the UE informs the upper layer of any new filtered RSRP every

200ms.

Conclusion: timeToTrigger value was not changed in the field




Handover Optimization: filterCoefficientRSRP

Parameter filterCoefficientRSRP is used to filter and average the RSRP measurements at the UE level at a sample of

200ms.

The higher this value, the less ping pongs and less HO rate but increases

filter time content and therefore increase HO delay

The current default value is 4.

It was tuned to a value of 11 for smoother handovers (i.e.

filterCoefficientRSRP = fc11).

Also, less ping pongs effect was seen in the field.

RSRP(n) = (1 – 1/2(k/4)) x RSRP(n-1) + (1/2(k/4)) x M(n)

Where

• k = filterCoefficientRSRP

• M(n) the latest measurement




Handover Optimization: filterCoefficientRSRP

Effect of filterCoefficient k on RSRP

-120

-110

-100

-90

-80

-70

-60

R S R P ( d B m

)

RSRP Measurements

Filtered RSRP (k=4)

Filtered RSRP (k=11)

Conclusion: filterCoefficientRSRP value was changed from fc4 to fc11




Intra-Frequency DL Measurements : sIntrasearch / sMeasure

Two parameters are used to trigger measurements of the neighbouring cells:

1) sIntrasearch, in idle mode:

• intra-frequency measurements are triggered when the signal value in the serving

cell fulfills the below formula:

RSRPserving cell ≤ qRxLevMin (in dBm) – sIntrasearch (indB)

2) sMeasure, in connected mode:

• measurements are triggered when the signal value in the serving cell is lower than

sMeasure.

• This parameter is set by default to more_than_minus44, i.e. -43dBm.

• Meaning that measurements are always performed in connected mode.




Paging 1/2

It can happen that the mobile receives a lot of paging messages.

This does not necessarily imply UL problems.

The first thing to look at is the TMSI (Temporary Mobile Subscriber Identity) value in the

paging message and compare it to the TMSI of the mobile we are using (see LLDM NAS

ATTACH request).

If both TMSI are matching, then and only then, there might be some UL problems due to

UL messages from the mobile not being received by the eNodeB.




Paging 2/2




Reselection parameters : qHyst / tReselectionEUTRAN – SIB3

Cell reselection is used for mobility in idle mode.

HysteresisCellReselection should theoretically be set to a value at least equal

to the handover hysteresis as cell reselection in idle mode is not as important as

handover in connected mode.

tReselectionEUTRAN configures the time during which specific criteria for the

event needs to be met in order to trigger a cell reselection.

Its default value is 2s.

The idle mode was only quickly tested in mobility, and therefore idle mode

mobility parameters were not optimized.

But qHyst could be fine tuned.

HysteresisCellReselection = qHyst + qOffsetCell

• qHyst is set by default to 4dB.

• qOffsetCell is set by default to 3dB for all cells (this parameter,

although present in LA1.0, will start to be used in LA1.1).




Reselection parameters : qHyst / tReselectionEUTRAN

Cell reselection between cells 32 and 31

-130.00

-125.00

-120.00

-115.00

-110.00

-105.00

-100.00

-95.00

-90.00

R S R P ( d B m )

RSRP (cell 32)

RSRP (cell 31)

qRxLevMin = -110 dBm

Start of the other-cells measurements

with sIntrasearch = 10 dB

Reselection hysteresis = 7 dB

tReselectionEUTRAN = 2 s




Handover Hysteresis Background: cellIndividualOffset

cellIndividualOffset can be used to fine tune the handover hysteresis on a cell-to-cell basis.

Handover can be made more difficult from cell1 to cell2 by decreasing the cellIndividualOffset of thecell2, or sooner by increasing the value.

The effect would be to decrease number of ping pongs between 2 cells – after having already tuned theparameter hysteresis -, or to have an earlier HO before loosing the source cell.

In the next slide, moving from cell 34 to 33, the call is dropped all the time.

In the opposite direction, more than 50% dropped call.

By setting cellIndividualOffset to dB2 in both direction, the dropped call disappeared as the HO ishappening sooner and without any ping-pong.




Handover Hysteresis Background: cellIndividualOffset

Drop call + Reselection on 33

Building doing shadowing




cellIndividualOffset setting in WiPS



Analysis of Handoff Failures at 50%

OCNS LoadingMurray-Hill Blockhouse Drive test run with G13 DE hardware




G13 DE run w/ 50% loading

11 failures

5 on the target cell right after HO

Typically displayed as T304 expiry on the LLDM Radio Problem Cause screen

6 (4+2) on the source cell itself before MR could be sent out (incl. 2 w/o MR)

Typically displayed as SRMax on the LLDM Radio Problem Cause screen

Counted as HO failures as MR should

have been sent

The HO failure rate

can be calculated in

various ways:

HO Failure Rate 1:

(5+6)/(94+2) = 11.5%

HO Failure Rate 2:

(5+4)/94 = 9.8%

SINR drops right after HO to weak




Failure – 1: HO Failure On Target - T304 Expire

Sudden / Momentary

drop in RSRP on 83 causing UE to send MR to

a weak Cell ID 82

SINR drops right after HO to weakcell. RSSI strong. Call drops because

UE can not acquire target cell




Failure – 1: HO Failure On Target - T304 Expire - Timeline

09:39:56.740 Cell 83: RSRP -67 dBm

09:39:56.750 SINR: 15.65 dB

09:39:56.910 SINR: 15.74 dB

09:39:56.940 Cell 83: RSRP -65 dBm

09:39:57.070 SINR: 14.75 dB

09:39:57.140 Cell 83: RSRP -150 dBm Sudden Drop in RSRP of 83

09:39:57.180 RRC MeasurementReport: (83, RSRP=38) ; (82, RSRP=42)

09:39:57.210 RRC Conn Reconfig – Handover Command to weak cell 82

09:39:57.240 SINR: -52 dB

09:39:57.240 Cell 82: RSRP -150 dBm; Cell 83: RSRP -150 dBm

09:39:57.250 RRC Reconfig Complete: Handover Confirm

09:39:57.310 Cell 83: RSRP -65 dBm; Cell 82: RSRP -150 dBm 83 strong again

09:39:57.240 SINR: -8.79 dB:

09:39:58.840 SINR: -17.45 dB

:

09:39:59.210 Radio Problem Cause (T304 Expiry) - Call drops

42 = ~ -98 dBm RSRP

Momentary plunge in 83’s RSRP -> causes HO to weak cell 82 -> leads to the drop call

What could causeUE to lose 83

momentarily ?

MR could not be sent out in timeb f DL SINR d d t




Failure – 2: HO Failure On Source - SRMax

before DL SINR degrades to apoint the call drops; strong RSSI

Last DL and UL grantbefore the dropped call– MR not ready by then




Failure – 2: HO Failure On Source - SRMax - Timeline


09:46:56.630 SINR 4.24

:


09:46:58.470 SINR -0.80


09:46:58.630 SINR 0.92

09:46:58.790 SINR -7.6


09:46:58.950 SINR 4.42


09:46:59.110 SINR 3.45


09:46:59.270 SINR -3.82


09:46:59.430 SINR -10.50

09:46:59.590 SINR -7.08


09:46:59.750 SINR -4.60


09:46:59.830 RRC MeasurementReport: (81, RSRP=69) ; (82, RSRP=72) Too late to send MR?

09:46:59.910 SINR -44.56

09:46:59.920 Radio Problem Cause (SRMax)

MR should be sent out aroundhere before SINR degrades

too much?






Sudden / Momentary

drop in RSRP on 82 causing UE to send MR toa weak Cell ID 81








Sudden / Momentary

drop in RSRP on 83 causing UE to send MR toa weak Cell ID 82



MR could not be sent out in timebefore DL SINR degrades to a





before DL SINR degrades to apoint the call drops; strong RSSI

Last DL and UL grant

before the dropped call– MR not ready by then

Both source and Target RSRPs drop to -150dBmmomentarily just around the time of HO

MR could not be sent out in timebefore DL SINR degrades to a point





before DL SINR degrades to a pointthere are no Grants decoded leading

to drop call; strong RSSI


before the dropped call– MR not ready by then









before the dropped call– No MR sent









before the dropped call– No MR sent






p gcell. RSSI strong. Call drops because


Sudden / Momentarydrop in RSRP on 112

causing UE to send MR toa weak Cell ID 111






p gcell. RSSI strong. Call drops because


Sudden / Momentarydrop in RSRP on 112

causing UE to send MR toa weak Cell ID 113






Last UL grant beforethe dropped call – MR

not ready by then



First MR due to momentary / sudden drop inRSRP of 113. Results in HO to 112. Next

when 113 becomes strong again, UEattempts to send MR to another weak pilot

82. The MR never gets sent out.




•Layer 3 Analysis Using eDAT




Call Flow 1/2




Call Flow 2/2




Successful Handover Call




Handover Failure Call Flow




Successful PS Origination




Open Issues

The following Issues were observed during Phase 4 trial

Admission Control

UL Fairness

HO Performance

Throughputs drop

Stuck IMSI issue

Admission Control

o Ensures that enough radio resources are available to support the dedicated bearer

throughput requirements before a new user is allowed to establish connection to the same

sector

o This capability was not fully functional in the eNB build delivered to the trial hence it was

not demonstrated

o The feature is planned for validation as part of the LE2.0 release

UL Link Budget

UE: USB detach, crashes seen under certain Scenarios




Open Issues – UL Fairness

It was observed that with 2 to 4 active UEs, the PRB allocations between userswere not quite fair

We observe that this fairness issue is not seen with higher number of users undervariable radio conditions such as with mobility

The main reason for this behavior was identified as the non-flat nature of SRS SINRprofile that causes some skew in the priority metric computations of the scheduler,which in turn are used by the PRB allocation algorithm.

Additional distortion of the SRS signal from fading that may bias the priority metric

evaluated at eNB in field environments UL scheduler has very limited choices for the PRB allocation, hence any bias in the

scheduler metrics will result in a noticeable skew in PRB allocation and impactfairness.

With larger number of users, PRB allocations are less restrictive and biases in themetrics are not apparent.




Open Issues – HO Performance

High Handover Failure Rate under 100% loading

PDCCH Performance at low SINR

– PDCCH has errors or UE could not decode under marginal conditions

– Signal quality on serving sector degrade noticeably as there is delay in detecting and

reporting the candidates

UE measurement performance at low SINR

RF Coverage Issues

Implementation of Aggregation Level 8 has been identified as key enhancement for

PDCCH performance. This is plan for LE2.0

Further enhancement to PDCCH Power Control introduced in LA1.1 will be madeavailable in LE2.0

Further UE enhancements will also be required to improve performance.




Open Issues – Throughput Drops

Bi-directional TCP traffic & Fairness with Bi-directional FTP

DL scheduler fairness and throughput with TCP traffic impacted by bi-directional traffic

TCP Acks and bearer data flowing in the UL direction potentially was causing the server to

throttle down on the bearer data going in the DL direction

Test executed with UDP traffic to evaluate the fairness aspect and RB utilization is quite

uniform indicating good fairness

Flow that suffers throughput degradation with bi-directional TCP can be improved to the

same level as if it is by itself through the TCP window size change




Open Issues – Stuck IMSI

Some error conditions causes the call status between the UE, MME andSGW to be out of sync leading to call processing failures.

eNB and MME could have released the UE context, but the SGW is still in astate where it has an active context for the same UE

UE tries to attach again, SGW would reject it due to an existing session.

Work around was to “CLEAR” the UE IMSI before UE could attach again

Mismatch in status between network elements is currently under test andis planned for LE2.0

Uplink Link Budget Uplink MCS and PRB would gracefully reduce to the minimum supported

value before the link breaks resulting in a call drop

UL MCS did not reach the minimum before the uplink performancedegraded and the call ultimately dropped.

This reduced the achievable range The issue remains under investigation

UE

USB detach, crashes seen under certain scenarios




RSRP & RSRQ Measurement Report Mapping



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