8 RF Optimisation

8/2/2019 8 RF Optimisation

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17/7/2005 1Company Confidential


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Troubleshooting

Blocked CallsPoor Quality and Drop callsAbnormal HandoversInterferenceTermination Failures


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Blocked Call

Troubleshooting


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Blocked Calls

Blocked Calls can occur due to :Access FailuresSDCCH CongestionSDCCH DropTCH Congestion

Trouble shooting cause :Use Layer 3 messages to analyze the causeDecode System Information Type 3 messages.

Note the parameter , max_retransmission ;CCCH CONF and BS_AG_BLKS_RES


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The best way of analyzing blocked calls, to identify the cause, is from a Layer III protocol log.* Paging Failure

A paging message always originates from the MSC and is sent to all the BSCs in theLocation Area of the MS to be paged. The BSC will then calculate the Paging group of the MS and

send a Paging Command to the BTSs controlling the Location Area of the MS. On the air interfacethere are two cases of Paging Failure, either the Mobile receives no Paging message or itreceives a Paging message, but is not able to respond (not able to send a RACH) which could bedue errors in the Paging message.* Access Failure

Irrespective of the purpose, for any communication required with the network, a mobilesends a Channel Request (for SDCCH) on a RACH and waits for some time for a response whichshould come from the BTS on an AGCH. A mobile will do several retransmission of RACHs (pre-defined) and if it still does not get a response, it goes back to idle mode and preferably does a cellreselection. At this stage we call it an Access Failure.* SDCCH Blocked

Once a mobile has sent a Channel Request on a RACH , it expects a response fromthe BTS on the AGCH. This should be an Immediate Assignment Command to an SDCCH. If an

Immediate Assignment Reject comes instead , then this is SDCCH blocking.* TCH Blocked

After the completion of call set-up signaling, a mobile expects an AssignmentCommand to a TCH so that speech can commence. If no Assignment occurs for a specific periodand the Mobile has to return to idle mode, then it is due to TCH congestion.


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Blocked Call Analysis - L3 messages

1 3

Channel Request Channel Request

RACH .

: Imm Assignment

RACH

max_retrans Service Request

NO RESPONSE FROM N/WACCESS FAILURE ! Signalling

:

Signalling

2

NO TCH ASSIGNMENT

Channel Request Mobile Returns To Idle

RACH TCH BLOCKED !

Imm Assign Reject

SDCCH BLOCKED !


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Blocked Call - Cause troubleshooting

Access Failures

- CCCH Overload at the Base Station

- Uplink Interference at the Base Station

- Low Rxlev at the Base Station

- Base Station TRX decoder malfunctioning

- Downlink Low Rxlev ( Coverage Hole )

- Downlink Interference

- Excess Cell Range


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Access Failure - Cause Troubleshooting

No Downlink Message after Channel Request

RR Channel RequestRR Channel RequestRR Channel RequestRR Channel Request

RR Channel RequestRR Channel RequestRR Channel RequestRR Channel RequestRR System Information 1RR Paging Request Type 2RR Paging Request Type 2RR System Information

1st RACH Transmission

Multiple Re- Transmissions

( number depends on max_retrans)

Phone back to Idle ModeCall Blocked !!!Access Failure !!!

Causes : Downlink problem ( Coverage Hole / Interference)


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Access Failure - Troubleshooting

Access Failure - Uplink Problem

Causes:

1. AGCH Overload at Base Station2. RACH Collisions3. MS out of Range4. Poor Uplink quality5. BTS Receiver Problem


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Access Failure - Uplink Problem -- Identification

RR Channel RequestRR Paging Request Type 1RR Immediate AssigmentRR Sytem Information Type 3RR Channel Request

RR Immediate AssignmentRR Immediate Assignment ExtendedRR Paging Request Type 1RR Channel RequestRR Immediate AssignmentRR Immediate AssignmentRR Paging Request Type 1RR System Information 1RR Paging Request Type 2RR Paging Request Type 2RR System Information

Return to Idle State

Blocked Call !!!Access Failure !!!

Cause : Downlink Messages are seen after Channel Request.Problem suspected in Uplink !!!!!

Channel Request

max retransmissions


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Uplink Problem - Cause : AGCH Overloading

RR Channel RequestRR Immediate Assignment ExtendedRR Immediate Assigment ExtendedRR Channel RequestRR Immediate Assignment Extended

RR Immediate Assignment ExtendedRR Channel RequestRR Immediate AssignmentRR Immediate Assignment ExtendedRR Paging Request Type 1RR System Information 1RR Paging Request Type 2RR Paging Request Type 2RR System Information

Multiple Immediate

Assignments seen afterChannel Requests


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AGCH Overloading - Root Cause Analysis

If Multiple Immediate Assignment Extended Messages are seen,problem could be AGCH overloading OR RACH Collisions/Non-detection

If max_retrans and Tx-Integer are set to a lower value,problem could be more towards RACH Collisions/ Non-detection

If set high, then possibility of overloading is high!!

Check for CCCH_CONF and BS_AG_BLKS_RES.If less blocks are reserved , problem is overloading.

Analyze the OMC data for the same period for the following stats:- No of Deletions- No of Successful RACHs- RACH Busy counts- No of RACHs with invalid establishment cause


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RACH Collisions/ Max Range - Root Cause Analysis

Max_Retrans and Tx-Integer set low - RACH Collisions Possible

Check for Distance from Base Station-- Plot a map for BCH ARFCN-- Export the Channel Request and CellID Label-- Import the Site IDs Raster Images on the Map-- Calculate distance between Channel Request and BTS-- Compare this distance with the Max_Distance_Allowed

set for thjs cell.-- Note: Max_Dist_Allowed is a BSC paramter ( not on Air )

-- If the MS distance is more than max_distance then problemwith Max Range

If both the above conditions dont meet, then problem is Non-Detection


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RACH Non-Detection - Root Cause Analysis

Downlink is fine !!! Parameters are well set for RACH control !!!

Problem could be Uplink Quality / Base Station

Analyze the following OMC Data-- No of Invalid RACHs-- Interference on Idle Channel-- SDCCH RF Loss / TCH RF Loss

If Interference and RF Losses are above normal, problem is Uplink Interference.

If RF Losses are high, but interference is low , problem is Uplink level

Uplink level poor indicates Link imbalance.

If all above conditions are satisfactory and still No of Invalid RACHs high, problem

could


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Downlink

Fine

AnalyzeOMC Data

No of InvalidRACHs

Interferenceon Idle Channels

SDCCH/TCHRF Losses

High HighUplinkIntrfer

YesYes

Uplink

Imbalance

No

Link ImbalanceTest

UL-InterferenceTest

No

High

ProblemFound

Identify CauseTroubleshoot

Yes

BTS TRXBTS Testing

No

AbisMonitoring

No

Yes

RACH Non-Detection


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Now let us go a step further in understanding the most probable causes behind call blockproblems.* Access Failures

It could simply be caused by coverage holes . Interference could however play animportant role. Uplink interference on a serving cell can result in RACH rejections and hence no

AGCH assignments. Improper channel distribution between AGCH and PCH (paging channel)can result in RACH/AGCH overloading. Paging Failures can be impacted by BCH pollution (co-channel and adjacent channel interference).* SDCCH Blocked

Heavy Traffic and excessive Location Updates can result in congestion of SDCCHresources. Interference can block the channels , so though resources are available they maynot be able to be used.

* TCH BlockedHeavy Traffic is the main cause of TCH congestion. The TCH can also be blocked

due to continuous interference in the uplink.

Solutions to access failures would be to ensure continuous coverage and optimization of CCCHconfiguration parameters . For TCH and SDCCH congestion, the hot spots need to be identified

and load sharing techniques implemented. Some techniques that have been used successfullyinvolve adjusting cell powers to vary the coverage and therefore the location where mobiles willhandover from one cell to the next.Interference management is essential for optimum network performance. Location updates canbe optimized by independent drive tests on the ALL BCH carriers. The delta is measured ofeach BCH with the current serving BCH and the Reselect Hysteresis parameters adjustedappropriately.


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Blocked Call Analysis

SDCCH Congestion Cause

Location Updatesto be analysed with OMC statistics first.If high, determine the source to target cell ratioDrive around the suspected area in the Idle ModeConfigure Delta LAC < > Constant 0 alarmsOptimize Location Updates

InterferenceAnalyze OMC statistics on Idle Channel InterferenceCarry out Uplink Interference Measurements using Viper

Heavy TrafficVerify from OMC statistics SDCCH CongestionCarry Call Setup Time measurementsOptimize set up time if high, else modify channel configuration


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Blocked Call

Solutions To Blocked CallsOptimize coverage

Optimize Cell loadingInterference managementChannel configurationsOptimize neighbors

TCH Blocked - Causes

Interference-- Verify Idle Channel Interference reports from OMC-- If suspected, carry out uplink interference measurements

Heavy Traffic-- Verify the TCH Holding time and no of attempts statistics from OMC-- During low traffic hours, Activate Cell barring in the cell-- Carry out Time slot testing , by setting Ignore Cell Barring.


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Blocked Call - Interference

Base Station Measures Uplink Interference on Idle TimeslotsAt regular intervals, categorizes Timeslots into Interference Bands.There are Five Interference Bands.Each Interference Band has a range of interference level.

Example : Interference Band 1 = -110 to -100 dbmInterference Band 2 = - 99 to -90 dbmInterference Band 3 = - 89 to -75 dbmInterference Band 4 = - 75 to -60 dbmInterference Band 5 = -59 dbm and above

Network will assign Timeslots starting from lower bandInterference Band 5 Timeslots are considered as BLOCKEDOMC reports Hourly average statistics for each timeslot.


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Timeslot - Testing

Activate Cell Barring from OMC.Remove this cell from the neighbor list ofother cells.

Get the cell configurationARFCNs and Timeslots configured for

TCH.

For BCH carrier select the Timeslot andcarry out the Testing

For TCH Carriers: Block the BCH Timeslotsfrom OMCCarry out Timeslot testing.If more than 1 TCH Carrier is activated,block all others .


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Congestion Relief - Redirection

Most of the manufacturers now provide this feature

BSC

MSC

Channel Request

Immd Assign

MM/CC Signaling

Assignment Request

ALL TCHs Busy

Allocates a Free TCH

Assign Command

*** Some of the systems may also do handover existing calls to strongneighbors and assign TCH for this call from the same cell


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Congestion Relief - Redirection

Role of Drive Testing

OMC statistics may not reveal the actual congestion in a cell.Drive Testing may be required in the Peak traffic hours to estimate howmany times this happens.Drive Testing may also be required to optimize the neighbor list foreffectiveness of this feature.


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Dropped CallTroubleshooting


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Call drops are identified through SACCH messages. A Radio Link Failure Counter value isbroadcast on the BCH. The counter value may vary from network to network. At theestablishment of a dedicated channel, the counter is set to the broadcast value (which will be

the maximum allowable for the connection). The mobile decrements the counter by 1 for everyFER (unrecoverable block of data) detected on the SACCH and increases the counter by 2 forevery data block that is correctly received (up to the initial maximum value). If this counterreaches zero, a radio link failure is declared by the mobile and it returns back to the idle mode.

If the counter reaches zero when the mobile is on a SDCCH then it is an SDCCH Drop. If ithappens on a TCH, it is a TCH drop.

Sometimes an attempted handover, which may in itself have been an attempt to prevent adrop, can result in a dropped call.

When the quality drops, a mobile is usually commanded to perform a handover. Sometimeshowever, when it attempts to handover, it finds that the target cell is not suitable. When this

happens it jumps back to the old cell and sends a Handover Failure message to the old cell. Atthis stage, if the handover was attempted at the survival threshold, the call may get droppedanyway. If on the other hand the thresholds were somewhat higher, the network can attemptanother handover.


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Drop Calls Analysis

1 2

Channel Request Channel Request

Imm Assignment Imm Assignment

Service Request Service Request

Signalling SDCCH Signalling

: :

Signalling SpeechTCH

RLT = 0 ; DROPS RLT = 0 ; DROPS

SDCCH DROP ! TCH DROP !

3 SDCCH / TCH

Handover Command

Hand Access

Handover Failure

HANDOVER FAILURE DROP !


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Dropped Call Analysis

SDCCH Drops - Causes Coverage

Interference & Multipath

BTS performance

TCH Drops - Causes

Coverage

Interference & Multipath

BTS performance

Pre-emption

Handover Failure - Causes

Threshold parameters

Missing neighbors

Solutions to Dropped Calls

Optimize Coverage

Interference Management

Optimize neighbors

Optimize handoverparameters

Effective Frequency Hopping

Use of DTX & Power control


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We will examine the potential causes behind call drops and some solutions to combat them.

* CoveragePoor non-contiguous coverage will reduce C/N and hence will reduce the Ec/No

and will result into call drops.

* InterferenceThis is one of the major causes of dropped calls. Interference could be co-channel,

adjacent channel or external. Under certain severe cell interference conditions, the call willbe dropped before a handover can be initiated.. Multipath interference can also add to theproblems. Strong signal reflections result in time dispersion issues resulting in a largecoherence bandwidth.

* Network initiated dropsCertain network features, like preemption, can kill an ordinary call to provide

connection to an emergency class subscriber.

A handover is the key to survival from dropping calls. But if there are problems in the

Handover process itself, then this will not avoid a drop.

Dropped calls can be effectively reduced by improving coverage, detecting and reducinginterference, setting appropriate Handover Margins , thresholds for handovers and thecorrect selection of neighbors. Use of DTX and dynamic downlink power control will alsoreduce average interference which should lead to some improvements.


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Drop Call - Troubleshooting

RLT=0

RxQual > 4

YES

YES

YES

YES

TroubleshootMultipath/

Co-Channel/BSIC decode

TCH=BCH Pri/Int


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Speech Quality Parameters

RxQUAL : Measured on the midamble.Indicates poor speech quality due to radio interface impairments

FER : Measured on the basis of BFI ( Ping -Pong effect on speech )Preferred under Frequency Hopping situation

Echo and distortion : Generally caused by the Transmission and switchingsystem.

Audio holes : Blank period of speech, due to malfunctioning of Transcoderboards or PCM circuits.

Voice Clipping : Occurs due to improper implementation of DTX.

Mean Opinion Score (MOS) : ITU standard for estimating speech quality.


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Speech Quality Parameters

Mean Opinion Score

Criteria for Voice Quality: A set value x at which y percent of customersrate the voice quality at Circuit Merits(CM) 4 - 5.

MOS Quality Scale

5 Excellent ( speech perfectly understandable)

4 Good ( speech easily understandable, some noise)

3 Fair ( speech understandable with a slight effort ,occasional repetitions needed)

2 Poor ( speech understandable only withconsiderable effort , frequent repetitions needed)

1 Unsatisfactory ( speech not understandable)


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Speech Quality - Estimation

Speech RxQual FER FH DTX Reason

BAD HIGH LOW NO NO Air Interface Impairments BAD HIGH HIGH NO NO Severe Air Interface Impairments

BAD LOW HIGH NO NO Transmission and Switching system , Transcoder

BAD LOW HIGH YES NO Air Interface Impairments

BAD GOOD HIGH YES NO Transmission and Switching system , Transcoder

GOOD HIGH LOW YES NO Hopping Implementation CLIPPING LOW HIGH YES YES Hopping Implementation and VAD

CLIPPING LOW LOW YES YES VAD

ECHO LOW LOW Y/N Y/N Transmission and Switching system , Transcoder

MOS up to a certain extent can easily estimated by configuring an algorithmusing the Alarms in the HP E74XX systems for the following elements, anexample of subset of which is illustrated above

- RxQual Full and Sub- RxLev Full and Sub- FER and RLT- L3 Measurement Report- L3 Handover specfic messages


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Speech Quality - Troubleshooting

RxQual> 6

FER > 1%

YES

YES

YES

YES

TroubleshootMultipath/

Co-Channel/BSIC decode

TCH=BCH Pri/Int


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Filtered Spots- RxQual > 4

Bad Spot 1


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Troubleshooting - Bad Spot 1


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Which is this CELL ??

Select QualPhoneData Layer.

Check on Labels

Select Labels

Select CellID.

Display it on the MAP


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Which is the cell ? 47450 !!


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Did the Call Drop ???

Create RLT.tabQuery for RLT < 8Import as LabelLayer

RLT = 0 , DROP CALL !!


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Conclusion

BAD Spot 1 has poor quality and Call DropThis spot is covered by Cell 47450Poor Coverage. Level below -97 dbmBut Call should not DropThe other Problem is Interference.

Mobile is Hopping on 99 and 84

99 is also the BCH.Co-Channel on BCH is very high.50% of the time quality will be poor.But Poor Quality is consistent.

Channel 84 is also suffering from Interference.No Adjacent Channel on 84 and 99

This means there is Co-Channel on 84 also.It could also be multipath issue on 84.

WHY NO HANDOVER


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Troubleshooting

Handover Problems


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Troubleshooting - No Handover

Create a Handover PLAN

Total Attempted CallsTotal Dropped CallsTotal Blocked CallsRxQual FullRxLeve Full

RLT Current ValueARFCNNeighbor Cell MeasurementsRR MessagePhone StateSequency Number

Weak Neighbors - Reported by Phone


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Analyzing - No Handover

No Neighbor( very weak)


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CH 40

CH 35

CH 27

CH 88

CH 29

CH 35

CH 98

CH 72BSIC23

BSIC 16

BSIC22

BSIC 75

BSIC 15

BSIC 21

BSIC57

BSIC 53

CH 25BSIC17PHONE REPORTS

CH RxQual RxLev27 1 -8035 -8540 -8325 -95

Frequency Re-use

'A' NET

'A' NET

'A' NET

'B' NET

'B' NET

'B' NET

'B' NET

'B' NET

'B' NET

Channel 29 is not in the neighbour list !

BCH Analyzer : TOP N = 7

Configure an Alarm for Missing NeighborSubset BCH TOP N not-subset Value ARFCN

Missing Neighbors

Real Time


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Missing Neighbor is a very critical problem in optimizing GSM neighbors.

Missing Neighbor as such doesnt means that there is no neighbor. Wedefine neighbors to a cell based on the geography, I.e which base stationsare nearby. In certain cases we may also use the Planning Tools to predictneighbors. It is such not desirable to use excessive neighbors also, sincethis reduces the samples collected per neighbor. Since Mobile has to lookfor each neighbor in one TDMA frame, and we have 104 frames for

measurement. This means if we define 32 neighbors , the number ofsamples per neighbor will be only 3-4, which means the authenticity ofHandovers will not be there. So we need to define appropriate neighborsonly. In certain cases the neighbors we define may not be able to cope upwith traffic or at certain significant patches of the cell, these neighborssignals would be weak and may result into no handovers or handover

failures. So in such kind of problem spots , we should be see the possibilitythat signals from any other cell is coming or not, so that we can define thatas a neighbor. So how do we find such missing neighbors.

T bl h i H d


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Troubleshooting - HandoverParameter Settings

Decide the Target Cell for handover in the Trouble SpotFilter the Handover.txt file on Handover Attempts ( before AND after numbers)Filter again on Neighbor ARFCN = Target Cell ARFCNCreate another column as HO_Margin , with Delta for Neighbor_Level to RxLevPlot this on the MAP and see wether Handover Margin can be reduced to improve

quality OR increased to avoid Ping-Pong effectsIf handover margin settings are proper, and still handover is not occurring thenit could be a problem with Handover decision and processing parameters at theBSC.


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Interference Measurementsand

Troubleshooting

T f I t f


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Types of Interference

We already know .

Co-Channel

BCH - BCHTCH - BCH

BCH - TCHTCH - TCH

Adjacent Channel Interference

BCH - BCHTCH - BCHBCH - TCHTCH - TCH

Multipath


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TroubleShooting


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Uplink Problems

L ti U li k lit B d


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Locating Uplink quality BadSpots

RX_Level is near P_Con Threshold

Ms Tx Level is max ( 5)

L ti g U li k B d Q lit


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Locating Uplink Bad QualitySpots.

Trend shows:High receive levelAnd High Ms Tx Levelsin this area.

Locating Uplink Bad Quality


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Locating Uplink Bad QualitySpots

At some areas for low signal level ( < -88), MS at full powerIn another area for high signal level ( -75 to -85 ) , MS full power

MS Tx Level is controlled by the network.Network will ask MS to increase power if Uplink level is poor orQuality is poor.

Link Imbalance will result into low uplink level even if DL Level isgood.Then MS will be at high power in all those spots where level isgood. BUT THIS IS NOT THE CASE !!! ( as seen from two spots)

This means this is a problem of Uplink quality being poor at good

level.

This can happen due to UPLINK INTERFERENCE.This interference seems to be burst in nature !!!!


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Termination FailuresTroubleshooting


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Troubleshooting - Termination Failures

Termination Failures could be due to:Paging Problems with NetworkNo response to page from MSBlocking of Resources

If Paging Success Rate = 100 %, but still the caller reportstermination failures, then this Blocking.

If Paging Success Rate < 100 % ,then this could be :- Paging Problems with Network Or

- No response to page from MS


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Troubleshooting-Termination Failures

Verify and Isolate the problem as:InterferenceExcessive Cell ReselectionExcessive Location UpdatesNone of above indicates Network Problem ( no paging)

Excessive Cell Reselection and Location Updates can occur due toInterference ( Downlink signaling Counter reaches 0)BCH PollutionOverlapping of Location Areas.

Carry out protocol analysis on Abis and A Interface to isolate network

problem further


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Cell Optimization


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Optimization

Cell Loading

C1

C3

Cells C1 and C2 are reporting BlockingCell C3 is added to cater for capacityStill C1 and C2 report blocking. Why ?????


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Optimizing - Cell Reselections

Mobile Monitors the Neighbor cells BCCH Carriers and decodes SCHof the neighbors in the idle mode.The list of the BCCH ARFCNs which the mobiles will monitor in the

idle mode is broadcasted in the System Information Type 2 Message .More the neighbors , less the samples taken by the mobile perneighbor to the calculate the value of C1.

More neighbors of good and near to equal power levels will result intoexcessive reselections.Excessive reselections can result into missing of paging messagesSolution to this problem would be to adjust power levels of basestations.Adjusting power levels most of the time either effects coverage orenhances interference, since the terrain is not flat.To optimize cell reselections, the best possible solutions are:

Optimizing the BA listImplement C2 Reselections


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Optimizing - Neighbor lists

Maximum 32 neighborLess samples, averaged reported value not authentic

Results in Ping-Pong effect of Handovers

Neighbor list must be optimized


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Mobile in the dedicated mode measures the BCH of the neighbor cells.The list of BCH to be measured is informed to the Mobile in the System

Information Type 5 message which can have maximum 32 neighbors BCHsMobile has to report the measurements of the neighbors to the network , every480ms which corresponds to 104 TDMA frames.Mobile will take several samples of the neighbors, average it and then send thereport as one value of Rxlev of the neighbor, with a minimum requirement ofmeasuring at least 1 neighbor in 1 TDMA Frame.

So , more the BCHs in the neighbor list, less will be the number of samples.If the Mobile just meets the minimum requirement, then for a neighbor list of 32,only 3 - 4 samples per neighbor will be available and 12-13 samples perneighbor with a list of 8 neighborsIf the samples collected are less, then average value reported will not beauthentic, and can result into Ping-Pong effect of handovers.So, the Neighbor list must be optimized with the minimum best possible

neighbors


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Optimizing -Handover Traffic

A B

C D


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As shown above, Cell B is a cell which is reporting Blocking due to heavy traffic.Cell B on OMC analysis is found to have traffic generated due to call setups

within the cell , as well as handovers, since it covers a major highway withheavy mobility traffic coming from Cell A.Instead of implementing an excess TRX or a new cell to resolve this issue, wecan optimize this problem, we can optimize the neigbbor lists, so as to drag theMobiles coming from A to handover to cells other than B, unless B is the onlycandidate for handover.

We can drive through the area between Cell A and Cell B and mark the spotswhere handover occurs.

Look for the continuity between BCHs. This will indicate to us, which are the

strong BCHs other than Cell B which could be potential targets for handovers

and with what handover margins, so in case we dont get an alternative for a

certain patch which is above the handover margin threshold, we can reduce the

handover margin .

Optimizing - Handover Margin


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Handover Margin is value set for all cells, which will allow mobiles to

handover to the cell, only when this cells power budget balanceexceeds the source cells power budget balance by this value.

Handover Margins main objective is to avoid ping-pong effects ofhandovers on cell periphery, when the power levels of two cells arenear to equal , so due to multipath and mobility, this will result intofrequent handovers between these two cells.

Setting this value low, will result into fast handover to the target cell,which may result in improvement in quality.

Setting this to high value will delay the handover to the target cell, andensure that when handover takes place, the probability of the mobilegoing back to the source cell is very low for some significant period.

Two examples of use of this parameter are illustrated ( next page)


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Adjacent Channel Interference Reduction

Tight Re-use patterns can permit the use of Adjacent ARFCN inadjacent cell.

This should not pose an problem if Handover margin is setappropriately low.

If Handover Margin is set to 6 dB, Handover will occur from C1 toC2 at spot B itself

C/Ia = -1 db C/Ia = -6 db C/Ia = -9 db

A B C

C1 C2



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H

Multipath solutions

Antenna Tilts Using Directional Antenna's

Shifting the BTS site

Handover Parameters

C1

C2

Reduce HO_margin

C3

Increase HO_margin

Reduce HO_margin



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Selecting an appropriate value of handover margin

is essential to ensure good quality communication .

Handover margin set for a cell is applicable to allcalls going on in the cells, to which this cell is aneighbor.


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In order to optimize interference the cells which are generating high level ofinterference should be located from the OMC Performance Database. It is essentialto locate the source of interference whether it is external or internal. If it is internal

then it could be due to multipath, adjacent, co-channel, inter-modulation orspurious. This can be located by analyzing the spectrum. There are variousmethods involved in optimizing interference. Appropriate Power Control of theinterfering entities could be done. For Co-channel Interference problems, Tilting ofantenna could be done with proper coverage aspects, for adjacent channel analysisshould be done to locate if any adjacent channel is allocated to the neighbors , If it

is then frequency allocation could help with this. It is very essential to carry outTransmitter and Mobile tests for Power v/s time output spectrum and inter-modulation products, generally a mobile should be tested for these basic testsbefore being dispatched. In the Uplink Space Diversity is the worldwide adoptedmethod for reducing the effect of Multipath fading, where as in the downlink it isthe mobiles equalizer which has to do this job. One very relevant solution tointerference is frequency hopping where the mobile constantly changes frequency

for every burst which reduces the effect of interference. Implementation ofDTX.DRX also reduces average interference.


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Co-Channel Optimization

Static Power Control

Antenna Tilting

Trade Off with EC/No

Frequency Reallocation

Implementing Features likeDynamic Power Control, DTX and

Frequency Hopping


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Antenna Tilts

x6 km 1 km

A

M N

50m

Point x is having problem of C/I from Cell M Estimate the C/I improvement required at Point x. Refer to the Antenna Vertical Pattern, and calculate the tilting angle required Example : To get an improvement of 3 dB , a tilt of 10 degrees is required.

Tilting of Antenna in certain cases may reduce coverage also. Tilting of Antenna should be done after proper study.

Documents

8 RF Optimisation