123107231 gsm rf optimization imp haaaaaaaaaaaaaaaaay

17/7/2005 1 Company Confidential


2

Troubleshooting

Blocked Calls

Poor Quality and Drop calls

Abnormal Handovers

Interference

Termination Failures


3

Blocked Call Troubleshooting


4

Blocked Calls

Blocked Calls can occur due to :

•Access Failures

•SDCCH Congestion

•SDCCH Drop

•TCH Congestion

Trouble shooting cause :

•Use Layer 3 messages to analyze the cause

•Decode System Information Type 3 messages.

•Note the parameter , “max_retransmission” ;

“CCCH CONF” and “BS_AG_BLKS_RES”


5

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 the

Location 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 interface

there are two cases of Paging Failure, either the Mobile receives no Paging message or it

receives a Paging message, but is not able to respond (not able to send a RACH) which could be

due errors in the Paging message.

* Access Failure

Irrespective of the purpose, for any communication required with the network, a mobile

sends a Channel Request (for SDCCH) on a RACH and waits for some time for a response which

should 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 cell

reselection. 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 from

the 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 Assignment

Command to a TCH so that speech can commence. If no Assignment occurs for a specific period

and the Mobile has to return to idle mode, then it is due to TCH congestion.


6

Blocked Call Analysis - L3 messages

1 3

Channel Request Channel Request

RACH .

: Imm Assignment

RACH

max_retrans Service Request

NO RESPONSE FROM N/W

ACCESS FAILURE ! Signalling

:

Signalling

2

NO TCH ASSIGNMENT

Channel Request Mobile Returns To Idle

RACH TCH BLOCKED !

Imm Assign Reject

SDCCH BLOCKED !


7

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


8

Access Failure - Cause Troubleshooting

No Downlink Message after Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR Channel Request

RR System Information 1

RR Paging Request Type 2


RR System Information

1st RACH Transmission

Multiple Re- Transmissions

( number depends on max_retrans)

Phone back to Idle Mode

Call Blocked !!!

Access Failure !!!

Causes : Downlink problem ( Coverage Hole / Interference)


9

Access Failure - Troubleshooting

Access Failure - Uplink Problem

Causes:

1. AGCH Overload at Base Station

2. RACH Collisions

3. MS out of Range

4. Poor Uplink quality

5. BTS Receiver Problem


10


Access Failure - Uplink Problem -- Identification

RR Channel Request


RR Immediate Assigment

RR Sytem Information Type 3

RR Channel Request

RR Immediate Assignment

RR Immediate Assignment Extended


RR Channel Request








Return to Idle State

Blocked Call !!!

Access Failure !!!

Cause : Downlink Messages are seen after Channel Request.

Problem suspected in Uplink !!!!!

Channel Request

max retransmissions


11


Uplink Problem - Cause : AGCH Overloading

RR Channel Request



RR Channel Request



RR Channel Request








Multiple Immediate

Assignment’s seen after

Channel Requests


12


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 RACH’s with invalid establishment cause


13


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 ID’s 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 problem

with Max Range

•If both the above conditions don’t meet, then problem is Non-Detection


14


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 RACH’s” high, problem

could


15


Downlink

Fine

Analyze

OMC Data

No of Invalid

RACH’s

Interference

on Idle Channels

SDCCH/TCH

RF Losses

High High Uplink

Intrfer Yes Yes

Uplink

Imbalance

No

Link Imbalance

Test

UL-Interference

Test

No

High

Problem

Found

Identify Cause

Troubleshoot Yes

BTS TRX

BTS Testing

No

Abis

Monitoring

No

Yes

RACH Non-Detection


16

Now let us go a step further in understanding the most probable causes behind call block

problems.

* Access Failures

It could simply be caused by coverage holes . Interference could however play an

important 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 SDCCH

resources. Interference can block the channels , so though resources are available they may

not be able to be used.

* TCH Blocked

Heavy 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 CCCH

configuration parameters . For TCH and SDCCH congestion, the hot spots need to be identified

and load sharing techniques implemented. Some techniques that have been used successfully

involve adjusting cell powers to vary the coverage and therefore the location where mobiles will

handover from one cell to the next.

Interference management is essential for optimum network performance. Location updates can

be optimized by independent drive tests on the ALL BCH carriers. The delta is measured of

each BCH with the current serving BCH and the Reselect Hysteresis parameters adjusted

appropriately.


17

Blocked Call Analysis

SDCCH Congestion Cause

Location Updates

to be analysed with OMC statistics first.

If high, determine the source to target cell ratio

Drive around the suspected area in the Idle Mode

Configure “Delta LAC < > Constant 0” alarms

Optimize Location Updates

Interference

Analyze OMC statistics on “ Idle Channel Interference”

Carry out Uplink Interference Measurements using Viper

Heavy Traffic

Verify from OMC statistics SDCCH Congestion

Carry Call Setup Time measurements

Optimize set up time if high, else modify channel configuration


18

Blocked Call

Solutions To Blocked Calls

Optimize coverage

Optimize Cell loading

Interference management

Channel configurations

Optimize 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.


19

Blocked Call - Interference

•Base Station Measures Uplink Interference on Idle Timeslots

•At 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 dbm

Interference Band “2” = - 99 to -90 dbm



Interference Band “5” = -59 dbm and above

•Network will assign Timeslots starting from lower band

•Interference Band “5” Timeslots are considered as “BLOCKED”

•OMC reports Hourly average statistics for each timeslot.


20

Timeslot - Testing

•Activate Cell Barring from OMC.

•Remove this cell from the neighbor list of

other cells.

•Get the cell configuration

•ARFCN’s and Timeslots configured for

TCH.

•For BCH carrier select the Timeslot and

carry out the Testing

•For TCH Carriers: Block the BCH Timeslots

from OMC

•Carry out Timeslot testing.

•If more than 1 TCH Carrier is activated,

block all others .


21

Congestion Relief - Redirection

•Most of the manufacturers now provide this feature

BSC

MSC

Channel Request

Immd Assign

MM/CC Signaling

Assignment Request

ALL TCH’s Busy

Allocates a Free TCH

Assign Command

*** Some of the systems may also do handover existing calls to strong

neighbors and assign TCH for this call from the same cell


22

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 how

many times this happens.

•Drive Testing may also be required to optimize the neighbor list for

effectiveness of this feature.


23

Dropped Call Troubleshooting


24

Call drops are identified through SACCH messages. A Radio Link Failure Counter value is

broadcast on the BCH. The counter value may vary from network to network. At the

establishment 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 every

FER (unrecoverable block of data) detected on the SACCH and increases the counter by 2 for

every data block that is correctly received (up to the initial maximum value). If this counter

reaches 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 it

happens on a TCH, it is a TCH drop.

Sometimes an attempted handover, which may in itself have been an attempt to prevent a

drop, can result in a dropped call.

When the quality drops, a mobile is usually commanded to perform a handover. Sometimes

however, 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. At

this stage, if the handover was attempted at the survival threshold, the call may get dropped

anyway. If on the other hand the thresholds were somewhat higher, the network can attempt

another handover.


25

Drop Calls Analysis

1 2

Channel Request Channel Request

Imm Assignment Imm Assignment

Service Request Service Request

Signalling SDCCH Signalling

: :

Signalling Speech

TCH

RLT = 0 ; DROPS RLT = 0 ; DROPS

SDCCH DROP ! TCH DROP !

3 SDCCH / TCH

Handover Command

Hand Access

Handover Failure

HANDOVER FAILURE DROP !


26

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 handover

parameters

– Effective Frequency Hopping

– Use of DTX & Power control


27

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

* Coverage

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

and will result into call drops.

* Interference

This 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 will

be dropped before a handover can be initiated.. Multipath interference can also add to the

problems. Strong signal reflections result in time dispersion issues resulting in a large

coherence bandwidth.

* Network initiated drops

Certain 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 reducing

interference, setting appropriate Handover Margins , thresholds for handovers and the

correct selection of neighbors. Use of DTX and dynamic downlink power control will also

reduce average interference which should lead to some improvements.


28

Drop Call - Troubleshooting

RLT=0

RxQual > 4

YES

YES

YES

YES

Troubleshoot

Multipath/

Co-Channel/

BSIC decode

TCH=BCH Pri/Int <= 9

C/IAa <= -9 C/IAb <= -9

Co-Channel /

Multipath

Interference!! No

YES

No

No Adjacent Channel

Interference!!

YES YES

Troubleshoot

Source Cell

Using

BCH Analyzer

L3 Message

Analysis

No

Rx_Lev < -95

No **

Coverage Hole

!!!

Uplink

Problem !!!

No

Troubleshoot

Link Imbalance

UL-Interference

TCH -Pri/Int <= 9

Optimize

Neighbors/

Coverage

YES

No

** If TCH is Hopping, then all causes needs to be verified further,

before concluding on the root cause and optimizing.

YES

DROP

CALL

Handover Attempted ?

Handover Failed ?

Area of concern

No

Multipath No


29

•SDCCH Drop - Coverage

•SDCCH Drop - Co- Channel Interference

•SDCCH Drop - Adjacent Channel Interference

•SDCCH Drop - Uplink Problem

•TCH Drop - Coverage

•TCH Drop - Co-Channel Interference

•TCH Drop - Adjacent Channel Interference

•TCH Drop - Uplink Problem

•Handover Failure


30

Poor Quality

•Poor Speech Quality could be due to

•Patchy Coverage ( holes)

•No Target cell for Handover

•Echo , Audio holes, Voice Clipping

•Interference ---:

•Co-channel

•Adjacent channel

•External

•Multipath

•Noise


31

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 switching

system.

Audio holes : Blank period of speech, due to malfunctioning of Transcoder

boards or PCM circuits.

Voice Clipping : Occurs due to improper implementation of DTX.

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


32

Speech Quality Parameters

Mean Opinion Score

Criteria for Voice Quality : A set value “x” at which “y” percent of customers

rate 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)


33

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 algorithm

using the Alarms in the HP E74XX systems for the following elements, an

example 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


34

Speech Quality - Troubleshooting

RxQual> 6

FER > 1%

YES

YES

YES

YES

Troubleshoot

Multipath/

Co-Channel/

BSIC decode

TCH=BCH Pri/Int <= 9

C/IAa <= -9 C/IAb <= -9

Co-Channel

Interference!!

No

YES

No

No Adjacent Channel

Interference!!

YES YES

Troubleshoot

Source Cell

Using

BCH Analyzer

No

Rx_Lev < -95

No **

Coverage

Hole !!!

Uplink

Problem !!!

Troubleshoot

Link Imbalance

UL-Interference

TCH -Pri/Int <= 9

Optimize

Neighbors/

Coverage

YES

No

** If TCH is in Hopping, then all interference causes needs to be verified further,

before concluding on the root cause and optimizing.

YES

Poor

Quality

Area of concern

Hopping ON

YES

RLT going down

DTX_DL ON

FER > 2% YES

No No

No No

Transmission/

Switching Systems

Transcoder

Patchy Quality

Multipath

High Probability

with Hopping ON

YES

No A

A

B B

A

MOS < 4


35

Troubleshooting Poor Speech Quality

Method 1 : Post Processing

Create the Following PLAN Wizards in Export

Quality-PHONEDATA

Rx_Lev

Rx_Qual

FER

RLT current Value

Timing Advance

Tx Level

ARFCN ( BCH and TCH)

Serving Cell Information

-- CellID

Quality-Cell Information

Serving Cell Information

-- CellID

-- BSIC

Neighbor Cell Measurements

Neighbor Meas Count()

Timeslot

ARFCN

Interference

Co-Channel Interference

-- Channel

-- Primary/Interferer

-- Primary BSIC

-- Secondary BSIC

-- Textual Meas Status

-- Numeric Meas Status

-- Fading

Adjacent Channel Interf

-- AdjA Channel

-- AdjA N/N-1

-- AdjA N/N+1

-- AdjB Channel

-- AdB N/N-1

-- AdjB N/N+1

ARFCN


36

Filtered Spots- RxQual > 4

Bad Spot 1


37

Troubleshooting - Bad Spot 1


38

Which is this CELL ??

•Select QualPhoneData Layer.

•Check on Labels

•Select Labels

•Select CellID.

•Display it on the MAP


39

Which is the cell ? 47450 !!


40

Did the Call Drop ???

•Create RLT.tab

Query for RLT < 8

•Import as Label

Layer

RLT = 0 , DROP CALL !!


41

Conclusion

•BAD Spot 1 has poor quality and Call Drop

•This spot is covered by Cell 47450

•Poor Coverage. Level below -97 dbm

•But Call should not Drop

•The 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


42

Troubleshooting Handover Problems


43

Troubleshooting - No Handover

Create a Handover PLAN

Total Attempted Calls

Total Dropped Calls

Total Blocked Calls

RxQual Full

RxLeve Full

RLT Current Value

ARFCN

Neighbor Cell Measurements

RR Message

Phone State

Sequency Number

Weak Neighbors - Reported by Phone


44

Analyzing - No Handover

No Neighbor( very weak)


45

CH 40

CH 35

CH 27

CH 88

CH 29

CH 35

CH 98

CH 72 BSIC

23

BSIC 16

BSIC

22

BSIC 75

BSIC 15

BSIC 21

BSIC

57

BSIC 53

CH 25 BSIC

17 PHONE REPORTS CH RxQual RxLev

27 1 -80

35 -85

40 -83

25 -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 Neighbor

“Subset BCH TOP N not-subset Value ARFCN”

Missing Neighbors

Real Time


46

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

Missing Neighbor as such doesn’t means that there is no neighbor. We

define neighbors to a cell based on the geography, I.e which base stations

are nearby. In certain cases we may also use the Planning Tools to predict

neighbors. It is such not desirable to use excessive neighbors also, since

this reduces the samples collected per neighbor. Since Mobile has to look

for each neighbor in one TDMA frame, and we have 104 frames for

measurement. This means if we define 32 neighbors , the number of

samples per neighbor will be only 3-4, which means the authenticity of

Handovers will not be there. So we need to define appropriate neighbors

only. In certain cases the neighbors we define may not be able to cope up

with traffic or at certain significant patches of the cell, these neighbors

signals would be weak and may result into no handovers or handover

failures. So in such kind of problem spots , we should be see the possibility

that signals from any other cell is coming or not, so that we can define that

as a neighbor. So how do we find such missing neighbors.


47

Troubleshooting - Handover Parameter Settings

•Decide the Target Cell for handover in the Trouble Spot

•Filter the Handover.txt file on Handover Attempts ( before AND after numbers)

•Filter again on Neighbor ARFCN = Target Cell ARFCN

•Create another column as HO_Margin , with Delta for Neighbor_Level to RxLev

•Plot this on the MAP and see wether Handover Margin can be reduced to improve

quality OR increased to avoid Ping-Pong effects

•If handover margin settings are proper, and still handover is not occurring then

it could be a problem with Handover decision and processing parameters at the

BSC.


48

Interference Measurements and

Troubleshooting


49

Types of Interference

We already know ….

Co-Channel

•BCH - BCH

•TCH - BCH

•BCH - TCH

•TCH - TCH

Adjacent Channel Interference

•BCH - BCH

•TCH - BCH

•BCH - TCH

•TCH - TCH

Multipath


50

TroubleShooting


51

Uplink Problems


52

Locating Uplink quality Bad Spots

RX_Level is near P_Con Threshold

Ms Tx Level is max ( 5)


53

Locating Uplink Bad Quality Spots.

Trend shows:

High receive level

And High Ms Tx Levels

in this area.


54

Locating Uplink Bad Quality Spots

•At some areas for low signal level ( < -88), MS at full power

•In 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 or

Quality is poor.

•Link Imbalance will result into low uplink level even if DL Level is

good.

•Then MS will be at high power in all those spots where level is

good. 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 !!!!


55

Troubleshooting - Uplink Interference

•Uplink Interference can be due to:

• Mobiles in reuse and adjacent channel cells

• Spurs and Intermods from base stations

• External sources

•Interference due to Mobiles will be bursty and intermittent.

•The behavior and its effect on quality will be time dependent.

•More interference during heavy traffic hours.

•Interference from external sources can be continous or also

time dependent if the source is not continously ON.

•Uplink Interference measurement needs long term monitoring ,

collection of data, processing the data and estimating the

probability of interference , and also estimating the source.


56

Termination Failures Troubleshooting


57

Troubleshooting - Termination Failures

•Termination Failures could be due to:

•Paging Problems with Network

•No response to page from MS

•Blocking of Resources

•If Paging Success Rate = 100 %, but still the caller reports

termination failures, then this Blocking.

•If Paging Success Rate < 100 % ,then this could be :

- Paging Problems with Network Or

- No response to page from MS


58

Troubleshooting-Termination Failures

•Verify and Isolate the problem as:

•Interference

•Excessive Cell Reselection

•Excessive Location Updates

•None of above indicates Network Problem ( no paging)

•Excessive Cell Reselection and Location Updates can occur due to

•Interference ( Downlink signaling Counter reaches 0)

•BCH Pollution

•Overlapping of Location Areas.

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

problem further


59

Cell Optimization


60

Optimization

Cell Loading

C1

C2C3

•Cells C1 and C2 are reporting Blocking

•Cell C3 is added to cater for capacity

•Still C1 and C2 report blocking. Why ?????


61

Optimizing - Cell Loading

More Traffic could be persuaded towards C3 by :

•Increasing the Power Output of C3 BTS

•Tilting Antenna’s towards Hot spots in C1 and/or C2

•Applying Reselect Offsets to certain less loaded neighbor cells

With this approach more MS's will camp on or do cell reselection to C3, thus

reduce load in C1 & C2.

These issues are very common to fast growing networks, because new cells

when are planned , we don't know the HOT SPOTS.

In most cases , new cells investment could also be minimized by understand

the RF environment and optimizing cell loading.


62

Optimizing - New Cell Site Location & Antenna Tilts

•New cell sites in the earlier phase of the network are implemented to

enhance coverage.

•Coverage enhancements takes priority over areas of predicted traffic

•Once coverage objectives are met, cell expansion takes place purely

on the basis of traffic.

•New BTS are sited near to the cells which are generating blocking

conditions.

•The objective behind new cell site investment is to take the load from

the existing congested cells and hence increase revenue.

•An existing congested cell would be covering an area of 1 sqKm.

•Traffic in this area would be concentrated in small patches known as

“HOT SPOTS”.

•Network Planner always intends to optimize the cell site location or the

antenna tilts to target on some of the Hot spots .

•But the question is do we precisely know where are the Hot spots ???


63

Optimizing - Cell Reselections

•Mobile Monitors the Neighbor cells BCCH Carriers and decodes SCH

of the neighbors in the idle mode.

•The list of the BCCH ARFCN’s 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 per

neighbor to the calculate the value of C1.

•More neighbors of good and near to equal power levels will result into

excessive reselections.

•Excessive reselections can result into missing of paging messages

•Solution to this problem would be to adjust power levels of base

stations.

•Adjusting power levels most of the time either effects coverage or

enhances interference, since the terrain is not flat.

•To optimize cell reselections, the best possible solutions are:

• Optimizing the BA list

•Implement C2 Reselections


64

Optimizing - Neighbor lists

•Maximum 32 neighbor

•Less samples, averaged reported value not authentic

•Results in Ping-Pong effect of Handovers

•Neighbor list must be optimized


65

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 BCH’s

Mobile has to report the measurements of the neighbors to the network , every

480ms which corresponds to 104 TDMA frames.

Mobile will take several samples of the neighbors, average it and then send the

report as one value of Rxlev of the neighbor, with a minimum requirement of

measuring at least 1 neighbor in 1 TDMA Frame.

So , more the BCH’s 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 per

neighbor with a list of 8 neighbors

If the samples collected are less, then average value reported will not be

authentic, and can result into Ping-Pong effect of handovers.

So, the Neighbor list must be optimized with the minimum best possible

neighbors


66

Optimizing -Handover Traffic

A B

C D


67

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 with

heavy mobility traffic coming from Cell A.

Instead of implementing an excess TRX or a new cell to resolve this issue, we

can optimize this problem, we can optimize the neigbbor lists, so as to drag the

Mobiles coming from A to handover to cells other than B, unless B is the only

candidate for handover.

We can drive through the area between Cell A and Cell B and mark the spots

where handover occurs.

Look for the continuity between BCH’s. This will indicate to us, which are the

strong BCH’s other than Cell B which could be potential targets for handovers

and with what handover margins, so in case we don’t get an alternative for a

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

handover margin .


68

Optimizing - Handover Margin

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

handover to the cell, only when this cells power budget balance

exceeds the source cells power budget balance by this value.

• Handover Margins main objective is to avoid ping-pong effects of

handovers on cell periphery, when the power levels of two cells are

near to equal , so due to multipath and mobility, this will result into

frequent 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, and

ensure that when handover takes place, the probability of the mobile

going back to the source cell is very low for some significant period.

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


69

Adjacent Channel Interference Reduction

Tight Re-use patterns can permit the use of Adjacent ARFCN in

adjacent cell.

This should not pose an problem if Handover margin is set

appropriately low.

If Handover Margin is set to 6 dB, Handover will occur from C1 to

C2 at spot B itself

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

A B C

C1 C2



70

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



71


• Selecting an appropriate value of handover margin

is essential to ensure good quality communication .

• Handover margin set for a cell is applicable to all

calls going on in the cells, to which this cell is a

neighbor.


72

Optimization for Interference

After drive test - Co-Channel benchmarking, we know which

cells are generating severe co-channel problem

We also know by decoding BSIC , the interefering source

Following processes could be adopted to optimize

interference

---- Power Control

---- Antenna Tilts

---- Frequency Reallocations

---- Transmitter Tests

---- Mobile dispatch inspection

---- Space Diversity

---- Frequency Hopping /DTX


73

In order to optimize interference the cells which are generating high level of

interference should be located from the OMC Performance Database. It is essential

to 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 or

spurious. This can be located by analyzing the spectrum. There are various

methods involved in optimizing interference. Appropriate Power Control of the

interfering entities could be done. For Co-channel Interference problems, Tilting of

antenna could be done with proper coverage aspects, for adjacent channel analysis

should 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 out

Transmitter and Mobile tests for Power v/s time output spectrum and inter-

modulation products, generally a mobile should be tested for these basic tests

before being dispatched. In the Uplink Space Diversity is the worldwide adopted

method for reducing the effect of Multipath fading, where as in the downlink it is

the mobiles equalizer which has to do this job. One very relevant solution to

interference is frequency hopping where the mobile constantly changes frequency

for every burst which reduces the effect of interference. Implementation of

DTX.DRX also reduces average interference.


74

Co-Channel Optimization

Static Power Control

Antenna Tilting

Trade Off with EC/No

Frequency Reallocation

Implementing Features like

Dynamic Power Control, DTX and

Frequency Hopping


75

Antenna Tilts

x 6 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.

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