module 3 -rf optimisation module

8/14/2019 Module 3 -RF Optimisation Module

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VEDANG Radio Technology Pvt. Ltd .

105, Nirman Industrial EstateLink Road Malad (W)

Mumbai -400064


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Module 3- RF Optimization GSM


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Understanding RF Network Cycle

Why do we need optimization??

Optimization Stages

Physical and Hardware Optimization

Database parameter optimization


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Understanding the RF Network Cycle

RF Network Cycle

CW Drive Test

Model Tuning

RF Planning

Spreadsheet Design

Link Budget

RF Optimization

Parametric Optimization

Neighbor List

Site ParametersFrequency Planning

PN Planning

RF Site Survey

RF Drive Test

In-Building Solutions

Traffic Engineering

Expansion Planning

Benchmarking

Downlink / Voice Quality


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Spreadsheet Design ...

Usually done during Initial Network Build

Link budget to calculate the number of sites.

Calculations based on

subscriber density,

traffic per subscriber,

expected growth in traffic, etc.


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CW Drive Test/ Model Tuning...

Purpose

Model Tuning is used to

Accurately allocate the sites.

To achieve more accurate results from the prediction/simulation

tool deployed.

Identification of hotspots/special coverage requirement areas.

Tuned model can be used as a benchmark for future expansions.


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Model Tuning Process

Setup consists of Test transmitter for the particular band (GSM

900/1800) usually 20W Antenna Omni/Panel, cables, accessories.

One candidate chosen to represent each type of clutter area in

the network.

The clutter types could be urban, suburban, rural, etc.

The test transmitter is setup on a suitable rooftop.

Test frequency chosen and transmitted

Drive test is carried out using receiver or TEMS equipment set to

scan mode.



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Model Tuning Process

Data collected Rxlev samples aggregated over 30-50 m bins.

The Rxlev measurements are processed and input to theprediction tool.

Clutter offset and other parameters are corrected.

Corrections are made to achieve lowest possible Standard

Deviation values.

Thus we have a tuned model, which can be applied to other

areas which have the same clutter type.


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RF Planning

The inputs received from spreadsheet design and modeltuning surveys, is used to prepare a Nominal Cell Plan akaHi Level Design.

The HLD has the following details

Distribution of the sites across the agreed geographical area.

Coverage/Capacity objective details.

Type of antennas to be used, sites where special

hardware(TMA/MHA) is required, etc.


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RF Planning

The output of the HLD is search rings which is defined foreach site to be built in the network.

Each search ring will have

Nominal site coordinates,

Search radius and

Specifications about antenna height requirements for each site, iorder that the site objectives are reasonably achieved.

Search rings form a basis for further surveys to be carriedout to hunt for site candidates and identify suitable ones.


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RF Site Survey/Drive Testing

Using the inputs provided by the nominal cell plan, the RFteam performs

Surveys for each search ring in the network to identify the

suitable candidates which can be used for building the sites.

Candidates identified are ranked on basis of their RF suitability

and other parameters such as structural stability, line of sight

clearance(for Tx), accessibility, costs, etc. Drive testing may be carried out in some cases, to assess the RF

suitability

Once suitable candidate(s) is identified..acquisition begins!!!

RF Pl i Th REAL Ch ll !!!


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RF Planning The REAL Challenge!!!

Acquisition of ideal candidate poses a real challenge to thenetwork design process.

More often than not candidates which are lower on priorityin terms of RF suitability are the ones which get acquired!!

Often due to acquisition constraints, search rings need to bemodified and sometimes even the nominal plan needs to bechanged.

Thus as an end result the network built is deviated from the

one which was originally designed in the nominal plan.@!@!!!!$

F Pl i


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Frequency Planning

GSM works on a frequency reuse pattern.

As the sites get acquired and the build process starts, theRF planners prepare a frequency plan for the network.

Different techniques available for frequency plan a) Fixed

Plan, b) Hopping Plan further divided into Baseband Hoppingand Synthesized Frequency Hopping

RF Planners either manually or by the use of anAFP(Automatic Frequency Planner) create a frequency plan

for the network.


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Frequency Planning

An optimal frequency is critical to ensure good RFperformance of the network.

Spectral challenges

Limited band allocation

Fast growth rate of subscribers/ traffic growth

Tighter reuse patterns


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RF Optimization/Parametric Optimization

During the network build initial RF optimization is done, toensure that the sites built are reasonably meeting theirobjectives.

During the network build phase it is also ensured that optimaparameter settings are done for all sites to ensure good

performance.

Detailed explanation of the above to follow!!


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Traffic Planning/Expansion Planning

Two stages for Capacity Planning I) Initial Network Build II) Future

Expansion.

1) Initial Capacity Plan

Spreadsheet design is used.

The expected traffic is calculated based on a certain amount of

traffic assigned per subscriber say 25 mE.

The total traffic requirement is traffic per subscriber X totalno of subscribers.

Network capacity is based on a certain GOS say 2 %.

Erlang B table used to calculate the no. of TRX, hence no of

sites.


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Traffic Planning/Expansion Planning

Two stages for Capacity Planning I) Initial Network Build IIFuture Expansion.

2) Future Expansion

This can also be done using spreadsheet design methodology,

using a figure of expected traffic growth.

Alternatively TRX additions are done on an ad-hoc basis by

studying the traffic trend on a weekly/monthly basis. In cases where no further TRX addition is practicable, capacity

sites are added in the existing network.

Separate planning is done for Traffic Channels(TCH) and Access

Channels (SDCCH).

I b ildi S l i


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Inbuilding Solutions

IBS is required in places where indoor coverage requirement iscritical and the possibility of providing coverage from outdoorsites is not practicable.

Usually implemented for places like corporate offices, hotels,hospitals, shopping complexes, etc., where both coverage andcapacity is essential.

IBS implementations may consist of

Repeaters Low cost solution for covering a small area withless traffic

Microcells/Macrocells Separate BTS sites which can be a

single carrier microcell or a multi carrier macrocell,

implemented in places where larger area needs to be covered

and has higher traffic requirement.

I b ildi S l ti


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Inbuilding Solutions

IBS implementations usually deploy a passive RF networkusing DAS(Distributive Antenna Systems). In some exceptionalcases active elements like Leaky Feeders might be used.

Cost of leaky feeder is comparatively very high, hence therequirement needs to be justified!!

IBS performance also needs to be monitored and optimized asit is critical to the performance of the whole network. A bad

performing IBS can skew the statistics of the BSC to which itbelongs.

Special handover algorithms are used for controlling handoversbetween IBS sites to outdoor network, in order to achievegood performance and for traffic management.

B h ki


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Benchmarking

Benchmarking is done for having a comparison of own networkwith competitors network in terms of coverage/voice quality.

Benchmarking is also done for comparing own networksperformance against certain set KPIs or previously achievedperformance targets.

Special tools like Qvoice equipment is available for voicequality benchmarking.

For coverage/quality benchmarking could be done using regulardrive test and post processing tools like TEMS and DESKCAT

Benchmarking


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Statistical data from benchmarking can be used as a valuableinput to the network optimization process.

The data is used to identify weak areas in the network, whichhelps in developing strategies for improving the networkperformance.

Benchmarking

Frequency Planning


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q y g

Objective

Optimum uses of Resources

Reduce Interference

Frequency Planning


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q y g

F=1

F=2

F=3

F=4,8F=5,9

F=6,10

F=7

F=1

F=2

F=5,9F=6,10

F=7F=1

F=2

F=3

F=4,8

F=5,9

F=6,10

F=7

F= 1,2,3,4,5,6,7,8,9,10

Clusters

Co-Channel ( Re-use ) Cells

GSM uses concept of cells One cell covers small part of network Network has many cells

Frequency used in one cell can be usedin another cells This is known as Frequency Re-use

Frequency Re-use

Co - Channel Re-use factor


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A

A

Q = DR

C / I = 9 d

Co - Channel Re-use factor

Q = Co-Channel Reuse ratio

Adjacent-Channel Re-use Criteria


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Adjacent ARFCN's can be used in adjacent cells, but as far as possshould be avoided.

As such separation of 200 Khz is sufficient, but taking into considethe propagation effects, as factor of protection 600 Khz should be

In the worst, Adjacent ARFCN's can also be used in adjacent cells setting appropriate handover parameters ( discussed later in optim

* Practically not possible in most of the networks due to tight reus

Adjacent Channel Re use Criteria

Cell Configuration


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Omnidirectional Cell

BTS

Sectorial Cell

BT

Low gain Antennas Lesser penetration/directivity Receives Int from all directions Lower implementation cost

High gain Antennas Higher penetration/directivity Receives Int from lesser direct Higher implementation cost

Cell Configuration

Interference in Omni-Cells


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3,6,9A

A

B

C

3,6,9B

3,6,9C

Receives Interference from all directions

Sectored Cells


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A1

A2

A33

69

B

B2

B3 3

96

C1

C2

C3 36

9

ReceivesInterferen

from lessedirections.

Re-use Patterns


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Re-use Patterns ensures the optimum separation between Co-Channe

Re-use pattern is a formation of a cluster with a pattern of frequendistribution in each cell of the cluster.

Same cluster pattern is then re-used.

Preferred Re-use Patterns

Omni - Cells : 3 cell, 7 cell, 12 cell, 14 cell, 19 cells etc

Sector - Cells : 3/9 , 4/12, 7/21

3/9 Re-use Pattern


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A1

A2A3 B1

B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3A1

A2A3 B1

B2B3C1

C2C3 A1

B1

B2B3

A1

A2A3

B2C1

C2C3

C2C3C2C3 C2C3

A1

Exercise !!!


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A1

A2A3 B1

B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3

A1

A2A3 B1

B2B3C1

C2C3A1

A2A3 B1

B2B3C1

C2C3 A1

B1

B2B3

A1

A2A3

B2C1

C2C3

C2C3C2C3 C2C3

A1

Using ARFCN's 1to9 , do the channel allocation for the below cells using3/9 pattern

Frequency Allocation in 3/9 patterns


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Adjacent Channel Interference is very difficult to avoid within thcluster itself.

1

4

3

2

85

7

96

Frequency Allocation in 3/9 patterns

4/12 Reuse Patterns


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D1

D3

B1

B3

C1

C2C3 D1

A1

A2A3 B1

B2B3C1

C2C3B1

B2B3 A1

A2A3C1

C2C3 C1

D1

D2D3

D2D3B2B3 B2B3

D2 C1

C3

B2

D2D3A1

A2A3B1

B2B3

C2D1

D2D3A1

Exercise


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Using ARFCN's 61 to72 do the channel allocation for the below cells us4/12 pattern.

D1

D2D3 C1

C3B1

B2B3

C1

C2C3 D1

D2D3A1

A2A3

A1

A2A3 B1

B2B3C1

C2C3B1

B2B3 A1

A2A3C1

C2C3 C1

D1

D2D3

B1

B2B3

C2D1

D2D3

D2D3B2B3 B2B3

A1

4/12 Pattern Channel Allocation


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1

35

24 6

7

9 1112

10 8

4/12 pattern avoids adjacent channels in adjacent cells

Reuse Patterns Conclusion


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Larger reuse patterns give reduction in interference

Re-use patterns becomes more effective with sectorial cellconfigurations.

To implement large patterns ( like 4/12, 7/21) , more channels required.

So with less resources, the best way to plan is :

1. Use optimum no of channels per cell.2. Thus, increase the pattern size.

Critical Factors for good RF Network


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Grid based RF design.

Maintain standard azimuths while sectorizing cells Thismakes frequency plan easier

Correct choice of antenna type for specific coveragerequirements.

Use of optimal antenna heights Should be sufficient to caterto the coverage area, but should not exceed the requirement,

else it results into large spillovers and interference, makingreuse difficult!!

Use optimal tilt Electrical tilt as far as possible. In somecases combination of electrical and mechanical tilts


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Quality of Service


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Effect of QOS !

Dissatisfied Customers--- Customers face describes your profit

curve--- 1 Dissatisfied customer prevents 10 new

Revenue--- Customer Switchovers--- Less New Customers--- Cost of Dropped Calls--- Cost of Blocked Calls

Importance of RF Optimization


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RF Optimization is a continuous and iterative process.

Main Goal To achieve performance levels to a certain setstandard.

Network subscribers expect wireline/near wireline quality.

Network subscribers also expect 100 % availability at all giventimes.

RF network optimization is a process to try and meet the

expectation of subscribers in terms of coverage, QoS, networkavailability.

RF optimization also aims to maximize the utility of the availablenetwork resources.

Each operator has a certain set of decided KPIs (Key

Performance Indicators) based on which the operator guages theperformance of his network.



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RF/Access Network KPIs can be broadly classified into threetypes

a) Access related KPI

b) Traffic/Resource Usage related KPI

c) Handover related KPI

Examples of access KPI

a)SDCCH Drop rate b) Call setup success rate

c)SDCCH Blocking, etc. Examples of Traffic KPI

a)TCH Drop Rate b) Call success rate

c)TCH Blocking, etc.

Examples of handover performance KPIa)Handover Success rate b) Handover failure rate.

c)Handover per cause, per neighbour, etc.



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Apart from the KPIs mentioned earlier the operator may havehis own set of custom KPIs which the operator feels is critical toguage the performance of his network.

RF optimization process drives the effort to achieve andmaintain the network performance KPI.

Optimization can be broadly divided into 3 categories, as follows

a) Hardware Optimizationb) Physical Optimization

c) Database/Parameter Optimization

Generally the activities mentioned above are done in parallel. Insome cases one may precede the other.


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Network Optimization Cycle


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Optimization Stages

RF Planning

Network Rollout

/Build Phase

Nominal Cell Design

RF Fine tuning

Database

parameter optimization

Physical/

Hardware

Optimization

Network Pre

Optimization

Traffic Optimization


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Hardware Optimization


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Hardware Optimization is a process in which ailing networkelements which affect the performance of BSS (AccessNetwork) are trouble-shooted.

The BSS maintenance team attends to hardware issues. Howeverthere is a substantial assistance taken from the RF team forisolating the problem to the specific hardware.

How is hardware optimization done??

Inputs for the process are Drive testing

OMCR statistics

Hardware Optimization - Typical Hardware Problems


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In most cases, hardware failures on a BTS/BSC or any part ofthe access network alarms are generated at the OMC, whichhelp in identifying the fault

In some cases, there are no alarms generated

Key statistics from OMCR could point towards hardware failuresTypical statistics which indicate such problems are

a) Poor Assignment Success/High Assignment failure rate

b) High TCH/SD RF Lossc) High handover failure rate

d) Lower call volume/traffic on the cell



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Faulty TRX One of the most common problems. This can beidentified from OMCR statistics as well as drive test. In somecases only a particular timeslot on a TRX could be faulty.

Immediate step to be taken is to lock the particulartimeslot/TRX from the OMC and escalate the fault to the BSSteam. For identifying this problem vide drive test, the RFengineer has to go to the site and conduct a timeslot test/makeseveral calls on the particular cell and also test handovers to and

from neighbour cells.

Sleeping TRX/Sleeping Cell Sometimes certain TRXs/Cells donot take any calls during the day these are referred to assleeping radios OR sleeping cells. Usually this is a temporary

problem and gets resolved by performing a Reset on theparticular site or by doing a Lock Unlock process on thes ecific TRX/sector.



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Path balance problems This is also one of the common causesfor poor cell performance.

path balance is pegged as an OMCR statistic on a cell basis

General formula is path balance=uplink pathloss downlinkpathloss.

Pathbalance= pathloss+110.

where pathloss = uplink pathloss downlink pathloss.

uplink pathloss = actual Ms Txpower rxlev_uldownlink pathloss = actual Bs Txpower rxlev_dl

It is desirable to have the pathloss value as 0 which representsa balanced path. However a deviation of +/- 10 is acceptable



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Path balance problems If the pathbalance is below 100 or above 120, it

indicates that there could be a problem in either downlink or uplink. PB value

above 120 represents a weaker uplink and stronger downlink, whereas PB value

below 100 would represent a weaker downlink.

If MHA/TMA is used or receive diversity is applicable,an additional 3 dB gain is

introduced in the uplink. In such case a deviation of 20 is acceptable, i.e, a PB

of 95 would be normal in such case.

Path Balance If the PB statistic indicates problem in the downlink/uplink the

RF path should be traced for possible hardware faults. Possible things that

could go wrong are

a) High VSWR due to faulty feeder cable

b) Improper connectorisationc) Faulty combiner


d) F lt t i i d t hi b t


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d) Faulty antenna improper impedance matching between

antenna and feeder cable (rare case)

Processor problems The present BTS equipment architecture is quite robust and with the

evolution of VLSI techniques, the different hardware modules have been

compacted into single units.

The current TRXs/TRUs are having inbuilt processing abilities apart from

also containing the RF physical channels.

However in places where older equipment are still in use, problems with

processor, could be encountered.

These problems are easily identifiable by drive test and usually also show

up degradation on OMCR statistics. However in the current scenario theseproblems have rare occurences.



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BSC/Transcoder Problems Although the occurrence is rare, there are

instances where some part of Transcoder or timeslot on the PCM link go

faulty. In such cases, the timeslot mapping needs to be identified and

appropriate troubleshooting steps need to be taken. These problems canseldom be identified by drive testing.

Steps for Hardware Optimization

a) Check from OMCR statistics for indications of hardware faults

b) Check event logs from OMCR to find out if any alarms were generated

c) Conduct call test on the site/cell in question check for assignment

failures, handover failures, from layer 3 messages.

Hardware OptimizationHardware Optimization Steps


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Steps for Hardware Optimization

d) Isolate the problem to the specific TRX. This can be done by locking

the suspicious TRX.

e) Check for downlink receive level on each TRX. In some cases the

downlink receive level on a particular TRX may be very low, due to faulty

radio.

f) Request VSWR test to be performed if the problem appears to be

related to poor path balance.

g) Check for improper connectorization, improper antenna installation.

One loose connector could skew the performance of the entire cell!!!

f) If the problem is not isolated to a bad TRX/ other BTS hardware

further investigations needed to check other possible faulty hardware in

the BSC/XCDR


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Physical RF Optimization

A ll d i d RF i k d k f


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A well designed RF is key to good network performance.

More often than not, the actual network built is deviated fromthe network designed from the desktop. The variations are

a) Actual site locations are away from the nominal plannedlocations.

b) It is not practical to build a grid-based network due toseveral constraints.

c) Antenna heights may differ from the planned antennaheights.

Physical RF optimization may be done at several stages ofnetwork rollout.


Ph i l RF O ti i ti i ti l i t d i th


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Physical RF Optimization is an essential requirement during thenetwork build/pre optimization stages. In most cases the OEMvendor is responsible for the network during this phase and he

carries out the process to ensure that the actual network is asnear good as the desktop designed one.

The process comprises of conducting a drive test for the entirecluster, which may comprise of one or several BSC areas.

The drive test results are plotted on a GIS map and deficiencies

in coverage/interference problems are identified by plottingRxlev/Rxqual values.

Most of the coverage deficiencies are fixed by making changes toantenna heights(rare), bore and tilts.

At later stages parametric optimization is done to bring thenetwork performance close to desktop design.


RF ti i ti i l i d t d i t k i


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RF optimization is also carried out during network expansionphase, i.e when new site or group of sites are added into thenetwork.

In many networks RF optimization is also done as a regularprocess to maintain good network performance.

RF optimization is helpful in resolving specific coverage problemsor interference problems, cell overreach, no dominant serverissues, etc.

Typical thumb rule to follow while carrying out physical RFoptimization for resolving coverage or interference issues -

Step 1:- Try tilting the antennas.

Step 2:- Try changing the orientation.

Step 3:- Increase or reduce the height if tilt/reorientationdoes not solve the problem

Step 4:- Change the antenna type as a last resort.


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The process starts the moment a GSM network goes on air and

Database/Parameter Optimization


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The process starts the moment a GSM network goes on air andcontinues on a day-to-day basis, till the network is operational.

Under GSM each vendor has hundreds of parameters which can

be played with to achieve different performance metrics underdifferent scenarios.

Usually most of the parameters are enabled with default settingsand are always kept unchanged. However there are some specificparameters which control the RF performance which can be

changed on a cell or even carrier-level, to achieve specificimprovements.


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Database OptimizationFrequency Hopping

Frequency hopping is one of the standardised capacity


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Frequency hopping is one of the standardised capacityenhancement features in GSM system. It offers a significantcapacity gain without any costly infrastructure requirements.

Frequency hopping can co-exist with most of the other capacityenhancement features and in many cases it significantly booststhe effect of those features.

Frequency hopping can be briefly defined as a sequential changeof carrier frequency on the radio link between the mobile and the

base station. When frequency hopping is used, the carrier frequency is

changed between each consecutive TDMA frame. This means thatfor each connection the change of the frequency may happenbetween every burst.


At first the frequency hopping was used in military applications


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At first, the frequency hopping was used in military applicationsin order to improve the secrecy and to make the system morerobust against jamming.

In cellular network, the frequency hopping also provides someadditional benefits such as frequency diversity and interferencediversity.



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Frequency

Time

F1

F2

F3

Call is transmitted through severalfrequencies in order to average the interference (interference diversity) minimise the impact of fading (frequency diversity)


There are two methods of frequency hopping in GSM Baseband


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There are two methods of frequency hopping in GSM, BasebandFrequency Hopping(BB FH) and Synthesised Frequency Hopping(RF FH).

In the baseband frequency hopping the TRXs operate at fixedfrequencies.

Frequency hopping is generated by switching consecutive burstsin each time slot through different TRXs according to theassigned hopping sequence.

The number of frequencies to hop over is determined by thenumber of TRXs


The first time slot of the BCCH TRX is not allowed to hop it


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The first time slot of the BCCH TRX is not allowed to hop, itmust be excluded from the hopping sequence.

This leads to three different hopping groups.

The first group doesnt hop and it includes only the BCCH timeslot.

The second group consists of the first time slots of the non-BCCH TRXs.

The third group includes time slots one through seven from everyTRX.

Database OptimizationBaseband Hopping


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B

RTSL 0 1 2 3 4 5 6 7

TRX-1

TRX-2

TRX-3

TRX-4

f1 B = BCCH timeslot. It does not hop.

f2

f3

f4

Time slot 0 of TRX-2,-3,-4 hop over f2,f3,f4.

Time slots 1...7 of all TRXs

hop over (f1,f2,f3,f4).

Baseband hopping (BB FH).

Database OptimizationRF Hopping


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In the synthesised frequency hopping all the TRXs except theBCCH TRX change their frequency for every TDMA frame

according to the hopping sequence. Thus the BCCH TRX doesnt hop.

The number of frequencies to hop over is limited to 63, which isthe maximum number of frequencies in the Mobile Allocation(MA) list.



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BTRX-1

Non-BCCH TRXs are hopping over

the MA-list (f1,f2,f3,...,fn) attached to the cell.

TRX-2

B = BCCH timeslot. TRX does not hop.

f1,

f2,

f3,

fn

f1,

f2,

f3,

fn

. . . .

Synthesised hopping (RF FH).



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The biggest limitation in baseband hopping is that the number ofthe hopping frequencies is the same as the number of TRXs.

In synthesised hopping the number of the hopping frequenciescan be anything between the number of hopping TRXs and 63.



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MSC

BB-FHF1(+ BCCH)

F2

F3Dig. RF

TRX-3

TRX-1

RF-FH

F1, F2,F3

Dig. RF

TRX-1

TRX-2

BSCTCSM

BCCH

Frequency

Time

F1F2F3

MS does not seeany difference

BB-FH is feasible with large configurationsRF-FH is viable with smaller configurations

The difference between BB and RF FH.

Database OptimizationRF HoppingCell Allocation

The Cell Allocation(CA) is a list of all the frequencies allocated


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( ) f f qto a cell. The CA is transmitted regularly on the BCCH.

Usually it is also included in the signaling messages that command

the mobile to start using a frequency hopping logical channel. Thecell allocation may be different for each cell.

The practical limit is 64, since the MA-list can only point to 64frequencies that are included in the CA list .

Database OptimizationRF HoppingMobileAllocation The MA is a list of hopping frequencies transmitted to a mobile


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every time it is assigned to a hopping physical channel.

The MA-list is automatically generated if the baseband hopping is

used. If the network utilises the RF hopping, the MA-lists have to be

generated for each cell by the network planner.

The MA-list is able to point to 64 of the frequencies defined inthe CA list

However, the BCCH frequency is also included in the CA list, sothe practical maximum number of frequencies in the MA-list is63.

The frequencies in the MA-list are required to be in increasingorder because of the type of signaling used to transfer the MA-

list.

Database OptimizationRF HoppingHSN

The Hopping Sequence Number(HSN) indicates which hoppingf h 64 il bl i l d


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sequence of the 64 available is selected.

The hopping sequence determines the order in which the

frequencies in the MA-list are to be used. The HSNs 1 - 63 are pseudo random sequences used in the

random hopping while the HSN 0 is reserved for a sequentialsequence used in the cyclic hopping.

The hopping sequence algorithm takes HSN and FN as an input

and the output of the hopping sequence generation is a MobileAllocation Index(MAI) which is a number ranging from 0 to thenumber of frequencies in the MA-list subtracted by one.

The HSN is a cell specific parameter.

Database OptimizationRF HoppingMAIO

When there is more than one TRX in the BTS using the same MA-li t th M bil All ti I d Off t (MAIO) i d t


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list the Mobile Allocation Index Offset(MAIO) is used to ensurethat each TRX uses always an unique frequency.

Each hopping TRX is allocated a different MAIO. MAIO is addedto MAI when the frequency to be used is determined from theMA-list.

MAIO and HSN are transmitted to a mobile together with theMA-list.

The MAIOoffset is a cell specific parameter defining the MAIOTRXfor the first hopping TRX in a cell. The MAIOs for the otherhopping TRXs are automatically allocated according to theMAIOstep-parameter

For thisTDMA frame the output from the algorithm is 1

Database OptimizationRF HoppingMAIO


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GSM Hopping algorithm

MAI(0...N-1)=

f1 f2 f3 f4 fNfN-1MA

0 1 2 3 N-1N-2MA INDEX(MAI)

TRX-1 TRX-2 TRX-3

FN & HSN

MAIOTRXTRX-1 0TRX-2 1

TRX-3 2

1

1

+ MAIOTRX

MAIOOFFUser defi

These param

are set

automaticall

Database OptimizationRF HoppingMAIO Step

The MAIOstepis a NSN specific parameter used in the MAIO allocation to

th TRX


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the TRXs.

The MAIO for the first hopping TRXs in each cell is defined by the cell

specific MAIOoffsetparameter MAIOs for the other hopping TRXs are assigned by adding the MAIOstep

to the MAIO of the previous hopping TRX

MAIOTRX(N) = MAIOoffset + MAIOstep(n-1)

Database OptimizationRF HoppingReusepatterns When RF Hopping is deployed the BCCH layer is planned using the

standard 4X3 or 7X3 or an intermediate suitable pattern


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standard 4X3 or 7X3 or an intermediate suitable pattern.

Maximum protection is assigned while planning to the BCCH layer

as it is critical to call setup procedure. For the TCH layer there are mainly three types of widely used

reuse patterns

1X1 All sectors in the network use a single MA list.

1X3 3 MA lists are created. Sec A of each cell uses MAL1,

Sec B uses MAL2 and Sec 3 uses MAL3

Ad-hoc/Mixed SFH Multiple MA lists are used. Can have as many MA

lists as the number of sectors in the network. The reuse is based on

fractional loading * with a maximum loading factor of 100 %.

Database OptimizationRF HoppingLoadingFactor Loading Factor This is the ratio of no of TRX to the no of

hopping frequencies in the MA list


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hopping frequencies in the MA list

Loading Factor = No of Hopping TRX/No of Frequencies.

For eg. Loading factor = 50 % if there are 2 TRX and 4hopping frequencies.

Lowest practically achievable loading factor is 33 %for 1X3,17 % for 1X1 and highest is 100 % .

Usually 100% loading factor is used in case of ad-hoc RF

hopping, for cells with higher configuration (6-6-6), howeverfor lower configuration like (2-2-2) 50 % loading factorcould be used.

In case of ad-hoc hopping the loading factor can be plannedto be specific to the cell configuration.


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Database OptimizationDTX & Power Control

In a non-hopping network these features provide some qualitygain for some users but this gain cannot be transferred


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gain for some users, but this gain cannot be transferredeffectively to increased capacity, since the maximuminterference experienced by each user is likely to remain thesame.

The power control mechanism doesnt function optimally becausethe interference sources are stable causing chain effects wherethe increase of transmission power of one transmitter causesworse quality in the interfered receiver, which in turn causes thepower increase in another transmitter and so on.

This means that, for example, one mobile located in a coveragelimited area may severely limit the possibility of several othertransmitters to reduce their power.


In a non-hopping network these features provide some qualitygain for some users but this gain cannot be transferred


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gain for some users, but this gain cannot be transferredeffectively to increased capacity, since the maximuminterference experienced by each user is likely to remain thesame.

The power control mechanism doesnt function optimally becausethe interference sources are stable causing chain effects wherethe increase of transmission power of one transmitter causesworse quality in the interfered receiver, which in turn causes thepower increase in another transmitter and so on.

This means that, for example, one mobile located in a coveragelimited area may severely limit the possibility of several othertransmitters to reduce their power.

In a random hopping network the quality gain provided by bothfeatures can be efficiently exploited to capacity gain because



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features can be efficiently exploited to capacity gain becausethe gain is more equally distributed among the users.

Since the typical voice activity factor (also called DTX factor) isless than 0.5, DTX effectively cuts the network load in half whenit is used.

The power control works more efficiently because each user hasmany interference sources. If, one interferer increases its

power, the effect on the quality of the connection is not seriouslyaffected. In fact, it is probable that some other interferers aredecreasing their powers at the same time. Thus, the system ismore stable and chaining effects mentioned earlier do not occurfrequently.

Database Optimization

DTX & Power Control

Reuse 3/9, TU 3km/h Reuse 3/9, TU 50km/h


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GAIN:

PC on1.4 dB

DTX on 2.3 dBPC on, DTX on 3.7 dB

GAIN:

PC on1.0 dB

DTX on 2.3 dBPC on, DTX on 3.5 dB

C/I improvement

The simulated gain of PC and DTX with FH.


DTX has some effect on the RXQual distribution.

Normally the BER is averaged over the duration of one SACCH


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Normally the BER is averaged over the duration of one SACCHframe lasting 0.48 seconds and consisting of 104 TDMA frames.

However, four of these TDMA frames are used formeasurements, so that only 100 bursts are actually transmittedand received.

When DTX is in use and there is no speech activity, only thebursts transmitting the silence descriptor frame (SID-frame)

and the SACCH are transmitted. When there are periods of no speech activity, the BER is

estimated over just the bursts carrying the silence descriptorframe and the SACCH. This includes only 12 bursts over whichthe BER is averaged (sub quality).


BER gets averaged much more effectively when DTX is not usedyielding to a quality distribution where the proportion of


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yielding to a quality distribution where the proportion ofmoderate quality values is enhanced.

The sub quality distribution is wider than the full qualitydistribution, meaning that more good and bad quality samples areexperienced.

The differences between full and sub quality distributions arelargest in frequency hopping networks utilising low frequency

allocation reuse, since in that kind of networks the interferencesituation may be very different from burst to burst.

A couple of severely interfered bursts may cause very bad qualityfor the sub quality sample when they happen to occur in the setof 12 bursts over which the sub quality is determined.


The full quality sample of the same time period has probably onlymoderate quality deterioration because of the better averaging


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q y f g gof BER over 100 bursts.

In a real network utilising DTX the quality distribution is amixture of full and sub quality samples.

The proportions of full and sub samples depend on the speechactivity factor also known as the DTX factor.

The differences in the BER averaging processes cause significant

differences in the RXQUAL distributions. These differencesshould be taken into account when the RXQUAL distributions ofnetworks utilising and not utilising DTX are compared.


Power Control what to optimize??

The parameters to optimize in case of power control are the


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The parameters to optimize in case of power control are thewindow settings.



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Downlink Power Control Typical Rxlev Window settings

Downlink Rxlev (dBm)- 75 -95

+ 42



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Downlink RxQual0 4

+ 42

Downlink Power Control Typical RxQual Window settings



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Uplink Rxlev (dBm)- 70 -90

+ 33

Uplink Power Control Typical Rxlev Window settings

5

Power Control Features

Objective is to reduce average interference



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j g

In case of uplink also helps in saving battery power

Algorithm works on measurement reports sent by the MS every480 ms (SACCH frame)

Downlink power control cannot be applied to BCCH carrier

Uplink power control is mandatory but downlink power control isnot mandatory. Feature selectable by the operator.

For controlling interference in the network the operator usesDTX, Power Control and Frequency Hopping. These featureseffectively act as combined forces in interference reduction andimproved call quality.


Typical problems which GSM subscribers experience are

Coverage issues


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g

Voice quality issues

Access issues/congestion Handover related issues

Dropped calls

BSS Parameters are broadly classified into the following groups

Access related parameters



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p

Call handling/Handover related parameters

Congestion related parameters



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Database OptimizationIDLE Mode Cell Selection

The MS uses a "path loss criterion" parameter C1 to determinewhether a cell is suitable to camp on [GSM 03.22]


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C1 depends on 4 parameters:

1. Received signal level (suitably averaged) 2. The parameter rxLevAccessMin, which is broadcast on the BCCH,

and is related to the minimum signal that the operator wants thenetwork to receive when being initially accessed by an MS

3. The parameter msTxPwrMaxCCH, which is also broadcast on the

BCCH, and is the maximum power that an MS may use when initiallyaccessing the network

4. The maximum power of the MS.


Cell Selection in IDLE Mode, based on C1


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Radio Criteria

A = Received Level Average - p1

C1 = (A - Max(B,0))

B = p2 - Maximum RF Power of the Mobile Station

p1 = rxLevelAccessMin

p2 = msTxPowerMaxCCH


Cell Reselection

In case of reselection from one cell to another in the same location area h C1 l f ll b hi h h ll


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the C1 value of target cell must be higher than source cell

In case of reselection to a target cell in a different location area the C1value must be greater than that of the source cell by a databaseparameter cell_reselect_hysteresis

Cell Reselection C2

C2 is an option GSM feature which can only be used for cell reselection, itcan be enabled or disabled on a cell basis.

If C2 parameters are not being broadcast the C1 process is used forreselection.

Cell Reselection C2

C2= C1 + cell_reselect_offset temporary offset * H



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(penalty_time T) (for penalty_time penalty_time H= 1 if T < penalty_time

C2= C1 cell_reselect_offset (for penalty_time= 31)

Why C2??

Cell Prioritisation

As a means of encouraging MSs to select some suitable cells inpreference to others


Example of C2 usage

In dualband network-- to give different priorities for different


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band

In multilayer-- to give priority to microcell for slow movingtraffic

Any other special case where specific cell required higherpriority than the rest

Cell Reselection Strategy

Positive offset-- encourage MSs to select that cell

Negative offset-- discourage MSs to select that cell for theduration penalty Time period


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Database OptimizationHandovers

Handover

The handover (HO) process is one of the fundamental principlesi ll l bil di i i i h ll i hil h


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in cellular mobile radio, maintaining the call in progress whilst themobile subscriber is moving through the network.

In idle mode the MS does a cell reselection, whereas in dedicatedmode the MS performs a handover.

Handovers are mainly classified into two types

A) Inter cell handovers

B) Intra cell handovers

Inter cell handovers further classified as

Inter BSS ie between two cells belonging to different BSCs

Intra BSS ie between two cells belonging to same BSC

Handover

Intra cell handovers is the switching of call from oneh l/TRX t th TRX ithi th ll/BTS Thi i



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channel/TRX to another TRX within the same cell/BTS. This is anoptional feature which can be enabled on a cell basis. Intra cellhandovers usually take place when the Rxqual on the sourcechannel deteriorates.

Handover process may be initiated due to the following main reasons

Radio Criteria

To maintain receive level/receive quality

Absolute MS-BS distance

Power Budget

Network Criteria

Traffic load (to manage traffic distribution)

Handovers also classified as imperative/non-imperative based on thereason for which the process is triggered.

The cause value contained in the handover recognized message will



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The cause value contained in the handover recognized message willaffect the evaluation process in the BSC.

Handover causes may be prioritized as follows

1. Uplink Quality

2. Uplink Interference

3. Downlink Quality

4. Downlink Interference

5. Uplink Level

6. Downlink Level

7. Distance

8. Power Budget


Power budget handover

If an MS on a allocated resource during its measurementrep rtin pr cess sees n ther ch nnel th t uld pr vide n


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reporting process sees another channel that would provide anequal or better quality radio link requiring a lower output powerthen a handover may be initiated.

Handovers due to power budget ensure that the MS is alwayslinked to the cell with minimum pathloss though the quality andlevel thresholds may not be exceeded.

Handover to the target cell takes place when PBGT>hoMarginPBGT

PBGT = (msTxPwrMax Av_Rxlev_DL_HO (btsTxPwrMax BTS_TXPWR)) (msTxPwrMax(n) Av_Rxlev_NCELL(n))

where n nth adjacent cell which is a handover candidate


Power budget handover

hoMarginPBGT is a parameter which can be set on a cell to cellbasis Each cell may have a different value for each neighbour


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basis. Each cell may have a different value for each neighbourcell which is a candidate for power budget handover.

hoMargin is expressed in dB and is usually set to 4. However thismay be reduced if the handover needs to be speeded orincreased to 6 or higher to prevent ping-pong or to delayhandovers

In some cases negative homargin may also be used.


Handover Algorithms

Handover algorithms are used in addition to default parametersto control the handover process


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to control the handover process

These algorithms assist in mobility management and are effectivein traffic distribution.

The algorithms have an important role to play in GSM networkswhich use multi-band or multi-layer architectures.


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Handover per neighbour

This statistic gives the value of no of handover attempts as wellas successes for each neighbour cell This statistic is also helpful



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as successes for each neighbour cell. This statistic is also helpfulin troubleshooting handover performance, it can be used toidentify neighbour relations which have a high handover failurerate

The handover per neighbour statistic can also be used forneighbourlist pruning.


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Database OptimizationTRHO/Congestion Related

ParametersTRHO What does it do??

TRHO effectively reduces the service area of the congestedcells


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cells

Increases service area of under-utilised target cells HO is triggered using a special parameter amhTrhoPbgtMargin

instead of hoMarginPbgt

General guideline:

Target cell Rxlevaccessmin should be set higher to avoidbad downlink Rxqual after HO

amhTrhoPbgtMargin must be lower than hoMarginPbgt


Parameters

TRHO/BSC Parameters

amhUpperloadthreshold This parameter determines minimumt ffi l d th h ld t hi h ll t t t i ti t TRHO


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traffic load threshold at which cell starts to intiate TRHO

default value 80 % amhMaxLoadOfTargetCell This parameter determines maximu

traffic load threshold beyond which target cell will not acceptTRHO hand-ins default value 60 %

TRHO/BTS Parameters

amhTrhoPbgtMargin This parameter is new Pbgt margin whencell exceeds amhUpperloadthresh. Its the revised power budgetmargin which replaces the normal Pbgt definition when the Trhocriteria are met default value is 5 dB.


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Directed Retry

A transition (handover) from SDCCH in one cell to a TCH inanother cell durin call setup due to unavailability of an empty


Parameters


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another cell during call setup due to unavailability of an empty

TCH within the first cell. To control traffic distribution between cells to avoid a call

rejection.

Can be used for both MOC and MTC

Setting guidelines: drThreshold should be higher than Rxlevmincell

(Rxlevaccessmin); else the improved target cell selectioncriteria will be ignored.

Congestion Relief

This procedure is initiated when an MS is assigned to an SDCCHrequires a TCH and none are available


Parameters


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requires a TCH and none are available.

Two options are offered for deciding how many handoverprocedures are actually initiated.

First Option The no. of HO procedures initiated is at most theno. of outstanding requests for a TCH.

Second Option This allows for initiation of a HO procedure foreach MS that meets the modified criteria to support thefeature.


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Things which normally subscribers normally experience

(common problems)

RF OptimizationAnalysis and troubleshooting


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No coverage/poor coverage issues.

Dropped calls.

Failed handovers/Dominant server issues.

Breaks in speech/crackling sound or bad voice quality.

Access related problems Network Busy.Often all the above problems are addressed to the RF optimization

for resolution

Poor Coverage Issues

Coverage problems are one of the most concerning issues.

S b ib i N t k N t k S h

RF OptimizationPoor Coverage Issues


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Subscribers experience a No network or Network Search

scenarios on the fringe area of the cells. Mostly these problems are experienced in suburban areas and also in

many cases inbuilding coverage problems occur.

Analysis is simple

TEMS equipment/test phone displays Rxlev of serving cell andneighbour cells Generally problem occurs when Rxlev drops below 95 dBm. When the Rxlev drops to 100 dBm or lower the subscriberexperiences a fluctuating single bar or a network search scenario

When Rxlev (DL) drops below 95 dBm its very difficult to havesuccessful call setup, as typically the uplink Rxlev would be much

lower.

Poor Coverage Issues (Steps to solve the problem)

Analyze the extent of area which is experiencing a coverageproblem

RF OptimizationPoor Coverage Issues


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problem

Can this be solved by physical optimization??

Possible steps would be to improve the existing serving cellstrength by proper antenna orientation or up-tilting the antenna

If it is an indoor coverage/limited area coverage issue, this coul

be resolved by deploying a repeater/micro cell if the trafficrequirement in the question area is high.

In case of rural/suburban cells where the concern is a weakuplink TMA could be installed.

RF OptimizationDrop Call Troubleshooting

Dropped Calls

Dropped calls may be attributed to several reasons.

Usually categorized as


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Usually categorized as

Drop during call setup aka SDCCH Drop.

Drop during call progress aka TCH Drop.

Drop due to failed handovers with no recovery.

Call drops may occur due to RF/non RF reasons.

RF Reasons attributing to dropped calls

Weak coverage RL timer times out.

Interference low C/I bad Rxqual RL timer times out.

Faulty TRX resulting in low C/I call may drop during setupor after TCH assignment RL timer may/may not time out.

RF OptimizationDrop Call Troubleshooting

Dropped Calls

Non RF Reasons

S it h l t d MS i D li k Di t


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Switch related MS experiences a Downlink Disconnect

abnormal release, usually with a Cause Value.

CV 47 is a common example Layer 3 message DLDisconnect.

Non RF related call drops need to be escalated to isolate the

fault which could be related to the switch/transcoder or atany point in the Abis/A Interface.

Handover Failures/Problems

Handover failures may also be attributed to different reasons.

U ll d t RF

RF OptimizationHandover Problems


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Usually occur due to RF reasons.

Common RF reasons for handover failures

Interference Co BCCH/Co BSIC issue.

Faulty hardware on target cell.

Improper neighbourlist definitionSteps to identify and solve Handover issues.

Use TEMS (layer 3 messages) to identify the cell to which theMS attempts handover and results in a failure


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Steps to identify and solve Handover issues.

The Handover Command message contains information about thBCCH and BSIC of the target cell to which the handover was


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g

attempted. Check for any possible Co BCCH/Co BSIC interferer Check for possible hardware faults on the target cell.

Neighbourlist problems

Sometimes handover problems occur due to improperneighbourlist definition.

Neighbour Rxlevel are reported to be strong, but HandoverCommand does not get initiated.

Call drags on the source cell and in some situation drops.

Most common cause is improper definition of neighbourBSIC/BCCH

Steps to identify and solve Handover issues.

Neighbourlist Problems

Crosscheck with RF BSC dump to confirm the BCCH/BSIC and



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Crosscheck with RF BSC dump to confirm the BCCH/BSIC and

other parameters of the target cell. Report any inconsistencies to the OMCR personnel.


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End of Module 3

Lets explore the drive testTool

module 3 -rf optimisation module

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