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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.
CW Drive Test/ Model Tuning...
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CW Drive Test/ Model Tuning...
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.
Importance of RF Optimization
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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.
Importance of RF Optimization
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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
Hardware Optimization - Typical Hardware Problems
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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.
Hardware Optimization - Typical Hardware Problems
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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
Hardware Optimization - Typical Hardware Problems
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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
Hardware Optimization - Typical Hardware Problems
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.
Hardware Optimization - Typical Hardware Problems
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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.
Physical RF Optimization
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.
Physical RF Optimization
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.
Database OptimizationFrequency Hopping
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.
Database OptimizationFrequency Hopping
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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)
Database OptimizationFrequency Hopping
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
Database OptimizationFrequency Hopping
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.
Database OptimizationRF Hopping
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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).
Database OptimizationRF Hopping
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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.
Database OptimizationFrequency Hopping
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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.
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 random hopping network the quality gain provided by bothfeatures can be efficiently exploited to capacity gain because
Database OptimizationDTX & Power Control
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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.
Database OptimizationDTX & Power Control
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).
Database OptimizationDTX & Power Control
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.
Database OptimizationDTX & Power Control
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.
Database OptimizationDTX & Power Control
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.
Database OptimizationDTX & Power Control
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Downlink Power Control Typical Rxlev Window settings
Downlink Rxlev (dBm)- 75 -95
+ 42
Database OptimizationDTX & Power Control
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Downlink RxQual0 4
+ 42
Downlink Power Control Typical RxQual Window settings
Database OptimizationDTX & Power Control
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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
Database OptimizationDTX & Power Control
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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.
Database Optimization
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
Database Optimization
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p
Call handling/Handover related parameters
Congestion related parameters
Database Optimization
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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.
Database OptimizationIDLE Mode Cell Selection
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
Database OptimizationIDLE Mode Cell Selection
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
Database OptimizationIDLE Mode Cell Selection
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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
Database OptimizationIDLE Mode Cell Selection
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
Database OptimizationHandovers
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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
Database OptimizationHandovers
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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
Database OptimizationHandovers
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
Database OptimizationHandovers
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.
Database OptimizationHandovers
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
Database OptimizationHandovers
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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
Database OptimizationTRHO/Congestion Related
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
Database OptimizationTRHO/Congestion Related
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
Database OptimizationTRHO/Congestion Related
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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RF OptimizationHandover Problems
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
RF OptimizationHandover Problems
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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