DT Training

Click to edit Master title styleNetwork Analysis through Drive Test
Instructor : Mudassir Abbas Shah
Huawei ID 75971
RF Optimization Engineer
21st February 2008
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Contents
How to Analyze the Network
Network Analysis Flow
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Purpose and Scope of Network Analysis
Finding and correcting any existing problems after site implementation and integration.
Meeting the network quality criteria agreed in the contract.
The results of Network Investigation forms the base for the network optimization.
Area of interest is divided in smaller areas called clusters to make optimization and follow up processes easier to handle.
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How to Analyze the Network ?
Traffic Statistical Data
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Network analysis Flow Chart
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Drive Test
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Network Problem Identification
Disabled GPRS
Coverage Problems
Overshooting sites
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Tems Information
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Coverage Problems
Different types of terrain structures.
C/I affected by drop of signal level.
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Coverage Problems Contd
Same cell always popping as the second strongest neighbor.
Overshooting coverage of a cell.
Rx levels too close to each other
Overlapping coverage of two or more cells
Can be reduced by Changing the Transmit power ,Down tilts.
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Coverage Problems Contd
Drop Calls due to Bad Coverage
Signal levels goes below the minimum Signal level required to Carry on a call.
This level is much lower than the Access Min Level required to Access the network.
Access Failures due to Bad Coverage
Rx Level goes Below the ACC MIN
ACC MIN -104dbm depending upon the sensitivity levels of equipment.
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Possible Solutions to Coverage Problems
Addition of New Sites
Sector Addition
Repeater Option
Repeater can also become a source of interference the received signal level in the repeater must be -80 dbm so it can be amplified or retransmitted.
Check Hardware Attenuation
Azimuths, Types of antennas.
Putting higher gain antennas, increased output power, changing orientations towards desired areas, reducing down tilts and use of diversity gain can help in improving coverage.
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Overshooting sites
MS is stuck in an overshooting cell Which is far away. MS can not Handover to an other cell because this overshooting cell was not defined as a neighboring cell in the cell nearby as expected.
How to get out of this overshooting cell ?
The only way to get out of this overshooting cell and restart calls on nearby cells will be possible when the call is dropped
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C/I Carrier to Interference Ratio
The Ratio B/W carrier and interferer is caused by carriers with the same frequency present in other cells.
C/I > 9 dB
However in modern GSM the C/I value must be greater than 12 dB.
In TEMS an acceptable value of C/I is ranging from 20 to 25 and a bad C/I
C/I > 9 dB Frequency Hopping
C/I> 12 dB No Frequency Hopping used.
Reasons for High C/I
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Adjacent Frequency Suppression Ratio C/A
The Primary interference issue addressed in Frequency planning is Carrier to Interference ratio C/I.
There are cases where the adjacent channels are used in neighboring cells resulting in bad Speech quality and some times even Handover failures.
According to GSM standards C/A must be greater than -9 dB
During cell planning the aim should be to keep the C/A target higher than 3 dB.
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Quality Problems
Delay more than 15 ms.
TA is too high even the mobile is closer to the site.
C/R must be larger than 9 dB.
More C/R resulting in bad quality.
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Bad Quality Due to Signal Strength
As the Rx Level drops the Quality of the call becomes worse being effected by low C/I and fading.
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Handover Problems
Late Handovers
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Handovers Problems Contd
Ping Pong Handovers
Why Ping Pong Handovers ?
Rx Levels of Serving and many neighboring cells are almost the same.
Two many cells having overlapping coverage.
Incorrect Handover margins.
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Handover Problems Contd
Missing Neighbor Relationship
If the Handover is not preformed to a neighboring cell that seems the best server.
If the Handover takes place in one direction but not in the second direction.
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Handover Problems Contd
Fake Neighbor Relationship
Good Handover Candidate and still no handover why?
The Network send the BCCH of the neighboring cells to the MS if the MS finds an other cell having the same BCCH it appears in the neighbor list of the Mobile.
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Handover Failures
Reasons for Handover Failures can be high traffic ,Congestion, low signal strength or bad quality on target cell.
However repeated Handover failures are caused by hardware problems in the target cell ,more likely TRx or Time slot problem.
When a handover failure occurs the MS attempts to camp on the old BCCH, if it cannot call drop occurs.
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Faulty TRXs
In a drive test the easiest way to check the TRXs is to lock all the TRXs except the one which has to be checked from the OMC.
In the same way each time slot of each TRX can be checked.
But the above solution is not always a feasible one as locking off the TRXs during day time in an urban area is not an affordable option.
The solution to this problem can be to make call tests and check the TRXs according to MAIOs.
To check the GSM 1800 MAIOs we must go close to the site and make test calls until we check all the MAIOs.If a certain MAIO is not found we must contact the OMC representative to check the status of channels on that TRX if the channels are in active state that means the TRX channels are busy.
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Sector Swap
Some times we see the strong coverage of one cell in the direction of the other cell such that one gets a feeling as if the antenna orientation data in the Tems cell file and in our data base is not correct.
If the antenna orientation in our cell file is correct then why do we see such coverage pattern ?
The Reason for such coverage pattern can be the phenomenon of Sector Swap.
In sector swap the feeder cables for a particular cell gets swapped with the
Feeder cables of some other cells .
In this way we get the BCCH of some other cell in the coverage area of some other cell.
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Case Study 1
A Drive test of Rawalpindi city’s cluster was conducted on 10th May 2007 to check the quality of service. So the DT was conducted with long calls.
Some Bad quality patches were found ,the reason for was found to be low value of C/I . Two dropped Calls and four Blocked calls were also found.
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Case Study 1 (Continued)
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Rx Level Case Study 1 (Continued)
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Rx Quality Case Study 1 Continued
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Carrier to interference ratio
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BCCH Plot
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Dropped Calls
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Blocked Calls
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Handover Failure
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Case Study 2
There were complaints of coverage problem at the Garhi dopatta site in Muzaffarabad A.J.K. The Customer SCO had complained that the coverage of the sector 2 i.e. SCOBS042 had reduced considerably and also there were lots of coverage holes in the area.
The site is a bisector site and is installed on Muzaffarabad-Srinagar Highway some 40 Kms from Muzaffarabad. The Terrain is highly mountainous and is quite difficult for site planning purposes.
Drive test was Conducted to check the state of the site’s Coverage.
After DT the Azimuths and tilts of the site were adjusted.
The site showed marked improvement in coverage.
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Case Study 2
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Case Study 2

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