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Drive Test Learning 2015

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Table of Contents

1) What is the motivation behind GSM Air Interface drive testing? .............................................................................. 3

2) Types of Drive Tests? ................................................................................................................................................... 3

3) How to do Coverage verification Drive test?............................................................................................................... 3

4) What is the information need to be collected and carry before to start any Drive testing work? ............................ 4

5) What need to be check when you reach to any new sites for Drive test? ................................................................... 4

6) What are the major causes behind bad quality during the call? ................................................................................ 5

7) What is BCCH Pollution? How to overcome from such problem? ............................................................................ 5

8) What is cell overshooting? ........................................................................................................................................... 5

9) What are the prominent results behind CALL DROP? .............................................................................................. 6

10) What to do when CALL Drop happen? ....................................................................................................................... 6

11) Hand over Failure reasons? ......................................................................................................................................... 6

12) What is difference between Electrical tilt and Mechanical tilt? ................................................................................. 7

13) What is Radio Link Timeout ( RLT )? ......................................................................................................................... 7

14) DATA LINK FAILURE? ............................................................................................................................................... 7

15) Why Rx Level is in negative? What is its unit? ........................................................................................................... 7

16) How can we reduce Co-channel interference? ............................................................................................................ 8

17) What are the parameters present in Current Channel Window?............................................................................... 8

18) What are the parameters present in Radio Parameter window? ............................................................................... 8

19) What is C/I? ................................................................................................................................................................. 9

20) What is Rx-Level? ........................................................................................................................................................ 9

21) What is Rx-Quality? ..................................................................................................................................................... 9

22) What is TA? .................................................................................................................................................................. 9

23) What is SQI? ............................................................................................................................................................... 10

24) How much is C/I if Frequency hopping is present or absent? ................................................................................. 10

25) What is swap cable antenna? ..................................................................................................................................... 10

26) What are MAIO and HSN? ........................................................................................................................................ 10


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1) What is the motivation behind GSM Air Interface drive

testing? Ans)

GSM Drive testing is traditional and best way to verify network performance for New Sites or Existing sites.

Drive testing can be asked for various objectives like:

Coverage verification

New site Performance Verification and Field optimization

Network Problem trouble shooting like Drop calls, Handover failure, Poor Coverage patches,

Poor RX Quality patches, etc

Benchmarking Drive test to find out Coverage and Quality comparison against competitors

networks.

2) Types of Drive Tests? Ans)

New Site Drive Test (Coverage Drive)

Bench-marking Drive Test (Comparative Drive)

Migration Drive Test (Night activity and Software Up-gradation)

Route Drive Test.

3) How to do Coverage verification Drive test? Ans)

Best way to do it by putting TEMS Phone in Idle mode and drive across targeted routes .

In Idle mode, MS will measure BCCH TRX’s Time slot 0 and TS 0 transmit always full power

which is consider as real foot print of BCCH ( which is call as Cell foot print ) and such RX

LEVEL is RX LEVEL Full because TIMESTOL 0 doesn’t have DTX and POWER CONTROL

But when you do Dedicate mode Drive testing then TCH TRX has most of the time DTX ON

and POWER Control too so that RX LEVEL Measurement is RX LEVEL SUB which is not

genuine Cell foot print.


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4) What is the information need to be collected and carry

before to start any Drive testing work? Ans)

we go for Drive test, we need to collect all require information for site or group of sites (cluster)

like:

Site Master Database, which has Antenna parameters like Azimuth, antenna tilts, Antenna

Type, Height, Tilt, Azimuth( Physical Verification and Optimization)

Digital Maps for DT

Frequency Plan for Sites including BCCH and Hopping ( Frequency Optimization and

Interference detection)

Neighbor List so we can compare who is missing for addition or unnecessary defined for

removal ( Neighbor Optimization)

Complete tools like DT Kits, Camera, Campass, etc

Rigger with require tool for Physical optimization changes

Drive test desired route if any specific

5) What need to be check when you reach to any new sites

for Drive test? Ans)

when we reach to site, 1st thing need to be done is Sites Antenna physical parameter verification

If any antenna parameter is wrongly implemented like Azimuth or Tilt then there is no use

of doing drive test

Once Antenna parameter check and all are as per require or planed, then we can start call

testing , handover testing and area coverage verification

Drive testing can be done with various testing scenario but widely done as per below :

Call testing cell wise to understand performance of cell and each TRX

Inter cell between same site handover testing

Inter cell between different sites for handover testing

Coverage verification drive till you get -95 dBm RX Level Full in idle mode

Frequency Plan check BCCH and Hopping C/I Verification

RX Quality Sub in Dedicated mode

During Drive test some time you might face, Call drop , Handover fail, Block call, …etc and in that’s

case we need to do some analysis and you have find out who is service cell when we had such

problem.


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6) What are the major causes behind bad quality during

the call? Ans)

Major cause behind Poor RX Quality is Interference and call can suffer interference due to various

reasons like:

Current serving cell RX Level( C )is poor so C is very close to Interference ( I )

Due to Missing neighbor, Call is dragging on current poor servicing cell and after sometime it

will suffer Interference

Due to closely reuse of Co_Ch, even Mobile is closer to Cell sites but interference ( I ) is very

strong which might reduce C/I margin

Hardware issues, and due that Receiver can lose sensitivity so it’s become more sensitive to

Interference

Sometime Target cell is strong but doesn’t have free capacity to take handover so call will

drag in serving cell and it will suffer interference

There might be more reason behind bad quality ….

7) What is BCCH Pollution? How to overcome from such

problem? Ans)

BCCH Pollution mean, more than at any location if MS is getting more than 3 GSM Cells and none of

them are dominating and most of their RX LEVEL difference is less than 3 dB, means Mobile

is unnecessary will be in “Cell reselection” in idle more or “Ping Pong handover” in Dedicated

mode Best way to overcome from such problem to Optimize Antenna parameter and remote

such heavy overlapping.

8) What is cell overshooting? Ans)

During GSM Cell planning, RF Planner has design each cells such way to serve specific area of target

,Base on Cell planning Frequency and Neighbor also planned, But sometime due to

propagation friendly clutter or wrong Antenna parameter planning Cell coverage goes beyond

desired area and create interference .

Such cell doesn’t add any value in network performance but they create more interference to

other cell and resulting in high drop call.

Using Drive test you can find out such overshoot Cell and fine tune Antenna parameters like

Height or Tilt or … and restrict their propagation to specific area.


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9) What are the prominent results behind CALL DROP? Ans)

Low Signal Strength (Bad Rx Level)

Missing Neighbor (Handover not define)

Bad RX Quality

Delayed Handover due to lack of capacity in target cell

Interference

BTS related hardware

TCH Activation Failures During Call

Hardware Problem

Radio Link Timer out (when becomes zero).

10) What to do when CALL Drop happen? Ans)

We need to understand who is serving cell

Call was in BCCH or TCH/Hopping TRX

We need to understand who is serving cell after call drop

If serving cell change after drop call then need to find out that and BSIC was there in

neighbor list, if not means this is missing neighbor

What was the C/I? is it better than 9 dB or bad, If it’s bad means there is Co_Ch interference

in DL

11) Hand over Failure reasons? Ans)

Low signal strength or bad quality on target cell.

Hardware problems in target cell

Interference in target cell

What to do when handover failure happen?

Find out who was serving cell

Find out who was target cell from layer 3 message ( HO Command )


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12) What is difference between Electrical tilt and

Mechanical tilt? Ans)

In urban area mostly now a day’s Electrical tilt used, and reason is when you give tilt using E

tilt , font and back lobe both get tilted which reduce interference in network, and also shape

of Main lobe doesn’t get change only size become small

In case of Mechanical tilt, if you give tilt then font lobe will get tilted and back lobe will up

tilted which might create interference to other cells, and also shape of main lobe will get wide

which again create interference in network so that’s why such antenna can be use in rular or

highway kind o sites where intra site distance is high and those kind of area are not much

sensitive to interference.

13) What is Radio Link Timeout ( RLT )? Ans)

Radio link time out is bi-directional counter, it will set as per parameter when call started in

Downlink in Mobile and uplink at TRX in that particular timeslot

RLT is applied only when call is on TCH During call in TCH, in Downlink Network send

System info 5 and 6 which is carrying very important information about neighbor , cell and

power control related in every 480 ms ( 1 complete SACCH Radio block), if Mobile cannot

able to decode 1 SACCH block then RLT will reduce by 1 and if it decode SACCH Block then it

will increase by 2 and if it’s become 0 means call is drop due to some downlink problem and

DT Engineer has to find out and co-related current RF Measurement and parameter with

Call drop .

14) DATA LINK FAILURE? Ans)

If you come across any data link failure in sites it means that packets are not sent properly. So from

RF side we have to check whether any interference is there or not.

15) Why Rx Level is in negative? What is its unit? Ans)

we know that:

Power (in Watt) = 10 log10 (P).

Since available power is in mill watts

= 10 log10 (P*10^-3)

= -3*10 log10 (P)

= Rx Level in negative.


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16) How can we reduce Co-channel interference? Ans)

1) Frequency Hopping

2) DTX mode operation

17) What are the parameters present in Current Channel

Window? Ans)

1) Cell Name

2) CGI ( MCC, MNC, LAC, CI)

3) Cell GPRS Support

4) BCCH ARFCN

5) TCH ARFCN

6) BSIC

7) Time Slot

8) Band

9) Channel Type

10) Channel Mode

11) Speech Codec

12) Ciphering Algorithm

13) Hopping Channel

14) Hopping Frequency

15) MAIO

16) HSN

18) What are the parameters present in Radio Parameter

window? Ans)

1) Rx Level

2) Rx Quality

3) FER (Frame Eraser Rate)

4) BER Actual (Bit Eraser Rate)

5) SQI (Speech Quality Index)

6) C/ I Worst

7) MS Power Control Level

8) RL Timeout Counter

9) MS Behavior Modified

10) DTX- Discontinuous Transmission.

11) TA – Timing Advance


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19) What is C/I? Ans)

Its Carrier to interference ratio. It is the ratio of power of carrier to the power of all the sources

producing interference.

Ranges:

12 o 32 Excellent

9 to 12 Average

5 to 9 Poor

–5 to 5 Worst

20) What is Rx-Level? Ans)

It will be the Received Signal Strenth. Measured in Mili Desible and it will be always in negative.

Ranges:

-60 to 0 Excellent

-70 to -60 Good

-80 to –70 Average

–90 to –80 Poor

–110 to –90 Worst

21) What is Rx-Quality? Ans)

It is the quality of signal means quality of voice we can say.

Ranges:

0 to 3 Excellent

3 to 4 Average

4 to 7 Worst

22) What is TA? Ans)

It is Timing Advance Which Shows How much meters we r away from BTS. If Ta=0 means we r

500m away from BTS.


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23) What is SQI? Ans)

It is the Speech Quality Index Which is based on No of frames Erased and No of Bits Erased.

Ranges:

25 to 31 Excellent

14 to 25 Good

5 to 14 Average.

–5 to 5 Worst

24) How much is C/I if Frequency hopping is present or

absent? Ans)

In case of Absence of Frequency Hopping C/I will be Greater than 12db.In case of Frequency

Hopping C/I will be Greater than 9db.

25) What is swap cable antenna? Ans)

Sometimes, TX/ Rx coming from GSM Antenna gets mismatched while getting connected with TRU

cards. So, SWAP get occurs. It’s of two types, Partial & Sectoral.

If completely TX/ Rx of one sector gets connected to TRU Card, sectoral swap is there.

If one of the TX or Rx gets mismatched with TRU, partial SWAP is there.

26) What are MAIO and HSN? Ans)

MAIO is Mobile Allocation Index Offset which shows from where the Hopping will starts.

HSN it is Hopping Sequence Number will shows the different algorithms used in Frequency

Hopping.

When an ongoing call the Hopping will be 217 times in 1 sec.

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