Drive Test Learning: 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.
By: FAISAL KHAN (RF ENGINEER)
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. 2) 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. 3) 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 4) What need to be check when you reach to any new sites for Drive test?
By: FAISAL KHAN (RF ENGINEER)
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 5) 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 loose 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 …. 6) 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. 7) 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. 8) What are the prominent results behind CALL DROP?
By: FAISAL KHAN (RF ENGINEER)
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).
What to do when CALL Drop happen? 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 9) 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 ) 10) 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. 10) What is Radio Link Timeout ( RLT )?
By: FAISAL KHAN (RF ENGINEER)
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 . 11) 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. 12) 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 so 10 log10 (P/10^3) = 10 log10 (P*10^-3) = -3*10 log10 (P) = Rx Level in negative. Q13) how can we reduce Co-channel interference? Ans) 1) Frequency Hopping 2) DTX mode operation Q14) what are the parameters present in Current Channel Window?
By: FAISAL KHAN (RF ENGINEER)
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 Q15) 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 16) 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 to 32 Excellent.
5 to 9 Poor.
9
–5 to 5 Worst.
to 12 Average.
17) 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 o Excellent.
–90 to –80 Poor.
-70 to -60 Good.
–110 to –90 Worst.
By: FAISAL KHAN (RF ENGINEER)
80 o –70 Average.
18) 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.
19) 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
20)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. 21)
What is M Com? For what purpose it is used?
Ans) it’s a planning and post processing tool. a) To check interference b) To know about handover relation definition. c) To find any site in map.
d) To make Ppt of Rx Level, Rx Quality, C/I worst, SQI, etc. 22) What is swap? 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.
By: FAISAL KHAN (RF ENGINEER)
23) What is Dongle? Ans) It is the license key for the network radio parameters. It is used with TEMS , PROBE and NEMO. 24) 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. 24) 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.
By: FAISAL KHAN (RF ENGINEER)
