TYPICAL RADIO PROBLEMS AND THIER OPTIMIZATION

TYPICAL RADIO PROBLEMS

Several sources of information for optimization: QoS indicators, Coverage predictions, Customer complaints, Drive tests, Abis/A traces, Other teams information (NSS statistics).

I. Coverage Problem

Bad coverage :

A network facing coverage problems has bad RxLev. RxQual can be bad at the same time. Sometimes the RxLev can look OK on the street (i.e. from drivetest) but coverage inside the buildings can be poor due to building losses. Building losses can range from 10 to 30 dB or more.

Indications :

• Customers complain about dropped calls and/or “No Network”

• OMC QoS indicators

– TCH failure rate

– Call drop rate

– Low proportion of better cell HO

– High rate of DL & UL Level HO (possibly also DL/UL quality HO)

• A interface indicators

– High rate of Clear Request message, cause radio interface failure

• Drive Test

– Poor RxLev ( < -95 dBm)

Investigation and Optimization :

• If actual coverage OK compared to predicted ones

– Improve coverage by adding Macro site, Microcell site (Border or Shadow areas), or IBC site (In-building coverage).

– Use parameter setting ex. Bi-layer Handover.

• If actual coverage is not the one predicted by planning tool

– Check antenna system (azimuth, crossed sector?)

– Check prediction in the tool (EIRP, Clutter Type, Obstructing building?). Most of the time the prediction will be incorrect, as it is only a computer simulation

– Increase or decrease antenna down-tilt

– Check BTS output power set in the OMC-R (BS_TXPWR_MAX: should always be set at 0)

From Drive test or Abis Interface

– RxLev < -95 dBm and

– RxQual > 4

II. Interference Problem

Interference:

A network facing interferences problems presents good RxLev and bad RxQual in the same time on some areas.

Indications:

• Customers complain about bad speech quality (noisy calls) and/or call drops

• OMC QoS indicators

– SDCCH/TCH Drop

– Low proportion of better cell HO

– High rate of DL/UL quality HO and interference HO

– Low HO success rate

• A interface indicators

– High rate of Clear Request message, cause radio interface failure

• Drive Test

– RxQual > 4 with RxLev > -85 dBm

Typical causes:

• GSM interferences: Co-channel, adjacent.

• Non GSM Interference: Other Mobile Network, Other RF sources.

Co-channel interference:

• C/I = Carrier-to-interference ratio

• +12 dB is taken into account for Alcatel ( +9 dB according to GSM standard) (i.e. interference can be 9  12 dB weaker than serving cell to cause poor RxQual).

• Indications: Downlink and/or Uplink Interference, High rate of quality handover, call drop, and call setup failure.

• Investigation: Drive test analysis, Lumpini: Co-channel checking, Check prediction to verify which cell could be causing the interference, Frequency plan C/I < 12 dB.

• Optimization: Antenna optimization, Change frequency, Reduce BTS power (not an option in TA Orange network), Concentric cell.

Adjacent channel interference:

• C/A = Carrier-to-adjacent ratio

• -6 dB is taken into account for Alcatel ( -9 dB according to GSM standard).

• Indications : Downlink and/or Uplink Interference, High rate of quality handover, call drop, and call failure

• Investigation: Adjacent HO statistics (if they are defined as neighbors). If they often handover, then adjacent channel interference could be a problem. Drive test analysis ; Lumpini: Adjacent channel checking ; Cell coverage prediction; Frequency plan C/I < -6 dB.

• Optimization : Antenna optimization, Change frequency, Reduce BTS power (not an option in TA Orange network), use Concentric cell

Non GSM interference :

• Other mobile network : Inter-modulation with GSM BTS/MS receiver

• Other RF interferers

– Radar

– Shop anti-theft mechanism

– Medical devices

– Other man-made radio transmission

– Illegal usage of GSM frequencies (e.g. illegal imported devices, illegal usage of other organization, …)

III. Unbalance power budget Problem

– A cell facing unbalanced power budget problems presents a too high path-loss difference between UL and DL (often DL>UL)

– Lower Rx diversity gain of cross polarized antenna in rural area compared to dense area

– Crossed feeders

– Traffic not directly in boresight of antenna, where cross-polar discrimination is lower

– Loose connectors

– Faulty antenna, feeder, TMA

– Rule : try to have delta as small as possible to avoid access network possible only in 1 direction (usually BTS->MS : OK and MS->BTS : NOK), i.e. uplink limited

Indicators:

• OMC QoS indicators

– High ratio of Uplink level Handover cause

– Low incoming HO success rate

– DL level looks OK for HO, but UL is too weak when HO is attempted

– Degradation of TCH failures and OC call drop indicators

• A interface indicators

– High rate of Clear Request message, cause radio interface failure

• O&M Alarms

– Voltage Standing Wave Ratio (VSWR)

– TMA

Investigation :

– Abis monitoring: ABS(delta Path loss) > 10Check if problem is occurring for 1 TRX or all.

– ABS(delta path loss) = ABS(UL Path loss – DL Path loss)

– UL Path loss = MS transmitted power – measured received UL level

– DL Path loss = BTS transmitted power – combiner loss – measured received DL level

– Large difference between uplink and downlink level triggered HO.

– Use PM observation counter (in OMC-R)

– RT11

Optimization : This equipment should be checked

– TRX types (MP, HP)

– Antennas or common RF components, TMA (common to all TRX of the BTS)

– TRX RF cables, LNA

IV. TCH congestion Problem

TCH Congestion :

• TCH Congestion rate (TCH Assignment Phase) is too high (more than 2%)

• Rule : try to meet the offered traffic (asked by users) by providing the right number of resources (TRX extension)

Indications:

– Customers complain about “Network busy”

– OMC QoS indicators: High “TCH Congestion rate”, “RTCH Assignment fail-Congestion”, Low “incoming Intra/Inter BSC HO success rate” (no TCH available), High “Directed Retry rate or Congestion Relief (Motorola)” if activated, A interface indicators: “BSS Congestion failure in OC”(High rate of Assignment Failure message, No radio resource available)

– Drive Test : Handover to better cell is slow, or never occurs

Optimization:

– Special events :

Foreseeable:  (footballs match, important meeting, Exhibition, etc): Add some extra TRXs, Add special mobile BTSs (Cell on Wheel), Activate HR (may not always be possible due to BSC capacity limitations)

Not foreseeable: car crash on the highway

– Daily periodic problems : At busy hour:

Hardware solution: Add TRX(s), concentric cell, new site, Optimize cell boundaries (i.e. antenna optimization) to share traffic with surrounding cells.

Software solution: Directed Retry, Traffic Handover, Half Rate (Alcatel), Congestion Relief (Motorola) Flow control (Motorola).

Problems and responsible parties

• Coverage problem :

If the measured RxLev does not match the prediction:

– Check the prediction, check panoramic pictures for obstructions

– Field Operation team to check physical configuration (tilt, azimuth, antenna height…). Also check for faults in the antenna system (VSWR, sweep antennas, DTF test, etc.)

– If Field Operation find no problem, antenna optimization may be required (if necessary)

If the field realities match the prediction:

RF team to add new sites (tri-sector, micro cellular, indoor cells) to improve poor/no coverage area (Optimizer requests for some new sites to RF team regarding to the drive test result).

• Interference problem :

Optimization team to identify the interference source and request Spectrum Management team to correct it in case of internal source (new frequency planning…)

• Unbalanced power budget problem :

Field Operation team to check the impacted BTS (Antenna, TMA, RF cables, LNA, diversity system…) according to request from Optimizer.

• TCH Congestion problem :

Regional RF to request TRX expansion.

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