network_optimisation.doc
TRANSCRIPT
CCELLULARELLULAR N NETWORKSETWORKS OOPTIMISATIONPTIMISATION
Course « Mobile Communications »
ALTTC (Ghaziabad) - February 18th – 22nd 2002
Sami Tabbane (ITU)
SSUMMARYUMMARY
- 1 – Network structure and basic equipment
- 2 – Cellular engineering basics- 3 – Network tuning- 4 – Quality of service monitoring process and equipment
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- 5 – Decision process
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- 1 GSM - 1 GSM NETWORKNETWORK STRUCTURESTRUCTURE ANDAND BASICBASIC
EQUIPMENTEQUIPMENT
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GSM GSM NETWORKNETWORK INFRASTRUCTUREINFRASTRUCTURE ANDAND INTERFACESINTERFACES
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AANTENNANTENNA CHARACTERISTICSCHARACTERISTICS
Antenna: one of the most critical elements in a RF communications system. Antenna used for receiving and transmitting: 1 dB gain = 2 dB gain for the system (1 dB in each way). It is a passive element: Cannot transmit more power than received (in each direction), Can concentrate energy in a specific direction and thus, shows a gain.
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RRADIATIONADIATION PATTERNSPATTERNS
The radiation pattern: polar diagram: Horizontal plane: azimuth radiations (i.e., according to the direction N-E-S-W), Vertical plane: radiations according to the height (i.e., up, down and horizontally). Antennas can be compared through their diagram characteristics:
Points at -3 dB, -6 dB, -10 dB, Front to back ratio, Zeros angles, minor lobes, etc.
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EEXAMPLESXAMPLES OFOF A ANTENNANTENNA D DIAGRAMSIAGRAMS (1) (1)
Sector antenna
EEXAMPLESXAMPLES OFOF A ANTENNANTENNA D DIAGRAMSIAGRAMS (2) (2)
Omnidirectional antenna
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GGAINAIN OBTENTIONOBTENTION
Optical-based methods:- Reflectors used to focus the radiations. Works well
for the micro-waves where the reflectors have small sizes (parabolic reflector).
Array-based antennas (discrete elements):- Power feeded to multiple elements,- The radiations of the elements are on-phase in
some directions.
AARRAYRRAY ANTENNASANTENNAS FORFOR SECTORSECTOR SITESSITESSector antennas: Dipoles, yagis, or log-periodic
elements vertical combinations with reflectors in the back.
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SSOMEOME ANTENNAANTENNA SELECTIONSELECTION CRITERIACRITERIA
Before chosing an antenna, check: The radiation patterns of the vendor:
Radiating pattern in the low, central and upper frequencies of the band, Minor beams and zeros, Symetry.
Test: Physical integrity, water resistance, Type of power, Measurements.
Ask other users.
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UUSUALSUAL ANTENNASANTENNAS
Type Gains (in dB)
Dipole 0Omni 0
Gain omni 3 to 12Helicoidal 5 to 15
Yagi 3 to 20Parabolic 10 to 30
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RRADIATINGADIATING CÂBLESCÂBLES
Classical solution to cover indoor tunnels.
Signal remains constant over several hundreds of meters.
The use of radiating cables allows to smooth the signal variations and to extend the coverage.
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BTS BTS ARCHITECTUREARCHITECTUREA BTS contains: A transmission board (TRU), A unit for combining, filtering and duplexing functions, … A TRX for: digital/analog processing, power amplifying, main receiver and diversity, frequency hopping (synthesized and baseband).
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BTS BBTS BASICASIC A ARCHITECTURERCHITECTURE
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BTS BTS TRANSMISSIONTRANSMISSION//RECEPTIONRECEPTION CHARACTERISTICSCHARACTERISTICS
Receiver sensitivityWith duplexerWithout duplexerWith diversity
- 106 dBm- 105 dBm- 111 dBm
Transmitter output powerWithout combinerWith combiner
42 dBm39 dBm
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RBS 2000 (ERBS 2000 (ERICSSONRICSSON) ) EXAMPLEEXAMPLEElements:- DXU, Distribution Switch Unit (interface between
PCM links and the TRU, LAPD multiplexing),- TRU, Transceiver Unit (management of the 8 time
slots, include A5/1 or A5/2 protocols),- CDU, Combining and Distribution Unit (combining
the signals transmitted by the different TRU),- PSU, Power Supply Unit (+24V),- BFU, Battery Fuse Unit (safety power),- ECU, Energy Control Unit (supervises and controls
the power and the cooling of the equipment).
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MMICROICRO-BTS (A-BTS (ALCATELLCATEL))
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TMA (TTMA (TOWEROWER M MOUNTEDOUNTED A AMPLIFIERMPLIFIER)) Amplifier which compensate the power losses in the
feeder link of the antenna and reduces the noise level.
If a usual antenna is provided for reception and transmission with a TMA installed on the mast, a duplexor filter must be installer on the mast.
External alarms: RBS2000 provided the necessary connections for the external alarms. These are defined by the operator and transmitted to the BSC through the LAPD signaling link on the Abis interface.
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MMICROICRO-W-WAVEAVE L LINKSINKS
BTSs to BSC links can be insured by micro-waves (MW).
Example of MINI-LINKs:
- Bitrates: 2, 22, 42 or 8, 28 or 82, 34+2 or 172 Mb/s.
- Frequency bands: 7, 14-15, 17-19, 21-23, 25-26 and 37-39 GHz.
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EEXAMPLEXAMPLE OFOF MW MW USEUSE
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BSCBSCExample of Ericsson BSC and TRC equipment
BSC/TRC BSC TRCNumber of TRX Up to 1020 Up to 1020 -Number of cabinets
2 to 8 1 to 5 3 to 11
Number of cells 512 512 -Number of BSCs 16 - 16BHCA 200 000 200 000 700
000Capacity in Erlangs 6 400 1 000 6 400
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Power 48 – 54.5 V - -
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RREPEATERSEPEATERS
Transmission-Reception equipment: To fill coverage holes (indoor, …). Amplify the signals in these areas. To extend the service area of a cell beyond its normal coverage.
Not visible by the system. Passive function of signal regeneration in both ways. Channel management remains under BTS complete control.
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RREPEATEREPEATER EXAMPLEEXAMPLE OFOF USEUSE
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- 2 C- 2 CELLULARELLULAR ENGINEERINGENGINEERING BASICBASIC
CONCEPTSCONCEPTS
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SSOMEOME BASICBASIC RULESRULES
Urban areas coverage: Define regular pattern (reals sites located within 10 to
20% radius distance from the theoretical position). Homogeneous antennas azimuths in the same area
(for example, 0°, 120° and 240°). Similar antennas heights in the same area (15 meters
for example). Choice of the BTSs sites: buildings of (n+1 or n+2)
floors compared to the mean neighbouring buildings height (n floors) in the area, that is, about 6 to 8 meters. Do not use quasi-bi or quasi-tri sites in urban areas. Use
only tri-sector sites.
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AAERIALERIAL INSTALLATIONINSTALLATION
Diversity techniques Horizontal space diversity: put two antennas
separated by a certain distance (1 to 2.5 meters typically). Gains: 5 dB on the uplink.
Vertical space diversity: put two antennas one above the other with a separation (1 meter typically, a too large distance gives rise to different coverages). Gains: 4 dB on the uplink.
Note: The BCCH should be transmitted on the lowest antenna.
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Polarisation diversity: use a cross-polar antenna which transmits and receive with polarisation angles of +45° and –45°.
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HHORIZONTALORIZONTAL DIVERSITYDIVERSITY
The most current one.If it cannot be used polarisation diversity in urban environment and vertical space diversity in rural environment.Diversity antennas must have:
Same height, Same azimuths, Same tilts, Coplanar (i.e., in the same plan).
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AANTENNASNTENNAS
In the transmission system, feeders must be of same type and length, antennas must be similar.Antennas types: 65° or 85° horizontal aperture and 4.5° and 7° vertical aperture with an electrical tilt. Mechanical tilt: physical inclinaison of the antenna. An important downtilt up shifts the back lobes of the antenna pattern. Electrical tilt: Allows to gather the lateral lobes of the horizontal radiations towards the center of the radiating area.
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AANTENNANTENNA INSTALLATIONINSTALLATION ONON THETHE ROOFTOPROOFTOP - - IINSTALLATIONNSTALLATION ONON AA CENTRALCENTRAL MASTMAST
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AANTENNANTENNA INSTALLATIONINSTALLATION ONON THETHE ROOFTOPROOFTOP - I- INSTALLATIONNSTALLATION ONON SEPARATESEPARATE 3 3 MASTSMASTS
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AAERIALERIAL CLEARINGCLEARING RULESRULES (1) (1)
Antennas close obstacles (horizontal or vertical plan) Importante reduction of the signal and thus of the site role.Horizontal clearing:Avoid obstacles located within an angle of 120° from the antenna.
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AAERIALERIAL CLEARINGCLEARING RULESRULES (2) (2)Vertical clearing: Angle between obstacles and the lower part of the antenna 30°.
Antenna height (Hm) = d.tg(30°) + h + L/2.
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AAERIALERIAL INSTALLATIONINSTALLATION
BTS to aerials connection includes: A jumper between the output of the BTS and the feeder connector. A feeder. A jumper between the feeder connector and the antenna connector. The connectors.To avoid important losses: Minimise the BTS to antennas distance. Maximum acceptable value: 3 dB.
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Curvature radius cables indicated by the vendor must be respected to avoid an increase in the value of the SWR (Stationary Wave Ratio).
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CCONNECTIONONNECTION OFOF THETHE BTS BTS TOTO THETHE AERIALSAERIALS
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FFEEDERSEEDERS SIZESSIZESFor a 3 dB maximum loss.
Feeder Link Maximum length½ super flex
1 feeder 17 m
½ 1 feeder + 1 jumper
23 m
7/8 1 feeder + 2 jumpers
35 m
1 ¼ 1 feeder + 2 jumpers
47 m
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jumpers
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BTS BTS INSTALLATIONINSTALLATION
Some basic rules: BTS cabinet must be installed close to each others with an easy access. Transmission equipement (MW or HDSL) installed on the same line as the BTS or the closest possible. A/C installed front of the BTS equipment. Backup power supply (rectifier and battery) installed the farest possible from the BTS cabinets. Cabling (energy, ground, transmission) on the cable tray. Patch pannel with multipoints connection to fixed half-way from the BTS equipment and power equipment, directly connected to the ground.
Feeder cables outgoing path must be waterproofed.
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BTS BTS SITESITE INSTALLATIONINSTALLATION PLANPLAN
BTS BTS CONNECTIONSCONNECTIONS WITHWITH NETWORKNETWORK EQUIPMENTEQUIPMENT
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- 3 N- 3 NETWORKETWORK PPARAMETERARAMETER SETTINGSETTING
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NNETWORKETWORK PARAMETERPARAMETER SETTINGSETTING
Working parameters setting is one of the main tasks to realize when putting a network into operation.
Essential task of the network operator for:
Activate or desactivate some fonctionnalities,
Quality of Service,
Network optimisation.
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DDIFFERENTSIFFERENTS TYPESTYPES OFOF PARAMETERSPARAMETERS Equipment related parameters: Specific to the equipment.
System parameters (activation of certain fonctionalities such as ciphering, power control, …).
Product related parameters (software versions). Engineering parameters (almost fifteen for optimising the network): Can be modified by the operators at the OMCs.
Numbering (BSC number, …). Network design (sites numbers, …).
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Optimisation for system tuning (handover margins, access thresholds, …).
Operation (barred cells, …).
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BSS BSS PARAMETERSPARAMETERS (1) (1)
Cell selection/reselection parameters
- Cell_Reselect_Offset: Favor the cells of a frequency band.
- Temporary_Offset: Avoid Ping-Pong cell reselection.
- Cell_Reselect_Hysteresis: Avoid the reselection of cells belonging to different LAs
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and reduces the unsuccessful paging rate. Example: 6 dB.
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BSS BSS PARAMETERSPARAMETERS (2) (2)
Access related parameters- Max_Number_Retransmission: Maximum
number of retransmissions on the access channel (example: 1, 2, 4, 7). Default value: 2.
- Number_of_Slots_Spread_Trans: Maximum number of slots between 2 successives retransmissions (3 to 12, 14, 16, 20, 25, 32, 50).
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- RXLEV_Access_Min: Defined the cell area. A change of 3 dB corresponds to 21% of the cell radius and 46% of the cell coverage area.
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BSS BSS PARAMETERSPARAMETERS (3) (3)
Handover
Their tuning is done to meet the following goals:
Minimise the number HO/distance, HO triggered off the closest to the cell border, Target cell correctly selected, Link quality maintained during the HO phase.
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BSS BSS PARAMETERSPARAMETERS (4) (4)
HandoverTo meet these constraints, we shall minimise: The number of HO attempts, The HO failure probability or call dropping rate, Ping-pong effect, Handover duration (handover triggering target BS link successful establishment), Resource consumption.
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BSS BSS PARAMETERSPARAMETERS (5) (5)Handover related parameters
- L_RXQUAL_H: Maximises the quality of the communication and minimises the HO rate. Typical value: 1.6 to 3.2%.
- L_RXLEV_XX_H: Determined by radio engineering, so that the HO triggering occurs at the cell border. If too low, the HO will be triggered too soon (ping-pong effect), if too large, HO triggered too late (call dropping). Default value: -101 to –110 dBm.
- MAX_MS_RANGE: Determined according to cell sizes (fixed during the cellular engineering phase).
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- HO_MARGIN(n): Hysteresis allowing to meet a tradeoff between the ping-pong handover rate and the quality of service.
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HO_MHO_MARGINARGIN ADJUSTEMENTADJUSTEMENT (1) (1)
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HO_MHO_MARGINARGIN ADJUSTEMENTADJUSTEMENT (2) (2)
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- 4 M- 4 MEASUREMENTSEASUREMENTS
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IINTERFACESNTERFACES TYPESTYPES
Air Interface (Um): Provide information on the downlink as well as on the exchanged messages during the protocols operation (calls, lcoation updates, …). Tools (mobiles with trace and associated tools) such as Ericsson TEMS. BTS-BSC Interface (Abis): Allows evaluate radio performance of one or several calls in both ways (uplink and downling). Allows observe resource allocation mechanisms (TCH or SDCCH) as well as intra-BSC handovers operation. Tools (protocol analysers) such as Siemens K11XX or K12XX series.
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BSC-MSC Interface (A): FoAllows capture additional information on the protocol operation and BSS - NSS problems. Tools such as Siemens K11XX or K12XX series.
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Radio interface analysis Radio interface analysis tools: essential to identify the origins of the problems (handover failures, coverage holes, bad quality due to interference, call drop, …). Mobiles with trace: display the serving cell frequency, the allocated time slot number, RXLEV and RXQUAL, neighbouring cells list, neighbouring cells BCCHs, timing advance, ... Data can be stored in a laptop. GPS receiver connection allows to display on a map (for instance in MAPINFO) the mobile trajectory and the evolutions of the indicated parameters.
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EERICSSONRICSSON TEMS TEMS TOOLTOOL EXAMPLEEXAMPLEA measurement chain using TEMS includes:
PC software with a serial port for the data, TEMS mobile with trace including the related software,
GPS receiver.
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TEMS collected information- Serving cell and neighbouring cells identities (BSICs) and BCCH frequencies.- Radio parameters: RXLev, RXQual, TXPower, DTX, Timing Advance, FER, SQI (voice quality), C1, C2, …- Current channel: CGI (MCC, MNC, LAC, CI), BSIC, BCCH ARFCN, TCH ARFCN, Time slot, Channel type, Channel mode (FR, EFR, HR), Hopping Channel, Hopping Frequencies, HSN (Hopping Sequence Number).- Map to display the measurement itinerary with: parameters values, main events (handover, call drop, …) and sites position. A GPS receiver is required for this feature.- Level 2 messages (RR-RSP, DISC-CMD, UA-RSP, SABM-CMD, …) and 3 (Synch Channel Information, System Information Type 6, Measurement Report, Synch Channel Information, Paging Request, Assignement Complete, Handover Complete, …).
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- Frequency scanning.
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Voice quality analysisQVOICE example (Ascom)
This tool allows:- To recognize FR and EFR types of coding.- To measure the voice quality of several networks in
parallel.- Store the differents voice effects (live recording),- To detect blank, mettalic voice, Ping-Pong effect, echos
problems.
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OMC-R counters analysis to partir des
Counters transmitted by the BSCs to the OMC-R. Essentials to analyse the quality, to detect problems, to realize statistics, … at the system side. Analysis tools use these counters (generally, these are specific). Example: Alcatel RNO or NPA, Metrica.
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EEXAMPLEXAMPLE OFOF AA QUALITYQUALITY REPORTREPORT TABLETABLE Operator quality monitoring teams may developp applications (Excel based typically) to extract the required information.
BSC OUARDIA1CELL TBLOCTCH TBLOCS
DTRUPTCH
TRUPTSD
THBETERC TEHOSintra
TEHOSinter
TEHOEintra
TEHOEinter
TEHO/cel
HODROP
Aeroport_Tunis_M_12_2
1,0% 0,1% 2,3% 0,3% 90,0% 2,2% 13,6% 6,5% 33,3% 2,1% 0,3%
Aeroport_Tunis_M_51
0,0% 0,0% 1,3% 0,0% 15,7% 3,0% 0,0% 10,1% 0,0% 0,0%
Asdrubal_13_3 0,0% 0,4% 1,0% 0,3% 64,6% 2,5% 72,5% 3,0% 5,9% 6,7% 0,6%Asdrubal_7_1 0,0% 0,0% 0,6% 0,3% 69,1% 3,3% 17,6% 2,5% 10,6% 4,5% 0,2%Asdrubal_7_2 0,4% 0,0% 0,8% 0,4% 59,7% 3,8% 19,4% 5,0% 8,9% 1,3% 0,2%Bardo_10_1 0,0% 0,3% 1,3% 0,5% 52,2% 12,3% 34,8% 9,3% 7,2% 7,1% 0,4%Bardo_10_2 5,9% 0,9% 0,7% 0,4% 26,5% 9,0% 42,5% 17,6% 13,9% 0,0% 0,2%Bardo_10_3 0,2% 0,0% 0,6% 0,3% 41,6% 12,4% 32,5% 5,2% 3,3% 0,0% 0,1%Belvedere1_1 0,0% 0,0% 0,5% 0,2% 48,4% 3,5% 10,3% 2,1% 5,0% 1,0% 0,2%Belvedere1_2 0,0% 0,0% 0,9% 0,2% 55,3% 4,1% 45,1% 4,7% 11,1% 2,2% 0,5%Belvedere2_3 0,0% 1,0% 0,6% 0,3% 46,1% 6,5% 14,4% 46,0% 95,5% 1,0% 0,2%
Quality of service indicators
Indicator Meaning Indicator MeaningTBLOCTCH TCH blocking rate (TCH congestion) TBLOCSD SDCCH bloking rate (SDCCH congestion)TRUPTCH TCH dropping rate TRUPTSD SDCCH dropping rateTHBETERC Best cell criterion HO rate TEHOSintr Intra-BSC outgoing HO failure rate
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aTEHOSinter Inter-BSC outgoing HO failure rate TEHOEintr
aIntra-BSC incoming HO failure rate
TEHOEinter Inter-BSC incoming HO failure rate TEHO/cel Intra-cell HO failure rateHODROP Call drop due to HO problem
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- 5 D- 5 DECISIONECISION PPROCESSROCESS
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AANALYSISNALYSIS PROCESSPROCESS
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DDECISIONSECISIONS EXAMPLESEXAMPLESProblem Detection Solutions
Coverage - Access failure rate- Call drop rate- Important RXLEV HO
rate
- New sites- Antennas (tilt, azimuths,
aperture)- TMA installation
Interference
- Communication quality - RXQUAL HO rate- Call dropping
- Frequency change- Power control tuning- Antennas action
Capacity - Blocking rate- HO failure rate
- TRX adjunction- Cell load distribution- HO thresholds and cell access
parameters adjustmentHandover
Ping Pong
- Bad quality- Micro-communication
interruption
- HO parameters adjustement- BTSs power adjustment
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CCONCLUSIONSONCLUSIONS
Network optimisation is a continuous process. Requires well-trained and experienced staff. Many problems can have various origins (maintenance, parameters tuning, features activation, frequency planning, equipment installation, …). Close interaction between Quality Monitoring Team, Engineering Department and Maintenance Teams is necessary.
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