Optimization: Introduction

Contents  Global indicators  Typical Radio Network Problem  Coverage Problem  Interference problem  Unbalanced power budget problem  Congestion problem  Problems and responsible parties

Optimization: Introduction Global Indicators 4 stages of a call establishment, 2 stages (1&2) for location update : 1. Radio link establishment Phase 2. SDCCH Phase then only “Circuit Switch call” 3. TCH assignment Phase 4. Alerting/Connect Phase

Optimization: Introduction Global Indicators 1. Radio Link Establishment Phase 2. SDCCH Phase 3. TCH Assignment Phase 4. Alert and Connect Phase

Optimization: Introduction Global Indicators  Handover : Intracell HO

Optimization: Introduction Global Indicators  Handover : Internal HO

Optimization: Introduction Global Indicators  Handover : External HO

Optimization: Introduction Global Indicators Important indicators :  SDCCH Congestion  SDCCH Assignment Failure  SDCCH Drop  TCH Assignment Failure  Call Drop  Call Setup Success  Call Success  Outgoing Handover  Incoming Handover  Handover Causes Distribution (Better/Level/Quality/MCHO/...)

Optimization: Introduction Global Indicators  SDCCH Congestion :  MS will try to request again for a certain of times  If still failures, it will  Wait for a certain of time without reception of immediate_assign message  Try an automatic cell_reselection in case of LU (no impact for subscriber)  Notify a network error for call establishment (call setup failure for subscriber)

Optimization: Introduction Global Indicators  SDCCH Congestion : Main causes  Too much SDCCH “normal” traffic for cell SDCCH design  Logical cell design, extra TRX, new site  “Common Transport Effect”  Difficult to avoid for small cells  LA border at crowded area  Abnormal SDCCH traffic  “phantom” channel requests  Inadequate LAC design, causing too much LU  Redesign LAC  Problem with neighbor cells belong to other LAC

Optimization: Introduction Global Indicators  SDCCH RF Failure : main causes  Real radio problems  Unbalanced power budget  Bad coverage  Interference In case of radio failure, MS will retry as in case of SDCCH congestion

Optimization: Introduction Global Indicators  SDCCH RF Failure : main causes  “Phantom/Ghost/Spurious/Dummy...RACH”  Channel request received but not sent : 3 causes  Noise decoding  Reception of channel request sent to a neighbor cell  Reception of HO_ACCESS sent to a neighbor cell

Optimization: Introduction Global Indicators  SDCCH RF Failure : main causes : Phantom RACH  Noise decoding  GSM 05.05: “0.02% of RACH Frame can be decoded without error without real input signal” (this extra-load has no impact for the system)  BCCH not combined: 156 phantom RACCH/hour (or about 0.08 Erlang SDCCH)  BCCH combined: 83 phantom RACH/hour  During period with low real traffic (night), high rate of phantom RACH  No impact for subscriber  But impact on indicator SDCCH RF access failure

Optimization: Introduction Global Indicators  SDCCH RF Failure : main causes : Phantom RACH  Channel request sent to neighbor cell  Subscriber is not impacted ( the real transaction is performed elsewhere)  But SDCCH RF access failure is impacted  Usual radio planning rules are sufficient to avoid the trouble (BSIC and BCCH frequency plan)

Optimization: Introduction Global Indicators  SDCCH RF Failure : main causes : Phantom RACH  Channel request due to handover  During HO, first message sent to target cell is HO access which is an access burst like channel_request  If received on BCCH, can be understood as a channel-request This case is the most dangerous  MS is sending usually a sequence of HO Access message, every frame !!  In some cases, this can create phantom RACH if the frequency of TCH is co- or adjacent to one of the interfered BCCH  Can block very easily SDCCH

Optimization: Introduction Global Indicators  SDCCH Drop :  Loss of connection during SDCCH phase = SDCCH Drop  3 causes of SDCCH drop  Radio problems when connected on SDCCH  BSS problems (Transcoder and Internal BSS problems)  Call lost during an SDCCH HO (handover failure without reversion to old channel)

Optimization: Introduction Global Indicators  TCH assign failure : Congestion  4 sub causes of congestion  A: Queuing is not allowed  B: Queue is full  C: T11 expired  D: RTCH requests is de-queued due to a high priority (i.e. emergency call) request to be queued

Optimization: Introduction Global Indicators  TCH assign failure : Radio

Optimization: Introduction Global Indicators  TCH assign failure : BSS

Optimization: Introduction Global Indicators  TCH Drop : Radio

Optimization: Introduction Global Indicators  TCH Drop : BSS  Transcoder reported problem  A bis transmission Problem: Microwave, Microwave antenna, fixed line problems.  Problem on TC boards (hardware problem)

Optimization: Introduction Global Indicators  TCH Drop : BSS  Other BSS problem (excluding TC)  Hardware or software failures of BSC/BTS

Optimization: Introduction Global Indicators  TCH Drop : Handover  Call drop during handover

Optimization: Introduction Global Indicators  Intracell HO Failures  Congestion  Reversion to Old Channel (ROC)  Drop Radio  BSS problem

Optimization: Introduction Global Indicators  Intracell HO Failures : Congestion  Intracell HO Failures : Drop

Optimization: Introduction Global Indicators  Intracell HO Failures : Reversion to Old Channel

Optimization: Introduction Global Indicators  Internal HO Failures : Reversion to Old Channel

Optimization: Introduction Global Indicators  Internal HO Failures : Drop

Optimization: Introduction Global Indicators  External HO Failures : Congestion  External HO Failures : A int. Congestion

Optimization: Introduction Global Indicators  External HO Failures : Reversion to Old Channel

Optimization: Introduction Global Indicators  External HO Failures : Drop

Optimization: Introduction Global Indicators Important indicators : SDCCH Congestion Rate  Definition: Rate of SDCCH not allocated during radio link establishment procedure due to congestion on Air interface  Comment: Check SDCCH Erlang : if not critical, SDCCH availability/allocation problem, or HO access on a nearby cell side effect or interference on the carrier handling SDCCH (the last 2 can lead to high rate if Phantom RACH

Optimization: Introduction Global Indicators Important indicators : SDCCH Assign Failure Rate  Definition: Rate of SDCCH seizure failed (BSS problem, radio access problem) during radio link establishment procedure over the total amount of SDCCH seizure requests during radio link establishment procedure

Optimization: Introduction Global Indicators Important indicators : SDCCH Drop Rate  Definition: Rate of dropped SDCCH (SDCCH is established for any transaction OC, TC, LU,...)  Comment: Drop radio + Drop HO + Drop BSS

Optimization: Introduction Global Indicators Important indicators : TCH Assign Failure/Congestion Rate  Definition:  TCH Assign Failure Rate: Rate of RTCH seizure failed (BSS problem, radio access problem) during normal assignment procedure over the total amount or RTCH request for normal assignment procedure  TCH Assign Congestion Rate: Rate of RTCH not allocated during normal assignment due to congestion on Air interface

Optimization: Introduction Global Indicators Important indicators : Call Drop Rate  Definition: Rate of dropped calls over the total amount of calls with a successful end  Comment:  Drop BSS + Drop radio + Drop HO  TCH drop occurring after successful assignment but before speech connection are considered as call drop even if from customer’s point of view it is a call setup failure

Optimization: Introduction Global Indicators Important indicators : Call Setup Success Rate  Definition: Rate of calls going until TCH successful assignment, that is not interrupted by SDCCH drop neither by assignment failures  Comment:  Subscriber : call not established at the first attempt

Optimization: Introduction Global Indicators Important indicators : Call Success Rate (BSS view)  Definition: Rate of calls going until normal release, that is not interrupted by SDCCH drop neither by assignment failures and neither by CALL DROP  Comment:  1 call success =  1 call successfully established  Without any call drop

Optimization: Introduction Global Indicators Important indicators : Outgoing Handover Success Rate  Definition: Rate of outgoing external and internal intercell TCH and SDCCH handover successes over the total amount of outgoing external and internal intercell TCH and SDCCH handover required

Optimization: Introduction Global Indicators Important indicators : Incoming Handover Success Rate  Definition: Rate of incoming external and internal intercell TCH and SDCCH handover successes over the total amount of outgoing external and internal intercell TCH and SDCCH handover requests

Optimization: Introduction Global Indicators Important indicators : Handover Cause Distribution  Definition: Distribution of Handover requests by cause: UL Qual, UL Lev, DL Qual, DL Lev, Better Cell, UL Interference, DL Interference, Interband, Micro cells HO, Concentric cell causes

Optimization: Introduction Typical Radio Problems i.Coverage Problem ii.Interference problem iii.Unbalanced power budget problem iv.Congestion problem

Optimization: Introduction Typical Radio Problems Several sources of information for optimization  QoS indicators  Coverage predictions  Customer complaints  Drive tests  A bis /A traces  Other teams information (NSS statistics)

Optimization: Introduction 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.

Optimization: Introduction i. Coverage Problem  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)

Optimization: Introduction i. Coverage Problem  Investigation and Optimization :  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)

Optimization: Introduction i. Coverage Problem  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.

Optimization: Introduction i. Coverage Problem Example on thresholds :  From Drive test or A bis Interface  RxLev < -95 dBm and  RxQual > 4

Optimization: Introduction ii. Interference Problem Interference : A network facing interferences problems presents good RxLev and bad RxQual in the same time on some areas.

Optimization: Introduction ii. Interference Problem  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

Optimization: Introduction ii. Interference Problem Typical causes :  GSM interferences  Co-channel  adjacent  Non GSM Interference  Other Mobile Network  Other RF sources

Optimization: Introduction ii. Interference Problem 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).

Optimization: Introduction ii. Interference Problem Co-channel interference :  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: Introduction ii. Interference Problem Co-channel interference :  Optimization  Antenna optimization  Change frequency  Reduce BTS power (not an option in TA Orange network)  Concentric cell

Optimization: Introduction ii. Interference Problem Co-channel interference: Optimization: Concentric cell  Concept  Conclusion from Alcatel trial on Orange’s network (Suburban area)  Concentric cell is difficult to optimize, and hence is rarely used in the network today.

Optimization: Introduction ii. Interference Problem Adjacent channel interference :  C/A = Carrier-to-adjacent ratio  -6 dB is taken into account for Alcatel ( -9 dB according to GSM standard).

Optimization: Introduction ii. Interference Problem Adjacent channel interference :  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: Introduction ii. Interference Problem Adjacent channel interference :  Optimization  Antenna optimization  Change frequency  Reduce BTS power (not an option in TA Orange network)  Concentric cell

Optimization: Introduction ii. Interference Problem Interference : Forced Directed Retry  MS should connect to Cell_A, but no TCH available.  MS connects to Cell_B with forced directed retry  MS is emitting at high level (due to long distance from Cell_B)  UL interference for Cell_C  Cell_B is emitting at high level (if DL PC is being used)  DL interference at Cell_C  MS is serving on the 2 nd or 3 rd best server: increases chances of interference problem (poor C/I & C/A as “C” is lower)

Optimization: Introduction ii. Interference Problem 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,...) ...

Optimization: Introduction 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

Optimization: Introduction  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 iii. Unbalance power budget Problem

Optimization: Introduction  Investigation :  A bis monitoring: –ABS(delta Path loss) > 10 »Check 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 iii. Unbalance power budget Problem

Optimization: Introduction  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 iii. Unbalance power budget Problem

Optimization: Introduction 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)

Optimization: Introduction iv. TCH congestion Problem  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: Introduction iv. TCH congestion Problem  Optimization :  Special events : –Foreseeable: football 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

Optimization: Introduction iv. TCH congestion Problem  Optimization :  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

Optimization: Introduction iv. TCH congestion Problem  Optimization :  Daily periodic problems : At busy hour: –Software solution : »Directed Retry »Traffic Handover »Half Rate (Alcatel) »Congestion Relief (Motorola) »Flow control (Motorola)

Optimization: Introduction 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)

Optimization: Introduction Problems and responsible parties  Coverage problem :  If the field reality 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)

Optimization: Introduction  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. Problems and responsible parties

Optimization: Introduction  TCH Congestion problem :  Regional RF to request TRX expansion. Problems and responsible parties