2G KPI

10. KPI & Optimisation

Summary • GSM Network Planning Process • GSM Overview • Radio Propagation & Interference • Network Dimensioning • Network Characteristics Characteristics • KPI & Optimisation


Parameter Planning • Parameter Plan (1) • Handovers (2) • Location Areas (3) • Frequency Plan (4) •BCC Plan (5)


Parameter Plan • Parameter planning means creating a default set of BSS parameters • It is performed right before the network launch • Relevant BSS parameter for NW planning • • • • • • •

frequency transmit power definition of neighbouring cells definition of location areas handover parameters power control parameters cell selection parameters


Handovers Handover Criteria & Priority 1. Interference (UL or DL) 2. UL quality 3. DL quality

Radio Resource HO

4. UL level 5. DL level 6. MS-BS distance (max or min) 7. Turn-around-corner Turn-around-corner MS 8. Rapid field drop

Imper ative ative HO

9. Fast/Slow-moving MS 10. Better cell (Power budget or Umbrella) 11. PC due to Lower quality thresholds (UL and DL) 12. PC due to Lower level thresholds (UL and DL) 13. PC due to Upper quality thresholds (UL and DL) 14. PC due to Upper level thresholds (UL and DL)


Location Areas Location Area Design 1/2 • Location updating affects all mobiles in network

major road

Location area 2

•LocUp in idle mode •LocUp after call completion

• Location updating causes signaling and processing load within the network Location area 1

• Avoid oscillating LocUpdate • Trade-off between Paging load and Location Update signaling


Location Areas Location Area Design 2/2

• Different MSC cannot use the same LAC. • Location areas are important input for transmission transmission planners • should be planned as early as possible

• Never define location area borders along major roads! • Dual band or microcellular networks require more attention on LAC planning • co-located DCS and GSM cells are defined to the same LAC • same MSC to avoid too much location updates which would cause very high SDCCH blockings


Frequency Plan Basics •

Tighter Tighter re-use of own frequencies more capacity  more interferences 

•

Target • to minimise interferences at an acceptable capacity level

•

First when a complete area has been finalised

• Automatic frequency planning tools

D

R


Frequency Plan Best Method •

Do not use • hexagon cell patterns • regular grids

• systematic frequency allocation

f2

Use • interference matrix calculation • calibrated propagation models

• minimise total interference in network

f3

f3 f5

•

f6

f7

f4 f2

f4

f7

f4 f2 f3

R f5

f7

f4

f2

f7

f3 f5

D

f4 f2

f6

f6

f3 f3

f5

f2

f6

f6

f3 f5

f5

f4

f5

f4


BCC Plan Basics NCC = Network Colour Code BCC = Base station Colour Code BSIC = Base station Identity Code = NCC + BCC

• For each mobile, BSIC and ARCFN identify unambiguously a cell in the whole network • BCC is made by 3 binary digits possible values



8

• The BCC plan can be done manually


Network Optimization

• Network Optimization (1) • Performance Evaluation (2) • Optimization Process (3)


Network Optimization • Improving network quality from a subscribers point of view • Improving network quality from an operators point of view


Performance Evaluation Basics

• Network is under permanent change detect problems and symptoms early!



OMC

It´s far too late when customers complain!

field tests customer complaints


Performance Evaluation Key Performance Indicators

• KPIs are figures used to evaluate Network performance • post processing of NMS data or • drive test measurements data

• Usually one short term target and one long term target • check the network evolution and which targets are achieved

• KPIs calculated with NMS data

• network performance on the operator side.

• KPIs from drive test

• performance on the subscribers side

• Usually turn key projects are evaluated according to some predefined KPIs figures like drop call rate


Performance Evaluation With Drive Tests

• Evaluate network performance from the subscriber point of view • KPIs information

• DL quality, call success rate, handover success rate, DL signal level • not statistically as reliable as NMS information

• Added value of drive drive test measurement measurement • find out the geographical position of problems like bad DL quality to look for a possible interference source in the area • compare the performance of different networks (benchmarking) • display the signal level on the digital maps to individuate areas with lack of coverage eventually improve the propagation model • verify the neighbour list parameter plan


Optimization Process • There are not strict processes for optimization because the activity is driven by the network evolution


Optimization Process Young Network Case

• In a young network the primary target is normally the coverage • In this phase usually there is a massive use of drive test measurement • check the signal and • the performance of the competitors

MMAC

GPS NMS X


Optimization Process Mature Network Case • In a mature network the primary targets are quality indicators • drop call rate, average quality, handover failures • Important use the information from NMS • a general view of the network performance • Drive test measurements are still used • but not in a massive way • in areas where new sites are on air • where interference and similar problems are pointed out by NMS data analysis

Drop Call Rate (%) 3.5 3 2.5 Call Bids / 10000

2

Average 1.5

Busy Hour

1 0.5 0 Mon

Tue

Wed

Thu

Fri

Sat

Sun

Mon

Tue

Wed


KPI


KPI Definitions


Indicators

Coverage

•

Signal Strength -outdoors

•

In building, In-Car penetration penetration signal levels

•

Uplink Voice Quality

•

Downlink Voice Voice Quality

•

Call Drops

•

Cell Power control


Indicators

Capacity

•

Erlangs per Cell

•

TCH success

•

TCH assignment failures

•

TCH Drop calls

•

TCH Blocking

•

Cell congestion

•

Congestion Relief Usage

•

BHCA against rated rated MSC limit limit

•

mErl/subs. against rated MSC limit

•

SMS/subs. Against MSC limit

•

MM values(HO,LU,Paging) values(HO,LU,Paging) against limits

•

Overload : Voice/Signalling/Processor Voice/Signalling/Processor


•

Indicators

RxLev

Handovers

•

•

Call Drops

Call Success Rate

•

Call set up success rate

•

Call completion rate

•

Call set up time

•

Voice quality(MOS)

•

RxQual

•

Echo

•

Quality

•

TCH success

TCH assignment failures

•

TCH Drop calls

•

SDCCH traffic blocking

•

SDCCH drop calls

•

SDCCH Success rates

•


Analysis - Typical Setup Failure Causes - Distribution


Analysis - Typical Dropped Call Causes Distribution


KPI Network Health Statistics

We have considered a typical network with BSC’s to analyse the network performance. To undertake this activity stastics of atleast 2 weeks should be analysed. The key parameters to be considered are; Call Success Rate  Call Setup Success Rate  Handover Success Rate  Dropped Call Rate  SDCCH Blocking Rate  TCH Blocking Rate 


Customer sets a bench mark for these parameters as a measure of quality of the network and expects the performance to be equal or better than this. The typical value of the bench marks for each of the above mentioned parameters are as follows,

Call Success Rate Call Setup Success rate Handover Success Rate Dropped Call Rate SDCCH Blocking Rate TCH Blocking Rate -

98 % 98 % 98 % 1% 0.5 % 2%


LET’S LET ’S ST START ART OPTIMIZA OPTIM IZATIO TION N


Blocked Calls

Blocked Calls can occur due to : • Access Access Failures Failures •SDCCH Congestion •SDCCH Drop •TCH Congestion Trouble shooting cause : •Use Layer 3 messages to analyze the cause •Decode System Information Type 3 messages. •Note the parameter parameter , “max_retransmission” “max_retransmission” ; “


Blocked Call Analysis -

1

3 Channel Request RACH . : RACH max_retrans NO RESPONSE FROM N/W ACCESS FAILURE !

Channel Request Imm Assignment Service Request Signalling : Signalling

2 Channel Request RACH Imm Assign Reject SDCCH BLOCKED !

NO TCH ASSIGNMENT Mobile Returns To Idle TCH BLOCKED !


Blocked Call - Cause troubleshooting

Access Failures - Uplink Interference at the Base Station - Low Rxlev at the Base Station - Downlink Low Rxlev ( Coverage Hole ) - Downlink Interference - Excess Cell Range


Access Failure Troubleshooting

Access Failure - Uplink Problem

Causes: 1. 2. 3. 4. 5.

AGCH Overload at Base Station RACH Collisions MS out of Range Poor Uplink quality BTS Receiver Problem


Blocked Call Analysis

SDCCH Congestion Cause Location Updates to be analysed with OMC statistics first. If high, determine the source to target cell ratio Drive around the suspected area in the Idle Mode Configure “Delta LAC < > Constant 0” alarms Optimize Location Updates Interference Analyze OMC statistics on “ Idle Channel Interference” Interference” Carry out Uplink Interference Int erference Measurements using Viper Heavy Traffic Verify Verify from OMC statistics st atistics SDCCH Congestion Carry Call Setup Time measurements Optimize set up time if high, else modify channel c hannel configuration


Blocked Call

TCH Blocked - Causes •Interference -- Verify Idle Channel Interference reports from OMC -- If suspected, carry out uplink interference measurements •Heavy Traffic -- Verify Verify the TCH Holding time and no of attempts statistics from OMC -- During low traffic hours, Activate Activate Cell barring in the cell -- Carry out Time slot testing , by setting Ignore Cell Barring. Solutions To Blocked Calls Optimize coverage Optimize Cell loading Interference management Channel configurations Optimize neighbors


Dropped Call Troubleshooting


Drop Calls Analysis

1

2

Channel Request

Channel Request

Imm Assignment

Imm Assignment

Service Request

Service Request

Signalling : Signalling

SDCCH Signalling : Speech TCH RLT = 0 ; DROPS TCH DROP !

RLT = 0 ; DROPS SDCCH DROP !

3

SDCCH / TCH Handover Command Hand Access Handover Failure HANDOVER FAILURE DROP !


Dropped Call Analysis

• SDCCH Drops - Causes •Coverage •Interference & Multipath •BTS performance

• TCH Drops - Causes •Coverage •Interference & Multipath •BTS performance •Pre-emption

• Handover Failure - Causes •Threshold parameters •Missing neighbors

• Solutions to Dropped Calls •Optimize Coverage •Interference Management •Optimize neighbors •Optimize handover parameters •Effective Frequency Hopping •Use of DTX & Power control


•SDCCH Drop - Coverage •SDCCH Drop - Co- Channel Channel Interference Interference •SDCCH Drop - Adjacent Channel Channel Interference •SDCCH Drop - Uplink Problem •TCH Drop - Coverage •TCH Drop - Co-Channel Interference •TCH Drop - Adjacent Channel Interference •TCH Drop - Uplink Problem •Handover Failure


Poor Quality

•Poor Speech Quality could be due to •Patchy Coverage ( holes) •No Target cell for Handover •Echo , Audio holes, Voice Clipping •Interference ---: •Co-channel • Adjacent Adjacent channel channel •External •Multipath •Noise


Speech Quality Parameters

RxQUAL : Measured : Measured on the midamble. Indicates poor speech quality due to radio interface impairments FER : Measured : Measured on the basis of BFI ( Ping -Pong effect on speech ) Preferred under Frequency Hopping situation

Audio holes : Blank : Blank period of speech, due to malfunctioning of Transcoder boards or PCM circuits. Mean Opinion Score (MOS) : ITU : ITU standard for estimating speech quality. quality.


Speech Quality Parameters

Mean Opinion Score Criteria for Voice Quality : A set value “x” at which “y” “y” percent of customers rate the voice quality at Circuit Merits(CM) 4 - 5.

M OS

Qual i t y Scal e

5

Excellent ( speech perfectly understandable)

4

Good ( speech easily understandable, some noise)

3

Fai r ( spe Fai pee ech understanda understandabl ble e wi witt h a sl i ght eff ort , occasional repetitions needed)

2

Poor ( speech understandable only with consii de cons derable rable effort eff ort , freq f reque uent nt repe repett i ti tions ons ne nee ede ded) d)

1

Unsatisfactory ( speech not understandable)


Troubleshooting Handover Problems


Troubleshooting - No Handover

Weak Neighbors - Reported by Phone Create a Hand Hand over PL AN

Total Attempted Calls Total Dropped Calls Total Blocked Calls RxQual Full RxLeve Full RLT Current Value ARFCN Neighbor Cell Measurements RR Message Phone State


Missing Neighbors

Real Time

Frequency Reuse

BCH Analyzer Analyzer : TOP N = 7 Configure an Alarm for Missing Neighbor “Subset BCH TOP N notnot -subset Value ARFCN”

'B' NET

CH 35

BSIC 16

'B' NET

CH 35

BSIC 21

'A' NET

CH 88

BSIC 53

'B' NET

CH 40

'A' NET

BSIC 15

CH 98

'B' NET

BSIC 57

'B' NET

CH 29

PHONE REPORTS

'B' NET

CH 27 35 40 25

CH 27

RxQual 1

RxLev -80 -85 -83 -95

BSIC 23

'A' NET

BSIC 22

CH 72

BSIC 75

Channel 29 is not in the neighbour list !

CH 25 BSIC 17


Troubleshooting - Handover Parameter Settings

•Decide the Target Cell for handover in the Trouble Spot •Filter the Handover.txt Handover.txt file on Handover Attempts ( before AND AND after numbers) •Filter again on o n Neighbor ARFCN = Target Cell ARFCN •Create another column as HO_Margin , with Delta for Neighbor_Level to RxLev •Plot this on the MAP and see whether Handover Margin can be reduced to improve quality OR increased to avoid Ping-Pong effects •If handover margin settings are proper, and still handover is not occurring then it could be a problem with Handover decision and processing parameters at the BSC.


Troubleshooting - Uplink Interference

•Uplink Interference can be due to: • Mobiles in reuse and adjacent channel cells •External sources •Interference due to Mobiles will be bursty and intermittent. •The behavior and its effect on quality will be time dependent. •More interference during heavy traffic hours. •Interference from external sources can be continous or also time dependent if the source is not continously ON. •Uplink Interference measurement needs long term monitoring , collection of data, processing the data and estimating the probability of interference , and also estimating the source.


Optimization for Interference è After drive test - Co-Channel benchmarking, we know which

cells are generating severe co-channel problem èWe also know by decoding BSIC , the interefering source èFollowing processes could be adopted to optimize interference ---- Power Control ---- Antenna Tilts ---- Frequency Reallocations ---- Transmitter Tests ---- Mobile dispatch inspection ---- Space Diversity ---- Frequency Hopping /DTX


Antenna Tilts M

N A

50m

x 6 km

l l l l

l l

1 km

Point x is having problem of C/I from Cell M Estimate the C/I improvement required at Point x. Refer to the Antenna Vertical Pattern, and calculate the tilting angle required Example : To get an improvement improvement of 3 dB , a tilt of 10 degrees is required. Tilting of Antenna in certain cases may reduce coverage also. Tilting of Antenna should be done after proper study. study.


Co-Channel Optimization H

Static Power Control

H Antenna

Tilting Tilting

H

Trade Off with EC/No

H

Frequency Reallocation

H

Implementing Features like Dynamic Power Control, DTX and Frequency Hopping


DRT & Queuing


Directed Retry (DR)

• DR used to avoid the loss of a call in call-setup if the accessed cell is congested • When no no TCH is available available in serving cell, TCH TCH can be allocated in an adjacent cell (SDCCH (SDCCH TCH HO) • Mobile Originated (MOC) and Mobile Terminated Terminated (MTC) Calls TCH • Target cell entry based on DR Method; SDCCH • Method 0 - RxLevAccessMin RxLevAccessMin • Method 1 - drThreshold congested •Imperative Handover (only equation 1) • Candidates ranked based on radio properties. • Steps through candidates (if congested) until MaxTimeLimitDR MaxTimeLimitDR expires expires • Queuing can take place in source cell, not not in in target cell.

Time

minTimeLimitDR Assignment Request

maxTimeLimitDR

DR not allowed : improves the reliability of the measurements of adjacent cells and gives the queuing process time DR allowed


Directed Retry macro cell DR DR congestion micro cells

congestion


Intelligent Directed Retry

•

Based on Directed Retry : Target Cell selection depends on • Classmark of the MS or MS Priority • Adjacent Cell Type • Subscribers Classified in GSM or MCN • Based on Classmark ( bitmap in BSC associates classmarks to GSM / MCN ) • Based on MS Priority ( bitmap in BSC associates MS Priorities to GSM / MCN ) • Criterion defined in the BSC • DR and IDR enabled / disabled independently on cell basis.

Parameters IdrUsed CellType AdjCellType AdjCellType

macro cell (GSM cell) IDR micro cells congestion (MCN cells) MCN subscriber

• No TCH Available on Accessed Cell • GSM or MCN subscriber ? • MCN => IDR in Use in the Cell ? • Yes => Directed Retry Only to MCN Cells • No => Reject Reject Call • GSM => DR in Use in the Cell ? • Yes Yes => Directed Directed Retry (any Cell) • No => Reject Reject Call

Value Yes/No GSM / MCN GSM / MCN

macro cell (GSM cell) DR congestion GSM subscriber

micro cells (MCN cells)


Queuing of Radio Resources •

Used to avoid rejecting call set-up or HO attempt by waiting for the release of a suitable TCH

•

Queuing Environment • queuing is a BTS specific procedure (controlled by the BSC) • each BTS has a queue of its own • individual queue parameters and queue management for each BTS • call attempts and handovers in the t he same queue • the maximum queue length is relative to the number of traffic channels • the maximum queuing time can be set individually for both queue types • the queuing can be de-activated by setting queuing time or queue length to zero • different priorities according to queue type (call/HO) and/or MS priority Entering the queue: • The queue is entered when there is no traffic channels available of requested kind and if • • •

queuing is allowed in the BTS queuing enabled in the assignment request from MSC queue is not full (of higher or equal priority requests)


Queuing of Radio Resources Prioritisation: • The placement in the queue is determined by: • queue type (priority) • call set-up • handover attempt (non-urgent) • urgent handover attempt • MS Priority level in the PIE element of Assignment Request or Handover Request • time of entering the queue Allocation of TCH • When a TCH is released (in either channel release or TSL/TRX TSL/T RX lock) the queue is checked from top to bottom in order to find a queuer whose requirements match with the released channel • If a suitable queued call is found f ound the TCH is allocated to the queued call and the queue is re-organised


Queuing of Radio Resources Leaving the Queue • A queuer is removed from the queue when when •

No suitable channel is released within queuing time limit => timer expires

•

Higher priority subscriber (queue type and/or MS priority) replaces a lower priority queued entry when the queue is full

•

The queuing TRX/TSL is blocked (call release)

•

Queue size is reduced due to removing TRX’s


Queuing of Radio Resources • Queuing reserves SDCCH • SDCCH occupation for call setup 7 seconds. • If maximum queuing time for calls is 10 s • => in case of queuing 50-60 % more load on SDCCH / call attempt!! • (2 TRXs cell) with Combined BCCH / SDCCH • If maximum queue is 50 % of TRX x 8 = 8 queuing positions If there are 4 call attempts in queue, SDCCHs are fully booked no space for short messages or for location updates on SDCCH

cell is fully overloaded!!


Queuing of Radio Resources Q u e u i n g a n d H an an d o v e r

• Internal inter cell Handover • • • •

Ranked list is produced by the Handover algorithm and passed to RR management Maximum sixteen cells as alternative target cells The best candidate with free traffic channel is selected If all BTSs in the list are congested

queuing possibility is checked in the candidates c andidates according to ranking • External inter cell Handover • The BTS identified by the MSC in a HANDOVER_REQUEST message is used as queuing target

• Averaging and processing for HO continues during during queuing • The timers for hoPeriodPBGT hoPeriodPBGT or or hoPeriodUmbrella hoPeriodUmbrella are are stopped during queuing


Queuing of Radio Resources Q u e u i n g a n d D i r ec e c t e d R e t r y ( D R) R)

• Directed retry and queuing are parallel procedures • When the queuing and DR processes are both active for active for a call; • If MaxTimelImitDR MaxTimelImitDR expires expires without any DR attempt, the queuing will be terminated even if the queuing timer(s) have not elapsed • If a TCH is released in the serving cell during the period when DR is not allowed/allowed, DR process is terminated


Queuing of Radio Resources Queuin g Parameters Parameters

Parameters

Values

maxQueueLength

0 ... 100 %

timeLimitCall timeLimitHandover

0 ... 15 (s) 0 ... 10 (s)

msPriorityUsedInQueuing queuePriori queuePr iorityUsed tyUsed

Yes / No Yes / No

queuingPriorityCall queuingPriorityHandover queuingPriorityNonUrgentHO

1 ... 14 1 ... 14 1 …14


Dropped Call Control

Radio Link Timeout • A counter is initialised at the value of the radioLinkTimeout • When SACCH is not correctly received •Counter decremented by 1 • When SACCH correctly received Counter incremented by 2 •When counter reaches 0 call is released • Same behaviour UL and DL

Parameters radioLinkTimeout callReestablishmentAllowed

Call re-establishment re- establishment • In case of a Radio Link Failure (radioLinkTimeout) (radioLinkTimeout) within 20 seconds. • Measurements averaged over 5 seconds for serving Cell BCCH adjacent Cell BCCHs • Strongest is considered BCCH data decoded (cell selection param.) If: C1 greater than 0, Cell not barred Cell belong to selected PLMN, Call re-establishment allowed • Call re-establishment attempted

Values 4 ... 64 SACCH Yes/No


SDCCH ASSIGNMENT


Thanks

Shashi B Tripathi Tripathi

2G KPI

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