Summary
•
GSM Network Planning Process
•
GSM Overview
•
Radio Propagation & Interference
•
Network Dimensioning
•
Network Characteristics
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
Handovers
Handover Criteria & Priority
1. Interference (UL or DL)
2. UL quality
3. DL quality
4. UL level
5. DL level
6. MS-BS distance (max or min)
7. Turn-around-corner MS
8. Rapid field drop
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)
Radio Resource HO
Location Areas
Location Area Design 1/2
•
Location updating
affects all mobiles in
network
•LocUp in idle mode
•LocUp after call completion
•
Location updating
causes signaling
and processing load
within the network
•
Avoid oscillating
LocUpdate
•
Trade-off between
Paging load
and Location Update
signaling
Location area 1
Location area 2
Location Areas
Location Area Design 2/2
•
Different MSC cannot use the same
LAC.
•
Location areas are important input for
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
•
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
R DFrequency Plan
Basics
•
Do not use
• hexagon cell patterns • regular grids • systematic frequency allocation•
Use
• interference matrix calculation • calibrated propagation models • minimise total interference in network R D f2 f3 f4 f5 f6 f7 f3 f4 f5 f6 f2 f3 f4 f5 f6 f2 f3 f4 f5 f6 f7 f2 f3 f4 f5 f7 f2 f3 f4 f5 f2 f3 f4 f5 f6 f7Frequency Plan
Best Method
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
8
possible values
•
The BCC plan can be done manually
BCC Plan
Network Optimization
•
Network Optimization (1)
•
Performance Evaluation (2)
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
field tests
customer
complaints
It´s far too late
when customers
complain!
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
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 test 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
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
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 (%)
0 0.5 1 1.5 2 2.5 3 3.5
Mon Tue Wed Thu Fri Sat Sun Mon Tue Wed
Call Bids / 10000 Average Busy Hour
Indicators – Coverage
•
Signal Strength -outdoors
• In building, In-Car penetration signal levels
• Uplink Voice Quality
• Downlink Voice Quality
• Call Drops
Indicators – Capacity
• Erlangs per Cell
• TCH success • TCH assignment failures
• TCH Drop calls
• TCH Blocking • Cell congestion
• Congestion Relief Usage
• BHCA against rated MSC limit
• mErl/subs. against rated MSC limit • SMS/subs. Against MSC limit
• MM values(HO,LU,Paging) against limits • Overload : Voice/Signalling/Processor
Indicators – Quality
• RxLev
• Handovers • Call Drops
• Call Success Rate
• Call set up success rate • Call completion rate • Call set up time • Voice quality(MOS) • RxQual • Echo • 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
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 -
98 %
Call Setup Success rate - 98 %
Handover Success Rate - 98 %
Dropped Call Rate -
1%
SDCCH Blocking Rate -
0.5 %
Blocked Calls
Blocked Calls can occur due to :
•Access 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 , “max_retransmission” ; “
Blocked Call Analysis -
1 3
Channel Request Channel Request
RACH .
: Imm Assignment
RACH
max_retrans Service Request
NO RESPONSE FROM N/W
ACCESS FAILURE ! Signalling
:
Signalling
2
NO TCH ASSIGNMENT Channel Request Mobile Returns To Idle
RACH TCH BLOCKED !
Imm Assign Reject
troubleshooting
Access Failures
- Uplink Interference at the Base Station
- Low Rxlev at the Base Station
- Downlink Low Rxlev ( Coverage Hole )
- Downlink Interference
Access Failure -
Troubleshooting
Access Failure - Uplink Problem
Causes:
1. AGCH Overload at Base Station
2. RACH Collisions
3. MS out of Range
4. Poor Uplink quality
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” Carry out Uplink Interference Measurements using Viper Heavy Traffic
Verify from OMC statistics SDCCH Congestion Carry Call Setup Time measurements
Blocked Call
Solutions To Blocked Calls
Optimize coverage
Optimize Cell loading
Interference management
Channel configurations
Optimize neighbors
TCH Blocked - Causes
•Interference
-- Verify Idle Channel Interference reports from OMC
-- If suspected, carry out uplink interference measurements
•Heavy Traffic
-- Verify the TCH Holding time and no of attempts statistics from OMC
-- During low traffic hours, Activate Cell barring in the cell
Dropped Call
Troubleshooting
Drop Calls Analysis
1 2
Channel Request Channel Request
Imm Assignment Imm Assignment
Service Request Service Request
Signalling SDCCH Signalling : : Signalling Speech TCH RLT = 0 ; DROPS RLT = 0 ; DROPS SDCCH DROP ! TCH DROP ! 3 SDCCH / TCH Handover Command Hand Access Handover Failure
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 Interference
•SDCCH Drop - Adjacent Channel Interference •SDCCH Drop - Uplink Problem
•TCH Drop - Coverage
•TCH Drop - Co-Channel Interference
•TCH Drop - Adjacent Channel Interference •TCH Drop - Uplink Problem
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 channel
•External
•Multipath
•Noise
Speech Quality Parameters
RxQUAL : Measured on the midamble.
Indicates poor speech quality due to radio interface impairments
FER : Measured on the basis of BFI ( Ping -Pong effect on speech ) Preferred under Frequency Hopping situation
Audio holes : Blank period of speech, due to malfunctioning of Transcoder boards or PCM circuits.
Speech Quality Parameters
Mean Opinion Score
Criteria for Voice Quality : A set value “x” at which “y” percent of customers
rate the voice quality at Circuit Merits(CM) 4 - 5.
MOS Quality Scale
5 Excellent ( speech perfectly understandable)
4 Good ( speech easily understandable, some noise)
3 Fair ( speech understandable with a slight effort, occasional repetitions needed)
2 Poor ( speech understandable only with
considerable effort, frequent repetitions needed)
Troubleshooting
Handover Problems
Troubleshooting - No
Handover
Create a Handover PLAN
Total Attempted Calls Total Dropped Calls Total Blocked Calls RxQual Full
RxLeve Full
RLT Current Value ARFCN
Neighbor Cell Measurements RR Message
Phone State
CH 40 CH 35 CH 27 CH 88 CH 29 CH 35 CH 98 CH 72 BSIC 23 BSIC 16 BSIC 22 BSIC 75 BSIC 15 BSIC 21 BSIC 57 BSIC 53 CH 25 BSIC 17 PHONE REPORTS CH RxQual RxLev 27 1 -80 35 -85 40 -83 25 -95 Frequency Re-use 'A' NET 'A' NET 'A' NET 'B' NET 'B' NET 'B' NET 'B' NET 'B' NET 'B' NET
Channel 29 is not in the neighbour list !
BCH Analyzer : TOP N = 7
Configure an Alarm for Missing Neighbor
“Subset BCH TOP N not-subset Value ARFCN”
Missing Neighbors
Troubleshooting - Handover
Parameter Settings
•Decide the Target Cell for handover in the Trouble Spot
•Filter the Handover.txt file on Handover Attempts ( before AND after numbers) •Filter again on 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
Optimization for Interference
After drive test - Co-Channel benchmarking, we know which
cells are generating severe co-channel problem
We also know by decoding BSIC , the interefering source
Following processes could be adopted to optimize
interference
---- Power Control
---- Antenna Tilts
---- Frequency Reallocations
---- Transmitter Tests
---- Mobile dispatch inspection
---- Space Diversity
Antenna Tilts
x
6 km 1 km AM
N
50m 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 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.
Co-Channel Optimization
Static Power Control
Antenna Tilting
Trade Off with EC/No
Frequency Reallocation
Implementing Features like
Dynamic Power Control, DTX and
Frequency Hopping
Directed Retry (DR)
• DR used to avoid the loss of a call in call-setup if the accessed cell is congested
• When no TCH is available in serving cell, TCH can be allocated in an adjacent cell (SDCCH TCH HO)
• Mobile Originated (MOC) and Mobile Terminated (MTC) Calls
• Target cell entry based on DR Method;
• Method 0 - RxLevAccessMin
• Method 1 - drThreshold
•Imperative Handover (only equation 1)
• Candidates ranked based on radio properties.
• Steps through candidates (if congested) until MaxTimeLimitDR expires
• Queuing can take place in source cell, not in target cell.
Time
Assignment Request
minTimeLimitDR
maxTimeLimitDR
DR not allowed : improves the reliability of
the measurements of adjacent cells and gives the queuing process time
DR allowed
SDCCH TCH
Directed Retry
macro cell
micro cells
DR
congestion
DR
congestion
Intelligent Directed Retry
congestion macro cell (GSM cell) micro cells (MCN cells) MCN subscriber IDR GSM subscriber congestionmacro cell (GSM cell)
micro cells (MCN cells) DR
NOKIA TELECOMMUNICATIONS
• 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. Value Parameters IdrUsed Yes/No CellType GSM / MCN AdjCellType GSM / MCN
• 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 Call
• GSM => DR in Use in the Cell ?
•Yes => Directed Retry (any Cell) • No => Reject Call
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 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
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
Queuing of Radio Resources
Allocation of TCH
• When a TCH is released (in either channel release or TSL/TRX 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 the TCH is allocated to the queued call and the queue is re-organised
Leaving the Queue
• A queuer is removed from the queue 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 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!!
• 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 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 queuing
• The timers for hoPeriodPBGT or hoPeriodUmbrella are stopped during queuing
Queuing of Radio Resources
• Directed retry and queuing are parallel procedures
• When the queuing and DR processes are both active for a call;
• If MaxTimelImitDR 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
maxQueueLength 0 ... 100 % timeLimitCall 0 ... 15 (s) timeLimitHandover 0 ... 10 (s) msPriorityUsedInQueuing Yes / No queuePriorityUsed Yes / No queuingPriorityCall 1 ... 14 queuingPriorityHandover 1 ... 14 queuingPriorityNonUrgentHO 1 …14 Parameters Values
Queuing of Radio Resources
Dropped Call Control
radioLinkTimeout 4 ... 64 SACCH
callReestablishmentAllowed Yes/No
Parameters Values
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
Call re-establishment
• In case of a Radio Link Failure
(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
