Download - 06 RA2006-13A Radio Network Optimization

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Module objectives
After completing this learning element, the participant will be able to:
Theory:
Explain the necessity of the optimization work: uncertainty of propagation models, uncertainty of expected traffic and limitation of frequency planning algorithms
List and explain the steps in the optimization flow
List and explain some key performance indicators and counters
Explain the planning and execution of the drive tests, evaluation of the measurements
results
Explain the optimization ides of cell reselection, power control, handover, adjacency plan
and signaling
Explain general the role of downtilt and height of antenna for coverage and interference
Content: Radio Network Optimization
Content: Optimization Principles
Need for Optimization
Discuss problems like uncertainty of propagation models, uncertainty of expected traffic and limitation of frequency planning algorithms
In most cases, the network will not run optimal from the very beginning due to
the following reasons:
Systematic errors of input data (e.g. wrong estimate of traffic)
Statistical nature of involved processes (e.g. average reflection / diffraction
coefficients instead of building material specific ones) – uncertainty of the
statistical values used in the propagation models
Dynamical nature of involved processes (e.g. increasing number of subscribers
and change of their behavior, e.g. SMS traffic increases more than voice traffic)
Installation errors (e.g. wrong cabling: transmitting into cell A, but receiving from
cell B)
Optimization Targets
Explain that network performance targets are specified by some general characteristics (KPI) like SDCCH/TCH blocking rate, SDCCH/TCH drop rate, HO failure rate (give also typical target values)
Optimisation needed to...
Maximise/Maintain network Quality
Achieve Quality Criteria set by
Operator:
Subscriber:
Performance Evaluation
when customers
The targets are always defined by the customer but the following figures can be considered as satisfactory results e.g.:
Item Target Lowest acceptable
Dropped calls <2 % 4 %
Handover success >98 % 96 %
Performance Evaluation
with NMS
Optimization Flow
Objectives
Explain general flow of monitoring (selection of bad cells based on KPI, than detailed analysis of counter statistics or even drive test) and that in dependence on the result certain actions are needed (e.g. capacity / coverage / interference problem, air interface / transmission problem)
Tools
KPI
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
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 OMC
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 OMC data analysis
Optimization Process
Operation & Maintenance
Parameters (OMC/BSS)
Field Tests (TIM, SAM, ISAR, ESVD, power meter, spectrum analyzer, transmitters, TEMS)
Drive tests
CW measurement
Customer Complaints
Parameter changes
Frequency changes
Content: Performance Measurements
OSS Statistics
Key performance indicators
Give examples for KPI (blocking, drop, success of SDCCH / TCH calls / HO, also Erl traffic), and for some present Network Doctor reports
Counters
Discuss shortly the most important types of counters (traffic / blocking / drops on CCCH / SDCCH / TCH, HO causes / adjacencies, RxLev, RxQual, timing advance) and for some present Network Doctor reports
OSS Statistics
Statistics obtained by O&M system
Cover a short or long time interval (e.g. one hour, one day, one week)
Cover a small or large part of the network (e.g. single TRX, single cell, all cells of a
BSS)
Are specified (GSM 12.04) or vendor specific
Each time a certain events occurs (e.g. SDCCH / TCH request / allocation /
rejection), the corresponding counter is updated
Call processing
Performance Indicators,
QoS Parameters
OSS Statistics – KPIs Concept 2/2
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
The most reliable KPIs to evaluate the network performance with NMS are:
SDCCH and TCH congestion
Call setup success rate
With NMS
With Drive Tests
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
Verify the neighbour list parameter plan
OSS Statistics – Performance Evaluation 2/2
OSS Statistics – KPIs
KPI
TCH drop rate
Handover failure rate
Rate of unsuccessful handovers without a loss of the MS connection because of reversion to the old cell
SDCCH drop rate
Blocking call percentage
MOC refused call percentage
CALL SUCCESS FACTORS (breakdown I)
====================
TCH access probability
(before DR) ......................................./csf_3i 116.11 %
(before re-est.) ................................/csf_41a 97.43 %
(after re-est) ..................................../csf_42a 97.56 %
CALL SUCCESS FACTORS (breakdown II)
====================
TCH success ratio after assignment ......../100-dcr_8h 99.04 %
OSS Statistics – KPIs – Examples:
OSS Statistics – Counters – Examples
SDCCH requests ................................./c1000 7967688
HO in..................................................../c1006 28324 ( 0.36 %)
To FACCH call setup .........................../c1099 0 ( 0.00 %)
LU ......................................................../c3019 2918916 ( 36.63 %)
supplementary service request (S9) … /c3044 104573 ( 1.31 %)
IMSI detach (S7) ................................../c3033 66587 ( 0.84 %)
call re-establishment............................./c3020 3568 ( 0.04 %)
other (fails, ghosts) .............................../sd_1b 607623 ( 10.25 %)
Counters Formulas KPIs
Monitoring – Reporting Suite
Before Reporting Suite can be started, connection to NetAct must be done
Reporting Suite can be used for KPI monitoring
Different KPI group can be seen here
Example of final KPI report
Monitoring – Reporting Suite
Drive Tests
Planning and execution
Discuss selection of the route, documentation of preparation and logging of the results, also discuss shortcomings like snap shot character and lack of information about interaction between the cells
Analysis
Evaluation of coverage holes, interfered spots, location with drops or unwanted HO on the basis of an example
Drive Tests 1/3
Statistics (counters) help to find bad cells but the information offered are insufficient to localize problematic areas inside of one cell. The counters can offer information about the distance dispersion of the MSs with a best case resolution of 550 m.
Field measurements give information with a resolution of less than 10 m about fading in the area, that can not be captured with the statistics.
Field measurements offer valuable conclusions about the interaction between the cells.
Field measurements are used in the optimization process but even in the planning phase e.g. to tune the propagation models.
An often used type of the field measurements is the drive test.
Drive tests are performed by the network operator for various reasons:
To check the coverage in a certain area
To check the quality of service in a certain area
To find the answer for customer complaints
To realise that the network is not properly running
To verify that the network is properly running
To verify that certain optimisation steps have been successful
...
Drive tests must be well prepared. Before, during and after the drive test the following steps should be performed:
Drive Tests 2/3
- Make back-up files of captured data
- Replay captured data and analyse them
- Find out problem areas and problem events
- Use further post-processing tools to display the captured data more
- clearly and to display further values graphically
- Perform statistics and summarise the results
After drive test
- Monitor test equipment
- Reconnect dropped calls
- Note interesting events separately
- Plan the route where to drive
- Plan the time when to drive
- Determine MS mode (idle mode/ connected mode) and also call strategy
(long / short calls)
- Decide which values to focus on (e.g. RXQUAL, RXLEV, SQI...)
- Select an appropriate test equipment and check it
- Think of notes to be inserted later on in recording file
Before drive test
Drive Tests 3/3
Drive Tests – Example 1
Content: Parameter Data Base Optimization
Cell Re-selection
Receive quality
Adjacency Plan
Cell Re-selection 1/2
C1 = A – max(B,0)
Path loss Criterion C1 for Cell Selection and Reselection
0..63
0..31
for PENALTY_TIME 640 s
C2 =
0 .. 126 dB
Path loss Criterion C2 for Cell Selection and Reselection
Properly cell re-selection helps to reduce the signaling in the air interface:
Cell Re-selection 2/2
cell 1
cell 2
cell 3
Power Control – Why?
BSC in command
Use power control in both uplink & downlink
Doesn't affect the Power Budget
PC not allowed
on BCCH carrier
Power Control – Measurements
BTS measures:
BSIC 6 best neighbors
Measurement reports
30 dB
msTxPwrMaxCCH1x00 0..30 dBm GSM 1800 CCH
(TXP2) 0..32 dBm GSM 1900 CCH
Minimum MS output power
0..36 dBm GSM 1800
0..32 dBm GSM 1900
bsTxPwrMax (PMAX1) 0..30 dB GSM 900
bsTxPwrMax1x00 (PMAX2) 0..30 dB GSM 1800/1900
Minimum MS output power (by maximum attenuation)
bsTxPowerMin 0..30 dB
bsTxPowerOffset (POFF) 0..30 dB
Fixed increment step size
- powerIncrStepSize (INC) 2,4,6 dB
Fixed decrement step size
- powerDecrStepSize (RED) 2,4,6 dB
Desired power level can be achieved in 1 or 2 commands
Yes
UL Level
UL Quality
threshold
No power change triggered
Nx 1..32
Px 1..32
Power Control Strategy
Exceeded threshold Action Reason
Power Control Strategy
pcLowerThresholdsLevelDL/UL
RXLEV_DL/UL > pcLowerThresholdLevelDL/UL - 2 powerIncrStepSize
pcUpperThresholdsLevelDL
variableDLStepUse (VDLS) = No
For BTS
pcUpperThresholdsLevelUL
For MS
pcLowerThresholdsQualDL/UL
Actual receive quality RXQUAL_DL/UL
PWR_INCR_STEP = (1 + Max (0,QUAL)) * powerIncrStepSize
QUAL = RXQUAL_DL/UL – pcLowerThresholdQualDL/UL
Take algorithm for power increase due to signal level
Take largest step size
pcUpperThresholdsQualDL/UL
Yes
No
Power Decrease Due to Signal Quality
Lower Level
Upper Level
Upper Quality
Lower Quality
Power Decrease Due to Signal Quality
Ping Pong Effect
Power Control Summary
AMR Power Control
AMR Progressive Power Control
AMR PPC provides mechanism to change quality thresholds depending on used power level so that it favours increase of power with low power levels and avoid increase of power with higher power levels. Following results are striven by this:
• Better power distribution - introducing less interference to the network
• Better quality distribution - yielding to better speech codec distribution
Handover
Objectives
Graduate drop of receive level (RxLev), RxQual and interference
Discuss HO due to RxLev, RxQual and interference, give typical values for HO thresholds in RxLev – RxQual - diagram
Rapid drop of receive level
Explain differences to RxLev HO (rapid field drop based on raw measurement reports, RxLev HO based on average over several reports; in case of rapid field drop only certain adjacencies checked against RxLev, in case of RxLev HO all adjacencies checked against RxLev and power budget)
Reasons for the optimisation of handover parameters:
To reduce the number of call drops
To reduce the number of handovers
To maximise the time duration the MS spends in the best cell
To improve the speech quality
Handover types: intra- / inter- cell, BTS, BSC, MSC handovers
Handover causes:
(Bad) RXQUAL
(Low) RXLEV
(far) DISTANCE
Handover – Power Budget HO
Handover triggered by power budget:
In an interference limited area (e.g. small cells in cities) most of the handovers should be power budget handovers:
For this type of handover not the level, quality, or distance is the handover cause, since all the corresponding thresholds are not exceeded in the serving cell, but a neighbour cell offers a better service (a smaller path loss, see link budget).
Since the power budget handover looks for the serving cell with the smallest path loss, this kind of handover will:
Reduce interference
Prolong MS battery time
The power budget is defined as the difference between the path loss in the serving cell and the path loss in the neighbour cell:
PBGT(n) = (BS_TXPWR – RXLEV_DL) – ( BS_TXPWR_MAX(n) – RXLEV_DL_NCELL
Handover – Receive Level, Receive Quality
RXQUAL
RXLEV
L_RXQUAL_XX_H
power budget handover or
due to interference
Handover – Receive Level, Receive Quality
Requirements for L_RXLEV_DL_H
Due to fading, for a stable connection the average RXLEV must exceed the receiver sensitivity by roughly 15 dB
With a MS receiver sensitivity of roughly -104 to -100 dBm one gets the condition L_RXLEV_DL_H = -90 dBm to –85 dBm
Requirements for L_RXLEV_UL_H
The receiver sensitivity of a normal BTS is –104 dB. This includes, however, already fast fading compensation due to RX diversity.
Therefore the average RXLEV must exceed the receiver sensitivity by about 10 dB only. Thus leads to the condition L_RXLEV_UL_H = -95 dBm
Ping-Pong effect
There should be a level hysteresis between the threshold RXLEV_MIN to accept a cell for incoming handover and the threshold L_RXLEV_XX_H triggering outgoing handover. Its size should be about the standard deviation of long term fading.
RXLEV_MIN = L_RXLEV_XX_H + 6..10 dB
To small hysteresis frequent not desired back handovers to bad old cell
To high hysteresis back handover required because of rapid signal drop in new cell not possible
Cell A
Cell B
Handover triggered due to L_RXLEV_XX_H (A)
Cell B accepted as target only, if RXLEV (B) > RXLEV_MIN (B) and PBGT (B) > LEVHOM (B)
Handover triggered due to L_RXLEV_XX_H (B)
Cell A accepted as target only,
if RXLEV (A) > RXLEV_MIN (A) and PBGT (A) > LEVHOM (A)
Handover – Triggered by RXLEV 1/2
Handover – Triggered by RXLEV 2/2
Street corner effect:
Sudden loss of more than 20 dB possible due to loss of LOS
Fast UL handover:
Triggered, if average UL receive level below threshold THR_RXLEV_FAST_UL_HO (default = -102 dBm
Shorter averaging period of measurements possible, defined by A_LEV_FUL_HO (default = 2 SACCH periods)
To speed up target cell selection, use of predefined target cell list possible (instead of ranking based on power budget)
Target cell must be good enough only according RXLEV, but not according power budget
Back handover allowed
Quality Handover
- Always inter cell handover
- Target cell selected on the basis of RXLEV and PBGT
Interference Handover
- Now high probability for the presence of interference
- Now intra cell handover allowed
- Does not help in case of RF hopping or if there is 1 TRX only
Handover – Triggered by RXQUAL
Adjacency Plan
Number of adjacencies
Explanation that compromise is needed (too few adjacencies -> HO failure, as no neighbor can be found; too many -> HO failure, as information about neighbor not up to date enough (FR MS can decode BSIC for 1-2 cells per SACCH period only)
Selection of best adjacency in case of HO
Discuss that this depends on the type of HO (in case of graduate problems RxLev, RxQual, interference priorities can be defined, RxLev and power budget is checked; in case of urgent problems like rapid field drop or directed retry no priorities and only RxLev checked)
Adjacency Plan
Otherwise use a mixed solution again!
Always cross-check FP and AP!!!
Automatic adjacency plan
Final result
Adjacency Plan – Rules
Changes of frequencies for a single cell often have an effect on its surroundings and require a re-planning for nearby cells also.
Too keep such changes as small as possible, the following rules should be obeyed during the planning process:
Strict separation of BCCH and TCH frequencies
Strict separation of macro and micro cell frequencies
Keep spare frequencies to enable an easy replacement of single strongly interfered frequencies
BA list transmitted to MS effects the number of nearby cells which can be measured
Missing neighbor cell -> call drop possible
Too many neighbors -> uncertain measurement values due to bad statistics, wrong power control or handover decisions possible
In practice 6-8 neighbors
SACCH multi-frame
Adjacency Plan – BCCH Allocation
Remarks to the pre-processing (averaging) of the measurements needed for power control and handover decisions:
In general:
Many measurements should be averaged in case that reliable decisions are necessary (better statistics).
Only a few measurements should be averaged in case that fast decisions are necessary.
For level / quality triggered handover / power control decisions:
To allow the system to ‘power up before handover’ usually the averaging process for the handover decisions should include more measurements than for power control decisions.
Usually for level triggered handover decisions more measurement values should be averaged than for quality triggered handover decisions since quality handovers must be executed quickly if sudden interference appears.
Adjacency Plan – Measurement averaging
Selection of Best Adjacency in Case of HO
Handover Strategy
(Handover Reasons)
Timing Advance
Adjacent Cells
Downlink Quality
Uplink Quality
Intelligent Underlay/Overlay (IUO)
Signaling
PCH, AGCH (especially when GPRS is launched)
SDCCH (remember also dynamic SDCCH)
RACH (impact of GPRS?)
Signaling Optimization
Location Area Design
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
Signaling Optimization – LA
Signaling Optimization – Paging vs. LAU
Paging
signalling
traffic
function of user density,
function of
user mobility
Content: Physical Optimization
Feeder cable
Diversity antennas
Antenna
Short explanation only, role of downtilt and height for coverage and interference already discussed
The antenna configuration is a key parameter of the optimization.
Has an impact on:
coverage of the cell,
It’s possible to change:
the antenna tilt,
Altering antenna tilt:
to reduce interference
to improve in-building penetration
Altering the Antenna tilt must be done very carefully to really improve the situation.
Typical down-tilts are between 0° and 10°, however even higher values (up to 25°) have already been used.
Altering antenna azimuth:
to reduce interference in certain directions
Increasing or decreasing antenna height:
to reduce or improve coverage
to reduce interference
Antenna 2/2
Other Equipment
If the link budget is not balanced (which leads to different coverage areas in uplink and downlink), there will be a bad overall performance of the cell.
This situation can be overcome using the TMAs, which will improve the link balance, noise figure and sensitivity thus leading to a better performance.
Other Equipment
Other Equipment – TMA
Different coverage areas for UL and DL
- too low RXLEV and/or RXQUAL at cell edge
- many call drops and handover failures
DL stronger than UL (usual case) Add tower mounted amplifier to improve UL (see chap.2)
UL stronger than DL Add booster or reduce downtilt to improve DL
Other Equipment – Repeater
Repeater to improve coverage:
Full duplex RF amplifier
Receives, amplifies and retransmits downlink signal from donor base station and uplink signal from
close mobile stations without any changes
Overcomes local coverage problems (tunnels, valleys, in buildings etc.)
Extends BTS coverage area
Document change history
Threshold DL Rx Qual
Threshold UL Rx Qual