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17/7/2005 1 Company Confidential
17/7/2005 2 Company Confidential
2
Troubleshooting
Blocked Calls
Poor Quality and Drop calls
Abnormal Handovers
Interference
Termination Failures
17/7/2005 3 Company Confidential
3
Blocked Call Troubleshooting
17/7/2005 4 Company Confidential
4
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” ;
“CCCH CONF” and “BS_AG_BLKS_RES”
17/7/2005 5 Company Confidential
5
The best way of analyzing blocked calls, to identify the cause, is from a Layer III protocol log.
* Paging Failure
A paging message always originates from the MSC and is sent to all the BSCs in the
Location Area of the MS to be paged. The BSC will then calculate the Paging group of the MS and
send a Paging Command to the BTSs controlling the Location Area of the MS. On the air interface
there are two cases of Paging Failure, either the Mobile receives no Paging message or it
receives a Paging message, but is not able to respond (not able to send a RACH) which could be
due errors in the Paging message.
* Access Failure
Irrespective of the purpose, for any communication required with the network, a mobile
sends a Channel Request (for SDCCH) on a RACH and waits for some time for a response which
should come from the BTS on an AGCH. A mobile will do several retransmission of RACHs (pre-
defined) and if it still does not get a response, it goes back to idle mode and preferably does a cell
reselection. At this stage we call it an Access Failure.
* SDCCH Blocked
Once a mobile has sent a Channel Request on a RACH , it expects a response from
the BTS on the AGCH. This should be an Immediate Assignment Command to an SDCCH. If an
Immediate Assignment Reject comes instead , then this is SDCCH blocking.
* TCH Blocked
After the completion of call set-up signaling, a mobile expects an Assignment
Command to a TCH so that speech can commence. If no Assignment occurs for a specific period
and the Mobile has to return to idle mode, then it is due to TCH congestion.
17/7/2005 6 Company Confidential
6
Blocked Call Analysis - L3 messages
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
SDCCH BLOCKED !
17/7/2005 7 Company Confidential
7
Blocked Call - Cause troubleshooting
Access Failures
- CCCH Overload at the Base Station
- Uplink Interference at the Base Station
- Low Rxlev at the Base Station
- Base Station TRX decoder malfunctioning
- Downlink Low Rxlev ( Coverage Hole )
- Downlink Interference
- Excess Cell Range
17/7/2005 8 Company Confidential
8
Access Failure - Cause Troubleshooting
No Downlink Message after Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR Channel Request
RR System Information 1
RR Paging Request Type 2
RR Paging Request Type 2
RR System Information
1st RACH Transmission
Multiple Re- Transmissions
( number depends on max_retrans)
Phone back to Idle Mode
Call Blocked !!!
Access Failure !!!
Causes : Downlink problem ( Coverage Hole / Interference)
17/7/2005 9 Company Confidential
9
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
5. BTS Receiver Problem
17/7/2005 10 Company Confidential
10
Access Failure - Troubleshooting
Access Failure - Uplink Problem -- Identification
RR Channel Request
RR Paging Request Type 1
RR Immediate Assigment
RR Sytem Information Type 3
RR Channel Request
RR Immediate Assignment
RR Immediate Assignment Extended
RR Paging Request Type 1
RR Channel Request
RR Immediate Assignment
RR Immediate Assignment
RR Paging Request Type 1
RR System Information 1
RR Paging Request Type 2
RR Paging Request Type 2
RR System Information
Return to Idle State
Blocked Call !!!
Access Failure !!!
Cause : Downlink Messages are seen after Channel Request.
Problem suspected in Uplink !!!!!
Channel Request
max retransmissions
17/7/2005 11 Company Confidential
11
Access Failure - Troubleshooting
Uplink Problem - Cause : AGCH Overloading
RR Channel Request
RR Immediate Assignment Extended
RR Immediate Assignment Extended
RR Channel Request
RR Immediate Assignment Extended
RR Immediate Assignment Extended
RR Channel Request
RR Immediate Assignment
RR Immediate Assignment Extended
RR Paging Request Type 1
RR System Information 1
RR Paging Request Type 2
RR Paging Request Type 2
RR System Information
Multiple Immediate
Assignment’s seen after
Channel Requests
17/7/2005 12 Company Confidential
12
Access Failure - Troubleshooting
AGCH Overloading - Root Cause Analysis
•If Multiple Immediate Assignment Extended Messages are seen,
problem could be AGCH overloading OR RACH Collisions/Non-detection
•If max_retrans and Tx-Integer are set to a lower value,
problem could be more towards RACH Collisions/ Non-detection
•If set high, then possibility of overloading is high!!
•Check for CCCH_CONF and BS_AG_BLKS_RES.
•If less blocks are reserved , problem is overloading.
•Analyze the OMC data for the same period for the following stats:
- No of Deletions
- No of Successful RACHs
- RACH Busy counts
- No of RACH’s with invalid establishment cause
17/7/2005 13 Company Confidential
13
Access Failure - Troubleshooting
RACH Collisions/ Max Range - Root Cause Analysis
•Max_Retrans and Tx-Integer set low - RACH Collisions Possible
•Check for Distance from Base Station
-- Plot a map for BCH ARFCN
-- Export the Channel Request and CellID Label
-- Import the Site ID’s Raster Images on the Map
-- Calculate distance between “Channel Request” and BTS
-- Compare this distance with the “Max_Distance_Allowed”
set for thjs cell.
-- Note: Max_Dist_Allowed is a BSC paramter ( not on Air )
-- If the MS distance is more than max_distance then problem
with Max Range
•If both the above conditions don’t meet, then problem is Non-Detection
17/7/2005 14 Company Confidential
14
Access Failure - Troubleshooting
RACH Non-Detection - Root Cause Analysis
•Downlink is fine !!! Parameters are well set for RACH control !!!
•Problem could be Uplink Quality / Base Station
•Analyze the following OMC Data
-- No of Invalid RACHs
-- Interference on Idle Channel
-- SDCCH RF Loss / TCH RF Loss
•If Interference and RF Losses are above normal, problem is Uplink Interference.
•If RF Losses are high, but interference is low , problem is Uplink level
•Uplink level poor indicates Link imbalance.
•If all above conditions are satisfactory and still “No of Invalid RACH’s” high, problem
could
17/7/2005 15 Company Confidential
15
Access Failure - Troubleshooting
Downlink
Fine
Analyze
OMC Data
No of Invalid
RACH’s
Interference
on Idle Channels
SDCCH/TCH
RF Losses
High High Uplink
Intrfer Yes Yes
Uplink
Imbalance
No
Link Imbalance
Test
UL-Interference
Test
No
High
Problem
Found
Identify Cause
Troubleshoot Yes
BTS TRX
BTS Testing
No
Abis
Monitoring
No
Yes
RACH Non-Detection
17/7/2005 16 Company Confidential
16
Now let us go a step further in understanding the most probable causes behind call block
problems.
* Access Failures
It could simply be caused by coverage holes . Interference could however play an
important role. Uplink interference on a serving cell can result in RACH rejections and hence no
AGCH assignments. Improper channel distribution between AGCH and PCH (paging channel)
can result in RACH/AGCH overloading. Paging Failures can be impacted by BCH pollution (co-
channel and adjacent channel interference).
* SDCCH Blocked
Heavy Traffic and excessive Location Updates can result in congestion of SDCCH
resources. Interference can block the channels , so though resources are available they may
not be able to be used.
* TCH Blocked
Heavy Traffic is the main cause of TCH congestion. The TCH can also be blocked
due to continuous interference in the uplink.
Solutions to access failures would be to ensure continuous coverage and optimization of CCCH
configuration parameters . For TCH and SDCCH congestion, the hot spots need to be identified
and load sharing techniques implemented. Some techniques that have been used successfully
involve adjusting cell powers to vary the coverage and therefore the location where mobiles will
handover from one cell to the next.
Interference management is essential for optimum network performance. Location updates can
be optimized by independent drive tests on the ALL BCH carriers. The delta is measured of
each BCH with the current serving BCH and the Reselect Hysteresis parameters adjusted
appropriately.
17/7/2005 17 Company Confidential
17
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
Optimize set up time if high, else modify channel configuration
17/7/2005 18 Company Confidential
18
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
-- Carry out Time slot testing , by setting Ignore Cell Barring.
17/7/2005 19 Company Confidential
19
Blocked Call - Interference
•Base Station Measures Uplink Interference on Idle Timeslots
•At regular intervals, categorizes Timeslots into Interference Bands.
•There are Five Interference Bands.
•Each Interference Band has a range of interference level.
Example : Interference Band “1” = -110 to -100 dbm
Interference Band “2” = - 99 to -90 dbm
Interference Band “3” = - 89 to -75 dbm
Interference Band “4” = - 75 to -60 dbm
Interference Band “5” = -59 dbm and above
•Network will assign Timeslots starting from lower band
•Interference Band “5” Timeslots are considered as “BLOCKED”
•OMC reports Hourly average statistics for each timeslot.
17/7/2005 20 Company Confidential
20
Timeslot - Testing
•Activate Cell Barring from OMC.
•Remove this cell from the neighbor list of
other cells.
•Get the cell configuration
•ARFCN’s and Timeslots configured for
TCH.
•For BCH carrier select the Timeslot and
carry out the Testing
•For TCH Carriers: Block the BCH Timeslots
from OMC
•Carry out Timeslot testing.
•If more than 1 TCH Carrier is activated,
block all others .
17/7/2005 21 Company Confidential
21
Congestion Relief - Redirection
•Most of the manufacturers now provide this feature
BSC
MSC
Channel Request
Immd Assign
MM/CC Signaling
Assignment Request
ALL TCH’s Busy
Allocates a Free TCH
Assign Command
*** Some of the systems may also do handover existing calls to strong
neighbors and assign TCH for this call from the same cell
17/7/2005 22 Company Confidential
22
Congestion Relief - Redirection
Role of Drive Testing
•OMC statistics may not reveal the actual congestion in a cell.
•Drive Testing may be required in the Peak traffic hours to estimate how
many times this happens.
•Drive Testing may also be required to optimize the neighbor list for
effectiveness of this feature.
17/7/2005 23 Company Confidential
23
Dropped Call Troubleshooting
17/7/2005 24 Company Confidential
24
Call drops are identified through SACCH messages. A Radio Link Failure Counter value is
broadcast on the BCH. The counter value may vary from network to network. At the
establishment of a dedicated channel, the counter is set to the broadcast value (which will be
the maximum allowable for the connection). The mobile decrements the counter by 1 for every
FER (unrecoverable block of data) detected on the SACCH and increases the counter by 2 for
every data block that is correctly received (up to the initial maximum value). If this counter
reaches zero, a radio link failure is declared by the mobile and it returns back to the idle mode.
If the counter reaches zero when the mobile is on a SDCCH then it is an SDCCH Drop. If it
happens on a TCH, it is a TCH drop.
Sometimes an attempted handover, which may in itself have been an attempt to prevent a
drop, can result in a dropped call.
When the quality drops, a mobile is usually commanded to perform a handover. Sometimes
however, when it attempts to handover, it finds that the target cell is not suitable. When this
happens it jumps back to the old cell and sends a Handover Failure message to the old cell. At
this stage, if the handover was attempted at the survival threshold, the call may get dropped
anyway. If on the other hand the thresholds were somewhat higher, the network can attempt
another handover.
17/7/2005 25 Company Confidential
25
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
HANDOVER FAILURE DROP !
17/7/2005 26 Company Confidential
26
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
17/7/2005 27 Company Confidential
27
We will examine the potential causes behind call drops and some solutions to combat them.
* Coverage
Poor non-contiguous coverage will reduce C/N and hence will reduce the Ec/No
and will result into call drops.
* Interference
This is one of the major causes of dropped calls. Interference could be co-channel,
adjacent channel or external. Under certain severe cell interference conditions, the call will
be dropped before a handover can be initiated.. Multipath interference can also add to the
problems. Strong signal reflections result in time dispersion issues resulting in a large
coherence bandwidth.
* Network initiated drops
Certain network features, like preemption, can kill an ordinary call to provide
connection to an emergency class subscriber.
A handover is the key to survival from dropping calls. But if there are problems in the
Handover process itself, then this will not avoid a drop.
Dropped calls can be effectively reduced by improving coverage, detecting and reducing
interference, setting appropriate Handover Margins , thresholds for handovers and the
correct selection of neighbors. Use of DTX and dynamic downlink power control will also
reduce average interference which should lead to some improvements.
17/7/2005 28 Company Confidential
28
Drop Call - Troubleshooting
RLT=0
RxQual > 4
YES
YES
YES
YES
Troubleshoot
Multipath/
Co-Channel/
BSIC decode
TCH=BCH Pri/Int <= 9
C/IAa <= -9 C/IAb <= -9
Co-Channel /
Multipath
Interference!! No
YES
No
No Adjacent Channel
Interference!!
YES YES
Troubleshoot
Source Cell
Using
BCH Analyzer
L3 Message
Analysis
No
Rx_Lev < -95
No **
Coverage Hole
!!!
Uplink
Problem !!!
No
Troubleshoot
Link Imbalance
UL-Interference
TCH -Pri/Int <= 9
Optimize
Neighbors/
Coverage
YES
No
** If TCH is Hopping, then all causes needs to be verified further,
before concluding on the root cause and optimizing.
YES
DROP
CALL
Handover Attempted ?
Handover Failed ?
Area of concern
No
Multipath No
17/7/2005 29 Company Confidential
29
•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
•Handover Failure
17/7/2005 30 Company Confidential
30
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
17/7/2005 31 Company Confidential
31
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
Echo and distortion : Generally caused by the Transmission and switching
system.
Audio holes : Blank period of speech, due to malfunctioning of Transcoder
boards or PCM circuits.
Voice Clipping : Occurs due to improper implementation of DTX.
Mean Opinion Score (MOS) : ITU standard for estimating speech quality.
17/7/2005 32 Company Confidential
32
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 withconsiderable effort, frequent repetitions needed)
1 Unsatisfactory ( speech not understandable)
17/7/2005 33 Company Confidential
33
Speech Quality - Estimation
Speech RxQual FER FH DTX Reason
BAD HIGH LOW NO NO Air Interface Impairments
BAD HIGH HIGH NO NO Severe Air Interface Impairments
BAD LOW HIGH NO NO Transmission and Switching system , Transcoder
BAD LOW HIGH YES NO Air Interface Impairments
BAD GOOD HIGH YES NO Transmission and Switching system , Transcoder
GOOD HIGH LOW YES NO Hopping Implementation
CLIPPING LOW HIGH YES YES Hopping Implementation and VAD
CLIPPING LOW LOW YES YES VAD
ECHO LOW LOW Y/N Y/N Transmission and Switching system , Transcoder
MOS up to a certain extent can easily estimated by configuring an algorithm
using the Alarms in the HP E74XX systems for the following elements, an
example of subset of which is illustrated above
- RxQual Full and Sub
- RxLev Full and Sub
- FER and RLT
- L3 Measurement Report
- L3 Handover specfic messages
17/7/2005 34 Company Confidential
34
Speech Quality - Troubleshooting
RxQual> 6
FER > 1%
YES
YES
YES
YES
Troubleshoot
Multipath/
Co-Channel/
BSIC decode
TCH=BCH Pri/Int <= 9
C/IAa <= -9 C/IAb <= -9
Co-Channel
Interference!!
No
YES
No
No Adjacent Channel
Interference!!
YES YES
Troubleshoot
Source Cell
Using
BCH Analyzer
No
Rx_Lev < -95
No **
Coverage
Hole !!!
Uplink
Problem !!!
Troubleshoot
Link Imbalance
UL-Interference
TCH -Pri/Int <= 9
Optimize
Neighbors/
Coverage
YES
No
** If TCH is in Hopping, then all interference causes needs to be verified further,
before concluding on the root cause and optimizing.
YES
Poor
Quality
Area of concern
Hopping ON
YES
RLT going down
DTX_DL ON
FER > 2% YES
No No
No No
Transmission/
Switching Systems
Transcoder
Patchy Quality
Multipath
High Probability
with Hopping ON
YES
No A
A
B B
A
MOS < 4
17/7/2005 35 Company Confidential
35
Troubleshooting Poor Speech Quality
Method 1 : Post Processing
Create the Following PLAN Wizards in Export
Quality-PHONEDATA
Rx_Lev
Rx_Qual
FER
RLT current Value
Timing Advance
Tx Level
ARFCN ( BCH and TCH)
Serving Cell Information
-- CellID
Quality-Cell Information
Serving Cell Information
-- CellID
-- BSIC
Neighbor Cell Measurements
Neighbor Meas Count()
Timeslot
ARFCN
Interference
Co-Channel Interference
-- Channel
-- Primary/Interferer
-- Primary BSIC
-- Secondary BSIC
-- Textual Meas Status
-- Numeric Meas Status
-- Fading
Adjacent Channel Interf
-- AdjA Channel
-- AdjA N/N-1
-- AdjA N/N+1
-- AdjB Channel
-- AdB N/N-1
-- AdjB N/N+1
ARFCN
17/7/2005 36 Company Confidential
36
Filtered Spots- RxQual > 4
Bad Spot 1
17/7/2005 37 Company Confidential
37
Troubleshooting - Bad Spot 1
17/7/2005 38 Company Confidential
38
Which is this CELL ??
•Select QualPhoneData Layer.
•Check on Labels
•Select Labels
•Select CellID.
•Display it on the MAP
17/7/2005 39 Company Confidential
39
Which is the cell ? 47450 !!
17/7/2005 40 Company Confidential
40
Did the Call Drop ???
•Create RLT.tab
Query for RLT < 8
•Import as Label
Layer
RLT = 0 , DROP CALL !!
17/7/2005 41 Company Confidential
41
Conclusion
•BAD Spot 1 has poor quality and Call Drop
•This spot is covered by Cell 47450
•Poor Coverage. Level below -97 dbm
•But Call should not Drop
•The other Problem is Interference.
•Mobile is Hopping on 99 and 84
•99 is also the BCH.
•Co-Channel on BCH is very high.
•50% of the time quality will be poor.
•But Poor Quality is consistent.
•Channel 84 is also suffering from Interference.
•No Adjacent Channel on 84 and 99
•This means there is Co-Channel on 84 also.
•It could also be multipath issue on 84.
•WHY NO HANDOVER
17/7/2005 42 Company Confidential
42
Troubleshooting Handover Problems
17/7/2005 43 Company Confidential
43
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
Sequency Number
Weak Neighbors - Reported by Phone
17/7/2005 44 Company Confidential
44
Analyzing - No Handover
No Neighbor( very weak)
17/7/2005 45 Company Confidential
45
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
Real Time
17/7/2005 46 Company Confidential
46
Missing Neighbor is a very critical problem in optimizing GSM neighbors.
Missing Neighbor as such doesn’t means that there is no neighbor. We
define neighbors to a cell based on the geography, I.e which base stations
are nearby. In certain cases we may also use the Planning Tools to predict
neighbors. It is such not desirable to use excessive neighbors also, since
this reduces the samples collected per neighbor. Since Mobile has to look
for each neighbor in one TDMA frame, and we have 104 frames for
measurement. This means if we define 32 neighbors , the number of
samples per neighbor will be only 3-4, which means the authenticity of
Handovers will not be there. So we need to define appropriate neighbors
only. In certain cases the neighbors we define may not be able to cope up
with traffic or at certain significant patches of the cell, these neighbors
signals would be weak and may result into no handovers or handover
failures. So in such kind of problem spots , we should be see the possibility
that signals from any other cell is coming or not, so that we can define that
as a neighbor. So how do we find such missing neighbors.
17/7/2005 47 Company Confidential
47
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 wether 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.
17/7/2005 48 Company Confidential
48
Interference Measurements and
Troubleshooting
17/7/2005 49 Company Confidential
49
Types of Interference
We already know ….
Co-Channel
•BCH - BCH
•TCH - BCH
•BCH - TCH
•TCH - TCH
Adjacent Channel Interference
•BCH - BCH
•TCH - BCH
•BCH - TCH
•TCH - TCH
Multipath
17/7/2005 50 Company Confidential
50
TroubleShooting
17/7/2005 51 Company Confidential
51
Uplink Problems
17/7/2005 52 Company Confidential
52
Locating Uplink quality Bad Spots
RX_Level is near P_Con Threshold
Ms Tx Level is max ( 5)
17/7/2005 53 Company Confidential
53
Locating Uplink Bad Quality Spots.
Trend shows:
High receive level
And High Ms Tx Levels
in this area.
17/7/2005 54 Company Confidential
54
Locating Uplink Bad Quality Spots
•At some areas for low signal level ( < -88), MS at full power
•In another area for high signal level ( -75 to -85 ) , MS full power
•MS Tx Level is controlled by the network.
•Network will ask MS to increase power if Uplink level is poor or
Quality is poor.
•Link Imbalance will result into low uplink level even if DL Level is
good.
•Then MS will be at high power in all those spots where level is
good. BUT THIS IS NOT THE CASE !!! ( as seen from two spots)
•This means this is a problem of Uplink quality being poor at good
level.
•This can happen due to UPLINK INTERFERENCE.
•This interference seems to be burst in nature !!!!
17/7/2005 55 Company Confidential
55
Troubleshooting - Uplink Interference
•Uplink Interference can be due to:
• Mobiles in reuse and adjacent channel cells
• Spurs and Intermods from base stations
• 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.
17/7/2005 56 Company Confidential
56
Termination Failures Troubleshooting
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Troubleshooting - Termination Failures
•Termination Failures could be due to:
•Paging Problems with Network
•No response to page from MS
•Blocking of Resources
•If Paging Success Rate = 100 %, but still the caller reports
termination failures, then this Blocking.
•If Paging Success Rate < 100 % ,then this could be :
- Paging Problems with Network Or
- No response to page from MS
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Troubleshooting-Termination Failures
•Verify and Isolate the problem as:
•Interference
•Excessive Cell Reselection
•Excessive Location Updates
•None of above indicates Network Problem ( no paging)
•Excessive Cell Reselection and Location Updates can occur due to
•Interference ( Downlink signaling Counter reaches 0)
•BCH Pollution
•Overlapping of Location Areas.
•Carry out protocol analysis on Abis and A Interface to isolate network
problem further
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Cell Optimization
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Optimization
Cell Loading
C1
C2C3
•Cells C1 and C2 are reporting Blocking
•Cell C3 is added to cater for capacity
•Still C1 and C2 report blocking. Why ?????
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Optimizing - Cell Loading
More Traffic could be persuaded towards C3 by :
•Increasing the Power Output of C3 BTS
•Tilting Antenna’s towards Hot spots in C1 and/or C2
•Applying Reselect Offsets to certain less loaded neighbor cells
With this approach more MS's will camp on or do cell reselection to C3, thus
reduce load in C1 & C2.
These issues are very common to fast growing networks, because new cells
when are planned , we don't know the HOT SPOTS.
In most cases , new cells investment could also be minimized by understand
the RF environment and optimizing cell loading.
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Optimizing - New Cell Site Location & Antenna Tilts
•New cell sites in the earlier phase of the network are implemented to
enhance coverage.
•Coverage enhancements takes priority over areas of predicted traffic
•Once coverage objectives are met, cell expansion takes place purely
on the basis of traffic.
•New BTS are sited near to the cells which are generating blocking
conditions.
•The objective behind new cell site investment is to take the load from
the existing congested cells and hence increase revenue.
•An existing congested cell would be covering an area of 1 sqKm.
•Traffic in this area would be concentrated in small patches known as
“HOT SPOTS”.
•Network Planner always intends to optimize the cell site location or the
antenna tilts to target on some of the Hot spots .
•But the question is do we precisely know where are the Hot spots ???
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Optimizing - Cell Reselections
•Mobile Monitors the Neighbor cells BCCH Carriers and decodes SCH
of the neighbors in the idle mode.
•The list of the BCCH ARFCN’s which the mobiles will monitor in the
idle mode is broadcasted in the System Information Type 2 Message .
•More the neighbors , less the samples taken by the mobile per
neighbor to the calculate the value of C1.
•More neighbors of good and near to equal power levels will result into
excessive reselections.
•Excessive reselections can result into missing of paging messages
•Solution to this problem would be to adjust power levels of base
stations.
•Adjusting power levels most of the time either effects coverage or
enhances interference, since the terrain is not flat.
•To optimize cell reselections, the best possible solutions are:
• Optimizing the BA list
•Implement C2 Reselections
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Optimizing - Neighbor lists
•Maximum 32 neighbor
•Less samples, averaged reported value not authentic
•Results in Ping-Pong effect of Handovers
•Neighbor list must be optimized
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Mobile in the dedicated mode measures the BCH of the neighbor cells.
The list of BCH to be measured is informed to the Mobile in the System
Information Type 5 message which can have maximum 32 neighbors BCH’s
Mobile has to report the measurements of the neighbors to the network , every
480ms which corresponds to 104 TDMA frames.
Mobile will take several samples of the neighbors, average it and then send the
report as one value of Rxlev of the neighbor, with a minimum requirement of
measuring at least 1 neighbor in 1 TDMA Frame.
So , more the BCH’s in the neighbor list, less will be the number of samples.
If the Mobile just meets the minimum requirement, then for a neighbor list of 32,
only 3 - 4 samples per neighbor will be available and 12-13 samples per
neighbor with a list of 8 neighbors
If the samples collected are less, then average value reported will not be
authentic, and can result into Ping-Pong effect of handovers.
So, the Neighbor list must be optimized with the minimum best possible
neighbors
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Optimizing -Handover Traffic
A B
C D
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As shown above, Cell B is a cell which is reporting Blocking due to heavy traffic.
Cell B on OMC analysis is found to have traffic generated due to call setups
within the cell , as well as handovers, since it covers a major highway with
heavy mobility traffic coming from Cell A.
Instead of implementing an excess TRX or a new cell to resolve this issue, we
can optimize this problem, we can optimize the neigbbor lists, so as to drag the
Mobiles coming from A to handover to cells other than B, unless B is the only
candidate for handover.
We can drive through the area between Cell A and Cell B and mark the spots
where handover occurs.
Look for the continuity between BCH’s. This will indicate to us, which are the
strong BCH’s other than Cell B which could be potential targets for handovers
and with what handover margins, so in case we don’t get an alternative for a
certain patch which is above the handover margin threshold, we can reduce the
handover margin .
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Optimizing - Handover Margin
• Handover Margin is value set for all cells, which will allow mobiles to
handover to the cell, only when this cells power budget balance
exceeds the source cells power budget balance by this value.
• Handover Margins main objective is to avoid ping-pong effects of
handovers on cell periphery, when the power levels of two cells are
near to equal , so due to multipath and mobility, this will result into
frequent handovers between these two cells.
• Setting this value low, will result into fast handover to the target cell,
which may result in improvement in quality.
• Setting this to high value will delay the handover to the target cell, and
ensure that when handover takes place, the probability of the mobile
going back to the source cell is very low for some significant period.
• Two examples of use of this parameter are illustrated ( next page)
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Adjacent Channel Interference Reduction
Tight Re-use patterns can permit the use of Adjacent ARFCN in
adjacent cell.
This should not pose an problem if Handover margin is set
appropriately low.
If Handover Margin is set to 6 dB, Handover will occur from C1 to
C2 at spot B itself
C/Ia = -1 db C/Ia = -6 db C/Ia = -9 db
A B C
C1 C2
Optimizing - Handover Margin
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H
Multipath solutions
Antenna Tilts
Using Directional Antenna's
Shifting the BTS site
Handover Parameters
C1
C2
Reduce HO_margin
C3
Increase HO_margin
Reduce HO_margin
Optimizing - Handover Margin
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Optimizing - Handover Margin
• Selecting an appropriate value of handover margin
is essential to ensure good quality communication .
• Handover margin set for a cell is applicable to all
calls going on in the cells, to which this cell is a
neighbor.
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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
---- Frequency Hopping /DTX
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In order to optimize interference the cells which are generating high level of
interference should be located from the OMC Performance Database. It is essential
to locate the source of interference whether it is external or internal. If it is internal
then it could be due to multipath, adjacent, co-channel, inter-modulation or
spurious. This can be located by analyzing the spectrum. There are various
methods involved in optimizing interference. Appropriate Power Control of the
interfering entities could be done. For Co-channel Interference problems, Tilting of
antenna could be done with proper coverage aspects, for adjacent channel analysis
should be done to locate if any adjacent channel is allocated to the neighbors , If it
is then frequency allocation could help with this. It is very essential to carry out
Transmitter and Mobile tests for Power v/s time output spectrum and inter-
modulation products, generally a mobile should be tested for these basic tests
before being dispatched. In the Uplink Space Diversity is the worldwide adopted
method for reducing the effect of Multipath fading, where as in the downlink it is
the mobiles equalizer which has to do this job. One very relevant solution to
interference is frequency hopping where the mobile constantly changes frequency
for every burst which reduces the effect of interference. Implementation of
DTX.DRX also reduces average interference.
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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
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Antenna Tilts
x 6 km 1 km
A
M 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.