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CELLULAR NETWORK
The geographical area to be covered by the Radio Network is divided into cells. A cell is the area cover
1. Type of Cells
There are two main types of cells: omni-cell and sector-cell
An omni cell is served by a BTS placed in its center. The antenna system transmits equally in all dire
A sector cell is served by a BTS placed on its edge and uses a sector antenna (e.g. 120 or 180). On
Typically, omni directional cells are used to gain coverage, whereas sector cells are used to gain ca
Example of a 32 sector site RBS2000 CDU A
BTS Configuration
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Antenna Configuration top view (with duplex filter)
2. Frequency Allocation
Since an operator only has access to a limited number of frequency carriers, the frequencies have t
Example: a reuse-factor 3.
The distance D between two cells using the same frequencies must be long enough to ensure that
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Ericsson recommends (C/I)min = 18 dB for analog systems (NMT, TACS) and 12 dB for GSM. But
Example MRP :
The operator has 37 frequencies. BCCH carriers have a frequency reuse factor 12. The second TR
3. Cell Capacity
Cell planning begins with traffic and coverage analysis. The analysis should produce information ab
Cost
Capacity
Coverage
Grade of Service (GoS)
Available frequencies
Speech quality
System growth capability
The basis for all cell planning is the traffic channel demand, i.e. how many subscribers use the net
A = n x T/3600 Erlang
Where,
A = offered traffic from one or more users in the system
n = numbers of calls per hour
T = average call time in seconds
The geographical distribution of traffic demand can be calculated by the use of demographic data s
Population distribution
Car usage distribution
Income level distribution
Land usage data
Telephone usage statistics
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Other factors like subscription/call charge and price of MSs.
Calculation of required number of BTSs.
To determine the number and layout of BTSs the number of subscribers and the Grade of Service (
If n = 1 and T = 90 seconds the traffic per subscriber is:A = 1 x 90/3600 = 25 mE
If the following data exist for a network:
Number of subscribers = 10000
Available frequencies = 24
Cell pattern = 4/12
Gos = 2%
Traffic per subscriber 25 mE
This lead to the following calculations:
Frequencies per cell = 24/12 = 2
Traffic channels per cell = 2 x 8 2 (control ch.) = 14 TCH
Traffic per cell = 14 TCH with a 2% GoS implies 8.2 Erlangs per cell (use erlang table)
The number of subscribers per cell = 8.2 E / 25 mE = 328 subscribers per cell
If there are 10000 subscribers then the number of cells needed is 10000 / 328 = 30 cells
Therefore the number of three sector sites needed is 30 / 3 = 10
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d by a set of frequencies (transceivers). Each cell has at least one control channel (for BCCH, PCH, S
tions (360) and can be constituted of an omni-antenna or an array of sector antennas.
e BTS site can serve one, two or three sector cells.
pacity. Each cell requires two RX antennas (for RX diversity) and a number of TX antennas (depending
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o be reused in the network. In order to prevent co-channel interference (C/I), the frequencies are reused
/I is superior to a certain threshold. With a reuse factor 9, C/I will be superior to 9 dB; with reuse factor
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dB is acceptable if interference-reducing features are used (such as frequency hopping, DTX, MS/BT
in each cell have a frequency reuse-factor 8, the third have a reuse-factor 6 and the fourth have a reu
out the geographical area and the expected capacity (traffic load). The types of data collected are:
ork and how much traffic they generate. The Erlang (E) is a unit of measurement of traffic intensity. It c
ch as:
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GoS)have to be known. The GoS is the percentage of allowed congested calls and defines the quality o
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CCH, etc)
on the type of TX combiner). It is possible to reduce the number of antennas by using duplex filters and
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in an organised way, according to certain re-use pattern.
12, C/I > 12 dB and with reuse factor 21, C/I > 18 dB. If we use 3-sector sites, the corresponding cell pa
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power control). This means that a 4/12 pattern must be used for BCCH carriers (which cannot use fea
e-factor 4.
n be calculated with the following formula.
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the service.
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X-polarised antennas, see below.
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tterns are called 3/9 (3 sites/9 cells), 4/12 and 7/21. The figure below shows 4/12 and 3/9.
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ures such as fhop, DTX, pwr ctrl). However, the other carriers can have tighter reuse factor such as 9 o
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r less. This is called Multiple Reuse Pattern (MRP).
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Handover Analysis
Handover is an important function, which shows the
integrity of the GSM network. If the handover
performance is poor, the subscriber will perceive the
quality of the network as bad.
Bad locating parameter settings
Uplink interference
Missing measurement frequency in BA-list
Extra measurement frequency
Co-Channel and Co-BSIC
Unnecessary neighbor relation
HW faults
Permitted NCC (network color code)
Wrong use of HCS parameters
Congestion
Probable Reason of Bad Handover Performance
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Handover Analysis Flowchart.
The following procedure should be performed for
handover analysis:
- Check the successful handover per cell
- Check for the handover success rate below certain cr
- Check the handover activity from the number of han
- Check the site location, whether the site to site
distance or co-sited. (Good map is needed here).
- Check whether the cell is isolated or not, if already k
- Check the handover flow whether is balance between
- Check the difference performance of incoming and o
- Focus on the bad direction.
- Check the worst relation and pick one cell.
- Check the cell whether it is external or internal. If
external, start inter-BSC analysis.
- If the cell is internal, check the high ratio of lost han
- If one of them is yes, check the uplink & downlink in
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- Check the frequency plan. There might be problem o
- Check the handover related parameters such as
relation, BA-list, BSIC, hysteresis, offsets etc.
- Check if many Ping-Pong handover.
- Check if assignment handover are used.
- Check if cell has HW problem. If problem found, swa
- Check if congested target cell. If the target cell is
congested and then solve the congestion by adding
TRU.
- Perform measures to improve HO performance. This
No or Few Handover Attempts
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No or Few Handover Attempt Flowchart.
The following procedure should be performed for no
or few handover attempts:
- Check the neighbor relation with low traffics (less th
- Check from the unbalance relation whether the targe
- Check the missing measurement frequency (MBCCH
- Check unnecessary neighbor cell relations and if
found, remove unnecessary relations.
- Check NCCPERM, if NCC of the neighbor is missing,
- Check the setting of the cells whether they are set
in different layer of HCS. (Layer 1 is the highest
priority)
- Check if the cell is at the inter BSC border, if so , ch
- Check locating parameters and list out.
- Check the poor setting of parameter and if so, corre
- Check BTS definition (RX commands especially
RXMOP).
- Check if BTS is defined but might not be in service. I
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Bad setting might cause the locating will not rank the
best cell as a candidateIncoming handover failed as the target cell could not
decode the handover burst messa e from the mobilePrevent any handover to affected neighbor cells
Can provide inaccuracy of measurement for handover
decisionsMeasurement result from neighbors can not be
distinguished and MS may perform a handover to the
wron cell.
None or few handover might indicate a unsuitablenei hbor relation.Can cause bad neighbor relation.
Missing NCC of the neighbors will not allow any
handover.Cause unnecessary handover
High congestion might lead to dragged calls (handover
performed at a not intended location) a lot of
unsuccessful handover.
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iteria for example X%. If it doesnt meet the criteria and change the X% to a higher val
over performed. If the number is above certain value for example Y and then take into
nown problem of no dominant serving cell and take other cell.
incoming and outgoing.
tgoing handover. This is to ensure the priority for more problem direction.
over, urgency handover and reversion.
erference problem.
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n co-channel or adjacent channel.
or repair hardware.
is the assumption comes from the above analysis.
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n 10% of average number of handover per relation) and unbalance relations.
t cell is congested, if congested, solves the congestion problem.
O) in Double BA-list in active list (if yes, add the measurement frequency in the BA-list
dd the NCC of the neighbor list into the NCCPERM list.
ck the inter BSC HO performance (SIEMENS System).
t the parameter setting
f yes, correct parameter setting or deblock it.
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ue and check for the next cell.
consideration.
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.
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TCH Assignment analysis
Successful assignments show the number
At unsuccessful assignment, the Assignme
The formula is defined as:
No dominant serving cell
Severe congestion on TCH
Low signal strength for call access
InterferenceFaulty transceiver
The following procedure should be perform
For TCH assignment success rate, the first
If there is congestion on TCH, it is recomm
If there is no congestion on TCH, check the
If the output power is ok, check the faulty
If hardware fault found, swap or repair HW.
Perform drivetests to check the coverage a
If no dominant cell or similar signal strengt
If there is no problem on the dominant cell,
Check the disturbance whether it is on SDC
Mostly, the problems of low TCH assignme
Traffic congestion is one of the major netw
Short term growth
If the high traffic related to an occasional e
Long term growth
% TCH ASSIGNMENT SUCCESS RATE
Probable Reason
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If there is a long-term growth the network
Type of Congestion
The congestion analysis begins by identifyi
Consider how many channels that are alloc
SDCCH Congestion
In R8, the time congestion should be used i
Ericsson recommends using the SDCCH Ti
where PERLEN is the measurement period i
SDCCH Congestion Analysis
The flowchart below, Figure 51, explains a
SDCCH TIME CONGESTION OF TOTAL
MEASUREMENT INTERVAL
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Diposkan oleh waveice di 00:20 0 komentar
Low Availability
What should I check?
SDCCH Availability.
Where do I look for it?
STS Counters on SDCCH availability. Th
Run BSC commands to see if the chann
Why do I need to check this?
Available SDCCHs of Total Number of Defined
SDCCHs
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We need to check this area to make sur
Location Area Border Cell &
What should I check?
Check site position and location area bo
Where do I look for it?
Refer to site database for site info.
Location Update performance should coYou can run BSC command RLSSPto ch
Why do I need to check this?
If the cell is situated on a misplaced Loc
The CRH is the hysteresis value used w
TCH Congestion
What should I check?
Check if TCH congestion exists and if th
Where do I look for it?
Use STS Counters and look into time co
Note that if the feature Adaptive configuration of l
What should I check?
Check if TCH congestion exists and if th
Where do I look for it?
Use STS Counters and look into time co
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If activated, check CLS and Assignment
Why do I need to check this?
TCH congestion causes the mobiles to s
SMS Usage
What should I check?
Check SMS activity.
Where do I look for it?
Check STS counters for SMS on SDCCH.
Why do I need to check this?
Extensive SMS usage increases the SDC
Too Frequent Periodic Regis
What should I check?
Check Random Access Distribution. Che
Where do I look for it?
STS Object Type RANDOMACCfor Rand
Why do I need to check this?
To prevent intervals of Location Updatin
Increasing Traffic Demand
What should I check?
Check if its short-term traffic growth an
Where do I look for it?
TCH TIME CONGESTION OF TOTAL
MEASUREMENT INTERVAL
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Compare the traffic trends and refer to
Why do I need to check this?
The high traffic could be related to an o
Cell Broadcast Used
What should I check?
Check if Cell Broadcast is active.
Where do I look for it?
Run BSC command RLCCP.
Why do I need to check this?
CBCHis the cell broadcast channel and i
Long Mean Holding time
What should I check?
Check SDCCH Mean Holding Time.
Where do I look for it?
STS counter for SDCCH Mean Holding ti
Why do I need to check this?
If the mean holding time is too long, thi
SDCCH Dimensioning
What should I check?
Check how SDCCH is dimensioned.
Where do I look for it?
Check the SDCCH settings from CNA pa
SDCCH MEAN HOLDING TIME
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Why do I need to check this?
As no of subscribers steadily increase, p
Bad use of Adaptive Configu
What should I check?
Check if ACSTATE is on or off. If its on,
Where do I look for it?
Run RLACPin BSC for ACSTATE status
Why do I need to check this?
By using the Adaptive Configuration of
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f successful TCH allocations at call setup.
t Complete message, sent by the MS, was never received by the BTS.
= TCASSAL
L
X 100 %
TASSALL
The serving cell cannot cope with the TCH traffic.
Failing TCH allocation for assignment or handover due to
con estionThe signal strength might be higher on the BCCH than on the
TCH.
Disturbance on SDCCH or target TCHFaulty equipment
d for TCH Assignment analysis:
hing, check the TCH Time Congestion.
nd doing the dimensioning and adding TRU based on carried TCH traffic demand.
output power of the BTS. If the output power is low, increase the output power.
TS by extracting BTS error log.
d received RxLEV.
hs of a few cells found during drivetests, it is recommended to add BTS.
check the interference whether co-channel or adjacent channel.
CH or target TCH. If disturbance found, improve the frequency plan.
t are TCH availability and interference.
rk problems in a mobile system. A high congestion deteriorates the overall performance of the
vent, like sports event, fairs, conference, a temporary solution might be considered.
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capacity has to grow according to the demand.
g if there is only SDCCH or TCH congestion or both. Congestion on both SDCCH and TCH may
ted in the cell. If possible, expand the capacity with new transceivers, otherwise a new site m
nstead of congestion based on access attempts as there is no way to estimate the number of a
e congestion as a KPI in R8. The formula is defined as:
= CTCONG
S
X 100 %
PERLEN
* 60
n minutes.
lowchart
eneral approach to investigate SDCCH Congestion. The next section describes the action point
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e formula is:
= CAVAAC
C
X 100 %
CAVASC
AN *
CNUCHC
NT
ls are manual, control or automatic blocked.
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that there is no hardware problem as faulty equipment can affect SDCCH availability. If need
RH
rder, Location Update Performance and parameter CRH.
e from the MSC. eck CRH value.
ation Area border, this means that unnecessary many normal Location Updating are performed
en the MS in idle mode crosses a LA border. The default value for this parameter is 4. If a high
SDCCH mean holding time is high. Check CLS and Assignment to Worse cell parameter settin
gestion to see how severe is the TCH Congestion. The formula is:
ogical channels is active the formulahave no meaning. Then S_AV_NR is recommended to be used instead.
SDCCH mean holding time is high. Check CLS and Assignment to Worse cell parameter settin
gestion to see how severe is the TCH Congestion. The formula is:
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= TFTCON
GS
X 100 %
PERLEN
* 60
to worse cells parameters from CNA.
ay extra long time on the SDCCH. Increase TCH capacity if required. Utilize the features for tra
Counters are CSMSDWNand CSMSUP.
CH traffic and could cause congestion if badly dimensioned SDCCH channels. Re-dimension the
ration
k the timer T3212 in the BSC.
om Access and look into CNA for T3212 setting.
g which is too short that it affects Random Access Performance to deteriorate.
d whether combined SDCCH is used.
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lanning on how SDCCH is dimensioned.
casional event or due to a long-term growth. If its a special one-time event of surge in traffic,
ndicates whether or not a CBCH shall be included in one of the SDCCH/8 sub-channels for the
e. The formula is:
= CTRALAC
C*PERLE
N*60
X 100 %
CNSCAN
*CMSES
TAB
s can possibly generates a higher traffic load.
ameters.
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revious SDCCH dimensioning plan might be insufficient. Review SDCCH dimensioning requirem
ation of Logical Channels
check the parameter settings.
nd use CNA to check these feature parameter settings.
ogical Channels feature, the basic SDCCH configuration in a cell will be under-dimensioned. If
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network and should be minimized.
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mean that the only way to get rid of the congestion is to add more physical capacity in ter
st be implemented. Frequency planning schemes such as MRP and FLP could be used to r
cess attempts a single mobile does.
in this flowchart. The reference to each action point is indicated on the flow chart as well
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d, the O&M procedures need to be reviewed.
. If the site is located close to major road or railway, consider moving the Location Area b
number of Location Updating occurs in a Location Area border cell, a higher CRH can be s
s.
s.
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ffic distribution such as CLS and Assignment to Worse Cell.
SDCCH channels with consideration taken to SMS usage.
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then it shouldnt cause too much concerned. But if the trends continue, then SDCCH migh
ell or channel group. It is defined per cell or per channel group. If it is active, check if the
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ent to existing demands.
his feature is not used correctly, it will cause SDCCH congestion.
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ms of transceivers or sites.
lieve congestion. Microcells cou
.
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rder.
t in order to reduce the numbe
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t need to be re-dimensioned
network uses it. If it is
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Frequency Planning (GSM)
CELL
Definition: Area Coverage of Radio Base of Station.
Type :
Sectored Cell, Omni Cell, etc
Coverage measure:
MacroCell (>5 km), MicroCell (3
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Reuse factor
Tighter Reuse : + higher capacity ; Interference between cells.
Design Frequency Reuse:
Basic Parameters:
C/I = Measure of signal quality
* FDMA (AMPS) : C/I > 18 dB ===>
====>
* TDMA (GSM) : C/I > 12 dB ===>
====>
* CDMA : Quality evaluated from Eb/Io
Cluster (K) = Frequency grouping
*Measure of Cluster depended from C/I system condition.
N ( Total Frequency Spare)
Example : Frequency Reuse pattern with N = 6so that: K = 4
SIGNAL TO INTERFERENCE RATIO :
1. Consider closest co-channel cells (SIRmin)
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2. Measure of Cluster (N)
3. Co-Channel reuse distance ratio
CELL SPILTTING
Is needed at the time of :
1. Overload traffic on cell mount.
2. TCH Congestion
To increase the capacity, the operator performs cell splittings:
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Strategy Frequency Reuse pattern for cell splitting
Type of Interference :
Co Channel Interference
Adjacent Channel Interference
Reasons is:
Poor frequency plan
No dominant cell > low C/I > increased Rxqual
Frequency Hopping is off
External interference e.g. jammer
Analysis :
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Identify areaswith Rxqual > 5
=>> Identify source using Map and STS
=>> New drive test around problem area?
Solution:
Enable Frequency Hopping or add frequencies to Hopping group.
Enable BTS Power Control, MS power control and make it less aggressive.
Change frequency of interferer or interfered cell (Possible to find new frequency by using TEMS Scan
Down tilt or change antenna of interferer.
Redo frequency plan.
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wer, etc.
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ning).
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Interference
Type of Interference :
Co Channel Interference
Adjacent Channel Interference
Reasons is:
Poor frequency plan
No dominant cell > low C/I > increased Rxqual
Frequency Hopping is off
External interference e.g. jammer
Analysis :
Identify areaswith Rxqual > 5
=>> Identify source using Map and STS
=>> New drive test around problem area?
Solution:
Enable Frequency Hopping or add frequencies to Hopping group.
Enable BTS Power Control, MS power control and make it less aggressive.
Change frequency of interferer or interfered cell (Possible to find new frequency by using TEMS Scan
Down tilt or change antenna of interferer.
Redo frequency plan.
Add new site.
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ning).
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Handover Ping Pong (Repeated)
Handover case :
1. Repeated handover (Ping Pong)
2. Repeated Intra-cell handover
3. Handover failure4. Unrealistic handover
Reasons :
1. Repeated HandOver (Ping pong)
No dominant server
Locating penalty settings e.g. PSSBQ, PTIMBQ
Too small hysterisis setting
Solution :
a. Change physical Antenna
Antenna direction
Antenna Tilts
Plan for new site Change
Locating Parameter e.g. KHYST or LHYST
b. Change Parameter, KHYST or LHYST
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2. Repeated Intra-Cell HandOver
Repeated Intra-Cell HandOver the following reasons is:
Wrong intra-cell handover settings
External Uplink Interference
Solution :
Consider to change parameters:
SSOFFSETUL, SSOFFSETDL
QOFFSETUL, QOFFSETDL
A wrong setting of parameters will cause unnecessary IHO
Turn off Intra-cell Handover?
3. HandOver Reversion
Check layer 3 message
Handover command BSC correct?
Check reason for handover SS, RXQUAL
Target cell?
Handover Reversion the following reasons is :
Duplicates of BSIC BCCH combinations in neighboring list
Solution:
Change one of the BSIC between BTS with same BSIC or Rotation Cell Parameters.
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Other Reasons:
Uplink interference in target cell
Downlink interference at MS
Solution:
Interference Investigation
Statistics
Scaning
4. Unrealistic HandOver
Reasons :
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Wrong locating settings, in Serving or Target cell
Note : Could be on purpose e.g. Dual Band network with HCS
Solution:
Fix locating settings e.g. MSRXSUFF, BSRXSUFF
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Call Setup Failure Analysis
Call Setup Failure the following reasons is :
1. Low Signal Strength
2. Interference3. High Congestion on SDCCH
4. High Congestion on TCH
5. Transmission Fault
6. Faulty TRU/Hardware
7. Central Processor Overload
8. Software File Congestion
9. Cell is not defined in MSC
Possible Solutions:
Low signal strength=> Signal strength analysis.
Interference => Interference analysis.High Congestion on SDCCH:
==> Define more SDCCHs.
==> Activate adaptive configuration of logical channel feature and immediate assignment to T
High Congestion on TCH:
==> Add capacity/Tune coverage array.
==> Activate assignment to worse cell.
Analysis :
SDCCH Blocking
Multiple Channel Requests.
Successful Random Access after the third try.
==> Congestion in SDCCH.
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From plot, identify which & where in logfile the dropped call occurs.
In the log file, Jump the drop call event.
Check if there is a disrupt in the recording. If yes ==> could be a false drop due to TEMS mobile disc
Check the Radio environment just before drop:
If High Rxqual for a longer period of time and RLINKTexpire ==> Interference problem.
If Low SS DL, SSDL < MS SENS ( -104dBm) ==> low signal strength problem.
If TA> 63 ==> too far from the cell.
If DL radio is good, check the TX power. If there is MS power down regulation when the MS is close t
Verify or isolate the problem using STS (Drop call and reasons).
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H.
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nnection during recording!!!
the cell. If full power ==> suspect uplink interference or antenna, TMA problem.
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Signalling protocols are decomposed into layers, each layer having a specific function.
Example :
The protocols used in BSS are :
On Layer 1 :
G.703. This protocol is used in the Transmission Network ( A, A-bis)
Signalling processing and Radio. This used in the Radio Network ( Um)
On Layer 2 :
LAPD (Link Access Procedure on D-Channel). This protocol is used on A-bis, for safe transport of BT
LAPDm. This protocol is use on Um, for safe transport of Traffic management messages between M
On layer 3 :
BTS O&M. This protocol is only used on A-bis (between BTS and BSC), for operation and maintenan
Traffic Management. This protocol is used on all interfaces ( between Mobile, BTS, BSC and MSC) to
Signalling used layer 1, 2 and 3 while speech and data only layer 1.
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S O&M and Traffic Management messages. BTS O&M use the OML Link, Traffic Management use the
bile and BTS.
e of the BTSs.
handle the phone calls.
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RSL Link.
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TCH Blocking maybe due to the following reasons
1. Handover and Power budget margins
2. Cells too large
3. Capacity Limitations (Congestion)
4. Incorrect or Inapproriate Timer
Optimization for tips :
1. Handover and Power budget Margins
Handover Margins should be properly optimised to move the traffic to neighbouring cell. Strict hand
TIPS :
6 dB handover margin is considered to be an approriate margin for handover. A strict handover mar
2. Cells too Large
If cell are too large meaning antenna too high or antenna too shallow, it will pull in out of area traffic
TIPS :
Consider reducing antenna height to reduce the footprint of the site. Also increase the antenna tilt (t
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ver margins can results in lower handovers and ultimatelly congestion in cell.
gin results in the strict criteria for Power Budget Handovers also. Setting a lower value of handover mar
again causing congestion in the cell.
he max tilt is 12)
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in will initiate ping pongs handovers, which are not considered good for network health. (handover mar
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in have already been discussed).
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SDCCH Blocking
SDCCH Blocking is probably due to one or more reasons
1. No Access to SDCCH
2. Failure before Assignment of TCH.
3. High Paging Load
4. Incorrect or Inapproriate timer values
Optimization for tips :
1. No Access to SDCCH
This cases before has already been discussed
2. Failure before Assignment of TCH
This cases before has already been discussed
3. High Paging Load
Irreguler paging distribution in location areas results in SDCCH Blocking. Higher paging load in cert
TIPS :
A location area with a high paging load needs to be reduced in size to relieve SDCCH Blocking. A l
4. Incorrect or Inapproriate timer values
Timer rr_t3 111 sets the amount of time allowed to delay the deactivation of a traffic channel (TCH)
TIPS :
The suitable value for this timer is 1200 ms (max being 1500 ms). The timer will cause the BSS to
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in location area mean higher location updates on SDCCH resulting in SDCCH Blocking.
cation area with low paging load need to be enlarged in size to reduced the overal number of location a
after the disconnection of the main signaling link.
ait before the channel in question is allocated another connection. A lower value of timer will result in hi
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res.
gher capacity since the channel is held for less time before being released.
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High Handover Failure Rate
High handover failures rate will probably be due to one or more of the following reason.
1. High Neighbour Interference
2. No Dominant Server
3. Database Parameters
Optimization for tips :
1. High Neighbour Interference
While handling off to the best neighbour the interference on the target cell frequency may result in t
TIPS:
When designing the cell frequencies care should be taken that there is proper frequency spacing b
2. No Dominant Server
If cell sites are designed poorly there might be areas where neighbour being received at the same l
TIPS :
Antenna tilts provide the good way to reduce the footprint of the sites. Efforts should be made that a
3. Database Parameters
Received level, receive quality and power budget algorithm are set in the system information to defi
TIPS:
Enable the per neighbour feature which dispalys the successful and unsuccessful handovers on a
All those cells can be identified which are problematics in terms of hand off so one can focus only s
a. Defines the lower threshold for uplink quality handovers.
ex : Range 0 to 1800
Step size = 0.01
a value of 500 defines the lower threshold value of 5 (BER) for a quality handover to be trigge
b. Defines the lower threshold for downlink quality handovers.
c. Defines the lower threshold for received level uplink handovers.
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Ex : A value of 20 defines the threshold value of -90 dBm for a level handover to be triggered
Range 0 to 63
Where 0 = -110 dBm
1 = -109 dBm
63 = -47 dBm
The optimum value for this threshold is 15 ( 95 dBm). If the signal level goes below that,
d. Defines the lower threshold for received level downlink handovers.
e. Defines the upper threshold for uplink interference handovers
f. Defines the upper threshold for downlink interference handovers
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e hand off failure.
tween the cells to avoid neighbour interference. In most of the cases Ping Pong Handover starts i.e the
vel and some neighbour randomly look good for hand off for a certain amount of time. Such situation is
single dominant server should serve the specific area. Timing advance limitation is applied to cell area
e the criteria for handover. Improper values for these criteria may result in poor handoff.
per cell basis. In optimization, monitor the following stats, which comes under cell statistic category.
ecific cell causing the major contribution towards poor HSSR. Ensure that handover margins are optimi
ed for uplink. The optimum for this threshold is 500.
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for uplink.
a level handover is initiated.
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mobile hand off to a cell for better level and due to interference (Quality issues) hand off again to origin
disastrous because handoff decision will be hard and mostly it will end up unsuccessful handovers.
where there is multiple servers.
sed. Rule of thumb is a 4 dB for adjecent frequencies and 6 dB per cell without adjecent frequencies. T
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l cell. A through drive test can determine the interfering frequencies which should be eliminated from t
e following parameters can be played for defining the threshold for imperative and non-imperative han
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he frequencies plan.
overs.
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High Call Drop Rate (HCR) Guidelines
For healthy network the drop call rate should be less than 1%. There are again number of reasons, whi
1. Drop on Handover
2. Low signal Level
3. Adjacent channel Interference
4. Co-channel Interference
5. Extraneous Interference
6. Link Imbalance
Optimization for Tips :
1. Drop on Handover
The call may drop on handover. Its mostly high neighbor interference on the target cell, which caus
TIPS :
Within optima, monitor the following statistic. Theses statistics are defined under the category of BS
a. total and successful handover on UL/DL quality
b. total and successful handover on UL/DL signal strength.
c. total and successful power budget handovers.
From the above statistics, quality or level must be estimated.
2. Low signal Level
Signal level below -95 dBm is considered to be poor. If the mobile is unable to handoff to a better c
TIPS:
First of all path balances should be checked. If path balances are deviating fro the standard value t
a. I_Rx Lev_DL_p
Defined the lower value for receive level for the power control to be triggered.
Range 0 to 63
Where 0 = -110 dBm
1 = -109 dBm
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63 = -47 dBm
Example : If the value of 20 is set it means that the BTS will start transmitting more if it sense
b. U_RxLev_DL_p
Defines the upper threshold value for receive level for the power control to be triggered (Rang
Example : On setting the value of 50 (equivalent to -60 dBm) BTS will lower down the power.
3. Adjacent and Co-channel Interference
Frequency planning plays a major role to combat adjecent channel and Co-channel Interference. C
TIPS :
An Optimization tools like Neptune could be helpful in identifying the interference on the particular a
a. TCH Interference level 1
b TCH Interference level 2
c. TCH Interference level 3
d. TCH Interference level 4
When a TCH timeslot is idle it is constantly monitored for an uplink ambient noise. During a SACCH
4. Extraneous Interference
Extraneous Interference might be from :
a. Others mobile network
b. Military communication
c. Cordless Telephones
d. Illegal radio communition equipment.
TIPS :
External interference is always measured through spectrum analyser which can scan the whole ban
5. Link Imbalance
Sometime the multifunctionality of vendor hardware becomes responsible for high Call Drop Rate.
a. Transmited and receiving antenna facing different direction
b. Transmited and receiving antennas with different tilts.
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c. Antenna feeder demage, crossion or water ingress.
d. Physical obstruction.
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h could contribute towards higher dropped call rate is :
s the main problem. Sometime the mobile is on the wrong source cell (not planed for that area but ser
C level statistics.
ll on level basis, the call will possibly be dropped. Topology or Morpology issues may also be there like i
en check the BTS transmited power with the help of wattmeter. BTS may transmit low power because
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that downlink receive level is below -90 dBm.
is same as described above).
channel is observed mostly when mobile is elevated and receives signals from cell far away but using
ea. Such frequencies can be cleaned from existing frequency plan. The following statistic can also be
Multiframe an idle timeslot is monitered 104 times. These samples are the processed to procedure a n
d. Some spectrum analyser can even decode voice from AMPS circuits or Cordless Phones.
ne of the possible scanarios could be :
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es due to the antenna overshoot) which may the result in the drop call.
f Mobile enters into a tunnel or a building , higher RF losses will be develoved.
f the malfunctioning of radio or higher combiner losses. Also check the feeder losses, antenna connect
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he same frequencies.
onitored to confirm that there interferences issues in the cell. These stats are defined in optima under t
ise level average per 480 ms. An interference band is allocated to an idle slot depending upon the inter
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rs. Enable Downlink power control. Power control is be directional. The lower and Upper recieve level d
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e category of BSC stats.
ference level. The threshold for these levels can be set in the system parameters. Interference level 1 b
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ownlink power control values should be properly defined.
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eing the least ambient and interference level 4 being the most ambient. While planning the Network car
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should be taken that the cell do have the proper frequency spacing.
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SDCCH Success Rate is percentace of MS Call Setup success due to TimeSlot at the first call.
Where :
SDCCH success rate = [(Call Setup Success - Call Setup Failure )/ Call setup success] x 100%
The call setup rate should be above 90% for a healthy network. However a cssr of 85% 90% is satisf
There could be so many reasons for a poor cssr. Some are described as follows :
1. No access to SDCCH
2. CM Service Reject
3. TCH Failure Assignment
4. Hardware Problem
Optimization Tips for :
1. No Access to SDCCH.
BSS detacts channel request (in the form of RACH) from a source, requesting resources for networ
Case :
a. Valid RACH ( SDCCH Congestion)
Due to unavailability of SDCCH, BSS will response to MS with immediate assignment reject, t
Invalid RACH (Invalid established cause detected in the received RACH)
b. Phantom RACHs
The received RACH is in fact genereted from an unknown source, wherby it fails to continue
TIPS:
Within the optima there are certain stats which can be monitored before coming to conclusion that t
a. SDCCH Blocking
b. SDDCH Congestion (Valid RACH)
If the SDCCH blocking greater than 1% or SDCCH Congestion greater than 2% than that mean tha
A TCH can be allocated by passing SDCCH. A parameter namely Immediate Assign Mode when en
2. CM Service Reject
CM Service Request (MOC) or Paging Response (MTC) to BSC/MSC. Inside the CM service requ
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3. TCH Failure Assignment
Upon completion of MS/BSC/MSC link established, MS issues Assignment Request to BSC, Reque
Case :
TCH Congestion
TIPS :
For TCH Congestion certain features can be enabled like TCH queuing , Directed Retry and Conge
Interference analysis on a particular carier can be done through an optimization tools like Neptune.
4. Hardware Problem
Hardware failures also play major role for poor CSSR. Improper functinality of any BTS hardware ca
TIPS :
If there are no capacity or RF issues then equipment needs to be checked. Before starting the drive
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ctory.
s transactions. After validation of the RACH, BSS will attempt to allocate a dedicated channel (SDCCH
rminating the transactions. In which case, call setup is termed as unsuccessful due to SDCCH congesti
the transaction after SDCCH has been allocated by the BSS. For instances, case of channel request de
here is SDDCH problem :
it is a capacity related issue and more slots should be assigned for SDCCH.
abled allocates TCH by passing SDCCH.
st message (MS initiated service request), MS informs the network the types of service it requires (i.e.
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sting TCH Assignment to the dedicated MS. Subsequently, BSS will attempt to allocated free TCH for M
tion Relief. In case of the TCH queuing feature is enable, MS will queue in the Original SDCCH , awaiti
nce interfering frequencies are determind, the frequency plan can be cleaned from such frequencies.
n affect the overall performance of sites.
est make sure that the cell site are free for any hardware alarms. The important parameter to check is t
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for the source. One the availability of SDCCH channel is confirmed, the BSS will send immediate assig
on.
tected by overshooting cells, handover access burst from distanced MS, hardware deficiency, UL/DL im
obile Originationl, Emergency Call, Short Message transfer or Suplementary services activity), wherby
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S voice messaging. Once Assignment Command is received by MS, stating the availability of TCH for t
ing for the next available TCH. It is to be remainded that once Queuing timer expires. BSS will also term
he path balance. If path balances are not fine then start checking the power from radio to connected ant
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nment to MS indicating the dedicated SDCCH sub-channel (via AGCH), wherby subsequent message e
balance path, MS moving out of range would carry the Phantom RACHs symptoms.
Paging response is specific to MTC. Subsequently, BSS embraces the information with its own initiated
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e MS, it will move to the dedicated TCH and responds with Assignment Complete. In turns, BSS will su
inates transactions, in which case, call setup is termed as unsuccessfull due to TCH Congestion. The s
ennas. If we take the examples of GSM 900 scenario, the link budget defines that the radio should trans
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xchange will be preformed over the dedicated SDCCH.
connection request BSSMAP message, send to MSC to approval. MSC will response with either conne
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bmit Assignment Complete to MSC as to complete the signal activity.
me situation also applies in situation where Congestion Relief feature is enabled. In the case of Directe
mit 40 watts power and at the top of the cabinet, 20 watts are recaived ( Considering the 3 dB loss of c
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tion confirmed, confirming the success in link establishment between MS-BSC-MSC, or connection Re
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d Retry feature is enabled, MS will perform Handover to TCH of another cell if a valid handover neighbo
mbiner). While checking the power, if any components seems to procedure more losses than expected
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used, Indicating the termination of the specific network transaction.
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r is detected. The best thing to do is to add more radios in the cell to remove congestion.
, change that component. Similarly check the power at antenna feeder ports. Some time due to the wate
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r ingress, connectors get rusty and needs to be replaced.
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TCH Call drop
Causes of TCH Call Drop:-
Usually, the typical causes for sending the Clear_Request mssg are as follows ;
1. Radio Interface Failure
2. O & M Intervention
3. Equipment Failure
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