2. Dheepak. M and Dr. S. V. Saravanan
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If radio resource utilization is high.
A main reason for handover in cellular networks is to overcome call drops due to
handoff failure.
If the mobile device moves out of the range of one cell (base station) and a different
base station can provide it with a stronger signal.
If all channels of one base station are busy then a nearby base station can provide
service to the device.
Handover process is an important one in any cellular network, must be completed
efficiently and without inconvenience to the user.
2. GSM NETWORK OVERVIEW
The GSM network is divided into various subsystems. Each of these systems is
comprised of a number of functional units which are individual components of the
mobile network.
Mobile Station (MS)
It consists of Mobile Equipment (ME) and Subscriber identity module (SIM).
Base Station Subsystem
It consists of Base transceiver station (BTS), Base station controller (BSC)
Network Switching Subsystem (NSS):
It consists of Mobile switching center (MSC), Home location register (HLR), Visitor
location register (VLR), Authentication centre (AuC) and Equipment Identity register
(EIR).
Public Switched Telephone Network (PSTN):
It is owned and deployed by mobile phone operators and allows mobile devices to
communicate with each other and telephones in the wider Public Switched Telephone
Network (PSTN) used for processing and switching.
Figure 1 GSM Cellular Network Architecture
Home Location Register (HLR) and Visitor Location Register (VLR) maintains
and manages the home and visiting subscriber datas ,it is the component of
a GSM system that carries out call switching and mobility management functions
for mobile phones roaming on the network of base stations.
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The NSS originally consisted of the circuit-switched core network, used for
traditional GSM services such as voice calls, SMS, and circuit switched data calls. It was
extended with overlay architecture to provide packet-switched data services known as
the GPRS core network. This allows mobile phones to have access to services such
as WAP, MMS, and the Internet.
Base station subsystem (BSS)
Base station subsystem (BSS) consists of Base Transceiver station (BTS) and Base
station controller (BSC). BSS manages the radio link between MS and CN. This
access network provides access to both Circuit switch (CS) and Packet switched (PS)
core networks. It is the section of a cellular network which is responsible for handling traffic
and signaling between a mobile station and network switching subsystem. The BSS carries
out transcoding of speech channels, allocation of radio channels to mobile station, paging,
transmission and reception of information in form of voice and data over the air interface.
• The Network Switching Sub-system (NSS) – comprising an MSC and associated
registers. Several interfaces are defined between different parts of the system.
• 'A' interface between MSC and BSC
• 'Abis' interface between BSC and BTS
• 'Um' air interface between the BTS (antenna) and the MS
3. KEY PERFORMANCE INDICATORS
Key Performance Indicators (KPI) is a measure of successful network performance
and its quality. With growing customer base and continuous addition of capacity and
coverage sites, operators need to continuously monitor the KPIs of their network to
assess service quality. To improve the performance of a network detection of problem
and symptoms in early stage is very important and it’s far too late when a customer
complaint arises.
3.1 Performance Evaluation
The most reliable KPIs to evaluate the network performance with NMS are:
SDCCH and TCH Block
Blocking percentage [%]
CDR (Call Drop Rate).
HSR (Handover Success Rate).
CSSR (Call Set up Success Rate).
TCH (Traffic Channel) Congestion Rate.
Average quality DL and UL
3.2 Quality of Service Objective
“Healthy Network provides Healthy Subscribers and Healthy Business”
Quality of Service (QOS) targets as per the Operators’ specifications:
Efficient network utilization
Coverage of Service Area: Urban, Suburban, Roads or the Rural areas
Call Success Rate
Minimizing Handover failures
Rx quality
4. Dheepak. M and Dr. S. V. Saravanan
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Improve Speech Quality Index for higher Network quality
Satisfied customers resulting in low churn
Reduced investment costs and increased revenue for the customers
4. TYPES OF HANDOVER
Two main types of handover
Hard hand over
Soft hand over
Figure 2 Hard and Soft handover in a wireless network
4.1 Hard Handover
Hard handover: “break before make” connection
Existing radio link must be dropped for a small period of time, and then taken over by
another base station. A call in progress redirected not only from a base station to
another base station but also from its current transmit–receive frequency pair to
another frequency pair. An ongoing call can not exchange data or voice for this
duration
4.2 Soft Handover
Soft handover: “make-before-break” connection.
Mobile station at the boundary of two adjacent cells does not suffer call drops due to
handover in the boundary region Gives seamless connectivity to a Mobile station An
offset to pseudo noise code method of soft handover Soft handover does not require
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breaking of the radio link for cell-to-cell transfer of a call. A mobile device can be
simultaneously connected to several base stations
4.3 Frequency related Hand Over
In network following handover can occur:
4.3.1 Intra-frequency hard handover
Due to interference of certain frequencies, the signal quality poor. The BSC can
handover the call to another Frequency of the cell in such cases
4.3.2 Inter-frequency hard handover
When the signal strength goes weak due to several reasons (for example, the mobile
moving away from the cell in which it is presently localized to the boundary region of
another cell), there is handover from a cell to another
4.4 Call drop in hard handover
Figure 3 Handoff success and Handoff failure
When an MS moves from one sector/cell to another, it breaks connection of
original sector/cell and makes new connection with the target sector/cell. In wireless
communications, during HO process, there is a high probability of call drop, which
directly affects the system performance (90% of radio call drop occurs during
handoff). Proper handoff algorithm can reduce system call drop and increase network
capacity.
Break in call transmission
Handover takes place in a few ms (at best in 60 ms)
Interruption is hardly discernible by the user
Handover to another cell is required when the signal strength is low and error rate is
high. GSM systems perform hard handovers
In network, one user needs to switch within
Same frequency
Different frequency
Soft handoff is possible only within the same frequency i.e. intra-frequency handoff.
Hard handoff is required for inter-frequency handoffs
Softer handoff is possible only within the same frequency and BTS.
Not all the intra-frequency handoffs are soft handoff.
6. Dheepak. M and Dr. S. V. Saravanan
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Figure 4 Connect to Base Station 1(BS1) and start conversation.
Figure 5 Moving out of Base Station 1’s coverage –connect to Base Station 2(BS2).
Figure 6 Moving out of BS2’s coverage-Connect to Base station 3(BS3).
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Figure 7 Conversation ended –Still within BS3’s Coverage
Figure 8 Handover process in GSM Network
The procedure for analyzing the handoff
Check the successful handover per cell
Check the handover activity from the number of handover performed
Check the site location, whether the site to site distance or co-sited are properly
located or not.
Check whether the cell is isolated or not
Check the handover flow whether is balance between incoming and outgoing.
Check if many Ping-Pong handover.
Check if assignment handover are used.
Check if cell has Hardware problem. If problem found, swap or repair hardware.
Check if congested target cell. If the target cell is congested then solve the congestion
by expanding the capacity of the cell.
Perform measures to improve HO performance. This is the assumption comes from
the above analysis.
8. Dheepak. M and Dr. S. V. Saravanan
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5. NETWORK OPTIMIZATION PROCESS
5.1. Young Network Case
In a young network the primary target is normally the coverage
In this phase usually there is a massive use of drive test measurement
check the signal and
the performance of the competitors
5.2 Mature Network Case
In a mature network the primary targets are quality indicators
drop call rate, average quality, handover failures
Important to use the information from Network Management System (NMS)
a general view of the network performance
Drive test measurements are still used
but not in a massive way
in areas where new sites are on air
where interference and similar problems are pointed out by NMS data analysis
6. DRIVE TEST ANALYSIS
6.1 Network Data Collection & Analysis
Poor Performance Sites and regions are identified based on the NMS data collection
and Analysis and Network indicating parameters are monitored.
6.2 Drive Test
Drive Test is conducted for checking coverage criteria of a cell site with RF drive test
tool. The data collected by drive test tool as Log files is analyzed to evaluate various
RF parameters of the network.
Drive Route and Site Data for the sites to be driven is taken from customer
Drive Test LOG File of the specified Drive Route / Site is saved and the same will be
handed over for analysis at the end of the day
Figure 9 Pre drive test status before correcting Handoff failure showing poor signal
strength and high call drop.
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Figure 10 Post drive test status after correcting Handoff failure showing good signal
strength and high call drop.
RECOMMENDATIONS FOR IMPROVEMENT OF QOS
Defining missing neighbor relations
Proposing antenna azimuth changes
Proposing antenna tilt changes
Re–tuning of interfered frequencies
Proposing antenna type changes
RF Parameters should be monitored and updated on periodic basis
Adjusting Handover Margins
Re-tuning of interfered frequencies
Avoid voltage standing wave ratio (VSWR)
Analysis of Pre and Post drive test results
Identify Coverage Holes and carryout optimization or new site proposal
Physical Audit and Optimization
Avoid Site Outage and Maintain Transmission Hardwares Good Condition
Identifying vital locations and understanding the geographical terrain.
Minimize the call drop and RF interference in the network
Increase the indoor & outdoor coverage and better speech quality
Daily KPI Report analysis
7. CONCLUSION
In this paper, we have recommended some of the techniques which can improve the
handoff success rate and reduce call drop which plays a vital role in maintaining the
quality of services (QoS) of GSM cellular network. These recommendations might be
helpful for telecom engineers during there operation and maintenance activity. A
network Operator can maintain a Healthy competition and Business Model only when
he fulfills customer’s requirements and reliable network in maintained.
10. Dheepak. M and Dr. S. V. Saravanan
http://www.iaeme.com/IJEET/index.asp 62 editor@iaeme.com
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