ABSTRACT
The study presents the management of handover process in
mobile communication network. Recent time there have been increase in number of
drop calls in mobile communication due to failure of handover process in mobile
communication networks. Data were obtained from Operation and Maintenance
Centre (OMC), such data were calls setup failure rate , handover failure rate,
call setup time and quality of service (QoS) from two different mobile networks
for period of one year. The data obtained were analysed using Micro Software
excel package. Additive model was developed based on handover parameters
highlighted and reservation of channel for handover process, based on the
analysis. Seven days were considered for call setup failure rate in percentage,
It is observed that high values were obtained from MTN compared to the GLO
network in the daily call setup failure rate. Corresponding call setup failure
rates in percentage were obtained for 24 weeks. It is observed that there is
discrepancies between the MTN and GLO network in terms of call setup failure
rate and handover failure rate. The call setup failure rate and handover
failure rate witnessed non –linear pattern over the day, week and year.
CHAPTER ONE
INTRODUCTION1.1 Background To The Study
A
handover is a process in telecommunications and mobile communications in which
a connected cellular call or data session is transferred from one cell site
(base station) to another without disconnecting the session. Cellular services
are based on mobility and handover, allowing the user to be moved from one cell
site range to another to be switched to the nearest cell site for better
performance Ekici (2012).
The
progressive trend of urbanization involving changes in the activities of a city
has created several problems. Addressing these problems requires reliable and
detailed information regarding the urban structure and its dynamics.
Previous
studies have tried to explore cellular networks data for urban analysis, yet
little attention has been given in exploring mobility related events of
cellular networks. This study uses handover, which is the process of
transferring an ongoing call from one cell to the other, to capture urban dynamics.
The
cellular network has gone through three generations. Advanced Mobile Phone
System (AMPS), the first generation of cellular networks are analog based which
is the current standard of U.S cellular network. To increase network capacity
new technologies Time Division Multiple Access (TDMA) and Code Division
Multiple Access (CDMA) were used in Global System for Mobile Communications
(GSM), the second generation of cellular networks to accommodate more mobile
terminals. The third generation cellular phone provides high speed data
transmission with voice transmission
Chussi, Khotimsky and Krishnan (2012).
The
growth in the field of cellular communication has led to intensive research and
development towards cellular systems. The unique characteristic of cellular
communication system is that it offers user maximum freedom of movement while
using cell phones.
A
cellular network is made up of numbers of cells (or radio cells). Each cell is
allocated to band of frequencies and served by base station consisting of
transmitter, receiver and control unit. Adjacent cells are assigned different
frequencies to avoid interference or cross talk Eastwood, Migaldi and Gupta
(2010).
As
more customers use the cellular network with single base station, traffic may
be built up so there are not enough frequency bands assigned to a cell to
handle its calls. An approach can be used to cope with this situation to use
the same radio frequency in which case it is reused in different areas for a
completely different transmission. The reuse of frequencies in different cells
is a form of space division multiple access and it requires the location of
each mobile agent to be known. This is provided through a service known
location management or mobility management. The obstruction in cellular network
involves the problem when a mobile user travels from one cell to another during
a call Chussi et al (2012).
Although
the concept of cellular handover or cellular hand off is relatively straight
forward, it is not an easy process to implement in reality. The cellular
network needs to decide when handover or hand off is necessary and to which
cell.
Also,
when the handover occurs, it is necessary to reroute the call to the relevant
base station along with changing the communication between the mobile and the
base station to a new channel. All of these need to be undertaken without any
noticeable interruption to the call. The process is quite complicated and in
early systems calls were often lost if the process did not work correctly.
Different
cellular standards handle handover/handoff in slightly different ways. The
terms handover and handoff are sometimes used interchangeably. However for the
purpose of clarity, American English use the term handoff, and this is most
commonly used within some American organizations such as 3GPP2 and in American
originated technologies such as CDMA2000. In British English the term hand over
is more common and is used within international and European organizations such
as ITUT, IETF, ETSI and 3GPP, and standardized within European originated
standards such as GSM and UMTS. The term handover is more common than handoff
in academic research publications and literature, while handoff is slightly
more common within the IEEE and ANSI organizations Dutta, Famolari and Ohba
(2012).
In
telecommunications there may be different reasons why a handover might be
conducted. When the phone is moving away from the area covered by one cell and
entering the area covered by another cell the call is transferred to the second
cell in order to avoid call termination when the phone gets outside the range
of the first cell. The most basic form of handover is when a phone call in
progress is redirected from its current cell (called source) to a new cell
(called target) Campbell, A.T. and Gomez, J. (2010).
There
are two major classes of handover: hard handover and soft handover.
Hard
handover is an instantaneous handover in which the existing connection is
terminated and the connection to the destination channel is made. It is also
known as a break-before-make handover. The process is so instantaneous that the
user does not hear any noticeable interruption Adulova and Aubay (2010).
Soft
handover is a substantial handover where the connection to the new channel is
made before the connection from the source channel is disconnected. It is
performed through the parallel use of source and destination channels over a
period of time. Soft handovers allow parallel connection between three or more
channels to provide better service. This type of handover is very effective in
poor coverage areas Adulova and Aybay
(2010).
Furthermore,
hard handovers are intended to be
instantaneous in order to minimize the disruption to the call. A hard handover
is perceived by network engineers as an event during the call. It requires the
least processing by the network providing service. When the mobile is between
base stations, then the mobile can switch with any of the base stations, so the
base stations bounce the link with the mobile back and forth. This is called
'ping-ponging' Campbell, Gomez and Kim (2010).
A soft
handover is one in
which the channel in the source cell is retained and used for a while in
parallel with the channel in the target cell. In this case the connection to
the target is established before the connection to the source is broken, hence
this handover is called make-before-break. The interval, during which
the two connections are used in parallel, may be brief or substantial. For this
reason the soft handover is perceived by network engineers as a state of the
call, rather than a brief event. Soft handovers may involve using connections
to more than two cells: connections to three, four or more cells can be
maintained by one phone at the same time. When a call is in a state of soft
handover, the signal of the best of all used channels can be used for the call
at a given moment or all the signals can be combined to produce a clearer copy
of the signal. The latter is more advantageous, and when such combining is
performed both in the downlink (forward
link) and the uplink (reverse
link) the handover is termed as softer.
Softer handovers are possible when the cells involved in the handovers have a
single cell site Campbell, Gomez and Kim (2012).
An advantage of the hard handover is
that at any moment in time one call uses only one channel. The hard handover
event is indeed very short and usually is not perceptible by the user. In the
old analog systems it could be heard as a
click or a very short beep; in digital systems it is unnoticeable. Another
advantage of the hard handover is that the phone's hardware does not need to be
capable of receiving two or more channels in parallel, which makes it cheaper
and simpler. A disadvantage is that if a handover fails the call may be
temporarily disrupted or even terminated abnormally. Technologies which use
hard handovers, usually have procedures which can re-establish the connection
to the source cell if the connection to the target cell cannot be made. However
re-establishing this connection may not always be possible (in which case the
call will be terminated) and even when possible the procedure may cause a
temporary interruption to the call Eastwood, L.; Migaldi, S.; Xie, Q. and
Gupta, V (2010).
One advantage of the soft handovers
is that the connection to the source cell is broken only when a reliable
connection to the target cell has been established and therefore the chances
that the call will be terminated abnormally due to failed handovers are lower.
However, by far a bigger advantage comes from the mere fact that simultaneously
channels in multiple cells are maintained and the call could only fail if all
of the channels are interfered or fade at the same time. Fading and
interference in different channels are unrelated and therefore the probability
of these taking place at the same moment in all channels is very low. Thus the
reliability of the connection becomes higher when the call is in a soft
handover. Because in a cellular network the majority of the handovers occur in
places of poor coverage, where calls would frequently become unreliable when
their channel is interfered or fading, soft handovers bring a significant
improvement to the reliability of the calls in these places by making the
interference or the fading in a single channel not critical. This advantage
comes at the cost of more complex hardware in the phone, which must be capable
of processing several channels in parallel.
While theoretically speaking soft
handovers are possible in any technology, analog or digital, the cost of
implementing these analog technologies is prohibitively high and none of the
technologies that were commercially successful in the past (e.g. AMPS, TACS, NMT, etc.) had this feature. Of the
digital technologies, those based on FDMA also face a higher cost for the
phones (due to the need to have multiple parallel radio-frequency modules) and
those based on TDMA or a combination of TDMA/FDMA, in
principle, allow not so expensive implementation of soft handovers. However,
none of the 2G (second-generation) technologies have this feature (e.g.
GSM, D-AMPS/IS-136, etc.). On the other hand, all CDMA
based technologies, 2G and 3G (third-generation), have soft handovers. On one hand, this
is facilitated by the possibility to design not so expensive phone hardware
supporting soft handovers for CDMA and on the other hand, this is necessitated
by the fact that without soft handovers CDMA networks may suffer from
substantial interference arising due to the so-called near-far effect Buddhikot,
Chandranmenon, Han and Lee (2011).
In all current commercial
technologies based on FDMA or on a combination of TDMA/FDMA (e.g. GSM, AMPS,
IS-136/DAMPS, etc.) changing the channel during a hard handover is realised by
changing the pair of used transmit/receive frequencies.
For the practical realization of handovers
in a cellular network each cell is assigned a list of potential target cells,
which can be used for handing over calls from this source cell. These potential
target cells are called neighbors and the list is called neighbor
list. Creating such a list for a given cell is not trivial and specialized
computer tools are used Campbell
and Gomez (2011).
During a call one or more parameters
of the signal in the channel in the source cell are monitored and assessed in
order to decide when a handover may be necessary. The downlink (forward link)
and/or uplink (reverse link) directions may be monitored. The handover may be
requested by the phone or by the Base
Station(BTS) of
its source cell and, in some systems, by a BTS of a neighboring cell. The phone
and the BTSes of the neighboring cells monitor each other's signals and the
best target candidates are selected among the neighboring cells. In some
systems, mainly based on CDMA, a target candidate may be selected among the
cells which are not in the neighbor list. This is done in an effort to reduce
the probability of interference due to the aforementioned near-far effect Akyildiz,
Altunbasak and Sivakumar (2012).
1.2 Justification of The Study
In analog systems the parameters
used as criteria for requesting a hard handover are usually the received signal power and the received signal-to-noise
ratio (the
latter may be estimated in an analog system by inserting additional tones, with
frequencies just outside the captured voice-frequency band at the transmitter
and assessing the form of these tones at the receiver). In non-CDMA 2G digital
systems the criteria for requesting hard handover may be based on estimates of
the received signal power, Bit
Error Rate(BER) and Block
Error/Erasure Rate
(BLER), received quality of speech (RxQual), distance between the phone and
the BTS (estimated from the radio signal propagation delay) and others. In CDMA
systems, 2G and 3G, the most common criterion for requesting a handover is Ec/Io ratio measured in the Pilot Channel (CPICH) and/or RSCP Ghaderi and Boutaba (2010).
Based
on the discussions and data related to the problem of call drop or improper
termination of calls in cellular communications, it is observed that handover
management in cellular communication requires adequate scientific research
attention for significant improvement in calls and data sessions without
disruptions in connections when a user moves from one cell site to another Akyildiz, Altunbasak and Sivakumar (2012).
Often
times the problem of call drop and disruptions in connections occur in cellular
communications when handover management is not properly implemented when a
mobile subscriber moves from once cell to another. Hasswa, Nasswer and
Hassanein (2012).
Replication
of this experience can adversely downplay the business integrity and reputation
of the service provider as an organization which can give the rival
organizations operating in the same domain a competitive advantage and perhaps
lead to eventual collapse or possibly, bankruptcy of the organization(s)
concerned.
Also
when there are call drops and disconnections in calls or data sessions, it
could also have legal implications from dissatisfied clients or contract
revocation from the appropriate engaging and regulatory authorities since
capacity utilization and service delivery is obviously reduced and compromised
as a result of the inefficiencies and ineffectiveness arising from such
operational challenges Ghaderi and
Boutaba (2010).
When the capacity for connecting new
calls of a given cell is used up and an existing or new call from a phone,
which is located in an area overlapped by another cell, it is transferred to
that cell in order to free-up some capacity in the first cell for other users,
who can only be connected to that cell.
In non-CDMA networks when the channel used by
the phone becomes interfered by another phone using the same channel in a
different cell, the call is transferred to a different channel in the same cell
or to a different channel in another cell in order to avoid the interference Hsieh, Zhou and
Seneviratne (2011).
Again in non-CDMA networks when the
user behaviour changes, e.g. when a fast-travelling user, connected to a large,
umbrella-type of cell, stops then the call may be transferred to a smaller
macro cell or even to a micro cell in order to free capacity on the umbrella
cell for other fast-traveling users and to reduce the potential interference to
other cells or users (this works in reverse too, when a user is detected to be
moving faster than a certain threshold, the call can be transferred to a larger
umbrella-type of cell in order to minimize the frequency of the handovers due
to this movement) Juha, k., (2003),
In CDMA networks a handover may be
induced in order to reduce the interference to a smaller neighboring cell due
to the "near-far" effect even when the phone
still has an excellent connection to its current cell.
There are occurrences where a
handoff is unsuccessful. Lots of research was conducted regarding this. In the
late 80's the main reason was found out. Because frequencies cannot be reused
in adjacent cells, when a user moves from one cell to another, a new frequency
must be allocated for the call. If a user moves into a cell when all available
channels are in use, the user’s call must be terminated. Also, there is the
problem of signal interference where adjacent cells overpower each other
resulting in receiver desensitization Kim,
Kim and Kim (2012).
Different systems have different
methods for handling and managing handover request. In such systems the
probability that the handover will not be served is equal to blocking
probability of new originating call. But if the call is terminated abruptly in
the middle of conversation then it is more annoying than the new originating
call being blocked. So in order to avoid this abrupt termination of ongoing
call handover request should be given priority to new call this is called as
handover prioritization Khaja Kamaluddin, Aziza Ehmaid. I. Omar (2011).
There are two techniques for this.
1) Guard Channel Concept: In this
technique, a fraction of the total available channel in a cell is reserved
exclusively for handover request from ongoing calls which may be handed off
into the cell.
2) Queuing: Queuing of handoffs is
possible because there is a finite time interval between the time the received
signal level drops below handoff threshold and the time the call is terminated
due to insufficient signal level. The delay size is determined from the traffic
pattern of a particular service area Misra,
Das and McAuley (2010).
Handover mechanism is extremely important in
cellular network because of the cellular architecture employed to maximize
spectrum utilization. Handover is the procedure that transfers an ongoing call
from one cell to another as the user’s moves through the coverage area of
cellular system. One way to improve the cellular network performance is to use
efficient handover prioritization schemes when user is switching between the
cells. This research work is presented on the platform of an analytical
framework that can enhance considerably the handover call mechanism in wireless
network. Some advance schemes namely, guard channels, and handover queuing are
discussed . All these of prioritizations schemes have a common characteristic
reducing the call dropping probability at the expense of increased call
blocking probability. Efficient prioritization scheme accommodates a number of
new calls while guarantees the quality of service (QoS) of handover call Mohanty
(2011).
This
is the process by which a mobile mode keeps its connection active when it moves
from one access point to another. There are stages in a Handover process.
First, the initiation of handover is triggered by either the mobile device or a
network agent or the changing network conditions. The second stage is for a new
connection generation when the network must find new resources for the handover
connection and perform an additional routing operations. Depending on the
movement of the mobile device, it may undergo various type of handover. In a
broad sense, handover may be intra-system (horizontal handover) which occurs in
homogenous networks. The type occurs when the signal strength or the serving BS
goes below a certain threshold value. Handover on the other hand may also be
inter-system (vertical handover) which takes place in heterogeneous networks
when a user moves out or the serving network and enters and overlying network
or to underlying network for service requirement.
In
essence, the design or handover management techniques in all wireless networks
must address the following issues: (1) signaling overhead and power requirement
for processing handover messages should be minimized. (ii) Qos guarantees must
be made (iii) network resources should be efficiently used and (iv) the
handover mechanism should be scalable viable and robust.
With
the convergence of the internet and wireless mobile communications and with the
rapid growth in the number of mobile subscribers, mobility management emerges
as one of the most important and challenging problems for wireless mobile
communication over the internet. Mobility management enables the serving
networks to locate a mobile subscriber’s point or attachment for delivering
data packets (ie location management) and maintain a mobile subscriber’s
connection as it to change its point of attachment.
Location
management enables the networks to track the location of mobile nodes. Location
management has two major sub-tasks (i) location registration on (ii) call
delivery or paging. In location registration procedure, the mobilizers node
periodically sends specific signals to inform the network of its current
location so that the location database it kept update.
The
call delivery proceedure is involved after the completion or the location
registration. Based on the information that has been registered in the network
during the location registration.
The
design and location management scheme must address the following issues:
Minimization
of signaling overhead and latency in the service delivery (ii) meeting the
guaranteed quality of service (Qos) or applications and (iii) in a fully
overlapping area where several wireless network co-exist, an efficient and
robust of rhythm must be desired so as to select the network through which a
mobile device should perform registration information and should be stored and
how to determine the exact location of a mobile device within a specific time
frame Pandya, Grillo and Lycksell (2010).
Due
to rapid change in technology the demand for better and faster cellular
communication also increases. This growth in field of cellular communication
has led to increase intensive research towards
development of cellular system. The main reason of this growth is the
new concept of mobile terminal and user mobility. The main characteristics of
cellular communication system offers user maximum freedom of movement while
using cell phones (mobiles). A cellular network is made up of number of cells
(or radio cells). Each cell is allocated a band of frequencies and served by
base station consisting of transmitter, receiver and control unit. Adjacent
cells are assigned different frequencies to avoid interference or cross talk.
As more customers use the cellular network with single base station traffic may
be built up so there are not enough frequency bands assigned to a cell to
handle the calls Osahenvenmwen, O.A and Emagbetere, J.O (2014).
1.3 Objective of the study
The
overall aim of this study is to enhance the concept and management of cellular
communication handover process (es) to support ongoing calls when mobile users
are switching between base stations.
In
order to achieve the aim of this study, the following objectives are set out as
outlined below:
(1)
Analyze call set up failure rate in
cellular network operations.
(2)
Investigate cellular handover process
failure rate.
(3)
Ascertain call set up time to enhance
efficiency and effectiveness of calls made by a mobile station.
(4)
Optimize the Quality of Service (QoS) as
a critical performance indicator of the network for efficient service delivery.
1.4 Scope
and limitation of the study
This
study is on analysis on handover process management in GSM system, where
different handover process and techniques were consideredin mobile communication
system. Various parameters associated with handover process were considered in
this study. Data were obtained from Operation and Maintenance Centre (OMC) unit
of the mobile communication system. Two
major mobile operators were
considered in this research such as MTN
and GLO network were the required data
were obtained. The additive model was developed on various parameter that
affect the handover process in GSM system.
1.5 Methodology of the
study
The
various methods and materials deployed in this research were listed as follow;
(1)
Literature review on handover process in
GSM system and various types of handover process were investigated.
(2)
Identification of various parameters in mobile communication
system that affect the handover process
(3)
Collection of relevant data from mobile
communication under investigation, from
two major mobile operators within one
period
(4)
Analysis of data collected.
(5)
Additive model was developed alongside
with relative parameters.
1.6 Thesis
Arrangement
Introduction
of this research is presented in this section with aim, objectives, scope and
limitation of the study and thesis arrangement in chapter one. Also, chapter
two involves Overview of mobile communication network, handover process and the
various techniques associated with handover process in GSM system.
Chapter three involves data
collection from OMC unit of the mobile network; two different mobile networks
were considered. While chapter four involves data
analysis, result and discussion on data obtained and analysis handover additive
model developed and conclusion, recommendations are presented in chapter Five.
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