1.1 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:
- Analyze call set up failure rate in cellular network operations.
- Investigate cellular handover process failure rate.
- Ascertain call set up time to enhance efficiency and effectiveness of calls made by a mobile station.
- 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;
- Literature review on handover process in GSM system and various types of handover process were investigated.
- Identification of various parameters in mobile communication system that affect the handover process
- Collection of relevant data from mobile communication under investigation, from two major mobile operators within one period
- Analysis of data collected.
- 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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