International Journal of Interactive Mobile Technologies (iJIM) - Volume 3, Special Issue 2: "Technical Basics", October 2009 SIMULATION AND PROPOSED HANDOVER ALERT ALGORITHM FOR MOBILE COMMUNICATION NETWORKS Simulation and Proposed Handover Alert Algorithm for Mobile Communication Networks doi:10.3991/ijim.v3s2.838 Muzhir Al-Ani1 and Wael Al-Sawalmeh 2 1Amman Arab University, Amman, Jordan 2Philadelphia University, Amman, Jordan Abstract—This paper deals with a novel approach to realize the handover process of a mobile systems network. It con- centrates on the existing challenges of Global System for Mobile communication (GSM) networks and how to over- come these challenges. The proposed algorithm extracts the received signal information features in order to track the significant coverage cell. The presented algorithm distin- guishes between real problems and false alarms. One of the important contributions of the algorithm is how it predicts the adequate signal of the effective coverage cell. Index Terms—Handover, Coverage Area Measurements, Mobile Positioning & Digital Cellular System I. INTRODUCTION During the early 1980s, analog cellular telephone sys- tems were experiencing rapid growth in Europe, particu- larly in Scandinavia and the United Kingdom, but also in France and Germany. Each country developed its own system, which was incompatible with everyone else's in equipment and operation. In 1989, GSM responsibility was transferred to the European Telecommunication Standards Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries. Although standardized in Europe, GSM is not only a European standard. Over 200 GSM networks (including DCS1800 and PCS1900) are operational in 110 countries around the world. In the beginning of 1994, there were 1.3 million subscribers worldwide, which had grown to more than 55 million by October 1997. With North America making a delayed entry into the GSM field with a deriva- tive of GSM called PCS1900, GSM systems exist on every continent, and the acronym GSM now aptly stands for Global System for Mobile communications. The developers of GSM choose an unproven digital system, as opposed to the then-standard analog cellular systems like AMPS in the United States and TACS in the United Kingdom. They had faith that advancements in compression algorithms and digital signal processors would allow the fulfillment of the original criteria and the continual improvement of the system in terms of quality and cost. The revolution of the telecommunication technology opens new doors in many countries all over the world to start development and investigation over mobile system fields starting from the analog systems and then to digital systems that allow the subscriber mobility and comfort. The cellular system as implemented in most countries demonstrates that the world is a small village. On the other hand, a substantial growth in the number of sub- scribers makes it necessary to evaluate large and extended networks to overcome the growth of mobile networks [1], thus GSM becomes the most popular network. Nowadays there are many different and potential types of GSM with different features and applications. The popularity of mo- bile phones and the number of mobile devices users is continuously increasing, so manufacturers are continually trying to introduce new features and services to attract new customers [2]. GSM and 2G networks have been started in many Arab countries during 1990s, while the transition from 2G to 3G started at the beginning of 2000s. Due to some finan- cial problems, some countries have not evolved to 2G or 3G technologies. Therefore, our work is concentrated on improving the GSM network performance, since it is still widely used. II. DIGITAL COMMUNICATION SYSTEM The usage of mobile communication systems keeps ex- panding, leading to large number of studies related to GSM services, techniques and algorithm improvement. A fundamental goal of mobile telephone network operators is to provide seamless service for their subscribers [3] & [4]. The report of the GSM Association in 2004 indicates that the number of GSM networks exceeds 500 in more than 180 countries worldwide; the Latina American and Asian Pacific regions have the largest share of the highest rates of growth in both number of subscribers and number of networks lunched [5]. As in [6], developing and proposing a new algorithm for receiving GSM broadcasted data is important because it allows subscribers to obtain the identification parame- ters and carry out an analysis of mutual interference ef- fects. Cellular Systems are moving from 2G to 3G, while 2G already brings high speed data transmission enabling wideband multimedia applications. The next generation of cellular systems will be increasingly similar to a data communication system, which not only transfers voice and multimedia, but will also be integrated with WLAN to access Internet [7]. Great effort has been devoted towards the design and development of mobile architectures and multiservice networks. High performance modeling and evaluation of General Packet Radio Service (GPRS) are adapted via 6 http://www.i-jim.org http://dx.doi.org/10.3991/ijim.v3s2.838� SIMULATION AND PROPOSED HANDOVER ALERT ALGORITHM FOR MOBILE COMMUNICATION NETWORKS GSM cell involving both voice and multiple class data services under a complete partitioning scheme [8]. GPRS provides packet switched data transfer to effi- ciently utilize the radio resources. GPRS is considered the main development step of GSM networks toward the next generation of mobile communication system like UMTS. GPRS allows a single mobile station to transmit data using multiple time slots to increase the transmission rate [9]. Existing GSM/GPRS base station site utilization can be sped up 3G wide band code division multiple access (WCDMA) deployment. It is possible to provide full cov- erage even for bit rates higher than 144 or 384 kbps when 3G WCDMA use GSM 1800 sites [10]. As operators continue to roll out networks, it is obvious that providing the depth of coverage equal to the mass- market population coverage achieved with GSM 900/1800 will not be possible for many years. Early release of UMTS enables circuit-switched users to access similar services via GSM or UMTS [11]. Detecting and explaining fully states in complex tele- communications systems such as GSM networks are men- tioned as challenging tasks. Mobile networks are hierar- chical cell-based systems with complex dynamics influ- enced by the stochastic user demand. The network operat- ing characteristic such as hand over algorithms, carrier frequency and the non-stationary influence of the envi- ronment [12] are considered as main areas of this field. III. DIGITAL CELLULER SYSTEM Digital cellular systems are implemented in numerous countries all over the world. This system consist of three main parts: mobile switching center (MSC), base station system (BSS) and mobile station (MS). Each division is divided into other entities. The overall system is con- nected to the normal public service telephone network (PSTN) as shown in Figure 1 [13]. In cellular systems, the coverage area of an operator is divided into cells. These cells are normally hexagonally shaped, but in practice, because of the influence of the terrain, the shape is irregu- lar as shown in Figure 2. The size of cells varies from a few hundred meters to many tens of kilometers, depending on the coverage area and the number of subscribers and transmitting power [14].Each base station (BS) is allo- cated to a different carrier frequency and each cell has a usable bandwidth associated with this carrier. The cellular radio permits the use of a limited part of the radio spec- trum, so the available number of carrier frequencies is limited [15]. Therefore, to provide sufficient channels for more sub- scribers, it is necessary to re-use the available frequencies many times, which creates possible interference between cells carrying the same frequencies. To avoid interference it is preferable to isolate the cells by employing different frequencies [1]. The cells with different carrier frequencies form a sec- tion which identifies the re-use distance, which can de- pend on many factors, including the number of co-channel cells in the vicinity of the center cell and the geography of the terrain and the transmitted power within each cell [16]. Mobile network providers installing thousands of base stations in each country implies using small cells, al- though it is more expensive than using big cells. Figure 1. Main parts of Cellular System The advantages of cellular systems with small cells are: • Higher capacity, they are limited to fewer possible users per km2. • Less transmission power, a receiver near the base sta- tion requires less power. • Only local interference occurs. • Robustness, if one cell fails, the problem occurs in small area. Small cells also have disadvantages: • They need a complex infrastructure to connect all base stations. • They require a fast handover because of mobility be- tween cells. • Frequencies have to be distributed carefully to avoid interference between transmitters. IV. GSM SYSTEM ARCHITECTURE GSM became popular rapidly because of the quality improvement and its use of international standards. The GSM communication network is divided into three main groups; the mobile station (MS), the base station subsys- tem (BSS) and the network subsystem (NS), which is il- lustrated in Figure 3. The MS or mobile equipment (ME) includes the sub- scriber identity module (SIM). Each SIM card has an identification number called international mobile sub- scriber identity (IMSI). Each MS is assigned to a hard- ware identification called international mobile equipment identity (IMEI). Besides providing transmission and re- ception of voice and data, the mobile performs other tasks such as authentication, handover encoding and channel encoding [17] & [18]. The BSS consists of the base station controller (BSC) and the base transceiver station (BTS). The BTS is used to connect the mobiles to a cellular network. Their tasks include channel coding/decoding and en- cryption/decryption. A BTS is comprised of radio trans- iJIM – Volume 3, Special Issue 2: Technical Basics, October 2009 7 SIMULATION AND PROPOSED HANDOVER ALERT ALGORITHM FOR MOBILE COMMUNICATION NETWORKS Figure 2. Effects of crossing on the coverage mitters and receivers, antennas and an interface to the PCM facility …etc. A group of BTS are connected to a particular BSC. The main function of the BSC is call maintenance. The MS sends a report of their received sig- nal strength to the BSC, so the BSC decides to initiate hand over to other cell [19]. The network subsystem includes the mobile switching center (MSC), the home location register (HLR), the visi- tor location register (VLR), the authentication center (AUC) and the equipment identity register (EIR). The main functions of MSC are registration, authentication, location updating, handovers and call routing to a roaming subscriber. MSC also has a gateway function for commu- nication with other networks. HLR is a data base used for management of mobile subscribers; it stores the IMSI, mobile station ISDN number (MSISDN) and current visi- tor location register (VLR) address. VLR contains the current location of the MS and se- lected administrative information from the HLR, neces- sary for call control and the provision of subscriber ser- vices, for each mobile currently located in the geographi- cal area controlled by the VLR, which is connected to one MSC. AUC is a protected database that holds a copy of the secret key stored in each subscriber’s SIM card which is used for authentication and encryption. The EIR is the database that contains a list of all valid mobile station equipment within the network, where each mobile station is identified by its IMEI. The operation and management center (OMC) is a management system which maintains the operation of the GSM network. The OMC is responsi- ble for controlling and maintaining the MSC, BSC and BTS. V. GEOGRAPHICAL AREAS OF GSM NETWORK In order to work properly, a cellular system must verify the following main conditions: • The power level of a transmitter within a single cell must be limited in order to reduce the interference with the transmitters of neighboring cells. The inter- ference will not produce any damage to the system if a distance of about 2.5 to 3 times the diameter of a cell is reserved between transmitters. • Neighboring cells can not share the same channels. In order to reduce the interference, the frequencies must be reused only within a certain pattern. • In GSM, five main functions must be defined: Transmission, Radio Resources management (RR), Mobility Management (MM), Communication Man- agement (CM), Operation, and Administration and Maintenance (OAM). The geographical area of a GSM network is illustrated in Figure 4. In a GSM system, a cell is identified by its Cell Global Identity number (CGI), corresponding to the radio coverage of a base transceiver station. A Location Area (LA), identified by its Location Area Identity (LAI) number, is a group of cells served by a single MSC/VLR. A group of location areas under the control of the same MSC/VLR defines the MSC/VLR area. A Public Land Mobile Network (PLMN) is the area served by one net- work operator. Figure 3. GSM System Architecture Figure 4. GSM Network Areas 8 http://www.i-jim.org SIMULATION AND PROPOSED HANDOVER ALERT ALGORITHM FOR MOBILE COMMUNICATION NETWORKS VI. SIGNAL POWER The selected area/zone for measurements is a multilevel geographical area/zone that needs a special solution for each case. Because an isotropic antenna has unity gain for both transmitter and receiver, hence the power received by such antenna is given by: 2)4/(Pr dPt πλ= (1) Where: Pr is the MS received power signal, Pt is the BS transmitted power signal, d is the distance between the BS and the MS and λ is the wavelength of the signal. The above expression indicates that the attenuation is inversely proportional to the square of the distance be- tween BS and MS [19] & [20]. In practice of real system as illustrated in Figure 5 exist direct, diffracted and re- flected paths. In this system a small difference between these three signals is mentioned. The received power can be written as: 22 |1|*)4/(Pr φρπλ jedPt += (2) where ρ is the reflection coefficient which is equal to -1 for an ideal reflector and φ is the phase difference between direct and reflected path. Let the height of the BS and MS are HBS &HMS re- spectively. The phase difference can be calculated using the following equation, dHH MSBS λπφ /4= (3) So, the received power can be written as, 22 |1|*)4/(Pr φπλ jedPt −= (4) Because the value of φ is very small, the second term is approximately equal to φ Therefore φφ =− |1| je (5) Substituting equations 3 & 5 in equation 4 so 22 )/(Pr dHHPt MSBS= (6) This equation is used in the frequency re-used calcula- tion [18]&[20]. Figure 5. Multiple paths of GSM VII. PROPOSED HANDOVER ALGORITHM The subscribers' movement across the cells results in the need to change the allocated channels, which also happens when interference occurs. These events affect the quality of the communication so it is necessary to change the resources; defined as handover (HO) that is mainly controlled by the MSC. The MS continuously monitors the strength of received signal strength either from its BS or the surrounded BSs, which depends on the list of all cells that must be moni- tored by its BS. This will help in deciding the destination of the MS, based on the best transmission of the surround- ing cells in order to maintain the quality of the communi- cation network. The unwanted case (the worst case) situation occurs when the mobile is placed on the boundary of its serving cell. In this case the interference is produced by all over- lapping surrounding cells. This case causes the strength and the quality of the signal to fall under a certain thresh- old. Therefore, HO must happen to maintain the call con- tinuity. In the GSM architecture shown in Figure (6) there are four possible types of HO which involve transferring a connection between two BSs: • HO1: Channels in the same cell named Intra-cell handover • HO2: Cells in the same BSC named Inter-cell, Intra- BSC handover • HO3: Cells in the different BSC named Inter BSC handover • HO4: Intra MSC handover and cells in the different MSC named Inter MSC handover Most cellular systems that employ GSM aim at maxi- mizing HO for 60ms. The proposed algorithm depends on the continuous measurement at any point of the received power and the carrier to interference ratio. Depending of the refused values, a trade off process will be used to choose the best suitable value between these two meas- urements. The scanning process is necessary to check the mentioned values concerned with the received signal around all the nodes related to the MS. These processing, are used to verify various situations of handover problem, whenever it occurs during the crossing cells or MSC. When the problem is defined correctly, as indicated in the first phase, then the processing can be pass to the second phase that deals with the keeping continuity of the call during a correct decision. The proposed algorithm is summarized by the follow- ing: 1. Management of received power measurements 2. Verification of the received power measurements 3. Management of carrier to interference ratio meas- urements 4. Verification of carrier to interference ratio measure- ments, 5. Comparison of measured and reference values 6. Selection of the optimal base station to connect the mobile 7. Handover decision depends on performance and the average value of the received signal iJIM – Volume 3, Special Issue 2: Technical Basics, October 2009 9 SIMULATION AND PROPOSED HANDOVER ALERT ALGORITHM FOR MOBILE COMMUNICATION NETWORKS Figure 6. Possible types of handover VIII. SIMULATION The Cellular systems require HO procedures to main- tain the entire service area within an effective coverage range. The smaller cells require fast HO to recover calls within an adequate time and typical levels of the received signal, but this is different in large cells. The behavior of the received signal level while a mobile device moves away from one BS to another seems to vary from one level to another. In this case, the hand over deci- sion does not depend on the actual value of the received signal level, but on the average value. The simulated results of the received power refers to the distance between the base station and the mobile station as demonstrated in Figure 7 (a) and (b) which are calculated at the same cell size. Graphs (c) and (d) in Figure 7 dupli- cate the cell size to determine its effects. Figure 8 illus- trates the variation of carrier to interference ratio with respect to re-use frequency distance. These results are affected intensively by the topographic terrain. IX. CONCLUSION The popularity of mobile phones and the number of mobile device users is continuously increasing, and at the same time mobile phone manufacturers are striving to introduce new feature-packed devices to hopefully attract potential new customers. This work is limited and applied in a selected area that is situated in crowded noisy zones based on using GSM systems. This is due to the limitations that are already mentioned earlier in the introduction. The presented HO algorithm is flexible and successful to select a proper significant path to take a correct deci- sion. It also provides the ability to distinguish between real problems and false alarm problems. One of the impor- tant conclusions of the obtained results is that the pro- Figure 7. The received power with respect to the antenna height Figure 8. The received power with respect to the re-use frequency distance proposed algorithm procedure maintains the ability to predict the adequate signal of the effective coverage cell. More sophisticated handover mechanisms are needed for the transition from 2G networks to the more advanced mobile systems such as 3G networks, which are not avail- able in all countries. The handover field offers a wide area for research and more challenges appear in the future when huge number of users access different types of wire- less networks and different generations of mobile systems. ACKNOWLEDGMENT We would like to express our appreciations to Dr. Omar Daoud for his valuable assistance in proofreading this work. REFERENCES [1] A. Jagoda, M. Viillepin, and De Viillepin, “Mobile Communica- tions”, John Wiley and Sons, 1993. [2] J.Dunlop and D.G. Smith, “Telecommunication Engineering”, Prentice Hall, 3rd edition, 1992. [3] A. Alexiou, P.Kastarakis and V.N. 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Campbell, “Design, Implementation and Evaluation of Programmable Handoff in Mobile Networks”, Mobile Networks and Applications, Kluwer Academic Publishers, 2001. [14] Pubudu N. Pathirana et al, “Mobility and Trajectory Prediction for Cellular Network with Mobile Base Station”, 4th international symposium on mobile Ad hoc networking and computing, 1-3, June 2003. [15] M. E. Kounavis et al, “Supporting Programmable Handoff in Mobile Networks”, 6th International workshop on Mobile Multi- media Communications (MoMuC’99) San Diego, Ca, (November 1999). [16] S. Seshan, et al, “Handoff in Cellular Network: the Daedalus Im- plementation and experience”, Kluwer International Journal on Wireless Communication Systems, 1996. [17] M. Dillenger and S. Buljore, “Reconfigurable Systems in Hetero- geneous Environments”, Software Defined Radio: Architectures, Systems and Functions, John Wiley and Sons, Ltd 2003. [18] Payam Taaghol, “Optimization of WCDMA”, Bechtel Telecom- munication Technical Journal, Vol.2, No.1, 2004. [19] S. Aust et al, “Policy Base Mobile IP Handoff Decision Using Generic Link Layer Information”, Proceedings of the IEEE Inter- national Conference on Mobile and Wireless Communication Networks (MWCN 2003), Singapore, 27-29 October 2003. [20] Gertie Alsenmyr et al, “Hand Over Between WCDMA and GSM “Ericson Review, No.1, 2003. AUTHORS Muzhir Al-Ani was born in Iraq in 1956. He received B. Sc. degree in Electrical Engineering from Sulaimania University, Iraq in 1979, Higher Diploma in Electronic and Communication Engineering from College of Engi- neering, University of Baghdad, Iraq in 1981, M. Sc. de- gree in Electronic and Communication Engineering from College of Engineering, University of Baghdad, Iraq in 1983. He received the Ph. D. degree in 1 July 1994 for his Thesis entitled Fast Algorithms of Digital Signal and Im- age Processing from Electronic Department of E.T.S.I.I. from University of Valladolid, Spain. He joined in 7 July 1984 the Technical Institute of Al- Anbar as an Assistant Lecturer. He worked as Assistant of the Dean in 1985, the Head of the Electrical Department during the years 1985 to 1988. He joined in 1994 the Electrical Engineering Depart- ment, College of Engineering, University of Al- Mustansiriya, Iraq, as a lecturer. He joined in 1996 Com- puter and Soft Engineering Department at the same Col- lege, working as the Head of Computer and Software En- gineering Department during the years 1997 to 2001. In 5 May 1999 he was promoted Assistant professor (equiva- lent to Associate professor in Jordan) at the same Depart- ment. In 2 October 2001 he joined the Department of Computer Science and Information Systems in the Uni- versity of Technology as the Head of the Department dur- ing the years 2001 to 2003. He joined in September 2003 Electrical Engineering and Computer Department Applied Science University, Amman, Jordan as Associate professor. He joined in September 2005 Management Information System Department Applied Science University, Amman, Jordan as Associate professor, then in September 2008 Computer Science Department at the same University. His research interests include Digital Signal Processing, Parallel Processing, Digital Filters, Digital Image Process- ing, Image Compression, Computer Vision, Information Systems, Information Hiding and Steganography, Wire- less Networks Mobile Communications and related work. (e-mail: muzhir@gmail.com) Wael Hassan Al-Sawalmeh received Ph.D. degree in 1998 for his thesis entitled Working Out and Investigating of checking method of Color Production with help of sen- sors Based on LEDs and Fiber Optics of University of Telecommunication, Saint- Petersburg State-Russia. He received M.Sc. degree in Electrical Engineering from Electro technical Institute of communication Lenin- grad, Bonch-Bruyevith, Russia in 1993. He joined in 1998 university of Omar Al-Mouktar in Libya as assistant professor. In September 2001 he joined the Higher Institute of Comprehensive Vocational in Al- Baida, Libya as a part time lecturer. In October 2001 he joined the Shahat Company for Computer Techniques in Al-Baida, Libya as a part time lecturer, in September 2002 he joined the Electrical Engineering and Computer De- partment Applied Science University, Amman, Jordan as Assistant professor. He joined in September 2004 Com- munication & Electronics Department Philadelphia Uni- versity, Amman, Jordan as Assistant professor. His research interests include TV, mobile communica- tions and Digital Signal Processing. (e-mail: waelalsawalme@hotmail.com) Submitted 17 February 2009. Published as resubmitted by the authors on 9 October 2009. iJIM – Volume 3, Special Issue 2: Technical Basics, October 2009 11 http://dx.doi.org/10.1007/s10669-005-4286-6� http://dx.doi.org/10.1007/s10732-007-9024-4� http://dx.doi.org/10.1065/lca2005.08.216� http://dx.doi.org/10.3103/S0735272709020010� http://dx.doi.org/10.1023/A:1023603308841� http://dx.doi.org/10.1007/s11277-007-9247-6� http://dx.doi.org/10.1007/s11277-007-9247-6�