International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                           1ISSN 222...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                           2ISSN 222...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                             3ISSN 2...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                            4ISSN 22...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                            5ISSN 22...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                            6ISSN 22...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                            7ISSN 22...
International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012                             8ISSN 2...
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Researchpaper improving channel-capacity-of-a-cellular-system-using-cell-splitting

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Researchpaper improving channel-capacity-of-a-cellular-system-using-cell-splitting

  1. 1. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 1ISSN 2229-5518IMPROVING CHANNEL CAPACITY OF A CELLULAR SYSTEM USING CELL SPLITTINGOhaneme C.O.1,Onoh G.N.2, Ifeagwu E.N.1, Eneh I.I.2Abstract-As the number of users in a cellular system increases, the traffic per unit time also increases. Theallocated spectrum becomes gradually congested and eventually becomes used up. Congestion of the spectrummeans that the call blocking probability has increased and this is not desired in the system. This paper presentscell splitting as a technique of improving channel capacity in cellular communication system. It also wentfurther to show that increase in channel capacity directly reduces call blocking probability and call delayprobability. The results are simulated and shown using Matlab .Key Words: Frequency reuse, footprint, cell ——————————  ——————————1.0 Introduction There are currently many different types of wirelessThe developments in wireless communication networks. Wireless local area networks and cellularnetworks over the past couple of decades have been networks are by far the most dominant wirelessenormous and have become ubiquitous in modern networks of our generation and have still sparkeddays. numerous research studies. [2]It is commonly assumed that the next generation of The design objective of early mobile radio systemswireless communication networks will be was to achieve a large coverageheterogeneous, with different types of wireless area by using a single, high powered transmitternetwork and technologies co-existing [1]. with an antenna mounted on a tall tower. While this _________________________ approach achieved very good coverage, it also1 .Department of Electronic and Computer Engineering, NnamdiAzikiwe University, Awka,( scotolysis@yahoo.com) meant that it was impossible to reuse those same2 Department of Electrical and Electronic Engineering, frequencies throughout the system, since anyEnugu State University of Science and Technology ,ESUT IJSER © 2012 http://www.ijser.org
  2. 2. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 2ISSN 2229-5518attempt to achieve frequency reuse would result in Cellular radio systems rely on an intelligentinterference. For example, the Bell mobile system allocation and reuse of channels throughout ain New York City in the 1970s could only support a coverage region. Each cellular base station ismaximum of twelve simultaneous calls over a allocated a group of radio channels to be usedthousand square miles. Faced with the fact that within a small geographic area called a cell. Basegovernment regulatory agencies could not make stations in adjacent cells are assigned channelspectrum allocations in proportion to the increasing groups which contain completely different channelsdemand for mobile services, it became imperative to than neighboring cells. The base station antennasre-structure the radio telephone system to achieve are designed to achieve the desired coverage withinhigh capacity with limited radio spectrum while at the particular cell. By limiting the coverage area tothe same time covering very large areas [3]. within the boundaries of a cell, the same group ofThe idea of cellular network goes back as early as channels may be used to cover different cells that1947, and it was thought that instead of using just are separated from one another by distances largeone high-powered antenna to cover an entire enough to keep interference levels within tolerablemetropolitan area, we should employ several lower limits.powered antenna base stations scattered throughout 2.0 Frequency Reuse concept in cellular Networkthe city, thereby breaking a macro-cell into several The design process of selecting and allocatingsmaller micro-cells. channel groups for all of the cellular base stationsThe spectrum is then divided such that the base within a system is called frequency reuse orstations of each of these micro-cells would be able frequency planning . Figure 1 illustrates theto use a certain frequency band or channel without concept of cellular frequency reuse, where cellsbeing affected too much by neighboring cells (i.e., labeled with the same letter use the same group ofto avoid inter-cell interference) [4]. channels. The hexagonal cell shape shown in Figure 1 is conceptual and is a simplistic model of the IJSER © 2012 http://www.ijser.org
  3. 3. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 3ISSN 2229-5518radio coverage for each base station, but it has been As the demand for wireless service increases, theuniversally adopted since the hexagon permits easy number of channels assigned to a cell eventuallyand manageable analysis of a cellular system. The becomes insufficient to support the required numberactual radio coverage of a cell is known as the of users. At this point, cellular design techniques arefootprint and is determined from field needed to provide more channels per unit coveragemeasurements or propagation prediction models. area. Techniques such as cell splitting, sectoring, and coverage zone approaches are used in practice to expand the capacity of cellular systems. Cell splitting allows an orderly growth of the cellular system. Sectoring uses directional antennas toFig1:Cellular Network with Frequency Re-use further control the interference and frequency reuseWhen using hexagons to model coverage areas, of channels. The zone microcell concept distributesbase station transmitters are depicted as either being the coverage of a cell and extends the cell boundaryin the center of the cell (center-excited cells) or on to hard-to-reach places. While cell splittingthree of the six cell vertices (edge-excited cells). increases the number of base stations in order toNormally, Omni directional antennas are used in increase capacity, sectoring and zone microcellscenter-excited cells and sectored directional rely on base station antenna placements to improveantennas are used in corner-excited cells[5]. capacity by reducing co-channel interference. CellPractical considerations usually do not allow base splitting and zone microcell techniques do notstations to be placed exactly as they appear in the suffer the trunking inefficiencies experienced byhexagonal layout. Most system designs permit a sectored cells, and enable the base station to overseebase station to be positioned up to one-fourth the all handoff chores related to the microcells, thuscell radius away from the ideal location [6]. reducing the computational load at the mobile2.1 Improving Capacity in Cellular Network switching centre (MSC) [7]. IJSER © 2012 http://www.ijser.org
  4. 4. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 4ISSN 2229-5518 probability has risen above an acceptable level.2.2. Cell Splitting Imagine if every cell in the cluster was reduced inCell splitting is the process of subdividing a such a way that the radius, R of every cell was cutcongested cell into smaller cells, each with its own in half, (R/2). In order to cover the entire servicebase station and a corresponding reduction in area with smaller cells, approximately four times asantenna height and transmitter power. Cell splitting many cells would be required. The increasedincreases the capacity of a cellular system since it number of cells would increase the number ofincreases the number of times that channels are clusters over the coverage region, which in turnreused. By defining new cells which have a smaller would increase the number of channels, and thusradius than the original cells and by installing these capacity, in the coverage area. In the examplesmaller cells (called microcells) between the shown in Figure 2, the smaller cells were added inexisting cells, capacity increases due to the such a way as to preserve the frequency reuse planadditional number of channels per unit area [8]. of the system. In this case, the radius of each newThe consequence of the cell splitting is that the microcell is half that of the original cell.frequency assignment has to be done again, whichaffects the neighboring cells. It also increases thehandoff rate because the cells are now smaller and amobile is likely to cross cell boundaries more oftencompared with the case when the cells are big.Because of altered signaling conditions, this alsoaffects the traffic in control channels [9]. Fig 2 : Each cell in a cluster is split intoA typical example of cell splitting is shown in approximately four smaller cells by reducing R byFigure 2. Here, it is assumed that the cell cluster is half.congested and as a result, the call blocking IJSER © 2012 http://www.ijser.org
  5. 5. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 5ISSN 2229-5518For the new cells to be smaller in size, the transmit In this paper we considered a cellular system shownpower of these cells must be reduced. The transmit in Figure 3, where the original cells have radius Rpower of the new cells with radius half that of the which equal to 2 km. The cellular system covers anoriginal cells can be found by examining the area of 6 km x 6 km whose call blocking probabilityreceived power ,Pr, at the new and old cell Pr[blocking] and call queuing (delay) Pr[delay] hasboundaries and setting them equal to each other. risen beyond an acceptable level. These cells needThis is necessary to ensure that the frequency reuse be split into smaller cells and must have equalplan for the new microcells behaves exactly as for number of channels no matter how small it becomesthe original cells. For Figure 2, after splittingPr[old cell] Pt1 (1)andP[new cell] Pt2 (2)where Pt1 and Pt2 are the transmit powers of thelarger and smaller cell base stations, respectively,and n is the path loss exponent. If we take n = 4 andset the received powers equal (that is, assume Fig 3: System Under Considerationperfect power control) to each other, then 3.1 System Parameters Pt2 =Pt1/16 (3) The various parameters which are directly related toIn other words, the transmit power must be reduced models under consideration in this work areby 12 dB in order to fill in the original coverage described in table 1 below. These parameters arearea with microcells, while maintaining the S/I constant which have been assumed to enable usrequirement [2]. successfully analyze the various mathematical3.0 Result and Analysis models used in this paper. IJSER © 2012 http://www.ijser.org
  6. 6. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 6ISSN 2229-5518Table 1: Numerical Parameters and their Values The probability that a call is delayed or queued given by the Erlang [10] was evaluated for different value of channels in ascending order using Matlab. The corresponding values of call delay probabilities are tabulated against the number of channels as shown below in table 3. Table 3: Call probabilities evaluated from channel The probability that a new call is blocked given capacity by the Erlang B formula [9] was evaluated for different value of channels in ascending order using Matlab. The corresponding values of call blocking probabilities are tabulated against the number of channels as shown below in table 2 Table 2: Blocking probabilities and number of channels Fig 4: Graph of blocking probability against number of channels3.2 Impact of Increase in Capacity on Call DelayProbability (Pr[delay]). IJSER © 2012 http://www.ijser.org
  7. 7. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 7ISSN 2229-5518 The probability that a new call is queued or delayed Pr[delayed] is plotted against channel capacity in Figure 5. The graph shows that as the channel capacity increases, the call delay probability decreases until it becomes constant as it tends toFig 5: Graph of delay probability against number of channels zero (0). From the graph, it could be seen that the number of calls delayed in the system is reduced as the channel capacity increases. The sum of all co-channel interfering signals is plotted against the desired signal power in Figure 6. The result shows a linear relationship between the desired signal power and the sum of co-channel interfering signals. Therefore, to ensure that theFig 6: Graph of interference against desired signal power system function properly, the power of the desired4.0 Summary of Results signal must chosen such that the resultant co-The probability that a new call is blocked channel interference is manageable.represented as Pr[blocked] is plotted against 5.0 Conclusionchannel capacity in Figure 4. The result shows that In conclusion, the capacity of a cellularas the channel capacity increases, the call blocking communication system can be increased using cellprobability Pr[blocked] reduces until a point when it splitting. The extent to which the capacitybecomes constant. Figure 4 proves that more calls increment can be achieved is dependent on signal-is allowed in the system as the channel capacity to-interference ratio of the system after the cellincreases. splitting. The increment in channel capacity after cell splitting helps to reduce the call blocking IJSER © 2012 http://www.ijser.org
  8. 8. International Journal of Scientific & Engineering Research Volume 3, Issue 5, May-2012 8ISSN 2229-5518probability and call delay or queuing probability. [5]. Lal C. Godara, ‘A handbook of antennas inThe results show that when properly and orderly wireless communications’, CRC Press LLC, 2009carried out, the cell splitting technique has the [6]. Yumin Wu, Kun Yang, and Hsiao-Hwa Chen,capability of increasing the capacity of a congested ‘An Adaptive Routing Protocol for Converged Ad-cellular system. Hoc and Cellular Networks’, KICS 2006.References. [7]. Ali Khawaja, ‘System Design in Wireless[1] .Chao-Wen Lin, ‘Scheduling in a Multi-Sector Communication’, University of Texas at Dallas,Wireless Cell’, University of Waterloo, Ontario, December 6, 2009.Canada, 2009. [8]. Jahangir khan, ‘Handover Management in GSM[2]. Rappaport T. S., ‘Wireless Communication Cellular System’, International Journal of ComputerPrinciple and Practice Second Edition’, Prentice Applications (0975 – 8887), Volume 8– No.12,hall, 2006 October 2010[3]. Sara Bavarian, ‘Symbol by Symbol Soft-input [9]. V. H. Mac Donald, ‘The Cellular Concept’,Soft-output Multiuser Detection for Frequency The Bell Technical Journal, Vol. 58, No. 1, JanuarySelective MIMO Channels’, Simon Fraser 2008.University fall, 2008. [10]. Nasıf Ekiz, Tara Salih, Sibel Küçüköner, and[4]. Kamil S. Zigangirov, ‘Theory of Code Kemal Fidanboylu, ‘An Overview of HandoffDivision Multiple Access Communication’, Techniques in Cellular Networks’, World AcademyInstitute of Electrical and Electronics Engineers, of Science, Engineering and Technology , 2005.Copyright © 2004. . IJSER © 2012 http://www.ijser.org

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