IJCER (www.ijceronline.com) International Journal of computational Engineering research


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IJCER (www.ijceronline.com) International Journal of computational Engineering research

  1. 1. Pooja bhat, Bijender mehandia /International Journal Of Computational Engineering Research / ISSN: 2250–3005 Analysis of Handover in Wimax for Ubiquitous connectivity Pooja bhat1, Bijender mehandia2 1,2 Gurgaon Institute of Technology and Management, Bilaspur, GurgaonAbstractWIMAX is Wireless Interoperability for Microwave the moving user switches from one base station to anotherAccess. It is a telecommunication technology that provides i.e. during handover. Handover is procedure when a mobilewireless data over long distances in several ways, from station changes the serving base station. The reason forpoint-to-point links to full mobile cellular type access. The handover could be relatively low signal strength or workmain consideration of Mobile Wimax is to achieve load of base station.seamless handover such that there is no loss of data. In Wimax is a state-of-the-art wireless technologyWimax both mobile station (MS) and base station (BS) which utilizes adaptive modulation and coding, supportsscans the neighbouring base stations for selecting the best single carrier (SC) and orthogonal frequency divisionbase station for a potential handover. Two types of multiplexing techniques (OFDM) and several frequencyhandovers in wimax are: Hard handover (break before bands for different operation environments.make) and Soft handover (make before break). To avoiddata loss during handover we have considered soft II. Materials and methodshandovers this research topic. We have proposed atechnique to select a base station for potential soft 1.1 Handovers in Wimaxhandover in wimax. We have developed a base station A special requirement of a mobile device is the ability toselection procedure that will optimize the soft handover change its serving base station if there exists another basesuch that there is no data loss; handover decision is taken station with better signal strength in the reach of mobilequickly and thus improving overall handover performance. station (MS). Handover is a procedure that providesWe will compare the quality of service with hard handover continuous connection when a MS migrates from the air-and soft handover. We have analysed the proposed interface of one BS to another air-interface provided bytechnique with an existing scheme for soft handover in another BS without disturbing the existing connections.wimax with simulation results. Handovers are needed to support mobility. For a handover to occur, one needs to have at least twoKeywords: Wimax, Topology, handover, QOS, base stations : serving base station(SBS) and target baseubiquitous connectivity. station(TBS). The handover is generally considered asI. Introduction change in serving base station but it does not necessarilyIEEE 802.16 standard defines the air interface for fixed mean that the base station must be changed. In some casesBroadband Wireless Access (BWA) systems to be used in there may be different reasons why a handover might beWMANs (Wireless Metropolitan Area Networks), conducted:commonly referred to as Wimax  When the MS is moving away from the area covered by one cell and enters the area covered by another cell(Worldwide Interoperability for Microwave Access). The the connection is transferred to the second cell in orderoriginal standard IEEE 802.16 does not support mobility to avoid data loss when the MS gets outside the rangeand for this purpose IEEE 802.16e-2005 was introduced. It of the first cell;is also known as Mobile Wimax .It is the new mobile  When the capacity for connections of a given cell isversion of the older Wimax specification known as IEEE used up, the new connection which is located in an802.16e-2004 which is wireless but fixed, it lacks the area overlapped by another cell, is transferred to thatability for user to move during data transmission. The main cell in order to free-up some capacity in the first cellpurpose of Wimax is to provide users in rural areas with for other users, who can only be connected to that cell;high speed communications as an alternative to expensive  When the channel used by the MS becomes interferedwired connections (e.g. cable or DSL). That is Wimax is with by another MS using the same channel in acapable to provide high speed internet to last mile different cell, the call is transferred to a differentconnections. But this is not the only purpose of Wimax channel in the same cell or to a different channel insystems. Mobile Wimax allows the user to move freely another cell in order to avoid the interference;during data transmission. The main consideration of  Signal strength is not enough for maintaining propermobile Wimax is that there should be no data loss when connection.IJCER | July-August 2012 | Vol. 2 | Issue No.4 |997-1000 Page 997
  2. 2. Pooja bhat, Bijender mehandia /International Journal Of Computational Engineering Research / ISSN: 2250–3005Behaviour of MS changes, for example in case of fastmoving MS suddenly stopping; the large cell size can beadjusted by a small size cell with better capacity.In CDMA networks a soft handover may be induced inorder to reduce the interference to a smaller neighbouringcell due to the "near-far" effect even when the phone stillhas an excellent connection to its current cell;1.1.1 Stages of Handover procedure: II. Fast Base Station Switching (FBSS)  Call restriction We are considering fast base station switching technique.  Handover decision/initiation In this method a diversity set is maintained for each mobile  Synchronization station. The serving base station and mobile station  Termination of service monitors the neighbouring base stations that can be added in diversity set. Diversity set is maintained by both mobileTypes of handovers station and serving base station. Diversity set is collectionThere are two types of handovers used in cellular network of base stations that can chosen as target base station for asystems: hard handover and soft handover handover. Handover decision can be taken by mobileHard handover station, base station or base station controller dependingThe hard handover is used when the communication upon the implementation.channel is released first and the new channel is acquiredlater from the neighbouring cell. For real-time users itmeans a short disconnection of communication. Thus,there is a service interruption when the handover occursreducing the quality of service. Modification in Efficient FBSS technique In the proposed technique, we are trying to modify the1.2.1 Methods of Soft Handovers in Wimax FBSS procedure to optimize target base station selection for soft handovers in wimax. We have introduced monitor I. Macro Diversity Handover (MDHO) base station which is selected from diversity set of mobileThe MDHO supported by MS and by BS, the “Diversity station. The function of monitor base station (MBS) is toSet” is maintained by MS and BS. The Diversity Set is a communicate with mobile station and maintain thelist of the BSs, which are involved in the handover database of potential target base stations for a handover forprocedure. The Diversity Set is maintained by the MS and mobile station. Another advantage of MBS is thatBS and it is updated via MAC (Medium Access Control) whenever ABS fails, mobile station can start datamanagement messages. A sending of these messages is communication with MBS without any loss of data byusually based on the long-term CINR (Carrier to Noise sending register message.plus Interface Ratio) of BSs and depends on two The mobile station sends its current location to MBS andthresholds: Add Threshold and Delete Threshold. according to history of mobile station movement and itsThreshold values are broadcasted in the DCD (Downlink current location, MBS sorts the TBS’s having maximumChannel Descriptor) message. The Diversity Set is defined div parameter.for each MS in the network. The MS continuously Div= s/w- dmonitors the BSs in the Diversity Set and defines an S= received signal strength“Anchor BS”. The Anchor BS is one of the BSs from w= work loadDiversity Set in MDHO. The MS is synchronized and d= distance between mobile station and base stationregistered to the Anchor BS, further performs ranging and d=√ (xs – xi)2 + (ys-yi)2monitors the downlink channel for control information. where (xs , ys) are coordinates of mobile station and (xi , yiThe MS communicates (including user traffic) with ) are coordinates of i’th base station where i=1,2,3…. NAnchor BS and Active BSs in the Diversity Set[1] N = total number of base station in diversity set s= (k* st )/d Where st= transmitted signal strength k= other factors affecting signal (interference)IJCER | July-August 2012 | Vol. 2 | Issue No.4 |997-1000 Page 998
  3. 3. Pooja bhat, Bijender mehandia /International Journal Of Computational Engineering Research / ISSN: 2250–3005The MBS scans the neighbouring base stations and Number of Base Station : 2calculates div parameter for each base station. Then MBS Physical layer : 802.11sorts the BS’s in diversity set using sorting algorithm indescending order such that the BS having maximum value Data link layer : 802.16-eof div is on the top of diversity set. Step 1 Fig. shows 3 nodes used in simulation of base stationSelection of MBS selection procedure for soft handover. Here node 1 isWhen a mobile station gets registered to a Serving Base mobile station and all the other nodes are baseStation (SBS), it sends scan_req message to SBS, it stations. The simulation shows the handoverresponds to this message by sending the data of its procedure as mobile station changes its positionneighboring base stations through scan_rsp message.With this data the mobile station will choose the MBShaving maximum value of div parameter. That is mobilestation will communicate with best suited target basestation so at any point if SBS goes down, the mobilestation can easily switch to MBS. As the mobile stationis moving continuously the diversity set is required toupdated according to current location of mobile station.If the div value of MBS goes below the threshold value.It will send the stored information to SBS and SBS willselect new MBS the mobile station. The Serving BaseStation (SBS) periodically broadcasts NeighborAdvertisement (NBR_ADV) message that containsnetwork topology information or channel information ofavailable neighbouring base stations. The mobile station Fig Nam file for soft handoversends Step 2(SCN_REQ) message to the serving base station to scan Above figure shows the ranging between node 0 and nodethe neighbouring base station according to the current 1. The node 0 acts as serving base station for mobilelocation (div) of mobile station. The serving base station station (node 1). The node 1 starts data communicationresponds to SCN_REQ message by sending the with node 0. As the mobile station moves, its distance frominformation of neighboring base station as per the serving base station increases and the mobile station lookscalculation, the base station with maximum value of div for another base station for soft handover ie. Target Baseparameter is selected as monitor base station. Station. The below figure shows handover when theWhenever mobile station requires a handover, it sends mobile station connects with target base station. Node 2 isHO_INIT target base station.(Handover Initiation) message to monitor base station thatsends the information of target base station to mobilestation. The mobile station synchronizes the downlink anduplink frequencies with target base station. The mobilestation can now start the data communication with targetbase stationIII. Simulation and ResultThe proposed technique is implemented in NS-2.34Simulator in Linux environment. We have modified ns-2.34 by adding mac802.16-e layer to it for supportingWimax. The wimax.tcl file is coded on c++, when Fig Signaling with serving base stationexecuted it generates a .nam file which can be viewed inNetwork Animator tool of ns2 simulator. Step 3 First result is comparison between the fast base stationParameters Used switching handover and our proposed technique. It shows that using the proposed technique the performance of softPacket size : 1500 bytes handover is improved.Time interval of data sent : 6 msTotal Number of nodes : 3IJCER | July-August 2012 | Vol. 2 | Issue No.4 |997-1000 Page 999
  4. 4. Pooja bhat, Bijender mehandia /International Journal Of Computational Engineering Research / ISSN: 2250–3005 standard, the BS selection procedure based on location predication algorithms and current load factors of the target BSs give an alternative way of deciding the target BS. Further, reducing the number of handovers is highly desirable from a system perspective. References: [1] Andrews, J. G., A. Ghosh, Fundamentals of WiMAX Understanding broadband wireless networking, Prentice Hall ,et al. (2007). [2] Zdenek Becvar, Jan Zelenka ,Handovers in the Mobile WiMAX.Fig Graph of soft handover with modified [3] N. P. Singh, Brahmjit Singh, PerformanceFBBS technique Enhancement of Cellular Network Using AdaptiveStep 4 Soft Handover Algorithm.The x axis denotes the time and y-axis denotes the packet [4] Zdenek BECVAR, Pavel MACH, Robert BESTAK,received Initialization of Handover Procedure in WiMAX Networks. [5] Kumar Mrinal , Chetan Aneja, Vikram Gupta, Swati Sharma , Mobility Improvement In IEEE 802.16. [6] Rajat Prakash and Venugopal V. Veeravalli, Locally Optimal Soft Handoff Algorithms. [7] IEEE Std: “Soft Handover and Fast BS Switching Procedure,” IEEE 802.16 Broadband Wireless Access Working Group, June 2004.Fig Graph for hard hand overIV. Conclusion:We simulated the more realistic handover in the mobileWiMAX using NS-2 with WiMAX and mobility modules.The goal of this simulation is to find out the relationshipbetween the handover latency and the velocity of mobilestation. It can be seen that the current handover mechanicsused in the NS-2 module meets the requirement ofseamless handover in mobile WiMAX when the mobilestation travel at the speed of 20 m/s. Although, using link-going down mechanism will dramatically reduce thehandover latency, it is still a challenge to achieve the fullmobility: up to 120 km/h, handover latency of less than 50ms with an associated packet loss that is less than 1percent.V. Future scope:As extension to this research work, two topics for futureresearch investigations are suggested. Since there is a tradeoff between handover threshold and margin, an adaptivethreshold window could be used to balance the load of basestation and the QoS of the mobile.Also, the current work isrestricted to hard handover only. Possibilities of extendingthis work to macro diversity and fast base station switchingcan be worthy of an investigation. Although these are softhandover techniques and currently optional in the WiMAXIJCER | July-August 2012 | Vol. 2 | Issue No.4 |997-1000 Page 1000