Hybrid Predictive Base Station (HPBS) Selection Procedure in IEEE 802.16e-Based WMAN Sayan K. Ray 1 , Arpan Mandal 2 , Krys Pawlikowski 1 and Harsha Sirisena 2 1 Dept. of Computer Science and Software Engineering 2 Dept. of Electrical and Computer Engineering University of Canterbury, Christchurch, New Zealand Email: {skr29@student., ama179@student., krys.pawlikowski@, harsha.sirisena@}canterbury.ac.nz Abstract— According to the mobility framework of IEEE 802.16e, a Mobile Station (MS) should scan the neighbouring Base Stations (BSs), for selecting the best BS for a potential handover activity. However, the standard does not specify the number of BSs to be scanned leaving room for unnecessary scanning. Moreover, prolonged scanning also interrupts data transmissions thus degrading the QoS of an ongoing connection. Reducing unnecessary scanning is an important issue. This paper proposes a scheme to reduce the number of BSs to scan (thus improving the overall handover performance). Simulation results have shown that this hybrid predictive BS selection scheme for potential scanning activities is more effective than the conventional IEEE 802.16e handover scheme in terms of handover delay and resource wastages. Keywords- IEEE 802.16e, handover, scanning, MOB_HO- REP, TMDB, PHDB, selection procedure, hybrid, predictive,WMAN I. INTRODUCTION Broadband wireless access (BWA) solutions based on IEEE 802.16 family of technologies (IEEE 802.16d and IEEE 802.16e) [1],[2] offer promising features for wireless metropolitan area networks (WMAN) in terms of high bandwidth, extended coverage, high speed multimedia services and low cost. In the case of IEEE 802.16e, this is the provisioning of mobility and handover scenarios. While Hard Handover (HHO) is the default and the most common scheme, the Fast Base Station Switching (FBSS) and the Macro-Diversity Handover (MDHO) are two other optional procedures. In our paper, we will concentrate on performance issues related with hard handovers. In IEEE 802.16e handover, Network Topology Acquisition Phase (NTAP) and the Actual Handover Phase (AHOP) are the two main phases. During the NTAP, the MS performs scanning and downlink synchronization activities with the advertised neighbouring BSs to select a new target BS to perform the handover activity. During the AHOP, the MS releases its connection with the current serving BS and performs synchronization and registration procedures with the newly selected target BS to successfully complete the handover process. However, the entire procedure is not free from ambiguities. Excessive scanning and synchronization activities may result in unwanted handover delays along with wastages of valuable resources. Hence, limiting the extent of scanning activities remains a challenging task in the IEEE 802.16e systems. Over the last few years, much research has been focused on limiting and / or reducing the disruptive effects of excessive scanning activities during selection of the new serving BS for a potential handover. An Adaptive Channel Scanning Algorithm (ACSA) [3] is used in IEEE 802.16e to estimate the total scanning time required by an MS and to allocate scanning intervals to multiple MSs by interleaving them with the data transmission intervals. However, instead of minimizing excessive scanning activities, this scheme actually focuses on minimizing the disruptive effects of excessive scanning activities on the different application traffics. Selection of the target BS based on the mean CINR and Arrival Time Difference (ATD) information of each neighbouring BSs (before any kind of synchronization and association activities are performed) was proposed in [4]. According to that scheme, an IEEE 802.16e-supported MS can get this neighbouring BSs-related information from the broadcasted advertisements of the serving BS. As this scheme does not consider the MS’s direction of motion as well as the current load of the selected BS, it might lead to unwanted ping-pong activities as well as call drops. In this paper, we have proposed a fast and hybrid handover approach for IEEE 802.16e systems that can result in significant reduction of unnecessary scanning activities. Selection of the new target BS for a potential handover not only depends on the signal strength but also on other parameters like the MS’s direction of motion, current load of the neighbouring BSs and the location of the neighbouring candidate BSs with respect to the current serving BS. Simulation studies have shown that in comparison to the standardized IEEE 802.16e handover approach, our scheme has the capabilities of reducing both the overall handover latency and the wastage of channel resources. The rest of the paper is organised as follows. Sections II and III respectively provides a brief discussion on HHO procedure and the scanning activity-related drawbacks in the conventional IEEE 802.16e handover scheme. Section IV describes our proposed scheme in detail. Section V contains the simulation methodology along with the results, followed by conclusion in section VI. II. IEEE 802.16e HANDOVER An 802.16e handover process starts when the communication signal strength drops below a certain threshold as perceived by the MS or the serving BS. NTAP and AHOP are the two main phases in the 802.16e handover. A. Network Topology Acquisition Phase (NTAP) During the NTAP, shown in Fig. 1, the MS and the serving BS, jointly with the help of the backbone network, 2007 Australasian Telecommunication Networks and Applications Conference December 2nd – 5th 2007, Christchurch, New Zealand 93