Relay Selection And Handoff Mechanism Research In Mobile WiMAX System

The rapid development of wireless broadband access driving force is the needs of users'broadband services. In recent years, various wireless broadband access technologies (Wi-Fi, WiMAX, Bluetooth,3G) enable people to shake off the shackles of telephone lines, and gradually from fixed to mobile. Broadband wireless access technology, with its low cost, without geographical constraints, support for user mobility and other advantages, which will become the important means of "last mile" access issues in communication and information technology is the fastest growing areas of the technologies.WiMAX broadband wireless network technology based on IEEE802.16 series of standards accelerates the "last mile" deployment for DSL or cable access inaccessible areas to provide convenient solutions which promotes the global broadband wireless access development. WiMAX can provide high-speed data services and high resource utilization in the spectrum, a large coverage area and so on.Next generation mobile communication system can support users'100Mbit/s-1Gbit/s transfer rate, such a high rate on the existing cellular network structure made many practical challenges. As the frequency resource constraints, most wireless systems will be greater than the 2GHz band applications, such a high frequency radio results to signal attenuation be more severe than the existing 1～2GHz band, cell edge users will not get the quality of service guarantee. Such a network will not be able to adapt to future wireless applications. The Ad-hoc networks are more intermediate nodes by means of a multi-hop packet of information which will be passed from the source node to destination node to achieve communication between the network nodes. Mobile Multi-hop relay (MMR) network integrates the advantages of cellular networks and Ad-hoc network to increase network interference and enhance network robustness through load balancing and path diversity advantages.IEEE802.16 Relay Task Group introduced relay station into IEEE802.16 system and formed a new standard IEEE802.16j. IEEE802.16j doesn't establish a multi-hop mesh network but expands single-hop 802.16d/e standards to support the capacity of multi-hop communications. Weak signal in a network go through "multi-hop" to reach the end by relay station. RS can expand the network coverage, increase data throughput and reduce network operating costs.In the mobile communication system, the user moves in the cellular coverage area and its ongoing call may be transferred from one cell coverage area to another cell coverage area. Handoff should be quickly and effectively, otherwise will affect the user experience in the district boundaries, but also may cause certain network load and reduce the system's overall quality. Mobile station under the IEEE802.16j has much choice of handoff means, the option to switch to a relay station or switch to a base station. Within the coverage area of the RS, MS switch more frequently because of MS's mobility and less coverage of the RS, Handoff control will be more complex.In this paper, several relay station selection algorithms in the MMR networks are researched, and we propose improved optimal relay selection algorithm based on path loss and SINR which are compared with existing algorithms. Simulation results show that the system is reduced disruption probability and improve the robustness of the network. In some cases, due to resource scheduling or load balancing and other reasons MS may not get the best relay services. In this case, we should look for second-best relay or the Nth-best relay, and we research the system performance such as interruption and bit error rate performance under the Nth-best relay selection strategy. Finally, in the mobile multi-hop relay network, we propose an improved handoff algorithm to adjustment MS handoff procedure on the handoff threshold and hysteresis level by considering QoS requirements of different service types. This handoff algorithm effectively improves the handoff efficiency, reduces the unnecessary handoff, reduces the handoff "ping-pong" effect and greatly reduces the handoff service interruptions to ensure continuity of services.