Handoff Technology Research In Heterogeneous Wireless Networks

With the deployment of 2G and 3G cellular networks and WLAN, WiMax world widely, the next generation wireless networks are envisioned as heterogeneous networks with all sorts of wireless networks coexisting. For the diversity of these networks, each network has its in point applications and scenarios. How to fully utilize ubiquitous heterogeneous networks' potential advantages has become many research personnel's pursued goals. Vertical handoff among heterogeneous networks and horizontal handoff are important contents of mobility management in mobile communications. Next generation wireless networks have been envisioned with all-IP-based loose-coupled architecture. Vertical handoff decision manner has transformed from network-controled to terminal-controled. In this thesis, we have studied the best network selection and handoff decision problems.Vertical handoff problem is more complex then horizontal handoff. Vertical handoff whole process can be divided into three main steps: best network selection, handoff decision and handoff signalling protocol interworking. The factors that contribute to best network selection are relatively long time network characteristics, including money cost, bandwidth, end to end delay and security etc. These factors have low correlation with mobile terminal's velocity, position and environment's short time variable. While the main considerations of handoff decision are the ping-pong handoff and possible outages caused by drastic change of received signal strength (RSS) for the movements and fading.Network condition, terminal and user perferences must be considered in best network selection decision. Any change in above three aspects may trigger one time network reselection process. This thesis analyses the features and drawbacks of published best network selection schemes and proposes a cost sensitive best network selection scheme. The RSS changes have not been added into this selection scheme, but are considered in handoff decision algorithm. According to this special money cost factor, we design a user accepted maximal cost level parameter, this parameter is inputted by user to accurately fit user's demand in cost. A fuzzy segmental normalization policy has been proposed when calculate the quality function value of different networks. The inner QoS demands of the application are considered in the marks of relative quality parameters. This normalization means has good fairness among all networks quality parameters.This thesis analyses handoff decision problem. Because the RSSs that are from different base stations vary greatly by distance and fading, mobile terminal has to keep connected and minimize the ping-pong times. Handoff decision should decide when to trigger the handoff and which is the best objective base station among many available base stations . For the traditional handoff decision algorithms have poor performance, for instance, too many ping-pong times or outage times. In heterogeneous networks, the overcome of vertical handoff is quite greater then horizontal handoff, the performance problem seems more crucial. This thesis proposes a coverage aware handoff decision algorithm, which uses the precise location, moving speed and direction information combined with downloaded coverage information opened by network operators. In order to implement the coverage aware strategy, we analyse the propagation model and the revising of propagation model parameters, and bring forward the algorithm to calculate the cell border. Simulation results prove the high performance compared with traditional RSS-based decision algorithms. By measuring signal coverage information in west area of Huazhong Univ. of Sci. and Tech., the practicality of proposed algorithm has been proved.When the mobile terminal can not receive GPS (Globe Positioning System) signal, we put forward a distance and velocity assisted handoff decision algorithm. By transforming the signal strength into distance, and continuous sampling the RSS, we gettwo estimated parameters that are initial distance (d|^l) and leaving velocity (v|^) from base station. The proposed algorithm uses these two estimated algorithms to assist the handoff decision with the help of previous downloaded base station coverage information. Simulation results demonstrate the better perfonnance then traditional RSS-base handoff decision algorithms.