Research On The Admission Control And Vertical Handoff In Heterogeneous Wireless Networks

With the rapid development of wireless communications, various emerging wireless technologies together provide ubiquitous and heterogeneous network environment for users. In order to realize the final goal of future wireless communications, accommodate rapidly increasing mobile users and offer multimedia traffic services with high quality, different wireless networks overlaid each other must be integrated to cooperate and form an uniform heterogeneous system. Effective radio resource management (RRM) technologies are needed to improve system performance by maximizing the overall system capacity and maintaining the quality of service (QoS). The admission control and vertical handoff are emphatically studied in this thesis.The heterogeneous wireless access technology, overlaid network architecture, multi-service traffic and user mobility pose great opportunities and challenges to radio resource management, and the traditional radio resource management technologies can not fulfill the requirements of heterogeneous networks. Exploiting the overlaid network architecture to balance the network load has always been a hot issue. Different access networks present perfectly complementary characteristics in terms of service capability and technology, and service diversity is becoming a trend in future, which together present great challenges to admission control. The admission control is based on the above characteristics and network states, and needs to guarantee the QoS of multi-service users while optimizing the utilization of radio resource. In addition, vertical handoff and joint radio resource management are in close relationship, optimizing radio resource management with vertical handoff to improve user satisfaction, network load balance and improve system throughput is also a focused issue. Spectrum efficiency is improved by cognitive radio technology to enhance the convergence of heterogeneous networks, and admission control is key technology to implement optimal spectrum resource utilization. This thesis focuses on these outstanding issues and key technologies, the research conducted in the following four aspects:(1) Ajoint admission control scheme with vertical handoff is proposed based on the overlaid network architecture. From the view of operator, vertical handoff is utilized as a resource management tool to enhance the joint admission control. To guarantee the performance of vertical handoff user, the network resources are captured by a link utility function, and a signaling cost is used to model the signaling load incurred on the network when vertical handoff is performed. The utility function is used to be the trigger of vertical handoff. The average reward and blocking probabilities are modeled to analyze the proposed scheme. Real-time and non real-time applications are considered in the performance evaluation, and the simulation results show that the proposed scheme can efficiently relieve the congestion, balance the load and improve the overall network reward.(2) Service-differentiated admission control is studied in heterogeneous wireless networks based on the complementary characteristics of each access network and multi-service traffic users. A theoretical model of admission control with differentiated services is proposed on basis of semi-markov decision processes theory, in which the admission actions are specified for each traffic service and system state transition probabilities are formulated. Furthermore, a fuzzy logic admission utility evaluation method is presented based on the analysis of relationship of QoS requirements and network state, then the optimal admission control policy that maximizes overall utility with new and handoff call blocking probability constraints is formulated. The simulation results reveal that network dynamics can be captured by the proposed utility evaluation method. And the average utility earned in the optimal admission control policy is significantly larger than two other schemes in which service differentiation and mobility are not considered, in addition, the new and handoff call blocking probability can be strictly constrained.(3) A utility-based vertical handoff scheme is proposed to fulfill the QoS requirements of handoff users, and to optimize the utilization of radio resource in heterogeneous networks. Performance metrics of networks are denoted by a reward function, and overhead of switching signaling and blocking rate are depicted by a loss function. The net reward of networks gain in the process of handoff is presented by the profit function. The characteristics of different traffics are considered in the vertical handoff trigger rules and decision algorithm, and the target is to find the network that obtains the maximum profit. Through real-time and non real-time simulation of the two traffics, the results show that the proposed scheme gains lower blocking rate, dropping rate and handoff times, furthermore, the system throughput is improved and network load is balanced. (4) Cognitive radio is expected to solve the spectrum scarcity problem by enabling unlicensed secondary users (SUs) to opportunistically utilize the licensed spectrum whenever it is left unused by legacy primary users (PUs). An optimal admission control scheme in cognitive radio networks is proposed, which maximizes the SUs network revenue with blocking probability constraints over the PUs. The optimal scheme is formulated as a semi-markov decision processes, the state space and action space of PUs and SUs are firstly defined, and then the state transition probabilities and reward function are derived, lastly the linear programming algorithm is presented to obtain the optimal admission control policy. The convergence of linear programming algorithm is also proved. Simulation results show that, the proposed scheme achieves more reward than the complete sharing scheme and threshold-based scheme, and the PUs blocking probability can be strictly constrained.