An Investigation On Group Handover Strategy Of Cellular Wireless Network Under High Mobility Scenarios

In cellular radio communication system, it is very likely that a large number of User Equipments (UEs) try to initiate handover almost at the same time while traveling across the edge of cells, which will lead to a rapid decrease of available access resource, causing network congestion and handover interruption. This phenomenon is known as group handover. When group handover occurs, how to allocate the limited system resources efficiently to current mobile users is a critical issue, especially in high mobility scenarios. Therefore, the research of group handover mechanism of cellular radio network under high speed is of great practical value for designing reasonable handover flow and optimizing radio resource allocation algorithm.At the first place, typical handover priority strategies in cellular radio network are presented. From the perspective of mobile users, the interruption of ongoing real-time service has much higher impact than that of blocking new service requests.As a result, in order to minimize forced interruption of existing real-time services, many resource reservation and priority allocation schemes are proposed, e.g. GCM with first-in-first-out (GCM-FIFO),Guard Channel Method (GCM) and Dynamic Priority Queuing (DPQ).The performance of these three schemes is analyzed theoretically, and their application scenarios are given in this thesis.In high-speed railway scenarios, as UEs travel very fast across the overlapping area of cells, smaller delay and shorter handover flow is required. On the other hand, due to the large number of passengers on the train, the number of UEs at the same time to initiate handover request increases rapidly. Consequently, queuing of handover UEs will results in the increasing delay when resource is insufficient, thus many UEs are not able to complete handover during the time crossing overlapping area, and the ongoing service might be interrupted. In this thesis, the restraining factors of group handover performance in LTE/LTE-A network are analyzed with respect to the random access resource allocation. According to the analysis, lack of preambles is the key factor. To alleviate the pressure of large number of handover users on system,a user grouping method is proposed. Simulation results show that this scheme can significantly improve the handover performance of system.During the handover process under high-speed scenarios,because the number of UEs initiating handover is large, even if user grouping scheme is applied, we cannot guarantee that all users initiating handover can get access to resource they applied for. Therefore, instead of new access users, handover users, particularly those handover users with high priority can get access to resource first. In this thesis, according to QCI (QoS Class Identifier) standard, the priority of four classes of handover users with different types of services is defined. Then we further propose a Guard Preamble Method (GPM) in order to provide preferential response for the requests of handover users and reserve different number of preamble codes when preamble resource is limited. It is shown that the combination of GPM and user grouping scheme can elevate the handover performance of LTE system in high-speed scenario, while achieving a tradeoff between the handover success rate of handover users with high priority and the fairness of the whole system.