Research On Ultra Dense Networks (UDN) Technologies For 5G Mobile Networks

As cellular network advances with cell densification techniques like cell splitting,wireless access points have become increasingly denser.Recent years we have witnessed the explosive growth of mobile data traffic due to the drastic increase of smart-phone usage and emerging applications like Internet of Things(IoTs).It is estimated that the growth will continue that new data-hungry applications like high-quality video streaming are expected in massive number of connections.In some special scenarios,it is expected that the traffic density can reach the magnitude of Tbps/Km2.Hence,the current degree of network densification cannot meet the requirements of future traffic growth.To cope with the demand of further densification on ultra dense network(UDN)deployment,it becomes a common view in the industry and research institutes to conduct relevant research on the solutions which can potentially become one of the key techniques in the 5th generation(5G)mobile network.Therefor this thesis focuses on the key research problems in UDN.Based on the extensive literature survey and related studies,we focus on the key issues of interference coordination and management,mobility management,network deployment and capacity improvement.Through the analysis on existing problems under UDN scenarios with more complicated interference characteristics,typical interference coordination schemes have been studied.We propose a method which performs cluster-based interference coordination and management on downlink channels for a group of base stations in a cluster.By using a game based method to model an appropriate scheduling algorithm,user performance is enhanced together with consideration on overall system performance.Non-coherent joint transmission scheme has been proposed to deliver data to a user simultaneously from multiple transmission points.With significant reduction on interference,this scheme provides benefits on overall cell average performance with substantial improvement on the cell edge throughput.This proposed non-coherent joint transmission scheme,which got well recognized in the industry,has enabled our leading role to establish the projects for standardization of this scheme in the 3~rdd Generation Partner Project(3GPP).In the aspect of uplink channels,methods on interference control on uplink often have the drawbacks of being unstable which may sometimes impose negative impact on uplink performance.Through an iterative method,it can help to converge to the target and avoid the instability.This can greatly improve overall uplink performance in UDN scenarios with emphasis on keeping more balanced cell edge user performance to ensure the system fairness.To cope with challenges on mobility management under UDN scenarios,a scheme which enhances dual connectivity is proposed.This scheme makes use of channel state information-reference Symbol(CSI-RS)resources to identify secondary cells and hence resolves the conflict issue of secondary cells.As a result,this effectively increases the flexibility of mobility management.In this dissertation,further enhancements on mobility management have been studied.Through this study,a novel concept of user-centric based cell virtualization scheme has been introduced.Starting from overall architecture of virtual cells,studies have been focused on the key design procedure of cell virtualization.For examples,we consider the procedure involving the aspects on how each TP joins a virtual cell,data exchange among TPs,change of control plane,etc.By adopting joint transmission with ideal backhaul among TPs,very good consistency of user experience on mobility has been proven in our evaluation.Sufficiently good performance can also be ensured even in the case of non-ideal channel estimation.Backhaul link is one of the key factors considering practical UDN deployment.In this dissertation,joint design of integrated access and backhaul links is considered.We first propose a design on flexible 5G frame structure for self-backhand.Joint resource allocation and optimization between access and backhaul links are considered for the evaluation of overall capacity improvement.Considering the loading of backhaul,we propose a scheme of caching on small cells to reduce the requirements of backhaul capacity.Through estimation on data content,caching is done on small cells for data sharing.By integrating the characteristics of UDN and large scale MIMO,some new techniques have been studied.For instances,these techniques include flexible switching to backhaul nodes among UDN nodes and joint broadcasting and multiple access scheme.Utilization of the spatial domain freedom degree in MIMO system is a promising technique to further improve the capacity of access links under UDN scenarios.In this dissertation,distributed MIMO techniques are studied to improve the network capacity.In reality,the less number of user device receive antennas has limited the rooms of improvement.In this dissertation,a virtual MIMO technique by combining multiple devices to form a virtual device with higher MIMO dimension has been studied,the method increases both uplink and downlink capacity.In summary,this dissertation first analyzes the issues expected in 5G UDN deployment.Based on the analysis,interference coordination and mobility management schemes have been proposed.Considering the realistic issues in practical deployment,our research is closely aligned with standardization activities.Including the detailed design on channel and signaling,we have provided a set of practical solutions covering full aspects which result in significant capacity improvement for UDN scenarios.These techniques,many of which have been adopted in standards,have together formed a positive driving force for the UDN development in the industry.