Area Spectral Efficiency And Interference Management In Ultra-dense Networks

Even though only four decades have passed since the emergence of the firstgeneration cellular network,wireless communication system has experienced tremendous changes to support such large traffic volume from a multitude of uses with diverse service requirements.In order to support the increasing number of smart devices with high data rate transmissions,ultra-dense networks(UDNs)have been regarded as a promising technology.In UDNs,small cell base stations(SBSs)are densely deployed,so that the transmission capacity per unit can be improved greatly,which is defined as area spectral efficiency(ASE).Even though the concept of ASE has been proposed in the 90 s,the existing analytical approaches are not suitable for UDNs due to the simplified assumptions.Meanwhile,the densification of SBSs also aggravates the interference among adjacent cells and the existing interference management schemes fail to manage the scenario with multi interference sources.In order to solve the aforementioned problems in UDNs,this dissertation analyzed ASE theoretically and proposed suitable interference management schemes.The contents and innovations of this dissertation are summarized as follows.Compared to the conventional heterogeneous networks,the main characteristic of UDN is that the density of SBSs is high.And it is important to understand the impact of ultra-densification on various system performances.Thus,a unified analytical approach is proposed which is effective to simplify system performance analysis based on different path loss models with a low computation complexity.The proposed approach can be applied to different path loss models,such as conventional,3D,multi-slope,exponential,and LOS/NLOS path loss models.The theoretical results of coverage probability and ASE are obtained and other performances,e.g.,transmission rate and system capacity,can also be analyzed using the proposed method.Apart from the influence of SBS density,user association scheme also has great influence on the analysis of system performances.Thus,different user association schemes are summarized.Then,an approximate approach is proposed which uses a log-normal distribution to denote the receive aggregated interfering power.The simulation results prove the effectiveness of this method in different path loss models and small-scale fadings.The proposed theoretical analyzing approaches can provide guidance in determining the SBS density and interference level during the stage of cell planning.Generally,UEs and BSs are assumed to be distributed uniformly within the coverage of the network,which is not consistent with the actual situation.Thus,considered the non-uniform distributions of SBSs and UEs and propose a heterogeneous network model based on service centers.Then,ASE is analyzed accordingly.To tackle the issue of computational complexity,several approximation methods have been presented to simplify the calculation processes.The simulation results are compared to the theoretical results and approximated results to verify the correctness of the proposed analytical model.By adjusting parameters based on the practical situation,the proposed analyzing approach can provide theoretical support for cell planning and performance analysis.Apart from the improvement of ASE,the inter-cell interferences also increase with the densification of SBSs.Thus,it is an important issue to effectively manage the intercell interferences.By using joint transmission,which is one specific technique to achieve coordinated multi-point transmission,the inter-cell interference can be reduced or even eliminated.Considering the deficiency of conventional methods in choosing cooperative set,a user-centric method is proposed based on the differences among received powers.Then,a unified approach to analyze different joint transmission schemes is presented,which analyzes the statistics of desired and interfering powers based on their averages,variances and moment generating functions.Moreover,the performances of different transmission schemes are compared thoroughly.For a practical application scenario of UDNs,transmission schemes can be determined by adjusting the parameters of the proposed analyzing approach.At last,a joint optimization problem of both ASE and fairness is constructed,which can achieve the optimization of ASE and fairness simultaneously by the allocation of resource blocks and transmission powers.ASE and fairness are two contradictory performance indicators,and thus the optimal values of ASE and fairness can not be reached simultaneously.However,there exist multiple solutions to reach the comprehensive optimum of ASE and fairness,and these kinds of solutions are defined as Pareto optimal solutions.There are multiple solutions to tackle this multi objective optimization problem,and both stratified sequencing method and NSGA-II method are adopted in this thesis.When using stratified sequencing method,the multi objective optimization can be transformed to two single objective optimizations based on the importance of different objective,and then ADMM and modified particle swarm optimization algorithms are applied respectively to solve these two optimization problems.However,it is worth noticing that only one Pareto optimal solution can be obtained by using stratified sequencing method.If more optimal solutions are needed,NSGA-II algorithm should be adopted.Then,a proper Pareto solution can be chosen based on the actual needs.