【作者】 孙颖；

【导师】 汪洋；

【作者基本信息】 哈尔滨工业大学 ， 信息与通信工程， 2018， 博士

【摘要】 日益普及的智能移动终端对便捷通信的需求推动着无线通信新技术的迅猛发展。云无线接入网(C-RAN,Cloud Radio Access Network)作为一种新型网络架构,将传统基站的基带处理和无线传输功能从地理位置上分隔开,构成一种远端射频单元通过前传链路连接至基带处理单元的集中式架构。该架构能够以虚拟多输入多输出的方式进行小区间协作多点传输,获得空间分集增益。但同时,C-RAN全新的架构也带来了一些技术应用层面的挑战,体现在如下几个方面:一是C-RAN的大规模部署倾向于使用低成本的无线收发机硬件,导致系统内存在收发机硬件减损,在协作信号的收发过程中引入硬件畸变噪声,严重影响系统性能的提升。二是C-RAN的协作多点传输需要在地理上分散的天线之间共享信道状态信息以及终端数据,当协作规模较大时上述数据量极为庞大,即使采用光纤前传链路作为传输通道也面临着极大的压力。三是在光纤前传链路不可达的区域,C-RAN通常使用无线通道作为前传链路,迫切需要灵活高效地分配有限的无线前传链路带宽,有效支撑信道状态信息以及终端数据在天线间的共享。鉴于此,本论文针对C-RAN的协作多点传输,通过对天线、前传链路带宽以及发射功率等资源的高效分配来优化系统性能,达到最大程度抑制干扰、增加容量、降低能耗的目的。具体开展如下研究:(1)C-RAN网络架构以及性能优化模型。对C-RAN以及传统移动通信网络的架构以及信号传输流程进行对比,以频谱效率和能量效率作为系统利用频谱以及能量的效率评价参数。分析小区间干扰对系统性能的影响,明确联合处理、协作波束成形、协作静默三类协作多点传输的工作机制。重点对联合处理的原理进行分析,将其建模为优化问题并求得最优解的结构,通过渐近性分析得到特殊场景的预编码优化策略,为后续研究提供理论基础。(2)针对使用低成本收发机硬件的C-RAN协作多点传输,提出存在收发机硬件减损的协作鲁棒性优化策略,解决硬件减损导致的系统性能下降问题。建立无线收发机的硬件减损模型,计算硬件畸变噪声,分析其对协作多点传输的信道估计和数据收发的影响,并基于信道容量下界构建了旨在提升系统最差能量效率性能的多目标优化问题。将该问题利用帕雷托思想转换为单目标优化问题,并提出基于KKT(Karush-Kuhn-Tucker)条件的迭代算法进行求解,得到协作功率的最优分配值,实现协作多点传输对于收发机硬件减损的鲁棒性。(3)针对使用光纤前传链路的C-RAN协作多点传输,提出高能量效率的非对称协作优化策略,解决大规模协作时光纤前传链路严重受限以及系统能耗高的问题。综合考虑协作复杂度以及低能耗的绿色通信需求,非对称地选择参与协作的天线。同时,引入适用于C-RAN的能耗模型,提出与协作复杂度相关的基带信号处理以及前传链路的能耗计算机制。在此基础上,搭建满足凸性的能量效率优化问题,利用詹森不等式(Jensen’s inequality)将目标函数转换为其下界值,并提出联合优化求解方法,通过分步迭代求得最优的协作天线数以及分配功率,实现前传链路带宽受限时高能量效率的大规模协作。(4)针对使用无线前传链路的C-RAN协作多点传输,提出基于背包算法的前传链路带宽分配优化策略,满足无线前传链路资源在协作天线间的灵活分配需求。搭建无线协作以及无线前传的链路模型,在二者之间建立耦合关系,通过定义链路协作贡献度将无线前传链路带宽的分配建模为以链路协作贡献度为收益、以无线前传链路带宽为体积的背包优化问题。利用虚拟天线将该问题转换为可求解的0-1背包问题,并提出基于递归分治的动态规划算法,引入强制背包与可选背包,在保障终端信号干扰噪声比要求的同时以低空间复杂度最优地分配无线前传链路带宽,实现C-RAN使用无线前传时协作多点传输性能的提升。更多还原

【Abstract】 The growing demand for broadband communication facilitates the rapid develop-ment of novel wireless communication technologies.The core feature of CoMP in C-RAN(Cloud Radio Access Network)is that the cooperation between the geographically dispersed antennas needs to exchange data and channel state information at the centralized baseband processing unit through fronthaul.This,however,lays a great burden to the data transmission of fronthaul transmission link.Meanwhile,in the areas where wired optical fronthaul link is unavailable,the microwave radio wireless link is usually deployed as the fronthaul.How to fully utilize the flexibility of the wireless fronthaul link to facilitate the base station coordinated transmission is critical to optimize the spectral and energy effi-ciency performance of C-RAN.In addition,the optimization of the system performance in C-RAN is affected by the transceiver hardware impairments,which will generate dis-tortion noise and needs to be compensated to achieve the robustness of coordinated trans-mission.Therefore,this thesis studies the optimization of coordinated multi-point transmis-sion in C-RAN,including the principle of pre-coding based beamforming,optimization of antenna clustering and fronthaul bandwidth allocation,etc.To sum up,the main work of this thesis are as follows:(1)C-RAN architecture and optimization model.The network architecture and the signal transmission process of the centralized multi-cell cooperation system and the tradi-tional cellular system are compared.The working mechanism of three kinds of multi-cell cooperative transmission modes,i.e,joint processing,cooperative beamforming and co-operative silence,are introduced.Then the principle of multi-cell joint processing is ana-lyzed,which is modeled as an optimization problem and solved by analyzing the structure of optimal solution and obtaining the optimal pre-coding strategy of the specific coverage scenarios.The above work provide the theoretical basis for the subsequent study.(2)When the system uses low-cost hardware,the transceiver hardware impairment will affect the performance of coordination.By establishing a hardware derivation model for transceivers in C-RAN,the transmitter and receiver distortions are obtained,and then the wireless transmission capacity and the impact of the hardware impairment on system performance are deduced.An KKT(Karush-Kuhn-Tucker)based optimization problem is then formulated to enhance the worst energy efficiency performance,which generates the robustness of C-RAN to the transceiver hardware impairments.(3)The concept of asymmetric cooperative transmission is proposed considering the multi-cell cooperative complexity and the demand for green communication with low energy consumption in the coverage scenario of fiber-optic fronthaulled C-RAN.Then,the energy efficiency enhancement of C-RAN is transformed into an optimization problem,whose objective function is transformed into its lower bound by using Jensen’s inequality to satisfy convexity.Furthermore,an iterative convex optimization scheme is proposed to optimize the number of antennas participating in the cooperation and the corresponding transmission power,thus realizing energy efficient CoMP under fronthaul constraints.(4)For wireless fronthaulled C-RAN,the wireless fronthaul is regarded as an addi-tional resource that can be allocated among antennas.The models of wireless cooperative transmission and wireless fronthaul link are established with a coupling relationship be-tween the two.Then the degree of contribution is defined to measure the enhancement to system spectral efficiency,based on which the wireless fronthaul bandwidth allocation is formulated into a knapsack optimization problem.In the knapsack problem,the degree of contribution of each link through cooperation is defined as the benefit,and the occupied wireless fronthaul bandwidth is defined as the cost.Then,the concept of virtual antenna is proposed to further convert the knapsack problem into a 0-1 knapsack problem,which is then solved by a recursive and divide-and-conquer algorithm.Meanwhile,the concepts of optional and mandatory knapsack are utilized to maximize the degree of contribution while guaranteeing the quality of service of mobile terminals,thus enhancing the perfor-mance of wireless fronthaulled C-RAN.更多还原