A Study Of Converse Proof Of The Downlink In Finite Backhaul System

With the rapid development of wireless communication and the arrival of mobile Internet era,the radio access network,as the important part of the cellular communication system,is becoming the bottleneck of performance improvement of communication system due to lack of cooperative service capability between multi-base stations.An effective means to solve this problem is C-RAN,which is considered to be the radio access network architecture of the future networks and has great significance in solving inter-cell interference and base station deploy-ment.However,when the capacity of the backhaul link is limited,how to use the backhaul link to complete the collaboration between the base stations is still unknown.In this thesis,C-RAN system is modeled as a finite backhaul system from the perspective of information theory.The backhaul links between the central processing unit and the base stations are modeled as links with finite capacity and These links are independent with each other.The wireless links between the base stations and the user terminals are modeled as Gaus-sian interference channels.Different from the related literatures which usually focus on the achievability of finite backhaul system,this thesis mainly studies the converse of this problem to obtain the fundamental limits of finite backhaul system.The known result of the converse is the simple cut-set bound,which has two defects:1)The backhaul link and the wireless link are separated.2)Lacks a description of the correlation between the correlated signals transmitted to the different base stations on the backhaul link.In this thesis,we first study the simplest finite backhaul system with two base stations and two receivers.The model can be regarded as an extension of the multiple access diamond channel(The number of receivers extends from one to two).This thesis proposes two novel methods to solve the problems mentioned above,that is,adding more cuts to combine the back-haul link and the wireless link in the converse proof and considering the correlation between signals transmitted on the backhaul link.The upper bound is derived by using the converse tools of the multiple access diamond channel and that of the Gaussian MIMO broadcast channel.The derived upper bound is expressed in terms of the sum capacity of the Gaussian MIMO broadcast channel given input covariance constraint.The converse results of system with two base stations are extended to the case of any num-ber of base stations.The derived upper bound is also expressed in terms of the sum capacity of the Gaussian MIMO broadcast channel given input covariance constraint.The main innovation of this part is that the method of describing the correlation between the correlated signals in the converse proof of the system with two base stations is extended to the case of any number of base stations.Finally,the numerical results show that the new upper bound has been improved greatly compared with the existing cut-set bound.The interval between the upper bound and the lower bound determined by the known achievability schemes is greatly reduced.