Reserch On Linear Precoding In Downlink Of Distributed Antenna Systems

Mobile Internet and Internet of things promote the vigorous development of the fifth-generation mobile communication system. As new network architecture, distributed antenna systems can be integrated in the current 5G key physical layer technology:millimeter wave technologies, heterogeneous network and large-scale MIMO technology and show vast potential. In this thesis, we study precoding technology and evaluate the performance of ultra-dense DAS.First, the main features of radio channel, statistical channel models and the performance of distributed antenna system are studied. The wireless channel consists of large scale fading and small scale fading, we also study the capacity and bit error rate of DAS. Through simulation, we study the downlink performance of DAS and get the conclusion that DAS has advantages in reducing transmission power and improving the cell coverage area, comparing with the collocated antenna system (CAS).Next, the linear precoding schemes which provide a promising tradeoff between performance and com-plexity are studied. The optimization problem of minimizing the total transmitted power while satisfying signalto-interference-and-noise ratio (SINR) constraints is formulated to optimize the beamforming vectors of all users jointly, which can be solved by conic optimization method. However, the problem is non-convex and can not be solved efficiently. Suboptimal methods are proposed by optimizing power allocation after given the direction of precoding. Through simulation, the performance of different linear precoding methods are analyzed for the downlink of DAS. Moreover, the performance of DAS and CAS are compared under the same antenna configuration and precoding scheme.Furthermore, a new weighted SLNR precoder is designed. We study the relation between ZF precoder, MRT precoder and SLNR precoder and designed weighted SLNR precoder. Through simulation, the weighted SLNR precoder can improve the performance of SLNR precoder.Finally, we study the performance of ultra-dense DAS. The influences of RAU and user distribution, precoding method, users number, cell radius and antenna number are analyzed. Through simulation, we measure and analyze the area spectral efficiency of ultra-dense DAS.