The aim of this paper is to investigate a linear precoding scheme design for a multiple-input multiple-output two-way relay system with imperfect channel state information. The scheme design is simplified as an optimal problem with precoding matrix variables, which is deduced with the maximum power constraint at the relay station based on the minimum mean square error criterion. With channel feedback delay at both ends of the channel and the channel estimation errors being taken into account, we propose a matrix decomposition scheme and a joint iterative scheme to minimize the average sum mean square error. The matrix decomposition method is used to derive the closed form of the relay matrix, and the joint iterative algorithm is used to optimize the precoding matrix and the processing matrix. According to numerical simulation results, the matrix decomposition scheme reduces the system bit error rate (BER) effectively and the joint iterative scheme achieves the best performance of BER against existing methods.
In this paper, an unmanned aerial vehicle (UAV) communication channel model is established by considering the propagation path loss, shadowing, and multi-path fading. Moreover, an efficient generation method for Gaussian random processes based on sum of sinusoids (SoS) theory is presented and is easy to realize by a graphics processing unit (GPU). Based on the proposed method, a new real-time generation method for multi-path shadowing composite fading is designed and implemented. The implementation results show that the proposed approach enables the easy generation of multi-path shadowing composite fading while reducing the processing time. Meanwhile, the impacts of flight altitude and communication scenarios on the performance of the UAV communication system are discussed. The results of this paper should have significant application value in UAV communication channel simulation. Index Terms-UAV communication channel, SoS, GPU, channel simulation. I. INTRODUCTION Due to their simple structure and low cost, unmanned aerial vehicles (UAVs) have found a wide range of applications such as communications, emergency assistance, and environmental monitoring [1]-[2]. An UAV communication channel simulator, which can be easily used to analyze the performance of an UAV communication system, can not only reduce project costs but also accelerate development cycles. Therefore, research on UAV communication channel model simulation has become a hot topic because such studies can improve the efficiency and quality of an UAV communication channel simulator by using a graphics processing unit (GPU). Channel fading includes shadowing and multi-path fading, which obey log-normal and Nakagami distributions, respectively [3]-[4]. Measured data for UAV communication channels in different scenarios are reported in [5] and [6], which show that shadowing and multi-path fading have a substantial influence on UAV communication channels. A three-dimensional geometric channel model is proposed for air-to-ground communication environments in [7]. In this work, the proposed model considers the air station and ground base station to be located at foci points of Manuscript
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