基于NOMA技术的大规模D2D通信系统可靠性性能和保障机制研究

The combination of device-to-device (D2D) communications and non-orthogonal multiple access (NOMA) is one of effective ways to match the trend of the next generation mobile communications that significantly boost system capacity, reduce packet delay, and support simultaneous multi-user communications, etc. As a result, it becomes vitally important and considerably urgent that how to apply NOMA to realize reliable and large-scale D2D communications. To this end, this project thoroughly investigates the relevant theories and the key technologies. The main objectives of this project can be summarized as follows:.1) To enable the analysis of the system reliability, a rigorous model of NOMA-based large-scale D2D communication system is established first. The principal factors that affect the system reliability are then examined and modeled accurately, including network aggregate interference, imperfect channel state information, error propagation of sequential decoding, and limitations of practical applications. Based on the models of communication system and the influence factors, relevant theoretical framework can be built to enable the performance evaluation of the system reliability of NOMA-based large-scale D2D communications..2) To guarantee the system reliability, hybrid automatic repeat request (HARQ) can be exploited to assist the application of NOMA to large-scale D2D communications, meanwhile a basic mechanism of HARQ-aided NOMA communications is set up. Considering diverse hardware performances of D2D terminals as well as different design objectives, different types of HARQ-aided reliability assurance mechanism should be properly selected. The performance of the system reliability of the selected mechanism is then analyzed to reveal its validity..3) With the analytical results of the system reliability, the settings of local system parameters and the configurations of wireless resources can be enabled by aiming at optimizing the performance of a single group of NOMA users and the performance of the whole D2D network, respectively. .The research achievements of this project can provide solid theoretical foundations and significant reference values for practical applications of large-scale D2D communications.

终端直通（D2D）通信结合非正交多址接入（NOMA）技术是满足下一代移动通信显著提高容量、降低延迟和支持多用户同时通信等的有效手段之一。如何通过NOMA技术来实现可靠性大规模D2D通信是迫在眉睫的科学研究问题。本课题针对上述挑战，尝试探索相应的理论和关键技术。具体概括为：.1）构建基于NOMA技术的大规模D2D通信系统模型，分析影响系统可靠性的主要因素，包括叠加干扰、非完美信道状态信息、序列译码错误等，探索相应的理论基础；.2）通过混合自动重发请求（HARQ）辅助NOMA技术的传输机制来保障大规模D2D通信系统可靠性，面向不同终端性能和设计目标，合理选择HARQ类型辅助机制，并对该机制进行可靠性性能分析；.3）基于可靠性分析，通过以单组NOMA用户性能和以全局网络性能为优化目标分别进行系统参数设置和无线资源配置。.本课题的研究工作对于大规模D2D通信的可靠性应用提供理论基础与参考。

为应对下一代移动通信面临数据量和用户数激增的挑战，联合终端直通（D2D）通信与非正交多址接入（NOMA）技术各自优势有望提高容量、降低延迟和支持多用户同时通信。由于存在叠加干扰、不完美信道、错误传播、空间相关性等负面因素影响可靠性，本项目研究如何基于NOMA实现大规模D2D可靠性通信。针对该课题，本项目主要研究内容和重要结果可以概括为三方面：..①精确构建基于NOMA技术的大规模D2D通信系统模型，定量分析叠加干扰、不完美信道特征、错误传播以及空间相关性等对中断性能的影响，运用渐进性分析挖掘这些因素的深层物理规律。研究发现通过引入NOMA用户间协作通信、多输入多输出（MIMO）以及虚拟簇可以有效提高大规模D2D通信的可靠性；通过渐进性分析还证明了MIMO-NOMA系统的分集阶数与接收天线数成正比、MIMO的空间相关性造成可靠性下降、渐进中断概率是速率的增且凸函数。..②进一步通过混合自动重发请求（HARQ）辅助NOMA来提升大规模D2D通信的可靠性，重点研究叠加干扰、不完美信道、有限长缓存等对NOMA-HARQ中断性能的影响，并分析NOMA-HARQ极限可靠性。不同于纯HARQ，NOMA-HARQ只在传输速率足够小时才能获得全分集增益，理论上证明了分集阶数是速率与功率比的阶跃下降函数；NOMA用户的分集阶数与其平均信道增益成反比，即信道条件越差，其分集性能越好；中断概率与有效容量分析中均揭示了固定速率HARQ方式中Type I HARQ性能表现最差，HARQ-CC次之，HARQ-IR表现最优，但是固定速率HARQ性能均次于可变速率HARQ和交叉HARQ两种方式。..③以高可靠性为目标，设计低复杂度的单组NOMA用户的功率分配因子；以提高网络总吞吐量为目标，面向不完美信道和统计信道状态条件分别设计多组NOMA用户功率和速率的联合分配。采用“分集阶数”替代“中断概率”不但有助于构建可靠性最大化问题，而且有利于对优化问题进行解析求解;此外，基于渐进性理论简化功率与速率的联合优化，通过KKT条件求解优化变量的闭合解，并且理论上证明了该方法有效保证了NOMA用户之间的公平性。