Dynamic time division duplex (D-TDD) dynamically allocates the transmission directions for traffic adaptation in each cell. D-TDD systems are receiving a lot of attention because they can reduce latency and increase spectrum utilization via flexible and dynamic duplex operation in 5G New Radio (NR). However, the advantages of the D-TDD system are difficult to fully utilize due to the cross-link interference (CLI) arising from the use of different transmission directions between adjacent cells. This paper is a survey of the research from academia and the standardization efforts being undertaken to solve this CLI problem and make the D-TDD system a reality. Specifically, we categorize and present the approaches to mitigating CLI according to operational principles. Furthermore, we present the signaling necessary to apply the CLI mitigation schemes. We also present information-theoretic performance analysis of D-TDD systems in various environments. As topics for future works, we discuss the research challenges and opportunities associated with the CLI mitigation schemes and signaling design in a variety of environments. This survey is recommended for those who are in the initial stage of studying D-TDD systems and those who wish to develop a more feasible D-TDD system as a baseline for reviewing the research flow and standardization trends surrounding D-TDD systems and to identify areas of focus for future works.

中文翻译：

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5G 及以后的动态 TDD 系统：交叉链路干扰缓解调查

动态时分双工 (D-TDD) 为每个小区中的流量自适应动态分配传输方向。D-TDD 系统受到了很多关注，因为它们可以通过 5G 新无线电 (NR) 中灵活和动态的双工操作来减少延迟并提高频谱利用率。然而，由于相邻小区之间使用不同传输方向而产生的交叉链路干扰(CLI)，D-TDD系统的优点难以充分利用。本文综述了学术界的研究以及为解决此 CLI 问题并使 D-TDD 系统成为现实而进行的标准化工作。具体来说，我们根据操作原则对缓解 CLI 的方法进行分类和介绍。此外，我们还介绍了应用 CLI 缓解方案所需的信令。我们还介绍了 D-TDD 系统在各种环境中的信息论性能分析。作为未来工作的主题，我们讨论了与各种环境中的 CLI 缓解方案和信号设计相关的研究挑战和机遇。建议处于研究 D-TDD 系统初始阶段的人以及希望开发更可行的 D-TDD 系统作为基线的人，以审查围绕 D-TDD 系统的研究流程和标准化趋势并确定未来工作的重点领域。