6G networks are envisioned to deliver a large diversity of applications and meet stringent quality of service (QoS) requirements. Hence, integrated terrestrial and non-terrestrial networks (TN-NTNs) are anticipated to be key enabling technologies. However, the TN-NTNs integration faces a number of challenges that could be addressed through network virtualization technologies such as Software-Defined Networking (SDN), Network Function Virtualization (NFV) and network slicing. In this survey, we provide a comprehensive review on the adaptation of these networking paradigms in 6G networks. We begin with a brief overview on NTNs and virtualization techniques. Then, we highlight the integral role of Artificial Intelligence in improving network virtualization by summarizing major research areas where AI models are applied. Building on this foundation, the survey identifies the main issues arising from the adaptation of SDN, NFV, and network slicing in integrated TN-NTNs, and proposes a taxonomy of integrated TN-NTNs virtualization offering a thorough review of relevant contributions. The taxonomy is built on a four-level classification indicating for each study the level of TN-NTNs integration, the used virtualization technology, the addressed problem, the type of the study and the proposed solution, which can be based on conventional or AI-enabled methods. Moreover, we present a summary on the simulation tools commonly used in the testing and validation of such networks. Finally, we discuss open issues and give insights on future research directions for the advancement of integrated TN-NTNs virtualization in the 6G era.

翻译：6G网络被设想为能够支持多样化的应用场景并满足严格的服务质量（QoS）要求。因此，天地一体化网络（TN-NTNs）被视为关键使能技术。然而，天地一体化网络融合面临诸多挑战，而网络虚拟化技术（如软件定义网络（SDN）、网络功能虚拟化（NFV）和网络切片）可有效应对这些挑战。本综述全面回顾了这些网络范式在6G网络中的适配过程。我们首先简要概述非地面网络（NTNs）与虚拟化技术，进而重点阐述人工智能在提升网络虚拟化中的核心作用，总结AI模型应用的主要研究领域。在此基础上，本综述识别了SDN、NFV和网络切片在天地一体化网络适配中的主要问题，并提出了一个涵盖相关研究贡献的天地一体化网络虚拟化分类体系。该分类体系基于四级分类标准，每项研究均按天地一体化网络融合层级、所用虚拟化技术、所解决问题、研究类型及解决方案（可基于传统方法或AI驱动方法）进行归类。此外，我们总结了常用于此类网络测试与验证的仿真工具。最后，探讨了开放性问题，并对6G时代天地一体化网络虚拟化发展的未来研究方向提出了见解。