The advent of the sixth-generation (6G) of wireless communications has given rise to the necessity to connect vast quantities of heterogeneous wireless devices, which requires advanced system capabilities far beyond existing network architectures. In particular, such massive communication has been recognized as a prime driver that can empower the 6G vision of future ubiquitous connectivity, supporting Internet of Human-Machine-Things for which massive access is critical. This paper surveys the most recent advances toward massive access in both academic and industry communities, focusing primarily on the promising compressive sensing-based grant-free massive access paradigm. We first specify the limitations of existing random access schemes and reveal that the practical implementation of massive communication relies on a dramatically different random access paradigm from the current ones mainly designed for human-centric communications. Then, a compressive sensing-based grant-free massive access roadmap is presented, where the evolutions from single-antenna to large-scale antenna array-based base stations, from single-station to cooperative massive multiple-input multiple-output systems, and from unsourced to sourced random access scenarios are detailed. Finally, we discuss the key challenges and open issues to shed light on the potential future research directions of grant-free massive access.

翻译：第六代无线通信（6G）的来临催生了连接海量异构无线设备的需求，这对现有网络架构的系统能力提出了远超以往的挑战。特别是，海量通信已被公认为实现6G万物智联愿景的关键驱动力，支撑人机物互联场景，其中大规模接入至关重要。本文系统综述了学术界与工业界在大规模接入领域的最新进展，重点聚焦极具前景的基于压缩感知的免授权大规模接入范式。我们首先指出现有随机接入方案的局限性，揭示海量通信的实际部署需要完全不同于当前以人为中心的随机接入范式。随后，详细阐述了基于压缩感知的免授权大规模接入技术路线图，包括从单天线基站向大规模天线阵列基站的演进、从单站向协作式大规模多输入多输出系统的演进，以及从无源随机接入向有源随机接入场景的演进。最后，我们探讨了关键挑战与开放性问题，为免授权大规模接入的未来潜在研究方向提供指引。