Ultradense cell-free massive multiple-input–multiple-output (CF-MMIMO) has emerged as a promising technology expected to meet the future ubiquitous connectivity requirements and ever-growing data traffic demands in sixth generation (6G). This article provides a contemporary overview of ultradense CF-MMIMO networks and addresses important unresolved questions on their future deployment. We first present a comprehensive survey of state-of-the-art research on CF-MMIMO and ultradense networks. Then, we discuss the key challenges of CF-MMIMO under ultradense scenarios such as low-complexity architecture and processing, low-complexity/scalable resource allocation, fronthaul limitation, massive access, synchronization, and channel acquisition. Finally, we answer key open questions, considering different design comparisons and discussing suitable methods dealing with the key challenges of ultradense CF-MMIMO. The discussion aims to provide a valuable roadmap for interesting future research directions in this area, facilitating the development of CF-MMIMO for 6G.

中文翻译：

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面向 6G 的超密集无蜂窝大规模 MIMO：技术概述和开放性问题


超密集无单元大规模多输入多输出 （CF-MMIMO） 已成为一项很有前途的技术，有望满足未来无处不在的连接要求和第六代 （6G） 不断增长的数据流量需求。本文对超高密度 CF-MMIMO 网络进行了现代概述，并解决了有关其未来部署的重要未解决的问题。我们首先对 CF-MMIMO 和超密集网络的最新研究进行了全面调查。然后，我们讨论了 CF-MMIMO 在超密集场景下的主要挑战，例如低复杂度架构和处理、低复杂度/可扩展的资源分配、前传限制、海量访问、同步和信道获取。最后，我们回答了关键的开放性问题，考虑了不同的设计比较，并讨论了应对超高密度 CF-MMIMO 关键挑战的合适方法。该讨论旨在为该领域未来有趣的研究方向提供有价值的路线图，促进 6G CF-MMIMO 的发展。