Ultra-dense 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 6G. This article provides a contemporary overview of ultra-dense 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 ultra-dense networks. Then, we discuss the key challenges of CF-MMIMO under ultra-dense 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 ultra-dense CF-MMIMO. The discussion aims to provide a valuable roadmap for interesting future research directions in this area, facilitating the development of CF-MMIMO MIMO for 6G.

翻译：超密集无蜂窝大规模多输入多输出（CF-MMIMO）已成为一种有望满足6G未来泛在连接需求及持续增长数据流量需求的前沿技术。本文对超密集CF-MMIMO网络进行了当代综述，并探讨了其未来部署中尚未解决的重要问题。我们首先全面调研了CF-MMIMO与超密集网络的最新研究进展，继而讨论了超密集场景下CF-MMIMO面临的关键挑战，包括低复杂度架构与处理、低复杂度/可扩展资源分配、前传链路限制、大规模接入、同步及信道获取。最后，我们通过比较不同设计方案并探讨应对超密集CF-MMIMO关键挑战的适宜方法，回答了各项核心开放问题。本文旨在为该领域未来有价值的研究方向提供路线图，从而推动面向6G的CF-MMIMO MIMO技术发展。