  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.
