Abstract
To meet the unprecedented mobile traffic demands of future wireless
networks, a paradigm shift from conventional cellular networks to
distributed communication systems is imperative. Cell-free massive
multiple-input multiple-output (CF-mMIMO) represents a practical and
scalable embodiment of distributed/network MIMO systems. It inherits not
only the key benefits of co-located massive MIMO systems but also the
macro-diversity gains from distributed systems. This innovative
architecture has demonstrated significant potential in enhancing network
performance from various perspectives, outperforming co-located mMIMO
and conventional small-cell systems. Moreover, CF-mMIMO offers
flexibility in integration with emerging wireless technologies such as
full-duplex (FD), non-orthogonal transmission schemes, millimeter-wave
(mmWave) communications, ultra-reliable low-latency communication
(URLLC), Â unmanned aerial vehicle (UAV)-aided communication, and
 reconfigurable intelligent surfaces (RISs). In this paper, we provide
an overview of current research efforts on CF-mMIMO systems and their
promising future application scenarios. We then elaborate on new
requirements for CF-mMIMO networks in the context of these technological
breakthroughs. We also present several current open challenges and
outline future research directions aimed at fully realizing the
potential of CF mMIMO systems in meeting the evolving demands of future
wireless networks.