  Densification of network base stations is indispensable to achieve the
stringent Quality of Service (QoS) requirements of future mobile networks.
However, with a dense deployment of transmitters, interference management
becomes an arduous task. To solve this issue, exploring radically new network
architectures with intelligent coordination and cooperation capabilities is
crucial. This survey paper investigates the emerging user-centric cell-free
massive Multiple-input multiple-output (MIMO) network architecture that sets a
foundation for future mobile networks. Such networks use a dense deployment of
distributed units (DUs) to serve users; the crucial difference from the
traditional cellular paradigm is that a specific serving cluster of DUs is
defined for each user. This framework provides macro diversity, power
efficiency, interference management, and robust connectivity. Most importantly,
the user-centric approach eliminates cell edges, thus contributing to uniform
coverage and performance for users across the network area. We present here a
guide to the key challenges facing the deployment of this network scheme and
contemplate the solutions being proposed for the main bottlenecks facing
cell-free communications. Specifically, we survey the literature targeting the
fronthaul, then we scan the details of the channel estimation required,
resource allocation, delay, and scalability issues. Furthermore, we highlight
some technologies that can provide a management platform for this scheme such
as distributed software-defined network (SDN). Our article serves as a check
point that delineates the current status and indicates future directions for
this area in a comprehensive manner.
