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.

中文翻译：

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以用户为中心的无蜂窝大规模 MIMO 网络：机遇、挑战和解决方案的调查

网络基站的密集化对于实现未来移动网络的严格服务质量 (QoS) 要求是必不可少的。然而，随着发射机的密集部署，干扰管理成为一项艰巨的任务。为了解决这个问题，探索具有智能协调和协作能力的全新网络架构至关重要。本调查报告研究了新兴的以用户为中心的无蜂窝大规模多输入多输出 (MIMO) 网络架构，该架构为未来的移动网络奠定了基础。此类网络使用分布式单元（DU）的密集部署来为用户提供服务；与传统蜂窝模式的关键区别在于，为每个用户定义了一个特定的 DU 服务集群。该框架提供宏分集、电源效率、干扰管理和强大的连接性。最重要的是，以用户为中心的方法消除了小区边缘，从而有助于为整个网络区域的用户提供统一的覆盖和性能。我们在此提供该网络方案部署面临的主要挑战的指南，并考虑针对无蜂窝通信面临的主要瓶颈提出的解决方案。具体来说，我们调查了针对前传的文献，然后我们扫描了所需的信道估计、资源分配、延迟和可扩展性问题的细节。此外，我们重点介绍了一些可以为该方案提供管理平台的技术，例如分布式软件定义网络 (SDN)。