User-centric Cell-free Massive MIMO Networks: A Survey of Opportunities, Challenges and Solutions

Abstract

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) and self-organizing network (SON). Our article serves as a check point that delineates the current status and indicates future directions for this area in a comprehensive manner.

RWP+: A New Random Waypoint Model for High-Speed Mobility

“…Moreover, by comparing our theoretical results with empirical data, we can clearly see that our new assumptions for the mobility model provide more accurate results compared to those used in the literature, while still maintaining mathematical tractability. This makes the proposed modifications useful for future research into designing handoff policies [11].…”

Cell-Free Massive MIMO for 6G Wireless Communication Networks

“…In this paper, we provide a comprehensive overview on enabling physical layer technologies for cell-free massive MIMO, especially from the signal processing perspective. Different from other survey papers [21][22][23] providing all research directions about cell-free massive MIMO in state-of-the-art literature, we pay attention to the existing signal processing algorithms for user association, pilot assignment, transmitter and receiver design, as well as power control and allocation. Furthermore, some future research directions, including machine learning (ML) and reconfigurable intelligent surfaces (RIS) for cell-free massive MIMO, are highlighted.…”

Analysis and Design of Distributed MIMO Networks with a Wireless Fronthaul

“…This means that the fronthaul capacity does not change with the problem variables such as the location of the RRHs. We note that this problem, under the scenarios considered in this paper, is still not studied even for other distributed MIMO architectures [21], such as the user-centric cell-free [massive] MIMO scheme [22].…”