A survey on user-centric cell-free massive MIMO systems.

The mobile data traffic has been exponentially growing during the last decades, which has been enabled by the densification of the network infrastructure, in terms of increased cell density (i.e., ultra-dense network (UDN)) and/or increased number of active antennas per access point (AP) (i.e., massive multiple-input multiple-output (mMIMO)). However, neither UDN nor mMIMO will meet the increasing data rate demands of the sixth generation (6G) wireless communications due to the inter-cell interference and large quality-of-service variations, respectively. Cell-free (CF) mMIMO, which combines the best aspects of UDN with mMIMO, is viewed as a key solution to this issue. In such systems, each user equipment (UE) is served by a preferred set of surrounding APs that cooperate to serve the UE in a CF approach. In this paper, we provide a survey of the state-of-the-art literature on CF mMIMO systems. As a starting point, we present the significance and challenges of improving the user-experienced data rates which motivate CF mMIMO, derive the basic properties of CF mMIMO, and provide an introduction to other technologies related to CF mMIMO. We then provide the canonical framework for CF mMIMO, where the essential details (i.e., transmission procedure and mathematical system model) are discussed. Next, we provide a deep look at the resource allocation and signal processing problems related to CF mMIMO and survey the state-of-the-art schemes and algorithms. After that, we discuss the practical issues when implementing CF mMIMO, including fronthaul limitations and hardware impairment. Potential future directions of CF mMIMO research are then highlighted. We conclude this paper with a summary of the key lessons learned in this field. The objective of this paper is to provide a starting point for anyone who wants to conduct research on CF mMIMO for future wireless networks.