Ubiquitous cell-free Massive MIMO communications

Abstract

Since the first cellular networks were trialled in the 1970s, we have witnessed an incredible wireless revolution. From 1G to 4G, the massive traffic growth has been managed by a combination of wider bandwidths, refined radio interfaces, and network densification, namely increasing the number of antennas per site. Due its cost-efficiency, the latter has contributed the most. Massive MIMO (multiple-input multiple-output) is a key 5G technology that uses massive antenna arrays to provide a very high beamforming gain and spatially multiplexing of users, and hence, increases the spectral and energy efficiency. It constitutes a centralized solution to densify a network, and its performance is limited by the inter-cell interference inherent in its cell-centric design. Conversely, ubiquitous cell-free Massive MIMO refers to a distributed Massive MIMO system implementing coherent user-centric transmission to overcome the inter-cell interference limitation in cellular networks and provide additional macro-diversity. These features, combined with the system scalability inherent in the Massive MIMO design, distinguishes ubiquitous cell-free Massive MIMO from prior coordinated distributed wireless systems. In this article, we investigate the enormous potential of this promising technology while addressing practical deployment issues to deal with the increased back/front-hauling overhead deriving from the signal co-processing.

Efficient Angle-Domain Processing for FDD-Based Cell-Free Massive MIMO Systems

“…Cell-free (CF) massive MIMO has recently been considered as a practical and useful embodiment of network MIMO that can potentially reduce such inter-cell interference through coherent cooperation between base stations [5]- [8]. In cellfree massive MIMO, the serving antennas are distributed over a large area.…”

“…According to [8], "cell-free" massive MIMO implies that, from a user perspective during data transmission, all access points (APs) cooperate to jointly serve the end-users; hence there are no cell boundaries and no inter-cell interference in the data transmission. The APs are connected to a central processing unit (CPU) via a backhaul link.…”

Dynamic Resource Allocation in Co-Located and Cell-Free Massive MIMO

“…Recently, Cell-Free (CF) Massive MIMO has emerged as an alternative implementation of the Massive MIMO concept, with the potential to provide better coverage probability. In such systems, the access points (APs) with either one or multiple antennas are distributed at different places and jointly serve all users simultaneously without cell boundaries [6]- [8]. Power control is a critical aspect of both co-located and CF Massive MIMO systems and has been extensively studied in many different scenarios [5], [9]- [13] under the common assumption of an infinite backlog, i.e., there that is an infinite amount of data waiting to be transmitted.…”

“…The Lyapunov drift measures the difference between the network congestion level in two consecutive slots. Intuitively, if the drift is minimized at every slot, meaning that the congestion will be reduced progressively, the queues will eventually be stabilized 6. The basic philosophy of the min-drift-plus-penalty technique is to make control decisions that balances the network congestion and the optimal network utility.…”

Optimal Design of Energy-Efficient Cell-Free Massive Mimo: Joint Power Allocation and Load Balancing

“…Cell-free Massive MIMO (multiple input multiple output) is a new promising wireless technology to deal with the mediocre cell-edge performance of cellular networks [1]. The system consists of many distributed access points (APs) that transmit coherently in the downlink and process the received signals coherently in the uplink [2,3], leading to a higher signal-to-noise ratio (SNR) without using more power.…”