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 [1]. 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 (see [2] and references herein). 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 [3] 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.