I. Introduction
Wireless communication technology has progressed dramatically in the last several decades. Wireless communication systems have impacted our society in profound ways and have become an integral part of our daily lives. The development of wireless communication technology is continuously driven by the requirements imposed by newly emerging use cases—such as aggregate/peak data rate, latency, cost and energy consumption, spectrum and energy efficiency, connectivity density, and many others. These ever more stringent key performance indicators (KPIs) have propelled innovations in both the physical and networking layer technologies from 1G to current 5G [1] in the past several decades, and will continue to do so into the era of future 6G wireless systems [2], [3], [4], [5], [6]. These innovations include but are not limited to: advanced massive multiple-input multiple-output (MIMO) technologies [7], [8], [9] such as coordinated/cooperative beamforming [10], [11], hybrid beamforming [12] and symbol-level precoding [13], new waveforms [14] ranging from time-division multiple access, code-division multiple access, orthogonal frequency-division multiple access to nonorthogonal multiple access, novel access protocols and paradigms [15], [16] such as grant-free multiple access, new networking architectures [17], [18] such as cloud radio access network (C-RAN) and cell-free (massive) MIMO, as well as advanced signal processing algorithms such as efficient compressed sensing techniques.