I. Introduction
MIMO technology has been demonstrated to be effective in providing reliable wireless links, and the advantages of MIMO communications are widely recognized [1]. MIMO systems utilizing a large number of antennas at the base station, referred to as massive MIMO systems, are emerging as a key technology for the design of high throughput and energy efficient systems for future wireless communications. Massive MIMO represents a paradigm shift in system configuration, wherein the power per antenna is reduced and relatively simple signal processing is performed, e.g., spatial matched filtering [2]. This is all enabled by exploiting advantageous properties of the propagation environment that arise from asymptotic random matrix analysis The large size of the transmit antenna array relative to the number of serviced users mitigates thermal noise, fast channel fading, and some forms of interference, all drawing in part at least from the law of large numbers [3].