The Massive Mimo Idea

To achieve the dramatic improvements in capacity and spectral efficiency needed for fifth generation (5G) systems to accommodate the ever increasing number of users who require ubiquitous access to high volumes of wireless data, three symbiotic technological directions are independently emerging [1]: • Increasing frequency reuse through the creation of smaller cells, referred to as picoand femtocells, with ranges on the order of 10–200 m • Millimeter wave (mmWave) communications around and above 30 GHz where the spectrum is less crowded and greater bandwidths are available • Base stations (BSs) equipped with a very large number of antennas (100 or more) that can simultaneously accommodate many co-channel users, an idea referred to as massive multiple-input multiple-output (MIMO) Individually, each of these approaches could offer an order of magnitude increase in wireless capacity or more compared to current broadband systems; in combination with one another, one can envision achieving the approximate thousand-fold increase in capacity that will be needed in the coming decades. An encouraging factor is the apparent symbiosis between these three concepts: smaller cell sizes are attractive for the mmWave spectral band where RF path loss (PL) increases with frequency, the large beamforming gains achievable with very large antenna arrays can conversely extend coverage at longer ranges to help overcome the high mmWave path loss, the reduction in channel coherence time at mmWave frequencies is offset by the lower mobility and hence the higher channel coherence bandwidth due to operation in small cells, and the shorter wavelength associated with higher frequencies is appealing for massive MIMO transceiver designs since the physical dimensions of the antenna array and associated electronics are reduced in size. There are, however, many questions and challenges that must be addressed before a system that leverages the above attributes could become a reality. These issues span the breadth of communications theory and engineering, from modulation, equalization, and interference management to antenna design and RF/analog/ baseband processing. In this article, we explore the issues surrounding the idea of mmWave massive MIMO, and offer some potential solutions and research directions to consider.