  This article presents a tutorial on multiuser multiple-antenna wireless
systems with a very large number of antennas, known as massive multi-input
multi-output (MIMO) systems. Signal processing challenges and future trends in
the area of massive MIMO systems are presented and key application scenarios
are detailed. A linear algebra approach is considered for the description of
the system and data models of massive MIMO architectures. The operational
requirements of massive MIMO systems are discussed along with their operation
in time-division duplexing mode, resource allocation and calibration
requirements. In particular, transmit and receiver processing algorithms are
examined in light of the specific needs of massive MIMO systems. Simulation
results illustrate the performance of transmit and receive processing
algorithms under scenarios of interest. Key problems are discussed and future
trends in the area of massive MIMO systems are pointed out.
