This book aids the reader in the understanding and evaluation of different Massive multiple-input, multiple-output (MIMO) architectures and approaches. It starts with a general introduction of the use of multiple antenna technologies in cellular radio access networks (RAN). The book then moves on to discuss the most promising radio access network (RAN)physical layer technique in 5G: Massive MIMO, or the intelligent use of a large number of antenna elements at the cellular base station antenna. This book shows how 5G Massive MIMO architectures are driven by the way the multitude of antenna elements are designed to work together, through signal processing and/or hardware synchronization and calibration techniques.