ent condition above is satisfied for Rayleigh fading channels, which are considered in the studies on massive MIMO, but approximate favorable propaga- tion can also be obtained in other conditions [16,52]. The conventional open-loop beamforming provides meaningful array gains for small arrays in LoS propagation environments; however, this scheme is not scalable and not able to han- dle isotropic fading (isotropic fading encompasses a broad range of fading channels with the common property that the transmitter is unable to track the directions of the users' time-varying channel vectors. Thus, from the transmitter standpoint all directions are statis- tically equivalent). In practice, the channel of a particular user terminal might not be isotro- pically distributed, rather it might have distinct statistical spatial properties. The codebook in open-loop beamforming cannot be tailored to a specific terminal, rather needs to explore all channel directions that are possible for the array. For large arrays with arbitrary propaga- tion properties, the channels must be measured by reference signals as is done in the mas- sive MIMO [16,52]. The studies on massive MIMO were mainly focused on the asymptotic regime where the number of service antennas Nantennas 00. Recent studies have derived closed-form achiev- able spectral efficiency expressions that are valid for any number of antennas and user term- inals, SNR, and choice of reference signals. Those expressions do not rely on idealized assumptions such as perfect CSI, rather on worst case assumptions regarding the channel acquisition and signal processing. Although the total spectral efficiency per cell is greatly improved with massive MIMO, the anticipated performance per user lies in the conventional range of 1-4 bps/Hz. This is part of the range where conventional channel codes perform close to the Shannon limits. There are no strict requirements on the relation between Nantennas and Nuser in massive MIMO systems. A simple definition of massive MIMO would be a system