  Hybrid Massive MIMO reduces implementation complexity but only supports
beamforming coefficients that are common across all subbands. However, in macro
cellular where the channel has limited degrees of freedom, the long-term
component of the channel can be decomposed into a set of subband-independent
beamforming basis functions referred to as eigenbeams. A Coherent Hybrid
Massive MIMO system can form arbitrary linear combinations of the eigenbeams at
every subband to mimic Digital Massive MIMO beamforming as observed across all
locations in the cell.
