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.
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