  Favorable propagation (FP) for massive multiple-input multiple-output (MIMO)
systems with uniform and non-uniform linear arrays is considered. A gap in the
existing FP studies of uniform linear arrays is identified, which is related to
the existence of grating lobes in the array pattern and which results in the FP
condition being violated, even under distinct angles of arrival. A novel
analysis and design of \textit{non-uniform} linear arrays are proposed to
cancel grating lobes and to restore favorable propagation for \textit{all}
distinct angles of arrival. This design is consistent with the popular hybrid
beamforming paradigm and extends to multipath channels and arrays of
directional elements.
