Favorable Propagation for Wideband Massive MIMO with Non-Uniform Linear Arrays

Favorable propagation (FP) for massive MIMO with uniform and non-uniform linear arrays is studied. 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 non-uniform linear arrays are proposed to cancel grating lobes and to restore favorable propagation for all distinct angles of arrival. This design is based on a sub array structure and fits well with efficient hybrid beamforming structures proposed for 5/6G systems. In addition, we show that the proposed design is robust in the frequency domain and can be used for wideband or ultra-wideband systems.