Hybrid analog-digital downlink beamforming for massive MIMO system with uniform and non-uniform linear arrays

In this paper, a hybrid analog-digital beamforming technique for downlink massive multiple input multiple output (MIMO) system is studied. The performances of fully analog beamforming, fully digital beamforming, and hybrid beamforming are to be examined when applying both uniform and non-uniform linear arrays. For this system, first perfect channel estimation is assumed at the base station and the procession time at each case is computed and compared. For non-uniform linear arrays, binomial and Dolph-Chebyshev arrays are applied to hybrid beamforming to reduce the sidelobes as well as maximize the directivity of array pattern, respectively. By varying the number of radio frequency (RF) chains, hybrid, fully digital and fully analog beamformings can be examined. Simulation results show that the performance of hybrid beamforming is very similar to that of fully digital beamforming. In addition, the program running time reflecting the complexity of the systems show that the hybrid case can be reduced it while achieving similar spectral efficiency to the fully digital one. The simulation results of applying non-uniform linear array to hybrid beamforming indicates that the performances increased significantly than the uniform linear arrays. Therefore, we can conclude that the hybrid beamforming can approach the fully digital beamforming solutions by applying non-uniform linear arrays, while requiring lower power complexity.