Optimal Design of Interleaved Subconnected Hybrid Structure for mmWave Massive MIMO Systems

Massive multiple-input multiple-output (MIMO) systems based on millimeter-wave (mmWave) technology commonly utilize a combination of hybrid analog and digital signal processing to reduce the number of radiofrequency (RF) chains, cutting down the hardware costs and power consumption. The subconnected structure is one of the architectures that realize hybrid analog and digital processing, in which each RF chain is connected to a subset of antennas through phase shifters so as to reduce the implementation complexity. In this paper, we investigate the best subconnected structure design for mmWave massive MIMO systems. First, we formulate the optimization problem based on a simplified connection model. Next, we derive a suboptimal solution in closed form by leveraging the mmWave channel characteristics and discuss the impact of different connection parameters on the spectral efficiency. The spectral efficiency is closely related to the structure of the analog processing and the interantenna spacing. When the interantenna spacing is an even a multiple of half wavelength, the localized structure, i.e., i = 1 achieves the maximum spectral efficiency. When the interantenna spacing is an odd multiple of half wavelength and the analog processing is accomplished by i = 2 interleaved structure, the best performance is obtained. Numerical results demonstrate the correctness of the theoretical analysis.