All-Analog Structures for AF Relaying in mmWave Massive MIMO Systems

Hybrid analog/digital (A/D) beamforming is preferred in the implementation of relays for mmWave massive multiple-input multiple-output (mMIMO) systems due to the smaller number of radio frequency (RF) chains required compared to fully digital (FD) beamforming. Although the hybrid structure reduces system cost and power consumption, it still requires expensive baseband processing while the unit-modulus constraint in the analog domain limits system performance. In this paper, motivated by these considerations, we propose and investigate the design of all-analog structures for amplify-and-forward (AF) half-duplex relaying in mMIMO systems, which are comprised of the conventional RF components, including: power dividers/combiners, phase-shifters, and delay elements. For the proposed structures, we consider a constrained data rate maximization problem and formulate the AF relay designs. The ensuing solution is not bound to the unit-modulus constraint and does not require RF chains for conversion between analog and baseband domains. Simulation results demonstrate that using the proposed analog structures for AF half-duplex relaying in mMIMO communications can achieve the same performance as the optimal FD relay design.