Spectral-Efficient TSB Scheme for Multiple Scatterer Clusters Channel in Massive MIMO

This letter presents a two-stage beamforming (TSB) scheme based on the multi-armed bandit (MAB) for multiple scatterer clusters (MSC) channel in frequency division duplex (FDD) massive MIMO. The existing TSB schemes heavily depend on the channel covariance matrix (CCM), necessitating a significant pilot overhead. The proposed scheme does not have the requirement to estimate the CCM, resulting in reduced overhead and improved spectral efficiency. Particularly, the prebeamforming matrix design problem is formalized using a MAB framework. Considering that the power angle spectrum (PAS) affects the pilot overhead, the prebeamforming matrix design is divided into two subproblems. The first subproblem involves selecting the arm to quickly determine the PAS, while the second subproblem focuses on selecting the arm to maximize the spectral efficiency. Moreover, the pilot length equals to the chromatic number of a graph which is expensive to calculate, and we propose a greedy algorithm for arm selection. We prove that the regret grows logarithmically with time. Simulations validate the good performance of the proposed MAB based TSB scheme.