Angle-Domain Frequency Synchronization for Massive MIMO Uplink with Adaptive MUI Suppression

In this paper, we design a novel angle-domain adaptive filtering (ADAF)-based frequency synchronization method for massive multiple-input multiple-output (MIMO) multiuser uplink, which is suitable for users with either separate or overlapped angle-of-arrival (AoA) regions. We first introduce the angle-constraining matrix (ACM), which consists of a set of selected match-filter (MF) beamformers pointing to the AoAs of the interested user. Then, the adaptive beamformer can be acquired by appropriately designing the ADAF vector. Such beamformer can achieve a two-stage adaptive multiuser interference (MUI) suppression, i.e., inherently suppressing the MUI from distant users by ACM in the first stage and substantially mitigating the MUI from adjacent overlapping users by ADAF vector in the second stage. For both separate and mutually overlapping users, the carrier frequency offset (CFO) estimation and subsequent data detection can be performed individually for each user, which reduces the computational complexity. Moreover, ADAF is rather insensitive to imperfect AoA knowledge, making itself promising for multiuser uplink transmission. Numerical results are provided to show the effectiveness of the proposed method, as well as its superiority over existing competitors.