  In this paper, we develop a novel angle-domain adaptive filtering
(ADAF)-based frequency synchronization method for the uplink of a massive
multiple-input multiple-output (MIMO) multiuser network, which is applicable
for users with either separate or overlapped angle-of-arrival (AoA) regions.
For each user, 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 vectors. Such beamformer can achieve two-stage
adaptive multiuser interference (MUI) suppression, i.e., inherently suppressing
the MUI from non-overlapping 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
independently for each user, which significantly reduces computational
complexity. Moreover, ADAF is rather robust to imperfect AoA knowledge and side
cluster of scatterers, making itself promising for multiuser uplink
transmission. We also analyze the performance of CFO estimation and obtain an
approximated closed-form expression of mean-squared error (MSE). The
effectiveness of the proposed method and its superiority over the existing
methods are demonstrated by numerical results.
