Frequency Synchronization for Massive MIMO Multi-User Uplink

In this paper, we propose a new frequency synchronization scheme for multiuser orthogonal frequency division multiplexing (OFDM) uplink with a large-scale uniform linear array (ULA) at base station (BS). Considering that the incident signal at BS from each user can be restricted within a narrow angular spread, the proposed scheme performs carrier frequency offset (CFO) estimation for each user individually through a joint spatial-frequency alignment procedure. The basic idea behind is that with sufficient spatial dimension, the multiuser interference (MUI) effect can be effectively mitigated via beamforming with the steering vectors corresponding to any direction of arrival (DOA) of each user. A multi-branch receive beamforming is further designed for each user which results in an equivalent single user transmission model and the conventional single-user channel estimation and data detection can be carried out. We show that the proposed CFO estimation can be applied efficiently with the aided of fast Fourier transform (FFT). The theoretical CFO estimation performance analysis is also conducted. Various numerical results are provided to verify the proposed studies.