# Frequency Synchronization for Uplink Massive MIMO Systems

## TL;DR

This paper introduces a frequency synchronization method for uplink massive MIMO OFDM systems that leverages angle information and FFT to efficiently estimate carrier frequency offsets for multiple users.

## Contribution

It proposes a joint spatial-frequency alignment scheme utilizing angle information and FFT for efficient CFO estimation in massive MIMO uplink systems.

## Key findings

- The scheme achieves accurate CFO estimation with reduced complexity.
- A multi-branch beamforming design simplifies multi-user channel estimation.
- Numerical results confirm the effectiveness of the proposed method.

## Abstract

In this paper, we propose a frequency synchronization scheme for multiuser orthogonal frequency division multiplexing (OFDM) uplink with a large-scale uniform linear array (ULA) at base station (BS) by exploiting the angle information of users. Considering that the incident signal at BS from each user can be restricted within a certain angular spread, the proposed scheme could perform carrier frequency offset (CFO) estimation for each user individually through a \textit{joint spatial-frequency alignment} procedure and can be completed efficiently with the aided of fast Fourier transform (FFT). A multi-branch receive beamforming is further designed to yield an equivalent single user transmission model for which the conventional single-user channel estimation and data detection can be carried out. To make the study complete, the theoretical performance analysis of the CFO estimation is also conducted. We further develop a user grouping scheme to deal with the unexpected scenarios that some users may not be separated well from the spatial domain. Finally, various numerical results are provided to verify the proposed studies.