Frequency Synchronization for Coherent Transmission in Distributed Massive MIMO

This paper addresses carrier frequency offset (CFO) estimation for frequency synchronization in downlink distributed massive multiple-input multiple-output (MIMO) systems with coherent transmission. Existing CFO estimation methods often require high computational complexity due to the joint estimation of CFO and channel matrices between base stations (BSs). To overcome this limitation, we propose a low-complexity CFO estimation method that leverages an orthogonal frequency division multiplexing (OFDM) training sequence with a dedicated structure. By estimating CFO based on the covariance of received signals at slave BSs, instead of relying directly on the received signals, our method avoids joint CFO and channel estimation. This results in a closed-form CFO estimator with significantly reduced complexity. We further derive the Cramer-Rao lower bound (CRB), showing that it decreases as the number of antennas at slave BSs increases. Simulation results demonstrate that our CFO estimator achieves lower mean squared error (MSE) and reduced computational cost compared to the bestknown existing methods.