This paper gives a summary of the principles, key techniques and state of the art of frequency synchronization in multi-input multi-output (MIMO) systems. The system model of frequency synchronization in MIMO systems is given and some assumptions are made first, and the principles and state of the art of the estimation and compensation of carrier frequency offsets (CFOs) are discussed for co-located MIMO and distributed MIMO systems, respectively. By analyzing the disadvantages of the existing research, we present some open issues worth of studying and some promising research interest. The results of analysis show that there has been adequate intensive study on the CFO estimation for co-located MIMO systems while the CFO estimation for distributed MIMO systems and the CFO compensation for MIMO-OFDM (orthogonal frequency division multiplexing) systems appear incomplete and still need further research.
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