Frequency synchronization in distributed MIMO systems

The problem of frequency synchronization in distributed multiple-input multiple-output (MIMO) systems is addressed. It is supposed that the transmit antennas are distributed while the receive antennas are still centralized, and the general case where both the time delays and the frequency offsets are possibly different for each transmit antenna is considered. The channel is supposed to be frequency flat, and the macroscopic fading is also taken into consideration. A carrier frequency offset estimator based on maximum likelihood (ML) is proposed, which can separately estimate the frequency offset for each transmit antenna and exploit the spatial diversity. Simulation results are given to illustrate the performance of the estimator and compare it with the Cramer-Rao Bound (CRB) for synchronous MIMO (i.e., the time delays for each transmit antenna are all equal). It is shown that the estimator can provide satisfactory frequency offset estimates and its performance is close to the CRB for the signal-to-noise ratio (SNR) below 20dB.