I. INTRODUCTION
Multi-Antenna transmission over multi-input multi-output (MIMO) channels has been proved to be effective in combating multipath fading, as well as increasing the channel capacity [1], [2]. In practice, coherent detection requires accurate channel and frequency offset information, thus channel and frequency offset estimation has become a critical task in modern wireless communication systems. In conventional MIMO systems, the transmit/receive antennas are colocated, thus they usually share one oscillator, and it is usually assumed that there is only one frequency offset within the system [3]–[9]. Recently, there is an increasing interest in the research of the distributed MIMO systems [10]–[12] where each of the transmit antennas is utilized by one user and the receive antennas are distributed in various locations in order to compensate for long-term shadowing fading. One typical distributed MIMO scenario is the cellular systems where several cell edge users communicate with several base stations. In this case, each transmit/receive antenna is equipped with its own oscillator, thus different transmit-receiver pair may have different frequency offset.