Robust uplink synchronization in MIMO-OFDM systems: Inter-TS-Interference analysis and superposition distortion mitigation

There are some new challenges in Multiple-input Multiple-output (MIMO) systems, e.g., uplink synchronization in full-multiplexing transmission mode systems where several packets are transmitted from different users simultaneously. In MIMO uplink transmission systems, different mobile nodes introduce independent carrier frequency offsets, which makes uplink carrier frequency offset estimation in MIMO systems more difficult than that in single antenna systems. The received training sequence at the base station (receiver) is a superposition of training sequences transmitted from more than one mobile node, which may degrade the synchronization performance. In addition, self-interference may also appear if different training sequences (TS) are transmitted at its different transmit antennas equipped in one mobile node. In this paper, both kinds of interferences in MIMO uplink synchronization are analyzed first, and then a superposition distortion mitigation method is given. For an unbiased carrier frequency offset estimator, its Cramer-Rao Lower Bound (CRLB) is also derived. A Joint Best Linear carrier frequency offset Estimator (JBLUE) is also proposed to explore receive antenna diversity in MIMO systems. A new group of training sequences called Round-Robin training sequence group are also proposed in this paper, which outperform conventional training sequences especially for uplink synchronization.