  In this paper, we consider the problem of preamble design in multiple-input
multiple-output (MIMO) systems employing offset quadrature amplitude modulation
based filter bank multicarrier (OQAM/FBMC) and propose a preamble optimization
method for the frequency division multiplexing (FDM)-structured preamble.
Specifically, we formulate an optimization problem to determine the frequency
division multiplexed preambles, where the objective is to minimize the mean
square error (MSE) of the channel estimation, subject to the constraint on the
transmit energy. For two transmit antennas, we find the relationship between
preambles and the intrinsic interference from neighboring symbols to achieve
the minimum channel estimation MSE, and derive the optimal closed-form
solution. For more than two transmit antennas, the constrained preamble
optimization problem is nonconvex quadratic. Therefore, we convert the original
optimization problem into a quadratically constrained quadratic program (QCQP)
and obtain the suboptimal solution by relaxing the nonconvex constraint.
Simulation results demonstrate that, in terms of MSE and bit error rate (BER)
performances, the proposed method outperforms the conventional FDM preamble
design method at all signal-to-noise ratio (SNR) regimes and outperforms the
interference approximation method-complex (IAM-C) preamble design method at low
to medium SNR regimes with lower preamble overhead.
