Optimal pilot sequence design for multiple-input multiple-output OFDM systems

In orthogonal frequency division multiplexing (OFDM) systems, some subcarriers at the borders of the allocated bandwidth are usually used as guard band. Since these subcarriers, which are often referred to as virtual subcarriers, are not used for transmission, approach of conventional uniformly placed pilot tones is not applicable any more in some situations. Therefore, it is necessary to derive the optimal pilot sequences based on the nonuniform placement of pilot tones. In this paper, we first compute the mean square error (MSE) of the least squares (LS) channel estimate for multiple-input multiple-output (MIMO) OFDM systems. Then, based on nonuniform pilot tone placement, we derive the optimal pilot sequences with respect to this MSE. Simulation results demonstrate the effectiveness of the proposed approach