Spectrally Efficient Time-Frequency Training OFDM for MIMO Systems

The large number of pilots commonly used in OFDM MIMO systems reduces the spectral efficiency in practice. This paper proposes the time-frequency training OFDM (TFT-OFDM) transmission scheme for MIMO systems to solve this problem. The transmission frame is composed of one preamble and the following TFT-OFDM symbols, where each TFT-OFDM symbol without cyclic prefix adopts the time-domain training sequence (TS) and the frequency-domain orthogonal grouped pilots as the time-frequency training information. At the receiver, the time-frequency joint channel estimation directly exploits the "contaminated" time-domain TS to estimate the path delays only, while the path gains are acquired by the frequency-domain grouped pilots. The Cramer-Rao lower bound (CRLB) of the proposed estimator is also derived. Compared with standard OFDM MIMO systems in typical applications, the proposed scheme has about 17% higher spectral efficiency, and has better performance over doubly selective fading channels as indicated by the simulation results.