MIMO-OFDM systems based on space-time coding and OFDM carrier block spreading

In this paper, we propose a space-time coding and carrier block spreading based multiple-input-multiple-output (MIMO) orthogonal-frequency-division-multiplexing (OFDM) system. The proposed system exploits the potential spatial and frequency diversity of MIMO systems in frequency-selective fading environments through a two-stage procedure. In the first stage, we use space-time coding to exploit the spatial diversity, and in the second stage, we adopt block spreading among blocks of OFDM carriers to obtain frequency diversity. Moreover, by using rotated Hadamard matrix or rotated discrete Fourier transform (DFT) matrix as the carrier spreading matrix, the system can also possess a certain degree of modulation diversity. Our proposed system enjoys the advantage of high flexibility and low complexity. It allows the use of any existing space-time coding scheme as its component space-time code, and the diversity gains are obtained with only moderate transmit and receive processing complexity. Simulation results show that our proposed system can achieve desired spatial and frequency diversity with low detection complexity. Comparing to some existing space-frequency coding schemes, our proposed system enjoys the merit of low complexity, although it could suffer from some performance degradation, which is caused by the separation of space-time coding and frequency-domain spreading process.