This paper describes a novel low complexity time-frequency spreading of orthogonal block codes for MIMO-OFDM systems in highly time and frequency selective channels. The space-time codes (STC) and space-frequency codes (SFC) proposed in literature for MIMO-OFDM systems require a quasi-static channel, where the channel is assumed to be constant over a number of OFDM time symbols or OFDM subcarriers. In orthogonal block codes (OBC) this is equal to the number of transmit antennas Nt employed. For higher order space-diversity systems with Ntges4, it is likely that the assumption of constant channel within the OBC to be violated in fast fading channels resulting performance degradation. We propose an adaptive time-frequency spreading strategy that minimizes the decoding error rate in different types of fading channels. The adaptation technique incorporated is based on a fading interference matric, which is evaluated at the transmitter using channel fading statistics. Simulation results demonstrate the effectiveness of the proposed adaptive time-frequency spreading of OBCs
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