Space-time coding with OFDM over correlated fading channels

To improve the performance of multiple antenna, orthogonal frequency division multiplexing (MIMO-OFDM) systems, space-time block codes (STBC) are studied. Two important aspects of STBC are considered: coding schemes and linear decoding methods. Coding schemes are explored to exploit inherent space, time and frequency diversities in the system and to facilitate low-complexity decoding. Alamouti's orthogonal space-time block code (OSTBC) design is implemented with space-frequency (SF) and space-time-frequency (STF) coding schemes for a system with two transmit antennas and one receive antenna (2x1) configuration. A proposed STF scheme combines the SF and ST coding of Alamouti's code, and enhances performance by using transmit diversity. A quasi-orthogonal space-time block code (QOSTBC) is used for a 3x1 or 4x1 configuration with SF and STF coding schemes. The STF scheme is designed to lower the decoding complexity by reducing signal coupling. Linear decoders are emphasized for simplicity and implementation compared with complicated maximum likelihood decoding. Linear decoding methods for OSTBC and QOSTBC are studied for increasing the adaptability of STBC to the correlated non-quasi-static channels of OFDM systems, by reducing undesired signal coupling with proper linear filtering on received signals and compensation for inter-symbol interference (ISI). Besides transmit diversity, receive diversity is also explored with linear processing in decoding for performance improvement and reliability. Simulations are provided, which exhibit the improvement on system performance when using the studied coding schemes in MIMO-OFDM systems.