Orthogonal frequency-division multiplexing (OFDM) is a popular method for high data- rate wireless transmission. OFDM may be combined with multiple antennas at both the access point and mobile terminal to increase diversity gain and enhance system capacity on a time-varying multipath fading channel, resulting in a MIMO-OFDM system In this book we have studied various method for improving spectral efficiency and link reliability of OFDM system, and also carried out study to the increase the performance of MIMO-OFDM system and to reduce the Peak to Average Power Ratio (PAPR).