A quasi-orthogonal space-frequency coded ofdm system with time spreading and constellation rotation

A constellation rotated quasi-orthogonal OFDM system could offer full rate and full diversity in a frequency selective fading channel. Time spreading can give additional time diversity gain in a fast fading channel. In this paper, we propose and verify a time-spread quasi-orthogonal space-frequency coded OFDM (TS-QO-SF-OFDM) system with constellation rotation. Assuming perfect channel state information at the receiver, we develop a coding scheme for 4times1 MISO transceiver and its BER performance is evaluated for different Doppler frequencies in an OFDM system. Monte-Carlo simulation results show that 2 dB gain can be achieved at BER of 10^-3 using the proposed scheme compared to a scheme without time spreading and constellation rotation when 512 subcarriers are used at maximum Doppler frequency of 300 Hz. The proposed scheme also lends itself to easy implementation making it more attractive for adaptive cognitive radio systems.