Frequency-time codes for OFDMA uplink applications

Orthogonal frequency division multiplexing (OFDM) is a multi-carrier block modulation scheme primarily used in high bit rate systems to mitigate inter-symbol interference (ISI) experienced in multipath wireless channels. The annoying effect of ISI is almost all removed by the insertion of a cyclic prefix (or postfix) extension. However, during the transmission, ISI may also provides a multipath diversity advantage if the path gains fade independently on the different OFDM subcarriers. In this case, diversity translates in frequency diversity when discrete Fourier transform (DFT) is performed at the receiver. The Doppler shift of each subcarrier frequency caused by relative motion of the receiver and transmitter also yields to frequency diversity due to independent frequency shifts on each subcarrier. In an uplink multi-access scenario, subcarriers used by different users inevitably exhibits frequency diversity due to their different locations and speeds. In this paper, we introduce a new frequency diversity scheme called frequency time Trellis code (FTTC) that exploit the channel diversity. The performance of the FTTC transmit diversity is analyzed by simulation on slow Rayleigh fading channels. The results show that the FTTC scheme SER performances are better than the uncoded version.