Pre-equalization in combination with transmit diversity for OFDMA code-division multiplexing systems in fading channels

The potential of combining channel pre-equalization with transmit diversity for orthogonal frequency-division multiple-access code-division multiplexing (OFDMA-CDM) systems is investigated. The pre-equalization exploits the channel knowledge which is presumed to be available at the transmitter (Tx). The transmission signal is pre-distorted according to the channel state information (CSI) which can be made available, for example, by using time division duplex (TDD) mode. In TDD mode and under the assumption that CSI changes slowly in time, CSI estimated from a received slot can be used for pre-equalization of the next transmission slot. Concerning transmit diversity, we consider space-frequency block coding (SFBC) and transmit selection diversity (TSD). It turns out that TSD significantly outperforms SFBC. Moreover, systems with pre-equalization and TSD exhibit lower complexity than systems with pre-equalization and SFBC, since in the former case CSI is needed only at Tx, while in the latter case CSI is required at both Tx and receiver (Rx). In addition, it is shown that TSD is the optimal transmit diversity technique in the sense of bit error rate (BER) minimization in the case when CSI is available at Tx.