Transmit diversity is an effective technique to improve the performance of wireless communication systems. We investigate different transmit diversity schemes for multicarrier code-division multiple-access (MC-CDMA) systems. More specifically we compare the performance of different transmit diversity methods, including delay diversity, phase diversity, code diversity, scrambling code diversity and space-time block coding diversity for MC-CDMA. All the signal processing is carried out in the frequency domain. The impacts of the system load on the bit error rate performance is presented in correlated fading channels. The performance of multiple transmit antennas and multiple receive antennas (MIMO) MC-CDMA is also considered. Based on the simulation results, the space-time block coding method achieves the best performance. All transmit diversity schemes perform worse in the heavily loaded system than no transmit diversity system. Based on this, we propose a switched transmit diversity method in which the transmit diversity methods are changed according to the system load.
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