Diversity-coding-orthogonality tradeoff for coded MC-CDMA with high level modulation

In MC-CDMA, the data rate can be increased by reducing the spreading factor SF or by allowing multicode transmission. In this paper, we evaluate by computer simulations which gives a better bit error rate (BER) performance lower SF or multicode operation - when high level modulation is used in addition to error control coding. For a coded system in a frequency selective channel, there is a tradeoff between frequency diversity gain due to the spreading of symbols, coding gain due to better frequency interleaving effect and orthogonality destruction. It is shown that for QPSK, the performance of OFDM (MC-CDMA with SF=1) is almost the same as that of a fully spread MC-CDMA system. However, for 16QAM and 64QAM, the BER performance is better for lower SF unlike the uncoded system, wherein higher SF gives a better BER