In this paper, the performance of OFDM-CDMA system in mobile communication channels with different detection algorithms is evaluated in terms of the probability of bit error as a function of processing gain. Both the frequency diversity and multi-user interference (MUI) are taken into account. The simulation results show that MUI is the dominant factor that affects system performance, and MMSE outperforms the other algorithms. Based on the simulation results, a modified system scheme is proposed which performs better than the conventional method.