Abstract: In multi-carrier code division access (MC-CDMA) systems, symbol-level frequency domain equalizer (FDE) can achieve better performance than chip-level FDE but suffers from higher complexity. Our studies indicates that the spreading sequences has two basic roles, i.e., separating parallel data streams to provide multiplexing gain and spreading signal spectrum to provide frequency diversity gain. These two roles are often coupled with each other, which leads to the high complexity of symbol-level FDE. In this paper, by decoupling the spreading sequences into an orthogonal code matrix and a spreading matrix that play different roles and judiciously designing the spreading matrix, a low-complexity symbol-level FDE and an optimal spreading matrix that maximizes diversity gain are proposed. Theoretical analysis and simulation results show that the proposed symbol-level FDE outperforms the existing chip-level FDE and achieves the matched filter bound when the processing gain exceeds the number of resolvable paths.