Relying on mutual orthogonality between subcarriers of different users in Orthogonal Frequency-Division Multiple Access (OFDMA) systems and exploiting both Linear Constellation Precoding (LCP) and Group layered space-time block code (GLSTBC), a novel LCP-GLSTBC-OFDM transmitter diversity technique is proposed for wireless communications over frequency-selective fading channels. The proposed approach is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on STBC) results in space diversity, and the lower level (based on LCP) results in frequency diversity. The proposed design only needs one receive antenna to suppress the group interference and can obtain half space diversity gain , half multiplexing gain and full frequency diversity gain simultaneously. The performance merits of this design are verified by simulations.