This article presents a novel multicarrier direct sequence code-division multiple-access (MC DS-CDMA)scheme based on the loosely synchronous (LS) codes, which can achieve very high spectral efficiency. Instead of the single subcarrier modulation in a conventional MC DS-CDMA scheme,this novel scheme introduces the nonorthogonal subcarrier-group modulation, whose interferences caused by the non-orthogonal spacing in a subcarrier group can be reduced by a multicode detector. A detailed analysis for this system architecture and also a multiple-input-multiple output (MIMO) MC DS-CDMA solution to satisfy the requirement of beyond 3G mobile communication system are proposed. Simulated results demonstrate that this scheme can obtain excellent bit error rate (BER) performance, as well as achieve more than twice the spectral efficiency as that of conventional MC DS-CDMA systems.