Abstract: In order to improve the spectral efficiency, this paper proposes a new framework for designing a modulation consisting of more than one modulation method. The new modulation method is able to provide better spectral efficiency while preserving the merits of Unitary Space-Time Modulation (USTM), such as low error rate and demodulation simplicity. The necessary condition for this set of modulations is presented and several different options satisfying the condition are given. Finally a novel modulation called Differential Phase Unitary Space-Time Modulation (DPUSTM) is designed as a special case. DPUSTM, which consists of USTM and Differential Phase-Shift Keying (DPSK) modulation, is not only easier to improve the spectral efficiency than USTM, but also simpler to demodulate. The novel modulation can achieve low bit error rate as UST signals and be exploited in multi-rate data communication. The simulation results not only verify the theoretical analysis, but also show that DPUSTM provides a better performance than USTM and DSTM.