Wireless communication systems equipped with multiple antennas can achieve high spectral efficiency, so it has been a hot research topic since the late 20th century. Among various multi-antenna techniques, spatial modulation (SM) uses a single antenna for transmission each time to avoid synchronization and interference among transmitter antennas. Differential SM (DSM) is a kind of SM technique which does not need pilot symbols and channel estimation. In our paper which was accepted recently, we propose low-complexity DSM. In this two-year project, we will do more advanced research on DSM. In the first year, we will research on increased-diversity DSM. By appropriately designing complex-valued antenna-index matrix and block codes, the desired diversity order can be achieved. On the other hand, massive multiple-input multiple-output (MIMO) systems where the base station is equipped with hundreds of antennas have attracted much attention in recent years. Existing DSM schemes are too complicated to be realized for massive MIMO systems. In the second year, we will research on extremely-low-complexity DSM which is suitable for massive MIMO systems.