Differential spatial modulation (DSM) is a multi-antenna technique which transmits additional data bits by selecting indexes of antennas and avoids pilot overhead. However, DSM is complicated in two aspects and we aim to reduce the complexity of DSM in this letter. First, the complexity of the noncoherent maximum-likelihood (ML) detector increases exponentially with the number of transmitter antennas. For this problem, we propose a new symbol-based ML detector whose complexity is roughly proportional to the number of transmitter antennas. The other problem is that the signal constellation of the transmitted signal has unlimited points due to complex-valued antenna-index matrices. We propose a systematic design of complex-valued antenna-index matrices for which the constellation of the transmitted signal has a few signal points only. Both the proposed techniques decrease the complexity without sacrificing error performance.