The performance of multiple access interference (MAI) cancellations for the asynchronous M-ary pulse position modulation (PPM) time hopping (TH) ultra-wideband (UWB) systems are investigated. Parallel interference cancellation (PIC) and successive interference cancellation (SIC) are the most popular interference cancellation schemes. However, both the PIC and SIC have some shortcomings that the PIC suffers an error-propagation effect due to the erroneous MAI cancellation and the SIC imports a long processing delay caused by the large number of user. Therefore, a low-complexity iterative interference cancellation (IIC) scheme is proposed in this paper. The IIC scheme is based on the concept of iteratively cancelling MAI from those users whose symbols have been reliably detected. The performances of these interference cancellation schemes are compared in terms of the symbol error rate (SER) for the asynchronous M-ary PPM TH-UWB systems. The simulation results show the proposed IIC scheme performs better than PIC and SIC detectors in a MAI dominant environment.