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A flexible time-hopping (TH) multiple-access (MA) ultrawideband (UWB) system employing M-ary orthogonal pulse position modulation (PPM) is implemented and analyzed in this paper. In contrast to the conventional TH method using the fixed partition of TH slots, a new flexible TH scheme that utilizes the whole frame to carry out TH and PPM is proposed. The advantage of the proposed scheme is to increase the modulation level of PPM and improve the capability of MA. Based on the analytical CF technique, we also derive the probability distribution of the MA interference (MAI) and the average symbol error probability of the proposed scheme. Simulation results are provided to demonstrate the effectiveness of the proposed TH system and verify the analytical frameworks in both the additive white Gaussian noise (AWGN) channel and the realistic UWB fading channels. Irrespective of the adopted modulation level of PPM, the proposed TH system outperforms the conventional TH scheme. The tradeoff of the system parameters for the proposed TH system under the constraints of fixed bit rate and signal bandwidth is also examined.