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A spread-spectrum pulse spacing modulation (SS-PSM) system, which combines a pulse spacing modulation system with the spread-spectrum (SS) communication schemes, is proposed. In the proposed system, the information to be transmitted is represented by the number of slots between the previous and the present spreading code. The preamble signal is used for block synchronisation, and the proposed system uses an even-shift orthogonal sequence as a spreading code. The following performance measures are analysed: the data transmission rate, the block error rate when the receiver detects the preamble signal correctly, the synchronisation performance (the probability of detecting a preamble signal and the probability of false alarm), the block error taking account of the synchronisation performances, and the efficiency. The data transmission rate of the proposed system is much higher than that of the conventional spread spectrum pulse position modulation (SS-PPM) system. The block error rate considering the synchronisation performance can reach the lower bound by making use of the preamble signal [-1, -1, -1, +1, 0]. The efficiency of the proposed system is about 1.5 times greater than that of the SS-PPM system. This shows the effectiveness of the proposed system for improving the performance of the conventional SS-PPM system.