A new scheme to enhance the random aspect of information symbols in Time Hopping Pulse Position Modulation is proposed to reduce the catastrophic effect of collisions in multiple access. It is evaluated through analytical expressions and simulations. Multiple access interference modeling methods have been proposed in the recent literature to carry out accurately the bit error performance of these systems. The characteristic function method has been used here to derive the analytical expressions. Monte-Carlo simulations help to confirm the accuracy of the obtained expressions. The performances of this scheme and of the classical case are compared in term of Bit-error rates. The results show that the proposed scheme is a good candidate to enhance MAI avoidance.

The objective of this work is to enhance performance of impulse radio ultra wide band transmissions in multiple-user environment, without implementing complex multiuser detectors. A new scrambling scheme, randomising the pulse width, is introduced at the transmitter. It allows both to improve the spectral occupancy of the transmitted signal and the resistance to interference. This informationless scrambler is analysed and optimised with an evolutionary algorithm to achieve, according to simulations, significant improvement.Index Terms-Multi access, IR-UWB, error probability , multiobjective optimisation, genetic algorithm.

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