In this work, the authors address the effect of spreading sequences and pulse waveforms on the performance of asynchronous direct-sequence ultra-wideband systems. Regarding spreading sequences, they consider the two cases of short and long sequences. The expression of the multi-user interference (MUI) variance is derived using long sequences at the output of a rake receiver, when propagating through multipath channels. The authors also propose the use of sequences generated with a family of spatiotemporal chaotic systems, namely piecewise coupled map lattices (PCML), as spreading sequences. Such sequences are shown to reduce the MUI variance with regard to independent identically distributed and Gold sets for both short and long sequences. Simulation results show that the use of long PCML codes in additive white Gaussian noise and multipath fading channels results in an improvement of the system average bit error rate, hence an increase of the system capacity in terms of number of active users.
It is known that in a direct-sequence (DS) ultrawideband (UWB) system, a single type of UWB pulse is used in all bits for all users. In this paper, we address a method to generate varying waveforms based on spatiotemporal chaotic approach. Regarding spreading sequences, we consider the two cases of short and long sequences. In particular, we propose the use of a family of spatiotemporal chaotic systems, namely Piecewise Coupled Map Lattices (PCML), as spatiotemporal chaotic waveforms and spreading sequences. Such sequences are shown to reduce the multi-user interference (MUI) variance with regard to i.i.d. and Gold sequences for both short and long cases. On the other hand, we evaluate and compare the DS-UWB system performance by using conventional waveform such as Gaussian monocycle pulse and PCML pulses. Finally, we show that the use of long PCML codes with PCML pulses, results in an improvement of the system average BER, hence an increase of the system capacity in terms of number of active users.Index Terms-direct sequence (DS) ultra-wideband (UWB) communications, spreading sequences, Piecewise Coupled Map Lattices (PCML) varying waveforms and sequences.
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