On the multi-user interference study for ultra wideband communication systems in AWGN and modified Saleh-Valenzuela channel

This paper studies the effects of multiple access interference (MAI) for ultra wideband (UWB) systems in AWGN and multipath fading channels. Time hopping (TH) and direct sequence (DS) based UWB spreading approaches are performed, utilizing binary pulse amplitude modulation (PAM) and pulse shape modulation (PSM). Both the possibilities of having synchronous and asynchronous users have been taken into account. In the fading channel case, different coherent and non-coherent detection methods have been considered. Simulated performance results are depicted for the above mentioned UWB systems when a variable number of equally powered users are operating in the channel. Results show that in both the AWGN and fading channel the DS system performance shows sensibly different results in the synchronous and asynchronous case. This is due to the continuous time transmission of the different users. This problem does not occur in systems using TH spreading since the different users are less overlapped in time, due to the spreading approach used. In the fading channel, TH systems show a general higher performance loss in all cases compared to DS, in contrast to what happens in the AWGN case. That is, the allocation of the TH codes creates a higher interfering effect due to MAI in the presence of multipath fading. In general, DS-based systems outperform the corresponding TH ones.