Multiuser Systems Implementations in Fading Environments

Abstract

The theory of multiuser detection technique has been developed during the 90s [Verdu, 1998], but its application gained a high potential especially for large mobile networks when the base station has to demodulate the signals coming from all mobile users [Verdu, 1998; Sakrison, 1966]. The performances of multiuser detection systems are affected mostly by the multiple access interference, but also by the type of channel involved and the impairments it might introduce. Therefore, important roles for improving the detection processes are played by the type of noise and interferences affecting the signals transmitted by different users. Selection of spreading codes to differentiate the users plays an important role in the system performances and in the capacity of the system [Halunga & Vizireanu, 2009]. There are important conclusions when the signals of the users are not perfectly orthogonal and/or when they have unequal amplitude [Kadous& Sayeed, 2002], [Halunga & Vizireanu, 2010]. In a wireless mobile communication system, the transmitted signal is affected by multipath phenomenon, which causes fluctuations in the received signal’s amplitude, phase and angle of arrival, giving rise to the multipath fading. Small-scale fading is called Rayleigh fading if there are multiple reflective paths that are large in number and there is no line-of-sight component. The small-scale fading envelope is described by a Rician probability density function [Verdu, 1998], [Marcu, 2007]. Recent research [Halunga & Vizireanu, 2010] led us to several conclusions related to the performances of multiuser detectors in different conditions. These conditions include variation of amplitudes, selective choice of (non) orthogonal spreading sequences and analysis of coding/decoding techniques used for recovering the original signals the users transmit. It is very important to mention that the noise on the channel has been considered in all previous simulations as AWGN (Additive White Gaussian Noise). This chapter implies analysis of multiuser detection systems in the presence of Rayleigh and Rician fading with Doppler shift superimposed over the AWGN noise. The goal of our research is to illustrate the performances of different multiuser detectors such as conventional detector and MMSE (Minimum Mean-Square Error) synchronous linear detectors in the presence of selective fading. The evaluation criterion for multiuser systems performances is BER (Bit Error Rate) depending on SNR (Signal to Noise Ratio). Several conclusions will be withdrawn based on multiple simulations.