Performance analysis of band-limited DS-CDMA systems over generalized-k fading channels.

• Main Author: Haider Mehdi.
• Other Authors: Teh Kah Chan
• Format: Theses and Dissertations
• Language: English
• Published: 2010
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Wireless communication systems
• Online Access: https://hdl.handle.net/10356/22677
• Institution: Nanyang Technological University

Direct-sequence spread-spectrum (DS-SS) has attracted considerable attention and has been adopted by many wireless communication vendors. Well-known communication systems employing DS-SS are based on the code-division multiple-access (CDMA) technique, such as global positioning system (GPS), wideband CDMA (WCDMA) system, etc. In practice, wireless channels are commonly modeled as a mixture of fading and shadowing. Various composite models have been reported in the literature. These models are based on lognormal distributions. The main disadvantage of these models is that they are mathematically complicated to analyze. Hence, the lognormal shadowing was approximated by a gamma shadowing leading to the K distribution and its generalized version, i.e., generalized-K distribution. This versatile distribution is useful in evaluating the performance of composite channels with simplicity and mathematical tractability. The BER performance of DS-CDMA systems has been extensively studied during the last two decades. However, most of the reported work is focused on DSCDMA systems with rectangular pulse-shaping, while in practice, band-limited pulse shapes are adopted. Therefore, the performance analysis of band-limited systems is of considerable interest, and has gained some interest in the recently published literature. The aim of this research is to analyze the BER performance of band-limited DS-CDMA systems over composite small-scale fading and shadowing channels. We consider generalized-K distribution in our BER analysis. We study conventional matched filterbased systems as well as decorrelator based multiuser detection systems. We consider both flat and frequency-selective multipath fading channels. Systems with single-dimensional technique based on multipath diversity as well as two-dimensional scheme based on the combination of space and multipath diversity are considered. We present analytical BER expressions as a function of multiple-access interference (MAI), multitone jamming (MTJ) due to narrowband interference source, and the shadowing and fading parameters of the wireless channels. In recent literature, there has been interest in the transmit diversity techniques in order to combat multipath fading and interference conditions. Such diversity schemes are commonly incorporated in the form of multiple transmitter and receiver antennas and are known as multiple-input multiple-output (MIMO) diversity schemes. We analyze MIMO systems based on the recently introduced space-time spreading scheme with two transmitter and multiple receiver antennas. We consider fast as well as slow frequency-selective multipath fading channels. The corresponding analytical BER expressions of space-time DS-CDMA systems with decorrelator based receivers are presented. In our numerical analysis, we consider two types of band-limited pulse shapes, namely, spectrum raised cosine (SRC) and Beaulieu-Tan-Damen (BTD) pulses. From the numerical results, we observe that the systems with BTD pulse show better BER performance as compared to the ones with SRC pulse. We further observe that the presence of MTJ and MAI introduces an irreducible noise floor at high signal-to-noise ratio (SNR) levels. We also observe that the systems with two-dimensional diversity outperform the ones with single-dimensional diversity. From our BER analysis, we demonstrate that by incorporating generalized-K distribution, various scenarios of shadowing and fading can be easily analyzed.