Multiple antenna systems : channel capacity and low-density parity-check codes.

Abstract

The demand for high data rate wireless communication systems is evident today as indicated by the rapid growth in wireless subscribers and services. High data rate systems are bandwidth intensive but bandwidth is an expensive and scarce commodity. The ability of future wireless systems to efficiently utilise the available bandwidth is therefore integral to their progress and development. The wireless communications channel is a harsh environment where time varying multipath fading, noise and interference from other users and systems all contribute to the corruption of the received signal. It is difficult to overcome these problems and achieve the high data rates required using single antenna technology. Multiple-input-multipleoutput (MIMO) systems have recently emerged as a promising technique for achieving very large bandwidth efficiencies in wireless channels. Such a system employs multiple antennas at both the transmitter and the receiver. These systems exploit the spatial dimension of the wireless channel to achieve significant gains in terms of capacity and reliability over single antenna systems and consequently achieve high data rates. MIMO systems are currently being considered for 3rd generations cellular systems. The performance of MIMO systems is heavily dependent on the environment in which the system is utilised. For this reason a realistic channel model is essential for understanding the performance of these systems. Recent studies on the capacity of MIMO channels have focused on the effect of spatial correlation but the joint effect of spatial and temporal correlation has not been well studied. The first part of this thesis proposes a new spatially and temporally correlated MIMO channel model which considers motion of the receiver and nonisotropic scattering at both ends of the radio link. The outage capacity of this channel is examined where the effects of antenna spacing, array angle, degree of scattering and receiver motion are investigated. It is shown that the channel capacity still increases linearly with the number of transmit and receive antennas, despite the presence of both spatial and temporal correlation. The capacity of MIMO channels is generally investigated by simulation. Where analytical expressions have been considered for spatially correlated channels, only bounds or approximations have been used. In this thesis closed form analytical expressions are derived for the ergodic capacity of MIMO channels for the cases of spatial correlation at one end and both ends of the radio link. The latter does not lend itself to numerical integration but the former is shown to be accurate by comparison with simulation results. The proposed analysis is also very general as it is based on the transmit and receive antenna correlation matrices. Low-density parity-check (LDPC) codes have recently been rediscovered and have been shown to approach the Shannon limit and even outperform turbo codes for long block lengths. Non-binary LDPC codes have demonstrated improved performance over binary LDPC codes in the AWGN channel. Methods to optimise non-binary LDPC codes have not been well developed where only simulation based approaches have been employed, which are not very efficient. For this reason, a new approach is proposed which is based on extrinsic information transfer (EXIT) charts. It is demonstrated that by performing curve matching on the EXIT chart, good non-binary LDPC codes can be designed for the AWGN channel. In order to approach the theoretical capacity of MIMO channels, many space-time coded, multiple antenna (MA) systems have been considered in the literature. These systems merge channel coding and antenna diversity and exploit the benefits of both. Binary LDPC codes have demonstrated good performance in MA systems but nonbinary LDPC codes have not been considered. Therefore, the application of non-binary LDPC codes to MA systems is investigated where the codes are optimised for the system of interest, using a simulation and EXIT chart based design approach. It is shown that non-binary LDPC codes achieve a small gain in performance over binary LDPC codes in MA systems.