Download files
Abstract
During the last decade there has been a steady increase in the demand for incorporation of high data rate and strong reliability within wireless communication applications. Among the different solutions that have been proposed to cope with this new demand, the utilization of multiple antennas arises as one of the best candidates due to the fact that it provides both an increase in reliability and also in information transmission rate. A Multiple Input Multiple Output (MIMO) structure usually assumes a frequency non-selective characteristic at each channel. However, when the transmission rate is high, the whole channel can become frequency selective. Therefore, the use of Orthogonal Frequency Division Multiplexing (OFDM) that transforms a frequency selective channel into a large set of individual frequency on-selective narrowband channels, is well suited to be used in conjunction with MIMO systems.
A MIMO system employing OFDM, denoted MIMO-OFDM, is able to achieve high spectral
efficiency. However, the adoption of multiple antenna elements at the transmitter for spatial transmission results in a superposition of multiple transmitted signals at the receiver, weighted by their corresponding multipath channels. This in turn results in difficulties with reception, and imposes a real challenge on how to design a practical system that can offer a true spectral efficiency improvement.
In addition, as wireless networks continue to expend in geographical size, the distance between the source and the destination precludes direct communication between them. In such scenarios, a repeater is placed between the source and the destination to achieve end-to-end communication.
New advances in electronics and semiconductor technologies have enabled and made relay based systems feasible. As a result, these systems have become a hot research topic in the wireless research community in recent years. Potential application areas of cooperation diversity are the next generation cellular networks, mobile wireless ad-hoc networks, and mesh networks for wireless broadband access. Besides increasing the network coverage, relays can provide additional diversity to combat the effects of the wireless fading channel.
This thesis is concerned with methods to facilitate the use of MIMO, OFDM and relay based systems. In the first part of this thesis, we concentrate on low complexity algorithms for detection of symbols in MIMO systems, with various degrees of quality of channel state information. First, we design algorithms for the case that perfect Channel State Information (CSI) is available at the receiver.
Next, we design algorithms for the detection of non-uniform symbols constellations where only partial CSI is given at the receiver. These will be based on non-convex and stochastic optimisation techniques.
The second part of this thesis addresses primary issues in OFDM systems. We first concentrate on a design of an OFDM receiver. First we design an iterative receiver for OFDM systems which performs detection, decoding and channel tracking that aims at minimising the error propagation effect due to erroneous detection of data symbols.
Next we focus our attention to channel estimation in OFDM systems where the number of channel taps and the power delay profile are both unknown a priori. Using Trans Dimensional Markov Chain Monte Carlo (TDMCMC) methodology we design algorithms to perform joint model order selection and channel estimation.
The third part of this thesis is dedicated to detection of data symbols in relay systems with non-linear relay functions and where only partial CSI is available at the receiver. In order to design the optimal data detector, the likelihood function needs to be evaluated at the receiver. Since the likelihood function cannot be obtained analytically or not even in a closed form in this case, we shall utilse a “Likelihood Free” inference methodology. This will be based on the Approximate Bayesian Computation (ABC) theory to enable the design of novel data sequence detectors.