Name:	Hung Tuan Nguyen
Title of Thesis:	Multiple Antenna Systems for Mobile Terminals
PhD programme:	Wireless Communications
Department:	Department of Electronic Systems (Department of Communication Technology)
Supervisor:	Professor Jørgen Bach Andersen
PhD degree awarded:	January 18, 2006

Abstract

Recently Multiple Input Multiple Output (MIMO) systems where antenna array applied at both ends of the transmission link have been a major focus of research. The main reason for such interest is that MIMO system has a potential of providing more spectral efficiency and better transmission quality. Hence, beyond the original purpose of transmitting and receiving speech, it is possible to provide more services which require higher data rate. The first part of the project is devoted to the characterization of the measured MIMO radio channel in various propagation environments. Several parameters of the measured MIMO radio channel are extracted and the system performance is interpreted. The influence of the propagation condition and some real-life usage effect to the channel capacity is evaluated. Based on the measurement data, a simple stochastic model for a spatio-temporally correlated MIMO radio channel is proposed. The capability of the model in replicating the spatial and temporal characteristic of the measured channel is assessed.
The second part of the project aims at analyzing and evaluating the performance of several link adaptation (LA) techniques for multiple elements antenna (MEA) systems. With a premise that the channel knowledge is available at both ends of the transmission link, the performances of the LA techniques for single user scenario narrowband MIMO systems are proposed and evaluated. It is shown that efficient uses of the channel information can improve the system performance significantly. However, the performance of the LA techniques strongly depends on the quality of the estimated channel coefficients. Imperfection in the estimation of the channel coefficient could lead to degradation in the performance of any MIMO system. We study the impact of the time-varying channel, as the only source of such imperfection, to the MIMO system performance. An expression of the signal to interference plus noise ratio (SINR) quantifying the effect of time-varying channel to the spatial multiplexing MIMO system is derived. It is shown that for a channel varying with time, increasing the transmitted power does not really help in improving the performance of a MIMO system. As a remedy for this problem, a precoder/decoder prediction scheme for the time-varying frequency flat MIMO channel is proposed and evaluated. Using the prediction scheme, in a slowly time-varying narrowband channel improvement of the MIMO system performance by a factor of two is shown to be possible.
In a wideband transmission, the frequency selective fading causing the inter symbol interference (ISI) remains a big challenge for point to point wireless communication in general. Moreover, interferences caused by other users in the multi-user environment also degrade the system performance significantly. It is especially true for MEA systems where the received signals are not only mixed in space but also in time. The last part of the project is devoted to the investigation an the performance of the time reversal technique (TR) in wireless communication. The feasibility of applying TR in MEA wireless system to combat the effect of time dispersion and interference is illustrated. The results show that TR is very promising solution for interference reduction and time dispersion in multi-user MISO system.