Mimo Communication Systems with Reconfigurable Antennas

RÉSUMÉ Les antennes reconfigurables sont capables d'ajuster dynamiquement les caractéristiques de leur diagramme de rayonnement, par exemple, la forme, la direction et la polarisation, en réponse aux conditions environnementales et exigences du système. Ces antennes peuvent aussi être utilisées en conjonction avec des systèmes à entrées multiples sorties multiples (MIMO) pour améliorer davantage la capacité et la fiabilité des systèmes sans fil. Cette thèse étudie certains des problèmes dans les systèmes sans fil équipés d'antennes reconfigurables et propose des solutions pour améliorer la performance du système. Dans les systèmes sans fil utilisant des antennes reconfigurables, la performance atteignable par le système dépend fortement de la connaissance de la direction d'arrivée (DoA) des signaux souhaités et des interférences. Dans la première partie de cette thèse, nous proposons un nouvel algorithme d'estimation de la DoA pour les systèmes à entrer simple et sortie simple (SISO) qui possèdent un élément d'antenne reconfigurable au niveau du récepteur. Contrairement à un système utilisant un réseau d'antennes conventionnelles à diagramme de rayonnement fixe, où la DoA est estimée en utilisant les signaux reçus par plusieurs éléments, dans le réseau d'antennes avec l'algorithme proposé, la DoA est estimée en utilisant des signaux reçus d'un élément d'antenne unique pendant qu'il balai un ensemble de configurations de diagramme de rayonnement. Nous étudions aussi l'impact des différentes caractéristiques des diagrammes de rayonnement utilisés, tels que la largeur du faisceau de l'antenne et le nombre d'étapes de numérisation, sur l'exactitude de la DoA estimée. Dans la deuxième partie de cette thèse, nous proposons un système de MIMO faible complexité employant des antennes reconfigurables sur les canaux sélectifs en fréquence pour atténuer les êtes de trajets multiples et donc éliminer l'interférence entre symboles sans utiliser la technique de modulation multiplexage orthogonale fréquentiel (OFDM). Nous étudions aussi l'impact de la propagation et de l'antenne angulaire largeur de faisceau sur la performance du système proposé et faire la comparaison avec la performance du système MIMO-OFDM. Dans la troisième partie de cette thèse, nous fournissons des outils analytiques pour analyser la performance des systèmes sans _l MIMO équipés d'antennes reconfigurables au niveau du récepteur. Nous dérivons d'abord des expressions analytiques pour le calcul de la matrice de covariance des coefficients des signaux reçus empiétant sur un réseau d'antennes reconfigurables en tenant compte de plusieurs caractéristiques de l'antenne tels que la largeur du faisceau, l'espacement d'antenne, l'angle de pointage ainsi que le gain de l'antenne. Dans cette partie, nous considérons un récepteur MIMO reconfigurable où le diagramme de rayonnement de chaque élément d'antenne dans le réseau peut avoir des caractéristiques différentes. Nous étudions également la capacité d'un système MIMO reconfigurable en utilisant les expressions analytiques dérivées.----------ABSTRACT Reconfigurable antennas are able to dynamically adjust their radiation pattern characteristics, e.g., shape, direction and polarization, in response to environmental conditions and system requirements. These antennas can be used in conjunction with multiple-input multiple-output (MIMO) systems to further enhance the capacity and reliability of wireless networks. This dissertation studies some of the issues in wireless cellular systems equipped with reconfigurable antennas and offer solutions to enhance their performance. In wireless systems employing reconfigurable antennas, the attainable performance improvement highly depends on the knowledge of direction-of-arrival (DoA) of the desired source signals and that of the interferences. In the first part of this dissertation, we propose a novel DoA estimation algorithm for single-input single-output (SISO) system with a reconfigurable antenna element at the receiver. Unlike a conventional antenna array system with fixed radiation pattern where the DoA is estimated using the signals received by multiple elements, in the proposed algorithm, we estimate the DoA using signals collected from a set of radiation pattern states also called scanning steps. We, in addition, investigate the impact of different radiation pattern characteristics such as antenna beamwidth and number of scanning steps on the accuracy of the estimated DoA. In the second part of this dissertation, we propose a low-complexity MIMO system employing reconfigurable antennas over the frequency-selective channels to mitigate multipath effects and therefore remove inter symbol interference without using orthogonal frequency division multiplexing (OFDM) modulation. We study the impact of angular spread and antenna beamwidth on the performance of the proposed system and make comparisons with that of MIMO-OFDM system equipped with omnidirectional antennas. In the third part of this dissertation, we provide an analytical tool to analyze the performance of MIMO wireless systems equipped with reconfigurable antennas at the receiver. We first derive analytical expressions for computing the covariance matrix coefficients of the received signals impinging on a reconfigurable antenna array by taking into account several antenna characteristics such as beamwidth, antenna spacing, antenna pointing angle, and antenna gain. In this part, we consider a reconfigurable MIMO receiver where the radiation pattern of each antenna element in the array can have different characteristics. We, additionally, study the capacity of a reconfigurable MIMO system using the derived analytical expressions

Residue number system coded differential space-time-frequency coding.

Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2007.The rapidly growing need for fast and reliable transmission over a wireless channel motivates the development of communication systems that can support high data rates at low complexity. Achieving reliable communication over a wireless channel is a challenging task largely due to the possibility of multipaths which may lead to intersymbol interference (ISI). Diversity techniques such as time, frequency and space are commonly used to combat multipath fading. Classical diversity techniques use repetition codes such that the information is replicated and transmitted over several channels that are sufficiently spaced. In fading channels, the performance across some diversity branches may be excessively attenuated, making throughput unacceptably small. In principle, more powerful coding techniques can be used to maximize the diversity order. This leads to bandwidth expansion or increased transmission power to accommodate the redundant bits. Hence there is need for coding and modulation schemes that provide low error rate performance in a bandwidth efficient manner. If diversity schemes are combined, more independent dimensions become available for information transfer. The first part of the thesis addresses achieving temporal diversity through employing error correcting coding schemes combined with interleaving. Noncoherent differential modulation does not require explicit knowledge or estimate of the channel, instead the information is encoded in the transitions. This lends itself to the possibility of turbo-like serial concatenation of a standard outer channel encoder with an inner modulation code amenable to noncoherent detection through an interleaver. An iterative approach to joint decoding and demodulation can be realized by exchanging soft information between the decoder and the demodulator. This has been shown to be effective and hold hope for approaching capacity over fast fading channels. However most of these schemes employ low rate convolutional codes as their channel encoders. In this thesis we propose the use of redundant residue number system codes. It is shown that these codes can achieve comparable performance at minimal complexity and high data rates. The second part deals with the possibility of combining several diversity dimensions into a reliable bandwidth efficient communication scheme. Orthogonal frequency division multiplexing (OFDM) has been used to combat multipaths. Combining OFDM with multiple-input multiple-output (MIMO) systems to form MIMO-OFDM not only reduces the complexity by eliminating the need for equalization but also provides large channel capacity and a high diversity potential. Space-time coded OFDM was proposed and shown to be an effective transmission technique for MIMO systems. Spacefrequency coding and space-time-frequency coding were developed out of the need to exploit the frequency diversity due to multipaths. Most of the proposed schemes in the literature maximize frequency diversity predominantly from the frequency-selective nature of the fading channel. In this thesis we propose the use of residue number system as the frequency encoder. It is shown that the proposed space-time-frequency coding scheme can maximize the diversity gains over space, time and frequency domains. The gain of MIMO-OFDM comes at the expense of increased receiver complexity. Furthermore, most of the proposed space-time-frequency coding schemes assume frequency selective block fading channels which is not an ideal assumption for broadband wireless communications. Relatively high mobility in broadband wireless communications systems may result in high Doppler frequency, hence time-selective (rapid) fading. Rapidly changing channel characteristics impedes the channel estimation process and may result in incorrect estimates of the channel coefficients. The last part of the thesis deals with the performance of differential space-time-frequency coding in fast fading channels

Signal processing techniques for mobile multimedia systems

Recent trends in wireless communication systems show a significant demand for the delivery of multimedia services and applications over mobile networks - mobile multimedia - like video telephony, multimedia messaging, mobile gaming, interactive and streaming video, etc. However, despite the ongoing development of key communication technologies that support these applications, the communication resources and bandwidth available to wireless/mobile radio systems are often severely limited. It is well known, that these bottlenecks are inherently due to the processing capabilities of mobile transmission systems, and the time-varying nature of wireless channel conditions and propagation environments. Therefore, new ways of processing and transmitting multimedia data over mobile radio channels have become essential which is the principal focus of this thesis. In this work, the performance and suitability of various signal processing techniques and transmission strategies in the application of multimedia data over wireless/mobile radio links are investigated. The proposed transmission systems for multimedia communication employ different data encoding schemes which include source coding in the wavelet domain, transmit diversity coding (space-time coding), and adaptive antenna beamforming (eigenbeamforming). By integrating these techniques into a robust communication system, the quality (SNR, etc) of multimedia signals received on mobile devices is maximised while mitigating the fast fading and multi-path effects of mobile channels. To support the transmission of high data-rate multimedia applications, a well known multi-carrier transmission technology known as Orthogonal Frequency Division Multiplexing (OFDM) has been implemented. As shown in this study, this results in significant performance gains when combined with other signal-processing techniques such as spa ce-time block coding (STBC). To optimise signal transmission, a novel unequal adaptive modulation scheme for the communication of multimedia data over MIMO-OFDM systems has been proposed. In this system, discrete wavelet transform/subband coding is used to compress data into their respective low-frequency and high-frequency components. Unlike traditional methods, however, data representing the low-frequency data are processed and modulated separately as they are more sensitive to the distortion effects of mobile radio channels. To make use of a desirable subchannel state, such that the quality (SNR) of the multimedia data recovered at the receiver is optimized, we employ a lookup matrix-adaptive bit and power allocation (LM-ABPA) algorithm. Apart from improving the spectral efficiency of OFDM, the modified LM-ABPA scheme, sorts and allocates subcarriers with the highest SNR to low-frequency data and the remaining to the least important data. To maintain a target system SNR, the LM-ABPA loading scheme assigns appropriate signal constella tion sizes and transmit power levels (modulation type) across all subcarriers and is adapted to the varying channel conditions such that the average system error-rate (SER/BER) is minimised. When configured for a constant data-rate load, simulation results show significant performance gains over non-adaptive systems. In addition to the above studies, the simulation framework developed in this work is applied to investigate the performance of other signal processing techniques for multimedia communication such as blind channel equalization, and to examine the effectiveness of a secure communication system based on a logistic chaotic generator (LCG) for chaos shift-keying (CSK)