Abstract The MIMO (Multiple Input Multiple Output) technology uses multiple antennas at the transmitter and at the receiver of a wireless communication system to improve the data rate. It exploits the multipath propagation of the signals radiated by the antennas of the transmitter. When the channel gains are correlated, the capacity gains are reduced. In this thesis, spatial correlation between the signals received by the dipole elements which are arranged as a uniform rectangular array is analytically found. It was found that the spatial correlation tends to be lower for certain antenna spacings when the mutual coupling is considered. When the elevation spread and the azimuth spread of the incoming waves decrease, the correlation of the channel gains increases. It has been demonstrated that a MIMO system equipped with reconfigurable antennas as elements can be made to shape the channel in order to obtain a channel conducive for MIMO. Through a ray tracing software, it has shown that the capacity offered by a MIMO system employing reconfigurable antennas is higher than that of omnidirectional antennas. In this context, the thesis proposes three different reconfigurable antennas. A beam reconfigurable three layer antenna was designed, fabricated and characterized. This fabricated antenna works as designed and simulated. As it is based on the Yagi antenna concept, the radiation behavior is explained using the Yagi antenna. This antenna can tilt its beam in the H-plane of the antenna in two directions. The second proposed antenna is a pattern and polarization reconfigurable antenna. This is designed to reconfigure ist beam in four directions. Two of the beams will have the same polarization, while the other two beams will have a polarization orthogonal to that of the former. A simple model involving halfwave dipoles was created to explain the pattern tilt. The model was created by investigating the current distribution on the antennas. The third of the proposed reconfigurable antennas is based on planar dipoles. This antenna is made to tilt the beam in the E-plane of the antenna. As a step forward, the role of switched beam antennas as elements of MIMO was also investigated at 60 GHz. It has been found that the switched beam antennas can indeed provide capacity gains over the unidirectional antennas. For fixed broadband wireless access such as a microwave relay, LOS MIMO (Line Of Sight) has been proposed in the literature for high data rate. A 4 × 4 LOS MIMO technique employing alternatively polarized antennas was analyzed in this thesis using the condition number and the capacity expressions. These expressions were derived in this thesis using fundamental principles. It was found that the antenna spacing required for the alternatively polarization is less than that of the single polarization case Kurzfassung Die MIMO (Multiple Input Multiple Output)-Technologie verwendet mehrere Antennen auf der Sende- und Empfangsseite eines drahtlosen Kommunikationssystems, um die Datenrate zu steigern. Dabei wird die Mehrwegeausbreitung der von den Sendeantennen ausgestrahlten Signale ausgenutzt. Wenn die Kanalgewinne der verschiedenen Ausbreitungswege korreliert sind, ergibt sich nur eine geringe Vergroßerung der Kanalkapazitat. Im Rahmen dieser Arbeit wird fur die raumliche Korrelation von Signalen, die von Dipol-Elementen in einem rechteckformigen Array empfangen werden, eine analytische Darstellung abgeleitet. Dabei lasst sich feststellen, dass die raumliche Korrelation fur bestimmte Antennenabstande geringer wird, wenn die gegenseitige Verkopplung der Antennen berucksichtigt wird.Wenn die Breiten derWinkelverteilungen der einfallenden Signale in Elevation und Azimut abnehmen, erhoht sich hingegen die Korrelation der Kanalgewinne. In der Arbeit wird demonstriert, dass ein MIMO-System mit Hilfe von rekonfigurierbaren Antennen so ausgelegt werden kann, dass es den U