| Popis: |
In this digital era, the usage of smart phones and mobile devices is becoming a norm in society with mobile communication quickly transitioned from voice oriented transmission to picture transmission to a more complex live video streaming. This latest development has demanded more capacity and higher bandwidth in communication links. Static links, which are the focus of this thesis, are an integral part of this mobile system in delivering high capacity data transmission using backhauls or nomadic links. Multi polarised antennas with multiple-input multiple-output (MIMO) multiplexing can be employed to greatly enhance the capacity of a mobile system, especially at frequencies lower than 6 GHz, using their compact size. A practical antenna inherently exhibits elliptical polarisation though it may be designed to form linear or circular polarisation. Little attention has been given to this aspect of polarised waves as they have always been deemed as unwanted polarisation, although in practice, any antenna is elliptically polarised as it can never be perfectly circularly or linearly polarised. This work therefore aims to deliberately exploit this opportunity by forming antennas with elliptical polarisation to identify the advantages of doing so in order to improve orthogonality in comparison with linear polarisation. It was found that in order to achieve perfect orthogonality, it was more practical to set the magnitudes and phases of the co-polar and cross-polar linear components, which resulted in an improved co to cross polar ratio more than 20 dB better than linear polarisation in free space. A dual elliptically polarised antenna prototype was designed and evaluated in this work, which was evaluated both in free space and within an indoor measurement campaign. Results concluded that at short distances with low scattering in the channel and directional antennas, elliptically polarised antennas provide improved multiplexing gain over dual linear polarisations. Key words: Elliptical Polarisation, MIMO, Multiplexing, static wireless links, degrees of freedom, dual polarised antenna. |
