Radio characterisation of single trees at micro- and millimetre wave frequencies

Wireless Communications are revolutionising personal and telecommunications services and the way in which they are utilised. Overall growth in cellular, fixed and satellite communication system markets in recent years has exceeded expectations. There is a widespread anticipation that customer demand for wireless telecommunication systems will continue to expand in the foreseeable future. Such systems rely in their planning, design and implementation on the availability of radiowave propagation models. These models are required to describe and characterise with sufficient accuracy the interaction of radiowaves with the environment, especially the various obstacles in the radio path. Prediction tools are highly desirable to radio planners in achieving appropriate coverage planning, determination of the propagation modes and the prediction and control of mutual cochannel interference between existing and new radio links. In the case of land mobile systems as well as wireless fixed access systems, trees, singly or in a group, are usually present in the radio cell environment, giving rise to both absorption and scatter of radio signals. An important part of the modelling process applied to vegetation effects is aimed at analysing the radio propagation modes and the identification of individual signal contributions to the scattered signal caused by various elements of the tree. Past research work on vegetation available in the literature does not account for the interaction between an incident plane wave and individual elements constituting the tree. The work reported in this thesis describes detailed studies aimed at the characterisation of propagation mechanisms arising in single trees of various types. It explores effects of geometrical and physical properties of the tree on radiowave propagation modes arising specifically, i.e. absorption, scatter and depolarisation. These have been addressed through a combination of analytical, computational and experimental modelling, based on thorough examination of the re-radiation functions of single trees. Appropriate measurements performed in both anechoic and outdoor environments at microwave and millimetre wave frequencies, covering frequency band from 2 to 62.4 GHz, provided both model validation and a deeper insight into the problem. Single tree scatter has been shown to be modelled adequately in terms of a re-radiation function with parameters which can be deduced from measured data. This is an important extension to the Radiative Energy Transfer (RET) model, which hitherto has been applied to a homogeneous forest half space. Forward, side and back scatter regions have been identified and characterised. Depolarisation effects were subjected to detailed study with the help of an idealised metallic structure. Analyses of measured results provided deep insight into the causes of depolarisation and the specific polarisation states likely to arise in vegetation. Reasons for signal fades or nulls in the re-radiated signals are established. Wideband channel measurements performed at 2 GHz provided valuable information on the dispersive effects of single trees, whose subsequent analyses revealed the sources of scattering, effects of tree elements, e.g. leaves and branches, and wind effects. The thesis provides also a novel method based on the Finite Difference Time Domain (FDTD) technique, used in studying the propagation modes due to interaction with single trees, starting from primary models for leaves, trunk and branches. Re-radiation fields in 3D of the entire tree are predicted by combining the effects of the single elements forming the tree. The model is shown to be capable of predicting the near-field radiated signal and the radar cross section (RCS). RCS predictions yielded good agreement with measurements and have provided a good basis for a planning model capable of accounting for single trees in the radio path. Overall, the thesis contributes new information, results and models which are very useful in radio system planning and design of broadband wireless communication services.