This thesis is concerned with the statistical studies of microwave signals received from beyond the radio horizon. A measurement campaign has been conducted in collaboration with Rutherford Appleton Laboratories and CNET (§) in France in order to collect propagation data across the English channel. The transmitter sites were located at Lannion and Cap d'Antifer in the North French coast and the receiver sites at HighcIiffe and Portsmouth in the South of England. Several original findings have been obtained by the research. For the two types of path considered of 155 and 250 km respectively, the cumulative distributions of transmission loss level L display two separate regions; one represents the troposcatter region and the other one represents the anomalous region. The separation between the two regions occurs between 10 and 20 dB below the free space path loss irrespective of the type of path considered. The two regions appear to be well modelled by two distinct normal distributions with their particular mean and standard deviation. The cumulative distributions of the transmission loss derived from measured data were also compared with prediction models available from Ii tera ture. The duration D of exceedance or enhancement above signal level threshold was investigated in terms of the joint statistics of transmission loss and duration of exceedances Prob (D' ~ DIL' s L). Two different distributions were analysed and modelled; the number distribution Pn and the time distribution PI. P,dDIL) represents the fractional number of cases where durations have been greater than D once the path loss is less than L, whereas PdDIL) 5ives the relative time with path loss less than L due to all durations greater than or equal to D. It is found that both Pn and PI are well modelled by log-normal distributions for all types of propagation conditions and these distributions are very skewed. For PI the (§) Centre Nationale D'Etudes des Tclecomunications, Paris and Lannion Laboratories mode computed can range from fractions to several minutes whereas the median can range from many tens of minutes towards the troposcatter levels down to a few minutes close to the free space loss threshold of L = 0 dBf. The logarithm of the median duration of Pt was found to be linearly related to L (dBt). As regards the standard deviation, an (lnD) and at(lnD) are very close in agreement with the theory and range from 1 to 4 when D is in minutes. They were also found to be linearly related to L (dBD. The research has shown that there is a strong seasonal dependence of the path loss and the monthly attenuation may vary over a wide range throughout the year, with variation being greatest on the longer path. The research has also shown that during a 24 hours period there is, in general, a rise in the probability of exceeding a given level at around 1000 GMT and this probability reaches its peak at around 1900 GMT to then begin to decrease at about 2000 GMT. This pattern is much more pronounced in the summer months than in the winter months. The daily variation is also more pronounced for the shorter paths than for the longer paths. The time between 11 no to 1900 GMT appears to be the strongest period of potential interference. The research has included a preliminary analysis of the correlation between signal level strength and the local surface meteorological parameters: temperature, water vapour pressure and atmospheric pressure. As expected good correlation with temperature was observed particularly with weekly averaged maximum daily values. However; atmospheric pressure is the most dominant parameter on signal enhancements. Finally, short term fading of clear air troposcatter signals was investigated and the power spectral density analyses show significant components up to about 5 Hz.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:281113 |
| Date | January 1991 |
| Creators | Tawfik, Abdulkarim Naji |
| Publisher | University of Portsmouth |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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