The presence of abnormally high signal levels beyond the horizon may be exploited for communication purposes. On the other hand, these high signal levels pose the threat of spatial co-channel interference. A long-term detailed investigation into the wideband characteristics of a typical transhorizon link was instigated by the Radiocommunications Agency (UK) to permit the compilation of the hitherto unknown channel parameter statistics. This thesis describes that investigation conducted at a frequency of 11.64 GHz on a 160 km transhorizon sea path between Cap d' Antifer (France) and Fort Widley (England). A channel sounder with a bandwidth of 31.25 MHz which allows the implementation of an automated time-critical continuous data acquisition strategy, is described in detail. The parametric estimation of time-domain model parameters, from measured channel transfer functions, in the context of multi path propagation is discussed. The Singular Value Decomposition Prony and Bayesian techniques are described in detail, having been chosen in preference to Fourier analysis because of their higher resolution potential. The deri vation and implementation of a novel Bayesian algorithm which incorporates prior knowledge concerning the channel parameters is presented. Multipath channel simulation data using differing channel models and varying signal-to-noise ratios has been generated and this data, together with field data, has been used to carry out a systematic and critical comparison between the Bayesian and the SVD_P approaches. It is shown that the Bayesian algorithm gives more accurate estimates of channel parameters, namely amplitudes and delays, especially when the signal-to-noise ratio is less than 30 dB. An 8 month long measurement campaign has generated a 110 Gbyte database of channel transfer functions. The estimated channel impulse response and the associated signal levels are used to discuss the underlying propagation phenomena on the link. It is shown that ducting propagation conditions occurred for a larger than expected period totalling 16% of the measurement time. This has been attributed to the dominance of surface ducting conditions in the summer. Troposcattering was found to be more prevalent in the winter. The risk of the channel being a source of interference was found to be greatest at about 20:00 GMT due to the presence of advection ducts and a minimum at about 10:00 GMT. Delay spread, Doppler spread, coherence bandwidth and fade depths have also been estimated from the database. The results show that there is a very strong correlation between these parameters and the wideband signal level. The median values of 35 ns and 6 Hz for the delay and Doppler spreads respectively, reveal that the transmission medium can be considered to be slowly varying. Coherence bandwidths greater than 250 MHz have been observed for 1 % of the time with a signal level of -2.7 dBf. Such a high value implies that to achieve acceptable interference levels on spatially aligned links, large frequency separations may well be required. Comparison with earlier CW investigation results (COST210 and follow-up research) show that the statistics of signal levels acquired from CW measurements also apply to wideband signals. However, there is a significant discrepancy between CW and wideband fading statistics due to frequency selective fading.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:242520 |
| Date | January 1998 |
| Creators | Ndzi, David Lorater |
| Publisher | University of Portsmouth |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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