Wideband VHF radio channel modelling and characterisation beyond line-or-sight.
2017-02-15T04:38:46Z (GMT) by
The dawn of the information age brought to light technical and economic challenges in delivering broadband telecommunication services to subscribers in rural and remote areas; challenges created by tiny pockets of subscribers scattered over a vast network service area. Dealing with such challenges calls for an appropriate system engineering approach and the utilisation of suitable network building blocks, which could differ significantly from those of the urban areas. The aim of this thesis, therefore, was to make a distinct contribution of knowledge to enable the delivery of broadband telecommunication services to rural and remote communities in a technically-effective and economically-viable manner. The thesis viewed the customer access line as the crucial network building block in determining the optimal architecture of a telecommunication network, based on the fact that maximising the length of customer access line results in minimising the number of network nodes required to serve a given area. A customer access line utilising wideband VHF radio channels, capable of bridging distances of up to 100 km, would make a significant economic difference in deploying telecommunication services in rural and remote areas. For many years, narrowband VHF radio channels have been used to carry voice and low-speed data over long distances in rural and remote areas, but very little or no data is available on the use of wideband VHF radio channels. Therefore, the research focused on wideband VHF radio channel modelling and characterisation over paths extending beyond line-of-sight, where diffraction is the dominant mode of radiowave propagation. This research was mainly concerned with aspects of radiowave propagation, rather than modulation and coding schemes. Conducting radiowave propagation studies over long distances extending beyond line-of-sight is often time consuming and can be extremely expensive. Traditionally, a powerful radio channel sounder is needed for conducting such studies. In this work, a novel inexpensive wide band VHF radio channel sounding technique was developed by the candidate. The technique makes use of a VHF digital television transmitter as a source for wideband sounding signals. The receiving end of the sounder is a special configuration of two synchronised modern swept-tuned spectrum analysers and a crossed-dipole antenna array. This enables the simultaneous measurement of the co-polarised and cross-polarised components of incident radiowaves. The power spectrum traces obtained from both the analysers were then mathematically processed to produce the channel parameters. A field measurement campaign was staged for collecting primary data for eight case studies using a sma1l4-wheel drive vehicle equipped with a 6 m telescopic antenna mast. A specially developed in-situ terrain path profiling tool played a pivotal role in the investigations. In line with the aim of the thesis, the measurements were conducted over ranges from 10 to 100 km. The signals from two transmitter sites, one transmitting on horizontal and the other on vertical polarisation, were used as sources for wide band sounding signals. Field measurements were conducted using antenna height of 7 m above ground level. In conclusion, using the developed radio channel sounding technique, and under the propagation conditions described in this thesis, showed that diffraction alone has little or no effect on the performance of a 7-MHz wide VHF radio channel. Diffraction by terrain obstacles seemed to reduce the effect of multi path. The investigation showed that low antenna height of approximately 7 m at the customer's premises can be used for distance of up to 100 km from the transmitter. The actual data transmission speed possible over a 7-MHz wide VHF radio channel will depend on the adopted modulation and coding schemes.