The fading of signals propagating in the ionosphere for wide bandwidth high-frequency radio systems.

Abstract

The use of High-Frequency (HF) radio-wave propagation in the ionosphere remains prevalent for applications such as long-range communication, target detection and commercial broadcasting. The ionosphere presents a challenging channel for radio-wave propagation as it is a varying medium dependent on a number of external factors. Of the many adverse effects of ionospheric propagation, signal fading is one of the most difficult to eliminate due to its unpredictable nature. Increase in the knowledge of how the ionospheric channel affects the propagating signals, in particular fading of the signals, will drive the continual improvements in the reliability and performance of modern wide-bandwidth HF systems. This is the underlying motivation for the study of signal fading of HF radio-waves propagating through the ionosphere, from both the theoretical and experimental perspectives, with the focus of application to modern wide bandwidth HF systems. Furthermore, it is the main objective of this investigation to address the lacking in the current literature of a simple analytical signal fading model for wideband HF systems that relates the physics of the ionospheric irregularities to the observable propagation effects due to the irregularities, and one that is verified by experimental observations. An original approach was taken in the theoretical investigation to develop an analytical model that combines the effects of signal fading and directly relating them to the ionospheric irregularities that are causing the fading. The polarisation fading model (PFM) is a combination of geometric optics, perturbation techniques and frequency offset techniques to derive expressions for the Faraday rotation of the radio-wave propagating in the ionosphere. Using the same notation as the PFM, the amplitude fading model (AFM) extends the Complex Amplitude concept using perturbation techniques and Green’s functions solution to arrive at a set of expressions that describes the focussing and defocussing effects of the wave. The PFM and AFM, together with expressions for combining the effects of multiple propagation paths, provide a simple analytic model that completely describes the fading of the signal propagating in the ionosphere. This theoretical model was implemented into an efficient ionospheric propagation simulator (IPS) from which simulations of wide bandwidth HF signals propagating through the ionosphere can be undertaken. As an example of the type of results produced by the IPS, for a typical 1200km path in the north-south direction with the ionospheric channel under the influence of a travelling ionospheric disturbance (TID), a 10 MHz radio-wave signal in one-hop path is shown to be affected by polarisation fading with fading periods in the order of minutes, and a fading bandwidth in the order of 100 kHz. Further results generated by the IPS have shown to be consistent with the results reported elsewhere in the literature. The experimental investigation involves the study of signal fading from observations of real signals propagating in the ionosphere, a major part of which is the development of a digital compact channel probe (CCP) capable of operating in dual-polarisation mode, and the characterisation of such systems to ensure that data collected are not compromised by the non-idealities of the individual devices contained within the system. The CCP was deployed in experiments to collect transmissions of HF frequency-modulated continuouswave (FMCW) radio signals from the Jindalee Over-the-Horizon radar (OTHR) in dualpolarisation. Analyses of the collected data showed the full anatomy of fading of signals propagating in the ionosphere for both horizontal and vertical polarisations, the results of which are consistent with that from the IPS and thus verifying the validity of the theoretical model of fading. Further experimental results showed that in majority of the observations polarisation fading is present but can be masked by multi-path fading, and confirming that periods of rapid signal fading are associated with rapid changes in the ionospheric channel. From the theoretical and experimental investigations, the major achievement is the successful development of an efficient propagation simulator IPS based on the simple analytical expressions derived in the PFM and AFM theoretical models of signal fading, which has produced sensible signal fading results that are verified by experimental observations. One of the many outcomes of this investigation is that polarisation diversity has the potential to bring improvements to the quality of wide-bandwidth HF signals in a fading susceptible propagation channel. The combination of an efficient propagation simulator IPS based on theoretical signal fading model and the experimental data collection by the dual-polarisation CCP is a major step in allowing one to fully understand the different aspects of fading of signals propagating in the ionosphere, which sets a solid foundation for further research into the design of wide bandwidth HF systems and the possible fading mitigation techniques.