Digital radio and its application in the HF (2-30 MHz) band
The propagation environment at high frequencies (HF, 2-30 MHz) has a significant impact on the performance of radio systems (especially data communications). However, the ability to communicate information over very long ranges using ionospheric propagation paths without any intermediate infrastructure makes the use of HF attractive for many applications. In order to increase the utility of HF communications there is a strong desire to increase HF data rates. Currently data rates of up to -2400 bps can be reliably achieved in standard 3 kHz HF channel allocations. Whilst further increases in data rate within the confines of these narrowband frequency allocations is likely, the use of larger bandwidths (contiguous or otherwise) appears to offer potential for much greater throughputs. This requires a greater understanding of the characteristics of wideband channels and also requires transmitting and receiving equipment capable of wideband/multi-channel operation. New waveforms have been proposed for the transmission of higher data rates in extended channel bandwidths (6 kHz). The results of laboratory measurements and analysis of data collected during on-air trials of a number of 16 kbps waveforms are presented. Analysis indicates that operation over surface wave and benign skywave channels is possible, demonstrating the benefit of exploiting greater channel bandwidths. Suitable architectures for the implementation of wideband and multi-channel digital HF radios (software radios) have been investigated. The work presented indicates that it is now possible, for the first time, to construct high performance, direct sampling wideband digital HF receivers. In such a receiver the entire HF band is digitised and then all subsequent processing is undertaken digitally. Conceptually this would allow an arbitrary number of channels to be simultaneously received using a single RF frontend and digitiser. With careful design performance comparable with that of the high performance conventional super-heterodyne single channel receivers can be obtained. A prototype wideband multi-channel digital HF transceiver with this architecture has been implemented and its performance shown to agree with that predicted. A particular challenge in complex systems such as software radios is the deployment of software across a number of heterogeneous processors. A new asynchronous, eventbased, processing architecture which employs messaging to allow processing tasks to be effectively distributed across a multiple processors and buses is proposed. It has been implemented on the digital transceiver platform and its effectiveness has been demonstrated. A new low-power pulse-compression oblique HF ionospheric sounder, known as WHISPER, has been developed. This sounder has been implemented as a software application on the wideband HF digital transceiver. Waveforms suitable for making wideband (-80 kHz) measurements of the channel time varying complex impulse response have been designed. These have been used to make measurements on a 170 km path in the UK during Spring 2001. The results of these measurements have been analysed and confirm the ability of the sounding instrument to measure the channel scattering function and the amplitude and phase within individual modes. A number of possible directions for further analysis, pertinent to the design of wideband HF modems, have been proposed.