The Radio Systems Research Group specialises in experimental studies of the influence of the propagation medium on radio systems and on the development of techniques to overcome or to take into account propagation related system performance limitations.

An underlying theme of the projects is the development of a better understanding of the effects of the atmospheric (principally the ionospheric) environment on the propagation characteristics of radio waves, and the development of techniques to account for these effects in radio system planning, development and operation. All of the projects are experimentally based and involve the development, deployment and operation of equipment at a number of sites, currently over-sea paths in the Channel Islands and very long distance paths in the Arctic.

Off-great circle HF propagation and direction finding

Experimental investigations of the accuracy and performance of HF direction finders as a function of signal frequency, propagation mode content, path geometry and ionospheric propagation conditions with particular reference to signals traversing the high latitude region is a long standing interest. A wealth of knowledge has resulted from these experimental investigations into gross deviations from the great circle path (often by up to around 100 degrees), and research is currently being undertaken aimed at incorporating this knowledge into techniques that will be of direct operational application. Recent activity in this area includes the development of ionospheric models coupled with ray tracing studies to enable the impact of the presence of the high latitude electron density features causing the deviations on paths other than those subjected to experimental investigation to be assessed. Since ray-tracing is computationally intensive, a set of rules are being developed through which the influence of the off-great circle propagation mechanisms on specified communications links, radiolocation systems and OTH radars may be estimated.

Combined channel sounding and direction finding

This research combines channel sounding studies undertaken for communications purposes with direction finding in order to investigate the detailed directional structure of the received signals. A novel technique has been developed whereby the directional characteristics of signals from a network of ionospheric channel sounders are measured. In initial tests, this equipment was operated in the UK, Canada and Sweden and several interesting results obtained. Further experimental studies are currently in progress with the receiving equipment located in Kiruna, northern Sweden, and transmitters at Kirkenes, Norway and at Longyearbyen, Svalbard.

Adaptive multi-channel reception

One of the major limitations in the performance of HF long range radio communications operating over paths subject to severe ionospheric distortion (e.g at northerly latitudes) is the imposition of large delay and Doppler spreads on the signal. The magnitude of these dispersions is such as to severely limit the data throughput capabilities of communication systems. Techniques are being investigated whereby the signals from several receivers connected to a spaced element antenna array are combined in such a way as to reduce the apparent level of dispersion by means of spatial filtering. Initial studies have yielded very promising results and a paper on this topic was awarded the 2002 IEE Maxwell Premium Prize. Future developments in this area will include the investigation of the applicability of MIMO techniques to the HF band.

Propagation along the mid-latitude trough

An experimental programme has been undertaken with a transmitter installed in Uppsala, Sweden and a DF system installed at Bruntingthorpe, near Leicester. Direction of arrival measurements, coupled with ray tracing studies, are being undertaken with the aim of improving the ionospheric models of the trough. In addition, the delay and Doppler spread characteristics of the received signals have been measured, information which is of direct relevance to the operation and planning of digital communications systems operating in the HF frequency band.

VHF / UHF propagation over sea paths

Measurements are currently being made to investigate the propagation characteristics of VHF and UHF signals propagating over the sea. Initial studies were undertaken at frequencies around 300 MHz, of particular interest for communications between ships at the extreme limits of propagation. Subsequently, the study has been extended to 2 GHz to aid the development of tools available for the planning and operation of UHF systems. VHF and UHF studies are a diversification of interest and were initially aimed at developing a better understanding of inter-ship communications at distances beyond the line of sight. Subsequently, substantial funding for studies at 2 GHz to answer an ITU requirement in connection with frequency planning requirements has been obtained from Ofcom (formerly the Radiocommunications Agency). As an enhancement to this project, collaborative studies have been established with Professor Ersin Tulunay (Middle East Technical University, Ankara, Turkey) on the use of neural networks to predict UHF signal amplitude based on past amplitude measurements and meteorological data.

COST271 AND COST-296

Over the past 4 years, the group has been actively involved in the EU COST-271 Action entitled The Effects of the Upper Atmosphere on Terrestrial and Earth-Space Communications. (COST- the acronym for European COoperation in the field of Scientific and Technical Research - is an EU intergovernmental network for cooperation in research.) Within this Action, Dr Warrington led Work Package 3.4 on the development of methods and algorithms to minimize the deleterious effect of the ionosphere on terrestrial communications. All of the group's ionospheric work during this period was of direct relevance to the aim of this COST action.

The COST-271 programme ended in August 2004 and, in February 2005, a new Action (COST-296) commenced entitled MIERS - Mitigation of Ionospheric Effects on Radio Systems. Dr Warrington is one of the UK representatives on the Management Committee for this action and is co-leader of a work package on Radar and Radiolocation within the Advanced Terrestrial Systems Working Group.