Trans-horizon Link Research Articles

Trans-horizon propagation at 42 GHz

Results from a measurement programme for trans-horizon links are presented and compared with the ITU-R REC452-8 model. Reasonably good agreement between measured results and the model is found, although there are some doubts over the contribution from ducting on short paths over hilly terrain.

A spread spectrum communications channel sounder

The design of a microwave channel probing analyzer is presented and discussed. Special attention is paid to the characteristics of the transmission path to be measured; the choice of spread spectrum probing signal; the signal processing methods used; modulation, demodulation, and synchronization at the transmitter and receiver sites; the path model to be fitted to the measured data and finally, the impact of all of these issues upon the necessary hardware and software. Following a formulation of the channel-probing problem, transfer function regression techniques are reviewed to illustrate the advantages of periodic averaging over established correlation methods. The construction of an experimental sounder based on these methods is described. The equipment has been used to measure the time-variant frequency-selective channel dynamics of a short (158 km) troposcatter transhorizon link at a frequency of 11.647 GHz over a 31.25-MHz bandwidth. Off-line, nonlinear spectrum analysis, based on Prony's method and singular value decomposition, is used to estimate parameters of a high-resolution delay model that accurately reflects the underlying ray structure of the transmission path.

Path loss statistics and mechanisms of transhorizonpropagation over a sea path

Carrier amplitude data at 11.6 GHz captured on four parallel transhorizon links over the sea have been divided into four groups according to the identified dominating propagation mechanisms. The statistics of path loss are each found to be lognormal. The rate of occurrence of the mechanisms is investigated relative to transmitter/receiver heights, diffraction and evaporation ducting.

Obstacle diffraction and site shielding under varying incident illumination

A trans-horizon microwave radio link is installed and signal levels have been monitored to provide data on obstacle-diffraction loss and site shielding, under conditions of varying field distribution incident on the shielding obstacle. Both spatial and temporal variations in the incident signal are observed, owing to changes in the refractive-index profile and the occurrence of ducts in the troposphere brought about by certain climatic conditions. Continuous monitoring of the received signal at points on the front of the shielding obstacle, and in its radio shadow, has yielded valuable data on the diffraction-loss behaviour in trans-horizon links. Results indicate that the instantaneous diffraction loss can attain much smaller values than those experienced in uniformly illuminated line-of-sight links. However, a more detailed analysis of the results shows that the shielding effectiveness of the obstacle remains basically unchanged, and that site shielding is an effective interference reduction technique.

Anomalous transhorizon propagation and meteorological processes of a multilink path

An event‐based approach is developed and applied to 3 years of data of a four link transhorizon path over the English Channel (La Manche). The signal enhancements (events) observed on the chart recordings during anomalous propagation are selected and, according to their dynamical signatures, categorized into three main patterns of ducting, layer reflection, and high k factor diffraction. The statistical analysis carried out on this event basis gives the correlation between the signal levels of different links and the cross correlation between signal levels and local N index derived from surface meteorological monitoring. The results reveal the fundamental properties of the anomalous propagation regimes. Using case studies of several interesting events, the meteorological processes and resultant impact on transhorizon transmission are also explained. This extensive work contributes to a better understanding of anomalous propagation mechanisms present in transhorizon links.

Dynamic behaviour of radio channels due to trans-horizon propagation mechanisms

To study the propagation mechanisms of trans-horizon radio links, the Letter proposes an event-based approach. Data from one year captured on a trans-horizon link of 156 km are divided into different groups according to the signal amplitude signature of the chart recordings. The propagation mechanisms for each group are investigated using the cross-correlation between received signal amplitudes along the same path. These amplitudes are obtained using two different transmitter antenna heights.

Measured statistics of transmission loss due to ducting on trans-horizon links at 6.4 and 7.4 GHz

During a period of 6 years, propagation measurements were performed by the Dr. Neher Laboratories in order to collect data of statistical evidence on trans-horizon microwave links. These data are of importance for the evaluation of frequency sharing possibilities, which are studied because of the increasing demand of frequency channels above 1 GHz. The results of the measurements are compared with the data on basic transmission loss, following from theCcirprediction methods.

Interference aspects of off-shore terrestrial microwave networks

In an off-shore environment, as a general rule, communications need to be established directly between terminals with no provision for intervening repeaters. In planning terrestrial microwave communication networks, this imposes a requirement to use trans-horizon links to a greater extent than in land-based communications.The attendant problems of interference and practical methods for reducing such interference in terms of frequency planning, equipment design, and network power control are discussed.

V.H.F and u.h.f. field-strength variations corresponding to river beds

The mechanisms of propagation over land and over sea, respectively, are reviewed to detect the causes of the field-strength maximum in the afternoon, in the former case, and of the corresponding minimum in the latter case. The analysis of the received power for two transhorizon links over land along a river bed indicates an effect of the river bed on the afternoon field-strength rise, and a three-part nocturnal trend of field strength. These properties are explained in terms of evaporation and condensation processes of water vapour near the ground, and also by-the superposition of the continental and oceanic types of diurnal variations of field strength. They are in agreement with fading characteristics on line-of-sight links along river beds.