The direct current (DC) in optical orthogonal frequency division multiplexing (DCO-OFDM) scheme is commonly adopted in light fidelity (Li-Fi) technology as it offers a spectrally efficient solution. A prior study adopted a machine learning (ML)-based solution to predict the optimum DC bias using key parameters, including the statistical properties of the OFDM transmitted signal and a polynomial regression model. However, the model's robustness decreased when the data structure was shuffled, indicating limited generalization to unseen data. This study builds upon that work by utilizing the same dataset and improving the prediction model with advanced ML tools, such as the LazyPredict algorithm (LPA), to systematically evaluate and select a regression model. A robust ML regressor selection process is proposed to ensure the reliability of predictions. Additionally, a comprehensive data analysis is conducted to assess the importance of features affecting the optimum DC bias. The results demonstrate that the ensemble learning algorithm, Random Forest (RF), outperforms other models with an R-squared of 0.953 and an RMSE of 0.233. A Friedman statistical test was applied to validate the results over five iterations of model training. Furthermore, hyperparameter tuning and bootstrap sampling were employed to conduct a deeper investigation into the model's performance and stability. The proposed model significantly enhances the accuracy and robustness of DC bias prediction compared to previous approaches, ensuring consistent performance across different data distributions.

This article proposes the design, simulation, fabrication and testing of a beam steerable antenna array using a 4 x 4 Butler Matrix feed network technique. The proposed antenna is developed to operate at the X band frequency range. The integrated antenna array and the Butler Matrix feed network operating at 10 GHz has a bandwidth of approximately 2 GHz with an achieved gain of 12 dBi. The main beam pattern of the antenna array is switched between four directions (+10 degrees, - 38 degrees, +38 degrees, - 10 degrees) providing an effective coverage of over 100 degrees. The proposed antenna has the advantages of low cost, easy fabrication, and simplicity.

High frequency Over-the-Horizon Radar (OTHR) provides an economical means to track non-cooperative air targets over large expanses of land and ocean. Because of dynamic ionospheric conditions in polar regions, any OTHR would necessitate a system where the operating frequencies and elevation angles change periodically to maintain constant detection of targets downrange. In this regard, an accurate electron density model is necessary for the purpose of improving operational OTHR and OTHR planning/design. Observations over recent years have established that large-scale electron density structures are a common feature of the polar cap F-region ionosphere. These structures take the form of convecting patches and arcs of enhanced electron density which form tilted reflection surfaces for HF radiowaves, allowing off-great circle propagation paths to be established. Numerical ray tracing has been employed to simulate the effects of these structures on the ray paths of the radiowaves. The effect of frequency monitoring system for OTHR due to the presence of patches of enhanced electron density within the polar cap ionosphere has been studied in this paper.

Vehicle to vehicle (V2V) communication plays a key role in the safety of autonomous vehicles. Therefore, practical tests in many environments are essential. Due to the relatively low altitude of antennas on connected vehicles, any change in the environment is expected to affect the performance of wireless systems from vehicle to vehicle (V2V). This study presents measurements of the impact of vehicle size on the Doppler spreads in V2V radio channel at 5.9 GHz. The measurements were conducted at an irregular three-way intersection where one side includes buildings and the other two sides consist of large open areas surrounded by a row of trees and a metal fence. Vehicles were classified into three groups: small passenger cars (Group A), medium vehicles (Group B), and larger vehicles (Group C). The results demonstrate that the size of the vehicle and the distance between transmitter and receiver are important factors in determining the impact on the V2V link.

Vehicle to vehicle (V2V) communication plays a key role in the safety of autonomous vehicles. Therefore, practical tests in many environments are essential. This study presents measurements of signal strength and Doppler of a 5.9 GHz V2V channel at a road junction in Leicester city centre. The signal shows variations which depend on the size, speed and location of vehicles relative to the line of sight between the stationary transmitter and receiver. Disturbances in the received signal from different types of vehicles have been studied as they pass through the V2V channel resulting from transmitter and receiver positioning opposite sides of the road. The results showed that the size of the vehicle is an important factor when looking at the impact on a V2V link.

Spacecraft communication systems operating at X band are strongly affected when the propagation path passes close to the Sun (which can be characterized by the Sun‐Earth‐Probe, SEP, angle). In this paper, a channel model that can generate a time series of signal amplitude and phase, and hence the signal in‐phase and quadrature values, is presented. For X band, the channel model reproduces the observations well for SEP > 0.8°. While the model has not been experimentally validated at SEP angles below 0.8°, the behavior is consistent with measurements reported in the literature and is therefore likely to be suitable for use, with caution, in that region too.

Some results so far achieved in the framework of the HELIOS (Highly rEliable Links during sOlar conjunctions) Project, founded by the European Space Agency (ESA), are presented. The purpose of the project is the definition of a TT&C communication subsystem architecture (including both ground and space segments, as well as operational methods) being robust to impairments due to superior solar conjunction, especially when the Sun-Earth-Probe angle is below 5 degrees.

HF communications can be difficult in the polar regions since they are strongly influenced by space weather events. Airline communications within the polar regions rely on HF communications and improved nowcasting and forecasting techniques in support of this are now required. Previous work has demonstrated that ray tracing through a realistic, historical ionosphere provides signal coverage in good agreement with measurements. This paper presents an approach to providing a real-time ionospheric model by assimilating TEC measurements and validates it against observations from ionosondes.

Commercial airlines began operations over polar routes in 1999 with a small number of proving flights. By 2014 the number had increased to in excess of 12,000 flights per year, and further increases are expected. For safe operations, the aircraft have to be able to communicate with air traffic control centres at all times. This is achieved by VHF links whilst within range of the widespread network of ground stations, and by HF radio in remote areas such as the Polar regions, the North Atlantic and Pacific where VHF ground infrastructure does not exist. Furthermore, the Russian side of the pole only has HF capability. This has created a demand for improved HF nowcasting and forecasting procedures to support the polar operations, which are the subject of this paper.

Spacecraft communication systems operating at X-band are strongly affected when the propagation path passes close to the sun (characterised by the Sun-Earth-Probe, SEP, angle). In this paper, a channel model that can generate a time-series of signal amplitude and phase is presented. The channel model reproduces the observations well for SEP>2° and, with some caveats, works at smaller values of SEP.

Evaporation ducts are a particular type of atmospheric duct that predominantly exist over large sections of water. These ducts can cause anomalous propagation of radio signals. The most popular method of evaluating the height or strength of evaporation duct is the Paulus Jeske Method. This research provides analysis of annual, seasonal, monthly, daily and hourly trends of occurrence of evaporation ducts in English Channel. It is believed that such extensive insight into these ducts has not been provided before. The annual mean duct height is 7.3m. The seasonal trend shows higher values in autumn and lower in summer. Further analysis shows that occurrence of maxima is possible at any time of the year. Also, there are indications of enhancements in received signal levels due to the occurrence of evaporation ducts, despite the fact that dominant propagation phenomenon was diffraction. The findings of this study improve the understanding of over-sea propagation.

The authors have previously reported on the development of an HF propagation model for signals reflected from the northerly regions of the ionosphere, and its validation by comparison with measurements made over a number of paths within the polar cap, crossing the auroral oval, and along the mid-latitude trough. The model incorporates various features (e.g. convecting patches of enhanced plasma density) of the polar ionosphere that are, in particular, responsible for off-great circle propagation and can lead to propagation at times and frequencies not expected from on-great circle propagation alone. Currently, the model drivers include ionosonde measurements and geomagnetic data from a period of several days spanning the time of interest. We have previously only examined the propagation effects on a historical basis, and have achieved good agreement between measurements and simulations.

Previous studies on anomalous, over-sea propagation have been either focused on single links employing space/antenna diversity or on point-to-multipoint links, usually involving single frequency. Measurements on two co-linear, trans-horizon paths (50km and 140 km long) over the English Channel have been made over periods in excess of a year in order to investigate the propagation characteristics of VHF and UHF signals propagating over the sea. The setup comprises a transmitter located on Jersey and receivers on Alderney and Portland. Signal strength enhancements have been observed on both paths, occurring for different percentages of time between 12% and 19%. UHF signals show greater magnitude of signal level enhancement and longer link shows greater degree of signal level enhancement.

This study focuses on hourly breakdown of received signal strength enhancement events, short-time fading characteristics during and outside enhancement and co-site and co-frequency enhancements on two over-sea, trans-horizon paths. Hourly breakdown of enhancement events reveals a diurnal pattern, with a greater percentage of enhanced signal strengths predominantly occurring during late afternoon/evening periods and the least during morning. Generally, signal strength enhancement tends to reduce the short-time fading. Concurrent as well as isolated signal strength enhancements at both the receiving sites/frequencies have been observed, which can have implications in radio network interference management.

Observations of the effect of polar patches (fast moving regions of strong ionization) on the Doppler and time of flight behavior of HF signals propagating in the polar cap ionosphere are presented. The observed patch-induced characteristics of the HF propagation have successfully been reproduced using a simple model of the movement of the patches, thereby allowing the trajectory and velocity of a patch to be estimated. The increased ionization in the patch allows higher frequencies to reflect than those that would normally be expected. A comparison of the fluctuations in the measured TEC data (a proxy for the presence of patches) with the detection of HF signals has provided some evidence for this relationship.

The northerly ionosphere is a dynamic propagation medium that causes HF signals reflected from this region to exhibit delay, Doppler shifts and Doppler spreads that significantly exceed those observed over mid-latitude paths. These Doppler effects are due to large-scale motion of the ionosphere and/ or the turbulent motion of plasma irregularities that are a common feature of the auroral and polar cap ionospheres. From the perspective of communications systems, such large delay and Doppler spreads can result in a significant degradation in system performance (e. g. in achievable data throughput).

Information on delay and Doppler spreading is of importance to designers and operators of digital communication systems within the HF band since modem performance rapidly deteriorates when delay and Doppler spreads exceed system dependent thresholds. The northerly ionosphere is a particularly challenging region in this regard as delay and Doppler spreads often significantly exceed those for similar length paths at midlatitudes. This paper presents results from an extensive set of measurements undertaken over three northerly paths: two subauroral paths aligned along the midlatitude trough (Uppsala to Bruntingthorpe and Nurmijärvi to Bruntingthorpe) and one path (Qaanaaq to Ny‐Ålesund) contained entirely within the polar cap. Measurements span the period from the 2009 solar minimum to July 2012, close to solar maximum and 2001, close to the previous solar maximum.