With the rapid evolution of communication technologies, high-speed optical wireless applications under the water surface as a replacement or complementary to the conventional radio frequency (RF) and acoustic technologies are attracting significant attention from the researchers. Since underwater turbulence (UWT) is an inevitable impediment for a long distance underwater optical wireless communication (UOWC) link, mixed RF-UOWC is being considered as a more feasible solution by the research community. This article deals with the secrecy performance of a variable gain relay-based mixed dual-hop RF-UOWC framework under the intercepting attempt of a potential eavesdropper. The RF link undergoes Generalized Gamma (GG) fading distribution, whereas the UOWC link is subjected to mixture Exponential Generalized Gamma (mEGG) distribution. The eavesdropper is capable of wiretapping via a RF link that also experiences the GG fading. The secrecy analysis incorporates the derivations of closed-form expressions for strictly positive secrecy capacity, average secrecy capacity, and exact as well as lower bound of secrecy outage probability in terms of univariate and bivariate Meijer's G and Fox's H functions. Based on these expressions, impacts of heterodyne and intensity modulation/direct detection techniques along with weak, moderate, and severe UWT conditions due to air bubbles, temperature, and salinity gradients are quantified. To the best of authors' knowledge, the proposed model is the first of its kind that addresses the secrecy analysis of a temperature gradient RF-UOWC system along with air bubbles, as opposed to the existing models that considered thermally uniform scenarios only. Finally, the derived expressions are verified via Monte-Carlo simulations.

Post commercial deployment of fifth-generation (5G) technologies, the consideration of sixth-generation (6G) networks is drawing remarkable attention from research communities. Researchers suggest that similar to 5G, 6G technology must be human-centric where high secrecy together with high data rate will be the key features. These challenges can be easily overcome utilizing PHY security techniques over high-frequency free-space or underwater optical wireless communication (UOWC) technologies. But in long-distance communication, turbulence components drastically affect the optical signals, leading to the invention of the combination of radio-frequency (RF) links with optical links. This article deals with the secrecy performance analysis of a mixed RF-UOWC system where an eavesdropper tries to intercept RF communications. RF and optical links undergo $\eta -\mu$ and mixture exponential generalized Gamma distributions, respectively. To keep pace with the high data rate of the optical technologies, we exploit the antenna selection scheme at the source and maximal ratio combining diversity at the relay and eavesdropper, while the eavesdropper is unaware of the antenna selection scheme. We derive closed-form expressions of average secrecy capacity, secrecy outage probability, and probability of strictly positive secrecy capacity to demonstrate the impacts of the system parameters on the secrecy behavior. Finally, the expressions are corroborated via Monte Carlo simulations.

This research focuses on the computational design, prototype development, and experimental validation of a compact microwave antenna for orthopaedic applications in biomedical regimes. The unique resonant structure comprises an embedded double negative (DNG) metamaterial (MTM)-inspired slotted stub, designed and simulated in the CST Microwave studio environment, as well as verified through simulation in Advanced Design Systems (ADS) software. This adaptation of the design technique improves the antenna’s performance, as demonstrated in the design and result analysis section of the research paper, which includes experimental comparison results from previously developed antenna prototypes. To make the design suitable for rapid industry prototyping, Rogers RT5880 substrates have been used with a standard substrate height of 0.79 mm (loss tangent of 0.009 and dielectric constant of 2.2). However, the antenna’s effectiveness was also compared with other commercially available variations of Rogers RT5880 substrate material (0.508 mm, 4.575 mm, and 3.175 mm) while transmitting the input power into the electromagnetic wave. Antenna performance for measuring bone tissue penetration was checked, and a gain of 3.5 dB and a distributed surface current value of 295 A/m were found to be satisfactory for the early stage bone fracture diagnosis application. The measured radiating efficiency was 97.5%. The far-field experiment measurements show an omnidirectional radiation pattern and a well-performed return loss well below -10 dB in the operating region of 3.8 GHz to 4.8 GHz. During simulation, the stacked bone phantom environment’s operational characteristics demonstrated satisfactory gain, directivity, and efficiency performance, which are also compared in this research analysis.

The underlay cognitive radio-based hybrid radio frequency/free-space optical (RF/FSO) systems have emerged as a promising technology due to their ability to eliminate spectrum scarcity and spectrum under-utilization problems. The physical layer security of such a network with a primary user, a secondary source, a secondary receiver, and an eavesdropper is therefore examined in this work. In this network, secret communication occurs between two reliable secondary peers over the RF and FSO links simultaneously, and the eavesdropper can only overhear the RF link. In particular, the maximum transmits power limitation at the secondary user as well as the permissible interference power restriction at the primary user are also taken into consideration. All the RF and FSO links are modeled with $\alpha$ - $\mu$ fading and Málaga turbulence with link blockage and pointing error impairments. At the receiver, the selection combining diversity technique is utilized to select the signal with the best electrical signal-to-ratio (SNR). Furthermore, to examine the secrecy performance taking into account the effects of each system parameter, closed-form expressions for the secrecy outage probability and effective secrecy throughput are derived. The resultant expressions are finally verified by Monte-Carlo simulations.

Osteoporosis, a medical condition distinguished by the reduction of bone mineral density and the consequent degradation of bone structure, poses significant health scrutiny, especially in elderly individuals. The fractures and hospitalisations that occur worldwide are a direct consequence of socioeconomic conditions. Prompt intervention and therapy are crucial for the early stage osteoporosis diagnosis and the detection of bone fractures. Several established methods, including single-energy X-ray absorptiometry (SXA), dual-energy X-ray absorptiometry (DEXA), non-invasive magnetic resonance imaging (MRI), quantitative CT (QCT), quantitative ultrasonography (QUS), and others, are now being employed for diagnostic purposes. Despite the availability of these technologies, microwave sensing has emerged as a feasible approach for early identification of osteoporosis and bone fractures, owing to its numerous advantages. This review paper aims to provide a comprehensive summary of recent advancements in microwave sensing techniques specifically aimed at detecting osteoporosis and diagnosing bone fractures. Various measuring, sensing, and antenna scattering parameter post-processing approaches are classified and assessed for their effectiveness in identifying alterations in bone characteristics associated with osteoporosis. These techniques encompass both traditional non-invasive and invasive scanning procedures, along with more sophisticated machine learning-based prediction approaches. The reliability and efficiency of microwave sensing as a tool for the early detection of osteoporosis stage and bone fractures has been demonstrated by this work. This generates prospects for enhanced patient care and more intricate diagnosis and management strategies.