This paper presents a new systematic survey of the free-space optical (FSO) system, highlighting its operations and applications in outdoor optical communication underlying its interaction with the environment. As the application portfolio of FSO technology grows, so does the need for a clear classification for FSO link configurations. Most existing surveys and classifications are single-level classifications using traditional survey methods, and thus not inclusive enough to accommodate recent and emerging changes and developments of different FSO link configurations and systems. To that end, this paper adopts a novel methodology based on statistical analysis using a natural language processing approach (NLP) to analyze, visualize, and interpret the researchers' trends of 5075 relevant literature published documents (journal papers, conference papers, books, etc.) from 2017 to Feb-2024. The process of summarizing and analyzing is encompassed by adopting hierarchal multi-level classifications for the key performance evaluations, motivations, challenges, and contributions related to FSO systems. The survey is started by understanding the key performance of FSO such as matrices models and mathematical models. Then, the survey process is followed by investigating the outdoor FSO applications (urban, satellite, smart city, military, healthcare, disasters, and sensing applications). We have relied on listing and dividing the outdoor FSO networks into sub-networks based on the FSO’s architectural structures (point-to-multipoint, mobile network, long-range transmission and heterogeneous). In turn, outdoor impairments are classified into two categories: 1) environmental factors (weather and atmospheric conditions) and 2) component systems (source types and mechanical issues). We demonstrate the impact of these impairments in terms of the optical phenomena (effects), such as scattering, reflection, refraction, absorption, shadowing, and consequences (outcomes), such as pointing error, misalignments, beam divergence, etc. Finally, multi-technical solutions are listed and analyzed based on the researchers' contributions, such as modulation, detection techniques, relay, array, and retroflectors. This paper is the first paper that adopts the statistical analysis approach for studying the FSO in outdoor applications, making this approach a good choice for future researchers to prioritize their decisions about future trends and target their improvements.

In this paper, which represents a fundamental step in ongoing research, a new smart low-energy dual-function half-mode substrate integrated waveguide cavity-interdigital capacitor (HMSIWC-DIC) antenna-based sensor is developed and investigated for remote frost and wildfire detection applications at 5.7 GHz. The proposed methodology exploits the HMSIW antenna-based sensor, a microfluidic channel (microliter water channel (50 μL)), interdigital capacitor technologies, and the resonance frequency parameters combined with machine learning algorithms. This allows for superior interaction between the water channel and the TE101 mode, resulting in high sensitivity (∆f/∆ε = 5.5 MHz/ε (F/m) and ∆f/∆°C = 1.83 MHz/°C) within the sensing range. Additionally, it exhibits high decision-making ability and immunity to interference, demonstrating a best-in-class sensory response to weather temperature across two ranges: positive (≥0 °C, including frost and wildfire) and negative (<0 °C, including ice accumulation). To address the challenges posed by the non-linear, unpredictable behavior of resonance frequency results, even when dealing with weak sensor antenna responses, an innovative sensory intelligent system was proposed. This system utilizes resonance frequency results as features to classify and predict weather temperature ranges into three environmental states: Early Frost, Normal, and Early Wildfire, achieving an accuracy of 96.4%. Several machine learning techniques are employed, including artificial neural networks (ANNs), random forests (RF), decision trees (DT), support vector machines (SVMs), and Gaussian processes (GPs). This sensor serves as an ideal solution for energy management through its utilization in RF-based weather temperature sensing applications. It boasts stable performance, minimal energy consumption, and real-time sensitivity, eliminating the necessity for manual data recording.

Beyond data transmission, antennas have recently been utilized as sensors, offering the advantage of reducing hardware requirements and power consumption compared to systems where sensors are separate from antennas. Patch antennas, in particular, are widely used across various applications, including sensing, due to their attractive features like compact size and conformability. In addition, they can be easily designed in different ways to sense variations in certain variables. Adding a slot to the patch antenna introduces several advantages, including multiband, wideband operation, and improved impedance bandwidth. Slots also provide a concentrated region of electromagnetic fields, which increases the antenna’s sensitivity for sensing and detection purposes. In this paper, a rectangular patch antenna with a cross slot is designed and proposed for water, ice, and frost detection. Detection is achieved by measuring variations in the resonant frequency in response to water, ice accumulation, and frost. The results indicate that the proposed antenna can detect both water and ice accretion with a frequency shift of up to 1.538, 0.358, and 0.056 GHz, respectively, which reflects good sensitivity levels of the antenna. The effect of the slot on strengthening the near electric field and antenna sensitivity is discussed in this paper. The antenna is fabricated and measured and the indicators of each detection scale have been extracted. The proposed antenna has a simple structure and a small size of (40 × 40 × 1.53 m³). In addition, it can be precisely used to sense different environmental parameters such as frost and ice. Thus, it can serve as a strong candidate for detecting natural disasters like frost damage. Furthermore, the findings in this paper offer valuable insights into how the presence and structure of slots influence the sensitivity response of patch antennas, supporting ongoing research in this field.

In this paper, a flexible rectangular loop antenna is designed and proposed for ice, frost and wildfire detection. The antenna is composed of two concentric rings made of a flexible conductor. The proposed antenna was responsive to different materials based on distinct shifts in the resonant frequency, which was employed to differentiate between these materials. The antenna provides a wide response and sensitivity range to detect ice or frost with relative permittivity close to 3 and water with relative permittivity close to 72 at the same time. This wide sensitivity level is attributed to the internal loop which works with the external ring to form a capacitor with a capacitance varying with the relative permittivity of the material under test. The internal loop also enhances coupling with the material under test and fine-tunes the antenna’s response. The antenna achieved a maximum radiation efficiency of 97.1% and gain of 2.83 dBi at 2.45 GHz across the tested scenarios involving frost and ice. It also obtains a maximum radiation efficiency and gain of up to 6.67% and −8.27 dBi, respectively, for water at 40 °C and 50 °C, respectively. Additionally, the antenna preserves the same direction of maximum radiation for all of the investigated materials, which minimizes constraints on the receiving antenna’s radiation pattern requirements. The proposed antenna features simplicity, robust performance and a wide sensitivity range over temperatures between 0 and 50 °C, which makes it a good candidate for environmental monitoring.

This paper proposes a free-space optical (FSO) system for early warning and detection of natural disasters. The system consists of multiple sensor nodes equipped with mirrors, motors, and controllers. Under normal weather conditions, signals are transmitted between nodes. However, in the presence of frost, signal reflection is directed to a base station (BS) by adjusting the mirrors’ orientation based on the reflection angle determined using Snell’s law. By monitoring the round-trip time of the signal to the BS, the frost-affected node can be identified. The power received at various nodes is analyzed, considering reflections from mirrors and path attenuation. The results indicate that sufficient power levels can be achieved for six nodes, covering an area of approximately 314.16 km². The total time required to send an alarm signal to the BS is calculated and compared with the systems proposed in the literature. The proposed system demonstrates a time reduction of up to 69.7% compared to systems where the signal traverses all nodes before reaching the BS and a 7% reduction compared to systems employing dedicated transmitters and receivers. The proposed system is easy to deploy in a crop area with a fast response and relatively low power consumption, making it an efficient solution for early warning of frost. To the author’s best knowledge, the proposed system is the first one to exploit mirrors in free-space optics for early warning of frost. The analysis presented in this paper is very helpful for further investigations into using mirrors in FSO systems for early warning and notification of natural disasters such as frost.