Effects of haze particles and fog droplets on NLOS ultraviolet communication channels

Non-line-of-sight ultraviolet transmission coverage in non-precipitating, foggy, and rainy weather

Sensitivity to weather conditions is the principal limitation of free-space optical communication. However, for the scattering based ultraviolet (UV) non-line-of-sight (NLOS) communication, the atmospheric scattering effect functions as an attenuation factor and potentially as a performance enhancer. To investigate the UV NLOS transmission coverage under different weather conditions, we employ the Mie Theory in conjunction with classical aerosol and hydrometeor particle models to estimate the absorption coefficient, the scattering coefficient, and the scattering phase function. We then use these atmospheric parameters combined with a range estimation model to determine the coverage of the UV NLOS communication for specified path loss. Simulation results reveal that in non-precipitating weather, poorer visibility correlates with broader coverage. In foggy conditions, the coverage range in light fog exceeds that in fog-free environments; however, as fog intensity increases, the coverage range decreases. Rain enhances the coverage range; and heavier precipitation results in a larger coverage area.

Ultraviolet communication with a large scattering angle via artificial agglomerate fog

Considering strong signal attenuation of the large-angle non-line-of-sight (NLOS) link achieved due to the ultraviolet (UV) scattering properties, we propose to increase the UV communication link gain under a large scattering angle via generating agglomerate fog within a certain range as a secondary light source. In this study, a channel model with locally strong scatterers from agglomerate fog is proposed based on Monte Carlo ray-tracing approaches. Mie theory is adopted to calculate the atmospheric channel parameters, to further evaluate the link gain of a channel under non-uniform atmosphere. The performance of scattering system in the presence of fog conditions depends on the relative positions of the light source to the fog and to the receiver. The link gain results reveal the transmission capabilities for different scattering channel geometries, and give the optimal spray point location within a 10 m communication range. We further establish a foggy NLOS system using an agglomerate fog generator to verify our research in the real environment. The results show that the received signal strength of the NLOS link can be effectively enhanced by constructing scatterers in the atmospheric channel, which significantly decreases the bit-error rate (BER).

Full-duplex wireless deep ultraviolet light communication

With recent advancements in deep ultraviolet (DUV) light-emitting diodes (LEDs) and solar-blind photodetectors, wireless DUV light communication is emerging as a novel technique, which can extend transmission ranges and avoid solar interference. Herein, a full-duplex, real-time wireless light communication system using 275 nm DUV LEDs is proposed. We adopted high-power DUV LEDs and designed a high-speed transmitter, a high-sensitivity receiver, and a main processing unit for the system. Furthermore, the DUV communication system, using a Reed-Solomon (RS) encoder and an on-off keying (OOK) modem with frequency control, achieves a 10 Mbit/s bidirectional data transmission rate within 5 m in free space, while a full-duplex video communication link is formed. The encapsulated DUV communication system described in this Letter provides a feasible scheme for confidential and anti-electromagnetic interference communication in Internet of Things (IoT) applications.

Characteristic Study of Non-Line-of-Sight Scattering Ultraviolet Communication System at Small Elevation Angle

Ultraviolet (UV) communication is considered an effective complement to traditional wireless communication. However, the scattering models of existing non-line-of-sight (NLOS) UV, which are complex, are difficult to combine with the test. In this paper, the single scattering isosceles model with a small elevation angle is proposed first. Then, the relationships between the path loss of single scattering isosceles and elevation angle, emission beam angle, receiving field angle, and transmission distance are studied. Finally, we consider outdoor NLOS UV solar-blind communications test at ranges of up to 100 m and 400 m, with different transmit and receive elevation angles. The results show that the isosceles model is in good agreement with the experiments. In addition, the UV isosceles model exhibits good properties compared with the existing scattering model. The proposed UV isosceles model can be employed as a reference for practical applications in outdoor tests.

An Enhanced VLC Channel Model for Underground Mining Environments Considering a 3D Dust Particle Distribution Model

Underground Mining (UM) is a hostile industry that generally requires a wireless communication system as a cross-cutting axis for its optimal operation. Therefore, in the last five years, it has been shown that, in addition to radio-frequency-based communication links, wireless optical communications, such as Visible Light Communication (VLC), can be applied to UM environments. The application of VLC systems in underground mines, known as UM-VLC, must take into account the unique physical features of underground mines. Among the physical phenomena found in underground mines, the most important ones are the positioning of optical transmitters and receivers, irregular walls, shadowing, and a typical phenomenon found in tunnels known as scattering, which is caused by the atmosphere and dust particles. Consequently, it is necessary to use proper dust particle distribution models consistent with these scenarios to describe the scattering phenomenon in a coherent way in order to design realistic UM-VLC systems with better performance. Therefore, in this article, we present an in-depth study of the interaction of optical links with dust particles suspended in the UM environment and the atmosphere. In addition, we analytically derived a hemispherical 3D dust particle distribution model, along with its main statistical parameters. This analysis allows to develop a more realistic scattering channel component and presents an enhanced UM-VLC channel model. The performance of the proposed UM-VLC system is evaluated using computational numerical simulations following the IEEE 802.1.5.7 standard in terms of Channel Impulse Response (CIR), received power, Signal-to-Noise-Ratio (SNR), Root Mean Square (RMS) delay spread, and Bit Error Rate (BER). The results demonstrate that the hemispherical dust particle distribution model is more accurate and realistic in terms of the metrics evaluated compared to other models found in the literature. Furthermore, the performance of the UM-VLC system is negatively affected when the number of dust particles suspended in the environment increases.

On-channel characteristics of wireless ultraviolet communication with mobile terminals

Most studies of ultraviolet communication do not consider the mobility of communication terminals. In this Letter, a single-scattering-channel model based on the Monte Carlo method is proposed for non-line-of-sight ultraviolet communication with mobile terminals. Our main contribution is to present an original method of evaluating the impulse response of communication channels given the relative moving speed and direction of the terminals. Using this method, the impact of the moving speed and direction of the terminals on the system response is simulated. Our simulation results show that the transmission distance plays a dominant factor in all scenarios, whether the communication terminals are coplanar or noncoplanar and with or without a height difference.

Research on dark channel dehazing of single-image based on non-dispersive infrared (NDIR) detection technology

This paper presents an experimental study on the non-dispersive infrared (NDIR) detection technology and dark channel dehazing technology. Based on the analysis of Beer-Lambert Law and differential carbon dioxide detection principle, this paper proposes an atmospheric light value estimation algorithm based on NDIR detection technology. First, the change characteristics of the gas concentration in indoor smoky environment are collected and analyzed. Then appropriate weighting coefficients are chosen based on the gas characteristics to estimate the atmospheric light value. Finally, the digital image dehazing technology through dark channel prior is used for calculation to obtain a haze-free image with high quality and high resolution. The experiment in this paper proves the feasibility of combining NDIR detection technology with dehazing technology, and its ability to improve image quality and achieve better restoration effect.

Single and double scattering event analysis for ultraviolet communication channels

This paper presents a channel analysis method for single and double scattering events in non-line-of-sight (NLOS) ultraviolet (UV) communication systems. In general, the calculations of path loss and impulse response of such systems require Monte Carlo random number generations. However, the high computational costs of Monte Carlo methods impose severe limitations on quick reliable evaluations of system performance under complex atmospheric conditions. This paper proposes a sample-based UV channel characterization approach that improves computational performance by multiple orders of magnitude. The proposed novel approach uses fixed probability-based sampling. The method focuses only on single and double scattering events which dominate the received signal. The effects of various fog and dust aerosols are discussed under non-planar realistic conditions. The results demonstrate reliable channel characterization with significantly lower complexity using the proposed approach.

Numerical analysis of light reflection and transmission in poly-disperse sea fog

The presence of sea fog greatly affects both the reflected and transmitted detections when radiation propagates through targets and maritime backgrounds. Thus, the maritime target detections and the remote sensing in oceanic environments would be disturbed by the sea fog. In our work, a poly-disperse sea fog system is introduced. Such a sea fog layer comprises spherical water particles of different radii, where the radii are divided into eight radius regions. The attenuation, asymmetry factors, and absorption probabilities of the radiation interacting with sea fog particles in each radius region are computed using Mie theory. The scattering processes of the radiation in the poly-disperse sea fog layer are traced in our improved Monte Carlo (MC) simulation. This paper presents a new method (named "our method" hereafter) with the intention to provide more accurate calculations on the reflection and transmission when radiation propagates through poly-disperse sea fog media of two different refractive indices. Therein, we investigated the influence of liquid water contents and thicknesses of the poly-disperse sea fog layer on the reflectance and transmittance of the radiation. The results using our MC method compared with those using the previous MC method are also presented. Besides, with three different MC methods along with our method and the previous method, we also inspected how different MC methods affect the calculations of reflectance and transmittance, and it shows manifestation that our method has an advantage over the previous method.

Gbit/s ultraviolet-C diffuse-line-of-sight communication based on probabilistically shaped DMT and diversity reception

We demonstrated a high-speed 1×2 single-input and multiple-output (SIMO) diffuse-line-of-sight (diffuse-LOS) ultraviolet-C (UVC) solar-blind communication link over a distance of 5 meters. To approach the Shannon limit and improve the spectral efficiency, we implemented probabilistically shaped discrete multitone modulation. As compared to a single-input and single-output (SISO) counterpart, we observed significant improvement in the SIMO link in terms of the angle of view of the receiver and the immunity to emulated weather condition. A wide angle of view of ± 9^° is achieved in the SIMO system, with up to a 1.09-Gbit/s achievable information rate (AIR) and a minimum value of 0.24 Gbit/s. Moreover, the bit error rate of the SIMO link in emulated foggy conditions is lowered significantly when compared to that of the SISO link. This work highlights the practicality of UVC communication over realistic distances and in turbulent environments to fill the research gap in high-speed, solar-blind communication.

Unmanned Aerial Vehicle-Borne Sensor System for Atmosphere-Particulate-Matter Measurements: Design and Experiments

An unmanned aerial vehicle (UAV) particulate-matter (PM) monitoring system was developed that can perform three-dimensional stereoscopic observation of PM2.5 and PM10 in the atmosphere. The UAV monitoring system was mainly integrated by modules of data acquisition and processing, wireless data transmission, and global positioning system (GPS). Particularly, in this study, a ground measurement-control subsystem was added that can display and store collected data in real time and set up measurement scenarios, data-storage modes, and system sampling frequency as needed. The UAV PM monitoring system was calibrated via comparison with a national air-quality monitoring station; the data of both systems were highly correlated. Since rotation of the UAV propeller affects measured PM concentration, this study specifically tested this effect by setting up another identical monitoring system fixed at a tower as reference. The UAV systems worked simultaneously to collect data for comparison. A correction method for the propeller disturbance was proposed. Averaged relative errors for the PM2.5 and PM10 concentrations measured by the two systems were 6.2% and 6.6%, respectively, implying that the UAV system could be used for monitoring PM in an atmosphere environment.

Power losses in diffuse ultraviolet optical communications channels

One of the most critical parameters in free-space optical communications systems operating in a non-line-of-sight regime are the optical losses. In this Letter, we numerically calculate these losses taking into account the scattering effects using the Monte Carlo simulation technique. The obtained results are compared with experimentally obtained data at 265 nm (solar-blind UV regime). A large set of measurements at distances up to 20 m, for different elevation angles of the transmitter (UV-LEDs) and receiver (photomultiplier tube) and for different atmospheric conditions has been taken for the characterization of the optical communications channel in terms of its loss properties.

Closed-form impulse response model of non-line-of-sight single-scatter propagation

For optical scattering communication, a closed-form expression of channel impulse response (CIR) is favorable for further system design and channel capacity analysis. Combining the mean value theorem of integrals and L'Hôpital's rule, the exact non-line-of-sight (NLOS) single-scatter propagation model is simplified to a closed-form CIR model for a laser source with a narrow beam. Based on this model, by joint geometrical and empirical approaches, a piecewise CIR expression is presented under certain system NLOS geometries. Through numerical results on CIR for various NLOS geometries, the proposed model is verified with the exact NLOS single-scatter propagation model and the previous Gamma fitting model, showing that our model agrees better with the former than the latter.