A single-scatter path loss model for non-line-of-sight ultraviolet channels

Theoretical and experimental studies on channel impulse response of short-range non-line-of-sight ultraviolet communications

A theoretical channel impulse response (CIR) model of short-range non-line-of-sight (NLOS) ultraviolet communications (UVC) in noncoplanar geometry under the single-scatter condition is proposed. Simulation results obtained from the widely accepted Monte-Carlo (MC)-based channel model of NLOS UVC are provided to verify corresponding theoretical results obtained from the proposed theoretical single-scatter CIR model. Additionally, an outdoor experiment with a light-emitting diode (LED) as the light source is first designed to measure the channel step response of NLOS UVC and to further validate the proposed theoretical single-scatter CIR model. By varying the different parameters of the transmitter and the receiver, such as the baseline range, the inclination angle, the azimuth angle, the beam divergence angle, and the field-of-view angle, the results of the proposed theoretical single-scatter CIR model and the MC-based channel model are exhibited and further analyzed in detail. Results indicate that the computational time cost by the proposed theoretical single-scatter CIR model is decreased to less than 0.6% of the MC-based one with comparable accuracy in assessing the temporal characteristics of NLOS UVC channels. Additionally, theoretical results obtained from the proposed theoretical single-scatter CIR model manifest satisfactory agreement with corresponding experimental measurements.

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.

Measurement system for ultraviolet channel modeling and communications

There has been an increasing interest in ultraviolet (UV) communications as a promising technology for non-line-of-sight (NLOS) networking by exploiting atmospheric scattering at UV wavelengths that enables a unique NLOS UV communication channel. While there has been significant theoretical and simulation-based investigation of the UV channel characteristics, there is limited work in terms of experimental research and validation of the analytical models. In this paper, we present a flexible experimental system for precise UV channel and communications measurements. Specifically, a transceiver system is developed that consists of a gimbal, UV light-emitting-diode array, and photomultiplier tube detector, node synchronization, and LabVIEW-based data acquisition subsystems. Novel techniques to precisely characterize the UV LED array radiation pattern, absolute transmit power, and field of view of the detector are also presented. The utility of the developed system is then demonstrated by performing a variety of experiments including UV channel model validation and steering optimization for UV communication links where the results were in very good agreement with theory and simulation.

Transmission characteristics of the rough surface scattering channel for wireless ultraviolet communication in a cemented ground scenario

During non-line-of-sight ultraviolet (UV) communication, as the elevation angle of the transmitter decreases, more UV photons can be scattered by the ground and reach the receiver, and those photons are important components of the received signals. In this paper, a new, to the best of our knowledge, non-line-of-sight noncoplanar UV scattering transmission model taking into account the UV rough surface scattering characteristics is proposed based on the theory of non-line-of-sight multiple scattering UV light transmission and the Monte Carlo method and Kirchhoff's dwell phase method. The proposed model is validated by experiment. Simulation and experiment show that the energy of UV light is mainly concentrated in the direction of mirror reflection after being scattered by the concrete floor, and the energy gets stronger as it gets closer to the mirror reflection direction. In the case of small elevation angle, the photon energy scattered by the ground and received by the receiver must be considered. The path loss obtained by the improved model better fits with experiment.

Optical data transmission through highly dynamic and turbid water using dynamic scaling factors and single-pixel detector

Free-space optical data transmission through non-static scattering media, e.g., dynamic and turbid water, is challenging. In this paper, we propose a new method to realize high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water using a series of dynamic scaling factors to correct light intensities recorded by a single-pixel bucket detector. A fixed reference pattern is utilized to obtain the series of dynamic scaling factors during optical data transmission in free space. To verify the proposed method, different turbidity levels, different strengths of water-flow-induced turbulence and a laser with different wavelengths are studied in optical experiments. It is demonstrated that the proposed scheme is robust against water-flow-induced turbulence and turbid water, and high-fidelity free-space optical information transmission is realized at wavelengths of 658.0 nm and 520.0 nm. The proposed method could shed light on the development of high-fidelity and high-robustness free-space optical data transmission through highly dynamic and turbid water.

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.

Single-collision-induced path loss model of reflection-assisted non-line-of-sight ultraviolet communications

By considering both scattering and reflection events as collision-induced events (CIEs), an analytical path loss model of reflection-assisted non-line-of-sight (NLOS) ultraviolet communications (UVC) is proposed with single CIE incorporated, which refers to the single-collision-induced (SCI) path loss model. More specifically, the analytical expressions of the received optical energy resulting from single-scatter and single-reflection events in reflection-assisted NLOS UVC are respectively derived. Then, in terms of those two expressions, the expression of the proposed SCI path loss model is obtained. Finally, Monte-Carlo (MC) simulations and experimental results are given to verify the correctness and effectiveness of the proposed SCI path loss model. The results manifest that the proposed SCI path loss model can work well in both coplanar and noncoplanar geometry of the reflection-assisted NLOS UVC.

Wireless ultraviolet scattering channel estimation method based on deep learning

Due to the strong scattering characteristics, there are serious problems of inter-symbol interference (ISI) and transmission attenuation in the none-line-of-sight (NLOS) wireless ultraviolet communication system. In this paper, a wireless ultraviolet scattering channel estimation method based on deep learning is presented. The learning model structure is designed by combining the one-dimensional convolutional neural network (1D-CNN) and the deep neural network (DNN). In the training stage, the network optimization process is improved by the differential evolution (DE) algorithm. The computer simulation results show that the proposed deep learning channel estimation scheme has better mean square error (MSE) performance and bit error rate (BER) performance compared with the traditional algorithms. Furthermore, we verify the stability of this scheme in different communication environments, and the constructed neural network model has good generalization ability.

Non-line-of-sight optical information transmission through turbid water

In this paper, a new and robust method is proposed to realize high-fidelity non-line-of-sight (NLOS) optical information transmission through turbid water around a corner. A series of 2D random amplitude-only patterns are generated by using the zero-frequency modulation method, which are used as optical information carriers. The laser beam modulated by random amplitude-only patterns propagates through turbid water, and the wave diffused by turbid water is further reflected around a corner. A single-pixel detector is used to collect light intensity at the receiving end. To demonstrate feasibility and effectiveness of the proposed NLOS free-space optical information transmission system, many optical experiments are conducted. The proposed method is fully verified by using different turbid water conditions, different separation distances around a corner and different detection angles of the single-pixel detector. Optical experimental results demonstrate that the proposed method is able to achieve high fidelity and high robustness for free-space optical information transmission through turbid water. Even when there is an obstacle behind turbid water, high-fidelity free-space optical information transmission is still realized by using the proposed method. In addition, the proposed method possesses a wide detection range at the receiving end, which is of great significance in practical applications. The proposed method is a promising application for NLOS free-space optical information transmission.

Single-scatter path loss model of LED-based non-line-of-sight ultraviolet communications

Light-emitting diodes (LEDs) are popularly used as light sources in non-line-of-sight (NLOS) ultraviolet communications (UVC). However, currently reported single-scatter path loss (PL) models of NLOS UVC links assume that the radiant intensity of the light source is uniformly distributed within the beam divergence angle, which cannot well characterize the light emission pattern of LEDs. In this Letter, we propose a single-scatter PL model for LED-based NLOS UVC systems, and the corresponding analytical expression is derived by modeling the LED emission pattern as a Lambertian distribution. Monte Carlo simulations and experimental results are provided to verify the effectiveness of the proposed PL model. Results show that the proposed model can accurately predict PL results in actual situations. The root-mean-square error of the proposed PL model is only about 1 dB in typical scenarios with experimental results as benchmarks. Also, the proposed model is much easier to implement than the currently reported PL model with the light source model of a uniformly distributed emission pattern.

Modeling of ultraviolet scattering propagation and its applicability analysis

The existing ultraviolet (UV) single scattering models do not incorporate the applicability of transceiver geometry, which makes them have certain errors even in short-range cases. To solve this issue, we propose a single scattering model in this Letter, which is suitable for the case where the transceiver field of view is above ground. This meets the requirements of general UV communication. For tractable analysis, we utilize the number of intersections between special rays on a transceiver cone to classify communication scenarios. Calculation results show that when the transceiver elevation angle exceeds certain values, the path loss difference between the single scattering model and Monte Carlo simulation model increases rapidly, which implies that single scattering approximation is not applicable to these conditions even in short-range cases. This work presents a new way to obtain the path loss of the UV channel and provides guidelines for experimental system design.

Research on pulse response characteristics of wireless ultraviolet communication in mobile scene

Based on the Riemann sum method, we propose the idea of studying the pulse response width in a moving scene by using the non-line-of-sight (NLOS) wireless ultraviolet (UV) single-scatter communication model. We simulated the effect of the transmitter moving in eight directions from the origin on the pulse response width, when the receiver is fixed at point (100, 0, 0). Furthermore, the pulse response characteristics of the receiver were analyzed with varying elevation angle, beam angle, field-of-view (FOV) angle, and other geometric parameters. The results show that the pulse response width is widened under the condition of movement. In addition, the influence of the elevation angle of the transmitter on the pulse width is larger than that of the receiver. However, the effect of the FOV and beam angles on the pulse response width is not obvious when the FOV angle is large.

Performance analysis of UV multiple-scatter communication system with height difference

Based on the Monte Carlo (MC) method, a non-coplanar ultraviolet (UV) multiple-scatter propagation model with a height difference between the transmitter (Tx) and receiver (Rx) was presented. We focused on the relationship between bit error rate (BER) and the height difference between the Tx and Rx. We also studied the impact of the elevation angle and the off-axis angle of the Tx and Rx on the BER under the condition that the height difference is not zero. In addition, an outdoor UV communication testbed was set up to provide support for the validity of the MC model. The simulation results show that when the height difference between the Tx and Rx increases, the BER first decreases and then increases, the BER can be reduced by adjusting the transceiver elevation angle, and the bigger the off-axis angle is, the bigger BER is. The experimental results are in good agreement with the simulation results.

Analytical link bandwidth model based square array reception for non-line-of-sight ultraviolet communication

An analytical model is presented firstly in this paper to formulate the link bandwidth of non-line-of-sight (NLOS) ultraviolet (UV) channel. The link bandwidth is characterized by three geometrical parameters including transmitter (Tx) elevation angle, receiver (Rx) field of view (FOV), and transceiver separation distance, and further expressed as a closed-form through software-aided numerical fitting. Comparison with the link bandwidth obtained via a Monte Carlo model is done to verify the feasibility of this model. Based on this model, we investigate the diversity reception on the NLOS UV communication from a new perspective. A spatially squared distributed Rx array is customized for the NLOS UV channel. Lower temporal broadening is enabled, leading to a higher link bandwidth. Numerical results suggest that over 100% improvement of the link bandwidth is predicted by the square array reception and the ratio grows rapidly with the narrowing of Tx beam divergence. Therefore, this paper provides a guide for link analysis and receiver design for NLOS UV communication.

Modeling of optical wireless scattering communication channels over broad spectra

The air molecules and suspended aerosols help to build non-line-of-sight (NLOS) optical scattering communication links using carriers from near infrared to visible light and ultraviolet bands. This paper proposes channel models over such broad spectra. Wavelength dependent Rayleigh and Mie scattering and absorption coefficients of particles are analytically obtained first. They are applied to the ray tracing based Monte Carlo method, which models the photon scattering angle from the scatterer and propagation distance between two consecutive scatterers. Communication link path loss is studied under different operation conditions, including visibility, particle density, wavelength, and communication range. It is observed that optimum communication performances exist across the wavelength under specific atmospheric conditions. Infrared, visible light and ultraviolet bands show their respective features as conditions vary.

Non-line-of-sight ultraviolet single-scatter propagation model in random turbulent medium

Non-line-of-sight (NLOS) ultraviolet communication (UVC) uses the atmosphere as a propagation medium. In previous literature, various scatter propagation models have been derived based on the premise that atmospheric turbulence was ignored and the atmosphere was considered as a turbid medium, also called random scatterers. In this Letter, a NLOS single-scatter propagation model is proposed to describe the singly scattered radiation in a turbulent medium, also called a random continuum, such as the clear atmosphere. The model is established based on the relationship between the scattered power and the characteristics of the random turbulent medium. The scattering cross section is further investigated in terms of different correlation distances and wavelengths. The received power dependence for NLOS UVC is also analyzed for different factors, including refractive-index structure parameter and transceiver range.

Closed-form path loss model of non-line-of-sight ultraviolet single-scatter propagation

Non-line-of-sight ultraviolet propagation models have been developed for both coplanar and noncoplanar geometries. Based on an exact integral-form single-scatter model, this Letter proposes an approximate closed-form model for tractable analysis applicable to noncoplanar geometries with a narrow transmitter beam or receiver field of view. Numerical results on path loss are presented for various system geometries. These results are verified with the integral-form model and a previous approximate model, showing our model agrees well with the former and outperforms the latter.