Non-line-of-sight Ultraviolet Communication Research Articles

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.

On the capacity of a SIM-based cooperative NLOS UVC system with best relay selection

Ultraviolet communication (UVC) is a promising technology due to its ability to operate in non-line-of-sight (NLOS) mode thereby eliminating the pointing acquisition and tracking (PAT) requirement as needed by infrared and visible light communications. However, NLOS UVC suffers from very high attenuation and turbulence-induced fading when operated over a long distance. Due to these limitations, the existing literature on the NLOS UVC is mostly restricted to short-distance communications only. Therefore, this paper addresses these challenges by proposing an outdoor subcarrier-intensity-modulation (SIM) based multi-relay cooperative communication system employing the best relay selection and decode-and-forward (DF) relaying protocol. The turbulence-induced fading is modelled using lognormal distribution under weak atmospheric turbulence conditions. We derive novel closed-form analytical expressions of outage probability and ergodic capacity. Correctness of the derived analytical expressions is validated through numerical simulations.

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.

Performance of Multipulse Pulse-Position Modulation in NLOS Ultraviolet Communications

The performance of multipulse pulse-position modulation (MPPM) in non-line-of-sight (NLOS) ultraviolet communications (UVC) with output signals of detectors following Poisson distribution and Poisson-triggered Gaussian (PTG) distribution has been investigated. Specifically, the probabilities of a given number of incorrectly identified pulsed or empty slots in an MPPM symbol have been derived with output signals Poisson and PTG distributed. Based on those probabilities, the expressions of symbol-error rate and achievable data rate of MPPM in the NLOS UVC systems have been further obtained. Finally, Monte-Carlo simulations are presented to verify the theoretical results.

A Power-Domain MST Scheme With BPPM in NLOS Ultraviolet Communications

Due to the high path loss of non-line-of-sight (NLOS) ultraviolet communications (UVC) channel, pulse modulation schemes, such as on-off keying (OOK) and M-ary pulse-position modulation (PPM), and photon-counting receivers are commonly adopted. Although M-ary PPM can be demodulated and decoded without the channel state information, its spectral efficiency is lower than OOK's. To improve the spectral efficiency of M-ary PPM in NLOS UVC, a power-domain multilayer-superposed transmission (MST) scheme with binary PPM (BPPM) is proposed in this work. Based on this scheme, the signal in each layer can be directly decoded without successive interference cancellation, which is necessary for other MST schemes in the power domain. Hence, a low complexity receiver in the NLOS UVC system can be realized. Additionally, the bit-error rate (BER) and achievable data rate (ADR) expressions of the proposed MST scheme are derived. Monte-Carlo simulations are performed to verify the analytical BER expression. The NLOS UVC system performance employing the proposed MST scheme is further demonstrated and compared with pure BPPM. The results indicate that with the optimal power allocation factor for each layer, the overall ADR of the NLOS UVC system with the proposed power-domain MST scheme can surpass that with pure BPPM.

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.

Reflection-Assisted Non-Line-of-Sight Ultraviolet Communications

Considering high path loss (PL) of the non-line-of-sight (NLOS) ultraviolet communications (UVC) realized by the atmospheric scattering effect of ultraviolet light, we propose to use a reflective plane to improve the received optical power and further reduce the PL. More specifically, a channel model for NLOS UVC with not only scattering but also reflection considered is proposed via Monte-Carlo (MC) photon-tracing method. Both scattering and reflective events are called collision-induced events (CIEs). The proposed channel model can obtain the PL and channel impulse response (CIR) performances of the NLOS UVC system with multiple-order CIEs. It is shown that the reflective plane could reduce the PL and the time delay of CIR, which could be a benefit for designing high data-rate NLOS UVC systems. Finally, experimental results in two representative scenarios are also given to verify the effectiveness of the proposed channel model for reflection-assisted NLOS UVC systems.

On the performance of a relay assisted hybrid RF-NLOS UVC system with imperfect channel estimation

Non-line-of-sight (NLOS) ultraviolet communication (UVC) is emerging as an attractive optical wireless communication technology that enables wireless connectivity in radio-frequency (RF) prohibited areas with no LOS availability. NLOS UVC, however, suffers from a very high path loss, thereby restricting its usage to smaller link distances. In this paper, we address the challenge of providing long-distance wireless connectivity to RF prohibited areas by mixing NLOS UVC with RF communication using a decode-and-forward relay. The RF link is modeled using Rayleigh distribution, and the NLOS UV link is modeled using lognormal distribution under weak turbulence conditions. A framework for analytical expressions of the outage probability and probability density function (PDF) of the end-to-end signal-to-noise ratio is presented by considering the practical scenario of imperfect channel state information (CSI) at the receiver. Subsequently, a PDF based novel closed-form analytical expression of the average symbol error rate is deduced for spectrally efficient higher-order modulation schemes, including rectangular quadrature modulation (RQAM), square QAM (SQAM), cross-QAM (XQAM), and hexagonal QAM (HQAM). Numerical investigations are conducted, and the impact of CSI imperfections on the system performance is evaluated. It is shown that the RF link is more vulnerable to channel estimation error (CEE) than the NLOS UV link. Further, it is illustrated that for constellation sizes greater than four, HQAM always performs better than the RQAM, SQAM, and XQAM schemes, irrespective of the amount of CEE present. Furthermore, it is shown that an elevation angle of 70° or less in the NLOS UVC link results in better outage performance. Correctness of the derived analytical expressions is supported through extensive Monte Carlo simulations.

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.

Performance of Multiple Relay DF NLOS UVC System With CSI Imperfections

Ultraviolet communication (UVC) is emerging as an attractive alternative to the existing optical wireless communication (OWC) technologies. UVC experiences negligible noise on the earth's surface, and also has the ability to operate in non-line-of-sight (NLOS) mode, thereby making it a perfect choice for outdoor communication. However, due to strong interaction of ultraviolet waves with atmospheric particles, it suffers from a very high path loss and turbulence-induced fading, which limits UVC system's performance. We consider a decode-and-forward based cooperative relaying technique to improve the performance of NLOS UVC system, and to extend its communication range. We consider the practical case of imperfect channel state information at the receiver and derive outage probability of the system. We also consider impact of elevation angles, receiver field-of-view (FOV), and turbulence strength on the system performance. We compute the relative diversity order of the system and demonstrate its convergence through asymptotic analysis. Next, we obtain the novel expression of probability density function of the end-to-end instantaneous signal-to-noise-ratio. We use single subcarrier intensity modulation employing quadrature amplitude modulation (QAM) and derive the novel generalized analytical expressions for rectangular QAM, cross QAM, and futuristic hexagonal QAM schemes. We carry out a detailed performance study considering different system configurations and several interesting insights are highlighted, which reinforces UVC as a futuristic OWC technology. Correctness of the derived analytical expressions is confirmed using Monte-Carlo simulations.

Performance analysis of MIMO communication system with NLOS UV channel

Performance analysis is carried out, of a multiple input, multiple output (MIMO) ultraviolet (UV) communication system with a non-line-of-sight (NLOS) UV channel. The achievable bit error coefficient is calculated using three spatial multiplexing methods for different bitrate values, azimuthal deviation between the directional diagrams of an optical transmitter and an optical receiver, and different noise levels. Full Text: PDF ReferencesZ. Xu, B. Sadler, "Ultraviolet communications: potential and state-of-the-art", IEEE Commun. Mag. 4667-73 (2009). CrossRef D. Han, Y. Liu, K. Zhang et al., "Theoretical and experimental research on diversity reception technology in NLOS UV communication system", Opt. Expr. 20(14), 15833 (2012). CrossRef Q. Guo, N. He, Z. He, "Research on the channel performances and transmission in UV-LED scatter communications", Study Opt. Comm. 3, 64 (2013). DirectLink G. Chen, L. Liao, Z. Li et al., "Experimental and simulated evaluation of long distance NLOS UV communication", Communication Systems, Networks and Digital Signal Processing (CSND-SP), 9th Int. Symp. on IEEE, 904-909 (2014). CrossRef M.A. El-Shimy, S. Hranilovic, "Spatial-Diversity Imaging Receivers for Non-Line-of-Sight Solar-Blind UV Communications", J. Lightwave Techn. 33(11), 2246 (2015). CrossRef G. Shaw, M. Nischan, M. Iyengar, S. Kaushik, M. Griffin, NLOS UV communication for distributed sensor systems, Proc. SPIE 412683, 96 (2000). CrossRef I.S. Konstantinov, G.S. Vasyliev, O.R. Kuzichkin, D.I. Surzhik, I.A. Kurilov, S.A. Lazarev, "AUV Link Mobile Ad-Hoc Network Examination", J. Eng. Adv. Techn. 8(5S) July 2019 CrossRef I.S. Konstantinov, G.S. Vasilyev, O.R. Kuzichkin, I.A. Kurilov, S.A. Lazarev, "Modeling and Analysis of the Characteristics of Ultraviolet Channels under Different Conditions of Radiation Propagation for the Organization of Wireless AD-HOC Network", J. Adv. Res. Dynam. Contr. Syst. 07, 1853 (2018) DirectLink I.S. Konstantinov, G.S. Vasyliev, O.R. Kuzichkin, D.I. Surzhik, I.A. Kurilov, S.A. Lazarev, "Development Of Uv Communication Channels Characteristics Modeling Algorithm In A Mobile Ad-Hoc Network", J. Adv. Res. Dynam. Contr. Syst. 11(08), 1920 (2019). CrossRef G. Chen, F. Abou-Galala, Z. Xu, B.M. Sadler, "Experimental evaluation of LED-based solar blind NLOS communication links", Opt. Expr. 16(19), 15059 (2008). CrossRef

Outage analysis of MIMO NLOS‐UV communication systems over atmospheric turbulence channels

Rich scattering of the signals in the ultraviolet (UV) band of electromagnetic spectrum enables non-line-of-sight (NLOS) outdoor wireless connectivity. It eliminates the line-of-sight requirement of the links which are using the signals in the infra-red band. Besides scattering losses, UV signals also suffer from atmospheric turbulence-induced fading. The turbulent atmospheric channel causes fluctuations in the received signal intensity of a NLOS ultraviolet (NLOS-UV) link, resulting in power loss and the performance degradation at the receiver. In this study, spatial diversity techniques are investigated as an appealing fading mitigation technique for NLOS-UV systems over atmospheric turbulence channels. In particular, the outage probabilities of multiple-input-single-output (MISO), single-input-multiple-output (SIMO), and multiple-input-multiple-output (MIMO) links are studied, and the outage performances are investigated for different system configurations. Closed-form expressions are derived for the outage probability of MISO, SIMO, and MIMO systems, and the accuracy of the derivations is confirmed through Monte Carlo simulations. The outage probability of single-input-single-output link is also derived as a benchmark. The results demonstrate significant performance improvements through deployment and proper configuration of MIMO systems in NLOS-UV communication links.

Analysis of the low-probability-of-detection characteristics of ultraviolet communications.

Deep ultraviolet wavelengths have been proposed for low-probability-of-detection (LPD) communications, particularly for non-line-of-sight (NLOS) links, because of the increased atmospheric absorption at these wavelengths. Motivated by this favorable feature, we develop a modeling framework to quantitatively study the LPD characteristics of ultraviolet communications (UVC). We then demonstrate the application of our modeling framework by considering various friendly and adversarial system configurations and quantifying the proposed LPD metric (the range at which an adversary can detect communications that uses the minimum power needed to meet given communications performance requirements), as well as investigating the sensitivity of the analysis to various scenario parameters. The results demonstrate the potential for this modeling and analysis approach to provide key insights into the design and operation of LPD NLOS UVC systems.

Performance analysis of cooperative relaying technique for non-line-of-sight UV communication system in the presence of turbulence

Ultraviolet communication (UVC) with its ability to operate in non-line-of-sight (NLOS) offers significant advantages as compared to conventional optical wireless communication systems. NLOS UVC is a short-distance communication system due to high path loss and turbulence-induced fading. We propose an effective cooperative multirelay system to combat the effect of attenuation and fading in NLOS UVC, thereby making it suitable for long-distance communication. We investigate the performance of the proposed system in terms of outage probability by considering both variable-gain and fixed-gain amplified and forward relaying. Further, we consider higher-order modulation schemes to improve the data rate of the system and derive innovative closed-form expressions of the average symbol error rate and diversity order for the variable-gain relaying. We demonstrate performance gains of the considered multirelay system over nonrelayed links for different system parameters. We present numerical results to confirm the accuracy of the derived analytical expressions.

Performance analysis of NLOS ultraviolet communications with correlated branches over turbulent channels

Ultraviolet communication (UVC) is becoming increasingly popular due to its ability to effectively communicate in non-line-of-sight (NLOS) mode, which offers alternative communication system advantages over visible light communication and infrared links. However, the performance of UVC is affected by atmospheric turbulence, which is generally ignored under the assumption of short-distance links. In this paper, we consider a NLOS UVC system experiencing turbulence due to variation in the refractive index of the atmosphere. The atmospheric turbulence results in fading due to fluctuations in the received signal strength, thus deteriorating the quality of communication. Spatial diversity is proven to mitigate the effect of fading by introducing multiple parallel communication paths between the transmitter and receiver. We present a spatial diversity reception in the form of ${N_r}$-branches selection combining at the receiver. The channel coefficients are assumed to be exponentially correlated, and turbulence is modeled using log-normal distribution under weak turbulence conditions. Closed-form expressions for the outage probability and average symbol error rate for general order rectangular quadrature amplitude modulation (RQAM), cross-QAM, and hexagonal QAM schemes are derived. Furthermore, the ergodic capacity of the system is computed as a function of the channel correlation coefficient. The numerical values are compared with computer simulations to validate the accuracy of the theoretical analysis.

Performance dependence of non-line-of-sight ultraviolet communications on atmospheric parameters of the ultraviolet channel

Non-line-of-sight (NLOS) ultraviolet (UV) communication systems are rapidly gaining popularity as an effective means of data transmission over long distance. UV communication systems offer huge unlicensed spectrum with low-cost implementation. However, the performance of the UV system depends strongly on atmospheric parameters of the UV channel, such as temperature, pressure, turbulence and altitude, between the transmitter and the receiver. In this paper, we evaluate the link performance of the NLOS UV communication under these varying atmospheric parameters. Specifically, the impact of temperature, pressure and altitude on the Rayleigh and Mie scatterings are investigated, as the scatterings provide the basis for transmitting information over the NLOS UV channel. As the turbulent atmospheric channel causes fluctuation in the received signal intensity, a closed-form expression for the average channel capacity as a function of temperature and pressure for a given transmit power is also derived. The atmospheric turbulence is modeled by a Gamma–Gamma distribution. Simulation results show that the received power decreases with increasing atmospheric pressure, while the data rate increases with increasing temperature.

Secrecy performance of noncoplanar SIMOME NLOS ultraviolet communications over turbulence channels

Abstract This paper considers improving the confidentiality of non-line-of-sight (NLOS) ultraviolet (UV) links within the framework of physical-layer security. We study an NLOS UV scenario with one transmitter, one legitimate receiver and an eavesdropper. The transmitter has only one receive aperture, while the legitimate and unauthorized receivers have multiple receive apertures. Maximum ratio combining (MRC) and equal gain combining (EGC) techniques are utilized to overcome the data rate bottleneck and security risk induced by scattered UV channels and atmospheric turbulence. Alignment errors are exit in the noncoplanar single-input multiple-output multi-apertures eavesdroppers (SIMOME) NLOS UV communication system. The expressions of the average secrecy capacity (ASC), the secrecy outage probability (SOP) and the probability of positive secrecy capacity (PPSC) are derived. It is founded that the smaller the transmit or receive elevation angles are, the better the secrecy performance is. Spatial diversity techniques can improve the confidentiality of NLOS UV links with smaller off-axis errors.

Performance analysis of multiple NLOS UV communication cooperative relays over turbulent channels.

Ultraviolet (UV) communication overcomes pointing and tracking errors and is superior to other modern optical wireless communication technologies at short range. Using effective wavelengths from 200 to 280 nm, enables the non-line-of-sight (NLOS) outdoor UV communication in the presence of strong molecular and aerosol scattering. Because of these characteristics, solar blind NLOS UV communications offers broad coverage and high security. In this paper, NLOS UV communication is considered with decode and forward (DF) relays in the presence of log-normal (LN) channels using the best relay selection technique according to the channel state information (CSI). Then the outage probability of the multi-relay UV system is discussed for the proposed model. Simulation results verify the effectiveness of our employed analytical model. The outage probability for both serial and cooperative relays is compared with a different number of relays. Numerical simulations are further presented for many factors influencing the functioning of the system such as elevation angle, atmospheric scattering parameters and receiver field of view (FOV) angles. The obtained results demonstrate that increasing the number of UV NLOS cooperative relays does not necessarily improve the system performance, but there are other factors that must be considered such as the value of the elevation angle and the number of relays.

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.

Multi‐user interference in a non‐line‐of‐sight ultraviolet communication network

On the basis of non-line-of-sight (NLOS) multiple-scattering channel characteristic of wireless ultraviolet (UV), the authors designed a UV transmit–receive link model and discussed many aspects of this model. The model is developed by a multiple-scattering Monte Carlo method. They focus on the performance study of NLOS UV communication system with multi-user interference. Numerical simulations are conducted for some application scenarios, where various parameters are changed in the three-dimensional space such as elevation angle of transmitter and receiver, field-of-view angle, full-beam angle and so on. System performance is analysed with simulation results including path loss, signal-to-interference ratio and probability of error (Pe). The results show that reasonable parameter configuration can reduce Pe and improve the performance of communication system.