Optical Wireless Links Research Articles

Time jitter influence on the performance of gamma–gamma turbulence FSO links with various modulation schemes

ABSTRACTThe research area of optical wireless communication links has attracted significant interest over the last years due to the significant advantages offered by this kind of technology. However, the performance of FSO communication systems depends strongly on the effects, which are related to the atmosphere along the propagation path such FSO links are using. More specifically, the weather conditions and atmospheric turbulence effects can deteriorate considerably the performance characteristics. In this work, we are studying the joint influence of atmospheric turbulence and time jitter effects on the average BER of an FSO link by presenting a new approach for various modulation schemes. Thus new closed-form mathematical expressions are derived for accurate estimation of the BER performance of the optical wireless communication systems. Finally, using the obtained expressions and typical parameter values for FSO links, the numerical results are presented and then verified through Monte Carlo simulations.

Modeling and performance investigation of 4 $$\times$$ 20 Gbps underwater optical wireless communication link incorporating space division multiplexing of Hermite Gaussian modes

In this work, we report the development of long-haul high-speed underwater optical wireless communication (UOWC) transmission link by incorporating space division multiplexing (SDM) of distinct Hermite Gaussian (HG) modes for enhanced channel-capacity. Four distinct 20 Gbps non-return to zero encoded information signals are transmitted over distinct HG modes of a single wavelength laser at 1800 m underwater link range to realize an 80 Gbps UOWC system. Also, we investigate the performance of the proposed UOWC transmission link under the influence of different operating wavelengths, aperture diameter, attenuation coefficient, and link range. Further, we optimize the proposed UOWC link performance in terms of spatial aperture to stabilize the bit-error rate (BER) performance of the link at high signal attenuation values and varying transmission link reach.

Entanglement transmission through dense scattering medium

Scattering effects are ubiquitous in practical wireless optical links. Here a transmission model with complete consideration of scattered light and beam wandering effects for underwater link is developed, with the aim to completely characterize the received quantum state of light through dense scattering medium. Based on this model, we show the influence of scattered photons on the improvement of the entanglement after transmission through turbid water may vary for different copropagation scenarios, i.e., the contribution of scattered light on entanglement transmission may be turned from positive to negative, with increase of the strength of underwater beam wandering. And the attenuation coefficient and aperture size are found to be the dominant factors affecting the entanglement through underwater link. While for the counterpropagation scenario, the scattered photons will severely deteriorate the entanglement transmission especially for the high-loss scattering links. These findings may shed light on quantum entanglement transmission and help to develop its applications through dense scattering medium.

Effect of anisotropy on performance of M-ary phase shift keying subcarrier intensity-modulated optical wireless communication links operating in strong oceanic turbulence

In strong oceanic turbulence, the performance of M-ary phase shift keying subcarrier intensity-modulated optical wireless communication (OWC) links is investigated in terms of the bit-error-rate (BER) by considering the effect of anisotropy of the oceanic channel. To calculate the BER of the OWC link, a gamma–gamma statistical channel model is adopted. The extended Huygens–Fresnel principle and the asymptotic Rytov theory are used to obtain the received optical power and the large-scale and small-scale log-intensity variances, respectively. Our graphical illustrations include the BER versus anisotropic factor for various system parameters such as the modulation order, filter bandwidth, link distance, peak amplitude of each subcarrier and the oceanic turbulence parameters.

On the performance of optical wireless communication links impaired by time jitter, M-turbulence and pointing errors

The interest for the optical wireless communication systems increases significantly the last years due to the very high advantages of this technology. More specifically, the FSO links can achieve very high data rate and secure communications with low installation and operational cost. However, the main disadvantage of the FSO links is that their performance is dominated by the weather conditions and the turbulence of the atmosphere, where the optical beam propagates. The atmospheric turbulence mitigates significantly the performance of the FSO links and causes the scintillation effect, which results in rapid irradiance fluctuations at the receivers. Furthermore, another factor which increases the influence of the atmospheric turbulence and causes further fast changes at the receivers’ irradianceis the pointing errors. Additionally, the performance of most of the high data rate communication systems, is reduced by the time jitter effect, which causes the detection of each digital pulse before or later than the ideal time spot. Thus, in this work, the performance of a terrestrial FSO link is investigated, for first time under the simultaneous action of the M(alaga) atmospheric turbulence, the pointing errors and the time jitter effects for various, commonly used in FSO links, modulation schemes. Taking into account their joint influence, new accurate closed form mathematical expressions are derived for the average BER estimation. Finally, the corresponding numerical results and conclusions are shown and derived.

Quantum Noise-Assisted Coherent Radio-Over-Fiber Cipher System for Secure Optical Fronthaul and Microwave Wireless Links

This article reports on a symmetric-key direct-data encryption technique that directly protects the interception of optical and microwave signals from physical layer in a coherent analog radio-over-fiber (RoF) system. Secrecy is realized by signal masking by quantum (shot) noise. The quantum noise masking for the encryption is achieved by converting data into extremely high-order signals at an optical frequency. After signal transmission over an optical fronthaul link, the frequency of the encrypted signal is shifted to a microwave frequency via an optical heterodyne process using a local oscillator for wireless transmission. The effect of the quantum noise masking is naturally and seamlessly kept in the heterodyne process. We experimentally demonstrate 12 Gbit/s coherent RoF cipher systems utilizing the quantum noise masking for a 30 GHz wireless band. Adequate signal quality and high security against interception with sufficient quantum noise masking are simultaneously achieved in the optical fronthaul and microwave wireless links.

Spiral spectrum of anomalous vortex beams propagating in a weakly turbulent atmosphere

ABSTRACTWe study the spiral spectrum of anomalous vortex beams propagating through a turbulent atmosphere. Based on the Huygens–Fresnel integral and the Rytov approximation, the integral expression and then the analytical expression for the spiral spectrum of anomalous vortex beams in the weakly turbulent atmosphere are derived. The capacity of wireless optical links using the anomalous vortex beam is obtained. It is found that the spiral spectrum of the anomalous vortex beam is less affected by turbulence than that of the Laguerre-Gaussian beam. And thus, the information capacity of wireless optical links using the anomalous vortex beam is larger than that using the Laguerre-Gaussian beam. The influence of beam order, wavelength, topological charge, propagation distance, refractive index structure constant and the radius of receiver aperture on spiral spectrum is investigated. These results contribute to reduce the disturbing effects of atmospheric turbulence on the orbital angular momentum of the vortex beam.

Networked multiple-input-multiple-output for optical wireless communication systems.

With the escalation of heterogeneous data traffic, the research on optical wireless communication (OWC) has attracted much attention, owing to its advantages such as wide spectrum, low power consumption and high security. Ubiquitous optical devices, e.g. light-emitting diodes (LEDs) and cameras, are employed to support optical wireless links. Since the distribution of these optical devices is usually dense, multiple-input-multiple-output (MIMO) can be naturally adopted to attain spatial diversity gain or spatial multiplexing gain. As the scale of OWC networks enlarges, optical MIMO can also collaborate with network-level operations, like user/AP grouping, to enhance the network throughput. Since OWC is preferred for short-range communications and is sensitive to the directions/rotations of transceivers, optical MIMO links vary frequently and sharply in outdoor scenarios when considering the mobility of optical devices, raising new challenges to network design. In this work, we present an overview of optical MIMO techniques, as well as the cooperation of MIMO and user/AP grouping in OWC networks. In consideration of the challenges for outdoor OWC, key technologies are then proposed to facilitate the adoption of optical MIMO in outdoor scenarios, especially in vehicular ad hoc networks. Lastly, future applications of MIMO in OWC networks are discussed. This article is part of the theme issue 'Optical wireless communication'.

Experimental demonstration and analysis of underwater wireless optical communication link: Design, BCH coded receiver diversity over the turbid and turbulent seawater channels

AbstractIn this article, we demonstrate an experimental underwater wireless optical communication (UWOC) system in the presence of air bubbles and weak turbulence for varying turbidity levels of the aquatic optical medium. The major limiting factors of the UWOC system are: absorption, scattering, and beam fluctuations; these effects can be mitigated by employing transmitter/receiver diversity schemes and channel codes. In this proposed system, we have employed receiver diversity (selection combining (SC), majority logic combining (MLC), and equal gain combining (EGC)) techniques augmented with Bose‐Chaudhuri‐Hocquenghem (BCH) codes to improve the performance of on‐off keying modulated UWOC system. The bit error rate (BER) expressions are derived for the proposed system and results are validated using analytic and experimental means. The results show that the proposed system, that is, the receiver employing SC, MLC, and EGC receiver combining techniques augmented with the BCH code provides a transmit power gain of 4, 6, and 8 dB respectively, when compared with the uncoded single‐input single‐output system, at a BER of 10−5.

A high gain and wideband on-chip hybrid plasmonic V-shaped nano-antenna

In this paper, a wideband hybrid plasmonic V-shaped nano-antenna is proposed based on coupled hybrid plasmonic waveguide (CHPW) feeding to increase the surface plasmon propagation length. The CHPW specifications are investigated analytically and numerically to obtain the dispersion relation and propagation length using the genetic algorithm and the finite element method, respectively. Moreover, the proposed V-shaped nano-antenna, with a fractional bandwidth of ∼86% and a maximum efficiency of 98%, is able to receive/transmit optical signals at three telecommunication wavelengths of 850, 1310, and 1550 nm with high realized gains of 10.5, 9.39, and 9.05 dB, respectively. The shape of the radiation pattern, with a main lobe along the antenna axis, makes this antenna appropriate for point-to-point connections in inter- or intra-chip optical wireless links and networks, which is studied comprehensively in this article. Furthermore, to obtain a high optical power signal and tune the antenna orientation, the performance of the antenna is investigated with two different types of array structure, single row and square, and its applications for energy harvesting and beam steering are studied. The fabrication feasibility of the nano-antenna is realizable based on complementary metal-oxide-semiconductor technology.

Effect of moving microalgae on underwater wireless optical links.

Underwater wireless optical communications is a promising technique for addressing short-range data networks, as it provides cost, performance, and complexity improvements as compared with other alternatives, such as acoustic communications or radio frequency links. It is a part of the optical wireless communications research area, since for these applications, broad optical sources such as visible LED lamps can be used. Unless those links are designed for short distances (about 1 m, as in data-muling services on internet-of-things submerged systems), they are still severely affected by channel perturbations, such as scattering due to the presence of particles. This effect is particularly important when considering sensing applications for algae or aquaculture farming, which are becoming a crucial economic resource in many maritime areas. In this work, the effects of moving microalgae on underwater short-range optical links are studied so as to estimate a model for this scattering under dynamic conditions. The statistical parameters over experimentally measured received signal level and signal-to-noise ratio (SNR) are calculated, and the experimental setup is described. Results show that in clear water (no-algae scenario), the water-pump-induced movement effect over the mean and variance of the received optical power can be neglected, while when microalgae are present, the average optical power value decreases and the variance increases with all measured wavelengths. Finally, the SNR penalty due to the movement of microalgae is statistically evaluated.

Experimental study of the effects of thermally induced optical turbulence on underwater wireless optical communication link parameters

In this experimental research, the effects of refractive index fluctuations resulting from temperature fluctuations on some of the most important underwater communication link parameters such as the average received power, signal to noise ratio (SNR) and normalized power variance (aperture averaged scintillation) has been experimentally investigated. Moreover, by introducing a new probability density function which is fitted more appropriately to the experimental data obtained from our simulator tank, the bit error rate (BER) behavior of an underwater wireless optical communication (UWOC) link is also studied. These investigations are essential due to the optical turbulence limiting effects on the performance of UWOC systems. The experimental results indicate that reduction of the aperture averaged scintillation effects by using of optical components in the transmitter and receiver of UWOC links can enhance their performance. In general, increasing the strength of the optical turbulence (by increasing the temperature gradient induced by the rod heater) or the distance of the localized turbulence screen generated by temperature gradient from the transmitter plane, the aperture averaged scintillation decreases and consequently the SNR raises whilst the BER declines. Also, expanding the aperture diameter of the receiver optics reduces the normalized power variance which leads to decrease and increase in the BER and the SNR amounts, respectively. The results are presented as experimental simulation graphs.

500 Gb/s PAM4 FSO-UWOC Convergent System With a R/G/B Five-Wavelength Polarization-Multiplexing Scheme

A 500-Gb/s four-level pulse amplitude modulation (PAM4) free-space optical (FSO)underwater wireless laser transmission (UWOC) convergent system over 100 m free-space transmission with either 10 m piped underwater link or 5 m turbid underwater link is established, employing a red/green/blue (R/G/B) five-wavelength polarization-multiplexing scheme as a demonstration for the first time. Integrating PAM4 modulation with five-wavelength polarization-multiplexing scheme, the channel capacity of FSOUWOC convergent systems is significantly increased with an aggregate data rate of 500 Gb/s [50 Gb/s PAM4/wavelength × 5 wavelengths × 2 polarizations (p- and s-polarizations)]. Results reveal that five R/G/B laser diode (LD) transmitters with two-stage light injection and optoelectronic feedback techniques are capably adopted for 500 Gb/s PAM4 signal transmission. Compared with prior FSO-UWOC convergence and visible light communication using polarization-multiplexing R/G/B LDs, it shows a prominent one with the benefits of high aggregate transmission rate and long-range optical wireless link. Excellent bit error rate performance and accepted PAM4 eye diagrams are attained over a 110-m/105-m FSO-UWOC link. This demonstrated five-wavelength polarization-multiplexing FSO-UWOC convergent system is promising because it not only integrates free-space backbone with underwater optical wireless feeder, but it also substantially multiplies total channel capacity.

Interference Analysis for Optical Wireless Communications in Network-on-Chip (NoC) Scenarios

Optical wireless (OW) communications, besides being of great interest for indoor and outdoor applications, have been recently proposed as a powerful alternative to the existing wired and wireless radio frequency (RF) interconnects in a network-on-chip (NoC). Design and analysis of networks with OW links require a careful investigation of cross-link interference, which impacts considerably the efficiency of systems that reuse the same channel for multiple transmissions. Yet, there is no comprehensive analysis of interference for OW NoCs, and the analyses of crosstalk in optical waveguide communications usually rely on synchronous data transmissions. A novel framework for the analysis of on-chip OW communications in the presence of cross-link cochannel interference and noise is proposed, where asynchronous data transmissions are considered. Self-beating of interfering signals is also considered, which was often neglected in previous literature. The bit error probability (BEP) for arbitrary number of interfering sources is derived as a function of signal-to-noise ratio (SNR), interference powers, detection threshold and pulse shaping, using both exact and approximation methods. The proposed analysis can be applied to both noise- and interference-limited cases, and enables a system designer to evaluate reuse distance between links that share the same optical carrier for simultaneous communication in NoCs.

Multi-Level Analysis of On-Chip Optical Wireless Links

Networks-on-chip are being regarded as a promising solution to meet the on-going requirement for higher and higher computation capacity. In view of future kilo-cores architectures, electrical wired connections are likely to become inefficient and alternative technologies are being widely investigated. Wireless communications on chip may be therefore leveraged to overcome the bottleneck of physical interconnections. This work deals with wireless networks-on-chip at optical frequencies, which can simplify the network layout and reduce the communication latency, easing the antenna on-chip integration process at the same time. On the other end, optical wireless communication on-chip can be limited by the heavy propagation losses and the possible cross-link interference. Assessment of the optical wireless network in terms of bit error probability and maximum communication range is here investigated through a multi-level approach. Manifold aspects, concurring to the final system performance, are simultaneously taken into account, like the antenna radiation properties, the data-rate of the core-to core communication, the geometrical and electromagnetic layout of the chip and the noise and interference level. Simulations results suggest that communication up to some hundreds of μm can be pursued provided that the antenna design and/or the target data-rate are carefully tailored to the actual layout of the chip.

Time Jitter, Turbulence and Chromatic Dispersion in Underwater Optical Wireless Links

The performance of an underwater optical wireless communication link is investigated by taking into account—for the first time and to the best of our knowledge—the simultaneous influence of the chromatic dispersion, the time jitter and the turbulence effects, by assuming chirped longitudinal Gaussian pulse propagation as information carriers. The estimation procedure is presented and a novel probability density function is extracted in order to describe the irradiance fluctuations at the receiver side. Furthermore, the availability of the link is investigated by means of its probability of fade and various numerical results are presented using typical parameters for the underwater optical wireless communication systems.

A high‐speed long‐haul wavelength division multiplexing–based inter‐satellite optical wireless communication link using spectral‐efficient 2‐D orthogonal modulation scheme

SummaryIn this work, we propose a novel multi‐bit/symbol spectral‐efficient optical orthogonal modulation scheme based on simultaneously modulating differential quadrature phase shift keying (DQPSK)‐polarization shift keying (PolSK) in a 16‐channel wavelength division multiplexing (WDM)‐based inter‐satellite optical wireless communication (IsOWC) system. Through numerical simulations, we demonstrate a reliable transportation of 16 × 100 Gbps information over 25 000 km of transmission range with acceptable bit error rate (BER) using the proposed system. Further, the impact of space turbulences (ie, pointing error losses) on the BER performance of the proposed IsOWC link has been evaluated using numerical simulations. The simulation results report a successful transportation of information up to 2.7 μrad receiver pointing error angle with acceptable performance.

Performance Analysis of the Shore-to-Reef Atmospheric Continuous-Variable Quantum Key Distribution

The effects of sea salt concluded in oceanic atmosphere are ubiquitous in practical wireless optical links. Here a shore-to-reef atmospheric continuous-variable quantum key distribution (CVQKD) model is established on the basis of Mie scattering theory, with the aim to characterize the complex case of beam propagation in the atmosphere caused by sea salt particles. The effects on performance of shore-to-reef atmospheric CVQKD under the sea salt particles and relative humidity are also studied. Simulation results show that the increase of particle radius and relative humidity will lead to the degeneration of secret key rate. Extending the channel distance also reduces the secret key rate. This paper provides a basis for the establishment of practical shore-to-reef atmospheric CVQKD model. The research of this paper also gives momentous reference for the study of optical communication channel models with other suspended particles over the ocean.

Hybrid Lightwave/RF Cooperative NOMA Networks

We propose an indoor lightwave downlink wireless communication network with the non-orthogonal multiple access (NOMA) technology, that consists of one visible light communication (VLC) access point (AP) and a pair of randomly located users. Although both users can directly receive information from the AP, the performance of the far user is degraded compared to the near one due to the asymmetrical channel gains. Thus, the user cooperation strategy is proposed to improve the performance of the far user by using mixed VLC/RF relaying technique in parallel with the wireless optical direct link. To efficiently exploit the available heterogeneous links, the concept of cross-band selection combining (CBSC) is introduced, according to which the far user is continuously served by either the mixed VLC/RF or the direct VLC link. Meanwhile, the performance of the proposed scheme is thoroughly investigated and compared to appropriate baselines. To this end, we derive closed-form expressions for the outage probability of each user as well as the system sum throughput. Finally, simulation results are provided to verify the effectiveness of the proposed scheme and the accuracy of the corresponding analysis.

Average capacity analysis of underwater optical wireless communication links over anisotropic strong oceanic turbulence channels.

The effect of anisotropy on the channel capacity of underwater optical wireless communication (OWC) links operating in strong oceanic turbulence is investigated. We consider a Gaussian beam wave propagating through a turbulent oceanic fading channel whose statistical distribution is modelled by a gamma-gamma function. To numerically calculate the channel capacity of the OWC system, related entities of the propagating beam such as coherence length, received signal intensity, and the scintillation index are formulated. Further, in this way, the received signal-to-noise ratio and fading distribution of the channel are obtained. The channel capacity examinations analyzed in this paper depend on the oceanic turbulence parameters, especially for the anisotropic factor of oceanic turbulence, and also depend on the other system parameters such as wavelength, link distance, noise variance, and the quantum efficiency of the photodetector.