OWC System Research Articles

Design and investigation of symmetric bidirectional VCSEL laser incorporated extensive reach OWC system using considering channel impairments for beyond 5G application

Design and investigation of symmetric bidirectional VCSEL laser incorporated extensive reach OWC system using considering channel impairments for beyond 5G application

Detailed investigation of the local flow-field in a Wells turbine coupled to an OWC simulator

One of the most promising technologies for sea-wave energy conversion is the one based on the OWC principle. In a system of this type, the oscillatory motion of the sea waves is converted into a bi-directional air flow which is commonly exploited by means of a self-rectifying turbine such as the Wells turbine, the simplest and most reliable device for this purpose. The vast majority of experiments on Wells turbines and OWC devices has analyzed their performance from a global point of view, often in experimental facilities where the turbine was operated under stationary flow conditions.This paper presents the results of the experimental investigation carried out on a Wells turbine, by measuring the flow field both upstream and downstream of the rotor, in a laboratory set-up capable to reproduce the bi-directional airflow typical of an OWC system. The investigation aims to evaluate the local performance of the Wells turbine under unsteady flow conditions.The experimental measurements allow the identification of the loss components that affect the performance of the turbine. Viscous losses, due to the aerodynamic of the rotor cascade, represent the main contribution to the total losses, and appear larger than kinetic energy losses at the machine exhaust.

Adaptive optics compensation of M-ary pulse position modulated communication systems in anisotropic non-Kolmogorov turbulent atmosphere

Adaptive optics compensation effect on the performance of an optical wireless communication system (OWC) employing M-ary pulse position modulation (PPM) scheme in anisotropic non-Kolmogorov turbulent atmosphere is investigated. Avalanche photodetector (APD) is used at the receiver side and log-normal channel that models the weak turbulence conditions is utilized. Anisotropy, generally resulting in better performance in OWC systems operating in the turbulent medium, combined with the adaptive optics applications will enhance the bit-error-rate (BER) of the OWC systems significantly. Results are obtained depending on various parameters for both the turbulent atmosphere and the receiver. Our work gives OWC system designers a perspective to optimize their design.

Signal-to-noise ratio with adaptive optics compensation in non-Kolmogorov weak turbulent atmosphere

This study investigates the average signal-to-noise ratio ⟨ SNR ⟩ at the shot-noise limited receiver of an optical wireless communication system (OWC) that uses collimated Gaussian beam with adaptive optics correction in non-Kolmogorov weak turbulent atmosphere. ⟨ SNR ⟩ and the variation in the percentage ⟨ SNR ⟩ are calculated and the results are presented against various adaptive optics correction modes, non-Kolmogorov power-law exponent, receiver aperture size, Gaussian beam source size, photodetector quantum efficiency, electronic bandwidth, link length, and turbulence strength. Adaptive optics correction is analyzed for tilt, defocus, astigmatism, coma and trefoil modes and their variations. The effects of adaptive optics compensation are examined that cause an increase in SNR, hence improve the performance of an OWC system using collimated Gaussian beam and operating in the non-Kolmogorov weak turbulent atmosphere.

Overall performance of a model OWC system under the free-spinning mode: An experimental study

The oscillating water column is the most widely used wave energy converting technology for onshore applications. In the previous studies, the performances of the chamber and the turbine were investigated separately, or some substitutions were employed to induce the damping effects. In this study, an OWC model employing the cubic chamber and the impulse turbine is studied experimentally in a wave flume. The model turbine operates under the free-spinning mode with the resistance connecting to the electrical generator as the external loads. Regular wave conditions are applied to generate different wave scenarios to investigate the hydrodynamic and aerodynamic performance of the chamber-turbine system of the OWC model. The primary-stage, secondary-stage, and overall efficiencies under the free-spinning mode for various wave conditions and load coefficients are presented. The peak efficiencies are compared with those under the fixed rotation-speed mode. Experimental results indicate that the chamber length should be designed according to the local predominant wave climate, and the proper external resistances should be chosen to enhance its overall efficiency.

Amplifier Operated Multiple OWC System to Improve Q-factors in Optical Communication

Amplifier Operated Multiple OWC System to Improve Q-factors in Optical Communication

Real-Time 2.2-Gb/s Water-Air OFDM-OWC System With Low-Complexity Transmitter-Side DSP

Underwater optical wireless communication (UOWC) is of great interest to the academic and the industry community. In this article, we propose a low-complexity and effective joint transmitter-side digital signal processing (DSP) including geometric shaping, time-domain tone reservation (TR), and clipping. The peak-to-average power ratio reduction performance and implementation complexity of the proposed time-domain TR are extensively analyzed. We then develop a real-time 2.2-Gb/s system, which is optimized for its shaping ratio and clipping ratio. With the help of the efficient DSPs, an 8-dB received optical power enhancement is realized under the bit-error-rate threshold of 3.8 × 10−3. We successfully demonstrate a time-multiplexed four 4K video transmission in real-time using the proposed scheme over a 3.6-m underwater and 8-m free-space channel. The implementation details, as well as the analyses of system stability, resource utilization, and latency, are presented. The results validate the feasibility and effectiveness of the proposed scheme and a 2.2-Gbit/s real-time OWC system is demonstrated for a water-air communication link.

Performance enhancement of the average spectral efficiency using an aperture averaging and spatial-coherence diversity based on the modified-PPM modulation for MISO FSO links

The performance evaluation and enhancement of the proposed model for a free-space optical (FSO) communication links using the modified pulse-position modulation (MPPM) and spatial pulse-position modulation (SPPM) is analyzed in this paper. The average spectral efficiency (ASE) is investigated for the FSO communication using MPPM, SPPM, and spatial-coherence diversity in a coherent optical wireless communication (coherent OWC) systems. We proposed the hybrid MPPM/SPPM as a new modulation to improve the ASE performance of pulse-position modulation. To reduce the atmospheric turbulence (AT) effects and scintillation, the use of an aperture averaging (AA) technique has been proposed in our work. Here, we analyze the pointing error (PE) effects on the performance of M−ary MPPM based on a multiple-input single-output (MISO) of the repetition codes (RCs) and FSO systems using SPPM. The enhancement in the ASE can be accomplished with an increase in the receiver aperture and decreasing the beam radius in the presence of PEs. The ASE performances of 53 bits/s/Hz and 37 bits/s/Hz are achieved with the average transmitted optical power of 10 dBm for an aperture diameter of 10 cm over the effect of without and with PEs for coherent OWC technique, respectively. The simulation results obtained for the bit-error-rate (BER) and ASE against various AT conditions are compared with the diversity techniques and proved that both of the hybrid SPPM and MPPM/MISO−RCs based on the AA and spatial diversity offers better performance in the average channel capacity (ACC). Numerical results show that the ASE performance can be improved using the spatial-coherence diversity in coherent OWC technique. The spatial-coherence diversity in a coherent OWC system significantly outperforms on-off keying and MPPM system even in the presence of PEs under strong turbulence. Efficient estimation of the signal-to-noise ratio also allows accurate quantification of the BER and ACC for adequate quality of service and network link resource allocation.

Dynamic interaction between OWC system and Wells turbine: A comparison between CFD and lumped parameter model approaches

Systems composed of an oscillating water column and a self-rectifying turbine are among the most well-known devices for wave energy conversion. These systems have been largely studied experimentally and numerically, focusing either on OWC hydrodynamic or power-unit performance. The turbine has been studied in experimental facilities that reproduced the periodic motion of the water column in the chamber by an artificially moved piston, while OWC performance has been generally studied neglecting or crudely simplifying the turbine interaction.The objective of this work is to introduce a lumped parameter model, able to reproduce the interaction of the turbine with the air mass in the OWC chamber. Results are compared with experimental and CFD data, and demonstrate that the often discussed hysteresis is caused by compressibility effects in the air chamber and not, as previously assumed, by turbine aerodynamics. The model is also used to estimate energy loss and phase delay in full-scale OWC systems, proving the importance of a correct sizing of OWC-turbine systems. The lumped parameter model allows to rapidly isolate parameters with the largest influence on system performance, and could be integrated with existing wave-to-wire models to improve the understanding of the hydrodynamic/aerodynamic interaction in the overall system.

Comparison of inflow and outflow radial air turbines in vented and bidirectional OWC wave energy converters

This study analyses the aerodynamic performance of two unidirectional-radial-air-turbine configurations; inflow and outflow. These turbines were studied as the Power-Take-Off unit for application on a vented-OWC and a conventional-bidirectional-OWC with a twin-turbine topology, forming four different turbine-OWC configurations. These configurations were evaluated in terms of full-scale power extraction using extrapolated hydrodynamic experimental data of irregular waves for a King Island test site. The power extraction capacity was evaluated by defining a lower and upper bound of power generation under fixed and controlled-RPM schemes and the energy produced in each configuration was then compared against a state-of-the-art twin-rotor turbine. It was found that the difference between these power extraction bounds was lower in case of the outflow turbine, which shows this turbine is less sensitive to RPM variations than the inflow turbine. In addition, due to its lower resistance to the flow in direct mode, the outflow turbine has a smaller full-scale size than the inflow turbine. It was concluded that the outflow turbine provides better efficiency in a twin-turbine-OWC system, while the inflow turbine yields higher conversion efficiency in a vented-OWC system. The vented OWC equipped with a radial inflow turbine can obtain comparable power to the bidirectional OWC system.

Comparative investigation of CSRZ‐DQPSK, DRZ‐DQPSK, and MDRZ‐DQPSK modulation techniques in MDM IS‐OWC system

AbstractThe article presents the comprehensive approach of designing MDM in Inter‐Satellite OWC system using CSRZ‐DQPSK, DRZ‐DQPSK, and MDRZ‐DQPSK modulation system. The 64 Laguerre Gaussian modes are integrated to enhance the system's capability. The proposed system is designed for 64 channels having distances varied from 900 to 4500 km at various data rates. The mode division multiplexing concept is implemented over WDM because it uses single laser source and enhance the capacity of optical networks. Performance of proposed system is examined by comparing the eye diagrams and Q‐factor. It is observed from the outcomes that MDRZ‐DQPSK modulation presents superior Quality factor of 19.42 then DRZ‐DQPSK and CSRZ‐DQPSK Modulation with the favorable distance upto 4500 km at 10 Gbps bit rate. MDRZ‐DQPSK modulation technique performs well till 40 Gbps whereas CSRZ‐DQPSK Modulation exhibits least Q‐factor due to high consumption of bandwidth but it saves cost and preferable for long reach transmission.

Average spectral efficiency analysis of FSO links over turbulence channel with adaptive transmissions and aperture averaging

In our paper, the impact of adaptive transmission schemes: (i) optimal rate adaptation (ORA) and (ii) channel inversion with fixed rate (CIFR) on the average spectral efficiency (ASE) are explored for free-space optical (FSO) communications with On–Off Keying (OOK), Polarization shift keying (POLSK), and Coherent optical wireless communication (Coherent OWC) systems under different turbulence regimes. Further to enhance the ASE we have incorporated aperture averaging effects along with the above adaptive schemes. The results indicate that ORA adaptation scheme has the advantage of improving the ASE performance compared with CIFR under moderate and strong turbulence regime. The coherent OWC system with ORA excels the other modulation schemes and could achieve ASE performance of 49.8 bits/s/Hz at the average transmitted optical power of 6 dBm under strong turbulence. By adding aperture averaging effect we could achieve an ASE of 50.5 bits/s/Hz under the same conditions. This makes ORA with Coherent OWC modulation as a favorable candidate for improving the ASE of the FSO communication system.

Bidirectional air turbines for oscillating water column systems: Fast selection applying turbomachinery scaling laws

The collector of an oscillating water column system (OWC) for wave energy utilization requires a bidirectional turbine that copes with pneumatic power while providing specified impedance or, in terms of an OWC designer, “damping”. Damping is realized by keeping to a specific flow rate through the turbine at a given pressure head due to the individual performance characteristic of the turbine. With the number of turbine designs increasing designers of OWC systems are facing more options to select and dimension a bidirectional turbine. Energy yield, size and hence cost of the turbine and electric generator, operational behaviour, envisaged control strategy and noise emitted by the turbine are possible criteria for selection.The primary objective of this paper is to describe a simple procedure for making a first choice of a turbine for a particular OWC application. Here we confine ourselves to a family of reaction type of turbines (axial-flow Wells and mixed-flow turbines by Moisel) with their approximately linear pressure head/volume flow rate characteristics. Starting point is the set of non-dimensional steady-state characteristics of each turbine in the family. Utilizing standard scaling laws and a very simple time domain model for the cyclic turbine operation (i.e. based one single sea state and turbine operation assumed to be fixed rotational speed), first estimates of turbine size and rotor speed, number for stages or flows, and performance curves can be determined. The resulting turbine may also serve as a starting configuration for a refined analysis, e.g. the optimization of the turbine and the complete OWC system, utilizing more complex stochastic models. Three case studies illustrate the application of the method: selection and scaling of turbines, effect of collector parameters, turbines in series and parallel.

Aperture averaging effects on the average spectral efficiency of FSO links over turbulence channel with pointing errors

The average spectral efficiency (ASE) is investigated for the free space optical (FSO) communications employing On-Off keying (OOK), Polarization shift keying (POLSK), and Coherent optical wireless communication (Coherent OWC) systems with and without pointing errors over the Gamma-Gamma (GG) channels. Additionally, the impact of aperture averaging on the ASE is explored. The influence of different turbulence conditions along with varying receiver aperture has been studied and analyzed. For the considered system, the exact average channel capacity (ACC) expressions are derived using Meijer G function. Results reveal that when pointing errors are introduced, there is a significant reduction in the ASE performance. The enhancement in the ASE can be achieved with an increase in the receiver aperture across various turbulence regimes and reducing the beam radius in the presence of pointing errors, but the rate of increment of ASE reduces with a larger diameter and it is saturated finally. The coherent OWC system provides better ASE performance of 49 bits/s/Hz at the average transmitted optical power of 5dBm with an aperture diameter of 10cm and 34 bits/s/Hz without and with pointing errors under strong turbulence respectively.

SER analysis of selective DF relaying based OWC system with generalised turbulence

AbstractIn this paper, we analyse the error performance of selective decode‐and‐forward based optical wireless communication system utilising subcarrier intensity modulation for arbitrary M‐ary signalling over generalised ‐distributed atmospheric turbulence. The ‐distribution is a generic statistical model for characterising the atmospheric turbulence‐induced fading and unifies several existing statistical models for atmospheric turbulence. In particular, we derive a closed‐form expression of the approximate average symbol error rate for the considered system with independent and non‐identically distributed optical links. Furthermore, we analytically evaluate the diversity order for various distributions (e.g. gamma–gamma distribution, K‐distribution and negative exponential distribution), characterised by the generalised ‐distribution model. The analytical results have been verified by simulations. Copyright © 2016 John Wiley & Sons, Ltd.

2.50 Gbit/s optical wireless communication system using PPM modulation schemes in HAP-to-satellite links

HAP-to-satellite, one of the important applications of FSO/OWC technology, which will be deployed in space in the near future. In this paper, we have presented analysis of 2.50Gbit/s OWC system using PPM modulation schemes and a distance of 2500km was achieved at BER 10−6 in HAP to satellite links.

Development of a novel nonlinear system identification scheme for the pressure fluctuation inside an oscillating water column-wave energy converter Part I: Theoretical background and harmonic excitation case

In this work, we present a nonlinear system identification method, modeling the pressure fluctuation inside the chamber of an oscillating water column wave energy converter (OWC–WEC) under monochromatic excitation. The systemic scheme, upon which the identification is based, is a Wiener–Hammerstein cascade and thus the functional analogue of the model is a truncated Volterra series.The Fourier spectrum of the fully nonlinear thermodynamic equations investigated for various combinations of excitation amplitudes and frequencies revealed that the first three harmonics are the most significant ones and explain adequately the response, while the relative significance of second and third harmonics varies strongly with respect to the excitation parameters.To tame such nonlinear characteristics, a novel approach is developed for the systemic modeling and identification of the OWC system. The key concept is the construction of filter banks and static-polynomial coefficient banks, associated with the excitation properties (amplitude and frequency) resulting in an accurate and robust systemic model sensitive to the inherent nonlinear characteristics of the actual system.Furthermore, we present how this novel systemic concept can be appropriately extended in order to be able to treat multi-chromatic excitations. This extension will be fully developed in a forthcoming work.

Performance analysis of a wave energy converter using numerical simulation technique

A general purpose viscous flow solver Ansys CFX was used to study a Savonius type wave energy converter in a 3D numerical viscous wave tank. This paper presents the results of a computational fluid dynamics (CFD) analysis of the effect of blade configuration on the performance of two Savonius rotors for wave energy extraction. A piston-type wave generator was incorporated in the computational domain to generate the desired incident waves. A complete OWC system with a 3-bladed Savonius rotor was modeled in a three dimensional numerical wave tank and the hydrodynamic conversion efficiency was estimated. The flow over the rotors was assumed to be two-dimensional (2D), viscous, turbulent and unsteady. The CFX code was used with a solver of the coupled conservation equations of mass, momentum and energy, with an implicit time scheme and with the adoption of the hexahedral mesh and the moving mesh techniques in areas of moving surfaces. Turbulence was modeled with the k-e model. The results indicated that the developed models are suitable to analyze the water flows both in the chamber and in the turbine. For the turbine, the numerical results of pressure and torque were compared for the two cases.