Performance Of Optical Communication Systems Research Articles

Code design and interception probability of optical fibre communication system based on optical coding

Traditional optical orthogonal code has small code cardinality, which is vulnerable to the brute-force attacks by eavesdroppers. In order to improve the anti-interception performance of optical fibre communication system based on optical coding, a new family of optical code with large code cardinality is proposed. Based on information-theoretic security, the interception probability of optical fibre coding system is proposed and defined, and the expression of the interception probability is derived for the first time. The influences of extraction distance, extraction ratio, the number of interfering users and the code length on the interception probability are analysed quantitatively. The anti-interception performance of the optical coding system is investigated by numerical analysis and OptiSystem simulation. It is shown that reliability and anti-interception performance can be achieved simultaneously by selecting appropriate code parameters and the number of interfering users.

Performance Enhancement of FSO System using Modified Exponentiated Weibull Channel Model

Background: In free space optics (FSO) communication system, turbulence in the atmosphere leads to the intensity fluctuations of the received signal at the receiver end. Due to this reason, the performance of the free space optical communication system gets affected and results in fading of the received signal. Objectives: Performance improvement of FSO system using sophisticated channel modelling and fading mitigation techniques. Methods: This paper presents the performance improvement of the FSO system using a modified Exponentiated Weibull (MEW) channel model. Moreover, the performance of the FSO system is compared using different channel models such as Gamma-Gamma and Exponentiated Weibull. Results: The results in bit error rate (BER) with respect to different signal to noise ratio (SNR) are obtained using MEW, Gamma-Gamma, and Exponentiated Weibull channel models. In addition, BER is calculated for different aperture sizes such as 1.8, 5, and 13 mm using the proposed channel model to improve the performance of the FSO system. Similarly, the relay assisted technique is utilized to calculate the BER using the proposed channel model. Conclusion: An improvement of free space optical communication system is presented in terms of fading using channel modelling at different atmospheric turbulence conditions. An appropriate channel model is proposed for improving the performance of the FSO communication system using fading mitigation techniques. The proposed MEW channel model best describes the strong atmospheric turbulence induced fading. In addition, the performance of the free space optics system is improved using fading mitigation techniques such as aperture averaging and relay assisted FSO system.

Modeling and performance analysis of oblique underwater optical communication links considering turbulence effects based on seawater depth layering.

Underwater wireless optical communication (UWOC) has been introduced to support emerging high-speed and low latency underwater communication applications. Most of the current studies on UWOC assume that the water temperature and salinity are constant, which can be justified only for horizontal links. In fact, as the temperature and salinity of seawater change with increasing depth, the seawater at different depths is bound to exhibit different optical properties. This implies that for the same link length, the communication system with the transmitter and receiver at different depths, will exhibit different performances. This paper first proposes an oblique optical link model considering turbulence effects, which is based on the layering of temperature and salinity with depth in realistic ocean water. Subsequently, the performance of the optical communication system with vertical and oblique links is analysed by adopting the oceanic power spectrum and seawater data from different ocean areas measured by the global ocean observation buoy, Argo. Our simulation shows that the performance of the underwater optical communication system is worse when the optical transmitter is located at the mixed layer than at the thermocline. When the transmitter is at the thermocline, the communication quality of the system will be worse at environments that temperature and salinity vary more slowly. When the tilt angle of the optical link in the vertical direction is less than 10°, the oblique link can be treated as a vertical link with the same link length.

Performance of Vertical Underwater Wireless Optical Communications With Cascaded Layered Modeling

This paper investigates the performance of vertical underwater wireless optical communication (UWOC) systems in the presence of air bubbles and temperature gradients. We consider a generalized UWOC channel model that contains <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$N$</tex-math></inline-formula> layers, each having the same distribution but with different parameters to consider the vertically inhomogeneous nature of the underwater environment. To capture the effects of air bubbles and temperature gradients on channel statistics, we model each layer with a mixture exponential-generalized Gamma distribution. A closed-form probability density function (PDF) is presented for the end-to-end signal-to-noise ratio. Based on the PDF, we derive the average bit-error-rate (BER) and the ergodic capacity. Moreover, we provide asymptotic BER and ergodic capacity results in simple forms. The performance and behavior of vertical UWOC systems are thoroughly analyzed, and all the derived expressions are verified via Monte Carlo simulations.

Optimal Prediction of Atmospheric Turbulence by Means of the Weather Research and Forecasting Model

The performance of ground-based astronomical observations and free-space optical communication (FSOC) systems suffers from atmospheric turbulence and meteorological conditions. The a priori knowledge of atmospheric conditions several hours before observations allows the programming of astronomical observations (flexible scheduling) to be optimized. In this paper, we present a prediction study based on the Weather Research and Forecasting (WRF) model. It allows the prediction and characterization of a useful set of meteorological parameters relevant to atmospheric physics (e.g., pressure, temperature, relative humidity, wind speed, and direction). Predicted parameters are then injected into an optical turbulence (OT) model to compute the refractive index structure constant C n 2. We performed sets of simulations for Cerro Pachon Observatory in Chile, using the data from the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR). The main goal is to quantify how accurately numerical weather prediction models can reproduce conditions over the complex terrain of the Cerro Pachon area. In order to produce a reliable forecast, meteorological prognostic skills need accurate representations of the physical parameterization options. Three widely used Planetary Boundary Layer (PBL) schemes and two Land Surface Models (LSM) were tested, analyzed, and compared in order to find the optimal WRF configuration. Predictions are compared to in situ measurements coming from balloon-borne radiosoundings. It is determined that the predicted C n 2 are in good agreement with the radiosoundings measurements with a mean relative error (MRE) under 6.4% at all altitudes when using balloon measurements to deduce some parameters such as the outer scale of turbulence L 0, which is used in the OT model. For a fully operational prediction, the MREs between the predictions and the measurements range from 1.4% to 8% according to the different ways to estimate the L 0 profiles. Seasonal statistics are also presented for different meteorological and turbulence parameters.

Effect of partial coherence on signal-to-noise ratio performance of free space optical communication system in weak turbulence

The effect of source coherence on the average signal-to-noise ratio SNR performance of free space optical communication (FSOC) systems operating in weak atmospheric turbulence is investigated with the help of the extended Huygens–Fresnel​ principle. For an FSOC system that uses a partially coherent laser source, first, the received power at the finite-sized receiver aperture is derived. Then, the power scintillation index is evaluated that reflects the aperture averaging. Using these derived optical entities, the variations of SNR are examined versus parameters such as the degree of source coherence, wavelength, link distance, source size, structure constant of atmosphere and the receiver aperture radius. Obtained results show that a decrease in the degree of source coherence has a positive effect on SNR.

Low-latency and efficient retiming and equalizing scheme for a 112-Gbps bandwidth-limited optical PAM-4 system.

The bandwidth limitation of optoelectronic devices is a key constraint for realizing higher capacity short-reach optical communication systems. In this work, we show that the performance of the timing recovery algorithm will significantly impact the performance of bandwidth-limited optical PAM-4 signals. To address this problem, we introduce a moving average filter (MAF) into the Gardner retiming loop to realize ultra-stable retiming without any pilot symbols. It enables a significantly reduced timing jitter, especially for bandwidth-limited conditions. Based on simulations, we show that the proposed retiming scheme will give a stable sampling phase, which is beneficial to the performance of the decision feedback equalizer. Compared to the system based on the traditional Gardner timing synchronization algorithm, the 2-dB power budget gain is realized. The proposed method improves the equalization performance of short-reach optical communication systems where complexity and processing latency are important concerns.

Comparative analysis of underwater wireless optical communication system

The performance of an underwater wireless optical communication (UWOC) system is affected by many factors, such as channel conditions, modulation and detection techniques, type of optical source, etc. The UWOC system performance was investigated in the presence of weak and strong turbulence. Pure sea was the water type used for the performance analysis. Binary phase shift keying, quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation, and non-return to zero-on–off keying were used for comparison. Further, a continuous wave laser was used as the optical source, and photodetectors are at the receiver. The performance of the UWOC system was analyzed in terms of the bit error rate (BER). The BER performance was evaluated using the log-normal probability density function (PDF) in weak turbulence and the gamma-gamma PDF in strong turbulence. In all modulation techniques, QPSK had the best BER performance. The BER was reduced further using avalanche photodiode (APD) as the photodetector instead of positive-intrinsic-negative diode for the same transmitter power and channel conditions. In weak turbulence, a BER of 10 − 8 was achieved at a transmitter power of 12.5 dBm using the QPSK modulation technique and APD as the photodetector. For the same conditions, a BER of 9 × 10 − 7 was achieved in strong turbulence. The multiplication gain, dark current, and carrier ionisation ratio of the photodetector were optimized for better BER performance in the UWOC system.

Evolution of temporal broadening of ultrashort optical pulse propagation in general ocean turbulence.

Theoretical and experimental explorations have demonstrated that both anisotropy and unstable stratification exist in general ocean turbulence. Recent analyses of temporal broadening of ultrashort optical pulses in oceanic turbulence have adopted the assumptions that the propagation path in the z direction was isotropic, or turbulent cells are simply premised on circular symmetry in the xy plane. In this paper, circular symmetry of turbulent cells in the xy plane is no longer maintained, and two anisotropic factors describing the anisotropic scales in the xy plane are introduced to study the temporal broadening of ultrashort optical pulses. Moreover, unstable stratification of oceanic turbulence indicates the eddy diffusivities of temperature and salt are no longer equal. By Rytov approximation and the temporal-moments method, a new model is proposed for the temporal broadening of ultrashort optical pulse propagation in general ocean turbulence. We focus on the effects of asymmetric turbulent cells, unstable stratification, and other characteristics in general ocean turbulence on the temporal broadening. This work provides a theoretical basis for improving the transmission quality of ultrashort optical pulses and the performance of underwater optical communication systems.

Impact of orientation-based solar light noise on the performance of underwater optical wireless communication system and noise cancellation

Absorption, scattering, turbulence, and ambient light affect the performance of underwater optical wireless communication (UOWC) systems. Solar light is a form of undersea ambient light that causes interference to the UOWC system severely, and various optical components have been used to suppress solar noise. Therefore, the UOWC channel realization must ensure suitable system-based parameters to offer optimal performance. However, UOWC system performance lacks verification with the system receiver (Rx) facing multiple orientations in a high intense solar light at undersea depth. This work discusses the impact of solar light on the UOWC system with its Rx facing different orientations. It is found that the Rx orientation plays a significant role in determining the system performance. Furthermore, we numerically demonstrate the cancellation of solar noise interference using back-to-back Rx with differential amplifier mechanisms and better noise cancellation for Rx horizontal orientation.

Effect of Partial Coherence on Signal-to-Noise Ratio Performance of Free Space Optical Communication System in Weak Turbulence

Effect of Partial Coherence on Signal-to-Noise Ratio Performance of Free Space Optical Communication System in Weak Turbulence

Influence of underwater composite channel on performance of GMSK wireless optical communication system

Influence of underwater composite channel on performance of GMSK wireless optical communication system

BER Performance of UWOC With APD Receiver in Wide Range Oceanic Turbulence

In this paper, the bit error rate (BER) performance of underwater wireless optical communication (UWOC) systems with on-off keying (OOK) modulation and an avalanche photodiode (APD) detector are analyzed in a wide range of oceanic turbulence environment, such as lognormal, Gamma, K, Weibull, and Exponentiated Weibull distributions. To achieving this goal, a new mathematical system model is established, which has not only considered the aggregated degrading phenomena, namely the absorption, scattering, and turbulence, but also been suitable for describing UWOC with APD receiver. Then the closed-form expressions of the BER performances of UWOC with APD for different turbulence distributions are derived in detail by subdividing the working situation into high and lower inter-symbol interference (ISI) scenarios. Theoretical analysis and computer simulations show that no matter what the ISI levels are, the derived closed-form expressions for system BER under various turbulences can approximate the accurate integral ones well. This is obviously helpful to the design and performance evaluation of UWOC systems.

Impulse response of underwater optical wireless channel in the presence of turbulence, absorption, and scattering employing Monte Carlo simulation.

Recently, compared with acoustic and radio methods, underwater optical wireless communications has been considered as a high-speed and high-bandwidth transmitting method at a lower cost. Absorption, scattering, and optical turbulence are three destructive phenomena that affect the performance of underwater optical communication systems. In this work, we use computer simulations to mimic the statistical behavior of underwater media employing the Monte Carlo method. Our simulation results for optical turbulence are in good agreement with the lognormal probability density function, which describes weak turbulence well, and they deviate as the turbulence moves away from weak. By considering the combined effect of absorption, scattering, and turbulence (AST) phenomena, we obtain the underwater channel's impulse response (IR). We demonstrate that there is no noticeable difference between the mean of ensemble IRs of the AST channel and the IR of the channel when turbulence is not taken into account. Moreover, our results predict that tripling the coastal link length from 10 to 30 m increases the average variance of sample IRs of the AST channel from their ensemble average by more than five times.

Subtraction dual‐wavelength for enhanced transmission performance of free‐space optical communication over turbulence effect

Performance of a free-space optical communication system is influenced by atmospheric turbulence which degrades signal transmission quality. A gamma–gamma channel model is employed to characterise this turbulence under weak-to-strong conditions. The proposed dual diffuser modulation using subtraction dual-wavelength concept is combined with a phase screen diffuser to improve signal transmission. Its performances are compared with conventional intensity modulation-direct detection on off keying and partial coherent beam on off keying techniques. In comparison, results show that the dual diffuser modulation demonstrates superior performances in both received power and bit rate transmission under different turbulence conditions.

Hardware optimization of dual-stage carrier-phase recovery for coherent optical receivers

We present an optimized dual-stage CPR method based on pilot symbols and a blind phase search algorithm (BPS) for M-QAM coherent optical transmission systems, targeting an efficient hardware implementation. A comprehensive optimization of the key CPR parameters is performed, including, pilot-rate, number of test phases, angle interval and moving average filter size. The optimization process is validated through numerical assessment of the optical communication system performance using 16-QAM, 64-QAM, and 256-QAM modulation formats at 64 GBaud. The optimized dual-stage CPR is demonstrated to operate with a very low number of test phases in the second stage BPS algorithm. In addition, it supports more than 10× higher laser linewidth than the standard CPR based on pilot symbols. Besides, a reduction of more than 90%, in terms of hardware complexity is achieved with respect to the standalone application of the BPS algorithm. The optimized dual-stage CPR is also validated through hardware implementation based on VHDL, and its gate-level complexity is assessed for a commercial off-the-shelf Xilinx Virtex-7 FPGA.

Adaptive optics technology and urban horizontal link laser communication system

The wavefront distortion caused by atmospheric turbulence seriously affects the performance of free-space optical communication (FSOC) system. Adaptive optics (AO) technology has been proven to be an effective method to suppress atmospheric turbulence. Two laser communication terminals equipped with AO system are designed and manufactured, which are used in the horizontal link near the ground. In the laboratory simulation, 100-Gbps data transmission can be realized under moderate turbulence (D / r0 < 10), the bit error rate (BER) is better than 1 × 10 − 6, and the measured single-mode fiber coupling efficiency is 42% to 48%. 10-Gbps data transmission can be realized under strong turbulence (D / r0 > 10), and the BER is better than 1 × 10 − 6. The BER is high in the practical communication test under the urban horizontal link of 1 km (D / r0 = 10), the BER is 1 × 10 − 3, and the coupling efficiency is 12% to 20%. Finally, to achieve the goal of 100-Gbps FSOC for near ground and long distance, based on the experimental performance of the existing two sets of laser communication system, the solution of laser communication system is proposed, which provides a reference for relevant researchers.

Effects of adaptive optics on bit error rate of M-ary PPM oceanic optical wireless communication systems with aperture averaging in strong turbulence

Scintillation is the result of oceanic turbulence reducing the bit error rate (BER) performance of oceanic optical wireless communication (OWC) systems. The scintillation, also known as intensity fluctuations, occurs due to the turbulence-induced wavefront deformations. The correction of deformations by adaptive optics (AO) reduces the scintillation effect of turbulence and results in improved BER performance. In this paper, an oceanic OWC (OOWC) system that has a Gaussian laser beam at the transmitter, finite-sized circular aperture at the receiver, employing M-ary pulse position modulation (PPM) and operating in strong oceanic turbulence, is considered. Improvement in the BER performance of the OOWC system is examined with the implementation of AO correction. Comparison of BER performances between the AO and non-adaptive optics OOWC systems is shown by calculating the metric defined. BER of M-ary PPM OOWC links is evaluated over gamma–gamma fading channels. The modified Rytov theory together with the Zernike filter functions is used to find the AO corrected aperture averaged scintillation index where extended Huygens–Fresnel technique is used to obtain the average received signal power.

Error performance optimization utilizing the dynamic detection cycle in a SPAD-based free space optical communication system.

Afterpulsing is a critical non-ideal factor of the single photon avalanche diode (SPAD) at telecommunication wavelength, which limits the performance of a SPAD-based free space optical communication (FSO) system. Afterpulsing probability (AP) is highly dependent on the SPAD detection cycle. In a conventional SPAD-based system, the detection cycle is set to a relatively large constant time length to mitigate afterpulsing. However, it will limit the SPAD counting rate and degrade system performance. In order to improve system performance, a new scheme of the dynamic detection cycle is proposed to adapt to different operation conditions. Then, a multi-exponential model of AP is built and fitted with the test data of the single photon detector QCD-300. Furthermore, based on the joint model of the multi-exponential model of AP and bit error rate (BER) model of the SPAD-based FSO system, a simple and effective optimization algorithm is developed to optimize the detection cycle. And the optimization of the detection cycle under different operation conditions is also investigated. The results indicate that optimal detection cycle is dominated by signal light, background radiation, and SPAD gate length. Compared with a conventional scheme with a constant detection cycle, the proposed scheme with the dynamic detection cycle can improve system error performance effectively.

Secrecy Performance of FSO Using HD and IM/DD Detection Technique Over F-Distribution Turbulence Channel With Pointing Error

This work is concerned with the secrecy performance of a free-space optical (FSO) communication system over Fisher-Snedecor <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {F}$ </tex-math></inline-formula> -distribution turbulence fading channel. Considering the influence of pointing error, atmospheric turbulence and different detection technique (i.e., the intensity modulation with direct detection and heterodyne detection), we derive the exact closed-form expressions for the average secrecy capacity (ASC), the secrecy outage probability (SOP) and the strictly positive secrecy capacity (SPSC) in terms of Meijer’s G-function. Furthermore, the asymptotic expression for SOP is analyzed as the electrical signal-to-noise ratio approaches infinity. Monte Carlo simulations are provided to validate analytical results.