Atmospheric Turbulence Channels Research Articles

Performance of Generalized QAM/FSO Systems With Pointing Misalignment and Phase Error Over Atmospheric Turbulence Channels

This article investigates the performance of free-space optical (FSO) systems using sub-carrier quadrature amplitude modulation (SC-QAM) signaling for broadband terrestrial applications. While previous studies ignored the influence of phase noise, we theoretically analyze and derive closed-form expressions of the system's average bit error rate (ABER) considering the combined effect of pointing misalignment, turbulence-induced fading, and phase error. The log-normal distribution is used to model the atmospheric turbulence in the weak regime while the gamma-gamma distribution is applied for modeling the turbulent atmosphere in the moderate and strong regimes. The phase error, which becomes complicated under the effect of atmospheric turbulence, is modeled by Tikhonov distribution. The numerical results reveal that the influence of the phase error on the system performance is dominant in the weak turbulence and high order modulation while the fading becomes more severe in the moderate-to-strong turbulence regime. The accuracy of analytical results is also validated by Monte-Carlo simulations and a good match can be confirmed.

聚焦贝塞尔光束在大气湍流中的螺旋相位谱

聚焦贝塞尔光束在大气湍流中的螺旋相位谱

Deep Learning for Improving Performance of OOK Modulation Over FSO Turbulent Channels

Free space optical (FSO) communication technology has become increasingly advanced with capabilities of high speed, high capacity, and low power consumption. However, despite the great potential of FSO, its performance is limited in a turbulent atmosphere. Atmospheric turbulence causes scintillation in the FSO propagated signals, leading to an increase in the bit error rate (BER) performance of the recovered signals at the receiver. In this paper, we demonstrate that the use of deep learning (DL) detection methods could overcome these limitations. We present a new detection method of on-off keying (OOK) modulated signals by using different models of DL over different strength FSO turbulent channels, without the need for prior knowledge of the parameters of the channel. The demonstrated DL decoders improve the performance of the FSO turbulent channel and decrease the power consumption. Moreover, the demonstrated DL models also work faster than maximum likelihood (ML) methods with perfect channel estimation decoders, with even slightly better performance because of the turbulence, thus enabling realization of FSO over turbulent atmospheric channels.

EVOLUTION OF FREE SPACE OPTICAL SYSTEM BASED ON SUBCARRIER INTENSITY MODULATION OVER SOME DEGREE OF TURBULENT

The use of Binary Phase Shift Keying (BPSK) Subcarrier Intensity Modulation (SIM) in a Gamma-gamma turbulent atmospheric channel was used to study the behavior of Free-Space Optical (FSO) communication system. Subcarrier Intensity Modulation was used to obviate a requirement for an adaptive threshold detector. At the receiver, direct detection was used and each subcarrier is subsequently demodulated clearly. The optimal Maximum Ratio Combining (MRC), the Equal Gain Combining (EGC), and the Selection Combining (SC) was used to combine the received signals linearly. Combining diversity made the effect of irradiance fading less severe. The Bit Error Rate (BER) equations are gotten by putting both the Additive White Gaussian Noise and Gamma-gamma intensity fluctuations into consideration. The simulation results show that at any given Bit Error Rate (BER), the performance of MRC was exceptional compared to the other two techniques. The overall results also put forward the model capability of reducing atmospheric turbulence especially for the 5G wireless network in the terrestrial link.

M-ary phase-shift keying-based single-input-multiple-output free space optical communication system with pointing errors over a gamma-gamma fading channel.

The performance of free space optical (FSO) communication systems is severely affected due to a number of reasons, such as atmospheric impairments, fading due to turbulence, and misalignment between transmitter and receiver sections. In this paper, we have analyzed the atmospheric turbulence and pointing error effect on M-ary phase-shift keying modulation (M-PSK)-based FSO system link performance over a gamma-gamma fading channel. In our analysis, the effect of a weak-to-strong atmospheric turbulence channel condition and pointing errors are taken into consideration. To deal with the performance degradation, a single-input-multiple-output (SIMO) link along with maximal ratio combining (MRC) diversity scheme is employed. The closed-form expressions for the estimation of average bit error rate (BER), outage probability, and spectral efficiency are derived in terms of the Meijer G-function. The analytical results are presented to indicate the effect of turbulence and pointing errors on BER. The outage probability and spectral efficiency of the proposed SIMO-FSO system with MRC technique improved significantly. The accuracy of all the analytical results is verified by Monte Carlo simulation.

Tractable Optical Channel Modeling Between UAVs

More recently, the research on potential use of free-space optical (FSO) link as a powerful communication link between unmanned aerial vehicles (UAVs) has created much interest in academia and industry. Due to the need of higher number of UAV-based FSO links relative to the conventional ground-based FSO links, the optimum design of UAV-based FSO system parameters (such as optimum values for beam divergence angle, photodetector size, receiver lens radius, and transmit power, among many others) is much more needful and challenging relative to the ground-based counterpart. Moreover, to avoid the time consumed in Monte-Carlo simulations, existence of a simple and tractable channel model is very important and necessary. To address this need, in this paper, for the weak turbulence conditions, we construct a novel channel model for the considered system model under the effect of log-normal atmospheric turbulence channel. Then, for moderate to strong turbulence conditions, a novel closed-form statistical channel model is derived for Gamma-Gamma turbulence channel. The provided channel models, despite being simple and tractable, include the combined effects of atmospheric turbulence as well as the pointing errors. These also include the effects of receiver field-of-view limitation and inherent position, and orientation deviations of UAVs. Subsequently, for better performance analysis, the closed-form expressions for the outage probability and bit error rate (BER) are derived. Finally, the validity of the proposed novel channel models as well as the closed-form expressions for the outage probability and BER are confirmed by employing Monte-Carlo simulations. The developed results can therefore be applied as a benchmark for finding the optimal tunable parameters of UAV-based FSO links under different channel conditions and different levels of UAV instability without resorting to time-consuming Monte-Carlo simulations.

Fading channel estimation for free-space continuous-variable secure quantum communication

We investigate estimation of fluctuating channels and its effect on security of continuous-variable quantum key distribution. We propose a novel estimation scheme which is based on the clusterization of the estimated transmittance data. We show that uncertainty about whether the transmittance is fixed or not results in a lower key rate. However, if the total number of measurements is large, one can obtain using our method a key rate similar to the non-fluctuating channel even for highly fluctuating channels. We also verify our theoretical assumptions using experimental data from an atmospheric quantum channel. Our method is therefore promising for secure quantum communication over strongly fluctuating turbulent atmospheric channels.

Improving Continuous-Variable Quantum Key Distribution in a Turbulent Atmospheric Channel via Photon Subtraction

The transmission performance of continuous-variable quantum key distribution (CVQKD) in turbulent atmospheric channel is lower than CVQKD in fiber channel due to the large loss of quantum entanglement in turbulent atmospheric channel. In this paper, we propose a new approach to prolong the transmission performance of CVQKD in a turbulent atmospheric channel by using photon subtraction operation since the proposed approach can effectively enhance the entangled state between quantum. Simulation analysis shows that photon subtraction operation can effectively improve the security key rate of continuous-variable quantum key distribution, and the optimal performance can be achieved when only one photon is subtracted.

A Unified Statistical Model for Atmospheric Turbulence-Induced Fading in Orbital Angular Momentum Multiplexed FSO Systems

This paper proposes a unified statistical channel model to characterize the atmospheric turbulence induced distortions faced by orbital angular momentum (OAM) in free space optical (FSO) communication systems. In this channel model, the self-channel irradiance of OAM modes as well as crosstalk irradiances between different OAM modes are characterized by a Generalized Gamma distribution (GGD). The latter distribution is shown to provide an excellent match with simulated data for all regimes of atmospheric turbulence. Therefore, it can be used to overcome the computationally complex numerical simulations to model the propagation of OAM modes through atmospheric turbulent FSO channels. The GGD allows obtaining very simple tractable closed-form expressions for a variety of performance metrics. Indeed, the average capacity, the bit-error rate, and the outage probability are derived for FSO systems using single OAM mode transmission with direct detection. Furthermore, we extend our study to FSO systems using OAM mode diversity to improve the performance. By using a maximum ratio combining (MRC) at the receiver, the GGD is also shown to fit the simulated combined received optical powers. Finally, space-time (ST) coding is proposed to provide diversity and multiplexing gains, and the error probability is theoretically derived under the newly proposed generic model.

BER Performance Analysis of an Orthogonal FDM Free Space Optical Communication System with Homodyne Optical Receiver over Turbulent Atmospheric Channel

Abstract Atmospheric turbulence induced fading may severely impair free-space optical (FSO) communication systems, affecting the quality of the propagated laser beam and lead to significant performance degradation. Recent research works reveal that performance can be improved by using orthogonal FDM. In this paper, an analytical approach is presented to evaluate the bit error rate performance of an OFDM FSO link with optical intensity modulation and coherent homodyne receiver taking into consideration the effect of strong atmospheric turbulence. The turbulence effect is modeled as gamma-gamma distribution and the performance results are evaluated in terms of average CNR and BER. Power penalty suffered by the system due to the effect of turbulence at a given BER is evaluated for several system parameters viz. link distance, turbulence parameter, local oscillator power etc. It is noticed that effect of turbulence can be significantly reduced by increasing the number of OFDM subcarrier. For example, power penalty for BER of 10–9 at a link distance of 3,600 m is 6 dB when number of subcarrier is 4 and can be reduced to 0.5 dB by increasing the number of subcarrier to 64. In addition, by utilizing coherent optical receiver and synchronous demodulation at the receiving end, we have introduced local oscillator (LO) for both Optical and RF state which have ability to track the signal’s phase changes over time relative to the LO’s phase and helps the system to remain stable.

Analytical Performance Evaluation of a STBC Coded OFDM FSO Communication System over Turbulent Atmospheric Channel

An analytical approach is presented for an optical IM/DD system with STBC coded OFDM RF subcarrier modulation over a free space optical turbulent channel. The analysis is developed to find the expressions for the output photodetector current and carrier to noise power ratio at the output of the STBC–OFDM demodulator. The average bit error rate is evaluated numerically considering log-normal distribution of the atmospheric turbulence. The numerical results show that STBC-OFDM optical link suffers power penalty due to atmospheric turbulence significantly. However, the power penalty can be reduced by increasing the number of OFDM subcarriers. For example, power penalty for (2X1) STBC with N = 16 OFDM subcarriers is 7.5 dB and the power penalty reduces to 0.55 dB when N is increased to 64. Results are evaluated for different link distance, OFDM subcarriers and various channel condition.

BER analysis of amplify-and-forward relaying FSO systems using APD receiver over strong atmospheric turbulence channels

<span lang="EN-US">In this paper, we theoretically analyze the performance of amplify-and-forward (AF) serial relaying free-space optical (FSO) systems using avalanche photodiodes (APD) and subcarrier quadrature amplitude modulation (SC-QAM) over strong atmospheric turbulence channels modelled by gamma-gamma distribution. Closed-form expression for average bit error rate (BER) of system is theoretically derived talking into account APD shot noise, thermal noise as well as the impact of atmospheric loss and turbulence. The numerical results show that using AF relay stations can extend the transmission distance and help to improve performance of FSO system significantly when compared with the direct transmission. Moreover, the selection of APD gain value is indispensable to the system performance. The proposed system could be achieved the best performance by selecting an optimal APD gain value. In addition, the optimal value of APD gain also significantly depends on various conditions, such as link distance, the number of relay stations and APD receiver noise.</span>

Analysis and evaluation of the performance between reciprocity and time delay in the atmospheric turbulence channel.

In order to better evaluate the relationship between reciprocity and time delay of the fiber receiving system in the atmospheric turbulence channel, a time-domain signal generation mathematical model is proposed for the first time. A numerical solution of Johnson SB probability density distribution (PDF) in time-domain is creatively given for evaluating the reciprocity of both communication ends, which relates to the normalized fluctuation variance of the light intensity and the Greenwood frequency. An experiment is then carried out for verifying the time-domain signal generation model and measuring reciprocity. It shows that the excellent fitting accuracy of Johnson SB PDF signal generation model is first experimentally verified. It also indicates that the system reciprocity is improved by 10% after eliminating the system time delay. Meanwhile, the relationship between time delay and reciprocity under different atmospheric environments are analyzed and the relationship between time delay and system reciprocity at different Greenwood frequencies are discussed. This work provides a time parameter reference for the design of adaptive system and free-space optical (FSO) communication system.

Physical-layer security in fractional orbital angular momentum multiplexing under atmospheric turbulence channel.

In this paper, the physical layer security (PLS) of fractional orbital angular momentum (OAM) multiplexing under atmospheric turbulence channels is studied. Based on the PLS theory, the secrecy capacities and the probabilities of positive secrecy capacities of fractional OAM (FrOAM) multiplexing systems with different topological charge intervals are analyzed. The influence of the eavesdropping ratio and the power allocation on secrecy capacities are compared. The simulation results show that, under the finite aperture limitation, the FrOAM multiplexing technique provides higher security over the integer OAM multiplexing in terms of the total secrecy capacities under weak and medium turbulence.

QC-LPDC Code modulation with probabilistic shaping for atmospheric turbulence channel

In this paper, we propose two succinct probabilistic shaping (PS) schemes with 18.8% and 37.5% overhead for quasi-cyclic low-density parity-check code to transmit polarization-multiplexed 16 quadrature amplitude modulation (PM-16QAM) signal over the turbulent free space optical (FSO) links. We theoretically analyze irradiance fluctuation within the cross section of received beam after propagating through the Gamma–Gamma channel, of which the refractive-index property is related to Kolmogorov spectrum. The Gamma–Gamma channel model is used to simulate the effects of atmosphere turbulence on PS PM-16QAM FSO communication system. Simulation results show that the proposed PS schemes provide a more efficient way to compensate scintillation effects compared with uniform distribution in PM-16QAM FSO communication system under different turbulence strength regimes. Furthermore, indoor short-range experiments have been carried out to verify the effectiveness of the proposed probabilistic shaping schemes

Adaptive Channel Coding and Power Control for Practical FSO Communication Systems Under Channel Estimation Error

Adaptive channel coding and power control for practical free-space optical communication systems are proposed in this paper. Particularly, we first assume that the channel state information (CSI) is perfectly known at the transmitter, and propose adaptive transmission schemes in which coding rate is adjusted either independently or jointly with transmit power according to the channel conditions. Moreover, an optimization problem is developed to attain power consumption minimization under free space optical (FSO) practical constraints, i.e., target bit-error rate, target outage probability, and maximum transmit power. We then solve the optimization problem and derive closed-form expressions for throughput and average transmit power for both adaptive schemes over the Gamma–Gamma atmospheric turbulence channels. Our results demonstrate the superiority of the proposed adaptive schemes over non-adaptive schemes, especially under strong turbulence conditions. In the second part of this paper, we move toward a more realistic scenario where the channel is not perfectly known to Tx/Rx and is estimated by the sequence of received signals. Therefore, we investigate the effect of channel estimation inaccuracies on the performance of the proposed schemes and show that the accuracy of the channel estimator depends on the length of the sequence and, for a sufficiently large length of the observation window, it achieves performance close to the receiver with known CSI. Extensive analytical derivations are provided to investigate the performance of the proposed adaptive schemes under channel estimation error. Our analytical results can be used for calculating and tuning of the optimum length of the observation window without resorting to Monte Carlo simulations.

Modeling and topology design for free-space optical networks

To date, free-space optical (FSO) networks play an important role in current network construction to support large-capacity transmission, where randomly distributed FSO terminals desire to exchange a tremendous amount of information over atmospheric turbulence channels. However, in the presence of atmospheric turbulence and misalignment fading channels, FSO network topology can be dynamic and disconnected. To mitigate the impact of dynamic network environments, appropriate higher-layer protocols should be designed. We explore a practical terrestrial mobile ad-hoc FSO network based on the bundle protocol of disruption-tolerant network, and the theoretical cross-layer system model between physical layer and network layer is derived. To design the topology, at the bundle layer, the distributed routing scheme centrality and probability (CAP) is proposed, where contact probability, sociocentric measure, and message replication strategy are considered simultaneously, and the joint forwarding decision rule is given. Simulation results on the opportunistic networking environment simulator are presented, which show that CAP can be better compared with the conventional end-to-end protocol-based routing scheme.

Comparison of adaptive algorithms for free space optical transmission in Málaga atmospheric turbulence channel with pointing errors

In this study, the authors investigate average capacity of free space optics communication over Málaga atmospheric turbulence channel with pointing errors and path loss, for intensity modulated/direct detection (IM/DD) and heterodyne detection. Various algorithms which use adaptive transmission with both types of detection are considered, such as: optimal rate adaption (ORA), optimal power and rate adaption (OPRA), channel inversion with fixed rate (CIFR) and truncated channel inversion with fixed rate (TIFR). Analytical closed-form expressions for channel capacities of ORA, OPRA and TIFR adaptive transmission are presented, and the authors prove that CIFR transmission is not feasible in the strict sense for the conditions considered. Obtained analytical results are numerically evaluated and graphically presented for different strengths of atmospheric turbulence (in weak, moderate and strong turbulence regime) for both types of detection (IM/DD and heterodyne), and for considered algorithms of adaptive transmission (ORA, OPRA and TIFR). The authors have developed expressions suitable for approximating high signal-to-noise ratio channel capacity, and they graphically present and compare the asymptotic approximations with the obtained analytical results for different strengths of turbulence for both types of detection. Also, obtained analytical results were confirmed by Monte-Carlo simulations, and graphically compared for different strengths of turbulence regimes.

Generic blind spectrum sensing scheme for all optical-wavelength multi-user free space optical communications

Spectrum sensing is a fundamental component in multi-user free space optical communication systems. In order to avoid interference among the users, it is required to periodically sense the optical frequency spectrum to detect if the channel is occupied by active transceiver pairs. Spectrum sensing allows the user to continuously sense the optical spectrum within their operating range to detect the channels. In this paper, we propose a generic novel blind spectrum sensing scheme for an unknown optical signal over strong atmospheric turbulence channel. By generic, we mean that the proposed scheme is applicable to all optical wavelengths, i.e. ultraviolet (UV), visible light, and infrared. The proposed blind spectrum sensing scheme is based on the estimated signal-to-noise ratio (SNR) and the noise power, for which we have derived expressions from a statistical ratio of the received signal. The atmospheric turbulence is the main reason behind the signal fading in free space optical links as it causes fluctuations in the received signal intensity. To mitigate this adverse effect, the proposed scheme is equipped with a spatial diversity scheme in the form of switch-and-stay combining. We use Gamma–Gamma distribution to model both small-scale and large-scale turbulence fadings. From the estimated SNR and noise power expressions, we formulate the novel closed-form expressions for the probabilities of detection and false alarm mathematically. Numerical and simulation results based on randomly generated optical signals are presented to verify the effectiveness of the proposed scheme.

A high SNR partially coherent beam source based on supercontinuum for free space data transmission

We experimentally demonstrate a high signal-to-noise ratio (SNR) partially coherent beam (PCB) source based on wide-spectrum produced by a filtered supercontinuum (SC) which can generate from a segment of highly nonlinear fiber (HNLF) pumped by a picosecond fiber laser. The SNR of PCB source can improve from 12.43 dB to 22.45 dB by filtering out amplified spontaneous emission (ASE) noise of erbium-doped fiber amplifier (EDFA) and using lower power pumped HNLF. We also conduct a 4 Gbit/s transmission experiment using the high SNR PCB source in 1km simulated atmospheric turbulent channel. The characteristics of scintillation indexes (SIs), eye patterns, SNRs and bit error rates (BER) based on CB source and PCB sources after turbulent channel are analyzed. In ΔT=240°C, atmospheric turbulent channel, the SI of the CB link can reduce by 31% by using PCB source. The SNR of PCB source is 8.15 dB higher than the 5.20 dB of the CB source, and the sensitivity of the PCB source has a 2.9 dB improvement compared with the CB source. The PCB source possess high SNR and turbulence resistance, which can improve the performance of free space data transmission.