Turbulence Channel Model Research Articles

Optical orthogonal frequency division multiplexing with differential index modulation

This paper proposes a differential index modulation (DIM) scheme to address the complex channel estimation challenges in optical orthogonal frequency division multiplexing with index modulation (OOFDM-IM). The main idea of the DIM scheme is to design a time-frequency dispersive matrix with unitary characteristics and perform index mapping based on the Lemer code principles. By applying differential operations, the DIM scheme enables decoding at the receiver without requiring channel estimation. The paper provides a detailed explanation of the design principles and theoretical bit error rate (BER) of the proposed scheme. Simulations based on the exponential Weibull atmospheric turbulence channel model are conducted to compare the DIM scheme with the existing OOFDM-IM scheme. Additionally, a carefully designed proof-of-concept experiment is performed to further validate the scheme’s effectiveness and feasibility. Both simulation and experimental results demonstrate that, compared to OOFDM-IM, the proposed scheme can entirely avoid complex channel estimation with a maximum signal-to-noise ratio (SNR) loss of no more than 4 dB, even under various turbulence intensities and higher-order modulation scenarios. This provides a valuable reference for OOFDM-IM in complex environments or where channel estimation is challenging.

Performance Evaluation of UOWC Systems from an Empirical Channel Model Approach for Air Bubble-Induced Scattering.

Underwater optical wireless communication (UOWC) systems provide the potential to establish secure high-data-rate communication links in underwater environments. The uniqueness of oceanic impairments, such as absorption, scattering, oceanic turbulence, and air bubbles demands accurate statistical channel models based on empirical measurements for the development of UOWC systems adapted to different types of water and link conditions. Recently, generalized Gamma and a mixture of two generalized Gamma probability density functions (PDF) were proposed to describe the statistical behavior of small and large air bubbles, respectively, when considering several levels of particle-induced scattering. In this paper, we derive novel closed-form analytic expressions to compute the bit error rate (BER) and outage performance using both proposed PDFs for various scattering conditions. Furthermore, simple asymptotic expressions are obtained to determine the diversity order of each scenario. Monte Carlo simulation results verify the obtained theoretical expressions. Our results also reveal that UOWC systems present lower BER and outage performance under more turbid water cases with respect to the tap water case due to the higher diversity order and despite the significant increases in pathloss at short link distances. Particle-induced scattering provides an inherent mechanism of turbid waters to mitigate air bubble-induced fluctuations and light blockages.

Enhancing Performance of Air–Ground OAM Communication System Utilizing Vector Vortex Beams in the Atmosphere

The modified uplink and downlink atmospheric turbulence channel models were established and employed to assess the system performance of air–ground orbital angular momentum (OAM) communication. The advantage of the vector vortex beam taking the place of the scalar one in the OAM communication system operated in the atmospheric turbulence was verified, that vector vortex beam can guarantee the more homogeneous energy in the circular hollow beam profile and the less phase distortion on signal OAM in the turbulence, which can reduce OAM crosstalk and improve OAM communication performance, especially small topological charge in strong turbulent regime. With the increase in turbulence strength, the vortex beam with a larger topological charge suffered more OAM mode crosstalk, and the average BER of the OAM communication system increased. Bessel–Gaussian (BG) beams with larger beam shape parameters had the strong capability of turbulence disturbance rejection in short-distance atmospheric applications, conversely, Laguerre–Gaussian (LG) beams with suitable parameter selection were preferred for long-distance atmospheric applications. Additionally, compared to the downlink channel, the transmission of OAM mode and the related communication system in the uplink channel are dramatically deteriorated due to atmospheric turbulent effects.

Performance of Integrated Ground-Air-Space FSO Links Over Various Turbulent Environments

We analyze the outage probability of integrated ground-air-space free-space optical (FSO) communication links for different atmospheric turbulence channel models. The performance of ground to high altitude platforms (HAPS), HAPS to geostationay Earth orbit (GEO) satellite, HAPS to HAPS and ground to GEO satellite is investigated for various link configurations (downlink, horizontal, and uplink) and parameters such as zenith angle, channel state, horizontal and vertical deviations, altitude, beam waist, receiver aperture diameter, wind speed, and visibility. The atmospheric attenuation (the effects of fog, cloud and volcanic activities), atmospheric turbulence, angle of arrival (AOA) fluctuations and pointing error are included in the considered model. Closed-form expressions for the probability density function (PDF) and cumulative distribution function (CDF) are obtained for Lognormal, Gamma and exponentiated Weibull distributed channel models. The benefit of using HAPS is shown by comparison of outage probabilities for direct ground to GEO satellite and HAPS assisted ground to GEO satellite links.

Note about Passive Continuous Variable Quantum Key Distribution over Turbulent Atmospheric Channel

Continuous variable quantum key distribution (CVQKD) has been implemented over the atmospheric channels over free space. However, atmospheric turbulence weakens the quality of the transmitting quantum signals and hence decreases the secret key rate of the system. Here, we suggest an atmospheric turbulence channel model that involves atmospheric turbulence bubbles and demonstrates the implementation feasibility of passive CVQKD with spectrum resources in the terahertz band over the atmospheric turbulence channel. We achieve the channel transmittance characterized by the refractive index and the wavefront distortions. Moreover, an adaptive optics (AO) unit is used for performance improvement while considering the effect of the thermal noise and excess noise on the atmospheric turbulence bubble-modeled channel. Numerical simulations show that the AO-involved detection scheme can result in reductions in excess noise when being faced with the floating clouds and mist in atmospheric turbulence, which results in performance improvements in terms of secret key rate, which confirms the utility of the high-rate and long-distance CVQKD in terahertz (THz) for practical implementations.

Statistical channel model to characterize turbulence‐induced fluctuations in the underwater wireless optical communication links

SummaryUnderwater wireless optical communication (UWOC) turns out to be a primary sought alternative to wireless technology, which can provide high data rate, enormous bandwidth, and low deployment cost and resolve the crowded radio frequency band issues. The performance of the UWOC link is significantly affected by the turbulent ocean environment. In this paper, the different turbulence scenarios such as salinity gradients, temperature gradients, and air bubbles are considered to model the turbulent nature of the ocean environment experimentally. These turbulent conditions induce severe intensity fluctuations in the received optical signal. These intensity fluctuations are modeled stochastically, and a novel turbulence channel model is proposed, which excellently fitted with the experimental data. The conformity of the proposed model is validated by performing a goodness of fit test in terms of R2 and root mean square error (RMSE) coefficients. The results show that for all the considered turbulent scenarios, the Gaussian mixture model better approximates the experimental observations than the Weibull distribution. As the strength of the turbulence increases, the chances of link outage and the probability of bit errors increase. At a flow rate of 4 L/min, the bit error rate (BER) of 10−20, 10−14, and 10−10 is recorded under clear water, salinity, and temperature gradient channel conditions, respectively. It is observed that the air bubbles have a significant impact on the propagating optical beam in comparison to temperature and salinity‐induced turbulence. Moreover, a close agreement between the proposed model and the experimental data is also observed, which makes the proposed model more appropriate to describe the turbulent ocean environment.

Hybrid FSO-RF Turbulent Link Performance under Misalignment Aperture Error

A novel hybrid Free Space Optics (FSO)/Radio Frequency (RF) model is proposed which is based on the single decision threshold feedback bit switching technique. The FSO and proposed hybrid FSO-RF links performance are investigated under strong turbulence and misalignment aperture error in terms of the average symbol error rate (ASER) and outage probability. The FSO-RF link is preferred in this work over the normal FSO link due to ease in switching among the link established with the help of decision threshold during failure of FSO link. In this proposed hybrid model, Gamma-Gamma distribution is considered as free-space optical turbulence channel and Nakagami-m model is for RF link. The analytical expressions for the outage probability and ASER have been derived for various Intensity Modulation - M-phase shift keying (PSK) schemes. A performance comparative study is reported for FSO link and hybrid FSO-RF link. The obtained result indicates the enhanced system performance with the hybrid model whereas the obtained results are validated through MC simulations and results are found in good agreement.

Proposed Different Signal Processing Tools for Efficient Optical Wireless Communications

Optical Wireless Communication (OWC) is a new trend in communication systems to achieve large bandwidth, high bit rate, high security, fast deployment, and low cost. The basic idea of the OWC is to transmit data on unguided media with light. This system requires multi-carrier modulation such as Orthogonal Frequency Division Multiplexing (OFDM). This paper studies optical OFDM performance based on Intensity Modulation with Direct Detection (IM/DD) system. This system requires a non-negativity constraint. The paper presents a framework for wireless optical OFDM system that comprises (IM/DD) with different forms, Direct Current biased Optical OFDM (DCO-OFDM), Asymmetrically Clipped Optical OFDM (ACO-OFDM), Asymmetrically DC-biased Optical OFDM (ADO-OFDM), and Flip-OFDM. It also considers channel coding as a tool for error control, channel equalization for reducing deterioration due to channel effects, and investigation of the turbulence effects. The evaluation results of the proposed framework reveal enhancement of performance. The performance of the IM/DD-OFDM system is investigated over different channel models such as AWGN, log-normal turbulence channel model, and ceiling bounce channel model. The simulation results show that the BER performance of the IM/DD-OFDM communication system is enhanced while the fading strength is decreased. The results reveal also that Hamming codes, BCH codes, and convolutional codes achieve better BER performance. Also, two algorithms of channel estimation and equalization are considered and compared. These algorithms include the Least Squares (LS) and the Minimum Mean Square Error (MMSE). The simulation results show that the MMSE algorithm gives better BER performance than the LS algorithm.

Parallel relay-assisted free-space optical communication using multi-pulse position modulation over the generalized turbulence channel model

Parallel relay-assisted free-space optical communication using multi-pulse position modulation over the generalized turbulence channel model

Experimental performance evaluation of weak turbulence channel models for FSO links

Outdoor wireless communication is subject to several outdoor environment conditions. Among them, atmospheric turbulence which happens on a daily basis, introduces signal fading and degrades the system performance. Many channel models have been proposed in the literature to model the turbulence fading in free space optic (FSO) communication systems. In this work, we aim to evaluate the performance of some key proposed channel models for FSO systems under weak turbulence condition. First, an outdoor experimental setup is utilized to measure the turbulence fading and build a dataset. The setup is all-optical, which gives us the advantage of isolating the solar radiation noise during measurements. Second, the obtained dataset is used to investigate the performance of five key FSO channel models claimed to perform well under weak turbulence conditions. The results show that for very weak turbulence with scintillation index (SI) <10−3, four models that claim to work well under such condition failed to fit the dataset. For weak turbulence with SI>10−3, all the models fit the dataset except I-K model. Third, we exploited the obtained dataset to propose a new empirical channel model that has good performance under weak turbulence conditions. Using this model, we studied the outage probability of FSO system under the joint effect of weak turbulence and fog attenuation.

Effects of Solar Scintillation on Deep Space Communications: Challenges and Prediction Techniques

A reliable deep space communications system plays a key role in extending the range of space communications and has become an indispensable part of space information networks. With the emergence of many deep space exploration projects, it can be envisioned that a new era of research on deep space communications has arrived. However, the performance of deep space communications is severely degraded by solar scintillation, which occurs frequently during superior solar conjunction. In this article, we first review the state of the art of solar scintillation research, including the definition of solar scintillation, influencing factors, assessment methods, and scintillation models. Then an effective solar scintillation prediction model is proposed to overcome the challenges related to solar scintillation for deep space communications. After that, a coronal turbulence channel model is investigated and discussed. Based on this coronal channel model and our proposed solar scintillation model, the link bit error rate performance is further analyzed. Finally, we identify some open problems and research directions that could be investigated for a deep space communications system during superior solar conjunctions, which will support space information networks.

Performance analysis of pulse position modulation-based hybrid technique for cellular backhaul free-space optical link

We investigate the performance of half-cosine pulse-shaped hybrid pulse position modulation-orthogonal frequency division multiplexing (HC-PPM-OFDM) under weak (i.e., log-normal) and strong (i.e., gamma–gamma) turbulence free-space optical channel. Joint impact of frequency and time offset on the performance parameters such as intercarrier interference, a symbol to interference ratio, and bit-error rate are also investigated. This research aims to improve the spectral efficiency performance of traditional PPM through the proposed hybrid technique. Further, the proposed HC-PPM-OFDM technique is compared with pulse position modulation-minimum shift keying (PPM-MSK), PPM, and hybrid PPM-OFDM. Obtained results show comprehensive improvement in the performance parameters under both weak and strong atmospheric turbulences. In addition to the above results, the upper-bound channel capacity expression is derived for log-normal as well as gamma–gamma turbulence channel models. Obtained results reveal that the proposed modulation technique outperforms PPM and PPM-MSK techniques under all turbulence conditions at a marginal cost of power penalty.

Performance and equilibrium experiment of a multiband CAP modulation system in wireless optical communication

Abstract Multiband carrierless amplitude and phase (MultiCAP) modulation is a modulation mode based on Carrierless Amplitude and Phase (CAP). To improve the capability of a short distance optical communication system for communication capacity and frequency band utilization, multiband CAP modulation technology is adopted. In this article, the diagram for MultiCAP modulation signal of signal-to-noise ratio and bit error rate was analyzed under the condition of Gamma–Gamma atmospheric turbulence channel model. The bit error rate of 4 the band CAP modulation system has been discussed under conditions of different intensity fluctuation variance and signal-to-noise ratio. The simulation results show that when the total bandwidth is constant, the larger the frequency band is, the lower bit error rate is. When the value of σ R 2 is small, the system error rate decreases with increasing signal-to-noise ratio. When the σ R 2 is larger, the system error rate decreases with decreasing σ R 2 . The experimental results of atmospheric laser communication show that the receiver branch signal phase shift are well compensated and intersymbol interference is effectively suppressed after using MMA algorithm for equilibrium, and the bit error rate of each branch signal is decreased from 1 0 − 3 to 2 . 4462 × 1 0 − 4 . Consequently, the system transmission performances improve.

CO-OFDM and DP-QPSK Based DWDM Optical Wireless Communication System

Abstract The 5 G technology provides a promising solution for future broadband networks. It requires access to high-speed wireless services for systems at anytime and anywhere. For adapting the 5 G services, this paper proposes an optical wireless system with hybrid scheme of dual polarized Gbps dense wavelength division multiplexed (DWDM) system. The multi-carrier transmission technique such as coherent optical orthogonal frequency division multiplexing (CO-OFDM) has also been incorporated into the system. The frequency spacing for the DWDM system is taken 25, 50 and 100 GHz according to the ITU-T standard. Here the subcarriers of each channel are modulated with the dual polarized-quadrature phase shift keying (DP-QPSK) sequence. The information is transmitted with an overall data rate of 0.32 Tbps. The system is successfully demonstrated at different turbulence regime from clear weather to severe turbulence weather under Gamma-Gamma atmospheric turbulence channel model. Due to the high optical signal-to-noise ratio (OSNR) tolerance and high spectrum efficiency of the dual polarized multi-channel and multicarrier transmission technique, the link distance achieved is 2.9 km in clear weather, 2.6 km in moderate turbulence regime, 2.3 km in high turbulence regime and 0.9 km in severe turbulence regime. A comparison study with related works has been carried out. Mathematical modeling is also incorporated for detail analysis of the proposed system.

Impact of pointing errors and turbulence effects on POLSK and coherent OWC-based FSO system over generalized turbulence channel model

The emerging technology in wireless communication, free space optics (FSO) offers a myriad of merits over current radio frequency links due to its wide license-free bandwidth, ease of installation, high security features and viable cost for short-distance communication. Its unparallel high-speed data rate and immunity against electromagnetic interference makes FSO the emerging technology of today. Atmospheric conditions like absorption, scattering, turbulence and pointing error prevail during wireless transmission. Through this paper we aim at elucidating the effect of the error introduced by misalignment between the transmitter and receiver that are ideally required to be in the line of sight rendering pointing error. Pointing error and turbulence effects are the main limitation parameters for our analysis. For this purpose, we have taken into consideration three different modulation techniques polarization shift keying, coherent OWC and on–off keying FSO communication system. We derived the novel expressions for the average spectral efficiency (ASE) over the generalized turbulence model for these modulation techniques. The ASE is analyzed against average transmitted optical power with the maximum ASE of 50 bits/s/Hz at the transmitted power of 10 dBm for coherent OWC technique.

HK distribution model for atmospheric turbulence channel under the influence of pointing errors

In this paper, HK statistical model is considered. The application of HK distribution in modeling atmospheric turbulence channel under the influence of strong and weak turbulences and all irradiance fluctuations are discussed. Expression for atmospherics turbulence channel model, pointing error models and path loss are given. Probability density functions (PDF) for both pointing error models (zero boresight and nonzero boresight) are calculated and graphically represented. Also, average bit-error-rate (BER) for zero boresight pointing error and nonzero boresight pointing error are calculated and graphically represented. Closed form expression for PDF and BER are given. Impact of zero boresight pointing error and nonzero boresight pointing error on transmission over atmospheric turbulence channel in FSO system is explained.

UV LED array based NLOS UV turbulence channel modeling and experimental verification.

In this paper, we comprehensively study the NLOV UV turbulence effect through simulated and experimental results. A Monte Carlo NLOS UV turbulence channel model, which incorporates the effects of multiple scattering and turbulence attenuation, is proposed based on previous work. To validate this model, a series of outdoor experiments is conducted to investigate the received-signal energy distribution and channel path loss under the turbulence circumstance for the first time. These experimental and simulated results are valuable for studying NLOS UV channel and communication system design.

A Novel Statistical Channel Model for Turbulence-Induced Fading in Free-Space Optical Systems

In this paper, we propose a new probability distribution function which accurately describes turbulence-induced fading under a wide range of turbulence conditions. The proposed model, termed double-generalized gamma (double GG), is based on a doubly stochastic theory of scintillation and developed via the product of two generalized gamma distributions. The proposed double GG distribution generalizes many existing turbulence channel models and provides an excellent fit to the published plane and spherical waves simulation data. Using this new statistical channel model, we derive closed form expressions for the outage probability and the average bit error as well as corresponding asymptotic expressions of free-space optical communication systems over turbulence channels. We demonstrate that our derived expressions cover many existing results in the literature earlier reported for gamma-gamma, double-Weibull and K channels as special cases.

A novel repetition space-time coding scheme for mobile FSO systems

Considering the influence of more random atmospheric turbulence, worse pointing errors and highly dynamic link on the transmission performance of mobile multiple-input multiple-output (MIMO) free space optics (FSO) communication systems, this paper establishes a channel model for the mobile platform. Based on the combination of Alamouti space-time code and time hopping ultra-wide band (TH-UWB) communications, a novel repetition space-time coding (RSTC) method for mobile 2×2 free-space optical communications with pulse position modulation (PPM) is developed. In particular, two decoding methods of equal gain combining (EGC) maximum likelihood detection (MLD) and correlation matrix detection (CMD) are derived. When a quasi-static fading and weak turbulence channel model are considered, simulation results show that whether the channel state information (CSI) is known or not, the coding system demonstrates more significant performance of the symbol error rate (SER) than the uncoding. In other words, transmitting diversity can be achieved while conveying the information only through the time delays of the modulated signals transmitted from different antennas. CMD has almost the same effect of signal combining with maximal ratio combining (MRC). However, when the channel correlation increases, SER performance of the coding 2×2 system degrades significantly.

Experiment and numerical evaluation of bit error rate for free-space communication in turbulent atmosphere

Influenced by atmospheric turbulence, performance of the free-space optical (FSO) communication system fluctuates greatly. Research on evaluating system error performance according to parameters of system and atmosphere is a subject of current interest. Based on the optical turbulence channel and photoelectric detection model, a mathematic simulation model of error performance for the FSO communication system was modified, and an expression of bit error rate (BER) for the FSO communication system through turbulent atmosphere was proposed. Results of simulation were compared with former models and experimental data obtained in weak turbulence, and the model characterizes factors in turbulence, such as intensity fluctuation, variable shot noise, etc. Simulations showed that the results simulated by the modified model were more consistent with experimental data in most cases, and the detection threshold should be adjusted according to real atmosphere and signal intensity. The presented model could lead to an efficient performance evaluation and provide reference to correlative researches.