Atmospheric Turbulence Conditions Research Articles

Adaptive Single-Mode Fiber Coupling Method Based on Coarse-Fine Laser Nutation

One of the major restrictions of the free-space optical communication systems is the limitation of single-mode fiber (SMF) coupling efficiency that impacts the system performance. In this paper, an adaptive coupling method for optical communication system based on coarse-fine laser nutation technique was presented. The scanning strategy and algorithm were set up and experimentally examined. The experimental results demonstrate that the coarse-fine laser nutation technique is capable of achieving a good coupling efficiency even the beam spot and the SMF do not coincide with each other at the start. The SMF center calculation accuracy of coarse and fine scanning stage were 0.25 and 0.01 μ m, respectively. And the average coupling efficiency of the system can increase from 0.351 in open loop to 0.624 in closed loop under a simulated atmospheric turbulence condition, the offset compensation error was 0.1245 μ m when turbulence induced. Finally, some factors influencing the systems were also discussed.

A Hardware Implemented Autocorrelation Technique for Estimating Power Spectral Density for Processing Signals from a Doppler Wind Lidar System.

A signal processing technique utilizing autocorrelation of backscattered signals was designed and implemented in a 1.5 µm all-fiber wind sensing Coherent Doppler Lidar (CDL) system to preprocess atmospheric signals. The signal processing algorithm’s design and implementation are presented. The system employs a 20 kHz pulse repetition frequency (PRF) transmitter and samples the return signals at 400 MHz. The logic design of the autocorrelation algorithm was developed and programmed into a field programmable gate array (FPGA) located on a data acquisition board. The design generates and accumulates real time correlograms representing average autocorrelations of the Doppler shifted echo from a series of adjustable range gates. Accumulated correlograms are streamed to a host computer for subsequent processing to yield a line of sight wind velocity. Wind velocity estimates can be obtained under nominal aerosol loading and nominal atmospheric turbulence conditions for ranges up to 3 km.

Concatenated LDPC-TCM Codes for Better Performance of OFDM-FSO System Using Gamma–Gamma Fading Model

Various channel coding techniques have been used for free space optical (FSO) links to alleviate the effects of fading, induced due to turbulence. In this paper, the error rate performance is scrutinized for orthogonal frequency division multiplexing based FSO system with 16-QAM modulation, and the effect of several atmospheric turbulence conditions has been analyzed. Due to atmospheric turbulence, the arbitrary fluctuations of optical irradiance are modelled by widely used Gamma-gamma distribution model. The various channel coding schemes has been analyzed and a new hybrid coding scheme (the outer low-density parity check codes efficiently integrated with interleaved trellis code modulation codes) is developed, which has shown superior performance under all turbulence conditions and outperform the other individual coding schemes. The proposed hybrid scheme is analyzed for single antenna fading channels to achieve better error rate performance.

A new switching scheme for hybrid FSO/RF communication in the presence of strong atmospheric turbulence

Hybrid FSO/RF communication systems make use of an RF link with OWC link to achieve higher reliability along with high data rate communication. Both these (OWC and RF) channels are complementary in nature. We propose and analyze a new switching scheme for the hybrid FSO/RF system, in which both FSO and RF links are emphatically activated or deactivated according to the channel conditions. A single threshold level is decided for the Rayleigh-faded RF link, and two threshold levels are defined for FSO link under strong atmospheric turbulence conditions. Closed-form analytical expressions have been obtained for the bit error rate and outage probability for the proposed system.

Temperature gradient in the solar photosphere. Test of a new spectroscopic method and study of its feasibility for ground-based telescopes

Context. The contribution of quiet-Sun regions to the solar irradiance variability is currently unclear. Some solar-cycle variations of the quiet-Sun physical structure, such as the temperature gradient, might affect the irradiance. The synoptic measurement of this quantity along the activity cycle would improve our understanding of long-term irradiance variations. Aims. We intend to test a method previously introduced for measuring the photospheric temperature gradient from high-resolution spectroscopic observation and to study its feasibility with ground-based instruments with and without adaptative optics. Methods. We used synthetic profiles of the FeI 630.15 nm obtained from realistic three-dimensional hydrodynamical simulations of the photospheric granulation and line radiative transfer computations under local thermodynamical equilibrium conditions. Synthetic granulation images at different levels in the line are obtained by convolution with the instrumental point spread function (PSF) under various conditions of atmospheric turbulence, with and without correction by an adaptative optics (AO) system. The PSF are obtained with the PAOLA software, and the AO performances are inspired by the system that will be operating on the Daniel K. Inouye Solar Telescope. Results. We consider two different conditions of atmospheric turbulence, with Fried parameters of 7 cm and 5 cm, respectively. We show that the degraded images lead to both a bias and a loss of precision in the temperature-gradient measurement, and that the correction with the AO system allows us to drastically improve the measurement quality. Conclusions. Long-term synoptic observations of the temperature gradient in the solar photosphere can be undertaken by implementing this method on ground-based solar telescopes that are equipped with an AO correction system.

Performance improvement of FSO system using multi‐pulse pulse position modulation and SIMO under atmospheric turbulence conditions and with pointing errors

Free space optics (FSO) communication links are impaired by the fading due to turbulence and misalignment. In this paper, we theoretically analyze the pointing error effects on performance of FSO systems using multi-pulse pulse position modulation (MP-PPM) over the Gamma-Gamma turbulence channel. We consider the moderate and strong atmospheric turbulence regimes with the combined effect of the pointing errors (PEs), and study the link average symbol error rate (ASER) and the outage probability. The numerical results are presented to show the impact of the pointing error on the ASER and the system outage. However, to overcome the channel degradation resulting from the turbulence effects and the PE, the single-input multiple-output (SIMO) system with maximal ratio combining (MRC) diversity is used. Exact close-form expressions for ASER and outage probability are derived and verified by using Monte Carlo simulation. We concluded that the optimal values of the system parameters are, the transmitted power is 20 dBm, jitter variance should be less than 0.5 m and laser beam width equals 0.2 cm. These values are significantly reduced the impacts of misalignment and fading whereas outage probability is less than at maximum jitter variance.

Demonstration of 40 GBaud intradyne transmission through worst-case atmospheric turbulence conditions for geostationary satellite uplink.

An optical communications system employing intradyne reception and offline digital signal processing is tested over a 10.45km link through the atmosphere. 40GBaud transmission using binary phase-shift keying in the C-band is demonstrated and compared with laboratory measurements. Simultaneous photodetector measurements show that the turbulence in the atmospheric channel is representative for relevant and worst-case conditions in the geostationary satellite uplink scenario.

A New Closed Form Approximation for BER for Optical Wireless Systems in Weak Atmospheric Turbulence

Abstract Weak atmospheric turbulence condition in an optical wireless communication (OWC) is captured by log-normal distribution. The analytical evaluation of average bit error rate (BER) of an OWC system under weak turbulence is intractable as it involves the statistical averaging of Gaussian Q-function over log-normal distribution. In this paper, a simple closed form approximation for BER of OWC system under weak turbulence is given. Computation of BER for various modulation schemes is carried out using proposed expression. The results obtained using proposed expression compare favorably with those obtained using Gauss-Hermite quadrature approximation and Monte Carlo Simulations.

Bessel Gaussian Beam Propagation through Turbulence in Free Space Optical Communication

This paper investigated the experimental demonstration of jitter analysis for Bessel Gaussian beam propagation through the atmospheric turbulence conditions using two different wavelength lasers such as red and green laser. Axicon lens is used to generate Bessel Gaussian beam experimentally and different modulation schemes such as PAM, PPM, PWM, ASK, BPSK and QPSK are used to analyze the phase and time jitter. The Plano-Convex type Axicon is used to create a ring shaped approximation of a Bessel Gaussian beam which increases in diameter over distance while retaining a constant ring thickness. In this BG beam propagation, for red laser the noted phase jitter value is 45.196 radians in ASK and for green laser 36.955 radians in with turbulence conditions. Hence the lower wavelength BG laser beam (green laser) is more energetic than the lower wavelength BG laser beam (red laser).

Two-Way Multiuser Mixed RF/FSO Relaying: Performance Analysis and Power Allocation

In this paper, the performance of two-way multiuser mixed radio frequency/free space optical (RF/FSO) relay networks with opportunistic user scheduling and asymmetric channel fading is studied. RF links are used to conduct data transmission between users and relay node, while a FSO link is used to conduct data transmission on the last-mile communication link between the relay node and base station. The RF links are assumed to follow a Rayleigh fading model, while the FSO links are assumed to follow a unified Gamma–Gamma atmospheric turbulence fading model with pointing error. First, closed-form expressions for the exact outage probability, asymptotic (high signal-to-noise ratio) outage probability, average symbol error rate, and average ergodic channel capacity are derived assuming a heterodyne detection scheme. The asymptotic results are used to conduct a power optimization algorithm where expressions for optimal transmission power values for the transmitting nodes are provided. Additionally, performance comparisons between the considered two-way-relaying (TWR) network and the oneway- relaying (OWR) network are provided and discussed. Also, the impact of several system parameters, including number of users, pointing errors, atmospheric turbulence conditions, and outage probability threshold on the overall network performance are investigated. All the theoretical results are validated by Monte Carlo simulations. The results show that the TWR scheme almost doubles the network ergodic capacity compared to that of the OWR scheme with the same outage performance. Additionally, it is shown that under weak-to-moderate weather turbulence conditions and small pointing error, the outage probability is dominated by the RF downlink with a neglected effect for the user selection process at the RF uplink transmission. However, for severe pointing error, the outage probability is dominated by the FSO uplink/downlink transmission.

24–26 GHz radio-over-fiber and free-space optics for fifth-generation systems

This Letter outlines radio-over-fiber combined with radio-over-free-space optics (RoFSO) and radio frequency free-space transmission, which is of particular relevance for fifth-generation networks. Here, the frequency band of 24-26GHz is adopted to demonstrate a low-cost, compact, and high-energy-efficient solution based on the direct intensity modulation and direct detection scheme. For our proof-of-concept demonstration, we use 64 quadrature amplitude modulation with a 100MHz bandwidth. We assess the link performance by exposing the RoFSO section to atmospheric turbulence conditions. Further, we show that the measured minimum error vector magnitude (EVM) is 4.7% and also verify that the proposed system with the free-space-optics link span of 100m under strong turbulence can deliver an acceptable EVM of <9% with signal-to-noise ratio levels of 22dB and 10dB with and without turbulence, respectively.

Outage Analysis of a SIMO FSO System Over an Arbitrarily Correlated $\mathcal {M}$ -Distributed Channel

In this letter, we present the outage performance analysis of a single-input multiple-output free-space optical system with selection combining over an arbitrarily correlated Malaga $\mathcal {(M)}$ -distributed channel. A novel and generalized analytical expression for the joint probability density function of the received signal-to-noise ratio, using Green’s matrix approximation, is derived in the form of power series. The proposed expression is valid for an arbitrary correlation model and can be mapped to most of the other existing distributions characterizing different atmospheric turbulence conditions. Moreover, mapping of correlated $\mathcal {M}$ -distribution to correlated Gamma–Gamma and $K$ distributions is illustrated with the help of derived analytical results; correctness of the mapped distributions is verified by simulation.

Transmitter Spatial Diversity for FSO Uplink in Presence of Atmospheric Turbulence and Weather Conditions for Different IM Schemes

AbstractWe investigate the error performance of an earth-to-satellite free space optical uplink using transmitter spatial diversity in presence of turbulence and weather conditions, using gamma–gamma distribution and Beer–Lambert law, respectively, for on–off keying (OOK),

Probability density function of turbulence fading in MRR free space optical link and its applications in MRR free space optical communications

Probability density function (PDF) of the modulating retro-reflector (MRR) turbulence fading channels is crucial for the performance analysis of MRR communication systems. In this study, closed-form expression for the PDF of normalised MRR free-space optical (FSO) turbulence fading coefficient is obtained first. Moreover, it is applied to the evaluations of the closed-form expressions for average capacity and outage probability of MRR FSO links. The effects of the parameters such as atmospheric turbulence conditions, communication distance, receiver aperture diameter and the average signal-to-noise ratio on the performance of MRR FSO links are discussed. Results show that in order to achieve successful MRR FSO communication, communication systems employing MRRs are suitable for short-distance communication in the case of low transmission power, besides quite high transmission power and large receiver aperture diameter are required simultaneously by long-distance MRR FSO communication.

A Study on the Application of Normalized Point Source Sensitivity in Wide Field Optical Spectrometer of the Thirty Meter Telescope

The image evaluation of an optical system is the core of optical design. Based on the analysis and comparison of the PSSN (Normalized Point Source Sensitivity) proposed in the image evaluation of the TMT (Thirty Meter Telescope) and the common image evaluation methods, the application of PSSN in the TMT WFOS (Wide Field Optical Spectrometer) is studied. It includes an approximate simulation of the atmospheric seeing, the effect of the displacement of M3 in the TMT on the PSSN of the system, the effect of the displacement of collimating mirror in the WFOS on the PSSN of the system, the relations between the PSSN and the zenith angle under different conditions of atmospheric turbulence, and the relation between the PSSN and the wavefront aberration. The results show that the PSSN is effective for the image evaluation of the TMT under a limited atmospheric seeing.

SIMO optical wireless links with nonzero boresight pointing errors over M modeled turbulence channels

Over the last years terrestrial free-space optical (FSO) communication systems have demonstrated an increasing scientific and commercial interest in response to the growing demands for ultra high bandwidth, cost-effective and secure wireless data transmissions. However, due the signal propagation through the atmosphere, the performance of such links depends strongly on the atmospheric conditions such as weather phenomena and turbulence effect. Additionally, their operation is affected significantly by the pointing errors effect which is caused by the misalignment of the optical beam between the transmitter and the receiver. In order to address this significant performance degradation, several statistical models have been proposed, while particular attention has been also given to diversity methods. Here, the turbulence-induced fading of the received optical signal irradiance is studied through the M (alaga) distribution, which is an accurate model suitable for weak to strong turbulence conditions and unifies most of the well-known, previously emerged models. Thus, taking into account the atmospheric turbulence conditions along with the pointing errors effect with nonzero boresight and the modulation technique that is used, we derive mathematical expressions for the estimation of the average bit error rate performance for SIMO FSO links. Finally, proper numerical results are given to verify our derived expressions and Monte Carlo simulations are also provided to further validate the accuracy of the analysis proposed and the obtained mathematical expressions.

Performance of 1‐D and 2‐D OCDMA systems in presence of atmospheric turbulence and various weather conditions

In this study, the authors compare the performance of one-dimensional (1D) and two-dimensional (2D) optical code division multiple access (OCDMA) systems in presence of turbulence and various weather conditions. Lognormal fading model is used for analysis of weak turbulence and gamma–gamma model for moderate and strong turbulence. It is observed that presence of moderate and strong turbulence may lead to a complete link failure in 2D OCDMA system depending upon the number of users and turbulence level. The various weather conditions considered are very clear air, light fog and thick fog. The results show that the performance degradation due to thick fog is more than the light fog and very clear air in all turbulence regimes.

Analysis of link availability and capacity in multiple‐input/single‐output‐Bose–Chaudhuri–Hocquenghem FSO systems under various turbulence conditions with pointing errors

Free-space optical (FSO) communication is highly sensitive to atmospheric turbulence. The gamma-gamma model has been used as it describes the behaviour of a wide range of atmospheric conditions with a good agreement. In this study, closed-form expressions for calculating the bit error rate, the outage probability and ergodic capacity have been proposed for the gamma-gamma atmospheric turbulence channel with pointing errors. The performance of FSO for three turbulence mitigation techniques: aperture averaging, multiple-input/single-output and Bose-Chaudhuri-Hocquenghem block code have been investigated. Analytical results are depicted by using a realistic set of system parameter values, to lend a helpful insight into the performance of source-destination link distance of 10km in FSO communication system under atmospheric turbulence conditions.

Demonstration of intradyne BPSK optical free-space transmission in representative atmospheric turbulence conditions for geostationary uplink channel.

Binary phase-shift keying optical transmission in the C-band with coherent intradyne reception is demonstrated over a long-range (10.45km) link through the atmosphere. The link emulates representative channel conditions for geostationary optical feeder uplinks in satellite communications. The digital signal processing used in recovering the transmitted data and the performed measurements are described. Finally, the bit error rate results for 10Gbit/s, 20Gbit/s, and 30Gbit/s of the outdoor experiments are presented and compared with back-to-back measurements and theory.

Adaptation of Mode Filtering Technique in 4G-LTE Hybrid RoMMF-FSO for Last-Mile Access Network

This paper demonstrates a hybrid radio over multimode fibre and free space optics (RoMMF-FSO) system that can be used to extend the transmission range of the fourth generation long-term evolution (4G-LTE) signal in access networks. A single mode filtering technique (SMFT) is used to enhance 4G-LTE performance. The proposed scheme is evaluated in terms of the system transfer function, laser beam profile, and error vector magnitude (EVM). We show that using SMFT increases the RoMMF-FSO system bandwidth by 2 GHz and improves the received optical power by 13.6 dB. Moreover, the proposed system enhances the EVM by 4%. The measured results show that by using a 1 km MMF instead of a 1 km SMF will marginally increase the measured EVM from ∼6.6% to ∼7% with a 0.2 dB power penalty with respect to the LTE EVM limit of 12.5% as is specified for 16-quadrature amplitude modulation. The proposed system is validated practically under atmospheric turbulence conditions to mimic the outdoor environment. Measured EVM results are verified theoretically through transmitting LTE signals with turbulent using log-normal model. We also show that for a FSO link span of 500 m to meet the EVM target of 12.5% the SNR power penalties are ∼2 dB and ∼11 dB for Rytov variance of 1.2 × 10−4 and 0.1, respectively, compared with no turbulence.