Atmospheric Optical Communication Research Articles

Development and characterization of a simulation module in the temporal domain for experimental optical atmospheric links affected by optical turbulence with gamma‐gamma probability density function

AbstractWe present the development and characterization of a simulation module in the temporal domain for optical atmospheric links affected by optical turbulence with Gamma‐Gamma probability density function. We generate time series for different turbulence levels, from weak to strong, to simulate the effect produced by the atmospheric optical channel over an optical carrier with binary‐modulated intensity. We added at the photoreceiver stage a time‐domain amplitude noise from a real‐world component. We carried out performance evaluation of the simulated links through eye pattern diagrams making a qualitative comparison of the waveforms obtained in some of our simulations with waveforms acquired in our long‐distance atmospheric optical links. Our module allows carrying out in a simple way the design and the characterization in the time domain by simulation models of optical links for atmospheric optical communications. Including parameters of physical components, we can obtain results closer to those obtained in the real world.

Research on the key technology of turbulence suppression for atmospheric optical laser communication based on OFDM

Based on the analysis of the two main problems of atmospheric channel in free space optics (FSO), aiming at the problem of laser atmospheric channel especially the problems of frequency selective fading and multipath effect in complex turbulent environment, the suppression method is proposed based on OFDM turbulence effect, the FSO-OFDM system is built, the baseband model of this system and the signal of multi-carrier modulation and demodulation method are studied. First of all, this article systematically analyzes the mechanism of turbulence effect of atmospheric channel, and discusses the model of plane wave laser communication system under the influence of atmospheric turbulence, the space optical communication system model of Gaussian beam of a logarithmic normal turbulent channel is established under the influence of atmospheric turbulence, the probability density function of light intensity is deduced, the methods for analyzing the effects of various atmospheric turbulence effects on system performance using the signal-to-noise ratio probability density function; the OFDM multi-carrier modulation scheme of FSO-OFDM system is designed, the baseband mode model of FSO-OFDM system is constructed, and the modulation and demodulation principle of its signal is studied using this model. Finally, FSO-OFDM system is realized by using MATLAB, and the FSO communication system under the multipath interference is simulated, experiments on bit error rate characteristics under different guard intervals are performed. It is confirmed that the FSO-OFDM system has a strong ability to resist multipath interference and frequency selective fading, as well as good bit error rate (BER) performance. It can effectively solve the problem of intersymbol interference and the reliable link, and has a very broad application prospect and using value.

ОБ ИСПОЛЬЗОВАНИИ БИСТАТИЧЕСКИХ ПОДВОДНЫХ ОПТИЧЕСКИХ СИСТЕМ СВЯЗИ

Atmospheric and underwater optical communication lines have much in common due to variable conditions for the transmission of laser radiation along the path of the communication line, both in time and in space. The passage of a laser beam through a cloudy medium is accompanied by a decrease in the radiation intensity and an increase in the intensity of the scattered flux, which forms illumination, the intensity of which decreases with distance from the laser beam. As a rule, at the scales of distances at which the radiation of the optical range decays by several orders of magnitude, water in natural systems is homogeneous. The analysis shows that water in natural reservoirs is a light-attenuating medium both due to attenuation on dispersed particles and due to scattering on nanobubbles. The article analyzes the capabilities of atmospheric and underwater optical communication systems (POS). The results of modeling bistatic PIC in a reservoir containing particles with a diameter of 0.8 ?m and a concentration of 2 х 107 cm-3 are presented. It was shown that the parameters of the bistatic PIC pulses are significantly affected by nanobubbles in water. In natural reservoirs, the use of systems with direct line of sight is preferable. It has been shown that bistatic PICs, in which the scattering region is located on the side of the receiver unit relative to the transmitter - receiver line, have the greatest energy potential and minimal intersymbol interference. In natural reservoirs, it is possible to use such bistatic PICs for communication with a frequency of less than 20 MHz at distances of not more than 20 m. As a scattering region in bistatic PIC, you can use the bottom of the reservoir, the surface of the water or the region of water with a higher turbidity than the rest of the reservoir.

Experimental results of atmospheric coherent optical communications with adaptive optics

Although there is an urgent need, it is still a significant challenge to establish high-speed laser communication links between satellites to ground stations owing to the influence of atmospheric turbulence. Adaptive optics (AO) is a considerable scheme to solve the problem. In this paper, the primary experiment of satellite-to-ground laser communication with the data rate of 5Gbps and BPSK modulation is carried out through a 500 m horizontal channel with relatively strong turbulence. A 137-element AO unit is used for correcting the phase distortion brought by the turbulence. When the AO unit is working, both the received optical power and bit error rate (BER) (All the eight experimental results of BER at different time are below 10−6.) are improved significantly. The primary results demonstrate that the ground station with AO technology has the capability to establish the satellite-to-ground laser communication links and provide a reference for the design of satellite terminals.

M-ary pulse position modulation performance in non-Kolmogorov turbulent atmosphere.

The performance of atmospheric optical wireless communication systems in terms of the bit error rate (BER) is investigated when a Gaussian laser beam propagating in non-Kolmogorov turbulence is M-ary pulse-position-modulated (PPM). BER variations against the changes in different parameters such as the non-Kolmogorov power law exponent, symbol number, data bit rate, avalanche photodetector gain, equivalent load resistor, detector quantum efficiency, wavelength, turbulence structure constant, and the Gaussian beam source size are analyzed. Making the design of the PPM optical wireless communication system able to operate in a non-Kolmogorov atmosphere will give better BER performance if the parameters are taken into account in line with the trends presented in our results.

Simulation of information transfer through atmospheric channels of scattered laser radiation propagation

Results of simulations of the pulse response of an atmospheric optical communication channel on scattered radiation at wavelengths of 0.3, 0.5, and 0.9 μm are considered. The performed analysis shows that the maximum power of the received information signal at base distances of 2–3 km and less between the source and receiving system is reached at λ = 0.3 μm; for longer base distances and low turbidity of the atmosphere, λ = 0.5 μm. The highest maximum power is observed at λ = 0.5 μm under high turbidity of the atmosphere and base distances of 3–10 km. However, if the base distances are longer than 10 km, the maximum of the information signal power is reached at λ = 0.9 μm. For an example of a laser source and receiving system, using calculation results for the pulse reaction of the atmospheric communication channel at λ = 0.5 μm, we estimate the limiting base distance between the source and receiver, as well as the limiting pulse frequency which is not filtered by the communication channel.

Performance Evaluation of Atmospheric Optical Communications Links Affected by Generalized Málaga Turbulence Model

In this paper, the performance analysis of atmospheric optical communications links is analyzed in terms of the average bit error rate. To this end, the optical irradiance scintillation due to the turbulence effects is modeled by a generalization of the Malaga or $${\mathcal {M}}$$ distribution. In particular, the behavior of the atmospheric optical channel is treated as a superposition of a finite number of Generalized-K distributed sub-channels, controlled by a discrete Negative-Binomial distribution dependent on the turbulence parameters. All previous results derived until now were restricted to natural numbers for those turbulence parameters. Here, new analytical formalisms for evaluating the average bit error rate performance are proposed. These formalisms are based on considering the most accurate scenario in which such turbulence parameters are extended to real numbers. Theses new expressions are a valuable tool for analysing the performance of atmospheric optical links.

Analysis to beam quality of partially coherent flat-topped vortex beams propagating through atmospheric turbulence

Based on the extended Huygens-Fresnel principle and the second-order moments of the Wigner distribution function (WDF), the analytical formula for the propagation factor (M2-factor) of partially coherent flat-topped vortex beams propagating through atmospheric turbulence are derived, and used to analyze beam quality of partially coherent flat-topped vortex beams propagating through atmospheric turbulence. It is shown that the smaller the structure constant and the correlation length are, the bigger the inner scale of turbulence, the beam order and the wave length are, the smaller the normalized propagation factor is, and the better the beam quality of partially coherent flat-topped vortex beams in atmospheric turbulence is. The partially coherent flat-topped vortex beams is less affected by atmospheric turbulence than that of partially coherent flat-topped beams under certain condition, and will be useful in atmospheric optical communications.

弱湍流条件下大气光通信的阈值优化方法

弱湍流条件下大气光通信的阈值优化方法

弱湍流条件下大气光通信的阈值优化方法

弱湍流条件下大气光通信的阈值优化方法

Adaptive optics for the free-space coherent optical communications

We present the results that the performances of the atmospheric coherent optical communications improved by a real 127-element adaptive optics system operating in 1550nm wavelength on a 1.8m telescope. The mixing efficiency and the BER of the homodyne free-space optical coherent communications have been analyzed with the correction of the adaptive optics system. It is shown that, the AO can expand the scope of the atmospheric turbulence condition D/r0(1550) from 1 to around 6.5 in which the coherent optical communications can function well. In order to acquire favorable results, the mixing efficiency should be larger than 0.4 and the residual wavefront error after the AO correction should be smaller than λ/6. We also find that the mixing efficiency of the coherent homodyne receiver is approximate to the Strehl ratio of the far field of the signal light. In conclusion, we have shown that the adaptive optics is a very promising technique that can largely improve the performances of the free-space coherent optical communications degraded by the atmospheric turbulence.

General closed-form bit-error rate expressions for coded M-distributed atmospheric optical communications.

In this Letter, general closed-form expressions for the average bit error rate in atmospheric optical links employing rate-adaptive channel coding are derived. To characterize the irradiance fluctuations caused by atmospheric turbulence, the Málaga or M distribution is employed. The proposed expressions allow us to evaluate the performance of atmospheric optical links employing channel coding schemes such as OOK-GSc, OOK-GScc, HHH(1,13), or vw-MPPM with different coding rates and under all regimes of turbulence strength. A hyper-exponential fitting technique applied to the conditional bit error rate is used in all cases. The proposed closed-form expressions are validated by Monte-Carlo simulations.

基于BPSK改进的大气光通信混合MPPM调制方案

基于BPSK改进的大气光通信混合MPPM调制方案

General analytical expressions for the bit error rate of atmospheric optical communication systems: erratum

General analytical expressions for the bit error rate of atmospheric optical communication systems: erratum

Spatially correlated gamma-gamma scintillation in atmospheric optical channels.

In this paper, novel analytical closed-form expressions are derived for the probability density function of the sum of identically distributed correlated gamma-gamma random variables that models an optical atmospheric channel communication with receiver spatial diversity. The mathematical expressions here proposed provide a general procedure to obtain information about the scintillation effects induced by turbulence over a diversity reception scheme implementing equal-gain combining method. Both, validity and accuracy of the obtained statistical distribution are corroborated by comparing the analytical results to numerical results obtained by Monte-Carlo simulations. These simulations are particularized for constant, exponential and circular correlation models, corresponding to three different receivers spatial configurations. In addition, the extreme situations of no correlation and fully correlated received signals are also studied. The presented expressions lead to a simple and easy-to-compute analytical procedure of analyzing atmospheric optical communications systems with correlated spatial diversity.

Evolution of branch points for a laser beam propagating through an uplink turbulent atmosphere

Evolution of branch points in the distorted optical field is studied when a laser beam propagates through turbulent atmosphere along an uplink path. Two categories of propagation events are mainly explored for the same propagation height: fixed wavelength with change of the turbulence strength and fixed turbulence strength with change of the wavelength. It is shown that, when the beam propagates to a certain height, the density of the branch-points reaches its maximum and such a height changes with the turbulence strength but nearly remains constant with different wavelengths. The relationship between the density of branch-points and the Rytov number is also given. A fitted formula describing the relationship between the density of branch-points and propagation height with different turbulence strength and wavelength is found out. Interestingly, this formula is very similar to the formula used for describing the Blackbody radiation in physics. The results obtained may be helpful for atmospheric optics, astronomy and optical communication.

On the capacity of ℳ-distributed atmospheric optical channels

In this Letter, closed-form expressions of ergodic capacity, outage probability, and outage rate are derived for an atmospheric optical communication link using intensity modulation and direct detection with unbounded optical wavefront propagating through a homogeneous and isotropic turbulent medium. The optical scintillation of the received signal is modeled with the recently proposed Málaga or M turbulence distribution. By taking advantage of this unifying statistical model, the expressions here presented are valid for all possible irradiance fluctuation conditions, leading to direct relationships between turbulence parameters and link capacity performance.

Performance Analysis of Coherent Atmospheric Optical Communications with Soft-Decision Forward Error Correction

Performance of coherent atmospheric optical communication system with heterodyne detection and LDPC codes was evaluated over atmospheric channel attenuations of which are about 20-30 dB/km. To reduce bit error and enhance the system performance LDPC code was implemented in system. Combining coherent detection and LDPC codes could reduce the received power requirement ~4 dBm at the BER of 10-9.

Diversity and Multiplexing for Near-Field Atmospheric Optical Communication

In this paper, the performance of multi-beam free-space optical (FSO) communication systems are studied through an accurate analytical approach which does not rely on far-field assumptions commonly used in the literature. A framework is presented for analytical and numerical performance analyses of multi-beam FSO systems employed in a diversity or multiplexing scheme. The performance analyses show that the far-field assumptions may not correctly estimate the system behavior in many geometrical scenarios within practical interest. The results demonstrate the degrading effects of diffraction and spatial correlation of atmospheric turbulence in the form of diversity gain reduction in diversity systems and crosstalk in multiplexing systems. These effects have been mostly neglected in the literature by applying far-field assumptions. Our results can thus be useful in the design of practical diversity or multiplexing FSO systems especially when a compact design is desired.

Application of LDPC codes in atmospheric optical communication with coherent detection

A novel scheme employing low-density parity-check (LDPC) codes in atmospheric optical communication system is proposed. We deploy coherent detection at the receiving side in the proposed scheme. To reduce bit error rate (BER) and enhance the system performance, LDPC codes are exploited and coherent receiver is used to improve the receiving sensitivity. Experiments are implemented to evaluate the performance of the transmission system. The atmospheric channel attenuations are set to 20–30 dB/km. The coherent detection with LDPC codes can reduce the received power requirement by ∼4 dBm at the BER of 10−9.