CNN neural network temporal feature storage structure fusion for the visible channel equalization algorithm.

The visible light communication channel has time-varying characteristics and is difficult to predict. This paper proposes an equalization algorithm based on the structure of a convolutional neural network (CNN), combining time series feature length and long short-term memory (LSTM), and adding a residual structure. It can be seen that the equalization coefficient vector of the optical channel is a time series, which can reflect the noise characteristics of the channel and has storage characteristics. The equalizer algorithm can accurately learn the complex channel characteristics and calculate the compensation coefficient according to the channel characteristics. It can also restore the original transmission signal. At the same time, this paper also examines the compensation method of the receiver in the mobile state. The long-term memory parameters of LSTM are used to represent the sequence causality in the memory channel, and CNN and residual structure are used to refine the results and improve the accuracy of the reconstruction. The simulation results show that the algorithm can effectively eliminate the influence of the fading characteristics of the visible optical channel, improve the bit error rate performance of system transmission, solve the overall problem of channel corruption, and precisely restore the original transmission signal with fast convergence speed. In addition, this method can achieve a better balance between performance and complexity compared to the traditional contention balancing method, which proves the potential and effectiveness of the proposed channel balancing method.

Vehicular visible light communications noise analysis and modeling.

Vehicular visible light communications (VVLC) is promising intelligent transportation systems technology with the utilization of light-emitting diodes. The main degrading factor for the performance of VVLC systems is noise. Traditional VVLC systems noise modeling is based on the additive white Gaussian noise assumption in the form of shot and thermal noise. In this paper, to investigate both time correlated and white noise components of the VVLC channel noise, we propose a noise analysis based on Allan variance, which provides a time-series analysis method to identify noise from the data. The results show that white noise and random walk are observed in the VVLC systems. We also propose a motion detection algorithm based on the adaptive Gaussian mixture (GM) model to generate a double Gaussian model of VVLC channel noise. We further present a study on the error performance of a VVLC system considering channel noise to be a mixture of Gaussian components. We derive the analytical expressions of probability of error for binary phase-shift keying and quadrature phase-shift keying constellations. It has been observed that, in the presence of GM noise, the system performance degrades significantly from the usual one expected in a Gaussian noise environment and becomes a function of the mixing coefficients of the GM distribution.

Equalization of camera-based channel to mitigate uncertain sampling for optical camera communications.

The asynchronous nature of optical camera communication makes uncertain sampling a fundamental and inevitable problem that degrades data transmission performance. Based on a parametric model describing the exposure effect of the camera-based channel as the source of uncertain sampling, a simple channel equalization method and implementation are proposed in this paper to mitigate the uncertain sampling by exploiting the space-time relationship of the transmitted spatial delayed pulse width modulation waveform on multiple light sources. In addition, the rate of equalization error caused by under-sampling and over-exposure is analyzed for different duty cycles and exposure times. Numerical simulation and experiment results demonstrate the availability and reliability in mitigating uncertain sampling by the proposed channel equalization.

Parametric modeling and experimental measurement of rolling shutter characteristics for optical camera communication using undersampled modulation.

The characteristics of a rolling shutter in a commonly used complementary metal-oxide-semiconductor (CMOS) camera are important for the demodulation of an undersampled OOK symbol in optical camera communication (OCC). Therefore, a parametric model is proposed to analytically characterize the rolling shutter transfer function that converts the intensity of the light source to the brightness of the output image using several parameters. To validate the model, an effective sampling technique was used to measure the rolling shutter effect using a low-frame-rate CMOS camera. The model and results provided insights into the threshold adaptation and error performance analysis of OCC using undersampled modulation.

Underwater blue-green LED communication using a double-layered, curved compound-eye optical system.

Optical receiving systems with single-lens structures have problems such as low receiving efficiency and small field of view when applied to underwater optical wireless communication systems. In this study, a design scheme for a double-layered fly-eye-lens optical system with wide-angle focusing is proposed. Based on the analysis of the LED light source transmission model and seawater channel, the optical-power receiving equation of the fly-eye lens system is deduced. The fly-eye-lens receiving system was designed and simulated using Zemax according to the geometrical optics principle of the lens array. An experimental device for measuring the insertion loss and receiving efficiency of an underwater blue-green LED communication system was built, and the optical power of the receiving optical system was experimentally measured. For the link distances of 1, 3, and 5 m, the received optical power of the double-layered-compound eye system was higher than that of the single-layered system, with a power increase of 72%, 65%, and 60%, respectively. The results show that the double-layered fly-eye-lens receiving antenna can significantly improve the optical power received by the receiving end; therefore, this antenna structure has strong practicability and good development prospects in the field of underwater optical wireless communication.

Fuzzy control algorithm for adaptive optical systems.

In this study, the direct wavefront gradient algorithm is used to calculate the control voltage of the deflection mirror. Considering the control voltage as the input of the fuzzy controller, the fuzzy rule base is established and the three parameters of the proportional-integral-derivative controller are obtained using dual fuzzy control. In addition, a weight factor and threshold judgment are included to correct the control voltage and realize parameter self-tuning. A simulation and experiments demonstrate that this method can effectively correct the wavefront distortion signal and improve the system response speed. Moreover, it can reduce the difficulty of parameter adjustment and meet the requirements of laser communication for wavefront correction.

Influence of source parameters and atmospheric turbulence on the polarization properties of partially coherent electromagnetic vortex beams.

In this paper, we introduce a vortex class of a partially coherent source of Schell type with an electromagnetic Gaussian Schell-model vortex (EGSMV) beam, which is the product of the partially coherent EGSM beam passing through a spiral phase modulator. The analytical expressions for the degree of polarization (DoP) and orientation angle of polarization (OAoP) of the EGSMV beam propagating through atmospheric turbulence are derived. The expressions are used to analyze the influence of topological charge, wavelength, and atmospheric turbulence on the DoP and OAoP of the EGSMV beam, and we come to some new conclusions. The larger the topological charge is, the bigger the dark spot in the center of the partially coherent EGSMV beam, the more dispersed the DoP distribution, and the wider the OAoP distribution are; therefore, there is more information received by the detector when the partially coherent EGSMV beam propagates in atmospheric turbulence. The longer the wavelength is, the smaller the near-surface refractive index structure constant and the larger the inner scale are and the more concentrated the DoP distribution is, but the influence of the outer scale is negligible. The number of petals, which is the shape of the OAoP distribution, is equal to just twice the topological charge. These results provide a theoretical basis for better control of coherent detection in optical communication when the partially coherent EGSMV beam propagating through atmospheric turbulence is used as the local oscillator in a wireless optical communication system.

Analysis of a heterodyne detection system affected by irradiance and phase fluctuations in slant atmospheric turbulence.

A mathematical model considering the effects of amplitude and phase random fluctuations on the signal-to-noise ratio (SNR) of heterodyne detection was established, and the effects of irradiance and phase fluctuations on the performance of heterodyne detection were investigated. The results show that the inner scale of turbulence significantly affects the SNR. The first 20 Zernike modes have a greater effect on the SNR of the heterodyne detection system. The SNR is affected more significantly by phase fluctuations than by irradiance fluctuations. The SNR decreases by four orders of magnitude because of amplitude and phase fluctuations.

Arbitrary shaped beam scattering from a chiral-coated conducting object with arbitrary monochromatic illumination.

An exact semi-analytical method of calculating the scattered fields from a chiral-coated conducting object under arbitrary shaped beam illumination is developed. The scattered fields and the fields within the chiral coating are expanded in terms of appropriate spherical vector wave functions. The unknown expansion coefficients are determined by solving an infinite system of linear equations derived using the method of moments technique and the boundary conditions. For incidence of a Gaussian beam, circularly polarized wave, zero-order Bessel beam and Hertzian electric dipole radiation on a chiral-coated conducting spheroid and a chiral-coated conducting circular cylinder of finite length, the normalized differential scattering cross sections are evaluated and discussed briefly.

Diffraction characteristics of a Laguerre-Gaussian beam through a Maksutov-Cassegrain optical system.

Maksutov-Cassegrain (MC) systems are widely used in long-range free-space optical (FSO) communication. In this study, analytical expressions for the diffraction field of a Laguerre-Gaussian (LG) beam passing through an MC system have been derived. The numerical results reveal that, in the long-range FSO system, the replacement of a Gaussian beam by the LG beam enhances the emission efficiency of the MC. Furthermore, the MC has a shaping effect on the LG beam, and the orbital angular momentum of the LG beam is not dispersed when the beam is diffracted by the MC.

Effect of angle-of-arrival fluctuation on heterodyne detection in slant atmospheric turbulence.

A mathematical model of the effect of random pointing errors on the mixing efficiency of heterodyne detection is established, and the effect of angle-of-arrival fluctuation on the mixing efficiency of heterodyne detection is investigated. The results show that the average mixing efficiency is significantly affected by the angle-of-arrival fluctuation in the outer scale. The larger the obscuration ratio and receiving aperture of the optical system, the lower the average mixing efficiency is. The closer the value of D/r0 is to 0.79, the closer the bit error rate of heterodyne detection is to 1×10-9 under weak turbulence.

The Study of UV Scattering Polarization Properties of Spherical Particles of Haze.

With the rapid development of economy and society in our country, atmospheric haze has become a prominent environmental issue. Measuring haze particles is also important. Polarized ultraviolet light is scattered by atmospheric haze particles, the changes of scattered light polarization state(Stokes vector and the degree of polarization) in atmospheric haze can reflect the physical properties of the particles (particle size and the complex refractive index, etc.). Based on Mie scattering theory, line-of-sight and non-line-of-sight of a single UV scattering model is established. On one hand, the effects of the physical properties of the single and chain-structure spherical particles for the UV light polarization properties are studied. On the other hand, based on Monte Carlo Simulation, the impacts of haze particle concentration on polarization state at a fixed particle size distribution are discussed. The simulation results show that: with the particle size of single spherical increasing, the scattered light intensity in Stokes vector (Is) is significantly enhanced. Is shows a trend of first increase then decrease with the increase of the complex refractive index's imaginary part. The degree of polarization is constantly increasing with the increase of the complex refractive index's imaginary part and the imaginary part of the complex refractive index is small, the degree of polarization trend of fast increase. With the concentration of particles in haze increasing when the distribution of particle size is a fixed value, haze particles scattering coefficient, extinction coefficient and absorption coefficient showed a linear increase, while Is is reduced after the first increases. For chain-structure spherical particles, with the increase of the number of particles, Is shows a tendency to increase. As the same time, degrees of polarization distinguish whether the chain-structure spherical particles are made up by the same spherical particle. In the chain-structure consisted of same spherical particles, Is increases linearly with the increase of the number of particles and the degree of polarization does not change. The Is under the chain-structure consisted of different spherical particles can distinguish particles' physical properties according to the changing trend of scattering light polarization state.

Spatial light coupled into a single-mode fiber by a Maksutov-Cassegrain antenna through atmospheric turbulence.

Maksutov-Cassegrains are widely used in free-space optical communication. The coupling efficiency and variance of a Maksutov-Cassegrain fiber (single-mode) system distorted by atmospheric turbulence are numerically evaluated using second-order and fourth-order moments under a Von Karman spectrum. Considering the limited cost and size of the equipment, the Maksutov-Cassegrain aperture should satisfy DA/ρS≈7 (ρS approximates the characteristic atmospheric coherence length), and the obscuration ratio should be no more than 0.2.