Proc. SPIE. 11048, 17th International Conference on Optical Communications and Networks (ICOCN2018)
KEYWORDS: Antennas, Telecommunications, Atmospheric optics, Transmittance, Free space optical communications, Signal attenuation, Air contamination, Atmospheric modeling, Systems modeling, Visibility through fog
Considering the weather’s influence to atmospheric transmittance, the paper modifies the attenuation matrix in the basement of original MIMO FSOC system channel model. And then, based on the modified channel model, the capacity in different given conditions is shown through numerical simulation. The paper simulates the capacity in different transmitting antenna’s height as well as in different weather. Considering that in most conditions, the optical signal doesn’t transfer vertically, the paper stimulates the capacity versus system slant angle. The paper also simulates difference value of capacity. At last, the paper designs a software to evaluate whether the given conditions are fit for communication or not.
The application of asymmetrically clipped optical-orthogonal frequency division multiplexing (ACO-OFDM) in free-space optical (FSO) communication system can increase the system channel capacity, while the use of channel estimation technology can ensure that ACO-OFDM has better performance. Since the channel estimation performance is directly affected by the selection of pilot patterns, especially when using compressed sensing (CS), it has been a major task in OFDM transmission schemes. In order to improve the channel reconstruction accuracy, a hybrid optimization algorithm based on genetic algorithm (GA) and particle swarm optimization (PSO) algorithm is proposed for two types of pilot design criterion. Simulation results show that compared with other three algorithms, the proposed algorithm has faster convergence speed and lower convergence value. With the pilots designed by the proposed algorithm, the system can achieve higher channel reconstruction accuracy.
We present the performance benefits of differential phase-shift keying (DPSK) modulation in eliminating influence from atmospheric turbulence, especially for coherent free space optical (FSO) communication with a high communication rate. Analytic expression of detected signal is derived, based on which, homodyne detection efficiency is calculated to indicate the performance of wavefront compensation. Considered laser pulses always suffer from atmospheric scattering effect by clouds, intersymbol interference (ISI) in high-speed FSO communication link is analyzed. Correspondingly, the channel equalization method of a binormalized modified constant modulus algorithm based on set-membership filtering (SM-BNMCMA) is proposed to solve the ISI problem. Finally, through the comparison with existing channel equalization methods, its performance benefits of both ISI elimination and convergence speed are verified. The research findings have theoretical significance in a high-speed FSO communication system.
An analytical formula model which is used to describe laser beam’s depolarization characteristics is solved based on multiple Rayleigh scattering model. Firstly, by using Stokes vector to characterize intensity and polarization, while at the same time using Mueller matrix and rotation matrix to characterize polarization changing in scattering procedure, a single scattering model is built. Then, a multiple scattering model is built considering the effects of atmospheric absorbing and scattering attenuation. The received light’s Stokes vectors through multiple scattering procedure are separately solved. At last, on the basis of multiple scattering vectors, the depolarization characteristics of laser beam propagation through atmosphere are estimated though calculating ratio of depolarization and polarized angle shifting. The numerical analysis based on analytical conclusion of this paper shows that for a horizontal polarized laser beam, its ratio of depolarization is about 1% and polarized angle shifting is about 0.3。 when propagates through atmosphere and arrives into the receiver on the ground, and both the above characteristics have only a small change compared with the change of cloud’s depth. The findings of these research show that Rayleigh scattering from atmosphere has a weak effect on the laser beam’s polarization status. The multiple scattering model and Stokes vector analytical formulas raised in the paper could also be used to study the depolarization characteristics of ellipse polarized laser beam and partially polarized laser beam propagating through atmosphere. The research findings of this paper will have theoretical guiding significances in the domain of laser communication, laser detection and laser imaging.
Laser reflective tomography is a long-range, high-resolution active detection technology, whose advantage is that the spatial resolution is unrelated with the imaging distance. Accompany satellite is a specific satellite around the target spacecraft with encircling movement. When using the accompany satellite to detect the target aircraft, multi-angle echo data can be obtained with the application of reflective tomography imaging. The feasibility of such detection working mode was studied in this article. Accompany orbit model was established with horizontal circular fleet and the parameters of accompany flight was defined. The simulation of satellite-to-satellite reflective tomography imaging with satellite-accompany was carried out. The operating mode of reflective tomographic data acquisition from monostatic laser radar was discussed and designed. The flight period, which equals to the all direction received data consuming time, is one of the important accompany flight parameters. The azimuth angle determines the plane of image formation while the elevation angle determines the projection direction. Both of the azimuth and elevation angles guide the satellite attitude stability controller in order to point the laser radar spot on the target. The influences of distance between accompany satellite and target satellite on tomographic imaging consuming time was analyzed. The influences of flight period, azimuth angle and elevation angle on tomographic imaging were analyzed as well. Simulation results showed that the satellite-accompany laser reflective tomography is a feasible and effective method to the satellite-to-satellite detection.
Orthogonal frequency division multiplexing (OFDM) technique applied to the atmospheric optical communication can improve data transmission rate, restrain pulse interference, and reduce effect of multipath caused by atmospheric scattering. Channel estimation, as one of the important modules in OFDM, has been investigated thoroughly and widely with great progress. In atmospheric optical communication system, channel estimation methods based on pilot are common approaches, such as traditional least-squares (LS) algorithm and minimum mean square error (MMSE) algorithm. However, sensitivity of the noise effects and high complexity of computation are shortcomings of LS algorithm and MMSE algorithm, respectively. Here, a new method based on compressive sensing is proposed to estimate the channel state information of atmospheric optical communication OFDM system, especially when the condition is closely associated with turbulence. Firstly, time-varying channel model is established under the condition of turbulence. Then, in consideration of multipath effect, sparse channel model is available for compressive sensing. And, the pilot signal is reconstructed with orthogonal matching tracking (OMP) algorithm, which is used for reconstruction. By contrast, the work of channel estimation is completed by LS algorithm as well. After that, simulations are conducted respectively in two different indexes -signal error rate (SER) and mean square error (MSE). Finally, result shows that compared with LS algorithm, the application of compressive sensing can improve the performance of SER and MSE. Theoretical analysis and simulation results show that the proposed method is reasonable and efficient.