Coherent Free-space Optical Communication System Research Articles

Carrier FOE Scheme Based on FSTS in Spatial Diversity PM Coherent FSO Communication

The efficient and cost-effective suppression of atmospheric turbulence effects in free-space optical (FSO) communication poses a significant challenge. To address this challenge, a carrier frequency offset estimation (FOE) scheme based on frame-synchronous training sequence (FSTS) is proposed, which can be applied to spatial diversity polarization multiplexing (PM) coherent FSO communication system. The proposed scheme integrates the functions of frame synchronization (FS) and FOE by designing distinct training sequences for the X and Y polarizations. Specifically, the FOE scheme achieves high accuracy, wide range, low complexity, and multi-format versatility while maintaining FS accuracy. Simulation results for PM 4/16-quadrature amplitude modulation (QAM) demonstrate the feasibility and generality of the proposed scheme. Moreover, when combined with the spatial diversity reception under strong and weak turbulence conditions, the proposed algorithm exhibits higher receiver sensitivity compared to the conventional training sequence (TS)-based FOE algorithm. The advantages of this scheme make it highly promising for high-speed coherent FSO communications across multiple modulation formats.

Performance analysis of a spatial diversity coherent free-space optical communication system based on optimal branch block phase correction.

Coherent optical communication systems have high receiver sensitivities, high spectral efficiencies, and high-capacity information transmission, which are widely used in free-space optical communications. However, atmospheric turbulence affects the power budget of coherent receiving systems. Diversity can effectively suppress atmospheric turbulence, but relative phase differences caused by phase asynchrony degrade the performances of diversity systems. Hence, spatial diversity reception based on optimal branch block phase correction is proposed herein and verified through simulations and experiments to improve diversity gain and reduce the complexity and outage probability of diversity systems effectively. This scheme is promising for application to high-speed low Earth orbit satellite-to-ground communications.

Application of AdamSPGD algorithm to sensor-less adaptive optics in coherent free-space optical communication system.

Sensor-less adaptive optics based on stochastic parallel gradient descent (SPGD) is effective for the compensation of atmospheric disturbances in coherent free-space optical communication systems. However, SPGD converges slowly and easily falls into local extremes. Combining adaptive moment estimation and SPGD, we propose the AdamSPGD algorithm for efficient wavefront correction. Theoretical analysis and numerical simulations demonstrate that AdamSPGD can significantly increase the convergence speed, robustness, and dynamic ability, thereby more efficiently suppress the negative effects of atmospheric turbulence on mixing efficiency, bit error rate, and outage probability. Experimental results show that AdamSPGD reduces ∼50% of iterations. The improved performances make the proposed algorithm suitable for SLAO to improve the quality of optical communications.

Design and Performance Analysis of NadamSPGD Algorithm for Sensor-Less Adaptive Optics in Coherent FSOC Systems

Sensor-less adaptive optics (SLAO) based on stochastic parallel gradient descent (SPGD) is effective for the compensation of atmospheric turbulence in coherent free-space optical communication (CFSOC) systems. However, SPGD converges slowly and easily falls into local extremes. Therefore, we propose a novel NadamSPGD algorithm for efficient wavefront correction that combines Nesterov-accelerated adaptive moment estimation (Nadam) and SPGD. Specifically, Nesterov’s accelerated gradient momentum (NAG) and adaptive gain coefficients are integrated to conventional SPGD to accelerate its convergence speed and avoid converging to extremum points. Theoretical analysis, numerical simulations and experimental results demonstrate that NadamSPGD can increase the convergence speed by ~50% and significantly improve the robustness of parameters, and thus more efficiently suppress the negative effects of atmospheric turbulence on mixing efficiency (ME) and bit error rate (BER). Our algorithm also presents better dynamic performance under strong turbulence and high Greenwood frequency conditions, and it is more suitable for real-time SLAO systems. This study proves that the NadamSPGD algorithm is suitable for SLAO in the CFSOC system and is a viable substitute for SPGD to improve the quality of optical communications.

Influence of atmospheric turbulence on coherent source in a horizontal long-distance laser link

In coherent optical communication, the application of linearly polarized light can greatly improve communication sensitivity and communication capacity. However, when a laser transmits in an inferior atmospheric layer, the polarization character is influenced by atmospheric turbulence. This study proposes a measurement experiment of the characteristics of linearly polarized light over an 11.16-km turbulent path. We also present a theoretical approach and its operation for investigating the polarization characteristics of a laser. The variations of polarization under different atmosphere link distances and turbulence intensities are also discussed. The experimental results were in agreement with the theoretical analysis results. The theoretical analysis and experimental results show that the effect of atmospheric turbulence on polarization direction is stronger than that of atmospheric turbulence on the polarization state. This work will benefit coherent free-space optical communication system design efforts.

Performance analysis of 349-element adaptive optics unit for a coherent free space optical communication system

As a continuation of our previous work [Optics Express.25, 15229(2017)] in which we have verified the performance of a coherent free space optical communication (FSOC) system with a 97-element adaptive optics (AO) system, in this paper, we evaluated the performance improvement of the coherent FSOC system using a large-scale high-speed AO system with a 349-element continuous surface deformable mirror. The mixing efficiency (ME) and bit-error-rate (BER) under different Greenwood frequency (GF) were calculated as the performance metric of coherent FSOC system. The performance of FSOC system using such a large-scale AO system was quantitatively verified for the first time. The obtained results showed that the performance was obviously improved when a larger-scale high-speed AO system is employed in coherent FSOC system. This analysis result provides a performance verification for large-scale high-speed AO systems used in FSOC system which is beneficial for coherent FSOC system parameters design.

An experimental study of wavefront correction for a heterodyne detection system for optical communication

The turbulence effect is the most significant factor limiting the performance of coherent optical communication systems. In this paper, we report on wavefront correction for a coherent free-space optical communication system comprising a Shack–Hartmann wavefront sensor (SH-WFS), a deformable mirror, and a wavefront controller. With this system, distorted wavefronts of laser beams transmitted through the atmosphere can be corrected in real time. To demonstrate this real-time correction, we simulated atmospheric turbulence using phase screens of different intensities, based on a modified Hill spectrum. These laboratory-based simulations indicate that the peak-to-valley value obtained using the wavefront correction system in a closed-loop configuration can be reduced to less than 0.8 μm, the root-mean-squared error to less than 0.05 μm, and the Strehl ratio can be increased to greater than 0.8. Finally, the adaptive optics system described above was applied to a heterodyne detection system in an experiment investigating optical communication at a distance of 1.3 km, from which we were able to demonstrate real-time video transmission. The results of this experiment verified that a closed AO system can improve signal beam coupling power and increase the amplitude of the intermediate frequency signal, making the waveform of the baseband signal more ideal.

基于数字相位恢复算法的正交相移键控自由空间相干光通信系统

基于数字相位恢复算法的正交相移键控自由空间相干光通信系统

OAM Mode Selection and Space–Time Coding for Atmospheric Turbulence Mitigation in FSO Communication

Orbital angular momentum (OAM) multiplexing has recently received considerable interest in free space optical (FSO) communications. Propagating OAM modes through free space may be subject to atmospheric turbulence (AT) distortions that cause intermodal crosstalk and power disparities between OAM modes. In this paper, we are interested in multiple-input-multiple-output (MIMO) coherent FSO communication systems using the OAM. We propose a selection criterion for the OAM modes to minimize the impact of the AT. To further improve the obtained performance, we propose a space-time (ST) coding scheme at the transmitter. Through numerical simulations of the error probability, we show that the penalty from AT is completely absorbed for the weak AT regime, and considerable coding gains are obtained in the strong AT regime.

Channel correlation and BER performance analysis of coherent optical communication systems with receive diversity over moderate-to-strong non-Kolmogorov turbulence.

In this paper, new expressions of the channel-correlation coefficient and its components (the large- and small-scale channel-correlation coefficients) for a plane wave are derived for a horizontal link in moderate-to-strong non-Kolmogorov turbulence using a generalized effective atmospheric spectrum which includes finite-turbulence inner and outer scales and high-wave-number "bump". The closed-form expression of the average bit error rate (BER) of the coherent free-space optical communication system is derived using the derived channel-correlation coefficients and an α-μ distribution to approximate the sum of the square root of arbitrarily correlated Gamma-Gamma random variables. Analytical results are provided to investigate the channel correlation and evaluate the average BER performance. The validity of the proposed approximation is illustrated by Monte Carlo simulations. This work will help with further investigation of the fading correlation in spatial diversity systems.

Average BER of coherent optical QPSK systems with phase errors over M turbulence channels

The average bit-error-rate (BER) performance is studied for a coherent free-space optical communication system employing differentially encoded quadrature phase-shift keying (QPSK) with the Mth-power phase estimation method. A closed-form expression, considering the combined effects of the Malaga (M) turbulence fading, pointing errors, and phase estimation errors, is derived in terms of Meijer’s G function. Numerical and Monte Carlo simulation results are presented to verify the derived expression.

Performance analysis of coherent free space optical communications with sequential pyramid wavefront sensor

Based-on the previous study on the theory of the sequential pyramid wavefront sensor (SPWFS), in this paper, the SPWFS is first applied to the coherent free space optical communications (FSOC) with more flexible spatial resolution and higher sensitivity than the Shack-Hartmann wavefront sensor, and with higher uniformity of intensity distribution and much simpler than the pyramid wavefront sensor. Then, the mixing efficiency (ME) and the bit error rate (BER) of the coherent FSOC are analyzed during the aberrations correction through numerical simulation with binary phase shift keying (BPSK) modulation. Finally, an experimental AO system based-on SPWFS is setup, and the experimental data is used to analyze the ME and BER of homodyne detection with BPSK modulation. The results show that the AO system based-on SPWFS can increase ME and decrease BER effectively. The conclusions of this paper provide a new method of wavefront sensing for designing the AO system for a coherent FSOC system.

Performance analysis of a coherent free space optical communication system based on experiment

Based on our previous study and designed experimental AO system with a 97-element continuous surface deformable mirror, we conduct the performance analysis of a coherent free space optical communication (FSOC) system for mixing efficiency (ME), bit error rate (BER) and outage probability under different Greenwood frequency and atmospheric coherent length. The results show that the influence of the atmospheric temporal characteristics on the performance is slightly stronger than that of the spatial characteristics when the receiving aperture and the number of sub-apertures are given. This analysis result provides a reference for the design of the coherent FSOC system.

BER Analysis of Coherent Free Space Optical Communication Systems with Holographic Modal Wavefront Sensor

Degradation of bit-error-rate (BER), caused by atmospheric turbulence, seriously hinders the performance of coherent Free Space Optical (FSO) communication systems. An adaptive optics system proves to be effective in suppressing the atmospheric turbulence. The holographic modal wavefront sensor (HMWFS) proposed in our previous work, noted for its fast detecting rates and insensitivity to beam scintillation, is applied to the coherent FSO communication systems. In this paper, based on our previous work, we first introduce the principle of the HMWFS in brief and give the BER of the coherent FSO with homodyne detection in theory, and then analyze the improvement of BER for a coherent FSO system based on our previous simulation works. The results show that the wavefront sensor we propose is better for weak atmospheric turbulence. The most obvious advantages of HMWFS are fast detecting rates and insensitivity to beam scintillation.

Performance of an adaptive phase estimator for coherent free-space optical communications over Gamma-Gamma turbulence

This paper investigates an adaptive phase estimator for coherent free-space optical (FSO) communication systems. Closed-form solutions for variance of phase errors are derived when the optical beam is subjected to Gamma-Gamma distributed turbulence. The adaptive phase estimator has improved upon the phase error performance in comparison to conventional phase estimators. We also demonstrate notable improvement in BER performance when applying our adaptive phase estimator to coherent FSO communication systems.

Principle and performance analysis of coherent tracking sensor based on local oscillator beam nutation.

By integrating coherent tracking sensor functionality into a coherent communication receiver, a dual-function receiver with coherent boresight error sensing is developed for tracking in coherent free-space optical communication systems. The sensor principles are analyzed according to optical interference theory, and the boresight error detection algorithm and small signal linear model are derived. Analysis of local-oscillator beam nutation on system performance shows that the best nutation half-angle is 0.5-1 μrad, the noise equivalent angle is less than 0.02 μrad, and the communication sensitivity degradation is 0.2-0.6 dB. This technology opens new avenues for free-space optical communication system design.

Performance of Coherent Differential Phase-Shift Keying Free-Space Optical Communication Systems in M-Distributed Turbulence

In this paper, we investigate the error rate performance of a coherent free-space optical communication system with differential phase-shift keying over M-distributed turbulence channels. In our derivations, we express the probability density function of M distribution in terms of a series expansion. The coefficients of this series only include elementary and gamma functions. For numerical evaluation, this power series is truncated to a finite number of terms. An upper bound for the associated truncation error is provided and used for the convergence analysis of the series expansion. In the next step, using the moment generating function approach, we derive a bit error rate (BER) expression as a generalized infinite power series and demonstrate that existing BER results in the literature reported for gamma-gamma channels and K channels can obtained as special cases.

Coherent Free-Space Optical Communications in Lognormal-Rician Turbulence

A coherent free-space optical communication system is analyzed for lognormal-Rician turbulence channels using Padé approximants. Highly accurate closed-form BER expressions are developed for binary phase-shift keying with maximum ratio combining and binary differential phase-shift keying with selection combing receptions. Asymptotic error rate analyses for these systems are also presented.

Error Rate Analysis of M-ary Coherent Free-Space Optical Communication Systems with K-Distributed Turbulence

Coherent free-space optical (FSO) communications is analyzed for long-range, high turbulence FSO systems operating under K-distributed turbulence conditions. Exact error rates are presented for M-ary phase-shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) using a closed-form moment generating function. Maximum ratio combining (MRC) reception is employed in the system with MPSK and MQAM to mitigate the turbulence effects, and asymptotic error rate analysis is presented for this MRC reception. The error rate performance degradation caused by phase compensation error is also studied, and it is found that the impact of phase compensation error on the error rate performance can be small when the standard deviation of the phase compensation error is kept below twenty degrees.

Exact error rate analysis of equal gain and selection diversity for coherent free-space optical systems on strong turbulence channels

Exact error rate performances are studied for coherent free-space optical communication systems under strong turbulence with diversity reception. Equal gain and selection diversity are considered as practical schemes to mitigate turbulence. The exact bit-error rate for binary phase-shift keying and outage probability are developed for equal gain diversity. Analytical expressions are obtained for the bit-error rate of differential phase-shift keying and asynchronous frequency-shift keying, as well as for outage probability using selection diversity. Furthermore, we provide the closed-form expressions of diversity order and coding gain with both diversity receptions. The analytical results are verified by computer simulations and are suitable for rapid error rates calculation.