Coherent Free-space Optical Communication Research Articles

Atmospheric Turbulence Phase Reconstruction via Deep Learning Wavefront Sensing

The fast and accurate reconstruction of the turbulence phase is crucial for compensating atmospheric disturbances in free-space coherent optical communication. Traditional methods suffer from slow convergence and inadequate phase reconstruction accuracy. This paper introduces a deep learning-based approach for atmospheric turbulence phase reconstruction, utilizing light intensity images affected by turbulence as the basis for feature extraction. The method employs extensive light intensity-phase samples across varying turbulence intensities for training, enabling phase reconstruction from light intensity images. The trained U-Net model reconstructs phases for strong, medium, and weak turbulence with an average processing time of 0.14 s. Simulation outcomes indicate an average loss function value of 0.00027 post-convergence, with a mean squared error of 0.0003 for individual turbulence reconstructions. Experimental validation yields a mean square error of 0.0007 for single turbulence reconstruction. The proposed method demonstrates rapid convergence, robust performance, and strong generalization, offering a novel solution for atmospheric disturbance correction in free-space coherent optical communication.

Real-time low-complexity diversity combining algorithm for free space coherent optical communication systems over atmospheric turbulence channel.

A novel diversity combining scheme, in conjunction with the complex-valued decision-directed least mean square (CV-DD-LMS) algorithm, is evaluated, and a real-time experimental validation is presented. This proposed scheme employs the CV-DD-LMS algorithm to concurrently perform beam combination and carrier phase recovery (CPR), thereby effectively reducing the overall complexity of digital signal processing. Furthermore, in the numerical simulation, under a low signal-to-noise ratio (SNR), a scheme utilizing the CV-DD-LMS algorithm effectively avoids cycle slips (CS) and outperforms schemes employing independent CPR modules. We experimentally validate this novel scheme by implementing it on an FPGA in a real-time 2.5Gb/s QPSK diversity-receiving system with three inputs. The back-to-back sensitivity is assessed using static received optical power, while the dynamic performance is evaluated by employing variable optical attenuators (VOAs) to simulate a power fluctuation at a frequency of 100kHz. The result proves that the implementation of the CV-DD-LMS algorithm yields stable performance while effectively reducing computational complexity.

Joint mitigation of frequency offset and phase noise for coherent free-space optical communication in atmospheric turbulence

In this paper, taking the atmospheric turbulence into account, two low-complexity algorithms which can jointly compensate the frequency offset (FO) and phase noise (PN) caused by laser line-width and atmospheric turbulence were proposed in coherent free-space optical (FSO) communications with polarization division multiplexing QPSK (PDM-QPSK) scheme. By introducing phase-conjugated counterparts for training symbols or information symbols on one of two orthogonal polarization states, the FO and PN were sensed simultaneously for a transmitted block or for each symbol. Simulations and experiments were performed to evaluate the BER performance. The results both manifested significant bit-error-rate (BER) performance improvement.

Performance investigation of multi-aperture digital combining algorithm for satellite-to-ground coherent optical communication

An important component of the future communication network will include satellite-to-ground communication technology. In the meantime, the community of communication network also has a huge capacity as one of its purposes. In this situation, the free-space optical (FSO) communication technology is a potential option for satellite-to-ground communications. A major drawback of FSO communication systems, nevertheless, is atmospheric turbulence. Multi-aperture receiver with digital combining architecture is an effective approach to overcoming the effect of atmospheric turbulence on the performance of free-space optical (FSO) communication. In this paper, a blind digital combining algorithm for coherent FSO communication is proposed. To verify the effectiveness of the proposed combining algorithm, a 10-Gbps data rate single polarization 16-ary quadrature amplitude modulation (SP-16QAM) offline four-aperture simulated turbulence experiment is performed, and the performance of multi-aperture spatial diversity and signal processing is emulated through the experimental data. The results confirm that the proposed blind digital combining algorithm has an excellent performance in average bit error rate (BER). Compared with the single-aperture system, the receiver sensitivity of the four-aperture is increased by 1.2, 4.6, and 6.8 dB for the cases of weak, moderate, and strong turbulence at the SD-FEC threshold, respectively.

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.

A Low-Complexity Joint Compensation Scheme of Carrier Recovery for Coherent Free-Space Optical Communication

In this paper, a low-complexity joint compensation scheme of carrier recovery (JCSCR) for coherent free-space optical (CFSO) communication is proposed. We applied the carrier recovery joint compensation approach to a CFSO communication system in the quadrature phase shift keying (QPSK) modulation format. A signal-preprocessing stage, which effectively avoided the repetitive operations found in traditional carrier recovery schemes, was proposed. Unlike in existing carrier recovery algorithms, the modulated phase of the received signal could be accurately removed using only the sum and subtraction of real absolute values in the signal-preprocessing stage, greatly reducing the complexity of the operation. Since this algorithm avoids the traditional fourth operation, the system’s complexity is reduced while additional noise generated by fourth cross-terms would be prevented and system noise immunity would be greatly enhanced. In addition, this algorithm uses joint compensation of phase errors in the final compensation stage, further reducing the complexity of the computation of the whole algorithmic scheme. A 10 Gbps QPSK CFSO communication transmission experiment was conducted in an atmospheric turbulence channel to verify the proposed technique and improvement in receiver sensitivity.

On the correlated Gamma–Gamma distribution and coherent detection performance of optical spatial modulation

The performance of optical spatial modulation (OSM) is severely damaged by correlated atmospheric turbulence (AT) channels for coherent free-space optical (FSO) communication systems. In this paper, we assume correlated AT channels comply with Gamma–Gamma distribution and present a coherent FSO-OSM system model. Based on this, we derive the closed-form approximated expressions of average symbol error probability (ASEP), and discuss the interaction of signal constellation diagram and spatial constellation diagram of the OSM under different conditions. We further illustrate relations of the ASEP caused by signal constellation and spatial constellation to AT strength and correlation. In addition, we explore the impact trends of the correlation coefficients respectively resulted from large-scale and small-scale eddies on the performance of system under different AT conditions. Through simulations, we prove theoretical results and demonstrate that the rise of AT severity and correlation coefficients can degrade the performance of coherent OSM.

Evaluation of a high-speed hybrid OAM-OFDM-MDM multiplexed coherent FSO system under desert conditions

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10–3 in strong turbulent conditions.

Experimental study on correction of non-common path aberrations in free-space coherent optical communication

Experimental study on correction of non-common path aberrations in free-space coherent optical communication

Demonstration of 100 Gbps coherent free-space optical communications at LEO tracking rates

Free-space optical communications are poised to alleviate the data-flow bottleneck experienced by spacecraft as traditional radio frequencies reach their practical limit. While enabling orders-of-magnitude gains in data rates, optical signals impose much stricter pointing requirements and are strongly affected by atmospheric turbulence. Coherent detection methods, which capitalize fully on the available degrees of freedom to maximize data capacity, have the added complication of needing to couple the received signal into single-mode fiber. In this paper we present results from a coherent 1550 nm link across turbulent atmosphere between a deployable optical terminal and a drone-mounted retroreflector. Through 10 Hz machine vision optical tracking with nested 200 Hz tip/tilt adaptive optics stabilisation, we corrected for pointing errors and atmospheric turbulence to maintain robust single mode fiber coupling, resulting in an uninterrupted 100 Gbps optical data link while tracking at angular rates of up to 1.5 deg/s, equivalent to that of spacecraft in low earth orbit. With the greater data capacity of coherent communications and compatibility with extant fiber-based technologies being demonstrated across static links, ground-to-low earth orbit links of Terabits per second can ultimately be achieved with capable ground stations.

Effect of multi-beam propagation on free-space coherent optical communications in a slant atmospheric turbulence

Based on the multi-beam propagation theory, a mathematic model of log-intensity fluctuation variance under multi-beam propagation in slant path atmospheric turbulence near the ground for free-space optical communication is established. Taking single beam, double beam, three beam, and four beams as examples, the intensity fluctuation characteristics of single beam and multi-beam are obtained, and the relationship between the log-intensity fluctuation and the bit error rate (BER) of the heterodyne detection coherent optical communications based on binary phase shift keying modulation is deduced. The results show that, with the increase of beam number, the variance of signal log-intensity fluctuation is significantly mitigated, and the BER of the system is significantly reduced. It proves that the multi-beam propagation method can be effectively applied to the long-distance coherent optical communications, and improve the performance of the system.

Coherent Free-Space Optical Communications: Opportunities and Challenges

The ever-increasing data rate demand for wireless systems is pushing the physical limits of standalone radio-frequency communications, thus fostering the blooming of novel high-capacity optical wireless solutions. This imminent penetration of optical communication technologies into the wireless domain opens up a set of novel opportunities for the development of a new generation of wireless systems providing unprecedented capacity. Unlocking the full potential of free-space optics (FSO) transmission can only be achieved through a seamless convergence between the optical fiber and optical wireless domains. This will allow taking advantage of the staggering progress that has been made on fiber-based communications during the last decades, namely leveraging on the latest generation of Terabit-capable coherent optical transceivers. On the other hand, the development of these high-capacity optical wireless systems still faces a set of critical challenges, namely regarding the impact of atmospheric turbulence and pointing errors. In this work, we provide an in-depth experimental analysis of the main potentialities and criticalities associated with the development of ultra-high-capacity FSO communications, ultimately leading to the long-term (48-hours) demonstration of a coherent FSO transmission system delivering more than 800 Gbps over <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 42 m link length, in an outdoor deployment exposed to time-varying turbulence and meteorological conditions.

Coherent Free-Space Optical Communications: Opportunities and Challenges

Coherent Free-Space Optical Communications: Opportunities and Challenges

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.

Performance analysis of coherent optical communication based on hybrid algorithm

The performance of coherent free-space optical communication (FSOC) is often affected by atmospheric turbulence. Sensorless adaptive optics are widely used in coherent FSOC to mitigate the effect of atmospheric turbulence. However, the control algorithm for sensorless adaptive optics systems remains weak. To enhance the various performance indicators of coherent FSOC, we propose a hybrid algorithm to improve the coupling efficiency and mixing efficiency (ME) of the system while reducing the bit error rate (BER). This algorithm combines the fast convergence rate of the simulated annealing (SA) algorithm at the initial cooling stage and the beneficial convergence effect of the stochastic parallel gradient descent (SPGD) algorithm, avoiding the algorithm’s need for more iterations and its tendency to enter local optimisation. The simulation results show that the performance of the proposed hybrid algorithm outperforms that of the traditional SPGD algorithm, regardless of the intensity of the atmospheric turbulence. The SA and hybrid algorithms display similar performances in strong atmospheric turbulence, while the hybrid algorithm shows better performance than that of the SA algorithm in weak atmospheric turbulence.

Application scheme and performance analysis of free space optical communication technology in INMARSAT

Abstract In this study, we propose an application scheme of free space optical communication technology in INMARSAT, and propose a 1.12 Tbit/s coherent free-space optical (FSO) communication system based on wavelength division multiplexing (WDM) and polarization-multiplexing quadrature phase shift keying (PM-QPSK) modulation technology. Based on optisystem software platform, the spectrum, bit error rate (BER), received power, error vector magnitude (EVM), and receiver sensitivity of the edge and middle channels of the system are analyzed. The simulation results show that the transmission rate and channel capacity of INMARSAT communication system are greatly improved by selecting the channel spacing and transmission environment reasonably.

High-speed modulating retro-reflectors with optical phase conjugation compensation

A modulating retro-reflector (MRR) with optical phase conjugation (OPC) compensation is presented and demonstrated to mitigate the turbulence-induced channel distortion for high-speed coherent free-space optical (FSO) communications. The impact of the turbulence on the beam spot, the phase fluctuation and the bit error rate (BER) is analyzed for the MRR FSO link carrying quadrature phase-shift keying (QPSK) signals with spatial or single-mode fiber (SMF)-based OPC. The mitigation of the atmospheric turbulence is verified by comparing the transmission performance of the MRR FSO links with and without OPC. The compensation using SMF-based OPC is experimentally demonstrated in a turbulence-tunable atmospheric chamber. The BER is improved by two orders of magnitude in the turbulent channel with the temperature difference of 250 °C, and the power penalty is reduced by about 2.0 dB, which agree well with the simulation results. A larger receiving aperture size will improve the compensation effect

Dual-mirror adaptive-optics fiber coupling for free-space coherent optical communication

The wavefront distortion caused by atmospheric turbulence leads to the decrease of the coupling efficiency of spatial light to optical fiber. Adaptive optics technology was employed to improve the coupling efficiency of free-space coherent optical communication. Accordingly, the effect of the discrete distorted wavefront on the coupling efficiency could be determined. The experimental results showed that the average coupling power was −31.97, −31.42, −30.79, −32.05, and −35.31 dBm when turbulence strength D / r0 was 0.2, 0.5, 1, 2, and 5, respectively. After eliminating the noncommon path aberration, the fiber coupling optical power increased from −35 to −27 dBm. After the optical wavefront was transmitted at indoor (0.5 m), 600 m, 1, 5, 10, and 100 km, the coupling efficiency increased from 10.1%, 1.3%, 11.4%, 2.8%, 7.2%, and 8.2% to 95.2%, 83.4%, 93.5%, 56.7%, and 42.4%, respectively, after wavefront correction. This will improve the performance of free-space coherent optical communication system.