For Free-space Optical Links Research Articles

Evaluation of atmospheric coherent length of free-space optical links by using phase fluctuation.

Atmospheric coherence length is one of the most crucial parameters for free-space optical (FSO) links, which can reflect the level of phase and amplitude fluctuations caused by the atmospheric turbulence. In this paper, we study the evaluation of the atmospheric coherence length of the FSO links. The analytical expression of atmospheric coherence length is rendered based on the phase fluctuation resulting from atmospheric turbulence by using the most realistic Bump model. The proposed method is validated theoretically with the Monte Carlo phase screen. Also, the experimental setup with respect to FSO links is established with the spatial light modulator to validate the method experimentally, wherein the fluctuated phase is collected by Shack-Hartmann sensor. The results show that the evaluation of atmospheric coherence length by the analytical expression is consistent with the theoretical prediction as well as the experimental measurement. Thus, the proposed method enables the accurate evaluation of atmospheric coherence length under various turbulence conditions, which can assist the performance analysis as well as design of free-space optical communication systems.

Finite-state Markov modeling of Gamma-Gamma fading FSO links with pointing errors

The knowledge of atmospheric channel is quite imperative for free-space optical (FSO) communications. To capture the time-varying essence of intensity fluctuations in an FSO connection, we propose a finite-state Markov (FSM) model for FSO links affected by Gamma-Gamma distributed atmospheric turbulence and pointing errors. First, the level crossing rate (LCR) of the unified intensity fluctuation is derived considering the non-generic channel autocorrelation characteristics, based on which the state transition probability (STP) matrix of the FSM model is calculated. Then, numerical simulations are presented for different strength parameters and frequency parameters of the channel. The results show that the misalignment strength does not affect the ratio of the level where the LCR reaches its maximum to the mean level. Finally, a series of well-designed experiments are carried out to verify the theory. As a result, the theoretical predictions conform to the experimental data, with averaged STP relative errors below 6.7%.

Self-Pilot Tone Based Adaptive Threshold RZ-OOK Decision for Free-Space Optical Communications

This paper studies a novel self-pilot tone based adaptive threshold return-to-zero on-off keying (RZ-OOK) decision for free-space optical (FSO) communications. RZ-OOK has the characteristics of impulse series in the spectrum. Therefore, these impulses can be utilized as the pilot tones of the transmitted signal to convey the channel state information (CSI) of FSO links. Then, the CSI signal is extracted using a local oscillator (LO) with the frequencies of the impulse series and low pass filter. Finally, the adaptive threshold decision (ATD) is realized by assigning optimized weight factors into the extracted CSI signal. The proposed adaptive threshold RZ-OOK decision was studied in simulation under various pulse durations of RZ-OOK signal and frequencies of LO. Simulation results demonstrated that RZ-OOK with the proposed impulse tone-extracted CSI signal under optimized weight factor performs close to the conventional ATD under precise CSI knowledge.

Validation of a hybrid RF/FSO communication system under foggy weather conditions

Abstract Future high-capacity data links are best suited for free space optical (FSO) communications since they have a larger bandwidth and secure transmission compared to RF links. FSO has several uses including indoor and outdoor settings. The impacts of weather are extensive, with fog being the main mitigating factor reducing link availability. The solution to the problem is to employ a hybrid network with RF/FSO and backup link. Different liquid water densities were used to analyze the performance of the hybrid RF/FSO system in foggy weather. In terms of BER, Q factor, and data rate, a comparison between RF and FSO lines is made. The findings demonstrated that in lower-medium of liquids with water density, the influence of fog weather on RF link is minimal. Under the same situations, an FSO link is useful as a backup link in higher liquid water density concentrations.

Performance evaluation of PDM/SAC-OCDMA-FSO communication system using DPS code under fog, dust and rain

A new hybrid polarization division multiplexing (PDM) spectral amplitude coding optical code division multiple access (SAC-OCDMA) is proposed for free space optical (FSO) for capacity enhancement. Two polarization signals are utilized; one is x-polarization and carries three different channels at 0° azimuthal angle while the other is y-polarization at 90° azimuthal angle, and carries the same three channels. Each channel is assigned with a diagonal permutation shift (DPS) code and carries 10 Gbps. The suggested system is simulated, and its performance is evaluated in terms of maximum allowable number of users, propagation range, bit error rate (BER), Q-factor, and received power for the different channels under various fog, dust storm, and rain scenarios. The reported results indicate that the system can support a signal travelling up to 2, 0.9, and 1.3 km, respectively, under light fog (LF), light dust (LD), and light rain (LR). As the level of these weather conditions is increased from light to moderate, the FSO link length decreases to 1.3, 0.25, and 1.8 km under medium fog (MF), medium dust (MD), and medium rain (MR), respectively. Furthermore, the shortest propagation range is achieved as the level of weather conditions becomes heavy, where the FSO link range becomes 1, 0.095, and 1.1 km under heavy fog (HF), heavy dust (HD), and heavy rain (HR), respectively. All these ranges are considered at BER ≤ 10–3 and a received power ≤ − 27 dBm with 60 Gbps overall data transmission. This new hybrid FSO system is suggested to be implemented in desert areas that affects by dust storms and in 5G wireless transmission communications.

Performance evaluation of free space optics communication system in the scenario of triple play service using probabilistic shaping scheme

An advanced triple-play-services (TPS) system based on probabilistic shaping (PS) scheme for free- space-optical (FSO) communication network is proposed and experimentally demonstrated for the first time. In this paper, the PS-32-quadrature amplitude modulation (QAM) 10 Gb/s data, PS-32-QAM 10 Gb/s voice and PS based 2.5 Gb/s HDTV signal are successfully transmitted over 600-m FSO link employing doublet lens scheme. The PS and polarization multiplexing technique both are helped to upgrade the spectral efficiency and improve the system performance. Normalized-generalized-mutual-information (NGMI) threshold is 0.92 at FEC code rate 9/10 for voice/data and 0.82 at FEC code rate 4/5 for video signal are selected to successfully execute the experiment. The spectral efficiency of our system is achieved by calculating the NGMI and shaping parameter (α), 13.6 bits/s/Hz (voice and data) and 10.41 bits/s/Hz (video) spectral efficiency are obtained at maximum α. Experimental results prove that PS based FSO communication system provides flexible data rate and greater spectrum efficiency and also be a powerful candidate to alleviate pressure on modern optical transport system.

Reduction of the beam pointing error for improved free-space optical communication link performance

Free-space optical communication is emerging as a low-power, low-cost, and high data rate alternative to radio-frequency communication in short- to medium-range applications. However, it requires a close-to-line-of-sight link between the transmitter and the receiver. This paper proposes a robust H∞ control law for free-space optical (FSO) beam pointing error systems under controlled weak turbulence conditions. The objective is to maintain the transmitter–receiver line, which means the center of the optical beam as close as possible to the center of the receiving aperture within a prescribed disturbance attenuation level. First, we derive an augmented nonlinear discrete-time model for pointing error loss due to misalignment caused by weak atmospheric turbulence. We then investigate the H∞-norm optimization problem that guarantees the closed-loop pointing error is stable and ensures the prescribed weak disturbance attenuation. Furthermore, we evaluate the closed-loop outage probability error and bit error rate (BER) that quantify the free-space optical communication performance in fading channels. Finally, the paper concludes with a numerical simulation of the proposed approach to the FSO link’s error performance.

Ergodicity of Phase Fluctuations for Free-Space Optical Link in Atmospheric Turbulence

The knowledge of atmospheric turbulence is quite imperative for free space optical (FSO) communications. This letter reveals the relationship between spatial and temporal phase fluctuations in an FSO link. We demonstrate that the phase fluctuation is an ergodic process in stationary and homogeneous turbulence. On the basis of wave structure which sets up a bridge between spatial and temporal phase characteristics, we theoretically derive the power spectrum and the autocorrelation function which predict the conditions of ergodicity. A series of experiments were carried out to test theoretical predictions where coordinate optical phase was measured for a wide range of turbulent strengths. As a result, we show that both the temporal and spatial phase spectra have low-pass characteristics and both their average and autocorrelation show ergodic behavior, following the same law with the variation of turbulent strength. This letter indicates that the spatial and temporal statistics of phase fluctuations could be interconverted for free-space optical communication and other applications related to the propagation of optical signals in the atmosphere.

Average spectral efficiency analysis of FSO links over turbulence channel with adaptive transmissions and aperture averaging

In our paper, the impact of adaptive transmission schemes: (i) optimal rate adaptation (ORA) and (ii) channel inversion with fixed rate (CIFR) on the average spectral efficiency (ASE) are explored for free-space optical (FSO) communications with On–Off Keying (OOK), Polarization shift keying (POLSK), and Coherent optical wireless communication (Coherent OWC) systems under different turbulence regimes. Further to enhance the ASE we have incorporated aperture averaging effects along with the above adaptive schemes. The results indicate that ORA adaptation scheme has the advantage of improving the ASE performance compared with CIFR under moderate and strong turbulence regime. The coherent OWC system with ORA excels the other modulation schemes and could achieve ASE performance of 49.8 bits/s/Hz at the average transmitted optical power of 6 dBm under strong turbulence. By adding aperture averaging effect we could achieve an ASE of 50.5 bits/s/Hz under the same conditions. This makes ORA with Coherent OWC modulation as a favorable candidate for improving the ASE of the FSO communication system.

Experimental demonstration of bidirectional up to 40 Gbit/s QPSK coherent free-space optical communication link over [formula omitted]1 km

Advanced multi-level modulation formats have shown their great potential in high-speed and high-spectral-efficiency optical communications. Using quadrature phase-shift keying (QPSK) modulation format for free-space optical (FSO) communication, a bidirectional high-speed FSO transmission link with the bit rates of up to 40 Gbit/s over ∼1 km, between two buildings in the campus of Changchun University of Science and Technology, Changchun, China, is experimentally demonstrated cooperating by capture and tracking systems. The eye-diagrams and constellation diagrams of the transmitted QPSK signals are clearly observed. By comparing the bit error rate (BER) curves before and after transmission, one can find that the receiving powers are both less than −16.5 dBm for the forward and backward transmissions of the bidirectional 20, 30, and 40 Gbit/s FSO links, and their power penalties due to the phase fluctuation of the atmospheric channel are both less than 2.6 dB, at the BER of 3.8×10−3.

Two-dimensional beam steering using a thermo-optic silicon photonic optical phased array

Many components for free-space optical (FSO) communication systems have shrunken in size over the last decade. However, the steering systems have remained large and power hungry. Nonmechanical beam steering offers a path to reducing the size of these systems. Optical phased arrays can allow integrated beam steering elements. One of the most important aspects of an optical phased array technology is its scalability to a large number of elements. Silicon photonics can potentially offer this scalability using CMOS foundry techniques. A phased array that can steer in two dimensions using the thermo-optic effect is demonstrated. No wavelength tuning of the input laser is needed and the design allows a simple control system with only two inputs. A benchtop FSO link with the phased array in both transmit and receive mode is demonstrated.

Rate-adaptive FSO links over atmospheric turbulence channels by jointly using repetition coding and silence periods

In this paper, a new and simple rate-adaptive transmission scheme for free-space optical (FSO) communication systems with intensity modulation and direct detection (IM/DD) over atmospheric turbulence channels is analyzed. This scheme is based on the joint use of repetition coding and variable silence periods, exploiting the potential time-diversity order (TDO) available in the turbulent channel as well as allowing the increase of the peak-to-average optical power ratio (PAOPR). Here, repetition coding is firstly used in order to accommodate the transmission rate to the channel conditions until the whole time diversity order available in the turbulent channel by interleaving is exploited. Then, once no more diversity gain is available, the rate reduction can be increased by using variable silence periods in order to increase the PAOPR. Novel closed-form expressions for the average bit-error rate (BER) as well as their corresponding asymptotic expressions are presented when the irradiance of the transmitted optical beam follows negative exponential and gamma-gamma distributions, covering a wide range of atmospheric turbulence conditions. Obtained results show a diversity order as in the corresponding rate-adaptive transmission scheme only based on repetition codes but providing a relevant improvement in coding gain. Simulation results are further demonstrated to confirm the analytical results. Here, not only rectangular pulses are considered but also OOK formats with any pulse shape, corroborating the advantage of using pulses with high PAOPR, such as gaussian or squared hyperbolic secant pulses. We also determine the achievable information rate for the rate-adaptive transmission schemes here analyzed.

Performance of PPM on terrestrial FSO links with turbulence and pointing errors

In recent work, the error performance of on-off keying has been investigated for free-space optical (FSO) links impaired by both turbulence and pointing loss. In the current letter, the analytical framework is extended to M-ary pulse-position modulation (PPM), providing exact results for M = 2. Since the approach is not directly applicable to PPM signals with M > 2, a closed-form approximation of the average symbol error probability is derived for this case, which is shown via simulation to be tight over a wide SNR range of interest.

Analysis of average capacity for free-space optical links with pointing errors over gamma-gamma turbulence channels

A novel closed-form expression for average capacity is derived for free-space optical (FSO) links over Gamma-Gamma turbulence channels by considering the effect of misalignment (pointing errors). The simulation results show that the average capacity of the FSO links can be analyzed with the effects of atmospheric turbulence condition, beam width, detector size, jitter variable, and transmitted optical power. Meanwhile, the results are further provided to verify the accuracy of our mathematical analysis. This work is useful for the FSO designer.

Error performance of coded FSO links in turbulent atmosphere modeled by gamma-gamma distributions

For free-space optical (FSO) links in turbulent atmosphere modeled by gamma-gamma distributions, an approximation of the pairwise error probability (PEP) was derived for on-off keying in a recently published paper. It has been assumed that the channel state is perfectly known to the receiver and that the fading samples are independent and identically distributed. Based on these conditions, we show in this letter that the approximation can be replaced by a closed-form solution, disregarding a finite-limit integral which forms also part of the approximation in the reference paper. The PEP is then used to evaluate the union upper bound for convolutional codes with Viterbi decoding. Finally, in order to identify the potential of powerful error correction algorithms, like turbo codes as most prominent example in this respect, the lower bound of the error probability is given via the converse of Shannon's coding theorem.

Do We Really Need OSTBCs for Free-Space Optical Communication with Direct Detection?

In this letter, the authors investigate spatial diversity techniques for free-space optical (FSO) links with intensity modulation and direct detection (IM/DD) over log-normal atmospheric turbulence-induced fading channels. We restrict our attention to the deployment of orthogonal space-time block codes (OSTBCs) and repetition codes both of which have been recently proposed for FSO links. Our performance analysis demonstrates that, although both schemes are able to extract full diversity, repetition codes outperform OSTBCs. The performance gap increases with the increasing number of transmit apertures. Our findings clearly point out that deployment of OSTBCs is not necessary for a FSO IM/DD link.

Adaptive divergence and power for improving connectivity in free-space optical mobile networks

A significant challenge for free-space optical (FSO) links is the restrictive alignment requirements, especially when the transceivers are moving. For moderate distances and rapid unpredictable motion, the receiver's field of view and the positioning system's dynamics become factors. We explore the use of adaptive transmitter power and beam divergence to improve the likelihood of maintaining a mobile FSO link by using Gaussian beam propagation theory and link budgets. We calculate the allowable misalignment between the transceivers' optical axes as a function of power, divergence, and transceiver distance. The maximum allowable error is independent of the distance, except when the field of view is a limiting factor. Certain combinations of divergence and power, while suboptimal for one distance, provide a relaxed misalignment limit for many distances. Based on the calculations, we make initial suggestions for system design.