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Free space optical communication (FSO) is designed using the orthogonal quadrature phase shift keying (OQPSK) modulation technique. The system performance is evaluated for various levels of atmospheric turbulence, transmitter power, beam divergence, transmitter and receiver aperture diameters, and atmospheric losses. In this paper, we evaluated the system parameters for atmospheric turbulence using the Gamma gamma turbulence model. The system is evaluated for atmospheric losses such as clear air, medium-heavy rain, haze, and fog with various levels of atmospheric turbulence using the Optisystem Software tool for 1550 nm optical waves for SISO and MIMO OQPSK modulation. The obtained results show that the proposed model with OQPSK modulation and MIMO technology provides the best outcomes in terms of the BER, Q factor and received power. A link distance up to 3 km is achieved in the presence of atmospheric losses and turbulence. [ABSTRACT FROM AUTHOR]

In the last two decades, chaos-based signals have achieved much popularity due to their useful waveform properties such as non-periodicity, wide-band nature, and robustness against eavesdropping by intruders. In this paper, we utilize differential chaos shift keying (DCSK) for free space optical (FSO) communication systems to improve their performance under the combined influence of atmospheric turbulence (AT) and non-zero boresight pointing errors (PEs). We employ the generalized Malaga distribution for modeling the channel gain under all turbulence regimes from weak to strong. Moreover, we utilize the modified Rayleigh distribution to incorporate the effect of jitter and boresight PEs. The computation of the exact average bit error rate (ABER) expression for the proposed FSO system using DCSK is quite tedious. Therefore, we propose two approximations for ABER of the proposed system which show a tight match with the exact analysis and are also valid for the homodyne detection (HMD), heterodyne detection (HD), and intensity modulation/direct detection (IM/DD) techniques. We also observe the impact of different AT parameters $(\alpha,\beta)$ , spreading factor $(M)$ , and non-zero boresight PE parameters $(k_{x},k_{y})$ on the ABER performance of the DCSK-based FSO system. Further, we derive asymptotic ABER expression to obtain insightful observations into the FSO system performance. We also compare the ABER performance of the proposed DCSK-based FSO system with an FSO system employing, correlation delay shift keying (CDSK), binary phase shift keying (BPSK), and code division multiple access (CDMA). We also derive the generalized ergodic capacity (EC) for the considered FSO communication system and show the effect of the spreading factor and boresight on the EC of the DCSK FSO communication system. We derive closed form expression of the average secrecy capacity (ASC) under the combined effect of AT and nonzero boresight PEs for the considered DCSK FSO system.

This paper investigates and enhances unmanned aerial vehicle (UAV) relay-assisted free-space optics (FSO) optical wireless communication (OWC) systems under the effects of pointing errors (PEs) and atmospheric turbulences (ATs). The incorporation of UAVs as buffer-aided moving relays in the conventional FSO (CFSO) relay-assisted systems is proposed for enhancing the performance of PEs through AT. Using M-PSK (phase shift keying) and M-QAM (quadrature amplitude modulation), the impact of PEs on transmission quality is evaluated in this work. We evaluate and optimize the symbol error rate, outage probability (OP), and signal-to-noise ratio (SNR) for the UAV-to-ground station-based FSO communications systems. The spatial diversity-based relay-assisted CFSO systems can enhance the performance of the UAV-UAV FSO links. In this paper, a new FSO (NFSO) channel model for the hovering UAV-FSO OWC fluctuations under the PEs, AT effects, jitter, deviation, receiving an error, and wind resistance effects are established. To improve the performance of hovering UAV-based FSO relay OWC systems. We reduce the influence of UAV-FSO OWC fluctuations under PEs and AT effects. By receiving incoherent signals at various locations, the spatial diversity-based relay-assisted NFSO systems can significantly increase the system's redundancy and enhance connection stability. Numerical results show that to achieve a bit-error-rate (BER) of ≤ 10 - 5 , the required SNR is ≥ 23 dB when the wind variance of the UAVs σ α 2 increases from 0 to 7 mrad with FSO link distance L = 2000 m. The required SNR is ≥ 25 dB when the wind variance σ α 2 is 1 mrad at an OP of 10 - 6 . To obtain an average BER of 10 - 6 , the SNR should be 16.23 dB, 17.64 dB, and 21.45 dB when σ α 2 is 0 mrad, 1 mrad, and 2 mrad, respectively. Using 8-PSK modulation without PEs requires 23.5 dB at BER of 10 - 8 while 16-QAM without PEs requires 26.5 dB to maintain the same BER of 10 - 8 . Compared with 16-QAM without PEs, the SNR gain of 8-PSK without PEs is 3 dB. The results show the relay-assisted UAV-FSO system with five stationary relays can achieve BER 10 - 8 at 25 dB SNR in the ideal case and 10 - 5 at 27 dB SNR with AT and PE at FSO length 1000 m. The results show the relay UAV-FSO system outperforms the CFSO at the BER and SNR performance. The effects of UAV'FSO s fluctuation increase when the UAV-FSO link length, L fso increases. The results of the weak turbulence achieve better SER compared with MT and ST. The obtained results show that decreasing L fso can compensate for the effects of UAV-FSO link fluctuation on the proposed system. Finally, we investigated the CFSO relay-assisted UAV-FSO system with aided NFSO-UAVs spatial diversity-based relay-based on NFSO OWC and revealed the benefits of the resulting hybrid architecture. [ABSTRACT FROM AUTHOR]

The helical phasefront and orbital angular momentum (OAM) of vortex electromagnetic waves (VEMW) have attracted extensive attention in expanding communication capacity and enhancing radar imaging resolution. However, the VEMW is sensitive to atmospheric turbulence (AT), which will cause helical phasefront distortion and OAM mode diffusion. Hence, a convolutional neural network (CNN) based method for AT compensation is proposed. Specifically, the developed CNN automatically learns the AT characteristic parameters from the distorted VEMW phasefront and reconstructs the equivalent AT phase screen, and then, compensation is achieved by loading the inverse of the predicted phase on the distorted VEMW. The simulations show that the trained CNN has strong generalization capability and can perform high-quality AT prediction for different turbulence strengths and OAM modes, after compensation, the distorted phase is repaired and the mode purity is significantly improved. Moreover, comparing with the conventional AT compensation method, the proposed method shows smaller computational burdens and higher compensation accuracy. [ABSTRACT FROM AUTHOR]

The atmospheric turbulence (AT) causes distortion of phase fronts of orbital angular momentum (OAM) beams, which hinders the recognition of OAM modes. Convolutional neural network (CNN), a deep learning (DL) technique, can be utilized to realize the effective recognition of OAM modes. In this article, we propose a properly designed six-layer CNN model that can achieve high recognition accuracy (RA) of OAM modes at a reasonable computing complexity. We used intensity images of Laguerre-Gaussian (LG) beams to train our CNN model and explored the relationship between the RA for different single OAM modes and many factors including the number of training epochs, AT intensity, transmission distance, and the number of single OAM modes. Our CNN model can obtain an RA of 97.1% under moderate turbulence and 80% under strong turbulence, which are better than some CNN models proposed previously. Compared with these previous CNN models, our CNN model can also reduce the time consumption for at most 70%. Our research could contribute in achieving higher data capacity in OAM-based free space optical (FSO) communication systems.

Practical orbital angular momentum (OAM)-based free-space optical (FSO) communications commonly experience serious performance degradation and crosstalk due to atmospheric turbulence. In this paper, we propose a wave-front sensorless adaptive optics (WSAO) system with a modified Gerchberg–Saxton (GS)-based phase retrieval algorithm to correct distorted OAM beams. We use the spatial phase perturbation (SPP) GS algorithm with a distorted probe Gaussian beam as the only input. The principle and parameter selections of the algorithm are analyzed, and the performance of the algorithm is discussed. The simulation results show that the proposed adaptive optics (AO) system can significantly compensate for distorted OAM beams in single-channel or multiplexed OAM systems, which provides new insights into adaptive correction systems using OAM beams. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence (AT) will cause the crosstalk between OAM (orbital angular momentum) beams, which makes it difficult to identify the OAM modes at the demodulation part. In order to reduce AT's interference, we propose to combine phase compression (PC) with an improved convolutional neural network (CNN) to achieve high-precision recognition of OAM modes. We have investigated the performances (OAM recognition accuracy) of our system with changes of AT's intensities, transmission distances, and PC ratios. The results demonstrate that compared with traditional recognition systems, our model still has higher accuracy in the case of strong AT and long-distance transmission, which can be attributed as the additional characteristics of the PC OAM beams. At the same time, we also investigate the performances of the multiplexed hybrid vortex beam under different AT intensities, and discuss the recognition of the multiplexed hybrid vortex beam with the same OAM mode but different PC ratios, where the results show that the multiplexed hybrid vortex beams with the same OAM mode but different PC ratios can also be recognized at the receiving part. In addition, we also used unknown turbulence to test our trained model, and our model demonstrates good generalization ability, which has superior robustness for the unknown AT environments. • We have proposed an efficient system for recognizing OAM modes by combining phase compression with Shufflenet V2 network model. • For single OAM in vortex beams with different AT intensity levels and transmission distances, the system can obtain high detection accuracy of 100% nearly. • In different AT intensity levels and transmission distances, the same multiplexed hybrid vortex beam with different compression ratios can also be identified. • Compared to the traditional CNN model, our system has better generalization ability and stronger robustness. [ABSTRACT FROM AUTHOR]

This paper proposes and analyzes the performance of the multihop free-space optical (FSO) communication links using a heterodyne differential phase-shift keying modulation scheme operating over a turbulence induced fading channel. A novel statistical fading channel model for multihop FSO systems using channel-state-information-assisted and fixed-gain relays is developed incorporating the atmospheric turbulence, pointing errors, and path-loss effects. The closed-form expressions for the moment generating function, probability density function, and cumulative distribution function of the multihop FSO channel are derived using Meijer's G-function. They are then used to derive the fundamental limits of the outage probability and average symbol error rate. Results confirm the performance loss as a function of the number of hops. Effects of the turbulence strength varying from weak-to-moderate and moderate-to-strong turbulence, geometric loss, and pointing errors are studied. The pointing errors can be mitigated by widening the beam at the expense of the received power level, whereas narrowing the beam can reduce the geometric loss at the cost of increased misalignment effects. [ABSTRACT FROM PUBLISHER]

We experimentally demonstrate a simple temporal- domain diversity reception (TD-DR) scheme to enhance the reliability of optical wireless transmission links corrupted by atmospheric turbulence (AT) effects. Both nonreturn-to-zero and Manchester encoded on-off-keying modulation schemes are studied in laboratory environments at a 1.25-Gb/s data rate. AT effects are emulated as a slowly varying log-normal intensity variation for the received signal. System bit-error ratio is tracked for the performance evaluation, and results show that the outage probability due to the AT-induced intensity fluctuation for a 1.25-Gb/s link using Manchester coding is reduced from 3.2%plusmn0.1% to <0.1%, when the TD-DR is employed. [ABSTRACT FROM PUBLISHER]