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This paper investigates the technical aspects of the 1550 nm free space optical (FSO) link in extreme Arctic weather, specifically in falling snow, which is currently understudied. We measured the time variation of the received optical power in many snowfall events and derived attenuation statistics. Continuous measurements allowed us to estimate attenuation under varying temperatures and humidity to examine the after-snow channel impairments. Our results show several shortcomings in the International Telecommunication Union-Radio Communication (ITU-R) snow attenuation model and verify that a correlation exists between snow features, snowfall rate, temperature, and humidity in determining the total snow attenuation. We demonstrated the relationship between snow properties, temperature, humidity, and attenuation under both in-snow and post-snow environments. Furthermore, based on the experimental results, we recommend several mitigation techniques to realize FSO communication during challenging Arctic weather conditions. Lastly, we present a feasibility study of FSO in Svalbard at 78°N latitude.

This paper explores the performance of a single input multiple output (SIMO) free space optical (FSO) communication system using differential chaos shift keying (DCSK) with diversity combining techniques. Specifically, the paper presents the use of maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) diversity techniques for the DCSK-FSO system. Adding chaos to FSO systems using diversity can significantly improve the reliability and the system performance, making them a promising option for various 5G/6G wireless applications. The inherent properties of chaos, such as security, wide band nature, and immunity to fading, make it an attractive option for FSO communication. The Gamma-Gamma turbulence model was used to characterize the FSO link, and the system’s performance was analyzed in terms of bit error probability across varying atmospheric turbulence levels, link lengths, and numbers of receivers. The results indicate that under the same system conditions, MRC/EGC performs 2–4 dB better in terms of error probability than the SC diversity technique. Furthermore, the SIMO-DCSK-FSO system presents a substantial improvement in diversity gain, with around 30 dB for moderate turbulence and 37 dB for strong turbulence.

Vortex beams' orbital angular momentum (OAM) has important application value in optical communication and other fields. Accurate measurement of their OAM topological charges is required for the application of vortex beams in the field of optical communication. As a type of convolutional neural network, DenseNet which combines the “Data Augmentation with Expansion” method, was introduced to measure the fractional topological charge of OAM images in the case of few samples. The OAM images used in this paper generally do not have high brightness, which simulates ultra-long-distance transmission situations in reality. The experiment shows that the classification accuracy of DenseNet network reaches 98.77%, with an average training time of about 1100 seconds, which is short. Our results showcase the potential of convolutional neural network approach to further study the OAM light in the application of free-space optical communication.

The use of multiple transmitters in a turbulent free-space optical (FSO) communication link can enhance the systems performance because greater received power can improve receiver sensitivity. Nevertheless, the effect of having more power at the receiver can be severe. This paper investigates the impact of using multiple transmitters in preamplified FSO communication links limited by strong atmospheric turbulence (AT), amplified spontaneous emission noise, fixed path loss and pointing errors (PEs) using different preamplifier operating modes and different decision thresholding schemes. Results obtained show that regardless of the number of transmitters used, the best bit error rate (BER) performances are obtained with normalised decision threshold levels of about 0.2 and 0.5 when the decision thresholding scheme at the receiver is non-adaptive and adaptive, respectively. Also, in the strong AT regime, an additional transmitted power of about 7dB is required for the FSO communication systems under minimal PE effects to have the same performance as FSO communication systems without PE. The results also show that when the effects of PE are absent or minimal, a larger receiver would require about 15dB less transmitted power to record the same performance as a smaller receiver. However, with a non-adaptive decision threshold, smaller receivers perform better than larger receivers when the PE effects are severe. Additionally, it is shown in this paper that when the effects of PE is severe, the BER performances consistently get better with more transmitters regardless of the decision thresholding scheme employed at the receiver. However, in the absence of PEs, increasing the number of transmitters indefinitely will not always guarantee improved BER performances when the receiver decision threshold is non-adaptive. The use of multiple transmitters is particularly advantageous for applications where it is either necessary or unavoidable to use lower transmitted power.

Wireless optical coherent communication can be used as the backbone supporting technology of air-space-ground-sea-integrated network. In wireless optical coherent communication, the information can be demodulated from the carrier correctly only when the phase synchronization is accurate. Operating in different systems and diverse environments presents challenges for phase synchronization, especially lasers with different line-widths and atmospheric turbulence channels that cause scintillation and wave-front distortion. We propose a combination of adaptive fractionally-spaced blind equalization and adaptive kalman filter to realize phase synchronization under different line-widths and different scintillation and wave-front distortion. This combination which changed the limitation that the original settings of each part is not directly applicable to atmospheric channel combines spatial diversity in a natural way, jointly removes amplitude noise and phase noise more thoroughly with the optimal bandwidth, and can realize phase synchronization under time-varying conditions. The signal quality of the proposed scheme with constant parameter output can be improved 1-2dB in both mean square error (MSE) and symbol to error ratio (SER) compared to traditional equal gain combining (EGC) followed by Viterbi-Viterbi phase estimation (VVPE) and the proposed constant parameter scheme has more laser line-width options. The proposed scheme with noise parameter corrected by Autoregressive method outperforms the constant parameter scheme 1-2dB at both MSE and SER and has higher and wider line-width selections. With this design, phase synchronization can be more flexible to adapt to dynamic changes, which will be more suitable for future applications of wireless optical networks with dynamic,noisy, diverse environment.

For the fronthaul link in the fifth-generation (5G) wireless networks, optical fiber is one of the most widely used medium, owing to its large bandwidth and low transmission loss. However, in some scenarios, especially the dense urban areas, the installation of fiber optic cables is costly and difficult to establish. Alternatively, the free space optical (FSO) link can be installed easily and spans complex terrain. In this context, we propose and experimentally demonstrate a hybrid FSO/Wireless transmission system based on bandpass delta-sigma modulator and real-time coherent transponder. Advantage of the proposed technique is twofold. Firstly, coherent detection is applied to improve the system sensitivity compared to conventional direct detection. Secondly, delta-sigma modulation (DSM) is utilized to greatly simplify the wireless transmitters and to enhance the bandwidth flexibility. Over a 50-m FSO and 1-m wireless link, a 64-QAM wireless signal at 1 GHz with 500 MHz bandwidth is successfully transmitted. With 16 wavelength division multiplexing channels, the total throughput of the system is up to 24 Gb/s. Meanwhile, the error vector magnitude (EVM) of all channels is around 2.68% and 6.51%, which are demonstrated after FSO and FSO/Wireless transmission, respectively. The results meet the requirement of the third-generation partnership project release 15 with EVM value of 8% for 64-QAM. Moreover, sensitivity of the real-time coherent receiver is lower than −48 dBm under the 7% forward error correction limit with bit error rate of 3.8e-3.

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding.

Accurate recognition of orbital angular momentum (OAM) modes is a major challenge for OAM-based optical communications over atmospheric turbulence channels. The turbulence-induced distortions cause difficulties for the receiver to distinguish between adjacent OAM modes. Deep learning, such as convolutional neural networks (CNN), has been a promising technique in solving this problem. To improve the recognition performance, we propose a vortex modulation method that can magnify the subtle differences between closely adjacent OAM states. This allows the CNN to capture the image features more effectively and to recognize the topological charges more accurately. Numerical results show high recognition accuracy for both integer topological charges and fractional ones even under strong turbulence intensity and long propagation distance, which demonstrate the utility of the proposed method.

The scheme of combining relay and optical code division multiple access (OCDMA) is one of the effective methods to improve the reliability and security of free space optical (FSO) communication system. We experimentally investigate a 10 Gb/s decode-and-forward (DF) relaying FSO-CDMA system and a relaying amplify-and-forward (AF) FSO-CDMA system, respectively. Bit error rates (BERs) of DF and AF relaying FSO-CDMA are measured and compared under different turbulence conditions. The experimental results show that the BER of DF relaying FSO-CDMA is one to two orders of magnitude lower than that of AF relaying FSO-CDMA when the receiving power is −5.6 dBm. However, the performance gap is decreasing with the increase of receiving power, especially when there is an interfering user. At the receiving power −2.8 dBm, two-user AF and DF relaying FSO-CDMA systems have almost the same BER. Under both weak and moderate turbulence conditions, at the receiving power −2.8 dBm, the measured BER of the eavesdropper exceeds 0.1, which is larger than the BER threshold of soft-decision forward error correction code. Hence, physical layer security of FSO relaying system can be achieved by optical coding. [ABSTRACT FROM AUTHOR]

Accurate recognition of orbital angular momentum (OAM) modes is a major challenge for OAM-based optical communications over atmospheric turbulence channels. The turbulence-induced distortions cause difficulties for the receiver to distinguish between adjacent OAM modes. Deep learning, such as convolutional neural networks (CNN), has been a promising technique in solving this problem. To improve the recognition performance, we propose a vortex modulation method that can magnify the subtle differences between closely adjacent OAM states. This allows the CNN to capture the image features more effectively and to recognize the topological charges more accurately. Numerical results show high recognition accuracy for both integer topological charges and fractional ones even under strong turbulence intensity and long propagation distance, which demonstrate the utility of the proposed method. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence is a major factor limiting the transmission distance of free space optics communication. A novel weighted clustering algorithm based on polar coordinates is proposed in this letter, which can not only solve the error convergence problem of existing Cartesian coordinate system-based algorithm, but also effectively increase the maximum achievable transmission distance of the FSO systems. The effectiveness of the proposed algorithm is verified using commercial simulation software, which can achieve up to 14.1% maximum transmission distance increasement, proving that the proposed scheme is a practical alternative to traditional decision algorithms in future long-reach FSO communications. [ABSTRACT FROM AUTHOR]

In this paper, cascaded least mean square-least mean square (LMS-LMS) adaptive equalization scheme is proposed based on fiber coupling system, in order to improve the transmission performance of quadrature phase shift keying (QPSK) modulated optical signals in medium and weak turbulence channels. First, the transmission performance of QPSK optical signal in different atmospheric turbulence channels is simulated and analyzed according to the characteristics of optical fiber coupling system and the characteristics of atmospheric turbulence channel. Secondly, cascaded LMS-LMS equalization scheme is proposed, and its distortion signal is corrected based on the original simulation results to verify the effectiveness and robustness of LMS-LMS. Finally, atmospheric turbulence simulator is used to verify the improvement effect of cascaded LMS-LMS adaptive equalizer under different turbulence conditions. The experimental results show that when the coherence length is $r_{0} =1.6 $ cm and the received optical power is −34 dBm, the system can transfer the information at a rate of 32 Gbps QPSK after using cascaded LMS-LMS adaptive equalizer, and the bit-error ratio (BER) is lower than the minimum limit of forward error correction (FEC, $3.8 \times 10 ^{-3}$ ). This paper can provide theoretical guidance for the performance optimization of free space QPSK optical signal transmission system.

There has been significant interest in free-space optical (FSO) communication by the research community in recent years. This is due to its high data rate, unlicensed spectrum, low cost, and immense security for FSO systems. Due to these advantages, FSO can have broader applications that extend from terrestrial to satellite communication. Atmospheric turbulence (AT) induced fading is a primary problem in the FSO link since it significantly impairs its performance. Atmospheric turbulence occurs due to the random variation of the air refractive index with time. Several statistical models are introduced to characterize the AT. The Log-normal (LN) model represents weak and moderate turbulence, and the Gamma-Gamma (G-G) model is employed for strong turbulence. These models are used with the effect of weather attenuation, geometric losses, and misalignment errors. One possible solution is channel coding, such as low-density parity-check (LDPC) codes. This paper proposed employing Weighted Bit Flipping (WBF) and Implementation Efficient Reliability Ratio Weighted Bit Flipping (IERRWBF) decoding techniques to improve FSO link performance. The results show a superior improvement in the bit error rate (BER) than the uncoded FSO system. In addition, the obtained results prove that the IERRWBF technique is more optimized than WBF from the point of the number of iterations, especially in weak and moderate turbulence FSO channels. In the calculation of decoders processing time, the WBF maintained lower decoding time than the IERRWBF technique, while in higher $E_{b}/N_{o}\text{s}$ , they have the same level. The same response for both techniques in the case of resultant throughput. Finally, both methods are evaluated from the point of convergence. IERRWBF technique achieved faster convergence than WBF in all FSO channels under study.

This paper investigates the performance of a dual-hop mixed free-space optical (FSO)/radio frequency (RF) system. To this end, the FSO link is subject to the Gamma-Gamma distribution effects that account for pointing error impairments and under both heterodyne detection and intensity modulation with direct detection (IM/DD) methods, while the RF link undergoes the $\alpha $ - $\mathcal {F}$ distribution fading. For amplify-and-forward (AF) and decode-and-forward (DF) relaying, we derived the closed-form expressions of various important performance metrics by using bivariate Fox’s $H$ -functions, such as the average bit-error rate (BER), end-to-end outage probability (OP), effective capacity, and ergodic capacity. We unveil the effects of system and channel parameters on performance. Furthermore, we provide an asymptotical analysis to discuss significant diversity gains. Note that our results include previous works as special cases, which provides a framework. Finally, our derived expressions have confirmed the correctness via exactly matching Monte Carlo simulation results.

Single-layer phase screen is a classical, simple and accurate approach for simulating the optical field under turbulence, and it is generally used in free space optical communication (FSOC). However, conventional single-layer phase screen studies have rarely considered the pointing errors from FSOC, which prevents the theoretical results for mobile FSOC from being verified using simulated optical fields. This work establishes a simulation method for the single-layer phase screen with pointing errors. Statistics on simulated optical field with pointing errors indicated that the previous theoretical results for the long-term average irradiance and irradiance fluctuation probability density function (PDF) for mobile FSOC can be modified to better fit the simulation results. In addition, the received power fluctuation PDF, average bit-error-rate (BER), and the influence of non-zero mean pointing errors are investigated through the simulated optical field with pointing errors. The proposed simulation method provides a powerful approach for understanding and evaluating the comprehensive influences of atmospheric turbulence and pointing errors on the FSOC.

Free space optical (FSO) communication has established a reputation for itself as a next-generation network capable of delivering high-speed data services. In light of the upcoming launch of 5G services, this paper aims to evaluate the performance of the FSO system for the implementation of 5G technology in diverse geographical locations of India. The proposed 16 × 10 $\text{Gbps}$ wavelength division multiplexing (WDM) based FSO link uses orthogonal frequency division multiplexing (OFDM) and digital signal processing (DSP) module to mitigate channel-induced limitations. Additionally, the proposed FSO design also employs Erbium-doped fiber amplifier (EDFA) with optimized gain characteristics. Using the data sourced from the Meteorological Department of India, the link analysis reveals that due to weak fog events in coastal areas effective link range of up to 10.75 $\text{km}$ can be achieved while maintaining bit error rate of $10^{-9}$. Despite dominant foggy conditions in hilly, plains and desert areas, the proposed FSO link significantly limits the degrading effects of channel. Hence, in pursuit of futuristic smart city models, a heterogeneous combination of FSO and 5G infrastructure can conveniently cover diverse geographical locations of India to support high-speed data connectivity.

Space dynamic communication base stations are characterized by high mobility, flexible deployment and wide coverage. It has a wide range of needs in the fields of emergency communication, earthquake relief and island communication. To solve the problem of a limited number of users due to the limited bandwidth of space base stations, so we should improve the reliability and efficiency of data transmission between space mobile base stations. According to the characteristics of the airship platform, a blimp-borne laser communication terminal applied to space dynamic base stations is designed to meet the demand for bi-directional transmission of 5G signal laser carriers in space between airships. The experimental results show that the blimp-borne laser communication terminal achieves an overall technical index of capture probability is better than 98%, the average capture time is 5s, the tracking accuracy is better than 6–15 μrad, the communication rate is 2.5 Gbps and BER is 1E-7. In order to realize the real-time evaluation of the performance of laser communication system under the turbulent channel and guide the research of anti-turbulence fading method, a real-time test method of communication channel turbulence characteristics based on the principle of light intensity scintillation is proposed. And the real-time test of turbulence characteristics of airship inclined link and the horizontal link is completed. The research results play a positive role in promoting the development of 5G mobile technology and space optical communication technology.

We propose and experimentally demonstrate a hybrid free space optical (FSO) communication and radio frequency (RF) multiple-input and multiple-output (MIMO) system for interference separation. The system switches between FSO and RF MIMO modes and makes use of the complementary advantages of the two modes. The MIMO receiver separates signal of interest from in-band interference with blind source separation (BSS), while the separation ratio is limited by the physical dimensions of the MIMO antenna array. With FSO channel, the system achieves over 30dB signal to interference plus noise ratio, so the wireless system can transmit signals with high orders of modulation. When the FSO channel is blocked and line-of-sight transmission condition is not satisfied, the system switches to the RF MIMO mode for interference cancellation. The wireless system maintains continuous transmission at a lower bit rate. Both the FSO mode and the RF MIMO mode can be seamless integrated with photonic signal processing system for wideband and real-time interference cancellation. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication has established a reputation for itself as a next-generation network capable of delivering high-speed data services. In light of the upcoming launch of 5G services, this paper aims to evaluate the performance of the FSO system for the implementation of 5G technology in diverse geographical locations of India. The proposed 16 × 10 $\text{Gbps}$ wavelength division multiplexing (WDM) based FSO link uses orthogonal frequency division multiplexing (OFDM) and digital signal processing (DSP) module to mitigate channel-induced limitations. Additionally, the proposed FSO design also employs Erbium-doped fiber amplifier (EDFA) with optimized gain characteristics. Using the data sourced from the Meteorological Department of India, the link analysis reveals that due to weak fog events in coastal areas effective link range of up to 10.75 $\text{km}$ can be achieved while maintaining bit error rate of $10^{-9}$. Despite dominant foggy conditions in hilly, plains and desert areas, the proposed FSO link significantly limits the degrading effects of channel. Hence, in pursuit of futuristic smart city models, a heterogeneous combination of FSO and 5G infrastructure can conveniently cover diverse geographical locations of India to support high-speed data connectivity. [ABSTRACT FROM AUTHOR]

Space dynamic communication base stations are characterized by high mobility, flexible deployment and wide coverage. It has a wide range of needs in the fields of emergency communication, earthquake relief and island communication. To solve the problem of a limited number of users due to the limited bandwidth of space base stations, so we should improve the reliability and efficiency of data transmission between space mobile base stations. According to the characteristics of the airship platform, a blimp-borne laser communication terminal applied to space dynamic base stations is designed to meet the demand for bi-directional transmission of 5G signal laser carriers in space between airships. The experimental results show that the blimp-borne laser communication terminal achieves an overall technical index of capture probability is better than 98%, the average capture time is 5s, the tracking accuracy is better than 6–15 μrad, the communication rate is 2.5 Gbps and BER is 1E-7. In order to realize the real-time evaluation of the performance of laser communication system under the turbulent channel and guide the research of anti-turbulence fading method, a real-time test method of communication channel turbulence characteristics based on the principle of light intensity scintillation is proposed. And the real-time test of turbulence characteristics of airship inclined link and the horizontal link is completed. The research results play a positive role in promoting the development of 5G mobile technology and space optical communication technology. [ABSTRACT FROM AUTHOR]

We propose a 30 GHz radio over free-space optical (FSO) communication system for indoor wireless communication applications using a newly developed high-speed photodetector array (PDA) device. The new device performs the role of a photodetector with high optical alignment robustness and also acts as a 30 GHz optical-to-radio converter. Small high-speed photodetectors of 30 mm size are arranged in a 6 × 6 square-shaped alignment to achieve −4.4 dBm RF output at 30 GHz with an external amplifier. A 3–5 dB in-plane RF power distribution in the 4 × 4 PDA region was evaluated. To confirm the proposed concept, a 1 m long radio transmission over a 1.5 m long FSO transmission setup was demonstrated by switching the optical paths. We successfully confirmed a high data rate of up to 21.1 Gbps using several modulation formats under a static optical path switching condition. Additionally, stable and reproducible error vector magnitudes with a range of approximately 9% were recognized during the 10 switching cycles under a dynamic optical path switching condition. [ABSTRACT FROM AUTHOR]

The cognitive relay networks can effectively mitigate the congestion problem of electromagnetic spectrum. In this context, data buffering at the relaying terminal and free space optical communication can be incorporated to further improve the performance of such networks. To explore this novel scenario, we develop a network model for the mixed RF/FSO cognitive relaying networks and design a transmission protocol to utilize the available resources. Furthermore, we derive the analytical outage probability for the secondary system under the proposed protocol. Monte-Carlo simulations demonstrate that the proposed model guarantees a reliable communication system and the presented theoretical analysis is correct. [ABSTRACT FROM AUTHOR]

High sensitivity, small size, light weight and low power are the most critical requirements for the lasercom terminals used in free space optical communication (FSOC) systems. Coherent receivers have an exceptionally high sensitivity but their optical complexity and power consumption are too high, especially for the space-borne lasercom terminals. Recently we propose a pseudo-single-side-band-signal based single-photodiode coherent receiver (P-SCR) with very low optical and algorithm complexity which can potentially satisfy the strict requirement of the FSOC systems. In this paper, we first introduce the working principles of the P-SCR based on the frequency and time domain field reconstruction algorithms (TD-FRA and FD-FRA). We then investigate the relationships between the computation complexity and sensitivity for the two kinds of P-SCR, respectively. We also provide a method to minimize the FD-FRA complexity and demonstrate that its power consumption is only 3.7%, 25.3% and 37.8% of that of the well-known conventional Kramers-Kronig (KK) algorithm, upsampling-free KK algorithm and TD-FRA. Based on the optimized FD-FRA, we develop a highly parallel DSP circuit for the real-time P-SCR using a FPGA chip. The experiments in a 10GBaud QPSK transmission system show that the sensitivity of the real-time P-SCR is only ~4.2 dB away from the theoretical limit.

The data transmission between 5G floating base stations will be an important application area of the free space optical (FSO) link, but the problem of atmospheric disturbance will directly affect the application effect. An atmospheric turbulence suppression algorithm based on avalanche photodiode adaptive gain control (APDAGC) for the laser transmission terminal was proposed to suppress atmospheric turbulence. The given design mechanism and experimental results demonstrate its function to improve the performance of FSO. When APDAGC is over 1 km FSO, the variance of intensity fluctuation is lowered from 0.046 to 0.009, with the bit error rate reduced from 4.82E-6 to 1E-12. Comparatively, as APDAGC is over 6.5 km FSO, the variance of intensity fluctuation is decreased from 1.767 to 0.376, with the bit error rate varying from 4.6E-2 to 2.4E-4. Moreover, it was used for the first FSO transmission of 5G floating base station signals on an airship platform.

Free space optical (FSO) communication has emerged to provide line of sight connectivity and higher throughput over unlicensed optical spectrums. Cognitive radio (CR), on the other hand, can utilize the radio frequency (RF) spectrum and allow a secondary user (SU) to share the same spectrum with the primary user (PU) as long as the SU does not impose interference on the PU. Owing to the potential of these emerging technologies, to provide full spectrum efficiency, this paper focuses on the mixed CR RF-FSO transmission scheme, where RF communication is employed at one hop followed by the FSO transmission on the other hop in a dual-hop decode-and-forward (DF) configuration. To quantify the performance of the proposed scheme, closed form error probability is derived over Rayleigh/Exponentiated Weibull (EW) fading distributions by considering the statistical and instantaneous feedback channel of the primary network. We also employed an asymptotic analysis to illustrate the diversity gain of the overall system. We believe that the proposed scheme can be applicable to the 5G+ networks where an unlicensed university student connects to the home computer with the aid of an FSO path.

Free space optical (FSO) communication technology has become increasingly advanced with capabilities of high speed, high capacity, and low power consumption. However, despite the great potential of FSO, its performance is limited in a turbulent atmosphere. Atmospheric turbulence causes scintillation in the FSO propagated signals, leading to an increase in the bit error rate (BER) performance of the recovered signals at the receiver. In this paper, we demonstrate that the use of deep learning (DL) detection methods could overcome these limitations. We present a new detection method of on-off keying (OOK) modulated signals by using different models of DL over different strength FSO turbulent channels, without the need for prior knowledge of the parameters of the channel. The demonstrated DL decoders improve the performance of the FSO turbulent channel and decrease the power consumption. Moreover, the demonstrated DL models also work faster than maximum likelihood (ML) methods with perfect channel estimation decoders, with even slightly better performance because of the turbulence, thus enabling realization of FSO over turbulent atmospheric channels.

Due to the effect of atmospheric turbulence, the transmission reliability and physical-layer security of free space optical (FSO) communication system are facing many challenges. In this paper, for the first time, we design and investigate a 10 Gb/s reconfigurable FSO wiretap channel based on spatial diversity and optical code division multiple access (OCDMA). Reconfigurable two-dimensional optical encoder and decoder are constructed by wavelength selective switch and optical delay lines, and reconfigurable one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Under three different turbulence conditions, bit error rates (BER) of legitimate user and eavesdropper are measured. It can be seen that under turbulence effects, BER of legitimate user in spatial- diversity is lower than that of non-diversity system. By the rapid code reconfiguration of legitimate users, the eavesdropper can only use unmatched decoder. Thus, the eavesdropping methods of code interception and brute-force searching can be avoided. Experimental results show that this scheme can ensure the reliable transmission of legitimate users, and the eavesdropper can't crack the information when the extraction ratio is 1%. Hence, reliability and physical-layer security can be enhanced simultaneously.

This paper proposes the enhancement of spectral efficiency (SE) by linear polarizer based multilevel linear polarization shift keying (MLPolSK) detection in free-space optical (FSO) communication. At the transmitter end, in order to improve SE of FSO links, the various linear state of polarizations (SOPs) with an equal magnitude-spacing of x- or y-polarization are modulated for MLPolSK signal. At the receiver end, the cascaded polarization-independent semiconductor optical amplifier (SOA) and linear polarizer are introduced to detect MLPolSK signal. Polarization-independent SOA is applied to effectively mitigate the turbulence induced scintillation effect by the gain saturation without the distortion of polarization. Linear polarizer with high extinction ratio (ER) is used to transform various SOPs into the signal with an equal intensity-spacing of x- or y-polarization. Besides, the amplified spontaneous emission (ASE) noises are reduced by the polarization of ASE noises by linear polarizer. Finally, MLPolSK is detected by a single photodiode (PD) and decided by multilevel-thresholdings. In experiments, the performance of the proposed technique is evaluated under turbulence channels, which are emulated using Mach-Zehnder modulator (MZM)-based fading simulator. Experimental results show that the SE is improved up to 3 bit/s/Hz under the effective scintillation mitigation using the proposed technique.

In this paper, 10 Gb/s free space optical (FSO) wiretap channel using optical code division multiple access (OCDMA) time-diversity reception is designed and demonstrated for the first time. Reliability and security are investigated under weak and strong turbulence conditions. Two-dimensional optical encoder and decoder are constructed by wavelength selective switches and optical delay lines, and one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Experimental results show that reliability and security can be enhanced simultaneously using OCDMA time-diversity reception. At the received power of 2.58 dBm in strong turbulence, compared to non-diversity system, bit error rate (BER) of legitimate user in OCDMA time-diversity system is reduced from 3.9 × 10-7 to 6.73 × 10-8. Furthermore, the reliability improvement is more obvious when the correlation between the two signals of OCDMA time-diversity is smaller. The secrecy capacity can be increased from 0.546 bit/symbol to 0.665 bit/symbol in strong turbulence when received power is 2.28 dBm. On the other hand, the secrecy capacity of strong turbulence is about 15% higher than that of weak turbulence.

A free-space optical (FSO) receiver with high optical alignment robustness configured by a single optical lens is presented, in addition to a newly developed 2-D photodetector array (2D-PDA). A 6 × 6 square-shaped aligned arrangement matrix structure in the 2D-PDA provides a large active area for photodetection. In a 1.5-m FSO demonstration, the optical alignment tolerance on the receiver's lens is evaluated to be as large as 12 mm. The multiple-output signals from the 2D-PDA are used for the maximum ratio combining method in space diversity. The system design and alignment tolerance without a mechanical beam-tracking system are presented and the bit error rate contributed by space diversity is discussed. [ABSTRACT FROM AUTHOR]

Atmospheric turbulence (AT) tends to impair the performance of free space optical (FSO) communication systems. Detecting AT strength is significant for turbulence effect mitigation, which can appropriately guide the selection of turbulence mitigation techniques and modulation formats. Orbital angular momentum (OAM) beam aberrations received through the turbulence channel are closely related to the turbulence strength. In this paper, we experimentally detect the AT strength using an OAM beam based on a convolutional neural network (CNN). We collect 8 kinds of superposed OAM beam intensity images after 5 levels of turbulence in the laboratory as datasets and test the AT detector performance with respect to the number of pixels, mode number of OAM beams, different AT sets and training set size. The results show that the AT detection accuracy is near 100% for 3 kinds of ATs, and the accuracy remains at approximately 85% for 5 kinds of ATs. In addition, using data augmentation methods or a hybrid dataset can improve the AT detection accuracy. The CNN-based method in this paper can help detect the AT strength in atmospheric channels and provide references for choosing appropriate techniques to mitigate turbulence effects and then enhance the OAM-FSO system performance.

In the paper, we present and demonstrate experimentally a 10 Gbit/s on-off keying (OOK) bidirectional wavelength-division-multiplexing (WDM) free space optical (FSO) communication architecture integrated in fiber network together with 5 m free space transmission length and 25 km fiber link, respectively. To achieve bidirectional FSO link, the all WDM wavelengths can be used simultaneously to serve as the downstream and upstream traffics in the proposed FSO-WDM system. Here, four pairs of line terminations (LTs) and optical wireless units (OWUs) are integrated arbitrarily in fiber access network for confidential and bidirectional FSO connection based on the requirement of practical environment. In the measurement, the relationship of FSO signal performance and detected tolerance are also discussed and analyzed. Moreover, the WDM-FSO transmission can also avoid the Rayleigh backscattering (RB) beat noise in fiber connection.

High sensitivity, small size, light weight and low power are the most critical requirements for the lasercom terminals used in free space optical communication (FSOC) systems. Coherent receivers have an exceptionally high sensitivity but their optical complexity and power consumption are too high, especially for the space-borne lasercom terminals. Recently we propose a pseudo-single-side-band-signal based single-photodiode coherent receiver (P-SCR) with very low optical and algorithm complexity which can potentially satisfy the strict requirement of the FSOC systems. In this paper, we first introduce the working principles of the P-SCR based on the frequency and time domain field reconstruction algorithms (TD-FRA and FD-FRA). We then investigate the relationships between the computation complexity and sensitivity for the two kinds of P-SCR, respectively. We also provide a method to minimize the FD-FRA complexity and demonstrate that its power consumption is only 3.7%, 25.3% and 37.8% of that of the well-known conventional Kramers-Kronig (KK) algorithm, upsampling-free KK algorithm and TD-FRA. Based on the optimized FD-FRA, we develop a highly parallel DSP circuit for the real-time P-SCR using a FPGA chip. The experiments in a 10GBaud QPSK transmission system show that the sensitivity of the real-time P-SCR is only ∼4.2 dB away from the theoretical limit. [ABSTRACT FROM AUTHOR]

The data transmission between 5G floating base stations will be an important application area of the free space optical (FSO) link, but the problem of atmospheric disturbance will directly affect the application effect. An atmospheric turbulence suppression algorithm based on avalanche photodiode adaptive gain control (APDAGC) for the laser transmission terminal was proposed to suppress atmospheric turbulence. The given design mechanism and experimental results demonstrate its function to improve the performance of FSO. When APDAGC is over 1 km FSO, the variance of intensity fluctuation is lowered from 0.046 to 0.009, with the bit error rate reduced from 4.82E-6 to 1E-12. Comparatively, as APDAGC is over 6.5 km FSO, the variance of intensity fluctuation is decreased from 1.767 to 0.376, with the bit error rate varying from 4.6E-2 to 2.4E-4. Moreover, it was used for the first FSO transmission of 5G floating base station signals on an airship platform. [ABSTRACT FROM AUTHOR]

In free space optical communication systems, transmit laser selection (TLS) has been proposed as an efficient fading-mitigation technique over turbulence channels. In TLS, the transmitter is equipped with a number of laser sources and the best laser source is selected for transmission. In practice, feedback error or outdated selection due to temporal changes in the channel may result in selecting another source rather than the best one. In this letter, we consider a FSOC system with generalized TLS (GTLS) where the ${n^{\text {th}}}$ best laser is selected among the available $N$ lasers. Under the assumption of lognormal turbulence channels in addition to pointing errors, we derive a closed-form expression for asymptotic bit error rate. We use our derived closed-form expression to determine the diversity gains. We finally present simulation results to corroborate our analytical findings. [ABSTRACT FROM AUTHOR]

In this paper, 10 Gb/s free space optical (FSO) wiretap channel using optical code division multiple access (OCDMA) time-diversity reception is designed and demonstrated for the first time. Reliability and security are investigated under weak and strong turbulence conditions. Two-dimensional optical encoder and decoder are constructed by wavelength selective switches and optical delay lines, and one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Experimental results show that reliability and security can be enhanced simultaneously using OCDMA time-diversity reception. At the received power of 2.58 dBm in strong turbulence, compared to non-diversity system, bit error rate (BER) of legitimate user in OCDMA time-diversity system is reduced from 3.9 × 10−7 to 6.73 × 10−8. Furthermore, the reliability improvement is more obvious when the correlation between the two signals of OCDMA time-diversity is smaller. The secrecy capacity can be increased from 0.546 bit/symbol to 0.665 bit/symbol in strong turbulence when received power is 2.28 dBm. On the other hand, the secrecy capacity of strong turbulence is about 15% higher than that of weak turbulence. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication systems experience atmospheric turbulence-induced fading, which is typically modelled by lognormal distribution. Since the decreasing rate of error probability over lognormal fading channels does not converge to a finite value, the conventional definition of diversity order yields infinity and does not provide any insight into system design. In this letter, we propose the so-called incremental diversity order (IDO), which is defined as the incremental change in the decreasing rate of bit error rate (BER) on a log-log scale. As an example, we consider multiple-input multiple-output (MIMO) FSO system over lognormal turbulence channels. We derive a novel closed-form BER expression in terms of Lambert-W function and obtain IDO as a function of the number of laser sources, the number of photodetectors and the variance of the log-amplitude coefficient. We further present numerical results to validate the derived BER expression and associated IDO. [ABSTRACT FROM AUTHOR]

We investigate adaptive transmission schemes over a hybrid radio frequency (RF)/free space optical (FSO) backhaul network for connecting macro-cell base station with small-cell base station through multiple parallel buffer-aided relay nodes. The proposed adaptive transmission schemes improve delay-throughput trade-off of backhaul network by exploiting the degrees-of-freedom over both RF and FSO links and capturing quality-of-service (QoS) requirements. We study two different system configurations, namely, single-carrier and multi-carrier hybrid RF/FSO systems. Our goal is to maximize the constant data arrival rate to the network such that the total queue occupancy in the network is bounded with certain acceptable queue-length bound violation probability. Towards this goal, we formulate novel optimization problems in order to maximize the end-to-end effective capacity over RF and FSO links of both system configurations. Efficient solutions are derived by employing the Lagrangian optimization technique. Some interesting insights are also revealed by considering special cases on link conditions and QoS requirements. Simulation results provide the following two observations: (i) the proposed adaptive transmission scheme outperforms the conventional switch-over hybrid RF/FSO transmission, especially in clear to moderately harsh weather conditions; and (ii) compared to several non-buffer-aided and buffer-aided relaying with non-adaptive transmission schemes, the proposed adaptive transmission scheme substantially improves the supportable data arrival for the hybrid RF/FSO backhaul network. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communication has emerged to provide line of sight connectivity and higher throughput over unlicensed optical spectrums. Cognitive radio (CR), on the other hand, can utilize the radio frequency (RF) spectrum and allow a secondary user (SU) to share the same spectrum with the primary user (PU) as long as the SU does not impose interference on the PU. Owing to the potential of these emerging technologies, to provide full spectrum efficiency, this paper focuses on the mixed CR RF-FSO transmission scheme, where RF communication is employed at one hop followed by the FSO transmission on the other hop in a dual-hop decode-and-forward (DF) configuration. To quantify the performance of the proposed scheme, closed form error probability is derived over Rayleigh/Exponentiated Weibull (EW) fading distributions by considering the statistical and instantaneous feedback channel of the primary network. We also employed an asymptotic analysis to illustrate the diversity gain of the overall system. We believe that the proposed scheme can be applicable to the 5G+ networks where an unlicensed university student connects to the home computer with the aid of an FSO path. [ABSTRACT FROM AUTHOR]

One of the major restrictions of the free-space optical communication systems is the limitation of single-mode fiber (SMF) coupling efficiency that impacts the system performance. In this paper, an adaptive coupling method for optical communication system based on coarse-fine laser nutation technique was presented. The scanning strategy and algorithm were set up and experimentally examined. The experimental results demonstrate that the coarse-fine laser nutation technique is capable of achieving a good coupling efficiency even the beam spot and the SMF do not coincide with each other at the start. The SMF center calculation accuracy of coarse and fine scanning stage were 0.25 and 0.01 μm, respectively. And the average coupling efficiency of the system can increase from 0.351 in open loop to 0.624 in closed loop under a simulated atmospheric turbulence condition, the offset compensation error was 0.1245 μm when turbulence induced. Finally, some factors influencing the systems were also discussed.

New high-data-rate multimedia services and applications are evolving continuously and exponentially increasing the demand for wireless capacity of fifth-generation (5G) and beyond. The existing radio frequency (RF) communication spectrum is insufficient to meet the demands of future high-datarate 5G services. Optical wireless communication (OWC), which uses an ultra-wide range of unregulated spectrum, has emerged as a promising solution to overcome the RF spectrum crisis. It has attracted growing research interest worldwide in the last decade for indoor and outdoor applications. OWC offloads huge data traffic applications from RF networks. A 100 Gb/s data rate has already been demonstrated through OWC. It offers services indoors as well as outdoors, and communication distances range from several nm to more than 10000 km. This paper provides a technology overview and a review on optical wireless technologies, such as visible light communication, light fidelity, optical camera communication, free space optical communication, and light detection and ranging. We survey the key technologies for understanding OWC and present state-of-the-art criteria in aspects, such as classification, spectrum use, architecture, and applications. The key contribution of this paper is to clarify the differences among different promising optical wireless technologies and between these technologies and their corresponding similar existing RF technologies.

Bit error rate (BER) performance for modulating retro-reflector (MRR) free space optical communication (FSOC) system with an adaptive threshold over correlated gamma-gamma fading channel is investigated. In order to do this analysis, we first derive two closed form expressions for the probability density function of the correlated MRR channel and our instantaneous BER using the optimal adaptive threshold without channel state information (CSI). Then based on these results, we develop a closed form expression for the average BER in the infinite series form whose convergence is confirmed by Cauchy’s ratio test. Simulation results show that we can achieve small SNR penalty of 0.32 dB between our system and the system with CSI by constructing our adaptive threshold using 3 bit pilot data corresponding to a slight throughput reduction. [ABSTRACT FROM AUTHOR]

We investigate resource optimization for the downlink cloud small cell network, where the baseband unit pool communicates with the buffer-aided small remote radio heads (SRRHs) through free space optical fronthaul, and SRRHs transmit to the user equipments (UEs) by using time division multiplexing-based millimeter wave access links. Our objective is to maximize the supportable aggregate data arrival rate in the network by exploiting the inter-dependence of fronthaul and access links. Toward this objective, we consider maximum acceptable end-to-end queue-length bound violation probability constraints, load-balancing constraints in the access link, fronthaul link selection constraints, and transmit power budget constraints of fronthaul and access links. Since the joint fronthaul and access link optimization is a non-convex and combinatorial problem, we develop an iterative solution by decomposing the original optimization problem into two sub-problems. The first sub-problem optimally obtains fronthaul and access link power allocation and fronthaul link selection by using Lagrangian dual decomposition and canonical one-to-one matching techniques. By employing the Lagrangian dual decomposition and alternating optimization techniques, the second sub-problem obtains near optimal data arrival rate for each UE, UE-SRRH associations, fronthaul rate allocation among the transmitted data for the UEs, and the transmission duration scheduling in millimeter wave access link. An algorithm of polynomial complexity is developed in order to determine the supportable aggregate data arrival rate by considering the statistical quality-of-service requirements, and its convergence is proved. The simulation results depict that the proposed scheme significantly improves the aggregate data arrival rate over several benchmark schemes. [ABSTRACT FROM AUTHOR]

For the first time, we experimentally demonstrate a 400-Gb/s [ $4\times100$ -Gb/s wavelength-division multiplexing (WDM)] pulse-amplitude-modulation-4 (PAM-4) transmission based on free space optics (FSO) utilizing the polarization modulation (PolM) and direct detection (DD) scheme. Each WDM channel comprises a polarization modulator, a polarizing beam splitter (PBS), and two photodetectors (PDs). Four wavelengths each carrying a 100-Gb/s PAM-4 signal (aggregate 400 Gb/s) are transmitted over the free space. At the receiver side, the PBS separates the two orthogonal polarizations, and then, the two polarized signals are detected by two PDs. Then, the two detected signals are combined by a subtractor, followed by digital signal processing (DSP). The PolM-DD system can achieve higher data rate while maintaining the required forward-error-correction (FEC) threshold compared with intensity modulation and direct detection (IM-DD) scheme. Moreover, owing to the stable polarization state in the free space and the mitigation of the common mode noise by the subtraction, the PolM-DD system provides enhanced tolerance to the amplitude fluctuations attributable to weather turbulence, which makes it more reliable than the conventional IM-DD FSO system. To the best of our knowledge, this is the first experimental demonstration of a 400-Gb/s FSO system using the PolM-DD scheme with the transmission distance of 1.2 m. [ABSTRACT FROM AUTHOR]

An indoor 100 Gb/s free space optical communication link using an injection locked ~1621 nm quantum-dash laser has been experimentally demonstrated. The results are achieved using dual-polarization quadrature phase shift keying (DP-QPSK) and coherent detection over a 4 m indoor free space link. The system exhibited a receiver sensitivity of -17.5 dBm to achieve the bit error rate forward error correction (BER FEC) threshold of 3.8 × 10-3. This displays the potential of tunable injection locked far L -band quantum-dash lasers, with less scattering loss compared to C-band counterpart, as promising light sources in both indoor and outdoor future high speed free space optical information and communication technology.

Optical space shift keying (OSSK) is a multiple transmit aperture based scheme which attains improved spectral efficiency in optical wireless communication (OWC). In addition to atmospheric turbulence (AT), pointing error (PE), which originates due to vibrations or building sway, has potential to deteriorate the performance of OWC link. This paper discusses the effect of AT and PE jointly on the performance of OSSK scheme. An analytical framework is proposed for evaluating average bit error rate (ABER), coding gain, and diversity order of OSSK scheme. It is found by analysis that for low PE, error performance of OSSK in weak AT condition is poorer than that in strong and moderate AT conditions. However, for high PE, the trend is the reverse. ABER, coding gain, and diversity order for pulse amplitude modulation (PAM) over AT and PE are also derived in order to compare its performance with the same rate OSSK scheme. The analysis shows that OSSK is less sensitive to PE severity as compared to PAM. Also, the diversity order of OSSK is independent of AT and PE, whereas the diversity order of PAM depends on both AT and PE, which makes OSSK a suitable modulation scheme for the practical scenario.

One of the major restrictions of the free-space optical communication systems is the limitation of single-mode fiber (SMF) coupling efficiency that impacts the system performance. In this paper, an adaptive coupling method for optical communication system based on coarse-fine laser nutation technique was presented. The scanning strategy and algorithm were set up and experimentally examined. The experimental results demonstrate that the coarse-fine laser nutation technique is capable of achieving a good coupling efficiency even the beam spot and the SMF do not coincide with each other at the start. The SMF center calculation accuracy of coarse and fine scanning stage were 0.25 and 0.01 μm, respectively. And the average coupling efficiency of the system can increase from 0.351 in open loop to 0.624 in closed loop under a simulated atmospheric turbulence condition, the offset compensation error was 0.1245 μm when turbulence induced. Finally, some factors influencing the systems were also discussed. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate, for the first time, a system which integrates millimeter-wave (MMW) and free-space-optics (FSO) analog radio-over-fiber (A-RoF) channels, and provides enhanced reliability in 5G and beyond mobile fronthaul (MFH) links. The integrated system utilizes a novel coordinated mapping and combining (CMC) technique that senses the signal quality in each data block and apportions the load accordingly. Furthermore, the technique takes advantage of the abundant bandwidth in both MMW and FSO links and is compatible with orthogonal frequency division multiplexing (OFDM). In the CMC design, the data blocks at the transmitter side are interleaved and repeated in both the frequency domain and the time domain, and then the duplicated blocks are combined with adaptive weight coefficients at the receiver side. We demonstrate experimentally that the proposed CMC t echnique improves power margins and enhances the robustness of the adaptive diversity combining technique (ADCT)-based FSO/MMW links by mitigating system impairments caused by frequency/time burst interference and power fading attributable to adverse weather conditions. In experiments, compared with the ADCT-based system, we find a 9 dB gain in received optical power tolerance and a 5.8 dB lower EVM floor under a deliberately injected 20-MHz LTE frequency interference burst. [ABSTRACT FROM AUTHOR]

Driven by the limitation that the intrinsic of conventional cooperative communication is utilizing the spare transceivers at the other nodes, we design a novel cooperative protocol in free-space optical communication. The designed protocol permits that the user with favorable channel conditions can receive its own message, and at the same time, it can be utilized as the relay for providing cooperative diversity for the others. A novel closed-form asymptotic expression of outage probability is derived, following a gamma–gamma model in the presence of pointing error and path loss. Based on the obtained result, the diversity gain is analyzed, which shows that the diversity gain is highly dependent on the location of users, and the proposed protocol can enhance the performance of users. Furthermore, the behavior of ergodic data rate is analyzed at large signal-to-noise ratio. All the results are corroborated by the Monte Carlo simulations, and the results show that the proposed system outperforms the conventional system in terms of ergodic sum rate. [ABSTRACT FROM AUTHOR]

Free-Space-Optical (FSO) communication has the potential to provide optical-level wireless communication speeds. It can also help solve the wireless capacity problem experienced by the traditional RF-based technologies. Despite its capacity advantages, FSO communication is prone to mobility. Since the FSO transceivers are highly directional, they require establishment and maintenance of line-of-sight (LOS) between each other. We consider two autonomous mobile nodes, each with one FSO transceiver mounted on a movable head capable of rotating 360 degree. We propose a novel scheme that deals with the problem of automatic maintenance of LOS alignment between the two nodes with mechanical steering of the FSO transceivers. We design protocols to maintain an FSO link between the mobiles satisfying a minimum received power or signal-to-noise ratio (SNR). We also present a prototype implementation of such mobile node with FSO transceivers. The effectiveness of the alignment protocol is evaluated by analyzing the results obtained from both simulations and also experiments conducted using the prototype. The results show that, by using such mechanically steerable transceivers and a simple auto-alignment mechanism, it is possible to maintain optical wireless links in a mobile setting with nominal disruption. [ABSTRACT FROM AUTHOR]