Free-space Optical Transmission Research Articles

2 × 10 Gbit/s–10 GHz Radio over Free Space Optics Transmission System Incorporating Mode Division Multiplexing of Hermite Gaussian Modes

Abstract In this paper, we report the modeling and simulative investigation of a mode division multiplexing (MDM)-based Radio over Free space optics (RoFSO) transmission system. Two separate 10 Gbit/s–10 GHz information signals are transported successfully over 30 km link range using distinct Hermite Gaussian (HG) modes. Also, the decomposition of distinct modes at the receiver terminal has been reported in this work. Furthermore, the impact of different fog conditions and scintillation effect on the performance of the proposed link has also been discussed in this paper.

Performance comparison of M-QAM and DQPSK modulation schemes in a 2 × 20 Gbit/s–40 GHz hybrid MDM–OFDM-based radio over FSO transmission system

This work is focused on the modeling and performance investigation of a 2 × 20 Gbit/s–40 GHz hybrid mode division multiplexing–orthogonal frequency division multiplexing-based radio over free space optics (RoFSO) transmission system under the influence of different weather conditions. The performance of the proposed system has been compared for 4-quadrature amplitude modulation (QAM), differential quadrature phase-shift keying, 16-QAM, and 32-QAM modulation schemes using error vector magnitude, optical signal-to-noise ratio requirement, and maximum link reach as the performance metrics. The results show that 4-QAM scheme demonstrates the best performance. The proposed RoFSO transmission system incorporating 4-QAM modulation demonstrates a successful transmission of 2 × 20 Gbit/s–40 GHz information over 104 km link range under clear weather conditions. Also, the maximum link range using the proposed system is reported as 4.52 km under light fog, 2.78 km under moderate fog, and 2.11 km under heavy fog conditions. Further, the performance of the proposed system has been compared with the previously reported literature which shows that the proposed system has a better figure of merit (information rate × transmission distance). The presented work can be used to implement a spectrum efficient, high-speed, long-haul information transmission system for future wireless networks.

A Framework on the Performance Analysis of Dual-Hop Mixed FSO-RF Cooperative Systems

Free space optical (FSO) communication systems provide line-of-sight wireless connectivity in the unlicensed optical spectrum and achieve higher data rates in comparison to their radio frequency (RF) counterparts. FSO systems are particularly attractive for the last mile access problem by establishing a connectivity bridge between optical fiber-based backbone and RF access networks. To address this practical deployment scenario, there has been an increasing attention on the so-called mixed (dual-hop) FSO-RF systems where FSO transmission is utilized at one hop followed by RF transmission at the other one. In this paper, we investigate the performance of mixed FSO-RF transmission systems where the FSO link is modeled by the double generalized Gamma distribution with generalized pointing error impairments and the RF link experiences the extended generalized-K shadowed fading. For both amplify-and-forward and decode-and-forward relaying, we derive closed-form expressions for outage probability, bit error probability, and ergodic capacity in terms of bivariate Fox-H function. Monte Carlo simulation results are provided to verify the accuracy of derived expressions. We further provide an asymptotic analysis and discuss the achievable diversity orders of mixed FSO-RF systems.

Comparison of adaptive algorithms for free space optical transmission in Málaga atmospheric turbulence channel with pointing errors

In this study, the authors investigate average capacity of free space optics communication over Málaga atmospheric turbulence channel with pointing errors and path loss, for intensity modulated/direct detection (IM/DD) and heterodyne detection. Various algorithms which use adaptive transmission with both types of detection are considered, such as: optimal rate adaption (ORA), optimal power and rate adaption (OPRA), channel inversion with fixed rate (CIFR) and truncated channel inversion with fixed rate (TIFR). Analytical closed-form expressions for channel capacities of ORA, OPRA and TIFR adaptive transmission are presented, and the authors prove that CIFR transmission is not feasible in the strict sense for the conditions considered. Obtained analytical results are numerically evaluated and graphically presented for different strengths of atmospheric turbulence (in weak, moderate and strong turbulence regime) for both types of detection (IM/DD and heterodyne), and for considered algorithms of adaptive transmission (ORA, OPRA and TIFR). The authors have developed expressions suitable for approximating high signal-to-noise ratio channel capacity, and they graphically present and compare the asymptotic approximations with the obtained analytical results for different strengths of turbulence for both types of detection. Also, obtained analytical results were confirmed by Monte-Carlo simulations, and graphically compared for different strengths of turbulence regimes.

Hybrid MMW-over fiber/OFDM-FSO transmission system based on doublet lens scheme and POLMUX technique

Abstract A hybrid 60-GHz millimeter wave (MMW) over fiber as well as 500-m/10 Gbps OFDM based free space optical transmission system using doublet lens scheme and POLMUX technique is proposed and demonstrated. In the present paper, 10 Gbps OFDM data stream, 10 Gbps/60-GHz MMW and 10 Gbps base band data signal are transmitted simultaneously over 50-km SMF through an optical wave by employing POLMUX technique which make our system cost effective. 10 Gbps/60-GHz MMW is successfully communicated over 20-m wireless distance with the assistance of 60-GHZ horn antenna at the transmitting and receiving section of the wireless link. Our proposed OFDM-FSO system employing with doublet lens scheme significantly enhances the transmission distance and greatly reduces the multipath fading, and dispersion effect in our FSO link. Excellent bit error rate, good EVM (

$4\times100$ -Gb/s PAM-4 FSO Transmission Based on Polarization Modulation and Direct Detection

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.

Performance comparison of high-speed long-reach mode division multiplexing-based radio over free space optics transmission system using different modulation formats under the effect of atmospheric turbulence

Performance comparison of high-speed long-reach mode division multiplexing-based radio over free space optics transmission system using different modulation formats under the effect of atmospheric turbulence

A framework on the performance analysis of relay-assisted FSO transmission systems

In this work, we tend to analyze the performance of dual-hop relayed free-space optical (FSO) transmission systems. To do this, the FSO links are modeled as the Double Generalized Gamma (DGG) distribution with generalized pointing error impairment and under both heterodyne detection (HD) and intensity modulation with direct detection (IM/DD) methods. Using amplify-and-forward (AF) fixed-gain relaying as well as channel state information (CSI)-assisted AF relaying along with decode-and-forward (DF) relaying, we derive closed-form expressions for the outage probability (OP), the average bit error rate (BER) and the ergodic capacity (EC) in terms of the bivariate Fox-H function. Additionally, we analyze high signal-to-noise-ratio (SNR) behavior of the OP and formerly we derive the diversity order of the considered system. To asses the accuracy of the obtained results, numerical and Monte-Carlo simulation results are provided. Based on the high SNR analysis, we further provide a diversity gain analysis and discuss the bottlenecks in the dual-hop FSO systems.

Impact of rain weather over free space optic communication transmission

<span>This paper focuses the rain effect over FSO link medium in Malaysia environment. In this work, a rain data samples that collected from Malaysian Meteorological Department (MMD) to determine the scattering coefficient, atmospheric attenuation and total attenuation. From the analysis, the precipitation rate give different impact over FSO link which can cause the attenuation and bit error rate increase. The results also show the comparison parameter for optimal geometrical loss such as beam divergence, aperture size and receiver sensitivity.</span>

Enhancing Performance of Hybrid FSO/Fiber Optic Communication Link Utilizing Multi-Channel Configuration

Abstract Optical wireless is the alternative approach to complete the deployment of optical cable networks in case of failure in implementation. In optical wave the attenuation due to atmospheric turbulence conditions causes a performance degradation of a free space optical (FSO) system. Thus, it will lead to the deterioration of the signal power in the optical cable. To overcome these challenges, multi-channel FSO transmitter and erbium doped fiber in optical link was adopted with high transmission power. In this paper, a hybrid FSO/fiber optic (FSO/FO) link is simulated and investigated under different weather conditions. The proposed hybrid link is evaluated in terms of BER and Q-factor. The proposed system shows a closely performance as compared with the conventional FSO for whole investigation range from 200 m to 2000 m. While the communication distance with the hybrid link is increased by 40 km as compared to conventional FSO. The results show good agreement with Kim model, especially at high input power of 160 mW.

Design and Analysis of Sliding Window ARQ Protocols With Rate Adaptation for Burst Transmission Over FSO Turbulence Channels

This paper studies a cross-layer design of error-control protocols with rate adaptation for a free-space optical (FSO) burst transmission over atmospheric turbulence channels. Specifically, sliding window automatic repeat request (ARQ) protocols are designed so that the window size is determined by the burst duration. This facilitates an effective operation for sliding window protocols with the adaptive rate (AR) transmission over atmospheric turbulence channels. To analyze performance of the design, the time-varying behavior of the atmospheric turbulence channels, modeled by the popular gamma–gamma (ΓΓ) distribution, is captured to develop the burst error model. An embedded Markov model, which is formulated in the discrete time defined by the burst duration, is analyzed based on the developed burst error model. Using the queueing analysis, the frame loss rate, average delay, and system throughput are analytically derived. Numerical results quantitatively demonstrate the impact of atmospheric turbulence on the system performance and support the optimal selection of system parameters over turbulence conditions. Monte Carlo simulations are also performed to validate the analytical results, and an excellent agreement between the analytical and simulation results is confirmed.

A novel ergodic capacity analysis of MISO OFDM –PDM for free space optical communication system

This paper proposes a novel method to improve the capacity of Multiple Input Single Output (MISO) Free Space Optical (FSO) networks transmission using Polarization Division Multiplexing (PDM). To improve the transmission capacity and to reduce the cross channel effects, the OFDM subcarriers are separated into even and odd and combined with PDM which supports two independent data channels on a single wavelength with orthogonal polarization states. The system performance is evaluated under various atmospheric turbulence conditions. We propose a system of 10 GB/s by using 2.5 GB/s for each SCM. Closed form equations are derived for Ergodic capacity of Even -Odd carriers separated OFDM-PDM and the graphs are plotted.

Effect of Pointing Errors on the Performance of Hybrid FSO/RF Networks

The performance of a free space optics (FSO) transmission suffers from the atmospheric turbulence and the attenuation in foggy environment. By employing relay nodes, the error rate and the coverage area of the FSO communication system can be significantly improved. However, the pointing errors, generated because of the building sway, have the potential to eradicate the benefits of the relay-based FSO communication system. The effect of pointing errors in the presence of Gamma Gamma atmospheric fading together with path loss attenuation is considered in this paper. To counteract the adverse affects of the FSO link, a reliable millimeter-wave radio frequency (MMW RF) link is used as a backup. In this context, this paper proposes a cooperative decode-and-forward (DF)-relaying-based hybrid FSO/RF system with maximal-ratio-combining (MRC) at the destination. The system consists of FSO and RF sub-systems, where FSO sub-system has the priority to transmit and RF sub-system serves as a back up when the FSO sub-system is in outage. The exact and asymptotic outage probability and average symbol error rate (SER) expressions for the proposed system are derived in closed-form and the diversity order is determined. The effect of pointing errors on the system performance is analyzed extensively. The optimum values of transmit beam waist and radius of receiver aperture are determined. The theoretical results, which are validated by Monte-Carlo simulations, show that the proposed cooperative hybrid FSO/RF system drastically improves the system performance compared to single hop (SH) hybrid FSO/RF and cooperative FSO systems especially for large pointing errors scenario.

Performance of Decode-and-Forward Relaying in Mixed Beaulieu-Xie and $\mathcal{M}$ Dual-Hop Transmission Systems With Digital Coherent Detection

Performance analysis is presented for a decode-and-forward protocol based mixed radio-frequency (RF) and free-space optical (FSO) dual-hop transmission system with digital coherent detection. The RF path is modeled by Beaulieu-Xie fading, while the FSO hop is characterised by the Malaga ( $\mathcal {M}$ ) distributed turbulence with pointing errors. We first derive novel and exact analytical expressions for the cumulative distribution function (CDF), the probability density function and the moment generating function (MGF) of the overall signal-to-noise ratio by means of Meijer’s G function, followed by the accurate infinite series expressions of the performance criterions, such as the outage probability, the average bit-error rate (BER) and the ergodic capacity (average channel capacity). Asymptotic analysis for the CDF, the MGF, the outage probability, the average BER, and the ergodic capacity is also provided. Monte Carlo simulations are performed to verify these derived expressions.

Preface to Special Issue on Development of Free Space Optical Transmission Technology in the Atmosphere

Preface to Special Issue on Development of Free Space Optical Transmission Technology in the Atmosphere

Measurements of Atmospheric Fluctuations for Optical Free-Space Transmission to Moving Objects

Measurements of Atmospheric Fluctuations for Optical Free-Space Transmission to Moving Objects

Highly Stable Optical Free-Space Transmission Technology Under Atmospheric Fluctuation

Highly Stable Optical Free-Space Transmission Technology Under Atmospheric Fluctuation

Performance Enhancement of an Orbital-Angular-Momentum-Multiplexed Free-Space Optical Link Under Atmospheric Turbulence Effects Using Spatial-Mode Multiplexing and Hybrid Diversity Based on Adaptive MIMO Equalization

In this paper, we propose an adaptive multiple-input–multiple-output (MIMO) free-space optical (FSO) links using an orbital-angular-momentum (OAM)-multiplexed based on the spatial-mode multiplexing (SMM) through the turbulent channel. We propose to use the SMM and spatial-mode diversity (SMD) combined with an adaptive MIMO technique to mitigate the atmospheric turbulence effects. In this paper, our objective is to design the adaptive MIMO–FSO links based on OAM–MIMO/SMM multiplexed and analyze its performance in the atmospheric turbulence conditions. The simulation results show four OAM modes-based MIMO/SMM and resulting in four OAM-multiplexed channels. Each OAM mode carries a 100-Gbit/s quadrature phase-shift keying signal (aggregate 400 Gbit/s) on a single wavelength channel ( $\lambda \sim 1550$ nm) and is transmitted for a 2-km link. The calculated received power and inter-channel crosstalk of an OAM–MIMO/SMM signal fluctuate by 4.5 −6 dB, respectively. The power penalties can be reduced by 1–4 dB for all channels after OAM–MIMO/SMM equalization at a bit-error rate (BER) of 10−9. The calculating results show that the system using an OAM-based MIMO/SMM and SMD multiplexing achieves superior BER performance using an adaptive MIMO/SMM equalization. The Numerical results show favorable transmission performance of OAM based on MIMO/SMM and the SMD multiplexing compared to the conventional-MIMO (CMIMO) of FSO transmission link. The Simulation models verified the superior BER performance of OAM–SMD-based MIMO/SMM. The SMM/MIMO–SMD technique performed better in the channel capacity and signal-to-noise ratios over other commonly used CMIMO algorithms. To sufficiently discuss the OAM–MIMO/SMM behavior-based SMD multiplexing, the performance of the novel SMM/MIMO–SMD technique is analyzed using simulations based on the Matlab/simulink program in order to verify the accuracy of the models and simulation results. This paper could be useful for the practical implementation of the SMM and the SMD using an adaptive MIMO equalization in the FSO systems.

Adaptive resource allocation in FSO/RF multiuser system with proportional fairness for UAV application

The combination of free space optic (FSO) and unmanned aerial vehicle (UAV) can be a promising solution to last-mile problem because of FSO high bandwidth and flexibility of UAV. However, FSO is vulnerable to the atmospheric turbulence and transmitter configuration is limited on UAV. Therefore, in this paper, we investigate an adaptive resource allocation strategy to provide relative fair and high transmission capacity for users. In the proposed network model, the downlink scenario is considered and ground station communicates with UAV and its local users by FSO and RF links, respectively. According to the channel conditions and rate requirements from users, channel and power assignments should satisfy the capacity demand in reasonable. To this end, we decompose the original channel and power allocation problem with high computational complexity into low-complexity subproblems, corresponding to channel allocation in RF transmission phase and channel matching and power allocation in FSO transmission phase. Then, a heuristic efficient resource assignment algorithm is proposed to achieve the optimal capacity distribution for users. Numerical results show that the proposed method can asymptotically achieve optimal throughput. Furthermore, system throughput is higher at low transmitting power requirement than those of other existing methods.

Coherently demodulated orbital angular momentum shift keying system using a CNN-based image identifier as demodulator

Abstract A coherently demodulated orbital angular momentum shift keying (OAM-SK) system over free-space turbulence channels using an image identifier based on convolutional neural network (CNN) as demodulator is proposed in this paper. Compared to the incoherent system, the proposed approach has the advantages of higher signal-to-ratio (SNR) of the detected image and fewer OAM topology charges employed for the same m-ary OAM-SK system. The CNN-based image identifier features in high recognition accuracy even under long free-space transmission and strong turbulence. In addition, the influence of the intensity of the local beam on the system performance, as well as the trade-off between the signal-to-noise ratio and the contrast of the received images, is fully investigated. Simulation results show that the demodulation accuracy of the proposed system with 16-ary OAM-SK system can be as high as > 99 % for 1500 m of free-space optical (FSO) transmission distance with strong atmospheric turbulence and appropriate intensity of local Gaussian beam.