Performance analysis of Free Space Optical links encoded using Luby Transform codes

Free Space Optical (FSO) communication is a low cost and license free high bandwidth access technique, is a primary research area from last few years. Atmospheric turbulence plays a vital role in degrading the link performance. This paper presents the model study on the effects of atmospheric turbulence in FSO links. The study is based on the most popular gamma-gamma model distribution. The theoretical model is simulated by varying the height of link and the results are plotted in Mathworks® MATLAB®. The research demonstrates that the effects of turbulence depend primarily on wind speed and height of established FSO link. After varying the height of link, the effects of atmospheric turbulence on link performance can be clearly seen. The result also plotted for the attenuation of signal with varying visibility range conditions.

Terrestrial Free Space Optical (FSO) links transmit information by using the atmosphere (free space) as a medium. In this paper, we have investigated the use of Luby Transform (LT) codes as a means to mitigate the effects of data corruption induced by imperfect channel which usually takes the form of lost or corrupted packets. LT codes, which are a class of Fountain codes, can be used independent of the channel rate and as many code words as required can be generated to recover all the message bits irrespective of the channel performance. Achieving error free high data rates with limited energy resources is possible with FSO systems if error correction codes with minimal overheads on the power can be used. We also employ a combination of Binary Phase Shift Keying (BPSK) with provision for modification of threshold and optimized LT codes with belief propagation for decoding. These techniques provide additional protection even under strong turbulence regimes. Automatic Repeat Request (ARQ) is another method of improving link reliability. Performance of ARQ is limited by the number of retransmissions and the corresponding time delay. We prove through theoretical computations and simulations that LT codes consume less energy per bit. We validate the feasibility of using energy efficient LT codes over ARQ for FSO links to be used in optical wireless sensor networks within the eye safety limits.

The performance of ground to satellite Free Space Optical (FSO) communication link is degraded due to the presence of atmospheric turbulence that causes fluctuations in both intensity and phase of the received optical signal. In this paper, bit error rate (BER) performance of FSO link using transmit diversity is investigated for coherent (sub-carrier BPSK and sub-carrier QPSK) and non-coherent (OOK and Q-PPM) modulation schemes in weak atmospheric turbulence channel. It is seen that multiple transmitting antennae help to overcome the turbulence induced fading effect. It is observed that sub-carrier BPSK modulation outperforms the other modulation schemes in terms of minimum SNR requirement for a given BER. Among non-coherent modulation schemes, Q-PPM performs better than OOK. However, as the level of Q in Q-PPM increases, its performance degrades and it becomes worse than OOK for certain values of atmospheric turbulence. Also, the effect of correlation among different transmitting antennae beams is analysed on the link performance.

Availability analysis of free-space-optical links based on rain rate and visibility statistics from tropical a climate

Channel characterization and empirical model for ergodic capacity of free-space optical communication link

In this paper, the distant source communicates with the underwater destination using High Altitude Platform Station (HAPS) and a triple-hop relay setup. In this setup, the source transmits information using hybrid Radio Frequency (RF)/Free Space Optical Communication (FSO) link to the decode-and-forward (DF) based relay at HAPS, which forwards the signal to the relay at the water surface using FSO link, which further transmits the signal to the underwater destination using Under Water Optical Communication (UWOC) link. Both the relays use DF relaying protocols. HAPS are used to increase the reliability of the system and range of transmission. FSO links offers a high bandwidth and better data rates than the RF links. But they often get affected by the atmospheric conditions, turbulence, and pointing errors. RF link on the other hand acts as a good complement to FSO links, thereby increasing the reliability of the system when both are used in parallel to transmit the information. UWOC link has several benefits like higher data rates, less attenuation and low power consumption in comparison to acoustic links. New statistical characteristics for the signal-to-noise-ration (SNR) of the proposed system are developed using which the outage probability of the system is derived. The numerical results shows that the outage probability of the system proposed gets affected by the RF shadowing, FSO turbulence parameters and turbulence parameters of UWOC links such as salinity, temperature gradient and bubble level concentration. Also the asymptotic analysis matched with the analytical results.

This paper demonstrates the effect of haze condition on free space optical (FSO) communication and to thin out the attenuation effects. The effects in the main focus on the atmospheric attenuation effects due the visibility and link range (length). Visibility is the main concern in utilizing the FSO communication system. The higher visibility, the longer transmission link. Hence, FSO performance could be enhanced and increased efficiency by using different wavelengths of laser beam. The conventions generally used wavelengths are 785 nm, 850 nm, and 1550 nm. The analysis result shows that the wavelength 1550 nm is better than 785 nm and 850 nm due to the less attenuation performance. Wavelength 1550 nm had improved 33%, 67%, 80% and 100% compared to the wavelength 850 nm and 785 nm due to the visibility and distance of 3 km, respectively. Higher wavelength able to thin out the attenuation effects on FSO links. Wavelength 1550 nm is sufficient enough to penetrate haze dense without giving harm to the eyes.

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.

An important consideration when deploying free-space optical (FSO) communication links over ultra long distances is the ability to actively steer the laser beam. FSO links are currently being researched as an attractive option for deep-space communication links or as a link to provide broadband communications to aircraft in-flight. In order to establish ultra long FSO links or to actively track FSO links between moving platforms, an active tracking system based on hybrid technology is essential. These hybrid systems are usually a combination of a mechanical gimbal and some array of active optical components. The presence of active optical components in a beam steering device is necessary to provide a high bandwidth while offsetting vibrations present on the mounting platform. This study compares three active beam steering elements that can be used in FSO transmitters and receivers. Performance characteristics of MEMS-based micro mirror arrays, acousto-optic modulators and steerable mirrors are analyzed and compared. A comparison of aperture size, range of motion, resolution and scanning speed is performed. Simulations in order to show the effects of vibration on various different length FSO links are run. A simulation in order to verify the ability of a fast steering mirror to offset vibration effects in a ground-to-UAV link is performed.

The foundation of any smart city requires an innovative and robust communication infrastructure. Many research communities envision free-space optical communication (FSO) as a promising backbone technology for the services and applications provided by such cities. However, the channel through which the FSO signal travels is the atmosphere. Therefore, the FSO performance is limited by the local weather conditions. The variation in meteorological variables leads to variations of the refractive index along the transmission path. These index inhomogeneities (i.e., atmospheric turbulence) can significantly degrade the performance of FSO systems. Thus, a practical implementation of the FSO link must carefully consider the atmospheric turbulence effect. This paper aims to investigate the feasibility of FSO communication for NEOM, a promising smart city in Saudi Arabia. We study the effect of weather conditions on FSO links using the micrometeorology model, taking into account actual weather data. The FSO performance in winter and summer was compared in terms of the bit error rate, signal-to-noise ratio (SNR), link availability, and transmission distance. The study shows that the atmospheric turbulence strength is moderate and strong in winter and summer, respectively. The temperature has the biggest impact on the FSO system when compared to the other meteorological elements included in this study. Furthermore, at transmission distances less than 300 m, atmospheric turbulence does not significantly affect the FSO for the operating wavelength of 1550 nm. Furthermore, it has been shown that at transmission distances greater than 300 m, the SNR in summer is more than 18% higher than in winter. The findings of this research enable understanding of the effect of turbulence caused by NEOM weather on the FSO link, thus assisting engineers in establishing a reliable FSO backbone link by adjusting the relevant parameters.

In this paper, a hybrid DWDM/multiple-beam free space optics (FSO) communication link has been proposed consisting of 16 channels with the channel spacing of 100 GHz (0.8 nm) each transmitting at 2.5 Gbps bit rate. At the receiver terminal, an erbium-doped fiber amplifier (EDFA) is used to amplify the received signal. The results show an improved performance of hybrid DWDM/multiple-beam FSO link in terms of Q factor, received power, and link distance under rain attenuation of 19.2 dB/km. The proposed link design was able to transmit data from 16 channels, each transmitting at a bit rate of 2.5 Gbps along a link length of 2,300 m in FSO link under heavy rain weather conditions.

This survey article discusses the revolutions of wireless communication technologies right from the prehistoric man's fire signals to today's space optical communications. A brief description of evaluations of communication technologies from the ancient Greece to today's mature telecommunication fields is discussed in this review paper. Principle, significance, demonstration and development of free space optical (FSO) communication technology over different decades are discussed. Major FSO channel limitations, temporal and spatial challenges of FSO communication system and their state‐of‐art mitigation techniques are presented. Classical quantitative analysis of reliability of FSO communication, developments on radio over FSO (RoFSO) and hybrid FSO/RF systems are also explained. Advanced developments of FSO communication techniques such as optical free space wavelength division multiplexing (WDM), sub‐carrier multiplexing (SCM), worldwide interoperability for microwave access (WiMAX), visible light communications (VLC) and vehicular visible light communications (VVLC) are reported. Deep space optical communication systems and next generation FSO wireless terrestrial/global network architecture are summarised. Research challenges of FSO systems for internet of things/everything (IoT/IoE), 5G communication, mobile‐network, terahertz spectrum, quantum communication and underwater optical applications are presented. Based on the review, we outlined the challenges that need to be addressed in near‐future researches to realize full potential FSO communication systems.

In free space optical (FSO) communication links, atmospheric turbulence has a significant impact on the quality of a laser beam propagating through the atmosphere. Turbulence results in intensity scintillation, which can severely impair the operation of target designation and FSO communications systems. Two important parameters for describing quality of FSO communications systems are the signal-to-noise ratio (SNR) and the bit error rate (BER). In this paper, using a modified model we analyze the effect of atmospheric turbulence on the SNR and BER for an FSO system in both weak and strong turbulence conditions.

This paper presents a new framework for long range Free Space Optical communication (FSO). Motivated by the ever-growing demand for communication in 5G networks, the suggested framework tackles the natural FSO link drawbacks, commonly: complicated installation, limited range, size and pricing. The new FSO concept assumes a "best effort" model. It uses a new control and aiming mechanism for the Tx laser beam based on COTS components. This methodology is mostly suitable for urban FSO links but might also be applicable for both short range (IoT) and long range (satellite) communication. Based on simulations and preliminary field results we expect that such FSO links will be widely used in "best-effort" 5G applications allowing an affordable alternative to fiber optics and standard FSO systems.

Free space optical (FSO) communication links offer a number of advantages over radio frequency links. Main advantages of FSO communication links are electromagnetic compatibility, license-free band and high data rate. However, reliability of these links is highly dependent on atmospheric and weather conditions. An optical wave propagating through the atmospheric transmission media is very sensitive on variety of atmospheric effects, e.g. beam wander, fluctuation of optical intensity and loss of spatial coherence. The purpose of this paper is to present the measurement of the distribution of the relative optical intensity variance in the Gaussian beam. The measurement of the spatial coherence is also presented.