MOEMS-based Lens-Assisted Beam Steering for Free-Space Optical Communications

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.

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.

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.

In free-space optical communication (FSOC) systems, atmospheric turbulence can bring about power fluctuations in receiver ends, restricting channel capacity. Relay techniques can divide a long FSOC link into several short links to mitigate the fading events caused by atmospheric turbulence. This paper proposes a Reinforcement Learning-based Relay Selection (RLRS) method based on Deep Q-Network (DQN) in a FSOC system with multiple transceivers, whose aim is to enhance the average channel capacity of the system. Malaga turbulence is studied in this paper. The presence of handover loss is also considered. The relay nodes serve in decode-and-forward (DF). Simulation results demonstrate that the RLRS algorithm outperforms the conventional greedy algorithm, which implies that the RLRS algorithm may be utilized in practical FSOC systems.

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

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.

In recent years, free space optical communication (FSO) has gained amorous importance and become an interesting alternative to radio frequency communication because of its numerous benefits over radio frequency communication. It is efficient, low power, high data rate, unlicensed spectrum, and large bandwidth wireless technology. Despite, these advantages FSO is prone to the atmospheric attenuation, but in order to compensate this problem the MIMO FSO system is considered to be a good solution. This paper analyzes the performance of 4*4 MIMO FSO system using different modulation schemes, NRZ, RZ, and CSRZ. The impact of varying, Link range, attenuation factor and transmits power on the performance of 4*4 MIMO FSO has been observed in terms of Quality factor and eye diagram while keeping the other parameters constant. The results show that the RZ modulation scheme using 4*4 MIMO FSO technique show better performance as compare to NRZ and CSRZ. For low attenuation and small link, RZ is superior to NRZ and CSRZ while for higher attenuation and larger link the performance of RZ and NRZ is equivalent. Overall RZ performs better than NRZ and CSRZ. It is also evident from the results that as the number of transmitter and receiver are increased, the efficiency of the system increases. With MIMO technology the performance of FSO system improves to a larger extent.

In this study, Mie and Rayleigh scattering in free space optics (FSO) communication systems were investigated in terms of the atmospheric attenuation. Because of the movement of the Earth, the communication distance and surrounding gas densities are inconsistent in each region. This change leads to atmospheric attenuation and then data losses and inefficient communication in FSO occur. Therefore, the density change and distance must be calculated in each communication once the data is transmitted. In the literature, it was observed that the atmospheric attenuation is regarding some FSO communication parameters such as transmission distance, visibility, and scatter particle size distribution, the number of particles per unit volume, scatter cross-sectional area, and wavelength. Besides, in real-time communication, it is necessary to update FSO parameters simultaneously. However, this updating process for all parameter takes a long time to adapt to a new position. This paper proposes the design of the experiment method (Doe) to determine the severity of the FSO parameters. And Taguchi’s Doe method allows analyzing of FSO communication system parameters to avoid long calculation time. Results show that the proposed method helps in understanding the priorities of the parameters in FSO and reducing the updating time.

An enhanced transmission is presented in a multiple‐input‐multiple‐output (MIMO) dense‐wavelength division multiplexed (DWDM) free‐space‐optical (FSO) communication link using diversity coding techniques under the effect of turbulent weather phenomenon. The findings show good performance with an (8 channels × 2.5 Gbps data rate/channel) 20 Gbps 1500 m transmission distance. The bit‐error‐rate (BER), outage probability (OP), and signal‐to‐noise ratio (SNR) of the diversity combining techniques using maximum‐ratio combining (MRC), selection combining (SC), and equal‐gain combining (EGC) technique are evaluated in this work. The obtained results illustrate that Alamouti, space‐time coding (STC), space‐time block coding (STBC), space‐time trellis code (STTC), orthogonal STBC (O‐STBC), and quasi‐orthogonal STBC ( QO‐STBC ) on the minimum mean‐square‐error, and MRC are worth implementing on the DWDM‐FSO wireless communication systems. The mitigation of atmospheric turbulence is achieved using MIMO diversity combining techniques coding. The simulation results for diversity coding techniques using QO‐STBC/STTC and SC/MRC in the MIMO‐DWDM FSO communication system can improve BER performance, OP, and SNR. The MRC exhibits the lowest OP and BER when compared with the SC and EGC. The numerical results demonstrate that the FSO communication link using DWDM QO‐STBC/STTC improves the power penalty at both BER values under varying atmospheric turbulence conditions for ST, MT, and WT, in comparison to FSO systems without DWDM QO‐STBC/STTC diversity coding techniques.

Performance of free space optical communication systems over exponentiated Weibull atmospheric turbulence channel for PPM and its derivatives

In this paper, we propose a simultaneous fine tracking and signal detection subsystem based on QD (Quadrant Detector) for FSO (Free Space Optical communication) system in 1550nm without beacon. Then we test the sensitivity, resolution, SNR characteristics and the influence the size of light spot on QD by building an indoor experiment. We select 1550nm waveband which suffers relatively less from background radicalization and atmospheric channel as the waveband of transmitter and receiver, also our selection catered for the trend of FSO system. We use the same QD for both fine tracking and signal detection for FSO in our design. Besides we use the scheme without beacon beam, for which we reduce the use of a beacon laser source, so we can obtain weight reduction and size decrease which is very important for FSO system.

In a free space optical communication (FSOC) system, atmospheric turbulence will increase the bit error ratio (BER) and impair FSOC link reliability. Since computational temporal ghost imaging (CTGI) has anti-interference, we present an FSOC system over atmospheric turbulence based on CTGI. The simulation results show that the BER performance of CTGI is better than on-off keying under different atmospheric turbulence regimes. To improve the performance of the CTGI scheme, the influence of the number of transmission samples and code length is analyzed. It is shown that BER performance improves with the increment of the number of samples, while code length has no impact. This scheme provides an idea for reliable communication over atmospheric turbulence and an important reference for improving wireless optical communication in an extreme environment.

A free-space optical (FSO) communication system that does not implement compensation strategies for the effects of beam wander may experience deep long-term channel fades or even severe communication outage when the laser spot wanders away from the receiver aperture due to the effects of atmospheric turbulence. The impacts of laser beam diverging angles on channel fading probability density function (pdf) and on bit-error-rate (BER) performance of both uncoded and coded FSO communication systems are studied for different receiver aperture sizes and propagation distances. The optimal diverging angles are determined according to the average uncoded and coded BER for both uncoded and coded system, respectively. For a fast-tracked FSO communication system, a focused beam always provides better channel fading, coded BER, and uncoded BER than beams with larger diverging angles. For an untracked system, smaller beam diverging angels have better BER performance and lower channel fading for short distances when beam wander is small compared with the receiver aperture size, and larger beam diverging angles are preferred for longer distances when beam wander becomes significant. The optimal beam diverging angles of the coded FSO communication systems are smaller than those of the uncoded systems.

In this paper, a rate-adaptive free-space optical (FSO) communication system using systematic Raptor codes is investigated based on experimental data measured over a 1.87 km terrestrial FSO link in different weather conditions. Using Raptor codes, the FSO communication system can adapt to the channel gain fluctuations caused by atmospheric turbulence and/or adverse weather conditions. To alleviate the burden of synchronizing between transmitter and receiver, messages are packetized and sent in sequence. For each packet, a cyclic-redundancy code (CRC) is added resulting in an effective packet erasure channel (PEC). Using experimental data and on-off-keying (OOK) modulation, the performance of the rate-adaptive FSO system is evaluated at different signaling rates. While an uncoded FSO system fails to efficiently operate during adverse weather conditions, the rate-adaptive technique provides reliable FSO communication under clear and rainy weather conditions.

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