Performance Optimization of FSO link via Intelligent Reflecting Surfaces (IRS)

On the maximal-ratio combining of FSO and RF links over generalized distributions and its applications in hybrid FSO/RF systems

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.

In this paper, we present and experimentally evaluate a real-time 10 Gbps free-space optical (FSO) link under varying atmospheric conditions. In bandwidth-craving wireless technologies due to the ubiquitous consumption by internet of things devices and requiring high data rate online services, unlicensed FSO systems can be a promising candidate to satisfy the network capacity of the existing data communications technologies. In this work, we verify the experiment using small form-factor pluggable transceivers mounted FPGA as a FSO transmitter and receiver. Here, a high-speed single FSO link is proposed and its performance under turbulence and fog conditions using the dedicated indoor atmospheric chamber is evaluated. We show that the proposed system under the turbulence condition with a scintillation index of 0.35 offers the same data rate as the link under a clear channel, while the bit error rate increases from 10−12 to 5 × 10−4.

We propose a framework for providing instantaneous backup to traffic in a hybrid RF/FSO mesh network. Free space optical (FSO) links have high bandwidth and security, making them suitable for use in backbone networks. RF links have low bandwidth, but offer high reliability in conditions where FSO links are obscured. Thus, RF links are primarily used to provide instantaneous backup to traffic flowing on FSO links. We propose a framework to model FSO link failures due to obscuration, and propose algorithms for integrated topology control of RF links and routing for maximizing the backup provided. We do extensive simulations to demonstrate that the performance of RF topology and routing computed using our algorithms is much better than the case of having the same RF and FSO topologies and routes.

Free-space optical (FSO) links offer an attractive alternative to the radio frequency channel for the purpose of transmitting data at very high rates. A major impairment in FSO links is the turbulence-induced scintillations of an optical wave. Spatial diversity, which involves the deployment of multiple laser transmitters/receivers, can be used over FSO links to reduce the turbulence-induced scintillations. In this paper, the channel of a multiple-transmitter-multiple-receiver FSO link considering the spatial correlation between the constituent sub-channels was mathematically modeled and then the joint distribution function of the channel gain coefficients was derived. Base on the joint distribution function, the model for the outage probability of a FSO link with spatial diversity was established. With Monte Carlo method, simulations for the outage probability were performed. The results show that spatial correlations have a significant influence on the outage probability, and the diversity-induced performance improvement reduces to zero as the spatial correlation coefficient increases to 1.

Free Space Optics (FSO) has become a viable, high-bandwidth wireless alternative to fiber optic cabling. The primary advantages of FSO over fiber are its rapid deployment time and significant cost savings. The disadvantage of FSO over fiber is that laser power attenuation through the atmosphere is variable and difficult to predict, since it is weather airports, the link availability as a function of distance can be predicted for any FSO system. These availability curves provide a good indication of the reasonable link distances for FSO systems in a particular geographical area. FSO link distances can vary greatly from desert areas like Las Vegas to heavy-fog cities like St. Johns NF. Another factor in determining FSO distance limitations is the link availability expectation of the application. For enterprise applications, link availability requirements are generally greater than 99%. This allows for longer FSO link ranges, based on the availability curves. The enterprise market is where the majority of FSO systems have been deployed. The carriers and ISPs are another potential large user of FSO systems, especially for last-mile metro access applications. If FSO systems are to be used in telecommunication applications, they will need to meet much higher availability requirements. Carrier-class availability is generally considered to be 99.999% (5 nines). An analysis of link budgets and visibility-limiting weather conditions indicates that to meet carrier-class availability, FSO links should normally be less than 140m (there are cities like Phoenix and Las Vegas where this 99.999% distance limitation increases significantly). This calculation is based on a 53 dB link budget. This concept is extended to the best possible FSO system, which would have a 10 W transmitter and a photocounting detector with a sensitivity of 1 nW. This FSO system would have a 100 dB link margin, which would only increase the 99.999% link distance to 286 m. A more practical solution to extending the high availability range would be to back up the FSO link with a lower data rate radio frequency (RF) link. This hybrid FSO/RF system would extend the 99.999% link range to longer distances and open up a much larger metro/access market to the carriers. It is important to realize that as the link range increases, there will be a slight decrease in overall bandwidth. To show the geographical dependence of FSO performance, the first map of FSO availabilities contoured over North America is presented. This map is the first step to developing an attenuation map for predicting FSO performance, which could be used in similar fashion to the International Telecommunication Union (ITU)/Crane maps for predicting microwave performance.

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.

This paper investigates a hybrid framework that combines free space optics (FSO) and terahertz (THz) technologies for a backhaul network. In this hybrid configuration, the FSO and THz systems are empowered by reconfigurable intelligent surface (RIS) and are combined at the receiver using either maximal ratio combining (MRC) or selection combining (SC). The performance of the RIS-assisted hybrid FSO/THz system is evaluated under multiple impairments for FSO and THz links. The exact closed-form expressions for probability density function (PDF), cumulative distribution function (CDF), and moment-generating function (MGF) of the output signal-to-noise ratio (SNR) of both RIS-assisted FSO and THz links are derived. Leveraging these SNR statistics, we compute the outage probability and the average symbol error rate (SER) of the proposed hybrid FSO/THz system with MRC and SC schemes. Additionally, this paper conducts an asymptotic analysis at high SNR levels, yielding asymptotic expressions for outage and average SER, which not only reduce computational complexity but also provide insights into the system’s diversity gain. In the numerical results, the performance of the proposed hybrid FSO/THz system is compared against other state-of-the-art wireless systems, including direct link FSO without RIS, RIS-aided FSO, and the hybrid FSO/THz systems without RIS using diversity combining techniques.

Internet of things (IoT) with continuous development allows multiple devices to be connected to each other through the external environment. With that, free space optical (FSO) transmission links provide high data transmission rates, so that enhanced quality of services, that is very suitable for deployment for fifth generation (5G) and IoT. FSO is known as license free, cost effective and line-of-sight green communication technology, which is employed in various circumstances, such as high data rate connections between buildings within campus or city. This study performs enhancement of FSO link for reconfigurable intelligent surfaces (RISs) aided over free space with gamma-gamma distribution channels. The paper analyzes the performance of FSO links affected by misalignment fading, RISs aided, and link distance with the subcarrier quadrature amplitude modulation scheme. Several numerical outcomes obtained for average electrical end-to-end signal-to-noise ratio, misalignment fading displacement standard deviation and link distance are shown to illustrate the average channel capacity of systems.

Free-space optics (FSO)-based, high-altitude plat form (HAP)-assisted backhaul network has recently attracted research efforts worldwide. Nevertheless, one of the critical concerns on HAP-assisted FSO links is cloud coverage, which may block the FSO connections completely. This paper explores a novel solution that uses multiple unmanned aerial vehicles (UAVs) equipped with an intelligent reflecting surface (IRS) array. These UAVs are deployed to diverse the FSO link from HAP-to-ground base station (BS) to avoid cloud coverage. We assume the Fisher-Snedecor $\mathcal{F}$ model for the atmospheric turbulence and use a selection combing (SC) receiver to obtain signals from multiple UAVs. We analytically derive the probability density function (PDF) of the received end-to-end signal-to-noise ratio (SNR) by employing the moment matching method, which can obtain an accurate approximation of PDF to the product of $\mathcal{F}$ variables. Using the derived statistics, we investigate different system performance metrics, including outage probability, outage capacity, and average bit error rate (BER). Finally, Monte Carlo simulations are provided to validate analytical results.

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

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.

We study the performance of multi-hop networks composed of millimeter wave (MMW)-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in the cases with and without hybrid automatic repeat request (HARQ). Taking the MMW characteristics of the RF links into account, we derive closed-form expressions for the network outage probability. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we present mappings between the performance of RF- FSO multi-hop networks and the ones using only the RF- or the FSO-based communication, in the sense that with appropriate parameter settings the same outage probability is achieved in these setups. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, the HARQ can effectively improve the outage probability#x002F;energy efficiency, and compensate the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual- hop networks, outage probability 10^{-4} and code rate 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to the cases with no HARQ, by 13 and 17 dB, respectively.

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.

Free space optical (FSO) wireless communication has emerged as a viable alternative to the existing fiber optics and radio frequency (RF) communications due to its ability to operate in unlicensed spectrum, offering huge data handling capacities and being cost effective with easy deployment. The underlying communication mechanism in the FSO link is based upon the laser beam propagation model. The propagating free space optical beam undergoes signal degradation due to refraction and diffraction caused by atmospheric turbulence measured by refractive index structure parameter (C n 2). In this work, a novel, robust analytical model for propagating Gaussian-beam through atmospheric turbulence is presented, by solving the space-fractional paraxial wave equation. We report analytical expressions for the intensity and long-term beam spreading of a Gaussian beam in terms of space-fractional parameter D, for the range 2 < D ≤ 3. This range of parameter D, defines the effective number of euclidean space corresponding to atmospheric turbulence levels faced by the propagating Gaussian beam in the FSO link. The results of the proposed fractional model and the existing models agreed well, therefore the D-dimensional parameter can be used to effectively express the value of refractive index structure parameter (C n 2). The classical values of C n 2 ranges form 10−13 to 10−16 for strong to weak fluctuations respectively. However, in fractional-dimension scale, these fluctuations can be express as D = 2.668 (strong fluctuations) to D = 2.999 (weak fluctuations). The ideal case of free space beam propagation can be expressed as D = 3, with no turbulence. Moreover, we have studied the fractional-dimension model performance for varying wavelengths. Further, based upon a practical example, we proposed a self-sustainable FSO link architecture to efficiently minimize the effect of weak and strong fluctuations on the propagating optical beam, based upon the D-dimension fractional parameter, hence ensuring reliability of FSO link.