MIMO Hybrid FSO/RF System Over Generalized Fading Channels

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

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.

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.

In this paper, the error performance analysis is performed for the decode-and-forward (DF) based dual hop mixed radio frequency-free space optical (RF-FSO) communication system with a direct RF link. The direct link between the source-destination (S-D) and the link between source-relay (S-R) are Nakagami-m distributed asymmetric radio frequency (RF) links and the R-D link is a free space optical (FSO) link characterized by the path loss, Gamma-Gamma distributed turbulence, and pointing errors. Selection combining diversity scheme is used to evaluate the performance of this dual branch cooperative system. For the considered system, novel closed-form mathematical expressions of cumulative distribution function and probability density function of instantaneous signal-to-noise ratio are derived. Using these, new mathematical expressions are obtained for the outage probability and the average bit error rate (BER) for various modulation techniques. The effects of fading, turbulence, and pointing errors are observed on the outage probability and the BER of the considered system. These performance metrics are also compared with those of a DF based dual hop mixed RF-FSO system without direct link. The improvement in the performance is observed due to the diversity introduced by addition of RF link.

In order to create a connection in a specific remote area of a skip zone where an optical fiber cable or wireline solution is difficult to build, free space optics (FSO) is essential. For 5G and future transmission networks, FSO connection has been selected as a potential option for secure, low-latency, license-free spectrum and less costly wireless backhaul connectivity. The main obstructions to FSO transmission are adverse weather conditions as well as the need for a direct line of sight (LoS) from Source to Destination. An innovative approach termed intelligent reflecting surface (IRS) based FSO transmission is a most reliable and efficient interesting approach, similar to first-last mile access connection, to relax the line of sight condition of FSO transmission. In this study, IRS-assisted radio frequency (RF), as well as optical IRS-assisted FSO connections are modeled. The FSO link via the IRS module achieves 10–6 value of symbol error rate (SER) at 30 dB SNR while the FSO link without IRS module achieves only 10–3 and 10–2 values of SER at the same SNR for the FSO link is modeled with Gamma-gamma and generalised Malaga (M) distributions, respectively. The SER value is enhanced up to a factor of 10–8 at 20 dB SNR in case of multiple optical-IRS-based FSO connections by increasing additional reflecting elements module (N).

In this paper a novel hybrid Free Space Optical (FSO)-Radio Frequency (RF) model is proposed which is based on the single decision threshold feedback bit technique. The FSO and proposed hybrid FSO-RF link performance is investigated under strong turbulence and misalignment aperture error or pointing error in terms of the average symbol error rate (ASER) and outage probability. Atmospheric turbulence and misalignment aperture are foremost factors that affect the performance of FSO link which entails the requirement of an RF link as a backup to enhance the system performance. The FSO-RF link is preferred in this work over the normal FSO link due to ease in switching among the link established with the help of decision threshold during failure of FSO link. In this proposed hybrid model, Gamma-Gamma distribution is considered as free-space optical turbulence channel and Nakagami-m model is for RF link. The analytical expressions for outage probability, ASER have been derived and results are given for various Subcarrier Intensity Modulation—M-Phase Shift Keying (PSK) schemes. A performance comparative study is reported for FSO link and hybrid FSO-RF link. The obtained result indicates the enhanced system performance whereas the obtained results are validated through Monte-Carlo simulations and results are found in good agreement.

Observation of the Earth from aerial and spaceborne platforms using synthetic aperture radar (SAR) has provided scientists and researchers with a deeper knowledge of the physical surface information of the planet. With the aim of launching our own SAR sensor into orbit for the purpose of remote sensing, we have a program to test our systems in an airborne platform. Prior to that, we need to first test all related on-board subsystems of SAR sensor on the ground. A semi-automated rail enabled ground-based SAR imaging test-bed was developed and is able to function carrying our L-band SAR systems. Part of the challenge in this project is the scale, as related L-band SAR system is relatively larger than systems of higher bands. The target and scan area covered in the ground measurement is also large, leading to a rail design that need to be extendable yet still mobile. This paper explains our system's radio frequency (RF) subsystem, which generates, transmits and receives the electromagnetic waves and also the rail-based Positioner Subsystem. In addition, the paper also shows the SAR system design specifications, how they are applied into the RF subsystem, and its implementation with the moving platform of the Positioner Subsystem in a field test.

Due to its low out‐of‐band emission and high spectral efficiency, the generalized frequency division multiplexing (GFDM) has been adopted as one of the most prominent waveform techniques for the 5G networks. To further combat the deep fading, the space time coding (STC) can be integrated with the GFDM system which provide high diversity gain that results into low symbol error rate at the receiver. In this article, a generalized model of space time coded GFDM system (STC‐GFDM) has been proposed. More precisely, a comprehensive analytical framework to compute the average symbol error rate (ASER) is introduced, which can be used for any MIMO configuration, code rate, and arbitrary fading channel. The proposed framework has wider applicability over generalized fading channels and having more accurate exact solution along with easy to evaluate approximate solution. Thus, the proposed framework extends the horizon of performance analysis of STC‐GFDM system over any fading scenario, that is not covered so far in the literature. In general, the closed from expressions of exact and approximate along with asymptotic ASER are derived for different fading channels using moment generating function (MGF) approach. Particularly, the ASER analysis has been performed analytically using derived expression over Rayleigh, TWDP, Nakagami‐q, and Nakagami‐m fading channels for‐ary quadrature amplitude modulation (‐QAM). This analysis confirms that with the increase in transmit and receive antenna and decrease in modulation order, the ASER performance improves for all fading channels. Also, the improvement in ASER is observed as the power of specular components (ie, LOS paths) increases. Furthermore, these analytical results are validated using Monte‐Carlo simulation.

Wireless radio-frequency (RF) technologies has had universal wide-scale deployment. Despite this, the practical limitations of the RF spectrum being unable to meet the challenges (e.g. lack of security, high interference, limited bandwidth, scalability) of RF based communication networks have become increasingly apparent over the past decade. With the ever-growing of data heavy wireless communications, especially on the last mile (e.g. wireless mesh network) and the backbone, methods are required to help addresses the problem RF have been facing for the next generation network. Faced with such daunting obstacles in RF-only networks, the use of Free Space Optical (FSO) for wireless communications was proposed. FSO is a promising solution to security, limited spectrum bandwidth, the scalability problem of wireless mesh networks, but also the advantage of large transmission distance, free license, interference immunity, and high-bandwidth. Despite the major advantages of FSO technology, its widespread use has been hampered by atmospheric turbulence-induced fading. However, FSO is still consider to be a practical solution to RF challenges. To maximize the potential of FSO networks, we study the problem of maximum path resource flow for a route and minimum linear delay while the FSO topology under atmospheric turbulence. Such methods can be easy adopted to SDN to manage the data flow at the link-or network-layer.

This paper proposes a coherent orthogonal frequency division multiplexing (OFDM) free space optical (FSO) communication system model consisting of parallel channel state information (CSI)-assisted amplify-and-forward (AF) relays over Exponential Weibull (EW) atmospheric attenuation channel. Using quadrature amplitude modulation (QAM) as a subcarrier mapping mode for OFDM signals. Considering the joint effects of path loss, pointing errors and atmospheric turbulence, the closed-form expressions of the Meijer G form of moment generating function (MGF), probability density function (PDF), and cumulative distribution function (CDF) are derived. Using these results, the closed-form expressions for the lower bounds of the outage probability and the symbol error rate (SER) of the proposed system are derived using Meijer's G-function. In addition, the influence of atmospheric turbulence intensity, number of subcarriers, mapping coefficient and other important factors on outage performance and SER are analyzed by simulations. The simulation results show that the relaying technology can increase the link range and improve the performance of the FSO communication systems.

The Fisher-Snedecor <tex>$F$</tex> distribution has been recently proposed as an experimentally verified and tractable turbulence induced fading model (TIFM) for free space optical (FSO) communications. This paper provides outage probability (OP) and higher-order (HO) performance analysis of the product of <tex>$N$</tex> independent but not identically distributed (i.n.i.d) Fisher-Snedecor <tex>$F$</tex> random variates (RVs). Accurate and closed-form (C-F) expressions for cumulative distribution function (CDF), level crossing rate (LCR) and average fade duration (AFD) of N-Fisher-Snedecor <tex>$F$</tex> distribution are successfully derived. The general property of a Laplace approximation approach for evaluation of N-folded complex integral-form (I-F) LCR expressions has been applied. The obtained statistical results are directly related to the performance evaluation of N-hop FSO communication links over weak, moderate and strong atmospheric turbulence conditions.

In this letter, we investigate the error rate performance of two-way amplify-and-forward (AF) relaying communication system over independently but not necessarily identically distributed (i.n.i.d.) cascaded Nakagami- <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$m$</tex></formula> fading channels. We have derived a closed-form expressions of cumulative distribution function (CDF) and moment generating function (MGF) for considering system. By using these expressions, we analyse the performance of two-way AF relaying system in terms of outage probability and symbol error rate (SER). Numerical and simulation results show the validity of the proposed mathematical analysis and point out the confirmation of the analytical results.

Performance study of wavelength diversity serial relay OFDM FSO system over exponentiated Weibull channels

Free Space Optical (FSO) communications is the only viable solution for creating a three-dimensional global communications grid of inter-connected ground and airborne nodes. The high amount of data exchange between satellites and ground stations demands enormous capacity that can not be provided by strictly regulated, scarce resources of the Radio Frequency (RF) spectrum. Free Space Optical (FSO) communications, on the other hand, has the potential for providing virtually unlimited bandwidth. Furthermore, due to the spatial confinement of laser beams, such links are very secure. In other words, security is guaranteed at the physical layer. However, the promised enormous data rates are only available under clear weather conditions, and atmospheric phenomena such as clouds, fog, and even turbulence can degrade the performance, dramatically. While scattering media such as clouds and aerosols cause pulse broadening in space and time, turbulence presents itself as scintillation and fading. Hence, to exploit the great potential of FSO at its best under all weather conditions, prudent measures must be taken in the design of transmitter and receiver. More specifically, multiple transmitters and receivers can be used to de-correlate the turbulence induced fading and to compensate for pulse attenuation and broadening caused by scattering. In this paper, Multiple-Input Multiple-Output (MIMO) transmitter and receiver designs in FSO communications are investigated and the achievable performance improvements are discussed.

SER performance investigation of a MPPM relay-aided FSO system with three decision thresholds over EW fading channel considering pointing errors