Reconfigurable intelligent surface (RIS) is an emerging technology that can achieve reconfigurable radio propagation environments for beyond 5G/6G wireless systems by smartly tuning the signal reflection via a large number of low-cost passive reflecting elements. In this letter, we investigate the effect of co-channel interference (CCI) on RIS-based dual-hop mixed free-space optical (FSO)-radio frequency (RF) communication systems. The source node, equipped with multiple FSO apertures, transmits subcarrier intensity modulated symbols on the FSO link subjected to Gamma-Gamma (GG) distributed atmospheric turbulence with pointing errors (PEs). It is assumed that the relay-RIS and RIS-destination links follow Rayleigh distribution. The destination node is corrupted by multiple CCI (each following Rayleigh distribution). The performance of the considered system is evaluated by deriving novel closed-form expressions for the outage probability (OP) and bit error rate (BER). It is shown through numerical results that RIS-based dual-hop mixed FSO-RF system provide a significant performance enhancement in comparison to a traditional dual-hop mixed FSO-RF system even in presence of CCI.

Intelligent reflecting surface (IRS)-assisted wireless communication has recently become a promising technology to relax the line-of-sight requirement and also to provide a communication link in uncovered and remote areas. In this work, an alternate implementation of unmanned aerial vehicle (UAV)-based IRS-assisted hybrid free space optics/radio frequency wireless communications is presented. Phase shift errors occur due to the presence of reflecting elements in the IRS and affect the reflected beam. We consider the effect of phase shift error along with atmospheric turbulence, which is modeled as gamma-gamma distribution. Furthermore, a statistical model is considered for pointing errors due to the position and orientation fluctuation of the mounted IRS on the UAV. We derive closed-form expressions of the average symbol error rate and spectral efficiency of the considered system, under the combined effects of atmospheric turbulence, pointing error, and phase shift error. In addition, we present asymptotic analysis of the average symbol error rate of the system in a high SNR region and evaluate the diversity order and coding gain. The derived results are further validated using Monte Carlo simulation.

Underwater wireless optical communication (UWOC) refers to the transmission of data in unguided water medium through optical carriers. Beam spread function (BSF) characterizes the amount of light irradiance received at the receiver as a function of receiver's distance from main beam axis for a particular link length. The existing form of BSF includes multiple integrals which are burdensome to use for mathematical analysis. Because of this, it gets very difficult to employ the existing form of BSF to derive non-integral expressions for bit error rate (BER), capacity, and outage probability of UWOC systems. Moreover, it is very difficult to find any insight into the UWOC systems based on this integral form of BSF. In this paper, we derive a simplified power series expression of BSF by solving the existing integral form. Closed-form expressions of BSF with some approximations, which simplify the derived power series expression, are also provided as special cases. Important insights regarding the behavior of BSF are also obtained from the derived simplified forms. Furthermore, closed-form expressions of BER, capacity, and outage probability of the UWOC system, taking the effect of scattering, absorption, and misalignment in consideration, are also derived.

In this paper, we investigate the effects of jamming caused by a malicious unmanned aerial vehicle (UAV) on the performance of a free-space optical (FSO) communication system using a legitimate UAV as a relay and utilizing an intelligent reflecting surface (IRS) to improve the coverage. We statistically characterize the combined effects of the three impairments on the performance of the considered system: atmospheric turbulence (AT), pointing error (PE), and angle of arrival (AoA) fluctuation of both the UAVs. Incorporating the aforementioned effects, closed-form expressions of the overall average bit error rate (ABER) and outage probability (OP) for the considered system are derived. We also present a comprehensive study of the proposed system by considering different locations of the malicious UAV jammer and IRS and draw some interesting conclusions. Useful insights into the system performance are obtained through the asymptotic analysis. Furthermore, the analytically derived expressions are verified through Monte-Carlo simulations.

In this paper, an artificial noise (AN) injection technique is incorporated in a free-space optical (FSO) communication system with the aim of enhancing the secrecy performance of the system. An intensity modulated direct detection (IM/DD) FSO link which is subjected to Malaga (M) distributed turbulence with pointing errors is considered in this paper. The performance of FSO systems is evaluated by deriving novel closed-form expressions for the secrecy outage probability (SOP), strictly positive secrecy capacity (SPSC), and throughput of the system. By formulating a constrained optimization problem, we discuss an optimal power allocation strategy for throughput maximization in the considered system. It is shown through the results that the proposed technique is very effective in improving the secrecy performance of FSO systems.

In this paper, we investigate relay-assisted visible light communications (VLC) where a mobile user acts as a relay and forwards data from a transmitter to the end mobile user. We analyse the utilization of the amplify-and-forward (AF) and decode-and-forward (DF) relaying schemes. The focus of the paper is on analysis of the behavior of the mobile user acting as a relay while considering a realistic locations of the receivers and transmitters on a standard mobile phone, more specifically with two photodetectors on both sides of a mobile phone and a transmitting LED array located upright. We also investigate dependency of the bit error rate (BER) performance on the azimuth and elevation angles of the mobile relay device within a typical office environment. We provide a new analytical description of BER for AF and DF-based relays in VLC. In addition we compare AF and DF-based systems and show that DF offers a marginal improvement in the coverage area with a BER < 10^–3 and a data rate of 100 Mb/s. Numerical results also illustrate that relay-based systems offer a significant improvement in terms of the coverage compared to direct non-line of sight VLC links.