Performance Analysis of IRS-Assisted Multi-Link FSO System Under Pointing Errors

Relay-assisted cooperative free space optical (FSO) communication system is a powerful technique, which offers advantages like fading mitigation and significant gain in spatial diversity due to shorter transmission hops. Even so, it can be hampered by jamming activities which in turn degrade the overall system performance. In this paper, the effect of relay jamming in the decode-and-forward (DF) protocol based cooperative FSO communication system, is introduced and studied thoroughly. The probability density function (pdf) of a random variable containing mixture of the negative exponential and Gaussian random variables is derived. The error performance of the considered FSO system is analysed over this new kind of additive mixture noise for different probabilities of jamming. By utilizing the derived pdf, a closed-form expression of the average bit error rate (BER) of the DF FSO relaying is obtained. A comparison of the BER performance of maximum-likelihood (ML) detector and a derived threshold based detector is also provided. It is observed that the derived threshold based detector offers almost similar performance as that of the ML detector. A thorough numerical study is also provided by considering different pointing error parameters of jammer's and user's channels.

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.

Free space optical (FSO) communication system is a groundbreaking technology in the field of communication systems. Non-orthogonal multiple access (NOMA), on the other hand, currently being used in fifth-generation (5G) wireless systems, is a powerful technique for establishing communication for multiple users using the same time and frequency resources. In this paper, the performances of NOMA and orthogonal multiple access (OMA) in an FSO-based communication system are studied. In the FSO communication system, two base station units are considered to be connected to a central unit for FSO backhauling using an uplink-fixed NOMA scheme. Bit error rate (BER), ergodic capacity, and energy efficiency (EE) performances of the NOMA-based FSO system are studied and compared with an OMA-based FSO system. It is found that the NOMA-based system provides approximately 10% of the ergodic capacity gain and increases EE by 37%-60% for a given BER compared to the OMA-based system.

In this work, we explore the use of intelligent reflecting surfaces (IRSs) as transparent relays in free-space optical (FSO) links for low earth orbit (LEO) satellites constellations. We introduce a novel IRS design featuring a slightly convex surface geometry to broaden the reflected beam, thereby mitigating the beam alignment challenges typically encountered in satellite-based FSO systems. We investigate the impact of IRS size, curvature, and transmitter beam width on end-to-end system performance, taking into account attenuation, atmospheric turbulence, and pointing errors. To accommodate the effects of surface curvature, we enhance and refine the channel pointing model and derive novel closed-form expressions for key performance metrics, including bit error rate, outage probability, and ergodic capacity. Numerical results based on realistic system parameters confirm the effectiveness of the proposed approach, albeit at the cost of increased geometric channel losses. Finally, we carry out numerical simulations to validate the new channel model and derived closed-form expressions, providing valuable insights for the design of future non-terrestrial networks architectures based on FSO systems.

The performance of a free space optical (FSO) communication system is significantly affected by various atmospheric turbulence conditions and pointing errors (PEs) apart from the additive noise, which is assumed to be Gaussian. Optical pre-amplifiers are an essential component of FSO systems for improving the receiver sensitivity. However, optical pre-amplification results in amplified spontaneous emission (ASE), which dominates the receiver thermal and shot noises. The square law photodetection process at the receiver in a FSO system necessitates the consideration of chi-square statistics for the decision variable contrary to the Gaussian approximation that is widely used in the literature. In this paper, we evaluate the bit error rate (BER) performance of a FSO system assuming non-return-to-zero on–off keying modulation in the presence of ASE noise under weak, moderate to strong, and very strong atmospheric turbulence regimes and PEs. We also derive asymptotic BER expressions for the considered FSO system for large values of the signal-to-noise ratio in terms of simple elementary functions. Further insight into the system is provided by performing a diversity analysis.

This paper presents a constructive discussion and inclusive review on advancements in the coded modulation free-space optical (FSO) communication system and corresponding techniques. These techniques mainly include channel model, forward error correction (FEC) and modulation schemes. Firstly, a complete view of FSO system is presented which contains the description, the current research situation along with the advantages over the traditional radio frequency (RF) wireless communication and fiber optical communications. Then, the channel model, FEC, modulation schemes and complete system of coded modulation FSO are analyzed successively. At last, the review work of coded modulation FSO system is summarized and further improvements are prospected.

Free-space optical (FSO) communication systems employ unguided light beams propagating through the atmosphere to carry a large volume of data. The reliability of such data transfer can be hampered by various atmospheric effects. Based on an analytical model of a differential phase-shift keying FSO system through exponentiated Weibull turbulence, we investigate the effectiveness of beam width optimization and improved beam alignment, along with aperture averaging on the average channel capacity. Our results show significant signal deterioration produced due to the aerosol-induced optical turbulence, which substantially shadows the performance gain achieved through beam width optimization. Strong aerosol-induced atmospheric heating and the consequent enhanced optical scintillations result in reduction of the channel capacity by as much as 50% of its value when these effects are not considered or negligible. FSO systems are more resilient to aerosol-induced optical turbulence when the normalized beam width is less, and the average channel capacity can be significantly improved by improved beam alignment. These variations are weakly dependent under poor transmitter-receiver alignment conditions. Furthermore, the receiver aperture has a strong control on the link performance. While FSO systems with higher magnitude of normalized beam width have improved performance under all aperture diameter conditions; for a given beam configuration, large aperture diameter ensures a significant improvement in the link performance due to reduction in effects of scintillations.

This article investigates physical-layer security (PLS) of a typical wireless-powered communication (WPC) system with the aid of intelligent reflecting surface (IRS) in the presence of a passive eavesdropper for Internet of Things (IoT), and proposes three IRS-aided secure WPC modes. Specifically, in mode-I, the IRS is deployed between hybrid access point (HAP) and wireless user (U) for co-located power station (PS) and access point (AP). In mode-II, the IRS is deployed between AP and U for separate PS and AP, while in mode-III, the IRS is deployed between PS and U for separate PS and AP. For each mode, the optimal phase shift is designed to maximize the reception of energy and information at the legitimate receiver. We comprehensively analyze the performance of each mode, and derive the closed-form expressions of connection outage probability (COP), secrecy outage probability (SOP), and effective secrecy throughput (EST) for each mode, respectively. The theoretical analysis and simulation results reveal that from the perspective of reliability, mode-I can achieve the best COP with the increased number of IRS elements, while mode-II and mode-III have a similar COP. From the perspective of security, mode-II can achieve the best SOP with the increased number of IRS elements, while mode-I and mode-III have a similar SOP. Moreover, under the condition of small transmission power at HAP/PS or small number of IRS elements, mode-I has the best EST, while as the power or the number increases, the EST of mode-II becomes the best one.

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.

An intelligent reflecting surface (IRS) assisted wireless communication system, with statistically correlated channels between the transmitter and the IRS pair and between the IRS and the receiver pair, is studied in the absence of channel state information (CSI). Assuming the phases added by IRS elements to be deterministic, the closed-form analytical expression for the symbol error probability (SEP) and the upper bound on the channel capacity are derived, with the transmitter employing binary phase-shift keying (BPSK) modulation for data transmission. The asymptotic SEP expression is derived and the achievable diversity order of the system is shown to be independent of the number of IRS elements. A search based algorithm is proposed to obtain the optimal phase at IRS minimizing the average SEP, as a equal phase configuration. The optimal equal phase is also found to maximize the average channel capacity. Numerical results are presented for the case of channels following a practical IRS correlation model.

In many intelligent reflecting surface (IRS) aided communication systems, the IRS is deployed to provide a line-of-sight (LOS) link between the transmitter and receiver. In order to maximize the efficiency of the system, deploying the IRS at the correct position and orientation is important. We propose a method to find the optimal IRS orientation that maximizes the received power or achievable rate at locations within a desired geographical area. As shown through both analytical and simulation results, the proposed methodology on optimizing IRS orientation based on the receiver or transmitter locations significantly improves the overall system performance. For example, there is an approximately 8.1 bps/Hz achievable rate improvement by installing the IRS at the optimal orientation compared to an arbitrary orientation in a typical deployment scenario.

Passive technologies like ambient backscatter communication (AmBC) and intelligent reflecting surface (IRS) hold enormous potential for ubiquitous connectivity of Internet of Things (IoT) devices in future wireless networks. However, despite many research efforts, AmBC has so far been unable to fulfil its potential for widespread deployment, due to the weakness of backscatter signal in the presence of strong direct-link interference from the original RF source. This limits the bit error rate (BER) and hence the transmission rate and range of AmBC systems. Meanwhile, the IRS offers new degrees of freedom in enhancing a variety of systems by transforming their propagation medium and signals. In this work, we devise a novel scheme to improve the detection performance of an AmBC system using an IRS located in its proximity. The IRS augments the backscatter signal quality at the receiver by adjusting its phase shifts to balance signal strengths, ultimately improving the performance of energy detection at the receiver. Our results clearly show that an IRS of reasonable size can considerably improve the BER performance of ambient backscatter, which is an important improvement for low power IoT systems.

This study investigates the integration of an intelligent reflecting surface (IRS) into an unmanned aerial vehicle (UAV) platform to utilize the advantages of these leading technologies for sixth-generation communications, e.g., improved spectral and energy efficiency, extended network coverage, and flexible deployment. In particular, we investigate a downlink IRS–UAV system, wherein single-antenna ground users (UEs) are served by a multi-antenna base station (BS). To assist the communication between UEs and the BS, an IRS mounted on a UAV is deployed, in which the direct links are obstructed owing to the complex urban channel characteristics. The beamforming at the BS, phase shift at the IRS, and the 3D placement of the UAV are jointly optimized to maximize the sum rate. Because the optimization variables, particularly the beamforming and IRS phase shift, are highly coupled with each other, the optimization problem is naturally non-convex. To effectively solve the formulated problem, we propose an iterative algorithm that employs block coordinate descent and inner approximation methods. Numerical results demonstrate the effectiveness of our proposed approach for a UAV-mounted IRS system on the sum rate performance over the state-of-the-art technology using the terrestrial counterpart.

As an emerging technology, intelligent reflecting surface (IRS) has attached attention of academia because it can customize a favorable wireless propagation environment. However, for the traditional passive structure, IRS can merely alter the phase of the incident signal, which limits the maximum attainable beamforming gain. In order to give full play to the potential of IRS, in this paper, we employ an active IRS, which is able to alter the phase and amplify the amplitude of the incident signals simultaneously because there exists an additional power supply. So as to improve the performance of active IRS-assisted communication system, the reflection matrix of the IRS and the beamforming vector of the base station (BS) are jointly considered and optimized to minimize the transmit power of the BS. Design the algorithm of power resource allocation usually be converted into an optimization problem, and must meet the maximum power margin of active IRS and satisfy the quality of service (QoS) requirements of users. Aiming at solving the nonconvex problem, we designed a high calculation efficiency algorithm on the strength of the internal approximation (IA) method and bilinear transformation. The algorithm guarantees convergence to the local optimal solution of the problem under consideration. Simulation indicates that this scheme is effective compared with the two blank control schemes. In addition, the results indicate that compared with passive IRS, deploying active IRS has great advantages for boosting the performance of communication systems, especially in the presence of strong direct links.

Nowadays, transmitting and receiving data with high speed and a high level of security are the main demands. So, a new model of spectral amplitude coding optical code division multiple access (SAC-OCDMA) is suggested in this paper, based on a free space optical (FSO) communication system using coherent sources. Three different codes: enhanced double weight (EDW), modified double weight (MDW) and multi-diagonal (MD) codes are assigned to our proposed model with the direct detection (DD) technique. Furthermore, the model is simulated under different weather conditions including clear air (CA), light mist (LM), very light fog, and light fog (LF). The system performance is evaluated through its bit error rate (BER), Q-factor, received power, and signal to noise ratio (SNR). Moreover, classification of the information received by the three different SAC-OCDMA using three different codes (EDW, MDW, and MD) is still challenging. So, two different machine learning (ML) algorithms are used, in this study, to classify the data received using the different codes. Detecting which code is received at the receiver end is important in order to reduce code error detection. Two algorithms: K-Nearest Neighbor (KNN) and Support Vector Machine (SVM) are adopted to classify different codes used for data transmission under four different weather conditions. The ML input dataset consists of the obtained simulation results, including Q-factor, BER, and SNR. Each feature is to be normalized before using ML. The obtained results show that the performance of the proposed FSO model achieved the longest propagation range under CA weather conditions, 7 km, while the shortest is under LF, which is 0.98 km. This is due to the attenuation of fog that causes signal degradation. The FSO system that uses EDW gives the best result under different weather conditions, while the system that uses MD code gives the worst performance. Also, the highest power is achieved when the EDW code is used at 5.5 km. The EDW has a received power of − 21.58 dBm, while the received power is − 22.04 dBm and − 23.8 dBm for MDW and MD codes, respectively. As for classification between the received information coming from three different codes under different weather conditions, both algorithms, KNN and SVM, achieve promising results in most cases. They showed more than 97% of classification accuracy under CA, LM, and LF weather conditions.