This paper presents the performance analysis of a fixed-gain amplify-and-forward dual-hop mixed radio frequency (RF)/free-space optics (FSO) communication system. To ensure broad applicability, the RF link is modeled using the κ -μ fading distribution, while the FSO link is characterized by the unified ℳ -distribution, which accounts for atmospheric turbulence and pointing errors. The FSO receiver employs both intensity modulation with direct detection and heterodyne detection techniques. In this study, we derive closed-form expressions for the average symbol error rate (ASER) of higher-order quadrature amplitude modulation (QAM) schemes, particularly coherent rectangular QAM and coherent hexagonal QAM, in terms of the Meijer-G function. Furthermore, the ASER for non-coherent modulation schemes, such as binary frequency shift keying and differential phase shift keying, is analyzed by deriving the moment-generating function of the end-to-end signal-to-noise ratio. To validate the analytical expressions, the numerical results are compared with Monte Carlo simulation results. Additionally, a comprehensive comparative study of different modulation schemes is conducted, highlighting the impact of pointing errors, κ -μ fading parameters, and atmospheric turbulence on the overall system performance.
A growing demand for high-capacity, reliable, and low-latency communication networks to support emerging technologies (like 5G, IoT, and smart cities) requires new solutions. Integrating optical and wireless networks through Radio-over-Fiber (RoF) and DAPSK–OFDMA (Differential Amplitude Phase Shift Keying–Orthogonal Frequency Division Multiple Access) modulation addresses the limitations of standalone wireless or optical systems. RoF extends the reach of wireless networks by leveraging optical fiber’s capacity, while DAPSK–OFDMA enhances spectral efficiency and transmission reliability. This research aims to develop a scalable, high-performance heterogeneous network that can meet bandwidth, latency, and coverage requirements of recent broadband applications, especially in densely populated areas. The performance of the proposed network is analyzed by transmitting bit stream, text, and image. The transmission of images and text is significant compared to bit streams because they represent real-world data types commonly used in communication systems. Performance is evaluated through Bit Error Rate (BER) and Error Vector Magnitude (EVM), comparing wireless, PON (Passive Optical Networks)-only, and PON-RoF systems over various transmission distances (b2b, 25 km, 50 km), considering text, image, and audio data transmission.
This paper introduces a four-channel optical demultiplexer employing a cascade connection of four compact photonic crystal ring resonators. The design of the ring resonator incorporates a two-dimensional square lattice of dielectric rods forming a photonic crystal structure. Optimization of the resonator ring shape, the 90-degree bend of the output, and the radius of the coupling rods are undertaken to achieve a four-channel demultiplexer. The normalized transmission characteristics are investigated using the two-dimensional finite difference time domain (FDTD) method. The results demonstrate a maximum normalized transmission efficiency of 98
In this paper, we model the unmanned aerial vehicles (UAVs)-based three-hop radio frequency (RF)/free space optics (FSO) wireless communication system with UAV as decode-and-forward (DF) relay. The first link from source to UAV relay R1 is the RF link and modeled using α - κ - μ , second link from UAV relay R1 to UAV relay R2 is FSO link and modeled using gamma–gamma fading model, and the third link from UAV relay R2 to destination with multiple mobile users is again the RF link and is modeled using Rayleigh fading model. The direct communication from source to destination is not possible due to natural and man-made obstacles. Closed-form analytical expression of outage probability (OP) is analyzed based on the system parameters. Outage probability at higher SNR regimes is also investigated to study in deep the proposed system parameters. Based on this, coding gain and diversity order is also calculated to study the system behavior. Also, location optimization is carried out by minimizing the asymptotic outage to improve the system quality.
Intent-based networking (IBN) is a natural trend in communications networks with increasing automation levels. IBN provides an intermediate layer that abstracts complex engineering rules from the operator, interpreting high-level business intents and translating them into network configurations. Although IBN has received considerable attention for packet and wireless networks, the research in the field of optical IBN is still in its early stages of development. Indeed, the examination of optical-layer survivability as an intent criterion has not been explored yet. Recently, we showcased an intent-based survivability system built around a failure management module (FMM) connected to a network digital twin (NDT). In this paper, we expand upon this initial work by detailing the FMM and quantitatively assessing the metrics used in the demonstration. In accordance with the current practice, IBN systems should offer the user the ability to request traditional survivability metrics, such as the existence of protection or availability. However, we suggest that an IBN system should also warn the user of the limitations of these approaches. In particular, we show that using traditional asymptotic (steady-state) availability as an intent metric incurs high non-compliance risks. Therefore, we recommend the implementation of risk calculations in intent translators as an auxiliary resource for intent-based survivability.
Elastic Optical Networks (EONs) offer a more efficient solution for handling variable traffic demands compared to traditional wavelength division multiplexing (WDM) networks. However, EONs face critical constraints of spectrum contiguity and continuity, which pose challenges in resource allocation. The inherent variability in link lengths and nodal degrees in physical network topologies leads to uneven link betweenness centrality (LBC) across the network. Links with higher LBC are more prone to congestion, potentially increasing request-blocking and bandwidth-blocking probabilities. This study proposes a novel optimization approach using a non-dominated sorting genetic algorithm (NSGA) to reduce the variance of LBC across network links by determining optimum link costs while preserving the benefits of adaptive modulation. Uniform distribution of traffic due to optimized link costs, the proposed method effectively reduces both request-blocking and bandwidth-blocking probabilities. The efficiency of the approach is validated through simulations on two practical topologies: the German network and NSFNET. The results demonstrate significant performance improvements compared to optimizations of link costs focusing solely on minimizing LBC variance.
Various natural disasters occur worldwide every year, causing network collapses in many areas. Additionally, due to the increase in broadband internet speed, high-speed and capacity-based network connectivity requires low-cost, secure, and rapidly deployable infrastructure to reconstruct the communication link and provide wireless communication. Considering these issues and with the advantages of Free Space Optics (FSO), this research successfully proposed a hybrid multiplexing technology that combines Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) with quadrature amplitude modulation (4-QAM) and polarization division multiplexing (PDM) to attain a data throughput of 344 Gbps per WDM channel. Wavelength division multiplexing (WDM) is also incorporated to increase the data rate and capacity of the whole system, and a high data rate of 1.376 Tbit/s for four distinct channels has been finally achieved. Multi-level multiplexing also increases spectral efficiency for this proposed system, which achieves the highest spectral efficiency of 3.44 bps/Hz. Despite this, the weather conditions, which include haze, rain, fog, dust and other similar phenomena, are the obstacles that significantly degrade in an FSO connection to achieve the desired level of performance. Also, various researchers have tried to enhance the transmission distance and capacity for FSO systems. And finally, for reliable, secure FSO communication, this article proposed a hybrid modulated system with high capacity, which has achieved a maximum transmission distance of 67 km for clear weather conditions and a minimum distance of 0.198 km for heavy dust weather conditions.
Simulations play a critical role in the planning and design of optical networks, offering a cost-effective and flexible means to evaluate network performance and explore design alternatives. Despite their importance, many existing optical simulators remain proprietary, limiting accessibility and extensibility, particularly in the context of emerging technologies. Notably, there is a significant gap in simulation tools that integrate machine learning techniques within the optical networking domain. Elastic Optical Networks (EONs) represent a major advancement over traditional Wavelength Division Multiplexing (WDM) systems, primarily due to their finer channel granularity and dynamic spectrum allocation capabilities, which substantially improve spectral efficiency. One of the central challenges in EONs is the efficient allocation of network resources, formalized as the Routing, Modulation, and Spectrum Allocation (RMSA) problem. This problem involves selecting optimal paths, modulation formats, and spectrum slots to satisfy connection requests while optimizing overall network utilization. Addressing RMSA effectively is essential for realizing the full potential of EONs and advancing intelligent, adaptive optical network design. SimEON stands out as a unique open-source simulation tool tailored for EON, adept at simulating an array of EON configurations and designing RMSA alongside regenerator placement/assignment algorithms. Moreover, it can be augmented with appropriate models to simulate CapEx, OpEx, and network energy consumption metrics. Deep learning (DL), a specialized branch of machine learning, leverages neural networks, extensive data sets, and algorithms to cultivate models adept at unraveling intricate challenges. In this paper, we extended the capabilities of SimEON by integrating the DeepRMSA algorithm into the existing simulator. We compared the performance of conventional RMSA and DeepRMSA algorithms and provided a convenient way for users to compare different algorithms’ performance and integrate other machine learning algorithms.
Adaptive combining-based hybrid free space optical (FSO)/radio frequency (RF) systems are employed to enhance system performance, particularly in scenarios where atmospheric turbulence severely impacts optical signal transmission over the FSO link. In this framework, the FSO link serves as the primary mode of data transmission, ensuring uninterrupted communication, while the RF link provides secondary backup connectivity, activated based on the reliability of the FSO link. At the receiver, maximal-ratio combining is used to combine signals from both the FSO and RF links. The FSO and RF links are modeled using Fisher-Snedecor ( ℱ ) and Nakagami-m distributions to analyze the performance of hybrid system with an adaptive combining scheme. The probability density function and cumulative distribution function of the instantaneous received signal-to-noise ratio are utilized to derive new analytical formulations for the outage probability (OP) and average symbol error rate. Further, asymptotic analysis for the OP provides deeper insights into system behavior. The analytical findings demonstrate that atmospheric turbulence considerably reduces the system’s performance. The results also revealed that small scale fading turbulence parameter can affect the diversity order by a factor of a/2 . Additionally, comparison between considered system and single FSO system is presented. Lastly, Monte-Carlo simulations are performed to verify the accuracy of the derived expressions.
With the recent trend toward open architectures in optical communication networks, the risk of physical eavesdropping on secret keys and data has increased. In particular, the growing adoption of reconfigurable optical add-drop multiplexing nodes and spatial multiplexing technologies, such as multicore fibers, has highlighted the eavesdropping risks posed by physical factors such as crosstalk. In this study, as part of efforts to enhance security in physical layer encryption technologies, a model was developed to evaluate the leakage of encrypted data and keys in digital coherent communication using phase encryption. This model assumes simultaneous crosstalk attacks on ciphertext and side-channel attacks on encryption keys by an eavesdropper and assesses the information recovery potential when the eavesdropper partially acquires secret key information, using normalized generalized mutual information (NGMI) as a measure. Simulations were conducted to evaluate the effectiveness of phase encryption on Nyquist dual-polarized M-level quadrature amplitude modulation signals, examining the decoding characteristics on the eavesdropper’s side based on variations in wiretap ratio and block lengths. The results confirmed that an increase in the wiretap ratio led to a rise in NGMI obtained by the eavesdropper, with the NGMI surpassing the forward error correction limit at certain signal-to-noise ratio (SNR) levels in the case of a 99
This paper presents a visible light communication (VLC) prototype that repurposes commercially available off-the-shelf LED bulbs to provide both illumination and Internet connectivity. The system is implemented on a BeagleBone Black single-board computer, integrating a Linux-based network driver with a software-defined VLC transceiver and a custom analog front-end. In terms of architecture, it comprises a TCP/IP-compatible network protocol interface, programmable real-time unit-based baseband processing, and visible light transmission and reception using intensity modulation with direct detection. Experimental validation was conducted to evaluate the feasibility of this low-cost VLC system, which simultaneously provides illumination and data communication. The measured bandwidth corresponds to a physical-layer limit in the sub-hundred-kHz range, while loop-back testing confirmed stable IP packet delivery at a net data rate of approximately a few kbit/s. The results indicate a viable approach for simple signaling or short text exchanges in IoT-oriented scenarios, while offering cost-effective VLC deployment without altering existing lighting fixtures.
This paper presents a novel transmission and detection scheme for optical orthogonal code (OOC)-aided multistream generalized spatial modulation (MS-GSM), which is applied to the downlink multiuser (MU) multiple-input multiple-output (MIMO) channel in visible light communication (VLC). By simultaneously activating multiple light-emitting diodes (LEDs), each carrying independent optical signals, MS-GSM achieves higher system throughput than conventional index modulation techniques. For multiuser downlink transmission, temporal symbols are modulated onto user-specific OOCs. At each user terminal (UT), chip-level processing is first performed using the intended user’s OOC to eliminate multiuser interference (MUI), followed by two detection schemes: The joint detection scheme employs the maximum likelihood (ML) algorithm to jointly estimate spatial and all temporal symbols. The second scheme, alternatively, uses subspace tracking and zero-forcing (ZF) algorithms to decode spatial and temporal symbols sequentially. Comprehensive analyses of system throughput, computational load, and average overall symbol error rate (SER) are conducted, and computer simulations are performed to evaluate the proposed schemes. The results verify that the proposed scheme offers advantages in both computational load and system throughput. Additionally, the sequential detection scheme outperforms the joint detection scheme in terms of SER performance and complexity.
This paper studies problems related to slice request embedding using optimization techniques in multi-cloud infrastructures for Network Slicing-based 5G/6G networks. Network slicing partitions the network’s physical infrastructure into several logical networks, allowing multiple virtual networks to run on top of the underlying physical network. These logical networks run independently and are required to meet the Quality of Service (QoS) demands of specific user groups. This work considers resource allocation for embedding a network slice’s virtual network functions (VNF) components on a cloud-based physical infrastructure consisting of multiple cloud providers. This virtual network embedding (VNE) involves two steps: selecting a substrate network and determining the substrate nodes for embedding the virtual nodes and the corresponding virtual links on the selected substrate network. This can be modeled as a resource allocation optimization problem. The proposed solution approaches are a greedy strategy, a Reinforcement learning (RL)-based scheme using Proximal Policy Optimization (PPO), followed by a state-of-the-art VNE-Q-HRL solution for performance comparison. The proposed Hierarchical RL model has also been compared with an Integer Linear Programming (ILP) formulation for the VNE problem, and the infeasibility of the exact solution for large-scale VNE is shown. The performance of the algorithms has been studied using a discrete-event simulation model for several performance metrics. The results show that the PPO-based Multi-VNE-HRL algorithm shows an improvement of 8.33
This paper proposes an unmanned aerial vehicles (UAV)-integrated reconfigurable intelligent surface (RIS)-assisted modulating retroreflector (MRR) free-space optical (FSO) communication system, enhanced with wavelength diversity (WD) and time diversity (TD) techniques, to address the challenges posed by dynamic channel conditions such as fog, gamma–gamma turbulence, and pointing errors. The integration of UAVs, programmable RISs, and passive MRRs forms a scalable and modular architecture—referred to as the UAV-integrated RIS-assisted MRR-FSO system—that significantly enhances system availability, adaptability, and energy efficiency. Analytical expressions for key performance metrics, including outage probability, average bit error rate (BER), and maximum effective bit rate, are derived and validated through Monte Carlo (MC) simulations, demonstrating a high degree of agreement with the theoretical results. Numerical evaluations show that the proposed system achieves up to 75 × higher effective data rate and a 2–5 × improvement in spectral efficiency, positioning the system as a strong candidate for reliable, energy-efficient, and adaptive UAV-based optical backhaul solutions in future 6 G networks.
In this paper, we have presented a simple scheme to generate multi-frequency dual-chirp microwave waveform with increased time-bandwidth product (TBWP) based on two cascaded polarization modulators. In this scheme, the optical frequency comb (OFC) with four lines, generated by RF signal modulating the lightwave via an optical polarization modulator (PolM), is modulated by a parabolic signal via a second PolM to introduce the parabolic phase difference between the tones. After heterodyne beating in balanced photodetector, the parabolic phase-shifted OFC is converted to three-band dual-chirp microwave waveform. In order to increase the TBWP, the parabolic signal is divided into multi-segments and phase encoded to multiply both the bandwidth and temporal duration of the generated dual-chirp microwave waveform, respectively. The simulation results show that dual-chirp microwave waveforms with frequency-bandwidth of 10–3.2 GHz, 20–3.2 GHz, and 30–3.2 GHz, covering X, K, and Ka bands, are generated simultaneously. The generated waveform has the TBWP of 5133 approximately. Their auto-correlation results manifest high peak-to-sidelobe ratio and good pulse compression capabilities. These agree well with our theoretical prediction. The generated multi-frequency dual-chirp microwave waveforms with high-frequency, large bandwidth, and large TBWP, are expected to be used in multi-band radar system to improve its range resolution and detection range.
The deployment of high-speed computations in the digital world has the advantage of using optical logic gates individually. By employing the theory of a metal–insulator-metal (MIM) waveguide-based power combiner with a footprint of 20 µm × 20 µm, an innovative method of building an optical half-subtractor is designed. The linear interference theory explains the operation of this device. The insertion loss and extinction ratio are observed as 1.13 dB and 2.78 dB, respectively. To analyze the layout, the finite-difference time-domain (FDTD) method is implemented. Electric field strengths at a specified wavelength of 1550 nm and a refractive index of 1.65 were calculated throughout the simulation for a range of possible combinations of half-subtractor. The simplified structure can be used to create any sophisticated logic circuit for future performance improvements.
Radio-frequency/Wireless Optical Communication (RF/WOC) systems are used for long-distance transmission where the optical signals are severely impacted by atmospheric turbulence. This article provides the performance analysis of a dual-hop RF/WOC system where RF and wireless optical channels undergo Nakagami-m and Fisher-Snedecor ( ℱ ) distributions with approximate Beckmann distribution respectively. This system can operate with either coherent heterodyne detection (HD) or non-coherent intensity modulation with direct detection (IM/DD) scheme. The mathematical formulation of the cumulative distribution function (CDF) is derived for instantaneous overall received signal-to-noise ratio (SNR) using the fixed gain amplify and forward (AF) relaying technique. Its CDF is being utilized to create new analytical formulas for outage probability (OP), average bit error rate (ABER), and ergodic capacity (EC). Moreover, asymptotic analysis for the performance metrics is performed to get more insights. Additionally, a single WOC system that was previously studied and modeled by ℱ fading distribution with pointing errors is compared with the proposed system. The analytical findings demonstrate that the performance of the system is exacerbated by essential factors, including atmospheric turbulence and pointing errors and the HD scheme is preferable to a IM/DD scheme. Moreover, ABER with CBPSK modulation is found better than other binary modulation schemes. Additionally, results reveal that the considered system has remarkable performance improvement over a single WOC system.
The implementation of 5G and the future deployment of 6G require the utilization of optical networks that possess substantial capacity and exhibit minimal latency. The dynamic arrival and departure of connection requests in optical networks result in particular central links experiencing more traffic and congestion than non-central links. The occurrence of congested links leads to service blocking despite the availability of resources within the network, restricting the efficient utilization of network resources. The available algorithms in the literature that aim to balance load among network links offer a trade-off between blocking performance and algorithmic complexity, thus increasing service provisioning time. This work proposes a congestion-aware routing, modulation, core, and spectrum assignment algorithm based on dynamic routing for space-division multiplexing elastic optical networks (SDM-EON). The algorithm finds alternative candidate paths based on real-time link occupancy metrics to minimize blocking due to link congestion under dynamic traffic scenarios. As a result, the algorithm reduces the formation of congestion hotspots in the network due to link-betweenness centrality. We have performed extensive simulations using two realistic network topologies to compare the performance of the proposed algorithm with the relevant RMCSA algorithms available in the literature. The simulation results verify the superior performance of our proposed algorithm compared to the benchmark Yen’s K-shortest paths and K-Disjoint shortest paths RMCSA algorithms in terms of connection blocking ratio and spectrum utilization efficiency. Furthermore, to expedite the route-finding process, we present a novel caching strategy that allows the proposed algorithm to demonstrate a much-reduced service delay time compared to the recently developed adaptive link weight-based load-balancing RMCSA algorithm.
