Enhancement of channel capacity of IRS-aided OWC system for 6G wireless communication

Optical Wireless Communication (OWC) technology has gained significant attention in recent years due to its potential for providing high-data-rate wireless connections through the large license-free bandwidth available. A key challenge in OWC systems, similar to high-frequency Radiofrequency (RF) systems, is the presence of dead zones caused by obstacles like buildings, trees, and moving individuals, which can degrade signal quality or disrupt data transmission. Traditionally, relays have been used to mitigate these issues. Intelligent Reflecting Surfaces (IRSs) have recently emerged as a promising solution, enhancing system performance and flexibility by providing reconfigurable communication channels. This paper presents an overview of the application of IRSs in OWC systems. Specifically, we categorize IRSs into two main types: mirror array-based IRSs and metasurface-based IRSs. Furthermore, we delve into modeling approaches of mirror array-based IRSs in OWC and analyze recent advances in IRS control, which are classified into system power or gain optimization-oriented, system link reliability optimization-oriented, system data rate optimization-oriented, system security optimization-oriented, and system energy optimization-oriented approaches. Moreover, we present the principles of metasurface-based IRSs from a physical mechanism perspective, highlighting their application in OWC systems through the distinct roles of light signal refraction and reflection. Finally, we discuss the key challenges and potential future directions for integrating IRS with OWC systems, providing insights for further research in this promising field.

Optical wireless communication (OWC) systems are a promising communication technology that can provide high data rates into the tens of Tb/s and can support multiple users at the same time. This paper investigates the optimum allocation of resources in wavelength division multiple access (WDMA) OWC systems to support multiple users. A mixed-integer linear programming (MILP) model is developed to optimise the resource allocation. Two types of receivers are examined, an angle diversity receiver (ADR) and an imaging receiver (ImR). The ImR can support high data rates up to 14 Gbps for each user with a higher signal to interference plus noise ratio (SINR). The ImR receiver provides a better result compared to the ADR in term of channel bandwidth, SINR and data rate. Given the highly directional nature of light, the space dimension can be exploited to enable the co-existence of multiple, spatially separated, links and thus aggregate data rates into the Tb/s. We have considered a visible light communication (VLC) setting with four wavelengths per access point (red, green, yellow and blue). In the infrared spectrum, commercial sources exist that can support up to 100 wavelengths, significantly increasing the system aggregate capacity. Other orthogonal domains can be exploited to lead to higher capacities in these future systems in 6G and beyond.

In today's world, staying connected is more important than ever, but achieving reliable wireless communication everywhere can be a challenge. This dissertation introduces a cutting-edge technology known as Intelligent Reflecting Surfaces (IRSs) that promises to revolutionize how we connect. Imagine a smart, invisible “mirror” that can bend and direct wireless signals precisely where needed, overcoming obstacles and ensuring your device always gets a strong connection. That is what the IRS does. IRS, at its core, is a sophisticated planar array, composed of numerous passive or active elements capable of individually manipulating electromagnetic waves to reshape the wireless signal propagation environment. By smartly adjusting the phase and amplitude of these elements, an IRS can seamlessly steer signals toward intended receivers, effectively creating optimized communication paths even in scenarios where direct Line-of-Sight (LoS) is obstructed. This ability to mold the propagation environment on demand, without additional energy for signal transmission, enables the IRS to enhance connectivity in diverse environments, from densely built urban areas to indoor spaces. Furthermore, the ability of the IRS to operate without the need for active power amplification allows for a significant reduction in energy consumption, making it an eco-friendly solution for extending and improving wireless network coverage. In this dissertation, IRS is presented as a key enabler for a myriad of advanced technologies, unlocking new potentials across various high-tech fields by enhancing their performance and efficiency. By strategically manipulating electromagnetic waves, IRS provides a solution to enhance power efficiency in multi-user Simultaneous Wireless Information and Power Transfer (SWIPT) networks. This capability allows for a steady flow of information and power transfer, illustrating the dual capability of the IRS to support energy harvesting and data transmission. Furthermore, the integration of IRS into Ultra-Reliable Low-Latency Communication (URLC) and Machine Type Communication (MTC) systems emerges as a game-changer, significantly reducing latency and increasing reliability. IRS can significantly benefit Virtual Reality (VR) users facing considerable path loss or blockages, ensuring immersive experiences without latency or loss of quality. IRS also enhances Mobile Edge Computing (MEC) by optimizing signal delivery for efficient edge data processing. These improvements are essential for critical applications requiring instantaneous feedback and high levels of data integrity, such as autonomous vehicles and industrial automation, underpinning the role of the IRS in facilitating the next wave of communication needs. This work delves into the strategic deployment of IRS across a broad frequency spectrum, from Frequency Range 1 (FR1) to Frequency Range 2 (FR2), extending into the higher frequency domains of millimeter-Wave (mmWave) and TeraHertz (THz) frequencies, illustrating its profound impact on the future of telecommunications. In order to investigate the performance of IRS-assisted networks, this dissertation defines a range of Key Performance Indicators (KPIs), such as data rate, power efficiency, energy efficiency, Signal-to-Interference-plus-Noise Ratio (SINR), transmit signal power budget, and received power strength. These KPIs serve as metrics to assess and optimize the network's performance based on designing an efficient resource allocation policy. Non-linear, nonconvex, and Mixed Integer Nonlinear Programming (MINLP) problems arise when addressing the resource allocation optimization problem. These problems are Non-deterministic Polynomial time (NP)-hard due to the complex relationship between variables and the system's constraints. Given the complexity of these optimization problems, different strategies are used to simplify and approach their solution. By relaxing the objective function (the NPs) and constraints that are non-convex to a more tractable format, the problems became more manageable. This relaxation approach often involved transforming the optimization problem into its convex equivalent or utilizing approximation techniques to linearize or convexify non-convex terms. Algorithms are developed that are capable of solving the main problem either globally or suboptimally but sufficiently close to the global optimum. These solutions employ optimization solvers and computer simulations, exploiting advanced mathematical tools and techniques such as the big-M method for linearizing product terms involving binary variables and Successive Convex Approximation (SCA) to obtain convex approximations of non-convex terms. The iterative nature of these solutions allowed for step-by-step refinement, gradually moving towards an optimal configuration of a resource allocation design despite the initial problem's complexity. Through exhaustive simulations, this dissertation unveils the diverse performance improvements achievable through resource allocation in IRS-assisted networks, providing rich insights into how IRS technology can improve wireless communication systems. These simulations serve as a critical bridge, connecting theoretical predictions with empirical evidence and validating the practical feasibility of the proposed IRS-enhanced network. By exploring various IRS configurations — examining both passive and active types and varying the number of reflective elements — and their implementation in different environments and settings, this study not only confirms the theoretical models' accuracy but also explains the conditions under which IRS deployments yield maximal performance gains, manifesting the IRS versatility in adapting new technologies. Collectively, this dissertation studies the impact of IRS across a broad range of technologies. By enhancing the performance of SWIPT networks, facilitating URLLC and MTC, enabling MEC, and revolutionizing VR, mmWave, and THz applications, IRS stands at the forefront of wireless communication innovation. This work demonstrates the diverse applications of IRS technology and lays the foundation for future research aimed at utilizing IRS to tackle the dynamic challenges of modern wireless networks. It charts a path toward the development or more robust, efficient, and engaging communication ecosystems.

An optical wireless communication (OWC) system based on a light-emitting-diode (LED) transmitter and a camera receiver has been developed for use in the automotive area. The automotive OWC system will require Mb/s-class data rates and the ability to quickly detect LEDs from an image. The key to achieving this is improvements to the capabilities of the image sensor mounted on the camera receiver. In this paper, we report on a novel OWC system equipped with an optical communication image sensor (OCI), which is newly developed using CMOS technology. To obtain higher transmission rates, the OCI employs a specialized “communication pixel (CPx)” capable of responding promptly to optical intensity variations. Furthermore, a new quick LED detection technique, based on a 1-bit flag image which only reacts to high-intensity objects, is formulated. The communication pixels, ordinary image pixels, and associated circuits (including 1-bit flag image output circuits) are then integrated into the OCI. This paper describes the design, fabrication, and capabilities of the OCI, as well as the development of the LED and image sensor based OWC system, which boasts a 20-Mb/s/pixel data rate without LED detection and a 15-Mb/s/pixel data rate with a 16.6-ms real-time LED detection.

Solar noise, when it interferes with the received signal at the system receiver (Rx) of an optical wireless communication (OWC) system, degrades the system’s performance. The detrimental effect of solar noise on OWC systems has been well established in the literature. This work experimentally demonstrates solar noise interference in the OWC system by pointing the system Rx in various orientations in air and water mediums, e.g., 0° (Rx pointing horizontally leftward), 45°, 90° (Rx pointing vertically downward), 135°, 180° (Rx pointing horizontally rightward), 225°, 270° (Rx pointing vertically upward), and 315°. The experimental outcomes depict the signal’s noise content, spectral leakage, and roll-off rate variation at multiple Rx orientations. We also demonstrate the solar noise interference in transmitting an image through the outdoor underwater OWC link by pointing the system Rx in various orientations. Experimental demonstration confirms that the same OWC system never behaves identically in the presence of solar noise if the system Rx keeps changing its orientation during the maneuver.

Wireless Powered Communication Networks (WPCNs) represent a transformative approach to address the energy demands of mobile and Internet of Things (IoT) devices. By integrating Nonorthogonal Multiple Access (NOMA) and Intelligent Reflecting Surfaces (IRS), we can significantly enhance system performance, extend coverage, and elevate the sum rate. NOMA efficiently utilizes the entire bandwidth by employing a power allocation strategy, whereas IRS, serving as an alternative to traditional relay amplification, further bolsters the sum rate. Despite these advancements, optimizing the sum rate introduces a nonconvex optimization challenge, primarily owing to the signal‐to‐interference‐plus‐noise ratio (SINR) complexities introduced by NOMA's Successive Interference Cancellation (SIC). Traditional convex optimization solvers, such as the CVX, struggle to address nonconvexity directly. Consequently, they were unable to produce the desired outcome. Moreover, the combination of multiple technologies to improve the sum rate complicates the optimization framework, necessitating a multitude of constraints that not only heightens the mathematical complexity but also induces errors through the requisite approximations for convexity conversion. To circumvent these hurdles, we advocate the application of a minimum constrained nonlinear multivariable function (Fmincon). This approach enables us to tackle the nonconvex problem head‐on, maintaining consistent simulation parameters while limiting constraints to two pivotal factors: joint optimization of the transmit power ( ) and transmit time ( ). This strategic simplification mitigates complexity and minimizes errors. Our numerical analyses confirmed the efficacy of the proposed model and optimization technique. By co‐optimizing the transmission power and time, we achieved a notable sum rate. Comparative evaluations with extant models underscored the superior performance of our proposed framework, marking a significant stride in WPCN advancement.

This study has presented spatial continuous wave laser and spatiotemporal vertical cavity surface emitting laser (VCSEL) for high speed long haul optical wireless communication channels. Possible data rates range from 40 Gb/s to 250 Gb/s over propagation distance ranges from 500 km to 2500 km. The optical wireless communication (OWC) system performance is tested through the measurement of maximum Q-factor, minimum bit error rate (BER) and signal to noise ratio (SNR). It is observed that spatiotemporal VCSEL has presented better performance than CW laser in the OWC system, especially for long haul transmission applications. It is observed that SNR improvement ratio ranges from 8.15 % to 19 % by using spatiotemporal VCSEL than CW laser for bit rate of 40 Gb/s over propagation distance ranges from 500 km to 2500 km. Max. Q-factor improvement ratio ranges from 4.62 % to 13.71 % by using spatiotemporal VCSEL than CW laser for data rate of 40 Gb/s over propagation distance ranges from 500 km to 2500 km. So it is clear that spatiotemporal VCSEL is more suitable for long haul OWC applications than other optical sources.

The combination of orthogonal frequency division multiplexing (OFDM) and spatial modulation (SM) can enhance the capacity of optical wireless communication (OWC) systems with low complexity. In this paper, we propose a novel SM scheme for intensity modulation/direct detection (IM/DD) OWC systems by employing discrete Hartley transform based OFDM (DHT-OFDM). Due to the use of DHT with one-dimensional constellations, the Hermitian symmetry constraint, which is generally imposed in conventional discrete Fourier transform based OFDM (DFT-OFDM) to obtain a real-valued output signal, is not required in DHT-OFDM. As a result, DHT-OFDM based SM can achieve much higher spectral efficiency than that of DFT-OFDM based SM in OWC systems. Simulation results show that, for an indoor 4×4 SM-OWC system with a spectral efficiency of 6 bits/s/Hz, DHT-OFDM achieves a remarkable 4.5-dB transmit signal-to-noise ratio reduction for an overall bit error rate of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> in comparison to conventional DFT-OFDM.

Experimental demonstration of 4-PAM for high-speed indoor free-space OW communication based on cascade FIR-LMS adaptive equalizer

The performance of an optical wireless communication (OWC) system subjected to atmospheric turbulence induced fading can be significantly improved by using spatial diversity reception. Diversity reception also suffers from correlation among diversity branches due to insufficient spacing between multiple photodetectors at the receiver. In this paper, average channel capacity of OWC system employing maximal ratio combing (MRC) and equal gain combining (EGC) diversity reception with correlated diversity branches is evaluated using a simple expression based on point estimate. A weak atmospheric turbulence condition known to be modeled by log-normal distribution is considered. In addition, the impact of correlation is also investigated considering identical as well as non-identical turbulence induced fading among diversity branches. This analysis reveals that the channel capacity decreases with the increase in correlation coefficient and impact of fading correlation is higher in case of identically distributed diversity branches as compared to non-identically distributed branches. Validity of the analysis is demonstrated with the results obtained using Monte-carlo simulation.

Amidst revolutionary communication technologies of recent decades such as optical fiber and wireless systems other forms of communication technologies are popping up in order to complete or complement the ever-increasing and insatiable need for communications links in today's society. Among many such niche technologies optical wireless communication technology, in particular, is begining to enjoy a wide range of applications and attention from many industries for short-range interchip applications to interplanetary space applications. Furthermore, optical wireless systems have attracted even further considerable growth in research and development since they can be appropriate alternatives for wireless or fiber-optic communication systems in some specific applications. Indoor wireless LANs, atmospheric optical links, and submarine optical wireless systems have grown in importance where lightwave communications is preferred to radio communications. This preference in various applications can be originated from security requirements, radio interference avoidance, no need for reserving frequency bands and cost of development [1]–[9].

Using re-configurable intelligent surfaces (RIS) in optical wireless communication (OWC) systems to solve the signal obstruction and skip-zones dilemmas modifies the transmission channel. It is thus necessary to analyze the capacity of such a system. To this end, the capacity lower- and upper-bounds of RIS-assisted single-input single-output (SISO) OWC systems are discussed in this letter, focusing on the intensity-modulation and direct-detection scheme. This analysis considers two main constraints, namely peak-intensity and average optical power constraints. It also considers two types of structures: the single-layer structure (SLS) and multiple-layer structure (MLS). By exploiting the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$QR$ </tex-math></inline-formula> -decomposition, the analysis is extended to RIS-assisted multiple-input multiple-output (MIMO) OWC systems. As a result, the RIS-based cascaded channel capacities bounds and the achievable rate at a high signal-to-noise ratio are given for free-space RIS-based SISO/MIMO OWC systems. These results show that the channel exhibits a high capacity when the RIS module is closer to the data source, and that the MLS provides a higher achievable rate when compared to the SLS.

The atmospheric foggy condition is a major bottleneck for the deployment of optical wireless communication (OWC) systems in outdoor environments. Although the attenuation coefficient in the foggy channel follows the simple Gamma distribution, the distribution of its channel state is quite complicated similar to the well-studied Gamma–Gamma turbulence channel. Furthermore, existing literature suggested schemes that require feedback of the channel state information (CSI) to improve the outage performance of the OWC system under foggy conditions. In this article, we study the average signal-to-noise ratio (SNR), ergodic rate, and energy consumption performance of OWC systems under foggy conditions. First, we show that a single-aperture OWC system suffers significantly from the fog attenuation by deriving analytical expressions on the ergodic rate and energy consumption. Then, we consider a multiaperture OWC system equipped with an opportunistic receiver beam selection (RBS) scheme. The RBS scheme is based on the simple selection combining diversity algorithm, which does not require the knowledge of CSI. We analyze the proposed scheme by deriving computable bounds on the average SNR, ergodic rate, and energy consumption using system parameters and show the performance improvement of the RBS scheme comparing the single-aperture system. We demonstrate the performance of the considered scheme and validate derived expressions using computer simulations.

Since wireless optical communication (WOC) systems can offer several potential advantages over their radio frequency counterparts, there has been a growing interest in WOC systems. Influenced by the complicated optical propagation environment, there exists diffuse propagation phenomena. In order to eliminate the effect of multipath propagation caused by diffuse reflection, much attention should be concentrated on the channel estimation in diffuse WOC systems. This paper focuses on the complementary sequences (CSs) for estimating a channel impulse response in WOC systems. Based on the training sequences (a preamble and a postamble sequence are included in the data frame), this paper studies the channel estimation in WOC systems. Further, a simple expression for Cramer-Rao bound (CRB) is proposed to evaluate the two-sided channel estimation errors. By minimizing the CRB, the CS-based channel estimation method is proposed. Moreover, the sequence detection performance measured by a bit error ratio is also investigated. Simulation results show that the channel estimation and sequence detection performance can be significantly improved by employing the CS-based channel estimation.

Toward high-speed visible laser lighting based optical wireless communications