Free-space optics (FSO) and radio frequency (RF)-based non-orthogonal multiple access (NOMA) are key enablers for 5G and beyond communication. This paper proposes a novel mixed FSO/RF communication system with the NOMA framework leveraging high-altitude platform stations (HAPS) and unmanned aerial vehicles (UAVs) as relays to enhance reliability and spectral efficiency. The proposed system employs decode-and-forward (DF) relaying, with FSO links modeled using a doubly inverted Gamma-Gamma (IGGG) distribution incorporating atmospheric turbulence, pointing errors, and angle-of-arrival (AoA) fluctuations, while RF links follow a Nakagami-m fading model. A relay selection scheme based on channel conditions is implemented to optimize UAV selection. Further, closed-form expressions for outage probability (OP), ergodic capacity (EC), and throughput are derived and validated through Monte-Carlo simulations. Numerical results show that system performance is dominated by the link reliability of FSO communication at shorter user-to-UAV distances and transitions to RF link dominance at longer distances. For the two-user scenario, the analysis reveals that user 1 experiences interference-limited saturation at a high signal-to-noise ratio (SNR), while user 2 achieves linear capacity growth due to successive interference cancellation (SIC). The proposed system model effectively mitigates channel impairments, offering valuable insights for the next-generation wireless networks.

With its ultra-high-capacity, free space optical (FSO) communication stands out as a powerful connectivity solution for airborne backhauling. In this paper, we consider typical backhauling scenarios for single-layer and multi-layer airborne networks and characterize the underlying multi-hop FSO transmission. In the first scenario, we consider a single-layer airborne backhaul system where a fleet of high-altitude platform stations (HAPSs) continuously rotates on a circular track at a specific altitude. It is assumed that the communication links are always established with the closest HAPS. In such a scenario, a dual-hop FSO transmission is required where the first hop is from the gateway node to HAPS and the second hop is from the HAPS to the base station. In the second scenario, we consider a two-layer system where HAPSs are assisted by rotary-wing unmanned aerial vehicles (UAVs) at lower operation altitudes to ease line of sight (LoS) requirement, which might be especially critical in urban scenarios. This mainly corresponds to a three-hop configuration where the first hop is from the gateway to HAPS, the second hop is from HAPS to UAV and the final hop is from UAV to the base station. For both scenarios under consideration, we first develop a channel model for the airborne link to describe atmospheric attenuation, geometrical loss, and pointing error. The transmission distance of the HAPS-based link is subject to continuous change due to movement and leads to a time-varying atmospheric attenuation loss and geometric loss. These mobility-induced variations in the average received power effectively introduce a fading effect. We develop a probability density function (PDF) to capture this effect. While pointing error can be ignored for HAPS-based links under certain conditions, rotary-wing UAVs introduce displacements in both$\mathcal {X}$and$\mathcal {Y}$directions. We further characterize this phenomenon and quantify the resulting variance of displacements. Based on the developed PDFs, we derive the end-to-end bit error rate (BER) for two-hop and three-hop airborne systems under consideration. We further propose a power allocation scheme to optimize the BER performance, ensuring that the overall performance is not dominated by a single hop.

Laser diode (LD) powered luminaires are already gaining traction in automotive applications and are expected to be increasingly deployed in homes and offices in the near future. This gives a unique opportunity to build visible light communication (VLC) systems with ultra-high-bandwidths offered by the LDs. However, LDs are subject to some inherent noise sources, notably signal-dependent shot noise (SDSN) and relative intensity noise (RIN). RIN basically describes the instability in the power level of a laser and quantifies the output power fluctuations. SDSN arises from variations in the photodetector’s response to optical signals. Our research explores the interplay between SDSN and RIN, revealing their combined adverse effect on channel estimation accuracy in a VLC system. Towards this direction, we first derive the Cramér-Rao lower bound (CLRB) in the presence of the SDSN and the RIN, which gives a lower estimate for the variance of an unbiased estimator. Then, we present the derivation of least square (LS) and maximum likelihood (ML) channel estimators. Furthermore, we present the optimal receiver in ML sense and compare it with a simple threshold detector as a sub-optimal solution, quantifying the impact of channel estimation accuracy on both receivers. Our study results reveal the adverse impact of the joint presence of SDSN and RIN on the channel estimation error; consequently, it degrades the BER of the two proposed receivers. Moreover, RIN emerges as the dominant noise source, especially at higher levels of transmitted power.

Non-terrestrial networks (NTNs) involve the use of unmanned aerial vehicles (UAVs), high-altitude platform stations (HAPSs), and low-earth orbit satellites (LEOs) and have emerged as a powerful tool to enable global connectivity. With their flexible deployment, rotary-wing UAVs are particularly useful to deliver versatile airborne wireless access and backhaul in areas where there is limited terrestrial infrastructure. In this paper, we consider a scenario where free space optical (FSO) terminals are mounted on rotary wing UAV to provide high capacity wireless backhaul for ground base stations. Due to the narrow divergence angle of laser tranmsitters, FSO systems are prone to pointing errors. In our study, we classify and analyze two primary sources of pointing errors in rotary-wing UAV-based links: horizontal movement and rotational stability. The former stems from the semi-fixed hovering nature of UAVs, introducing uncertainties in horizontal displacement that influence the positioning of the received beam along the$x$and$y$directions. Additionally, rotational stability issues in rotary-wing UAVs, including rolling, yawing, and pitching, lead to shifts in the center of the received beam. We classify these pointing errors as altitude-dependent and altitude-independent and analyze their distinct impacts on the bit error rate (BER) of UAV-based FSO links. Our findings contribute to a better understanding of pointing errors in UAV-based FSO communication, paving the way for enhanced airborne connectivity solutions.

With its high capacity and immunity to electro-magnetic interference, free space optical (FSO) communication is positioned to be a key connectivity solution for airborne backhauling. In this paper, we consider a scenario where a fleet of high-altitude platform stations (HAPSs) follow a predefined circular trajectory and provide airborne backhauling to ground base stations. Unlike terrestrial FSO links that are primarily limited by atmospheric turbulence-induced fading, aerial links between a HAPS and a ground station are subject to fading effects induced by mobility. The instantaneous transmission distance continuously changes due to movement, leading to a time-varying atmospheric attenuation loss and geometric loss. These mobility-induced variations in the average received power effectively introduce a fading effect. In this paper, we present a statistical model for the aggregate airborne channel coefficient. First, we derive a probability density function (PDF) to describe the instantaneous changes in the propagation distances as a result of mobility. Then, we derive the PDF for the aggregate channel coefficient that includes both geometric losses and atmospheric attenuation. We present numerical results to corroborate our analytical findings and discuss the effects of several system and channel parameters on the severity of fading.

In this paper, we explore the use of multiple photodetectors for vehicular visible light communication (VLC) systems with a focus on the so-called Angle-Diversity Receiver (ADR). ADR builds upon the principle of using multiple photodetectors oriented at different reception angles to enable multidirectional signal reception. With ADR, it is possible to receive light rays from the vehicle headlight in challenging mobility conditions such as in the cases of U-turn, left-turn, and right-turn. In our work, we present preliminary results of an experimental verification of ADR-based vehicular VLC system implemented using software-defined radio platforms. Our results demonstrate that a packet delivery ratio (PDR) of more than 99 % is achieved even for T-junction road scenarios where the link is likely to get lost in the case of conventional single transmitter/receiver configurations.

This paper investigates streetlight-to-vehicle visible light communication (VLC) system performance for outdoor broadcasting applications. We adopt streetlight lamps as optical internet-of-thing (IoT) devices broadcasting internet services and safety messages to road vehicles. With their asymmetrical radiation patterns, Streetlight antennas are exceedingly different from indoor lighting modules, which deploy ceiling luminaries with ideal Lambertian ones. Therefore, a realistic channel modelling for streetlight-to-vehicle VLC system should be deployed for precise performance insights. We consider a streetlight-to-vehicle VLC system in a two-lane road with multiple light poles uniformly distributed on both sides. Based on that, we investigate the system performance of the streetlight-to-vehicle VLC system in terms of the bit-error-rate (BER) and outage distance and explore the effect of different transceivers and system parameters on the performance. These consider the transmission modulation order, receiver size, height of the streetlight poles, and their corresponding intermediate distances.

In vehicular visible light communication (VLC), the reliable communication distance may be relatively short due to its dependence on many factors including the weather condition. This necessitates the use of relay-assisted systems to extend the transmission range. In this paper, we investigate experimentally the performance of vehicular multi-hop VLC systems in outdoor environments. Particularly, we first examine the effect of ambient noise on signal-to-noise ratio (SNR) during a whole day. Then, we conduct two multi-hop experiments, i.e., one at daytime and one at nighttime. We measure the bit error rate (BER) for each individual hop and then obtain the overall performance of the multi-hop system. We further compare experimental BER with theoretical BER to confirm our results.

Underwater visible light communication (UVLC) has been introduced to support emerging high data rate applications such as real-time image and video transmission. Initial works on UVLC build upon the assumption of fixed turbulence strength through the transmission range which can be justified only for horizontal links. In vertical underwater links, the gradient of temperature and salinity changes with depth. This effectively results in ocean stratification where water with different values of salinity and temperature form non-mixing layers. In this paper, we first model the vertical underwater link as a cascaded fading channel where fading coefficients associated with different layers are modeled as independent and non-identical distributed. Based on the cascaded lognormal and Gamma-Gamma distributions respectively for weak and moderate/strong turbulence conditions, we first derive closed-form expressions for the bit error rate (BER) performance of UVLC systems. Then, we analyze the asymptotic BER performance and determine the diversity orders. In addition, we derive closed-form expressions for the average ergodic capacity of underwater cascaded fading channels under consideration. We present simulation results to confirm the analytical findings.

Visible light communication (VLC) has the potential to serve as a high-speed underwater wireless connectivity solution to support real-time image and video transmission. Underwater propagation medium imposes additional challenges on the design of VLC systems which were originally proposed for indoor applications. Temperature and salinity fluctuations result in fluctuations of the refractive index of seawater and eventually introduce turbulence-induced fading. In this paper, to mitigate the effects of fading, we propose transmit laser selection for a diver-to-diver underwater VLC link where the transmitter has multiple laser sources and the receiver has one photodetector. The source with the highest received instantaneous signal-to-noise ratio is selected for transmission while the remaining sources remain idle. We derive a closed-form expression for asymptotical bit error rate over log-normal distributed underwater turbulence channels and derive the achievable diversity order. We further present simulation results to confirm our derivations and diversity gain analysis.

In this paper, we investigate the performance limits of underwater visible light communication (UVLC) systems. We first develop a closed-form path loss expression as a function of transceiver parameters and water type. We then utilize this new expression to determine the maximum achievable link distance for UVLC systems in pure sea, clear ocean, coastal water, and harbor water. Our results demonstrate that the maximum achievable distance is limited to a few tens of meters. This necessitates the deployment of relay-assisted UVLC systems to extend the transmission range. We consider both detect-and-forward and amplify-and-forward relaying. For each relaying method, we first consider a dual-hop UVLC system and determine optimal relay placement to minimize the bit error rate (BER). Then, we consider a multi-hop system with equidistant relays and determine the maximum achievable distance for a given number of hops to satisfy a targeted end-to-end BER.

In Intelligent Transportation Systems, visible light communication (VLC) has emerged as a powerful candidate to enable wireless connectivity in vehicle-to-vehicle (V2V) and vehicle-to- infrastructure (V2I) links. While VLC has been studied intensively in the context of indoor communications, its application to vehicular networking is relatively new. In this paper, we carry out a comprehensive channel modeling study to quantify the effect of rain and fog on a V2V link with a high-beam headlamp acting as the transmitter. Taking advantage of advanced ray tracing features, we first develop a path loss model for V2V link as a function of distance under different weather conditions. Then, we use this expression to determine the maximum achievable distance to ensure a given bit error rate. We further investigate the deployment of relay- assisted systems to extend transmission ranges. Extensive numerical results are presented to corroborate our findings.