The increasing proliferation of internet-connected devices, together with the widespread reliance on wireless communication in daily activities, are limiting the capabilities of the radio frequency (RF) spectrum to meet data traffic demands. In particular, the rise in connectivity requirements to realize next generation networks, coupled with the growth in utilization of artificial intelligence (AI) and its associated transmission of massive amounts of data, contribute to the need to leverage the optical spectrum to complement the already-congested RF bands. Optical wireless connectivity encompasses a range of solutions that can support data transfer in various use cases, ranging from intra-chip interconnects to deep-space communication. They benefit from advantages in terms of operation within a broad unlicensed spectrum, minimization of RF interference, and inherent security. On the other hand, optical wireless communication (OWC) systems might experience a decline in performance as a result of factors like atmospheric conditions and link blockage. This requires customizing system designs according to the considered use cases and investigating new solutions for realizing optical wireless connectivity. In this article, we present an overview of OWC, encompassing both its conceptual frameworks and the scientific and technological advancements that hold the potential for shaping its future in next generation networks.

Beam misalignment induced by platform motion severely limits the reliability of narrow-beam underwater optical wireless communication (UOWC) links. This paper presents a camera-feedback micro-electro-mechanical systems (MEMS) mirror automatic beam alignment system for UOWC employing a laser diode transmitter and a photomultiplier tube receiver. An entropy-guided gradient-direction voting method is proposed to improve laser outlet estimation under distorted, elongated, and overexposed beacon-spot conditions. The estimated outlet position is converted into MEMS-mirror steering commands to realize closed-loop beam tracking. Water-tank experiments show that the proposed method provides more stable tracking performance and lower bit error rate (BER) than the principal component analysis based baseline under different receiver speeds and beacon imaging conditions. A 50-m swimming-pool experiment further verifies that the closed-loop system maintains BER below the forward error correction threshold under dynamic longitudinal and lateral misalignment.

To enable reliable and efficient communication under time-varying channel conditions, this paper proposes a feedback-based symbol rate adaptation scheme for ultraviolet (UV) communication. The receiver periodically reports channel state information, including the mean detected photon counts of signal and background components, to the transmitter after a fixed number of frames. Based on this feedback and predefined rate-switching thresholds obtained from off-line low-density parity-check (LDPC) performance simulations, the receiver selects appropriate symbol rates from a set of discrete levels and conveys the update decisions through a feedback channel. A lightweight decision logic is adopted to enable timely rate adjustments, and a feedback frame structure with frame indices is designed to ensure reliable symbol rate synchronization under frame loss. Indoor experiments with controlled link distance and noise variations verify the predictable adaptation behavior of the proposed scheme, while outdoor daylight experiments demonstrate stable communication and effective symbol rate tracking under time-varying channel conditions.

Delay jitter is a fundamental metric of communication link determinism and stability. This work introduces a path-ablation and timestamp-embedding feedback methodology, supported by a purpose-built static visible light communication (VLC) testbed, to systematically evaluate delay jitter. Two transmission paths are compared: a direct physical-layer connection and a protocol stack-based transmission. Real transmission results show that the direct connection constrains end-to-end average jitter tightly within 0.565–0.789 µs, achieving two-to-three orders of magnitude improvement over the stack-based transmission, where jitter ranges from 22.057 to 200.589 µs. Under diverse environmental perturbations and incidence angles, the direct connection jitter remains stable between 0.588–0.663 µs, confirming robustness and angular insensitivity. Furthermore, optical power sweeps at the receiver reveal a usable-power regime, within which valid-packet jitter converges consistently to 0.594–0.611 µs. Collectively, these results establish the deterministic and robust nature of the direct-connection-based visible-light links, providing experimental evidence for their application in high-reliability optical communications.

To address varying environmental conditions, this paper proposes a transmitter-side adaptive power updating mechanism for ultraviolet (UV) communication. The receiver performs channel estimation on the forward link and sends back the signal and background radiation components to the transmitter via the feedback link. We begin by generating a bit error rate (BER) performance map via offline simulation for various signal and noise conditions. The transmitter combines real-time channel information from the feedback link with this performance map to dynamically update the number of active transmitting elements (LEDs), ensuring the signal strength required for an ultra-low BER. Furthermore, to reduce power consumption, we have also defined a power-saving threshold, which is intended to proactively limit and decrease the number of active LEDs when the channel conditions are favorable, such as in short-range communication, thereby avoiding unnecessary energy loss. Outdoor experiments are conducted to verify the performance in long-range scenarios where the transceiver distance increases. Experimental results demonstrate the dual advantage of the proposed mechanism: it ensures high power efficiency by operating at low power in favorable channel conditions, while also guaranteeing communication reliability and robustness by increasing power in long-distance, weak-channel scenarios.

Using a unified free-space experimental platform, this paper presents a direct comparison of the same photovoltaic (PV) receiver in indoor and outdoor scenarios. The results demonstrate that strong sunlight does not degrade PV-based visible light communication (PV-VLC). Instead, sunlight-induced optical self-bias stabilizes the operating point, shifting the bit error rate (BER)–frequency curve downward/rightward and extending high-frequency usability. On the sensing side, we target wavelength discrimination with the same PV front end. The self-bias improves symbol stability against baseline drift and short-term fluctuations, while a window-accumulation strategy effectively suppresses noise and improves decision robustness. Even without data windowing, outdoor symbol-by-symbol classification achieves approximately 93.8% accuracy; and with moderate windows, the accuracy at both outdoor symbol rates reaches 100%. In the indoor scenario, the accuracy increases from about 53.0% to 97.3% with larger windows. Overall, these findings show that sunlight-induced self-bias combined with window accumulation enables robust PV-VLC and integrated sensing and communication, providing practical guidance for energy-autonomous terminals in vehicular, unmanned aerial vehicle, and industrial applications.

This paper demonstrates an ultraviolet (UV) time-of-flight (TOF) ranging system. We propose three enhanced approaches to overcome the non-perfect transceiver characteristics and validate their effectiveness, including an adaptive windowing method employing photon-counting statistics to dynamically adjust synchronization thresholds and enhance robustness against variable signal intensities, a waveform-matched synchronization approach with quantized coefficients to mitigate errors from non-ideal UV pulse shapes, and a light-intensity-assisted distance correction technique to compensate for hardware delays and channel-induced biases. The performance gain of improved methods for non-perfect transmitter and receiver are experimentally validated, demonstrating its ability to enhance both the reliability and accuracy of the ranging system. Finally, we realize a real-time hardware system and conduct two field tests under both daytime and night, resulting in root-mean-square errors (RMSEs) of 0.96 m and 1.01 m, respectively-closely approaching the theoretical lower bound dictated by current hardware constraints.

In this paper, we propose a novel computational method, termed Monte Carlo Integration with Common Volume Sampling (MCI-CVS), which enables direct photon sampling within the common scattering volume. This approach significantly accelerates the convergence of optical wireless scattering channel simulations. Although each sample in MCI-CVS incurs approximately 3.74 times the computational cost of conventional methods, numerical results for ultraviolet scattering channels demonstrate that it reduces the required number of samples by a factor of 1,000 compared with Monte Carlo Integration with Importance Sampling (MCI-IS). As a result, MCI-CVS achieves an overall efficiency gain of approximately 267.3 times faster than MCI-IS and about 1,206 times compared to the baseline Monte-Carlo Simulation (MCS) method. Furthermore, the proposed method effectively addresses inefficient photon path sampling in infrared scattering channels, where low scattering and absorption coefficients degrade sampling efficiency and thus result in slow convergence. By incorporating photon path reuse, MCI-CVS also enables fast generation of two-dimensional scattering fields on a standard personal computer.

We propose a covert ultraviolet communication scheme assuming that the eavesdropper does not know the transmission time slot of Alice-Bob link. The covertness and achievable rate of Alice-Bob link are analyzed. Under the covertness constraints on Warden, we propose to optimize the frame structure to maximize the achievable rate. Numerical and experimental results demonstrate the effectiveness and reliability of the designed system.

This article proposes a state-counting model to uniformly characterize photomultiplier tube (PMT) outputs across all operating regimes in underwater optical wireless communication (UOWC). Based on this model, a signal detection method with optimized state decision threshold is developed to improve adaptability in dynamic environments. Experiments show that the proposed method yields better bit error rate (BER) performance than the benchmark methods, achieving a BER of 10(-3) with only five samples per symbol and supporting data rates up to 50 Mbps over 30m underwater channel. A real-time FPGA-based UOWC system implementing the detection algorithm demonstrates its feasibility in handling large signal fluctuations while maintaining acceptable complexity, which highlights the method's practical applicability.

For challenging environments where conventional line-of-sight (LOS) systems are not available, we propose a novel non-line-of-sight (NLOS) ultraviolet (UV) positioning system based on time difference of arrival (TDOA). The system operates through two complementary procedures: constructing a geometric model by optimizing the reception angle according to signal intensity and transmitter beam characteristics, and estimating the time difference of arrival via UV synchronization to resolve the positioning equations. To further improve positioning accuracy, a symbol broadening matching synchronization scheme is developed, which employs the pulsed response function of the UV single-scattering channel obtained from initial positioning results. An upper bound on the mean square error (MSE) applicable to this synchronization process is also derived. Simulation results confirm that the proposed approach significantly enhances the synchronization estimation accuracy and effectively constrains the final positioning error within the theoretical bounds. We also verify the robustness of the algorithm under different environmental conditions. An average decrease of 34.03% for the synchronization standard deviation is achieved, and an average decrease of 32.67% for the positioning error is achieved. Additionally, the proposed positioning algorithm effectively compensates for the systematic bias present in the initial estimates, bringing the corrected center closer to the ground truth. To optimize system deployment, we propose efficient impulse response approximation instead of time-consuming Monte Carlo simulations, achieving a 28.64% reduction in positioning error.

To construct time slot synchronization for further resource management of ultraviolet (UV) networks, we propose a slot synchronization protocol based on flooding time and power optimization. Beacon sequence is applied to broadcast slot information without exchanging messages. Firstly, we propose slot synchronization schemes based on beacon sequence for two cases, with both multiple initialization nodes and a single initialization node. Then, we investigate the network synchronization accuracy with non-overlapping and overlapping beacons, as well as the synchronization accuracy of the entire network. Based on the network synchronization accuracy results, we propose power optimization to expand the synchronization range and enhance the synchronization probability. Numerical results show that power optimization can approximately achieve 18% higher synchronization success probability and 15% larger network synchronization range. Moreover, the proposed synchronization scheme is approximately 10 time periods shorter than meshed emergent firefly synchronization (MEMFIS), with approximately 40% lower network synchronization error.

To address varying environmental conditions, this paper proposes an adaptive threshold updating algorithm for photomultiplier tube (PMT) signals in ultraviolet (UV) communication. The receiver maintains a set of pulse detection thresholds and performs channel estimation for each threshold. Using the channel estimation of both signal and background radiation components, we introduce a method to estimate the bit error rate (BER) corresponding to each threshold. Additionally, an adaptive threshold selection system is developed to dynamically update signal detection in response to changing environments. The accuracy of the optimal detection threshold is experimentally evaluated. The effectiveness of the proposed adaptive threshold algorithm is validated through indoor experiments, where trends in channel estimation results, detection thresholds, and frame error rates (FER) are recorded. Moreover, an outdoor experiment is conducted to verify the algorithm's performance under sunlight. The experimental results demonstrate that the adaptive threshold algorithm effectively adjusts the pulse detection threshold based on the current channel state, ensuring robust communication performance in dynamic channel conditions.

We investigate the effect of Analog-To-Digital Converter (ADC) sampling rate on the synchronization and positioning error under ultra-violet (UV) photon-level detection. Taking into account the response pulse of the current photomultiplier tube (PMT) devices to incident UV photons, we investigate the photon-level synchronization error both numerically and experimentally. It is shown that increasing the analog-to-digital converter (ADC) sampling rate to 200 MHz results in significant positioning accuracy improvement, achieving a good balance between synchronization accuracy and hardware cost. From both 2-dimensional and 3-dimensional time-difference-of-arrival (TDOA) positioning simulations, increasing the ADC sampling rate to 200 MHz leads to enhanced positioning accuracy. The gain of further increasing the sampling rate to 500 MHz becomes less noticeable.

Beam steering using high-speed micro-electro-mechanical system (MEMS) mirrors can support mobility between optical wireless communication (OWC) terminals. However, optimization is necessary to enhance the alignment accuracy and reduce the response time. In this study, we design a real-time MEMS-based tracking system and investigate different scanning patterns to achieve satisfactory tracking performance. We propose a pattern-based scanning technique that employs non-repetitive fast scanning to significantly reduce the tracking latency without compromising detection efficiency. The system is further evaluated in terms of bit error rate (BER) performance and alignment error under various motion conditions. Experimental results demonstrate that the tracking system can effectively maintain reliable communication links in diverse scenarios. The proposed system achieves a data rate of 31.25 Mbps at a maximum moving speed of 1 m/s, with an average BER below the forward error correction limit within a 10-meter range.

Considering the scattering characteristics of wireless ultraviolet (UV) channels, the double-scatter assumption is revealed to well characterize the scattering-enhanced channel over a range of less than 100 m. Based on the cuboid scatterers with representative shapes, we present a ray-tracing algorithm under scattering enhancement conditions and give the probability density function of the free path in a non-uniform atmosphere. Then, we propose a semi-analytical (SA) link gain model with scattering enhancement, via generating a set of coordinates for a series of conical integral paths using a rotation matrix. On this basis, a one-dimensional integral expression for the single-scatter of the proposed SA link gain model is obtained, and the expression for the double-scatter is further derived. The link gain evaluation results show that the presence of scattering region can improve the received power of the non-line-of-sight UV system at a receiver. For all considered transceiver geometry, the results of the proposed SA model match well with the counterparts of Monte-Carlo (MC) based numerical calculation, with a gap of around 2 dB for both narrow and wide beam. In terms of computational efficiency, the execution time of the proposed SA model for the single-scatter and double-scatter is significantly reduced to 0.065% and 9.69% of those of the MC simulations, respectively. We further establish experiments using a fog-filled quartz cubic cavity in an outdoor environment to verify the correctness of the proposed model. The link gain from the numerical evaluation assuming double-scatter can improve the agreement with the experimental results by 1 dB compared with that of the single-scatter assumption.

This paper demonstrates an automatic beam alignment system for a laser diode (LD)-photomultiplier tube (PMT) based underwater optical wireless communication (UOWC) system. Initially, the imaging characteristics of the underwater laser spot are described. Subsequently, a laser outlet estimation algorithm based on principal component analysis (PCA) is proposed. The direction of the emitted light beam is rapidly determined by reflecting it with a micro-electro-mechanical system (MEMS) mirror to align with the receiver. Experimental results validate the feasibility of the proposed beam alignment method, indicating an acceptable alignment error. Moreover, the proposed system effectively enhances the bit error rate (BER) performance, meeting the BER requirements of the forward error correction (FEC) limit when the receiver moving speed is below 0.45 m/s.

In this article, we design and demonstrate a beam tracking system on the receiver side for ultraviolet (UV) communication using a single photomultiplier tube (PMT). Specifically, we investigate the properties of the angle-domain channel for UV communication. We characterize the effects of metallic housing and the spatial nonuniformity of the PMT on the angle-domain channel, both of which can significantly impact the channel's performance. To adjust the orientation of the PMT and maintain satisfactory communication in mobile UV systems, we propose a beam alignment algorithm based on coordinate search and a beam tracking algorithm based on pattern-search. A real-time system is developed to test the performance of these algorithms. Experimental results show that the proposed beam alignment algorithm can quickly search for the incident direction, while the beam tracking algorithm effectively tracks a moving transmitter at a rate of 5.5 degrees/s, ensuring a reliable communication link. The proposed beam alignment and tracking scheme uses only a single PMT, making it lighter and more cost-effective compared to the PMT array scheme, and is thus well-suited for deployment in compact UV communication terminals.

Due to its multiple advantages, ultraviolet (UV) communication exhibits significant potential for outdoor mobile communication. This paper proposes novel multi-access protocols for UV mobile communication networks, where each secondary node employs a single directional UV transceiver and tracks the primary node. The proposed protocols are evaluated through simulations based on a network model that accounts for node movement, node tracking, and UV link gain. Simulation results demonstrate that the proposed protocols outperform the conventional Carrier Sense Multiple Access protocol.