Direct current (DC) is added into intensity modulation and direct detection (IM/DD) signals in time domain to make most negative signals be positive in DC biased-optical orthogonal frequency division multiplexing (DCO-OFDM) systems for optical wireless communication (OWC). To the best of the authors' knowledge, this is the first work to utilize DC inherent in DCO-OFDM signals to allow semi-blind joint synchronization and channel estimation for DCO-OFDM OWC systems. We propose a novel semi-blind DC-based joint estimation (SDJE) approach, where sampling time offset (STO) as one of timing synchronizations and channels can be jointly estimated based on DC inherent in DCO-OFDM signals. The proposed SDJE approach is spectral-efficient, requiring no pilot. This is different from state-of-the-art works that employ a number of pilots to perform synchronization and channel estimation. We formulate a cost function by exploring the difference between the received signals and DC signals. The joint STO and channel estimations can be conducted by minimizing the formulated cost function. Furthermore, we propose a dimensionality reduction approach, and design an offline database to reduce complexity. Simulation results show that the proposed SDJE approach provides bit error rate (BER) performance close to the ideal case with no STO and perfect channel state information (CSI).

Laser-based optical wireless communication (OWC) can provide ultra-high-speed mobile connectivity for future networks. To address the misalignment challenges inherent in laser-based OWC systems, this paper introduces a novel intelligent reflecting surface (IRS)-assisted indoor OWC system. This system employs a multi-cell architecture and leverages passive IRS elements to enhance user coverage. The concept of angular coverage in indoor laser-based communication is introduced, and an analytical framework is proposed to calculate the angular coverage probability of users. Additionally, an innovative IRS deployment strategy, inspired by circle-packing principles, is proposed to enhance the angular coverage of users with varying locations and orientations. Numerical results validate the proposed IRS deployment strategy, demonstrating its superior performance compared to a conventional laser-based OWC architecture, achieving at least a threefold improvement in angular coverage probability.

Optical wireless communication (OWC) emerges as a pivotal solution for achieving terabit-level aggregate throughput in next-generation wireless networks.With the mature high-speed transceivers and advanced (de)multiplexing techniques designed for fiber optics, fiber-coupled OWC can be seamlessly integrated into existing ultra-high-speed networks such as data centres. In particular, OWC leveraging spatial mode multiplexing (SMM) and few-mode fiber (FMF) coupling can significantly increase capacity, though misalignment may reduce performance. This paper presents a thorough investigation into the SMM-enabled FMF coupling OWC systems affected by link misalignment, specifically focusing on systems with intensity modulation with direct detection (IM/DD) receivers. A theoretical analysis is conducted to assess the fiber coupling efficiency of the considered system in the presence of both pointing error and angle of arrival (AOA) fluctuations caused by random device vibrations. Our model elucidates the dependence of coupling efficiency to the order of the incident modes, highlighting the critical role of beam properties in system performance. To mitigate the intermodal crosstalk arising from link misalignment, we employ zero-forcing beamforming (ZF) to enhance the overall aggregated data rate. Through extensive numerical results, we identify optimal system configurations encompassing aperture design and mode selection, leading to a capacity boost exceeding 200% when compared with system performance without considering the ZF.

Laser-based optical wireless communication (OWC) is a key enabler for achieving terabit aggregate throughput for future indoor wireless networks. Fiber-coupled optical transceivers are the most mature technology to realize ultra-high-speed transmission links. However, fiber-coupled OWC links are vulnerable to misalignment issues due to device vibrations. To alleviate the impact of misalignment, the single-mode fiber can be replaced by the few-mode fiber (FMF). This paper presents a refined model to assess the impact of FMF coupling on indoor OWC channels in the presence of misalignment. Moving beyond the traditional use of Gaussian beams, our approach encompasses a broader range of spatial beams, offering a more realistic perspective on fiber coupling losses. We diverge from the conventional far-field assumptions and idealized channel conditions, instead offering detailed theoretical insights into the effects of both pointing errors and angle of arrival (AOA) fluctuations caused by random device vibrations. Our simulations reveal varied coupling efficiencies for different incident modes across FMF modes when misalignments occur. Furthermore, we provide insights into the achievable data rates of indoor fiber-coupled OWC systems, showing the variation of the throughput with different transmitted mode orders.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution to achieve high-speed mobile communications. To address the misalignment challenges encountered in laser-based LiFi, this study introduces an innovative full-coverage indoor LiFi system with integrated communication and positioning capabilities, leveraging intelligent reflected surfaces (IRSs). By design, the proposed system ensures successful wireless downlink connectivity, irrespective of the user's random location and orientation status. An algorithm is developed to ascertain the optimal deployment of both access points (APs) and IRS layers. Moreover, this study introduces a machine learning (ML)-based OWC positioning approach designed to enhance the accuracy of the user positioning, thereby effectively boosting the performance of the IRS-assisted communication system. Numerical results demonstrate the superiority of the proposed positioning approach over traditional methods in terms of average data rate.

Visible light communication (VLC) is an emerging mode of wireless communication that supports both illumination and communication. Dimming control is one essential function for such systems that requires a sensitive receiver for low-light conditions. The use of an array of single-photon avalanche diodes (SPADs) is one promising approach to enhancing receivers’ sensitivity in a VLC system. However, an increase in the light brightness can impair the performance of SPAD-based receivers due to non-linear effects caused by the SPAD dead time. In this paper, an adaptive SPAD receiver is proposed to ensure reliable operation at a wide dynamic range of channel conditions and different dimming levels. In the proposed receiver, a variable optical attenuator (VOA) is used to optimize the SPAD’s incident photon rate at each channel condition. Two dimming control methods of IEEE 802.15.7 standard based on analog and digital dimming for on-off keying (OOK) modulation are studied and compared for the proposed SPAD-based VLC system. Our detailed performance investigation shows that the proposed adaptive receiver outperforms traditional PIN PD and non-adaptive SPAD array receivers in terms of the bit error rate (BER) performance. Moreover, the considered analog dimming can provide a significantly higher achievable data rate compared to that of digital dimming at the expense of lower power efficiency.

In the context of beyond the fifth-generation (B5G), free-space optics (FSO) is a vital technology for high data rate applications because of its low cost and licence-free high bandwidth. However, during adverse weather, the FSO link’s capacity can be significantly impacted, and establishing parallel radio frequency (RF) backup links just for occasional utilization is both complex and costly. In this study, in order to improve the availability of FSO link in adverse weather conditions, we propose to use unmanned aerial vehicles (UAVs) as hybrid relays to intercept an FSO link. The UAVs construct an intermediate millimeter wave (MMW) link, while ensuring that the approach has no impact on the source and destination. The end-to-end system capacity is maximized by optimizing UAVs' location based on the negatively correlated FSO and MMW link length. The performance of the proposed model is compared to benchmark systems such as single FSO, fixed relay-assisted FSO, and fixed hybrid FSO/RF systems. The numerical results clearly demonstrate the proposed scheme’s superiority. Finally, using weather statistics from the cities of Edinburgh and London in the United Kingdom, the performance of the proposed system and its counterparts in realistic scenarios is demonstrated, which illustrates that the proposed system may significantly improve link availability towards the carrier-class criterion.

In recent years, there has been a growing interest in exploring the application of single-photon avalanche diode (SPAD) in optical wireless communication (OWC). As a photon counting detector, SPAD can provide much higher sensitivity compared to the other commonly used photodetectors. However, SPAD-based receivers suffer from significant dead-time-induced non-linear distortion and signal dependent noise. In this work, we propose a novel SPAD-based OWC system in which the non-linear distortion caused by dead time can be successfully eliminated by the pre-distortion of the signal at the transmitter. In addition, another system with joint pre-distortion and noise normalization functionality is proposed. Thanks to the additional noise normalization process, for the transformed signal at the receiver, the originally signal dependent noise becomes signal independent so that the conventional signal detection techniques designed for AWGN channels can be employed to decode the signal. Our numerical results demonstrate the superiority of the proposed SPAD-based systems compared to the existing systems in terms of BER performance and achievable data rate.

Optical wireless communication (OWC) has attracted more and more attention in recent decades. One promising method of improving the sensitivity of the receivers in OWC system is the utilization of the single-photon avalanche diode (SPAD). However, due to the non-linear effects induced by the dead time, the performance of SPAD-based receivers is significantly degraded in high incident power regime. Recently, we proposed a novel hybrid SPAD/PD receiver in which the receiver can adaptively switch between the SPAD- and PD-mode based on the instantaneous received optical power. In addition, a variable optical attenuator (VOA) is applied to optimize the performance of the SPAD unit. In this work, the superiority of the proposed hybrid receiver is experimentally demonstrated. For a data rate transmission of 450 Mbps, it is demonstrated that the proposed receiver significantly outperforms the traditional PD and SPAD array receiver in terms of the BER performance and can greatly extend the receiver dynamic range to reliably operate at both extremes of the incident light levels.

In this work, a hybrid RF/FSO system employing a game theoretic spectrum trading process is investigated. Different from the hybrid systems in the literature, in the proposed system no RF spectrum is preallocated to the FSO link. Only under infrequent adverse weather conditions where the availability of the FSO link is severely impaired, the source borrows a portion of the licensed RF spectrum from the surrounding RF nodes. The competition among RF nodes is considered and the Stackelberg game is employed to model the spectrum trading game between the source of FSO link and the RF nodes. Using the leased spectrum, the source can establish a hybrid link containing not only the original FSO link but also an RF link to improve its throughout to the destination. Our performance analysis demonstrates that the proposed scheme improves the average capacity of the system in adverse weather conditions remarkably, thereby enhancing the availability of FSO communications.

Free-space optical (FSO) communication offers wireless connectivity with high data rates and low system complexity; however, it suffers from the infrequent adverse weather conditions. This paper proposes a novel hybrid RF/FSO system based on a game theoretic spectrum trading process to enhance the reliability of FSO links. The proposed system is considered to be both spectrum- and power-efficient. It is assumed that no RF spectrum is preallocated to the FSO link and only when the link availability is severely impaired by the infrequent adverse weather conditions, i.e., fog, and so on, the source can borrow a portion of licensed RF spectrum from one of the surrounding RF nodes. A market-equilibrium-based pricing process is proposed for the spectrum trading between the source and RF nodes. By using the leased spectrum, the source is able to establish a dual-hop RF/FSO hybrid link to maintain its throughput to the destination. Our extensive performance analysis illustrates the effectiveness of the proposed communication system. It is demonstrated that the proposed scheme can significantly improve the average capacity of the system, especially when the surrounding RF nodes are with low traffic loads. In addition, the system benefits from involving more RF nodes into the spectrum trading process by means of diversity. Finally, the application of the proposed system in a realistic scenario is presented based on the weather statistics in the city of Edinburgh, U.K., which demonstrates that the system can substantially enhance the link availability toward the carrier-class requirement.

In this paper, a free-space quantum key distribution (QKD) system based on BB84 protocol is studied assuming that the transmitted quantum bits are multiplexed spatially based on orthogonal orbital angular momentum (OAM) carrying beams. The performance of such a system is estimated in terms of quantum bit error rate, outage capacity, and effective data rate taking into account the impact of error correction and privacy amplification. Based on the simulation of OAM-carrying beam propagating through atmospheric channels impaired by turbulence, the crosstalk among the multiplexed channels are investigated and its effect on the performance of the QKD system is evaluated. The presented results provide insight into the efficient design of such multiplexed QKD systems particularly in terms of the choice of the optimal average photon number per pulse and the multiplexing mode set.

In this paper, the spatial-mode multiplexing (SMM) free-space optical communication (FSO) systems with direct detection are considered. Unlike systems studied in several works in the literature where mutually incoherent channels are assumed, we investigate high-speed SMM FSO systems with mutually coherent channels where a single laser source with narrow linewidth is employed at the transmitter. An analytical model for such mutually coherent channels is developed and expression for aggregate achievable rate (AAR) is derived. Through numerical simulations, it is shown that an optimal transmitted mode set can be determined which results in the maximal asymptotic AAR at high transmitted power and for different turbulence conditions, the optimal mode set varies.

This paper investigates spatial-mode multiplexing (SMM) for practical free-space optical communication (FSO) systems using direct detection. Unlike several works in the literature where mutually incoherent channels are assumed, we consider mutually coherent channels that accurately describe SMM FSO systems employing a single laser source at the transmitter with a narrow linewidth. We develop an analytical model for such mutually coherent channels and derive expressions for aggregate achievable rate (AAR). Through numerical simulations, it was shown that there exist optimal transmit mode sets which result in the maximal asymptotic AAR at high transmitted power. Moreover, in order to resolve the reliability issues of such SMM FSO systems in the presence of turbulence, a so-called mode diversity scheme is proposed that can be easily implemented along with SMM FSO systems. It is demonstrated that mode diversity can significantly improve the outage probability and the E-outage achievable rate performance of the multiplexed channels in SMM FSO systems degraded by turbulence.

In this paper, the impairments of free-space optical (FSO) communication systems caused by angular fluctuations including beam misalignment and angle-of-arrival (AOA) fluctuations are modeled in the presence of both atmospheric turbulence and transceiver vibrations. In particular, assuming FSO receivers with a limited field-of-view (FOV), the fading caused by AOA fluctuations is studied. The outage probability expressions for both coherent and direct detections are derived in both shot-noise-limited and thermal-noise-limited regimes. For direct detection, the optimal receiver FOV that achieves the minimum outage probability is considered. Furthermore, the issue of imperfect phasefront tracking in practical coherent receivers is investigated.

In this paper, different optical detection schemes are investigated in the presence of turbulence-induced effects. Both log-amplitude fluctuations and angle-of-arrival (AOA) fluctuations introduced by atmospheric turbulence are taken into account in channel modelling. The impact of the angular spread can be thus determined based on the performance of the FSO receivers. For direct detection, we find the optimal receiver field-of-view (FOV) to improve the performance. We further show the benefit of coherent detection over direct detection in the presence of angular spread. Moreover, spatial diversity receivers which are able to significantly improve the performance of atmospheric optical systems are discussed in detail.