New high-data-rate multimedia services and applications are evolving continuously and exponentially increasing the demand for wireless capacity of fifth-generation (5G) and beyond. The existing radio frequency (RF) communication spectrum is insufficient to meet the demands of future high-data-rate 5G services. Optical wireless communication (OWC), which uses an ultra-wide range of unregulated spectrum, has emerged as a promising solution to overcome the RF spectrum crisis. It has attracted growing research interest worldwide in the last decade for indoor and outdoor applications. OWC offloads huge data traffic applications from RF networks. A 100 Gb/s data rate has already been demonstrated through OWC. It offers services indoors as well as outdoors, and communication distances range from several nm to more than 10 000 km. This paper provides a technology overview and a review on optical wireless technologies, such as visible light communication, light fidelity, optical camera communication, free space optical communication, and light detection and ranging. We survey the key technologies for understanding OWC and present state-of-the-art criteria in aspects, such as classification, spectrum use, architecture, and applications. The key contribution of this paper is to clarify the differences among different promising optical wireless technologies and between these technologies and their corresponding similar existing RF technologies.

A Comparative Survey of Optical Wireless Technologies: Architectures and Applications

Optical wireless communications (OWCs) refer to wireless communication technologies which utilize optical carriers in infrared, visible light, or ultraviolet bands of electromagnetic spectrum. For the sake of an OWC link design and performance evaluation, a comprehensive understanding and an accurate prediction of link behavior are indispensable. Therefore, accurate and efficient channel models are crucial for the OWC link design. This paper first provides a brief history of OWCs. It also considers OWC channel scenarios and their utilization trade-off in terms of optical carrier, range, mobility, and power efficiency. Furthermore, the main optical channel characteristics that affect the OWC link performance are investigated. A comprehensive overview of the most important OWCs channel measurement campaigns and channel models, primarily for wireless infrared communications and visible light communications, are presented. OWCs channel models are further compared in terms of computation speed, complexity, and accuracy. The survey considers indoor, outdoor, underground, and underwater communication environments. Finally, future research directions in OWCs channel measurements and models are addressed.

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Optical Wireless Communication Channel Measurements and Models

Optical wireless communication (OWC) is an excellent complementary solution to its radio frequency (RF) counterpart. OWC technologies have been demonstrated to be able to support high traffic generated by massive connectivity of the Internet of Things (IoT) and upcoming 5th generation (5G) wireless communication systems. As the characteristics of OWC and RF are complementary, a combined application is regarded as a promising approach to support 5G and beyond communication systems. Hybrid RF/optical and optical/optical wireless systems offer an excellent solution for recovering the limitations of individual systems as well as for providing positive features of each of the technologies. An RF/optical wireless hybrid system consists both RF and optical-based wireless technologies, whereas an optical/optical wireless hybrid system consists two or more types of OWC technologies. The co-deployment of wireless systems can improve system performance in terms of throughput, reliability, and energy efficiency of individual networks. This study surveys the state of the art and key research directions regarding optical wireless hybrid networks, being the first extensive survey dedicated to this topic. We provide a technology overview of existing literature on optical wireless hybrid networks, such as RF/optical and optical/optical systems. We consider the RF-based macrocell, small cell, wireless fidelity, and Bluetooth, as well as optical-based visible light communication, light fidelity, optical camera communication, and free-space optical communication technologies for different combinations of hybrid systems. Moreover, we consider underwater acoustic communication for hybrid acoustic/optical systems. The opportunities brought by hybrid systems are presented in detail. We outline important challenges that need to be addressed for successful deployment of optical wireless hybrid network systems for 5G and IoT paradigms.

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Optical Wireless Hybrid Networks: Trends, Opportunities, Challenges, and Research Directions

Orthogonal frequency division multiplexing (OFDM) is a modulation technique which is now used in most new and emerging broadband wired and wireless communication systems because it is an effective solution to intersymbol interference caused by a dispersive channel. Very recently a number of researchers have shown that OFDM is also a promising technology for optical communications. This paper gives a tutorial overview of OFDM highlighting the aspects that are likely to be important in optical applications. To achieve good performance in optical systems OFDM must be adapted in various ways. The constraints imposed by single mode optical fiber, multimode optical fiber and optical wireless are discussed and the new forms of optical OFDM which have been developed are outlined. The main drawbacks of OFDM are its high peak to average power ratio and its sensitivity to phase noise and frequency offset. The impairments that these cause are described and their implications for optical systems discussed.

OFDM for Optical Communications

The inherent broadcast characteristics of the visible light communication (VLC) channel makes VLC downlinks susceptible to unauthorized terminals in many actual VLC scenarios, such as offices and shopping centers. This paper considers a multiple-input-single-output (MISO) VLC scenario with multiple light fixtures acting as the transmitter, a VLC receiver as the legitimate user, and an eavesdropper attempting to intercept the undisclosed information. To improve the confidentiality of VLC links, a physical-layer anti-eavesdropping framework is proposed to obscure the unauthorized eavesdroppers and diminishes their capability of inferring the information through smart beamforming over the MISO VLC wiretap channel. To cope with the intractable problem of finding the theoretically optimal solution of the secrecy rate and utility for the MISO VLC wiretapping channel, a reinforcement learning (RL)-based VLC beamforming control scheme is proposed to achieve the optimal beamforming policy against the eavesdropper. Furthermore, a deep RL-based VLC beamforming control scheme is proposed to handle the curse of dimensionality for both observation space and action space and avoid the quantization error of the RL-based algorithm. Simulation results show that the proposed learning-based VLC beamforming control schemes can significantly decrease the bit error rate of the legitimate receiver and increase the secrecy rate and utility of the anti-eavesdropping MISO VLC system, compared with the benchmark strategy.

Deep Reinforcement Learning-Enabled Secure Visible Light Communication Against Eavesdropping

In this letter, we present a novel hybrid intensity modulation and direct detection communication system, which integrates asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and on-off keying (OOK) modulation schemes. First, the negative ACO-OFDM is proposed based on the conventional ACO-OFDM system to accommodate the on case with direct current in the OOK modulation, while the ACO-OFDM can match the off case in the OOK modulation. Therefore, the novel hybrid system combines the ACO-OFDM and OOK modulation schemes, while the signals can be recovered at the receiver to support the different qualities of service with high spectral efficiency and can be adapted to various receivers with different complexities. Simulation results are reported for the visible light channel and show that both the ACO-OFDM and OOK signals can be well recovered.

Novel Visible Light Communication Approach Based on Hybrid OOK and ACO-OFDM

In this paper, three hybrid modulation schemes for visible light communication based on orthogonal frequency division multiplexing (OFDM) are compared, which are asymmetrically clipped dc-biased optical OFDM, hybrid asymmetrically clipped optical OFDM, and layered asymmetrically clipped optical OFDM (LACO-OFDM) The following aspects are analyzed and compared, such as the probability distribution functions, the peak-to-average power ratio, and the bit error ratio performance in terms of optical bit energy to noise power under different conditions Furthermore, the optimal proportion of the optical power on different layers for LACO-OFDM is also investigated Simulation results show that LACO-OFDM has better performance than other hybrid schemes in high signal-to-noise ratio (SNR) scenario, and the improvement is slight when more than four layers signals are utilized, while in low SNR condition, LACO-OFDM with two layers performs better than that with more layers

Comparison of Hybrid Optical Modulation Schemes for Visible Light Communication

In this paper, in order to accommodate dimming demands for illumination, which is an essential requirement for visible light communication (VLC), a dimming control scheme with high power and spectrum efficiency is proposed. In the proposed method, a negative hybrid asymmetrically clipped optical orthogonal frequency division multiplexing (NHACO-OFDM) is first investigated and then combined with HACO-OFDM signal for multiplexed transmission. By adjusting the proportion of these two signals, the novel dimming control scheme can be achieved. Simulation results conducted that the proposed scheme can support optical wireless communication with higher power and spectrum efficiency and carry out the dimming control with broad dimming range for illumination, compared with conventional counterparts under various illumination conditions.

Dimming Control Scheme With High Power and Spectrum Efficiency for Visible Light Communications

In this paper, the adaptive layered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM) with variable layer is investigated. The channel capacities of the adaptive LACO-OFDM with different layers are formulated to determine the optimal layer amount under different optical signal-to-noise ratio scenarios with electrical or optical power constraint, respectively. Computer simulations verify the validity of the theoretical analysis for the proposed layer assignment scheme.

Adaptive LACO-OFDM With Variable Layer for Visible Light Communication

As the shortage of spectrum is becoming a serious problem to the wireless communication, visible light communication (VLC) is attracting widespread attention. However, the access to the backbone network is likely to completely modify the communication network, thus leading to a great cost. Fortunately, the technology of power line communication (PLC) is a perfect match to VLC, which provides power supply and connection for VLC to the backbone network. In this study, the authors take an integration of PLC and VLC into consideration and mainly focus on a communication framework with multi-service. In the case of not making a big modification to the origin network, a direct re-transmission concept is considered and three implementation schemes which differ in frequency domain, time domain, and bit division multiplexing (BDM) are proposed. The demonstration platform and the simulation results suggest that the proposed schemes have the ability to support multi-service transmission with no big modification of current power line network. Moreover, the BDM scheme can accommodate different requirements of quality of service with better performance than conventional multi-service schemes.

Junnan Gao

Papers

Novel Visible Light Communication Approach Based on Hybrid OOK and ACO-OFDM

Comparison of Hybrid Optical Modulation Schemes for Visible Light Communication

Dimming Control Scheme With High Power and Spectrum Efficiency for Visible Light Communications

Adaptive LACO-OFDM With Variable Layer for Visible Light Communication

Integrated power line and visible light communication system compatible with multi-service transmission