Visible light communication (VLC) enables access to huge unlicensed bandwidth, a higher security level, and no radio frequency interference. With these advantages, VLC emerges as a complementary solution to radio frequency communications. VLC systems have primarily been designed for indoor scenarios with typical transmission distances between 2 and 5 m. Different designs would be required for larger distances. This paper proposes for the first time the use of a liquid crystal (LC)based re-configurable intelligent surface (RIS) on improving the VLC signal detection and transmission range. An LC-based RIS presents multiple advantages, including the tunability of its photorefractive parameters. Another advantage is its light amplification capabilities when under the influence of an externally applied field. In this paper, we analyze an LC-based RIS structure to amplify the detected light and improve the VLC signal detection and transmission range. Results show that mixing LC with 4 to 8 wt% concentration of a dye such as the terthiophene (3T-2MB) improves the VLC transmission range of about 0.20 to 1.08 m. This improvement can reach 6.56 m if we combine 8 wt% concentration of 3T-2MB and 0.1 wt% concentration of trinitrofluorenone. Index Terms-Visible light communication (VLC), reconfigurable intelligent surface (RIS), dye-doped and polymer liquid crystals, terthiophene (3T-2MB), trinitrofluorenone (TNF), VLC transmission range expansion, radiative transfer equation (RTE), Beer-Lambert law.

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This article reviews the visible light communications (VLC), a technology in which the visible spectrum is modulated to transmit data. It presents the VLC communication system: the transmitter, the channel, and the receiver. The single and multichannel transceivers are presented. The channel for a system that uses a single light‐emitting diodes (LED) and the matrix representing the multicolor channel are discussed. Various modulation schemes, basic techniques used to implement a VLC system, and different causes of dimming are discussed. In addition, standardization of the VLC technology, applications, as well as challenges for VLC practical implementation and commercialization are reviewed.

One of the main problems faced by communication systems is the presence of skip-zones in the targeted areas. With the deployment of the fifth-generation mobile network, solutions are proposed to solve the signal loss due to obstruction by buildings, mountains, and atmospheric or weather conditions. Among these solutions, reconfigurable intelligent surfaces (RIS), which are newly proposed modules, may be exploited to reflect the incident signal in the direction of dead zones, increase communication coverage, and make the channel smarter and controllable. This paper tackles the skip-zone problem in terrestrial free-space optical (T-FSO) systems using a single-element RIS. Considering link distances and jitter ratios at the RIS position, we carry out a performance analysis of RIS-aided T-FSO links affected by turbulence and pointing errors, for both heterodyne detection and intensity modulation-direct detection techniques. Turbulence is modeled using the Gamma-Gamma distribution. We analyze the model and provide exact closedform expressions of the probability density function, cumulative distribution function, and moment generating function of the end-to-end signal-to-noise ratio. Capitalizing on these statistics, we evaluate the system performance through the outage probability, ergodic channel capacity, and average bit error rate for selected binary modulation schemes. Numerical results, validated through simulations, obtained for different RIS positions and link distances ratio values, reveal that RIS-based T-FSO performs better when the RIS module is located near the transmitter.Index Terms-Free-space optical communications, reconfigurable intelligent surfaces, unified Gamma-Gamma turbulence channels with pointing errors, average bit error rate, ergodic channel capacity, outage probability. I. INTRODUCTIONThe recent extensive investigation of optical wireless communications in the outdoor environment, also called free-space optical (FSO), is motivated by its advantages compared to its radio frequency (RF) counterpart, especially in point-to-point networks. These advantages include larger bandwidth, higher channel capacity, and cost-effectiveness due to an unlicensed environment [1], which can be leveraged to solve the bandwidth limitation in the RF technology. Its most prominent applications are satellite-to-ground, satellite-to-satellite, and terrestrial FSO (T-FSO) systems such as building-to-building (B2B) 1 communications. Besides turbulence, pointing errors,

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In this paper, relay selection is considered to enhance security of a cooperative system with multiple threshold-selection decode-and-forward (DF) relays. Threshold-selection DF relays are the relays in which a predefined signal-to-noise ratio is set for the condition of successful decoding of the source message. We focus on the practical and general scenario where the channels suffer from independent non-identical Rayleigh fading and where the direct links between the source and destination and source and eavesdropper are available. Based on channel state information knowledge, three relay selection strategies, namely traditional, improved traditional, and optimal, are studied. In particular, the secrecy outage probability of all three strategies are obtained in closed-form. It is found that the diversity of secrecy outage probability of all strategies can improve with increasing the number of relays. It is also observed that the secrecy outage probability is limited by either the source to relay or relay to destination channel quality.

Visible-light communication (VLC) has emerged as a prominent technology to address the radio spectrum shortage. It is characterized by the unlicensed and unexploited high bandwidth, and provides the system with cost-effective advantages because of the dual-use of light bulbs for illumination and communication and the low complexity design. It is considered to be utilized in various telecommunication systems, including 5G, and represents the key technology for light-fidelity. To this end, VLC has to be integrated into the existing telecommunication networks. Therefore, its analysis as a network technology is momentous. In this article, we consider the feasibility of using VLC as a network technology and discuss the challenges related to the implementation of a VLC-based network, as well as the integration of VLC into existing conventional networks and its inclusion in standards.Visible light communication (VLC) is a data transmission technology which exploits the light beam as a communication medium. It is a variant of optical wireless communications, which uses light-emitting diodes (LEDs) as antennas and is characterized by short transmission range. In the indoor environment, VLC provides both data transmission and illumination [1]. Its main applications include light-fidelity (Li-Fi), indoor positioning, as well as vehicle-to-vehicle and infrastructure-to-vehicle communications. It has recently been demonstrated that using laser diodes (LDs), VLC can also be utilized in access networks [2]. The efficient utilization of VLC for such applications requires its analysis as a network technology, which is the focus of this article.The interest in the VLC technology can be evidenced by the growing number of research works in the literature [1,3,4,5,6] (and references therein), the prototypes and field trials proposed by researchers and industry, and by the standardization activities. Following these efforts, the effective deployment is seen as the next key step. Several challenges are to be overcome when practically deploying communication systems using VLC in both simplex and duplex configurations. Among these challenges, we underline the fact that the return path in a VLC-based communication system may use a different technology, such as radio frequency (RF), power line communication, fiber optics, and free space optical. If using VLC, the return path can be based on a different wavelength. Another important challenge is the illumination coverage. The VLC transmission range is limited, as it is imposed by the lighting coverage of the source, which is short by nature [7]. LEDs and LDs are the two main types of light sources considered in VLC. Compared to LEDs, whose lighting range is only a couple of hundred meters, LDs provide an illumination coverage in the order of a couple of kilometers. On the other hand, since only positive and real signals can be successfully transmitted in VLC systems, the modulation schemes that can be employed should produce a non-complex and asymmetric signal. * Alain R. Ndjiongue is with ...