Toward the Use of Re-configurable Intelligent Surfaces in VLC Systems: Beam Steering

Ndjiongue, Alain R.; Ngatched, Telex M. N.; Dobre, Octavia A.; Haas, Harald

doi:10.1109/mwc.001.2000365

Cited by 52 publications

(39 citation statements)

References 13 publications

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“…where (•) is the ℎ -order modified Bessel function of second kind. The values of and can be calculated using the Rytov variance, 2 2 = 0.492 2 7 6 11 6 , which depends on the altitude-dependent index, 2 , characterized by the transmission environment, the angular wavenumber, = 2 , the aperture diameter, , and the transmission distance, . They are respectively given by [13]…”

Section: B the Fso Channelmentioning

confidence: 99%

Analysis of RIS-Based Terrestrial-FSO Link Over G-G Turbulence With Distance and Jitter Ratios

Ndjiongue

Ngatched

Dobre

et al. 2021

J. Lightwave Technol.

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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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Section: B the Fso Channelmentioning

confidence: 99%

Analysis of RIS-Based Terrestrial-FSO Link Over G-G Turbulence With Distance and Jitter Ratios

Ndjiongue

Ngatched

Dobre

et al. 2021

J. Lightwave Technol.

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“…Figure 3 depicts the structure and perspective view of an LCbased RIS element for VLC receivers, in which the construction is similar to most proposed designs [3], [9]. It shows a light conditioning unit using an LC cell and about twelve layers of thin materials located on both sides of the cell.…”

Section: Low-light Amplifier Using An Lc-based Rismentioning

confidence: 99%

“…offering LCs their wave-guiding potentials [3]. It includes two main components' contribution, namely, the ordinary refractive index, n o , and the extraordinary index, n e .…”

Section: Lc Concentration and Dopant Weighted Percentage Wt%mentioning

confidence: 99%

“…Even though the amplification gain is tunable, this type of system does not allow direct light re-transmission. In addition to its tuning smoothness, an LC-based RIS low-light amplifier does pure light amplification and presents a twofold-impact on the incident beam, namely, steering and amplification [3]. Employing this type of amplifier in VLC receivers has not yet been proposed or explored in the open literature.…”

Section: Introductionmentioning

confidence: 99%

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Re-configurable Intelligent Surface-based VLC Receivers Using Tunable Liquid-crystals: The Concept

Ndjiongue,

Ngatched,

Dobre

et al. 2021

Preprint

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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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“…Examples include plasmonics and graphene, to mention only two. Liquid crystals (LCs) have been identified as an excellent substance for RISs in both reflection and refraction phenomena, as the direction and amplitude of light can be controlled by altering the refractive index of the LC [41], [48], [49]. LC-based RISs have been successfully employed in a VLC receiver to improve the signal-to-noise ratio (SNR) at the receiver [41], [48], [49] and to realize an active RIS for FSO systems [32], [48].…”

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confidence: 99%

RIS-Assisted Visible Light Communication Systems: A Tutorial

Aboagye¹,

Ndjiongue²,

Ngatched³

et al. 2022

Preprint

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Recent intensive and extensive development of the fifth-generation (5G) of cellular networks has led to their deployment throughout much of the world. As part of this implementation, one of the challenges that must be addressed is the skip-zone problem, which occurs when objects such as trees, people, animals, and vehicles obstruct the transmission of signals.In free-space optical (FSO) and radio frequency (RF) systems, dead zones are most often caused by buildings and trees, while in visible light communications (VLC), obstructions are caused by individuals moving around a room or objects placed in the room. A signal obstruction can significantly reduce the signal-to-noise ratio in RF and indoor VLC systems, whereas in FSO systems, where the transmitted signals are directional, the obstruction can completely disrupt data transmission. Therefore, the skipzone dilemma must be resolved to ensure the smooth and efficient operation of 5G and beyond networks. By placing a relay between a transmitter and a receiver, the effects of obstacles can be mitigated. As a result, the signal from the transmitter will reach the receiver. In recent years, reconfigurable intelligent surfaces (RISs) that are more efficient than relays have become widely accepted as a method of mitigating skip-zones and providing reconfigurable radio environments. However, there have been limited studies on RISs for optical wireless communication (OWC) systems. Through the RIS technology, OWC and RF communication channels can be reconfigured. This paper aims to provide a comprehensive tutorial on indoor VLC systems utilizing RIS technology. The article discusses the basics of VLC and RISs and reintroduces RISs for OWC systems, focusing on RIS-assisted indoor VLC systems. We also provide a comprehensive overview of optical RISs and examine the differences between optical RISs, RF-RISs, and optical relays. Furthermore, we discuss in detail how RISs can be used to overcome line-of-sight blockages and the device orientation issue in VLC systems while revealing key challenges such as RIS element orientation design, RIS elements to access point/user assignment design, and RIS array positioning design problems that need to be studied. Moreover, we discuss and propose several research problems on integrating optical RISs with other emerging technologies, including non-orthogonal multiple access, multiple-input multiple-output systems, physical layer security, and simultaneous lightwave and power transfer in VLC systems. Finally, we highlight other important research directions that can further improve the performance of RISassisted VLC systems.

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Toward the Use of Re-configurable Intelligent Surfaces in VLC Systems: Beam Steering

Cited by 52 publications

References 13 publications

Analysis of RIS-Based Terrestrial-FSO Link Over G-G Turbulence With Distance and Jitter Ratios

Analysis of RIS-Based Terrestrial-FSO Link Over G-G Turbulence With Distance and Jitter Ratios

Re-configurable Intelligent Surface-based VLC Receivers Using Tunable Liquid-crystals: The Concept

RIS-Assisted Visible Light Communication Systems: A Tutorial

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