Hybrid RF/VLC Systems: A Comprehensive Survey on Network Topologies, Performance Analyses, Applications, and Future Directions

The Visible Light Communication (VLC) has the potential to provide dense and fast connectivity at low cost. In this paper, we propose WSWDC, a novel VLC enabled wireless small-world data center. It employs VLC links to achieve a fully wireless data center network (DCN) across racks for the first time. The using of VLC links eliminates hierarchical switches and inter-rack cables, and thus reducing hardware investment, as well as maintenance cost. More precisely, to simplify the configuration and control operations, we propose three DCN design rationales: (1) fully-wireless, all inter-rack links are wireless; (2) easy-deployable, it is not necessary to change the existing infrastructure inside data center; (3) plug-and-play, no extra centralized control operations are required. Previous proposals, however, cannot achieve the three rationales simultaneously. To this end, we first use regular VLC links to interconnect racks as a regular grid DCN. To further exploiting the benefits of VLC links, a few random VLC links are carefully introduced to update the wireless grid DCN as a wireless small-world DCN. To avoid the potential interference among VLC links, we deploy VLC transceivers at different height on the top of each rack. In this way, VLC links would not interfere with others at each height level. Moreover, we design a greedy but efficient routing method for any pair of racks using their identifiers as inputs. Comprehensive evaluation results indicate that our WSWDC exhibits good network performance.

The quest to meet diverse services and applications that are emanating from the increasing inter-connected devices including machine-to-human communication has brought about consideration of massive deployment of small cells for fifth generation (5G) and Beyond network. With densification of small cells, mm-wave and THz frequencies play significant role in delivering mobile services. Consequently, supporting radio frequency (RF) communication with visible light communications (VLC) link exhibits potentials to ameliorate the challenges of RF communication especially bandwidth limitations and interference. Capitalising on the strength of VLC, we propose a basic end-to-end system to experimentally evaluate the suitability of VLC as fronthaul using real-time mobile traffic. Successful downlink (DL) transmission was achieved via VLC. To investigate the performance of the VLC link, transmission signal-to-noise ratio (SNR) and throughput of the VLC and RF Links are measured and compared. The light emitting diodes (LEDs) and Photo-diodes (PD) have shown to have significant effects on the performance of the VLC link. However, the results establish a fundamental premise for the suitability of VLC as fronthaul for small cell network.

Visible light communication (VLC) offers the opportunity to provide services with new requirements, when compared to the ones offered so far by radio wireless technologies almost exclusively. The idea behind VLC is to re-use the light signal, currently utilized for illumination purposes only, to transfer information wirelessly. Light-emitting diodes (LEDs), which are the natural sources of light for VLC due to their fast time response and high energy-efficiency, are devices with a notable non-linear response. A current trend to improve the achievable data rate of the VLC links relies on the use of linear waveforms, instead of just using On-Off Keying, which is robust against LED non-linearities but difficult to adapt to the variable VLC link conditions. Since having a linear transfer function is important when using efficient waveforms such as orthogonal frequency division multiplexing (OFDM), this paper starts by modelling the voltage-versus-current response of commercial LEDs. Then, the derived model is used to identify the guidelines that new VLC prototypes should fulfill. In particular, this paper focuses on the strength of the communication signal, compared to the light source optical power, and shows the way in which these parameters affect the coverage range of a multi-cell VLC system. The effect that these key design parameters have on the VLC link are identified, based on a real experimental setting, as they are difficult to model accurately due to the complex non-linear nature of LEDs when working in highly energy-efficient regimes.

In this letter, we consider a dual-hop heterogeneous visible light communication (VLC)/radio frequency (RF) communication system to extend the coverage of VLC systems. Besides detecting the information over VLC link, the relay is able to harvest energy from the first-hop VLC link, by extracting the direct current component of the received optical signal, and uses the harvested energy to retransmit the data to a mobile terminal over the second-hop RF link. We investigate the optimal design of the hybrid system in terms of data rate maximization.

In order to counteract the explosive escalation of wireless tele-traffic, the communication spectrum has been gradually expanded from the conventional radio frequency (RF) band to the optical wireless (OW) domain. By integrating the classic RF band relying on diverse radio techniques and optical bands, the next-generation heterogeneous networks (HetNets) are expected to offer a potential solution for supporting the ever-increasing wireless tele-traffic. Owing to its abundant unlicensed spectral resources, visible light communications (VLC) combined with advanced illumination constitute a competent candidate for complementing the existing RF networks. Although the advantages of VLC are multi-fold, some challenges arise when incorporating VLC into the classic RF HetNet environments, which may require new system architectures. The user-centric (UC) design principle for VLCVLC environments constitutes a novel and competitive design paradigm for the super dense multi-tier cell combinations of HetNets. The UC concept may be expected to become one of the disruptive techniques to be used in the forthcoming fifth-generation era. This paper provides a comprehensive survey of visible-light-aided OW systems with special emphasis on the design and optimization of VLC networks, where the radically new UC design philosophy is reviewed. Finally, design guidelines are provided for VLC systems.

Recently, visible light communication (VLC) system has gained attention due to its higher data rate, wider spectrum, higher security, lower weight, extensive lifespan of LEDs, smaller sizes of apertures and lower power consumption, lower cost, and no health issues etc. The VLC is used to strengthen the radio frequency (RF) communication and it also improves the communication performance in short range such as homes, offices, corridors, hospitals, aircrafts etc. The VLC provides dual functionality, that is, illumination and communication through the same LED simultaneously. It needs fast switching of LED (which is not detectable by human eyes) along with some appropriate modulation schemes for VLC to provide communication and illumination simultaneously. Meanwhile, LED should be powered on for the duration of communication, which conversely creates two challenges. First, the LED would consume more electrical power, which makes the system expensive. Second, LED can also be used in a dimming mode to save the power. Dimming of LED decreases the data rate. The data rate is inadequate due to modulation bandwidth of high brightness LED. The power and the data rate can be achieved by using different modulation schemes or line encoding schemes. There are a number of other factors such as interference of sun light, reflection of mirror or other fluorescent or candescent light disturbing the communication due to visible light usage in communication, interference to other VLC devices, integration of VLC with existing architecture of communication system, that is, Wi-Fi. The rest of the chapter is organized as follows. Section 1.1 gives an overview of VLC, followed by the brief discussion on the VLC system architecture in Section 1.2. Section 1.3 highlights the applications of the VLC. The general modulation schemes are given in Section 1.4. Then, Section 1.5 discusses the power efficient techniques, spectral efficient techniques are covered in Section 1.6, and Section 1.7 demonstrates the interference cancellation techniques. This chapter also discusses a comparison of different modulation techniques in terms of power efficiency, spectral efficiency, and interference cancellation and concludes with the best suited technique for power, spectrum, and interference.

Visible light communication (VLC) is a technology that uses unlicensed light spectrum resources and high spatial reuse rates for communication. Because it does not occupy any resource allocation in wireless communication, it has fully alleviated the problem of spectrum scarcity in radio frequency (RF) communication and gradually become a new development direction. However, owing to the inherent broadcasting nature of the VLC channel, the VLC link is vulnerable to eavesdropping by unexpected or unauthorized users in spacious public places. Therefore, enhancing the security of the VLC system has attracted extensive attention. This paper studies the security optimization scheme of the VLC system based on the space shift keying (SSK) technology in the free space optical environment called the SSK-VLC system. The antenna selection (AS) technology and artificial noise (AN) cancellation method are adopted to enhance the confidentiality of the SSK-VLC system. In this paper, we presume that the SSK-VLC system includes three parts: a transmitter containing multiple light-emitting diodes, a legitimate receiver, and an eavesdropper, respectively. By using the designed AS and AN method, the transmitted valid information can be demodulated at the legitimate receiver, and at the same time, the received information by the eavesdropper will be disturbed. The simulation results prove that the proposed optimization scheme can further improve the security performance, including the secrecy rate (SR) and the bit error ratio (BER), compared with the traditional SSK-VLC scheme.

In this paper, we establish an air-to-water (A2W) visible light communication (VLC) link adopting one green light emitting diode (LED) as the transmitter and one white LED as the ambient light source. The effect of the ambient light on the A2W-VLC link gain is investigated under calm and wavy water surface conditions. By transmitting a high-frequency sinusoid signal, we extract the variation of overall A2W-VLC link gain from the received signal in the presence of ambient light. The experimental results show that the strong ambient light can cause the saturation of photodiode (PD) and yield no communication possibility under calm water surface condition. Under wavy water surface condition, the effect of strong ambient light can be alleviated to some extent, yielding some communication possibilities.

De-Noising Scheme for VLC-Based V2V Systems; A Machine Learning Approach

Wireless communication technologies have the potential to significantly contribute toward a safer and more efficient road network. In this area, Visible Light Communications (VLC) are on the way of making the transition from an emerging technology to a confirmed technology. In the upper mentioned context, this paper presents the results of an experimental demonstration of Infrastructure-to- Vehicle VLC link in outdoor conditions. For these field tests, a commercial traffic light has been used as a VLC emitter, whereas a photodiode-based VLC receiver has been used to transform the optical beam into an electrical signal. The experimental results demonstrate a communication range of up to 188 meters at a BER of 10–3, with BERs as low as 10– <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> for distances below 170 meters. As far as we know, this is the longest I2V VLC link based on standard road side unit equipment reported. Thus, the 188 m I2V VLC link delivered in this paper provides extremely encouraging evidence concerning the use of the VLC technology in automotive applications.

The Visible Light Communication (VLC) has the potential to provide dense and fast connectivity at low cost. In this paper, we propose SFNet, a novel VLC-enabled hybrid data center network that extends the design of wireless DCN into three further dimensions: (1) fully wireless in the inter-rack level; (2) no need for centralized control mechanism on wireless links; (3) no need for any infrastructure-level alteration to data center. Previous proposals typically cannot realize the three rationales simultaneously. To achieve this vision, the proposed SFNet augments Fat-tree by organizing all racks into a wireless small-world network via VLC links. The using of VLC links eliminates hierarchical switches and cables in wireless network, and thus reduces hardware investment and maintenance cost. To fully exploit the benefits of the topology of SFNet, we further propose the topology design and optimization method, and routing scheme for SFNet. Comprehensive experiments indicate that SFNet exhibits good network performance.

5G networks have to offer extremely high capacity for novel streaming applications. One of the most promising approaches is to embed large numbers of cooperating small cells into the macro-cell coverage area. Alternatively, optical wireless based technologies can be adopted as an alternative physical layer offering higher data rates. Visible light communications (VLC) is an emerging technology for future high capacity communication links (it has been accepted to 5GPP) in the visible range of the electromagnetic spectrum (~370–780 nm) utilizing light-emitting diodes (LEDs) simultaneously provide data transmission and room illumination. A major challenge in VLC is the LED modulation bandwidths, which are limited to a few MHz. However, myriad gigabit speed transmission links have already been demonstrated. Non line-of-sight (NLOS) optical wireless is resistant to blocking by people and obstacles and is capable of adapting its’ throughput according to the current channel state information. Concurrently, organic polymer LEDs (PLEDs) have become the focus of enormous attention for solid-state lighting applications due to their advantages over conventional white LEDs such as ultra-low costs, low heating temperature, mechanical flexibility and large photoactive areas when produced with wet processing methods. This paper discusses development of such VLC links with a view to implementing ubiquitous broadcasting networks featuring advanced modulation formats such as orthogonal frequency division multiplexing (OFDM) or carrier-less amplitude and phase modulation (CAP) in conjunction with equalization techniques. Finally, this paper will also summarize the results of the European project ICT COST IC1101 OPTICWISE (Optical Wireless Communications - An Emerging Technology) dealing VLC and OLEDs towards 5G networks.

Visible light communication (VLC) has recently gained significant academic and industrial attention. VLC has great potential to supplement the functioning of the upcoming radio-frequency (RF)-based 5G networks. It is best suited for home, office, and commercial indoor environments as it provides a high bandwidth and high data rate, and the visible light spectrum is free to use. This paper proposes a multi-user full-duplex VLC system using red-green-blue (RGB), and white emitting diodes (LEDs) for smart home technologies. It utilizes red, green, and blue LEDs for downlink transmission and a simple phosphor white LED for uplink transmission. The red and green color bands are used for user data and smart devices, respectively, while the blue color band is used with the white LED for uplink transmission. The simulation was carried out to verify the performance of the proposed multi-user full-duplex VLC system. In addition to the performance evaluation, a cost-power consumption analysis was performed by comparing the power consumption and the resulting cost of the proposed VLC system to the power consumed and resulting cost of traditional Wi-Fi based systems and hybrid systems that utilized both VLC and Wi-Fi. Our findings showed that the proposed system improved the data rate and bit-error rate performance, while minimizing the power consumption and the associated costs. These results have demonstrated that a full-duplex VLC system is a feasible solution suitable for indoor environments as it provides greater cost savings and energy efficiency when compared to traditional Wi-Fi-based systems and hybrid systems that utilize both VLC and Wi-Fi.

Enormous applications which are enabled using Vehicular Ad Hoc Networks (VANETs) require a both efficient and reliable data delivery. Visible Light Communication (VLC), which has high transmission rate without using Radio Frequency (RF) spectrum, is regard as a candidate for data transmission in VANETs. Since a VLC system requires a line-of-sight communication, the position and the posture of LEDs and receivers play key roles in VLC. Unfortunately the position and the posture always change with vehicle motion and may make the performance of VLC become severe and even result in the outage of VLC. In this paper, we present an improved Vehicle-to-Vehicle (V2V) model and an improved Vehicle-to-Infrastructure (V2I) model in consideration of the position and the posture. In order to get diversity gains and reduce the VLC interruption caused by vehicle motion, we propose a VLC based cooperative diversity scheme. Finally simulation results show that the cooperative diversity scheme can decrease the Bit-Error-Rate (BER) significantly and reduce the VLC interruption caused by vehicle motion.

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.