The evolving explosion in high data rate services and applications will soon require the use of untapped, abundant unregulated spectrum of the visible light for communications to adequately meet the demands of the fifth-generation (5G) mobile technologies. Radio-frequency (RF) networks are proving to be too scarce to cover the escalation in data rate services. Visible light communication (VLC) has emerged as a great potential solution, either in replacement of, or a complement to, existing RF networks, to support the projected traffic demands. Despite the prolific advantages of VLC networks, VLC faces many challenges that must be resolved in the near future to achieve full standardization and to be integrated to future wireless systems. Here, we review the emerging research in the field of VLC networks and lay out the challenges, technological solutions, and future work predictions. Specifically, we first review the VLC channel capacity derivation and discuss the performance metrics and the associated variables. The optimization of VLC networks are also discussed, including resources and power allocation techniques, user-to-access point (AP) association and APs-to-clustered-users-association, APs coordination techniques, non-orthogonal multiple access (NOMA) VLC networks, simultaneous energy harvesting and information transmission using the visible light, and the security issues in VLC networks. Finally, we propose several open research problems to optimize the various VLC networks by maximizing either the sum rate, fairness, energy efficiency, secrecy rate, or harvested energy.
This paper provides, in Section II, an overview of VLC technology, defines and discusses the objectives and constraints that must be taken into account when optimizing VLC networks. Special emphasis is placed on channel capacity derivations, and the unique properties of VLC. We also discuss the variables, parameters, and constraints having an impact on the performance of VLC networks. All optimization techniques are reviewed in Section III, including power and resource allocation, users-to-APs association, cell formation, and AP cooperation used for mitigating the disadvantages of VLC networks to improve performance. This important topic was previously investigated by Li @cite_202 . However, their study was focused on the difference between user-centric and network-centric cell formations, and the interference reduction techniques, whereas in this paper, we place our attention on the techniques, used in RF VLC and in VLC standalone networks, that are aimed at alleviating the limitations in VLC networks. In other words, we show how to formulate optimization problems, what are the techniques used for solving these optimization problems, how the different objectives, limitations, constraints are evaluated, and how added RF APs can remove stand-alone VLC network limitations.
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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 VLC 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. Visible light communication VLC is a newly emerging technology, which integrates communications and lighting purposes, and has become a very active research topic in the areas of wireless communications. It is expected to become an important part of the next generation wireless communications because of its unique features in using unlicensed spectrum, support of high data rate, and its resistance to electromagnetic interferences. In this survey paper, we begin with a review on the basis of photometry, which is used to establish channel models of VLC systems. Then, we will continue to address various issues on the fundamental characteristic features of VLC systems, the impact of indoor environments on system performance, and the analysis and discussions of five different types of typical VLC channel models and other related parameters in VLC channel models. Finally, in terms of the future works, we will show the possible follow-up research focuses and directions as an effort to identify some new research topics on VLCs. Copyright © 2016 John Wiley & Sons, Ltd. Due to severe inter-cell interference (ICI), the signal-to- interference-and-noise ratio (SINR) suffers from very significant fluctuation in indoor multi-cell visible light communication (VLC) systems, which greatly limits the overall system performance. In this paper, we propose and evaluate a generalized angle diversity receiver (ADR) structure to reduce SINR fluctuation in indoor multi-cell VLC systems. The generalized ADR consists of one top detector and multiple inclined side detectors. In an indoor multi-cell VLC system using the generalized ADR, the inclination angle of the side detectors is optimized in order to minimize the SINR fluctuation over the receiving plane, where the impact of receiver random rotation is considered. An optimized ADR with different numbers of detectors is analyzed with different LED layouts and diversity combining techniques in a typical indoor environment. Analytical results show that the SINR fluctuation is gradually reduced when more detectors are equipped in the optimized ADR. In an indoor two-cell VLC system, much more significant SINR fluctuation reduction is achieved by using select-best combining (SBC). However, nearly the same SINR fluctuations are obtained in an indoor four-cell VLC system when using SBC and maximal-ratio combining (MRC). By applying the optimized ADR, up to 15.9 and 32.4-dB SINR fluctuation reductions can be achieved in comparison to a conventional single-element receiver (SER) in indoor two-cell and four-cell VLC systems, respectively. In the context of an increasing interest toward reducing the number of traffic accidents and of associated victims, communication-based vehicle safety applications have emerged as one of the best solutions to enhance road safety. In this area, visible light communications (VLC) have a great potential for applications due to their relatively simple design for basic functioning, efficiency, and large geographical distribution. This paper addresses the issues related to the VLC usage in vehicular communication applications, being the first extensive survey dedicated to this topic. Although VLC has been the focus of an intensive research during the last few years, the technology is still in its infancy and requires continuous efforts to overcome the current challenges, especially in outdoor applications, such as the automotive communications. This paper is aimed at providing an overview of several research directions that could transform VLC into a reliable component of the transportation infrastructure. The main challenges are identified and the status of the accomplishments in each direction is presented, helping one to understand what has been done, where the technology stands and what is still missing. The challenges for VLC usage in vehicle applications addressed by this survey are: 1) increasing the robustness to noise; 2) increasing the communication range; 3) enhancing mobility; 4) performing distance measurements and visible light positioning; 5) increasing data rate; 6) developing parallel VLC; and 7) developing heterogeneous dedicated short range communications and VLC networks. Addressing and solving these challenges lead to the perspective of fully demonstrating the high potential of VLC, and therefore, to enable the VLC usage in road safety applications. This paper also proposes several future research directions for the automotive VLC applications and offers a brief review on the associated standardization activities.