The next generation of light fidelity (LiFi) networks will be based on laser-based optical wireless communication (OWC) to meet the demands for multi-Gigabit/s ($\text{Gb} / \mathrm{s}$) per user data rates. In this paper, we consider a double-tier access point (AP) architecture using an array of arrays of vertical cavity surface emitting lasers (VCSELs) to provide grid-of-beam (GoB) multi-user access for sixth generation (6G) laser-based optical wireless networks. In particular, we develop a novel dynamic beam clustering (DBC) algorithm based on graph theory with the aim to perfectly cancel inter-cluster interference (ICI) by dynamically adapting the clustering strategy according to network variations. We then carry out performance analysis for key performance metrics including the multi-user sum rate and fairness, as well as energy efficiency (EE). Simulation results demonstrate remarkable performance gains of DBC with respect to static beam clustering (SBC) strategies using fixed cluster sizes especially in terms of the sum rate and EE.

In this paper, we put forward a proof of concept for sixth generation (6G) Terabit infrared (IR) laser-based indoor optical wireless networks. We propose a novel double-tier access point (AP) architecture based on anarray of arraysof vertical cavity surface emitting lasers (VCSELs) to provide a seamless grid-of-beam (GoB) coverage with multi-Gb/s per beam. We present systematic design and thorough analytical modeling of the AP architecture, which are then applied to downlink system modeling using non-imaging angle diversity receivers (ADRs). We propose static beam clustering with coordinated multi-beam joint transmission (CoMB-JT) for network interference management and devise various clustering strategies to address inter-beam interference (IBI) and inter-cluster interference (ICI). Non-orthogonal multiple access (NOMA) and orthogonal frequency division multiple access (OFDMA) schemes are also adopted to handle intra-cluster interference, and the resulting signal-to-interference-plus-noise ratio (SINR) and achievable data rate are derived. The network performance is studied in terms of spatial distributions and statistics of the downlink SINR and data rate through extensive computer simulations. The results demonstrate that data rates up to 15 Gb/s are achieved within the coverage area and a properly devised clustering strikes a balance between the sum rate and fairness depending on the number of users.

To achieve multi-Gb/s data rates in 6G optical wireless access networks based on narrow infrared (IR) laser beams, a high-speed receiver with two key specifications is needed: a sufficiently large aperture to collect the required optical power and a wide field-of-view (FOV) to avoid strict alignment issues. This paper puts forward the systematic design and optimisation of multi-tier non-imaging angle diversity receivers (ADRs) composed of compound parabolic concentrators (CPCs) coupled with photodiode (PD) arrays for laser-based optical wireless communication (OWC) links. Design tradeoffs include the gain-FOV tradeoff for each receiver element and the area-bandwidth tradeoff for each PD array. The rate maximisation is formulated as a non-convex optimisation problem under the constraints on the minimum required FOV and the overall ADR dimensions to find the optimum configuration of the receiver bandwidth and FOV, and a low-complexity optimal solution is proposed. The ADR performance is studied using computer simulations and insightful design guidelines are provided through various numerical examples. An efficient technique is also proposed to reduce the ADR dimensions based on CPC length truncation. It is shown that a compact ADR with a height of$\leq 0.5$cm and an effective area of$\leq 0.5$cm2 reaches a data rate of 12 Gb/s with a half-angle FOV of 30° over a 3 m link distance.

To enable intelligent network management and various 6G smart services, the precise estimation of user location and device orientation is required. Light fidelity (LiFi) based on vertical cavity surface emitting lasers (VCSELs) can not only respond to the needs of 6G communication networks in terms of ultra-high data rate, connection density and area capacity, but also enable high precision position and orientation estimation. However, this problem of joint position and orientation estimation is a non-convex optimization problem. Therefore, in this paper, we design deep neural networks (DNNs) for joint position and orientation estimation of user devices in a VCSEL-based LiFi access network. Simulation results demonstrate that the proposed framework outperforms state-of-the-art methods by significantly reducing position and orientation estimation errors while maintaining a lower complexity. We illustrate the effectiveness of the proposed DNN solution by considering two types of network deployment including distributed VCSELs and collocated VCSELs. In addition, we present the convergence and complexity analysis for the proposed learning framework. It is shown that the proposed DNN provides at least 69% and 27.9% improvements in the mean estimation error for position and orientation, respectively, over the baseline method.

We propose a multi-beam access point (AP) design comprising a vertical cavity surface emitting laser (VCSEL) array followed by a plano-convex lens as a cost-effective and scalable angle diversity transmitter (ADT) solution to realize multi-Gb/s downlink data rates in sixth generation (6G) optical wireless networks. The AP structure is elaborated using the VCSEL array and lens parameters including inter-element spacing in the array, the array-to-lens distance, and the diameter, thickness and radius of curvature of the lens. The design parameters are properly configured to ensure that the beam spots are sufficiently separated from one another on the receiver plane. To address the eye safety for infrared (IR) laser emission, we conduct a thorough eye safety analysis of a VCSEL array when combined with a lens to compute the maximum permissible transmit power (MPTP) for each transmitter element. The coverage performance of the AP is evaluated as a function of the array-to-lens distance based on a non-imaging angle diversity receiver (ADR), identifying a tradeoff between the MPTP value and the coverage performance. Simulation results demonstrate downlink data rates > 11 Gb/s while achieving a 100% coverage over an area of$1\times 1\ \mathbf{m}^{2}$on the receiver plane at 3 m under the AP based on a 2 GHz bandwidth and coordinated multi-beam joint transmission (CoMB-JT).

In this paper, we propose an imaging angle diversity receiver (ADR) that can support multi-Gb/s data rates for 6G optical wireless networks. We derive accurate mathematical expressions for the optical gain and field-of-view (FOV) of the imaging receiver in terms of its various geometrical parameters. Additionally, we identify the design tradeoffs and formulate a non-convex optimisation problem to maximise the data rate under the required FOV constraint. We also propose a low-complexity optimal solution for the optimisation problem, based on which we demonstrate the design of an imaging ADR with a$10^{\circ}$half-angle FOV, providing a data rate of 12 Gb/s for a 3 m link distance.

The design of a compact high-speed and wide field of view (FOV) receiver is challenging due to the presence of two well-known trade-offs. The first one is the area-bandwidth trade-off of photodetectors (PDs) and the second one is the gain-FOV trade-off due to the use of optics. The combined effects of these two trade-offs imply that the achievable data rate of an imaging optical receiver is limited by its FOV, i.e., a rate-FOV trade-off. In this paper, we propose an imaging receiver design in the form of an array of (PD) arrays. To control the area-bandwidth trade-off, small PDs are used in an array of arrays structure instead of a single large PD. Moreover, to achieve a reasonable receiver FOV, we use an array of focusing lenses that focus the light individually on each inner PD array. The proposed array of arrays structure provides an effective method to control both gain-FOV trade-off (via an array of lenses) and area-bandwidth trade-off (via arrays of small PDs). We first derive a tractable analytical model for the signal-to-noise ratio (SNR) of an array of PDs that is equipped with a focusing lens assuming maximum ratio combining (MRC). Then, we extend the model to the proposed array of arrays structure and the accuracy of the analytical model is verified based on several Optic Studio-based simulations. Next, we formulate an optimization problem to maximize the achievable data rate of the imaging receiver subject to a minimum required FOV. The optimization problem is solved for two commonly used modulation techniques, namely, on-off keying (OOK) and direct current (DC) biased optical orthogonal frequency division multiplexing (DCO-OFDM) with variable rate quadrature amplitude modulation (QAM). Our results show the limits of high speed wide-FOV imaging receivers that can support mobility. For example, it is demonstrated that a data rate of$\sim 24$Gbps with a FOV of 15° is achievable using OOK with a total receiver size of 2 cm$\!\times \!\,\,2$cm.

To achieve multi-Gb/s data rates in 6G optical wireless networks based on narrow infrared (IR) laser beams, a high-speed receiver with two key specifications is needed: a sufficiently large aperture to collect the required optical power and a wide field of view (FOV). This paper investigates the design tradeoffs for a non-imaging angle diversity receiver (ADR) coupled with photodiode (PD) arrays for laser-based optical wireless communication (OWC) with the aim to achieve data rates higher than 10 Gb/s and a half-angle FOV of$30^{\circ}$. The design tradeoffs include the gain-FOV tradeoff for each element of the receiver and the area-bandwidth tradeoff for each PD array. Taking both tradeoffs into account, the rate maximisation is formulated as a non-convex optimisation problem to find the optimal configuration of the receiver bandwidth and FOV under the minimum FOV constraint. A low-complexity optimal solution is proposed and the ADR performance is studied using computer simulations.

Light amplification by stimulated emission of radiation (laser) sources has many advantages for use in high-data-rate optical wireless communications (OWCs). In particular, the low-cost and high-bandwidth properties of laser sources, such as vertical-cavity surface-emitting lasers (VCSELs), make them attractive for future indoor OWCs. In order to be integrated into future indoor networks, such lasers should conform to eye safety regulations determined by the International Electrotechnical Commission (IEC) standards for laser safety. In this article, we provide a detailed study of beam propagation to evaluate the received power of various laser sources, based on which and the maximum permissible exposure (MPE) defined by the IEC 60825-1:2014 Standard, we establish a comprehensive framework for eye safety analyses. This framework allows us to calculate the maximum allowable transmit power, which is crucial in the design of a reliable and safe laser-based wireless communication system. Initially, we consider a single-mode Gaussian beam and calculate the maximum permissible transmit power. Subsequently, we generalize this approach for higher mode beams. It is shown that the$M$-squared-based approach for analysis of multimode lasers ensures the IEC eye safety limits; however, in some scenarios, it can be too conservative compared to the precise beam decomposition method. Laser safety analyses with consideration of optical elements, such as lens and diffuser, as well as for the VCSEL array, have been also presented. Skin safety, as another significant factor of laser safety, has also been investigated in this article. We have studied the impacts of various parameters, such as wavelength, exposure duration, and the divergence angle of laser sources on the safety analysis by presenting insightful results.

In this paper, the design of a multiple-input multiple-output (MIMO) optical wireless communication (OWC) link based on vertical cavity surface emitting laser (VCSEL) arrays is systematically carried out with the aim to support data rates in excess of 1 Tb/s for the backhaul of sixth generation (6G) indoor wireless networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) and a spatial multiplexing MIMO architecture. For such an ultra-high-speed line-of-sight (LOS) OWC link with low divergence laser beams, maintaining alignment is of high importance. In this paper, two types of misalignment error between the transmitter and receiver are distinguished, namely, radial displacement error and orientation angle error, and they are thoroughly modeled in a unified analytical framework assuming Gaussian laser beams, resulting in a generalized misalignment model (GMM). The derived GMM is then extended to MIMO arrays and the performance of the MIMO-OFDM OWC system is analyzed in terms of the aggregate data rate. Novel insights are provided into the system performance based on computer simulations by studying various influential factors such as beam waist, array configuration and different misalignment errors, which can be used as guidelines for designing short range Tb/s MIMO OWC systems.

This paper presents the design of a multiple input multiple output (MIMO) optical wireless communication (OWC) system based on vertical cavity surface emitting laser (VCSEL) arrays. The objective is to realize data rates in excess of 1 Tb/s for the backhaul of next generation indoor networks. The proposed design combines direct current optical orthogonal frequency division multiplexing (DCO-OFDM) with spatial multiplexing to attain the target data rate subject to eye safety limitations. Following a bottom-up approach, the signal-to-interference-plus-noise (SINR) per channel is modeled using a Gaussian emission profile for laser beams by taking into account crosstalk in the MIMO channel. According to the SINR expression, the aggregate data rate is derived. The performance of the system is studied using computer simulations, and insightful remarks are provided on the configuration of various system parameters including the beam waist, the MIMO size and array dimensions.

This paper presents a proof-of-concept for Tb/s infrared (IR) indoor optical wireless networks. We introduce a novel double tier access point architecture based on array of arrays of vertical cavity surface emitting lasers (VCSELs) to deliver beyond Tb/s aggregate capacity. For a given indoor environment, the optimal access point architecture is designed. The downlink performance is analysed throughout the coverage area and the spatial distribution of signal-to-interference-plus-noise ratio (SINR) and data rate are obtained. Numerical results demonstrate that with a single access point in a 25 m2indoor area, data rates of at least 10 Gb/s per beam are achieved almost everywhere and the aggregate data rate can exceed 2 Tb/s.

Sojourn time is an important parameter in the analysis and design of mobile cellular networks. It shows the expected time that a user equipment (UE) remains connected to the serving access point (AP) while moving in the network, i.e. stays within one cell. Therefore, an accurate estimation of the sojourn time is an essential element for mobility management. In this study, an analysis of the sojourn time for indoor light-fidelity (LiFi) cellular networks is presented based on the random waypoint (RWP) mobility model. It is initially assumed that the UE is oriented vertically upward and closed-form analytical expressions are derived. Monte-Carlo simulations are also provided to validate the analytical derivations and to gain more insight into the performance with different orientations. It is shown that the sojourn time mainly varies depending on the speed of the user. However, other factors such as the device orientation and the coverage area of cells also play a role in determining the sojourn time. The results offer several insights and design guidelines in consideration of this parameter for indoor LiFi networks.

The problem of backhauling for optical attocell networks has been approached by a number of wired solutions such as in-building power line communication (PLC), Ethernet, and optical fiber. In this paper, an alternative solution is proposed based on the wireless optical communication in visible light and infrared (IR) bands. A thorough analysis of signal-to-noise-plus-interference ratio (SINR) is elaborated for a multi-user optical attocell network based on the direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and decode-and-forward (DF) relaying, taking into account the effects of inter-backhaul and backhaul-to-access interferences. Inspired by concepts developed for radio frequency (RF) cellular networks, full-reuse visible light (FR-VL) and in-band visible light (IB-VL) bandwidth allocation policies are proposed to realize backhauling in the visible light band. The transmission power is opportunistically minimized to enhance the backhaul power efficiency. For a two-tier FR-VL network, there is a technological challenge due to the limited capacity of the bottleneck backhaul link. The IR band is employed to add an extra degree of freedom for the backhaul capacity. For the IR backhaul system, a power-bandwidth trade-off formulation is presented. Closed form analytical expressions are derived for the corresponding power control coefficients. Finally, the network sum rate performance is studied using extensive Monte Carlo simulations.

The backhaul of tens and hundreds of light fidelity (LiFi)-enabled light bulbs constitutes a major challenge. In this paper, a multi-hop wireless backhaul configuration is investigated for optical attocell networks. The backhaul system proposed in this work uses visible light communication (VLC) to realize wireless connections between adjacent LiFi base stations (BSs). By using a tree topology for the backhaul system, the central BS is collocated with the gateway, and the other BSs are connected to the core network via multi-hop wireless backhaul links with the gateway based on decode-and-forward (DF) relaying. The system-level modeling and analysis is presented for the end-to-end sum rate of multiple user equipment (UE) devices with random coordinates in the network. The closest backhaul link to the gateway needs to support multiple independent data flows at the same time. To this end, novel bandwidth scheduling policies are proposed and evaluated. In addition, in order to opportunistically reduce the transmission power of the backhaul system, novel power control schemes are derived. Performance gains of the proposed schemes and their impact on the average sum rate of the downlink optical attocell network are studied using Monte Carlo simulations.

Light-fidelity (Li-Fi) is an emerging technology for wireless optical networking using the principle of visible light communication (VLC). Li-Fi attocells are smaller in size than the radio frequency (RF) femtocells, suitable for deploying ultradense cellular networks. In this paper, a novel wireless backhaul solution is proposed for indoor Li-Fi attocell networks using VLC, which is already embedded in the Li-Fi base station (BS) units. Since the backhaul links operate in the visible light spectrum, two methods are proposed for bandwidth allocation between the access and backhaul links, namely, full frequency reuse (FR) and in-band (IB). In order to realize dual-hop transmission over the backhaul and access links, both amplify-and-forward (AF) and decode-and-forward (DF) relaying protocols are analyzed. Considering a direct current optical orthogonal frequency division multiplexing (DCO-OFDM)-based multiple access system, novel signal-to-interference-plus-noise ratio (SINR) and spectral efficiency expressions are then derived for user equipment (UE) randomly distributed in each attocell. Downlink performance of the optical attocell network is assessed in terms of the average spectral efficiency using Monte Carlo simulations. Guidelines are given for the design of the proposed wireless backhaul system.

Light-fidelity (Li-Fi) is an emerging technology for wireless networking based on visible light communication (VLC). As a licence free, high speed, bidirectional and secure wireless access solution, Li-Fi is a complementary building block for fifth generation (5G) heterogeneous mobile networks. By modulating data on the light intensity emitted by light emitting diode (LED) luminaires which already exist in indoor lighting infrastructure, Li-Fi cellular networks are formed. Such networks are termed optical attocell networks, where the optical attcells are smaller in size than the radio frequency (RF) femtocells. This paper focuses on the problem of handover for downlink in an indoor optical attocell network. A fundamental approach is proposed for handover modeling by taking into account the effects of both mobility and rotation for a connected user equipment (UE). By using a random waypoint (RWP) model for the receiver movement and a geometric model for the receiver orientation, the probability of handover and the handover rate are calculated. Novel insights are provided into the handover performance in indoor optical attocell networks using Monte Carlo simulations.

In this paper, downlink cooperation is proposed for indoor visible light communication (VLC) cellular networks, also termed as optical attocell networks, with the aim of improving the spectral efficiency performance. An orthogonal frequency division multiple access (OFDMA) system based on direct current optical orthogonal frequency division multiplexing (DCO-OFDM) with fractional frequency reuse (FFR) planning is considered, and a system level analysis is carried out. A line-of-sight (LOS) light propagation model is used for the indoor VLC channel. The downlink cooperation is realized by means of a relay-assisted transmission with the aid of neighboring base stations (BSs) using a non-orthogonal amplify-and-forward (NAF) protocol. The links between the source BS and the relay BSs are provided by perfectly aligned VLC connections. Four relaying schemes with a different number of relays are considered, and for each scheme analytical signal-to-interference-plus-noise ratio (SINR) and spectral efficiency expressions for a user equipment (UE) with random coordinates in an optical attocell are derived. Also, average spectral efficiency performance of the system is studied using Monte-Carlo simulations.

In this paper, novel spectral efficient cooperative transmission schemes are proposed for downlink in indoor light-fidelity (Li-Fi) cellular networks, also termed as optical attocell networks. Downlink cooperation builds upon dual-hop decode-and-forward (DF) relaying with the aid of one or two neighboring base stations (BSs). The connections between the source BS and the relay BSs are provided by perfectly aligned visible light communication (VLC) links. Two cooperation protocols are introduced and evaluated, namely, non-orthogonal DF (NDF) and joint transmission with DF (JDF). A multiple access system based on direct current optical orthogonal frequency division multiplexing (DCO- OFDM) with fractional frequency reuse (FFR) is considered. Also, a line-of-sight (LOS) Lambertian propagation model is used for the indoor VLC channel. For each scheme, the signal-to- interference-plus-noise ratio (SINR) and spectral efficiency for user equipment (UE) with random coordinates in an attocell are derived. The average spectral efficiency within an optical attocell is studied using Monte Carlo simulations.