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.

In this paper the information transfer rate of a single-photon avalanche diode (SPAD) array is investigated. The SPAD array is modelled as a discrete-time Gaussian channel with signal-dependent mean and variance. The SPAD dead time is a parameter which affects the extent of this signal dependency. The SPAD array channel capacity and the properties of the capacity-achieving input distributions are studied. Using a numerical algorithm, the capacity and the optimal input distributions subject to peak and average power constraints are obtained for various array sizes, dead times and background count levels.

In this paper the photon counting characteristics, the information rate and the bit error performance of single-photon avalanche diode (SPAD) arrays are investigated. It is shown that for sufficiently large arrays, the photocount distribution is well approximated by a Gaussian distribution with dead-time-dependent mean and variance. Because of dead time, the SPAD array channel is subject to counting losses, part of which are due to inter-slot interference (ISI) distortions. Consequently, this channel has memory. The information rate of this channel is assessed. Two auxiliary discrete memoryless channels (DMCs) are proposed which provide upper and lower bounds on the SPAD array information rate. It is shown that in sufficiently large arrays, ISI is negligible and the bounds are tight. Under such conditions, the SPAD array channel is precisely modelled as a memoryless channel. A discrete-time Gaussian channel with input-dependent mean and variance is adopted and the properties of the capacity-achieving input distributions are studied. Using a numerical algorithm, the information rate and the capacity-achieving input distributions, subject to peak and average power constraints are obtained. Furthermore, the bit error performance of a SPAD-based system with on-off keying (OOK) is evaluated for various array sizes, dead times and background count levels.

A single photon avalanche diode (SPAD) receiver can provide a significantly improved detection sensitivity over conventional photodiodes. However, upon detecting a photon, the SPAD receiver is unable to respond to subsequent incident photons for a certain period of time called dead time. When the SPAD receiver is counting in consecutive intervals and its dead time is relatively long compared to the counting interval, the dead time in one interval may overlap the next intervals, causing counting losses in those intervals. In this paper, the photocount statistics of an active quenching SPAD receiver with a long dead time is investigated. While detecting in consecutive time intervals, the SPAD receiver can be modeled as a Markov chain. It is shown that the SPAD experiences a transient period, before eventually reaching its steady state. The duration of the transient period increases with both the dead time and the signal photon rate. The bit error performance of the system with an on-off keying (OOK) modulated signal is also evaluated. Our results show that the SPAD's dead time significantly degrades the error performance of high data rate links.

In this paper, a comprehensive analytical approach is presented for modeling the counting statistics of active quenching and passive quenching single-photon avalanche diode (SPAD) detectors. It is shown that, unlike ideal photon counting receiver for which the detection process is described by a Poisson arrival process, photon counts in practical SPAD receivers do not follow a Poisson distribution and are highly affected by the dead time caused by the quenching circuit. Using the concepts of renewal theory, the exact expressions for the probability distribution and moments (mean and variance) of photocounts in the presence of dead time are derived for both active quenching and passive quenching SPADs. The derived probability distributions are validated through Monte Carlo simulations and it is demonstrated that the moments match with the existing empirical models for the moments of SPAD photocounts. Furthermore, an optical communication system with on-off keying and binary pulse position modulation is considered and the bit error performance of the system for different dead time values and background count levels is evaluated.

In this paper, the information rate of single photon avalanche diode (SPAD) receivers with active quenching circuits for optical communication systems is studied. The SPAD receiver is modeled as a discrete memoryless channel (DMC), and the information transfer rate is studied using an information theoretic approach. To assess the input-output information transfer rate of the SPAD optical receiver, a channel capacity metric is proposed which is a function of the physical parameters including average signal photon count, SPAD dead time, and SPAD average background counts. The proposed metric can be used for theoretical optimization of the device structure and operating conditions. Performance results are presented based on the proposed metric.

This paper presents the characterization of single photon avalanche diodes (SPADs) for optical communication applications. SPAD-based optical receivers can provide a significantly improved single-photon detection sensitivity compared with conventional photodiodes. However, an undesirable dead time introduced by the quenching circuit decreases the performance of these receivers. Using a precise analysis, we show that in the presence of dead time, the photon arrival process does not follow a Poisson process. Also, the effective count rate is evaluated and shown to depend critically on dead time. The effect of SPAD dead time on the error performance of an on-off keying (OOK) modulation optical communication system is also investigated. It is shown that, when background counts due to dark current and afterpulsing are small, the performance degradation of the SPAD-based receiver due to dead time losses is negligible. However, for systems with considerable background counts, the bit error rate (BER) degrades rapidly with increasing dead time.