In optical wireless communication (OWC) networks, managing interference and enhancing data rates are critical challenges, particularly in environments with multiple users. This paper investigates the impact of the receiver's field of view (FoV) on interference reduction and signal-to-interference-plus-noise ratio (SINR) improvement in light-fidelity (LiFi), which is a networked OWC system. A narrower FoV can effectively limit the reception of unwanted signals, thereby mitigating inter-user interference and enhancing SINR. This work explores how FoV optimization contributes to interference suppression while maximizing SINR. Additionally, the integration of liquid crystal lenses (LCL) to the receiver permits a dynamic FoV for achieving interference-resistant communication. LCLs adjust the focus by applying an electric field to them, and have faster response time in comparison to the coherence time of LiFi. By incorporating realistic scenarios, including both line-of-sight (LoS) and non-LoS (NLoS) propagation, as well as random device orientations, and formulating an optimization problem to determine the optimal FoV, this study provides valuable insights for designing interference-resistant and high-performance LiFi systems. The results emphasize that employing a dynamic FoV enhances the average SINR by approximately 4 dB across the entire room, even under random device orientations. Moreover, configuring the receiver with a dynamic FoV range between 40° to 55° consistently yields high SINR throughout the room.

Indoor optical wireless communication (OWC) systems such as light fidelity (LiFi) face a number of challenges when it comes to providing seamless connectivity between the transmitter and the receiver. For example, establishing a line of sight (LOS) link that achieves uninterrupted optical wireless connectivity in a realistic indoor environment can be challenging. Some of the important factors that contribute to the obstruction of the LOS link include the presence of the human body, random device orientation and the limited field of view (FOV) of optical receivers. In most previous studies, only blockage by the human body are considered. However, in practical scenarios, it is also important to take into account the effect of random orientation of the user equipment (UE) as well as the receiver's limited FOV. In this study, we present an analysis of link blockage by developing an analytical framework to calculate the blockage probability of indoor OWC systems considering all of the aforementioned issues. In particular, we derive novel analytical expressions of the blockage probability for a single source scenario considering the effects of blockage due to the limited FOV of the receiver, random orientation of the UE and blockage by the user body and by other external blockers. Using the proposed analytical framework, we investigate the characteristics of link blockage of indoor OWC by varying the receiver FOV, user directions and the distance between the source and the receiver. The analytical results, that are computed much faster than simulation results, are shown to be perfectly matched with the simulation.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution to achieve multi-Gbps data rates for mobile optical communications. Unlike typical radio frequency (RF) wireless systems, the OWC channel is not isotropic, which means that the device orientation considerably affects the channel gain, especially for mobile users. In this study, a practical uplink LiFi scenario is analysed in which a number of mobile devices equipped with eye-safe laser-based transceivers transmit multi-Gbps data streams wirelessly to receivers on the ceiling. Ten models for five different types of user mobility with and without stabiliser have been developed on the basis of experimental measurements. Using the proposed models, the capability of stabilisers to mitigate the pointing error due to user mobility is investigated. It is shown that the device orientation may be modelled as Laplace, Gaussian, uniform distributions, or sinusoidal functions. Moreover, an orientation-based random waypoint (ORWP) mobility model is used to assess the performance of the LiFi system in terms of outage probability. The results show that using a stabiliser on mobile devices improves outage performance significantly, and it is feasible to reduce the number of uplink receivers while retaining the required outage performance.

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.

Optical wireless communication (OWC) is considered to be a promising technology which will alleviate traffic burden caused by the increasing number of mobile devices. In this study, a novel vertical-cavity surface-emitting laser (VCSEL) array is proposed for indoor OWC systems. To activate the best beam for a mobile user, two beam activation methods are proposed for the system. The method based on a corner-cube retroreflector (CCR) provides very low latency and allows real-time activation for high-speed users. The other method uses the omnidirectional transmitter (ODTx). The ODTx can serve the purpose of uplink transmission and beam activation simultaneously. Moreover, systems with ODTx are very robust to the random orientation of a user equipment (UE). System level analyses are carried out for the proposed VCSEL array system. For a single user scenario, the probability density function (PDF) of the signal-to-noise ratio (SNR) for the central beam of the VCSEL array system can be approximated as a uniform distribution. In addition, the average data rate of the central beam and its upper bound are given analytically and verified by Monte-Carlo simulations. For a multi-user scenario, an analytical upper bound for the average data rate is given. The effects of the cell size and the full width at half maximum (FWHM) angle on the system performance are studied. The results show that the system with a FWHM angle of 4° outperforms the others.

Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless communication (OWC) technology that is considered as a promising solution for high-speed indoor connectivity. In this paper, the joint estimation of user 3D position and user equipment (UE) orientation in indoor LiFi systems with unknown emission power is investigated. Existing solutions for this problem assume either ideal LiFi system settings or perfect knowledge of the UE states, rendering them unsuitable for realistic LiFi systems. In addition, these solutions consider the non-line-of-sight (NLOS) links of the LiFi channel gain as a source of deterioration for the estimation performance instead of harnessing these components in improving the position and the orientation estimation performance. This is mainly due to the lack of appropriate estimation techniques that can extract the position and orientation information hidden in these components. In this paper, and against the above limitations, the UE is assumed to be connected with at least one access point (AP), i.e., at least one active LiFi link. Fingerprinting is employed as an estimation technique and the received signal-to-noise ratio (SNR) is used as an estimation metric, where both the line-of-sight (LOS) and NLOS components of the LiFi channel are considered. Motivated by the success of deep learning techniques in solving several complex estimation and prediction problems, we employ two deep artificial neural network (ANN) models, one based on the multilayer perceptron (MLP) and the second on the convolutional neural network (CNN), that can map efficiently the instantaneous received SNR with the user 3D position and the UE orientation. Through numerous examples, we investigate the performance of the proposed schemes in terms of the average estimation error, precision, computational time, and the bit error rate. We also compare this performance to that of the k-nearest neighbours (KNN) scheme, which is widely used in solving wireless localization problems. It is demonstrated that the proposed schemes achieve significant gains and are superior to the KNN scheme.

Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless communication (OWC) technology that is considered as a promising solution for high-speed indoor connectivity. Unlike in conventional radio frequency wireless systems, the OWC channel is not isotropic, meaning that the device orientation affects the channel gain significantly. However, due to the lack of proper channel models for LiFi systems, many studies have assumed that the receiver is vertically upward and randomly located within the coverage area, which is not a realistic assumption from a practical point of view. In this paper, novel realistic and measurement-based channel models for indoor LiFi systems are proposed. Precisely, the statistics of the channel gain are derived for the case of randomly oriented stationary and mobile users. For stationary users, two channel models are proposed, namely, the modified truncated Laplace (MTL) model and the modified Beta (MB) model. For mobile users, two channel models are proposed, namely, the sum of modified truncated Gaussian (SMTG) model and the sum of modified Beta (SMB) model. Based on the derived models, the impact of random orientation and spatial distribution of users is investigated, where we show that the aforementioned factors can strongly affect the channel gain and the system performance.

Light fidelity (LiFi) is an emerging communication technology that utilizes light intensity modulation in order to transfer data from light-emitting diode (LED) to users. Due to the vast visible light spectrum, LiFi can support high data rates; however, its coverage is limited. In contrast to LiFi, WiFi works in radio frequency and is capable of providing ubiquitous coverage with limited data rates. Since the spectrum of LiFi does not overlap with WiFi, both can co-exist to form a hybrid LiFi and WiFi network. The advantage of hybrid LiFi and WiFi network is that it provides high data rates and better connectivity. The performance of a hybrid LiFi and WiFi network significantly depends upon the load balancing strategies. Therefore, in this paper, gradient descent-based reinforcement learning (RL) has been proposed to determine an optimal access point (AP) assignment policy that aims to maximize the average network throughput while ensuring user's satisfaction. The performance of the proposed method is then compared against conventional signal strength strategy (SSS); the results are presented in terms of the average network throughput, user satisfaction, and outage probability. Based on the results, it was observed that the proposed RL method provides a significant improvement in all the performance metrics over the SSS based method.

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.

Energy efficiency (EE) is a major performance metric for fifth generation (5G) and beyond 5G(B5G) wireless communication systems, especially for ultra dense networks. This paper proposes an end-to-end (e2e) power consumption model and studies the energy efficiency for a heterogeneous B5G cellular architecture that separates the indoor and outdoor communication scenarios in ultra dense networks. In this work, massive multiple-input-multiple-output (MIMO) technologies at conventional sub-6 GHz frequencies are used for long-distance outdoor communications. Light-Fidelity (LiFi) and millimeter wave (mmWave) technologies are deployed to provide a high data rate service to indoor users. Whereas, in the referenced non-separated system, the indoor users communicate with the outdoor massive MIMO macro base station directly. The performance of these two systems are evaluated and compared in terms of the total power consumption and energy efficiency. The results show that the network architecture which separates indoor and outdoor communication can support a higher data rate transmission for less energy consumption, compared to non-separate communication scenario. In addition, the results show that deploying LiFi and mmWave IAPs can enable users to transmit at a higher data rate and further improve the EE.

In conventional visible light communication (VLC) systems with multiple light-emitting diodes (LEDs) and multiple photodiodes (PDs), high data rate transmission with limited modulation bandwidth can be achieved via spatial multiplexing (SMP) or wavelength division multiplexing (WDM). However, the number of multiplexing channels is limited by the strong spatial correlation in SMP and by the inter-colour crosstalk in WDM. In this paper, we propose a multiple-input multiple-output (MIMO) hybrid multiplexing (HMP) VLC system which avoids the disadvantages of SMP/ WDM and explores the degrees-of-freedom (DoFs) in space and wavelength domains jointly. With appropriate system configuration, a MIMO channel matrix with a better channel condition in HMP can be obtained. Eventually, it is able to increase the number of multiplexing channels and support higher data rate transmission.

The increasing number of mobile devices challenges the current radio frequency (RF) networks, e.g. wireless fidelity (WiFi) networks. Light Fidelity (LiFi) is considered as a promising complementary technology, which operates within the visible light spectrum and infrared spectrum. In an indoor scenario, a hybrid LiFi/WiFi network (HLWN) provides a potential solution to future wireless communications where LiFi augments WiFi in providing ultra-high speed and low latency wireless connectivity. In this paper, dynamic load balancing (LB) with handover in HLWNs is studied. The orientation-based random waypoint (ORWP) mobility model is considered to provide a more realistic framework to evaluate the performance of HLWNs. Based on the low-pass filtering effect of the LiFi channel, we firstly propose an orthogonal frequency division multiplexing access (OFDMA)-based resource allocation (RA) method in LiFi systems. Also, an enhanced evolutionary game theory (EGT)-based LB scheme with handover in HLWNs is proposed. In the EGT scheme, each user adapts their strategy to improve the payoff until LB is achieved across LiFi and WiFi. Then, the LiFi system uses the proposed OFDMA-based RA method while the WiFi system applies the carrier sense multiple access with collision detection (CSMA/CA). Simulation results show that in the LiFi system the OFDMA-based RA scheme outperforms the time division multiple access (TDMA) scheme in terms of both user data rate and fairness. Regarding LB in HLWNs, the proposed EGT scheme can achieve a remarkable enhancement in throughput compared to benchmark schemes, such as hard threshold (HT) scheme and random access point assignment (RAA) scheme.

As an emerging technology for wireless communications based on the visible light communication (VLC), Light-fidelity (LiFi) is considered as a complementary building block for fifth generation (5G) mobile networks. In this paper, we investigate the optimum field of view (FOV) for the angle diversity receiver (ADR) in LiFi cellular networks. The probability of visibility and the visible area of the ADR is defined. There is a trade off between the channel gain and visibility. An optimisation problem is formulated to maximise the channel gain, and the lower bound of the FOV of photodiodes (PDs) mounted on an ADR is derived. The simulation results show that the data rate is maximised when the FOV is minimised and the structure for the pyramid receiver (PR) achieving highest data rate is the one consisting of 6 PDs each with a FOV of 30 degree.

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.

Light-Fidelity (LiFi) has been considered as a complementary technology to radio frequency (RF) communications. The reliability of a LiFi channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this study, we investigate the effect of receiver orientation including both polar and azimuth angles on the LOS channel gain in a LiFi system. The optimum tilt angle is calculated, which depends on both the user's location and direction. The probability density function (PDF) of signal-to-noise ratio (SNR) is derived for on-off keying (OOK) modulation. Using the derived PDF of SNR, the bit-error ratio (BER) of OOK in an additive-white Gaussian noise (AWGN) channel with random orientation of the receiver is evaluated. It is shown that the effect of random orientation is negligible if the optimum tilt angle is chosen. Finally, we assess the effect of random orientation on the Shannon-Hartley upper bound capacity.

Light Fidelity (LiFi) is an emerging technology for future high-speed indoor wireless communications. Co-channel interference (CCI) caused by the dense deployment of LiFi access points (APs) can be effectively mitigated by using angle diversity receivers (ADRs). ADRs require signal combining where the combining weights depend on the selection of serving APs. In this paper, the AP selection (APS) strategy considering handover is studied. A novel APS scheme based on evolutionary game theory (EGT) is proposed for the LiFi network using ADRs. The performance of the proposed scheme is comprehensively analysed and compared with the APS scheme based on signal strength strategy (SSS). The result shows that, in terms of ADRs with SBC/MRC, the EGT-based APS scheme achieves more than 5% improvement in quality of service (QoS) compared with the SSS-based APS scheme. With the sub-optimum weights of maximum ratio combining (MRC) for ADRs, the EGT-MRC scheme can achieve more than 20 Mbps data rate improvement compared with LiFi systems using single photodiode (PD) receiver.

Among the challenges of realizing the full potential of light-fidelity (LiFi) cellular networks are user mobility, random device orientation, and blockage. In this paper, we study the impact of those challenges on the performance of LiFi networks in an indoor environment using measurement-based channel models, unlike existing studies that rely on theoretical channel models. In our paper, we adopt spatial modulation (SM) and consider two configurations for the user equipment (TIE). A multidirectional receiver (MDR) structure is proposed, in which the PDs are located on different sides of the TIE, e.g., a smartphone. This configuration is motivated by the fact that conventional structures exhibit poor performance in the presence of random device orientation and blockage. In fact, we show that the MDR outperforms the benchmark structure by over 10 dB at bit-error ratio (BER) of 3.8 × 10-3. Moreover, an adaptive access point (AP) selection scheme for the SM is considered, where the number of APs is chosen adaptively in an effort to achieve the lowest energy requirement for a target BER and spectral efficiency. The user performance with random orientation and blockage in the entire room is evaluated for sitting and walking activities, for which the orientation-based random waypoint (ORWP) mobility model is invoked. Furthermore, we demonstrate that the proposed adaptive technique with SM outperforms the conventional spatial multiplexing system. We also study the performance of the underlying system on the uplink channel where we apply the same techniques used for the downlink channel. It is shown analytically that the multidirectional transmitter (MDT) with adaptive SM is highly energy efficient.

Light-fidelity (LiFi) is a networked optical wireless communication (OWC) solution for high-speed indoor connectivity for fixed and mobile optical communications. Unlike conventional radio frequency wireless systems, the OWC channel is not isotropic, meaning that the device orientation affects the channel gain significantly, particularly for mobile users. However, due to the lack of a proper model for device orientation, many studies have assumed that the receiver is vertically upward and fixed. In this paper, a novel model for device orientation based on experimental measurements of 40 participants has been proposed. It is shown that the probability density function (PDF) of the polar angle can be modeled either based on a Laplace (for static users) or a Gaussian (for mobile users) distribution. In addition, a closed-form expression is obtained for the PDF of the cosine of the incidence angle based on which the line-of-sight (LOS) channel gain is described in OWC channels. An approximation of this PDF based on the truncated Laplace is proposed and the accuracy of this approximation is confirmed by the Kolmogorov-Smirnov distance. Moreover, the statistics of the LOS channel gain are calculated and the random orientation of a user equipment (UE) is modeled as a random process. The influence of the random orientation on signal-to-noise-ratio performance of OWC systems has been evaluated. Finally, an orientation-based random waypoint (ORWP) mobility model is proposed by considering the random orientation of the UE during the user's movement. The performance of ORWP is assessed on the handover rate and it is shown that it is important to take the random orientation into account.

Light-fidelity (LiFi) is a wireless communication technology that employs both infrared and visible light spectra to support multiuser access and user mobility. Considering the small wavelength of light, the optical channel is affected by the random orientation of user equipment (TIE). In this paper, a random process model for changes in the TIE orientation is proposed based on the data measurements. We show that the coherence time of the random orientation is in the order of hundreds of milliseconds. Therefore, an indoor optical wireless channel can be treated as a slowly varying channel as its delay spread is typically in the order of nanoseconds. A study of the orientation model on the performance of direct-current-biased orthogonal frequency-division multiplexing (DC-OFDM) is also presented. The performance analysis of the DC-OFDM system incorporates the effect of a diffuse link due to reflection and blockage by the user. The results show that the diffuse link and the blockage have significant effects, especially if the TIE is located relatively far away from an access point (AP). It is shown that the effect is notable if the horizontal distance between the TIE and the AP is greater than 1.5 m in a typical 5 × 3.5 × 3 m3indoor room.

This paper focuses on the handover probability as a result of the random rotation of a user equipment (UE) in hybrid light-fidelity (LiFi) and radio frequency (RF) networks. A received signal strength indicator (RSSI)-based handover algorithm is considered in this study. Using RSSI as the user association rule does not guarantee that the randomly-oriented UE is always associated with the nearest access point (AP). In some cases, depending on the orientation of the UE, the received signal powers from the LiFi APs are very weak and unreliable. Therefore, a vertical handover from the LiFi AP to the RF AP is required to maintain the user quality of service. Hence, it is essential to study the handover probability due to the change of orientation. A theoretical analysis of the handover probability based on a Markov chain model is provided. The analytical results are confirmed by the Monte Carlo simulation. The effects of some parameters, such as the threshold, hysteresis level and the trade-off between the frequency and the delay of handover are presented in this paper.

Visible light communications (VLC) and Millimetre wave (mmWave) systems are two emerging technologies for short-range mobile communications. One of the limiting factors of both VLC and mmWave systems is the random orientation of mobile devices that can significantly affect the channel gain in both systems. Since there is no proper model for device orientation, many studies have assumed that device orientation is fixed or modelled as a uniform distribution. To address this issue, an experimental study of mobile user behaviour is conducted and a statistical orientation model is proposed in this paper. The results show that the probability density function (PDF) of the elevation angle follows a Laplace distribution. Based on the statistical orientation model, Monte-Carlo simulations are carried out to analyse the system performance of VLC and mmWave systems. The statistics of signal-to-noise-ratio (SNR) are compared with the experiment-based simulation results in both VLC and mmWave systems.

Visible light communication and its use in wireless networks, which is referred to as light fidelity (LiFi), can potentially provide ubiquitous indoor broadband connections. Most of the existing studies on LiFi consider a device that always faces upward, disregarding the importance of the orientation of the device. This paper presents experimental results on the randomly-oriented devices and a statistical model of the LiFi channel considering the random orientation. The distribution of the cosine of the incidence angle is shown to have a Laplacian form. This distribution is then applied to study the signal-to-noise ratio of the randomly-oriented and the upward-facing devices. Finally, we show that neglecting the random orientation can lead to inaccurate conclusions. Hence, it is important to take the random orientation into account in future LiFi network analyses.

Channel adaptive signaling, which is based on feedback, can result in almost any performance metric enhancement. Unlike the radio frequency channel, the optical wireless communication (OWC) channel is relatively deterministic. This feature of OWC channels enables a potential improvement of the bidirectional user throughput by reducing the amount of feedback. Light-Fidelity (LiFi) is a subset of OWCs, and it is a bidirectional, high-speed, and fully networked wireless communication technology where visible light and infrared are used in downlink and uplink, respectively. In this paper, two techniques for reducing the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback scheme based on reducing the content of feedback information and 2) limited-frequency feedback scheme based on the update interval. Furthermore, based on the random waypoint mobility model, the optimum update interval, which provides maximum bidirectional user equipment throughput, has been derived. Results show that the proposed schemes can achieve better average overall throughput compared with the benchmark one-bit feedback and full-feedback mechanisms.

The access point (AP) in many cases requires information about the channel condition of user equipment (UE) for better resource allocation and scheduling purposes. Although this feedback information can contribute to enhanced performance, it is at a cost of uplink throughput degradation. Limited feedback (LF) techniques offer the ways to reduce the amount of feedback at the AP. Light-Fidelity (Li-Fi) is a new high-speed and fully networked optical wireless technology where its channel is fairly static. Considering this feature, we proposed a new LF method based on the channel update interval. The optimum value for the update interval has been derived and it can provide maximum sum throughput of uplink and downlink. Results confirm that the proposed LF method can yield enhanced overall throughput compared to the traditional LF techniques.

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.

The cellular Light-Fidelity (Li-Fi) network is considered as a promising approach for high speed indoor data access. The conventional metric, based on which an access point (AP) is selected for each user, is signal strength. This metric offers the best channel quality for each user but does not guarantee the achievable data rate, since the resource of an AP is limited. In this paper, we propose a new metric for AP selection to improve the load balancing among APs by considering both the received signal-to-interference-plus-noise-ratio (SINR) and the traffic of AP. The orientation of mobile stations (MS) is also taken into account and its effect on users' performance is evaluated. In reality, receivers have random angles with the coordinate axes. We consider three standard angles similar to those used in mobile devices to model the device orientation. Based on this model, the effect of arbitrary orientation on user's throughput and satisfaction is investigated. Simulation results show that when the orientation of users is considered, the proposed AP selection metric outperforms the conventional metric.