The rise of intelligent reflecting surfaces (IRSs) is opening the door for unprecedented capabilities in visible light communication (VLC) systems. By controlling light propagation in indoor environments, it is possible to manipulate the channel conditions to achieve specific key performance indicators. In this paper, we investigate the role that IRSs can play in boosting the secrecy capacity of non-orthogonal multiple access (NOMA) VLC systems. More specifically, we propose an IRS-based physical layer security (PLS) mechanism that mitigates the information leakage risk inherent in NOMA. Our results demonstrate that the achieved secrecy capacity can be enhanced by up to 105% for a number of 80 IRS elements. To the best of our knowledge, this is the first paper that examines the PLS of NOMA-based IRS-assisted VLC systems.

In this paper, a vehicular light-fidelity (LiFi) system, which supports mobility for seamless broadband data transmission is proposed for multipoint-to-point (M2P) communication. To this end, a novel digital mirror device (DMD) based receiver (RX) is investigated that consists of two functionalities, to sense the transmitters and sense mobility. Relying on the proposed hybrid waveform, the proposed RX has the ability to detect and distinguish multiple transmitter (TX) sources under free movement conditions within three-dimensional space. Computer simulation results for the ‘TX sensing platform’ show that a high TX detection and identification performance is achieved at an extremely low channel signal-to-noise-ratio (SNR) of −11 dB and a bit error ratio (BER) of 10−4. Furthermore. the simulation results for the ‘mobility sensing platform’ was able to trace the positions of light sources simultaneously with the error rate of below 2% relative to the diameter of light spots on the DMD for all considered mobility scenarios.

This paper presents a color dimming (CD) method for Light Fidelity (LiFi), enabling the ability to change the color of a light-emitting-diode (LED) and adjust the brightness of the light. The proposed CD scheme targets both high spectral efficiency and high signal-to-noise ratio (SNR). In order to achieve a high spectral efficiency, a superposition modulation scheme called augmented spectral efficiency discrete multi-tone (ASE-DMT) is used with multi-layer modulation and iterative detection. In order to achieve a high SNR, different modulation layers of ASE-DMT are loaded onto different LEDs. This is called a multi-layer multi-output modulation (MLMO) scheme. The proposed MLMO scheme supports single photo diode (PD) demodulation, thus reducing the receiver complexity. Results show that red/green/blue channels can adjust their brightness levels independently within a brightness range of 16 % to 84 % to produce a mixed color. Within the operating color plane, the proposed MLMO ASE-DMT provides a consistently high SNR of up to 33 dB, which is 8.3 dB higher than the typical ASE-DMT with multi-layer single-output modulation (MLSO).

Light-fidelity (LiFi) is recognised as a promising technology for next generation wireless access networks. However, limited research efforts have been spent on the uplink (UL) transmission system in LiFi networks. In this article, a wireless infrared (IR)-based LiFi UL system is investigated. In particular, we focus on the performance of a single static user under the influence of random device orientation and link blockage. Simulations and mathematical analysis have been used to evaluate the UL system performance. The analytical expressions for the UL optical wireless channel and signal-to-noise ratio (SNR) statistics under the effects of random device orientation and link blockage are derived. The results show that the effects of random orientation and link blockage may lead to a decrease in coverage probability by 10% - 40% with various SNR thresholds.

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.

In this paper, a cyclic-prefixed (CP) system with pulse amplitude modulation (PAM) for uplink transmission in light-fidelity (LiFi) is discussed. LiFi supports bi-directional transmission, i.e., downlink and uplink transmissions. However, there have been only a few studies on uplink LiFi despite the fact that the uplink traffic demand increases due to frequent use of, for example, video call and live streaming services. The uplink transmission is designed such that a transmitter has a low peak-to-average ratio as well as a lower computational complexity using a CP in conjunction with PAM. We investigate three different schemes, namely linear equalization (LE) where only a feedforward filter is applied, decision feedback equalization (DFE) where a feedback filter is added at the receiver and Tomlinson-Harashima precoder (THP) where a feedback filter is added at the transmitter. Both feedforward and feedback filters are jointly optimized. In addition, we consider an optical wireless channel with reflections, randomly-located and randomly-oriented user equipment, a human body acting as a reflector and a limited linear dynamic range of a light emitting diode. We conclude that a DFE system is suitable in a low spectral efficiency region, where, compared to an LE system, up to 4 dB gain is achieved. A THP system is suitable for a high spectral efficiency region, where up to 5 dB gain compared to the LE and DFE systems is achieved.

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.

With the significant increase in wireless data traffic in recent years, the scarce radio frequency (RF) spectrum can no longer guarantee the capacity needed to meet the future growth in mobile traffic demand. New applications such as virtual reality and augmented reality further increase this demand. The emergence of high speed visible light communication (VLC) offers an opportunity to mitigate the looming spectrum crisis. However, the power consumption of a VLC link due to incoherent transmission is a major concern for system designers. Only a few studies have addressed this issue. In this paper, a theoretical study on the relationship between the power consumption and the data rate of a high speed VLC link is presented. The considered VLC link is based on direct-current-biased optical orthogonal frequency division multiplexing (DCO - OFDM) and on-off keying (OOK). The effects of spatial modulation (SM) are also investigated. The presented results show that the optimal operating bandwidth can be determined for a SM DCO-OFDM system to achieve the highest power efficiency possible. The study provides a guideline to configure a VLC system in order to optimize power efficiency for different data rate requirements.

Hybrid light fidelity (LiFi) and wireless fidelity (WiFi) networks (HLWNets) have been recently proposed as a promising technique for indoor wireless communications. Such a network combines the high-speed transmission of LiFi and the ubiquitous coverage of WiFi. Meanwhile, in addition to user mobility, intermittent light-path blockages make the handover issue intricate in HLWNets. Also, load balancing (LB) is necessary because the coverage areas of LiFi and WiFi networks are completely overlapped. This further increases the difficulty in handover as the decision for a vertical or a horizontal handover in a mobile environment with ultra-small cells is non-trival. Aimed at providing an optimal solution to the access point selection (APS) with given channel state information (CSI), the conventional LB method might cause frequent and unnecessary handovers. In this paper, a joint optimisation problem is formulated to simultaneously consider LB and handover in HLWNets. Results show that compared with the conventional LB method, the proposed method can improve the system throughput by up to 60%.

Light-fidelity (LiFi) extends visible light communication into high-speed and multiuser wireless networks. As in cellular networks, co-channel interference (CCI) stemming from other access points (APs) sharing the same transmission resources is the main limiting factor in the performances of the LiFi networks; therefore, it is important to study characteristics of CCI. This paper studies the statistical characteristics of aggregate signal interference iaggin downlink LiFi networks where LiFi-enabled APs are randomly deployed following a Poisson point process (PPP). One of the advantages of considering iaggcompared to the aggregate power interference is that parameters of modulation schemes and room dimensions can both be incorporated. The paper's key finding is that the energy of iaggis not monotonically decreasing as the vertical distance of user of interest and the AP increases. In addition, we show that the distribution of iaggrapidly approaches a Gaussian distribution as PPP intensity increases.

One of the important issues in an indoor visible light communication (VLC) system is the multi-path (MP) effect due to reflections. Generally, the MP effect can be characterised by the channel impulse response (CIR). Methods based on ray-tracing/Monte Carlo simulations are primarily used to obtain these impulse responses, but this generally leads to time consuming computer simulations. In this study, an analytical approach is proposed to directly and accurately calculate the non-line-of-sight (NLOS) components of the CIR due to the interior surface reflections in a room. The proposed method could be used in system-level simulations of a networked VLC system (also referred to as a LiFi attocell network), which requires a large number of channels with different transmitter and receiver deployments to be generated in a computationally efficient manner.

In this paper, the downlink signal-to-interference- plus-noise ratio (SINR) statistics are analysed in a direct current-biased optical orthogonal frequency division multiplexing (DCO-OFDM) based optical attocell network with a Poisson point process (PPP) cell deployment. An optical attocell system utilises existing lighting fixtures in an indoor environment to function as a small-cell cellular network. It uses each luminary as a base station (BS) to serve multiple nearby mobile users. Similar to a conventional radio frequency (RF) cellular system, the grid cell deployment is restricted by many practical issues in an optical attocell network. Therefore the performance of the system with PPP cell deployment is considered in this study in order to identify a lower bound for practical attocell networks with irregular cell deployment. An analytical framework is presented and compared with the computer simulations. Also the SINR statistics with different cell deployments are compared and discussed.

In this paper, an indoor visible light communication (VLC) cellular network, referred to as an optical attocell network, is analysed at system level. A line-of-sight (LOS) ray-tracing model is used to characterise the light propagation and its effect on the performance of an intensity modulation (IM) and direct detection (DD) communication system. Orthogonal frequency division multiple access (OFDMA) based on direct-current optical orthogonal frequency division multiplexing (DCO-OFDM) is used as a multi-user access scheme. The signal-to-interference-plus-noise ratio (SINR) for a user with a random location in an optical attocell is studied. An analytical approach to calculate the statistics of the SINR is presented and verified by Monte Carlo simulations. Moreover, average spectral efficiency is also studied in order to estimate the downlink wireless capacity of the optical attocell network. The spectral efficiency of the system has been found to be strongly dependent on the radius of an optical attocell and on the half-power semi-angle of the light transmission profile. Guidelines for the configuration of the relevant attocell parameters are provided. An optical attocell with an average spectral efficiency of 5.9 bits/s/Hz is demonstrated for an appropriate set of attocell parameters.

Interference coordination in optical wireless cellular networks using different frequency reuse techniques are discussed and compared in this paper. On the one hand, full frequency reuse maximises the system throughput at the cost of poor cell-edge user performance. On the other hand, cluster-based static resource partitioning offers good cell-edge user performance at the cost of low system throughput. Fractional frequency reuse (FFR) is introduced as a compromise between cell-edge user performance and the system throughput with low system complexity. Simulation results show that a guaranteed user throughput of 5.6 Mbps and an average area spectral efficiency (ASE) of 0.3389 bps/Hz/m2are achieved by the FFR optical wireless system with appropriate power control factors. These results show considerable throughput improvement compared to both benchmark systems. It is also shown that by adjusting the LED transmission optical power of a system using visible light spectrum, the illumination requirement for an office room can be satisfied without extra lighting facilities.