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We present a novel indoor communication network using 1-mm core diameter step-index plastic optical fibres as a feeder line, and using the light emitted by the POF-end for wireless data transmission, creating a fully passive access point (AP). This has the advantage that it does not require electrical powering, resulting in low maintenance. Due to the large numerical aperture, the light emitted by the POF is extremely divergent, thus a lens must be placed in front of the POF-end to adjust the size of the coverage area to enable users moving within the area or to increase the number of users served by the wireless link. We present a theoretical model for both multiple-input multiple-output (MIMO) transmission modes, spatial diversity (SD) and spatial multiplexing (SM) concepts, using orthogonal frequency division multiplexing (OFDM). We report the system throughput, that is estimated using the singular value decomposition (SVD) of the experimentally measured channel matrix H(f) . We then present the experimental results applying both SD and SM concepts and make a comparison between theoretical and experimental results. We demonstrate that this passive AP concept using OFDM achieves throughputs of around 5.2 Gbps when SM is applied, and 3.8 Gbps when SD is applied. Furthermore, we use eye-safe visible light laser at the beginning of the POF feeder which, due to optical losses by POF and components, limited the wireless link to 1 m only and the coverage area to 45 cm diameter. We believe the proposed system represents a low-cost, high-capacity, highly secure, interference free wireless system, suitable for next generation for industrial and home applications.

In this paper, we study the spectral efficiency (SE) and energy efficiency (EE) of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) for visible light communication (VLC). Firstly, we derive the achiev-able rates for Gaussian distributions inputs and practical finite-alphabet inputs. Then, we investigate the SE maximization problems subject to both the total transmit power constraint and the average optical power constraint with the above two inputs, respectively. By exploiting the relationship between the mutual information and the minimum mean-squared error, an optimal power allocation scheme is proposed to maximize the SE with finite-alphabet inputs. To reduce the computational complexity of the power allocation scheme, we derive a closed-form lower bound of the SE. Also, considering the quality of service, we further tackle the non-convex EE maximization problems of ACO-OFDM with the two inputs, respectively. The problems are solved by the proposed Dinkelbach-type iterative algorithm. In each iteration, the interior point algorithm is applied to obtain the optimal power allocation.The performance of the proposed power allocation schemes for the SE and EE maximization are validated through numerical analysis., 14 pages, 10 figures, accepted by IEEE Trans. Wireless Commun

In an optical wireless communication system, the LEDs require high bandwidth to achieve data rates compare to RF communications. Therefore, it is preferred to use high current densities to drive the LED, even beyond its maximum efficiency point. Nevertheless, the LED is a self-heating device where part of the electrical power is converted to optical power and the rest into heat. This increment of temperature in the LED will reduce its optical power and efficiency, causing a degradation of the optical system SNR. Firstly, we start by derivingthe relationship between the LED voltage with its junction temperature using the Shockley equation. Then, we measure the forward voltage at different temperatures to prove a linear relation between them. The optical power, LED bandwidth, and efficiency can be calculated using the ABC parameters from the rate equation. Therefore, we propose a method to obtain these parameters by measuring the emitted optical power and its rise time when varying the driving current. Finally, the ABC parameters are calculated by solving an MMSE problem with the measurements previously done. The throughput of an optical wireless link depends on the efficiency and the LED bandwidth, thus it is temperature dependent. We calculate the degradation of the rate caused by the temperature increment.

In this paper we report, for the first time, measuredperformance results of the concatenation between two differentplastic optical fibre (POF) fronthaul approaches and distributedmultiple-input multiple-output (D-MIMO) LiFi using commercialG.hn chipsets. These integrated circuits (IC)s are capable ofperforming, among other features, MIMO signal processing,orthogonal frequency-division multiplexing (OFDM) modulation,and adaptive bit-loading, which are important features forLiFi systems. The use of commercially available ICs with theaforementioned features, accelerates the mass market adoptionof LiFi technology. We consider the POF fronthaul approaches:spatial division multiplexing (SDM) and wavelength division multiplexing (WDM). POFs are interesting for industrial scenariosdue to its electromagnetic interference immunity, their low weightand cost. The measured throughput results show that a WDMPOF fronthaul, using a red light-emitting diode (LED) and agreen laser diode (LD), has ample throughput to support the DMIMO LiFi link, which proves the viability of the use of POFsas fronthaul for D-MIMO LiFi systems. Our results give inputfor possible enhancements to the ITU G.vlc standard G.9991.

Optical transmission is attractive for future wireless communication because signals can be confined in a specific coverage area. In designing a system based on LEDs, the trade-off between transmission bandwidth and optical power is important. As a common practice with micro-LEDs, one may drive the LED at a high current density, beyond the optimum efficiency point, to boost the bandwidth. Bit rates of multiple gigabit/s have been demonstrated, but coverage is small. However, using high current densities is discouraged by self-heating, as only a part of the electrical power is converted to optical power. A rising temperature reduces efficiency, causing a degradation of the SNR. We combine multiple models, not only the ABC photon generation and the LED dynamic responses subject to a rapidly varying current but also, the junction voltage method to estimate its temperature. We quantify the achieved throughput in a DCO-OFDM link at different temperatures. This leads to the insight that there is an optimal size of the active region and an optimal current density that maximizes the throughput of the wireless link at different temperatures. The work presented here builds upon work that we published earlier, and we place this in the perspective of a combined system model.

This paper studies the response of a Light Emitting Diode (LED) to a stepwise change of the power delivered by a current or voltage supply. This study contributes to the design of efficient electronic circuits to generate On-Off Keying modulation for optical wireless communication using LEDs as an emitter. We combine the Shockley diode equation with the ABC model for electron-hole recombination to propose a new model for rise and fall curves. The fall curve differs for an open or shunted LED, as in the latter case, the carriers (electrons and holes) are pulled out of the LED junction. We quantify these effects and exploit this difference to propose a tristate optimized modulator circuit, which is highly power efficient. In this design, we allow modulating currents to be switched a bit earlier or later than then the bit transition itself.

In this paper, a nonlinear model for Light Emitting Diodes (LEDs) inspired by semiconductor physics, and a corresponding post-compensator are implemented in a Field Programmable Gate Array (FPGA) for real-time Visible Light Communications (VLC). Our experiments demonstrate that the LED model effectively characterizes the dynamic LED nonlinearity, including the memory effects. The output signal of this nonlinear LED model shows a good resemblance with the measured LED output. In addition, a dedicated nonlinear equalizer, say, a post-compensator, inspired by this LED physical model can mitigate the nonlinear distortion substantially. Thereby it facilitates high data rate over the bandwidth-limited LED. It shows that the nonlinear compensator is attractive for practical real-time digital signal processing systems due to its high performance and low complexity.

Communication for the Internet of things (IoT) currently is predominantly narrowband and cannot always guarantee low latency and high reliability. Future IoT applications such as flexible manufacturing, augmented reality and self-driving vehicles rely on sophisticated real-time processing in the cloud to which mobile IoT devices are connected. High-capacity links that meet the requirements of the upcoming 6G systems cannot easily be provided by the current radio-based communication infrastructure. Light communication, which is also denoted as LiFi, offers huge amounts of spectrum, extra security and low-latency transmission free of interference even in dense reuse settings. We present the current state-of-the-art of LiFi systems and introduce new features needed for future IoT applications. We discuss results from a distributed multiple-input multiple-output topology with a fronthaul using plastic optical fibre. We evaluate seamless mobility between the light access points and also handovers to 5G, besides low-power transmission and integrated positioning. Future LiFi development, implementation and efforts towards standardization are addressed in the EU ELIoT project which is presented here.

A spatial multiplexing WDM transmission comprising a wired link of thick core step-index POF and a wireless link of 1m is presented. Applying singular value decomposition on experimental data, 4.8 Gbps throughput can be achieved.

This paper addresses the question of whether an optical wireless communication (OWC) LiFi system preferably uses LEDs near their most efficient operating point, which is preferred for environmentally-friendly highly efficient illumination, or that one should preferably drive the LED into its droop regime to speed up its response. In fact, additional non-radiative recombination speeds up the LED response to modulation but spoils efficiency. We introduce the differential (Internal and External) Quantum Efficiency (dIQE; dEQE), as a particular derivative of quantum efficiencies. It quantifies the efficiency at which a current modulation translates into an optical signal. We optimize the current density, thus how deeply the LED is to be driven into droop, using DC-offset (DCO) OFDM throughput expressions for the first--order low-pass LED response. Short-range small-coverage systems prefer high current densities to achieve very high throughputs. LiFi with wide opening angles, e.g. covering tens of square meters are better off with a high dIQE, thus lower current densities.

A single-input and single-output (SISO) frequency-modulated continuous wave (FMCW) radar architecture is proposed and in vivo demonstrated for remote 2-D localization (range and angular information) and vital signs monitoring of multiple subjects. The radar sensor integrates two frequency scanning antennas which allow angular separation and enable determining a 2-D map (range versus angle) of a room environment from which people can be distinguished from objects and clutter. After technical tests to validate the functionality of the proposed architecture and data processing algorithm, a practical setup was successfully demonstrated to locate human volunteers, at different absolute distances and orientations, and to retrieve their respiratory and heart rate information. Experimental results show that this radar sensor can monitor accurately the vital signs of multiple subjects within typical room settings, reporting maximum mean absolute errors of 0.747 breaths-per-minute and 2.645 beats-per-minute, respectively, for respiration rate and heartbeat. Practical applications arise for Internet of Things (IoT), ambient-assisted living, healthcare, geriatric and quarantine medicine, automotive, rescue and security purposes.

It is long believed that the phase information in a complex signal is lost in visible light communications (VLC) with intensity modulated and direct detection (IMDD). While Kramers-Kronig (KK) relation is recently explored in coherent optical communications to reduce the receiver complexity, and a KK receiver performs phase retrieval from the amplitude received by the square-law photodiode (PD). In principal, once a modulated waveform is generated with minimum phase condition, the phase information can be reconstructed only from its amplitude. This manuscript extends KK relation into an IMDD system, and a quadrature signaling is generated with only amplitude. The single amplitude stream is used to drive the light-emitting diode (LED), where the optical intensity instead of the electrical field is modulated. A KK receiver is used to retrieve the phase and thus demodulate the quadrature signal. Simulation results show that the proposed systems for single- and multi-carrier quadrature signaling can achieve high spectrum and power efficiency with low complexity. In particular, it is attractive for VLC where the DC prescribed for illumination can be utilized for constructing a minimum phase condition. Moreover, the data rate with KK relation in VLC is doubled compared to other modulations that only encode information in amplitude.

This work investigates handover in hybrid light fidelity (LiFi) and wireless fidelity (WiFi) networks (HLWNets). In such a network, the handover process becomes challenging due to two main factors: i) the relatively short coverage range of a single access point (AP), and ii) the largely overlapping coverage areas of different networks. As a result, HLWNets are susceptible to frequent handovers. To reduce the handover rate, the concept of handover skipping (HS) was introduced, which enables handovers between non-adjacent APs. However, conventional HS methods rely on knowledge about the user’s trajectory, which is not readily available at the AP. In this paper, a novel HS scheme is proposed on the basis of reference signal received power (RSRP) and its rate of change, with an adaptive network preference adopted. Since RSRP is commonly used in the existing handover schemes, the proposed method requires no additional signalling between the user and the AP. Simulation results show that the new method can effectively reduce unnecessary handovers, especially those between LiFi and WiFi. Compared to the standard and trajectory-based handover schemes, the proposed method improves network throughput by up to about 120% and 30%, respectively.

Multiple-input multiple-output (MIMO) is a promising technology to efficiently improve the achievable rate of white light-emitting diodes (LEDs) enabled visible light communication (VLC) systems. Nevertheless, for conventional MIMO techniques such as spatial diversity (SD) and spatial multiplexing (SMP), the working mode of each LED transmitter is independent of users' spatial positions. In this article, we propose and investigate three user-centric MIMO techniques, including SD/SMP switching, adaptive SMP (aSMP), and SD-aided aSMP (SD-aSMP), for achievable rate improvement of indoor MIMO-VLC systems, by exploiting users' spatial positions as a new degree of diversity. The analytical and simulation results show that the achievable rate of a 4 × 4 MIMO-VLC system in a typical indoor environment can be significantly improved by applying the user-centric MIMO techniques compared with the conventional ones. More specifically, SD/SMP switching can enjoy both high diversity gain of SD and high multiplexing gain of SMP at different signal-to-noise ratio (SNR) regions, while aSMP mitigates the adverse effect of noise amplification in conventional SMP and SD-aSMP can benefit from additional diversity gain. Furthermore, we also show that the proposed user-centric MIMO techniques outperform conventional MIMO techniques under the condition of only slightly increased computational complexity with limited feedback information.

This paper extends the 'go-with-the-flow' method to explore enclosed environments, like oil reservoirs, pipe lines that transport liquids, and industrial tanks for processing chemicals, where sensing nodes cannot establish communication with the external world. Nonetheless, large quantities of highly miniaturized, thus power-constrained sensor nodes are injected into these environment and flow through them along with the medium, monitoring their environment but also reconstructing their time-varying position from mutual communication, but without any communication to external base stations or beacons. The relative trajectories of nodes yield essential insights of the fluid flow in the otherwise inaccessible environment. We present a functional implementation of a ranging protocol accommodating size and energy constraints. Our simulation chain models node movement from different types of flow dynamics. It comprehensively assesses not only the performance of the communication and ranging protocols, but also of the reconstruction algorithm. Our assessments cover a wide range of different environments and flow profiles, including highly dynamic ones.

This paper builds a model for the benchmarking and the selection of a suitable LED for wireless optical communication, in particular for indoor LiFi Infrared or visible light communication. It reviews LED measurements and theoretical models for such trade-off and applies these into communication bit-rate throughput expressions. While illumination LEDs are chosen for a large quantum efficiency, for communications also a large 3 dB bandwidth is preferred. In the LED, electron hole pairs recombine radiatively (thereby emitting a photon) or non-radiatively (causing a leakage current and reducing EQE). Non-radiative recombination also contributes to the response speed of the LED and increases its 3 dB bandwidth. On the other hand, a reduction in effective optical power may counterproductively lead to an inadequate signal-to-noise ratio. A trade-off is postulated empirically, in the form of a rule of thumb: “transmit power raised to the power alpha times bandwidth raised to the power one minus alpha” appears to be an LED constant. This semi-empirical model gives straight lines on a log-log scale. This paper searches for a theoretical justification for such a model, where current density acts as a parameter to make the trade-off.According to communication theory, the achievable bit rate grows approximately linearly with an increasing bandwidth but approximately logarithmically with the received energy per bit. However, this needs to be reviewed for a gentle low-pass roll-off of the LED response, as it allows modulation far beyond the 3 dB bandwidth. These lead to a perspective on how to operate the LED: a system design faces the challenge to trade-off power versus bandwidth according to the physics LED properties, to optimize a communication throughput target.

The bound of the information transmission rate of direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) for visible light communication (VLC) with finite-alphabet inputs is yet unknown, where the corresponding spectral efficiency (SE) and energy efficiency (EE) stems out as the open research problems. In this paper, we derive the exact achievable rate of {the} DCO-OFDM system with finite-alphabet inputs for the first time. Furthermore, we investigate SE maximization problems of {the} DCO-OFDM system subject to both electrical and optical power constraints. By exploiting the relationship between the mutual information and the minimum mean-squared error, we propose a multi-level mercury-water-filling power allocation scheme to achieve the maximum SE. Moreover, the EE maximization problems of {the} DCO-OFDM system are studied, and the Dinkelbach-type power allocation scheme is developed for the maximum EE. Numerical results verify the effectiveness of the proposed theories and power allocation schemes., 14 pages, 14 figures, accepted by IEEE Transactions on Wireless Communications

The Internet is not only accessed by computers and smartphones. With the rise to the Internet-of-Things (IoT), a very wide variety of smart objects can be connected to the network. Today, there are already billions of wearables, sensors, televisions, medical and industrial machines, and other devices transmitting and receiving information through the network. To achieve better management of interference among nearby RF access points, and to densify the system, research in radio frequency (RF) technologies has moved towards to the use of shorter wavelengths, thereby achieving very small cells. Extending the choice of higher carrier frequencies further, optical wireless communications has also attracted more and more attention. To accelerate mass market adoption, LiFi system designers have considered the use of inexpensive light-emitting diodes (LED)s. However, some challenges must be overcome for designing high-speed data transmission systems using them, such as their low-pass effect. Besides that, LEDs are anyhow limited in power. To exploit its best performance and provide reliable signal transmission for a long time, their power limitation must be respected. The simultaneous transmission of multiple signals through multiple emitters and multiple receivers using multiple-input multiple-output (MIMO) schemes help to improve performance against the low-pass channel by exploring the MIMO multiplex gain. Using orthogonal-frequency-division-multiplexing (OFDM), multiple independent subcarriers are simultaneously transmitted over the low-pass frequency response of every LED. OFDM allows the use of power loading strategies for finding the best power-spectrum density for throughput improvement . This paper gives an overview on recent results and an outlook towards improved power allocation algorithms for LED-based D-MIMO OWC systems.

Optical Wireless Communication (OWC) is gaining popularity as it potentially offers interference-free communication in dense environments. By using LEDs, cells of very small size can be created with sharp boundaries thus bandwidth can be reused very densely. Building an effective communication systems starts from understanding the channel. In scientific literature, various models for the LED response are in use. This overview paper starts from the non-linear recombination of electrons and holes in the LED semiconductor junction, which then appears to lead to a very tractable and simple signal processing model. In fact, the LED channel substantially differs from a radio channel. The paper summarizes recent progress in developing an LED channel model, based on the physics of photon generation in the active region. The LED Quantum Well (QW) creates a first-order low-pass effect and the hole-electron re-combinations are inherently non-linear. Thereby the LED exhibits dynamic distortion, that is, non-linearities that are are intertwined with memory effects. The model can be used to reduce power consumption and to increase throughput of LED OWC systems.

Multiple-input multiple-output (MIMO) systems have been widely studied for improving throughput and robustness of optical wireless communication (OWC) systems. Indeed, with MIMO, the likelihood of simultaneous interruption of all optical beams is greatly reduced. For throughput improvement, many power loading strategies are obtained for a total power constraint while allowing power to be freely exchanged among light-emitting diodes (LEDs). In scenarios with unequal distances between LEDs and the user terminal, more power is allocated to the LED that has a better channel than to others. In practice, every optical front-end has its own power limitation and a per-LED power constraint is more meaningful than a total power constraint. This paper studies to what extent this has an impact on the performance of LED-based distributed MIMO (D-MIMO) OWC links. We formulate the achievable rate optimization problem for both cases, when the total power is constrained or when under a per-LED power constraint. Results show that, the achievable rate under a per-LED power constraint is lower than under a total power constraint, but it is higher than if the power is uniformly spread over the entire bandwidth. It also allows for transmit power saving, at a cost of higher computational complexity. In addition, under a per-LED power constraint, LEDs with better channel condition tends to use a modulation bandwidth far above the 3-dB bandwidth.

Visible light communication (VLC) is a short-range optical wireless communication (OWC) utilizing white light-emitting diode (LED) lighting, so that the VLC systems can provide both illumination and communication. Multiple-input multiple-output (MIMO) is an attractive technology to efficiently improve the achievable rate of VLC with multiple LED luminaries which experience a low-pass effect in practical channels. In this paper, we investigate the performance of MIMO-VLC over three general low-pass channels, including exponential, first-order and Gaussian low-pass channels. Over frequency domain, two power loading strategies for multi-subcarrier orthogonal frequency division multiplexing (OFDM) are considered, namely uniform power loading and water-filling power loading. Expressions on the achievable rate to the corresponding link power budget and bandwidth are derived. Low-pass MIMO-OFDM VLC with a matrix channel decomposition has not extensively been treated theoretically in literature, to the best of our knowledge.

In this paper, we propose an energy-efficient nonorthogonal multiple access (NOMA) technique for multi-user visible light communication (VLC) systems, by adopting a low-complexity quality of service (QoS)-guaranteed power allocation strategy. From the perspective of energy saving, we design the energy-efficient NOMA-enabled multi-user VLC system with the goal to achieve maximal energy efficiency (EE). The closed-form QoS-guaranteed optimal power allocation strategy is obtained and the analytical EE of the proposed NOMA-enabled multiuser VLC system is derived. The analytical and simulation results show that, for a VLC system with ten users, the average EE can be improved by 19% when adopting the proposed NOMA in comparison to NOMA with gain ratio power allocation (GRPA), and the EE improvement becomes much more significant when the users have more diverse QoS requirements.

The nonlinear effect of Light-Emitting Diodes (LEDs) is one of the important factors that hamper the bit rate of Visible Light Communications (VLC). To mitigate the nonlinearity, we propose a Legendre-polynomials-based post-compensator derived from a post-distorter deduced by a physical-based nonlinear LED model. We represent the formulation of the post-distorter with Legendre series expansion to ease the computation burden for training coefficients. Since only feed-forward structure is embodied in the series representation, the coefficients of the proposed compensator are easy to access with adaptive algorithms of low complexity. To validate the effectiveness of our proposed model, we adopt Recursive Least Square (RLS) and Least Mean Square (LMS) algorithms to train the coefficients of our proposed compensator with the same iteration steps. In particular, RLS achieves lower Mean square Errors (MSEs) with faster convergence speed. Due to the relatively low complexity of these nonlinear algorithms, our proposed compensator is more actual for implementation on VLC hardware platforms like Field-Programmable Gate Array (FPGA).

In this paper, we propose two orthogonal frequency division multiplexing (OFDM)-based generalized spatial modulation (GSM) techniques, i.e., frequency domain GSM (FD-GSM) and time domain GSM (TD-GSM), for indoor multiple-input multiple-output optical wireless communication (MIMO-OWC) systems. Specifically, FD-GSM selects a subset of parallel OFDM modulators to transmit the same quadrature amplitude modulation (QAM) constellation symbol in the subcarrier level, while TD-GSM selects a subset of light-emitting diode (LED) transmitters to transmit the same time domain sample after OFDM modulation. Compared with the existing OFDM-based spatial modulation (SM) scheme, OFDM-based GSM can provide additional transmit diversity. For both FD-GSM and TD-GSM, maximum-likelihood (ML) detectors are employed to recover the spatial and constellation symbols. Moreover, TD-GSM usually requires a secondary direct current (DC) bias to work properly. The simulation results show that in a typical indoor 4times 4 MIMO-OWC system, OFDM-based TD-GSM outperforms OFDM-based FD-GSM by adding a proper secondary DC bias.

In this paper, a nonlinear autoregressive exogenous (NARX) model with a wavelet network is applied to model and compensate the nonlinearity of the LED in Visible Light Communications (VLC). The NARX model shows the ability to accurately describe the response of the LED. PAM-4 signal with a symbol rate of 5 Msym/s is used to demonstrate the performance of the NARX adaptive compensator. The eyediagrams show that this compensator can substantially improve the distorted signal. The complexity of the NARX adaptive compensator is relatively low, with only 15 units. This also facilitates the adaptive parameters updating process due to the small number of parameters in the NARX adaptive compensator.

Illumination LEDs, but also infrared LEDs have limited bandwidth. To achieve high throughput, one needs to modulate the LED significantly above its 3 dB bandwidth. Orthogonal Frequency Division Multiplexing (OFDM) is a popular modulation technique to cope with the frequency selectivity of the LED channel. In this article, we challenge whether its large Peak-to-Average-Power Ratio (PAPR) and resulting large DC bias are justified. We compare systems using the same power and derive how PAM and OFDM variants reach their optimum throughput at different bandwidths and differently shaped spectral densities, thus at very different Signal to Noise Ratio (SNR) profiles but nonetheless the same transmit power.When corrected for the path loss and normalized to the noise power in the 3 dB bandwidth of the LED, we call this the Normalized Power Budget (NPB). OFDM can exploit the low-pass LED response using a waterfilling approach. This is attractive if the NPB exceeds 60 dB. OFDM will then have to spread its signal over more than ten times the LED bandwidth and requires a DC bias of more than 4 times the rms modulation depth. Second-order distortion and LED droop may then become a limitation, if not compensated. At lower power (NPB between 30 and 60 dB), DCO-OFDM outperforms PAM, provided that it significantly reduces its bias and only if it uses an appropriate adaptive bit and power loading. Without adaptive bit loading, thus using a frequency–constant modulation order, for instance made feasible by a pre-emphasis, OFDM always shows lower performance than PAM; about 2.5 dB at a NPB around 60 dB. Below 30 dB of NPB, even waterfilling cannot outweigh the need for a larger bias in OFDM, and PAM should be preferred. We argue that a mobile system that has to operate seamlessly in wide coverage and short–range high–throughput regimes, needs to adapt not only its bandwidth and its bit–loading profile, but also its DCO-OFDM modulation depth, and preferably falls back from OFDM to PAM.

Commercial-off-the-shelf (COTS) devices enabled visible light communication (VLC) for Internet of things (IoT) applications has attracted extensive attentions from both academic and industrial communities, thanks to the pervasive deployments of light emitting diode (LED) lighting infrastructure. However, due to the limitation of frequency response and non-linearity of the commercial illuminating LED light consisting of multiple LED chips, the achievable data rate is far less than that provided by the experimental VLC system with a single LED with specialized devices, e.g., lens. To this end, we propose a power-of-2 arrangement scheme for LED chips to generate spatial summing modulation with low control complexity, and demonstrate its availability in an orthogonal frequency division multiplexing (OFDM) VLC system purely built upon COTS devices. It significantly differs from a conventional OFDM VLC system relying on digital-to-analog converter (DAC) and analog signal chain, which is complex and confined by LED’s non-linearity, thanks to we design a novel digital-to-light converter (DLC) which can output 256 light intensities linearly and be directly controlled by the discrete digital signals generated by the OFDM modulator. An experimental demonstration with employing the QAM-OFDM modulation scheme successfully confirms the effectiveness of the proposed spatial summing VLC system, which can achieve low BERs of below the forward error correct (FEC) threshold of 3.8×10−3 for both QAM8 and QAM16 running transmission frequency of 300 kHz under a communication distance of 0.8 m. It demonstrates the promising potential for delivering a data rate at hundred kbps level with this novel spatial summing based OFDM VLC system, which is sufficient for many IoT applications.

This work addresses the power penalty in a Visible Light Communication (VLC) emitter that re-uses illumination Light Emitting Diodes (LEDs) for high-speed communications. We quantify the effect of modulation depth for two widely explored modulation schemes in VLC, namely Orthogonal Frequency Division Multiplexing (OFDM) and Pulse Amplitude Modulation (PAM). Extra power is dissipated in the LED and in the modulator due to such modulation. Two popular highspeed VLC transmitter topologies, namely a Bias Tee (Bias-T) and a Serial FET (Serial-F), are compared in terms of efficiency, Extra Energy Per Symbol/Bit (EEPS/B) and complexity. This work presents a theoretical model and compares it to simulationresults. We show that PAM outperforms OFDM in terms of better Bit Error Rate (BER), larger dynamic range for modulation depth, less extra power loss, thus higher power efficiency for communication over an Additive White Gaussian Noise (AWGN) channel.

The Internet of Things currently is predominantly narrowband and cannot always guarantee high reliability and low latency. Future IoT applications such as flexible manufacturing, augmented reality and self-driving vehicles need sophisticated real-time processing units in the cloud to which mobile IoT devices are connected. These high-capacity links meet the requirements of the upcoming 6G systems and cannot be facilitated by the current mobile communication infrastructure. Light communication, which is also denoted as LiFi, offers huge amounts of spectrum, extra security and interference-free transmission. We present the current state-of-the-art of LiFi systems and introduce new features needed for future IoT applications. We propose a distributed Multiple-Input Multiple-Output topology with a fronthaul using plastic optical fiber. Such a system offers seamless mobility between the light access points and also to 5G, besides low latency and integrated positioning. Future LiFi development, implementation and efforts towards standardization are addressed in the EU ELIoT project which is presented here.

Human cognitive functioning shows circadian variations throughout the day. However, individuals largely differ in their timing during the day of when they are more capable of performing specific tasks and when they prefer to sleep. These interindividual differences in preferred temporal organization of sleep and daytime activities define the chronotype. Since a late chronotype is associated with adverse mental and physical consequences, it is of vital importance to study how lighting environments affect chronotype. Here, we use a mathematical model of the human circadian pacemaker to understand how light in the built environment changes the chronotype distribution in the population. In line with experimental findings, we show that when individuals spend their days in relatively dim light conditions, this not only results in a later phase of their biological clock but also increases interindividual differences in circadian phase angle of entrainment and preferred sleep timing. Increasing daytime illuminance results in a more narrow distribution of sleep timing and circadian phase, and this effect is more pronounced for longer photoperiods. The model results demonstrate that modern lifestyle changes the chronotype distribution towards more eveningness and more extreme differences in eveningness. Such model-based predictions can be used to design guidelines for workplace lighting that help limiting circadian phase differences, and craft new lighting strategies that support human performance, health and wellbeing.

In this paper, we introduce a human-centric lighting control system optimized to support sleep and circadian coordination. We present an approach to optimize lighting that combines a “digital siblings” approach, i.e., a stochastic extension of a digital twin. It estimates and optimizes parameters in experimentally validated models of circadian and sleep regulation with a novel optimization algorithm for optimal timing of light exposure. We acknowledge that people have varying preferences for the lighting levels throughout the day based on their personal preferences and schedules and explicitly include this as a key parameter in our optimization strategy. Our results show that with the suggested lighting schedules, alignment of circadian rhythmicity to the desired sleep-wake schedule can be achieved with minimal disruption to people’s daily lives.

Intensity Modulated (IM) optical communication over an LED channel requires the use of a non-negative signal that can also cope with the low-pass nature of LEDs. For this purpose, dedicated schemes such as Flip-OFDM and Assymetrically Clipped Optical (ACO)-OFDM have been proposed. We derive a common mathematical description on which both schemes rely. Exploiting this insight, we propose Continuous Phase Flip-OFDM (CP-Flip-OFDM) as an enhancement to Flip-OFDM. It ensures phase continuity at the transition between the two successive copies of the OFDM blocks, thereby it obviates the Cyclic Midfix between the first OFDM block and its flipped copy. Simultaneously, we derive a less compute-intensive way to generate and detect ACO-OFDM. Instead of creating an Hermitian-symmetry at the transmit Inverse FFT input, which is common in optical IM OFDM, we use zero padding. After the transmit IFFT, a phase ramp-up, i.e., a multiplication with a complex-valued exponential, is applied before truncating to a real and non-negative signal.

Objective: In the near future, real-time estimation of peoples unique, precise circadian clock state has the potential to improve the efficacy of medical treatments and improve human performance on a broad scale. Human-centric lighting can bring circadian-rhythm support using biodynamic lighting solutions that sync light with the time of day. We investigate a method to improve the tracking of individual's circadian processes. Methods: In literature, the human circadian physiology has been mathematically modeled using ordinary differential equations, the state of which can be tracked via the signal processing concept of a Particle Filter. We show that substantial improvements can be made if the estimator not only tracks state variables, such as the phase and amplitude of the circadian pacemaker, but also fits specific model parameters to the individual. In particular, we optimize model parameter $\tau _{x}$ , which reflects the intrinsic period of the circadian pacemaker ( $\tau$ ). We show that both state and model parameters can be estimated based on minimally-invasive light exposure measurements and sleep-wake state observations. We also quantify the effect of inaccuracies in sensing. Results: We demonstrate improved performance by estimating $\tau _{x}$ for every individual, both with artificially created and human subject data. Prediction accuracy improves with every newly available observation. The estimated $\tau _{x}$ -s correlate well with the subjects’ chronotypes, in a similar way as $\tau$ correlates. Conclusion: Our results show that individualizing the estimation of model parameters can improve circadian state estimation accuracy. Significance: These findings underscore the potential improvements in personalized models over one-size fits all approaches.

In this paper, we consider a light fidelity (LiFi)-enabled bidirectional Internet of Things (IoT) communication system, where visible light and infrared light are used in the downlink and uplink, respectively. In order to efficiently improve the energy efficiency (EE) of the bidirectional LiFi-IoT system, non-orthogonal multiple access (NOMA) with a quality-of-service (QoS)-guaranteed optimal power allocation (OPA) strategy is applied to maximize the EE of both downlink and uplink channels. We derive closed-form OPA sets based on the identification of the optimal decoding orders in both downlink and uplink channels, which can enable low-complexity power allocation. Moreover, we propose an adaptive channel and QoS-based user pairing approach by jointly considering users' channel gains and QoS requirements. We further analyze the EE and the user outage probability (UOP) performance of both downlink and uplink channels in the bidirectional LiFi-IoT system. Extensive analytical and simulation results demonstrate the superiority of NOMA with OPA in comparison to orthogonal multiple access (OMA) and NOMA with typical channel-based power allocation strategies. It is also shown that the proposed adaptive channel and QoS-based user pairing approach greatly outperforms individual channel/QoS-based approaches, especially when users have diverse QoS requirements.

This article presents a modulator for optical wireless communication (OWC) via LEDs, controlled by a dual closed loop. While the potential communication speed and performance of OWC have received a lot of research attention, the design of appropriate modulator circuits has not been studied so widely. This article addresses the control aspects to minimize power losses and to ensure automatic adaptation to component spreads or aging, for “high-power” LED emitters. We initially address a “low-bandwidth– system, particularly suited for indoor camera-based detection, and then a “higher bandwidth” one for Li-Fi data communication. In contrast to maximizing data throughput as it applies to high-speed modulators, low-speed modulators aim at high power and high efficiency in general lighting applications. Specifically, for the high-power modulator, we propose a control strategy that allows a wide range of combinations of LED types and LED power supplies. The power consumption of the modulator is minimized by voltage margin setting that is automatically adapt to the modulation and the LED type using the nonlinear detector. The common industry practice is to use constant-current LED drivers for powering LEDs. After creating a theoretical framework of some generality to evaluate designs and control loops with rationals and verified models, we realize modulator designs that are supplied by constant current LED drivers and verify their performance by simulation and measurements.

Interconnecting networks adopting Fast Optical Switches (FOS) can achieve high bandwidth, low latency, and low power consumption. We propose and demonstrate a novel interconnecting topology based on FOS (FOSquare) with distributed fast flow control which is suitable for HPC infrastructures. We also present an Optimized Mapping (OPM) algorithm that maps the most communication-related processes inside a rack. We numerically investigate and compare the network performance of FOSquare with Leaf-Spine under real traffic traces collected by running multiple applications (CG, MG, MILC, and MINI_MD) in an HPC infrastructure. The numerical results show that the FOSquare can reduce &gt, 10% latency with respect to Leaf-Spine under the scenario of 16 available cores.

Light Fidelity (LiFi), using light-emitting diodes (LEDs) to communicate wirelessly via light, predominantly uses orthogonal frequency division multiplexing (OFDM) with adaptive bit loading. In contrast to Radio (RF) communications, on the optical wireless communications (OWC) the gains are substantial while the signalling overhead can be limited. An example is the International Telecommunication Union (ITU) G.9991 standard. This paper shows that the bit loading scheme can be further optimized for LED communication. The LED channel is characterized by a transfer function that is predominantly declining with frequency and does not exhibit severe multipath nulls. In this paper, we describe an algorithm that allows the exchange of a signal-to-noise ratio (SNR) margin on some subcarriers, to improve the SNR on subcarriers that experiences poorer channel responses. It combines a number of subcarriers to get a more equally distributed SNRs. Subcarriers with an excess margin in SNR are used to help subcarriers with insufficient SNR.

DC-Offset Orthogonal Frequency Division Multiplexing (DCO-OFDM) is a popular modulation method for Optical Wireless Communication (OWC) as it can assign specific power levels and constellations to different subcarrier frequencies across the low-pass response of the LED. However, LEDs exhibit dynamic non-linearities to which OFDM is very sensitive. As LED signals are subject to non-linear memory effects in photon generation, different OFDM subcarriers experience different amounts of distortion. This letter translates the LED behavior into a communication channel model for OFDM modulation. It gives an experimentally verified expression for the Signal-to-Distortion Ratio (SDR) as it varies per subcarrier in OFDM systems.

This paper shows the feasibility of a networked LiFi system using a distributed multiple-input multiple-output (MIMO) link for optical wireless transmission and a plastic optical fiber (POF) link as a fixed front-haul between distributed optical front-ends, and a centralized signal processing unit. The concatenation of POF and optical wireless links yields an easy-to-install all-optical LiFi system which is robust against both, blockage of individual light beams and electromagnetic interference. A significant cost-down appears by the use of colored LEDs to feed the POF link with multiple optical signals, and wavelength division demultiplexing filters. The spatial crosstalk in the wireless link and the spectral crosstalk over the POF link can be jointly compensated by the same end-to-end MIMO processing. A common signal model, which includes the combined effects of both links are provided to characterize the proposed all-optical LiFi system. We report the first experimental findings when using space-division multiplexing (SDM), i.e., multiple POFs, and wavelength-division multiplexing (WDM), i.e., multiple colors in the front-haul, indicating that the performance is mostly limited by the wireless link. Moreover, we show that the positions of mobile users in the wireless link, as well as gain variations and spectral crosstalk in the front-haul link, influence the singular values and the achievable data rates of the LiFi system.

In the recent years, we saw a rapidly increasing interest in monitoring the human circadian rhythm. Yet, accurate measurement of an individual's retinal light exposure appears to be the Achilles heel of many applications based on circadian physiology. In fact, available body-worn and infrastructure-based devices typically exhibit very large inaccuracies and discrepancies, depending for instance on the sensor position. In this paper, we study the feasibility of available sensors for circadian rhythm monitoring and whether the reported light exposure inaccuracies are reflected in the estimations of the actual circadian state. We applied the dynamic circadian oscillator model by Kronauer to compare the phase shift resulting from body-worn and ceiling-based light measurements to reference retinal light exposure measurements. Our results stress out that despite their low accuracy, the efficacy of body worn and infrastructure-based solutions is adequate for circadian rhythm monitoring as no significant phase-shift errors were observed. In fact, the maximum daily phase shift error was-12 ± 3 min.

In the last two decades, an unprecedented spread of communication systems has been witnessed. While at the beginning these systems were only able to support a small number of devices with limited data services, they have now matured to high speed networks that are densely populated. Society is increasingly connected, with different types of applications running on, by now Billions of devices, and this trend drives the use of communication systems. The growth is so fast that the Radio Frequency (RF) spectrum is already overcrowded. In future, it is expected that many applications will require speeds far beyond a Gbit/s. In order to achieve this capacity and, at the same time, to off load the pressure on RF systems, higher spectral bands and optical frequencies are currently being explored. Exploring higher frequencies in the electromagnetic spectrum, optical wireless communication (OWC) systems have recently gained great interest [1,2]. Due its many advantages, such as low cost, high energy efficiency, and minimal heat generation, LEDs are commonly used for illumination and are strong candidates to drive data transmission in OWC systems [2-4]. However, the modulation bandwidth of this source is limited and there is still the need to increase data throughput [4,5]. An alternative is to deploy multiple LEDs in a Multiple Input Multiple Output (MIMO) scheme [2-6]. MIMO is a well-known technology which explores the additional spatial dimension in order to provide a degree-of-freedom gain. By transmitting multiple data-streams over the light channel in a Spatial Multiplexing (SM) scheme from multiple spatially separated locations, Distributed-MIMO technology offers higher data throughput without the need of additional power or bandwidth. An important additional advantage of MIMO in OWC systems is that communication still works even when one line-of-sight link is blocked. In further boosting the bits rate, the low-pass frequency response of the LEDs poses further limitations. The low-pass behaviour of this source was pointed out in [6-9], but its impact on the performance of LED-based MIMO OWC systems still not fully addressed. To compensate the low-pass effect, Orthogonal Frequency Division Multiplexing (OFDM) is often used. OFDM is a robust and effective technology commonly used in RF systems to suppress inter-symbol interference (ISI) and to convert a frequency-selective fading channel into multiple parallel flat-fading, i.e., non-dispersive channels. In an OFDM scheme the spectrum bandwidth is divided into a set of orthogonal subcarriers in order to support high data rates through parallel transmission. By using OFDM, power loading strategies can be used to appropriately distribute power over the subcarriers in order to reduce the performance degradation caused by the low-pass effect of the LEDs [8]. Different power loading strategies are proposed to allocate power resources in the frequency domain, mainly the uniform loading and the optimized waterfilling loading [6-8]. In this paper, we consider the transmission mode of an indoor LED-based MIMO OWC system with SM and OFDM. We present an analytical model for the channel and we derive expressions for the achievable rate of the system considering common low-pass channel frequency responses: Gaussian, exponential and first-order [6-9]. Based on an indoor LED-based MIMO OWC setup, we investigate through analytical and simulation results the system performance for different power loading strategies. Through simulation results, we point out that the resource allocation optimization only in the frequency domain may not be satisfactory and we propose a new algorithm that considers both spatial and frequency domains to load power over the MIMO channels and OFDM subcarriers. With the singular value decomposition (SVD) applied to the channel frequency response matrix, the proposed space-frequency power allocation algorithm allocates more power to subchannels with larger gains considering all subchannels available for transmission in space and frequency domains.

In modern society, the average person spends more than 90% of their time indoors. However, despite the growing scientific understanding of the impact of light on biological mechanisms, the existing light in the built environment is designed predominantly to meet visual performance requirements only. Lighting can also be exploited as a means to improve occupant health and well-being through the circadian functions that regulate sleep, mood, and alertness. The benefits of well-lit spaces map across other regularly occupied building types, such as residences and schools, as well as patient rooms in healthcare and assisted-living facilities. Presently, Human Centric Lighting is being offered based on generic insights on population average experiences. In this paper, we suggest a personalized bio-adaptive office lighting system, controlled to emit a lighting recipe tailored to the individual employee. We introduce a new mathematical optimization for lighting schedules that align the 24-hour circadian cycle. Our algorithm estimates and optimizes parameters in experimentally validated models of the human circadian pacemaker. Moreover, it constrains deviations from the light levels desired and needed to perform daily activities. We further translate these into general principles for circadian lighting. We use experimentally validated models of the human circadian pacemaker to introduce a new algorithm to mathematically optimize lighting schedules to achieve circadian alignment to the 24-hour cycle, with constrained deviations from the light levels desired for daily activities. Our suggested optimization algorithm was able to translate our findings into general principles for circadian lighting. In particular, our simulation results reveal: (1) how energy constrains drive the shape of optimal lighting profiles by dimming the light levels in the time window that light is less biologically effective, (2) how inter-individual variations in the characteristic internal duration of the day shift the timing of optimal lighting exposure, (3) how user habits and, in particular, late-evening light exposure result in differentiation in late afternoon office lighting.

In modern society, humans spend over 90% or their time indoors. However, despite the growing scientific understanding of the impact of light on biological mechanisms, benefits of this understanding are not harvested in practical systems. Existing light in the built environment, offices in particular, is designed predominantly to meet visual performance requirements. Increasing attention is being given to the biological effects of light, especially how it could be used to promote occupants' health and well-being through the circadian functions that regulate sleep, mood, and alertness. While Human Centric Lighting is being offered based on generic insights on population average experiences, in this paper, we suggest a personalized bio-adaptive office lighting system, controlled to emit a lighting recipe tailored to the individual employee. We introduce a novel optimization algorithm that finds the best office lighting profile to achieve circadian alignment to the 24-hour cycle. The system aims to support employee's circadian rhythm and ensure that they receive the right light at the right time of the day. In particular, we use existing, commonly accepted chronobiologic models to find the most effective light exposure pattern.

Smart Buildings with connected lighting and sensors have become one of the first large-scale applications of the Internet of Things. However, existing efforts to make buildings smarter focus mainly on energy conservation and cutting costs. In this paper, we further address the beneficial effects of light upon humans. People nowadays spend more than 90% indoors and as such the indoor environment becomes paramount for people's health and wellbeing. We present a Human Centric Lighting solution that supports human health through the estimation of the biological effects of light. It exploits the possibility that IoT offers to monitor humans and their experience and to control internet-connected lights. Despite the existence of well-established and extensively tested models of the circadian mechanism, benefits of those models are not yet harvested in practical applications. We envision an application that controls the lighting system based on a suitable model that predicts the human response to light. Our work confirms that the statistical signal processing approach of a Particle Filter can account for sensor and model uncertainties. We show how the response of the biological clock to light depends largely on individual characteristics, such as the intrinsic characteristic duration of the day, and therefore we include such person characterization into our system design.

We formulate the lighting control system as a mathematical optimization problem which is highly nonlinear and acts of a longer time interval. To this end we introduce an iterative optimization algorithm to achieve a solution. This method can be used to develop scalable human-centric smart lighting control systems centered towards bringing support to the biological rhythm of individual employees. We test the inter-person variability of the optimum light exposure, and concluded that theory predicts that there is no one-size-fits-all light recipe. In particular, individual differences in late-evening light exposure require differentiation in the late afternoon. Humans are different. Our evaluations show that standard deviations in biological difference (intrinsic period) and social behavior require different light recipes. That is, human centric lighting control based on population averages have much less effect, compared to algorithms that take human variation into account. Possible one-size-fits-all human centric lighting is inadequate to achieve the desired impact or may be counterproductive for specific humans.

Plastic optical fiber (POF) is considered as an attractive candidate for next-generation networks, for fiber-to the-home or for fronthaul of wireless access points in offices. Inside such spaces, visible light communication (VLC) can be an attractive interference-free communication approach. Such system can offer high data rates with potential cost reduction compared to existing techniques. Wavelength division multiplexing (WDM) can not only increase the link capacity, but also offers the ability to share the POF media with multiple services. In VLC systems, WDM can increase capacity. However, the use of narrow emitters and narrow bandpass filters to demultiplex the WDM signals are relative costly components in such systems. This work focuses on inexpensive light emitting diodes (LEDs), to accelerate mass-market applications, and we explore the potential for relaxing the wavelength filters at the detector. In these scenarios, multiple-input multiple-output (MIMO) technology can efficiently suppress interference or automatically fall back to common signalling if the crosstalk is so large that parallel channels would be insufficiently independent. We calculate the MIMO crosstalk channel coefficients due to relatively wide LED spectra. We derive analytical expressions for the maximum achievable rate of the system and we conclude that relatively large amount of crosstalk can be tolerated without jeopardizing performance, provided that dedicated signal processing is used. We show that without such signal processing, the choice of optical filters is very critical.

This paper addresses reading failures of a barcode scanner interfered by light emitting diode (LED) lamps. It quantifies the reading performance in terms of timing signal-to-interference ratio (TSIR), in particular, as a function of modulation depth and frequency of the interference from modulated LED lighting. At the decision variable, the interference typically generated in the LED driver by a switched mode power supply (SMPS) is neither additive nor Gaussian. It has frequencies up to several MHz that can seriously affect the barcode reading performance. To calculate the signal and interference power for TSIR, the laser scanner physical channels are analyzed, including the laser beam path and the LED interference path. Since barcode scanners usually use peak or edge detection, the reading reliability is subject to the first and second derivatives of the LED inference, respectively. We validate the proposed TSIR for predicting scanner reading performance by experiments. For instance, we found that typical scanners reach a specified sensitivity at a TSIR value of around 50. We further investigate the reading performance under multi-frequency interference, e.g., interference from the harmonics of the SMPS control signal. In general, this paper presents a simple, yet realistic model to quantify the reading performance in terms of TSIR for barcode scanner subject to single-frequency LED interference, and it proposes an empirically verified flicker interference metric (FIM) for multi-frequency interference.