Indoor Optical Wireless Channel Research Articles

Radio environment maps for indoor visible light communications aided by machine learning

VLC has been proposed as reliable high-capacity wireless optical communication system for indoor access networks. This technology comprises a high bandwidth unlicensed solution and has been suggested for ultra-reliable environments like hospitals. In this study, Machine Learning is used to investigate indoor wireless optical channels where ultra-reliable communications and signal coverage are required. Specifically, we assess the precision of different Decision Trees ML algorithms based on their mean absolute error metric in comparison to the anticipated simulated outcomes for optical channel estimation in various rooms that could be used in medical care. We demonstrate that Decision Trees models are capable of making fast and accurate predictions. It is shown that the Stack model is the most appropriate Decision Trees-based method, from the ones we studied, for predicting indoor VLC RSS values and constructing the corresponding REMs. These models are 300–65,000 times faster than traditional calculation methods, depending on the number of reflections bounces considered, and deliver accurate predictions, with less than 0.015 mean absolute percentage error, of the Radio Environment Map of a communication system that supports VLC. This is the first implementation of constructing optical REMs for Indoor VLC systems to the best knowledge of the authors.

Optimized Multi-Primary Modulation for Visible Light Communication

Multi-primary modulation scheme has the potential to provide robust visible light communication links and mitigate any color fluctuations by providing identical chromaticity modulation alphabets. This work describes constellation optimization technique for multi-primary modulation (MPM), which shows that using different power levels for the primary lights can lead to obtain the same color and, furthermore, enhance the Euclidean distances between data symbols in the multi-dimensional color space. The bit error rate performances of the optimized MPM and the original non-optimized MPM schemes are evaluated considering indoor optical wireless channel models with additive white Gaussian noise. The results show that the optimized MPM provides better spectral efficiency and outperforms the original non-optimized MPM scheme by achieving up to 3.5 dB Signal-to-noise ratio gain for the same spectral efficiency and, therefore, opens doors for implementation of robust visible light communication links exploiting multi-color light sources.

Channel Characterization and Realization of Mobile Optical Wireless Communications

Link instability induced by users’ mobility is one of the challenges of optical wireless communications (OWC) inherited from the propagation nature of light. Hence, good understanding of the optical channel characteristics in dynamic environments plays a vital role in developing robust resource management strategies in OWC networks. Unfortunately, it is quite difficult to collect accurate indoor optical channel data in dynamic environments. In addition, indoor trajectory dataset is not publicly available. To overcome such limitations, this paper proposes a mobile terminal-centric analytical framework that captures the propagation channel characteristics in a mobile OWC network whose downlink is based on visible light and uplink is based on infrared light. We abstract the nature of human behavior by integrating both macro and micro mobility patterns. These patterns are then used to realize the spatio-temporal characteristics of optical wireless channels under long-term environment-confined mobility. The statistics derived from the developed framework indicate that the mobile line-of-sight (LOS) channel gain follows space-time-dependent multiple-peak Nakagami distributions, whereas the non-line-of-sight (NLOS) channel gain adheres to various space-time-dependent single peak distributions under different indoor layouts. The overall distribution of NLOS bandwidth follows space-time-dependent multiple-peak log-logistic distributions in downlinks and space-time-dependent generalized log-logistic distributions in uplinks. Our investigation demonstrates that the indoor layout and the user’s environment-confined mobility pattern significantly impact the LOS dynamics but present limited impact on NLOS components. Motivated by the need for better channel models for mobile OWC, the proposed framework fills up an important gap in literature and help the research community to understand better the indoor optical wireless channel characteristics.

Performance Limits for Fingerprinting-Based Indoor Optical Communication Positioning Systems Exploiting Multipath Reflections

In this paper, we introduce an uplink optical wireless positioning system for indoor applications. This technique uses fingerprints based on the indoor optical wireless channel impulse response for localization. Exploiting the line of sight peak power (LOS), the second power peak (SPP) of the impulse response, and the delay between the LOS and SPP, we present a proof of concept design and theoretical analysis for localization employing a single fixed reference point, i.e., a photodetector (PD) on the ceiling. Adding more PDs leads to more accurate transmitter position estimation. As a benchmark, we present analytical expressions of the Cramer-Rao lower bound (CRLB) for different numbers of PDs and features. We further present closed form analytical approximations for the chosen features of the channel impulse response. Simulation results show a root mean square (RMS) positioning accuracy of 25\,cm and 5\,cm for one and four PDs, respectively, for a typical indoor room at high SNR. Numerical results verify that the derived analytic approximations closely match the simulations.

Multipath Diversity for OFDM Based Visible Light Communication Systems Through Fractional Sampling

In this paper, we propose fractional sampling for optical orthogonal frequency division multiplexing (O-OFDM) in order to achieve diversity gain for indoor visible light communication systems. Fractional sampling (FS) is a diversity technique that generates different replicas of the received OFDM symbol by sampling the received signal higher than the baud rate. Indoor optical wireless channels exhibit frequency selectivity and FS increases the reliability of the transmission by exploiting the multipath diversity. Our numerical results show that for a realistic indoor channel model, proposed FS receiver provides significant performance improvement over conventional OFDM receiver.

A Novel Localization Technique Using Luminous Flux

As global navigation satellite system (GNNS) signals are unable to enter indoor spaces, substitute methods such as indoor localization-based visible light communication (VLC) are gaining the attention of researchers. In this paper, the systematic investigation of a VLC channel is performed for both direct and indirect line of sight (LoS) by utilizing the impulse response of indoor optical wireless channels. In order to examine the localization scenario, two light-emitting diode (LED) grid patterns are used. The received signal strength (RSS) is observed based on the positional dilution of precision (PDoP), a subset of the dilution of precision (DoP) used in global navigation satellite system (GNSS) positioning. In total, 31 × 31 possible positional tags are set for a given PDoP configuration. The values for positional error in terms of root mean square error (RMSE) and the sum of squared errors (SSE) are taken into consideration. The performance of the proposed approach is validated by simulation results according to the selected indoor space. The results show that the position accuracy enhanced is at short range by 24% by utilizing the PDoP metric. As confirmation, the modeled accuracy is compared with perceived accuracy results. This study determines the application and design of future optical wireless systems specifically for indoor localization.

Terminal Orientation in OFDM-Based LiFi Systems

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 (UE). In this paper, a random process model for changes in the UE 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 UE is located relatively far away from an access point (AP). It is shown that the effect is notable if the horizontal distance between the UE and the AP is greater than 1.5 m in a typical $5\times 3.5\times 3\,\,\text{m}^{3}$ indoor room.

Optical Wireless Communication Based Indoor Positioning Algorithms: Performance Optimisation and Mathematical Modelling

In this paper, the performance of the optimal beam radius indoor positioning (OBRIP) and two-receiver indoor positioning (TRIP) algorithms are analysed by varying system parameters in the presence of an indoor optical wireless channel modelled in line of sight configuration. From all the conducted simulations, the minimum average error value obtained for TRIP is 0.61 m against 0.81 m obtained for OBRIP for room dimensions of 10 m × 10 m × 3 m. In addition, for each simulated condition, TRIP, which uses two receivers, outperforms OBRIP and reduces position estimation error up to 30%. To get a better understanding of error in position estimation for different combinations of beam radius and separation between light emitting diodes, the 90th percentile error is determined using a cumulative distribution frequency (CDF) plot, which gives an error value of 0.94 m for TRIP as compared to 1.20 m obtained for OBRIP. Both algorithms also prove to be robust towards change in receiver tilting angle, thus providing flexibility in the selection of the parameters to adapt to any indoor environment. In addition, in this paper, a mathematical model based on the concept of raw moments is used to confirm the findings of the simulation results for the proposed algorithms. Using this mathematical model, closed-form expressions are derived for standard deviation of uniformly distributed points in an optical wireless communication based indoor positioning system with circular and rectangular beam shapes.

Mobility-Aware Resource Allocation in VLC Networks Using T-Step Look-Ahead Policy

Visible light communication (VLC) uses huge license-free spectral bandwidth of visible light for high-speed wireless communication. Since each VLC access point covers a small area, handovers of mobile users are inevitable. In order to deal with these handovers, developing fast and effective resource allocation algorithms is a challenge. This paper introduces the problem of mobility-aware optimization of resources in VLC networks using the T-step look-ahead policy and employing the notion of handover efficiency. It is shown that the handover efficiency can correlate the overall performance of the network with future actions based on the mobility of users. Due to the stationary nature of indoor optical wireless channels, future channel state information (CSI) can be predicted by anticipating the future locations of users. A resource allocation algorithm is thus proposed, which uses CSI to dynamically allocate network resources to users. To solve this mathematically intractable problem, a novel relaxation method is proposed, which proves to be a useful tool to develop efficient algorithms for network optimization problems with a proportional fairness utility function. The resulting algorithm is extremely faster than the previous method and awareness of mobility enhances the overall performance of the network in terms of rate and fairness utility function.

A generalized multi‐wavelength propagation model for VLC indoor channels using Monte Carlo simulation

AbstractIn this paper, we introduce the Matrix‐MMC propagation model for indoor wireless optical channels. This proposed method is evolved from the modified Monte Carlo (MMC)–based ray tracing algorithm, but in addition to the properties of MMC, it has the capability of capturing reflection phenomena implying changes on wavelength (eg, fluorescence, phosphorescence, or iridescence) with a simplified analytical expression. The model provides unprecedented information about the mechanism of surface reflection in a visible light communication channel, which influences the propagation of the light waves, that depends on wavelength.

Coherence distance in indoor optical wireless communication channels.

The coherence distance (CD) is the spatial distance over which the channel does not change appreciably. It is a common and important wireless communication channel parameter. While adoption of this parameter in the realm of radio-frequency is well established, it is a less familiar concept in the realm of optical wireless communication (OWC), where it can be beneficial as a measure for spatial optical channel changes. In this Letter, the average CD is proposed as a metric to characterize indoor OWC channels and a method for its evaluation is outlined.

BRDF Models for the Impulse Response Estimation in Indoor Optical Wireless Channels

Indoor wireless optical communications (IWOCs) are highly conditioned by the communication channel, which depends on factors, such as the endpoints’ positions and the reflective surfaces inside the room. Reflections have an important impact on the channel properties, since both channel gain and multipath interference are directly affected by reflections. Therefore, the selection of the reflection model used in the channel characterization is a critical decision in order to achieve realistic and accurate channel estimation. Simulation software, for the IWOC channel impulse response estimation, usually simplifies reflections surfaces using basic models, such as the ideal diffuse reflector (Lambertian model) or Phong’s reflection scheme. However, these models do not provide accurate results when are used with some surfaces, like those with a high-level of specular reflection components. In this letter, the integration of new reflection models into the impulse response estimation of the IWOC channels is proposed. These models are based on bidirectional reflectance distribution function theories, such as Blinn’s or Lafortune’s models, which are frequently used in 3D image rendering.

MCMC Methods for Realistic Indoor Wireless Optical Channels Simulation

This paper presents two new simulation algorithms of the optical wireless channel based on the Markov Chain Monte Carlo method. They allow to generate correlated three-dimensional light paths, keeping into account the nature of the simulation environment, taking advantage of its complexity such as transmitter and receiver position and orientation, the geometry, and the nature of the surfaces (diffuse, specular, etc.). Like Markov Chain Monte Carlo methods, these new algorithms are adaptive. They automatically do more calculation where light paths transport more power. Hence, they optimize their convergence speed, reducing the required number of paths to achieve a given accuracy threshold for the channel impulse response calculation. This paper shows that, compared to previous classical Monte Carlo solutions, these new algorithms reduce the computation time, up to a factor 225 for a fixed quality according to our experiments.

Adaptive Statistical Bayesian MMSE Channel Estimation for Visible Light Communication

Visible light communication (VLC) is considered to be one of the promising technologies for future wireless systems and has attracted an increasing number of research interests in recent years. Optical orthogonal frequency division multiplexing has been proposed for VLC systems to eliminate the multipath interference while also facilitating the frequency domain equalization. In comparison with the conventional radio frequency (RF) based wireless communications, there has been limited considerations on channel estimation (CE) for VLC, where the indoor optical wireless channel model differs from the traditional RF case. In this paper, we present a new CE algorithm for indoor downlink VLC systems, referred to as the adaptive statistical Bayesian minimum mean square error CE (AS-BMMSE-CE). Furthermore, a so-called variable statistic window (VSW) mechanism is designed for exploiting past channel information within a window of adaptively optimized size, such that the CE performance can be significantly improved. Detailed theoretical analysis is provided and verified by extensive numerical results, demonstrating the superior performance of the proposed AS-BMMSE-CE scheme.

Frequency-Domain Simulation of the Indoor Wireless Optical Communication Channel

The indoor wireless optical communication channel is characterised by the multiple reflections of the light from the surrounding walls. We present a new frequency-domain simulation method for this channel. In contrast to the known time-domain simulation techniques, our method is not restricted to a finite order of reflections. We show that the contributions of an infinite number of reflections can be summed up analytically to a formula that can be handled by standard numerical methods. We illustrate the method by comparing it to the results of previous time domain simulations, and we apply it to the numerical evaluation of some further model scenarios with more than one transmitter.

Indoor Optical Wireless Channel Characteristics With Distinct Source Radiation Patterns

In conventional indoor optical wireless communication, the transmitter is usually empirically viewed as a source with a Lambertian radiation pattern. This paper comprehensively studies the impact of practical source radiation patterns on channel characteristics, including light-emitting diodes (LEDs) from Nichia and Lumileds Philips. Compared with the Lambertian pattern, differences in impulse and frequency responses, spatial distributions of optical path loss (OPL), and root mean square (RMS) delay spread based on different patterns are clearly observed from simulation. The standard deviation reduction of OPL and RMS delay spread can be up to more than 3.1 dB and 0.138 ns for two practical sources, whereas the respective dynamic ranges of the conventional Lambertian pattern are just 5.1 dB and 0.67 ns. Meanwhile, the radiation pattern from Lumileds Philips provides almost uniform transmission characteristics that are independent of the receiver position.

Optical Wireless Channel Characterization for Indoor Visible Light Communications

The ­visible Light Communications (VLC) has gained a lot of interest for indoor communications resulting in several research challenges. The use of Light Emitting Diodes (LEDs) as light source poses many modulation issues for Line Of Sight (LOS) Optical Wireless Channel (OWC). This paper investigates the line of sight optical wireless channel characterization for visible light communications designing a realistic indoor environment adding path loss along with Additive White Gaussian Noise (AWGN) to the channel. A comprehensive mathematical model is derived and simulated to investigate the performance of the proposed system considering the effect of different parameters such as Eb/No, Signal to Noise Ratio (SNR), distance between transmitter and receiver, signal power on the Bit Error Rate (BER), propagation delay, and spectral efficiency. The propose indoor OWC for VLC enhances the transmission range from 4 meters to 6 meters having optimal quality of transmission in a channel representing real indoor environment.

Performance Improvement of ACO-OFDM Indoor Optical Wireless Transmissions Using Partial Pre-Equalization

This paper analyzes the performances and presents the benets of partial pre-equalization for indoor optical wireless transmissions based on asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) with intensity modulation and direct detection (IM/DD). In particular, for diffuse indoor optical wireless channels, partial pre-equalization can reduce the optical transmit power over post-equalization at the same target bit error rate (BER) for point-to-point transmissions even with imperfect channel knowledge. To further im-prove its performance, bit loading is considered to minimize the optical transmit power of ACO-OFDM while maintaining a constant target BER. In addition, broadcast transmissions to multiple users with possibly dierent channel qualities are considered, where pre-equalization is not applicable. Finally, we specify an appropriate channel estimate at the transmitter for such broadcast transmissions.

An optimized Gaussian beam for multicell indoor optical wireless communications

ABSTRACTThe characteristics of a multipath indoor optical wireless channel can be estimated by two parameters: optical channel path loss (OCPL) and the root mean square (RMS) delay spread. In this letter, an optimized Gaussian beam (OGB) is proposed for a cellular indoor optical wireless communications (CI‐OWC) system. The expression for OGB with an optimized beam waist radius for OGB is derived. OCPL and the RMS delay spread (Drms) for four different configurations are simulated and analyzed. Results show that, for a CI‐OWC system, a minimum OCPL can be achieved by using the OGB and the maximum transmission data rate can be significantly extended by increasing the number of cells. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1049–1052, 2015

Li-Fi: Light fidelity-a survey

Visible light communication (VLC), which uses a vast unregulated and free light spectrum, has emerged to be a viable solution to overcome the spectrum crisis of radio frequency. Light fidelity (Li-Fi) is an optical networked communication in the subset of VLC to offload the mobile data traffics which offers many advantages at indoor scenario. In this article, we survey the key technologies for realizing Li-Fi and present the sate-of-the-art on each aspect, such as: indoor optical wireless channel model, the VLC modulation techniques with user satisfaction, OFDM in VLC, optical MIMO, optical spatial modulation, multiple user access, resource allocation, interference management and hybrid Li-Fi schemes. Some challenges and future work that need to be solved in the area are also described.