Farshad Miramirkhani

Channel Modeling and Characterization for Visible Light Communications

In this paper, we present a comprehensive channel modeling and characterization study for visible light communications. Our study is based on ray tracing, which allows for an accurate description of the interaction of rays emitted from the lighting source within a specified confined space. Contrary to existing works, which are mainly limited to ideal Lambertian sources and purely diffuse reflections, our approach is capable of obtaining channel impulse responses (CIRs) for any nonideal sources, as well as specular and mixed specular-diffuse reflections. Furthermore, we can precisely reflect the presence of objects (e.g., furniture) and wavelength-dependent reflection characteristics of surface materials (e.g., ceilings, floor, walls, and furniture) in a channel study. As case studies, we consider a number of indoor environments with various dimensions and different surface materials, i.e., plaster, gloss paint, wood, aluminum metal, and glass. We further consider various scenarios with different transmitter specifications (i.e., single versus multiple transmitters and array type) and receiver specifications (i.e., location and rotation). For each environment, we obtain CIRs and present a channel characterization study where channel parameters, such as channel DC gain, root mean square (RMS) delay spread, coherence bandwidth, and mean excess delay, are obtained. We also make one-to-one comparisons between infrared and visible-light CIRs for the same environments to emphasize the differences between two optical bands.

A European view on the next generation optical wireless communication standard

Optical wireless technology uses light for mobile communications. The idea is to simultaneously combine the illumination provided by modern high-power light-emitting diodes (LEDs) with high-speed wireless communications. There have been numerous practical demonstrations of this concept, and the technology is now well matured to be deployed in practice. Independent market analysts forecast a high-volume market for mobile communication devices connected to the ubiquitous lighting infrastructure. This paper aims to make optical and wireless industries aware of the requirement for standardization in this area. The authors present the view of the European COST 1101 research network OPTICWISE towards a next-generation optical wireless standard aiming at data rates from 1 Mbit/s to 10 Gbit/s. Besides key technical insights, relevant use cases and main features are described that were recently adopted by the IEEE 802.15.7r1 working group. Moreover, a channel model is introduced to enable assessment of technical proposals.

Novel channel models for visible light communications

In this paper, we investigate channel modeling for visible light communications (VLC) using non-sequential ray tracing simulation tools. We create three dimensional realistic simulation environments to depict indoor scenarios specifying the geometry of the environment, the objects inside, the reflection characteristics of the surface materials as well as the characteristics of the transmitter and receivers, i.e., LED sources and photodioes. Through ray tracing simulations, we compute the received optical power and the delay of direct/indirect rays which are then used to obtain the channel impulse response (CIR). Following this methodology, we present CIRs for a number of indoor environments including empty/furnished rectangular rooms with different sizes and wall/object materials (e.g., plaster, gloss paint, wood, aluminum metal, glass) assuming deployment of both single and multiple LED transmitters. We further quantify multipath channel parameters such as delay spread and channel DC gain for each configuration and provide insights into the effects of indoor environment parameters (e.g., size, wall/object materials, etc.), transmitter/receiver specifications (e.g., single vs. multiple transmitters, location, rotation etc.) on the channel.

IEEE 802.15.7r1 Reference Channel Models for Visible Light Communications

The IEEE has established the standardization group 802.15.7r1 “Short Range Optical Wireless Communications”, which is currently in the process of developing a standard for visible light communication (VLC). As with any other communication system, realistic channel models are of critical importance for VLC system design, performance evaluation, and testing. This article presents the reference channel models that were endorsed by the IEEE 802.15.7r1 Task Group for evaluation of VLC system proposals. These were developed for typical indoor environments, including home, office, and manufacturing cells. While highlighting the channel models, we further discuss physical layer techniques potentially considered for IEEE 802.15.7r1.

Optical MIMO-OFDM With Generalized LED Index Modulation

Visible light communications (VLC) is a promising and uncharted new technology for the next generation of wireless communication systems. This paper proposes a novel generalized light emitting diode (LED) index modulation method for multiple-input-multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM)-based VLC systems. The proposed scheme avoids the typical spectrum efficiency losses incurred by time- and frequency-domain shaping in OFDM signals. This is achieved by exploiting spatial multiplexing along with LED index modulation. Accordingly, real and imaginary components of the complex time-domain OFDM signals are separated first, then resulting bipolar signals are transmitted over a VLC channel by encoding sign information in LED indexes. As a benchmark, we demonstrate the performance analysis of our proposed system for both analytical and physical channel models. Furthermore, two novel receiver designs are proposed. Each one is suitable for frequency-flat or selective channel scenarios. It has been shown via extensive computer simulations that the proposed scheme achieves considerably better bit error ratio versus signal-to-noise-ratio performance than the existing VLC-MIMO-OFDM systems that use the same number of transmit and receive units [LEDs and photo diodes (PDs)]. Compared with the single-input single-output (SISO) DC biased optical (DCO)-OFDM system, both spectral efficiency and DC bias can be doubled and removed respectively simply by exploiting a MIMO configuration.

A Mobile Channel Model for VLC and Application to Adaptive System Design

In this letter, we propose a realistic channel model for visible light communication (VLC) assuming a mobile user. Based on non-sequential ray tracing, we first obtain channel impulse responses for each point over the user movement trajectories, and then express path loss and delay spread as a function of distance through curve fitting. Our results demonstrate large variations in received power. In system design, this necessitates the use of adaptive schemes, where transmission parameters can be selected according to channel conditions. To demonstrate the benefits of link adaptation over a mobile VLC channel, we propose an adaptive system with luminary selection and demonstrate improvements in spectral efficiency over non-adaptive systems.

Channel Modeling for Visible Light Communications

In this chapter, we present a novel and realistic channel modeling approach for visible light communications that overcomes the limitations of previous works. In our work, we consider wavelength dependency, effect of realistic light sources as well as different types of reflections such as specular and mixed cases of diffuse and specular. We use nonsequential ray tracing algorithms to calculate the detected power and path lengths from source to detector for each ray. These are then processed to yield the channel impulse responses for various indoor environments. We further present a channel characterization study where channel parameters such as channel DC gain, root mean square delay spread, coherence bandwidth, mean excess delay are calculated for different environments.

Cooperative Visible Light Communications

In this chapter, we explore the concept of cooperative transmissions in the context of visible light communications (VLC). An indoor office space is considered with two light sources; the one at the ceiling is connected to the backbone network and provides ambient light to the environment while the other one is mounted on the desk and used for task lighting. The system architecture builds upon DC-biased optical orthogonal frequency-division multiplexing (DCO-OFDM). The task light performs relaying operation in either amplify-and-forward (AF) or decode-and-forward (DF) mode. Illumination constraints for task lighting are further considered in order to design a cooperative VLC system that provides satisfactory lighting. The communication performance of the system is optimized through a subcarrier-based power allocation mechanism. Numerical results incorporating practical issues such as band-limited channel process and imperfect channel estimation reveal that cooperative VLC systems can significantly outperform the conventional point-to-point VLC systems.

Performance of MIMO enhanced unipolar OFDM with realistic indoor visible light channel models

Visible light communication (VLC) involves the dual use of illumination infrastructure for high speed wireless access. Designing such optical based communication systems, realistic indoor optical channel modeling becomes an important issue to be handled. In this paper, first we obtain new realistic indoor VL channel characterizations and models, in a multiple-input multiple-output (MIMO) transmission scenario, using non-sequential ray tracing approach for the channel impulse responses (CIRs). Practical issues such as number of light emitting diode (LED) chips per luminary, spacing between LED chips, objects inside the room and cabling topology are also investigated. On the other hand, since indoor optical channels exhibit frequency selectivity, multi-carrier communication systems, particularly orthogonal frequency division multiplexing (OFDM) is used to handle the resulting inter-symbol interference in VLC systems. Hence, we propose a new MIMO-OFDM based VLC system, called MIMO enhanced unipolar OFDM (MIMO-eU-OFDM) by combining MIMO transmission techniques with the recently proposed eU-OFDM scheme. The bit error rate (BER) performance of the proposed system is investigated in the presence of the 2 χ 2 and 4 χ 4 realistic MIMO VLC channels and its BER performance is compared with the reference optical MIMO-OFDM systems.

Visible Light Channel Modeling for Gas Pipelines

In this paper, we explore the use of visible light communication as a means of wireless monitoring in gas pipelines. In an effort to shed light on the communication limits in the presence of gas, we create a three-dimensional simulation platform where the pipeline size/shape, the reflection characteristics of the interior coating, gas specifications (i.e., temperature, density, refractive index, transmittance, etc.) and the specifications of the light sources and detectors (i.e., field of view, lighting pattern, etc.) are precisely defined. Based on ray tracing, we obtain channel impulse responses within the gas pipeline considering the deployment of different colored LEDs with various viewing angles. We further investigate the maximum achievable link range to ensure a given bit error rate.