Omer Narmanlioglu

Relay-Assisted OFDM-Based Visible Light Communications

In this study, we investigate a relay-assisted visible light communication (VLC) system where an intermediate light source cooperates with the main light source. Specifically, we consider two light sources in an office space: one is the information source employed on the ceiling and the other one is a task light. Our system uses dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) where the task light performs relaying to assist the communication and operates in half-duplex mode for both amplify-and-forward and decode-and-forward relaying. We investigate the bit error rate (BER) performance of the relay-assisted VLC system and optimize the performance through optimal ac/dc power allocation under illumination constraints. The dc power allocation is determined by sharing the number of LED chips between the terminals to satisfy a desired luminance ratio. The ac power allocation decides the fraction of the information signal power to be consumed at the terminals in order to minimize the BER. Numerical results reveal that cooperation brings significant performance gains over direct transmission. We further provide comparisons of two relaying techniques under consideration and discuss the effect of clipping noise on the error rate performance.

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.

Relay-assisted OFDM-based visible light communications over multipath channels

In this paper, we investigate the performance of relay-assisted visible light communication (VLC) system over multipath channels. We consider an indoor environment where the ceiling light serves as the information source and a desk light helps the source forward the information to the destination. The VLC system is built on orthogonal frequency division multiplexing (OFDM) with either amplify-and-forward or decode-and-forward relaying. A realistic channel impulse response of the indoor environment generated through ray tracing is used. Through simulations, we present the bit error rate performance of OFDM-based relay-assisted VLC system over indoor multipath channels and quantify the performance degradation with respect to idealistic single tap channels. We further discuss the impact of imperfect channel estimation on the performance.

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.

Software-defined networking based network virtualization for mobile operators

Software-defined networking (SDN) paradigm provides many features including hardware abstraction, programmable networking and centralized policy control. One of the main benefits used along with these features is core/backhaul network virtualization which ensures sharing of mobile core and backhaul networks among Mobile Operators (MOs). In this paper, we propose a virtualized SDN-based Evolved Packet System (EPS) cellular network architecture including design of network virtualization controller. After virtualization of core/backhaul network elements, eNodeBs associated with MOs become a part of resource allocation problem for Backhaul Transport Providers (BTPs). We investigate the performance of our proposed architecture where eNodeBs are assigned to the MOs using quality-of-service (QoS)-aware and QoS-unaware scheduling algorithms under the consideration of time-varying numbers and locations of user equipments (UEs) through Monte Carlo simulations. The performances are compared with traditional EPS in Long Term Evolution (LTE) architecture and the results reveal that our proposed architecture outperforms the traditional cellular network architecture.

Physical Layer Implementation of Standard Compliant Vehicular VLC

Visible light communication (VLC) has recently gained popularity as a complementary technology to radio frequency (RF) based alternatives for vehicular communications as a low-cost, secure and RF interference free technology. In this paper, we propose IEEE 802.15.7 standard-compliant physical layer (PHY) implementation and experimental evaluation, using commercial off-the-shelf (COTS) automotive light emitting diode (LED) fog light for the purpose of low-latency safety message dissemination. We first show that the standard is applicable to line of sight (LoS) vehicle-to-vehicle (V2V) VLC. We then demonstrate that the proper selection of modulation coding schemes (MCS) plays an important role in order to minimize bit-error- rate (BER) for the reliable transmission with varying inter-vehicle distances. We also addressed the angular limitations of COTS automotive LED light for viable vehicular VLC.

Comparative performance evaluation of MIMO visible light communication systems

In this paper, we consider multiple-input multiple-output (MIMO) visible light communication (VLC) systems. The physical layer of VLC system builds upon optical orthogonal frequency division multiplexing (O-OFDM). Repetition code (RC), spatial multiplexing (SMUX) and spatial modulation (SMUD) are considered as MIMO techniques. We evaluate the performance of three MIMO O-OFDM VLC systems under consideration through the derivation of bit error rate expressions.

Broadcasting brake lights with MIMO-OFDM based vehicular VLC

Inter-vehicular connectivity to enhance road safety and enable highly autonomous driving is increasingly becoming popular. Despite the prevalent works on radio-frequency (RF) based vehicular communication schemes, visible light communication (VLC) is considered to be a promising candidate for vehicular communications due to its low complexity and RF interference-free nature. This paper investigates applicability of VLC to enhance road safety based on real world measurements. Deployment of multiple light emitting diodes (LEDs) enables multiple-input multiple-output (MIMO) transmission in the context of vehicular VLC. We consider direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) based MIMO transmission scheme and evaluate the performances of different MIMO modes including repetition code (RC) and spatial multiplexing (SM), different modulation orders and different transmitter-receiver selection. The results reveal that selection of the closest transmitters to the receivers, provide better performance due to high signal-to-noise-ratio (SNR) requirements for RC mode. However, usage of all possible transmitters does not always yield better performance due to power division at the transmitter side. Moreover, SM suffers from channel correlation whereas the performance of RC shows more degradation on higher-order modulations that are required to yield the same throughput with SM.

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.

Centralized Light Access Network (C-LiAN): A Novel Paradigm for Next Generation Indoor VLC Networks

Visible light communication (VLC) builds upon the idea of using existing lighting infrastructure for wireless data transmission. In a conventional VLC network, each light fixture acts as an access point (AP) which are connected to each other through electrical grid as well as data backbone. These VLC-enabled fixtures consist baseband unit (BBU) followed by the optical front-end (OFE). In this paper, we propose the so-called centralized light access network (C-LiAN) which aggregates all AP computational resources into a central pool that is managed by a centralized controller. Unlike the distributed architecture where each light fixture performs both baseband processing and optical transmission/reception, the centralized architecture employs “dummy” fixtures with a VLC OFE. Moving the baseband processing to a central pool reduces the associated cost and complexity of each VLC-enabled LED luminary. It further enables joint processing of signals from different APs making possible an efficient implementation of joint processing, offloading, handover, interference management, scheduling, and resource management algorithms. As an example to demonstrate the virtues of C-LiAN, we further present the performance of coordinated multi-point transmission and enhanced inter-cell interference coordination with almost blank subframe techniques originally proposed for Long Term Evolution-Advanced in the context of indoor VLC networks.