Michael B. Rahaim

A hybrid Radio Frequency and broadcast Visible Light Communication system

Wireless network data consumption is experiencing drastic increases due to growing demands of mobile services and applications. Deployed networks using Radio Frequency (RF) communications are characterized by a shared medium, limited available spectrum and limited ability to scale with increasing demand. Directional communications, including Visible Light Communications (VLC), can limit contention in controlled environments and provide scalability through spatial reuse of the medium. This solution can provide massive aggregate data capacity in indoor scenarios if properly distributed. In this paper we propose an indoor hybrid system that integrates WiFi and VLC luminaries. Our system utilizes (i) broadcast VLC channels to supplement RF communications and (ii) a handover mechanism between WiFi and VLC to dynamically distribute resources and optimize system throughput. This approach offers the bandwidth density benefits of VLC, provides a non-intrusive RF back-channel, maintains compatibility with conventional RF devices, and demonstrates excellent scalability. Analytical and simulation results show improvements in aggregate performance (i.e., throughput and delay) of the hybrid, when compared to either system acting alone.

Coexistence of WiFi and LiFi toward 5G: concepts, opportunities, and challenges

Smart phones, tablets, and the rise of the Internet of Things are driving an insatiable demand for wireless capacity. This demand requires networking and Internet infrastructures to evolve to meet the needs of current and future multimedia applications. Wireless HetNets will play an important role toward the goal of using a diverse spectrum to provide high quality-of-service, especially in indoor environments where most data are consumed. An additional tier in the wireless HetNets concept is envisioned using indoor gigabit small-cells to offer additional wireless capacity where it is needed the most. The use of light as a new mobile access medium is considered promising. In this article, we describe the general characteristics of WiFi and VLC (or LiFi) and demonstrate a practical framework for both technologies to coexist. We explore the existing research activity in this area and articulate current and future research challenges based on our experience in building a proof-of-concept prototype VLC HetNet.

Spotlighting for visible light communications and illumination

The trend toward solid state lighting with white LEDs has motivated much research for using these devices to provide wireless broadband data communications. Much work in this area has attempted to fit VLC into currently dominant indoor lighting modes, which broadcast the light in a wide field to achieve uniform coverage throughout a room. In this paper we explore spotlighting, which is appropriate lighting for many scenarios, as an alternative for implementing high datarate VLC. We find that spotlighting VLC has several benefits over uniform lighting implementations, including enabling higher datarate densities within a room and less channel distortion. We also introduce a hybrid scheme that combines spotlighting with uniform lighting to provide wide area data coverage as well as high-datarate “white hot spots” where needed.

An Indoor Hybrid WiFi-VLC Internet Access System

Visible light communications (VLC) is emerging as a new alternative to the use of the existing and increasingly crowded radio frequency (RF) spectrum. VLC is unlicensed, has wide bandwidth, supports new levels of security due to the opacity of walls, and can be combined to provide both lighting and data communications for little net increase in energy cost. As part of a lighting system, VLC is ideal as a downlink technology in which data are delivered from overhead luminaries to receivers in the lighting field. However, realizing a symmetric optical channel is problematic because most receivers, such as mobile devices, are ill-suited for an optical uplink due to glare, device orientation, energy constraints. In this paper we propose and implement a hybrid solution in which the uplink challenge is resolved by the use of an asymmetric RF-VLC combination. VLC is used as a downlink, RF is used as an uplink, and the hybrid solution realizes full duplex communication without performance glare or throughput degradation expected in an all-VLC-based approach. Our proposed approach utilizes a software defined VLC platform (SDVLC) to implement the unidirectional optical wireless channel and a WiFi link as the back-channel. Experiments with the implemented prototype reveal that the integrated system outperforms conventional WiFi for crowded (congested) multiuser environments in term of throughput, and demonstrate functional access to full-duplex interactive applications such as web browsing with HTTP.

Design and analysis of a visible-light-communication enhanced WiFi system

Visible light communication (VLC) has wide unlicensed bandwidth, enables communication in radiofrequency-sensitive environments, realizes energy-efficient data transmission, and has the potential to boost the capacity of wireless access networks through spatial reuse. On the other hand, WiFi provides more coverage than VLC and does not suffer from the likelihood of blockage due to the line-of-sight requirement of VLC. In order to take the advantages of both WiFi and VLC, we propose and implement two heterogeneous systems with Internet access. One is the hybrid WiFi-VLC system, utilizing a unidirectional VLC channel as the downlink and reserving the WiFi backchannel as the uplink. The asymmetric solution resolves the optical uplink challenges and benefits from the full-duplex communication based on VLC. To further enhance the robustness and increase throughput, the other system is presented, in which we aggregate WiFi and VLC in parallel by leveraging the bonding technique in the Linux operating system. We also theoretically prove the superiority of the aggregated system in terms of average system delay. Online experiment results reveal that the hybrid system outperforms the conventional WiFi for crowded environments in terms of throughput and Web page loading time, and also demonstrate the further improved performance of the aggregated system when considering the blocking duration and the distance between the access point and the user device.

Toward practical integration of dual-use VLC within 5G networks

Visible light communication (VLC), a form of indoor optical wireless access, has the potential to be an integral part of 5G networks due to the massive available spectrum in the visible light band. VLC has desirable high bandwidth density (b/s/m2) stemming from the directionality of optical signals, and can exist as part of installed infrastructure in LED-based luminaires. In spite of these benefits, it is best paired with complementary RF technologies to overcome limitations of line of sight, glare in optical uplink, and use cases requiring darkness. The combination of VLC and RF shows promise to overcome limitations of each medium if successfully paired. In this article we describe motivating factors for VLC usage in environments with a high density of user devices, present models for VLC integration with RF technologies, define system components required for practical deployment, and evaluate scenarios where VLC integration is most beneficial. We show that by selecting appropriate operating conditions for each of the RF and VLC solutions, we can realize substantial improvement in the resulting heterogeneous network design.

State estimation and motion tracking for spatially diverse VLC networks

Improvements in solid-state/LED lighting are driving increased capabilities of lighting systems - offering potential for applications such as Visible Light Communications (VLC) and indoor localization using the lighting medium. In this paper, we motivate the adoption of localization for both the support of handover between arrays of VLC-equipped luminaires as well as for indoor positioning and beam steering. Our approach uses a state estimation model to achieve localization and motion tracking in spatially diverse VLC networks while considering user mobility and dynamic device orientation. Under these conditions, we show simulation results with location and velocity errors of 5cm and 10cm/s, respectively.

SINR analysis and cell zooming with constant illumination for indoor VLC networks

Visible Light Communication offers potential for dual purpose devices providing both wireless data and illumination; however much of the early research in the area has focused primarily on link capacity. In this work, an RF cell zooming method is adapted for VLC such that the additional requirement of constant illumination is met. This method has the potential to improve traffic distribution in multi-user environments and handover at cell edges when considering mobile users. SINR is observed and analyzed in a range of scenarios to show coverage variability when transmitters are capable of dynamically changing their signal power.

“Lights-off” visible light communications

In order to implement visible light communications (VLC) through indoor lighting, a set of challenges arise due to conflicting requirements from the two missions. In this paper we examine one of these challenges: how to communicate when the lights are “off.” We investigate VLC with limits on transmit power, which we define to be low enough so that users will accept that the lights are in their “off” state. We argue that these limits vary based on levels of natural illumination already present in the environment. Our analysis shows that we can meet the limits while providing robust data coverage by using VLC devices of low complexity. The result is an important step toward ensuring acceptance and adoption of VLC technology.

Wireless access test-bed through visible light and dimming compatible OFDM

Visible light communications (VLC) technology has a potential to complement the RF wireless technology for indoor coverage. It promises concurrent illumination control and wireless data transmission capabilities. The potential of advanced multi-carrier modulation techniques, i.e. orthogonal frequency-division multiplexing (OFDM), introduces new challenges in the development of driver circuits. The reverse polarity optical OFDM (RPO-OFDM) is a recent approach to realize compatibility between the analog OFDM signal and the industry well-known pulse-width modulation (PWM) for dimming control. In this paper, we present the implementation of a wireless access test-bed through visible light and dimming compatible RPO-OFDM. A driver topology capable of driving a string of light-emitting diodes (LEDs) using a RPO-OFDM signal is proposed. The driver alleviates the nonlinearity of LEDs by proper biasing and maintaining a quasi-linear range of operation, i.e. minimizes the OFDM signal clipping. Using a software defined radio (SDR) platform for online modulation and demodulation, and a range of un-coded quadrature amplitude modulation (QAM) constellations, the experimentally obtained measurements demonstrate linear wide-range dimming while independently maintaining a bit-error performance below the forward error correction (FEC). Error free bit-error rate (BER) performance without symbol level equalization is obtained for binary phase shift keying (BPSK) and 4-QAM.