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Towards 10 Gb/s orthogonal frequency division

multiplexing-based visible light communication

using a GaN violet micro-LED

MOHAMED SUFYAN ISLIM,

,†

RICARDO X. FERREIRA,

2,†

XIANGYU HE,

2,†

ENYUAN XIE,

STEFAN VIDEV,

SHAUN VIOLA,

SCOTT WATSON,

NIKOLAOS BAMIEDAKIS,

RICHARD V. P ENTY,

IAN H. WHITE,

ANTHONY E. KELLY,

ERDAN GU,

HARALD HAAS,

AND MARTIN D. DAWSON

Li–Fi R&D Centre, the University of Edinburgh, Institute for Digital Communications, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, UK

Institute of Photonics, Department of Physics, University of Strathclyde, Glasgow G1 1RD, UK

Institute for Digital Communications, Li–Fi R&D Centre, the University of Edinburgh, King’s Buildings, Mayfield Road, Edinburgh EH9 3JL, UK

School of Engineering, University of Glasgow, Glasgow G12 8LT, UK

Centre for Advanced Photonics and Electronics, Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge

CB3 0FA, UK

*Corresponding author: m.islim@ed.ac.uk

Received 28 November 2016; revised 9 February 2017; accepted 9 February 2017; posted 10 February 2017 (Doc. ID 280671);

published 28 March 2017

Visible light communication (VLC) is a promising solution to the increasing demands for wireless connectivity.

Gallium nitride micro-sized light emitting diodes (micro-LEDs) are strong candidates for VLC due to their high

bandwidths. Segmented violet micro-LEDs are reported in this work with electrical-to-optical bandwidths up to

655 MHz. An orthogonal frequency division multiplexing-based VLC system with adaptive bit and energy loading

is demonstrated, and a data transmission rate of 11.95 Gb/s is achieved with a violet micro-LED, when the non-

linear distortion of the micro-LED is the domi nant noise source of the VLC system. A record 7.91 Gb/s data

transmission rate is reported below the forward error correction threshold using a single pixel of the segmented

array when all the noise sources of the VLC system are present.

OCIS codes: (060.4510) Optical communications; (060.2605) Free-space optical communication; (230.3670) Light-emitting diodes;

(230.3990) Micro-optical devices.

https://doi.org/10.1364/PRJ.5.000A35

1. INTRODUCTION

The increasing demands of communication services are chal-

lenging radio frequency (RF) wireless communications technol-

ogies. The overall number of networked devices is expected to

reach 26.3 billion in 2020 [1]. Visible light communication

(VLC) is a promising solution to the limited availability of the

RF spectrum as the visible light spectrum offers abundant

bandwidth that is unlicensed and free to use. VLC improves

the spectral efficiency per unit area, which enhances the quality

of service in crowded environments and allows for secure and

localized services to be provided.

General lighting is under a rapid transformation to become

semiconductor based due to huge energy savings. This trans-

formation has already enabled applications such as active energy

consumption control and color tuning. Solid state lighting

devices such as gallium nitride (GaN)-based inorganic light

emitting diodes (LEDs) are ubiquitous power-efficient devices

to enable illumination and communications. Commercially

available LEDs have a limited frequency response due to the

yellow phosphor coating on top of the blue LED chips. How-

ever, the slow response of the yellow phosphor can be filtered

out using a blue filter in front of the receiver. Recent results

for VLC using a phosphorescent white LED with adaptive bit

and energy loading were reported at 2.32 Gb/s aided by a two-

staged linear software equalizer [2].

Micro-LEDs are promising candidates in enabling lighting

as a service (LaaS) and Internet of things (IoT). The introduc-

tion of micro-LEDs has enabled high-performance value-added

lighting functions such as VLC and indoor positioning and

tracking [3]. Micro-LEDs are known for their small active areas

enabling high current density injection, which drives the

modulation bandwidth to hundreds of megahertz [4,5]. At

450 nm, micro-LEDs have set the standard for high-speed

VLC. A 60 μm diameter pixel has achieved 3 Gb/s [6], and

more recently a single pixel of a new segmented array has

demonstrated 5 Gb/s [7]. The novel micro-LEDs emitting at

Research Article

Vol. 5, No. 2 / April 2017 / Photonics Rese arch A35

400 nm featured in the current work offer a number of advan-

tages over the 450 nm devices previously reported [7]. From

typical trends concerning the internal quantum efficiency

(IQE) of indium GaN-based active regions, comparable IQEs

are expected at 400 and 450 nm, whereas the IQE decreases

steeply at shorter emission wavelengths [8]. For generation of

white light for illumination, the use of violet-emitting LEDs

exciting tricolor (red, green, and blue) phosphors also offers

advantages over the widely used method of combining blue

direct LED emission with a yellow-emitting phosphor. These

include much superior color rendering indices [9,10] and the

absence of a direct blue component, which has proven to be

disruptive to the human circadian rhythm [11]. The micro-

LED die shapes employed in this work are also expected to

be advantageous for efficient light extraction, by analogy with

previous designs employing non-circular emitting areas [12].

VLC is enabled by incoherent illumination from the light

sources. Therefore, only real and positive modulating wave-

forms can be realized. Single carrier modulation schemes such

as on–off keying (OOK), pulse amplitude modulation (PAM),

and pulse width modulation (PWM) are straightforward to

implement. However, the performance of these modulation

schemes degrades as the transmission speed increases due to

the increased inter-symbol interference (ISI). Equalization

techniques can be used to improve the system performance

at significant computation cost [13]. Multi-carrier modulation

techniques such as orthogonal frequency division multiplexing

(OFDM) are promising candidates for VLC. Computationally

efficient single-tap equalizers are straightforward to realize in

OFDM. Adaptive bit and energy loading in OFDM allows

the channel utilization to approach the information capacity

limit. In addition, multiple access can be easily supported in

OFDM by assigning groups of subcarriers to multiple users,

which is known as orthogonal frequency division multiple

access (OFDMA).

Previously, a 40 μm diameter micro-LED at 405 nm

achieved a data rate of 3.32 Gb/s at an optical power of

2.5 mW with electrical–optical bandwidth up to 307 MHz

[14]. In this paper, we present a high bandwidth VLC link at

400 nm. The emitter consists of a single pixel of the segmented

micro-LED array design introduced in Ref. [7 ]. This device

achieves 2.3 mW of optical output power while maintaining

an electrical-to-optical (E-O) bandwidth of 655 MHz.

A VLC system is realized with a modulation bandwidth of

1.81 GHz, evaluated beyond the 3 dB bandwidth of the sys-

tem. A transmission rate of 11.95 Gb/s is presented, when the

nonlinear distortion noise of the micro-LED is the major

source of noise in the system. A record transmission rate at

7.91 Gb/s is presented when all the noise sources of the

VLC system are considered.

2. VIOLET MICRO-LED

A. Design and Fabrication

The design of standard GaN LEDs is based on a large-area chip

assembled on a package that maximizes heat extraction through

an n-pad at the bottom for a flip-chip configuration. This cre-

ates two limitations: a large capacitance due to the package con-

tact area and an upper limit on the current density due to the

rapid self-heating of a large-area chip. The design and fabrica-

tion process of the micro-LED array used in this work is as

reported in our previous work [7]. It consists of two circular

micro-LED arrays, an inner and an outer, containing 5 and

10 pixels, respectively. Originally designed to match the geom-

etry of plastic optical fiber, the inner and outer pixels have

active areas of 435 and 465 μm

, respectively. This compares

with the 1256 μ m

active area for the 405 nm device in Ref. [14].

Figure 1 shows optical images of this micro-LED array, together

with a schematic of the pixel layout.

The wafer used in this work is for a commercially available

GaN-based LED emitting at 400 nm. In order to fabricate

these arrays, micro-LEDs emitters are etched by inductively

coupled plasma to expose n-type GaN. An annealed Pd layer

is used as a metal contact to p-type GaN. Each emitter is iso-

lated by a layer of SiO

. The metallization on the n-type GaN is

formed by depositing a Ti/Au metal bilayer, which fills the area

between each micro-LED and enables an improved current

spreading. This bilayer connects each micro-LED emitter in

order to individually address them. The micro-LED array

allows increasing the total output power with minimal reduc-

tion in performance due to mutual heating between pixels. The

low optical power per pixel in micro-LEDs is a challenge when

combined illumination and communication is considered. This

problem can be addressed by using large arrays of pixels, where

a system capable of handing the communication link over

multiple pixels can be designed to reduce the duty cycle, reduce

the junction temperature on individ ual pixels, and maintain

high efficiency. These investigations are subject to future work.

B. Performance Measurements

The electrical performance of the micro-LED arrays was mea-

sured by a semiconductor analyzer (HP 4155). The optical

power of the arrays under direct current (DC) conditions was

measured using a Si detector placed in close proximity to the

polished sapphire substrate. A spectrometer and a charge

coupled device detection system were used for the collection

Fig. 1. Plan view micrographs of the segmented micro-LED arrays. The magnified micrographs on the right show the array configuration and

individual pixel design. A diagram is also included noting the inner and outer pixels (dimensions in micrometers).

A36 Vol. 5, No. 2 / April 2017 / Photonics Research

Research Article

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