LED Modulation Characteristics in a Visible-Light Communication System

doi:10.4236/opj.2013.32B034

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Optics and Photonics Journal, 2013, 3, 139-142

doi:10.4236/opj.2013.32B034 Published Online June 2013 (http://www.scirp.org/journal/opj)

LED Modulation Characteristics in a Visible-Light

Communication System*

Yanrong Pei, Shaoxin Zhu, Hua Yang, Lixia Zhao, Xiaoyan Yi, Junxi Wang, Jinmin Li

Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China

Email: [email protected]

Received 2013

ABSTRACT

This paper conducts a research on modulation characteristics of blue light-emitting diodes (LEDs) used in a visible-light

communication (VLC) system. Through analysis of the modulation characteristics of LEDs with different emitting sizes,

we find that there is a similar linear relationship between LED’s 3dB bandwidth and the operation current density. This

experiment also shows that high series resistance is one major issue that limits our LED's modulation speed. To further

improve the LED bandwidth, the resistance can be reduced by optimizing device layout as well as reducing material

bulk resistance. Clearly, this study provides an approach to increase the modulation bandwidth of GaN-based LEDs for

VLC systems.

Keywords: LED; Visible-light Communication (VLC); Modulation Characteristics; 3dB Bandwidth; Current Density

1. Introduction

With the rapid development of modern solid-state light-

ing, light-emitting diodes (LEDs) are increasingly used

in a wide range of display, signaling and illumination

applications [1]. Because of the long lifetime and high

energy efficiency, LED is becoming one of the dominant

illumination technologies [1,2]. In addition, these

small-sized and energy-efficient devices tend to be used

in both illumination and communication: visible-light

communication (VLC) is attracting a lot of research in-

terests in Asia, Europe and the U.S. [1,3]. Using GaN-

based LEDs as the signal sources in VLC systems, the

modulated signal is able to transmit digital data beyond

the perception speed of human eyes [4]. Therefore, the

double functions of LEDs make them popular in recent

researches on the area of free space VLC system.

A nonreturn-to-zero (NRZ) system using a postequal-

ized white LED at 100 Mb/s has been reported in [1], and

a discrete multi-tones (DMT) modulation of white LED

has reached a data rate of 513 Mbit/s in [5]. Furthermore,

a single RGB LED luminary with the DMT modulation

has achieved a rate of 813 Mbit/s in [6]. A research of

white LED modulation bandwidth and modulation char-

acteristics has been reported in [4]. And also the LED

spatial light intensity effect on the amplitude-frequency

characteristics of VLC has been reported in [7]. Re-

sponse frequency of LED is directly affecting the avail-

able bandwidth of VLC system [4]. Wherefore, increas-

ing the frequency response of LED is the primary event

to achieve high speed VLC, when chasing high-power

LED simultaneously [4]. In this paper, we demonstrate

that the current density on LED affects the response fre-

quency of LED, which determines the modulation band-

width of LED.

2. Experimental Setup

As Figure 1 inset shows that the wavelength of LED in

this experiment is approximately 450 nm, which is the

common wavelength of blue LEDs. This curve is meas-

ured with the integrating sphere using 45 * 45 mil blue

LED.

Figure1. The P-I curve of LED modulation. Inset: The spectra

of the light sources in this experiment.

*This work supported by the National High Technology Research and

Development Program of China (863 Program), No. 2011AA03A105.

Y. R. PEI ET AL.

140

A typical LED’s current-voltage (I-V) curve is clearly

known. The LED is single conduction and when the bias

voltage exceeds a certain turn-on value VA, the LED can

operate in the linear region (work area). The turn-on

voltage of the LED in this study is about 2.8 V. There-

fore, in order to ensure the LED work in the linear region,

the bias voltage should be higher than 3 V. The modula-

tion capability of a LED is described with optical power

and electrical current (P-I curve). In Figure1, the P-I

curve was approximately linear without a threshold cur-

rent, so the LED’s optical power output can be linearly

modulated with a small input voltage signal that is biased

above VA.

The bandwidth measurement setup for the VLC sys-

tem is shown in Figure 2 and the parameters of each

component were listed in Table 1. The VLC transmitter

consists of an amplifier a power supply and a bias-T. The

receiver comprises a photo-detector that was Positive-

Intrinsic Negative diode (PIN diode) or Avalanche Photo

Diode (APD). LED that was the light source in transmit-

ter emits visible light and then absorbed by receiver

through free space spread. The two-port network ana-

lyzer works as a signal source and also a terminal ana-

lyzer, providing a small sine wave as a function genera-

tor and measuring the received amplitude as well.



Figure 2. The experimental setup of VLC system.

Table 1. VLC system parameters.

Parameters Values

Photo-detector

detection area 0.8 mm2

Photo-detector

bandwidth 150 MHz

Amplifier gain 25 dB

Amplifier frequency 500 MHz

Bias-T current (max.) 500 mA

Bias-T frequency 4200 MHz

Load 50 Ω

The modulation bandwidth of LED is restricted with

the response frequency. However, the minority carriers’

lifetime in semiconductors affect this response frequency.

Therefore, the theoretical bandwidth of LED was limited

below 2 GHz [8]. Currently, the bandwidth of LED is far

below this theoretical value. Hence, the lower modula-

tion bandwidth of LED affects its application in the field

of high-speed communications. In the following parts,

the research for modulation characteristics of LED based

on the above experimental setups of VLC system is de-

scribed.

Table 1 shows the parameters in the experimental set-

ups of VLC system. These parameters ensure the accu-

racy and reliability of the experimental results.

3. Results and Discussion

In the following parts, the research for modulation char-

acteristics of LED based on the above experimental set-

ups of VLC system is described.

The optical source in this experimental was some dif-

ferent sizes LEDs (blue LED without yellow phosphor).

The chip sizes of LEDs A, B and C are 07 * 09 mil, 10 *

23 mil and 45 * 45 mil, respectively. Or, in other words,

the active area of LED A is 0.04 mm2; and the active area

of LED B is 0.15 mm2; and lastly, the active area of LED

C is 1.3 mm2.

The three different experimental LEDs were divided

into three groups, which were measured under the same

light intensity, and then current density became the only

variable. Therefore, the results can directly reflect the

relationship between current density and the 3 dB band-

width of LED. In Figure 3, the Y coordinates, which is

S21 Amplitude, means that the ratio between output

power and input power and it reflects the 3 dB bandwidth

of LED. Because of the background noise in this experi-

ment, the curves in Figure 3 have large fluctuations.

However, these non-smooth curves are not affecting the

overall trend of the experimental results.

As Figure 3 shows that the 3 dB bandwidth of LED A

was 10.5MHz in the bias current of 20 mA and the value

rapid increased to 44MHz with the bias current of 100mA.

In addition, the LED B and LED C also demonstrated

this rule and the results were shown in Figures 3(b) and

(c), respectively. Therefore, the measured 3dB bandwidth

of three different sizes LEDs were all improved signifi-

cantly with the increased current density. This phe-

nomenon can be demonstrated with the probability of

bimolecular recombination, which was proportional to

the injected carrier density into the active volume [9,10].

In Figure 4, it was clear that there was a similar linear

relationship between current density and the 3 dB band-

width at each size of LEDs, which were 07 * 09 mil, 10

* 23 mil and 45 * 45 mil. Although 45 * 45 mil LED C

possesses largest capacitance due to its larger active area,

Y. R. PEI ET AL. 141

(a)

(b)

(c)

Figure 3. The normalized frequency response of (a) LED A,

(b) LED B and (c) LED C measured under different bias

currents.

Figure 4. The diagram of the relationship between current

density and 3 dB bandwidth.

the smaller resistance, which helps to reduce more para-

sitic, thus improve the high speed performance at low

current density. Therefore, high series resistance is one

major issue that limits our LED's modulation speed. To

further improve the LED bandwidth, the resistance can

be reduced by optimizing device layout as well as reduc-

ing material bulk resistance.

These experimental results from the above setups that

further evidence the relationship between 3dB bandwidth

of LED and the current density into LED.

4. Summary and Conclusions

In this paper, a measurement setup of modulation char-

acteristics for VLC systems is described and the band-

widths of different-sized blue LEDs have been reported.

The results clearly reveal a similar linear relationship

between LED current density and its 3dB bandwidth.

This phenomenon can be attributed to the bimolecular

recombination probability that is proportional to the in-

jected carrier density into the active volume. Therefore,

increasing the LED current density is a feasible method

in VLC systems to enhance the data transmission rate.

Moreover, this experiment shows that high series resis-

tance is one major issue that limits our LED's modulation

speed. Thus, further study will focus on optimizing de-

vice layout as well as reducing material bulk resistance to

reduce the resistance.

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