Design of a Visible Light CommunicationTransmitter for the Evaluation of a Wide Range of

Modulation Techniques

Steven De Lausnay, Lieven De Strycker, Jean-Pierre Goemaere, Bart Nauwelaers and Nobby StevensDraMCo Research Group, TELEMIC, Faculty of Engineering Technology, ESAT, KU Leuven

Gebroeders De Smetstraat 1, B-9000 Gent, Belgium

Email: steven.delausnay@kahosl.be and nobby.stevens@kahosl.be

Abstract—In this paper, we describe the development of adedicated transmitter to be used in Visible Light Communicationapplications. The transmitter has a high power efficiency, so thecommunication functionality does not lead to significant powerconsumption. For this reason, a Trans-Conductance Amplifier(TCA) was designed making use of a class AB amplifier. The finaldesign has an electrical bandwidth from 20 kHz until 25 MHzand is able to power LEDs with a driving current up to 700 mA.Results show that the bandwidth of the driver electronics ismuch larger than the bandwidth of the commonly used powerLEDs that are designed for illumination purposes. Because thetransmitter has the ability to implement different modulationtechniques, as well digital as analogues in nature, it can be usedin a wide range of applications depending on the needs of datarate and bandwidth.

I. INTRODUCTION

The Light Emitting Diode (LED) will replace conventional

lighting equipment, especially incandescent lamps. LEDs offer

a lot of advantages over conventional lighting, such as low

power consumption, low cost, high luminance efficiency, long

lifetime, etc [1]. Besides these advantages, a LED can also

be directly modulated with a data signal. This way, the LED

combines its function of lighting, with that of a data commu-

nication system [2]–[4]. This type of communication configu-

ration is known as Visible Light Communication (VLC). A lot

of VLC search goes out to high performance applications like

streaming of high definition video content, where high bit rates

are necessary [5], [6]. But VLC also finds its applications for

low data rate applications like for example indoor localization

systems and location based services. For an indoor localization

application, the goal is to receive a number of IDs of the

different LEDs and to run localization algorithms to calculate

the position of the mobile device or object in general. In order

to accelerate the rapid implementation of the technology, it is

important that the modifications of the already existing lighting

facilities is minimized. This implies low complexity, plug-and-

play electronics, especially at the transmitter side. This papers

describes the design of a VLC transmitter which can drive

high power LEDs with a wide range of modulation types to

evaluate the communication possibilities of VLC for low bit

rate, broadcast applications.

There are several ways to construct white LEDs, which have

an significant impact on the optical bandwidth. In a first way,

only semiconductor technology is used, where white light is

produced by different color LED-chips, typical Red, Green

and Blue LEDs are combined (RGB-LED). A second method

uses Phosphor Converted LEDs (PC-LED). These LEDs use

a phosphor layer to convert the light emitted by a blue LED

to other regions of the spectrum to produce white light. The

third method is a combination of the previous two methods. A

blue LED with phosphor layer is combined with LEDs without

phosphor layers [7].

For lighting applications, PC-LEDs are most widely used

instead of RGB-LEDs because of the easy construction and

simple electrical control. RGB-LEDs suffer from color shifts

due to aging and different junction temperatures of the

LEDs [8]. When PC-LEDs are used in a VLC system, previous

research has proven that the phosphor layer will limit the

maximum modulation bandwidth [4], [9]–[11]. For high data

rate applications, a blue filter is used at the receiver or

an equalizer at the transmitter or receiver, to enlarge the

optical bandwidth of the LED. This is however at the cost of

degradation of the received power and the introduction of more

complex electronics [9], [12]. For low data rate applications

(10 kbps to several 100 kbps), none of these requirements are

necessary. The final design of the VLC transmitter was based

on the report published by the IEEE P802.15.7 Task Group

VLC [13].

The paper is organized as follows. In Section II, a general

description is given of the working principle of Phosphor

Converted LEDs to understand the band limiting factors of

the phosphor layer. This will give an idea of the electrical

bandwidth of the transmitter necessary to drive the LEDs. In

Section III, a short discussion is given about the realization of

the transmitter according to the requirements for low data rate

applications. The results of the measurements are discussed in

Section IV. At the end, a summarized overview of the main

topics covered in this paper can be found in Section V.

II. PHOSPHOR CONVERTED LEDS

LEDs which are mostly used in lighting applications, are

Phosphor Converted LEDs (PC-LEDs). PC-LEDs consist of

a LED which produces a short wavelength (blue region of

the spectrum) and a phosphor layer which will perform a

wavelength transformation. Depending on the phosphor, more

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Fig. 1. Spectrum of White LEDs with different color temperatures

or less of the emitted blue light will be converted to other

wavelengths and other color temperatures of white light will

be produced (Figure 1).

There are two processes in the phosphor layer namely

fluorescence (the dominate process) and phosphorescence.

Figure 2 gives an overview of the fluorescence process which

takes place in the phosphor layer. The process is based on

the fact that some atoms and molecules can absorb light at a

particular wavelength and subsequently emit light of longer

wavelength after a brief interval, termed the fluorescence

lifetime. The fluorescence process consists of three important

events which all have their own time frame to occur. When a

blue LED emits photons in the phosphor layer, molecules in

this layer will absorb the energy of the photons. This energy

will cause an excitation of the molecules form the lowest vi-

brational level, the electrical ground state (most stable state), to

a higher vibrational level. This process happens in the order of

femtoseconds. Quickly, vibrational relaxation will take place

where the molecule will fall back to the lowest vibrational

level of the excited electrical state (internal conversion). This

relaxation takes longer, in the order of picoseconds [15].

The molecule now stays some time in the lowest vibrational

level of the excited electrical state with a period in the order

of nanoseconds. Afterwards the process of fluorescence takes

place when the molecule falls back from the excited electrical

state to a particular vibrational level of the electrical ground

state. This process can cause radiation (also non radiative fall

back is possible) with a stoke shift. As an example, the fluo-

rescence lifetime, from excitation to emission by the phosphor

layer, for a YAG : Ce3+ is specified to be ≈ 62 ns. The stoke

shift describes the change in wavelength of the photons emitted

after the fluorescence process compared to the wavelength of

the photons emitted by the LED. Figure 2 shows a smaller

energy gap when the fluorescence process takes place in

Fig. 2. Fundamental concept of fluorescence

comparison with the energy gap which caused the excitation.

This leads to larger wavelengths of the photons emitted by

the fluorescence process then the original wavelengths emitted

by the blue LED. The presence of a phosphor layer over a

blue LED leads to a wavelength translation of a part of the

blue spectrum to higher wavelengths. The color temperature

of white light depends on the amount of incident photons

that experience a wavelength shift. The main disadvantage

of the use of PC-LED for visible light communication, is

the fluorescence lifetime. This will limit the communication

bandwidth when used in VLC applications [15], [16].

III. VLC TRANSMITTER

The transmitter of a VLC link is shown in Figure 3. The

critical part of the transmitter is the combination of the DC

bias current IDC and the AC communication current coming

from a Trans-Conductance Amplifier (TCA). The TCA will

convert the data signal (UAC), which is a voltage signal, to a

current (IAC) so it can be added to the bias current ( IDC)

to drive the LED ( ILED(t) ≥ 0). Therefore, the TCA should

meet a number of requirements [13]:

• High linearity

• Adjustment of the LED bias current independent of the

VLC AC signal

• Medium to high output power over large baseband band-

widths

• Small footprint and short wiring (Inductance of the wiring

can limit the bandwidth)

• Matched to the very low differential impedance of the

LEDs

• High power efficiency

Figure 4 shows the proposed architecture of a VLC trans-

mitter [13]. One can see two stages, namely a buffer (1) used

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Fig. 3. Block diagram of a VLC transmitter

Fig. 4. The proposed architecture of a VLC transmitter

as a voltage amplifier and input impedance matching, and a

TCA (2) which amplifies the current. For power efficiency

reasons, the proposed TCA can be constructed of a class AB

or class D amplifier. The AC current coming from the TCA is

added in a final stage to a DC current by use of a bias Tee (6).

For the design of the bias Tee, one should keep in mind the

low differential impedance of LEDs. When the current-voltage

characteristic is measured of a LED, the differential impedance

can be calculated in function of the driving current (Equa-

tion 1). Figure 5 shows the differential impedance of the LEDs

which are used during the measurements in Section IV. From

these measurements one can see small differential impedances,

varying between 0.5 Ω and 9.5 Ω).

Rd =d(VLED)

d(ILED)(1)

For low data rate application, the transmitter could have a

bandwidth approximately the same as the LED. In Section II,

the working principle of the phosphor layer was discussed and

an example was given for YAG : Ce3+. This phosphor layer

has a fluorescence lifetime of ≈ 62 ns. For the transmitter, this

would imply a bandwidth of ≈ 25 MHz. LEDs currently used

in lighting applications, have a driving current that is typically

350 mA or 700 mA. For the largest driving current, when the

data signal drives the the LED current from 0 mA to 700 mA,

the transmitter would need to deliver an AC current which has

an amplitude of 350 mA and a bias current of 350 mA. When

smaller data signals are used, the maximum of the data signal

should still reach the maximum driving current, which implies

an increased DC component. This has two advantages namely

Fig. 5. Differential Impedance Rd in function of the driving current for theLEDs used in Section IV

the working point lays in a more linear part of the current-

voltage curve of the LED and there is a higher light output

for illuminating the room. As a consequence, if the transmitter

is designed to deliver a maximum DC current of 700 mA, all

potential scenarios are covered. From these specifications, the

transmitter is constructed according to the principle design

shown in Figure 4.

For the first stage, the buffer (1), an OPAMP is used to

perform the voltage amplification and the impedance matching

for the 50 Ω output impedance of the function generator. For

the implementation, the LMH6672 from Texas Instruments

was used which has a bandwidth of 90 MHz and can drive low

impedance loads. The OPAMP is configured as a non-inverting

amplifier with an amplification of 2.

For the TCA (2), a class AB amplifier is implemented. The

circuit consist of two current mirror sinks (4) which delivers

the current for the bias circuit (5) namely a Rubber Diode.

The output stage (3) is a complementary transistor pair which

has to drive a maximum current of 350 mA and has a large

transition frequency. The BC807 and BC817 are used for this

stage which could drive currents up to 500 mA and has a

transition frequency of 100 MHz. The output of the TCA is

added to a DC bias current source which can be modified

independently of the data signal. The bias Tee (6) which is

used to add the DC current with the AC data signal is designed

to drive a low output impedance load and have a bandwidth

between 10 kHz and 40 MHz. For these low frequencies and

impedances, the inductance of the coil can become large. High

inductance coils with a high current rate, will have a low self

resonance frequency, therefor the bias Tee can consist of two

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Fig. 6. The block diagram of the measuring setup

stages in the DC path to get the the low frequency bandwidth.

IV. EVALUATION OF THE DESIGNED TRANSMITTER

A. Measurement set-up

The block diagram of the measurement set-up is show in

Figure 6. At the receiver end, the PDA10A is used which has

an electrical bandwidth of 150 MHz and an optical response

in the Visible Light region (200-1100 nm) [17]. The data input

of the transmitter was connected to an RF function generator

(Marconi Signal Generator 2024) which produces a sine wave

with a frequency sweep from 10 kHz until 100 MHz. The input

signal (function generator), output current and voltage (LED

Current/Voltage) and received optical voltage were measured

with a Tektronix DPO3012 oscilloscope which has a band-

width of 100 MHz. The signals where sampled at a sample

rate of 2.5 MSps and the data from the scoop was send to

Matlab for further analysis. Regarding these measurements, the

term modulation index is used to refer to the amplitude of the

AC current with respect to the maximum driving current of the

LED. For a LED with a maximum driving current of 350 mA,

a modulation index of 1, gives an AC current amplitude of

175 mA and bias current of 175 mA. A modulation index

of 0.25 corresponds to an AC current amplitude of 44 mA

with bias current of 306 mA so the maximum driving current

is still 350 mA. All LEDs used in the measurements have a

maximum driving current of 350 mA.

B. Evaluation of the transmitter design

For this test, a resistor of 4.71 Ω was connected to the output

of the bias Tee. The resistor was driven by a DC current of

200 mA and an input amplitude of 500 mV which corresponds

to an AC current amplitude of 100 mA. This resistance value

was chosen from Figure 5 where one can see that for a

DC driving current of 200 mA the differential impedance

Rd is approximately 4.5 Ω. For the electrical bandwidth of

the transmitter, the input voltage UAC and output current

IAC were considered (Figure 3). The bode plot is given in

Figure 7.The results show a bandwidth of the transmitter of

approximately 25 MHz.

Fig. 7. Electrical bandwidth of the transmitter

C. Experimental Evaluation of the Receivers Bandwidth

In a second test, the optical response is measured with the

PDA10A. To show that the optical bandwidth of the receiver

is much larger then the bandwidth of the transmitter, a laser

diode is connected to the transmitter. Figure 8 gives the results

and one can see that the optical bandwidth of the receiver is

much larger then the bandwidth of the transmitter.

D. Overall System Bandwidth

Based on the transmitter design and the measurements on

the electrical bandwidth of the transmitter on one hand, and the

optical bandwidth of the receiver on the other hand, one can

evaluate experimentally the bandwidth of the three LEDS. The

BXRA-30E0740-A-00 and LXK9-PW30-008 are warm white

LEDs (CCT = 3000 K), while the BXRA-56E0700-A-00 is

a cool white LED (CCT = 5600 K). The results are show in

Figure 9 which shows a smaller optical bandwidth for warm

white LEDs with respect to the cool white LED. It is clear

that the bandwidth limitation in our system is imposed by

the three commercially available LEDs. This also implies that

the transmitter is designed correctly, as well as from a power

delivery point of view as with regard to the bandwidth.

V. CONCLUSION

In this paper we have presented a VLC transmitter which

can be used for low data rate applications. The transmitter has

an electrical bandwidth from 20 kHz to approximately 25 MHz

which is large enough when used in a VLC system where the

optical bandwidth is not increased with the use of an optical

blue filter or equalizers. The lower frequency can be enlarged

when an lager coil is used in the bias Tee. The transmitter is

capable of independently setting a DC bias current and add it

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Fig. 8. Current vs Optical bandwidth for different modulation indexes

Fig. 9. Current vs Optical bandwidth for LEDs with different white colortemperatures

with a time varying data signal which can be any analogues or

digital modulation technique. The bandwidth of the transmitter

and the obtained output power levels are sufficient to drive

commercially available LEDs that are currently solely used

for lighting purposes.

ACKNOWLEDGMENT

The authors would like to thank the Agency for Innovation

by Science and Technology in Flanders (IWT).

REFERENCES

[1] McKinsey & Company, Lighting the way: Perspectives on the globallighting market, http://img.ledsmagazine.com/pdf/LightingtheWay.pdf .August, 2011

[2] T. Komine and M. Nakagawa, Fundamental Analysis for Visible-LightCommunication System using LED Lights, IEEE Transactions on Con-sumer Electronics Vol. 50. February, 2004

[3] K. Cui and G. Chen and Z. Xu and R.D. Roberts, Fundamental Anal-ysis for Visible-Light Communication System using LED Lights, IEEETransactions on Consumer Electronics Vol. 50. July, 2010

[4] D. C. OBrien and L. Zeng and H. Le-Minh and G. Faulkner and J. W.Walewski, Visible Light Communications: challenges and possibilities,Proc. of SPIE Vol. 7091. 2008

[5] Siemens, 500 Megabits/second with White LED Light, Siemens ResearchNews 2010. http://www.siemens.com/innovation/en/news events/ctpressreleases/e research news/2010/e 22 resnews 1002 1.htm

[6] J. Vucic, C. Kottke, S. Nerreter, K. Langer and J. Walewski, 513 Mbit/sVisible Light Communication Link Based on DMT-Modulation of a WhiteLED, Journal of Lightwave Technology Vol. 28 No.24 . December,2008

[7] E. Fred Schubert and J. Kyu Kim, Solid-state light sources getting smart,Science 27 . May, 2005

[8] P. Deurenberg, C Hoelen, J. Van Meurs and J. Ansems, Achieving colorpoint stability in RGB Multi-chip LED modules using various colorcontrol loops, Fifth International Conference on Solid State Lighting .2005

[9] H. Le Minh, D. OBrein, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y.Oh, High-Speed Visible Light Communication Using Multiple-ResonantEqualization, IEEE Photonics Technology Letter Vol. 20 No.14 . July,2008

[10] K. Cui, G. Chen, Z. Xu and R. D. Roberts, Line of sight Visible-light communication system Design and Demonstration, IEEE GlobecomWorkshop on Optical Wireless Communications . 2010

[11] J. Grubor, O.C. Geate Jamett, J.W. Walewski, S. Randel and K.D.Langer, High Speed Wireless Indoor Communication via Visible Light,ITG Fachbericht. 2007

[12] O. Bouchet, P. Porcon, M. Wolf, L.Grobe, J. W. Walewski, S. Nerreter,K. Langer, L.Fernandez, J. Vucic, T. Kamalakis, G. Ntogari and E. Gueu-tier, Visible-light communication system enabling 73Mb/s data streaming,IEEE Globecom Workshop on Optical Wireless Communications . 2010

[13] R. Baumgartner, A. Kornbichler, J. W. Walewski, High-power high-bandwidth linear driving circuit for VLC applications, IEEE P802.15.7Task Group Visible-Light Communication. March 2010

[14] Bridgelux, Datasheet of the BXRA-30E0740-A-00, Bridgelux ES Star Ar-ray Series. http://www.bridgelux.com/assets/files/DS23%20Bridgelux%20ES%20Star%20Array%20Data%20Sheet%20DS23%20120312.pdf

[15] Peter TC So and Chen Y Dong, Fluorescence spectrophotometry,Macmillan Publishers Ltd. . 2002

[16] B. Herman, V. E. Centonze Frohlich, J. R. Lakowicz, D. B. Murphy,K. R. Spring and M. W. Davidson Basic Concepts in Fluorescence,Microscopy Resource center Olympus . http://www.olympusmicro.co/primer/techniques/fluorescence/fluorescenceintro.html

[17] Thorlabs, PDA10A http://www.thorlabs.de/thorproduct.cfm?partnumber=PDA10A

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