VISIBLE LIGHT COMMUNICATIONSYSTEMS FOR OPTICAL VIDEOTRANSMISSION

J. Rufo, F. Delgado, C. Quintana, A. Perera, J. Rabadan,and R. Perez-JimenezPhotonics and Communications Division, Technological Center forInnovations in Communications (CeTIC), Universidad de LasPalmas de Gran Canaria, 35017 Las Palmas de Gran Canaria,Canary Islands, Spain; Corresponding author: rperez@cetic.eu

Received 12 September 2009

ABSTRACT: This article describes an optical wireless system based onvisible light communication (VLC), which allows a video broadcasting

to reach a bit rate of 1 Mbps, although this system can also be used forlow-speed sensor interconnections. The main advantages of thistechnology are the robustness against EM interference, safety for human

eye, and security against undesired network access. In this article, wepresent the electronic structure of a low-cost VLC transceiver, the

modulation process and the Ethernet interface that has beenimplemented in each AP (access point). Finally, some conclusions aredrawn. VC 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52:

1572–1576, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.25236

Key words: optical wireless communications; visible light

communication; video transmission

1. INTRODUCTION

Nowadays, there is a growing interest in using illumination

devices based on visible LED lamps on several different scenar-

ios (as could be in-house domestic applications, traffic lights,

hospitals, hotels, etc.). These lamps combine very low-power

consumption with an extremely long operational life, maintain-

ing during all their operation the same chromaticity without sig-

nificant changes. Moreover, they can also be used as communi-

cation emitters without losing their main functionality as

illumination devices. This technique (known as visible light

communication or VLC [1, 2]) does not only maintain the usual

capabilities of wireless optical transceivers (robustness against

EM interference, absence of EM compatibility constraints and

secure transmissions as radiation is confined by walls) but is

also eye-safe (as it uses visible wavelengths). Additionally, as

they are used as lamps, the amount of optical emitted power is

enough to provide full transmission coverage over a regular size

room. We have a huge amount of low-cost COTS optical emit-

ters available that can be used wherever we need to provide illu-

mination and to broadcast audio and video jointly.

The illumination channel is nice for broadcasting transmis-

sion [3] or for addressing low-speed communications for sensors

(i.e., by means of polling protocols) [4]. It can be considered for

several simultaneous OCDMA transmissions as well. Some

application scenarios can be easily imagined. The most promis-

ing area seems to be video/audio transmission for domestic in-

home applications. The use of sensors/actuators should be com-

manded by VLC links (even for transmitting medium baud rate

channels). We can also think on monitoring patients in hospitals

by means of several low-speed sensors while they perform their

regular activities or when they are under medical testing (using

strategies as described in the ISO/IEEE 11073 standard) [5].

However, there are some constraints that must be considered.

First, we should keep in mind that the primary use of the lamps

is to provide illumination, so they should be able to transmit as

maintaining the same optical emitted power (when the lamp is

switched on or even when the lamp is off) emitting narrow

pulses under the eye detection capability. The second problem

relies on the uplink channel for commands (in the case of broad-

casting transmission), for acknowledging the transmission or for

providing the network with the required measured parameters

(when considering sensors, actuators, or addressed devices).

There are several studies testing low/medium speed infrared

channels [6, 7] not to be interfered by the visible ones. We can

also use one of the huge varieties of available commercial radio-

frequency links (e.g., using commercial ZigBee strategies).

In this article, we present the results derived from the BAL-

DUR project (see acknowledgements for details). This project is

currently under development so as to obtain real-time video

transmission through visible light LED lamps. The final objec-

tive is to obtain an asymmetrical communications link which

can be connected to a structured TCP/IP network, based on

VLC for the downlink, while using infrared low speed (up to

115 kb/s) for the uplink. This device can be easily modified to

be used for sensor networking by using a polling strategy or op-

tical CDMA too.

2. VISIBLE LIGHT COMMUNICATIONS FUNDAMENTALS

A basic indoor VLC geometry system is shown in Figure 1.

Lighting and communication sources are provided by a number

of LED arrays mounted on the room ceiling. An optical receiver

is placed on a receiving plane, such as a desk, and data is

received from the illuminating sources.

Natural white light contains all the colors in visible light

spectrum. In LED illumination (also known as SSL—solid state

lighting), a quasi-white light source is typically generated by

using a combination of red/green/blue (RGB) or yellow-phos-

phor/blue (YB) LEDs [8]. Yellow phosphor/blue LED emitters

are being developed as an alternative white light source. Blue

light is emitted from a single LED chip, which is coated with a

yellow phosphor layer [9]. The optical spectrum in Figure 2

shows the combination of these two components, which creates

a white light source. The advantages of this YB-LED are low-

power consumption, high-illumination efficiency, and low-pro-

duction cost. However, the main drawback of this type of LED

for communications is the narrow modulation bandwidth caused

by the slow temporal response of phosphor emission compared

with the blue LED response [10].

It is envisaged that a single chip YB-LED is the most likely

candidate for the next generation of lighting technology due to

Figure 1 Schematic view of indoor VLC communications system with

a LED-array

1572 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 7, July 2010 DOI 10.1002/mop

its low cost, long lifetime, and low-power consumption. A typi-

cal VLC transmitter is designed for both lighting and data com-

munication. SSL ‘‘lamps’’ [E1] typically use a number of LEDs

connected in a chain to provide an appropriate resistance for the

drive circuit. Assemblies may include LEDs in series and in par-

allel to achieve this [11]. In addition, a means to convert the

AC main voltage into a high-current low-voltage DC source is

required. Oscillator type converters are typically used to achieve

this [12]. This voltage is typically fed directly to the devices, as

the integrating effect of the eye removes the need to smooth

these voltages.

Normally, in this kind of systems, intensity control is

achieved using pulse width modulation (PWM) typically at fre-

quencies of 10s of kHz. A VLC transmitter is usually designed

to provide both sufficient lighting and data communication.

As in conventional infrared wireless communication systems,

VLC uses intensity modulation and direct detection for data

transmission and detection. The basic modulation schemes,

which are widely used in optical wireless communications, are

pulse-position modulation (PPM) and on-off keying (OOK). De-

spite the bandwidth limitation in VLC, we have chosen PPM

instead of OOK because of its capability to avoid the effects of

symbol interference produced by the phosphor LEDs. As the

received SNR is greater than 40 dB, the difficulty of obtaining

the correct output levels will be reduced.

3. MODULATION TECHNIQUES, VIDEO CODING, ANDETHERNET INTERFACE

As widely extended for these kind of devices, we are using

PPM to ensure correct data transmission in both operating

modes: when the LED lamps are switched on and when they are

turned off. This is achieved by means of a variation in the pulse

duration. In this case, 90% of symbol duration has been selected

for the ‘‘turn on’’ mode and 10% for the ‘‘turn off’’ one. More-

over, this modulation technique reduces (almost completely in

the ‘‘turn off’’ state) the lowest frequency components of the

spectrum, making it suitable for avoiding interferences from in-

candescent and/or fluorescent lamps. The main drawback of this

modulation technique is the necessity of complex synchronism

system to ensure correct detection. To alleviate this, constant

rate-differential pulse position modulation (CR-DPPM) has been

developed. This technique combines the noncoherent detection

capability of classical DPPM schemes with a constant bit rate.

Figure 3 shows the waveforms corresponding to the pulse posi-

tion modulations mentioned earlier.

This technique allows noncoherent detection at the receiver,

ensuring that a symbol is received each clock period. Symbol

information is contained in distance between two adjacent

pulses, so detection is achieved counting clock samples (with a

frequency four times higher than the PPM one) between rising

edges. Correspondence between symbol and distances is shown

in Table 1.

Both blocks have been programmed using VHDL over a

Spartan 3A development kit. Ceiling LED array implement also

the subnet access point. Additional functions of this net are the

following: Flow control between Ethernet network and VLC

one, MAC identification of all the room optical nodes, and mod-

ulation and demodulation of the optical signal. As the downlink

VLC bit rate is limited to 1 Mbps (due to lamp characteristics),

the access point should extract video components from the

Ethernet frames so as to deliver them to any optical subnet

node.

Moreover, an interface of the Ethernet media access control-

ler/-physical (MAC/PHY) and the FPGA has been developed so

as to extract Ethernet frames payload and transmit them through

the optical link. Only Ethernet packets addressed to nodes at the

optical subnet are transmitted. The Ethernet payload is re-encap-

sulated using a start-of-frame code to ensure packet reception.

As the system frequency is 50 MHz and the available speed

of the bus used to communicate both modules is 62.5 MHz,

this application can be easily extended to higher optical baud

rates.

The transmitted video used for testing uses MPEG-TS for-

mat, to allow video streaming transmission (presenting the

received video while receiving the following incoming frames).

4. OPTICAL TRANSCEIVER

One of the biggest problems found in this implementation was

driving enough current to the emitting LED (which use a com-

mercial battery fed lamp). Different configurations were tested,

all of them based on SN75452 open collector-logical gate chips.

They are able to switch current values up to 100s of mW, as

required for the illumination LED (even in a parallel configura-

tion, as is shown in Fig. 4).

Receiver structure is depicted in Figure 5. After the photo-

diode (HAMAMATSU S5107), the first stage is a preamplifierFigure 3 Different pulse position modulations waveforms

TABLE 1 DPPM-CR Symbols and Their CorrespondingDistances

Symbol Distance (Chip Periods)

00 5

01 2 or 6

10 3 or 7

11 4 or 8

Figure 2 Optical spectrum of a yellow-phosphor/blue LED (adapted

from [1])

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 7, July 2010 1573

in a trans-impedance configuration, with a boost-trap circuit

used to reduce the effect of spurious capabilities in the photodi-

odes. The second stage is composed by an amplification block

and an active filter in sallen-key configuration for noise reduc-

tion. Finally, the received signal is delivered to a ML detector.

5. RESULTS

To test the system performance, we have transmitted a conven-

tional video signal, coded in TS-MPEG4, as a first approach. It

was extracted from a PC through serial port at a baud rate of 1

Mbps. These data were CR-DPPM modulated and optically

transmitted. Encouraged by the excellent performance of the op-

tical link, we implemented the routines needed to extract

directly the data from the Ethernet PC card.

The following figures show some of the preliminary results

of the ‘‘on the table’’ laboratory testing. The performed experi-

ment is shown in Figure 6. It consists on the commercial phos-

phor LED lamp shown in Figure 7 emitting at a distance over 2

m to an array of PIN photodiodes. Figures 8 and 9 show the

transmitted and received electrical signal (after electro-optical

conversion). Figure 8 shows for both transmission states (‘‘turn

off’’ and ‘‘turn on,’’ corresponding to a 10% and a 90% of the

nominal amount of optical power emitted by the lamp), the

noisy PPM signal before the comparison stage on the receiver

(compared with the transmitted). Figure 9 shows how this signal

is rectified to be delivered to the video presentation device.

The baud error rate was estimated comparing emitted and

received sequences of coded video. As we have transmitted sev-

eral video fragments of 108 length and zero errors were found

on them, we can estimate that error probability should be below

10–8 on a 3-m link in the previous conditions.

An uplink channel should be implemented to transmit com-

mands so as to control the broadcasting or several low-speed

channels for sensor interconnection. Other possibilities can be

considered such as RF or infrared links. The latter is a more

suitable option with the aim of avoiding EM compatibility

restrictions. Moreover, time hopping spread spectrum (THSS)

techniques could be used without generating any type of inter-

ference between both systems [4].Figure 5 Optical receiver block diagram

Figure 4 Driver scheme for the LED lamps

1574 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 7, July 2010 DOI 10.1002/mop

Figure 8 Experimental testing of the VLC prototype, received signal

for the link at 2 m distance, directly taken at the photodiode output

(before the comparison stage). (a) ‘‘Turn off’’ state (light is 10% of

nominal emitted power and (b) ‘‘Turn on’’ state (90% of the maximum

emitted power). Emitted (up) and received (down) signal

Figure 9 Experimental testing of the VLC prototype, received signal

for the link at 2 m distance, (after being rectified), Emitted (up) and

received (bottom) signal, ‘‘Turn on’’ stateFigure 7 Commercial phosphor LED lamp used for testing

Figure 6 Experimental testing of the VLC prototype: (a) Optical link

at 2 m distance and (b) array of PIN photodiodes used as optical

receiver

DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 7, July 2010 1575

6. CONCLUSIONS

The design of a visible light system suitable for transmitting

video files has been shown. It is now able to transmit encrypted

video at a baud rate up to 1 Mbps. The same structure allows

audio or raw data transmission without loss of generality. At a

link distance over 3 m, the received SNR is over 40 dB, with a

bit error rate extremely low (and even these remaining errors

are corrected by the protocol itself). The system is suitable for

video transmission as providing illumination levels from 90% to

10% of the nominal lamp emitted optical power value. The

future work is oriented to test other lamps (e.g., RGB Lamps

instead of phosphor ones) to achieve real-time video transmis-

sion or even symmetrical 10 Mbps Ethernet transmission.

ACKNOWLEDGMENTS

The authors thank CeDInt-UPM for their technical support. BAL-

DUR is a joint project of CeTIC, 07GLOBALAN and the regional

health service of Canary Islands (SCS). This work was supported in

part by the Spanish Industry and Commerce Ministry, (projects

TSI-020100-2008-660 and 2009-175). C. Quintana has a PhD grant

of the Regional Canary Island Research Administration (ACIISI)

and the support of INNOVA program.

REFERENCES

1. T. Komine and M. Nakagawa, Fundamental analysis for visible-

light communication system using LED lights, IEEE Trans Con-

sum Electron, 50 (2004).

2. T. Komine, Y. Tanaka, S. Haruyama, and M. Nakagawa, Basic

study on visible-light communication using light emitting diode

illumination, In: Proceedings of 8th International Symposium on

Microwave and Optical Technology, 2001, pp. 45–48.

3. Le-Minh, et al. Short-range visible light communications, World

Wireless Research Forum (2007).

4. C. Quintana, et al. Time-hopping spread-spectrum system for wire-

less optical communications, IEEE Trans Consum Electron,

submitted.

5. ISO/IEEE 11073.

6. F.J. Lopez-Hernandez, R. Perez-Jimenez, and A. Santamarıa, Ray-

tracing algorithms for fast calculation of the channel impulse

response on diffuse IR-wireless indoor channels, SPIE Opt Eng 39

(2000), 2775–2780.

7. O. Gonzalez, S. Rodrıguez, R. Perez-Jimenez, A. Ayala, and

B.R. Mendoza, Error analysis of the simulated impulse response

on indoor wireless optical channels using a Monte Carlo based

ray tracing algorithm, IEEE Trans Commun 53 (2005), 124–

130.

8. M.O. Holcomb, R. Mueller-Mach, G.O. Mueller, D. Collins, R.M.

Fletcher, D.A. Steigerwald, S. Eberle, Y.K. Lim, P.S. Martin, and

M. Krames, The LED Lightbulb: Are we there yet? Progress and

challenges for solid state illumination, In: Proceedings of Confer-

ence on Lasers and Electro-Optics (CLEO’03), 2003.

9. K.H. Lee and S.W.R. Lee, Process development for yellow phos-

phor coating on blue light emitting diodes (LEDs) for white light

illumination, In: Proceedings of 8th Electronics Packaging Tech-

nology Conference (EPTC’06), 2006.

10. J.K. Sheu, S.J. Chang, C.H. Kuo, Y.K. Su, L.W. Wu, Y.C. Lin,

W.C. Lai, J.M. Tsai, G.C. Chi, and R.K. Wu, White-light emission

from near UV InGaN-GaN LED chip precoated with blue/green/

red phosphors, IEEE Photon Technol Lett 5 (2003), 18–20.

11. Luxeon LED, Application brief AB20–3.

12. FAN semiconductor, F., FAN5608 LED driver, In Book FAN5608

LED driver, 2006.

VC 2010 Wiley Periodicals, Inc.

INTEGRATED THE INDUCTORS ONULTRA-THIN Si SUBSTRATE TO IMPROVETHE RF PERFORMANCE FOR LOW-NOISEAMPLIFIER APPLICATIONS

H. L. Kao1 and T. Chang2

1 Department of Electronic Engineering, Chang Gung University,Tao-Yuan, Taiwan, Republic of China2Chung-Shan Institute of Science and Technology, Taoyuan,Taiwan, Republic of China; Corresponding author:snoopy@mail.cgu.edu.tw

Received 20 September 2009

ABSTRACT: This article presents the Q-factor improvement as high as

31% of 0.8 nH spiral inductors of 90-lm-thick silicon substrate onplastic. The improvement of Q-factor is due to reducing the parasitic

effect from Si substrate. The loss mechanisms of parasitic effect ofinductor have been studied by thinned down the Si substrate to 90 lmand transfer to plastic. The inductance of the inductors before and after

thinned down to 90 lm Si substrates mounted on plastic are almostidentical for radio frequency circuit design. A low 0.2 dB minimum

noise figure (NFmin) of low-noise amplifier circuit with 90 lm Sisubstrate on plastic was obtained due to the improvement of highQ-factor inductors in the first stage for radio frequency identification

applications. VC 2010 Wiley Periodicals, Inc. Microwave Opt Technol

Lett 52: 1576–1579, 2010; Published online in Wiley InterScience

(www.interscience.wiley.com). DOI 10.1002/mop.25249

Key words: inductor; Q-factor; minimum noise figure; LNA

1. INTRODUCTION

The integration of radio frequency integrated circuits (RFICs) on

plastic substrates is useful technology for radio frequency identifi-

cation (RFID) [1–4] and wireless display applications. The radio

frequency (RF) performance of Silicon RF MOSFETs (radio fre-

quency metal-oxide-semiconductor field-effect transistors) has

improved rapidly due to aggressive downscaling of Si technology

[5–10]. Advanced CMOS technology is one of the candidates for

System-on-Chip (SOC) due to integration and low cost. However,

the key factor of RFICs is the quality of inductors due to the para-

sitic effects of the coupling capacitance and lossy related to the Si

substrate. The VLSI-standard Si substrates have a much lower re-

sistivity of 10 X cm than the highly insulating plastic substrates of

the resistivity �108 to 109 X cm. The substrate loss may be larger

than the RF MOSFETs, which has a typical minimum noise figure

(NFmin) of less than 1 dB for 0.18 lm technology at high frequen-

cies [11]. Therefore, the signal/noise ratio from lossy Si substrates

is becoming the limiting factor for RF circuits. For minimize the

substrate loss, many approaches have been proposed, e.g., micro-

electromechanical system technology [12], thick-porous Si [13],

high-resistivity Si [14–16], or substrate modification through pro-

ton implantation [11], etc. However, these could interfere also

with the current CMOS IC process. In this article, we reported the

Q-factor of inductors on ultra-thin Si substrate with high resistivityand bonding on plastic can be improved �31% but the inductance

without changes. The silicon substrate removing is using CMP

process after CMOS IC processes. Also, the active devices after

thinned down and transfer on plastic do not suffer the performance

had been reported [9]. Then, we integrated the low-noise amplifier

(LNA) with active device and passive device on plastic which is

the front-end of the receiver. The NFmin of the LNA circuit can be

improved �0.2 dB due to the high Q-factor of inductors. Thesechanges in the RF performance could be useful in RF circuit

designs for flexible electronics and RFID applications.

1576 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 52, No. 7, July 2010 DOI 10.1002/mop