Design of Wireless Optical Access System using LED

doi:10.4236/opj.2013.32B036

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Optics and Photonics Journal, 2013, 3, 148-152

doi:10.4236/opj.2013.32B036 Published Online June 2013 (http://www.scirp.org/journal/opj)

Design of Wireless Optical Access System using LED

Liwei Ding, Fang Liu, Yingjie He, Hongbo Zhu, Yongjin Wang

Institute of Communication Technology, Nanjing University of Posts and Telecommunications, Jiangsu, China

Email: [email protected]

Received 2013

ABSTRACT

This paper develops a prototype to demonstrate a full-duplex wireless visible light communication (VLC) system based

on Universal Serial Bus (USB) port. Combing with a USB 2.0 port on one board, it can achieve up to 2 Mbps bit rates

error free, which is mainly limited to the USB bridge circuit, between two computers for data transmission, such as text,

audio and video. Hyper Terminal and self-written software are used to allow setting the transmission data rate, control

bits, check bits and achieve real-time transmission between any two mobile devices with a USB port. The work is based

on the research of VLC local access network. This paper also shows the experimental results and the relationship be-

tween system transmission eye diagram and bit rate.

Keywords: Visible Light Communications; Visible Light Access Networking; USB Interface; Real-time Transmission;

Full-duplex

1. Introduction

Looking into the future, traditional home and office illu-

mination tools will soon be substituted by the lower en-

ergy consuming, longer lifetime and health harmless

LED devices. Meanwhile, the very fast changing capa-

bility in the output power makes LED devices possib le to

transmit a large amount of data at high speeds in the

visible range, which is an added advantage of LED be-

sides its illumination purpose. Visible light communica-

tion (VLC) using white LEDs has aroused global atten-

tion. At present, many famous research institutions and

universities, such as the Visible Light Communications

Consortium (VLCC) [1-2], the European OMEGA pro-

ject [3], the Wireless World Research Forum (WWRF) [4]

and so on, have dedicated much to area. And it has wit-

nessed increasing interest in the research community with

a forthcoming IEEE standard [5]. High speed transmis-

sion rate [6,8], channel characteristics [9,10], modulation

and demodulation methods [11-13], etc [14,15] are the

main focuses of research.

USB is a universal standard of external bus to specify

the connection and communication through computer

and peripheral electronic devices. USB port as the inter-

face to PC with its popularity and support to hot plugging.

VLC with a variety of practical applications in short-

range transmission scenarios is promised to enter into

people’s daily life. The channel between two computers

via VLC has been shown in [16-19], where RS-232 pro-

tocol or RS-485 protocol is used to connect visible light

module with a PC. Design of an integrated optical re-

ceiver using USB interface is introduced in [20]. The

data rate and transmission distance can surely be im-

proved by increasing the number of LEDs and adopting

modulation schemes. An application scenario for the

developed system is indoor short distance wireless

transmission where LED with communication function is

enveloped as a desk lamp and when turned on, data

stream flows wirelessly in both ways.

In this paper, a full-duplex access system using white

LED based on USB port with 2 Mbps data rate is de-

signed as a basic visible light access network. Hyper

Terminal and self-written software are used to achieve

the transmission of data and real-time audio transmission

between two PCs. The rest of this paper is organized as

follows. Section 2 gives the system design and system

characteristics. In section 3, the testing and results of

transmission link is discussed. Finally, Section 4 con-

cludes the paper.

2. System Design and System Characteristics

2.1. System Design

A CH340 is used to connect USB interface for realizing

the conversion of signal from differential to TTL, or in-

verse. USB signal after conversion has a 4V peak-to-

peak voltage value. A voltage follower then plays the

role of buffer to amplify the signal current to an adequa te

level, driving LED for lig hting. A h igh p ower white LED

of 1W and with the beam angel 120°is used as optical

transmission source. OOK NRZ data for Intensity modu-

lation (IM) and direct detection (DD) scheme is chosen

L. W. DING ET AL. 149

for the widely reception in VLC research. The circuit

design of transmitter is given as Figure 1.

The receiver employs a commercially available Si PIN

(Hamamatsu S6968) photodiode for optical to electrical

conversion with 150 mm2 effective active area, 14mm

active area size and 35°half angle. The received optical

signal is sent to a low pass filter followed by a custom

amplifier for recovery. A decision circuit (LT1715) is

used as a single threshold voltage comparator to achieve

the signal decision with a reference voltage level to ob-

tain the original signal. The TTL signal after decision is

sent to CH340 to achieve the conversion from TTL sig-

nal to USB serial signal. The whole circuit of receiver

design is given in Figure 2. It needs to be noted that the

transceiver module is composed of a transmitter and a

receiver to realize a full-duplex wireless communication.

Transceiver design is shown in Figure 3.

Figure 1. Transmitter circuit diagram.

Figure 2. Receiver circuit diagram.

Figure 3. Transceiver block diagram.

2.2. System Characteristic

All the system power supply is supported by the 5V DC

from USB port. The driver of CH340 must be firstly in-

stalled on PC before being connected to a transceiver

module. After transmission rate being set and Com port

being selected in Hyper Terminal, which is a communi-

cation tool available in computer’s self-carried enclo-

sures, PC treats the transceiver module as a visual serial

port via which all types of the data can be transmitted,

such as text, audio, video and so on. Another application

example is developed using Visual C#. Program is cre-

ated to achieve real time audio transmission between two

transceiver-settled computers. The input voice from mi-

L. W. DING ET AL.

150

crophone or music of wave waveform type is captured by

one computer, and then audio data is sent to the selected

Com port and transmitted by our transceiver module.

After a light of sight channel, the opposite transceiver

module interfaced on the second PC received the bit

streams and played it real time. The physical map of the

system model is shown in Figure 4. The transmission

between two computers using Hyper Terminal is shown

in Figure 5.

3. Testing and Results

With the VLC system described above, performance of

the transceiver module is measured. On one hand, we

studied the eye pattern of the TTL signal before decision

circuit versus data rate, and the relationship between data

rate and eye diagram is investigated. On the other hand,

the text transmission using program based on Visual C#

is also given. The system is tested using the configura-

tion shown in Figure 6, as the arbitrary waveform gen-

erator and oscilloscope instead of PCs.

Figure 7 presents the eye diagrams for the receiver ar-

bitrary OOK NRZ data (PN23) at speeds of 1 Mbps and

2 Mbps after optical signal conversation. The eye dia-

grams of the signal receive without amplify and filtering

are clear, thus show error free performance. The eye dia-

grams indicate that the signal can be correct recovered

after deci s ion circu i t.

Figure 4. Physical map of system design.

Figure 5. Prototype of the full-duplex system.

Figure 6. Testing block diagram.

L. W. DING ET AL. 151

Figure 7. Eye diagram waveforms after direct detection without filtering and being amplified at (a) 1 Mbit/s data rate, (b) 2

Mbit/s data rate.

012345

Q-Factor

Frequency(MHz)

（

）

0123456

-15

-12

-9

-6

-3

Normalized response(dB)

Frequency(MHz)

（

）

Figure 8. (a) Eye diagram Q-factor Vs data rate, (b) fre-

quency response curves of the whit light LED.

We obtained the Q-factor of eye diagram at different

transmission rates and estimated the BER from Q-factor.

All BERs are below 10-6 when data rate goes up to 4.5

Mbit/s. The curve of Q-factor vs data rate is showed in

Figure 8(a). We can find that the BER performance is

mainly restricted to 3 dB bandwidth of the white LED

emitter, where the normalized response is shown in Fig-

ure 8(b).

In this experiment, the system of VLC was demon-

strated for applying to data transmission because the

concept of VLC can be used in marine, automotive, un-

derwater, home network and other scenarios. The green

waveforms stand for the transmission signal and the yel-

low waveforms come from the receiver after decision

circuit, as Figure 9 shown. It is clear that no bit error

Figure 9. Captured data waveforms after decision circuit at

(a) 1 Mbit/s data rate, (b) 2 Mbit/s data rate.

occurs after visible light transmission through the system

and the decision circuit can correctly complete the signal

decision.

4. Conclusions and Future Work

We developed a full-duplex optical wireless system pro-

totype to achieve the transmission between two com-

puters using white light LED. The upper limit transmis-

sion rate is 2 Mbit/s, which is restricted to USB bridge

circuit CH340. The obtained Q-factor of eye diagram at

different transmission rates and estimated the BER from

Q-factor. All BERs are below 10-6 when data rate goes

up to 4.5 Mbit/s. The higher USB bridge circuit, the more

optimizing circuit designs, efficient coding method, ad-

vanced modulation techniques and mechanism for cor-

recting errors will be considered to improvement the

transmission system in our future work.

5. Acknowledgements

This work is jointly supported by NSFC (11104147),

Jiangsu 973 project (BK2011027), and research project

(NY211001, BJ211026) .

L. W. DING ET AL.

152

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