Real-time Bi-directional Visible Light Communication System

Hui Han1, Min Zhang1, Pengfei Luo2, Weishu Xu1, Dahai Han1, Menglong Wu3

1State Key Laboratory of Information Photonics and Optical Communications, Beijing Univ. of

Posts & Telecom, Beijing, China, 2Research Department of HiSilicon, Huawei Technologies Co., Ltd, Beijing, P. R. China,

3North China University of Technology, Beijing 100141, China *Email: mzhang@bupt.edu.cn

ABSTRACT

A real-time bi-directional LED-to-LED VLC system is

developed. The test results show that the proposed

system can realize a downlink and uplink speed at

2.6Mbps and 2.55Mbps over a transmission range up to

2m.

Keywords: real-time, bi-directional, LED-to-LED

1. INTRODUCTION

Visible light communication (VLC) using light-

emitting Diodes (LEDs) for wireless communication as

well as general illumination is becoming an extremely

attractive area and has numerous practical applications.

There are several advantages with LED-based VLC

system: Firstly, lighting LEDs are low-cost and the

emitted light is harmless to the human eyes; Secondly,

VLC is electromagnetic interference (EMI) free and

license-free compared with radio-frequency (RF)

communication [1]. Although competitive advantages

have been found in VLC system, one main drawback for

most LED-to-photodetector (PD) VLC systems is that

they could only provide uni-directional link. In order to

establish a bi-directional communication, another LED-

to-PD link has to be introduced. At the moment, there

have been some attempts to implement the bi-directional

VLC link by means of utilizing two different LEDs [1]

or using splitters to separate the downstream and

upstream [2]. However, all of them deploy many devices

to realize the bi-directional link and make the realization

complicated.

In this paper, a simplified real-time LED-to-LED

VLC system utilizing high-brightness LED both as

transmitter and receiver is proposed and realized. To

achieve a LED-to-LED bidirectional link, a time-

division multiple access protocol is designed to switch

the operating mode of the LEDs between transmitting

and receiving. The system bit error rate (BER)

performance is measured with On-Off-Keying (OOK)

modulation scheme as well.

The rest of the paper is organized as follows. In Section

2, characteristics of duplexer circuit is introduced, the

communication protocol to support bi-directional LED-

to-LED VLC link is detailed designed, then a bi-

directional VLC prototype is built and tested. Finally,

conclusion and future work are described in Section 3.

2. BI-DIRECTIONAL LED-TO-LED VLC

SYSTEM DEMONSTRATION

2.1 Subsection heading

A conversion circuit was developed to achieve the

proposed LED-to-LED bi-directional VLC system. Fig.

1 shows the conversion circuit diagram we designed and

Fig. 2 shows the circuit we implemented. The circuit has

three parts: a transmitter, a receiver and a switch. The

transmitter part is a common LED direct current (DC)

driver circuit. As we all know, using a LED as a receiver

requires a properly reverse voltage [3]. When the LED is

irradiated, it transforms the light energy to electrical

energy. For the receiver part, it is a high speed, very high

slew rate operational amplifier to convert the LED

generated current to voltage. And a Field Programmable

Gate Array (FPGA) board is employed to further process

the voltage signal. Fig. 3 presents the normalized

frequency response of the receiver part (I-V converter).

Through the picture we could see that the 3dB-

bandwidth is 8.2MHz. and the gain of the receiver part is

higher than 8dB. The switch is constituted by a single-

pole double-throw (SPDT) analog switch (TS5A2053).

In our prototype, both LEDs are placed at the single-pole

of the switch, and the LED driver and the I-V converter

is connected to the two throw ports respectively of the

switch. Therefore, by changing the input level of the

switch’s in port, we could set the LED to be an emitter

or PD.

2.2 Time-division multiple access protocol

Figure 4 shows the self-designed time-division multiple

access protocol in the system we test. The frame format

in the communication is plain. The frame header is 13-

bit barker code, and we use the 7-bit Pseudo-Noise

Sequence to form the information bits. Prior to the

connection established, both LEDs are in the receiving

state. There are three steps to establish a connection

between two communication sides:

2016 15th International Conference on Optical Communications and Networks (ICOCN)

978-1-5090-3491-8/16/$31.00 ©2016 IEEE

Fig. 1. Diagram of the conversion circuit

Fig. 2. The circuit we tested

1. the initiator sends a request data with special

information bits, then the initiator changes the state to

receiving.

2. Once the receiver (the other part of the parties)

received the request data, it transforms to be a transmitter

and send a confirm data with the special frame format

also. After that, it switches to the receiving state again.

3. When the initiator receives the confirm data, the

link between the two sides is established. If the initiator

does not receive the confirm data, the connection is failed

and the initiator keeps sending a link establishment

request after a certain time.

Note that both sides are exactly the same including

all circuits, FPGA boards and codes. Therefore any side

can be an initiator or a receiver. Once the connection is

established, the initiator and the receiver will

communicate under a time division multiplexing mode.

The initiator converts to the reception state each time

when the transmitting is completed. The connection

termination is convenient. As long as any side sends an

end frame to the other, and the other part detect the signal

and change its state to receiver. At this point, both sides

are in receiving mode, and the system returns back to the

original state.

2.3 Real-time Bi-directional LED-to-LED system

experimental setup

Figure 5 shows the system block diagram. Both LEDs

are CREE C503B-RAN [4]. Two lens are employed to

focus the light and increase the received optical power

for the conversion circuit in the receiving mode. And

both FPGAs work as mode controllers of conversion

circuit, signal generator and real-time signal processor.

Fig. 3. Frequency response of the receiver part at 2m

Fig. 4. Time-division multiple access protocol

2.4 Experimental results

Figure 6 shows the real system we implemented. Figure

7 depicts the measured BER performance as a function

of the transmission rate for both uplink and downlink at

the distance of 2m. When using a forward error

correction (FEC) with an overhead of 7%, the

International Telecommunication Union’s (ITU’s)

recommended BER floor is 3.8×10-3 [5]. As can be

observed from the figure 5, the maximum transmission

data rate of uplink is ~2.55Mbps and the downlink is

~2.6Mbps for the FEC limit. From the figure 5, we

confirmed that our real-time bi-directional VLC system

has a good performance when the data rate is lower than

2.4Mbps. In the applications such as smart home and

indoor localization system, the performance of the

proposed system is sufficient to meet the requirements.

3. CONLUSION AND FUTURE WORK

In this work, a real-time half-duplex LED-to-LED VLC

link utilizing self-designed circuit and protocol was

proposed. The measurement results demonstrate that the

prototype could communication in the rate of 2.6Mbps

downlink and 2.55Mbps uplink. As far as future work is

concerned, more high performance LEDs could be used

to increase the data rate, also the white LED with the

same performance is under our concern. Additionally, to

2016 15th International Conference on Optical Communications and Networks (ICOCN)

978-1-5090-3491-8/16/$31.00 ©2016 IEEE

Fig. 5. Diagram of the real-time bi-directional LED-to-

LED VLC system

Fig. 6. Real-time bi-directional LED-to-LED VLC

system

extend the system bandwidth, the circuits and the

program in FPGA need to be redesigned.

11. ACKNOWLEDGMENTS

This study is supported by NSFC Project No.61471052.

Fig. 7. BER as a function of transmission rates for uplink

and downlink

12. REFERENCES

[1] Shi, Jianyang, et al. "Real-time bi-directional visible light

communication system utilizing a phosphor-based LED

and RGB LED." Sixth International Conference on

Wireless Communications and Signal Processing 2014:1-

5.

[2] Chun, Hyunchae, et al. "Demonstration of a Bi-directional

visible light communication with an overall sum-rate of

110 Mb/s using LEDs as emitter and detector." Photonics

Conference IEEE, 2014:132-133.

[3] Siuzdak, Jerzy. "Influence of reverse bias on the LEDs

properties used as photo-detectors in VLC systems."

XXXVI Symposium on Photonics Applications in

Astronomy, Communications, Industry, and High-Energy

Physics Experiments (Wilga 2015) International Society

for Optics and Photonics, 2015.

[4] Kowalczyk, M., and J. Siuzdak. "VLC link with LEDs

used as both transmitters and photo-detectors."

(2015):893-897.

[4] ITU-T, “G.975.1, Forward error correction for high bit-rate

DWDM submarine systems,” July, 2004, p. Appendix I.9

2016 15th International Conference on Optical Communications and Networks (ICOCN)

978-1-5090-3491-8/16/$31.00 ©2016 IEEE