Spread Spectrum-based White Light Communication with Experiments

Xiaobin Niu1, Min Zhang1, Dahai Han1, Zabih Ghassemlooy3, Qing Li1, Pengfei Luo2, Bo Zhang1

1. State Key Laboratory of Information Photonics and Optical Communication,

Beijing University of Post and Telecommunication, Beijing 100876, China

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

3. Optical Communications Research Group, NCRLab, Faculty of Engineering and Environment

Northumbria University, Newcastle upon Tyne NE18ST, U.K. E-mail: niuxiaobin@bupt.edu.cn

ABSTRACT

In this paper a Direct Sequence Spread Spectrum (DSSS)

based offline wireless white light communication system

is established. Field measurements are conducted to

compare the performance with and without DSSS. The

experimental results show that the DSSS is able to

improve performance obviously.

Keywords: White light communication, DSSS, Wireless, Offline processing

1. INTRODUCTION

White light LED has been widely used in visible light

wireless communication system and networks [1,2].

Spread-spectrum is a technique by which a

telecommunication signal is transmitted on a bandwidth

considerably larger than the frequency content of the

original information. Frequency-hopping spread

spectrum (FHSS), direct sequence spread spectrum

(DSSS), time-hopping spread spectrum (THSS), chirp

spread spectrum (CSS), and combinations of these

techniques are forms of spread spectrum [3]. DSSS is

a kind of common spread-spectrum technique. With

strong anti-interference ability, strong resistance to

multipath interference, good confidentiality and access

to multiple communication easily, DSSS has been

widely used. So we use DSSS technology in our

experiment [4].

In the paper, we design and implement white light

communication system with spread-spectrum technique.

Our main contribution lies in the fact that to the best of

our knowledge this is the first report of spread-spectrum

based offline white light communication system. The

feasibility of spread-spectrum technique in improving

the white light communication is verified through offline

experiments with different transmission speeds. Besides,

the influence of LED output power on system bit error

rate (BER) performance is also studied.

2. SYSTEM STRUCTURE AND DSSS

IMPLEMENTATION

Fig.1 presents a DSSS-based white light communication

system. Regarding our experiment, a block of 1000

random bits is packed as a payload and a sequence of

“10101011” is set a frame-head. Therefore, we get 1008

bits of binary data for each frame. In our DSSS system,

the pseudo random (PN) sequence (PN0) and its inverse

(PN1) are used to represent data bits “0” and “1”

respectively. So we get 1008 × 7=7056 chips of PN

sequences after the frame is being spread spectrum

processed in total.

Fig.1 Block diagram of system configuration

Fig.2 shows the whole experiment system. We first

upload the generated 7056 chips of PN sequence to

arbitrary waveform generator (AWG) (TGA12104).

After that the AWG is used to send high and low

electrical level according to the 7056 chips of PN

sequences to the LED. On the receiver side, a

photoelectric sensor (PIN) (PDA10A) is employed to

receive the emitted light. The distance between the LED

and the PIN is about 1.2 meters. In order to get greater

intensity, we use lens to collect light. Beside a blue

optical filter is adopted to block the slow yellow light. As

we can see from Fig. 2 that the PIN is connected to a

digital oscilloscope (RIGOL MSO4024), which is used

to record and store the received waveforms. Since the

data rate of our transmitter can be adjusted, the

relationship between data rate and BER performance of

the proposed DSSS-based white light communication

system can be measured. In the similar manner, the same

relationship of the system without DSSS technique can

be also measured by directly sending the 1008 bits of the

original sequences.

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

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

Fig.2 Experimental system of DSSS-based white light

communication

3. THE METHOD OF DATA PROCESSING AT

THE RECEIVER SIDE

Since the techniques employed for data processing at the

receiver side directly influence the experiment results. In

this section, the process of data processing which can be

divided into three steps is clearly explained.

Step 1 chip recovery. Since we set the transmission

rate as “m”, the sampling rate of digital oscilloscope as

“n”, then we can get “p” numbers of sample from every

chip (p = n / m). As “n” and “m” are specially set, p is an

integer. Therefore, we could use a moving average filter

and a threshold decider to preform chip recovery

(recover digital bits or chips from the sampled

waveforms).

Step 2 clock synchronization. As shown in Fig. 3 that

the cross-correlation between the recovered chips and

PN1 are computed. If the cross-correlation result is

higher than the threshold, we set frame_clc0 as 1

otherwise 0. After that frame_clc0 is shifted by one bit

position each time, therefore, frame_clc1, frame_clc2,…,

frame_clcm are generated. Finally, by adding them

together, a peak will be found at a certain place of this

sequence, by using this peak we can detect the end of

each spread spectrum sequence. So the clock could be

synchronized accurately.

Step 3 data recovery. In this step, the recovered PN

chips are decoded into bit “0” and “1” according to DSSS

demodulation scheme (e.g., the modulation scheme in

[5]) with the accurate synchronized clock. After the

DSSS demodulation process, the original data bits could

be recovered. At last the BER value can be calculated .

Fig.3 Method of finding accurate synchronous clock

4. RESULT AND ANALYZE

In this section, we first calculate the error rate of three

different modulation schemes (OOK, 3DSSS, 7DSSS)

under different transmission speed. The measured results

are presented in Fig.4

Fig.4 BER of OOK, 7DSSS, 3DSSS with different

data rate.

As we can see from Fig.4 that the DSSS can decrease

BER obviously at the cost of bandwidth consumption.

However, the DSSS improves the computational

complexity.

It should notice that the speed of 7DSSS, 3DSSS is not

the effective transmission speed. And the effective speed

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

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

can be obtained by dividing the raw transmission speed

by the length of PN sequences. Fig.5 illustrates the

effective transmission speed of OOK, 7DSSS, 3DSSS.

Fig.5 BER v.s. the effective transmission

speed of OOK, 7DSSS, 3DSSS

Therefore, we draw a conclusion: DSSS can improve

the efficiency of transmission at cost of high

computational complexity.

Besides, we also measured the influence of the LED

output power on system BER. We set three kind of power

level: low, middle, strong by adjusting the amplitude of

AWG and get three curve of error rate growth. The

results is shown in Fig.6.

Fig.6 Three power level curve of error rate growth

We can learn that the performance of high power is

better than of middle power. The performance of middle

power is better than of low power.

5. CONCLUTIONS

In conclusion, we analyzed the performance of white

light communication with spread-spectrum technique.

We conducted experiments of 3DSSS, 7DSSS and

compare the system performances without spread-

spectrum technique. The results show that the spread-

spectrum technique is able to improve the performance

of white light communication system evidently, and the

improvement is more significant when the PN sequences

get longer. Besides, the power of transmission terminal

LED has influence on the performance of

communication system. At a certain level, the greater the

power, the better the performance.

6. REFERENCES

[1] Grobe L, Paraskevopoulos A, Hilt J, et al. High-speed

visible light communication systems[J]. IEEE

Communications Magazine, 2013, 51(12): 60-66. [2] G. Cossu, A. M. Khalid, P. Choudhury, R. Corsini & E.

Ciaramella, Long distance indoor high speed visible light

communication system based on RGB LEDs, Asia

Communications and Photonics Conference,AS3C.2

(2012). [3] Y. Peng, Z. Fang, and E. Zang, Chin. Opt. Lett. 10,

S11405 (2012). [4] J. J. Puschell and R. Bayse, in Proceedings of the Tactical

Communications 253 (1990).

[5] M. Zhang, P. Luo, X. Guo, X. Zhang, D. Han, and Q. Li,

"Spread spectrum-based ultraviolet communication with

experiments," Chinese Optics Letters, vol. 12, p. 100602,

2014/10/01 2014.

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

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