spread spectrum-based white light communication with
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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
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