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Vol. 10 No. 1 January 2012 International Journal of Computer Science and Information Security Publication January 2012, Volume 10 No. 1 . Copyright � IJCSIS. This is an open access journal distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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(IJCSIS) International Journal of Computer Science and Information Security,
Vol. 10, No. 1, January, 2012
Adaptive Optical PIC Applied in VLC For Multi-user
Access Interference Reduction
Peixin, Li Ying Yi
Department of Electronics and Radio Engineering Department of Electronics and Radio Engineering
Kyung Hee University Kyung Hee University
Suwon, Korea Suwon, Korea
peixin@khu.ac.kr yiying@khu.ac.kr
Abstract—Optical wireless data transmission systems for indoor A lighting device is used as a transmitter without any
application are usually affected by optical interference induced traces of embellishment in the wireless communication
by sun light and artificial ambient lights. This paper presents a environment.
characterization of the optical interference produced in visible
light communication (VLC) systems and proposes an effective The visible light spectrum does not occupy the radio
scheme to solve it. Regarding the sun light noise and some frequency spectrum; therefore the electromagnetic
artificial light noises reduction, the common method is to adapt interference (EMI) can be avoided by VLC.
the optical bandpass filter which can distinguish the wavelength
VLC is suitable for high-speed data transmission, especially
between interference lights and information lights. However, for
some photo-electric systems, the visible lights from the in an indoor environment. Though VLC system has distinct
transmitters occupy the same wavelength range, in this case, the advantages as mentioned above, the performance of VLC is
optical bandpass filter would not reduce the interference noise limited by several aspects, for example, an inevitable issue is
from the other user, for example, the optical interference caused the optical interference noise that induced by both natural and
by multi-user access of the optical medium. Therefore, we artificial light on the receiving photodiode (PD) and the optical
proposed a novel scheme, adaptive optical parallel interference interference from the multi-user access of the optical medium.
cancellation (AOPIC) to reduce the multiple access interference In addition, few studies have examined the effects of optical
(MAI) and multiple user interference (MUI) induced by multi- interference from the multi-user access of the optical medium.
user access of the optical medium, the conventional parallel Actually, in the realistic communication environment,
interference cancellation (PIC) is analyzed as the comparison. numerous users transmitting signals are inevitable existing,
Through the simulation results, we can conclude that the AOPIC Motivated by this, we investigate the utilization of Multi-
scheme shows much better bit error rate (BER) performance Carrier (MC)-code division multiple access (CDMA)
than the conventional PIC with the increasing number of user. technology in VLC system. However, we also found that the
interference signals from the other users will cause the multi-
Keywords-component; AOPIC; MAI; MUI;MC-CDMA user interference (MUI) and multi-access interference (MAI)
that have a significant impact on the MC-CDMA
I. INTRODUCTION communication performance. We further proposed an effective
Recently, visible light communication (VLC) systems have scheme, AOPIC, to reduce both MAI and MUI induced by
attracted attentions due to the growing progress in the field of multi-user access of the optical medium, the conventional
visible light technology [1]. Visible light has several attractive parallel interference cancellation (PIC) is analyzed as the
features distinct from those of radio frequency (RF) and comparison. Our essential target is to improve the end-to-end
infrared (IR) [2]. Though both LED and laser diodes (LD) are optical wireless communication performance.
usually used as optical sources, LEDs are preferred as strong The remainder of this paper is organized as follows. In
candidates for the next generation lighting technology [3] for Section Ⅱ, the solution scheme for the natural light noise is
several reasons including fewer safety concerns, a relatively described, and the performance comparisons and analyses are
long useful life time, and a wider emission angle than those of given for the proposed system using the AOPIC technique and
LDs [4]. As an emitter for optical wireless communication, a typical PIC technique through computer simulations in
LED lights emit visible rays as the medium of optical data Section Ⅲ. Section Ⅳ provides the concluding remarks.
transmission. Nevertheless, with the development of VLC
systems, both the industrial and scientific communities have
recognized that visible light also can be used in the high data II. NATURAL LIGHT NOISE REDUCTION
rate transmission systems, since it has the following advantages
compared to those of RF: A. Sunlight interference
Sunlight that produces interference to the desired lights is
VLC is harmless to our health.
the dominant noise source to induce power penalties in the
A friendly user interface. performance of transmission systems [5-8], and usually this
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power penalties are very large. The effects of optical 2pcos(θk)nkdk ⁄ λ, where λ is the wavelength of the light in a
interference have been included in the performance analysis of vacuum [11]. Starting with ηk= Nk, (4) can be applied
VLC high data rate transmission systems by considering that recursively to arrive at η2, which when substituted into (2)
the optical interference power is proportional to the average yields ρTE or ρTM, depending on the initialization of the {Nk} as
surface area on the PD. Actually, the sun light produces the either TE or TM in (3).
highest levels of power spectral density around the wavelength
area of visible light, therefore, it is the major source of optical A PIN silicon PD is suitable for the outdoor environment
interference on the receiver PD. Moreover, the wavelength of because of its fast switching capability. Regarding the
the other artificial ambient lights (such as, incandescent lights preamplifier, we design a low noise field-effect-transistor
and fluorescent lights) and the wavelength of the transmitted (FET)-based transimpedance inside [14]. So the total received
visible light overlap in some area. Thereby shot noise and noise variance is the sum of contributions from the shot noise
interference are induced. and thermal noise, given by [14]:
The optical interference induces a power penalty that in s total
2
s s
shot
2
thermal
2
some cases may be very large. Therefore, optical filtering is
used in most systems to overcome some of the problems 2qgP I B {8 pkT
bg 2
k
hAI 2 B 2
produced by the ambient light interference. The higher g (6)
efficiency of the optical filter is achieved for sunlight
interference reduction due to the differences in the optical 16 p 2 kTkG 2 2 3
spectrum of each light source [9-10]. Usually, optical filter h A I 3B }
gm
includes two types, long-pass filter and band-pass filter (or
interference filters). The use of optical filters reduces the where the first term is shot noise, and second term is thermal
amount of ambient light that reaches the PD, thus reducing the noise variance. q is the electronic charge (1.6× -19 C), and B is
10
undesirable effects. The transmission gain obtained by the use the equivalent noise bandwidth corresponding to the data rate. γ
of an optical filter depends on its efficiency in attenuating the is the O/E conversion efficiency, and Pbg is the optical power of
ambient light while keeping intact the transmitted signal. the background light, which varies with time and reaches its
Clearly, interference (band-pass) optical filters are more peak at noon. Tk is the absolute temperature of the environment,
efficient in that operation, provided that the transmitted signal g is the open-loop voltage gain, η is the fixed capacitance of the
is not attenuated as well. PD per unit area, A is the physical area of the PD, Г is the FET
channel noise factor, gm is the FET transconductance, k is
Boltzmann's constant. We defined the noise bandwidth factor
B. PIN PD Receiver
I2=0.562 following [15-16], and the noise bandwidth factor
The front-end of PIN PD receiver is constructed from an I3=0.0868. We choose the average temperature and background
optical bandpass filter, a concentrator, a positive-intrinsic- noise power according to the time of day from [17], and choose
negative (PIN) silicon PD, and a preamplifier. Optical filter the other parameter values of the referred symbols from [15-16]
could reduce the optical interference without the bandpass and list them in Table Ⅰ.
wavelength at visible light spectrum but it could not eliminate
the interference light that are over the same wavelength as Sun light produces interference due to the time variations
desired light. The total fraction of power transmitted through on its intensity as shown in Fig. 1. In the simulation as depicted
the filter, assuming lossless dielectrics, is given by: in Fig. 1, the proposed receiver including PIN PD can signi-
ficantly reduce the optical interference from the sun light.
1 2 2 When the transmitted date rate is 10 Mbps, the PIN receiver
T ( 1) 1 ( TE TM ) (1)
2 can reduce nearly 10 dBm interference power as compared to
where the reflection coefficients ρTE and ρTM are defined by the the original sunlight power without handpass filter. Therefore
following set of recursive equations [11]—[13] the simulation result as shown in Fig. 1 can illustrates that
optical handpass filter is effectively used to reduce the optical
N 1 2 interference noise with the different wavelength to desired
(2)
N 1 2 lights.
nk / cos k , for TE
Nk , k {2,..., K } (3) TABLE I. PARAMETERS FOR OPTICAL INTERFERENCE REDUCTION
nk cos k ,
CALCULATION.
for TM
open-loop voltage gain, g 10
k 1 cos k jNk sin k
k Nk , k {2,..., K } (4) fixed capacitance, η 112 [pF/cm2]
Nk cos k jk 1 sin k
FET transconductance, gm 30 [mS]
nk 1
1
k sin ( sin k 1) , k {2,..., K} (5) Noise bandwidth factor, I2 I2=0.562
nk Noise bandwidth factor, I3 I3 =0.0868
Here, θk is the angle made by the light ray as it passes from FET channel noise factor, Г 1.5
medium k to medium k + 1, ηk is the effective complex valued
Data rate, Rb 10, 50 [Mbit/s]
index ―seen‖ by the light wave as it enters medium k, and βk =
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whom have the same number of subcarriers M and the same
spreading factor. Because the carrier frequency of visible light
is very high, the multipath fading can be ignored in the optical
channels [19]. Consequently, the received signals can be
written as:
K M
r (t) = 2 Pk , mbk , m(t k )ck (t k )
k 1 m 1 (9)
cos(2fmt k , m) n(t )
where n(t) is the additive white Gaussian noise (AWGN). τk is
the time delay for the k-th user. φk,m express the uniform
random variables over [0, 2π]. Pk , m is the received power, the
relationship between received power and transmitted power are
Figure 1. Optical power of interference noise over the day and the reduction given by:
effects by the proposed receiver for different transmission data rate.
n 1
Pk , m Pk , m( ) A cos n Ts ( )G cos (10)
III. PROPOSED SYSTEM MODEL 2 d 2
A. MC-CDMA Where A is the physical area of PD, d is the distance between
Different from the frequency division multiple access the emitter and the receiver, Ts(Ψ) is the gain of the optical
(FDMA) and time division multiple access (TDMA), filter, Ψ is the angle of incidence, and G is the optical
conventional CDMA techniques use spread codes to identify concentrator gain [20], as shown as follows:
each user separately, however, all users in a CDMA system
n2
interfere with each other. Take an example of MC-CDMA, G (11)
MC-CDMA is a combination access techniques of CDMA and sin 2 c
orthogonal frequency division multiplexing (OFDM) [18].
Regarding the signals from the other user, it is always where n is the material refractive index and Ψc denotes half of
considered as noise, for example, MAI and MUI. These the concentrator FOV, usually Ψc≤π/2.
interferences cause communication performance degradation
and limit the capacity of CDMA systems. Conventional The sampled output of the match filter for the k-user in
CDMA systems independently detect each user in parallel typical MC-CDMA systems can be expressed as follows:
using a matched filter which consists of the unique spreading
rk (t) = r (t)ck (t ) cos(2fmt k , m) dt
Tc
code used by that user. In the MC-CDMA, the transmitted 0
signal of the k-th user is given by: Tc
ck (t ) cos(2fmt k , m)
M 0
(12)
sk (t) = 2 Pk , mbk , m(t )ck (t ) cos(2 fmt k , m) (7) K M
m 1 [ 2 Pk , mbk , m(t k )
k 1 m 1
where M is the total number of sub-carriers, Pk,m represents the
ck (t k ) cos(2fmt k , m) n(t )]dt
transmitted power over m-th sub-carrier for the k-th user. The
subcarriers in MC-CDMA are orthogonal over the chip
duration, hence, m-th sub-carrier frequency is fm=f0+m/Tc, If the time delay is limited in a small value, the (12) can be
where Tc is chip duration. θk,m is the phase angle introduced in written as:
the carrier modulation process which distributes over [0, 2π].
M K M
bk,m(t) and ck(t) are the data sequence and spreading waveform,
respectively, given as follows: rk (t) 2 Pk , mbk , m(t ) 2 Pj , mbj , m(t )kj
m 1 j 1 m 1
jk (13)
bk , m(t ) b
i
k, m b(t iTs ) Tc
ck (t )n(t ) cos(2fmt k , m)dt
0
(8)
ck (t ) c (t iT )
i
k c c
rk (t) consists of three terms. The first is the desired signal
where bk,m and ck are independent random variables with equal which gives the sign of the information bit bk. The second term
probability of +1 or -1. While Пb is the rectangular symbol is the result of the MAI, and the last is due to noise. The cross-
waveform that is defined over the symbol duration Ts, and Пc is correlation of the spreading codes between k-user and j-user is:
the rectangular chip waveform over the interval [0, Tc].
Tc
We consider K asynchronous MC-CDMA users, all of kj ck (t )cj (t )dt (14)
0
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(IJCSIS) International Journal of Computer Science and Information Security,
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The decision made by the conventional single-user receiver is Wavelength Control
Optical signals
given as:
IM
…
…
LED User #1
Multi-user Frequency
…
bk sign[rk (t)]
…
(15) data stream Mapping Spread IFFT
LED User #K
Received electrical
where sign [.] is the sign function. Hence, the single-user signal
Output Frequency AO Filter and
matched filter receiver takes the MAI as noise and it can’t Information Demapping PIC
Despread FFT
Concentrator
suppress MAI. So we have to propose the interference PD
cancellation scheme to further reduce the MAI. Channel
Receiver
Decision
Estimation
B. AOPIC
The conventional PIC detector cancels the estimates of the Figure 2. Simplified block diagram of proposed system model.
MAI from the outputs of the matched filters in a parallel
manner. It follows an iterative process. Thus,
Wk(,zm1) Wk(,zm) ˆ( , )
sk zm [e( z ) ]* (20)
(z) 2
K
ˆ
sign[rk 2 Pjb ]
z 1 z 1 sk ,m
bk j kj (16)
jk
ˆ (z)
where α is a step size, sk , m denotes the input vector of the LMS
PIC detects all users simultaneously, and parallel detection equalizer, and it is defined as:
can be repeated. This process can be repeated over several
stages. With the increase of the stage in PIC process, the better ˆ( , ) ˆ ) (
sk zm bk( ,zm ck z ) (21)
BER performance can be obtained, but at the cost of high
complexity. On the contrary, AOPIC is based on mean square And * denotes complex conjugate, e(z) is the error between the
error (MSE) criteria, the cost function is given as follows: desired response and the output of the LMS filter, so that,
e( z ) r r ( z )
ˆ (22)
min E[ r (t ) r ( z )(t ) ]
2
ˆ (17)
W
(z)
Weight vector Wk , m is updated iteratively via minimize the e
Where r(t) is defined in (9), W is the weight vector. given as follows:
ˆ
r ( z )(t ) represents the estimate of the received signal at the z-th
K M 2
sequence of iterations that is defined as follows:
r (t) bk( ,zm ck z ) cos(2fmt k , m)Wk(,zm) dt (23)
ˆ ) (
Tc
e( z )
0
K M k 1 m 1
r ( z )(t ) = b c
ˆ ˆ (z) (z)
k ,m k cos(2fmt k , m)W(z)
k ,m (18)
k 1 m 1
Consider the k-th user, the interference cancellation can be
performed as:
ˆ )
bk( ,zm is the estimate of bk , m at the (z)-th iteration.
K M
r k (t) r ( z ) (t) sk zm cos(2fmt k , m)Wk(,zm)
(z)
The proposed system is depicted by Fig. 2. Frequency ˆ( , )
mapping accomplishes data transmission within the visible (24)
j 1 m 1
jk
light wavelength range. Intensity modulation (IM) and photo-
detector (PD) complete the conversion between the electrical ˆ )
signal and the optical signals. Spread codes are used to Therefore, the decision bk( ,zm in (21) becomes more reliable,
distinguish different users’ data, since users’ data are separated since it is based on the less interfered signal r k .
(z)
on the basis of their signature waveforms. The entire concept of
AOPIC is based on the premise that the received signal can be
reliably estimated. Decision, as shown in Fig. 2, follows an TABLE II. PARAMETERS FOR MULTI-USERS BER CALCULATION.
iterative process and subtracts the interference from other users.
Channel estimation evaluates all users simultaneously and then Modulation: BPSK
AOPIC can be repeated to update the weight vector. Many Noise Model: AWGN
algorithms can effectively reduce the MSE, for example, least Spread code: Walsh code
mean square (LMS) and recursive least square (RLS). Since the
LMS algorithm has a slower complexity as compared to the IFFT/FFT Size: 64
RLS, we propose a LMS algorithm in the AOPIC approach. Number of users: 5, 16, 48
The input of the first stage of AOPIC is defined as:
Spread factor: 32
ˆ
b(z)
sign[rk (t)] (19) Original Data rate: 100 Mbps
k
where rk (t) is defined in (13). The optimum weights are O/E Conv. Efficiency: 0.53 [A/W]
derived via a LMS algorithm which operates as follows: Background Light Noise: 0 [dBm]
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Figure 3. Comparison of BER of AOPIC, conventional PIC and no-PIC Figure 5. Comparison of BER of AOPIC, conventional PIC and no-PIC
scheme versus SNR for 5 users. scheme versus SNR for 48 users.
Figure 4. Comparison of BER of AOPIC, conventional PIC and no-PIC Figure 6. Comparison of BER of AOPIC, conventional PIC and no-PIC
scheme versus SNR for 16 users. scheme versus the number of user for SNR=18dB.
performance as compared to the 2-stage PIC.
C. Simulation Analysis
As analyzed above, we adopted Walsh spread code in the In Fig. 3, the BER performance penalty can be
AOPIC scheme and chose multi-stage PIC scheme for compensated with the increase of received optical power in
comparison purposes. The received electrical signal-to-noise both PIC scheme and AOPIC scheme if the number of user is
ratio (SNR) from [12] is: small. However, with the increase of the number of user, the
degree of MUI and MAI caused by the multiple users becomes
( Pr) 2 larger, as shown in Figs. 4-5, the scheme without PIC has been
SNR (25)
total 2
totally failed. Though the PIC can compensate the BER
performance penalty with the SNR increases, we can find that
where γ is the O/E conversion efficiency. total is defined in (6). AOPIC and 2-stage PIC are shown to achieve better
Pr represents the received optical power. The simulation performance than the conventional PIC. AOPIC shows a
parameters are listed in Table Ⅱ. Based on Table 1 and 2, the similar BER performance to 2-stage PIC. When the number of
users becomes much larger, 48 users, as shown in Fig. 5,
simulation results are given in Figs 3-6.
AOPIC provides a relative better performance than does 2-
It is shown in Figs. 3-6 that significant performance stage PIC.
improvement was obtained by employing the AOPIC and the
Finally, we assume that the SNR at the receiver is 18dB,
2-stage cancellation into the PIC receiver. It is clear from Figs.
and from Fig. 6, we can further observe that the AOPIC shows
3-6 that the 2-stage PIC shows a significantly better
an excellent performance among all schemes with an increase
performance than the 1-stage PIC regardless of the number of
in the number of users. Therefore, we can conclude that the
users. As a comparison, 2-stage PIC obtains the better BER
Walsh code has a better orthogonal quality in distinguishing
performance, however, at a cost of high complexity. Our
different users’ data, and AOPIC can further suppress the MUI
proposed AOPIC scheme is much easier operated, and from
and MAI effectively. It is shown that AOPIC can retain a
Figs. 3-6, we can find that AOPIC scheme shows a closed BER
performance advantage over conventional PIC.
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IV. CONCLUSIONS [8] G.W. Marsh and J.M. Kahn, 50-Mb/s diffuse infrared free-space link
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[13] J. D. Rancourt, ―Optical Thin Films‖ (Macmillan, New York, 1987).
In this paper, a conventional technique to overcome the
[14] S.D.Personick, "Receiver design for digital fiber optic communications
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a MC-CDMA system, using band-limited spreading waveforms outdoor visible-light communication system using selective combining
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[18] S. Hara and R. Prasad, ―Overview of multi-carrier CDMA,‖ IEEE Com.
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REFERENCES AUTHORS PROFILE
Peixin Li received bachelor degree in College of
[1] D.C.O’Brien et al, " Visible lightcommunication: state of the art and Materials Science and Engineering from Jiamusi
prospects," published in Proc. Wireless World Research Forum 2007. University, in Heilongjiang Province, China. He is
[2] M.Z.Afgani, H.Haas, H.Elgala, D.Knipp, ―Visible light communication currently pursuing the Master degree of Engineering in
using OFDM,‖ Proc. IEEE Symp. on Wireless Pervasive Computing, Department of Electronics and Radio Engineering, Kyung
TRIDENTCOM 2006. Hee University, Korea. His current research interests are
[3] C.P.Kno, R. M. Fletcher, T. D. Owentowski, M.C.Lardizabal and visible light communication, MIMO-OFDM and MC/DS
M.G.Craford, ―High performance ALGaInP visible light-emitting CDMA.
diodes," Appl. Phys. Lett., vol. 57, no.27, pp. 2937-2939, 1990.
[4] K.D.Langer and J.Grubor, "Recent Developments in Optical Wireless
Communications using Infrared and Visible Light", ICTON, 2007,
pp.146-151.
[5] A.M.R. Tavares, A.J.C. Moreira, C. Lomba, L. Moreira, R.T. Valadas
and A.M. de Oliveira Duarte, Experimental results of a 1 Mbps IR Ying Yi received the B.S degree in Information
transceiver for indoor wireless local area networks, in: COMCON V- Technology from HeBei Normal University, in HeBei
Intern. Conf. on Advances in Communications & Control, Crete, Greece Province, China, and M.E. degrees from the Department
(June 26–30, 1995). of Electronics and Radio Engineering, Kyung Hee
University, Korea, in 2008 and 2010, respectively.
[6] R.T. Valadas, A.J.C. Moreira, C. Oliveira, L. Moreira, C. Lomba, A.M.R.
Currently, he is a research associate in Department of
Tavares and A.M. de Oliveira Duarte, Experimental results of a pulse
Electronics and Radio Engineering, Kyung Hee
position modulation infrared transceiver, in: Proceedings of the Seventh
University, Korea. Meanwhile, he is doing the projects for
IEEE International Symposium on Personal, Indoor and Mobile Radio
Communications (PIMRC ’96), Taipei, Taiwan (October 15–18, 1996). IT Research and Development Program of the Korean
Ministry of Knowledge Economy and Korea Evaluation Institute of Industrial
[7] M.D. Audeh and J.M. Kahn, Performance evaluation of baseband OOK Technology (MKE/KEIT) as a researcher. His research interests are optical
for wireless indoor infrared LAN’s operating ant 100 Mb/s, IEEE Trans. wireless communication systems, Ad-hoc/Mesh network, and LTE.
Comm. 43(6) (1995) 2085–2094.
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