Investigation on Performance of Passive Optical
Network Based on OCDMA
Chongfu Zhang, Kun Qiu, and Bo Xu
Key Lab. of Broadband Optical Fiber Communication and Communication Networks Technology,
University of Electronic Science and Technology of China, Chengdu, P. R. CHINA
Email: chfzhang5178163.com
Abstract-In this paper, a Passive Optical Network (PON)
based on Optical Code Division Multiple Access (OCDMA) is
presented, and analyzed in detail. The proposed system can be
applied to an optical access network with full services on
demand, such as internet protocol, video on demand, telepresence and high quality audio. An OCDMA-PON combines
advantages of PON and OCDMA technology. Simulation
result elicits the proposed scheme is feasible. In this study, the
novel design improves the optical access network performance
and enhances the system flexibility and scalability.
INTRODUCTION
Since the internet and broad-band access network were
I.
introduced during the last decade, emerging applications,
such as video on demand (VOD), digital cinema, telepresence, and high-quality audio transmission, demand
high-throughput optical access networks with stringent
quality of service (QoS) requirements. However, the
infrastructure of current access networks suffers from
limited bandwidth, high network management cost, low
assembly flexibility and bad network security, which
obstructs the network from delivering integrated services to
users. Owing to the maturity of optical components and
electronic circuits, optical fiber links have become practical
for access networks. Passive optical network (PON) and
some multiplexing technologies, including wavelength
division multiplexing (WDM) [1], time division
multiplexing (TDM) [2], and optical code division
multiplexing (OCDM) for access networks, have been
proposed [3].
PON is one of the most promising solutions for fiber-tothe-office (FTTO), fiber-to-the-home (FTTH), fiber-to-thebusiness (FTTB), and fiber-to-the-curb (FTTC), since it
breaks through the economic barrier of traditional point-topoint solutions. PON has been standardized for FTTH
solutions and is currently being deployed in the field by
network service providers worldwide. Even though timedivision multiple-access (TDMA)-PON utilizes effectively
the bandwidth of fiber, it has limitations in its increased
transmission speed, difficulty in burst synchronization, low
security, dynamic bandwidth allocation (DBA) requirement
0-7803-9584-0/06/$20.00O2006 IEEE.
and inaccurate Ranging [4-5]. WDM technology has also
been proposed for PON. The emerging WDM-PON
becomes more favorable as the required bandwidth
increases, but the technology comes at an extravagant price
[6]. In addition, the effect of statistical multiplexing is
insignificant in multimedia communications environments
for WDM-PON. Even though WDM-PON has several
advantages over TDMA-PON, it has failed to attract
attention from industries because of its high cost. Now other
schemes for optical access network are being studied
worldwide.
Optical code division multiplexing access (OCDMA)
system has attracted increasing attention in recent years due
to the following advantages: asynchronous access capability,
accurate time of arrival measurements, flexibility of user
allocation, ability to support variable bit rate, busty traffic
and security against unauthorized users. OCDMA is a very
attractive multi-access technique that can be used for local
area network (LAN) and the first one mile [3].
We present a PON configuration based on OCDMA
technology in this paper. Our OCDMA-PON combines the
advantages of PON and OCDMA. The rest of the paper is
organized as follows. In Section II, we describe the
The performance of the system is
configuration.
investigated in detail in Section III. Section IV presents the
simulation results. We conclude this paper in Section V.
SYSTEM DESCRIPTION
Before introducing our OCDMA-PON, we give short
descriptions on the classical PON and the OCDMA
technology with their main properties. We then show how
the two are combined in our proposed OCDMA-PON.
II.
A. PON
A classical PON model is a next-generation optical
access network technology that deploys optical transmission
lines (fiber) between optical line terminator (OLT) and some
optical network units (ONU) and / or optical network
terminator (ONT). Passive optical splitter (POS) is used to
handle connection between OLT and ONU or ONT. The
OLT interfaces over the service node interface (SNI) to the
service nodes, and to the optical distribution network (ODN)
1851
which provides the optical transmission media between the
OLT to the users. The OLT broadcasts frames from services
nodes to all ONU or ONT via ODN by wavelength
multiplexing of 1.55,um wavelength in the downstream
direction and 1.31,um wavelength in the upstream direction
over a single fiber channel. The ONU receives video, audio
and data from the OLT and passes the data to the end-users.
B OCDMA
Since its birth, the OCDMA technology has attracted many
researchers' attention. The signature processing is limited to
optical field and the fiber-optic system acts as a non-negative
media, thus the signature sequences for the system are
constructed by l's and 0's. The address codes used in
OCDMA systems must be unipolar codes with good autoand cross-correlation properties. Optical orthogonal code is a
preferred code which satisfies these requirements, and it is
commonly used as the address code in OCDMA systems.
In this work, we assume that an optical orthogonal code
(OOC) C, denoted by a 4-tuple (v, k, ta,, >tc), is a family of
binary (0, 1) sequences of length v and weight k satisfying
the following two properties [3]:
1) The auto-correlation property:
v-1
('xx
t=O
<A,
fnr r=n
for O<r<v-1
sequence is re-processed via optical decoder from which the
users receive their desired information signal.
C OCDMA- PON
Fig. 1 depicts the schematic diagram of the proposed
OCDMA-PON system. The system is composed of OLT
and ONUs, based on OCDMA, and every ONU is identified
by its own code address. The presented schematic is
different from the classical PON in that OCMDA
technology is adopted in our system instead of TDMA or
WDM. The signal is modulated with not only frame
information but also address code sequence. The former is
used to accomplish data load switching and the latter is used
identify different users. The performance of the presented
system is analyzed in the next section.
-
(1)
for any X= (x0, xi, ..., xv ) E C and any integer Xc . 0
(mod v).
2) The cross-correlation property:
v-1
=
Zx,y±.2<A,
for
0<Tr<v -
Fig. 1 Schematic diagram of presented OCDMA-PON
(2)
t=O
for any Xe C and Ye C with X.Y, Xc is any integer.
OOC has mainly concentrated on the case when
a,=2c=A in the literature, in which an optical orthogonal
code is denoted by a 3-tuple (v, k, A), or briefly (v, k, A)OOC. ICI denotes the size of the code, i.e., the number of
code words contained in C. The largest possible size of the
(v, k, A)-OOC C is denoted by P (v, k, A). By the wellknown Johnson bound, we know that ' satisfies the
following inequality:
k(k-1). (k-k)
Using the above inequality, for a (v, k, 1)-OOC, if
Ic =L(v-f1t(k(k-1))]
Downstreamn
(4)
then C is called an optimal OOC.
After introducing the OOC for an OCDMA system, we
give short description of an OCDMA system in the following.
The system consists of optical pulse source, optical
en/decoders and detectors. At the transmitter, the signal is
modulated by optical source, and the carrier optical wave is
enclosed by OOC sequence at the optical encoder. At the
receiver, the optical pulse with the signal and the OOC
SYSTEM ANALYSIS
This section analyzes the performance of the system
presented in the previous section. Since the scalability of the
network is a key for network design, we focus on this issue
in this paper. The constraints for the network scalability
include the number of ONU/ONT and channel link length
which is affected by link's power budget, the number of
available codes and the cost of the system.
We assume that (1) the system supports users based on
OCDMA technology, (2) the joint of network deploys an
AWG which manages all ONUs/ONTs, and (3) every joint
has up to 64 ONUs/ONTs. Reference to [7], we can deduce
that the total number of network users Niotal and the number
ofjoint in the network Xji,,, have the following relationship
III.
X jiont =LAN totai/64 ]
(5)
In order for signal to be exactly detected, the downstream
traffic power budget must satisfy the following equation
(6)
P tr aADAX joint aFL a A WG a else R sen
The notations are explained in Table I.
Similar to the above, the upstream traffic power budget
must satisfy
1852
p
c log 2(S)-aADX joint
ou
(7)
a FL a AWG a else R sen
where aelse is the extra loss introduced by the en/decoder and
other optical components. Other parameters in the system
are listed in Table I.
As follow, the system transmission quality is analyzed.
Since the signal is encoded at the ONUs, and decoded at the
OLT, the signal encoded can be defined as
Since we take into consideration the system additive noise
power, the thermal noise 0th2, the beat noise bea,2 [8], the
relative intensity noise Yre2 and the link noise ujink , which are
given respectively by
2th
where M(co) is denoted as the encoding function, Sim(ow) is
the incident optical pulse, which are expressed as [9]
m (co)
=
F (H t(x)C,(t
(9)
00
EIXexp(-t<2)dSpT(t-IT)) (10)
Sin ()== F(Z
I=-oo
H1(x) is the transmission ratio of en/decoder at the ONUs or
OLT,
and
Cn(t) =
00
E X (n)
j=-o0
pT(t-jTi ) XJ (n)
is the
GOCs sequence set, P and d, are respectively the optical
power, data stream, p7-(t) reflects the spectral shape of optical
source.
TABLE I.
beat2 = s 2(2 + m 2) /(8p
J
re
07
lnk
+ 2R
Ptr
Pout
Rsen
L
C
aAD
AA WG
AF
S
Aelse
TC
description
System transmission power
Output of ONUs
Receiver sensitivity
Length of fiber link
filtering Index
Insertion loss of add/drop
Insertion loss of AWG
Propagation loss of fiber
Splitter's splitting ratio
Budget of the system
Chip duration
=
1
,-
i
eR85Ntoalpifr
(i7 sp(G -I)hvu) 1
(16)
G )2w
Jth +Tbeat +0re +0link
=
S
error
(17)
ratio (BER)
/T
(18)
Assuming the system noise is Gaussian-distribution, with
unipolar capacity, the corresponding BER is given by
BER
--e
2
.ota-l
ij=a Re m =oi [s(S en(t), C i(m))]
en(t),C j(M ))
hf ij / 7
where ac, h, and q is the scale factor index, Plank's constant,
optical frequency, APD quantum efficiency.
=
2
The signal to noise ratio (SNR) and bit
given as
SNR
Then the current ij at the detector, and the desired signal
S2 is educed as follows
S (S
(15)
reS 2Rb
m, Pc, Bs, 41e, Rb, s, pj fr, R, 'isp, GC
h,u, qa, and W are the Boltzmann constant, ration of the
equivalent receiver bandwidth to the signal bandwidth,
receiver noise temperature, chip duration, electric charge,
receiver load resistance, modulated index, the processing
gain, base-band signal bandwidth, PSD of the relative
intensity noise, ratio of data bit, response index of detector,
optical power per pulse, receiving bandwidth, response of
photo diode, spontaneous emission factor, gain of optical
amplifier, photo energy, quantum efficiency and optical
components bandwidth.
Thus total noise of the system c&2 iS given as
are
power
N
(14)
B s2)
22
=
2
C
value
5dBm
-4dBm
-4OdBm
5-20km
3
1dB
5dB
0.3dB/km
16-64
3dB
0. Ins
(13)
where kB, K, Ti, Tc, e, Re,
THE SYSTEM PARAMETER FOR SIMUNICATION
symbol
S
(7
(8)
S en(t) = F [ M ( )S in( )]
(2kKTrT) /(e 2Re)
rfc (- .SNR
/8 )
IV. NUMERICAL RUSULTS AND DISCUSSIONS
In this section, we present our numerical results for
proposed OCDMA-PON analyzed in the above section.
(19)
our
First, we show the maximum achievable length of the
fiber link as a function of the number of ONUs / ONTs. The
result based on formula (5)-(7) is shown in Fig. 2. The
maximum length of the fiber link that the system can support
is shortened with increased number of ONUs / ONTs in the
networks, and the length can be increased with increased
input optical power. For example, when the input power is 5
mW, the network can support over 300 ONUs / ONTs up a
fiber link length of 25 km, while when the input power is
reduced to 4 mW, the network can support only 200 ONUs /
ONTs at the same fiber link length. From the plot, we also
1853
observe a step per about 64 ONUs/ONTs which comes from
formula (5).
there is about 2dB penalty at input power p= -lOdBm, for
BER= 10-9, the former must be - 12dBm for input power,
while the latter must be -14dBm. So we take in consider
design of OCDMA-PON system, noise affect the system that
is neglected.
35
u-1
-J
P=4mW
r) P=5mW
0~~~~~~
30
fse
c
49
L-
E
u1
Y
-0
0
25
0
0
0
D
20
L)~~
D
CD
a}
fD0
C
_
D
9
4-J
_
0
C)
,@,
c
500
200
300
400
Number of ONUs / ONTs
5
30
|
-~~~~~
_~~~~~~~~~
itk
D
2
4
6
8
10
Number of Xjoint
12
144
Fig.4 The length of fiber link and number of number of XjAi,n for two
different signal power levels
100
7
10-
lassical PON
OCDMA-PON
cl
0
10
Fig. 3 compares the maximum achievable length of the
fiber link as a function of the number of XjAi,, for the classical
PON and our proposed OCDMA-PON system. From the
plot, our proposed OCDMA-PON clearly has a better
performance than the classical PON.
A
,').
0
Fig.2 The length of fiber link as a function of the number of ONUs
/ONTs for two different signal power levels
321
0
A
w
m
100
-
A
7
a)
E 28
io.
lie
c 26
v~~~
w_ 1 02
A
24
0
1 0 .o
1
22
a) 20
1 o-251l
-2C
18
bu
0
2
4
6
8
10
Number of Xjoint
12
v
A,with interference
back to back
-15
-10
Input power (dBm)
-5
0
Fig.5 The BER and input power for two system
14
Fig.3 The length of fiber link and number of XjAi>n for a classical PON
and OCDMA-PON
V.
Fig.4 shows the maximum achievable length of the fiber
link as a function of the number of Xj0i, for two different
input power of 4 and 5 mW with our proposed OCDMAPON. The effect of increasing the fiber link length by
increasing input power is clearly seen in the plot. Both Fig.
3 and Fig. 4 indicate that the length of fiber link must be
reduced with increased number of Xj,i,,.
In Fig. 5, we show the BER as a function of the input
power for both back-to-back configuration and the case with
interference. The system based on the case with interference
(with noise) and back to back is considered. the system with
interference is very worse than the back to back system, just
CONCLUSION
We present and analyze an OCDMA-PON system, even
though which is composed of an OLT at a center office and
ONU or ONT at user terminator, a PON is vitally different
from the OCDMA-PON, up/downstream in OCDMA-PON
is deal with by en/decoder to take with 0OCs, and there are
some advantages of PON and OCDMA technology. In the
paper, a classical PON and OCDMA scheme are introduced,
and an OCDMA-PON is presented. The performance of the
system is analyzed in detail, including the scalability of the
network, i.e., number of ONUs / ONTs and number of
network joint, and the relation of the system BER and
number of ONUs / ONTs for different input power. The
simulation results show that The maximum length of the
1854
fiber link that the system can support is shortened with
increased number of ONUs / ONTs in the networks. The
proposed OCDMA-PON clearly has a better performance
than the classical PON. The effect of increasing the fiber
link length by increasing input power. So the system is
logical and feasible.
ACKNOWLEDGMENT
The authors would thank to X. J. He, X. Wang, H. B.
Cheng, Ch. Li, and Y. Ling for discussion and suggestion.
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