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Chapter 12 Optical CDMA Network
12.1 The Main Concerns
12.2 Fiber-Optic Code-Division
Multiple-Access
12.3 Code Families for Optical CDMA
• 12.3.1 Walsh-Hadamard Codes
• 12.3.2 Binary M-sequence Codes
12.4 FBG-Based Optical CDMA Codecs
12.5 AWG-Based Optical CDMA Codecs
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12.1 The Main Concerns
Figure 12.01 Critical
factor limiting TDM
performance.
 The main concern for users of the new DWDM system
is its reliability and stability over time. We can
examine the critical factors limiting the performance
of a time-domain multiplexing (TDM) system by
placing them on a two-dimensional power-versus-time
representation.
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12.1 The Main Concerns
 On the power axis, tile critical issues are laser power,
fiber attenuation, and component loss.
 On the time axis, the critical factors are fiber PMD,
chromatic and (for multimode fibers) modal dispersion,
as well as signal jitter and transmission rate.
 At the junction of the power and time axes, new factors
come into play: laser modulation depth, fiber nonlinearity, relative intensity noise (RIN), and bit error
rate (BER).
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12.1 The Main Concerns
 WDM adds wavelength as a new dimension and
complicates considerably the representation of the
critical factors (Figure 12.02) .
Figure 12.02 Addition of
the wavelength dimension
in WDM system.
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12.1 The Main Concerns
 On the wavelength axis, critical elements as spectral
stability, EDFA spectral range, central wavelength,
and bandwidth.
 At the corner of wavelength and time, we encounter
laser chirp, chromatic dispersion, stability of the optical
frequency, and phase noise (self-phase modulation and
cross-phase modulation).
 At the corner of wavelength and power, we find EDFA
amplified spontaneous emission (ASE), EDFA gain,
crosstalk, four-wave mixing, and stimulated Raman
forward scattering. Where all three axes meet, we
encounter stimulated Brillouin backscattering.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 With rapid increases of utility rate in optical
communications nowadays, the capacity and the
security of one optical system have become a critical
issue.
 The development of optical CDMA (OCDMA)
technique is such attractive since it is possible that
allows multiple users in a local area network (LAN)
environment to access the same fiber channel
asynchronously at all times.
 Besides, CDMA systems also offer a security
advantage over other multiple access systems.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 In FO-CDMA, data of the users is spread by a
signature sequence, which uses to distinguish the
desired user from other users.
 Major CDMA techniques are direct-sequence (DS),
time-hopping (TH), frequency-hopping (FH) and
frequency-encoded (FE).
 Most of OCDMA systems previously proposed are DS
systems, in which short optical pulses are needed. In
the case, the dispersion should be taken into
consideration and the sequences used can’t be changed
dynamically.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 In order to avoid the dispersion, TH or FH patterns
are on the basis of different construction of codes
that represent signature sequences.
 As shown in Figure 12.1, time-hopping technique
divides a data bit slot to several chip slots and
represents the code by the slot position of pulse.
 On the other hand, frequency-hopping technique
represents the code by the wavelength it uses and the
similar example is shown in Figure 12.2.
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12.2 Fiber-Optic Code-Division
Multiple-Access
(01320)
Time
slot
1 0 0 0 0
0 1 0 0 0
0 0 0 1 0
0 0 1 0 0
1 0 0 0 0
Fig. 12.1 Graphical representation of an appropriate
optical code (time hopping pattern).
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12.2 Fiber-Optic Code-Division
Multiple-Access
( 0 1 4 2 )
Wavelength
λ
4
λ
λ
λ
3
2
1
Time
Figure 12.2 Graphical representation of
frequency hopping code pattern.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 Besides, signature sequences used simultaneously in
a communication system should be mutually
orthogonal or pseudo-orthogonal or have good
autocorrelation and cross-correlation properties.
 Furthermore, in order to utilize the large bandwidth
offered by optical fiber channels, the encoding and
detection operations must be performed optically to
circumvent the speed limitations of electronic
systems.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 The FE-CDMA is similar to the DS-CDMA, but the
coding is done in the frequency domain while in the
DS-CDMA, the code multiplies the modulation signal
in the time domain.
 Different wavelength represents different element in
the codeword, and the presence of wavelength means
“1” while the absence of wavelength means “0”.
 A new class of signature sequences known as optical
orthogonal codes (OOCs), which has low off-peak
autocorrelation and cross-correlation has been
proposed for such systems.
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12.2 Fiber-Optic Code-Division
Multiple-Access
Electrical
or Opt ical
Data
source
#1
Optical
CDMA
Decoder
Electrical
or Opt ical
Data
Recovery
#1
Optical
CDMA
Decoder
Data
Recovery
#N
Optical
Optical
CDMA
Encoder
NxN
Optical
Star Coupler
Data
source
#N
Optical
CDMA
Encoder
Optical
Fig. 12.3 Schematic diagram of FO-CDMA system with all-optical
encoder and decoder connected in a star configuration.
 Asynchronous multiplexing schemes are more efficient
than synchronous ones.
 Figure 12.3, a typical FO-CDMA communication
system is described each receiver correlates its own
address code with the received sequence.
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12.2 Fiber-Optic Code-Division
Multiple-Access
 Selecting the appropriate code sequence will maximize
the autocorrelation and minimize the cross-correlation
function and then the receiver would be able to
distinguish the correct address.
 In order to extract data with the desired optical pulse
sequence, we therefore have to design sequences that
satisfy some conditions：
 Each sequence can be distinguished from a shifted
version of itself easily.
 Each sequence can be distinguished from (a
possibly shifted version of) every other sequence
in the same set.
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12.3 Code Families for Optical CDMA
 In most of the earliest proposed OCDMA systems,
OOCs were popular with good autocorrelation and
cross-correlation properties.
 The use of OOCs makes a large number of
asynchronous users to transmit information
efficiently.
 OOCs also have applications in mobile radio,
frequency-hopping spread-spectrum communications,
and radar-sonar signal design.
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12.3.1 Walsh-Hadamard Codes
 Walsh-Hadamard code is obtained by selecting as
codes the row of a Hadamard matrix.
 All rows except one contains N/2 zeros and N/2 ones.
This code can be constructed using the iterative
procedure:
H1  1
 H1
H2  
 H1
H1  1 1 
  1 0
H1  

H
2 n 1
H 2n

 H 2 n


H n
2 
H
2n
where H n is derived from Hn by replacing all entries
with their complements.
For example：
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1
1
H4  
1

1
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1
0
1
0
1
1
0
0
1
0

0

1
H4
0
0

0

0
0
1
0
1
0
0
1
1
0
1

1

0
12.3.1 Walsh-Hadamard Codes
 Taking X as being row i and Y as being row j with i  j ,
it is easy to verify that
Rxy k   Rxy k   0
 We can apply this result to design our proposed
receiving decoder for balanced detection for an N x N
Hadamard matrix, N - 1 subscriber can be
accommodated since the codeword containing all 1’s
has to be rejected.
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12.3.2 Binary M-sequence Codes
 Maximal-length sequences (M-sequences) were
used mainly because of their excellent periodic
autocorrelation properties.
 The cross-correlation properties of such sequences
are as important as autocorrelation properties.
 Consider the sequence Y as being Y = TkX, where
Tk is the operator that cyclically shifts the
sequence X = (x0, x1, …, xN-1). For example, TX =
(x1, x2, …, xN-1, x0), T2X = (x2, x3, …, xN-1, x0, x1),
and so on.
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12.3.2 Binary M-sequence Codes
 The periodic cross-correlation of sequences X and Y is
RXY k  
N 1
 xi xi  k
i 0
RXY k    N  1 2 ,
RXY k    N  1 4 ,
for k  0
for k  1 ~ N  1
Note that the sum i + k is taken modulo N.
 In other words, half the 1’s in TkX coincide with the 1’s
of X while the other half coincides with the 0’s.
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12.3.2 Binary M-sequence Codes
 We can compute correlation difference
Rxy k   Rxy k  

N 1
 xi xi  k

 xi xi  k

i 0
N 1
i 0
N 1
 xi xi  k
i 0
N 1
 1  x i x i  k
i 0
 2 Rxy k   Rxy 0 
 N 1


2

0

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,
,
for k  0
for k  0
12.3.2 Binary M-sequence Codes
 We can apply this result to design our proposed
receiving decoder for balanced detection in the
OCDMA network.
Rxy k   Rxy k  will be used in our CDMA systems.
 By assigning the N cyclic shifts of a single M-sequence
to N subscribers and we have a network that can
support N simultaneous users without any interference.
 We have therefore obtained complete orthogonality
between each user in the OCDMA network.
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12.4 FBG-Based Optical CDMA Codecs
 The fiber-optic CDMA encoder/decoder is structured
with the spectral coding of incoherent light sources by
fiber gratings.
 The signature address code imposed on the data
encoder in the transmitter end should be matched to
the signature code of the decoder in the receiver end.
 The coded information sequences are then linked to a
common star coupler, and are shared with the same
channel bandwidth.
 The whole FO-CDMA system is as shown in Fig. 12.4.
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12.4 FBG-Based Optical CDMA Codecs
Decoder#1
Encoder#1
LED
EOM
user#1'
user#1
ASE
EOM
user#2'
KxK
star
coupler
user#2
Encoder#K
SLD
Decoder#2
Encoder#2
Decoder#K
EOM
user#K'
user#K
Fig. 12.4 Fiber-optic CDMA network.
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12.4 FBG-Based Optical CDMA Codecs
 The encoders (Fig. 12.5) consist of a series of fiber
Bragg gratings.
 The broadband optical wave is directed to a fiber
Bragg grating. If the central wavelength of the
narrowband pulse of the incoming optical wave is
equal to the Bragg wavelength, it will be reflected
by the FBG, or it will be transmitted.
 With a proper written CDMA coding pattern, the
reflected light field from FBG will be spectrally
encoded onto an M-sequence address code.
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12.4 FBG-Based Optical CDMA Codecs
Input spectra
Fiber Bragg Grating
1  2  3  4  5  6  7

FBG
transfer function

Light source
spectrum


Coded
spectral chips
1 2 3 4 5 6 7
Fig. 12.5 FBG-based optical CDMA encoder.
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12.4 FBG-Based Optical CDMA Codecs
 The incoherent light sources used in encoder module
include Edge-emitting LED, Superluminescent
Diodes (SLD), and Erbium-Amplified Spontaneous
Emissions (Er-ASE).
 These light sources provide broad spectrum, high
transmitted power, less driving circuits, low
temperature sensitivity, and low cost due to high
yields.
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12.4 FBG-Based Optical CDMA Codecs
 The decoder architecture in the receiver is as shown
in Fig. 12.6, which consists of a star coupler, a
matched series of FBGs, a balanced photo-detector,
and a decision circuitry unit.
 Every decoder needs to extract the desired sequences
from the interference of other users. The decoder can
reject the multiple-access interference (MAI) coming
from other users. It decreases the data error rate,
and thus increases the system performance of the
optical CDMA.
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12.4 FBG-Based Optical CDMA Codecs
Reflected chips
Received spectra
from star coupler
Transmitted
chips
7 6  5 4  3  2 1
Fig. 12.6 FBG-based optical CDMA decoder.
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12.5 AWG-Based Optical CDMA Codecs
 An optical CDMA encoder/decoder over arrayedwaveguide grating (AWG) routers is configured with
m-sequence coded spectral- amplitude of broadband
light sources. In such OCDMA system, each user
shares the same AWG routers in the transmitters and
receivers.
 The signature address imposed on the transmitencoder should be matched to the signature of the
receiver-decoder.
 The coded information sequences are linked to a
common star coupler to share the same channel
bandwidth.
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12.5 AWG-Based Optical CDMA Codecs
 In Fig. 12.7, the broadband optical signals of each user
are directed to the corresponding input port of the
grating router.
 M-sequence signature will determine the links between
AWG router and the star coupler.
Encoder
User#1
User#2
AWG
N×N
User#3
router
star
coupler
N×N
User#N
Fig. 12.7 AWG-based optical CDMA encoder.
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12.5 AWG-Based Optical CDMA Codecs
 The receiver-decoder architecture is as shown in Fig.
12.8.
 Each user has a balanced photo-detector and a
decision circuitry unit connected to a pair of
common grating routers.
 User’s M-sequence signature and complement codes
are adopted to determine the links between the star
coupler and the upper/lower AWG routers.
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12.5 AWG-Based Optical CDMA Codecs
Decoder
Upper
AWG
router
N×N
User#1'
Lower
AWG
router
N×N
User#N-1'
User#2'
N×N
star
coupler
User#N'
Fig. 12.8 AWG-based optical CDMA decoder.
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12.5 AWG-Based Optical CDMA Codecs
 The operating principle of the AWG- based OCDMA
coder/decoders is as illustrated in Fig. 12.9.
 The grating router demultiplexes the incident
wavelength into all the output ports, and the same
wavelength signals that are incident from different
input ports will go to different output ports in a
cyclic manner.
 By utilizing the cyclic property of m-sequence code,
we can obtain all users’ spectral chips in the selected
output ports, which are designed to be of different
central wavelength.
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12.5 AWG-Based Optical CDMA Codecs
(1)
1
(2)
1
(1)
(1)
2 . . . N
(1)
1
(2)
2
3
(2)
(2)
2 . . . N
(1)
(2)
(N)
2 . . .  N
2
(1)
(2)
(N)
3 . . .  1
N
(2)
(N)
1 . . .  N - 1
1
(1)
(2)
(N)
N
2
. . .
. . .
4
(1)
(N)
Fig. 12.9 Operation principle of waveguide grating routers.
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12.5 AWG-Based Optical CDMA Codecs
 In the receiver end, the decoder can reject the
multiple-access interference (MAI) coming from
other users and decreases the data error rate, thus
increases the system performance of the optical
CDMA.
 In addition, the total system is compact and low cost,
thus can be upgraded to larger capacity easily.
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