Download ps3.e.7. all-optical fo-cdma network

ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS
CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009
ALL-OPTICAL FO-CDMA NETWORK: PERFORMANCE ANALYSIS
E. S. Shivaleela S. Ganesh and T. Srinivas
Indian Institute of Science, Bangalore, India-560012.
[email protected], [email protected] , [email protected]
Abstract: Optical CDMA technology is being explored as a multiplexing technology for high speed access networks, with
optical fibre as transmission medium and also processing ie. the spreading/dispreading of the CDMA sequences. For fibre-optic
code division multiple access (FO-CDMA) scheme to be a practically implement able one as a high speed access network, two
main challenges are, design of (1) codes with high spectral efficiency and cardinality and (2) high speed encoders/decoders
which are simple and efficient to implement. As a solution to the first requirement, we have formulated wavelength/time singlepulse-per-row (W/T SPR) codes which are energy efficient and of weight/row WP = 1, for a given weight of the code W, in
addition have good 1) cardinality 2) spectral efficiency and minimal 3) correlation values, for given wavelength and time
dimensions. In this paper, we have simulated W/T SPR network and also Optical orthogonal codes (OOC) network using a
commercially available optical simulation package.
Keywords: Fibre optics communication, optical code division multiple access, incoherent optical communications,
wavelength/time codes
I.
INTRODUCTION
Spread sequences1 used in FO-CDMA use ultra short
pulses which are prone for nonlinear effects in optical fibres.
To overcome this problem of one dimensional codes in FOCDMA, several types of encoding of the CDMA sequences
such as frequency-hopping (FH), time-space (T/S) and W/T
have been proposed. FH codes2 are not suitable for FOCDMA3, as systems using Frequency-Encoding CDMA
technique suffer from optical beat noise that appears between
the frequency slices at the photo detector 4, 5.
Simulation results show that T/S systems are limited by
the skew in associated ribbon fibers6, 7. W/T coding of the
CDMA sequence is suitable and several W/T codes have
been proposed, either by crossing one type of sequence with
another such as prime-hop8, and extended quadratic
congruence/prime9 (eqc/prime) or converting 1-D sequences
to 2-D sequences.
Mendez et al. 10 have described the design of pseudoorthogonal codes based on folding of optimum Golomb rulers
and these codes have arbitrary cardinality. Yang and Kwong 11
have constructed 2-D Optical Orthogonal signature pattern
codes (OOSPC) from 1-D OOCs. The cardinality and
correlation properties of OOSPCs are same as that of 1-D
OOCs. Raymond12 et al. have described a depth-first search
algorithm for the generation of W/T 2-D codes, which have
unit out-of-phase autocorrelation and peak cross correlation.
We reported earlier the design of Addition Modulo
L_T (AML) codes13, where L is the temporal length. AML
codes are single-pulse-per-row (SPR) codes with and unit
peak cross correlation values. As T/S systems are limited by
the skew in associated ribbon fibres, AML codes can be
encoded in W/T dimensions instead of T/S, where wavelength
encoding is done in place of space. AML codes encoded in
W/T are called W/T SPR codes. We have also reported the
basic principles of W/T MPR. We report in this paper, the
simulation of OOC and also W/T SPR networks for 4 users.
In Figure 1 the schematic diagram of the all-optical
CDMA network with optical encoding of W/T SPR codes is
shown. Wavelengths derived from a laser source are given to
an external modulator. A data source (PRBS) modulates a
NRZ pulse generator, which in turn is used as the control
signal in external modulator, Mach-Zhender interferometer
and thus the amplitude modulated signal is obtained. Optical
encoding of the OOCs and W/T SPR codes is done using
optical delay lines. The AM signals from all the users in the
broadcast network get combined in the passive star.. On the
receiver side after demultiplexing the wavelengths,
corresponding inverted (optical) delay lines (which functions
as a matched filter) are used to recover the desired signal at a
receiver.
II. SIMULATION
We verify the autocorrelation and cross correlation
properties of OOCs and W/T SPR codes by simulation using
optisys commercial software. The four
OOC codes are
shown below Table 1 which is a representation of the actual
codes. The simulation has been run for number of users
ranging from 1 to 4. The sequence length has been taken as
16 and 32 for all the OOC codes. The weight has been chosen
as 2, temporal length = 16(case 1) or 32 (case 2) for all the
users.
Table 1: The number in bracket indicates the position of optical
pulse in corresponding chips in length of 16 and 32.
(1,2)
(1,5)
(1,8)
(1,11)
Similarly, the code description for 2 D codes is
explained below. The two W/T SPR codes, in the Table 2 are
ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS
CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009
chosen. Each row of a code is encoded by distinct 2; the number of coincidences between the pulses is greatly
wavelengths, which is equal to R and the columns represent reduced. So by increasing the temporal length, the Bit Error
the time, L. The 1s in code (1} and code (2}, in the Table 2 Rate (BER), reduces.
indicate the presence of optical pulses.
Compare Figure 2.B and D, for number of users
equal to four and code weight equal to two, temporal length is
Table 2: W/T SPR codes 1 and 2 with R = 2, L = 8.
increased from 16 to 32, and similar observations are made.
Compare Figure 2.C and D, for temporal length is
λ1
1 0 0 0 0 0 0 0 λ1 1 0 0 0 0 0 0 0
equal to 32 equal and code weight equal to two, when number
λ2 1 0 0 0 0 0 0 0 λ2 0 1 0 0 0 0 0 0
of users are increased from 2 to 4, the received signal has
In Figure 1 schematic diagram of All Optical (AO) more noise due to the increased number of interfering users.
CDMA network, for W/T SPR codes is shown. Two So for the given number of users, by increasing the temporal
transmitters, of different wavelengths of 1 mw power, length, the Multiple Access Interference (MAI) can be
operating at 1 Giga bits per second, for different rows are used, reduced, for better performance. In Figure 3.A and B the auto
whereas for OOC, it is one transmitter of wavelength 1550 nm and cross correlation plots are obtained respectively, for W/T
SPR codes. Further a detailed study is under progress on
and 1550.1 nm.
The optical pulses from two transmitters are passed similar lines as that of OOCs codes and to compare the
through different fibre delay lines, depending on the pulse performances of 1D and 2D codes.
sequence to be produced. Then the sequences from two rows
are multiplexed and sent over the channel. The signals after
passing through the broad cast channel are demultiplexed and
then fed to matched filters (inverted delay lines of the
encoder), to obtain the required signal (despread signal). The
unmatched signal will appear as noise (spread signal). The
received signal from matched filter is photo detected to get the
data in electrical signal. The electrical signal is analysed in
Eye Diagram Analyzer.
( A ) L = 16 , N = 2
Figure 1: Schematic diagram of the all-optical CDMA network with
W/T SPR code.
In the plots shown in Figure 2, we have compared
OOCs for different parameter variations. In Figure 2. (A)
shows the received signal for two users, code length of 16 and
weight equal to 2.
In Figure 2.B, shows the received signal for four
users, code length of 16 and weight equal to 2. In Figure 2.C,
shows the received signal for two users, code length of 32 and
weight equal to 2.
In Figure 2.D, shows the received signal for four
users, code length of 32 and weight equal to 2. Compare
Figure 2.A and C, we see that when temporal length is
increased from 16 to 32 for the same number of users equal to
( C ) L = 32 , N = 2
Figure 2. (A) (B) (C) (D):
( B ) L = 16 , N = 4
( D ) L = 32 , N = 4
W=2 for OOCs codes.
( A ) Auto Correlation
( B ) Cross correlation
Figure 3: L = 8 , W =2 for W/ T SPR codes.
ICOP-2009: INTERNATIONAL CONFERENCE ON OPTICS AND PHOTONICS
CHANDIGARH, INDIA, 30TH OCT – 1ST NOV.2009
8. L. Tancevski and I. Andonovic, "Wavelength
Hopping/Time Spreading code division multiple access
III. SUMMARY
systems,"
One of the challenges of FO-CDMA to be practically Electron. Lett., vol.30, No.17, pp.1388-1390, Aug 1994.
successful in high-speed access networks is the availability of
codes with good cardinality and spectral efficiency. W/T SPR 9. L. Tancevski and I. Andonovic, "Hybrid Wavelength
is one such family of codes suitable for FO-CDMA networks. Hopping/Time Spreading Schemes for Use in Massive Optical
Networks with Increased Security", IEEE/OSA J. Lightwave
We have analysed the Optical CDMA network for
Technol.,
vol.14, No.12, pp.2636-2647, Dec 1996.
OOCs codes in detail and verified the autocorrelation and
cross correlation properties of W/T SPR codes, by simulation
using optisys, and shown to be minimal for unipolar codes, 10. A. J. Mendez, R M. Gagliardi, V. J. Hernandez, C. V.
which is desired to keep multiple access interference low in Bennett, and W. J. Lennon, "Design and Performance
broadcast networks. As a result of which, better performance Analysis of Wavelength/Time (W/T) Matrix Codes for
is obtained in broadcast networks, wherein MAI is the main Optical CDMA", IEEE/OSA J. Lightwave Technol., Special
Issue on Optical Networks, vol.21, No.11, pp.2524-2533, Nov
cause for errors.
2003.
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