Proceedings of 2014 RAECS UIET Panjab University Chandigarh, 06 – 08 March, 2014

978-1-4799-2291-8/14/$31.00 ©2014 IEEE

LINK LENGTH AUGMENTATION OF OPTICALLY CODED MULTI-USER NETWORK BY USING ELECTRONIC

EQUALIZATION TECHNIQUE

Pankaj Sharma1, Sandeep Kaushal2, Anurag Sharma3, Jagjit Singh Malhotra4

Department of E.C.E 1, 2, 3, 4

Amritsar College of Engineering & Technology, Amritsar Punjab, India1, 2

CT Institute of Engineering, Management & Technology, Jalandhar, Punjab, India 3 D.A.V Institute of Engineering & Technology, Jalandhar, Punjab, India 4

er.pankajsharma@rocketmail.com1,er.anurags@gmail.com3

Abstract Dispersion in an optical fiber is one of the

major limitations which affect its data carrying

capacity and causes overall performance

degradation. In this paper, Electronic

Equalization Technique (EET) has been

employed in a multiuser Optical Code Division

Multiple Access (OCDMA) network to

compensate the chromatic dispersion. It has

been observed that in the proposed network the

Bit Error Rate (BER) is significantly reduced

by e-16 and Q2db factor reported is 19dB with

EET.

Keywords- BER; EET; ISI; OCDMA; SNR

I. Introduction With the massive boom in information technology

and telecommunication, ultra high bit rate

communication systems are becoming very

popular and OCDMA technology has emerged as

one of the promising and achievable methods

employed in time and wavelength based multiple

access methods for the high speed fiber optics [1]

[2]. OCDMA system is popular due to the

availability of excess bandwidth in the fiber optic

medium as multiple users can send their data on a

single optical cable by various techniques. But, at

high data rates, the Inter Symbol Interference (ISI)

is generated due to the effect of dispersion of

optical signal [5][ 6]. This dispersion can be of any

type like chromatic dispersion, polarization

dispersion and electrical dispersion. The main

focus in case of optical communication is to negate

chromatic dispersion. Researchers have proposed

many solutions to mitigate the effect of dispersion

but with certain pitfalls within as these are

generally focused on one kind of dispersion and

complex to execute in existing network [5].

The solution of these problems is Electronic

Equalization Technique (EET). The EET focuses

on all types of dispersions and is easy to

implement. The EET networks are typically based

on linear equalizers (LEs) and Non-Linear

equalizers (NLEs) [7]. The Linear equalizers are

generally based on feed forward equalizers (FFE)

and non linear equalizers are based on decision

feedback equalizers (DFE). In EET, digital signal

processing is employed to self-adjust the feed

forward and feedback taps in order to achieve the

pre and post compensation. The feed forward taps

helps to mitigate the effect of precursor ISI

whereas the feedback taps are used to remove the

effect of post cursor ISI. The two algorithms

generally applied for EET are Maximum

Likelihood Sequence Estimation (MLSE) and

Minimum Mean Square Error (MMSE) [7]. MLSE

is effective in presence of Self Phase Modulation

(SPM) and generally a digital signal approach,

whereas, MMSE is more suitable for long haul

optical communication links.

The goal of this paper is to minimize the effect of

chromatic dispersion over a multi-user OCDMA

network by adjusting the feed forward taps and

feedback taps in a optimal manner. The paper is

organized as follows: simulation setup of OCDMA

network with EET is outlined in section II and the

section III is focused on the results and discussion

followed by conclusion in section IV.

II. Proposed Design

The proposed OCDMA based, 24 user, network

model using electronic dispersion compensation

is shown in figure 1. It consists of 6 closely

spaced optical signal wavelengths with narrow

inter-channel spacing of 0.4 nm ranging from

1550 nm – 1552 nm and on every wavelength

four different Orthogonal Optical Codes (OOCs)

have been employed by using various codes

generators.

Continuous Wave (CW) laser produces one or

more optical signal outputs commonly used with

external modulator to encode the binary data

signal upon CW laser source. Optical WDM

multiplexer (combiner) accepts multiple optical

signals at its input and generate a single

stream which includes all the input WDM

optical signals The taps of Feed forward and

feedback in EET is adjusted to obtain optimum

performance and then the same OOC are used to

extract data from modulated signal. Link length

has been fixed at 100 km. Signal Analyzers, Eye

Diagram analyzers, Spectrum analyzers and BER

testers have been employed to gauge the

performance of proposed network.

Figure 2 shows a close look of OCDMA network

with EET module which has been used to

compensate the chromatic dispersion. The

functioning of this module is based on FFE and

DFE to negate the accumulated dispersion in

electrical domain. It uses both linear and non-

linear equalization to mitigate the effect of

chromatic dispersion.

Figure 1. Simulation setup for 24 users OCDMA network with EET

Figure 2. A close look of EET in simulation setup

III. Performance Analysis The performance of proposed network with

application of optical dispersion compensation

has been deliberated. Even after implementation

of optical dispersion compensation, significant

inter symbol interference (ISI) is seen in the

signal which limits the performance of the

system. Electronic Equalization Technique is an

effective way to nullify the dispersion in electrical

domain. As shown in figure 1 and figure 2,

MMSE based EET is applied after the receiver to

further increases the performance of the system.

EET used based on FFE and DFE. Using digital

signal processing, the performance of FFE- DFE

has been optimized on the basis of feed forward

and feed backward taps. The output has been

recorded on sample basis from Node-1 and Node-

24, which is first and last node respectively.

The values of BER reported at receiver side with

& without using EET and corresponding effect on

Q2dB factor has been tabulated in table-I.

The performance of proposed system has been

enhanced and optimized by varying the number of

taps of FFE and DFE, we compared the various

BER values to define an optimal point for both

feed forward and feedback in OCDMA network.

Table II represents the various achieved values of

BER at different taps-values in feed forward and

feedback respectively. It is evident here that the

minimum BER of the order of 1.07×e-16 has been

reported when feed-forward-tap value is 10 and

minimum BER of 6.62×e-16 has been recorded

when feed-backward-tap value is 7.

The figure 3 represents the Eye Pattern of received

signal at node 1 and node 24 without EET and

with EET.

From figure 3 we can clearly see the effect of

inter symbol interference is reduced and clear

eye formation is achieved when the EET is

applied in OCDMA network.

Table I: Performance parameters of received signal with and without EET

Table II: Effect on BER by changing the feed forward taps and feed backward taps

Figure 3 Eye Diagrams comparison of Node 1 and 24 with and without EET

IV. Conclusion

It has been observed that by using EET, there is a

significant improvement in eye-opening, BER and

Q2(dB) as it minimized the effect of ISI and

Chromatic dispersion. Optimal performance of the

network viz. BER of the order of e-16 and Q2 (dB)

as high as 19dB is reported with values of FFE and

DFE as 10 and 7 respectively.

References [1] Anurag Sharma, Vikrant Sharma, Dalvir Kaur and H.P Singh, “Performance Enhancement of 16 Channels Back Haul DWDM OADM Ring Network using Electronic Dispersion Compensation” AICECE, Kingston, CANADA IEEE Proceedings, Vol. 1, Page 150-153, May 2013 [2] M. Ravi Kumar, S.S Pathaky and N.B Chakrabarti,“ A New Multi Wavelenght- Optical code division multiple access code design based

S. No. Parameter Without EET With EET User 1 User 24 User 1 User 24

1 BER 1.23×e-10 1.78×e-12 6.62×e-14 4.68×e-16

2 Q2dB 18 dB 16dB 19 dB 17dB

S. No.

Feed Forward Feed back

No. of Taps BER No. of Taps BER

1 6 5.15×e-11 3 2.11×e-8

2 8 1.47×e-13 5 1.07×e-14

3 10 1.07×e-16 7 6.62×e-16

4 12 4.68×e-14 9 5.37×e-13

5 14 1.93×e-9 11 1.57×e-11

on balanced incomplete block design” IEEE transactions on communication vol.55 no. 2, Feb 2007. [3] M.Irfan Anis, Naveed Ahmed and Saifuddin, “Design and performance analysis of OCDMA system using different filters” IEEE transactions on communication, Feb 2009 [4] Ivan Glesk, Tululope B. Osadola, Siti K. Idris, Kensuke Sasaki and Gyanshewar C, Gupta, “Evaluation of OCDMA system deployed over commercial network Infrastructure” IEEE Tu.C5.4 ICTON, pp1-4, April 2011 [5] Parambir Singh, Manoj Kumar, Anurag Sharma, “Design and Performance investigation of multiuser OCDMA network” International journal of Scientific & Engineering Research, Vol. 4, Issue 7, pp 2549-2552, July 2013 [6] Pankaj Sharma, Sandeep Kaushal and Anurag Sharma, “To analyze the performance of various digital filters in OCDMA multiuser environment with 3D codes” International journal of Electronics and Communication Engineering &Technology, Vol. 4, Issue 5, Page 80-89,Sep 2013 [7] Mahdi Karimi and M. Nasiri-Kenari, “An internally coded TH/OCDMA scheme for fiber optics communication system and its performance analysis- part 1: using optical orthogonal codes” IEEE transactions on communication. Vol. 55,no. 2,pp333-344,Feb. 2007.

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