effects of dispersion & mai on optical codedivision multiple access systems

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International Journal of Advances in Electrical and Electronics Engineering 283

Available online at www.ijaeee.com & www.sestindia.org ISSN: 2319-1112

ISSN: 2319-1112 /V2N2:283-291 IJAEEE

Effects of Dispersion & MAI on Optical Code

Division Multiple Access Systems

Irfan Ali 1 ,Ankit Agarwal 2

M.Tech. Scholar1,Department Of Electronics and Communication, Jagannath University, Jaipur (India)

Assistant Professor2,Department OfElectronics and Communication, BMIT (East), Jaipur (India)[email protected] [email protected]

ABSTRACT - Optical Code Division Multiple Access (OCDMA) is an optical processing system which allows multiple

users to share the same bandwidth simultaneously without interfering with each other using unique optical codes. As

the number of user increases, the dispersion and multiple access interference begin to rise, which led to high bit error

rate and low quality of service of the system. This paper presents the effect of Dispersion and Multi Access

Interference (MAI) of optical fiber on the Bit Error Rate (BER) performance of a Direct Sequence Optical Code

Division Multiple Access (DS-OCDMA) network. By using Matlab simulations, Signal-to-Noise Ratio (SNR) versusReceived Optical Power (ROP) of an OCDMA transmission system can be evaluated for different numbers of system

users. Mat lab simulations can be performed in order to illustrate the reduction of the dispersion index gamma, or to

visualize different scenarios, e.g., what amount of transmitted power is required in order to obtain a BER of 10 -9

when the length of the optical fiber is increased.

Keywords:OCDMA, BER, MAI, OOC, ROP, SNR

I. INTRODUCTION

Fiber optics is a particularly popular technology for local area networks. In addition, telephone companies are

steadily replacing traditional telephone lines with fiber optic cables. In the future, almost all communications

will employ fiber optics. Multi- access techniques are required to meet the demand for high speed, large

capacity communications in optical networks, which allow multiple users to share the fiber bandwidth. Multipleaccess schemes available for optical LANS include Time division multiple access (TDMA), Wavelength-

division multiple access (WDMA), and Code division multiple access (CDMA). Code Division Multiple Access

(CDMA) is a well known scheme for multiplexing communication channels that is based on the method of

direct-sequence spread spectrum [1]. In CDMA, every channel is identified by a unique pseudo noise key,

whose bandwidth is much larger than that of the input data. Ideally, the key should mimic the correlation

properties of white noise and should be as long as possible in order to minimize the interference noise

introduced by other channels; thus, a great deal of effort is invested in finding practical keys with good

autocorrelation and cross-correlation properties [1]. Optical CDMA is a technology to realize multiplexing

transmission and multiple access by coding in the optical domain, which supports multiple simultaneous

transmissions in the same time slot and the same frequency. It is another technology of multiplexing and

multiple access besides OTDM and WDM and a potentially promising technique for optical networks in thefuture, and especially, due to its easy access and flexible network structure, it is very applicable to the access

network [2]. Now a days, OCDMA systems are highly interesting as they offer several sought-after features

such as asynchronous access, privacy, secure transmissions, and ability to support variable bit rates and busy

traffic and provide high scalability of the optical network [3]. In 1986, Prucnal, Santoro and Fan proposed to

realize the fiber-optic LAN by using optical signal processing, and used prime codes to carry out the experiment

of electronic encoding and fiber-optic delay line decoding, verifying the feasibility to implement incoherent

OCDMA system by encoding in the time domain. In 1988, Weiner, Heritage and Salehi demonstrated how to

spread the femto-second optical pulse into pico-second duration pseudo noise bursts. The spread frequency was

achieved by encoding the light spectrum into pseudorandom binary phase and then by decoding the spectrum


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IJAEEE ,Volume 2 , Number 2Irfan Ali and Ankit Agarwal

ISSN: 2319-1112 /V2N2:283-291 IJAEEE

phase encoded to recover the original pulse. They proposed that the coherent ultra-short pulse coding and

decoding could be applied to the fast reconfigurable OCDMA communication networks. Both breakthrough

studies were milestones for the development of OCDMA [2]. Optical orthogonal codes (OOC) defined by

Salehi, Chung, and Wei are a family of (0,1) sequences with desired autocorrelation and cross-correlation

properties providing asynchronous multi-access communications with easy synchronization and goodperformance in OCDMA communication networks [4].

II. OPTICAL ORTHOGONAL CODESAn optical orthogonal code is a family of (0, 1) sequences with good auto and cross-correlation properties.

Thumbtack-shaped auto-correlation enables the effective detection of the desired signal and low-profiled cross-

correlation makes it easy to reduce interference due to other users and channel noise. The use of optical

orthogonal codes enables a large number of asynchronous users to transmit information efficiently and reliably.

The lack of a network synchronization requirement increases the flexibility of the system. The codes considered

here consist of truly (0,1) sequences and are intended for unipolar environments that have no negative

components since you either have light, or you don't, while most documented correlation sequences are actually

(+1, -1) sequences intended for systems having both positive and negative components. An optical orthogonal

code (n, w, a ,c) is a family C of (0, 1) sequences of length n and weight w which satisfy the following twoproperties [5].

i) The Auto-Correlation Property:

< (1)for anyx Cand any integer t, 0


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285Effects of Dispersion & MAI on Optical Code Division Multiple Access Systems

ISSN: 2319-1112 /V2N2:283-291 IJAEEE

assignment require network synchronization at high speed (optical speed), and frequent conversions between the

optical domain and the electronic domain. These requirements limit the efficiency of such an optical multiple

access system. But if a code division multiple access system with optical orthogonal codes is applied, it

simplifies greatly the complexity of the system, and achieves potentially higher transmission efficiency [6].

III. Proposed System Description

Although in the Code Division Multiple Access (CDMA) system soft capacity is obtained, the system faces

interference in case of two users simultaneously access the communication channel which, in turn, degrades the

performance of the CDMA system. Consequently, the main shortcoming of the CDMA system is multiple users

access of the communication channel. For this reason, scientists and researchers are looking at systems that

enable transmission without interference. Nevertheless, there are several differences between the electrical and

the optical CDMA. The optical CDMA is very important and becoming increasingly popular due to its high

available bandwidth and elimination of cross talks. In the OCDMA system, multiple users can access the same

channel with help of various coding techniques. In OCDMA, the transmission signal may be subjected to

conversion from electrical-to-optical, optical-to-optical or optical-to-electrical signal domain. The OCDMA

system consists of five main sections:

a. Data source (i.e., transmitting computer).b. Optical CDMA encoder.c. Optical star coupler: Device that accepts one input signal and is able to output to several. At last, using the

PN sequence receiver can receive his desired signal. However star coupler has a loss. But this is very poor.

d. The 4th section is the optical CDMA decoder.e. Data sink (i.e., receiving computer).The schematic block diagram of an OCDMA communication system is depicted in Figure 1 and 2, for an

OCDMA transmitter and for an Optical Correlator Receiver (OCR) with switched sequence inversion keying,

respectively [7]. In the OCDMA transmitter, every user preserves different signature codes modulated as binary.

Data are actually electrical signals sent to the optical drive which converts the electrical signals into optical

signals. The encoded signal is further sent to the star coupler. The star coupler used depends on the topology of

the network which can be either a LAN or an access network. In case of a LAN, the star coupler is N:N, while inan access network, the star coupler is 1:N. Further, in OCDMA every user shares the same channel. For this

reason, crosstalk which is interference due to multiple accesses is introduced here. In order to reduce this

unwanted interference, every user uses various signature sequences. On the other hand, in the OCR with

switched sequence inversion keying, an optical switched correlator is used. Consequently, a bipolar reference

sequence is correlated directly with the channels unipolar signature sequence in order to recover the original

data [7]. The unipolar-bipolar correlation is practically realized in an optical correlator, by spreading the bipolar

reference sequence into two complementary unipolar reference sequences. In addition, the optical correlator

provides unipolar switching functions for de-spreading the optical channel signal [8]. The PIN photodiode is

also known as the p-i-n photo-receiver. Here, i is the intrinsic region which is un-doped between the doped

regions of n and p. Finally, the PIN photodiode cancels the de-spreaded signal integrated with the periodic data.

This occurs before the detection of the zero threshold voltage [9].

B1tOptical Fiber

A1t

Sout Optical Amplifier

Optical Source

Figure 1: Transmitter of Optical CDMA

Optical

Drive

SequenceInversionKey C

oupler


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A1(t)

1:2 CouplerR 1(t)

Sout(t)

Zout(t)A1(t) R 1(t)

no(t)

Figure 2: Receiver of Optical CDMA

IV.PROPOSED SYSTEM ANALYSIS

In the Optical code division multiple access (OCDMA) transmitter, the Sequence Inversion Keying (SIK)

modulated signal is passed through the optical drive to a laser diode. Mathematically, the expression for Kthusers can be written as [3].

N-1

Sk(t) = PTBk(t) Ak(t - l Tc) (3)l=0

In (3), Sk(t) provides information about the transmitted output pulse shape for different users in single modefiber while l is the period of the chip and PT is the optical power of the chip. Furthermore, Bkand Ak are the

users binary signal and signature codes, respectively. The operator describes the sequence inversion keymodulation, where Ak is transmitted for a1, Ak is transmitted for a 0, respectively. Sk(t) is transmittedthrough the single-mode fiber, undergoing dispersion; it gives the output Sk(t) at the end of the fiber. For the K

thuser, it is given as

Sk(t) =

PRBk(t) Sout (t) Ak(t - l Tc) (4)

l=0Where PR is the received optical power which is the difference between transmitted power and fiber loss. T c is

the pulse interval, Sout(t) stands for the output pulse shape due to fiber chromatic dispersion can be expressedmathematically as [3].

Sout(t) = e

()

() (5)

l=0

Here, indicates the index of chromatic dispersion of the optical fiber which can be expressed mathematicallyas [8].

= ()()()D bc2 L (6)In the equation (6), , c,L and D describes wavelength of the optical carrier, velocity of light, length of fiber andcoefficient of chromatic dispersion respectively of optical fiber, while the rate of the chip is bc. The signal is

sent to the photo detector and is integrated in the output of the correlator for the ith

user which is mathematicallyexpressed as [3].

K N-1

Zi(t) = RPRT BK(t) Sout(t) AK (t-lTc)*{Ai(t-lTc)- Ai(t-lTc)}*dt+ ))

2 K=1 l=0 (7)

Here, R, Kand n0shows the responsivity of the photodiode, multiple subscriber of the system and noise in thechannel respectively. PR Represents the optical received power given by [8].

PR = PT - Pf (8)

+ dt


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The mean of Zi(t) is given by [3]

T N-1

U= RPR Sout (t-lTc) dt (9)

4T 0 l=0The multiple accesses interference variance is given by [10]

2

= U2 2(K-1) (10)

3NThe thermal noise NTH and the shot noise NSH of the photo detector are given by [8]

NTH = (4KBTr) * Br (11)RL

NSH = 2qRKPR (12)4T

KB and Br define Boltzmann constant, receivers bandwidth and T r temperature respectively. q and RL denotethe electrons charge and the load resistor of the receiver section.The signal to noise ratio (SNR) and bit error rate (BER ) of the OCDMA system are given by [7]

SNR = U2 (13)

2+NO

BER = erfc ( ) (14)2

V. SIMULATIONTOOLMATLAB (matrix laboratory) is a calculating environment and fourth-generation programming language.Developed by Math Works, MATLAB allows matrix manipulations, plotting of functions and data,

implementation of algorithms, creation of user interfaces, and interfacing with programs written in otherlanguages, including C, C++, Java, and Fortran. Although MATLAB is intended primarily for numericalcomputing, an optional toolbox uses the MuPAD symbolic engine, allowing access to symbolic Computingcapabilities. An additional package, Simulink, adds graphical multi-domain simulation and Model-Based Designfor dynamic and embedded systems. In 2004, MATLAB had around one million users across industry andacademia. MATLAB users come from various backgrounds of engineering, science, and economics. MATLABis widely used in academic and research institutions as well as industrial enterprises.

VI. RESULTS AND DISCUSSIONThe OCDMA system performance is validated with a rate of 10*109 chips per second. We evaluate theOCDMA system performance by looking at the BER for various users and at the eye diagram penalty for 7 chipm-sequence signature (m-signature chip used in our simulations was 1110010. In our simulations we haveconsidered single mode optical fiber at 1550 nm wavelength, with coefficient of chromatic dispersion of

17ps/km-nm and a receiver load resistance of 50 . Table 1 below presents the parameters of the evaluatedOCDMA system.

Table 1: Simulation parameters

Symbo

l

Significance Value

Operating Wavelength 1550 nm

Bc Chip Rate 10 G chip/s

Q Electron charge 1.6 e-19

c

K Boltzmann constant 1.38e-23

W/K. Hz

Tr Receiver Temperature 300 K


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RL Load resistance of

receiver

50

R Responsivity of each p-i-n

photo diode

0.85

L Length of fiber 245.05 km

Ps dBm Received optical power

gain

-20

Idk Dark Current 10 nA

Nth Thermal Current 1 pA2Hz

-1

D Coefficient of chromatic

dispersion

17 ps/km-nm

Figure 3:BER Vs ROP for 3 and 6 users


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Figure 4: BER Vs ROP for 12/17 users

Figure 5: BER Vs ROP for different users


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Figure 6: Performance comparisons -SNR vs. ROP

Figure 3, 4 & 5 below presents the system performance for BER versus received optical power (ROP) for 3 and6 users, 12 and 17 users & illustrates the BER performance versus ROP for up to 23 users. We observe thatBER decreases when the ORP and the number of users is increased. For instance, when we consider a 10-5 BER,the ROP is - 14.8dBm for 19 users while for 23 users, this becomes - 14.2dBm. The eye diagram for theevaluated OCDMA system is also simulated in Mat lab. We used two signal levels in our simulations: signallevel 1 and signal level 0. The top line shows the output level of the signal level 1. The bottom line representsthe output level of signal level 0. When the eye is opened and the line is spiky, it means a better performanceof the OCDMA system. On the other hand the eye is distorted when dispersion occurs in the system. Weevaluated the OCDMA performance by looking at the eye diagram for a chip rate of 10 G chip/s and acoefficient of fiber dispersion of 17ps/km-nm and different indices of chromatic dispersion . 35.

Figure 7: Eye-diagram - gamma = 0.1


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Figure 8: Eye-diagramWe observed from all our simulations that the eye is more closed for longer fiber lengths with the same index ofchromatic dispersion. We also observed that, in order to maintain a better performance of the OCDMA systemwe need also to reduce the index of the chromatic dispersion of the optical fiber. We also considered the shotand the thermal noise with MAI. We observed that a higher power of the optical transmitter is required in order

to maintain a 10-9

BER for increasing number of users. We also observed the behaviour of the OCDMA systemby looking at the eye diagram of the OCDMA network with a coefficient of fiber dispersion of 17ps/km-nm andan index of chromatic dispersion of the optical fiber = 0.05 and when considering different lengths of theoptical fiber. The more closed the eye-diagram is, the worse performance the OCDMA system has. We noticedalso that when the fiber length is decreased, the index of chromatic dispersion of the optical fiber increases. Inaddition, BER performance degrades due to dispersion effects in the OCDMA system. The BER be reduced byadding the chips while the effect of the chromatic dispersion is reduced by sinking the power of the opticaltransmitter.

REFERENCES

[1] A. J. Viterbi, CDMA: Principles of Spread Spectrum Communication. Reading, MA: Addison-Wesley, 1995.

[2] Hongxi Yin, David J. Richardson, Optical Code Division Multiple Access Communication Networks:Theory and

Applications Tsinghua University Press, Beijing and Springer-Verlag GmbH Berlin Heidelberg.[3] S.P. Majumder, Member, IEEE, Afreen Azhari, and F.M. Abbou,Impact of fiber chromatic dispersion on the BER

performance of an Optical CDMA IM/DD transmission system, IEEE Photonics Technology Letters, Vol.17, No. 6,June 2005.

[4] J.A. Salehi, Code division multiple-access techniques in optical fiber networks-Part I: Fundamental principles,IEEETransactions on Communications, vol. 37, no. 8, pp. 824-833,August1989.

[5] J.A. Salehi, Emerging Optical Code-Division Multiple Access Communications Systems, IEEE Network, vol. 3, no.2, pp. 31-39, Mar. 1989.

[6] L. TanCevski, I. Andonovic, M. Tur, J. Budin, Hybrid wavelength hopping/time spreading code division multipleaccess systems, iee proc. Optoelectron, vol. 143, no. 3, june 1996, pp. 161-166.

[7] S.P. Majumder and Md. Forkan Uddin, The effect of four wave mixing on bit error rate performance of a Directsequence optical code division multiple access system, 2005 Asia-Pacific Conference on Communications, Perth,Western Australia, 3-5 October 2005.

[8] S. P. Majumder, Afreen Azhari, Performance Limitations of an Optical CDMA Sys-tem Impaired by Fiber ChromaticDispersion, 0-7803-8783-X/O4/$20.00 0 2004 IEEE.

[9] Abdul Gafur, Dr. Doru Constantinescu, Dispersion Effects on OCDMA system performance Blekinge Institute ofTechnology, School of Computing , September 2009,Sweden

[10] T. O'Farrell and S. I. Lochmann, Switched correlator receiver architecture for optical CDMA networks with Bipolarcapacity,Electron. Lett, vol. 31, pp. 905-906, May. 1995.

effects of dispersion & mai on optical codedivision multiple access systems

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