Investigation on Performance of Passive OpticalNetwork Based on OCDMA

Chongfu Zhang, Kun Qiu, and Bo XuKey Lab. ofBroadband Optical Fiber Communication and Communication Networks Technology,

University of Electronic Science and Technology of China, Chengdu, P. R. CHINAEmail: chfzhang5178163.com

Abstract-In this paper, a Passive Optical Network (PON)based on Optical Code Division Multiple Access (OCDMA) ispresented, and analyzed in detail. The proposed system can beapplied to an optical access network with full services ondemand, such as internet protocol, video on demand, tele-presence and high quality audio. An OCDMA-PON combinesadvantages of PON and OCDMA technology. Simulationresult elicits the proposed scheme is feasible. In this study, thenovel design improves the optical access network performanceand enhances the system flexibility and scalability.

I. INTRODUCTION

Since the internet and broad-band access network wereintroduced during the last decade, emerging applications,such as video on demand (VOD), digital cinema, tele-presence, and high-quality audio transmission, demandhigh-throughput optical access networks with stringentquality of service (QoS) requirements. However, theinfrastructure of current access networks suffers fromlimited bandwidth, high network management cost, lowassembly flexibility and bad network security, whichobstructs the network from delivering integrated services tousers. Owing to the maturity of optical components andelectronic circuits, optical fiber links have become practicalfor access networks. Passive optical network (PON) andsome multiplexing technologies, including wavelengthdivision multiplexing (WDM) [1], time divisionmultiplexing (TDM) [2], and optical code divisionmultiplexing (OCDM) for access networks, have beenproposed [3].PON is one of the most promising solutions for fiber-to-

the-office (FTTO), fiber-to-the-home (FTTH), fiber-to-the-business (FTTB), and fiber-to-the-curb (FTTC), since itbreaks through the economic barrier of traditional point-to-point solutions. PON has been standardized for FTTHsolutions and is currently being deployed in the field bynetwork service providers worldwide. Even though time-division multiple-access (TDMA)-PON utilizes effectivelythe bandwidth of fiber, it has limitations in its increasedtransmission speed, difficulty in burst synchronization, lowsecurity, dynamic bandwidth allocation (DBA) requirement

and inaccurate Ranging [4-5]. WDM technology has alsobeen proposed for PON. The emerging WDM-PONbecomes more favorable as the required bandwidthincreases, but the technology comes at an extravagant price[6]. In addition, the effect of statistical multiplexing isinsignificant in multimedia communications environmentsfor WDM-PON. Even though WDM-PON has severaladvantages over TDMA-PON, it has failed to attractattention from industries because of its high cost. Now otherschemes for optical access network are being studiedworldwide.

Optical code division multiplexing access (OCDMA)system has attracted increasing attention in recent years dueto the following advantages: asynchronous access capability,accurate time of arrival measurements, flexibility of userallocation, ability to support variable bit rate, busty trafficand security against unauthorized users. OCDMA is a veryattractive multi-access technique that can be used for localarea network (LAN) and the first one mile [3].We present a PON configuration based on OCDMA

technology in this paper. Our OCDMA-PON combines theadvantages of PON and OCDMA. The rest of the paper isorganized as follows. In Section II, we describe theconfiguration. The performance of the system isinvestigated in detail in Section III. Section IV presents thesimulation results. We conclude this paper in Section V.

II. SYSTEM DESCRIPTIONBefore introducing our OCDMA-PON, we give short

descriptions on the classical PON and the OCDMAtechnology with their main properties. We then show howthe two are combined in our proposed OCDMA-PON.

A. PONA classical PON model is a next-generation optical

access network technology that deploys optical transmissionlines (fiber) between optical line terminator (OLT) and someoptical network units (ONU) and / or optical networkterminator (ONT). Passive optical splitter (POS) is used tohandle connection between OLT and ONU or ONT. TheOLT interfaces over the service node interface (SNI) to theservice nodes, and to the optical distribution network (ODN)

0-7803-9584-0/06/$20.00O2006 IEEE. 1851

which provides the optical transmission media between theOLT to the users. The OLT broadcasts frames from servicesnodes to all ONU or ONT via ODN by wavelengthmultiplexing of 1.55,um wavelength in the downstreamdirection and 1.31,um wavelength in the upstream directionover a single fiber channel. The ONU receives video, audioand data from the OLT and passes the data to the end-users.

B OCDMASince its birth, the OCDMA technology has attracted many

researchers' attention. The signature processing is limited tooptical field and the fiber-optic system acts as a non-negativemedia, thus the signature sequences for the system areconstructed by l's and 0's. The address codes used inOCDMA systems must be unipolar codes with good auto-and cross-correlation properties. Optical orthogonal code is apreferred code which satisfies these requirements, and it iscommonly 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 ofbinary (0, 1) sequences of length v and weight k satisfyingthe following two properties [3]:

1) The auto-correlation property:v-1

('xx <A, for O<r<v-1t=O

for any X= (x0, xi, ..., xv ) E C and any integer Xc . 0

sequence is re-processed via optical decoder from which theusers receive their desired information signal.

C OCDMA- PONFig. 1 depicts the schematic diagram of the proposed

OCDMA-PON system. The system is composed of OLTand ONUs, based on OCDMA, and every ONU is identifiedby its own code address. The presented schematic isdifferent from the classical PON in that OCMDAtechnology is adopted in our system instead of TDMA orWDM. The signal is modulated with not only frameinformation but also address code sequence. The former isused to accomplish data load switching and the latter is usedidentify different users. The performance of the presentedsystem is analyzed in the next section.

- Downstreamn

(1)

(mod v).2) The cross-correlation property:

v-1= Zx,y±.2<A, for 0<Tr<v - (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 orthogonalcode 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 ofcode words contained in C. The largest possible size of the(v, k, A)-OOC C is denoted by P (v, k, A). By the well-known Johnson bound, we know that ' satisfies thefollowing inequality:

k(k-1). (k-k)

Using the above inequality, for a (v, k, 1)-OOC, if

Ic =L(v-f1t(k(k-1))] (4)then C is called an optimal OOC.

After introducing the OOC for an OCDMA system, wegive short description of an OCDMA system in the following.The system consists of optical pulse source, opticalen/decoders and detectors. At the transmitter, the signal ismodulated by optical source, and the carrier optical wave isenclosed by OOC sequence at the optical encoder. At thereceiver, the optical pulse with the signal and the OOC

Fig. 1 Schematic diagram of presented OCDMA-PON

III. SYSTEM ANALYSISThis section analyzes the performance of the system

presented in the previous section. Since the scalability of thenetwork is a key for network design, we focus on this issuein this paper. The constraints for the network scalabilityinclude the number of ONU/ONT and channel link lengthwhich is affected by link's power budget, the number ofavailable codes and the cost ofthe system.

We assume that (1) the system supports users based onOCDMA technology, (2) the joint of network deploys anAWG which manages all ONUs/ONTs, and (3) every jointhas up to 64 ONUs/ONTs. Reference to [7], we can deducethat the total number of network users Niotal and the numberofjoint in the network Xji,,, have the following relationship

X jiont =LAN totai/64 ] (5)

In order for signal to be exactly detected, the downstreamtraffic power budget must satisfy the following equation

P tr aADAX joint aFL (6)a A WG a else R sen

The notations are explained in Table I.

Similar to the above, the upstream traffic power budgetmust satisfy

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fnr r=n

p ou c log 2(S)-aADX joint (7)

a FL a AWG a else R sen

where aelse is the extra loss introduced by the en/decoder andother optical components. Other parameters in the systemare listed in Table I.

As follow, the system transmission quality is analyzed.Since the signal is encoded at the ONUs, and decoded at theOLT, the signal encoded can be defined as

S en(t) = F [ M ( )S in( )] (8)

where M(co) is denoted as the encoding function, Sim(ow) isthe incident optical pulse, which are expressed as [9]

(9)

00

Sin ()== F(ZEIXexp(-t<2)dSpT(t-IT)) (10)I=-oo

H1(x) is the transmission ratio of en/decoder at the ONUs or00

OLT, and Cn(t) = E X (n) pT(t-jTi ) XJ (n) is thej=-o0

GOCs sequence set, P and d, are respectively the opticalpower, data stream, p7-(t) reflects the spectral shape of opticalsource.

Since we take into consideration the system additive noisepower, the thermal noise 0th2, the beat noise bea,2 [8], therelative intensity noise Yre2 and the link noise ujink , which aregiven respectively by

2th (2kKTrT) /(e2Re)(7 beat2 = s 2(2 +m 2) /(8p B s2)22

J re= reS 2Rb

2 1 i07 lnk

=,-eR85Ntoalpifr

(13)

(14)

(15)

(16)

+2R2

(i7 sp(G -I)hvu) 1 G )2wwhere kB, K, Ti, Tc, e, Re, m, Pc, Bs, 41e, Rb, s, pj fr, R, 'isp, GCh,u, qa, and W are the Boltzmann constant, ration of theequivalent receiver bandwidth to the signal bandwidth,receiver noise temperature, chip duration, electric charge,receiver load resistance, modulated index, the processinggain, base-band signal bandwidth, PSD of the relativeintensity noise, ratio of data bit, response index of detector,optical power per pulse, receiving bandwidth, response ofphoto diode, spontaneous emission factor, gain of opticalamplifier, photo energy, quantum efficiency and opticalcomponents bandwidth.

Thus total noise ofthe system c&2 iS given as

TABLE I. THE SYSTEM PARAMETER FOR SIMUNICATION

descriptionSystem transmission power

Output of ONUsReceiver sensitivityLength of fiber link

filtering IndexInsertion loss of add/dropInsertion loss ofAWGPropagation loss of fiberSplitter's splitting ratioBudget of the system

Chip duration

Then the current ij at the detector, and thepower S2 is educed as follows

N .ota-l

ij=a Re i [s(S en(t), C i(m))]m =o

S (S en(t),C j(M ))

S = hfij / 7

value5dBm-4dBm-4OdBm5-20km

31dB5dB

0.3dB/km16-643dB0. Ins

desired signal

where ac, h, and q is the scale factor index, Plank's constant,optical frequency, APD quantum efficiency.

(17)C Jth +Tbeat +0re +0linkThe signal to noise ratio (SNR) and bit error ratio (BER)

are given as

SNR = S /T (18)

Assuming the system noise is Gaussian-distribution, withunipolar capacity, the corresponding BER is given by

BER --erfc(-.SNR /8 )2

(19)

IV. NUMERICAL RUSULTS AND DISCUSSIONSIn this section, we present our numerical results for our

proposed OCDMA-PON analyzed in the above section.

First, we show the maximum achievable length of thefiber link as a function of the number of ONUs / ONTs. Theresult based on formula (5)-(7) is shown in Fig. 2. Themaximum length of the fiber link that the system can supportis shortened with increased number of ONUs / ONTs in thenetworks, and the length can be increased with increasedinput optical power. For example, when the input power is 5mW, the network can support over 300 ONUs / ONTs up a

fiber link length of 25 km, while when the input power isreduced to 4 mW, the network can support only 200 ONUs /ONTs at the same fiber link length. From the plot, we also

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symbolPtrPoutRsenLCaADAAWGAFS

AelseTC

m (co) = F (H t(x)C,(t

observe a step per about 64 ONUs/ONTs which comes fromformula (5).

c-Ju-1fse49L-

u1

200 300 400Number of ONUs / ONTs

there is about 2dB penalty at input power p= -lOdBm, forBER= 10-9, the former must be -12dBm for input power,while the latter must be -14dBm. So we take in considerdesign ofOCDMA-PON system, noise affect the system thatis neglected.

35

30EY 25

-0 20

4-Jc

10

5

500

Fig.2 The length of fiber link as a function of the number ofONUs/ONTs for two different signal power levels

Fig. 3 compares the maximum achievable length of thefiber link as a function ofthe number ofXjAi,, for the classicalPON and our proposed OCDMA-PON system. From theplot, our proposed OCDMA-PON clearly has a betterperformance than the classical PON.

30

E 28lie

c 26

240

122

a) 20

18

0 2 4 6 8 10 12 14Number of Xjoint

Fig.3 The length of fiber link and number ofXjAi>n for a classical PONand OCDMA-PON

Fig.4 shows the maximum achievable length of the fiberlink as a function of the number of Xj0i, for two differentinput power of 4 and 5 mW with our proposed OCDMA-PON. The effect of increasing the fiber link length byincreasing input power is clearly seen in the plot. Both Fig.3 and Fig. 4 indicate that the length of fiber link must bereduced with increased number ofXj,i,,.

In Fig. 5, we show the BER as a function of the inputpower for both back-to-back configuration and the case withinterference. The system based on the case with interference(with noise) and back to back is considered. the system withinterference is very worse than the back to back system, just

P=4mW0~~~~~~r) P=5mW

0 00 0

L)~~D CD

D a}_ CfD0

D9 C)

0 0_ ,@, ,'). -~~~~~

0 cl0

_~~~~~~~~~itk

D 2 4 6 8 10 12 14Number of Xjoint

Fig.4 The length of fiber link and number of number ofXjAi,n for twodifferent signal power levels

100

10-wm100a)

io.w_ 1 02

1 0 .o

1 o-251l-2C -15 -10

Input power (dBm)-5 0

Fig.5 The BER and input power for two system

V. CONCLUSION

We present and analyze an OCDMA-PON system, even

though which is composed of an OLT at a center office andONU or ONT at user terminator, a PON is vitally differentfrom the OCDMA-PON, up/downstream in OCDMA-PONis deal with by en/decoder to take with 0OCs, and there are

some advantages of PON and OCDMA technology. In thepaper, a classical PON and OCDMA scheme are introduced,and an OCDMA-PON is presented. The performance of thesystem is analyzed in detail, including the scalability of thenetwork, i.e., number of ONUs / ONTs and number ofnetwork joint, and the relation of the system BER andnumber of ONUs / ONTs for different input power. Thesimulation results show that The maximum length of the

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A lassical PON| OCDMA-PON -

7

A

A7

v~~~A

A,with interferencev back to back

321

bu

4

fiber link that the system can support is shortened withincreased number of ONUs / ONTs in the networks. Theproposed OCDMA-PON clearly has a better performancethan the classical PON. The effect of increasing the fiberlink length by increasing input power. So the system islogical 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.

REFERENCES

[1] M.S.Goodman, H.Kobrinski, and K..W.Loh, "Application ofwavelength division multiplexing to communication networkarchitectures," in Proc. ICC'86,1986.

[2] D.M.Sirit, A.D.Ellis, and P.E.Barnsley, "Optical time divisionmultiplexing: System and networks," IEEE Commum. Mag., vol. 32,pp.56-62, Dec.1994.

[3] JA.Salehi, "Code division multiple-access techniques in optical fibernetworks -Parts 1: Fundamental principles," IEEE Trans. Commum.,vol. 37, pp. 824-833, Aug. 1989.

[4] K.Ohara, et al., "Traffic analysis of Ethernet-PON in FTTH trialserice," in Optical Fiber Commum. Tech. Dig., Anaheim, CA, pp.607-608, Mar. 2003.

[5] C.Assi, Y.Ye and S.Dixit, et al, "Dynamic bandwidth allocation forquality of service over Ethernet PONs," IEEE Select. Areas Commun.,vol. 21, pp. 1467-1477, Nov. 2003.

[6] K. Iwatsuki, J. I. Kani and H. Suzuki, et al., "Access and metronetworks based on WDM technologies," J. Lightwave Technol., vol.22, pp. 2623-2630, Nov. 2004

[7] F.T. An, K. S. Kim and D.Gutierrez, et al., "SUCCESS: A next-generation hybrid WDM/TDM optical access network architecture," JLightwave Technol., vol. 22, pp. 2557-2569, Nov. 2004

[8] B.G.Ahn, Y.Park, "A symmetric-structure CDMA-PON system andits implementation," IEEE Photon. Technol. Lett., Vol. 14, pp. 138 1-1383, Sept. 2002.

[9] Ch.F. Zhang, K.Qiu, "Investigation on realization of optical CDMAusing division frequency channel and biploar codes," ACTAPHOTONICA SINICA., vol. 33, pp. 229-332. Mar. 2004.

[10] J.R.Stern, et. al, "Passive optical local networks for telephonyapplication and beyond," Elec. Lett., vol. 23, pp. 1255-1257, nov.1987.

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