International Conference on Information and Communication Technology ICICT 2007, 7-9 March 2007, Dhaka, Bangladesh

PERFORMANCE COMPARISON OF WAVELENGTH SHIFT KEYING AND CONVENTIONAL ON OFF KEYING SCHEMES IN A MULTI-HOP OPTICAL RING NETWORK IN PRESENCE OF FOUR WAVE MIXING CROSS TALK Meer Nazmus Sakib, Sanjoy Dey*, S.P Majumder, Md Nazmul Alam, Redwan Noor Sajjad Department of Electrical and Electronic Engineering, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh, Department of Electrical Engineering, The University of Texas at Dallas, Texas, USA E-mail: [email protected], sanjoy.dey(@student.utdallas.edu, spmajumder(@eee.buet.ac.bd

Complementary keying has a 3 dB signal to noise advantage over on-off keying; symmetric assignment of symbol wavelengths with respect to the zero dispersion wavelengths cancels FWM interference to first order and dispersion shifted fiber permits higher transmission rates. In this paper the transmission performance of a multi-wavelength metropolitan optical ring network is analyzed in presence of four wave mixing and ASE noise for WSK-WDM and conventional 00K-WDM schemes. The results are presented in terms of BER, maximum allowable and minimum required power and maximum number of traversable hops at a given BER.

ABSTRACT A transmission performance study is carried out for an all-optical multi-wavelength metropolitan optical ring network (MON). Effect of using a wavelength shift keying (WSK) scheme is analyzed and compared to that of the conventional on off keying system (OOK) in respect of four wave mixing (FWM) cross talk. The performance results are evaluated at a bit rate of 10Gb/s taking into considerations the combined influence of photo detector shot noise, beat noise components arising out of the beating of the signal with accumulated amplifier's spontaneous emission (ASE) and cross talk introduced at each node due to four wave photon mixing. The computed results show that for 50 hops the bit error rate (BER) of the standard wavelength division multiplexing (WDM) system is higher than 1012 while for the wavelength shift keying system, for the same received power the BER is lower than 1016 for the hop length of 100 km. It is also shown that for a given BER, WSK-WDM technique provides higher values of both the maximum allowable number of hops and minimum

2. SYSTEM ARCHITECTURE The network shown in Fig. 1 is based on an a o t annltatureof W dM. ccss to the soisia =

required power.EW

1. INTRODUCTION The optical packets in a slotted ring network accumulate amplifier spontaneous emission (ASE) noise, four wave mixing and power losses that can* severely degrade the transmission performance [1]._..

w

_

A number of methods can be used to alleviate the effect of FWM cross talk [2-6]. While wavelength Fig. 1. Access points in multi-wavelength metropolitan shift keying requires twice as many wavelengths as opiarng standrd WDto spporta givn numer ofthe access points (APs) as shown in Fig. 1. In this channels, it has a number of advantages that can paper, a single ring is considered. As shown in the compensate for this wavelength usage. 150

figure the rings can be interconnected with KP IK{p/8rPpqr+1/4EPppr} intelligent bridges [7-8]. To put it in brief, III bandwidth of each channel is divided in the time Q= JG(mn)2 +G2 +G2 +Nt+NS + 2 +Nth domain into an equal and constant number of slots per wavelength that circulate around the ring. One of (4) the WDM channels is reserved for carrying control 2 information and header-slots. Slots on this channel ap and Sp-Sp (4b -1)( ) are called control slots. The remaining slots on the where,s- Sp rx other channels carry the payload (data-packets) and are called payload-slots. The header-slots contain the are the signal-ASE and ASE-ASE beat noise addresses of all destination APs on all corresponding variances respectively. Nth and Nsh are the variances payload slots. Furthermore, each AP on the ring is of thermal noise and shot noise given by Nth=4K TBe/RL and Nsh 2qBeRPr. Where Be is the able to transmit and receive at any wavelength. electrical bandwidth of the receiver, K/ is the Boltzman's constant, T is the temperature in Kelvin, RL is the terminating load resistance of the receiver, 3. ANALYSIS In a multi-channel optical system, new optical and K is the photo detector responsivity. It has been found that the FWM spectrum is signals are generated at frequency fpqr exiting the fiber, where ft= fp + fq - fr. Assuming that the input symmetric around the zero-dispersion wavelength the channels symmetrically around By assigning signals are not deplete by by the o[3]. . *> thle gene are not mixing dlepletedl generationn Of signals products, the power of a generated FWM signal is zero dispersion wavelength and using balanced detection, FWM cross talk can be minimized given by [9] significantly. In WSK-WDM system, each user is assigned two specific wavelengths symmetric with ,%zz pppqpre (1) respect to the zero-dispersion wavelength. One ,, 1024Z6 P = 42 2 77 wavelength is used to transmit the bit '1' and the n Ac L other to transmit bit '0'. The Fig. 2 shows the 'iffej

e_pffe-

of wavelength shift keying is the refractive index, X is the Schematic diagram N wavele h of te lWDM system with channels and the Fig. 3 shows nonlinear waelength Of the light, X, iS the decision statistic for WSK-WDM technique. susceptibility, c is the speed of light, L is hop length and Pp ,Pq ,Pr are the input powers of the channels. Leff is the effective length of the fiber, given by 'Vs os > Leff =(/x)(1-eUL). With ax the fiber loss, here ciaireLL( LR n- tO taken to be 0.25dB/km. Aeff is the effective area of serll...userl.u .49e.N' the fiber, D=3 for two-tone products and 6 for threeH tone products. | 1 Lhan.ne_ If noise component due to FWM is donated as E-1t.RPs bt 1'C _j> the mean value and variance of F(in) are Nxl F , IXN COUPLER CIOUPLIER expressed as [10]

where

n

er

c

n1(m)

b

/n(m)\_0

(2)

-

6(mTI)2

Rlbi t

(m(lTI)2)_(m(1TI)

=2K2PrD1/8

Fig. 2. Schematic diagram of wavelength shift keying WDM system with N channels. Each channel is assigned an upper wavelength and a lower wavelength for transmitting Is and Os.

2

DP1/AVD Ppqr

P1 rjL

(3)

The term Ppqr is the accumulated FWM power up to the nth' hop and given by Ppqr- Ppqs= Pppr- Pg.n assuming line spectra and all channels having same/ transmitting power level. If Gaussian distribution is lrlr assumed the signal to noise ratio (SNR) Q with OOK-WDM scheme for a multi-wavelength optical Fig. 3. Decision statistic for WSK-WDM technique. ring network is given by [1] 151

The modulated signals of all users are multiplexed and transmitted through the long haul low dispersion fibers, where they experience attenuation and spectral deformation due to FWM. At the receiver, two narrow band filters of each user select the two wavelengths and a balanced receiver then demodulates these wavelengths. The received signal of each wavelength of a user is positive for bit '1' and negative for bit '0'. If the signal level has Gaussian probability distribution, the mean value of the detected signal level is given by [4] KSm) = KPr for "mark" (5)

Sm) = -KPr for "space"

BER = 0.5erfc(Q) RESULTS AND DISCUSSION

The bit error rate (BER) performance of both the WSK-WDM and OOK-WDM network are evaluated using the theoretical formulations developed in the previous section. The parameters used in calculation are given in table 1. The parameters are chosen for comparing the systems under investigation with an equally spaced WDM system with 8 users.

(6)

TABLE I PARAMETERS USED IN CALCULATIONS

The variances of noise are given by 2rn

&

2 for "mark" (rm)2 +N th +N sh +2&S-SpJ +G SP_SPJ

= F

,CT2&s2 =G (s)2 +N +N +G_2 +G +G 2 +Nth +NSh

for "space"

Symb Meaning

Value/Range

(7)

ol R

Bit rate

10 Gb/s

(8)

Af K

Channel separation Responsivity

L

Hoplength Fiber attenuation Spontaneous emission factor EDFA output saturation power Refractive index Wavelength Degeneracy factor Nonlinear susceptibility

125 GHz 0.85 AW' 5/100km

where om)2 and oy(2 are the variances of FWM signal for 'mark' and 'space', respectively. Assuming Gaussian distribution for the FWM noise of the WSK-WDM system, the variance of this noise can be calculated as

a

nsp Psat N

1|

,(m)2 2K 2Pr I/ 8 Ppq +l l 4 Pppr ppr r KPr /Ppqr + =

(s)2 4 Y'pprr l F -=2K2Pr fl/8VP I I pqr +1/4V

(13)

D

()

0.25 dB/km

0.8

10 dBm 1.46

1.55 pim 3 4 x 10-5esu

Fig. 4system and fig. 5 shows the BERofofconventional the WSK(10) WDM as compared to that

OOK-WDM system for 10,20,30,40,50 hops with hop length of 100 km and 5 km respectively. At low power levels, noise due to FWM is small and BER decreases as received power is increased. Above a certain value of input power four-wave mixing becomes significant and BER increases with received power. In Fig. 5, we see that the BER is decreased considerably for hop length of 5km. But for all power levels the performance of the WSKWDM exceeds that of the OOK-WDM scheme. This is true for any number of hops. For example if the number of hop is 20 for a hop length of 100 km at 12dBm power level has a BER of 1 0-31 whereas the BER of the OOK-WDM scheme is 10o25. InFg4, fra fixed number of hp,theBE vs. received power curve crosses the 10-9 mark KPr twice. The power corresponding to the first point is = (12) the minimum required power for achieving BER of \I NFWM +Nth +N51, +G525 +(5p5 10-9 or less and the power corresponding to the Finally, the BER of either of the two schemes second point is the maximum allowable power. The can be determined as power should be kept between

where Ppqr is the accumulated FWM power up to the nih hop as described before. If all the noises have Gaussian distribution profile, the SNR is derived applying the Eqs. from (5)-(10) to the equation of SNR which is given by K (mf)) - (s) Q G(m) + K(S) +(1) After a series of manipulations the equation of SNR of the network with WSK-WDM scheme is found out to be Q= KPr - (-KPr) +Nth + Nhh+2 s+(ps + Nth +Nsh +( s+( s

+\<()

G((s2+

~~~received/transmitted

152

I40

HoeHI1oLp.1

io-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0 L

Hops 3~~~~~~~~~~~~~0

\

10

B

1 0-

10+ ~tr; / 10'

m

30

g

I.a

|

Hops~~~~~~~~~~~~~~~-1

iKW1ll 0

10

H

-20

1/ ops/W

1~~~~~~~~~~~~ 0 1

-10 U5

-15

Received Power (dBm)

Fig. 4. BER vs. received power for different number of hops for a hop length of 100 km.

10

a ]j t I_

11

05

5

16

20

15

25 30 35 40 Number of Hops

45

50

55

66

Fig. 7. BER vs. number of hops for different power levels, hop length is 100 km.

10~~~~~~~~~~~~~~~~~~~~~~8 50 Hops

10

-WSK-WDM

Fig. 8. M i

10 -60

n

b

of70

6

l

W 1

10

---SK-WDM

s

~ ~ ~~~~~0A

10Hops

-120

hops for a hop length of 5 km.~~~~~~~~3 --OOK-WDM -140 10

Hos

L

3

-WSK-WDM

o160 18

16

14

12

10

-8

6

Received Power (dBm)

4

-2

0

Fig. 5. BER vs. received power for different number of

a-

hopsfor hop

0

lengthiff

-20

-16

-16

-14

-12

-16

-6

-6

-4

power(dmini) Fig. 8. Maximum number of achievable hops vs. received power for a of BER of 10d for hop length of 100 km. Received

these two power levels for achieving BER of 10O9 or less. The variations of these power levels are shown in fig. 6. The difference between these two power levels reduces with the increase of number of hops. WSK-WDM has higher hop foraho length of500km. r=OadP 1 I hthatag Here it is also seen Fg 6. M _ maximum allowable and lower minimum required M~iximni AIow~bIePoweipower than that of OOK-WDM scheme. 1 2 effect of increase of number of hops on BER is ~~~~~~~~~~~The shown in fig. 7. for different power levels and hop ~~-14 ~~ Miiinimun Requirred Power length of 100Okm. In the range Pr s0 and Pr -12 dBm, as received/transmitted power is decreased, -18~~~~~~~~different BER vs. number of hops curves shift BRFW Beyon~~~~~~~~~~~~~~~dPr poe fo tusimsteallowable aBE of1-orlswihhp downward. Maintainin0~~~ =-1 2dBml these4inptwpwer cur1-ves4 will

16~ ~~

5 km.

~

~

~ ~ ~ ~~~5

[2] T. Numai, 0. Kubota, "Analysis of repeated unequally spaced channels for FDM lightwave systems, " IEEE J Lightwave Technol. 18(5), 2000. [3] M.F. Uddin, A.B.M. Nasirud Doulah, A.B.M. Isteak Hossain, M.Z. Alam, M.N. Islam, "Reduction of fourwave mixing effect in an optical wavelength-division

the extra bandwidth is provided for any given number of hops. Therefore, this scheme ultimately yields more bit rate-distance product compared to that for conventional OOK-WDM technique. In figure 8, maximum number of traversable hops for BER of 10-9 or less is shown against

multiplexed sustem by utilizing different channel spacing

power levelumforchoplengthbof100 km.pI is reticeid that noticed maximum achievable number of hops iS approximately 84 and 56 corresponding to received power level of -12dBm and at a of BER 10 respectively for WSK-WDM and OOK-WDM schemes. This certainly reflects how the network's transmission performance can be enhanced by WSK encoding

and chromatic dispersion schemes, " Opt. Eng. 42(9), pp.

2761-2767,2003.

[4] M. Faisal, M.N. Islam, M.S. Alam, "Comparative study of wavelength shift keying and repeated ununequal channel spacing schemes in reducing the four-wave mixing effect in optical wavelength-division-multiplexed system, " Opt. Eng. 45(1), pp. 0 15002-1-6, 2006. [5] C. Xiang, J.F. Young, "Wavelength shift keying technique to reduce four-wave mixing crosstalk in WDM,"

Proceedings of IEEE LEOS Annual Meeting, Paper WZ2, CONCLUSION pp. 609-610, San Francisco, CA, 1999. A detailed analytical investigation is carried out to [6] J. Lee, D. Lee, and C. Park, "Periodic allocation of a set evaluate the transmission performance study of the of unequally spaced channels for WDM systems adopting multi-wavelength optical ring network in respect of dispersion shifted fibers, " IEEE Photonics Technol. Lett. WSK-WDM and conventional OOK-WDM schemes 10(6), pp. 825-827, 1998. in presence of FWM cross-talk For a wide range of [7] D. Dey, A. M. J. Koonen, D. Geuzebroek, M. R. power levels the WSK-WDM technique shows that Salvador, "FLAMINGO: a packet-switched IP over WDM this technique substantially improves performance metro optical network," Proceedings of Networks and relative to standard on-off WDM encoding in terms of BER, maximum allowable and minimum required power and maximum number of achievable hops at a given BER though wavelength shift keying requires twice as many wavelengths as standard WDM to support a given number of channels. The results Of this paper can be easily extended to any similar practical multi wavelength optical ring network with varying length and other system parameters.

Optical Communications Conference (NOC), pp. 400-406,

June 2001. [8] D. Dey, A. M. J. Koonen, M. R. Salvador, "Network architecture of a packet-switched WDM LAN/MAN,"

Proceedings of the IEEEILEOS Symposium, Benelux Chapter, pp. 251-254, October 2000. [9] N. Shibata, R. P. Braun, and R. G. Waarts, "Phasemismatch dependence of efficiency of wave generation through four-wave mixing in a single-mode optical fiber, " IEEE J Quantum Electron., vol. 23, no. 7, pp. 1205-1210,

July 1987.

[10] K. Inoue, K. Nakanishi, K. Oda, "Crosstalk and power penalty due to fiber four-wave mixing in multichannel transmissions," J. Lightwave Technol., vol. 12, no. 8, pp. 1423-1439, August 1994.

REFERENCES [1] S.P. Majumder, Redwan Noor Sajjad, Meer Nazmus Sakib, Md. Nazmul Alam, "Impact of Four Wave Mixing and Accumulated ASE on the Performance of a Metropolitan Optical Network, " Proceedings of IEEE Conference on Networks (ICON), pp. 262-265, 2006.

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