Intrnational Journal of Enginring and Tchnology Volum No., Dcmbr, Analysis th rformanc of Codd WSK-DWDM Transmission Systm Bobby Barua Assistant rofssor, Dpartmnt of EEE, Ahsanullah Univrsity of Scinc and Tchnology, Dhaka, Bangladsh ABSTRACT Dns Wavlngth Division Multiplxing (DWDM) is th systm with mor than ight activ wavlngths pr fibr. Again high data rats as wll as long spans btwn amplifirs in a chain rquir high optical powr pr channl to satisfy th signal to nois ratio (SNR) rquirmnts. So th DWDM systms with long rpatr-lss spans, th simultanous rquirmnts of high launchd powr and low disprsion fibrs lad to th gnration of nw wavs by four-wav mixing (FWM), which dgrads th prformanc of a multi-channl transmission systm. Svral mthods hav bn proposd to mitigat th ffct of FWM crosstalk. All ths works ar prformd considring only binary WSK schm. Although M-ary WSK (M>) schms hav highr spctral fficincy than binary WSK systm. Again, th BER prformancs for M-ary WDM systm ar not satisfactory with th ffct of FWM. Thrfor, in this papr w includ th ffct of FWM on th prformanc of an M- ary WDM systm and try to mitigat th ffct by mploying th nrgy fficint convolution cod in a normal disprsiv fibr as wll as in a disprsion shiftd fibr (DSF). Kywords: Dns Wavlngth Division Multiplxing (DWDM); four-wav mixing (FWM); disprsion shiftd fibr (DSF; signal to nois ratio (SNR); bit rror rat (BER); Multipl Ary(M-ary).. INTRODUCTION Communication is th procss of transfrring information or mssag lik voic, vido, txt, data, pictur, tc. from on distanc to anothr. Th function of communication systm is to convy th signal from th information sourc ovr th transmission mdium to th dstination. In gnral, th information carrying capacity should b high so that it can mt th rquirmnt of th usr. Th information carrying capacity is closly rlatd to th bandwidth supportd by th communication systm. Th gratr th bandwidth, th highr is th information carrying capacity of th communication systm. In this rspct optical fibr communication systm has bn dvlopd. Th hug potntial bandwidth of optical fibr can b afficintly utilizd by multiplxing a numbr of channls and transmitting thm through th fibr simultanously. Th transmission bandwidth of fibr is dividd into a numbr of nonovrlapping frquncy (or wavlngth) bands and ach of ths bands is associatd to support a singl communication channl. Two principl kinds of multichannl systms ar common in practical applications, namly, frquncy division multiplxing (FDM) and wav lngth division multiplxing (WDM). Th two schms diffr from ach othr in rspct of transmittr/rcivr configuration. Optical dns wavlngth division multiplxing (DWDM) systms using low disprsion fibrs and rbium-dopd fibr amplifir (EDFA) ar vry attractiv to mt up th growing dmand for broadband information distribution ntworks. A larg numbr of wavlngth channls, oprating at Gb/s, can b multiplxd at svral gigahrtz intrvals if fibr low disprsion rgion around.55µm (~.5 THz bandwidth) is fully utilizd [-3]. For DWDM systms with long rpatr-lss spans, th simultanous rquirmnts of high launchd powr and low disprsion fibrs lad to th gnration of nw wavs by four-wav mixing (FWM) [4-5]. Th FWM is th dominant nonlinar ffct that svrly dgrads th prformanc of a multi-channl transmission systm. Svral mthods hav bn proposd to mitigat th ffct of FWM crosstalk, namly, arrangmnt of transmission fibr disprsion, unqual channl spacing (US) schm, rpatd unqual channl spacing (RUS) schm, wavlngth Shift Kying (WSK) [6-8]. Th improvmnt of WSK-WDM systm rlativ to convntional on-off wavlngth division multiplxing (WDM) systm has bn studid [8]. Th works on WSK-WDM considrd only binary WSK although M-ary WSK (M>) schms hav highr spctral fficincy than binary WSK systm. In th abov rsarch works, th BER prformanc for M- ary uncodd WDM systm prformanc has not bn considrd whil mitigating th ffct of FWM. It is, thrfor, vry much important to includ th ffct of FWM as th prformanc of a M-ary WDM systm is highly dpndant on th intrplay btwn th cross-phas modulation and four-wav mixing in a normal disprsiv fibr as wll as in a disprsion shiftd fibr (DSF) [9-].. SYSTEM MODEL Th modl of th WSK-DWDM systm considrd for analysis is shown in Fig. Th systm is usd to combin ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 974
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, diffrnt signal carrir wavlngths onto a singl fibr at on nd and sparat thm onto thir corrsponding dtctors at th othr nd. Th Encodr ncodd th data and at th rciving nd th dcodr dcodd that data to rcivd sam signal. Th convolution coding is usd in this systm. Hr MUX combin all of th signals and crat a composit signal. This signal passs through th optical fibr and optical amplifir. Optical amplifir amplifis an optical signal dirctly, without th nd to first convrt it to an lctrical signal. An optical amplifir may b thought of as a lasr without an optical cavity, or on in which fdback from th cavity is supprssd. Stimulatd mission in th amplifir's gain mdium causs amplification of incoming light and DMUX sparat thm. M U X OA SMF D M U X N TX RX Encodr Dcodr Data input Data output Fig.. Block diagram basic codd WSK-WDM transmission systm. Thn at th rcivr w gt all th signals individually. In orthogonal MWSK, th M=k district symbols ar rprsntd by M-WSK wavforms S m( m m t) ACos ( W t ), whr, m =,,..,M () whr C Wm f m m initial phas A = Signal amplitud T s = KT b = Signaling intrval T b = bit duration E s = A T s m Signal Enrgy = Enrgy/symbol To driv th avrag probability of symbol rror, w assum that th signal (S, H) was snt and rcivd signal r t S t n t () T s m m (3) n n( t) S ( t) dt 3. CHANNEL MODELLING With th incras of data rats on optical fibr, transmission lngth, numbr of channls and optical powr lvls, non linar ffcts of fibr bcoms dominant. Th hug potntial bandwidth of optical fibr can b afficintly utilizd by multiplxing a numbr of channls and transmitting thm through th fibr simultanously. Th transmission bandwidth of fibr is dividd into a numbr of nonovrlapping frquncy (or wavlngth) bands and ach of ths bands is associatd to support a singl communication channl. Two principl kinds of multichannl systms ar common in practical applications, namly, frquncy division multiplxing (FDM) and wav lngth division multiplxing (WDM). Th two schms diffr from ach othr in rspct of transmittr/rcivr configuration. Th probability dnsity function of S m, givn that S (t) was snt is Whr, n(t)=awgn with zro man. S ES n S m = n m,m=,3..m [( Sm mes ) EN s S m S EN s Now, (4) ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 975
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, SM ES N S S M dsm ds S M S EN (5) S lt x S, SM EN s x S M dsm dx S E S NS (6) Th prformanc of a multichannl WSK-WDM transmission systm in prsnc of FWM has bn valuatd in this sction for binary and M-ary systm. Th systm prformanc dpnds on various systm paramtrs such as transmission powr pr channl, numbr of channl, fibr lngth; diffrnt channl spacing will b xamind. Finally, th prformanc of WSK- WDM systm is compard for binary and M-ary uncodd systm with diffrnt paramtrs. 4. THEORETICAL ANALYSIS 4. Bit Error Rat (BER) of Uncodd Systm In th transmittr, th data of Gbps is usd to dirctly modulat a lasr to gnrat th WSK signal which is transmittd through a singl-mod fibr. At th rciving nd, th rcivd optical signal is dtctd by a Mach- Zhndr intrfromtr. In th WSK dirct dtction rcivr with MZI, th MZI act as an optical filtr and diffrntially dtct th mark and spac of rcivd WSK signal. Th st MZI rcivd th multipl channl signals thn it diffrntially dtcts th odd and vn signals. Thos ar thn dirctly fd to a pair of MZIs. Th Odd signals go to on MZI and th vn signals go to anothr MZI. Aftr that th signals ar sparatd gradually, which ar thn dirctly fd to a photo dtctor. All photo currnts ar applid to th amplifir which is followd by a Maximum Liklihood Dtctor (MLD). Aftr passing through th MLD, th signal is dtctd at th dcision circuit by comparing it with a thrshold of zro valu. In orthogonal MWSK, th M=k district symbols ar rprsntd by M S E S ES N S ds EN S EN S (7) M y E S y dy N (8) M y ES log M y dy N (9) Lt, y E M s Q y N Rd s Q y M () W can writ, th probability of BER for WSK-WDM systm, Rd S [ Q( y )] O M y ( ) dy () ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 976
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, whr,. th shot fwm s fwm and ( R )( R ) s fwm d fwm d s Rd fwm s. Q( y E s ) M y ( ) dy () W can writ, R d s E y s Q( y ) s M E dy 4. Bit Error Rat (BER) of Codd Systm Thr typs of nois also considr in this analysis, thrmal nois, shot nois and FWM and all othr nois ar assum to ngligibl. Th allowabl input powr is dtrmind for various transmission distancs to achiv a BER of -9 for th bit rat of Gbps limiting ach systm to th sam total bandwidth. It is indicating that for a givn BER of -9, WSK-WDM givs longr rpatr spacing. Th probability of bit rror rat (BER) for a codd (Convolution Coding) systm is givn by hd f W h) b h (. (4) (3) By taking valus for various valus of k which is a constant lngth, lik if tak k=6 thn w ar considring th hamming wight from and if tak k=7 thn w ar considring th hamming wight from and so on. 5. RESULTS AND DISCUSSION Following th analytical approach prsntd in sction 4, w valuatd th BER prformanc of WSK-DWDM systm considring th ffct of both binary and M-ary systm. For th convninc of th radrs th paramtrs usd for computation in this papr ar shown in tabl. Tabl : Nominal aramtrs of Optical Communication link Whr, ( h) ( ) h aramtr Nam Bit Rat, B r Valu Gbps and robability also of R d rfc s n th BER shot rat for ucodd fwm s fwm systm. Tmpratur 3 Fibr attnuation, α Rsponsivity, R Channl Spacing, D ch Load Rsistanc, R l.4 db/km.85 A/W GHz 5 ohm In this sction, w obsrv th ffct of BER prformanc for an uncodd systm and compar th BER prformanc ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 977
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, for binary and M-ary systm with four wav mixing (FWM). Fig : BER vs. in (dbm), codd and uncodd systm for WSK-DWDM systm. (Br = Gbps) Fig. shows th plots of BER vs. in (dbm) with diffrnt constraint lngth (k). Th bit rror rat prformanc rsults ar valuatd at a data rat of Gbps pr channl. Kping th othrs paramtrs ar constant, w compar th prformanc of th systm with and without coding (convolution). It is found that, significant improvmnt of BER prformanc is achivd by applying convolution coding. For convolution cod of rat ½, th coding gain is 5dB for constraint lngth k=6 and 6dB for k=7 at an uncodd BER of -9. Fig 3: BER vs. No. of channl (N) at constant input powr for WSK-DWDM systm. (Br=Gbps, in=-dbm, r=/). Fig 3.shows th plot of BER vs. No. of channl (N) for codd and uncodd systm at a constant input powr (in=-dbm) and diffrnt constraint lngth (k) at a rat of ½. Th plot shows that th rmarkabl incras in numbr of channl for codd systm than uncodd systm. At a BER of -9 th numbr of channl usd is ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 978
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, mor pronouncd for in= -dbm for codd systm than uncodd systm. Fig: 4: Rcivr snsitivity (inmax) vs. No. of channl (N) with and without coding and with four wav mixing for WSK-DWDM systm. (Br=Gbps, BER= -9, L=5km, Dch=GHz, r=/) Fig.4 shows th rcivr snsitivity vs. no. of channl for codd and uncodd systm at constant bit rror rat ( -9 ) and constant fibr lngth (L=5 km) with four wav mixing (FWM). Th plot shows that at a constant channl, mor input powr is rquird for uncodd systm than a codd systm. As a rsult th rcivr snsitivity is improvd for th codd systm. Fig: 5: BER vs. in (dbm) for codd and uncodd systm with four wav mixing. (N=4, L=km, Br=Gbps, r=/, Dch=GHZ) Fig: 5.Shows th plots of BER vs. in (dbm) for uncodd and codd systm with four wav mixing. It is noticd that BER prformanc is much bttr for codd systm than uncodd systm up to crtain valu of input powr. Aftr that th BER prformanc will start to dgrad for both th systm. For xampl at db input powr, th BER for uncodd systm is about -3 whil for codd (k=6, r=/) systm th BER is - and for codd (k=7, r=/) systm th BER is -5. ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 979
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, Fig: 6: Bit Error Rat (BER) vs. Input powr pr channl, in(dbm) for diffrnt no. of channl for WSK-DWDM. (L=km, Rb=Gbps, Dch=GHz, r=/, k=6) Fig: 6.shows th Bit Error Rat (BER) prformanc vs. Input powr, in (dbm) with diffrnt no. of channl (N) at constant fibr lngth (L) and constant bit rat (R b ) with four wav mixing for codd systm. It is obsrvd that th bit rror rat prformanc is dgrading with incras in channl with constant fibr lngth. For, xampl, at db input powr, th BER in th ordr of -5, -9, -6 for N=4, 8 and 6 rspctivly. Fig: 7: BER vs. in (dbm) for diffrnt fibr lngth for WSK-DWDM with FWM. (N=6, R b =Gbps, r=/, k=6, Dch=GHz) Fig: 7 shows th plots of BER vs. Input powr pr channl, in (dbm) for diffrnt fibr lngth at a constant no. of channl and constant bit rat. It is obsrvd that th systm nd mor powr with th incras of fibr lngth for sam amount of BER. ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 98
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, Fig. 8: BER vs. in(dbm) for binary and 4-ary uncodd systm with FWM. (L=km, R b = Gbps, N=4, uncodd, Dch=GHz) From Fig 8.it is obsrvd that, significant improvmnt of BER prformanc is achivd for M-ary systm than binary systm without coding at constant fibr lngth, channl and bit rat. Fig. 9 : BER vs. Input powr pr channl (in) for multipl ary without coding for WSK-WDM. (L=75km, N=6, R b = Gbps, Dch=GHz, uncodd systm) Fig: 9: shows th BER vs. input powr pr channl (in) with M=4, 8 and 6 at constant fibr lngth, channl and constant bit rat. W analyz th BER prformanc with diffrnt M valu for uncodd systm. It is obsrvd that BER prformanc is mor pronouncd with incras in no. of ary ( M) and rsults in BER floor. For xampl, at - 5dB input powr th BER for binary systm is - whil for uncodd multipl ary th BER in ordr of -4, -8 and -8 for M=4, 8, 6 rspctivly. ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 98
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, Fig. : BER vs. in(dbm) for WSK-DWDM systm at diffrnt channl spacing. (M=6, N=6, L=km, R b = Gbps, uncodd systm) Fig. : BER vs. input powr pr channl, in (dbm) with diffrnt channl spacing. It is noticd that th WSK- WDM systm suffrs almost th sam amount of powr pnalty for lowr valus of input powr at constant BER. But at highr valus of input powr th amount of powr pnalty incras with incras in channl spacing. Fig. : Rcivr snsitivity vs. M for WSK-WDM systm at diffrnt fibr lngth. (BER= -9, R b = Gbps, N=6, D ch = GHz, uncodd systm.) Fig.. illustrat th rcivr snsitivity or powr pnalty comparison with diffrnt fibr lngth for multipl ary. Th plots shows that th amount powr pnalty with incras in fibr lngth for a constant ary systm. Th tabl 5.3.4. shows th amount of powr pnalty with diffrnt fibr lngth at 4-ary systm. Th prformanc analyz is for uncodd systm at constant BER ( -9 ) with four wav mixing (FWM). ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 98
Intrnational Journal of Enginring and Tchnology (IJET) Volum No., Dcmbr, 6. CONCLUSION A dtaild analytical approach is prsntd to valuat th bit rror rat prformanc of WSK-WDM in prsnc of FWM. Th WSK-WDM systm xhibits promising faturs that will b usful for futur high spd, long distanc optical ntworks. FWM bcoms th major sourc of non-linar ffcts causing intrchannl crosstalk and channl powr dpltion and thrby dgrading th systm prformanc. It has bn obsrvd that th prformanc of WSK-WDM systm with out coding is not satisfactory; rathr th prformanc is furthr dtrioratd whn input powr pr channl is high. WSK- WDM can provid bttr prformanc by applying convolution coding. A rmarkabl improvmnt in systm prformanc can b achivd vn for uncodd systm for M-ary systm than binary systm. This systm offrs much bttr prformanc: it givs much lowr BER, maximum allowabl input powr pr channl is much highr than that of othr systms. REFERENCES [] A. R. Chraplyvy, Optical powr limits in multichannl wavlngth-division-multiplxd systm du to stimulatd Raman scattring, Elctron. Ltt., vol., no.9, pp. 58-59, 984. [] H. Toba, K. Oda, K. Nakanishi, N. Shibata, K. Nosu, N. Takato and M. Fukuda, A - Channl Optical FDM Transmission/ Distribution at 6 Mb/s ovr 5 Km, IEEE J. Lightwav Tchnol., Vol.8, No.9, pp. 396-4, Spt 99. [3] F. Forghiri, R. W. TKach, A. R. Chraplyvy, A. H. Gnauck, R. M. Drosir, Four-photon mixing and high spd WDM systms, IEEE J. Lightwav Tchnol., vol. 3, no. 5, pp-84-849, 995. [4] F. Forghiri, R. W. Tkach, A. R. Chraplyvy and D. Marcus, Rduction of four-wav mixing in WDM systms using unqually spacd channls, IEEE hoton. Tchnol. Ltt, vol. 6, no. 6, pp.754-756, 994. [5] K. Inou, K. Nakanishi, K. Oda, and H. Toba, Crosstalk and powr pnalty du to fibr four-wav mixing in multichannl transmission, IEEE J. Lightwav Tchnol., vol., no. 8, pp. 43-439, 994. [6] C. Xiang, and J. F. Young, Wavlngth Shift Kying tchniqu to rduc four-wav mixing crosstalk in WDM, proc. IEEE LEOS Annual Mting, papr WZ, pp. 69-6, San Francisco, CA, Nov 999. [7] T. Numai, and O. Kubota, Analysis of rpatd unqually spacd channls for FDM lightwav systms, IEEE J. Lightwav Tchnol., vol. 8, no. 5, May. [8] M. Faisal, Analysis of wavlngth shift kying Tchniqu with Disprsion Managmnt Schm to Rduc Four-Wav Mixing ffct in optical WDM systms. M.Sc. Thsis, Dpt. of EEE, BUET, 3. [9] R. Hui, K. R. Dmarst and Alln C. T., Cross has Modulation in Multispan WDM optical systms,. Journal of lightwav Tchnology, vol. 7, no. 6, pp 8-6, Jun 999. [] K. Hoon, Cross has Modulation inducd nonlinar phas nois in WDM dirct dtction DSK systms, Journal of lightwav Tchnology, vol., no. 8, pp 6-37, August 3. [] K. Song, and M. rmarantn, Effct of SM, XM and four wav mixing in L-band EDFAs on fibr optic signal transmission, IEEE hotonics Tch. Lttrs, vol., no., Dcmbr, pp. 63-63. ISSN: 49-3444 IJET ublications UK. All rights rsrvd. 983