Optimization of Single-Sideband DCS-RZ Format for Long-Haul 43-Gb/s/channel Ultradense WDM Systems

JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 2, FEBRUARY 2007 481

Optimization of Single-Sideband DCS-RZ Formatfor Long-Haul 43-Gb/s/channel

Ultradense WDM SystemsNataša B. Pavlovic and Adolfo V. T. Cartaxo, Senior Member, IEEE

Abstract—The optimization of the single-sideband (SSB) duo-binary carrier-suppressed (DCS) return-to-zero (RZ) format forlong-haul ultradense wavelength-division multiplexing (UDWDM)systems with a 43-Gb/s/channel and a channel spacing of 50 GHz isinvestigated numerically. It is shown that the optimized SSB-DCS-RZ format with an electrical transmitter bandwidth of 0.35 ofthe bit rate has about 1 dB of Q-factor improvement relative tothe conventional SSB-DCS-RZ format (an electrical transmitterbandwidth of 0.25 of the bit rate) due to the reduction of noisebandwidth, smaller linear crosstalk, and better tolerance to theintra-channel fiber nonlinear effects. No substantially differentUDWDM system performance is observed when varying the dutycycle of the DCS-RZ signal with optimized SSB filters settings.The UDWDM transmission performance of SSB-DCS-RZ for-mats is compared with the bandwidth-limited (BL)-duobinaryformats. It is shown that generally, the SSB-DCS-RZ formats havepoorer Q-factor and tolerance to the total residual dispersion butmuch higher tolerance to the in-line dispersion compensation andintra-channel nonlinear effects than the BL-duobinary formats.

Index Terms—Crosstalk, duobinary carrier-suppressed return-to-zero (DCS-RZ), fiber nonlinearity, in-line dispersion, opticalsingle-sideband (SSB) filtering, residual dispersion, ultradensewavelength-division multiplexing (UDWDM).

I. INTRODUCTION

CURRENT research on increasing the capacity of metroand long-haul (LH) ultradense wavelength-division mul-

tiplexing (UDWDM) systems aims at achieving very high spec-tral efficiency close to 1 b/s/Hz [1]. This has been accomplishedby using the highly reduced channel spacing in the order ofthe bit rate per channel, e.g., a channel spacing of 50 GHz foran information bit rate of 40 Gb/s. In such UDWDM systems,the spectra of adjacent channels may overlap significantly ifnot properly bounded, remarkably increasing the performancedegradation caused by linear crosstalk [2]. Forward-error cor-rection (FEC) techniques have been indicated for metro and LHsystems [1] to improve the performance or increase the system

Manuscript received June 22, 2006; revised October 24, 2006. This work wassupported by Fundação para a Ciência e a Tecnologia (FCT) from Portugal,FEDER, and POSC within Project POSC/EEA-CPS/56959/2004—SHOTS.The work of N. B. Pavlovic was supported by FCT under Contract SFRH/BD/10162/2002.

The authors are with the Optical Communications Group of Lisbon Pole,Instituto de Telecomunicações, and the Department of Electrical and ComputerEngineering, Instituto Superior Técnico, 1049-001 Lisboa, Portugal (e-mail:[email protected]; [email protected]).

Digital Object Identifier 10.1109/JLT.2006.889357

margin. On the other hand, its use leads to a larger transmittedsignal bandwidth, causing greater pre-FEC decoding perfor-mance degradation in UDWDM systems due to linear crosstalkenhancement.

Spectrally efficient signaling formats, such as duobinary,together with tight multiplexer (MUX) and demultiplexer(DMUX) optical filtering, and suitable pulse-shaping filteringhave been proposed to achieve such a high spectral efficiencywith acceptable linear crosstalk levels [1], [3]. For metroand LH transmission systems, signaling formats with low-costtransmitters and receivers are recommended. Thus, the useof signaling formats that require the use of interferometricdetection, such as those of the differential phase-shift keyingfamily, is questionable, particularly if direct-detection signalingformats also show good performance for those systems.

Two kinds of signals can be generated from tight MUXoptical filtering. In the first kind, the optical filtering is tunedto the channel optical carrier, and both sidebands are present inthe bandwidth-limited (BL) signal. The BL-duobinary formathas been reported as the direct-detection format with one of thebest transmission performances [1], [4], [5]. In the second kind,the optical filtering is suitably detuned from the optical carrierso that one of the sidebands is greatly suppressed, leadingto single-sideband (SSB) signals. SSB filtering of duobinary-carrier-suppressed (DCS) return-to-zero (RZ) format, whichpresents low spectral content around the carrier frequency,has been proposed to generate a signal suitable for use inUDWDM systems [6]. The transmission performance of theSSB-DCS-RZ format was first tackled in [6]. However, onlya dispersion map with perfect dispersion compensation, wherethe amount of transmission fiber dispersion per span is ex-actly compensated by dispersion-compensating fibers (DCFs)located at the end of each span, was considered. It is known thatsuch dispersion map is not the optimum when fiber nonlineareffects affect the transmission performance, which is the caseof LH transmission systems with typical average power levelsat the transmission fiber input of about 0 dBm [1], [3]. For a fairanalysis of transmission performance, the optimum dispersionmap (ODM) of each signaling format should be considered, andthe resilience to variations of the dispersion map and of nonlin-ear fiber effects strength should be analyzed [1]. Moreover, in[6], just a particular pulse shape and duty cycle of the DCS-RZ format was considered. Therefore, substantial performanceimprovement of a LH UDWDM system can be expected byoptimization of the DCS-RZ pulse shape and duty cycle.

0733-8724/$25.00 © 2007 IEEE

482 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 25, NO. 2, FEBRUARY 2007

Fig. 1. Schematic diagram of the 5 × 43 Gb/s UDWDM system over 10 × 80 km SSMF with an in-line post-dispersion compensation scheme and post-compensation.

In this paper, we perform an extensive numerical optimiza-tion of the SSB-DCS-RZ signaling format for LH UDWDMsystems with a bit rate of 43 Gb/s/channel and a channelspacing of 50 GHz and analyze its transmission performanceand tolerance to in-line dispersion compensation (IDC) andtotal residual dispersion (TRD), as well as to fiber nonlinearity.This bit rate was chosen to take into account the impact ofemploying FEC techniques on the system performance beforeFEC decoding corresponding to an information bit rate of40 Gb/s (SONET OC-768) and a worst-case FEC overhead bitrate of 3 Gb/s. The remainder of this paper is organized asfollows: In Section II, the system model is presented. Section IIIdeals with the optimization of the generation scheme of SSB-DCS-RZ signaling format. In Section IV, the transmissionperformance of different SSB-DCS-RZ formats is analyzed andcompared with the performance of BL-duobinary formats withseveral electrical transmitter bandwidths. Section V presentsthe main conclusions.

II. SYSTEM MODELING

The DCS-RZ signals are generated in the transmitter, asdescribed in [7]. The first Mach–Zehnder modulator (MZM)generates the optical duobinary signal. The 43-Gb/s non-RZ(NRZ) complementary signals are first precoded and then con-verted into electrical duobinary signals by five-pole Bessel low-pass filters (LPFs). The 3-dB electrical bandwidth of theseBessel LPFs is varied in order to optimize the SSB-DCS-RZ signaling format performance. In the following, this 3-dBelectrical transmitter bandwidth normalized to the bit rate is de-noted as bel,Tx,n. The second MZM induces carrier suppressionon the optical duobinary signal. It is driven by a sinusoidal clockwaveform with a frequency of half the bit rate. Both MZMsare the push–pull type, and they are biased at the transmissionminimum point. The pulsewidths of such DCS-RZ signals canbe set to 67% or 53% of the bit duration, i.e., a duty cycleof 67% or 53%, by choosing adequately the driving voltagesswing. However, increased modulator insertion loss occurs forthe duty cycle of 53%. The conventional DCS-RZ signal hasbel,Tx,n = 0.25 and the duty cycle of 53% [6], [7]. For smaller

TABLE ISSMF AND DCF PARAMETERS

duty cycles, a third MZM is inserted after the second MZM,which is driven by a clock signal with a frequency of half thebit rate, and biased at the transmission maximum point. Withthis scheme, a DCS-RZ signal with a duty cycle of 33% [8] oreven 27%, but at the expense of increased modulator insertionloss, can be generated.

The UDWDM system has five channels spaced by 50 GHzwith the center channel located at 1552.52 nm. All channels areassumed to be copolarized for worst case performance evalu-ation. The system setup is shown in Fig. 1. Each transmittergenerates a different “deBruijn” binary sequence of 212 bits ata bit rate of Rb = 43 Gb/s. This long sequence length allowsassessing the UDWDM system performance rigorously. Thiswas confirmed by checking that almost the same performanceis computed with a larger number of bits. The optical filteringaccomplished by the MUX is modeled by a second-order Super-Gaussian transfer function [9].

The LH link consists of ten spans of 80 km of standardsingle-mode fiber (SSMF). The spans are separated by repeatersincorporating dual-stage erbium-doped fiber amplifiers (EDFA)with a DCF between the two stages. Each EDFA has a noisefigure of 6 dB. The gain of each EDFA is set to compensate forthe total loss per span and impose a power level at a DCF inputof −6 dBm in order to reduce the nonlinear effects in the DCF.The fiber parameter values are shown in Table I. Each of the firstnine spans has the same DCF length, which is appropriatelychosen to set the IDC, whereas the TRD is set by choos-ing the DCF length of the tenth span. No pre-compensationat the transmitter side is employed to mitigate dispersive and

PAVLOVIC AND CARTAXO: OPTIMIZATION OF SSB-DCS-RZ FORMAT FOR LH 43-Gb/s/CHANNEL UDWDM SYSTEMS 483

nonlinear distortions because in metro and LH systems, and itsuse is only considered if really necessary.

Transmission along the SSMF and DCF is modeled by thegeneralized nonlinear Schrödinger equation (NLSE) [10]. Inorder to determine rigorously the joint impact of group veloc-ity dispersion (GVD), intra-channel nonlinear effects, cross-phase modulation (XPM), and four-wave mixing (FWM) onthe UDWDM system performance, the total field approach isadopted for the simulation model, and the simulated spectralrange is more than three times the bandwidth occupied by theUDWDM channels, as is usually required for rigorous fibertransmission simulation [11], [12]. The NLSE is solved usingthe symmetrized split-step Fourier method with a maximumstep size of 100 m. At the receiver side, the DMUX filter-ing is modeled also by a second-order Super Gaussian filter.Each 43-Gb/s receiver is modeled by a p-i-n photodiode withresponsivity of 1 A/W, an electrical filter modeled by a third-order Bessel filter with a 3-dB bandwidth of 0.7 Rb, and adecision circuit. The average power per channel at the p-i-ninput is 0 dBm.

To evaluate the system performance, a semianalytic sim-ulation technique is used. At the decision circuit input, theexact noise-free electrical signal waveform resulting from thesimulation of signal propagation along the transmission systemand the exact expression of the time-varying beat noise variancepresented in [13], with the noise accumulation of all EDFAstaken into account, are used. The Gaussian approach presentedin [13] is used to compute the bit-error probability associatedwith each individual bit of the “deBruijn” sequence. The av-erage bit-error probability is calculated by averaging the bit-error probability over all bits of the sequence. A very accurateestimate of the bit error rate (BER) before FEC decodingis obtained by minimizing the average bit-error probabilitythrough the joint optimization of the decision threshold andsampling instant. The high accuracy of this method of BERcomputation was experimentally shown in [14]. The equivalentQ-factor is used to categorize the system performance and isunivocally related to the BER before FEC decoding as

Q =√

2 · erfc−1(2 · BER) (1)

where erfc−1(x) is the inverse complementary error functionof x. For BER = 10−13, the required Q-factor is 17.3 dB. Toaccurately assess the UDWDM transmission performance, par-ticularly, the impact of inter-channel effects, several realizationscorresponding to different interfering conditions (relative timeshifts and optical phases) between the center channel and otherchannels are considered. The equivalent Q-factor is calculatedfrom the average BER using (1). The average BER is computedas the average of the BERs associated with each realization.Five realizations are used in this paper, as in [19]. Q-factorestimate variations of the order of only 0.1 dB were observedfor a larger number of realizations.

In order to illustrate the performance achieved by using FECand the gain insight of its ability to provide an acceptablesystem margin of operation, an FEC coding scheme based on

a concatenated1 RS (247,239) + RS (255,247) is used as areference. Therefore, the performance after FEC decoding canstill be improved whether a FEC scheme with larger overheadbut within 3 Gb/s of bit rate is used. With that FEC scheme,the required Q-factor achieving a BER of 10−13 after FECdecoding without burst errors is 9.2 dB [15].

III. OPTIMIZATION OF THE SSB-DCS-RZFORMAT GENERATION

In this section, the results of the joint optimization of thebandwidths and detunings of MUX and DMUX for several3-dB electrical transmitter bandwidths (bel,Tx,n) and signalduty cycles are presented and discussed. This optimizationis obtained, considering the same MUX and DMUX filtersettings in a back-to-back configuration. Therefore, it takesinto account the intersymbol interference (ISI), linear crosstalk,and amplified spontaneous emission noise from EDFAs. It wasconfirmed that a similar performance is obtained by varying theMUX and DMUX filters settings independently. The amplifiedspontaneous emission noise introduced by the EDFAs is suchthat an optical signal-to-noise ratio (OSNR) measured beforethe DMUX over a noise bandwidth of 0.4 nm is 17.6 dB, takinginto account the noise on both polarizations. Note that thisis the OSNR that is obtained when the DCF lengths are setto exact dispersion compensation in all spans, and the fibersin the transmission scheme shown in Fig. 1 are replaced byattenuators with the same loss as the fibers.

Fig. 2 shows the typical contour plots of the Q-factor penaltywith respect to the maximum Q-factor achieved for eachplot as a function of the 3-dB bandwidths and detunings ofMUX/DMUX SSB filters for different bel,Tx,n (left-hand sideand middle subplots) and duty cycles (middle and right-handside subplots). For all cases, a large tolerance to variationsof bandwidth and detuning is observed (a range of variationsof 10% for a Q-factor penalty not exceeding 1 dB) due tothe spectral characteristics of DCS-RZ signals. The DCS-RZsignals with higher bel,Tx,n have steeper but wider main lobes ofthe spectrum than the ones with smaller bel,Tx,n. Smaller signaldistortion is observed with narrower optical filtering due to thesteeper main lobes of the signal spectrum. Thus, smaller opti-mum 3-dB bandwidths and detunings of the MUX and DMUXoccur for the DCS-RZ format with higher bel,Tx,n. Slightlylarger Q-factor penalty with small MUX/DMUX bandwidthand detuning variations from their optima occurs for SSB-DCS-RZ formats with higher bel,Tx,n, due to the wider main lobes ofthe signal spectrum, than for the conventional SSB-DCS-RZformat.

Furthermore, Fig. 2 shows very similar optimum filter set-tings for different duty cycles. The DCS-RZ signals withsmaller duty cycle have smaller Q-factor penalty for higherbandwidths and detunings of the MUX and DMUX filters, dueto the slightly smaller 20-dB bandwidth of the main lobes,than the signal with larger duty cycle. Furthermore, the slightvariation of optimum MUX/DMUX bandwidths and detuningsbetween the DCS-RZ signals with different duty cycles causes

1RS stands for Reed–Solomon coding.


Fig. 2. Contour plots of the Q-factor penalty (in decibels) as a function of the MUX/DMUX 3-dB bandwidths and detunings for DCS-RZ with a duty cycle of67% and bel,Tx,n = 0.25 (left-hand side), a duty cycle of 67% and bel,Tx,n = 0.35 (middle), and a duty cycle of 33% and bel,Tx,n = 0.35 (right-hand side).

TABLE IIOPTIMAL BANDWIDTH Bo AND DETUNING deto OF MUX AND

DMUX FOR EACH SSB-DCS-RZ FORMAT OBTAINED IN

BACK-TO-BACK CONFIGURATION

only a small influence on the signal characteristics. The opti-mum bandwidths and detunings of MUX and DMUX for eachDCS-RZ format are shown in Table II. The optimum bandwidthand detuning for conventional DCS-RZ format are consistentwith the results shown in [6].

Fig. 3 shows the Q-factor as a function of the bel,Tx,n for theoptimum MUX and DMUX filter settings shown in Table IIand the different duty cycles of the DCS-RZ signal. Single-channel and UDWDM system performance is shown to stressthe influence of the linear crosstalk. Contrary to the single-channel system, a very similar Q-factor is found by varyingthe duty cycle of the DCS-RZ signal for the UDWDM system.Hence, SSB-DCS-RZ signals with the conventional duty cycleof 53% is considered in Section IV for the comparison ofthe UDWDM transmission performance of the SSB-DCS-RZformats with different bel,Tx,n. The different performances notexceeding 0.1 dB between the DCS-RZ signals with differentduty cycles found in the single-channel system were lessenedin the UDWDM system by the different influences of linearcrosstalk on the DCS-RZ signals with different duty cycles.

For all duty cycles, in single- and multichannel systems,SSB-DCS-RZ formats with higher bel,Tx,n than the conven-tional one have higher Q-factor than the conventional SSB-DCS-RZ format. The optimum system performance results

Fig. 3. Q-factor (in decibels) for single-channel (dashed lines) and UDWDM(solid lines) systems as a function of the normalized electrical transmitter filterbandwidth bel,Tx,n for optimum optical SSB filters and different duty cycles.

from a compromise between opposite effects that occur withthe increase of bel,Tx,n: the slight eye-opening decrease andthe decrease of optimum MUX and DMUX bandwidths clearlyshown in Table II, which leads to the noise power reduction atthe receiver input, and the increase of the tolerance to the linearcrosstalk. The decrease of the influence of linear crosstalk withbel,Tx,n for SSB-DCS-RZ formats was confirmed by varyingrandomly the phase and time shifts within 64 blocks of 27 bitsbetween two interfering channels and the center channel, asproposed in [2]. Small Q-factor variation (less than 0.7 dB)within 500 realizations was observed for all investigated SSB-DCS-RZ formats.

Fig. 3 shows that the maximum Q-factor for the UDWDMsystem is found with bel,Tx,n between 0.3 and 0.35 for all dutycycles. Thus, the SSB-DCS-RZ format with bel,Tx,n = 0.3 isconsidered in Section IV for the comparison of the transmissionperformance between SSB-DCS-RZ signals with different dutycycles. It is interesting to note that for the UDWDM system,a similar optimum electrical transmitter bandwidth was previ-ously found for the BL-duobinary format [1], [5]. However,


several important differences between the SSB-DCS-RZ andBL-duobinary formats should be stressed.

1) The SSB-DCS-RZ signal has slightly more degraded eye-opening due to the narrow filtering.

2) The SSB-DCS-RZ signal has narrower 20-dB spectrumbandwidth than the BL-duobinary signal, with about 40and 47 GHz for the SSB-DCS-RZ and BL-duobinaryformats with bel,Tx,n = 0.25, respectively. The 20-dBsignal bandwidth slightly increases with the increase ofbel,Tx,n for both formats.

3) Contrary to the BL-duobinary format, the SSB filteringimposes a certain chirp on the SSB-DCS-RZ signal [16],which may improve the transmission performance.

In the following, the optimized bandwidths and detuningsshown in Table II are considered in the comparison of trans-mission performance of the different SSB-DCS-RZ signalingformats.

IV. TRANSMISSION PERFORMANCE

A main issue in choosing a signaling format for LHUDWDM systems is the impact of the fiber transmission on theformat performance. Two aspects are relevant to that respect,namely: 1) the optimization of dispersion and power mapsin order to maximize the performance (Q-factor) and 2) theresilience to fiber propagation, particularly the tolerances toIDC and TRD and to fiber nonlinearity. In the following, first,the dispersion map is optimized for the same level of averagepower at the SSMF input for each signaling format, allowingassessment of the maximum Q-factor. Then, the tolerances toIDC and TRD are calculated from the ODM. Last, the toleranceto fiber nonlinearity is assessed for each signaling format.The analyses of tolerances are performed for single-channeland UDWDM systems in order to gain insight on the impactof crosstalk on the reduction of tolerances. The transmissionperformances of different SSB-DCS-RZ formats are comparedwith the ones of the BL-duobinary format generated as in [4]but for several bel,Tx,n.

A. Dispersion Map Optimization

In this section, the dispersion map optimization is carried outby varying the amount of IDC and TRD and computing theQ-factor corresponding to each pair of them. The IDC ex-pressed in percentage is defined as

IDC(%) =|LDCF · DDCF|LSSMF · DSSMF

× 100 (2)

where LSSMF and LDCF are the lengths of the SSMF and DCFin each of the first nine spans, respectively, and DSSMF andDDCF are the dispersion parameters of the SSMF and DCF,respectively. The TRD is the total link-cumulated dispersionand is defined as

TRD = (Nsp − 1) · LSSMF · DSSMF ·(

1 − IDC(%)100

)

+ LSSMF · DSSMF + (LDCF)ls · DDCF (3)

Fig. 4. Contour plots of the Q-factor penalty (in decibels) as a function ofthe IDC and TRD for the SSB-DCS-RZ format with a duty cycle of 53% andbel,Tx,n = 0.35 for single-channel (left-hand side) and UDWDM (right-handside) systems.

where Nsp is the number of spans (10), and (LDCF)ls is theDCF length of the last span.

In the case of a UDWDM system, the dispersion map opti-mization is performed for the center channel. For single- andmultichannel systems, the optical power at the SSMF input isset to 0 dBm per channel. In Section IV-C, it is shown that forthis power level at the SSMF input, a small system performancedegradation is observed in comparison with the performanceachieved with the optimum power.

Fig. 4 shows the typical contour plots of the Q-factor as afunction of the IDC and TRD for single-channel and UDWDMsystems. Fig. 4 refers to the SSB-DCS-RZ format with abel,Tx,n of 0.35 (as one of the optimal bel,Tx,n) and the dutycycle of 53%. Similar contour plots were drawn for otherinvestigated signaling formats. About 1000 different dispersionmaps were analyzed to draw each contour plot. The compu-tation error of the optimums and tolerances of IDC and TRDis mostly due to the IDC and TRD step used in simulations,which are 0.5%, and 5 ps/nm, respectively. Similar shapesof contour plots were obtained for all formats. The type ofshape shown in Fig. 4 indicates that the fiber nonlinearityconsiderably constrains the range of IDC and TRD in bothsingle-channel and UDWDM systems. Further analysis showedthat considering linear propagation in the SSMFs and DCFs,


TABLE IIIOPTIMUM IDC, TRD, AND MAXIMUM Q-FACTOR FOR SINGLE-CHANNEL AND UDWDM SYSTEMS FOR THE SSB-DCS-RZ FORMAT

WITH A DUTY CYCLE OF 53% AND BL-DUOBINARY FORMAT WITH DIFFERENT bel,Tx,n

the Q-factor is nearly constant in a much wider range of IDCthan the one shown in Fig. 4. On the other hand, with linearpropagation, the Q-factor degradation around the maximumQ-factor due to TRD variation is somehow similar to the oneshown in Fig. 4. Therefore, the IDC optimum and the IDCtolerance are mainly influenced by the intra- and inter-channelfiber nonlinear effects.

From the contour plots of the Q-factor, the ODM and themaximum Q-factor of each signaling format can be easilyextracted. Table III presents the optimum IDC and TRD, andthe maximum Q-factor in single-channel and UDWDM sys-tems for the SSB-DCS-RZ format with the duty cycle of 53%and the BL-duobinary format, for different bel,Tx,n. Furtheranalysis showed that in single-channel and UDWDM systems,very similar Q-factor and ODM are achieved by SSB-DCS-RZ formats with different duty cycles. Comparison of figuresshown in Table III indicates that the SSB-DCS-RZ formats havesimilar ODMs (IDC about 106% and TRD about 40 ps/nm).For all the formats, in the ODM, the amount of dispersion in theSSMF is not exactly compensated by the DCF. This is requiredto compensate for the distortion due to fiber nonlinear effects(intra-channel in single-channel system and intra-channel andXPM in the UDWDM system) as the FWM effect is expectedto be very weak with such high local dispersion fibers. A signalwith a wider spectrum is potentially more sensitive to pulsedistortion at nonzero residual GVD than a signal with narrowerspectrum [17]. Thus, for single-channel and UDWDM systems,a slight decrease of TRD optima for the SSB-DCS-RZ andBL-duobinary formats with the increase of bel,Tx,n are observed(see Table III) due to the slight increase of the signal spectrumwidth.

A Q-factor improvement of about 0.7 dB is observed whenSSB-DCS-RZ and BL-duobinary formats with a bel,Tx,n of0.35 are used, instead of conventional SSB-DCS-RZ andBL-duobinary formats (bel,Tx,n = 0.25) for the UDWDM sys-tem. For the ODM, the Q-factor degradation of the UDWDMsystem does not exceed 0.8 dB, in comparison with the single-channel system for all investigated signaling formats, anddecreases with the increase of bel,Tx,n. Thus, the SSB-DCS-RZ and BL-duobinary formats have large tolerance to crosstalk,

especially with higher bel,Tx,n. The BL-duobinary formats havehigher maximum Q-factor than the SSB-DCS-RZ formats inboth single-channel and UDWDM systems due to strongerintra-channel limitations of the SSB-DCS-RZ formats, as theQ-factor degradation due to crosstalk is similar in both formats.

B. Analysis of Dispersion Tolerance

The tolerance of any signaling format to incomplete disper-sion compensation is of great importance in the design of LH40-Gb/s/channel terrestrial systems [18]. In the following, thedispersion tolerances of each signaling format are the ranges ofdispersion where the Q-factor penalty remains within 1 dB ofvariation from the optimum performance shown in Table III.Table IV presents the IDC and TRD tolerances in single-channel and UDWDM systems for SSB-DCS-RZ (duty cycleof 53%) and BL-duobinary formats for different bel,Tx,n. Thesefigures were extracted from Q-factor contour plots similar tothose of Fig. 4 for each signaling format.

In terms of tolerance to IDC, the BL-duobinary format isworse than any investigated SSB-DCS-RZ format for bothsingle-channel and UDWDM operations due to the smallertolerance to the fiber nonlinear effects, as will be confirmedin the next section. It should be emphasized that the SSB-DCS-RZ format presents an improvement of the IDC toler-ance with respect to the BL-duobinary format for bel,Tx,n of0.25 and 0.35 of about 40% in the single-channel system andabout 20% in the UDWDM system. Similar IDC tolerance ofBL-duobinary formats is observed in single-channel andUDWDM systems, thus indicating very large tolerance to XPM.Furthermore, the IDC tolerance decreases with the increase ofthe bel,Tx,n for single-channel and UDWDM systems. In thecase of the SSB-DCS-RZ format, a significant IDC tolerancereduction between single-channel and UDWDM systems isobserved. This reduction increases with bel,Tx,n (from 1% to3%). Thus, the IDC tolerance of the SSB-DCS-RZ formats withhigher bel,Tx,n is more influenced by XPM degradation thanthat of the conventional SSB-DCS-RZ format.

In terms of tolerance to TRD, the BL-duobinary formatwith bel,Tx,n = 0.35 shows an improvement of about 30%, in


TABLE IVIDC AND TRD TOLERANCES FOR SINGLE-CHANNEL AND UDWDM SYSTEMS FOR THE SSB-DCS-RZ FORMAT WITH A

DUTY CYCLE OF 53% AND BL-DUOBINARY FORMAT WITH DIFFERENT bel,Tx,n

Fig. 5. Q-factor (in decibels) as a function of the TRD for the SSB-DCS-RZformat with bel,Tx,n = 0.4, a duty cycle of 53%, and IDC of 106% for single-channel and UDWDM systems.

comparison with the SSB-DCS-RZ format with bel,Tx,n = 0.35for both single-channel and UDWDM operations, due to bettertolerance to GVD of the BL-duobinary formats, which occursalso for linear transmission. It should be emphasized that theimprovement of TRD tolerance in multichannel in compari-son with the single-channel operation of SSB-DCS-RZ andBL-duobinary formats with higher bel,Tx,n results from theincrease of robustness of these formats to XPM degradationfor low and negative TRD. This can be seen in Fig. 5, wherethe Q-factor as a function of the TRD for the IDC of 106% ispresented for the SSB-DCS-RZ format with bel,Tx,n = 0.4 anda duty cycle of 53%.

Further analysis showed that for SSB-DCS-RZ signals withduty cycles of 27% and 33%, better TRD tolerance is observed,compared with the SSB-DCS-RZ signal with duty cycles of53% and 67% due to better tolerance to XPM for negative andlow TRD, which is similar to the one shown in Fig. 5.

The optimum TRD and TRD tolerances were analyzedfor each of the five UDWDM channels with an IDC of106% for the SSB-DCS-RZ with the duty cycle of 53%

Fig. 6. Q-factor (in decibels) as a function of the average power per channelat the SSMF input for SSB-DCS-RZ signals with a duty cycle of 53% andfor bel,Tx,n = 0.25 (left-hand side) and bel,Tx,n = 0.35 (right-hand side) forsingle-channel and UDWDM systems.

and the BL-duobinary format, both with bel,Tx,n = 0.35. Verygood UDWDM system performance uniformity, with less than0.5 dB of Q-factor variation between the channels, was ob-served in the range of TRD within 1 dB of the Q-factor penaltyof the center channel for both signaling formats.


TABLE VNLT AND MAXIMUM Q-FACTOR FOR SINGLE-CHANNEL AND UDWDM SYSTEMS FOR IDC = 106% AND TRD = 40 ps/nm, FOR THE SSB-DCS-RZ

FORMAT WITH DUTY CYCLE OF 53% AND BL-DUOBINARY FORMAT, WITH DIFFERENT bel,Tx,n

C. Nonlinearity Tolerance

In order to assess the impact of fiber nonlinearity on thesystem performance, the dependence of the Q-factor on theaverage power per channel at the SSMF input is investigatedfor all the formats considering the same dispersion map ofIDC = 106% and TRD = 40 ps/nm, which is very close tothe ODM for all signaling formats. Fig. 6 shows the Q-factorfor single-channel and UDWDM systems for conventionalSSB-DCS-RZ (bel,Tx,n = 0.25) and optimized SSB-DCS-RZ(bel,Tx,n = 0.35) signaling formats with the same duty cycle(53%). Similar Q-factor variation with the average power at theSSMF input is observed for other formats but with differentQ-factor values and slightly different optimum powers at theSSMF input. Small Q-factor reduction (less than 0.5 dB) isobserved for 0 dBm of the average power at the SSMF input, incomparison with the best Q-factor obtained with the optimumpower. Fig. 6 shows that the nonlinear effects are alreadyimportant for 0 dBm of the average power at the SSMF input,which is in agreement with the results of the previous section.Further simulations showed that the ODM for the optimumaverage power at the SSMF input is not very different fromwhat has been achieved with 0 dBm of the average power atthe SSMF input.

The robustness of different signaling formats to fiber nonlin-earity is characterized by the nonlinear threshold (NLT) definedas the average power per channel at the SSMF input that leads to1 dB of Q-factor degradation due to the nonlinear transmission.We stress that we use a slightly different NLT definition fromthe one used by Klekamp et al. [20]. In [20], the dispersionmap is optimized for each power level, while in this paper,we consider the same dispersion map (optimized for the powerlevel of 0 dBm) for all power levels and no compensation ofdispersion at the transmitter side. Table V shows the NLT andthe maximum Q-factor (extracted from plots similar to thoseof Fig. 6 for each format) in single-channel and UDWDM sys-tems for SSB-DCS-RZ (duty cycle of 53%) and BL-duobinaryformats for different bel,Tx,n. For the single-channel system,higher NLT and, thus, larger tolerance to intra-channel non-linear effects occur for the SSB-DCS-RZ formats with higherbel,Tx,n, compared with the conventional SSB-DCS-RZ format.Further analysis showed that the SSB-DCS-RZ formats withdifferent duty cycles show similar NLT and optimum Q-factor.

Table V shows that the NLT degradation between single-channel and UDWDM systems is higher for the SSB-DCS-RZformats with higher bel,Tx,n. Thus, the SSB-DCS-RZ formatswith higher bel,Tx,n are less tolerant to the XPM degradation(a similar conclusion was drawn in the previous section). Forsingle-channel and UDWDM systems, higher Q-factor occursfor the SSB-DCS-RZ formats with higher bel,Tx,n, especiallyfor bel,Tx,n = 0.35, compared with the conventional SSB-DCS-RZ format.

A Q-factor improvement not exceeding 1.1 dB is observedfor BL-duobinary formats in comparison with SSB-DCS-RZformats with the same bel,Tx,n due to the lower ISI. How-ever, larger NLT occurs with SSB-DCS-RZ formats than withBL-duobinary formats for high bel,Tx,n in the single-channelsystem and for all SSB-DCS-RZ formats in the UDWDM sys-tem, e.g., with the SSB-DCS-RZ format with bel,Tx,n = 0.35,1.2 dB of the NLT improvement for the UDWDM system isobserved, in comparison with the BL-duobinary format with thesame bel,Tx,n. Note that contrary to the SSB-DCS-RZ format,with higher bel,Tx,n, the BL-duobinary format has smallertolerance to the intra-channel nonlinear effects. Thus, theNLT improvement of the SSB-DCS-RZ format relative to theBL-duobinary format with higher bel,Tx,n is attributed mainlyto the interplay between the intra-channel nonlinear effects andthe chirp of the SSB-DCS-RZ signal.

The figures in Table V show that for the ODM and optimumaverage power at the SSMF input, the maximum Q-factor ofthe conventional SSB-DCS-RZ format in the UDWDM systemis 17.9 dB. This gives a system margin of only 0.6 dB withrespect to the required Q-factor without FEC. Thus, FEC isrequired, which gives 8.7 dB of the system margin. In caseof the conventional BL-duobinary format, the system marginwithout FEC coding is only 1.5 dB, and with FEC, it improvesto 9.6 dB. The improvement by 1 dB achieved by formatoptimization is not enough to avoid using FEC and have asystem margin of several decibels.

V. CONCLUSION

Systematic numerical optimization of the electrical transmit-ter bandwidth and duty cycle of the DCS-RZ signal in com-bination with optimization of MUX and DMUX SSB filteringhas been performed for 10 × 80 km of SSMF transmission for


the UDWDM system at a 43-Gb/s/channel and with 50 GHzof channel spacing. Nearly 1 dB of the Q-factor improve-ment is achieved by the SSB-DCS-RZ format with an elec-trical transmitter filter bandwidth of 35% of the bit rate, incomparison with the conventional one due to the smaller op-timum SSB bandwidths and, consequently, smaller influenceof linear crosstalk and lower noise power. Due to the smalldifference between the SSB-DCS-RZ signals with differentDCS-RZ duty cycles, no substantial improvement occurs byvarying the duty cycle of the DCS-RZ signal.

The tolerances to the IDC, TRD, and fiber nonlinearity ofseveral SSB-DCS-RZ formats have been analyzed and com-pared with the ones of several BL-duobinary formats. Similardispersion tolerances are observed for different SSB-DCS-RZ formats. An improvement of the IDC tolerance by about40% for the single-channel system and by about 20% for theUDWDM system and reduction of the TRD tolerance by about30% for both systems are observed for SSB-DCS-RZ formatswith higher electrical transmitter bandwidths, in comparisonwith BL-duobinary formats. Furthermore, it has been shownthat the SSB-DCS-RZ formats with higher electrical transmitterbandwidth have better tolerance to the intra-channel nonlineareffects than the conventional SSB-DCS-RZ (NLT improvementof 0.6 dB with bel,Tx,n = 0.4) or BL-duobinary formats (NLTimprovement of 0.9 dB with bel,Tx,n = 0.35).

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Nataša B. Pavlovic was born in Belgrade, Serbia,on December 26, 1977. She received the B.S. de-gree in electro-technical engineering with a ma-jor in telecommunications and the M.Sc. degreein telecommunications from Belgrade University in2001 and 2003, respectively. She is currently work-ing toward the Ph.D. degree in optical communi-cations at the Instituto Superior Técnico, Lisboa,Portugal. Her doctoral thesis is in the field of ad-vanced modulation formats for high spectral efficientoptical communication systems.

She spent one year in the DWDM/ODC project as a Beginning Researcher,working on the performance assessment of optical dispersion compensationschemes in conventional intensity modulation direct detection systems. Herresearch interests are new modulation formats for ultradense wavelength-division multiplexing systems.

Adolfo V. T. Cartaxo (S’89–A’89–M’98–SM’02)was born in Montemoro-o-Novo, Portugal, in 1962.He received the “Licenciatura” degree in electricalengineering, the M.Sc. degree in telecommunica-tions and computers, and the Ph.D. degree in electri-cal engineering from the Instituto Superior Técnico(IST), Lisbon Technical University, Lisboa, Portugal,in 1985, 1989, and 1992, respectively

In 1985, he joined the Department of Electricaland Computer Engineering, IST, where he becamean Assistant Professor in 1992 and was promoted to

Associate Professor in January 2002. Over those years, he lectured on severalcourses on telecommunications. He joined the Optical Communications Groupof Lisbon Pole, Institute for Telecommunications, IST, as a Researcher in 1992.He is currently a Senior Researcher conducting research on dense-wavelength-division-multiplexed systems and networks. He has been a Leader of the ISTparticipation in three projects of the European Union programs on research anddevelopment in the optical communications area and two national projects.In the past few years, he has acted as a Technical Auditor and Evaluatorfor projects included in several European Union research and developmentprograms, and he has served as a Reviewer for several publications in the areaof optical communications. He has authored more than 45 journal publicationsand more than 70 international conference papers. His current research interestsinclude fiber-optic communication systems and networks.

Dr. Cartaxo is a Senior Member of the IEEE Laser and Electro-OpticsSociety.

Documents

Optimization of Single-Sideband DCS-RZ Format for Long-Haul 43-Gb/s/channel Ultradense WDM Systems