Performance of DCS-RZ single-sideband signal in40 Gbit=s long-haul transmission systems

P.M.A. Charrua and A.V.T. Cartaxo

Abstract: Optical sideband suppression (SS) by means of optical filtering is proposedfor duobinary-carrier-suppressed return-to-zero (DCS-RZ) coding format. The performance ofDCS-RZ with SS format generated using three different SS filters is investigated through numericalsimulation. The power of the nonsuppressed sideband to the power of the suppressed sideband ratio(SSPR) is used to categorise the level of sideband suppression. A minimum level of SSPR of 20 dBis required to generate single-sideband (SSB) signals. It is shown that, for all three SS filters,an SSPR higher than 20 dB can be accomplished by choosing properly the SS filter bandwidthand detuning. Hence, the SS filter can generate single-sideband (SSB) signals. Transmissionperformance over different distances is assessed numerically. Results reveal that a double-sideband(DSB) modulation format shows better performance (Q-factor improvement of �3 dB; foroptimum input power levels), even after 8 transmission spans of standard single-mode fibre, thanthe SSB formats. However, the optically generated SSB signals increase the DSB signal toleranceto dispersion compensation ratio variation more than three times, showing remarkable robustness tooptical filter parameter variation, revealing that the DCS-RZ-SSB format is a good candidate forlong-haul transmission systems.

1 Introduction

To meet the demand for increasing data traffic, it is necessaryto increase the transmission capacity of ultra-dense wave-length-division-multiplexing (UDWDM) systems. In such acontext, the determination of the proper coding format andmodulation technique is crucial for achieving high perform-ance in ultra-high-capacity long-haul UDWDM systems.Duobinary NRZ [1] coding format has been shown to besuitable for UDWDM multi-terabit transmission systems forits low spectral occupancy and high dispersion tolerance [1].On the other hand, RZ formats have low intersymbolinterference (ISI) and have proven higher tolerance to self-phase-modulation (SPM) [2, 3] than NRZ formats, enablingultra-long-haul transmission systems [4]. These results seemto indicate that the optimum coding formats would be thosethat combine the duobinary and RZ formats.

One format that has those properties was proposed and, forthat, deserves our attention in this paper: duobinary-carrier-suppressed return-to-zero (DCS-RZ) [5]. This format ischaracterised by optical p-phase jumps, absence of discretespectral tones, and a spectrum null at the carrier frequencywith remarkably reduced spectral content around the carrierfrequency. A spectrum such as of that signalling format canresult in reduced transmission degradation due to groupvelocity dispersion (GVD) and SPM. Furthermore, it couldpotentially facilitate the generation of signals with sidebandsuppression (SS), single-sideband (SSB) or vestigial side-

band (VSB), by optical filtering. One of the purposes of suchoptical filtering is to increase the dispersion tolerance due tothe bandwidth reduction, therefore enhancing its perform-ance in long-haul transmission systems. Another purpose isto reduce the signalling format bandwidth, enhancing itsapplication in UDWDM systems. Other studies wereaccomplished with the DCS-RZ signal and SS filtering[5–8]. The studies presented in [5] and [7, 8] implementedthe SS optical filtering only at the receiver (Rx) side. Just incase of linear fibre transmission, similar performance isobtained for the SS implemented at the transmitter (Tx) andRx sides. For nonlinear fibre transmission, different per-formance is achieved, since the time and spectral features ofthe signal at the Tx output are substantially different when theSS filtering is accomplished at the Tx and Rx sides. In [6],the performance of the DCS-RZ format with VSB filtering atthe Tx side was assessed. However, a significantly highpower level in the suppressed sideband is considered foroptimum VSB filter settings [6]. In this paper, our purpose isto deeply investigate the level of sideband suppression inorder to achieve a very reduced power level in the suppressedsideband for the optically filtered DCS-RZ format, thusgenerating a SSB signal. To the authors’ knowledge, acomprehensive study of DCS-RZ with sideband suppressionby means of optical filtering at the Tx side has not beenperformed, giving special relevance to the detailed study ofits generation and performance.

In this paper, the generation of DCS-RZ SS signal and itsperformance in a 40 Gbit=s single-channel optical com-munication system are investigated. The settings (23 dBbandwidth and detuning frequency) of three different SSfilters are optimised. The transmission performanceobtained by different filters allows concluding aboutthe influence of filter features on the format performance.Thus, we can conclude which modulation format (SS ordouble-sideband (DSB)) provides the best performance inlong-haul transmission and tolerance to dispersioncompensation ratio (DCR) variation.

q IEE, 2005

IEE Proceedings online no. 20055004

doi: 10.1049/ip-opt:20055004

The authors are with the Optical Communications Group, Instituto deTelecomunicacoes, Department of Electrical and Computers Engineering,Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

E-mail: adolfo.cartaxo@lx.it.pt

Paper received 27th April 2004

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 2005 33

2 40 Gbit=s system set-up and performanceassessment

The DCS-RZ coding format is generated following [5] witha rectangular NRZ pulse shape. Figure 1 shows the systemconfigurations considered along this study: (a) single-channel set-up for SS filter optimisation; (b) single-channelset-up for long-haul transmission over standard single-modefibre (SSMF) with optical dispersion compensation usingdispersion compensating fibre (DCF) and tolerance to DCRvariation. Fibre parameters and lengths in each span areshown in Table 1. All EDFAs have a spontaneous emissionfactor nsp ¼ 2: In Fig. 1a, the EDFA gain is set to 30 dB.In Fig. 1b, the EDFAs fully compensate fibre loss andimpose some prescribed power level at the SSMF input and24 dBm at the DCF input, to minimise nonlineartransmission effects in the DCF. At the Rx side, a second-order super-Gaussian optical filter is considered. A PINdetector with responsivity of 1 A=W is considered. Theelectrical filter is assumed to be a 26 GHz bandwidth 4thorder Bessel filter. An operating signal wavelength of1550 nm is considered.

A 29 deBruijn bit sequence is used. With this sequencelength, accurate single-channel performance is assessed [9].To simulate fibre nonlinear transmission, where GVD andSPM effects are considered, the symmetrised split-stepFourier method [10] was used. The bit error probability iscomputed using the method presented in [11] because itprovides rigorous enough description of the effect of tightoptical filtering on signal-amplified spontaneous emissionbeat noise [11]. The results are presented in terms of powerpenalty and Q-factor. The power penalty is defined as thedifference of receiver sensitivity, in dB, at 10�12 of bit errorratio relative to a rectangular unfiltered NRZ signal with

12 dB extinction ratio. For this signal, �30 dBm of receiversensitivity is obtained at the input of the EDFA of Fig. 1a.

3 SS filter characteristics and systemoptimisation

The impact of three different SS filters on the generation ofDCS-RZ-SS format is investigated. Gaussian [12], second-order super-Gaussian [13] and commercially availableconventional ‘flat-top’ arrayed waveguide grating (AWG)[13] filters are analysed. The Gaussian filter is consideredbecause the main lobe of many of today’s commercial filtershas approximately this shape [12]. Because second-ordersuper-Gaussian filters match very well the filtercharacteristics of conventional ‘flat-top’ AWGs, it is alsoconsidered.

Different performances for different filters are due mainlyto main lobe and filter slope differences. Since the filtersconsidered in this study have a real transfer function, delaydistortion does not occur. Figure 2 shows the amplituderesponses of the three filters considered for a 23 dBbandwidth of 50 GHz. As one can see, the Gaussian filterhas the less uniform response in the passband, andconventional ‘flat-top’ AWG and super-Gaussian filtershave similar flat amplitude response behaviours up to��5 dB: Below this, the conventional ‘flat-top’ AWG filterslope decreases compared to the super-Gaussian. TheGaussian filter has the smaller filter slope, meaning that itis the less selective filter of our study. The conventional‘flat-top’ AWG has a level of selectivity between the twoGaussian filters. It is easier to filter out one of the sidebands,generating an optically filtered SS signal, with a filter that isvery selective. All these factors are accounted for when theSS filter settings are optimised and receiver performance isestimated.

3.1 SS signal generation criteria

In our study, the purpose of the SS filter is to filter out one ofthe sidebands, making our filtered signals similar to SSBsignals. This is accomplished by means of filter detuningrelative to the signal optical carrier frequency, assuring alevel of power in the suppressed sideband not exceedingsome prescribed figure. Following the SSB criterion used in[14], a signal is considered SSB when the sidelobesuppression ratio exceeds 30 dB=40 dB: This criteriondoes not take into account the level of residual power inthe suppressed sideband, which can be high when the signalspectrum has significant spectral content near the carrierfrequency, as in conventional NRZ and RZ signals, or evenduobinary signals. To avoid this limitation and to achieveSS signals more similar to ideal SSB signals than the ones

Fig. 1 System configurations

a Single channel setup for SS filter optimisationb Single-channel setup for long haul transmission performance and assessment of dispersion tolerance, with optical dispersion compensation

Table 1: SSMF and DCF parameters

SSMF DCF

Length, km 80 14.22

Fibre attenuation

parameter, dB=km 0.22 0.5

Dispersion parameter

at 1550 nm, ps=(nm km) 16 290

Dispersion slope at

1550 nm, ps=ðnm2 kmÞ 0.08 20.45

Effective core area, mm2 80 30

Nonlinear refractive index

n2; m2=W 3:2 � 10�20 2:67 � 10�20

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 200534

generated following the criterion of [14], we use thesuppressed sideband power ratio (SSPR) to categorise thelevel of sideband suppression. The SSPR is defined as

SSPR ¼ 10 log10

PNSSB

PSSB

� �

where PNSSB is the power in the nonsuppressed sideband andPSSB is the power in the suppressed sideband.

A minimum level of SSPR of 20 dB is required for SSBsignals. Below 20 dB, the signal is considered VSB. ForSSPR of 0 dB we get a DSB signal. Following this criterion,the SS filter can operate as a VSB or SSB filter. For theoptimum VSB filter settings presented in [6] and for theDCS-RZ signal, the level of SSPR does not exceed 11 dB,meaning that, following our definition, the signal generatedin [6] is a VSB signal.

The SS filtering optimisation has two main objectives:making the filtered signals more robust to fibre dispersionand compacting the signals’ spectra. Thus, we use thefollowing criteria to select the filter settings: (i) the powerpenalty relative to a rectangular unfiltered NRZ signal with12 dB of extinction ratio does not exceed 0.5 dB, guarantee-ing good system performance; (ii) the SSPR is higher than20 dB; and (iii) the 220 dB signal spectral width isminimised. Thus, implicitly, the selected SS signals willshow higher tolerance to GVD.

3.2 SS filter optimisation

In this Section, the SS filter parameters are optimised ina single-channel system in a back-to-back configuration(Fig. 1a). To minimise the influence of receiver filteringon the SS filter optimisation, a 23 dB bandwidth of320 GHz for the optical receiver filter is assumed, so thatreceiver filtering induced ISI and signal distortion areminimised. Figures 3–5 show the power penalty, SSPRvalues and 220 dB spectral widths, for DCS-RZ andeach SS filter type (Gaussian, super-Gaussian andconventional ‘flat-top’ AWG). Figures 3–5 show thatDCS-RZ format shows high penalties when the detuningfrequency is higher than 20 GHz, meaning that theoptical power is concentrated at �20GHz: Figures 3–5also show that SSPR and 220 dB spectral widths havedifferent behaviours according to the SS filter character-istics. Following the previous SS signal generationcriteria, optimum filter settings can be extracted fromFigs. 3–5. Table 2 shows the optimum SS filter settings,for DCS-RZ and each SS filter type.

Table 2 shows that the main difference between differentSS filters is the 23 dB bandwidth, since the frequencydetuning is � 20GHz (i.e. half bit-rate) in all cases. As it

Fig. 3 Power penalty, in dB (left), SSPR, in dB (middle), and�20 dB spectral widths, in GHz (right), for filtered DCS-RZ byGaussian SS filter

Fig. 4 Power penalty, in dB (left), SSPR, in dB (middle), and�20 dB spectral widths, in GHz (right), for filtered DCS-RZ byflat-top AWG SS filter

Fig. 5 Power penalty, in dB (left), SSPR, in dB (middle), and�20 dB spectral widths, in GHz (right), for filtered DCS-RZ bysecond-order super-Gaussian SS filter

Fig. 2 Amplitude responses of SS filters with a �3 dB bandwidthof 50 GHz

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 2005 35

was discussed in Section 3 (Fig. 2), the main featureresponsible for these differences is the filter selectivity.To guarantee the same spectral compression, a less selectivefilter needs a narrower 23 dB bandwidth, as seen for theGaussian filter (Fig. 2 and Table 2). The super-Gaussianfilter, which is the more selective filter, allows broader23 dB bandwidth.

SSPR figures presented in Table 2 allow us concludingthat the filtered signals can be considered SSB signals and,therefore, the SS filter operates as a SSB filter. This can bealso seen analysing the filtered SSB spectra. Since the

conventional ‘flat-top’ AWG SSB filter shows an eye-opening penalty between the other two filters, withrespective filter settings presented in Table 2, this SSBfilter output eye-diagram (Fig. 6) is used for analysis.Analysing the Tx output eye-diagram before SSB filtering(Fig. 7) and after SSB filtering (Fig. 6), we notice that theRZ-DSB signal turned into an NRZ-SSB signal. The changeof RZ to NRZ signalling after narrowband optical filteringhas been widely reported (for example, in [12, 13]).Furthermore, as an important contribution of this work,Fig. 6 shows that SSB filtering impairs slightly thehorizontal and vertical eye-opening, compared with anunfiltered NRZ signal. The same behaviour is also observedwith Gaussian and super-Gaussian filtering. However, withthe Gaussian filter, the SSB signal shows higher eye-opening penalty. On the other hand, second-order super-Gaussian and conventional ‘flat-top’ AWG filters have verysimilar eye-diagrams. This fact reveals that the generatedSSB signals are good candidates for high bit-rate long-haultransmission.

3.3 Optical Rx filter optimisation

The optical receiving filter is also optimised so thatoptimum system performance can be assessed. This wasaccomplished considering four transmission spans (N ¼ 4in Fig. 1b), and nonlinear transmission in the optical fibre.The input power was set to Pin ¼ 0 dBm; to ensure goodperformance even after transmission over four spans ofSSMF. Thus, it is possible to determine the impact ofnarrowband filtering, transmission effects and accumulatedspontaneous emission (ASE) noise on the optimum filtersettings in the Rx. This was done for both DSB (Fig. 8) andSSB (Fig. 9) signals. Additional investigations were doneconsidering one transmission span and similar optimumfilter settings are observed. It can thus be concluded that theincrease on the number of spans has a marginal impact onthe optimum filter settings. In [13], similar investigationswere accomplished for NRZ, RZ and duobinary-NRZcoding formats, and the same conclusions were obtained.This means that the effect of imposing the optimumreceiving filter settings is not the accumulation of ASEnoise, but the levels of ISI and signal-depending factors.For the SSB signals, it is noticeable that the system is veryrobust to bandwidth and detuning variations. In general, Rxfilter bandwidth and detuning can vary by >5GHz withperformance degradation �0:5 dB: Table 3 summarises theoptimum filter settings, for each modulation format. Table 3shows that the optimum Rx filter settings and performancewith the conventional ‘flat-top’ AWG and the second-ordersuper-Gaussian are very similar. After four transmissionspans, the Gaussian SSB filter showed the worstperformance, mainly due to its low selectivity and resultingpoorer eye-opening of the SSB signal.

As expected, the DSB signal showed better performancebecause of using perfect dispersion compensation. Asknown, RZ formats show better performance, in both

Table 2: Optimum SS filter settings, SSPR values and �20 dB spectral widths for DCS-RZ with sidebandsuppression

Filter 23 dB bandwidth, GHz Detuning, GHz SSPR, dB 220 dB spectral width, GHz

Gaussian [Note 1] 26 19 20.1 52.5

Flat-top 32 20 25.4 40

Super-Gaussian 35.5 20 27 40

Note 1: In this case, it was not possible to guarantee a maximum system power penalty of 0.5 dB

Fig. 6 Power spectrum (left) and Tx output eye-pattern (right),for DCS-RZ coding format after conventional ‘flat-top’ AWG SSBfiltering with 0 dBm average power (32 GHz of �3 dB bandwidth,20 GHz of detuning, SSPR of 25.4 dB)

Fig. 7 Power spectrum (left) and Tx output eye-pattern (right),for DCS-RZ coding format before SSB filtering with 0 dBm averagepower

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 200536

back-to-back [6] and after transmission [2]. DCS-RZ-DSBsignals showed larger 23 dB optimum receiver bandwidththan the SSB signals. The DCS-RZ-DSB broader spectrumis responsible for this fact. Since the 220 dB spectral widthis approximately the same for all SSB signals, their optimalreceiving filter settings are similar.

4 Transmission results for single-channel40 Gbit=s system

The optimised filter settings for the DCS-RZ format withDSB and SSB modulation formats are considered fornonlinear transmission performance estimation andassessment of the optimum power per channel. Furthermore,the dispersion tolerance is also assessed.

4.1 Long-haul transmission

In this study, the DCR is set to 100% and the dispersionslope is also compensated. The Q-factor for transmissionover 4 � 80 km; 6 � 80 km and 8 � 80 km of SSMF ispresented in Fig. 10. All numerical simulations are realisedconsidering a cost-effective transmission system (allEDFA-, SSMF- and DCF-based). Figure 10 shows thatDCS-RZ-DSB modulation format has much better perform-ance than DCS-RZ-SSB modulation formats, especially forhigh input powers where tolerance to SPM plays animportant role. The SSB modulation formats are NRZ-shaped, as explained before, while DSB signal is RZ-shaped. In this context, it was shown that RZ formats havemuch higher tolerance to SPM, in opposition to NRZsignals, because of its reduced ISI and due to the fact that thesignal pulse widths are independent of the transmittedsequence [2, 3]. For the DSB signal, even after 8 � 80 kmand if proper input power levels are set, a Q-factor of�22 dB is obtained. Therefore, DCS-RZ-DSB seems to be agood choice for long-haul transmission.

The DCS-RZ-SSB signals show different power leveldependence and worse performance than the DCS-RZ-DSB,even after eight transmission spans. The optimum averageinput power is lower ð� 4 dBÞ for the NRZ-shaped SSBsignals, in comparison with the corresponding RZ-shapedDSB signal, in agreement with [15]. This indicates that theDCS-RZ-DSB signal is more tolerant to SPM than the SSBmodulation formats.

For DCS-RZ-SSB, a Q-factor higher than 18.3 dB isattainable even after 8 compensated spans of 80 km ofSSMF, when the SSB filter is either a conventional ‘flat-top’AWG or a second-order super-Gaussian. The Gaussian filtershows the worst performance for all power levels. As hasbeen seen in Section 3.3, this is due to its worst selectivityand because of smaller Tx output eye-opening. With aGaussian SSB filter, a Q-factor higher than 17 dB isaccomplished after 8 � 80 km of SSMF.

Additionally, the quasi-absence of suppressed sidebandrecovery due to the SPM effect on the DCS-RZ-SSB signalswas observed, in contrast to what has been reported by otherauthors for VSB signals ([16], for example). This occurs forall the investigated SSB filters, even after 8 � 80 km ofSSMF, for optimum SSMF input power levels. Figure 11shows the power spectrum of the DCS-RZ-SSB modulationformat, after transmission over 8 � 80 km of SSMF with2 dBm of average power at the SSMF input, when the SSB

Table 3: Optimum Rx filter settings, in GHz, and optimum Q-factor, in dB, after 4 transmission spans, forDCS-RZ coding format, with SSB or DSB modulation format

Rx filter settings

Q-factor, dB, for 4 spansModulation format SSB filter 23 dB bandwidth, GHz Frequency detuning, GHz

DSB – 65 – 22.7

SSB Gaussian 44 17 21

Flat-top 37.5 18 22

Super-Gaussian 37.5 18 22.4

Fig. 8 Q-factor, in dB, as a function of optical receiver �3 dBbandwidth, for DCS-RZ-DSB format, after 1 and 4 transmissionspans

Fig. 9 Q-factor, in dB, as a function of optical receiver �3 dBbandwidth and frequency detuning, for DCS-RZ format after 4transmission spans for three filters

a Gaussian SSB filterb Conventional ‘flat-top’ AWG SSB filterc Second-order super-Gaussian SSB filter after 4 transmission spans forthree filters

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 2005 37

filter is the conventional ‘flat-top’ AWG filter. Figure 11shows a 220 dB spectral width increase of � 2GHz; thusproving the very reduced effect of the SPM on thesuppressed sideband recovery of the DCS-RZ-SSB format.It was also checked that even for 8 � 80 km of SSMF and8 dBm of input power level, where the eye-diagram iscompletely closed due to SPM, the DCS-RZ-SSB signalspreserved their SSB feature. This allows concluding that theDCS-RZ-SSB signal spectrum is tolerant to the SPM effectand that optical Rx filter parameters may still be consideredoptimum after nonlinear transmission over 8 � 80 km ofSSMF.

4.2 Assessment of tolerance to DCR variation

Collecting low system penalty due to DCR variation is oneof the key issues for achieving high efficient performancetransmission systems [17]. Usually, the narrower the signalspectral width, the higher the tolerance to DCR variationbecomes. However, fibre nonlinearity-induced effects such

as SPM and cross-phase modulation limit the transmissioncapacity. Concerning the accommodation of SPM, RZ-shaped transmission formats have several advantages overNRZ-shaped formats [2]. However, RZ pulses have arelatively broad spectral width in comparison with theoptical duobinary or NRZ formats and, thus, smallertolerance to DCR variation. To improve the DCR variationtolerance significantly, the proposed SSB modulationformat seems to be the solution because of its narrowerspectrum width, therefore compensating for the weakertransmission performance seen in Section 4.1.

The DCR is defined in this paper as the ratio betweenthe absolute dispersion in the DCF and the dispersion inthe SSMF. The DCR penalty is evaluated relative to thesituation of DCR ¼ 100%: A maximum DCR penalty of1 dB is considered to assess the allowable DCR variation.The same DCR is assumed in all spans of the link. Figure 12shows the DCR penalty of the DCS-RZ signal, with DSBor SSB modulation formats. This was accomplished

Fig. 10 40 Gbit=s single-channel performance over three SSMF lengths for DCS-RZ coding format, with DSB and SSB modulation formats

a 4 � 80 kmb 6 � 80 kmc 8 � 80 km

Fig. 11 Power spectrum of DCS-RZ-SSB modulation format,after 8 � 80 km of SSMF with 2 dBm of average power at SSMFinput

Fig. 12 DCR penalty, in dB, as a function of the DCR, for DCS-RZ coding format, with DSB and SSB modulation formats, after6 transmission spans

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 200538

considering optimum input power level for each modulationformat (see Section 4.1) and six transmission spans. Theallowable DCR variation for each SSB filter and modulationformat is presented in Table 4 (extracted from Fig. 12).

We can see in Fig. 12 and analysing Table 4 that all theSSB generated signals increased more than three times theallowable DCR variation relative to the DCS-RZ-DSBsignal. Two important conclusions can be taken from theseresults. First, the worst performance (see Section 4.1) andtolerance to DCR variation of SSB signals is obtained whenthe SSB filter is Gaussian. Second, the conventional ‘flat-top’ AWG filter and the second-order super-Gaussian show,in general, similar results. This was checked for bothtransmission performance and DCR variation tolerance.Furthermore, it can be seen in Fig. 12 that optimumperformance is obtained in the case of under-compensation(DCR of 99:5%). This is due to the SS filtering of theDCS-RZ signal. The SS filtering induces chirp into theDCS-RZ-SSB signal, meaning that, for optimum perform-ance, this chirp has to be compensated. Figure 13 shows thechirp of the DCS-RZ-SSB format as a function of time, andshows that the chirp is well above the average chirp whenthe DCS-RZ-SSB signal power levels are very low, meaningthat, for bits ‘0’, it marginally affects the performancedependence on the DCR variation. On the other hand, Fig. 13shows that the chirp is below the average chirp every timethe DCS-RZ-SSB power levels are high. Thus, to approxi-mate the amount of chirp, after transmission, to its average,the system must be working under-compensated. This factexplains the result obtained in Fig. 12 by varying the DCR.A similar result has been reported in [8].

Results presented so far enable the comparison of bothmodulation format (DSB and SSB) performances in a long-haul transmission system. Thus, if high system performance

is desirable, instead of robustness to system parameters anddispersion, the DSB signal is preferable to the SSB signals,as seen in Section 4.1. On the other hand, if systemperformance can be sacrificed slightly, in exchange for highsystem robustness to optical filter parameter variations andremarkable tolerance to DCR variation, then the SSBsignals generated in this paper show a very good trade-off.Thus, the DCS-RZ-SSB signal appears to be a veryinteresting candidate for long-haul transmission. Through-out the study, the differently filtered DCS-RZ-SSB signalsshowed very similar transmission performance (seeSections 3.3 and 4.1), especially when the SSB filter iseither a conventional ‘flat-top’ AWG filter or a second-ordersuper-Gaussian. All the facts presented before reveal thatthe SSB signal that shows the best trade-off between long-haul transmission and tolerance to DCR variation is theDCS-RZ-SSB signal, when the SSB filter is a second-ordersuper-Gaussian. The choice of the SSB filter is notstraightforward, but in all cases, even slightly, thesecond-order super-Gaussian showed best performanceand tolerance to DCR variation. Even so, considering thatthe conventional ‘flat-top’ AWG filter considered in thispaper is a feasible filter, we can conclude that this filter is agood choice leading just to a marginal degradation incomparison with the second-order super-Gaussian.

5 Conclusions

In this paper, SSB modulation has been proposed, by meansof optical filtering at the transmitter side, for the DCS-RZsignalling format. The SS signal generation capabilities andperformance of various SS filters (Gaussian, conventional‘flat-top’ AWG and second-order super-Gaussian) have beeninvestigated. For all cases, SSPR values are higher than20 dB, with a power penalty �0:5 dB: These figures showthat, with very low power penalty, we are able to generatefiltered SSB signals, and that the SS filter performs with aSSB filter.

After SSB filtering, the RZ-DSB signal turns intoNRZ-shaped signals and their �20 dB spectra widths are�40GHz (the same as the bit-rate). The optical receiverfilter settings optimisation has been accomplished consider-ing nonlinear transmission over four transmission spans.We conclude that the increase of the number of spans doesnot have much impact on the determination of the optimumfilter settings. The DSB signal has shown better trans-mission performance than the SSB signals due to its RZ-shape (Q-factor improvement of �3 dB; for optimum inputpower levels).

The allowable DCR variation has also been assessed. Wehave shown that the easily generated SSB signal allowableDCR variation increases by more than three times, in general,the allowable DCR variation of the DSB signal. This is animportant contribution of this paper and demonstrates themain advantage of the generation of SSB signals.

The gathering of all results and their analysis allow us toconclude that the DCS-RZ-SSB signal appears to be a veryattractive candidate for long-haul transmission, because itshows a very good trade-off between system performanceand system robustness to optical filter parameter variationsand allowable DCR variation.

6 Acknowledgment

This work was supported by Fundacao para a Ciencia eTecnologia, FEDER, and POSI within project POSI/CPS/35576/1999 – DWDM/ODC, Portugal.

Table 4: Allowable DCR variation of DCS-RZ signals fora maximum DCR penalty of 1 dB, for each modulationformat and SSB filter

Modulation

format SSB filter

Allowable DCR variation at

1 dB DCR penalty, %

DSB – 0.67

SSB Gaussian 2.62

Flat-top 2.96

Super-Gaussian 2.97

Fig. 13 Chirp of DCS-RZ-SSB format (solid line) and intensity ofDCS-RZ-SSB signal (dashed line), as a function of time

Average chirp of DCS-RZ-SSB signal (dash-dotted line) is shown forcomparison

IEE Proc.-Optoelectron., Vol. 152, No. 1, February 2005 39

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