ICTON 2005 385 We.P.3

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ This work was supported by FCT from Portugal, FEDER and POSI within project POSI/EEA-CPS/56959/2004 - SHOTS. N. Pavlović would like to thank FCT for supporting this work also under contract SFRH/BD/10162/2002. 0-7803-9236-1/05/$20.00 ©2005 IEEE

Optimized Bandwidth-Limited Duobinary Coding Format for Ultra Dense WDM Systems

Nataša B. Pavlović, Adolfo V. T. Cartaxo* Instituto de Telecomunicações, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal

Tel: (+351) 21 8418480, Fax: (+351) 21 8418172, e-mail: natasa.pavlovic@lx.it.pt * Tel: (+351) 21 8418476, Fax: (+351) 21 8417164, e-mail: adolfo.cartaxo@lx.it.pt

ABSTRACT By varying simultaneously the bandwidths of electrical transmitter filter, multiplexer and demultiplexer, the system performance of several duobinary coding formats is optimized for 40 Gbit/s/channel ultra dense wavelength division multiplexing (UDWDM) system with 50 GHz of channel spacing after 4×80 km of transmission. It is shown that bandwidth-limited (BL) duobinary generation schemes with optimized electrical transmitter filter bandwidth can achieve a Q-factor improvement of 1.4 dB over BL-conventional duobinary coding schemes. Moreover, the optimized BL-duobinary format achieves a better UDWDM system performance than BL-phase-shaped binary transmission, for which a capacity×distance record was demonstrated. Keywords: channel spacing, crosstalk, duobinary coding format, electrical and optical filtering, ultra dense

WDM system.

1. INTRODUCTION Advanced coding formats are needed to improve the performance, reach and robustness of high-capacity ultra dense wavelength division multiplexing (UDWDM) systems. Duobinary coding format is one such a coding format. Narrow optical filtering as multiplexer (MUX) and demultiplexer (DMUX) with optimized bandwidths is another technique used to improve the UDWDM transmission capacity by improving the spectral efficiency [1-4]. A capacity record of 6.3 Tb/s over 17×100 km using bandwidth-limited (BL)-phase-shaped binary transmission (PSBT) in a cost optimized UDWDM system without polarization multiplexing was experimentally demonstrated in [3]. The generation scheme of PSBT signal [5] is very similar to the low-pass filter (LPF) duobinary generation scheme [6], except for the electrical transmitter 3-dB bandwidth. PSBT uses an electrical transmitter 3-dB bandwidth of 0.28×Rb, and conventional LPF duobinary coding format uses 0.25×Rb, where Rb is the bit rate. Therefore, it is important to understand if some UDWDM performance improvement can be achieved by optimization of duobinary generation scheme (electrical filter bandwidth, type of duobinary encoder: LPF or delay-and-add (DAA) [7]) and what are the optimum MUX and DMUX bandwidths using these duobinary generation schemes. In many papers, the electrical transmitter filter for duobinary format was optimized, but only in [8], both the electrical and optical filters were optimized for LPF duobinary coding format in back-to-back UDWDM system. However, the optimization of different duobinary generation schemes in long transmission UDWDM system, together with optimization of MUX and DMUX bandwidths still remains.

In this paper, the optimization of 3-dB electrical filter bandwidths of LPF and DAA duobinary generation schemes and 3-dB bandwidth of MUX and DMUX is achieved. This optimization is performed for 5×40 Gbit/s/channel (0.8 bit/s/Hz) UDWDM system after 4×80 km of nonlinear transmission along standard single mode fiber (SSMF). The system performance with optimized BL-duobinary coding formats is compared with BL-conventional duobinary and BL-PSBT formats, with optimized MUX and DMUX bandwidths.

2. SYSTEM DESCRIPTION For generating duobinary signals, two techniques are well known: LPF [6] and DAA technique [7]. The 40 Gbit/s LPF and DAA duobinary generation schemes are generated as shown in Fig. 1a and 1b, respectively. In both generation schemes, the 5th order Bessel electrical filter reshapes the signal at the arms of Mach-Zehnder modulator (MZM). In the following, bel,n is the normalized 3-dB electrical filter bandwidth to the bit rate. The conventional technique of duobinary generation is the LPF technique shown in Fig. 1a with bel,n = 0.25 [6]. The duobinary generation scheme shown in Fig. 1b has the duobinary encoder made of logic circuits achieving the 3-level signal at the electrical filter input. Hence, bel,n with DAA generation scheme is much larger than with LPF technique. The conventional DAA duobinary format uses bel,n = 0.5 [7] and it has remarkable narrow bandwidth of about 35 GHz at 20-dB of signal spectrum [1], while the conventional LPF duobinary has a bandwidth of about 50 GHz at 20-dB of signal spectrum, for the bit rate of 40 Gbit/s. Thus, DAA duobinary coding format seems suitable for high spectral efficient UDWDM systems.

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duobinary precoder

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Fig. 1. a) LPF and b) DAA duobinary generation schemes. Rb is the bit rate.

Five WDM channels spaced by 50 GHz are considered. Different deBruijn binary sequences of 212 bits are considered to rigorously assess the performance of UDWDM system [2]. To achieve so small channel spacing, duobinary signals are filtered by a narrow optical filter, acting as MUX. The system set-up is shown in Fig. 2.

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The transmission line consists of 4 × 80 km of SSMF (Dλ = 16 ps/nm/km, S = 0.08 ps/nm2/km, γ = 1.62 (W.km)-

1, α = 0.22 dB/km). Dispersion and dispersion slope in SSMF are fully compensated by a dispersion compensating fiber (DCF) (Dλ = -90 ps/nm/km, S = -0.45 ps/nm2/km, γ = 4.32 (W.km)-1, α = 0.5 dB/km) in a post-compensating scheme. Each erbium doped fiber amplifier (EDFA) of the two-stage amplification has a noise figure of 6 dB. The gain of each amplifier is adjusted to compensate for the total loss per span and impose a power level at DCF input of -4 dBm. At the SSMF input, the power level is set to 0 dBm. This level was seen to be very close to the optimum for several coding formats [1, 4]. The central channel at 1550 nm is the channel investigated in the paper. 2nd order super Gaussian optical filters are considered for MUX and DMUX in UDWDM system due to similar filter characteristics to the “flat-top” arrayed waveguide grating filter [1]. The electrical receiver filter was assumed a 26 GHz bandwidth 4th order Bessel filter. The Q-factor is used to categorize system performance. The Gaussian approach presented in [9] is used to compute the Q-factor.

3. FILTER OPTIMIZATION Narrow optical filtering usually degrades the eye-opening of the signal. Therefore, it increases the intersymbol interference in the system. However, in the same time, it may improve UDWDM system performance because it reduces the signal bandwidth, which leads to lower crosstalk. Moreover, it reduces signal-amplified spontaneous emission beat noise at the receiver. Thus, a compromised optimum solution should be achieved by using narrow optical filtering. Therefore, improvement of the UDWDM system performance can be achieved by optimization of the 3-dB bandwidth of MUX and DMUX filters.

In this section, the normalized 3-dB electrical filter bandwidth (bel,n) of the generation scheme of both LPF and DAA duobinary formats, and normalized 3-dB MUX (bMUX) and DMUX (bDMUX) bandwidths of the 2nd order super Gaussian optical filter are optimized for 4 spans of nonlinear fiber transmission in UDWDM systems. Due to the easier comparison of optimized filters bandwidths for different bit rate, the normalized filter bandwidths to the bit rate are analyzed. The bel,n, bMUX, and bDMUX are varied independently.

3.1 LPF duobinary coding format The Q-factor and optimized bel,n for achieving the maximum attainable Q-factor are shown in Fig. 3a and 3b, respectively, as a function of bMUX and bDMUX of 2nd order super Gaussian filters for LPF duobinary coding format. In Fig. 3a, the maximum Q-factor of 15.53 is achieved for bMUX of 1.13, and bDMUX of 0.69. The optimum bel,n is 0.34, shown in Fig. 3b, which is higher than the one used in conventional duobinary format. The reason for achieving better UDWDM system performance with optimized BL-LPF duobinary format than with the other investigated BL-LPF formats can be clearly seen in Fig. 4, where the eye-diagrams and power

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spectral densities (PSD) of LPF and BL-LPF duobinary formats with bel,n = 0.25 (conventional), 0.28 (PSBT), and 0.34 (optimized) are shown. Even with the smallest eye-opening, and with a similar PSD 20-dB bandwidth, the UDWDM system improvement happened due to better robustness to the DMUX filtering for optimized bel,n and bMUX of BL-LPF duobinary format.

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The MUX bandwidths used are the optimum shown in Tab. 1.

The optimization in this section showed several important points. The optimization of electrical and optical filter bandwidths is very important to achieve system performance improvement. Moreover, the optimized bel,n is much larger than the conventional one. However, the optimized bel,n, achieved after 4 spans of transmission is smaller than the one optimized in back-to-back UDWDM system [8] (0.4) due to the transmission constraints. Thus, the BL-LPF duobinary format with bel,n of 0.34 is demonstrated as the optimal for high spectrally efficient UDWDM system due to the narrower signal spectrum.

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The Q-factor and optimized bel,n for achieving the maximum attainable Q-factor for DAA duobinary coding format are shown in Fig. 5a and 5b, respectively. In Fig. 5a, the maximum Q-factor of 11.72 is achieved for bMUX of 1.25, and bDMUX of 0.94. The optimum bel,n occurs 0.96, shown in Fig. 5b, which is much higher than the one used in conventional DAA duobinary scheme.

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4. COMPARISON OF OPTIMIZED CODING FORMATS WITH CONVENTIONAL ONES The UDWDM system performance with optimized and conventional BL-duobinary and BL-PSBT formats are compared. The optimized filter settings for all investigated coding formats are given in Table 1.

Table 1. Optimized filter settings and Q-factor for optimized, and conventional BL-LPF and BL-DAA duobinary, and BL-PSBT formats.

Coding formats ⇒ Optimized duobinary

Conventional duobinary

System parameters ⇓ LPF DAA LPF DAA

PSBT

bel,n 0.34 0.96 0.25 0.5 0.28 bMUX 1.13 1.25 1.5 1.38 1.13 bDMUX 0.69 0.94 0.75 1 0.75

UDWDM system

Q-factor 15.53 11.72 13.19 11.24 14.58

Table 1 shows, also, the maximum attained Q-factor of optimized and conventional BL-duobinary formats for UDWDM system. Due to better robustness to the DMUX filtering, the optimized BL-LPF, and BL-DAA duobinary coding formats for UDWDM system after 4×80 km of transmission along SSMF are found to be the ones with bel,n of 0.34, and 0.96, respectively. The optimized BL-LPF duobinary format has 1.4 dB higher Q-factor than the conventional BL duobinary format. The maximum achieved Q-factor for all investigated coding formats occurs for optimized BL-LPF duobinary coding format.

5. CONCLUSIONS By varying simultaneously the electrical transmitter, MUX and DMUX 3-dB bandwidths, the optimization of 40 Gbit/s LPF and DAA duobinary generation schemes is achieved for 0.8 bit/s/Hz UDWDM systems after 4×80 km of transmission along SSMF. It has been shown that the optimal bel,n is notably different from the conventional one in UDWDM systems, leading to a different generation scheme requirement.

It has been shown that BL-DAA duobinary generation schemes with optimized bel,n have better UDWDM system performance than BL-DAA conventional duobinary coding format. Moreover, the optimized BL-LPF duobinary format achieves 1.4 dB higher Q-factor than BL-LPF conventional duobinary format. Furthermore, better UDWDM system performance than with the capacity×distance recorder PSBT format has been achieved. For all investigated coding formats, the optimized BL-LPF duobinary format has shown the best Q-factor for UDWDM system.

REFERENCES [1] A. Hodžić, et al.: Optimized filtering for 40-Gb/s/ch-based DWDM transmission systems over standard single-mode

fiber, IEEE Photon. Technol. Lett., vol. 15, no. 7, pp. 1002-1004, July 2003. [2] G. Bosco, et al.: On the use of NRZ, RZ, and CSRZ modulation at 40 Gb/s with narrow DWDM channel spacing,

J. Lightwave Technol., vol. 20, no. 9, pp. 1694-1704, Sept. 2002. [3] G., Charlet, et al.: Cost-optimized 6.3 Tbit/s-capacity terrestrial link over 17×100km using phase-shaped binary

transmission in a conventional all-EDFA SMF-based system, in Proc. OFC 2003, Atlanta, GA, March 2003, paper PD25.

[4] N. B. Pavlović, A.V.T. Cartaxo: Optimized optical filtering for 40 Gb/s/channel enhanced phase-shaped binary transmission in ultra dense WDM systems, in Proc. ICTON 2004, Wroclaw, Poland, July 2004, vol. 1, pp. 221-224.

[5] D. Penninckx, et al.: The phase-shaped binary transmission (PSBT): A new technique to transmit far beyond the chromatic dispersion limit, IEEE Photon. Technol. Lett., vol. 9, no. 2, pp. 259-261, Feb. 1997.

[6] A.J. Price, N. Le Mercier: Reduced bandwidth optical digital intensity modulation with improved chromatic dispersion tolerance, Electron. Lett., vol. 31, no. 1, pp. 58-59, Jan. 5, 1995.

[7] K. Yonenaga, S. Kuwano: Dispersion-tolerant optical transmission system using duobinary transmitter and binary receiver, J. Lightwave Technol., vol. 15, no. 8, pp. 1530-1537, Aug. 1997.

[8] G. Bosco, et al.: Modulation formats suitable for ultrahigh spectral efficient WDM systems, IEEE J. Sel. Top. Quant. El., vol. 10, no. 2, pp. 321-328, March/April 2004.

[9] J.L. Rebola, A.V.T. Cartaxo: Gaussian approach for performance evaluation of optically preamplified receivers with arbitrary optical and electrical filters, IEE Proc.-Optoelectron., vol. 148, no. 3, pp. 135-142, June 2001.