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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 9 — May. 6, 2013
  • pp: 10496–10501
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Analyzing the benefit of optical transmission systems based on Root Raised Cosine PS-QPSK and a flexible channel grid

A. Seck, P. Ramantanis, J. Vuong, D. F. Bendimerad, C. Lepers, B.-E. Benkelfat, and Y. Frignac  »View Author Affiliations


Optics Express, Vol. 21, Issue 9, pp. 10496-10501 (2013)
http://dx.doi.org/10.1364/OE.21.010496


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Abstract

We numerically investigate the multi-channel transmission performance of Polarization Switched Quadrature Phase Shift Keying (PS-QPSK) and we compare it to the performance of Polarization-Division-Multiplexed QPSK (PDM-QPSK), using Root Raised Cosine (RRC) spectral shaping, in the context of a flexible channel grid. We point out the impact of the roll-off factor and the potential influence of different dispersion compensation scenarios. Finally, the advantage of PS-QPSK against PDM-QPSK is presented as a function of the system parameters, while we also discuss the benefit of a RRC spectral shaping against a tight filtering at the transmitter side with a 2nd order super-Gaussian-shaped filter.

© 2013 OSA

1. Introduction

In the context of coherent optical communications, the potential of several multi-level modulation formats has been extensively investigated. As shown in [1

1. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express 17(13), 10814–10819 (2009). [CrossRef] [PubMed]

], PS-QPSK appears as the most power efficient modulation format, while several numerical and experimental investigations have demonstrated its high resistance to non-linear effects [2

2. C. Behrens, D. Lavery, D. S. Millar, S. Makovejs, B. C. Thomsen, R. I. Killey, S. J. Savory, and P. Bayvel, “Ultra-long-haul transmission of 7×42.9 Gbit/s PS-QPSK and PDM-BPSK,” Opt. Express 19(26), B581–B586 (2011). [CrossRef] [PubMed]

5

5. P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for non-linear fiber propagation effects,” Opt. Express 18(11), 11360–11371 (2010). [CrossRef] [PubMed]

]. Nevertheless, since PS-QPSK may be seen as a coded version of PDM-QPSK [6

6. B. Krongold, T. Pfau, N. Kaneda, and S. C. J. Lee, “Comparison between PS-QPSK and PDM-QPSK with equal rate and bandwidth,” IEEE Photon. Technol. Lett. 24(3), 203–205 (2012). [CrossRef]

], the increased minimum distance between symbols comes at the price of a reduced Information Spectral Density (ISD). On the other hand, the ever-increasing demand for spectrally efficient transmission has paved the way towards a channel spacing below the “standard” 50 GHz, while at the same time, network considerations introduce the need for a flexible channel grid. Finally, techniques based on spectral shaping [7

7. G. Bosco, “Spectral shaping in ultra-dense WDM systems: optical vs. electrical approaches,” in Proc. of OFC, OM3H.1 (2012). [CrossRef]

] such as Root Raised Cosine (RRC) filtering [8

8. P. Ramantanis, A. Seck, J. Vuong, D. Bendimerad, and Y. Frignac, “Spectral shaping tradeoffs in root-raised-cosine PDM-QPSK nonlinear transmission,” in Proc. of ECOC, P04.01 (2012). [CrossRef]

10

10. B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of nyquist pulses for coherent optical communications,” Opt. Express 20(8), 8397–8416 (2012). [CrossRef] [PubMed]

] have also been employed in order to further increase the ISD of optical transmission systems.

2. Simulation setup

The simulation setup is shown in Fig. 1
Fig. 1 Simulation setup for our transmission system including RRC spectral shaping description. p(t) is the fundamental pulse in the case of RRC spectral shaping.
. The transmitter generates 1 or 9 signals with a symbol rate R of 32 Gbaud, based on a modulation format that is either filtered NRZ-PS-QPSK (referred to as “fNRZ_PS” in the following) using Non Return to Zero (NRZ) pulses with a 20% raised time or RRC-PS-QPSK (referred to as “RRC_PS”). The PS-QPSK modulation is obtained following the scheme represented in [1

1. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express 17(13), 10814–10819 (2009). [CrossRef] [PubMed]

] and based on a PDM-QPSK transmitter. For each channel, b1, b2 and b3 are derived from different random sequences of Ns = 4096 symbols while b4 = xor{xor(b1, b2), b3}. The channels are multiplexed using a frequency spacing Δν of 32, 36, 40 or 50 GHz with no time decorrelation and the same States Of Polarization (SOPs). Filtering is considered before multiplexing in the “fNRZ_PS” case by using a usual 2nd order super-gaussian optical filter with a variable 3 dB Bandwidth W, within the range {25-64} GHz. Similarly, in the “RRC_PS” case, a signal with a RRC spectral shape was generated for each of the 9 channels (as shown in Fig. 1), with roll-off factors ρ within the range {0-1}. In our simulations the signal complex envelopes for the two PDM tributaries Ax and Ay are numerically represented by Nt = 64 samples per symbol, achieving a total of Nfft = 4096x64 samples. For the generation of the transmitted pulses in the RRC spectral shaping case, we do not take into account potential imperfections caused by either non-ideal optical filtering or limited resolution of Digital-To-Analog Converters [7

7. G. Bosco, “Spectral shaping in ultra-dense WDM systems: optical vs. electrical approaches,” in Proc. of OFC, OM3H.1 (2012). [CrossRef]

]. We also indicate in Fig. 1 (as it is well-known for RRC signals) that higher values of ρ lead to narrower pulse widths and larger spectral occupancies.

3. Simulation results on RRC-PS-QPSK

In Figs. 2(a)
Fig. 2 Q2 versus Pin,avg for “RRC_PS” with different roll-off factors for either 1 or 9 channels (Δν = 50GHz) in the scenarios “wDCF” (a) and “w/oDCF” (b). Illustration of adjacent, spectrally-shaped RRC pulses, in the context of polarization switching (c). Q2max as a function of ρ (d) for 1 (solid line) or 9 channels (dashed line) in the case “w/oDCF”.
and 2(b) we show the transmission quality of RRC-PS-QPSK (“RRC_PS” scenario) in terms of Q2 factor as a function of Pin,avg, for both of the dispersion compensation scenarios, i.e. “wDCF” and “w/oDCF” respectively. Each curve corresponds to a different roll-off factor ρ, in either a single channel or a WDM configuration, for Δν = 50 GHz. For each value of ρ, the highest Q2 value (noted as Q2max) is observed for a power level, referred to in the following as nonlinear threshold (NLT). According to Fig. 2(a), in the single channel configuration, all values of ρ yield approximately the same performance in the linear regime, while increasing ρ improves the Q2 factor in the nonlinear regime. In addition, commenting on the variation of Q2max as a function of ρ, we observe a performance degradation, especially for lower values of ρ. To explain this behavior, we suggest that when applying a RRC spectral shaping with a bandwidth approaching the modulation rate, pulses are spread out the symbol time slot Ts, thus inducing power fluctuations due to Inter-Symbol Interference (ISI) during propagation. This is illustrated in Fig. 2(c) for the signal envelopes on the two polarization tributaries: Ax and Ay. These ISI induced power fluctuations are reduced when increasing ρ. Moreover, the NLT and Q2max increase with ρ, yielding a NLT and Q2max difference of 2 dB between the best and worst performance in single channel configuration.

Furthermore we observe a NLT and Q2max difference of 3 dB and 4 dB respectively between the best performance in WDM and single channel configuration. Nevertheless in the “w/oDCF” scenario (Fig. 2(b)), this difference is reduced to 1 dB for the NLTs and Q2max. The transmission quality difference between single-channel and WDM can be explained by the signal impairments due to cross-nonlinearities between channels, known to be more detrimental for systems with in-line DCFs. In order to analyze the impact of the roll-off factor, we show in Fig. 2(d), the evolution of Q2max as a function of ρ in the scenario “w/oDCF” for single channel and WDM configuration. We note that the optimal transmission performance in single-channel without DCFs slightly increases with ρ. We also show in Fig. 2(d) the influence of ρ for WDM systems in the scenario “w/oDCF” (appearing with dashed lines), for Δν = 32, 40 or 50 GHz. It turns out that the corresponding Q2max values as a function of ρ are roughly constant when considering a channel spacing Δν = 50GHz. On the other hand, for Δν = 40 GHz, Q2 remains constant for ρ within the range {0-0.4} and then decreases, while for Δν = 32 GHz, Q2 directly drops for ρ>0. As for each channel the spectral occupancy is higher when increasing ρ, it seems reasonable to attribute these Q2 evolutions to the cross-talk between different channels, appearing when ρ is higher than a given value, with this latter depending on Δν.

4. RRC-PS-QPSK transmission performance comparison

In this section we compare the above-analyzed RRC-PS-QPSK format against the Tx-filtered version of PS-QPSK and the two corresponding PDM-QPSK solutions, with the RRC-PDM-QPSK noted as “RRC_PDM” and the Tx-filtered PDM-QPSK noted as “fNRZ_PDM”. In what follows we have optimized ρ, while an optimal filter bandwidth Wopt, found to be roughly about Δν/1.25, was also employed for the “fNRZ_PS” and “fNRZ_PDM” scenarios.

As a first step, analyzing the gain of applying a PS-QPSK coding on PDM-QPSK, in Fig. 4(a)
Fig. 4 “Coding gain” (a) and “RRC spectral shaping gain” (b) as a function of the channel spacing Δν in the “w/oDCF” (solid lines) and in the “wDCF” DC scheme (dashed lines).
we show their performance difference ΔQ2max = Q2max(PS-QPSK) - Q2max(PDM-QPSK), noted as “coding gain”, for either RRC spectral shaping (triangle markers) or Tx-filtering (square markers) and both DC schemes, as a function of Δν. As a second step, in Fig. 4(b) we show ΔQ2max between solutions using a RRC spectral shaping against Tx-filtering, noted as “RRC Spectral Shaping gain”.

Commenting on Fig. 4(a) we note that the “coding gain” is roughly constant as a function of Δν for both DC schemes. Yielding similar performance for both RRC spectral shaping and a Tx-filtering, the “coding gain” is about 2.5dB in the “w/oDCF” case and about 1.5 dB lower for the “wDCF” DC scheme. We may generally conclude that the PS-QPSK coding advantage over PDM-QPSK is not particularly influenced by the channel spacing. Nevertheless, it has to be noted that PS-QPSK coding seems to be more efficient in a system configuration without in-line DCFs. Focusing on Fig. 4(b), we note that for both PDM- and PS-QPSK modulations and both DC schemes, RRC spectral shaping brings a minor improvement of only about 0.5 dB for Δν = 50 GHz, whereas it becomes a particularly profitable option for lower channel spacings, reaching its higher value of about 2.7 dB for Δν = R. The above results could be of interest when considering the tradeoff between the performance gain brought by a RRC spectral shaping and the cost of this implementation, for various values of Δν.

5. Conclusion

In this work we have numerically investigated the potential of PS-QPSK format at 32 Gbaud with a RRC spectral shaping in system configurations with variable dispersion management and a flexible channel grid. After a preliminary optimization of the roll-off factors of RRC-PS-QPSK in all cases, we compare RRC-PS-QPSK against solutions previously presented in literature by first showing that the gain of an ideal RRC spectral shaping with respect to a rough spectral shaping achieved by a practical filter at the transmitter side reaches a maximum value of 2.7 dBs in a Nyquist WDM condition (32 GHz channel spacing), while this gain vanishes for a channel spacing of 50GHz. Finally, our numerical simulations indicate a transmission performance improvement of RRC-PS-QPSK compared to RRC-PDM-QPSK of 2.5 dBs for example in the DCF-free configuration. We emphasize on the fact that this coding gain presents a weak dependence on the considered channel spacing.

Acknowledgments

We would like to thank G. Charlet from Alcatel-Lucent and A. Bononi from the University of Parma for discussions on PS-QPSK. This work has been based on the Optilux Toolbox [15].

References and links

1.

M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express 17(13), 10814–10819 (2009). [CrossRef] [PubMed]

2.

C. Behrens, D. Lavery, D. S. Millar, S. Makovejs, B. C. Thomsen, R. I. Killey, S. J. Savory, and P. Bayvel, “Ultra-long-haul transmission of 7×42.9 Gbit/s PS-QPSK and PDM-BPSK,” Opt. Express 19(26), B581–B586 (2011). [CrossRef] [PubMed]

3.

P. Serena, A. Vannucci, and A. Bononi, “The performance of polarization switched-QPSK (PS-QPSK) in dispersion managed WDM transmissions,” in Proc. of ECOC p. Th.10.E.2 (2010). [CrossRef]

4.

M. Sjödin, B. J. Puttnam, P. Johannisson, S. Shinada, N. Wada, P. A. Andrekson, and M. Karlsson, “Transmission of PM-QPSK and PS-QPSK with different fiber span lengths,” Opt. Express 20(7), 7544–7554 (2012). [CrossRef] [PubMed]

5.

P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for non-linear fiber propagation effects,” Opt. Express 18(11), 11360–11371 (2010). [CrossRef] [PubMed]

6.

B. Krongold, T. Pfau, N. Kaneda, and S. C. J. Lee, “Comparison between PS-QPSK and PDM-QPSK with equal rate and bandwidth,” IEEE Photon. Technol. Lett. 24(3), 203–205 (2012). [CrossRef]

7.

G. Bosco, “Spectral shaping in ultra-dense WDM systems: optical vs. electrical approaches,” in Proc. of OFC, OM3H.1 (2012). [CrossRef]

8.

P. Ramantanis, A. Seck, J. Vuong, D. Bendimerad, and Y. Frignac, “Spectral shaping tradeoffs in root-raised-cosine PDM-QPSK nonlinear transmission,” in Proc. of ECOC, P04.01 (2012). [CrossRef]

9.

J. Fickers, A. Ghazisaeidi, M. Salsi, G. Charlet, F. Horlin, P. Emplit, and S. Bigo, “Design rules for pulse shaping in PDM-QPSK and PDM-16QAM nyquist-WDM coherent optical transmission systems,” in Proc. of ECOC, p. We.1.C.2 (2012). [CrossRef]

10.

B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of nyquist pulses for coherent optical communications,” Opt. Express 20(8), 8397–8416 (2012). [CrossRef] [PubMed]

11.

R. I. Killey, H. J. Thiele, V. Mikhailov, and P. Bayvel, “Reduction of intrachannel nonlinear distortion in 40 Gb/s-based WDM transmission over standard fiber,” IEEE Photon. Technol. Lett. 12(12), 1624–1626 (2000). [CrossRef]

12.

G. P. Agrawal, Lightwave Technology Telecommunication Systems (John Wiley & Sons, Inc, 2005).

13.

P. Johannisson, M. Sjödin, M. Karlsson, H. Wymeersch, E. Agrell, and P. A. Andrekson, “Modified constant modulus algorithm for polarization-switched QPSK,” Opt. Express 19(8), 7734–7741 (2011). [CrossRef] [PubMed]

14.

A. Seck, P. Ramantanis, J. Vuong, D. F. Bendimerad, C. Lepers, and Y. Frignac, “ Novel carrier phase estimation scheme for polarization switched-QPSK-based transmission systems,” in Proc. of OFC, OTu3I. (2013).

15.

P. Serena, M. Salsi, M. Bertolini, A. Vannucci, N. Rossi, and F. Vacondio, Optilux Toolbox, http://optilux.sourceforge.net

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: April 3, 2013
Revised Manuscript: April 8, 2013
Manuscript Accepted: April 8, 2013
Published: April 22, 2013

Citation
A. Seck, P. Ramantanis, J. Vuong, D. F. Bendimerad, C. Lepers, B.-E. Benkelfat, and Y. Frignac, "Analyzing the benefit of optical transmission systems based on Root Raised Cosine PS-QPSK and a flexible channel grid," Opt. Express 21, 10496-10501 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-9-10496


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References

  1. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express17(13), 10814–10819 (2009). [CrossRef] [PubMed]
  2. C. Behrens, D. Lavery, D. S. Millar, S. Makovejs, B. C. Thomsen, R. I. Killey, S. J. Savory, and P. Bayvel, “Ultra-long-haul transmission of 7×42.9 Gbit/s PS-QPSK and PDM-BPSK,” Opt. Express19(26), B581–B586 (2011). [CrossRef] [PubMed]
  3. P. Serena, A. Vannucci, and A. Bononi, “The performance of polarization switched-QPSK (PS-QPSK) in dispersion managed WDM transmissions,” in Proc. of ECOC p. Th.10.E.2 (2010). [CrossRef]
  4. M. Sjödin, B. J. Puttnam, P. Johannisson, S. Shinada, N. Wada, P. A. Andrekson, and M. Karlsson, “Transmission of PM-QPSK and PS-QPSK with different fiber span lengths,” Opt. Express20(7), 7544–7554 (2012). [CrossRef] [PubMed]
  5. P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for non-linear fiber propagation effects,” Opt. Express18(11), 11360–11371 (2010). [CrossRef] [PubMed]
  6. B. Krongold, T. Pfau, N. Kaneda, and S. C. J. Lee, “Comparison between PS-QPSK and PDM-QPSK with equal rate and bandwidth,” IEEE Photon. Technol. Lett.24(3), 203–205 (2012). [CrossRef]
  7. G. Bosco, “Spectral shaping in ultra-dense WDM systems: optical vs. electrical approaches,” in Proc. of OFC, OM3H.1 (2012). [CrossRef]
  8. P. Ramantanis, A. Seck, J. Vuong, D. Bendimerad, and Y. Frignac, “Spectral shaping tradeoffs in root-raised-cosine PDM-QPSK nonlinear transmission,” in Proc. of ECOC, P04.01 (2012). [CrossRef]
  9. J. Fickers, A. Ghazisaeidi, M. Salsi, G. Charlet, F. Horlin, P. Emplit, and S. Bigo, “Design rules for pulse shaping in PDM-QPSK and PDM-16QAM nyquist-WDM coherent optical transmission systems,” in Proc. of ECOC, p. We.1.C.2 (2012). [CrossRef]
  10. B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of nyquist pulses for coherent optical communications,” Opt. Express20(8), 8397–8416 (2012). [CrossRef] [PubMed]
  11. R. I. Killey, H. J. Thiele, V. Mikhailov, and P. Bayvel, “Reduction of intrachannel nonlinear distortion in 40 Gb/s-based WDM transmission over standard fiber,” IEEE Photon. Technol. Lett.12(12), 1624–1626 (2000). [CrossRef]
  12. G. P. Agrawal, Lightwave Technology Telecommunication Systems (John Wiley & Sons, Inc, 2005).
  13. P. Johannisson, M. Sjödin, M. Karlsson, H. Wymeersch, E. Agrell, and P. A. Andrekson, “Modified constant modulus algorithm for polarization-switched QPSK,” Opt. Express19(8), 7734–7741 (2011). [CrossRef] [PubMed]
  14. A. Seck, P. Ramantanis, J. Vuong, D. F. Bendimerad, C. Lepers, and Y. Frignac, “ Novel carrier phase estimation scheme for polarization switched-QPSK-based transmission systems,” in Proc. of OFC, OTu3I. (2013).
  15. P. Serena, M. Salsi, M. Bertolini, A. Vannucci, N. Rossi, and F. Vacondio, Optilux Toolbox, http://optilux.sourceforge.net

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