## Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems |

Optics Express, Vol. 22, Issue 15, pp. 18770-18777 (2014)

http://dx.doi.org/10.1364/OE.22.018770

Acrobat PDF (1742 KB)

### Abstract

In this work we experimentally investigate the improved intra-channel fiber nonlinearity tolerance of digital subcarrier multiplexed (SCM) signals in a single-channel coherent optical transmission system. The digital signal processing (DSP) for the generation and reception of the SCM signals is described. We show experimentally that the SCM signal with a nearly-optimum number of subcarriers can extend the maximum reach by 23% in a 24 GBaud DP-QPSK transmission with a BER threshold of 3.8 × 10^{−3} and by 8% in a 24 GBaud DP-16-QAM transmission with a BER threshold of 2 × 10^{−2}. Moreover, we show by simulations that the improved performance of SCM signals is observed over a wide range of baud rates, further indicating the merits of SCM signals in baud-rate flexible agile transmissions and future high-speed optical transport systems.

© 2014 Optical Society of America

## 1. Introduction

1. E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol. **26**(20), 3416–3425 (2008). [CrossRef]

3. E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express **19**(2), 570–583 (2011). [CrossRef] [PubMed]

4. Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol. **29**(17), 2570–2576 (2011). [CrossRef]

6. Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in *Proc. OFC* (2014), paper Th4D.7. [CrossRef]

7. N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in *Proc. Opt. Commun. Conf.* (2011), paper OWW6. [CrossRef]

8. 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]

9. X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in *Proc. OFC* (2013), paper JTh2A.38. [CrossRef]

10. L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express **19**(9), 8079–8084 (2011). [CrossRef] [PubMed]

13. Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. **22**(16), 1250–1252 (2010). [CrossRef]

12. W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J. **2**(3), 276–283 (2010). [CrossRef]

12. W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J. **2**(3), 276–283 (2010). [CrossRef]

14. Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in *Proc. OFC* (2012), paper OTh1B.3. [CrossRef]

14. Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in *Proc. OFC* (2012), paper OTh1B.3. [CrossRef]

15. Y. Zhu, J. Wang, Q. Guo, Y. Cui, C. Li, F. Zhu, and Y. Bai, “Experimental comparison of terabit Nyquist superchannel transmissions based on high and low baud rates,” in *Proc. OFC* (2013), paper JW2A.37. [CrossRef]

16. M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in *Proc. OFC* (2014), paper Tu3J.2. [CrossRef]

16. M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in *Proc. OFC* (2014), paper Tu3J.2. [CrossRef]

## 2. Subcarrier multiplexing and de-multiplexing

*K*denotes the number of subcarriers in the SCM signal. Note that the SC signal can be regarded as a special SCM signal when

*K*= 1, so Fig. 2 applies to the SC signal as well. Independent data sequences are used in each subcarrier and mapped to QPSK or 16-QAM symbols. After the symbols are interpolated to 2 samples per symbol in each subcarrier, the samples, denoted as

*S*,

_{k}*k*= 1,2…

*K*, are transformed to the frequency domain and filtered by a RRC filter

*H(f)*with a roll-off factor of 0.1. Then the outputs are up-sampled by

*K*(essentially frequency-domain zero padding), shifted to different frequencies in the spectrum and multiplexed. These operations are also done in the frequency domain. After the re-sampling to the DAC sampling rate, the signals are pre-compensated to combat the limited bandwidth of the transmitter based on the pre-measured transmitter frequency response. Finally the time-domain samples are sent to DACs for digital-to-analog conversion. Correspondingly, for subcarrier de-multiplexing, we use a configuration as shown in Fig. 2(b). The SCM signal is first digitized by analog-to-digital converters (ADC) and re-sampled to twice the total baud rate. All subcarriers are captured simultaneously in a single detection. After the transform to the frequency domain, each subcarrier is successively shifted to the baseband and filtered out using a digital low-pass matched filter

*G(f)*. Finally the low-baud-rate signal in each subcarrier is down-sampled by

*K*(essentially truncating zeros in the frequency domain to 2 samples per symbol), transformed to time domain and processed in parallel. The other DSP blocks are similar to those in conventional SC systems and are not shown in Fig. 2.

## 3. Setup, results and discussions

17. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. **16**(5), 1164–1179 (2010). [CrossRef]

18. M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun. **36**(5), 605–612 (1988). [CrossRef]

19. Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express **20**(17), 19599–19609 (2012). [CrossRef] [PubMed]

20. L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in *Proc. OFC* (2009), paper OMT2. [CrossRef]

^{5}symbols are extracted for offline processing. The BER or Q

^{2}factor of the SCM signals is an average value over all subcarriers. Note that for the SCM signals the DSP blocks before matched filtering (with black color in Fig. 3) are applied to the multiplexed signal and the rest of the processing (with red color in Fig. 3) is done independently for each subcarrier.

^{−3}is ~0.2 dB if we transmit the SCM signal with 8 subcarriers in place of the SC signal. Similarly, in the 16-QAM case in Fig. 5(b), the penalty at a BER threshold of 2 × 10

^{−2}is ~0.3 dB if the SCM signal with 4 subcarriers is transmitted. This is because the SCM signals are more susceptible to certain types of system imperfections, e.g., the limited effective DAC resolution, the FO estimation error and the laser phase noise.

^{−3}and in Fig. 7(b) for 16-QAM transmission with a BER threshold of 2 × 10

^{−2}. Even though we can only transmit signals for integer number of loops in the experiments, the achievable transmission distance in Fig. 7 is estimated using interpolation at the BER threshold when necessary. In accordance with the results in Fig. 6, the achievable transmission distances of different signals are similar in the linear regime with low launch powers. However, if we investigate the maximum transmission distance of different signals with their respective optimum launch powers, we see that the achievable reach can be extended by transmitting the SCM signals. Specifically, in the QPSK case shown in Fig. 7(a), the optimum launch power for the SC signal is –2 dBm which enables a transmission distance of approximately 5900 km. All the three SCM signals can increase the maximum reach. In particular, the SCM signal with 8 subcarriers enables a transmission of 7250 km with its optimum power of –1 dBm. Therefore, an extension of ~23% can be achieved. In the 16-QAM case as shown in Fig. 7(b), the optimum launch power for all signals is –1 dBm, but the maximum reach is extended from 2030 km to 2200 km with this launch power if we transmit the SCM signal with 4 subcarriers in place of the SC signal. This means an extension of ~8%.

^{2}factors of different signals with various total baud rates. The transmission distance is 6400 km in the QPSK case in Fig. 8(a) and 1920 km in the 16-QAM case in Fig. 8(b). The Q

^{2}values are obtained with the optimum launch power in each case. One thing to observe is the optimum number of subcarriers tends to increase as the total baud rate becomes larger. For example, the optimum number of subcarriers is close to 8 in the 16-GBaud QPSK system and increases to 16 when the total baud rate becomes 40 GBaud as shown in Fig. 8(a). In addition, in both figures we can see the system performance can be improved significantly within a wide range of baud rates (from 16 GBaud to 56 GBaud) by transmitting properly designed SCM signals. This result further indicates the merits of SCM signals in baud-rate flexible networks and future high-speed transmission systems.

## 4. Conclusion

^{−3}and of 8% for a 24-GBaud DP-16-QAM system with a BER threshold of 2 × 10

^{−2}. Besides, the advantage of SCM signals is demonstrated within a wide range of baud rates, which further indicates their potential applications in baud-rate adaptive transmissions and future high-speed communication systems.

## References and links

1. | E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol. |

2. | L. B. Du and A. J. Lowery, “Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems,” Opt. Express |

3. | E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express |

4. | Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol. |

5. | Y. Gao, A. S. Karar, J. C. Cartledge, S. S.-H. Yam, M. O’Sullivan, C. Laperle, A. Borowiec, and K. Roberts, “Simplified nonlinearity pre-compensation using a modified summation criteria and non-uniform power profile,” in |

6. | Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in |

7. | N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in |

8. | 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 |

9. | X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in |

10. | L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express |

11. | A. Bononi, N. Rossi, and P. Serena, “Performance dependence on channel baud-rate of coherent single-carrier WDM systems,” in |

12. | W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J. |

13. | Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. |

14. | Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in |

15. | Y. Zhu, J. Wang, Q. Guo, Y. Cui, C. Li, F. Zhu, and Y. Bai, “Experimental comparison of terabit Nyquist superchannel transmissions based on high and low baud rates,” in |

16. | M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in |

17. | S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. |

18. | M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun. |

19. | Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express |

20. | L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.2330) Fiber optics and optical communications : Fiber optics communications

**ToC Category:**

Optical Communications

**History**

Original Manuscript: May 19, 2014

Revised Manuscript: July 7, 2014

Manuscript Accepted: July 9, 2014

Published: July 25, 2014

**Citation**

Meng Qiu, Qunbi Zhuge, Mathieu Chagnon, Yuliang Gao, Xian Xu, Mohamed Morsy-Osman, and David V. Plant, "Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems," Opt. Express **22**, 18770-18777 (2014)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-18770

Sort: Year | Journal | Reset

### References

- E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol.26(20), 3416–3425 (2008). [CrossRef]
- L. B. Du and A. J. Lowery, “Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems,” Opt. Express18(16), 17075–17088 (2010). [CrossRef] [PubMed]
- E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express19(2), 570–583 (2011). [CrossRef] [PubMed]
- Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol.29(17), 2570–2576 (2011). [CrossRef]
- Y. Gao, A. S. Karar, J. C. Cartledge, S. S.-H. Yam, M. O’Sullivan, C. Laperle, A. Borowiec, and K. Roberts, “Simplified nonlinearity pre-compensation using a modified summation criteria and non-uniform power profile,” in Proc. OFC (2014), paper Tu3A.6. [CrossRef]
- Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in Proc. OFC (2014), paper Th4D.7. [CrossRef]
- N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in Proc. Opt. Commun. Conf. (2011), paper OWW6. [CrossRef]
- 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]
- X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in Proc. OFC (2013), paper JTh2A.38. [CrossRef]
- L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express19(9), 8079–8084 (2011). [CrossRef] [PubMed]
- A. Bononi, N. Rossi, and P. Serena, “Performance dependence on channel baud-rate of coherent single-carrier WDM systems,” in Proc. ECOC (2013), paper Th.1.D.5. [CrossRef]
- W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J.2(3), 276–283 (2010). [CrossRef]
- Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett.22(16), 1250–1252 (2010). [CrossRef]
- Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in Proc. OFC (2012), paper OTh1B.3. [CrossRef]
- Y. Zhu, J. Wang, Q. Guo, Y. Cui, C. Li, F. Zhu, and Y. Bai, “Experimental comparison of terabit Nyquist superchannel transmissions based on high and low baud rates,” in Proc. OFC (2013), paper JW2A.37. [CrossRef]
- M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in Proc. OFC (2014), paper Tu3J.2. [CrossRef]
- S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron.16(5), 1164–1179 (2010). [CrossRef]
- M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun.36(5), 605–612 (1988). [CrossRef]
- Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express20(17), 19599–19609 (2012). [CrossRef] [PubMed]
- L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in Proc. OFC (2009), paper OMT2. [CrossRef]

## Cited By |
Alert me when this paper is cited |

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.

« Previous Article | Next Article »

OSA is a member of CrossRef.