## Joint digital signal processing for superchannel coherent optical communication systems |

Optics Express, Vol. 21, Issue 7, pp. 8342-8356 (2013)

http://dx.doi.org/10.1364/OE.21.008342

Acrobat PDF (1516 KB)

### Abstract

Ultra-high-speed optical communication systems which can support ≥ 1Tb/s per channel transmission will soon be required to meet the increasing capacity demand. However, 1Tb/s over a single carrier requires either or both a high-level modulation format (i.e. 1024QAM) and a high baud rate. Alternatively, grouping a number of tightly spaced “sub-carriers” to form a terabit *superchannel* increases channel capacity while minimizing the need for high-level modulation formats and high baud rate, which may allow existing formats, baud rate and components to be exploited. In ideal Nyquist-WDM superchannel systems, optical subcarriers with rectangular spectra are tightly packed at a channel spacing equal to the baud rate, thus achieving the Nyquist bandwidth limit. However, in practical Nyquist-WDM systems, precise electrical or optical control of channel spectra is required to avoid strong inter-channel interference (ICI). Here, we propose and demonstrate a new “super receiver” architecture for practical Nyquist-WDM systems, which jointly detects and demodulates multiple channels simultaneously and mitigates the penalties associated with the limitations of generating ideal Nyquist-WDM spectra. Our receiver-side solution relaxes the filter requirements imposed on the transmitter. Two joint DSP algorithms are developed for linear ICI cancellation and joint carrier-phase recovery. Improved system performance is observed with both experimental and simulation data. Performance analysis under different system configurations is conducted to demonstrate the feasibility and robustness of the proposed joint DSP algorithms.

© 2013 OSA

## 1. Introduction

1. E. Torrengo, R. Cigliutti, G. Bosco, G. Gavioli, A. Alaimo, A. Carena, V. Curri, F. Forghieri, S. Piciaccia, M. Belmonte, A. Brinciotti, A. La Porta, S. Abrate, and P. Poggiolini, “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” ECOC 2010, We.7.C.2.

5. A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-Guard-Interval coherent optical OFDM for 100-Gb/s long-haul WDM Transmission,” J. Lightwave Technol. **27**(16), 3705–3713 (2009). [CrossRef]

1. E. Torrengo, R. Cigliutti, G. Bosco, G. Gavioli, A. Alaimo, A. Carena, V. Curri, F. Forghieri, S. Piciaccia, M. Belmonte, A. Brinciotti, A. La Porta, S. Abrate, and P. Poggiolini, “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” ECOC 2010, We.7.C.2.

7. R. Schmogrow, M. Winter, M. Meyer, D. Hillerkuss, S. Wolf, B. Baeuerle, A. Ludwig, B. Nebendahl, S. Ben-Ezra, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM,” Opt. Express **20**(1), 317–337 (2012). [CrossRef] [PubMed]

8. R. Ryf, S. Randel, A. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. Burrows, R. J. Essiambre, P. J. Winzer, D. Peckham, A. McCurdy, and R. Lingle Jr., “Mode-division multiplexing over 96km of few-mode fiber using coherent 6 x 6 MIMO processing,” J. Lightwave Technol. **30**(4), 521–531 (2012). [CrossRef]

*frequency*overlap of the neighboring subchannels. Additionally, in carrier phase-locked Nyquist-WDM systems, we propose a jointly estimated carrier-phase methodology that exploits the carrier information of the multiple subchannels.

## 2. Background and practical issues of superchannels

*superchannel*. Nyquist-WDM and CO-OFDM are the two main approaches to generate superchannel signals optically. We note that electrical OFDM has features similar to CO-OFDM although the sub-carriers are initially generated in the electrical domain [9, 10]. Theoretically, both Nyquist-WDM and CO-OFDM can achieve baud-rate channel spacing without inducing inter-channel interference (ICI) or inter-symbol interference (ISI). However, in practical systems, neither of the two schemes is perfectly ICI or ISI free.

*sinc*shape in the time domain with zero-crossing points at integer multiples of the symbol period T, thus a Nyquist pulse (inter-symbol interference (ISI) free) is achieved. In contrast, for CO-OFDM system, each carrier is

*sinc*shape in the frequency domain, and rectangular in the time domain. Therefore, theoretically both schemes can be ICI and ISI free simultaneously [7

7. R. Schmogrow, M. Winter, M. Meyer, D. Hillerkuss, S. Wolf, B. Baeuerle, A. Ludwig, B. Nebendahl, S. Ben-Ezra, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM,” Opt. Express **20**(1), 317–337 (2012). [CrossRef] [PubMed]

1. E. Torrengo, R. Cigliutti, G. Bosco, G. Gavioli, A. Alaimo, A. Carena, V. Curri, F. Forghieri, S. Piciaccia, M. Belmonte, A. Brinciotti, A. La Porta, S. Abrate, and P. Poggiolini, “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” ECOC 2010, We.7.C.2.

7. R. Schmogrow, M. Winter, M. Meyer, D. Hillerkuss, S. Wolf, B. Baeuerle, A. Ludwig, B. Nebendahl, S. Ben-Ezra, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM,” Opt. Express **20**(1), 317–337 (2012). [CrossRef] [PubMed]

## 3. “Super receiver” principle and design

17. Z. Dong, J. Yu, H.-C. Chien, N. Chi, L. Chen, and G.-K. Chang, “Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection,” Opt. Express **19**(12), 11100–11105 (2011). [CrossRef] [PubMed]

## 4. Joint DSP algorithms

### 4.1 Adaptive linear-ICI cancellation

_{12}, W

_{22}, W

_{32}) are jointly and adaptively updated based on the LMS algorithm. Each filter coefficient Wij represents the weighted crosstalk from subchannel i to subchannel j. As mentioned previously, each Wij is comprised of a number of time-domain taps to allow for the compensation of any timing skew between the subchannels. Intra-channel ISI can also be partially compensated by these taps. The number of taps depends on the timing offset between the subchannels, it is found that 10 ~20 taps are usually sufficient, which corresponds to ~5cm of path mismatch. After the ICI equalizer, ISI equalizers are applied to each polarization to compensate any residual ISI effects induced by optical or electrical filtering in the link. Note that the same procedure is applied to all subchannels of a superchannel signal with the edge subchannels experience only one-side crosstalk since we presume a small guard band between superchannels.

19. J. Pan, C. Liu, T. Detwiler, A. Stark, Y.-T. Hsueh, and S. E. Ralph, “Inter-channel crosstalk cancellation for Nyquist-WDM superchannel applications,” J. Lightwave Technol. **30**(24), 3993–3999 (2012). [CrossRef]

### 4.2 Joint carrier-phase recovery

16. A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory **29**(4), 543–551 (1983). [CrossRef]

## 5. Experimental and simulation results and analysis

### 5.1 Joint adaptive ICI cancellation algorithm results (experimental and simulation)

^{−3}was determined, Fig. 8. The performance of both the conventional independent demodulation method and the LMS ICI cancellation algorithm is compared. The corresponding simulation results are shown for comparison. The conventional algorithm includes a long-memory ISI equalizer (15 taps) to mitigate the filtering induced ISI effects and is a typical algorithm used for Nyquist-WDM superchannel signal demodulation without joint DSP [17

17. Z. Dong, J. Yu, H.-C. Chien, N. Chi, L. Chen, and G.-K. Chang, “Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection,” Opt. Express **19**(12), 11100–11105 (2011). [CrossRef] [PubMed]

_{elec}(twice the electrical BW of the sampling scope), is ~15.5dB for both experiment and simulation indicating good performance for 28Gbaud DP-QPSK as well as good simulation accuracy in the low ICI regime. Decreasing the channel spacing to less than the electrical bandwidth results in increased ICI and increased required OSNR, Fig. 8. However, with the joint ICI equalizer applied, much of the ICI penalty is recovered. For this two-subchannel case, the 30GHz channel spacing has a 2dB penalty using the conventional independent process yet the joint ICI approach reduces this penalty to 1dB, demonstrating the capabilities of the joint ICI cancellation approach. We note that the experimental penalties are larger than the simulation penalties likely resulting from slightly different spectral shape and the high sensitivity to ICI. Note in this particular system setup, since no optical filter is applied at multiplexing, there is significant ICI and the conventional algorithm experiences large ICI penalties.

^{−2}and 10

^{−3}BER vs. channel spacing. We compared the conventional method with our ICI cancellation algorithm under different optical filter bandwidth configurations, Fig. 9. For all the optical filter bandwidth cases ((a) to (d)), the absolute performance deteriorates as the channel spacing decreases. The joint LMS ICI cancellation algorithm always shows performance gain over conventional methods at narrow channel spacing in all the filter bandwidth cases. For the very narrow optical filter case (30GHz, Fig. 9(a)), the joint DSP method provides less benefits due to the reduced ICI from narrow filtering. However, very narrow filtering induces ISI penalties, which can be seen by comparing the performance floors in Fig. 9(a) and 9(d) at wide channel spacing cases (near 50GHz). In particular, the required OSNR to achieve BER = 10

^{−3}for the 30GHz optical filter case (15.8dB) is about 1dB higher than for the 50GHz optical filter case (14.8dB) while both are in the ICI-free regimes (near 50GHz channel spacing). Narrower filtering results in dramatically larger ISI penalties. Therefore, a trade-off between ISI and ICI exists, which requires optimal design of the optical filter bandwidth for a given channel spacing.

^{−2}and 10

^{−3}) vs. optical filter bandwidth under different channel-spacing conditions, Fig. 10.

^{−3}. The LMS ICI equalizer yields ~18dB OSNR requirement for any filter bandwidth. Thus, joint LMS ICI cancellation methods produce better absolute performance, and relax the optical filter bandwidth requirement.

^{−3}. After 12 spans (960km) of SSMF transmission, the single channel performance shows only a small penalty compared to the BTB case. On the other hand, the 12-span multi-channel conventional DSP case reveals a penalty >5dB with respect to the single-channel BTB case, and ~1dB penalty compared to the multi-channel BTB case. With the ICI equalizer applied, the penalty is reduced by ~1.5dB, which demonstrates the effectiveness of the joint DSP in both linear and nonlinear transmission regimes.

### 5.2 Joint carrier-phase recovery algorithm results (simulation)

## 6. Conclusions

## Acknowledgments

## References and links

1. | E. Torrengo, R. Cigliutti, G. Bosco, G. Gavioli, A. Alaimo, A. Carena, V. Curri, F. Forghieri, S. Piciaccia, M. Belmonte, A. Brinciotti, A. La Porta, S. Abrate, and P. Poggiolini, “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” ECOC 2010, We.7.C.2. |

2. | Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and N. S. Bergano, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” ECOC 2010, We.7.C.4. |

3. | X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “Transmission of a 448-Gb/s reduced-guard-interval CO-OFDM signal with a 60-GHz optical bandwidth over 2000 km of ULAF and five 80-GHz-grid ROADMs,” OFC 2010, PDPC2. |

4. | S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express |

5. | A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-Guard-Interval coherent optical OFDM for 100-Gb/s long-haul WDM Transmission,” J. Lightwave Technol. |

6. | Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and N. S. Bergano, “Achieving high spectral efficiency in long-haul transmission with pre-filtering and multi-symbol detection,” ACP 2010, page 349–350. |

7. | R. Schmogrow, M. Winter, M. Meyer, D. Hillerkuss, S. Wolf, B. Baeuerle, A. Ludwig, B. Nebendahl, S. Ben-Ezra, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM,” Opt. Express |

8. | R. Ryf, S. Randel, A. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. Burrows, R. J. Essiambre, P. J. Winzer, D. Peckham, A. McCurdy, and R. Lingle Jr., “Mode-division multiplexing over 96km of few-mode fiber using coherent 6 x 6 MIMO processing,” J. Lightwave Technol. |

9. | Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access,” OFC 2009, Paper PDPC1. |

10. | R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/s/Hz over 400 km of SSMF,” OFC 2009, Paper PDPC2. |

11. | G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and CO-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett. |

12. | S. Chandrasekhar and X. Liu, “Terabit superchannels for high spectral efficiency transmission,” ECOC 2010, Tu.3.C.5. |

13. | D. Hillerkuss, M. Winter, M. Teschke, A. Marculescu, J. Li, G. Sigurdsson, K. Worms, W. Freude, and J. Leuthold, “Low-complexity optical FFT scheme enabling Tbit/s all-optical OFDM communication,” ITG Symposium on Photonic Networks, 2010Paper 5. |

14. | K. Takiguchi, M. Oguma, T. Shibata, and H. Takahashi, “Optical OFDM demultiplexer using silica PLC based optical FFT circuit,” OFC 2009, OWO3. |

15. | G. Gavioli, E. Torrengo, G. Bosco, A. Carena, V. Curri, V. Miot, P. Poggiolini, M. Belmonte, F.Forghieri, C. Muzio, S. Piciaccia, A. Brinciotti, A. La Porta, C. Lezzi, S. Savory, and S. Abrate, “Investigation of the impact of ultra-narrow carrier spacing on the transmission of a 10-Carrier 1Tb/s superchannel,” OFC 2010, OThD3. |

16. | A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory |

17. | Z. Dong, J. Yu, H.-C. Chien, N. Chi, L. Chen, and G.-K. Chang, “Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection,” Opt. Express |

18. | H.-C. Chien, J. Yu, Z. Dong, Z. Jia, “Offset RZ pulse shape for 128 Gb/s PDM-QPSK subject to aggressive filtering,” OFC 2012, Paper OM3H.3. |

19. | J. Pan, C. Liu, T. Detwiler, A. Stark, Y.-T. Hsueh, and S. E. Ralph, “Inter-channel crosstalk cancellation for Nyquist-WDM superchannel applications,” J. Lightwave Technol. |

20. | C. Liu, J. Pan, T. Detwiler, A. Stark, Y.-T. Hsueh, G.-K. Chang, and S. E. Ralph, “Joint digital signal processing for superchannel coherent optical system: joint CD compensation for joint ICI cancellation,” ECOC 2012, Paper Th.1.A.4. |

**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: November 5, 2012

Revised Manuscript: January 23, 2013

Manuscript Accepted: February 11, 2013

Published: March 29, 2013

**Citation**

Cheng Liu, Jie Pan, Thomas Detwiler, Andrew Stark, Yu-Ting Hsueh, Gee-Kung Chang, and Stephen E. Ralph, "Joint digital signal processing for superchannel coherent optical communication systems," Opt. Express **21**, 8342-8356 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-7-8342

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### References

- E. Torrengo, R. Cigliutti, G. Bosco, G. Gavioli, A. Alaimo, A. Carena, V. Curri, F. Forghieri, S. Piciaccia, M. Belmonte, A. Brinciotti, A. La Porta, S. Abrate, and P. Poggiolini, “Transoceanic PM-QPSK terabit superchannel transmission experiments at baud-rate subcarrier spacing,” ECOC 2010, We.7.C.2.
- Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and N. S. Bergano, “High spectral efficiency long-haul transmission with pre-filtering and maximum a posteriori probability detection,” ECOC 2010, We.7.C.4.
- X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “Transmission of a 448-Gb/s reduced-guard-interval CO-OFDM signal with a 60-GHz optical bandwidth over 2000 km of ULAF and five 80-GHz-grid ROADMs,” OFC 2010, PDPC2.
- S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express17(24), 21350–21361 (2009). [CrossRef] [PubMed]
- A. Sano, E. Yamada, H. Masuda, E. Yamazaki, T. Kobayashi, E. Yoshida, Y. Miyamoto, R. Kudo, K. Ishihara, and Y. Takatori, “No-Guard-Interval coherent optical OFDM for 100-Gb/s long-haul WDM Transmission,” J. Lightwave Technol.27(16), 3705–3713 (2009). [CrossRef]
- Y. Cai, J. X. Cai, C. R. Davidson, D. Foursa, A. Lucero, O. Sinkin, A. Pilipetskii, G. Mohs, and N. S. Bergano, “Achieving high spectral efficiency in long-haul transmission with pre-filtering and multi-symbol detection,” ACP 2010, page 349–350.
- R. Schmogrow, M. Winter, M. Meyer, D. Hillerkuss, S. Wolf, B. Baeuerle, A. Ludwig, B. Nebendahl, S. Ben-Ezra, J. Meyer, M. Dreschmann, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “Real-time Nyquist pulse generation beyond 100 Gbit/s and its relation to OFDM,” Opt. Express20(1), 317–337 (2012). [CrossRef] [PubMed]
- R. Ryf, S. Randel, A. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. Burrows, R. J. Essiambre, P. J. Winzer, D. Peckham, A. McCurdy, and R. Lingle., “Mode-division multiplexing over 96km of few-mode fiber using coherent 6 x 6 MIMO processing,” J. Lightwave Technol.30(4), 521–531 (2012). [CrossRef]
- Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s per channel coherent optical OFDM transmission with subwavelength bandwidth access,” OFC 2009, Paper PDPC1.
- R. Dischler and F. Buchali, “Transmission of 1.2 Tb/s continuous waveband PDM-OFDM-FDM signal with spectral efficiency of 3.3 bit/s/Hz over 400 km of SSMF,” OFC 2009, Paper PDPC2.
- G. Bosco, A. Carena, V. Curri, P. Poggiolini, and F. Forghieri, “Performance limits of Nyquist-WDM and CO-OFDM in high-speed PM-QPSK systems,” IEEE Photon. Technol. Lett.22(15), 1129–1131 (2010).
- S. Chandrasekhar and X. Liu, “Terabit superchannels for high spectral efficiency transmission,” ECOC 2010, Tu.3.C.5.
- D. Hillerkuss, M. Winter, M. Teschke, A. Marculescu, J. Li, G. Sigurdsson, K. Worms, W. Freude, and J. Leuthold, “Low-complexity optical FFT scheme enabling Tbit/s all-optical OFDM communication,” ITG Symposium on Photonic Networks, 2010.
- K. Takiguchi, M. Oguma, T. Shibata, and H. Takahashi, “Optical OFDM demultiplexer using silica PLC based optical FFT circuit,” OFC 2009, OWO3.
- G. Gavioli, E. Torrengo, G. Bosco, A. Carena, V. Curri, V. Miot, P. Poggiolini, M. Belmonte, F.Forghieri, C. Muzio, S. Piciaccia, A. Brinciotti, A. La Porta, C. Lezzi, S. Savory, and S. Abrate, “Investigation of the impact of ultra-narrow carrier spacing on the transmission of a 10-Carrier 1Tb/s superchannel,” OFC 2010, OThD3.
- A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory29(4), 543–551 (1983). [CrossRef]
- Z. Dong, J. Yu, H.-C. Chien, N. Chi, L. Chen, and G.-K. Chang, “Ultra-dense WDM-PON delivering carrier-centralized Nyquist-WDM uplink with digital coherent detection,” Opt. Express19(12), 11100–11105 (2011). [CrossRef] [PubMed]
- H.-C. Chien, J. Yu, Z. Dong, Z. Jia, “Offset RZ pulse shape for 128 Gb/s PDM-QPSK subject to aggressive filtering,” OFC 2012, Paper OM3H.3.
- J. Pan, C. Liu, T. Detwiler, A. Stark, Y.-T. Hsueh, and S. E. Ralph, “Inter-channel crosstalk cancellation for Nyquist-WDM superchannel applications,” J. Lightwave Technol.30(24), 3993–3999 (2012). [CrossRef]
- C. Liu, J. Pan, T. Detwiler, A. Stark, Y.-T. Hsueh, G.-K. Chang, and S. E. Ralph, “Joint digital signal processing for superchannel coherent optical system: joint CD compensation for joint ICI cancellation,” ECOC 2012, Paper Th.1.A.4.

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