## No-guard-interval coherent optical OFDM with self-tuning receiver |

Optics Express, Vol. 19, Issue 3, pp. 2181-2186 (2011)

http://dx.doi.org/10.1364/OE.19.002181

Acrobat PDF (1309 KB)

### Abstract

We propose and experimentally demonstrate a no-guard-interval (No-GI) coherent optical orthogonal frequency division multiplexed (CO-OFDM) system that uses Fractionally-Spaced Time-Domain Equalizers (FS-TDE) to simultaneously demultiplex and equalize each subcarrier. Least-mean-squares algorithms (LMS) tune each FS-TDE to a subcarrier. A short unique training sequence is transmitted on each subcarrier, allowing each FS-TDE to lock onto its subcarrier. After the initial training, the adaptive blind equalizers remain tuned to their respective subcarriers. Unlike previous systems, this system does not require digital filtering or mixing of each subcarrier to baseband, so is more computationally efficient. Error-free transmission was measured over 800 km of fiber with a three-subcarrier 30 Gb/s system and a five-subcarrier 33.33 Gb/s system. The required OSNRs for a BER of 10^{−3} were 8.6 dB and 9.3 dB respectively, which are within 1.5 dB of the theoretical limit for coherent systems.

© 2011 OSA

## 1. Introduction

1. K. Roberts, M. O'Sullivan, K.-T. Wu, H. Sun, A. Awadalla, D. J. Krause, and C. Laperle, “Performance of dual-polarization QPSK for optical transport systems,” J. Lightwave Technol. **27**(16), 3546–3559 (2009). [CrossRef]

2. A. D. Ellis, Z. Jian, and D. Cotter, “Approaching the non-linear Shannon limit,” J. Lightwave Technol. **28**(4), 423–433 (2010). [CrossRef]

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]

6. B. Zhu, X. Liu, S. Chandrasekhar, D. W. Peckham, and R. Lingle, “Ultra-long-haul transmission of 1.2-Tb/s multicarrier no-guard-interval CO-OFDM superchannel using ultra-large-area fiber,” IEEE Photon. Technol. Lett. **22**(11), 826–828 (2010). [CrossRef]

6. B. Zhu, X. Liu, S. Chandrasekhar, D. W. Peckham, and R. Lingle, “Ultra-long-haul transmission of 1.2-Tb/s multicarrier no-guard-interval CO-OFDM superchannel using ultra-large-area fiber,” IEEE Photon. Technol. Lett. **22**(11), 826–828 (2010). [CrossRef]

7. D. Hillerkuss, A. Marculescu, J. Li, M. Teschke, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Novel optical fast Fourier transform scheme enabling real-time OFDM processing at 392 Gbit/s and beyond,” in *Optical Fiber Communication Conference* (OSA, San Diego, California, 2010), p. OWW3.

8. A. J. Lowery, “Design of Arrayed-Waveguide Grating Routers for use as optical OFDM demultiplexers,” Opt. Express **18**(13), 14129–14143 (2010). [CrossRef] [PubMed]

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]

7. D. Hillerkuss, A. Marculescu, J. Li, M. Teschke, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Novel optical fast Fourier transform scheme enabling real-time OFDM processing at 392 Gbit/s and beyond,” in *Optical Fiber Communication Conference* (OSA, San Diego, California, 2010), p. OWW3.

8. A. J. Lowery, “Design of Arrayed-Waveguide Grating Routers for use as optical OFDM demultiplexers,” Opt. Express **18**(13), 14129–14143 (2010). [CrossRef] [PubMed]

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]

^{−3}, the three- and five-subcarrier systems require OSNRs of 8.6 dB and 9.3 dB (0.1 nm) respectively.

## 2. Equalizer description

*et al.*[5

**27**(16), 3705–3713 (2009). [CrossRef]

*N*-taps and decreases the sampling rate by

*S*, where

*S*is the number of samples per symbol [11]. Because mixers, DFTs and FIR filters are all linear, it is possible to combine all three operations as shown in Fig. 1b. The FIR filters can be tuned to give any response with a maximum impulse response of

*N*, where

_{taps}*N*is the number of taps used in the filter. A DFT of size

_{taps}*N*has an impulse response length

_{DFT}*N*. Therefore, for cases where

_{DFT}*N*>

_{taps}*N*, it is possible to incorporate the response of the DFT into an FIR filter. A mixer can also be incorporated into the FS-TDE by multiplying all the taps in the FIR filter by the mixing frequency, which transforms a low-pass filter into a band-pass filter. The down-sampling in the FS-TDE will then cause subcarriers not centered on DC to be aliased back to baseband. Therefore, no additional computations are required in the FS-TDE for systems where the number of equalizer taps is greater than the number of subcarriers; this has been the case for all previously demonstrated No-GI CO-OFDM systems with digital PMD compensation [5

_{DFT}**27**(16), 3705–3713 (2009). [CrossRef]

12. R. Johnson Jr, P. Schniter, T. J. Endres, J. D. Behm, D. R. Brown, and R. A. Casas, “Blind equalization using the constant modulus criterion: a review,” Proc. IEEE **86**(10), 1927–1950 (1998). [CrossRef]

12. R. Johnson Jr, P. Schniter, T. J. Endres, J. D. Behm, D. R. Brown, and R. A. Casas, “Blind equalization using the constant modulus criterion: a review,” Proc. IEEE **86**(10), 1927–1950 (1998). [CrossRef]

8. A. J. Lowery, “Design of Arrayed-Waveguide Grating Routers for use as optical OFDM demultiplexers,” Opt. Express **18**(13), 14129–14143 (2010). [CrossRef] [PubMed]

## 3. Long-haul experimental setup

^{®}14-GHz microwave amplifiers were used to drive a Sumitomo 40-Gbps C-MZM. The optical source was an Agilent External Cavity Laser (ECL) with a linewidth of ~100 kHz. The modulated optical signal was split with a polarization beam splitter (PBS) with its input polarization aligned so that the power was split evenly between the two outputs. A one-meter long polarization maintaining fiber patch lead was used as a delay line to decorrelate the two signals before they were recombined with another PBS to generate a polarization multiplexed signal. DACs were used to generate the subcarriers because of the cost of five C-MZMs. We expect the receiver to work for subcarriers generated from separate optical modulators at higher bit-rates as long as they are orthogonal.

^{®}using the equalizer described in Section 2. Firstly, the digital signal was down sampled to 10 Gsample/s to match the sampling rate of the transmitter’s DACs. The bulk of the CD was removed from all subcarriers with a frequency domain equalizer using the overlap-add algorithm [5

**27**(16), 3705–3713 (2009). [CrossRef]

12. R. Johnson Jr, P. Schniter, T. J. Endres, J. D. Behm, D. R. Brown, and R. A. Casas, “Blind equalization using the constant modulus criterion: a review,” Proc. IEEE **86**(10), 1927–1950 (1998). [CrossRef]

13. 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]

## 4. Experimental results

*Q*, shown for each subcarrier in Fig. 5, is calculated from the spread in the equalized constellations [14

_{const}14. A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” J. Lightwave Technol. **25**(1), 131–138 (2007). [CrossRef]

*Q*. Degradation from linear crosstalk would be most significant on the middle subcarrier because it has the maximum number of neighbors. This suggests linear crosstalk is not a significant source of degradation and the proposed equalizer is effective in subcarrier demultiplexing after 800 km of transmission. The degradation of the outer channels could be due to the truncation of their sinc responses, due to the DAC’s response, the subsequent image rejecting electrical LPFs and other bandwidth limitations, as studied in [8

_{const}**18**(13), 14129–14143 (2010). [CrossRef] [PubMed]

*Q*is plotted against the OSNR in Fig. 6 for both systems after 80 km (

_{BER}**◊**) and 800 km (◯).An 80-km system was used instead of an optical back-to-back system because the 80-km link was required to decorrelate the phases of the transmitter laser and LO. The received

*Q*was calculated from the average BER count of all the subcarriers assuming a Gaussian distribution. The FEC limit is shown at a

_{BER}*Q*of 9.8 dB which corresponds to a BER of 10

^{−3}. The ASE-limited

*Q*(green dashed lines) are calculated from the OSNR assuming that the only source of degradation is additive white Gaussian noise, generated by Amplified Spontaneous Emission (ASE). Note that the ASE-limited

*Q*is 0.5 dB lower for the five-subcarrier system because of the higher bit rate.

*Q*after the 800-km link, compared to the 80-km link, for both systems was very small, indicating that the fiber impairments are almost fully compensated by the digital equalizer. For the three-subcarrier 30 Gb/s system, the required OSNR for a BER of 10

_{BER}^{−3}was 8.5 dB after 80 km and 8.6 dB after 800 km, around 1-dB higher than the ASE-limited

*Q*of 7.6 dB. For the five subcarrier 33.33 GB/s system, OSNRs of 9.5 dB and 9.3 dB were required after 80 km and 800 km respectively, less than 1.5-dB above the ASE-limited

*Q*of 8.1 dB. This suggests that the OSNR penalty from linear crosstalk was negligible after 800 km for a BER of 10

^{−3}.

## 5. Conclusion

^{−3}was 8.6 dB for the three-subcarrier system and 9.3 dB for the five-subcarrier system. This shows that the proposed demultiplexing and equalization method is suitable for long-haul optical systems with multiple orthogonal subcarriers.

## Acknowledgements

## References and links

1. | K. Roberts, M. O'Sullivan, K.-T. Wu, H. Sun, A. Awadalla, D. J. Krause, and C. Laperle, “Performance of dual-polarization QPSK for optical transport systems,” J. Lightwave Technol. |

2. | A. D. Ellis, Z. Jian, and D. Cotter, “Approaching the non-linear Shannon limit,” J. Lightwave Technol. |

3. | B. Goebel, S. Hellerbrand, and N. Hanik, “Link-aware precoding for nonlinear optical OFDM transmission,” in |

4. | Y. Tang, W. Shieh, and B. S. Krongold, “Fiber nonlinearity mitigation in 428-Gb/s multiband coherent optical OFDM systems,” in |

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. | B. Zhu, X. Liu, S. Chandrasekhar, D. W. Peckham, and R. Lingle, “Ultra-long-haul transmission of 1.2-Tb/s multicarrier no-guard-interval CO-OFDM superchannel using ultra-large-area fiber,” IEEE Photon. Technol. Lett. |

7. | D. Hillerkuss, A. Marculescu, J. Li, M. Teschke, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Novel optical fast Fourier transform scheme enabling real-time OFDM processing at 392 Gbit/s and beyond,” in |

8. | A. J. Lowery, “Design of Arrayed-Waveguide Grating Routers for use as optical OFDM demultiplexers,” Opt. Express |

9. | X. Liu, S. Chandrasekhar, B. Zhu, and D. W. Peckham, “Efficient digital coherent detection of a 1.2-Tb/s 24-carrier no-guard-interval CO-OFDM signal by simultaneously detecting multiple carriers per sampling,” in |

10. | K. Takiguchi, T. Kitoh, A. Mori, M. Oguma, and H. Takahashi, “Integrated-optic OFDM demultiplexer using slab star coupler based optical DFT circuit,” in |

11. | J. G. Proakis, |

12. | R. Johnson Jr, P. Schniter, T. J. Endres, J. D. Behm, D. R. Brown, and R. A. Casas, “Blind equalization using the constant modulus criterion: a review,” Proc. IEEE |

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

14. | A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” J. Lightwave Technol. |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

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

(060.4080) Fiber optics and optical communications : Modulation

(060.4230) Fiber optics and optical communications : Multiplexing

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: November 16, 2010

Revised Manuscript: January 12, 2011

Manuscript Accepted: January 12, 2011

Published: January 20, 2011

**Citation**

Liang B. Du and Arthur J. Lowery, "No-guard-interval coherent optical OFDM with self-tuning receiver," Opt. Express **19**, 2181-2186 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-3-2181

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### References

- K. Roberts, M. O'Sullivan, K.-T. Wu, H. Sun, A. Awadalla, D. J. Krause, and C. Laperle, “Performance of dual-polarization QPSK for optical transport systems,” J. Lightwave Technol. 27(16), 3546–3559 (2009). [CrossRef]
- A. D. Ellis, Z. Jian, and D. Cotter, “Approaching the non-linear Shannon limit,” J. Lightwave Technol. 28(4), 423–433 (2010). [CrossRef]
- B. Goebel, S. Hellerbrand, and N. Hanik, “Link-aware precoding for nonlinear optical OFDM transmission,” in Optical Fiber Communication Conference (OSA, San Diego, California, 2010), p. OTuE4.
- Y. Tang, W. Shieh, and B. S. Krongold, “Fiber nonlinearity mitigation in 428-Gb/s multiband coherent optical OFDM systems,” in Optical Fiber Communication Conference (OSA, San Diego, California, 2010), p. JThA6.
- 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]
- B. Zhu, X. Liu, S. Chandrasekhar, D. W. Peckham, and R. Lingle, “Ultra-long-haul transmission of 1.2-Tb/s multicarrier no-guard-interval CO-OFDM superchannel using ultra-large-area fiber,” IEEE Photon. Technol. Lett. 22(11), 826–828 (2010). [CrossRef]
- D. Hillerkuss, A. Marculescu, J. Li, M. Teschke, G. Sigurdsson, K. Worms, S. Ben Ezra, N. Narkiss, W. Freude, and J. Leuthold, “Novel optical fast Fourier transform scheme enabling real-time OFDM processing at 392 Gbit/s and beyond,” in Optical Fiber Communication Conference (OSA, San Diego, California, 2010), p. OWW3.
- A. J. Lowery, “Design of Arrayed-Waveguide Grating Routers for use as optical OFDM demultiplexers,” Opt. Express 18(13), 14129–14143 (2010). [CrossRef] [PubMed]
- X. Liu, S. Chandrasekhar, B. Zhu, and D. W. Peckham, “Efficient digital coherent detection of a 1.2-Tb/s 24-carrier no-guard-interval CO-OFDM signal by simultaneously detecting multiple carriers per sampling,” in Optical Fiber Communication Conference (OSA, San Diego, California, 2010), p. OWO2.
- K. Takiguchi, T. Kitoh, A. Mori, M. Oguma, and H. Takahashi, “Integrated-optic OFDM demultiplexer using slab star coupler based optical DFT circuit,” in European Conference on Optical Communication (2010), p. PD1.4.
- J. G. Proakis, Digital Communications (McGraw Hill, 2001).
- R. Johnson, P. Schniter, T. J. Endres, J. D. Behm, D. R. Brown, and R. A. Casas, “Blind equalization using the constant modulus criterion: a review,” Proc. IEEE 86(10), 1927–1950 (1998). [CrossRef]
- 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]
- A. J. Lowery, L. B. Du, and J. Armstrong, “Performance of optical OFDM in ultralong-haul WDM lightwave systems,” J. Lightwave Technol. 25(1), 131–138 (2007). [CrossRef]

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