## 512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver |

Optics Express, Vol. 20, Issue 21, pp. 23383-23389 (2012)

http://dx.doi.org/10.1364/OE.20.023383

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### Abstract

We demonstrate a marked performance improvement in a 512 QAM transmission by employing frequency-domain equalization (FDE) instead of an FIR filter. FDE enables us to compensate for distortions due to hardware imperfections in the transmitter with higher precision, which successfully reduced the power penalty by 4 dB in a 54 Gbit/s (3 Gsymbol/s)-160 km transmission. FDE also allows the transmission distance to be extended up to 240 km.

© 2012 OSA

## 1. Introduction

2. E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. **25**(8), 2033–2043 (2007). [CrossRef]

3. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express **16**(2), 804–817 (2008). [CrossRef] [PubMed]

4. M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett. **22**(3), 185–187 (2010). [CrossRef]

6. R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express **20**(6), 6439–6447 (2012). [CrossRef] [PubMed]

7. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag. **40**(4), 58–66 (2002). [CrossRef]

9. A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7.

## 2. Capability of FDE for higher-order QAM

*n*

_{FDE}, is estimated as follows. It is known that an FFT involves

*n*

_{FFT}= 4

*N*

_{FFT}log

_{2}(

*N*

_{FFT}) real-valued multiplications [10]. Since FDE employs both FFT and IFFT, it includes 2

*n*

_{FFT}multiplications. Furthermore, since one symbol is represented by two samples, an FFT with a size of

*N*

_{FFT}accounts for

*N*

_{FFT}/2 symbols. Therefore, the number of multiplications per symbol is estimated as

*n*

_{FDE}= 2

*n*

_{FFT}/ (

*N*

_{FFT}/2) = 8log

_{2}(

*N*

_{FFT}), i.e., it increases only logarithmically. The relationship between

*n*

_{FDE}and

*N*

_{FFT}is shown in Fig. 2(b). Figure 2(c) shows a comparison of

*n*

_{FIR}and

*n*

_{FDE}as a function of Δ

*f*. This clearly shows the advantage of FDE in terms of the lower computation complexity especially for Δ

*f*values as low as 1 MHz and below, whereas with FIR such a low Δ

*f*is very difficult to realize due to the rapid increase in the computation complexity. For example, if we set Δ

*f*= 1 MHz, FDE requires

*N*

_{FFT}= 8192 and thus

*n*

_{FDE}= 104. On the other hand, the required number of taps with FIR is

*N*

_{FIR}= 4000, which corresponds to

*n*

_{FIR}= 16000.

## 3. Experimental setup

11. C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett. **21**(7), 459–461 (1996). [CrossRef] [PubMed]

_{3}(LN) IQ modulator in the present experiment. Specifically, surface acoustic waves generated by the piezoelectric effect in the LN crystal degraded the low-frequency response of the modulator [12

12. R. L. Jungerman and C. A. Flory, “Low-frequency acoustic anomalies in lithium niobate Mach-Zehnder interferometers,” Appl. Phys. Lett. **53**(16), 1477–1479 (1988). [CrossRef]

## 4. Experimental result

## 5. Conclusion

## References and links

1. | M. Nakazawa, K. Kikuchi, and T. Miyazaki, |

2. | E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. |

3. | S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express |

4. | M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett. |

5. | S. Okamoto, K. Toyoda, T. Omiya, K. Kasai, M. Yoshida, and M. Nakazawa, “512 QAM (54 Gbit/s) coherent optical transmission over 150 km with an optical bandwidth of 4.1 GHz,” ECOC’10, PD2.3. |

6. | R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express |

7. | D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag. |

8. | K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, E. Yamada, H. Masuda, and Y. Miyamoto, “Coherent optical transmission with frequency-domain equalization,” ECOC’08, We.2.E.3. |

9. | A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7. |

10. | J. C. Geyer, C. R. S. Fluger, T. Duthel, C. Schulien, and B. Schumauss, “Efficient frequency domain chromatic dispersion compensation in a coherent polmux QPSK-receiver,” OFC’10, OWV5. |

11. | C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett. |

12. | R. L. Jungerman and C. A. Flory, “Low-frequency acoustic anomalies in lithium niobate Mach-Zehnder interferometers,” Appl. Phys. Lett. |

**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: June 29, 2012

Revised Manuscript: September 4, 2012

Manuscript Accepted: September 9, 2012

Published: September 26, 2012

**Citation**

Yuki Koizumi, Kazushi Toyoda, Tatsunori Omiya, Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa, "512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver," Opt. Express **20**, 23383-23389 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-21-23383

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### References

- M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technologies (Springer 2010).
- E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol.25(8), 2033–2043 (2007). [CrossRef]
- S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express16(2), 804–817 (2008). [CrossRef] [PubMed]
- M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett.22(3), 185–187 (2010). [CrossRef]
- S. Okamoto, K. Toyoda, T. Omiya, K. Kasai, M. Yoshida, and M. Nakazawa, “512 QAM (54 Gbit/s) coherent optical transmission over 150 km with an optical bandwidth of 4.1 GHz,” ECOC’10, PD2.3.
- R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express20(6), 6439–6447 (2012). [CrossRef] [PubMed]
- D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag.40(4), 58–66 (2002). [CrossRef]
- K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, E. Yamada, H. Masuda, and Y. Miyamoto, “Coherent optical transmission with frequency-domain equalization,” ECOC’08, We.2.E.3.
- A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7.
- J. C. Geyer, C. R. S. Fluger, T. Duthel, C. Schulien, and B. Schumauss, “Efficient frequency domain chromatic dispersion compensation in a coherent polmux QPSK-receiver,” OFC’10, OWV5.
- C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett.21(7), 459–461 (1996). [CrossRef] [PubMed]
- R. L. Jungerman and C. A. Flory, “Low-frequency acoustic anomalies in lithium niobate Mach-Zehnder interferometers,” Appl. Phys. Lett.53(16), 1477–1479 (1988). [CrossRef]

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