## Experimental demonstration of improved fiber nonlinearity tolerance for unique-word DFT-spread OFDM systems |

Optics Express, Vol. 19, Issue 27, pp. 26198-26207 (2011)

http://dx.doi.org/10.1364/OE.19.026198

Acrobat PDF (1238 KB)

### Abstract

In this paper we experimentally demonstrate transmission performance of optical DFT-spread OFDM systems in comparison with conventional OFDM systems. A 440.8-Gb/s superchannel consisting of 8 x 55.1-Gb/s densely-spaced DFT-S OFDM signal is successfully received after 1120-km transmission with a spectral efficiency of 3.5 b/s/Hz. It is shown that DFT-S OFDM can achieve an improvement of 1 dB in Q factor and 1 dB in launch power over conventional OFDM. Additionally, unique word aided phase estimation algorithm is proposed and demonstrated enabling extremely long OFDM symbol transmission.

© 2011 OSA

## 1. Introduction

1. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express **17**(11), 9421–9427 (2009). [CrossRef] [PubMed]

5. S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “121.9-Gb/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1000 km of SSMF,” J. Lightwave Technol. **27**(3), 177–188 (2009). [CrossRef]

1. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express **17**(11), 9421–9427 (2009). [CrossRef] [PubMed]

5. S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “121.9-Gb/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1000 km of SSMF,” J. Lightwave Technol. **27**(3), 177–188 (2009). [CrossRef]

6. M. Nazarathy, J. Khurgin, R. Weidenfeld, Y. Meiman, P. Cho, R. Noe, I. Shpantzer, and V. Karagodsky, “Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links,” Opt. Express **16**(20), 15777–15810 (2008). [CrossRef] [PubMed]

7. 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]

7. 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]

9. W. Shieh, “OFDM for Flexible High-Speed Optical Networks,” J. Lightwave Technol. **29**(10), 1560–1577 (2011). [CrossRef]

## 2. Principle of unique-word (UW) DFT-S OFDM

10. D. Falconer, S. 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]

_{1}and GI

_{2}are 16-point GIs for UW

_{1}and UW

_{2,}respectively.), which results in an 80-point UW pattern at each side of OFDM symbol. An optional 8-point GI (GI

_{sym}) is appended at the start of the OFDM symbol, which is a copy of the last 8-point of UW

_{2}. The purpose of this optional 8-point is to keep data symbol length the same as training symbols. This GI is needed for training symbols so that no extra interpolation required when calculating channel matrix. But it is only optional for data symbols and thus can be dropped for data symbols without affecting any performance. The total length of one DFT-S OFDM symbol is 2056 points.

16. W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. **20**(8), 605–607 (2008). [CrossRef]

_{〈〉}represents ensemble average,

_{ϕi}represents the carrier phase estimated for the

*i*-th segment,

*are respectively received and transmitted*

_{Aij0}*j*-th point in the

*i*-th block of a OFDM symbol,

*M*is the OFDM symbol length, and

*K*is the block length for phase estimation.

*and*

_{Aij}_{Aij′}are the

*j*-th point in the

*i*-th block of a OFDM symbol before and after phase compensation, respectively. After constellation de-rotation, the symbol decision will be made to the phase compensated symbols

_{Aij′}. Then another iteration of phase estimation and compensation will be made by applying Eqs. (1) and (2) (for arbitrary

*i*), which will be subsequently passed to the following block. The propagation will continue until the end of payload blocks. It is noted that the phase estimation can also be performed using UW

_{2}and propagate the phase estimation backward. In this paper, we use

*K*of 16 if not otherwise mentioned.

*H*matrix) by using short training symbols. This coarse channel matrix will be interpolated and then expanded to the same length as long symbols. The next step is to perform frequency-domain equalization to the long symbols by using coarse

*H*matrix. The equalized long symbols are further transformed back to time domain to estimate intra-symbol phase noise. This estimated phase noise is then used to apply phase compensation on the original long training symbols. After the phase noise compensation, the conventional maximum-likelihood channel estimation can be performed to obtain updated

*H*matrix [16

16. W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. **20**(8), 605–607 (2008). [CrossRef]

*H*matrix can be obtained. In this paper, no iteration is used since it is found a single round of channel estimation is sufficient. After the frequency-domain equalization, M-point IDFT is applied to the equalized signal to rewind the DFT spreading at the transmitter. The UWs are employed to seed the DF aided phase estimation and compensation as described previously. Finally, symbol decision is made to the phase compensated OFDM symbols.

7. 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]

9. W. Shieh, “OFDM for Flexible High-Speed Optical Networks,” J. Lightwave Technol. **29**(10), 1560–1577 (2011). [CrossRef]

**22**(16), 1250–1252 (2010). [CrossRef]

9. W. Shieh, “OFDM for Flexible High-Speed Optical Networks,” J. Lightwave Technol. **29**(10), 1560–1577 (2011). [CrossRef]

**22**(16), 1250–1252 (2010). [CrossRef]

**29**(10), 1560–1577 (2011). [CrossRef]

## 3. Experimental results and nonlinear performance analysis

^{−3}is about 6-dB, 11-dB, and 21-dB for 18.4-Gb/s, 55.1-Gb/s, and 440.8-Gb/s system respectively.

*k*of 64. Since the conventional OFDM is also using phase estimation window of 64 points, the phase noise has the same impact on both DFT-S and conventional OFDM. The Q factor difference between DFT-S and conventional OFDM is shown in Fig. 8 (b) when both have the same phase estimation window size of 64. As can be seen in the figure, when the launch power is higher than 1 dBm, with the increasing power level, the advantage of DFT-S OFDM increases, e.g., 2.8 dB at the launch power of 8 dBm. Because DFT-S and conventional OFDM shall have the same linear phase noise impact due to using the same phase estimation window size, the Q factor difference between these two formats in Fig. 8 (b) is clearly caused by nonlinearity tolerance.

1. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express **17**(11), 9421–9427 (2009). [CrossRef] [PubMed]

## 4. Conclusion

## Acknowledgment

## References and links

1. | Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express |

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

3. | 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,” |

4. | X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guardinterval CO-OFDM transmission over 2000 km of ultra-large-area fiber and five 80-GHz-grid ROADMs,” J. Lightwave Technol. |

5. | S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “121.9-Gb/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1000 km of SSMF,” J. Lightwave Technol. |

6. | M. Nazarathy, J. Khurgin, R. Weidenfeld, Y. Meiman, P. Cho, R. Noe, I. Shpantzer, and V. Karagodsky, “Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links,” Opt. Express |

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

8. | H. G. Myung, J. Lim, and D. J. Goodman, “Peak-to-average power ratio of single carrier FDMA signals with pulse shaping,” In |

9. | W. Shieh, “OFDM for Flexible High-Speed Optical Networks,” J. Lightwave Technol. |

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

11. | M. Huemer, H. Witschnig, and J. Hausner, “Unique Word Based Phase Tracking Algorithms for SC/FDE Systems”, In |

12. | L. Deneire, B. Gyselinckx, and M. Engels, “Training Sequence vs. Cyclic Prefix: A New Look on Single Carrier Communication,” In |

13. | D. Ly-Gagnon, K. Katoh, and K. Kikuchi, “Unrepeated 210-km transmission with coherent detection and digital signal processing of 20-Gb/s QPSK signal,” Tech. Dig. OFC’05, Anaheim, CA, USA, 2005, p. OTuL4. |

14. | M. E. Mousa-Pasandi and D. V. Plant, “Zero-overhead phase noise compensation via decision-directed phase equalizer for coherent optical OFDM,” Opt. Express |

15. | S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser linewidth tolerance of decision-aided maximum likelihood phase estimation in coherent optical |

16. | W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. |

17. | D. C. Chu, “Polyphase Codes with Good Periodic Correlation Properties,” IEEE Trans. Inf. Theory |

18. | 3GPP TR 25.814 V7.0.0, “Technical Specification Group Radio Access Network; Physical Layer Aspects for Evolved UTRA”, (2006). |

**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: August 25, 2011

Revised Manuscript: October 16, 2011

Manuscript Accepted: October 29, 2011

Published: December 8, 2011

**Citation**

Xi Chen, An Li, Guanjun Gao, and William Shieh, "Experimental demonstration of improved fiber nonlinearity tolerance for unique-word DFT-spread OFDM systems," Opt. Express **19**, 26198-26207 (2011)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-27-26198

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### References

- Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission over 600-km SSMF fiber with subwavelength bandwidth access,” Opt. Express17(11), 9421–9427 (2009). [CrossRef] [PubMed]
- 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]
- 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,” Optical Fiber Communication Conference (OFC), San Diego, USA, 2009, p. PDPC2.
- X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guardinterval CO-OFDM transmission over 2000 km of ultra-large-area fiber and five 80-GHz-grid ROADMs,” J. Lightwave Technol.29(4), 483–490 (2011). [CrossRef]
- S. L. Jansen, I. Morita, T. C. W. Schenk, and H. Tanaka, “121.9-Gb/s PDM-OFDM transmission with 2-b/s/Hz spectral efficiency over 1000 km of SSMF,” J. Lightwave Technol.27(3), 177–188 (2009). [CrossRef]
- M. Nazarathy, J. Khurgin, R. Weidenfeld, Y. Meiman, P. Cho, R. Noe, I. Shpantzer, and V. Karagodsky, “Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links,” Opt. Express16(20), 15777–15810 (2008). [CrossRef] [PubMed]
- 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]
- H. G. Myung, J. Lim, and D. J. Goodman, “Peak-to-average power ratio of single carrier FDMA signals with pulse shaping,” In IEEE 17th Int. Symp. Personal, Indoor and Mobile Radio Communications, Sep. 11–14, 2006, 1–5.
- W. Shieh, “OFDM for Flexible High-Speed Optical Networks,” J. Lightwave Technol.29(10), 1560–1577 (2011). [CrossRef]
- D. Falconer, S. 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]
- M. Huemer, H. Witschnig, and J. Hausner, “Unique Word Based Phase Tracking Algorithms for SC/FDE Systems”, In Proceedings of the IEEE International Conference on Global Communications (GLOBECOM), 2003, 1, 70–74.
- L. Deneire, B. Gyselinckx, and M. Engels, “Training Sequence vs. Cyclic Prefix: A New Look on Single Carrier Communication,” In Proceedings of the IEEE International Conference on Global Communications (GLOBECOM), 2000, 1056–1060.
- D. Ly-Gagnon, K. Katoh, and K. Kikuchi, “Unrepeated 210-km transmission with coherent detection and digital signal processing of 20-Gb/s QPSK signal,” Tech. Dig. OFC’05, Anaheim, CA, USA, 2005, p. OTuL4.
- M. E. Mousa-Pasandi and D. V. Plant, “Zero-overhead phase noise compensation via decision-directed phase equalizer for coherent optical OFDM,” Opt. Express18(20), 20651–20660 (2010). [CrossRef] [PubMed]
- S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Laser linewidth tolerance of decision-aided maximum likelihood phase estimation in coherent optical M-ary PSK and QAM systems,” IEEE Photon. Technol. Lett.21(15), 1075–1077 (2009). [CrossRef]
- W. Shieh, “Maximum-likelihood phase and channel estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett.20(8), 605–607 (2008). [CrossRef]
- D. C. Chu, “Polyphase Codes with Good Periodic Correlation Properties,” IEEE Trans. Inf. Theory18(4), 531–532 (1972). [CrossRef]
- 3GPP TR 25.814 V7.0.0, “Technical Specification Group Radio Access Network; Physical Layer Aspects for Evolved UTRA”, (2006).

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