## The validity of “Odd and Even” channels for testing all-optical OFDM and Nyquist WDM long-haul fiber systems |

Optics Express, Vol. 20, Issue 26, pp. B445-B451 (2012)

http://dx.doi.org/10.1364/OE.20.00B445

Acrobat PDF (958 KB)

### Abstract

We investigate experimentally the validity of testing all-optical OFDM and Nyquist WDM systems using interleaved test channels derived from only two data sources. These “odd and even” channels are insufficiently decorrelated, so experiments underestimate the inter-carrier interference (ICI). Additionally, numerical simulations demonstrate that using odd and even channels generates stronger nonlinear distortions during transmission, causing an unrealistically large penalty in the nonlinearity-limited region.

© 2012 OSA

## 1. Introduction

1. S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express **17**(24), 21350–21361 (2009). [CrossRef] [PubMed]

4. X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval 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]

1. S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express **17**(24), 21350–21361 (2009). [CrossRef] [PubMed]

5. 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). [CrossRef]

2. Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. **28**(4), 308–315 (2010). [CrossRef]

3. D. Hillerkuss, R. Schmogrow, T. Schellinger, M. Jordan, M. Winter, G. Huber, T. Vallaitis, R. Bonk, P. Kleinow, F. Frey, M. Roeger, S. Koenig, A. Ludwig, A. Marculescu, J. Li, M. Hoh, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, A. Oehler, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moeller, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “26 Tbit/s line-rate super-channel transmission utilizing all-optical fast Fourier transform processing,” Nat. Photonics **5**(6), 364–371 (2011). [CrossRef]

6. S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Towards a practical implementation of coherent WDM: analytical, numerical, and experimental studies,” IEEE Photonics J **2**(5), 833–847 (2010). [CrossRef]

*odds-and-evens*[3

3. D. Hillerkuss, R. Schmogrow, T. Schellinger, M. Jordan, M. Winter, G. Huber, T. Vallaitis, R. Bonk, P. Kleinow, F. Frey, M. Roeger, S. Koenig, A. Ludwig, A. Marculescu, J. Li, M. Hoh, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, A. Oehler, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moeller, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “26 Tbit/s line-rate super-channel transmission utilizing all-optical fast Fourier transform processing,” Nat. Photonics **5**(6), 364–371 (2011). [CrossRef]

4. X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval 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]

*odds-and-evens*for AO-OFDM [1

1. S. Chandrasekhar and X. Liu, “Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection,” Opt. Express **17**(24), 21350–21361 (2009). [CrossRef] [PubMed]

3. D. Hillerkuss, R. Schmogrow, T. Schellinger, M. Jordan, M. Winter, G. Huber, T. Vallaitis, R. Bonk, P. Kleinow, F. Frey, M. Roeger, S. Koenig, A. Ludwig, A. Marculescu, J. Li, M. Hoh, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, A. Oehler, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moeller, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “26 Tbit/s line-rate super-channel transmission utilizing all-optical fast Fourier transform processing,” Nat. Photonics **5**(6), 364–371 (2011). [CrossRef]

7. 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]

8. X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “PDM-Nyquist-32QAM for 450-Gb/s per-channel WDM transmission on the 50 GHz ITU-T Grid,” J. Lightwave Technol. **30**(4), 553–559 (2012). [CrossRef]

*odds-and-evens*ensures that the received channel is different to its closest neighbors, the neighbors on either side are identical with one another. This was shown to under-estimate inter-carrier-interference (ICI) of a three-subcarrier coherent WDM system [6

6. S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Towards a practical implementation of coherent WDM: analytical, numerical, and experimental studies,” IEEE Photonics J **2**(5), 833–847 (2010). [CrossRef]

*odds-and-evens*in AO-OFDM and N-WDM superchannels. Two cases are considered: (a) without any cyclic prefix (CP) for OFDM or guard band (GB) for N-WDM; (b) with 20% CP/BG, which was sufficient to prevent ICI. Our results show that using

*odds-and-evens*overestimates the performance of both AO-OFDM and N-WDM without a CP/GB, as in a coherent WDM system [6

6. S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Towards a practical implementation of coherent WDM: analytical, numerical, and experimental studies,” IEEE Photonics J **2**(5), 833–847 (2010). [CrossRef]

*Odds-and-evens*underestimated the performance of AO-OFDM with a 20% CP. Of the cases tested,

*odds-and-evens*only accurately predicted the performance of N-WDM with a 20% GB [9].

*odds-and-evens*on degradation caused by fiber nonlinearity. Our results show that using

*odds-and-evens*causes a penalty in the nonlinearity limited region. At 5 Gbaud, this penalty is ~2.3 dB, but reduces with increasing channel spacing and baud rate.

## 2. Experimental description

^{2}T balanced photodiode pairs and another ECL tuned to the frequency of the desired channel. A polarization controller enabled a single-polarization receiver to be used; however, these results could be generalized to a dual-polarization system. An Agilent 40 GSample/s real-time sampling oscilloscope was used as the analogue to digital converter (ADC). After digitization, the signal was resampled in MATLAB to 10 GSample/s for N-WDM (two-times oversampling) and to 20 GSample/s for AO-OFDM (four-times oversampling). Next, a blind self-tuning equalizer separated the channels. A constant modulus algorithm (CMA) was used to roughly converge each channel, which was initialized with a filter response to reject neighboring channels [11]. This was followed by a decision-directed radius directed equalizer (DD-RDE) to fine-tune the equalizer taps [12

12. I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. **27**(15), 3042–3049 (2009). [CrossRef]

12. I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. **27**(15), 3042–3049 (2009). [CrossRef]

*Q*, of 20.4 dB with our experimental setup. Throughout this paper,

*Q*is the signal-to-noise ratio (SNR) measured from the received constellation [13

13. 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*of 16.5 dB for a BER of 10

^{−3}.

## 3. Experimental results for back-to-back systems

### 3.1 5-GHz channel spacing

*odds-and-evens*under-estimates ICI for both AO-OFDM and N-WDM if the channels are spaced at the baud-rate.

### 3.2 6-GHz channel spacing

### 3.3 Summary

## 4. Simulation results for an 800-km link

*i*) the total superchannel bandwidth increases; and (

*ii*) the optimum channel granularity for an 800-km link without inline CD compensation is ~6 Gbaud [16

16. 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]

17. 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]

## 7. Conclusions

*odds-and-evens*to test a design can significantly affect the accuracy of linear performance estimates for both AO-OFDM and N-WDM superchannels. In the case of no CP/GB, using odd and even channels provides a significant, but unrealistic, benefit for both AO-OFDM and N-WDM, as it decreases the estimated ICI. Therefore,

*odds and evens*does not accurately indicate the likely penalty from ICI.

*odds-and-evens*increases the degradation due to fiber nonlinearity. This overestimate of degradation is reduced for larger channel spacings (when the baud rate is also increased) because of the increased decorrelation caused by CD. This second result is consistent with the findings in [14

14. Q. Yang, Y. Tang, Y. Ma, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol. **27**(3), 168–176 (2009). [CrossRef]

## Acknowledgments

## References and links

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

2. | Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol. |

3. | D. Hillerkuss, R. Schmogrow, T. Schellinger, M. Jordan, M. Winter, G. Huber, T. Vallaitis, R. Bonk, P. Kleinow, F. Frey, M. Roeger, S. Koenig, A. Ludwig, A. Marculescu, J. Li, M. Hoh, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, A. Oehler, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moeller, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “26 Tbit/s line-rate super-channel transmission utilizing all-optical fast Fourier transform processing,” Nat. Photonics |

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

5. | 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. |

6. | S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Towards a practical implementation of coherent WDM: analytical, numerical, and experimental studies,” IEEE Photonics J |

7. | 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. |

8. | X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “PDM-Nyquist-32QAM for 450-Gb/s per-channel WDM transmission on the 50 GHz ITU-T Grid,” J. Lightwave Technol. |

9. | L. B. Du and A. J. Lowery, “Experimental investigation of the effect of using 'Odd and Even' channels in all-optical OFDM and Nyquist WDM system comparisons ” in |

10. | V. A. J. M. Sleiffer, M. S. Alfiad, D. van den Borne, M. Kuschnerov, V. Veljanovski, M. Hirano, Y. Yamamoto, T. Sasaki, S. L. Jansen, T. Wuth, and H. de Waardt, “10x224-Gb/s POLMUX-16QAM transmission over 656 km of large-L |

11. | L. B. Du, J. Schroeder, and A. J. Lowery, “Blind subcarrier equalization without pre-filtering for optical OFDM systems,” in |

12. | I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. |

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

14. | Q. Yang, Y. Tang, Y. Ma, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol. |

15. | M. S. Alfiad, D. van den Borne, T. Wuth, M. Kuschnerov, and H. de Waardt, “On the tolerance of 111-Gb/s POLMUX-RZ-DQPSK to nonlinear transmission effects,” J. Lightwave Technol. |

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

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

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.4080) Fiber optics and optical communications : Modulation

(060.4230) Fiber optics and optical communications : Multiplexing

**ToC Category:**

Transmission Systems and Network Elements

**History**

Original Manuscript: October 1, 2012

Revised Manuscript: November 12, 2012

Manuscript Accepted: November 12, 2012

Published: November 30, 2012

**Virtual Issues**

European Conference on Optical Communication 2012 (2012) *Optics Express*

**Citation**

Liang B. Du and Arthur J. Lowery, "The validity of “Odd and Even” channels for testing all-optical OFDM and Nyquist WDM long-haul fiber systems," Opt. Express **20**, B445-B451 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-26-B445

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### References

- 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]
- Y. Ma, Q. Yang, Y. Tang, S. Chen, and W. Shieh, “1-Tb/s single-channel coherent optical OFDM transmission with orthogonal-band multiplexing and subwavelength bandwidth access,” J. Lightwave Technol.28(4), 308–315 (2010). [CrossRef]
- D. Hillerkuss, R. Schmogrow, T. Schellinger, M. Jordan, M. Winter, G. Huber, T. Vallaitis, R. Bonk, P. Kleinow, F. Frey, M. Roeger, S. Koenig, A. Ludwig, A. Marculescu, J. Li, M. Hoh, M. Dreschmann, J. Meyer, S. Ben Ezra, N. Narkiss, B. Nebendahl, F. Parmigiani, P. Petropoulos, B. Resan, A. Oehler, K. Weingarten, T. Ellermeyer, J. Lutz, M. Moeller, M. Huebner, J. Becker, C. Koos, W. Freude, and J. Leuthold, “26 Tbit/s line-rate super-channel transmission utilizing all-optical fast Fourier transform processing,” Nat. Photonics5(6), 364–371 (2011). [CrossRef]
- X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval 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]
- 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). [CrossRef]
- S. K. Ibrahim, J. Zhao, F. C. Garcia Gunning, P. Frascella, F. H. Peters, and A. D. Ellis, “Towards a practical implementation of coherent WDM: analytical, numerical, and experimental studies,” IEEE Photonics J2(5), 833–847 (2010). [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]
- X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “PDM-Nyquist-32QAM for 450-Gb/s per-channel WDM transmission on the 50 GHz ITU-T Grid,” J. Lightwave Technol.30(4), 553–559 (2012). [CrossRef]
- L. B. Du and A. J. Lowery, “Experimental investigation of the effect of using 'Odd and Even' channels in all-optical OFDM and Nyquist WDM system comparisons ” in European Conference on Optical Communication, (Optical Society of America, 2012), Tu.4.C.5.
- V. A. J. M. Sleiffer, M. S. Alfiad, D. van den Borne, M. Kuschnerov, V. Veljanovski, M. Hirano, Y. Yamamoto, T. Sasaki, S. L. Jansen, T. Wuth, and H. de Waardt, “10x224-Gb/s POLMUX-16QAM transmission over 656 km of large-Leff PSCF with a spectral efficiency of 5.6 b/s/Hz,” IEEE Photon. Technol. Lett.23(20), 1427–1429 (2011). [CrossRef]
- L. B. Du, J. Schroeder, and A. J. Lowery, “Blind subcarrier equalization without pre-filtering for optical OFDM systems,” in Optical Fiber Communication Conference, (Optical Society of America, 2012), OM2H.6.
- I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol.27(15), 3042–3049 (2009). [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]
- Q. Yang, Y. Tang, Y. Ma, and W. Shieh, “Experimental demonstration and numerical simulation of 107-Gb/s high spectral efficiency coherent optical OFDM,” J. Lightwave Technol.27(3), 168–176 (2009). [CrossRef]
- M. S. Alfiad, D. van den Borne, T. Wuth, M. Kuschnerov, and H. de Waardt, “On the tolerance of 111-Gb/s POLMUX-RZ-DQPSK to nonlinear transmission effects,” J. Lightwave Technol.29(2), 162–170 (2011). [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]
- L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express19(9), 8079–8084 (2011). [CrossRef] [PubMed]

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