## Experimental demonstration of non-iterative interpolation-based partial ICI compensation in100G RGI-DP-CO-OFDM transport systems |

Optics Express, Vol. 20, Issue 14, pp. 14825-14832 (2012)

http://dx.doi.org/10.1364/OE.20.014825

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### Abstract

We experimentally investigate the performance of a low-complexity non-iterative phase noise induced inter-carrier interference (ICI) compensation algorithm in reduced-guard-interval dual-polarization coherent-optical orthogonal-frequency-division-multiplexing (RGI-DP-CO-OFDM) transport systems. This interpolation-based ICI compensator estimates the time-domain phase noise samples by a linear interpolation between the CPE estimates of the consecutive OFDM symbols. We experimentally study the performance of this scheme for a 28 Gbaud QPSK RGI-DP-CO-OFDM employing a low cost distributed feedback (DFB) laser. Experimental results using a DFB laser with the linewidth of 2.6 MHz demonstrate 24% and 13% improvement in transmission reach with respect to the conventional equalizer (CE) in presence of weak and strong dispersion-enhanced-phase-noise (DEPN), respectively. A brief analysis of the computational complexity of this scheme in terms of the number of required complex multiplications is provided. This practical approach does not suffer from error propagation while enjoying low computational complexity.

© 2012 OSA

## 1. Introduction

1. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express **16**(2), 841–859 (2008). [CrossRef] [PubMed]

1. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express **16**(2), 841–859 (2008). [CrossRef] [PubMed]

4. S. L. Jansen, B. Spinnler, I. Morita, S. Randel, and H. Tanaka, “100GbE: QPSK versus OFDM,” J. Opt. Fiber Technol. **15**(5-6), 407–413 (2009). [CrossRef]

11. Q. Zhuge, C. Chen, and D. V. Plant, “Dispersion-enhanced phase noise effects on reduced-guard-interval CO-OFDM transmission,” Opt. Express **19**(5), 4472–4484 (2011). [CrossRef] [PubMed]

11. Q. Zhuge, C. Chen, and D. V. Plant, “Dispersion-enhanced phase noise effects on reduced-guard-interval CO-OFDM transmission,” Opt. Express **19**(5), 4472–4484 (2011). [CrossRef] [PubMed]

12. M. E. Mousa-Pasandi and D. V. Plant, “Non-iterative interpolation-based partial phase noise ICI mitigation for CO-OFDM transport systems,” IEEE Photon. Technol. Lett. **23**(21), 1594–1596 (2011). [CrossRef]

14. P. Rabiei, W. Namgoong, and N. Al-Dhahir, “A non-iterative technique for phase noise ICI mitigation in packet-based OFDM systems,” IEEE Trans. Signal Process. **58**(11), 5945–5950 (2010). [CrossRef]

^{−3}as the forward error correction (FEC) threshold, for the case with weak DEPN, a transmission distance of 2300 km over single mode fiber (SMF) was achieved demonstrating a 24% increase in transmission reach with respect to a system employing a conventional equalizer (CE). In the presence of strong DEPN, a reach of 2300 km was achieved when our ICI compensator was combined with a grouped-maximum-likelihood algorithm [14

14. P. Rabiei, W. Namgoong, and N. Al-Dhahir, “A non-iterative technique for phase noise ICI mitigation in packet-based OFDM systems,” IEEE Trans. Signal Process. **58**(11), 5945–5950 (2010). [CrossRef]

## 2. The concept of interpolation-based ICI compensation

*x*,

*p*,

*h*and

*r*are the subcarrier-specific transmitted data symbol in frequency-domain, the phase noise spectral components, the subcarrier-specific channel frequency response and the subcarrier-specific received data symbol in frequency-domain, respectively.

*k*and

*m*denote the subcarrier (frequency) and symbol (time) indexes, respectively and

*w*represents the additive noise. As seen in Eq. (1),

**Similar to the CE, scattered pilot subcarriers are used to estimate the CPE in every OFDM data symbol asThis estimated CPE value is set equal to the phase noise corresponding to the middle time-domain sample of the same data symbol.**

*Step1.***The phase noise values of the remaining intermediate samples of each symbol in the time-domain are determined by a linear interpolation using the CPE estimates of the previous, current and next OFDM symbols. This linear interpolation provides the optimum MSE interpolation as long as**

*Step 2.**βT*1, where

_{s}<<*β*and

*T*are the two-sided 3-dB bandwidth of the phase noise process and the symbol duration, respectively [14

_{s}14. P. Rabiei, W. Namgoong, and N. Al-Dhahir, “A non-iterative technique for phase noise ICI mitigation in packet-based OFDM systems,” IEEE Trans. Signal Process. **58**(11), 5945–5950 (2010). [CrossRef]

*m*

^{th}received symbol can be completed only after reception of the

*(m + 1)*

^{th}symbol. Therefore, the interpolation-based ICI compensator requires a one-symbol buffer, resulting in a one-symbol latency.

**After the time-domain phase noise vector is approximated by a linear interpolation between consecutive OFDM symbols in step 2, we can derive the spectral components of phase noise estimation by the Fourier transform operation. Then, the received symbol can be equalized aswhere the notion of ^ indicates that the corresponding parameter is based on an estimation.**

*Step 3.**Q*. As Eq. (3) indicates, instead of de-rotating the OFDM samples in time-domain, the receiver can simply convolve the received symbol by the spectral components of the estimated phase noise followed by a one-tap frequency-domain equalizer. Since laser phase noise can be approximately expressed as a Wiener process and most of the energy of a Wiener process is concentrated in the first few harmonics, a small value for

*Q*can be adopted to reduce the required number of complex multiplications in Eq. (3).

## 3. Performance of interpolation-based ICI compensator in RGI-DP-CO-OFDM systems

^{17}-1 is first divided and mapped onto 224 frequency subcarriers with QPSK modulation format and subsequently transferred to the time-domain by an IFFT of size 256 while zeros occupy the remainder, fixing the value 1.14 as the oversampling ratio. In this RGI-DP-CO-OFDM system, a cyclic prefix of length 8, 2 pilot symbols for every 50 data symbols and 4 pilot subcarriers are employed. The in-phase (I) and quadrature (Q) parts of the resulting digital OFDM signal are then loaded separately on two field-programmable gate arrays (FPGAs) to electrically generate the electrical I and Q via two digital to analogue convertors (DACs), operating at 32 GS/s. Considering the oversampling ratio of 1.14, the analogue electrical I and Q signals at 28 Gbaud OFDM are generated and then fed into an IQ Mach-Zehnder modulator (IQ-MZM). Right after the IQ-MZM, a dual polarization emulator is used to imitate a dual-polarization multiplexed transmitter. The optical transmission link consists of a 4-span optical recirculating loop with uncompensated SMF with the dispersion parameter of 17 ps/nm.km, the nonlinear coefficient of 1.2 W

^{−1}.km

^{−1}and the loss parameter of 0.18 dB/km. Spans are 80 km long and separated by erbium-doped-fiber-amplifiers (EDFAs) with a noise figure of ~6 dB. At the optical receiver, two optical filters with the bandwidths of 0.4 nm and 0.8 nm are applied before and after the preamplifier, respectively, to reject the out-of-band accumulated spontaneous emission (ASE) noise. The receiver is based on the intradyne scenario in which the received signal beats with the optical LO signal in an optical polarization-diversity 90° hybrid to obtain the signal I and Q components. The LO is tuned to within the range of approximately tens of MHz of the received signal’s center frequency. The four pairs of balanced outputs from the hybrid are then detected by four balanced photodetectors and then electrically sampled and asynchronously digitized at 80 GSamples/s using two commercial 4-channel real-time oscilloscopes, equipped with analog-to-digital converters (ADCs) characterized by 33 GHz of analogue bandwidth, a nominal resolution of 8-bit and a frequency-dependent effective number of bits (ENoB) between 4 and 5. Four signals are then transferred to PC for off-line processing. In off-line processing, an inter subcarrier frequency averaging (ISFA) algorithm with an averaging parameter of 9 and a ML phase estimation were incorporated [1

1. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express **16**(2), 841–859 (2008). [CrossRef] [PubMed]

2. X. Liu and F. Buchali, “Intra-symbol frequency-domain averaging based channel estimation for coherent optical OFDM,” Opt. Express **16**(26), 21944–21957 (2008). [CrossRef] [PubMed]

16. X. Chen, A. Al Amin, and W. Shieh, “Characterization and monitoring of laser linewidths in coherent systems,” J. Lightwave Technol. **29**(17), 2533–2537 (2011). [CrossRef]

*f*using

*T*represents the symbol duration, assuming the laser phase noise as a Wiener process [15,16

16. X. Chen, A. Al Amin, and W. Shieh, “Characterization and monitoring of laser linewidths in coherent systems,” J. Lightwave Technol. **29**(17), 2533–2537 (2011). [CrossRef]

^{−3}, demonstrating a transmission reach improvement of 24%. In Fig. 5 , we again study the BER performance of the ICI compensator and the CE versus transmission distance however, this time, the ECL is used at transmitter and the DFB laser is employed at receiver, stimulating the strong DEPN effect. Blue and red curves correspond to the equalization without and with ICI compensation, respectively. Comparing Fig. 4 and Fig. 5, a significant degradation in the transmission reach is observed due to the strong DEPN effect. However, the ICI compensator, red curve, still provides a better performance than the CE, blue curve, achieving a transmission reach of 1800 km at the BER threshold of 1 × 10

^{−3}, demonstrating a transmission reach improvement of 13%.

11. Q. Zhuge, C. Chen, and D. V. Plant, “Dispersion-enhanced phase noise effects on reduced-guard-interval CO-OFDM transmission,” Opt. Express **19**(5), 4472–4484 (2011). [CrossRef] [PubMed]

## 4. System complexity

18. B. Spinnler, “Equalizer Design and Complexity for Digital Coherent Receivers,” IEEE J. Sel. Top. Quantum Electron. **16**(5), 1180–1192 (2010). [CrossRef]

18. B. Spinnler, “Equalizer Design and Complexity for Digital Coherent Receivers,” IEEE J. Sel. Top. Quantum Electron. **16**(5), 1180–1192 (2010). [CrossRef]

## 5. Conclusion

## Acknowledgments

## References and links

1. | W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express |

2. | X. Liu and F. Buchali, “Intra-symbol frequency-domain averaging based channel estimation for coherent optical OFDM,” Opt. Express |

3. | F. Buchali, R. Dischler, and X. Liu, “Optical OFDM: a promising high-speed optical transport technology,” Bell Syst. Tech. J. |

4. | S. L. Jansen, B. Spinnler, I. Morita, S. Randel, and H. Tanaka, “100GbE: QPSK versus OFDM,” J. Opt. Fiber Technol. |

5. | S. L. Jansen, I. Morita, T. Schenk, N. Takeda, and H. Tankada, “Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF,” J. Lightwave Technol. |

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

7. | C. Yang, F. Yang, and Z. Wang, “Orthogonal basis expansion-based phase noise estimation and suppression for CO-OFDM systems,” IEEE Photon. Technol. Lett. |

8. | Q. Zou, A. Tarighat, and A. H. Sayed, “Compensation of phase noise in OFDM wireless systems,” IEEE Trans. Signal Process. |

9. | L. Tomba, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun. |

10. | Y. Mostofi and D. C. Cox, “ICI mitigation for pilot-aided OFDM mobile systems,” IEEE Trans. Wirel. Comm. |

11. | Q. Zhuge, C. Chen, and D. V. Plant, “Dispersion-enhanced phase noise effects on reduced-guard-interval CO-OFDM transmission,” Opt. Express |

12. | M. E. Mousa-Pasandi and D. V. Plant, “Non-iterative interpolation-based partial phase noise ICI mitigation for CO-OFDM transport systems,” IEEE Photon. Technol. Lett. |

13. | M. E. Mousa Pasandi and D. V. Plant, “Non-iterative interpolation-based phase noise ICI mitigation for CO-OFDM transport systems,” in Signal Processing in Photonic Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper SPMB6. |

14. | P. Rabiei, W. Namgoong, and N. Al-Dhahir, “A non-iterative technique for phase noise ICI mitigation in packet-based OFDM systems,” IEEE Trans. Signal Process. |

15. | K. Kikuchi and K. Igarashi, “Characterization of semiconductor-laser phase noise with digital coherent receivers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OML3. |

16. | X. Chen, A. Al Amin, and W. Shieh, “Characterization and monitoring of laser linewidths in coherent systems,” J. Lightwave Technol. |

17. | Q. Zhuge, M. E. Mousa-Pasandi, M. Morsy-Osman, X. Xu, M. Chagnon, and Z. A. El-Sahn, and D. V. Plant, “Demonstration of dispersion-enhanced phase noise in RGI CO-OFDM systems,” IEEE Photon. Technol. Lett. ((submitted to). |

18. | B. Spinnler, “Equalizer Design and Complexity for Digital Coherent Receivers,” IEEE J. Sel. Top. Quantum Electron. |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.4080) Fiber optics and optical communications : Modulation

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: May 1, 2012

Revised Manuscript: May 28, 2012

Manuscript Accepted: May 29, 2012

Published: June 18, 2012

**Citation**

Mohammad E. Mousa-Pasandi, Qunbi Zhuge, Xian Xu, Mohamed M. Osman, Ziad A. El-Sahn, Mathieu Chagnon, and David V. Plant, "Experimental demonstration of non-iterative interpolation-based partial ICI compensation in100G RGI-DP-CO-OFDM transport systems," Opt. Express **20**, 14825-14832 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-14825

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### References

- W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express16(2), 841–859 (2008). [CrossRef] [PubMed]
- X. Liu and F. Buchali, “Intra-symbol frequency-domain averaging based channel estimation for coherent optical OFDM,” Opt. Express16(26), 21944–21957 (2008). [CrossRef] [PubMed]
- F. Buchali, R. Dischler, and X. Liu, “Optical OFDM: a promising high-speed optical transport technology,” Bell Syst. Tech. J.14, 127–148 (2009).
- S. L. Jansen, B. Spinnler, I. Morita, S. Randel, and H. Tanaka, “100GbE: QPSK versus OFDM,” J. Opt. Fiber Technol.15(5-6), 407–413 (2009). [CrossRef]
- S. L. Jansen, I. Morita, T. Schenk, N. Takeda, and H. Tankada, “Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF,” J. Lightwave Technol.26(1), 6–15 (2008). [CrossRef]
- 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]
- C. Yang, F. Yang, and Z. Wang, “Orthogonal basis expansion-based phase noise estimation and suppression for CO-OFDM systems,” IEEE Photon. Technol. Lett.22(1), 51–53 (2010). [CrossRef]
- Q. Zou, A. Tarighat, and A. H. Sayed, “Compensation of phase noise in OFDM wireless systems,” IEEE Trans. Signal Process.55(11), 5407–5424 (2007). [CrossRef]
- L. Tomba, “On the effect of Wiener phase noise in OFDM systems,” IEEE Trans. Commun.46(5), 580–583 (1998). [CrossRef]
- Y. Mostofi and D. C. Cox, “ICI mitigation for pilot-aided OFDM mobile systems,” IEEE Trans. Wirel. Comm.4(2), 765–774 (2005). [CrossRef]
- Q. Zhuge, C. Chen, and D. V. Plant, “Dispersion-enhanced phase noise effects on reduced-guard-interval CO-OFDM transmission,” Opt. Express19(5), 4472–4484 (2011). [CrossRef] [PubMed]
- M. E. Mousa-Pasandi and D. V. Plant, “Non-iterative interpolation-based partial phase noise ICI mitigation for CO-OFDM transport systems,” IEEE Photon. Technol. Lett.23(21), 1594–1596 (2011). [CrossRef]
- M. E. Mousa Pasandi and D. V. Plant, “Non-iterative interpolation-based phase noise ICI mitigation for CO-OFDM transport systems,” in Signal Processing in Photonic Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper SPMB6.
- P. Rabiei, W. Namgoong, and N. Al-Dhahir, “A non-iterative technique for phase noise ICI mitigation in packet-based OFDM systems,” IEEE Trans. Signal Process.58(11), 5945–5950 (2010). [CrossRef]
- K. Kikuchi and K. Igarashi, “Characterization of semiconductor-laser phase noise with digital coherent receivers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper OML3.
- X. Chen, A. Al Amin, and W. Shieh, “Characterization and monitoring of laser linewidths in coherent systems,” J. Lightwave Technol.29(17), 2533–2537 (2011). [CrossRef]
- Q. Zhuge, M. E. Mousa-Pasandi, M. Morsy-Osman, X. Xu, M. Chagnon, and Z. A. El-Sahn, and D. V. Plant, “Demonstration of dispersion-enhanced phase noise in RGI CO-OFDM systems,” IEEE Photon. Technol. Lett. ((submitted to).
- B. Spinnler, “Equalizer Design and Complexity for Digital Coherent Receivers,” IEEE J. Sel. Top. Quantum Electron.16(5), 1180–1192 (2010). [CrossRef]

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