## Experimental Study of a novel adaptive decision-directed channel equalizer in 28 GBaud RGI-DP-CO-OFDM transport systems |

Optics Express, Vol. 20, Issue 15, pp. 17017-17028 (2012)

http://dx.doi.org/10.1364/OE.20.017017

Acrobat PDF (850 KB)

### Abstract

We report and experimentally investigate the performance of an adaptive decision-directed channel equalizer (ADDCE) in reduced-guard-interval dual-polarization coherent-optical orthogonal-frequency-division-multiplexing (RGI-DP-CO-OFDM) transport systems. ADDCE retrieves an estimation of the phase noise value after an initial decision making stage by extracting and averaging the phase drift of all OFDM sub-channels. Moreover, it updates the channel transfer matrix on a symbol-by-symbol basis. We experimentally compare the performance of the ADDCE and the conventional equalizer (CE) combined with maximum-likelihood (ML) phase noise compensation and inter-subcarrier-frequency-averaging (ISFA) algorithms. The study is conducted at 28 GBaud for RGI-DP-CO-OFDM systems with quadrature-phase-shift-keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) formats. Using ADDCE, zero-overhead laser phase noise compensation is accomplished and the overhead due to training symbol (TSs) insertion is significantly reduced. In addition, ADDCE offers a superior performance over the CE in the presence of synchronization timing errors and residual chromatic dispersion (CD). We also achieve a longer transmission distance than when using the CE. At a forward-error-correction (FEC) threshold of 3.8 × 10^{−3}, using a cumulative overhead of less than 2.6%, transmission distances of 5500 km and 400 km were achieved for the cases of QPSK and 16-QAM RGI-DP-CO-OFDM, respectively.

© 2012 OSA

## 1. Introduction

3. M. E. Mousa-Pasandi and D. V. Plant, “Data-aided adaptive weighted channel equalizer for coherent optical OFDM,” Opt. Express **18**(4), 3919–3927 (2010). [CrossRef] [PubMed]

9. M. E. Mousa-Pasandi and D. V. Plant, “Zero-overhead phase noise compensation via decision-directed phase equalizer for coherent optical OFDM,” Opt. Express **18**(20), 20651–20660 (2010). [CrossRef] [PubMed]

11. M. Rim, “Optimally combining decision-directed and pilot-symbol-aided channel estimation,” Electron. Lett. **39**(6), 558–560 (2003). [CrossRef]

^{−3}, transmission distances of 5500 km and 400 km were achieved for the case of QPSK and 16-QAM RGI-DP-CO-OFDM, respectively, using zero-overhead phase compensation and a cumulative overhead of less than 2.6%. We also study the effect of the synchronization timing error and the residual dispersion on the ADDCE and the CE and demonstrate the superior performance of the ADDCE. It is notable that since ADDCE operates on a symbol-by-symbol basis and considering that OFDM symbol rates can be much lower than the actual transmitted bit-rate, implementing ADDCE does not necessarily require very high-speed and high power consuming electronics. A brief analysis of the computational complexity of this scheme in terms of the number of required complex multiplications is provided, showing a complexity of only 28%.

## 2. ADDCE for dual-polarization transmission

*n*and

*k*denote the indexes for the received symbol (time index) and the OFDM subcarrier (frequency index), respectively.

*X*and

*Y*represent the two optical polarizations. In RGI-DP-CO-OFDM systems, the subcarrier-specific received complex value vector,

*n*

^{th}received OFDM vector aswhere

*N*is the total number of OFDM subcarriers. As one can see, (3) tries to extract the phase drift of the OFDM sub-channels in the time interval of the

*n*

^{th}received vector, assuming that the optical channel drift due to other impairments such as CD and PMD is negligible. This is a good assumption since CD and PMD variations are believed to be low-speed in comparison to the typical CO-OFDM symbol rate [3

3. M. E. Mousa-Pasandi and D. V. Plant, “Data-aided adaptive weighted channel equalizer for coherent optical OFDM,” Opt. Express **18**(4), 3919–3927 (2010). [CrossRef] [PubMed]

4. F. Buchali, R. Dischler, and X. Liu, “Optical OFDM: A Promising High-Speed Optical Transport Technology,” Bell Labs Tech. J. **14**(1), 125–148 (2009). [CrossRef]

3. M. E. Mousa-Pasandi and D. V. Plant, “Data-aided adaptive weighted channel equalizer for coherent optical OFDM,” Opt. Express **18**(4), 3919–3927 (2010). [CrossRef] [PubMed]

*n*

^{th}received OFDM vector and consequently, includes not only the common-phase-error (CPE) information of the laser phase noise process but also any drift in channel response. A low-pass filter (LPF) is applied to

*n*

^{th}received vector can then be updated as where

**18**(4), 3919–3927 (2010). [CrossRef] [PubMed]

## 3. Performance of ADDCE in RGI-DP-CO-OFDM systems

^{17}-1 is first divided and mapped onto 112 frequency subcarriers with QPSK (16-QAM) modulation format and subsequently transferred to the time-domain by an IFFT of size 128 while zeros occupy the remainder, fixing the oversampling ratio of 1.14. In this RGI-DP-CO-OFDM system, a cyclic prefix of length 3 is employed, resulting in 2.34% of CP overhead. The CE employs 4 pilot subcarriers which can be translated in 3.57% of PSC overhead; however, ADDCE has no pilot subcarrier featuring the zero-overhead phase noise compensation. The CE and ADDCE use 2 training symbols (TSs) for every 100 and 1000 data symbols, equivalently 2% and 0.2% of TS overhead, respectively. This results in a cumulative overhand of ~8% (2.34% + 3.57% + 2%) and ~2.6% (2.34% + 0% + 0.2%) for the CE and the ADDCE, respectively. 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. Using 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). After the IQ-MZM, a dual polarization emulator is used to imitate a dual-polarization multiplexed transmitter which results in 112-Gb/s and 224-Gb/s for QPSK and 16-QAM RGI-DP-CO-OFDM signals, respectively. 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 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 local oscillator (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 the off-line processing section, to further characterize the capabilities of ADDCE, we compare the performance of the ADDCE with the CE combined with two other commonly-reported compensation schemes: the maximum-likelihood (ML) phase noise compensation [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. W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw. **7**(3), 234–255 (2008). [CrossRef]

13. 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]

### 3.1. BER vs. OSNR and launch power

### 3.2. BER vs. synchronization timing error

14. C. J. Youn, X. Liu, S. Chandrasekhar, Y. H. Kwon, J. H. Kim, J. S. Choe, D. J. Kim, K. S. Choi, and E. S. Nam, “Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols,” Opt. Express **19**(17), 16174–16181 (2011). [CrossRef] [PubMed]

### 3.3. BER vs. residual dispersion

16. 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]

### 3.4. BER vs. transmission distance

^{−3}, the ADDCE achieves a transmission distance of 5500 km and 400 km for QPSK and 16-QAM RGI-DP-CO-OFDM, respectively. These demonstrate 8% and 20% improvement in the transmission reach versus the CE with ML and ISFA algorithm for QPSK and 16-QAM RGI-DP-CO-OFDM, respectively. The ADDCE’s capacity in overhead reduction, improving the transmission reach and resilience to the synchronization timing error and the residual dispersion, makes it an attractive alternative equalization algorithm.

## 4. System complexity

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

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

## 5. Conclusions

## Acknowledgments

## References and links

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

2. | W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw. |

3. | M. E. Mousa-Pasandi and D. V. Plant, “Data-aided adaptive weighted channel equalizer for coherent optical OFDM,” Opt. Express |

4. | F. Buchali, R. Dischler, and X. Liu, “Optical OFDM: A Promising High-Speed Optical Transport Technology,” Bell Labs Tech. J. |

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

6. | X. Yi, W. Shieh, and Y. Tang, “Phase estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. |

7. | S. L. Jansen, I. Morita, N. Takeda, and H. Tanaka, “20-Gb/s OFDM transmission over 4,160-km SSMF enabled by RF-Pilot tone phase noise compensation,” in |

8. | M. E. Mousa-Pasandi and D. V. Plant, “Improvement of phase noise compensation for coherent optical OFDM via data-aided phase equalizer,” in |

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

10. | J. Ran, R. Grunheid, H. Rohling, E. Bolinth, and R. Kern, “Decision-directed channel estimation method for OFDM systems with high velocities,” in |

11. | M. Rim, “Optimally combining decision-directed and pilot-symbol-aided channel estimation,” Electron. Lett. |

12. | X. Liu and F. Buchali, “A novel channel estimation method for PDM-OFDM enabling improved tolerance to WDM nonlinearity,” in |

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

14. | C. J. Youn, X. Liu, S. Chandrasekhar, Y. H. Kwon, J. H. Kim, J. S. Choe, D. J. Kim, K. S. Choi, and E. S. Nam, “Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols,” Opt. Express |

15. | S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol. |

16. | 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. |

17. | 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 18, 2012

Revised Manuscript: June 28, 2012

Manuscript Accepted: July 3, 2012

Published: July 11, 2012

**Citation**

Mohammad E. Mousa-Pasandi, Qunbi Zhuge, Xian Xu, Mohamed M. Osman, Mathieu Chagnon, and David V. Plant, "Experimental Study of a novel adaptive decision-directed channel equalizer in 28 GBaud RGI-DP-CO-OFDM transport systems," Opt. Express **20**, 17017-17028 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-15-17017

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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]
- W. Shieh, X. Yi, Y. Ma, and Q. Yang, “Coherent optical OFDM: has its time come?” J. Opt. Netw.7(3), 234–255 (2008). [CrossRef]
- M. E. Mousa-Pasandi and D. V. Plant, “Data-aided adaptive weighted channel equalizer for coherent optical OFDM,” Opt. Express18(4), 3919–3927 (2010). [CrossRef] [PubMed]
- F. Buchali, R. Dischler, and X. Liu, “Optical OFDM: A Promising High-Speed Optical Transport Technology,” Bell Labs Tech. J.14(1), 125–148 (2009). [CrossRef]
- S. L. Jansen, I. Morita, T. C. W. Schenk, N. Takeda, and H. Tanaka, “Coherent optical 25.8-Gb/s OFDM transmission over 4160-km SSMF,” J. Lightwave Technol.26(1), 6–15 (2008). [CrossRef]
- X. Yi, W. Shieh, and Y. Tang, “Phase estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett.19(12), 919–921 (2007). [CrossRef]
- S. L. Jansen, I. Morita, N. Takeda, and H. Tanaka, “20-Gb/s OFDM transmission over 4,160-km SSMF enabled by RF-Pilot tone phase noise compensation,” in Optical Fiber Communication Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper PDP15.
- M. E. Mousa-Pasandi and D. V. Plant, “Improvement of phase noise compensation for coherent optical OFDM via data-aided phase equalizer,” in Optical Fiber Communication Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2010), paper JThA10.
- 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]
- J. Ran, R. Grunheid, H. Rohling, E. Bolinth, and R. Kern, “Decision-directed channel estimation method for OFDM systems with high velocities,” in Proceedings of IEEE Vehicular Technology Conference, (Institute of Electrical and Electronics Engineers, New York, 2003), 2358–2361.
- M. Rim, “Optimally combining decision-directed and pilot-symbol-aided channel estimation,” Electron. Lett.39(6), 558–560 (2003). [CrossRef]
- X. Liu and F. Buchali, “A novel channel estimation method for PDM-OFDM enabling improved tolerance to WDM nonlinearity,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OWW5.
- 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]
- C. J. Youn, X. Liu, S. Chandrasekhar, Y. H. Kwon, J. H. Kim, J. S. Choe, D. J. Kim, K. S. Choi, and E. S. Nam, “Channel estimation and synchronization for polarization-division multiplexed CO-OFDM using subcarrier/polarization interleaved training symbols,” Opt. Express19(17), 16174–16181 (2011). [CrossRef] [PubMed]
- S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol.27(16), 3699–3704 (2009).
- 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]
- 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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