## The application of cost-effective lasers in coherent UDWDM-OFDM-PON aided by effective phase noise suppression methods |

Optics Express, Vol. 22, Issue 6, pp. 6276-6286 (2014)

http://dx.doi.org/10.1364/OE.22.006276

Acrobat PDF (1030 KB)

### Abstract

Digital coherent passive optical network (PON), especially the coherent orthogonal frequency division multiplexing PON (OFDM-PON), is a strong candidate for the 2nd-stage-next-generation PON (NG-PON2). As is known, OFDM is very sensitive to the laser phase noise which severely limits the application of the cost-effective distributed feedback (DFB) lasers and more energy-efficient vertical cavity surface emitting lasers (VCSEL) in the coherent OFDM-PON. The current long-reach coherent OFDM-PON experiments always choose the expensive external cavity laser (ECL) as the optical source for its narrow linewidth (usually<100 KHz). To solve this problem, we introduce the orthogonal basis expansion based (OBE) phase noise suppression method to the coherent OFDM-PON and study the possibility of the application of the DFB lasers and VCSEL in coherent OFDM-PON. A typical long-reach coherent ultra dense wavelength division multiplexing (UDWDM) OFDM-PON has been set up. The numerical results prove that the OBE method can stand severe phase noise of the lasers in this architecture and the DFB lasers as well as VCSEL can be used in coherent OFDM-PON. In this paper, we have also analyzed the performance of the RF-pilot-aided (RFP) phase noise suppression method in coherent OFDM-PON.

© 2014 Optical Society of America

## 1. Introduction

1. E. Wong, “Next-generation broadband access networks and technologies,” J. Lightwave Technol. **30**(4), 597–608 (2012). [CrossRef]

2. X. Yi, W. Shieh, and Y. Ma, “Phase noise effects on high spectral efficiency coherent optical OFDM transmission,” J. Lightwave Technol. **26**(10), 1309–1316 (2008). [CrossRef]

1. E. Wong, “Next-generation broadband access networks and technologies,” J. Lightwave Technol. **30**(4), 597–608 (2012). [CrossRef]

6. E. Wong, M. Mueller, and M. C. Amann, “Characterization of energy-efficient and colorless ONUs for future TWDM-PONs,” Opt. Express **21**(18), 20747–20761 (2013). [CrossRef] [PubMed]

6. E. Wong, M. Mueller, and M. C. Amann, “Characterization of energy-efficient and colorless ONUs for future TWDM-PONs,” Opt. Express **21**(18), 20747–20761 (2013). [CrossRef] [PubMed]

## 2. The principles of OBE and RFP phase noise suppressions

### 2.1 The OBE phase noise suppression

*N*, the pilots

*i*th OFDM transmitted data block

*M*pilots and

*D*is the density. The superscript

*T*denotes transpose. In the time domain, laser phase noise induces a phase distortion

*j*th sampling point of the

*i*th received OFDM symbol. Define the phase noise vector

*i*th received OFDM symbol. Then it can be expanded by orthogonal basis aswhere

*L*-dimensional space.

*i*th received OFDM symbol

12. X. Fang, C. Yang, and F. Zhang, “Time-domain maximum-likelihood channel estimation for PDM CO-OFDM systems,” IEEE Photon. Technol. Lett. **25**(6), 619–622 (2013). [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]

*H*denotes Hermitian transpose. Subsequently, the pilots can be picked up to estimate the conjugation of the phase noise coefficient vector aswhereHere

*N*× 1 vector with

**x**as the diagonal. The final output

*M*and the dimension of the orthogonal basis

*L*play crucial roles for the accuracy of phase noise estimation. Generally, large

*M*can bring about more efficient phase noise suppression, but leads to lower spectral efficiency. For the dimension of the orthogonal basis, it couldn’t exceed the pilot number

*M*. Although a large

*L*should be used for serious phase noise, increasing

*L*may degrade the accuracy of the least square (LS) estimation in Eq. (5) when

*M*is fixed.

### 2.2 The RFP phase noise suppression

*i*th received OFDM symbol, will be filtered out by a digitallow pass filter (LPF). The RF-pilot is supposed to get the same phase distortions induced by laser phase noise as the OFDM signal and can be used to remove the phase distortions from the OFDM signal. The phase noise suppression can be operated asAfter that, the preamble is used to estimate the channel frequency response matrix

*2.1*. So the final output

### 2.3 Complexity analysis

*N*, the multiplications needed are

_{cp}*O*(

*N*log(

*N*)) +

*O*(

*N*+

*N*). Generally speaking, The OBE method needs more computation complexity than the RF-pilot method. Note that phase noise is usually narrow band, in other words,

_{cp}*L*or

## 3. Theoretical analysis about the laser linewidth tolerance enhancement by the OBE method

*L-*dimensional space,

*L*discrete Fourier transform (DFT) coefficients of the phase noise can be estimated by the OBE method and the phase noise on the relevant frequency can be compensated accordingly.

*k*th subcarrier can be written asHere,

*i*th OFDM symbol and the

*k*th subcarrier. The frequency-domain phase-noise complex exponential is given bywhere

*n*th sample within the studied

*i*th OFDM symbol. In particular,

*k*th subcarrier is defined asAs deduced in [15], if the power spectral density of the laser phase noise is known, the SIR of the received signal in the effect of phase noise can be written asHere

*2.1*,

*L*DFT coefficients of the phase-noise complex exponential vector

*L*DFT coefficients of the phase noise vector

*L*DFT coefficients of

*L*when the OBE method is used. But if

*L*is too large, the SIR becomes worse instead of better. Large

*L*will decrease the accuracy of the least squares (LS) estimation in calculating the DFT coefficients of the phase noise. The red curve in Fig. 4 stands for the SIR improvement when the OBE method (without DE suppression,

*L =*7) is used. Although Eq. (20) is derived under the approximation that the laser linewidth is relatively narrow, the results show the OBE method is still effective when the laser linewidth is large. The SIR can be improved 5.5dB when the linewidth is 200KHz and 3.5dB for 2MHz linewidth.

## 4. Simulation setup and results

*ζ*transceivers, an optical distribution network (ODN) and multiple optical network units (ONU). The OLT serves

*ζ*long-reach high-split ratio PONs and is capable of transmitting and receiving any one of the

*ζ*wavelengths. Each of the

*ζ*PONs can only deal with one wavelength, so the local exchange exists in the ODN for wavelength multiplexing/de-multiplexing. Each PONs may target a different transmission reach, i.e.

*U*km of straight SSMF to the local exchange for wavelength de-multiplexing. Then the data on

^{TM}V8.6 to simulate the long-reach coherent UDWDM-OFDM-PON described above and analyze the system performance when DFB lasers or VCSEL are used as optical sources in this architecture. The sample rate is 10GS/s. For simplicity but without loss of generality, the OLT sends data on 8 different wavelengths simultaneously whose frequency interval is 10GHz. The total transmission distance is 130km with

*U*of 80km and

12. X. Fang, C. Yang, and F. Zhang, “Time-domain maximum-likelihood channel estimation for PDM CO-OFDM systems,” IEEE Photon. Technol. Lett. **25**(6), 619–622 (2013). [CrossRef]

*L*has an important influence on the performance of the OBE method, just as the influence of PSR to RFP. So we choose different

*L*(

*L*= 5, 7, 9) and PSR (PSR = −12dB, −10dB, −8dB, −2dB) for the OBE and RFP methods when the performances of the corresponding systems are evaluated. As a comparison, the performance of the system utilizing the CPE correction for phase noise suppression proposed in [2

2. X. Yi, W. Shieh, and Y. Ma, “Phase noise effects on high spectral efficiency coherent optical OFDM transmission,” J. Lightwave Technol. **26**(10), 1309–1316 (2008). [CrossRef]

*L*in this situation. Although

*L*can be continuously increased and more DFT coefficients of the laser phase noise can be estimated, the limited pilots and other channel noise will decrease the estimation accuracy. So the performance will not improve when

*L*increases beyond 7. More pilots may be needed to guarantee the estimation accuracy at this time. Besides increasing the pilots, we can also use the DE phase noise suppression to improve the performance of the OBE method. At this time, error-free transmission can be achieved even when the laser linewidth is large to 7MHz. As a result, more computational complexity is induced.

*L*for OBE can’t estimate the phase noise accurately. And, limited by the pilot number, enlarging

*L*is meaningless. By contrast, the LPF has large bandwidth (625MHz here) and can realize better phase noise recovery especially for high frequency phase noise. So the RFP method shows better performance when the linewidth is large. To improve the performance of RFP, we can optimize the PSR and LPF according to the linewidth and channel information. In addition to them, more unmodulated subcarriers around the RF-pilot can guarantee better recovery of the phase distortion by LPF. Of course, if all parameters are optimized to the real-time channel information, higher complexity and lower spectral efficiency are following.

## 5. Conclusion

## Acknowledgments

## References and links

1. | E. Wong, “Next-generation broadband access networks and technologies,” J. Lightwave Technol. |

2. | X. Yi, W. Shieh, and Y. Ma, “Phase noise effects on high spectral efficiency coherent optical OFDM transmission,” J. Lightwave Technol. |

3. | D. Lavery, M. Paskov, and S. Savory, “Spectral shaping for mitigating backreflections in a bidirectional 10 Gbit/s coherent WDM-PON,” in |

4. | X. Pang, A. Lebedev, J. J. Vegas Olmos, I. T. Monroy, M. Beltran, and R. Llorente, “Performance evaluation for DFB and VCSEL-based 60 GHz radio-over-fiber system,” In |

5. | R. Gaudino, V. Curri, G. Bosco, G. Rizzelli, A. Nespola, D. Zeolla, S. Straullu, S. Capriata, and P. Solina, “On the use of DFB Lasers for Coherent PON,” in |

6. | E. Wong, M. Mueller, and M. C. Amann, “Characterization of energy-efficient and colorless ONUs for future TWDM-PONs,” 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. | C. Yang, F. Yang, and Z. Wang, “Phase noise suppression for coherent optical block transmission systems: A unified framework,” Opt. Express |

9. | C. Zhao, C. Yang, F. Yang, F. Zhang, and Z. Chen, “A CO-OFDM system with almost blind phase noise suppression,” IEEE Photon. Technol. Lett. |

10. | S. 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 |

11. | D. Lavery, R. Maher, D. Millar, B. Thomsen, P. Bayvel, and S. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol. |

12. | X. Fang, C. Yang, and F. Zhang, “Time-domain maximum-likelihood channel estimation for PDM CO-OFDM systems,” IEEE Photon. Technol. Lett. |

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

14. | B. Inan, S. Randel, S. Jansen, A. Lobato, S. Adhikari, and N. Hanik, “Pilot-tone-based nonlinearity compensation for optical OFDM systems,” in |

15. | V. Syrjala, M. Valkama, Y. Zou, N. Tchamov, and J. Rinne, “On OFDM link performance under receiver phase noise with arbitrary spectral shape,” in |

16. | N. Cvijetic, M. Huang, E. Ip, Y. Shao, Y. Huang, M. Cvijetic, and T. Wang, “Coherent 40Gb/s OFDMA-PON for long-reach (100+ km) high-split ratio (> 1: 64) optical access/metro networks,” in |

17. | ITU-T Recommendation G.975. 1, Appendix 1.9. |

**OCIS Codes**

(060.1660) Fiber optics and optical communications : Coherent communications

(060.4250) Fiber optics and optical communications : Networks

(060.4510) Fiber optics and optical communications : Optical communications

**ToC Category:**

Optical Communications

**History**

Original Manuscript: January 2, 2014

Revised Manuscript: February 27, 2014

Manuscript Accepted: February 27, 2014

Published: March 10, 2014

**Citation**

Yue Liu, Chuanchuan Yang, Feng Yang, and Hongbin Li, "The application of cost-effective lasers in coherent UDWDM-OFDM-PON aided by effective phase noise suppression methods," Opt. Express **22**, 6276-6286 (2014)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-6-6276

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### References

- E. Wong, “Next-generation broadband access networks and technologies,” J. Lightwave Technol. 30(4), 597–608 (2012). [CrossRef]
- X. Yi, W. Shieh, Y. Ma, “Phase noise effects on high spectral efficiency coherent optical OFDM transmission,” J. Lightwave Technol. 26(10), 1309–1316 (2008). [CrossRef]
- D. Lavery, M. Paskov, and S. Savory, “Spectral shaping for mitigating backreflections in a bidirectional 10 Gbit/s coherent WDM-PON,” in Proc. OFC, paper. OM2A.6 (2013).
- X. Pang, A. Lebedev, J. J. Vegas Olmos, I. T. Monroy, M. Beltran, and R. Llorente, “Performance evaluation for DFB and VCSEL-based 60 GHz radio-over-fiber system,” In Proc. ONDM, paper. 252–256 (2013).
- R. Gaudino, V. Curri, G. Bosco, G. Rizzelli, A. Nespola, D. Zeolla, S. Straullu, S. Capriata, and P. Solina, “On the use of DFB Lasers for Coherent PON,” in Proc. OFC, paper. OTh4G.1 (2012).
- E. Wong, M. Mueller, M. C. Amann, “Characterization of energy-efficient and colorless ONUs for future TWDM-PONs,” Opt. Express 21(18), 20747–20761 (2013). [CrossRef] [PubMed]
- C. Yang, F. Yang, 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]
- C. Yang, F. Yang, Z. Wang, “Phase noise suppression for coherent optical block transmission systems: A unified framework,” Opt. Express 19(18), 17013–17020 (2011). [CrossRef] [PubMed]
- C. Zhao, C. Yang, F. Yang, F. Zhang, Z. Chen, “A CO-OFDM system with almost blind phase noise suppression,” IEEE Photon. Technol. Lett. 25(17), 1723–1726 (2013). [CrossRef]
- S. 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 Proc. OFC, paper. PDP15 (2007).
- D. Lavery, R. Maher, D. Millar, B. Thomsen, P. Bayvel, S. Savory, “Digital coherent receivers for long-reach optical access networks,” J. Lightwave Technol. 31(4), 609–620 (2013). [CrossRef]
- X. Fang, C. Yang, F. Zhang, “Time-domain maximum-likelihood channel estimation for PDM CO-OFDM systems,” IEEE Photon. Technol. Lett. 25(6), 619–622 (2013). [CrossRef]
- X. Liu, F. Buchali, “Intra-symbol frequency-domain averaging based channel estimation for coherent optical OFDM,” Opt. Express 16(26), 21944–21957 (2008). [CrossRef] [PubMed]
- B. Inan, S. Randel, S. Jansen, A. Lobato, S. Adhikari, and N. Hanik, “Pilot-tone-based nonlinearity compensation for optical OFDM systems,” in Proc. ECOC 2010, paper. Tu.4.A.6 (2010).
- V. Syrjala, M. Valkama, Y. Zou, N. Tchamov, and J. Rinne, “On OFDM link performance under receiver phase noise with arbitrary spectral shape,” in Proc. WCNC, 1948–1953 (2011).
- N. Cvijetic, M. Huang, E. Ip, Y. Shao, Y. Huang, M. Cvijetic, and T. Wang, “Coherent 40Gb/s OFDMA-PON for long-reach (100+ km) high-split ratio (> 1: 64) optical access/metro networks,” in Proc. OFC, paper. OW4B.8 (2012).
- ITU-T Recommendation G.975. 1, Appendix 1.9.

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