## Digital non-data-aided symbol synchronization in optical coherent intradyne reception

Optics Express, Vol. 16, Issue 19, pp. 15097-15103 (2008)

http://dx.doi.org/10.1364/OE.16.015097

Acrobat PDF (177 KB)

### Abstract

Digital symbol synchronization of optical binary phase shift keying signals is experimentally demonstrated. Algorithm of timing error feedback is proposed for coherent intradyne reception. The feedback loop operates stable in the range of symbol rate, 9.7~0.3 GSymbol/s with 20 GSampling/s at relatively high bit error rate of ~2 × 10^{-2}.

© 2008 Optical Society of America

## 1. Introduction

2. S. J. Savory, G. Gavioli, R. I. Killey, and P. Bayvel, “Electronic compensation of chromatic dispersion using a digital coherent receiver,” Opt. Express **15**, 2120–2126 (2007). [CrossRef] [PubMed]

4. E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express **16**, 753–791 (2008). [CrossRef] [PubMed]

4. E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express **16**, 753–791 (2008). [CrossRef] [PubMed]

6. K. Kikuchi, “Electronic post-compensation for nonlinear phase fluctuations in a 1000 km 20 Gbit/s optical quadrature phase-shift keying transmission system using the digital coherent receiver,” Opt. Express **16**, 889–896 (2008). [CrossRef] [PubMed]

7. A. Leven, N. Kaneda, U. Koc, and K. Chen, “Frequency estimation in intradyne reception,” IEEE Photon. Technol. Lett. **19**, 366–368 (2007). [CrossRef]

8. T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noe, “Coherent optical communication: towards realtime systems at 40Gbit/s and beyond,” Opt. Express **16**, 866–872 (2008). [CrossRef] [PubMed]

## 2. Theory

7. A. Leven, N. Kaneda, U. Koc, and K. Chen, “Frequency estimation in intradyne reception,” IEEE Photon. Technol. Lett. **19**, 366–368 (2007). [CrossRef]

*x*(

*n*

*T*

*s*) where

*T*

_{S}is sample interval. The output signal samples equal to

*h*

_{n}(

*µ*) is interpolating FIR(finite impulse response) filter [9, 10

10. F. M. Gardner, “Interpolation in digital modems-part I: fundamentals,” IEEE Trans. Commun. **41**, 501–507 (1993). [CrossRef]

*N*=

*I*

_{1}+

*I*

_{2}+1 is a order of interpolating filter. We assumed two samples per symbol, and

*T*

_{1}=

*T*/2 where

*T*is symbol interval. Since

*T*

_{I}is not equal to

*T*

_{S}, the samples in the transmitter time reference have to be mapped onto the time scale of receiver as shown in Fig. 2.

*k*

*T*

_{I}+

*ε*

_{I}

*T*

_{I}represents the time scale at the transmitter time reference, and

*m*

_{k}

*T*

_{s}+

*µ*

_{k}

*T*

_{s}T is on the receiver time axis. From this, one has to compute corresponding basepoint index

*m*

_{k}and fractional interval

*µ*

_{k}.

*α*= 0.5) and cubic interpolator are applied and the results are compared in the processing. Farrow coefficients of each interpolator are given in [11

11. L. Erup, F. M. Gardner, and R. A. Harris, “Interpolation in digital modems-part II: implementation and performance,” IEEE Trans. Commun. **41**, 998–1008 (1993). [CrossRef]

*m*, and the corresponding fractional interval

_{k}*µ*

_{k}based on the output of timing processor in Fig. 1.

*y*(

*m*

_{k}

*T*

_{S}+

*µ*

_{k}

*T*

_{S})

^{2}[9, 11

11. L. Erup, F. M. Gardner, and R. A. Harris, “Interpolation in digital modems-part II: implementation and performance,” IEEE Trans. Commun. **41**, 998–1008 (1993). [CrossRef]

*m*

_{k}(

*k*= 2

*n*) and produces error signals at symbol rate. The error signal of Eq. (2) is further processed in a loop filter, and the output of loop filter is given as

*K*

_{P}and Ki are constants, and

*q*is a number of error signals in the summation. The output error signal of loop filter is used to adjust the control word (

*w*=

*T*

_{I}/

*T*

_{S}) of the timing processor

*w*(

*m*

_{2n+1})=

*w*(

*m*

_{2n})at every odd basepoint. Basepoint and fractional interval are recursively computed in the timing processor as followings [9].

*floor*[] denotes the nearest integer less than or equal to the number in the bracket, []

_{mod-1}is remainder after divided by 1. The algorithm can be applied to non-data-aided symbol synchronization for M-ary PSK signals.

## 3. Experiments and results

_{3}mach-zehnder modulator (MZM) for generating BPSK signals. The modulator is biased at its transmission null and two neighboring transmission maxima have 180° phase shift which is required for binary phase modulation. MZM introduces intensity dips in modulated optical signals at the time of data transition of two symbols. MZM-based optical BPSK or QPSK transmitter is preferred because of highly accurate phase modulation [12

12. A. H. Gnauck and P. J. Winzer, “Optical phase-shift-keying transmission,” J. Lightwave Technol. **23**, 115–130 (2005). [CrossRef]

^{11}-1 was used. 2-stage EDFA with variable attenuator was used for amplifying the optical signal and adjusting optical signal-to-noise ratio (OSNR). It was filtered with demux filter with a 3-dB bandwidth of 55 GHz.

15. D. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. **24**, 12–21 (2006). [CrossRef]

7. A. Leven, N. Kaneda, U. Koc, and K. Chen, “Frequency estimation in intradyne reception,” IEEE Photon. Technol. Lett. **19**, 366–368 (2007). [CrossRef]

*K*

_{P}and

*K*

_{i}were determined with the experimental data by trial and error. The choice of the constants was crucial for the stability of the feedback loop. When the constants went away from the optimal value, the error signal grew fast or the locking of loop was broken. The parameter

*q*determined the speed of convergence. If it was big, the summation in Eq. (3) was done with many error signals in long time, therefore the variance of the output parameters was slow. The constants could be used in all the experiments regardless of the experimental conditions if once they were determined. The same values of

*K*

_{P},

*K*

_{i}, and

*q*were used in all the following results.

*α*= 0.5. Fig. 4 shows the parameter response of digital symbol synchronization when the symbol rate was 10 GSymbol/s. Fig. 4(a) shows the variation of control word (

*w*) that settles to the number of 1. It was the expected value because

*T*

_{I}equals to

*T*

_{S}in this case. The magnitude of error signal was suppressed after ~250

*T*in Fig. 4(b). Time interval for the convergence of the parameters was mainly dependent on the number of q in Eq. (3), and

*q*was set to be 50 in all the results. Basepoint index

*m*

_{k}and fractional interval

*µ*

_{k}is shown in Fig. 4(c) and Fig. 4(d). Basepoint index continuously changed by 1 after the feedback had converged because control word was 1. We observed the value of

*µ*

_{k}varied continuously by ~0.5 in the measurement time of 100 µsec. It meant that the clock of PPG was different from the sampling clock of the oscilloscope by ~2.5 × 10

^{-7}in the ratio. Feedback loop was stable over the measurement time, though Fig. 4 shows the results in relatively small time interval to reveal the transient behavior of the parameters clearly.

*T*

_{I}= 0.99

*T*

_{S}at 10.1 GSymbol/s. The magnitude of error signal in Fig. 5(b) was nearly the same as that of Fig. 4(b). The parameters in Fig. 5(c) and Fig. 5(d) show repetitive responses, and the period was ~100

*T*

_{I}that agrees with

*T*

_{I}= 0.99

*T*

_{S}. There was no error detected after the feedback had converged.

*T*

_{I}after they had settled. The convergence time was increased to ~500

*T*. The convergence time could be reduced by decreasing the parameter of

*q*or optimally guessing the initial conditions. Moreover, convergence time of ~500

*T*was short enough compared with the time for stabilizing commercial optical transceivers. Bit error rate was 2.2×10

^{-2}in this measurement. The results demonstrated that the algorithm of digital symbol synchronization worked well even at high bit error rate with OSNR of 10 dB.

11. L. Erup, F. M. Gardner, and R. A. Harris, “Interpolation in digital modems-part II: implementation and performance,” IEEE Trans. Commun. **41**, 998–1008 (1993). [CrossRef]

## 4. Conclusions

## Acknowledgments

## References and links

1. | H. Sun, K. Wu, and K. Roberts, “Real-time measurements of a 40 Gb/s coherent system,” Opt. Express |

2. | S. J. Savory, G. Gavioli, R. I. Killey, and P. Bayvel, “Electronic compensation of chromatic dispersion using a digital coherent receiver,” Opt. Express |

3. | S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express |

4. | E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, “Coherent detection in optical fiber systems,” Opt. Express |

5. | K. Ho and J. M. Kahn, “Electronic compensation technique to mitigate nonlinear phase noise,” J. Lightwave Technol. |

6. | K. Kikuchi, “Electronic post-compensation for nonlinear phase fluctuations in a 1000 km 20 Gbit/s optical quadrature phase-shift keying transmission system using the digital coherent receiver,” Opt. Express |

7. | A. Leven, N. Kaneda, U. Koc, and K. Chen, “Frequency estimation in intradyne reception,” IEEE Photon. Technol. Lett. |

8. | T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noe, “Coherent optical communication: towards realtime systems at 40Gbit/s and beyond,” Opt. Express |

9. | H. Meyr, M. Moeneclaey, and S. A. Fechtel, |

10. | F. M. Gardner, “Interpolation in digital modems-part I: fundamentals,” IEEE Trans. Commun. |

11. | L. Erup, F. M. Gardner, and R. A. Harris, “Interpolation in digital modems-part II: implementation and performance,” IEEE Trans. Commun. |

12. | A. H. Gnauck and P. J. Winzer, “Optical phase-shift-keying transmission,” J. Lightwave Technol. |

13. | W. R. Leeb, “Realization of 90- and 180 degree hybrids for optical frequencies,” Arch. Elek. Ubertragung |

14. | L. G. Kazovsky, L. Curtis, W. C. Young, and N. K. Cheung, “All-fiber 90o optical hybrid for coherent communications,” App. Optics |

15. | D. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. |

**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: July 8, 2008

Revised Manuscript: August 12, 2008

Manuscript Accepted: September 6, 2008

Published: September 10, 2008

**Citation**

Sun Hyok Chang, Hwan Seok Chung, and Kwangjoon Kim, "Digital non-data-aided symbol synchronization in optical coherent intradyne reception," Opt. Express **16**, 15097-15103 (2008)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-19-15097

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### References

- H. Sun, K. Wu, and K. Roberts, "Real-time measurements of a 40 Gb/s coherent system," Opt. Express 16, 873-879 (2008).
- S. J. Savory, G. Gavioli, R. I. Killey, and P. Bayvel, "Electronic compensation of chromatic dispersion using a digital coherent receiver," Opt. Express 15, 2120-2126 (2007). [CrossRef] [PubMed]
- S. J. Savory, "Digital filters for coherent optical receivers," Opt. Express 16, 804-817 (2008). [CrossRef] [PubMed]
- E. Ip, A. P. T. Lau, D. J. F. Barros, and J. M. Kahn, "Coherent detection in optical fiber systems," Opt. Express 16, 753-791 (2008). [CrossRef] [PubMed]
- K. Ho and J. M. Kahn, "Electronic compensation technique to mitigate nonlinear phase noise," J. Lightwave Technol. 22, 779-783 (2004). [CrossRef]
- K. Kikuchi, "Electronic post-compensation for nonlinear phase fluctuations in a 1000 km 20 Gbit/s optical quadrature phase-shift keying transmission system using the digital coherent receiver," Opt. Express 16, 889-896 (2008). [CrossRef] [PubMed]
- A. Leven, N. Kaneda, U. Koc, and K. Chen, "Frequency estimation in intradyne reception," IEEE Photon. Technol. Lett. 19, 366-368 (2007). [CrossRef]
- T. Pfau, S. Hoffmann, O. Adamczyk, R. Peveling, V. Herath, M. Porrmann, and R. Noe, "Coherent optical communication: towards realtime systems at 40Gbit/s and beyond," Opt. Express 16, 866-872 (2008). [CrossRef] [PubMed]
- H. Meyr, M. Moeneclaey, and S. A. Fechtel, Digital communication receivers (John Wiley & Sons Inc, 1998).
- F. M. Gardner, "Interpolation in digital modems-part I: fundamentals," IEEE Trans. Commun. 41, 501-507 (1993). [CrossRef]
- L. Erup, F. M. Gardner, and R. A. Harris, "Interpolation in digital modems-part II: implementation and performance," IEEE Trans. Commun. 41, 998-1008 (1993). [CrossRef]
- A. H. Gnauck and P. J. Winzer, "Optical phase-shift-keying transmission," J. Lightwave Technol. 23, 115-130 (2005). [CrossRef]
- W. R. Leeb, "Realization of 90- and 180 degree hybrids for optical frequencies," Arch. Elek. Ubertragung 37, 203-206 (1983).
- L. G. Kazovsky, L. Curtis, W. C. Young, and N. K. Cheung, "All-fiber 90? optical hybrid for coherent communications," Appl. Opt. 26, 437-439 (1987). [CrossRef]
- D. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, "Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation," J. Lightwave Technol. 24, 12-21 (2006). [CrossRef]

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