## SSII cancellation in an EAM-based OFDM-IMDD transmission system employing a novel dynamic chirp model |

Optics Express, Vol. 21, Issue 1, pp. 533-543 (2013)

http://dx.doi.org/10.1364/OE.21.000533

Acrobat PDF (1766 KB)

### Abstract

We develop a novel subcarrier-to-subcarrier intermixing interference (SSII) cancellation technique to estimate and eliminate SSII. For the first time, the SSII cancellation technique is experimentally demonstrated in an electro-absorption modulator- (EAM-) based intensity-modulation-direct-detection (IMDD) multi-band OFDM transmission system. Since the characteristics of SSII are seriously affected by the chirp parameter, a simple constant chirp model, we found, cannot effectively remove the SSII. Therefore, assuming that the chirp parameter linearly depends on the optical power, a novel dynamic chirp model is developed to obtain better estimation and cancellation of SSII. Compared with 23.6% SSII cancellation by the constant chirp model, our experimental results show that incorporating the dynamic chirp model into the SSII cancellation technique can achieve up to 74.4% SSII cancellation and 2.8-dB sensitivity improvement in a 32.25-Gbps OFDM system over 100-km uncompensated standard single-mode fiber.

© 2013 OSA

## 1. Introduction

1. T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE **94**(5), 911–934 (2006). [CrossRef]

1. T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE **94**(5), 911–934 (2006). [CrossRef]

6. D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol. **25**(3), 685–693 (2007). [CrossRef]

11. W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. **27**(24), 5723–5735 (2009). [CrossRef]

12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express **20**(23), 25774–25789 (2012). [CrossRef] [PubMed]

13. D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express **18**(26), 27758–27763 (2010). [CrossRef] [PubMed]

## 2. SSII theory

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express **19**(18), 17546–17556 (2011). [CrossRef] [PubMed]

13. D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express **18**(26), 27758–27763 (2010). [CrossRef] [PubMed]

10. C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. **36**(2), 151–153 (2011). [CrossRef] [PubMed]

*V*is the modulation voltage,

*V*is the bias voltage,

_{b}*N*is the OFDM subcarrier number,

*v*is the encoded complex information of the

_{n}*n*subcarrier, and

^{th}*E*can be approximated aswhere

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express **19**(18), 17546–17556 (2011). [CrossRef] [PubMed]

10. C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. **36**(2), 151–153 (2011). [CrossRef] [PubMed]

*SSII*in this work. From the SSII theory, not only the chirp parameters of

_{T}*v*are necessary to calculate the SSII. Although the transmitted data are never exactly known at the receiver, the received data after hard decision are used as

_{n}*v*to estimate SSII, and an iterative process could be employed to improve the accuracy of SSII estimation.

_{n}## 3. SSII cancellation technique

*v*to calculate the SSII based on the SSII theory, and the calculated result can be further feedback to carry out the SSII cancellation. Since the SSII of each subcarrier will be calculated individually, the SSII cancellation is performed after the FFT. Then the OFDM data after the SSII cancellation are demodulated again, and the iterative process would get the more correct detected data.

_{n}*v*by the detected data,

_{n}*v*

_{n}^{(}

^{i}^{-1)}, where the superscript,

*i*, denotes the number of iteration. The detail of the block of SSII calculation of Fig. 1 is shown in Fig. 2(a) , and SSII calculation is composed of three parts, transmitted signal reconstruction, emulated SSII calculation, and calculated SSII modification. In the block of the transmitted signal reconstruction, the detected OFDM data

*v*

_{n}^{(}

^{i}^{-1)}is multiplied with the frequency response of optical transmitter,

*X*

_{1}

^{(}

^{i}^{)}of the OFDM signal can be reconstructed, and then be sent to the block of emulated SSII calculation.

7. D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express **19**(18), 17546–17556 (2011). [CrossRef] [PubMed]

12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express **20**(23), 25774–25789 (2012). [CrossRef] [PubMed]

## 4. Experimental set-up and results

**19**(18), 17546–17556 (2011). [CrossRef] [PubMed]

^{®}AWG7122) using Matlab

^{®}programs. The signal processing of the OFDM transmitter consists of serial-to-parallel conversion, QAM symbol encoding, inverse fast Fourier transform (IFFT), cyclic prefix (CP) insertion, and DAC. The sampling rate and DAC resolution of the AWG are 12 GS/s and 8 bits, respectively. Other detailed parameters of generated OFDM signals include the FFT size of 512, the CP size of 8. The channel-1 OFDM signal is located in the 1st passband, and it consists of 23.4375-MSym/s 64-QAM and QPSK symbols, which are encoded at the 2nd-124th and 125th-164th subcarriers, respectively. Thus, the channel-1 signal contains 163 subcarriers with 3.82-GHz bandwidth, yielding a total data rate of 19.1718 Gbps. The channel-2 OFDM signal of the same symbol rate but with 32-QAM and QPSK formats are encoded at the 2nd-44th and 45th-76th subcarriers, and then it is up-converted to 7.382 GHz, which is equivalent to the frequency of the 315th subcarrier. Thus, the up-converted OFDM signal occupies the 239th-313rd and 317th-391st subcarriers located in the 2nd passband, and it contains 150 subcarriers with 3.5-GHz bandwidth. Notably, since the channel number of the AWG is only 2, this up-converted signal is used to emulate a 13.0781-Gbps signal which should be realized by independent I- and Q-channels in practical. Through a power coupler, both OFDM bands are then combined, and a total data rate of 32.25 Gbps is achieved. The insets (a)-(d) of Fig. 3 exhibit the electrical spectra of the channel-1 signal, the channel-2 signal, and the up-converted channel-2 signal, and the combined 2-band signal, respectively. The combined OFDM signal is then sent to the EAM to generate an optical DSB OFDM signal. After 100 km of SSMF transmission and direct-detection, the received electrical signal is captured by a digital oscilloscope (Tektronix® DPO 71254) with a 50-GS/s sampling rate and a 3-dB bandwidth of 12.5 GHz, and the spectrum is shown in the inset (e) of Fig. 3. Due to the limited buffer size of oscilloscope, 1000 OFDM symbols with 1.707 Megabits are captured. An off-line Matlab

^{®}DSP program is used to demodulate the OFDM signals and the demodulation process includes synchronization, FFT, one-tap equalization, and QAM symbol decoding. Lastly, BERs are counted by bit-by-bit comparison between the detected data and transmitted data. In order to accurately estimate the required parameters of SSII cancellation, enough training symbols must be adopted in the beginning, and 50 training symbols are adopted in this work. Thus, SSII can be calculated and cancelled based on SSII cancellation technique as shown Fig. 1. Moreover, although the iterative process is expected to reduce the influence of the decision errors of

*v*

_{n}^{(}

^{i}^{-1)}on estimating SSII, our experimental results do not show further improvement provided by iteration, so that the SSII cancellation technique is applied without iteration throughout this work. The reason would be the SSII is contributed by a lot of subcarriers, and a few decision errors may not affect the estimation much around the BER of interest [12

12. C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express **20**(23), 25774–25789 (2012). [CrossRef] [PubMed]

**20**(23), 25774–25789 (2012). [CrossRef] [PubMed]

15. F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol. **11**(12), 1937–1940 (1993). [CrossRef]

*n*

^{th}subcarrier, if the received signal power, the noise power and the SSII power without the SSII cancellation are denoted as

*P*(

_{S}*n*),

*P*(

_{N}*n*) and

*P*

_{SSII}_{,}

*(*

_{W}*n*), respectively, the corresponding SNR could be written as Γ

*(*

_{W}*n*) =

*P*(

_{S}*n*)/[

*P*

_{SSII}_{,}

*(*

_{W}*n*) +

*P*(

_{N}*n*)]. Since we assume no SSII for the case of the upper bound, its SNR will be Γ

*(*

_{U}*n*) =

*P*(

_{S}*n*)/

*P*(

_{N}*n*). Furthermore, the power of residual SSII after SSII cancellation is set as

*P*

_{SSII}_{,}

*(*

_{C}*n*) or

*P*

_{SSII}_{,}

*(*

_{D}*n*), where the subscript

*C*or

*D*indicates the cancellation based on the constant chirp model or the dynamic chirp model. Hence, the SNR after the SSII cancellation will be Γ

*(*

_{C}*n*) =

*P*(

_{S}*n*)/[

*P*

_{SSII}_{,}

*(*

_{C}*n*) +

*P*(

_{N}*n*)] or Γ

*(*

_{D}*n*) =

*P*(

_{S}*n*)/[

*P*

_{SSII}_{,}

*(*

_{D}*n*) +

*P*(

_{N}*n*)]. Consequently, the power of SSII could be estimated by the SNRs and the signal power,where the subscript

*P*

_{SSII}_{,}

*(*

_{C}*n*) =

*P*

_{SSII}_{,}

*(*

_{W}*n*)–

*P*

_{SSII}_{,}

*(*

_{C}*n*) or Δ

*P*

_{SSII}_{,}

*(*

_{D}*n*) =

*P*

_{SSII}_{,}

*(*

_{W}*n*)–

*P*

_{SSII}_{,}

*(*

_{D}*n*). Then the average amount of SSII elimination in percentage can be written asIf the constant chirp is considered, the proposed SSII cancellation technique can only achieve 23.6% SSII elimination. However, employing the new dynamic chirp model, 74.4% SSII elimination can be achieved which demonstrates a considerable effectiveness of the new model.

^{−3}(the FEC limit) of cases (ii), (iii) and (iv) can be obtained at the received powers of –8.4 dBm, –10.2 dBm, and –11.2 dBm. Accordingly, after using the SSII cancellation technique with considering the constant and dynamic chirp parameters, the receiver sensitivities are improved by 1.8 dB and 2.8 dB, respectively. In addition, assuming the full knowledge of the transmitted data is known and used for the SSII estimation and cancellation, the BER performance with such ideal SSII cancellation is also plotted as the case (v) in Fig. 7(a). Compared with the ideal SSII cancellation, the proposed SSII cancellation without iteration almost shows identical performance, and therefore, the iteration process can be dropped in this experiment. Moreover, corresponding to the cases (ii) and (iv) at the received power of –6 dBm, the constellations of the 32-QAM and QPSK in the 2nd passband before and after SSII cancellation are shown in Fig. 7(b) for comparison.

## 5. Conclusion

**19**(18), 17546–17556 (2011). [CrossRef] [PubMed]

## References and links

1. | T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE |

2. | G. Talli, C. W. Chow, E. M. MacHale, C. Antony, R. Davey, P. D. Townsend, T. De Ridder, X. Z. Qiu, P. Ossieur, H. G. Krimmel, D. W. Smith, I. Lealman, A. Poustie, S. Randel, and H. Rohde, “Long reach passive optical networks,” in |

3. | R. Lin, “Next generation PON in emerging networks,” in |

4. | R. P. Davey, D. B. Grossman, M. Rasztovits-Wiech, D. B. Payne, D. Nesset, A. E. Kelly, A. Rafel, S. Appathurai, and S. H. Yang, “Long-reach passive optical networks,” J. Lightwave Technol. |

5. | K. Y. Cho, K. Tanaka, T. Sano, S. P. Jung, J. H. Chang, Y. Takushima, A. Agata, Y. Horiuchi, M. Suzuki, and Y. C. Chung, “Long-reach coherent WDM PON employing self-polarization-stabilization technique,” J. Lightwave Technol. |

6. | D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol. |

7. | D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express |

8. | D. Z. Hsu, C. C. Wei, H. Y. Chen, Y. C. Lu, and J. Chen, “A 40-Gbps OFDM LR-PON system over 100-km fiber employing an economical 10-GHz-based transceiver,” in |

9. | A. Gharba, P. Chanclou, M. Ouzzif, J. L. Masson, L. A. Neto, R. Xia, N. Genay, B. Charbonnier, M. Hélard, E. Grard, and V. Rodrigues, “Optical transmission performance for DML considering laser chirp and fiber dispersion using AMOOFDM,” in |

10. | C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett. |

11. | W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol. |

12. | C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express |

13. | D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express |

14. | E. O. Brigham, |

15. | F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol. |

**OCIS Codes**

(060.0060) Fiber optics and optical communications : Fiber optics and optical communications

(060.2330) Fiber optics and optical communications : Fiber optics communications

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: November 1, 2012

Revised Manuscript: December 14, 2012

Manuscript Accepted: December 21, 2012

Published: January 7, 2013

**Citation**

Dar-Zu Hsu, Chia-Chien Wei, Hsing-Yu Chen, Yi-Cheng Lu, Cih-Yuan Song, Chih-Chieh Yang, and Jyehong Chen, "SSII cancellation in an EAM-based OFDM-IMDD transmission system employing a novel dynamic chirp model," Opt. Express **21**, 533-543 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-1-533

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### References

- T. Koonen, “Fiber to the home/fiber to the premises: what, where, and when?” Proc. IEEE94(5), 911–934 (2006). [CrossRef]
- G. Talli, C. W. Chow, E. M. MacHale, C. Antony, R. Davey, P. D. Townsend, T. De Ridder, X. Z. Qiu, P. Ossieur, H. G. Krimmel, D. W. Smith, I. Lealman, A. Poustie, S. Randel, and H. Rohde, “Long reach passive optical networks,” in The 20th Annual Meeting of the IEEE Lasers and Electro-Optics Society, 2007. LEOS 2007, (IEEE-LEOS, 2007), pp. 868–869.
- R. Lin, “Next generation PON in emerging networks,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America (2008), paper OWH1.
- R. P. Davey, D. B. Grossman, M. Rasztovits-Wiech, D. B. Payne, D. Nesset, A. E. Kelly, A. Rafel, S. Appathurai, and S. H. Yang, “Long-reach passive optical networks,” J. Lightwave Technol.27(3), 273–291 (2009). [CrossRef]
- K. Y. Cho, K. Tanaka, T. Sano, S. P. Jung, J. H. Chang, Y. Takushima, A. Agata, Y. Horiuchi, M. Suzuki, and Y. C. Chung, “Long-reach coherent WDM PON employing self-polarization-stabilization technique,” J. Lightwave Technol.29(4), 456–462 (2011). [CrossRef]
- D. Shea and J. Mitchell, “A 10 Gb/s 1024-way-split 100-km long-reach optical-access network,” J. Lightwave Technol.25(3), 685–693 (2007). [CrossRef]
- D. Z. Hsu, C. C. Wei, H. Y. Chen, W. Y. Li, and J. Chen, “Cost-effective 33-Gbps intensity modulation direct detection multi-band OFDM LR-PON system employing a 10-GHz-based transceiver,” Opt. Express19(18), 17546–17556 (2011). [CrossRef] [PubMed]
- D. Z. Hsu, C. C. Wei, H. Y. Chen, Y. C. Lu, and J. Chen, “A 40-Gbps OFDM LR-PON system over 100-km fiber employing an economical 10-GHz-based transceiver,” in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2012), paper OW4B.
- A. Gharba, P. Chanclou, M. Ouzzif, J. L. Masson, L. A. Neto, R. Xia, N. Genay, B. Charbonnier, M. Hélard, E. Grard, and V. Rodrigues, “Optical transmission performance for DML considering laser chirp and fiber dispersion using AMOOFDM,” in 2010 International Congress on Ultra Modern Telecommunications and Control Systems and Workshops (ICUMT) (2010), pp. 1022–1026.
- C. C. Wei, “Small-signal analysis of OOFDM signal transmission with directly modulated laser and direct detection,” Opt. Lett.36(2), 151–153 (2011). [CrossRef] [PubMed]
- W. R. Peng, B. Zhang, K. M. Feng, X. Wu, A. E. Willner, and S. Chi, “Spectrally efficient direct-detected OFDM transmission incorporating a tunable frequency gap and an iterative detection techniques,” J. Lightwave Technol.27(24), 5723–5735 (2009). [CrossRef]
- C. C. Wei, “Analysis and iterative equalization of transient and adiabatic chirp effects in DML-based OFDM transmission systems,” Opt. Express20(23), 25774–25789 (2012). [CrossRef] [PubMed]
- D. Z. Hsu, C. C. Wei, H. Y. Chen, J. Chen, M. C. Yuang, S. H. Lin, and W. Y. Li, “21 Gb/s after 100 km OFDM long-reach PON transmission using a cost-effective electro-absorption modulator,” Opt. Express18(26), 27758–27763 (2010). [CrossRef] [PubMed]
- E. O. Brigham, Fast Fourier Transform and Its Applications, 1st ed. (Wiley, New York, 1997).
- F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol.11(12), 1937–1940 (1993). [CrossRef]

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