## Demonstration of DFT-spread 256QAM-OFDM signal transmission with cost-effective directly modulated laser |

Optics Express, Vol. 22, Issue 7, pp. 8742-8748 (2014)

http://dx.doi.org/10.1364/OE.22.008742

Acrobat PDF (3240 KB)

### Abstract

We experimentally demonstrated a 256-ary quadrature amplitude modulation (256QAM) direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) transmission system utilizing a cost-effective directly modulated laser (DML). Intra-symbol frequency-domain averaging (ISFA) is applied to suppress in-band noise while the channel response estimation and Discrete Fourier Transform-spread (DFT-spread) is used to reduce the peak-to-average power ratio (PAPR) of the transmitted OFDM signal. The bit-error ratio (BER) of 15-Gbit/s 256QAM-OFDM signal has been measured after 20-km SSMF transmission that is less than 7% forward-error-correction (FEC) threshold of 3.8 × 10^{−3} as the launch power into fiber is set at 6dBm. For 11.85-Gbit/s 256QAM-OFDM signal, with the aid of ISFA-based channel estimation and PAPR reduction enabled by DFT-spread, the BER after 20-km SSMF transmission can be improved from 6.4 × 10^{−3} to 6.8 × 10^{−4} when the received optical power is −6dBm.

© 2014 Optical Society of America

## 1. Introduction

14. Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. **22**(16), 1250–1252 (2010). [CrossRef]

4. H. Yang, S. C. J. Lee, E. Tangdiongga, C. Okonkwo, H. P. A. van den Boom, F. Breyer, S. Randel, and A. M. J. Koonen, “47.4 Gb/s transmission over 100 m graded-index plastic optical ﬁber based on rate-adaptive discrete multi-tone modulation,” J. Lightwave Technol. **28**(4), 352–359 (2010). [CrossRef]

15. F. J. Effenberger, “The XG-PON System: Cost Effective 10 Gb/s Access,” J. Lightwave Technol. **29**(4), 403–409 (2011). [CrossRef]

6. R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Experimental demonstration of record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM transmission over 25 km SMF in a simple EML-based IMDD system,” Opt. Express **20**(18), 20666–20679 (2012). [CrossRef] [PubMed]

15. F. J. Effenberger, “The XG-PON System: Cost Effective 10 Gb/s Access,” J. Lightwave Technol. **29**(4), 403–409 (2011). [CrossRef]

3. X. Q. Jin, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express **17**(22), 20484–20493 (2009). [CrossRef] [PubMed]

3. X. Q. Jin, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express **17**(22), 20484–20493 (2009). [CrossRef] [PubMed]

9. J. Silva, A. Cartaxo, and M. Segatto, “A PAPR reduction technique based on a constant envelope OFDM approach for ﬁber nonlinearity mitigation in optical direct-detection systems,” IEEE/OSA J. Opt. Commun. Netw. **4**(4), 296–303 (2012). [CrossRef]

14. Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. **22**(16), 1250–1252 (2010). [CrossRef]

^{−3}. The intra-symbol frequency-domain averaging (ISFA) [12

12. Q. Yang, N. Kaneda, X. Liu, and W. Shieh, “Demonstration of frequency-domain averaging based channel estimation for 40 Gb/s CO-OFDM with high PMD,” IEEE Photon. Technol. Lett. **21**(20), 1544–1546 (2009). [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]

14. Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. **22**(16), 1250–1252 (2010). [CrossRef]

^{−3}to 6.8 × 10

^{−4}when the received optical power is −6dBm.

## 2. Simulation

### 2.1 ISFA based channel estimation

12. Q. Yang, N. Kaneda, X. Liu, and W. Shieh, “Demonstration of frequency-domain averaging based channel estimation for 40 Gb/s CO-OFDM with high PMD,” IEEE Photon. Technol. Lett. **21**(20), 1544–1546 (2009). [CrossRef]

12. Q. Yang, N. Kaneda, X. Liu, and W. Shieh, “Demonstration of frequency-domain averaging based channel estimation for 40 Gb/s CO-OFDM with high PMD,” IEEE Photon. Technol. Lett. **21**(20), 1544–1546 (2009). [CrossRef]

**21**(20), 1544–1546 (2009). [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]

### 2.2 PAPR reduction with DFT-spread

*k*signal-carrying subcarriers in the positive frequency bins are firstly processed by

*k*-point DFT and thus the signal length becomes

*N/2*after zero padding. Afterward the complex conjugation operation is implemented to satisfy the Hermitian symmetry and the result signal with the length of

*N*is mapped from frequency domain to time domain by

*N*-point IDFT. The second

*N*-point DFT operation in the receiver is similar to the conventional OFDM scheme. We evaluate the PAPR performance by complementary cumulative distribution function (CCDF). CCDF denotes a probability distribution of the PAPR of current OFDM symbol is over a certain threshold. Figure 2(b) gives the calculated CCDF curves for traditional OFDM signal and DFT-spread OFDM signal. The PAPR of DFT-spread OFDM signal outperforms that of traditional OFDM signal. Accordingly, 2.5-dB PAPR improvement is attained at the probability of 1 × 10

^{−4}.

## 3. Experimental setup

^{®}and then uploaded into an arbitrary waveform generator (AWG, Tektronix 7122B) with 4-GSa/s sample rate. Here the FFT size for electrical OFDM generation is 256. 100 subcarriers in the positive frequency bins are used to convey data and other 100 subcarriers in the negative frequency are filled with Hermitian symmetric data to generate real value OFDM signal, the first subcarrier is set to zero for DC-bias and the rest 55 null subcarriers at the edge are reserved for oversampling. The subcarrier symbol rate is 15.625MSa/s without considering CP. All the 100 information-bearing subcarriers in the positive frequency bins are mapped via 256QAM. A 14-sample CP is added to the 256 samples and thus each OFDM symbol contains 270 samples. One TS is inserted into every 160 OFDM symbols in order to realize time synchronization and channel estimation. The total bit rate is 11.85Gb/s (4 × 100/270 × 8Gb/s≈11.85Gb/s) and the signal bandwidth is 1.56GHz (100/256 × 4GHz≈1.56GHz). A LPF is used after digital to analog convertor (DAC) to suppress residual alias spectrum and then the OFDM signal is amplified to 2.4V (peak-to-peak) by an electrical amplifier (EA) before injected into a distributed feedback (DFB) based DML. For optical OFDM modulation, the DML with 10-GHz bandwidth and 20-MHz linewidth is biased at 52mA to produce 6-dBm average output power. Since the bandwidth of OFDM signal is narrow and the optical fiber transmission distance is short, intensity noise converted form phase noise induced by the CD in fiber link [10

10. W.-R. Peng, “Analysis of laser phase noise effect in direct-detection optical OFDM transmission,” J. Lightwave Technol. **28**(17), 2526–2536 (2010). [CrossRef]

16. S. Yamamoto, N. Edagawa, H. Taga, Y. Yoshida, and H. Wakabayashi, “Analysis of laser phase noise to intensity noise conversion by chromatic dispersion in intensity modulation and direct detection optical-fiber transmission,” J. Lightwave Technol. **8**(11), 1716–1722 (1990). [CrossRef]

11. Z. Cao, J. Yu, W. Wang, L. Chen, and Z. Dong, “Direct-detection optical OFDM transmission system without frequency guard band,” IEEE Photon. Technol. Lett. **22**(11), 736–738 (2010). [CrossRef]

## 4. Experimental results and discussions

*H*as a function of the index of modulated subcarriers without and with ISFA, respectively. The received optical power is maintained at −9dBm and the adopted ISFA tap number is 7. Evidently, the ISFA can significantly make both amplitude and phase ñuctuations to be smoother. In order to find the optimized ISFA taps, Fig. 4(c) shows the measured Q-factor versus the number of ISFA taps. The Q-factor can be derived from the BER and DFT-spread is not adopted. The measurements are given with the received optical power of −6, −9 and −11dBm in both optical back-to-back (OBTB) and after 20-km SMF. The optical power launched into fiber is maintained at 6dBm and the received optical power is adjusted by the attenuator. It can be seen that the optimal ISFA taps should be 5 and in the following discussion the ISFA taps are fixed at 5. Figure 4(d) gives the measured Q-factor versus the launch power into 20-km SSMF. The received optical power is maintained at −6dBm. Both ISFA and DFT-spread are adopted. It can be seen that the optimal launch power into fiber is 6dBm. When the launch power is larger than 6dBm, the system performance is degraded due to the enhanced nonlinear fiber transmission effects. The received 256QAM constellations at the launch power of −4, 6 and 10dBm are inserted in Fig. 4(d), respectively. The constellation at the launch power of 6dBm exhibits the best performance.

*w*and

*w/o*represent the implementation with and without DFT-spread, respectively. With the aid of ISFA based channel estimation and PAPR reduction with DFT-spread, the BER of 11.85-Gbit/s 256QAM-OFDM signal after 20-km SSMF transmission can be improved from 6.4 × 10

^{−3}to 6.8 × 10

^{−4}with −6dBm received optical power. Nonlinearity during the electrical-to-optical conversion by the DML driven by the signal with high voltage after EA cannot be avoided and it leads to the error floor. Moreover, the significant receiver sensitivity improvement due to the DFT-spread in the OBTB case is quite similar to that after 20-km fiber transmission. When the launch power into fiber is only 6-dBm, the nonlinear noise for fiber transmission compared the transmitter nonlinearity (mainly including DML modulation nonlinearity and EA nonlinearity) can be neglected. Which means the DFT-spread is mainly used to suppress the nonlinear noise from transmitter. If the launch power is increased further, the fiber nonlinearity will become more and more obvious. At that situation the DFT-spread can suppress not only the nonlinearity in transmitter but also the nonlinearity in optical fiber. Figure 5(b) shows the BER of the 11.85-Gbit/s 256QAM-OFDM signal versus fiber span at 6-dBm launch power into fiber. The BER performance degrades with the increase of fiber span, but the BER is still no more than 7% HD-FEC threshold of 3.8 × 10

^{−3}after more than 35-km fiber transmission. Figure 5(c) shows the BER versus the raw bit rate of the DAC after 20-km SSMF transmission and the raw bit rate is obtained by adjusting the sample rate of AWG. The received optical power is maintained at −6dBm. The BER is below the 7% HD-FEC threshold of 3.8 × 10

^{−3}when the raw bit rate is no more than 15Gbit/s, and below the 20% soft-decision FEC (SD-FEC) threshold of 2.7 × 10

^{−2}[17] when the raw bit rate is no more than 28Gbit/s. The received 256QAM constellations at the bit rates of 6, 15 and 24Gbit/s are inserted in Fig. 5(c), respectively.

## 5. Conclusion

^{−4}with −6 dBm received optical power. After 20-km SSMF transmission, the BER of 15Git/s 256QAM-OFDM signal is less than 3.8 × 10

^{−3}. We believe this scheme is cost-effective and easy to be implemented for the next generation short-reach fiber-optics access system with high capacity and SE.

## References and links

1. | D. Qian, J. Yu, J. Hu, P. N. Ji, and T. Wang, “11.5Gb/s OFDM transmission over 640km SSMF using directly modulated laser,” in |

2. | R. P. Giddings, X. Q. Jin, and J. M. Tang, “First experimental demonstration of 6Gb/s real-time optical OFDM transceivers incorporating channel estimation and variable power loading,” Opt. Express |

3. | X. Q. Jin, R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express |

4. | H. Yang, S. C. J. Lee, E. Tangdiongga, C. Okonkwo, H. P. A. van den Boom, F. Breyer, S. Randel, and A. M. J. Koonen, “47.4 Gb/s transmission over 100 m graded-index plastic optical ﬁber based on rate-adaptive discrete multi-tone modulation,” J. Lightwave Technol. |

5. | M. F. Huang, J. Yu, D. Qian, N. Cvijetic, and G. K. Chang, “Lightwave centralized WDM-OFDM-PON network employing cost-effective directly modulated laser,” in |

6. | R. P. Giddings, E. Hugues-Salas, and J. M. Tang, “Experimental demonstration of record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM transmission over 25 km SMF in a simple EML-based IMDD system,” Opt. Express |

7. | M. Beltrán, Y. Shi, C. Okonkwo, R. Llorente, E. Tangdiongga, and T. Koonen, “In-home networks integrating high-capacity DMT data and DVB-T over large-core GI-POF,” Opt. Express |

8. | J. Yu, M. Huang, D. Qian, L. Chen, and G. K. Chang, “Centralized lightwave WDM-PON employing 16-QAM intensity modulated OFDM downstream and OOK modulated upstream signals,” IEEE Photon. Technol. Lett. |

9. | J. Silva, A. Cartaxo, and M. Segatto, “A PAPR reduction technique based on a constant envelope OFDM approach for ﬁber nonlinearity mitigation in optical direct-detection systems,” IEEE/OSA J. Opt. Commun. Netw. |

10. | W.-R. Peng, “Analysis of laser phase noise effect in direct-detection optical OFDM transmission,” J. Lightwave Technol. |

11. | Z. Cao, J. Yu, W. Wang, L. Chen, and Z. Dong, “Direct-detection optical OFDM transmission system without frequency guard band,” IEEE Photon. Technol. Lett. |

12. | Q. Yang, N. Kaneda, X. Liu, and W. Shieh, “Demonstration of frequency-domain averaging based channel estimation for 40 Gb/s CO-OFDM with high PMD,” 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. | Y. Tang, W. Shieh, and B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. |

15. | F. J. Effenberger, “The XG-PON System: Cost Effective 10 Gb/s Access,” J. Lightwave Technol. |

16. | S. Yamamoto, N. Edagawa, H. Taga, Y. Yoshida, and H. Wakabayashi, “Analysis of laser phase noise to intensity noise conversion by chromatic dispersion in intensity modulation and direct detection optical-fiber transmission,” J. Lightwave Technol. |

17. | D. Chang, F. Yu, Z. Xiao, N. Stojanovic, F. N. Hauske, Y. Cai, C. Xie, L. Li, X. Xu, and Q. Xiong, “LDPC convolutional codes using layered decoding algorithm for high speed coherent optical transmission,” OFC 2012, OW1H.4 (2012). |

**OCIS Codes**

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

(060.2360) Fiber optics and optical communications : Fiber optics links and subsystems

**ToC Category:**

Optical Communications

**History**

Original Manuscript: January 23, 2014

Revised Manuscript: March 28, 2014

Manuscript Accepted: March 28, 2014

Published: April 4, 2014

**Citation**

Fan Li, Jianjun Yu, Yuan Fang, Ze Dong, Xinying Li, and Lin Chen, "Demonstration of DFT-spread 256QAM-OFDM signal transmission with cost-effective directly modulated laser," Opt. Express **22**, 8742-8748 (2014)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-7-8742

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### References

- D. Qian, J. Yu, J. Hu, P. N. Ji, and T. Wang, “11.5Gb/s OFDM transmission over 640km SSMF using directly modulated laser,” in Proc. ECOC 2008, paper Mo3E4 (2008).
- R. P. Giddings, X. Q. Jin, J. M. Tang, “First experimental demonstration of 6Gb/s real-time optical OFDM transceivers incorporating channel estimation and variable power loading,” Opt. Express 17(22), 19727–19738 (2009). [CrossRef] [PubMed]
- X. Q. Jin, R. P. Giddings, E. Hugues-Salas, J. M. Tang, “Real-time demonstration of 128-QAM-encoded optical OFDM transmission with a 5.25bit/s/Hz spectral efficiency in simple IMDD systems utilizing directly modulated DFB lasers,” Opt. Express 17(22), 20484–20493 (2009). [CrossRef] [PubMed]
- H. Yang, S. C. J. Lee, E. Tangdiongga, C. Okonkwo, H. P. A. van den Boom, F. Breyer, S. Randel, A. M. J. Koonen, “47.4 Gb/s transmission over 100 m graded-index plastic optical ﬁber based on rate-adaptive discrete multi-tone modulation,” J. Lightwave Technol. 28(4), 352–359 (2010). [CrossRef]
- M. F. Huang, J. Yu, D. Qian, N. Cvijetic, and G. K. Chang, “Lightwave centralized WDM-OFDM-PON network employing cost-effective directly modulated laser,” in Proc. OFC 2009, paper OMV5 (2009).
- R. P. Giddings, E. Hugues-Salas, J. M. Tang, “Experimental demonstration of record high 19.125 Gb/s real-time end-to-end dual-band optical OFDM transmission over 25 km SMF in a simple EML-based IMDD system,” Opt. Express 20(18), 20666–20679 (2012). [CrossRef] [PubMed]
- M. Beltrán, Y. Shi, C. Okonkwo, R. Llorente, E. Tangdiongga, T. Koonen, “In-home networks integrating high-capacity DMT data and DVB-T over large-core GI-POF,” Opt. Express 20(28), 29769–29775 (2012). [CrossRef] [PubMed]
- J. Yu, M. Huang, D. Qian, L. Chen, G. K. Chang, “Centralized lightwave WDM-PON employing 16-QAM intensity modulated OFDM downstream and OOK modulated upstream signals,” IEEE Photon. Technol. Lett. 20(18), 1545–1547 (2008). [CrossRef]
- J. Silva, A. Cartaxo, M. Segatto, “A PAPR reduction technique based on a constant envelope OFDM approach for ﬁber nonlinearity mitigation in optical direct-detection systems,” IEEE/OSA J. Opt. Commun. Netw. 4(4), 296–303 (2012). [CrossRef]
- W.-R. Peng, “Analysis of laser phase noise effect in direct-detection optical OFDM transmission,” J. Lightwave Technol. 28(17), 2526–2536 (2010). [CrossRef]
- Z. Cao, J. Yu, W. Wang, L. Chen, Z. Dong, “Direct-detection optical OFDM transmission system without frequency guard band,” IEEE Photon. Technol. Lett. 22(11), 736–738 (2010). [CrossRef]
- Q. Yang, N. Kaneda, X. Liu, W. Shieh, “Demonstration of frequency-domain averaging based channel estimation for 40 Gb/s CO-OFDM with high PMD,” IEEE Photon. Technol. Lett. 21(20), 1544–1546 (2009). [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]
- Y. Tang, W. Shieh, B. S. Krongold, “DFT-spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. 22(16), 1250–1252 (2010). [CrossRef]
- F. J. Effenberger, “The XG-PON System: Cost Effective 10 Gb/s Access,” J. Lightwave Technol. 29(4), 403–409 (2011). [CrossRef]
- S. Yamamoto, N. Edagawa, H. Taga, Y. Yoshida, H. Wakabayashi, “Analysis of laser phase noise to intensity noise conversion by chromatic dispersion in intensity modulation and direct detection optical-fiber transmission,” J. Lightwave Technol. 8(11), 1716–1722 (1990). [CrossRef]
- D. Chang, F. Yu, Z. Xiao, N. Stojanovic, F. N. Hauske, Y. Cai, C. Xie, L. Li, X. Xu, and Q. Xiong, “LDPC convolutional codes using layered decoding algorithm for high speed coherent optical transmission,” OFC 2012, OW1H.4 (2012).

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