## A novel transform domain processing based channel estimation method for OFDM radio-over-fiber systems |

Optics Express, Vol. 21, Issue 6, pp. 7478-7487 (2013)

http://dx.doi.org/10.1364/OE.21.007478

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### Abstract

In this paper, a transform domain processing (TDP) based channel estimation method for orthogonal frequency-division multiplexing (OFDM) Radio-over-Fiber (RoF) systems is proposed. Theoretically investigation shows that TDP can greatly reduce the number of required training symbols. An 8 x 4.65 Gb/s multi-user OFDM RoF system over 40 km fiber link and 60 GHz wireless link is experimentally demonstrated utilizing TDP scheme. Compared with conventional time domain averaging (TDA) scheme, the overhead can be reduced from several tens of training symbols to merely one symbol and the receiver sensitivity has been improved by 1.8 dB at BER of 3.8 x 10^{−3}. The calculated BER performance for 8 wireless users clearly validates the feasibility of this TDP-based channel estimation method.

© 2013 OSA

## 1. Introduction

1. J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems,” J. Lightwave Technol. **28**(16), 2376–2397 (2010). [CrossRef]

6. W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express **16**(2), 841–859 (2008). [CrossRef] [PubMed]

7. 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. Principle

### 2.1 Theoretical analysis of TDP-based channel estimation

_{T}is often set before searching

_{T}is small, it would lose some useful “low frequency” components in the transform domain and obtain an estimated channel function with distortion. If R

_{T}is very large, more noise would be added into the estimated channel function. Usually, it can be set to 0.9~0.98.

### 2.2 Comparison of three channel estimation methods

9. Q. Yang, S. Chen, Y. Ma, and W. Shieh, “Real-time reception of multi-gigabit coherent optical OFDM signals,” Opt. Express **17**(10), 7985–7992 (2009). [CrossRef] [PubMed]

9. Q. Yang, S. Chen, Y. Ma, and W. Shieh, “Real-time reception of multi-gigabit coherent optical OFDM signals,” Opt. Express **17**(10), 7985–7992 (2009). [CrossRef] [PubMed]

## 3. Experimental setup and results

11. T. Nakasyotani, H. Toda, T. Kuri, and K. Kitayama, “Wavelength-division-multiplexed millimeter-waveband radio-on-fiber system using a supercontinuum light source,” J. Lightwave Technol. **24**(1), 404–410 (2006). [CrossRef]

12. H. Toda, T. Yamashita, T. Kuri, and K. Kitayama, “Demultiplexing using an arrayed-waveguide grating for frequency-interleaved DWDM millimeter-wave radio-on-fiber systems,” J. Lightwave Technol. **21**(8), 1735–1741 (2003). [CrossRef]

13. L. Tao, J. Yu, Y. Fang, J. Zhang, Y. Shao, and N. Chi, “Analysis of noise spread in optical DFT-S OFDM systems,” J. Lightwave Technol. **30**(20), 3219–3225 (2012). [CrossRef]

15. 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]

16. L. Tao, J. Yu, Q. Yang, M. Luo, Z. He, Y. Shao, J. Zhang, and N. Chi, “Spectrally efficient localized carrier distribution scheme for multiple-user DFT-S OFDM RoF- PON wireless access systems,” Opt. Express **20**(28), 29665–29672 (2012). [CrossRef] [PubMed]

1. J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems,” J. Lightwave Technol. **28**(16), 2376–2397 (2010). [CrossRef]

^{5}bits.

^{−3}(7% FEC threshold) are observed respectively (the constellations are shown as the insets in Fig. 8). It can be also found that TDA-based scheme with only one TS performs worst. Therefore, TDP-based scheme only needs one TS for the initial channel estimation and revises the transfer function with the help of the lowpass filter in the transform domain. Furthermore, it is observed that the improvement is almost the same when changing the received optical power level. Because noise in the transmission system is the main factor that impacts the accuracy of channel estimation. The improvement would be similar when received power is changing in a low level range. If received optical power level is very high, the improvement would not be considerable, because TDA processing can also obtain accurate channel estimation with less training symbol. We have tested all 8 channels and the calculated BER of DFT-S OFDM signal for each wireless user after 40 km fiber link and 60 GHz wireless link is shown in Fig. 9 , and find that the BER for all channels are all below 3.8 x 10

^{−3}. This clearly shows that TDP-based scheme is feasible and it could reduce the overhead of DFT-S OFDM signal and improve transmission performance. It should be noted that there are some leakage in Fig. 7(d) when choosing signal and beat carrier for the nonperfect filter effect of WSS. The BER performance can be improved further when this leakage is suppressed by using the well designed bandpass filters.

## 4. Conclusion

^{−3}are observed respectively. The calculated BER of all 8 channels are below 3.8 x 10

^{−3}and this clearly shows the feasibility of TDP-based channel estimation.

## Acknowledgments

## References and links

1. | J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems,” J. Lightwave Technol. |

2. | Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, “Reduction of intersubcarrier interference and frequency-selective fading in OFDM-ROF systems,” J. Lightwave Technol. |

3. | G. K. Chang, Z. Jia, J. Yu, A. Chowdhury, T. Wang, and G. Ellinas, “Super-broadband optical wireless access technologies,” |

4. | B. Liu, X. Xin, L. Zhang, K. Zhao, and C. Yu, “Broad convergence of 32QAM-OFDM ROF and WDM-OFDM-PON system using an integrated modulator for bidirectional access networks,” |

5. | C. Wei, C. Lin, M. Chao, and W. Jiang, “Adaptively modulated OFDM RoF signals at 60 GHz over-long-reach 100-km transmission systems employing phase noise suppression,” IEEE Photon. Technol. Lett. |

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

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

8. | Y. Zhao and A. Huang, “A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing,” IEEE Vehicular Technology Conference, |

9. | Q. Yang, S. Chen, Y. Ma, and W. Shieh, “Real-time reception of multi-gigabit coherent optical OFDM signals,” Opt. Express |

10. | Q. Yang, N. Kaneda, X. Liu, S. Chandrasekhar, W. Shieh, and Y. Chen, “Real-Time coherent optical OFDM receiver at 2.5-GS/s for receiving a 54-Gb/s multi-band signal,” |

11. | T. Nakasyotani, H. Toda, T. Kuri, and K. Kitayama, “Wavelength-division-multiplexed millimeter-waveband radio-on-fiber system using a supercontinuum light source,” J. Lightwave Technol. |

12. | H. Toda, T. Yamashita, T. Kuri, and K. Kitayama, “Demultiplexing using an arrayed-waveguide grating for frequency-interleaved DWDM millimeter-wave radio-on-fiber systems,” J. Lightwave Technol. |

13. | L. Tao, J. Yu, Y. Fang, J. Zhang, Y. Shao, and N. Chi, “Analysis of noise spread in optical DFT-S OFDM systems,” J. Lightwave Technol. |

14. | Q. Yang, Z. He, Z. Yang, S. Yu, X. Yi, and W. Shieh, “Coherent optical DFT-spread OFDM transmission using orthogonal band multiplexing,” Opt. Express |

15. | Y. Tang, W. Shieh, and B. S. Krongold, “DFT-Spread OFDM for fiber nonlinearity mitigation,” IEEE Photon. Technol. Lett. |

16. | L. Tao, J. Yu, Q. Yang, M. Luo, Z. He, Y. Shao, J. Zhang, and N. Chi, “Spectrally efficient localized carrier distribution scheme for multiple-user DFT-S OFDM RoF- PON wireless access systems,” Opt. Express |

17. | J. Lee, F. Breyer, S. Randel, J. Zeng, F. Huijskens, H. P. van den Boom, A. M. Koonen, and N. Hanik, “24-Gb/s transmission over 730 m of multimode fiber by direct modulation of an 850-nm VCSEL using discrete multitone modulation,” |

**OCIS Codes**

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

(060.5625) Fiber optics and optical communications : Radio frequency photonics

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: December 3, 2012

Revised Manuscript: February 22, 2013

Manuscript Accepted: March 4, 2013

Published: March 19, 2013

**Citation**

Li Tao, Jianjun Yu, Qi Yang, Yufeng Shao, Junwen Zhang, and Nan Chi, "A novel transform domain processing based channel estimation method for OFDM radio-over-fiber systems," Opt. Express **21**, 7478-7487 (2013)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-6-7478

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### References

- J. Yu, G. K. Chang, Z. Jia, A. Chowdhury, M. F. Huang, H. C. Chien, Y. T. Hsueh, W. Jian, C. Liu, and Z. Dong, “Cost-effective optical millimeter technologies and field demonstrations for very high throughput wireless-over-fiber access systems,” J. Lightwave Technol.28(16), 2376–2397 (2010). [CrossRef]
- Z. Cao, J. Yu, M. Xia, Q. Tang, Y. Gao, W. Wang, and L. Chen, “Reduction of intersubcarrier interference and frequency-selective fading in OFDM-ROF systems,” J. Lightwave Technol.28(16), 2423–2429 (2010). [CrossRef]
- G. K. Chang, Z. Jia, J. Yu, A. Chowdhury, T. Wang, and G. Ellinas, “Super-broadband optical wireless access technologies,” Opt. Fiber Conf. (OFC 2008), San Diego, USA, OThD1, Feb. 2008.
- B. Liu, X. Xin, L. Zhang, K. Zhao, and C. Yu, “Broad convergence of 32QAM-OFDM ROF and WDM-OFDM-PON system using an integrated modulator for bidirectional access networks,” Opt. Fiber Conf. (OFC 2010), San Diego, USA, JThA26, Mar. 2010.
- C. Wei, C. Lin, M. Chao, and W. Jiang, “Adaptively modulated OFDM RoF signals at 60 GHz over-long-reach 100-km transmission systems employing phase noise suppression,” IEEE Photon. Technol. Lett.24(1), 49–51 (2012).
- W. Shieh, H. Bao, and Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express16(2), 841–859 (2008). [CrossRef] [PubMed]
- 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]
- Y. Zhao and A. Huang, “A novel channel estimation method for OFDM mobile communication systems based on pilot signals and transform-domain processing,” IEEE Vehicular Technology Conference, 3, 2089–2093 (1997).
- Q. Yang, S. Chen, Y. Ma, and W. Shieh, “Real-time reception of multi-gigabit coherent optical OFDM signals,” Opt. Express17(10), 7985–7992 (2009). [CrossRef] [PubMed]
- Q. Yang, N. Kaneda, X. Liu, S. Chandrasekhar, W. Shieh, and Y. Chen, “Real-Time coherent optical OFDM receiver at 2.5-GS/s for receiving a 54-Gb/s multi-band signal,” Opt. Fiber Conf. (OFC 2009), San Diego, USA, PDPC, Mar. 2009.
- T. Nakasyotani, H. Toda, T. Kuri, and K. Kitayama, “Wavelength-division-multiplexed millimeter-waveband radio-on-fiber system using a supercontinuum light source,” J. Lightwave Technol.24(1), 404–410 (2006). [CrossRef]
- H. Toda, T. Yamashita, T. Kuri, and K. Kitayama, “Demultiplexing using an arrayed-waveguide grating for frequency-interleaved DWDM millimeter-wave radio-on-fiber systems,” J. Lightwave Technol.21(8), 1735–1741 (2003). [CrossRef]
- L. Tao, J. Yu, Y. Fang, J. Zhang, Y. Shao, and N. Chi, “Analysis of noise spread in optical DFT-S OFDM systems,” J. Lightwave Technol.30(20), 3219–3225 (2012). [CrossRef]
- Q. Yang, Z. He, Z. Yang, S. Yu, X. Yi, and W. Shieh, “Coherent optical DFT-spread OFDM transmission using orthogonal band multiplexing,” Opt. Express20(3), 2379–2385 (2012). [CrossRef] [PubMed]
- 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]
- L. Tao, J. Yu, Q. Yang, M. Luo, Z. He, Y. Shao, J. Zhang, and N. Chi, “Spectrally efficient localized carrier distribution scheme for multiple-user DFT-S OFDM RoF- PON wireless access systems,” Opt. Express20(28), 29665–29672 (2012). [CrossRef] [PubMed]
- J. Lee, F. Breyer, S. Randel, J. Zeng, F. Huijskens, H. P. van den Boom, A. M. Koonen, and N. Hanik, “24-Gb/s transmission over 730 m of multimode fiber by direct modulation of an 850-nm VCSEL using discrete multitone modulation,” Opt. Fiber Conf. (OFC 2007), Anaheim, USA, PDP 6, Mar. 2011.

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