## 2x2 MIMO-OFDM Gigabit fiber-wireless access system based on polarization division multiplexed WDM-PON |

Optics Express, Vol. 20, Issue 4, pp. 4369-4375 (2012)

http://dx.doi.org/10.1364/OE.20.004369

Acrobat PDF (12421 KB)

### Abstract

We propose a spectral efficient radio over wavelength division multiplexed passive optical network (WDM-PON) system by combining optical polarization division multiplexing (PDM) and wireless multiple input multiple output (MIMO) spatial multiplexing techniques. In our experiment, a training-based zero forcing (ZF) channel estimation algorithm is designed to compensate the polarization rotation and wireless multipath fading. A 797 Mb/s net data rate QPSK-OFDM signal with error free (<1 × 10^{−5}) performance and a 1.59 Gb/s net data rate 16QAM-OFDM signal with BER performance of 1.2 × 10^{−2} are achieved after transmission of 22.8 km single mode fiber followed by 3 m and 1 m air distances, respectively.

© 2012 OSA

## 1. Introduction

2. M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol. **25**(11), 3301–3320 (2007). [CrossRef]

3. K. Tsukamoto, T. Nishiumi, T. Yamagami, T. Higashino, S. Komaki, R. Kubo, T. Taniguchi, J.-I. Kani, N. Yoshimoto, H. Kimura, and K. Iwatsuki, “Convergence of WDM access and ubiquitous antenna architecture for broadband wireless services,” PIERS Online **6**(4), 385–389 (2010). [CrossRef]

4. S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol. **27**(16), 3699–3704 (2009). [CrossRef]

5. A. Agmon, B. Schrenk, J. Prat, and M. Nazarathy, “Polarization beamforming PON doubling bidirectional throughput,” J. Lightwave Technol. **28**(17), 2579–2585 (2010). [CrossRef]

6. G. L. Stuber, J. R. Barry, S. W. Mclaughlin, Y. Li, M. A. Ingram, and T. G. Pratt, “Broadband MIMO-OFDM wireless communications,” Proc. IEEE **92**(2), 271–294 (2004). [CrossRef]

8. S. L. Jansen, I. Morita, T. C. Schenk, and H. Tanaka, “Long-haul transmission of 16x52.5 Gbits/s polarization-division multiplexed OFDM enabled by MIMO processing (Invited),” J. Opt. Netw. **7**(2), 173–182 (2008). [CrossRef]

9. A. Kobyakov, M. Sauer, A. Ng’oma, and J. H. Winters, “Effect of optical loss and antenna separation in 2x2 MIMO fiber-radio systems,” IEEE Trans. Antenn. Propag. **58**(1), 187–194 (2010). [CrossRef]

11. M. B. Othman, L. Deng, X. Pang, J. Caminos, W. Kozuch, K. Prince, X. Yu, J. B. Jensen, and I. T. Monroy, “MIMO-OFDM WDM PON with DM-VCSEL for femtocells application,” Opt. Express **19**(26), B537–B542 (2011). [CrossRef] [PubMed]

12. S.-H. Fan, H.-C. Chien, A. Chowdhury, C. Liu, W. Jian, Y.-T. Hsueh, and G.-K. Chang, “A novel radio-over-fiber system using the xy-MIMO wireless technique for enhanced radio spectral efficiency,” in *36th European Conference and Exhibition on Optical Communication (ECOC)*, 2010 (2010), Paper Th.9.B.1.

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. X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval CO-OFDM transmission over 2000km of ultra-large-area fiber and five 80-GHz-Grid ROADMs,” J. Lightwave Technol. **29**(4), 483–490 (2011). [CrossRef]

## 2. Training-based PDM-MIMO-OFDM composite channel estimation

*t*and

_{x}*r*are respectively transmitted and received X branch optical signals, so as

_{x}*t*and

_{y}*r*for Y branch signals. The symbol

_{y}*ө*is the rotational angle.

*H*represents the combined effect of fiber chromatic dispersion and polarization dependent loss. In our transmitter, after two photodetectors, two antennas are used to radiate two radio signals, respectively. The wireless channel response could be represented by a matrix

_{F}*H*. Notice that, the polarization rotation in fiber does not change so fast compared to the wireless channel, so it is not difficult to estimate the channel. The hybrid optical and wireless response for our MIMO-OFDM signal can be represented as Eq. (2), where

_{MIMO}*n*and

_{x}*n*are the random noises, and

_{y}*h*,

_{xx}*h*,

_{xy}*h*and

_{yx}*h*represent the elements in the combined channel response matrix.

_{yy}*T*= [

_{X}*T*, 0]

_{1}^{T},

*T*= [0,

_{Y}*T*]

_{2}^{T}in the two tributaries. The received training sequences in two consecutive training durations can be expressed as:where

*RT*and

_{x1}*RT*stand for the received training symbol from X branch at the first and second training duration, respectively, so do

_{x2}*RT*and

_{y1}*RT*for the Y branch. And

_{y2}*n*,

_{x1}*n*,

_{x2}*n*and

_{y1}*n*are the random noises. The estimated channel transfer matrix then can be easily calculated as Eq. (4). From Eq. (4) we see that even with perfect channel estimation an error term will occur due to the random noises. More advanced algorithm such as minimum mean-squared-error (MMSE) algorithm could be used to improve the performance. However, in our experiment, zero forcing (ZF) instead of MMSE algorithm is used for channel estimation due to its lower computational complexity [4

_{y2}4. S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol. **27**(16), 3699–3704 (2009). [CrossRef]

## 3. Experimental setup

^{15}-1 is mapped onto 129 subcarriers, of which 64 subcarriers carry real QPSK/16-QAM data and one is unfilled DC subcarrier. The remaining 64 subcarriers are the complex conjugate of the aforementioned 64 subcarriers to enforce Hermitian symmetry in the input facet of 256-point inverse fast Fourier transform (IFFT). The cyclic prefix is 1/10 of the IFFT length resulting in an OFDM symbol size of 281. To facilitate time synchronization and MIMO channel estimation, 3 training symbols are inserted at the beginning of each OFDM frame that contains 7 data symbols. Each channel has a net data rate of 398.5 Mb/s (1.25 GSa/s × 2 × 64/281 × 7/10) for QPSK case and 797.1 Mb/s for 16QAM case with a bandwidth of 629.8 MHz (1.25 GSa/s × 129/256). For simplicity, one frame delay is applied in one channel to decorrelate the two channel signals in the AWG. These two-channel OFDM signals are then separately up-converted to 5.65 GHz. The two RF OFDM signals are used to modulate a 100 kHz-linewidth continuous-wave (CW) external cavity laser (ECL, λ

_{1}= 1550 nm) at two Mach-Zehnder modulators (MZMs), respectively. A pair of polarization controllers (PCs), namely PC

_{X1}and PC

_{Y1}are used to optimize the response of the MZMs. PC

_{X2}and PC

_{Y2}are inserted at the MZMs outputs to align the optical OFDM signal in each channel to the X and Y axis of the following polarization beam combiner (PBC), which then combines the two orthogonal polarizations. Subsquently PC

_{F}is introduced to roughly adjust the polarization of optical signal in the trunk fiber and set the variable power splitting ratio for equal SNR at each transmitter antenna. An erbium-doped fiber amplifer (EDFA) and an optical filter with 0.8 nm bandwidth are used to boost the optical OFDM signal and filter out the outband noise. The optical spectrum and the poincaré sphere of the combined optical signal are shown in the insets of Fig. 2. After excluding the overhead from cyclic prefix and training sequences, the output signal from the PBC is at a net data rate of 797 Mb/s with a spectral efficiency of 1.26 bits/s/Hz for QPSK case and 1.59 Gb/s with a spectral efficiency of 2.52 bits/s/Hz for 16-QAM case.

## 4. Experimental results and discussions

^{−3}) is achieved at −17.2 dBm and −14.3 dBm for the X polarization OFDM signal (Pol-x) and Y polarization OFDM signal (Pol-y), respectively. This 2.9 dB power penalty between Pol-x and Pol-y could be attributed to the different performances of optical and electrical components, particularly the responsivity of the two photodiodes used in these two branches. Negligible power penalty (around 0.5 dB) is induced after 22.8 km SMF transmission by using training-based MIMO OFDM channel estimation algorithm. The received constellations of Pol-x and Pol-y signal after 22.8 km SMF transmission are shown in the insets of Fig. 3 as well.

^{−4}and 1.28 × 10

^{−2}for 16-QAM MIMO-OFDM Pol-x and Pol-y signal after 1 m air distance and 22.8 km SMF transmission, respectively.

## 5. Conclusion

## Acknowledgments

## References and links

1. | J. Zhang and G. de la Roche, |

2. | M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol. |

3. | K. Tsukamoto, T. Nishiumi, T. Yamagami, T. Higashino, S. Komaki, R. Kubo, T. Taniguchi, J.-I. Kani, N. Yoshimoto, H. Kimura, and K. Iwatsuki, “Convergence of WDM access and ubiquitous antenna architecture for broadband wireless services,” PIERS Online |

4. | S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol. |

5. | A. Agmon, B. Schrenk, J. Prat, and M. Nazarathy, “Polarization beamforming PON doubling bidirectional throughput,” J. Lightwave Technol. |

6. | G. L. Stuber, J. R. Barry, S. W. Mclaughlin, Y. Li, M. A. Ingram, and T. G. Pratt, “Broadband MIMO-OFDM wireless communications,” Proc. IEEE |

7. | W. Shieh and I. Djordjevic, |

8. | S. L. Jansen, I. Morita, T. C. Schenk, and H. Tanaka, “Long-haul transmission of 16x52.5 Gbits/s polarization-division multiplexed OFDM enabled by MIMO processing (Invited),” J. Opt. Netw. |

9. | A. Kobyakov, M. Sauer, A. Ng’oma, and J. H. Winters, “Effect of optical loss and antenna separation in 2x2 MIMO fiber-radio systems,” IEEE Trans. Antenn. Propag. |

10. | M. B. Othman, L. Deng, X. Pang, J. Caminos, W. Kozuch, K. Prince, J. B. Jensen, and I. T. Monroy, “Directly-modulated VCSELs for 2x2 MIMO-OFDM radio over fiber in WDM-PON,” in |

11. | M. B. Othman, L. Deng, X. Pang, J. Caminos, W. Kozuch, K. Prince, X. Yu, J. B. Jensen, and I. T. Monroy, “MIMO-OFDM WDM PON with DM-VCSEL for femtocells application,” Opt. Express |

12. | S.-H. Fan, H.-C. Chien, A. Chowdhury, C. Liu, W. Jian, Y.-T. Hsueh, and G.-K. Chang, “A novel radio-over-fiber system using the xy-MIMO wireless technique for enhanced radio spectral efficiency,” in |

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

14. | X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval CO-OFDM transmission over 2000km of ultra-large-area fiber and five 80-GHz-Grid ROADMs,” J. Lightwave Technol. |

**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

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

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: January 4, 2012

Revised Manuscript: January 30, 2012

Manuscript Accepted: January 30, 2012

Published: February 7, 2012

**Citation**

Lei Deng, Xiaodan Pang, Ying Zhao, M. B. Othman, Jesper Bevensee Jensen, Darko Zibar, Xianbin Yu, Deming Liu, and Idelfonso Tafur Monroy, "2x2 MIMO-OFDM Gigabit fiber-wireless access system based on polarization division multiplexed WDM-PON," Opt. Express **20**, 4369-4375 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-4-4369

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### References

- J. Zhang and G. de la Roche, Femtocells: Technologies and Deployment (Wiley, 2010), Chaps. 2, 4, 9.
- M. Sauer, A. Kobyakov, and J. George, “Radio over fiber for picocellular network architectures,” J. Lightwave Technol.25(11), 3301–3320 (2007). [CrossRef]
- K. Tsukamoto, T. Nishiumi, T. Yamagami, T. Higashino, S. Komaki, R. Kubo, T. Taniguchi, J.-I. Kani, N. Yoshimoto, H. Kimura, and K. Iwatsuki, “Convergence of WDM access and ubiquitous antenna architecture for broadband wireless services,” PIERS Online6(4), 385–389 (2010). [CrossRef]
- S. Chen, Q. Yang, Y. Ma, and W. Shieh, “Real-time multi-gigabit receiver for coherent optical MIMO-OFDM signals,” J. Lightwave Technol.27(16), 3699–3704 (2009). [CrossRef]
- A. Agmon, B. Schrenk, J. Prat, and M. Nazarathy, “Polarization beamforming PON doubling bidirectional throughput,” J. Lightwave Technol.28(17), 2579–2585 (2010). [CrossRef]
- G. L. Stuber, J. R. Barry, S. W. Mclaughlin, Y. Li, M. A. Ingram, and T. G. Pratt, “Broadband MIMO-OFDM wireless communications,” Proc. IEEE92(2), 271–294 (2004). [CrossRef]
- W. Shieh and I. Djordjevic, OFDM for Optical Communications (Springer, 2009), Chap. 2.
- S. L. Jansen, I. Morita, T. C. Schenk, and H. Tanaka, “Long-haul transmission of 16x52.5 Gbits/s polarization-division multiplexed OFDM enabled by MIMO processing (Invited),” J. Opt. Netw.7(2), 173–182 (2008). [CrossRef]
- A. Kobyakov, M. Sauer, A. Ng’oma, and J. H. Winters, “Effect of optical loss and antenna separation in 2x2 MIMO fiber-radio systems,” IEEE Trans. Antenn. Propag.58(1), 187–194 (2010). [CrossRef]
- M. B. Othman, L. Deng, X. Pang, J. Caminos, W. Kozuch, K. Prince, J. B. Jensen, and I. T. Monroy, “Directly-modulated VCSELs for 2x2 MIMO-OFDM radio over fiber in WDM-PON,” in 37th European Conference and Exhibition on Optical Communication (ECOC), 2011 (2011), Paper We.10.P1.119.
- M. B. Othman, L. Deng, X. Pang, J. Caminos, W. Kozuch, K. Prince, X. Yu, J. B. Jensen, and I. T. Monroy, “MIMO-OFDM WDM PON with DM-VCSEL for femtocells application,” Opt. Express19(26), B537–B542 (2011). [CrossRef] [PubMed]
- S.-H. Fan, H.-C. Chien, A. Chowdhury, C. Liu, W. Jian, Y.-T. Hsueh, and G.-K. Chang, “A novel radio-over-fiber system using the xy-MIMO wireless technique for enhanced radio spectral efficiency,” in 36th European Conference and Exhibition on Optical Communication (ECOC), 2010 (2010), Paper Th.9.B.1.
- 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]
- X. Liu, S. Chandrasekhar, B. Zhu, P. J. Winzer, A. H. Gnauck, and D. W. Peckham, “448-Gb/s reduced-guard-interval CO-OFDM transmission over 2000km of ultra-large-area fiber and five 80-GHz-Grid ROADMs,” J. Lightwave Technol.29(4), 483–490 (2011). [CrossRef]

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