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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 21 — Oct. 8, 2012
  • pp: 23383–23389
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512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver

Yuki Koizumi, Kazushi Toyoda, Tatsunori Omiya, Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa  »View Author Affiliations


Optics Express, Vol. 20, Issue 21, pp. 23383-23389 (2012)
http://dx.doi.org/10.1364/OE.20.023383


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Abstract

We demonstrate a marked performance improvement in a 512 QAM transmission by employing frequency-domain equalization (FDE) instead of an FIR filter. FDE enables us to compensate for distortions due to hardware imperfections in the transmitter with higher precision, which successfully reduced the power penalty by 4 dB in a 54 Gbit/s (3 Gsymbol/s)-160 km transmission. FDE also allows the transmission distance to be extended up to 240 km.

© 2012 OSA

1. Introduction

Digital coherent technology with a multi-level modulation format has played a critical role in increasing spectral efficiency and expanding fiber capacity toward the 100 Tbit/s regime [1

1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technologies (Springer 2010).

]. Advances in DSP technologies have enabled not only the coherent detection of high-speed multi-level optical signals with high precision, but also compensation for linear and nonlinear transmission impairments in the electrical domain, both in a static and adaptive way [2

2. E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. 25(8), 2033–2043 (2007). [CrossRef]

, 3

3. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008). [CrossRef] [PubMed]

]. These equalizers are also very useful for dealing with distortions caused by hardware imperfections in individual components, such as a non-ideal frequency response or skew in optical or electrical devices.

Time-domain equalization techniques using a finite impulse-response (FIR) filter are already commonly used in digital coherent receivers. However, the frequency resolution of FIR filters is typically limited to around a few tens of MHz, which is determined mainly by the finite number of filter taps in order to avoid complex calculations. Insufficient resolution in the equalizer becomes disadvantageous for signals with higher-order multiplicity levels such as 256 and 512 QAM [4

4. M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett. 22(3), 185–187 (2010). [CrossRef]

6

6. R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express 20(6), 6439–6447 (2012). [CrossRef] [PubMed]

], as these formats typically contain non-negligible frequency components even below 10 MHz.

Recently, a frequency-domain equalization (FDE) scheme, which was originally developed for wireless communication [7

7. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag. 40(4), 58–66 (2002). [CrossRef]

], has received a lot of attention in relation to optical communication [8

8. K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, E. Yamada, H. Masuda, and Y. Miyamoto, “Coherent optical transmission with frequency-domain equalization,” ECOC’08, We.2.E.3.

]. FDE has been successfully applied to ultrahigh capacity WDM transmission with 16 QAM [9

9. A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7.

]. It features an equalization capability with less computational complexity than FIR filters by virtue of the FFT operation. FDE is also expected to improve the frequency resolution significantly compared with that obtained with FIR filters without the expense of calculation complexity.

In this paper, an FDE technique is applied to extremely high-order optical QAM for the first time, and a marked improvement is demonstrated in 512 QAM, 54 Gbit/s (3 Gsymbol/s) transmission performance over 160 km. The power penalty was greatly reduced as a result of the ability to compensate for a non-ideal frequency response especially in a lower frequency regime.

2. Capability of FDE for higher-order QAM

Figure 2 shows a comparison of the computation complexity for time-domain equalization with an FIR filter and FDE. With FDE, the number of real-valued multiplications per symbol, nFDE, is estimated as follows. It is known that an FFT involves nFFT = 4NFFT log2(NFFT) real-valued multiplications [10

10. J. C. Geyer, C. R. S. Fluger, T. Duthel, C. Schulien, and B. Schumauss, “Efficient frequency domain chromatic dispersion compensation in a coherent polmux QPSK-receiver,” OFC’10, OWV5.

]. Since FDE employs both FFT and IFFT, it includes 2nFFT multiplications. Furthermore, since one symbol is represented by two samples, an FFT with a size of NFFT accounts for NFFT/2 symbols. Therefore, the number of multiplications per symbol is estimated as nFDE = 2nFFT / (NFFT/2) = 8log2(NFFT), i.e., it increases only logarithmically. The relationship between nFDE and NFFT is shown in Fig. 2(b). Figure 2(c) shows a comparison of nFIR and nFDE as a function of Δf. This clearly shows the advantage of FDE in terms of the lower computation complexity especially for Δf values as low as 1 MHz and below, whereas with FIR such a low Δf is very difficult to realize due to the rapid increase in the computation complexity. For example, if we set Δf = 1 MHz, FDE requires NFFT = 8192 and thus nFDE = 104. On the other hand, the required number of taps with FIR is NFIR = 4000, which corresponds to nFIR = 16000.

3. Experimental setup

We applied the FDE technique to 512 QAM transmission. The experimental setup is shown in Fig. 3
Fig. 3 Experimental setup for polarization-multiplexed 512 QAM, 54 Gbit/s (3 Gsymbol/s) transmission.
. At the transmitter, coherent light emitted from an acetylene frequency-stabilized CW fiber laser at 1538.8 nm with a 4 kHz linewidth was modulated by an optical IQ modulator driven with a 3 Gsymbol/s, 512 QAM baseband signal generated by an arbitrary waveform generator (AWG) with a pattern length of 4096. The AWG was operated at 12 Gsample/s with a 10-bit resolution. The bandwidth of the 512 QAM signal was reduced to 4.05 GHz by employing a Nyquist filter with a roll-off factor of 0.35 in the AWG.

On the receiver side, after passing through a 0.7 nm optical filter and an EDFA for preamplification, the transmitted QAM data were combined with a local oscillator (LO) and received by a polarization-diversity coherent receiver. The LO was a frequency-tunable fiber laser, whose phase was locked to the transmitted pilot tone via an OPLL. The detected signals were digitized at 40 Gsample/s and processed with an offline DSP. Here, the polarization demultiplexing of the X- and Y-polarization signals was carried out by using a polarization controller in front of the polarization diversity coherent receiver. Specifically, a transmitted tone signal was maximized or minimized along the two polarization principal axes of the polarization diversity coherent receiver. We employed polarization demultiplexing in the optical domain instead of MIMO processing, because the DSP for polarization demultiplexing a 512 QAM signal is very complex due to its ultra-high multiplicity.

4. Experimental result

Figure 5
Fig. 5 Constellation diagrams of 512 QAM signal (a) before and (b) after transmission. Left and right figures correspond to equalized QAM data with FIR and FDE, respectively.
shows the constellation diagrams of the 512 QAM signal before and after a 160 km transmission. The corresponding BER characteristics are shown in Fig. 6
Fig. 6 BER characteristics for polarization-multiplexed 3 Gsymbol/s, 512 QAM (54 Gbit/s) transmission over 160 km.
. In Fig. 6, the gray and pink curves are the results obtained with FIR. As shown in Fig. 5(a), the back-to-back constellation had error vector magnitudes (EVMs) of 1.0 and 0.85% when equalized with FIR and FDE, respectively. This improvement is a consequence of the ability of FDE to eliminate distortions caused by hardware imperfections in the transmitter and receiver with a resolution better than FIR. This improvement is also apparent in the back-to-back BER performance, as shown by the gray and black curves in Fig. 6. After a 160 km transmission, the EVM was improved from 1.50 to 1.24% by employing FDE as shown in Fig. 5(b), corresponding to the pink and red curves in Fig. 6. As can be seen in Fig. 6, the power penalty was greatly reduced from 7.0 to 3.0 dB as a result of FDE.

We also evaluated the possibility of extending the transmission distance from 160 km, and calculated the maximum transmission reach. The relationship between the transmission distance and BER is shown in Fig. 7
Fig. 7 Relationship between transmission distance and BER in 54 Gbit/s, 512 QAM transmission with FDE and FIR.
. As shown by the red symbols, a BER below the FEC threshold is still obtained after 240 km with FDE, which is difficult to achieve with FIR.

5. Conclusion

We have successfully demonstrated the excellent potential of FDE for reducing impairments in QAM transmissions with multiplicity levels as high as 512. Because of its high frequency resolution, FDE enables us to realize frequency response compensation down to a low frequency range, which is responsible for impairments in such an extremely high-order QAM. As a result, the power penalty was greatly reduced from 7.0 to 3.0 dB in a 512 QAM, 54 Gbit/s (3 Gsymbol/s)-160 km transmission. We also showed that the maximum distance of a 512 QAM transmission could be extended to 240 km with FDE.

References and links

1.

M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technologies (Springer 2010).

2.

E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol. 25(8), 2033–2043 (2007). [CrossRef]

3.

S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16(2), 804–817 (2008). [CrossRef] [PubMed]

4.

M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett. 22(3), 185–187 (2010). [CrossRef]

5.

S. Okamoto, K. Toyoda, T. Omiya, K. Kasai, M. Yoshida, and M. Nakazawa, “512 QAM (54 Gbit/s) coherent optical transmission over 150 km with an optical bandwidth of 4.1 GHz,” ECOC’10, PD2.3.

6.

R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express 20(6), 6439–6447 (2012). [CrossRef] [PubMed]

7.

D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag. 40(4), 58–66 (2002). [CrossRef]

8.

K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, E. Yamada, H. Masuda, and Y. Miyamoto, “Coherent optical transmission with frequency-domain equalization,” ECOC’08, We.2.E.3.

9.

A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7.

10.

J. C. Geyer, C. R. S. Fluger, T. Duthel, C. Schulien, and B. Schumauss, “Efficient frequency domain chromatic dispersion compensation in a coherent polmux QPSK-receiver,” OFC’10, OWV5.

11.

C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett. 21(7), 459–461 (1996). [CrossRef] [PubMed]

12.

R. L. Jungerman and C. A. Flory, “Low-frequency acoustic anomalies in lithium niobate Mach-Zehnder interferometers,” Appl. Phys. Lett. 53(16), 1477–1479 (1988). [CrossRef]

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: June 29, 2012
Revised Manuscript: September 4, 2012
Manuscript Accepted: September 9, 2012
Published: September 26, 2012

Citation
Yuki Koizumi, Kazushi Toyoda, Tatsunori Omiya, Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa, "512 QAM transmission over 240 km using frequency-domain equalization in a digital coherent receiver," Opt. Express 20, 23383-23389 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-21-23383


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References

  1. M. Nakazawa, K. Kikuchi, and T. Miyazaki, High Spectral Density Optical Communication Technologies (Springer 2010).
  2. E. Ip and J. M. Kahn, “Digital equalization of chromatic dispersion and polarization mode dispersion,” J. Lightwave Technol.25(8), 2033–2043 (2007). [CrossRef]
  3. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express16(2), 804–817 (2008). [CrossRef] [PubMed]
  4. M. Nakazawa, S. Okamoto, T. Omiya, K. Kasai, and M. Yoshida, “256-QAM (64 Gb/s) coherent optical transmission over 160 km with an optical bandwidth of 5.4 GHz,” IEEE Photon. Technol. Lett.22(3), 185–187 (2010). [CrossRef]
  5. S. Okamoto, K. Toyoda, T. Omiya, K. Kasai, M. Yoshida, and M. Nakazawa, “512 QAM (54 Gbit/s) coherent optical transmission over 150 km with an optical bandwidth of 4.1 GHz,” ECOC’10, PD2.3.
  6. R. Schmogrow, D. Hillerkuss, S. Wolf, B. Bäuerle, M. Winter, P. Kleinow, B. Nebendahl, T. Dippon, P. C. Schindler, C. Koos, W. Freude, and J. Leuthold, “512QAM Nyquist sinc-pulse transmission at 54 Gbit/s in an optical bandwidth of 3 GHz,” Opt. Express20(6), 6439–6447 (2012). [CrossRef] [PubMed]
  7. D. Falconer, S. L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag.40(4), 58–66 (2002). [CrossRef]
  8. K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, E. Yamada, H. Masuda, and Y. Miyamoto, “Coherent optical transmission with frequency-domain equalization,” ECOC’08, We.2.E.3.
  9. A. Sano, H. Masuda, T. Kobayashi, M. Fujiwara, K. Horikoshi, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, H. Yamazaki, Y. Sakamaki, and H. Ishii, “69.1-Tb/s (432 x 171-Gb/s) C- and extended L-Band transmission over 240 Km using PDM-16-QAM modulation and digital coherent detection,” OFC’10, PDPB7.
  10. J. C. Geyer, C. R. S. Fluger, T. Duthel, C. Schulien, and B. Schumauss, “Efficient frequency domain chromatic dispersion compensation in a coherent polmux QPSK-receiver,” OFC’10, OWV5.
  11. C. Paré, A. Villeneuve, P.-A. Bélanger, and N. J. Doran, “Compensating for dispersion and the nonlinear Kerr effect without phase conjugation,” Opt. Lett.21(7), 459–461 (1996). [CrossRef] [PubMed]
  12. R. L. Jungerman and C. A. Flory, “Low-frequency acoustic anomalies in lithium niobate Mach-Zehnder interferometers,” Appl. Phys. Lett.53(16), 1477–1479 (1988). [CrossRef]

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