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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 18 — Aug. 27, 2012
  • pp: 19815–19821
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Marked performance improvement of 256 QAM transmission using a digital back-propagation method

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


Optics Express, Vol. 20, Issue 18, pp. 19815-19821 (2012)
http://dx.doi.org/10.1364/OE.20.019815


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Abstract

We demonstrate substantial performance improvements in 256 QAM transmission in terms of both data rate and distance that we realized by using a digital back-propagation (DBP) method. 160 Gbit/s-160 km and 64 Gbit/s-560 km transmissions were successfully achieved with a polarization-multiplexed 256 QAM signal, in which the symbol rate and transmission distance were greatly increased by compensating for the interplay between dispersion and nonlinearity, which is responsible for the transmission impairment especially for a higher symbol rate and longer distance.

© 2012 OSA

1. Introduction

Recently, a number of electrical compensation schemes have been proposed in accordance with advances in digital coherent technologies, and applied to the compensation of linear and nonlinear impairments. Among them, the digital back-propagation (DBP) method [8

8. 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]

,9

9. X. Li, X. Chen, G. Goldfarb, E. F. Mateo, I. Kim, F. Yaman, and G. Li, “Electronic post-compensation of WDM transmission impairments using coherent detection and digital signal processing,” Opt. Express 16(2), 880–888 (2008). [CrossRef] [PubMed]

], in which the nonlinearity is digitally compensated for by inverse fiber propagation using the nonlinear Schrödinger equation, has the potential to provide significantly improved performance as a result of enhanced OSNR by allowing a higher transmission power. This is because DBP is capable of compensating for the interplay between dispersion and nonlinearity, which is responsible for the transmission impairment especially for the higher symbol rate. DBP has been used to extend the transmission distance in 112 Gbit/s 16 QAM [10

10. S. Makovejs, D.S. Millar, V. Mikhailov, G. Gavioli, R.I. Killey, S.J. Savory, and P. Bayvel, “Experimental investigation of PDM-QAM16 transmission at 112 Gbit/s over 2400 km,” in Optical Fiber Communication Conference (Los Angeles, Calif., 2010), OMJ6.

] and 120 Gbit/s 64 QAM [11

11. T. Kobayashi, A. Sano, A. Matsuura, E. Yamazaki, E. Yoshida, Y. Miyamoto, T. Nakagawa, Y. Sakamaki, and T. Mizuno, “120-Gb/s PDM 64-QAM transmission over 1,280 km using multi-staged nonlinear compensation in digital coherent receiver,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OThF6.

] transmissions. Numerical simulations have also shown the benefit of DBP, especially for higher-order QAM [12

12. D. Rafique, J. Zhao, and A. D. Ellis,”Performance improvement by fibre nonlinearity compensation in 112 Gb/s PM M-ary QAM,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OWO6.

]. However, there have been no experimental demonstrations of the adoption of DBP for extremely high-order QAM transmission such as 256 QAM.

This paper reports the first application of DBP to 256 QAM, and describes the improved performance in terms of both a higher symbol rate and an extended maximum transmission distance.

2. Experimental setup

Our experimental setup is shown in Fig. 1
Fig. 1 Experimental setup for 256 QAM transmission.
. The 256 QAM transmitter comprises a coherent light source, an arbitrary waveform generator (AWG), and an IQ modulator. As a CW laser source, we used an acetylene frequency-stabilized fiber laser emitting at 1538.8 nm with a linewidth of 4 kHz. Part of the laser output was split and its frequency was downshifted by 2.7 GHz, which was co-propagated with a QAM signal and used as a pilot tone for OPLL in the receiver. The remaining CW light was QAM modulated with a 256 QAM baseband signal generated by the AWG, which was operated at 12 Gsample/s with a 10-bit resolution. The bandwidth of the 256 QAM signal was reduced by using a digital Nyquist filter with a roll-off factor of 0.35. In addition, digital pre-equalization was employed to compensate for the non-ideal frequency response of individual components by using a finite impulse response (FIR) filter with 99 taps. The 256 QAM optical signal was then polarization multiplexed andlaunched into a transmission link, which was composed of 80 km of SSMF per span. The loss of each span was compensated for by EDFAs.

After the transmission, the signal was preamplified via a 0.7 nm optical filter, and then coupled into a polarization-diversity coherent receiver together with a local oscillator (LO). Here, MIMO processing for the polarization demultiplexing of a 256 QAM signal is very complex due to its ultra-high multiplicity. Therefore, polarization demultiplexing was carried out with a polarization controller (PC) located before the polarization diversity coherent receiver instead of MIMO processing. In this scheme, a transmitted tone signal was maximized or minimized along the two polarization principal axes of the polarization diversity coherent receiver by controlling the PC so that X- and Y-polarization signals were completely demultiplexed. As the LO, we used a frequency-tunable fiber laser with a 4 kHz linewidth, which was phase-locked to the transmitted pilot tone via OPLL. The received signals were A/D converted at 40 Gsample/s using a digital oscilloscope, and fed into an offline digital signal processor (DSP). In the DSP, we employed the simultaneous compensation of dispersion and nonlinear impairments with DBP. We carried out a split-step Fourier analysis of the Manakov equation, which describes pulse propagation in the presence of dispersion, SPM, and XPM between the two orthogonal polarizations under a randomly varying birefringence [13

13. P. K. A. Wai, C. R. Menyuk, and H. H. Chen, “Stability of solitons in randomly varying birefringent fibers,” Opt. Lett. 16(16), 1231–1233 (1991). [CrossRef] [PubMed]

]:
{iAxz=β222Axt2+89γ|Ax|2Ax+89γ|Ay|2Ax+iα2AxiAyz=β222Ayt2+89γ|Ay|2Ay+89γ|Ax|2Ay+iα2Ay
(1)
where Ax and Ay represent the amplitude of the x and y polarization components, and α, β2, and γ are the loss, dispersion, and nonlinear coefficients, respectively. In DBP, we solve Eq. (1) with the reversed sign of α, β2, and γ. Here we chose an FFT size of 8192, and set the step size at a distance of 10 km. Finally, the compensated QAM signal was demodulated into binary data, and the bit error rate (BER) was evaluated.

3. 64 Gbit/s (4 Gsymbol/s), 256 QAM transmission over 560 km

Figure 2
Fig. 2 Relationship between launched power and BER after 160 km transmission. The squares and diamonds correspond to the two orthogonal polarization channels.
shows a comparison of BER as a function of the launched power. With DBP, the optimum launched power was increased by 2 dB, which resulted in an improved BER as a consequence of the increased OSNR. The BER characteristics against the received power with the optimum launched power is shown in Fig. 3
Fig. 3 BER characteristics in 64 Gbit/s, 256 QAM transmission over 160 km. The squares and diamonds correspond to the two orthogonal polarization channels.
, and the constellation diagrams after 160 km transmission with and without DBP are plotted in Fig. 4
Fig. 4 Constellation diagrams for 64 Gbit/s, 256 QAM signal after 160 km transmission. (a) without DBP, (b) with DBP.
. The blue curve in Fig. 3 is the result reported in [3

3. 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]

]. It can be seen that the power penalty at the FEC threshold (BER = 2x10−3) was greatly reduced from 5.3 to 0.8 dB by employing DBP. The error vector magnitude (EVM) was also improved from 2.10% to 1.82%. We confirmed that the interplay between dispersion and nonlinearity is still not significant at a symbol rate of 4 Gsymbol/s, and therefore this improvement is mainly attributed to the compensation of XPM between two orthogonal polarizations (the third term on the right hand side of Eq. (1)) with DBP.

4. 160 Gbit/s (10 Gsymbol/s), 256 QAM transmission over 160 km

5. Conclusion

We have successfully demonstrated substantial improvement in 256 QAM performance as regards both data rate and transmission distance by using DBP. By using EDFAs and Raman amplifiers, the transmission distance was extended from 160 to 560 km at a symbol rate of 4 Gsymbols (64 Gbit/s), and the symbol rate was increased from 4 to 10 Gsymbol/s in a 160 km transmission. These substantial improvements were successfully achieved as a result of the increased OSNR and the compensation of the interplay between dispersion and nonlinearity. Improved WDM transmission performance is also expected by reducing XPM between different WDM channels using DBP.

References and links

1.

D. Qian, M. Huang, E. Ip, Y. Huang, Y. Shao, J. Hu, and T. Wang, “101.7-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in National Fiber Optic Engineers Conference (Los Angeles, Calif., 2011), PDPB5.

2.

A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 x 548-Gb/s) C- and extended L-band all-Raman transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot tone,” in Optical Fiber Communication Conference, (Los Angeles, Calif., 2012), PDP5C.3.

3.

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]

4.

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,” in the 36th European Conference and Exhibition onOptical Communication (2010), PD2.3.

5.

Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express 20(11), 12508–12514 (2012). [CrossRef] [PubMed]

6.

M.-F. Huang, D. Qian, and E. Ip, “50.53-Gb/s PDM-1024QAM-OFDM transmission using pilot-based phase noise mitigation,” in Optical Fiber Communication Conference (2011), PDP1.

7.

X. Liu, S. Chandrasekhar, T. Lotz, P. J. Winzer, H. Haunstein, S. Randel, S. Corteselli, B. Zhu, and D. W. Peckham, “Generation and FEC-decoding of a 231.5-Gb/s PDM-OFDM signal with 256-iterative-polar-modulation achieving 11.15-b/s/Hz intrachannel spectral efficiency and 800-km reach,” National Fiber Optic Engineers Conference (Los Angeles, Calif., 2012), PDP5B.3.

8.

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]

9.

X. Li, X. Chen, G. Goldfarb, E. F. Mateo, I. Kim, F. Yaman, and G. Li, “Electronic post-compensation of WDM transmission impairments using coherent detection and digital signal processing,” Opt. Express 16(2), 880–888 (2008). [CrossRef] [PubMed]

10.

S. Makovejs, D.S. Millar, V. Mikhailov, G. Gavioli, R.I. Killey, S.J. Savory, and P. Bayvel, “Experimental investigation of PDM-QAM16 transmission at 112 Gbit/s over 2400 km,” in Optical Fiber Communication Conference (Los Angeles, Calif., 2010), OMJ6.

11.

T. Kobayashi, A. Sano, A. Matsuura, E. Yamazaki, E. Yoshida, Y. Miyamoto, T. Nakagawa, Y. Sakamaki, and T. Mizuno, “120-Gb/s PDM 64-QAM transmission over 1,280 km using multi-staged nonlinear compensation in digital coherent receiver,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OThF6.

12.

D. Rafique, J. Zhao, and A. D. Ellis,”Performance improvement by fibre nonlinearity compensation in 112 Gb/s PM M-ary QAM,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OWO6.

13.

P. K. A. Wai, C. R. Menyuk, and H. H. Chen, “Stability of solitons in randomly varying birefringent fibers,” Opt. Lett. 16(16), 1231–1233 (1991). [CrossRef] [PubMed]

14.

K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, and Y. Miyamoto, “Frequency-domain equalization for coherent optical single-carrier transmission systems,” IEICE Trans. Comm. E 92-B(12), 3736–3743 (2010).

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: July 2, 2012
Revised Manuscript: August 9, 2012
Manuscript Accepted: August 9, 2012
Published: August 14, 2012

Citation
Kazushi Toyoda, Yuki Koizumi, Tatsunori Omiya, Masato Yoshida, Toshihiko Hirooka, and Masataka Nakazawa, "Marked performance improvement of 256 QAM transmission using a digital back-propagation method," Opt. Express 20, 19815-19821 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-18-19815


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References

  1. D. Qian, M. Huang, E. Ip, Y. Huang, Y. Shao, J. Hu, and T. Wang, “101.7-Tb/s (370×294-Gb/s) PDM-128QAM-OFDM transmission over 3×55-km SSMF using pilot-based phase noise mitigation,” in National Fiber Optic Engineers Conference (Los Angeles, Calif., 2011), PDPB5.
  2. A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 x 548-Gb/s) C- and extended L-band all-Raman transmission over 240 km using PDM-64QAM single carrier FDM with digital pilot tone,” in Optical Fiber Communication Conference, (Los Angeles, Calif., 2012), PDP5C.3.
  3. 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]
  4. 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,” in the 36th European Conference and Exhibition onOptical Communication (2010), PD2.3.
  5. Y. Koizumi, K. Toyoda, M. Yoshida, and M. Nakazawa, “1024 QAM (60 Gbit/s) single-carrier coherent optical transmission over 150 km,” Opt. Express 20(11), 12508–12514 (2012). [CrossRef] [PubMed]
  6. M.-F. Huang, D. Qian, and E. Ip, “50.53-Gb/s PDM-1024QAM-OFDM transmission using pilot-based phase noise mitigation,” in Optical Fiber Communication Conference (2011), PDP1.
  7. X. Liu, S. Chandrasekhar, T. Lotz, P. J. Winzer, H. Haunstein, S. Randel, S. Corteselli, B. Zhu, and D. W. Peckham, “Generation and FEC-decoding of a 231.5-Gb/s PDM-OFDM signal with 256-iterative-polar-modulation achieving 11.15-b/s/Hz intrachannel spectral efficiency and 800-km reach,” National Fiber Optic Engineers Conference (Los Angeles, Calif., 2012), PDP5B.3.
  8. 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]
  9. X. Li, X. Chen, G. Goldfarb, E. F. Mateo, I. Kim, F. Yaman, and G. Li, “Electronic post-compensation of WDM transmission impairments using coherent detection and digital signal processing,” Opt. Express 16(2), 880–888 (2008). [CrossRef] [PubMed]
  10. S. Makovejs, D.S. Millar, V. Mikhailov, G. Gavioli, R.I. Killey, S.J. Savory, and P. Bayvel, “Experimental investigation of PDM-QAM16 transmission at 112 Gbit/s over 2400 km,” in Optical Fiber Communication Conference (Los Angeles, Calif., 2010), OMJ6.
  11. T. Kobayashi, A. Sano, A. Matsuura, E. Yamazaki, E. Yoshida, Y. Miyamoto, T. Nakagawa, Y. Sakamaki, and T. Mizuno, “120-Gb/s PDM 64-QAM transmission over 1,280 km using multi-staged nonlinear compensation in digital coherent receiver,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OThF6.
  12. D. Rafique, J. Zhao, and A. D. Ellis,”Performance improvement by fibre nonlinearity compensation in 112 Gb/s PM M-ary QAM,” Optical Fiber Communication Conference, (Los Angeles, Calif., 2011), OWO6.
  13. P. K. A. Wai, C. R. Menyuk, and H. H. Chen, “Stability of solitons in randomly varying birefringent fibers,” Opt. Lett. 16(16), 1231–1233 (1991). [CrossRef] [PubMed]
  14. K. Ishihara, T. Kobayashi, R. Kudo, Y. Takatori, A. Sano, and Y. Miyamoto, “Frequency-domain equalization for coherent optical single-carrier transmission systems,” IEICE Trans. Comm. E 92-B(12), 3736–3743 (2010).

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