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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 15 — Jul. 28, 2014
  • pp: 18770–18777
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Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems

Meng Qiu, Qunbi Zhuge, Mathieu Chagnon, Yuliang Gao, Xian Xu, Mohamed Morsy-Osman, and David V. Plant  »View Author Affiliations


Optics Express, Vol. 22, Issue 15, pp. 18770-18777 (2014)
http://dx.doi.org/10.1364/OE.22.018770


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Abstract

In this work we experimentally investigate the improved intra-channel fiber nonlinearity tolerance of digital subcarrier multiplexed (SCM) signals in a single-channel coherent optical transmission system. The digital signal processing (DSP) for the generation and reception of the SCM signals is described. We show experimentally that the SCM signal with a nearly-optimum number of subcarriers can extend the maximum reach by 23% in a 24 GBaud DP-QPSK transmission with a BER threshold of 3.8 × 10−3 and by 8% in a 24 GBaud DP-16-QAM transmission with a BER threshold of 2 × 10−2. Moreover, we show by simulations that the improved performance of SCM signals is observed over a wide range of baud rates, further indicating the merits of SCM signals in baud-rate flexible agile transmissions and future high-speed optical transport systems.

© 2014 Optical Society of America

1. Introduction

Digital signal processing (DSP) has been widely employed in coherent optical (CO) communication systems and enables effective compensation for many types of linear impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD). However, fiber nonlinearities remain a factor that limits the achievable transmission distance in optical communication systems. In the digital domain, several methods have been proposed to mitigate the nonlinear impairments such as digital back propagation (DBP) with inverse fiber parameters [1

1. E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol. 26(20), 3416–3425 (2008). [CrossRef]

3

3. E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express 19(2), 570–583 (2011). [CrossRef] [PubMed]

], perturbation-based nonlinear pre-compensation [4

4. Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol. 29(17), 2570–2576 (2011). [CrossRef]

6

6. Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in Proc. OFC (2014), paper Th4D.7. [CrossRef]

] and maximum-likelihood sequence estimation (MLSE) [7

7. N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in Proc. Opt. Commun. Conf. (2011), paper OWW6. [CrossRef]

]. However, the computational complexity of these approaches is usually high. Pulse shaping was proposed in [8

8. B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of Nyquist pulses for coherent optical communications,” Opt. Express 20(8), 8397–8416 (2012). [CrossRef] [PubMed]

, 9

9. X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in Proc. OFC (2013), paper JTh2A.38. [CrossRef]

] as an alternative to mitigate fiber nonlinearities by increasing the power of the high-frequency components in the signal spectra. The drawback of this method is the reduced spectral efficiency (SE) because an increased bandwidth is usually required for a noticeable performance improvement.

In this work, we discuss in more details our work in [16

16. M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in Proc. OFC (2014), paper Tu3J.2. [CrossRef]

]. The optimum design of SCM signals is also investigated. Moreover, we extend our study to 16-QAM transmissions and demonstrate the advantage of SCM signals in more schemes. The remainder of the paper is organized as follows. In Section 2 we introduce the transmitter and receiver DSP for the multiplexing and de-multiplexing of subcarriers. Then in Section 3, we numerically and experimentally compare the transmission properties of SC signals and SCM signals for QPSK and 16-QAM, showing that properly designed SCM signals outperform SC signals in terms of intra-channel nonlinearity tolerance. Finally we conclude in Section 4.

2. Subcarrier multiplexing and de-multiplexing

Figure 1
Fig. 1 Power spectra of the SC signal and SCM signals.
illustrates the simulated transmitter-side spectra of the SC signal and the SCM signals with 2, 4 and 8 subcarriers. The total baud rate is 24 GBaud, so the baud rate of each subcarrier is 12 GBaud, 6 GBaud and 3 GBaud respectively for the three SCM signals. The modulation format is QPSK. Root raised cosine (RRC) pulse shaping with a roll-off factor of 0.1 is used in this work. Note that no guard band is left between subcarriers in the SCM signal spectra, so the optical bandwidth and SE are the same for all signals.

The subcarriers are multiplexed using the configuration shown in Fig. 2(a)
Fig. 2 Illustration of (a) the transmitter-side DSP for subcarrier multiplexing and (b) the receiver-side DSP for subcarrier de-multiplexing.
. K denotes the number of subcarriers in the SCM signal. Note that the SC signal can be regarded as a special SCM signal when K = 1, so Fig. 2 applies to the SC signal as well. Independent data sequences are used in each subcarrier and mapped to QPSK or 16-QAM symbols. After the symbols are interpolated to 2 samples per symbol in each subcarrier, the samples, denoted as Sk, k = 1,2…K, are transformed to the frequency domain and filtered by a RRC filter H(f) with a roll-off factor of 0.1. Then the outputs are up-sampled by K (essentially frequency-domain zero padding), shifted to different frequencies in the spectrum and multiplexed. These operations are also done in the frequency domain. After the re-sampling to the DAC sampling rate, the signals are pre-compensated to combat the limited bandwidth of the transmitter based on the pre-measured transmitter frequency response. Finally the time-domain samples are sent to DACs for digital-to-analog conversion. Correspondingly, for subcarrier de-multiplexing, we use a configuration as shown in Fig. 2(b). The SCM signal is first digitized by analog-to-digital converters (ADC) and re-sampled to twice the total baud rate. All subcarriers are captured simultaneously in a single detection. After the transform to the frequency domain, each subcarrier is successively shifted to the baseband and filtered out using a digital low-pass matched filter G(f). Finally the low-baud-rate signal in each subcarrier is down-sampled by K (essentially truncating zeros in the frequency domain to 2 samples per symbol), transformed to time domain and processed in parallel. The other DSP blocks are similar to those in conventional SC systems and are not shown in Fig. 2.

3. Setup, results and discussions

Figure 3
Fig. 3 Experimental setup. ECL: external cavity laser, PC: polarization controller, PBS/PBC: polarization beam splitter/combiner, ODL: optical delay line, VOA: variable optical attenuator, SW: switch, CRx: coherent receiver.
illustrates the experimental setup. ECLs with linewidths <100 kHz are used at both the transmitter and the receiver. The SC or SCM signals are generated offline with a total baud rate of 24 GBaud, converted to analog domain via two DACs and drive an IQ-modulator for electrical-to-optical conversion. The DACs operate at a sampling rate of 32 GSa/s with a physical resolution of 6 bits. The DP signal is emulated by splitting the signal into orthogonal polarizations and recombining them after delaying one polarization by 26 ns. This time duration corresponds to a delay of 624 symbols in the SC case. A VOA is used to control the launch power before the signals enter a re-circulating loop. The loop consists of 320 km of standard single mode fiber (SMF-28e + ) and in-line EDFAs with a noise figure of 5 dB are employed after every 80 km of fiber. The output from the loop is filtered, pre-amplified and filtered again. After coherent detection by an integrated coherent receiver, the signals are digitized by a 4-channel real-time oscilloscope with a sampling rate of 80 GSa/s per channel. Finally the digital signals are processed offline in MATLAB, including IQ imbalance compensation, re-sampling, frequency offset (FO) and CD compensation [17

17. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

], matched filter followed by down-sampling in the frequency domain as shown in Fig. 2(b), timing recovery [18

18. M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun. 36(5), 605–612 (1988). [CrossRef]

], blind equalization and the carrier phase recovery described in [19

19. Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express 20(17), 19599–19609 (2012). [CrossRef] [PubMed]

]. For QPSK a modified version of constant modulus algorithm (CMA) [20

20. L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in Proc. OFC (2009), paper OMT2. [CrossRef]

] is used for blind equalization and for 16-QAM we use a training-symbol-aided decision-directed least mean square (DD-LMS) algorithm for better convergence with CMA for pre-convergence. A total of ~2 × 105 symbols are extracted for offline processing. The BER or Q2 factor of the SCM signals is an average value over all subcarriers. Note that for the SCM signals the DSP blocks before matched filtering (with black color in Fig. 3) are applied to the multiplexed signal and the rest of the processing (with red color in Fig. 3) is done independently for each subcarrier.

In experiments the back-to-back performance is first investigated. Figures 5(a)
Fig. 5 Back-to-back performance of different signals with a modulation format of (a) QPSK and (b) 16-QAM.
and 5(b) summarize the BER versus OSNR (0.1 nm) curves of different signals when the modulation format is QPSK and 16-QAM respectively. The OSNR is swept by using receiver-side noise loading and controlling the noise power using a VOA. The theoretical curves are also plotted as references. As shown in both figures, small penalties are observed when we increase the number of subcarriers. Specifically, in the QPSK case shown in Fig. 5(a), the OSNR penalty at a BER threshold of 3.8 × 10−3 is ~0.2 dB if we transmit the SCM signal with 8 subcarriers in place of the SC signal. Similarly, in the 16-QAM case in Fig. 5(b), the penalty at a BER threshold of 2 × 10−2 is ~0.3 dB if the SCM signal with 4 subcarriers is transmitted. This is because the SCM signals are more susceptible to certain types of system imperfections, e.g., the limited effective DAC resolution, the FO estimation error and the laser phase noise.

Finally we extend our work to other baud rates in simulations. Figure 8
Fig. 8 Simulated Q2 factors (with the optimum launch power) of different signals with various total baud rates (a) at 6400 km in the QPSK system and (b) at 1920 km in the 16-QAM system.
shows the Q2 factors of different signals with various total baud rates. The transmission distance is 6400 km in the QPSK case in Fig. 8(a) and 1920 km in the 16-QAM case in Fig. 8(b). The Q2 values are obtained with the optimum launch power in each case. One thing to observe is the optimum number of subcarriers tends to increase as the total baud rate becomes larger. For example, the optimum number of subcarriers is close to 8 in the 16-GBaud QPSK system and increases to 16 when the total baud rate becomes 40 GBaud as shown in Fig. 8(a). In addition, in both figures we can see the system performance can be improved significantly within a wide range of baud rates (from 16 GBaud to 56 GBaud) by transmitting properly designed SCM signals. This result further indicates the merits of SCM signals in baud-rate flexible networks and future high-speed transmission systems.

4. Conclusion

In this work we experimentally generate subcarrier multiplexed signals using transmitter DSP and high-speed DACs and investigate the improved intra-channel nonlinearity tolerance of the SCM signals in long-haul transmission systems. By transmitting an SCM signal with a nearly-optimum number of subcarriers, we demonstrate an extended reach of 23% for a 24-GBaud DP-QPSK system with a BER threshold of 3.8 × 10−3 and of 8% for a 24-GBaud DP-16-QAM system with a BER threshold of 2 × 10−2. Besides, the advantage of SCM signals is demonstrated within a wide range of baud rates, which further indicates their potential applications in baud-rate adaptive transmissions and future high-speed communication systems.

References and links

1.

E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol. 26(20), 3416–3425 (2008). [CrossRef]

2.

L. B. Du and A. J. Lowery, “Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems,” Opt. Express 18(16), 17075–17088 (2010). [CrossRef] [PubMed]

3.

E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express 19(2), 570–583 (2011). [CrossRef] [PubMed]

4.

Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol. 29(17), 2570–2576 (2011). [CrossRef]

5.

Y. Gao, A. S. Karar, J. C. Cartledge, S. S.-H. Yam, M. O’Sullivan, C. Laperle, A. Borowiec, and K. Roberts, “Simplified nonlinearity pre-compensation using a modified summation criteria and non-uniform power profile,” in Proc. OFC (2014), paper Tu3A.6. [CrossRef]

6.

Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in Proc. OFC (2014), paper Th4D.7. [CrossRef]

7.

N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in Proc. Opt. Commun. Conf. (2011), paper OWW6. [CrossRef]

8.

B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of Nyquist pulses for coherent optical communications,” Opt. Express 20(8), 8397–8416 (2012). [CrossRef] [PubMed]

9.

X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in Proc. OFC (2013), paper JTh2A.38. [CrossRef]

10.

L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express 19(9), 8079–8084 (2011). [CrossRef] [PubMed]

11.

A. Bononi, N. Rossi, and P. Serena, “Performance dependence on channel baud-rate of coherent single-carrier WDM systems,” in Proc. ECOC (2013), paper Th.1.D.5. [CrossRef]

12.

W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J. 2(3), 276–283 (2010). [CrossRef]

13.

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]

14.

Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in Proc. OFC (2012), paper OTh1B.3. [CrossRef]

15.

Y. Zhu, J. Wang, Q. Guo, Y. Cui, C. Li, F. Zhu, and Y. Bai, “Experimental comparison of terabit Nyquist superchannel transmissions based on high and low baud rates,” in Proc. OFC (2013), paper JW2A.37. [CrossRef]

16.

M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in Proc. OFC (2014), paper Tu3J.2. [CrossRef]

17.

S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron. 16(5), 1164–1179 (2010). [CrossRef]

18.

M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun. 36(5), 605–612 (1988). [CrossRef]

19.

Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express 20(17), 19599–19609 (2012). [CrossRef] [PubMed]

20.

L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in Proc. OFC (2009), paper OMT2. [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Optical Communications

History
Original Manuscript: May 19, 2014
Revised Manuscript: July 7, 2014
Manuscript Accepted: July 9, 2014
Published: July 25, 2014

Citation
Meng Qiu, Qunbi Zhuge, Mathieu Chagnon, Yuliang Gao, Xian Xu, Mohamed Morsy-Osman, and David V. Plant, "Digital subcarrier multiplexing for fiber nonlinearity mitigation in coherent optical communication systems," Opt. Express 22, 18770-18777 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-15-18770


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References

  1. E. Ip and J. M. Kahn, “Compensation of dispersion and nonlinear impairments using digital backpropagation,” J. Lightwave Technol.26(20), 3416–3425 (2008). [CrossRef]
  2. L. B. Du and A. J. Lowery, “Improved single channel backpropagation for intra-channel fiber nonlinearity compensation in long-haul optical communication systems,” Opt. Express18(16), 17075–17088 (2010). [CrossRef] [PubMed]
  3. E. F. Mateo, X. Zhou, and G. Li, “Improved digital backward propagation for the compensation of inter-channel nonlinear effects in polarization-multiplexed WDM systems,” Opt. Express19(2), 570–583 (2011). [CrossRef] [PubMed]
  4. Z. Tao, L. Dou, W. Yan, L. Li, T. Hoshida, and J. C. Rasmussen, “Multiplier-free intrachannel nonlinearity compensating algorithm operating at symbol rate,” J. Lightwave Technol.29(17), 2570–2576 (2011). [CrossRef]
  5. Y. Gao, A. S. Karar, J. C. Cartledge, S. S.-H. Yam, M. O’Sullivan, C. Laperle, A. Borowiec, and K. Roberts, “Simplified nonlinearity pre-compensation using a modified summation criteria and non-uniform power profile,” in Proc. OFC (2014), paper Tu3A.6. [CrossRef]
  6. Q. Zhuge, M. Reimer, A. Borowiec, M. O’Sullivan, and D. Plant, “Aggressive quantization on perturbation coefficients for nonlinear pre-distortion,” in Proc. OFC (2014), paper Th4D.7. [CrossRef]
  7. N. Stojanovic, Y. Huang, F. N. Hauske, Y. Fang, M. Chen, C. Xie, and Q. Xiong, “MLSE-based nonlinearity mitigation for WDM 112 Gbit/s PDM-QPSK transmission with digital coherent receiver,” in Proc. Opt. Commun. Conf. (2011), paper OWW6. [CrossRef]
  8. B. Châtelain, C. Laperle, K. Roberts, M. Chagnon, X. Xu, A. Borowiec, F. Gagnon, and D. V. Plant, “A family of Nyquist pulses for coherent optical communications,” Opt. Express20(8), 8397–8416 (2012). [CrossRef] [PubMed]
  9. X. Xu, B. Châtelain, Q. Zhuge, M. Morsy-Osman, M. Chagnon, M. Qiu, and D. Plant, “Frequency domain M-shaped pulse for SPM nonlinearity mitigation in coherent optical communications,” in Proc. OFC (2013), paper JTh2A.38. [CrossRef]
  10. L. B. Du and A. J. Lowery, “Optimizing the subcarrier granularity of coherent optical communications systems,” Opt. Express19(9), 8079–8084 (2011). [CrossRef] [PubMed]
  11. A. Bononi, N. Rossi, and P. Serena, “Performance dependence on channel baud-rate of coherent single-carrier WDM systems,” in Proc. ECOC (2013), paper Th.1.D.5. [CrossRef]
  12. W. Shieh and Y. Tang, “Ultrahigh-speed signal transmission over nonlinear and dispersive fiber optic channel: the multicarrier advantage,” IEEE Photon. J.2(3), 276–283 (2010). [CrossRef]
  13. 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]
  14. Q. Zhuge, B. Chatelain, and D. Plant, “Comparison of intra-channel nonlinearity tolerance between reduced-guard-interval CO-OFDM systems and Nyquist single carrier systems,” in Proc. OFC (2012), paper OTh1B.3. [CrossRef]
  15. Y. Zhu, J. Wang, Q. Guo, Y. Cui, C. Li, F. Zhu, and Y. Bai, “Experimental comparison of terabit Nyquist superchannel transmissions based on high and low baud rates,” in Proc. OFC (2013), paper JW2A.37. [CrossRef]
  16. M. Qiu, Q. Zhuge, X. Xu, M. Chagnon, M. Morsy-Osman, and D. Plant, “Subcarrier multiplexing using DACs for fiber nonlinearity mitigation in coherent optical communication systems,” in Proc. OFC (2014), paper Tu3J.2. [CrossRef]
  17. S. J. Savory, “Digital coherent optical receivers: algorithms and subsystems,” IEEE J. Sel. Top. Quantum Electron.16(5), 1164–1179 (2010). [CrossRef]
  18. M. Oerder and H. Meyr, “Digital filter and square timing recovery,” IEEE Trans. Commun.36(5), 605–612 (1988). [CrossRef]
  19. Q. Zhuge, M. Morsy-Osman, X. Xu, M. E. Mousa-Pasandi, M. Chagnon, Z. A. El-Sahn, and D. V. Plant, “Pilot-aided carrier phase recovery for M-QAM using superscalar parallelization based PLL,” Opt. Express20(17), 19599–19609 (2012). [CrossRef] [PubMed]
  20. L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. Rasmussen, “Initial tap setup of constant modulus algorithm for polarization de-multiplexing in optical coherent receivers,” in Proc. OFC (2009), paper OMT2. [CrossRef]

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