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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 4 — Feb. 13, 2012
  • pp: 3877–3882
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Performance comparison of MSK and QPSK optical long haul DWDM transmission with coherent detection

A. Hachmeister, M. Nölle, L. Molle, R. Freund, and M. Rohde  »View Author Affiliations


Optics Express, Vol. 20, Issue 4, pp. 3877-3882 (2012)
http://dx.doi.org/10.1364/OE.20.003877


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Abstract

We performed long-haul WDM transmission experiments to compare 10 Gbit/s MSK and QPSK modulation with a channel grid of 12.5 GHz. A standard link setup with inline dispersion compensation was applied in combination with coherent detection and following offline signal processing. Both modulation formats showed nearly equal performance bridging about 4000 km at a BER of 10−3.

© 2012 OSA

1. Introduction

To satisfy the demands for higher bandwidth of optical transmission systems, advanced optical modulation formats have been intensely investigated. For long haul transmission, the higher order modulation formats minimum shift keying (MSK) and differential quadrature phase shift keying (DQPSK) are aspirants to replace the conventional modulation formats differential binary phase shift keying (DBPSK) or on-off keying (OOK). They provide a doubled spectral efficiency at moderate additional complexity.

The authors experimentally demonstrated the performance of single channel optical minimum shift keying (MSK) [1

1. M. Rohde, R. Freund, C. Caspar, A. Hachmeister, and M. Gruner, “Long haul transmission of optical minimum shift keying format,“ in Proc. Eur. Conf. of Optical Comm, Sept. 2008, paper Mo.4.E.4.

] and compared MSK and DQPSK through numerical simulations [2

2. A. Hachmeister, M. Rohde, and R. Freund, “Long haul transmission of optical minimum shift keying format with narrow channel spacing,” in Proc. Asia Comm. and Phot. Conf., Nov. 2009, paper ThT6.

]. The results show that, with appropriate optical filtering, both modulation formats are applicable in a wavelength division multiplex (WDM) setup with a channel grid of 12.5 GHz at a data rate of 10 Gbit/s resulting in a symbol rate of 5 GBaud. This grid ensures the same spectral efficiency for the modulation formats in question, although the main lobe of MSK (15 GHz) is broader then the main lobe of DQPSK (10 GHz) as can be seen in Fig. 1
Fig. 1 5 GBaud MSK and QPSK in the frequency domain. The dotted rectangle represents the 12.5 GHz channel.
.

Because the channel spacing of 12.5 GHz can be generated through symmetrical splitting of ITU-T standard channels and optical interleavers are available on the market, 10 Gbit/s MSK or DQPSK may replace today’s 10 Gbit/s DBPSK format for long haul transmission links.

2. Experimental setup

The system setup used for this 5 GBaud QPSK and MSK experiment with WDM channel spacing of 12.5 GHz is illustrated in Fig. 2
Fig. 2 Experimental setup of 5 GBaud MSK resp. QPSK six channel WDM transmission.
. As laser sources, six external cavity lasers (ECL) with a linewidth of about 100 kHz spaced at 12.5 GHz were used. These six carriers were grouped in odd and even channels, individually modulated by two IQ-modulators (IQM) and then recombined by a 12.5 GHz interleaver.

The electrical driving signal is generated by an arbitrary waveform generator (AWG) operating at its maximum sample rate of 20 GSa/s. Due to the lack of a second AWG, the quadrature driving signal is generated out of the inphase signals, delayed by 11 symbols to ensure independent data, plus, in case of MSK an additional delay by half a symbol duration. The sinusoidal pulse shaping for MSK is done at 4 samples per symbol in the AWG. The driving signals for the second IQM are generated in the same manner out of the inverted data output of the AWG. These signals are delayed by approx. 1.25 ns to decorrelate the driving signals of the two individual IQMs. The electrical signals were amplified to Vpp ~5V, which is approx. Vπ of the IQMs. The IQMs were modulated symmetrically around the minimum power transmission point. Figure 3
Fig. 3 left: Power transfer function of the 12.5 GHz interleaver, right: Electrical inphase and quadrature driving signals (X: 50ps/div; Y: 2.5V/div) (upper diagrams) and optical WDM spectrum with 12.5 GHz channel spacing (X: 0.1nm/div; Y: 10 dB/div) (bottom diagrams) for MSK and QPSK.
shows the power transfer function of the interleaver on the left hand side and the electrical driving signals together with the optical WDM spectra at the transmitter output on the right hand side.

The optical attenuator in front of the EDFA in the coherent receiver allows noise loading to set arbitrary OSNR values for back-to-back experiments. Another interleaver followed by a narrow optical channel filter is used to select the central channel of the even band for measuring. The optical front end is composed of a 90° hybrid and two balanced detectors. The transmitter ECL for the analyzed channel is used as local oscillator (LO) signal as well, however for decorrelation in a back-to-back setup 4 km SSMF is assembled between transmitter and receiver. Two polarization controllers (PC) are applied to maximize the optical power of the signal and the LO manually.

A digital storage oscilloscope with 16 GHz bandwidth digitizes the two photocurrents synchronously at 10 samples per symbol. The high oversampling was chosen to keep the opportunity to regain the optical signal most accurately in order to calculate, for example, the performance of direct detection receivers.

Data is recovered offline in a personal computer and the following signal processing blocks are applied: hybrid mean- and amplitude correction, low pass filtering, resampling to one (QPSK) respectively two (MSK) samples per symbol at the optimum sampling instant, phase estimation and correction [3

3. M. Seimetz, “Digital phase estimation,” in High-Order Modulation for Optical Fiber Transmission (Springer, 2009), 99–111.

] and data recovery. A frequency offset was no issue in this experiment, because the same ECL is utilized in the transmitter and receiver. The final error counting was averaged over 3 blocks of 10 million samples (3 million symbols), allowing a reliable bit error ratio (BER) measurement down to approx. 2⋅10−6.

3. Experimental results

Figure 4
Fig. 4 left: Back-to-back results for experiment (QPSK represents single + WDM) and simulation (dashed lines; for WDM only), right: Constellation plots for MSK and QPSK, OSNR ~4 dB in case of WDM, red circles represent error symbols.
shows the optical signal-to-noise ratio (OSNR) requirements of experiments and simulations in a back-to-back setup. While QPSK and MSK theoretically show equal performance [4

4. F. Xiong, Digital Modulation Techniques (Artech House, 2006).

], the required measured OSNR for MSK turned out to be slightly higher in the simulation as well as in the experimental investigation. This is due to the higher electrical bandwidth of MSK. Comparing the simulation results for the back-to-back case with the experimental performance, Fig. 4 shows approximately 2 dB implementation penalty for QPSK. While the implementation penalty is slightly higher for the MSK case, we observed nearly no additional OSNR penalty for the WDM case in comparison to a single channel setup for both modulation formats. The same system setup, including realistic interleaver filter functions, was applied for the simulations. The 2 dB penalty of the experimental results is due to imperfect electrical signal generation and bandwidth limitations.

Reducing SPM by nonlinearity compensation with signal processing leads to a distance gain of approx. 380 km for QPSK and 260 km for MSK at a BER of 10−3 (Fig. 5). Reducing the WDM degradation effects cross phase modulation (XPM) and four wave mixing (FWM) by switching off the modulation of the neighboring channels (odd band) and thereby doubling the channel spacing results in a gain of approx. 420 km for QPSK and 300 km for MSK, also at a BER of 10−3. These measurement results show, that the distance loss caused by SPM and XPM/FWM respectively is within the same order of magnitude.

4. Discussion

The experimental results show, that MSK performs somewhat better than QPSK in a dispersion-compensated long haul transmission setup. With compensation of fiber nonlinearities they perform nearly alike. Obviously, implementation aspects play a major role for the decision, which of the both modulation formats is suitable to replace DBPSK.

Looking at the transmitter, the differential encoding for direct detection systems is less complex for MSK than for QPSK. It can be accomplished just as in a DBPSK system as a feedback XOR gate. A DQPSK transmitter needs parallel implementation with 10 gates [6

6. R. A. Griffin and A. C. Carter, “Optical differential quadrature phase shift key (oDQPSK) for high capacity optical transmission,” in Proc. Optical Fiber Comm. Conf., Mar. 2002, paper WX6.

]. However, considering the effort to implement FEC, the additional effort for differential encoding is not relevant. On the other hand, the necessity of the sinusoidal weighting of the driving signals for MSK increases the complexity of the transmitter and the bandwidth requirements of the components. Shifting this procedure to the optical domain prevents the drawback of enhanced bandwidth requirements at the cost of an extra optical modulator, but lately a highly integrated MSK transmitter has been demonstrated using a so called quad Mach-Zehnder inphase quadrature modulator [7

7. T. Sakamoto, G. W. Lu, A. Chiba, T. Kawanishi, and T. Miyazaki, “Coherent demodulation of 10-Gb/s optical minimum shift keying,” in Proc. Opticl. Fiber Comm. Conf., Mar. 2010, paper JThA2.

, 8

8. G. W. Lu, T. Sakamoto, A. Chiba, T. Kawanishi, T. Miyazaki, K. Higuma, and J. Ichikawa, “80-Gb/s optical MSK generation using a monolithically integrated quad Mach-Zehnder IQ modulator,” in Proc. Optical Fiber Comm. Conf., Mar. 2010, paper OWN5.

].

The hardware effort of a coherent receiver is identical for both modulation formats. In direct detection systems a standard DBPSK receiver composed of a delay line interferometer (DLI) and a balanced detector (BD) can be applied for MSK. A DQPSK receiver requires twice this hardware. Due to the 50% broader spectrum in comparison to QPSK, MSK is more critical with respect to optical filtering or laser emission frequency maladjustments.

Overall, for direct detection systems, MSK has an advantage over QPSK because the compensation of nonlinearities on the receiver side is not possible and MSK performs better under these circumstances [2

2. A. Hachmeister, M. Rohde, and R. Freund, “Long haul transmission of optical minimum shift keying format with narrow channel spacing,” in Proc. Asia Comm. and Phot. Conf., Nov. 2009, paper ThT6.

]. Furthermore the receiver for MSK is easier to implement. For coherent detection, this advantage is reduced. The receivers are identical and the performance of QPSK gains through the mitigation of nonlinear phase distortion. Coherent optical transmission systems have the big advantage, that optical inline dispersion compensation is no longer necessary. The overall system noise is reduced because there is no more need for optical amplifiers to compensate for the attenuation of the DCF and longer distances are achievable.

5. Conclusion

Acknowledgments

This work was supported by the BMOD project of the German Federal Ministry of Education and Research (BMBF).

References and links

1.

M. Rohde, R. Freund, C. Caspar, A. Hachmeister, and M. Gruner, “Long haul transmission of optical minimum shift keying format,“ in Proc. Eur. Conf. of Optical Comm, Sept. 2008, paper Mo.4.E.4.

2.

A. Hachmeister, M. Rohde, and R. Freund, “Long haul transmission of optical minimum shift keying format with narrow channel spacing,” in Proc. Asia Comm. and Phot. Conf., Nov. 2009, paper ThT6.

3.

M. Seimetz, “Digital phase estimation,” in High-Order Modulation for Optical Fiber Transmission (Springer, 2009), 99–111.

4.

F. Xiong, Digital Modulation Techniques (Artech House, 2006).

5.

K.-P. Ho and J. M. Kahn, “Electronic compensation technique to mitigate nonlinear phase noise,” J. Lightwave Technol. 22(3), 779–783 (2004). [CrossRef]

6.

R. A. Griffin and A. C. Carter, “Optical differential quadrature phase shift key (oDQPSK) for high capacity optical transmission,” in Proc. Optical Fiber Comm. Conf., Mar. 2002, paper WX6.

7.

T. Sakamoto, G. W. Lu, A. Chiba, T. Kawanishi, and T. Miyazaki, “Coherent demodulation of 10-Gb/s optical minimum shift keying,” in Proc. Opticl. Fiber Comm. Conf., Mar. 2010, paper JThA2.

8.

G. W. Lu, T. Sakamoto, A. Chiba, T. Kawanishi, T. Miyazaki, K. Higuma, and J. Ichikawa, “80-Gb/s optical MSK generation using a monolithically integrated quad Mach-Zehnder IQ modulator,” in Proc. Optical Fiber Comm. Conf., Mar. 2010, paper OWN5.

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4080) Fiber optics and optical communications : Modulation

ToC Category:
Fiber Optics and Optical Communications

History
Original Manuscript: November 2, 2011
Revised Manuscript: December 9, 2011
Manuscript Accepted: December 10, 2011
Published: February 1, 2012

Citation
A. Hachmeister, M. Nölle, L. Molle, R. Freund, and M. Rohde, "Performance comparison of MSK and QPSK optical long haul DWDM transmission with coherent detection," Opt. Express 20, 3877-3882 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-4-3877


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References

  1. M. Rohde, R. Freund, C. Caspar, A. Hachmeister, and M. Gruner, “Long haul transmission of optical minimum shift keying format,“ in Proc. Eur. Conf. of Optical Comm, Sept. 2008, paper Mo.4.E.4.
  2. A. Hachmeister, M. Rohde, and R. Freund, “Long haul transmission of optical minimum shift keying format with narrow channel spacing,” in Proc. Asia Comm. and Phot. Conf., Nov. 2009, paper ThT6.
  3. M. Seimetz, “Digital phase estimation,” in High-Order Modulation for Optical Fiber Transmission (Springer, 2009), 99–111.
  4. F. Xiong, Digital Modulation Techniques (Artech House, 2006).
  5. K.-P. Ho and J. M. Kahn, “Electronic compensation technique to mitigate nonlinear phase noise,” J. Lightwave Technol.22(3), 779–783 (2004). [CrossRef]
  6. R. A. Griffin and A. C. Carter, “Optical differential quadrature phase shift key (oDQPSK) for high capacity optical transmission,” in Proc. Optical Fiber Comm. Conf., Mar. 2002, paper WX6.
  7. T. Sakamoto, G. W. Lu, A. Chiba, T. Kawanishi, and T. Miyazaki, “Coherent demodulation of 10-Gb/s optical minimum shift keying,” in Proc. Opticl. Fiber Comm. Conf., Mar. 2010, paper JThA2.
  8. G. W. Lu, T. Sakamoto, A. Chiba, T. Kawanishi, T. Miyazaki, K. Higuma, and J. Ichikawa, “80-Gb/s optical MSK generation using a monolithically integrated quad Mach-Zehnder IQ modulator,” in Proc. Optical Fiber Comm. Conf., Mar. 2010, paper OWN5.

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