## Comparison of 128-SP-QAM with PM-16-QAM |

Optics Express, Vol. 20, Issue 8, pp. 8356-8366 (2012)

http://dx.doi.org/10.1364/OE.20.008356

Acrobat PDF (1733 KB)

### Abstract

In this paper we investigate an interesting modulation format for fiber optic communications, set-partitioning 128 polarization-multiplexed 16-QAM (128-SP-QAM), which consists of the symbols with even parity from the symbol alphabet of polarization-multiplexed 16-QAM (PM-16-QAM). We compare 128-SP-QAM and PM-16-QAM using numerical simulations in long-haul transmission scenarios at bit rates of 112 Gbit/s and 224 Gbit/s, and at the same symbol rates (14 and 28 Gbaud). The transmission link is made up of standard single-mode fiber with 60, 80 or 100 km amplifier spacing and both single channel and WDM transmission (25- and 50 GHz-spaced) is investigated. The results show that 128-SP-QAM achieves more than 40% increase in transmission reach compared to PM-16-QAM at the same data rate for all cases studied for a bit error rate of 10^{−3}. In addition, we find that in single channel transmission there is, as expected, an advantage in terms of transmission distance when using a data rate of 112 Gbit/s as compared to 224 Gbit/s. However, when comparing the two different WDM systems with the same aggregate data rates, the reach is similar due to the smaller impact of nonlinear crosstalk between the WDM channels in the systems with 50 GHz spacing. We also discuss decoding and phase estimation of 128-SP-QAM and implement differential coding, which avoids error bursts due to cycle slips in the phase estimation. Simulations including laser phase noise show that the phase noise tolerance is similar for the two formats, with 0.5 dB OSNR penalty compared to the case with zero phase noise for a laser linewidth to symbol rate ratio of 10^{−4}.

© 2012 OSA

## 1. Introduction

1. D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. **24**(1), 12–21 (2006). [CrossRef]

3. J.-X. Cai, Y. Cai, C. R. Davidson, D. G. Foursa, A. J. Lucero, O. V. Sinkin, W. W. Patterson, A. N. Pilipetskii, G. M. Mohs, and N. S. Bergano, “Transmission of 96x100-Gb/s bandwidth-constrained PDM-RZ-QPSK channels with 300% spectral efficiency over 10610 km and 400% spectral efficiency over 4370 km,” J. Lightwave Technol. **29**(4), 491–498 (2011). [CrossRef]

4. P. J. Winzer, A. H. Gnauck, C. R. Doerr, M. Magarini, and L. L. Buhl, “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM,” J. Lightwave Technol. **28**(4), 547–556 (2010). [CrossRef]

5. 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, “Ultra-high capacity WDM transmission using spectrally-efficient PDM 16-QAM modulation and C- and extended L-band wideband optical amplification,” J. Lightwave Technol. **29**(4), 578–586 (2011). [CrossRef]

6. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express **17**(13), 10814–10819 (2009). [CrossRef] [PubMed]

7. E. Agrell and M. Karlsson, “Power-efficient modulation formats in coherent transmission systems,” J. Lightwave Technol. **27**(22), 5115–5126 (2009). [CrossRef]

8. X. Liu, T. H. Wood, R. W. Tkach and S. Chandrasekhar “Demonstration of record sensitivity in an optically pre-amplified receiver by combining PDM-QPSK and 16-PPM with pilot-assisted digital coherent detection,” in *National Fiber Optic Engineers Conference*, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB1.

11. G. Ungerboeck, “Channel coding with multilevel/phase signals,” IEEE Trans. Inf. Theory **28**(1), 55–67 (1982). [CrossRef]

12. P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for nonlinear fiber propagation effects,” Opt. Express **18**(11), 11360–11371 (2010). [CrossRef] [PubMed]

^{−3}after coherent detection. The formats are compared for fiber span lengths of 60, 80, and 100 km, and it is found that 128-SP-QAM allows for significantly increased transmission distances compared to PM-16-QAM in all cases.

## 2. Power efficiency and decoding of 128-SP-QAM

6. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express **17**(13), 10814–10819 (2009). [CrossRef] [PubMed]

^{−3}was 1.3 dB.

### 2.1 Non-differential coding

14. J. H. Conway and N. J. A. Sloane, “Fast quantizing and decoding algorithms for lattice quantizers and codes,” IEEE Trans. Inf. Theory **28**(2), 227–232 (1982). [CrossRef]

- 1. Find two 16-QAM symbols independently by using the signals in the two polarizations. Decode the Gray-coded 16-QAM symbols to obtain eight bits.
- 2. Manipulate the detected 4D signal by moving it over the closest decision threshold [14]. (This inverts the most uncertain bit, i.e. the only one of the eight bits affected by this operation.) Decode the 4D signal as above.
14. J. H. Conway and N. J. A. Sloane, “Fast quantizing and decoding algorithms for lattice quantizers and codes,” IEEE Trans. Inf. Theory

**28**(2), 227–232 (1982). [CrossRef] - 3. Check the parity of the two bit sequences derived from step 1 and step 2 (which will be different). Keep only the one with even parity. Discard the parity bit.

### 2.2 Differential coding

15. T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. **27**(8), 989–999 (2009). [CrossRef]

^{−3}[15

15. T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. **27**(8), 989–999 (2009). [CrossRef]

15. T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. **27**(8), 989–999 (2009). [CrossRef]

### 2.3 DSP for 128-SP-QAM

16. P. Johannisson, M. Sjödin, M. Karlsson, H. Wymeersch, E. Agrell, and P. A. Andrekson, “Modified constant modulus algorithm for polarization-switched QPSK,” Opt. Express **19**(8), 7734–7741 (2011). [CrossRef] [PubMed]

4. P. J. Winzer, A. H. Gnauck, C. R. Doerr, M. Magarini, and L. L. Buhl, “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM,” J. Lightwave Technol. **28**(4), 547–556 (2010). [CrossRef]

**27**(8), 989–999 (2009). [CrossRef]

## 3. Numerical simulations

18. D. Wang and C. R. Menyuk, “Polarization evolution due to the Kerr nonlinearity and chromatic dispersion,” J. Lightwave Technol. **17**(12), 2520–2529 (1999). [CrossRef]

### 3.1 Transmitter

*d*

_{par}, is used to make the parity of the symbols even. For the data signals we used sequences of 2

^{16}bits with random data that were different for each WDM channel. The driving signals to the modulator are bandwidth-limited by a 5th order Bessel filter (3-dB bandwidth of 0.75 times the symbol rate). Before wavelength multiplexing, each channel was filtered by a super-Gaussian filter of the second order and a 3-dB bandwidth of 22 and 44 GHz for data rates of 112 Gbit/s and 224 Gbit/s, respectively, resulting in good agreement with the measured transfer functions of commercial interleaver filters. The same type of filter was used in the receiver to demultiplex the channels.

_{p}

^{2}= 2π∆ν∙

*T*, where ∆ν is the combined linewidth of the transmitter and LO lasers and

*T*is the symbol period.

### 3.2 The link

*α*= 0.20 dB/km, dispersion coefficient

*D*= 17 ps/nm/km, dispersion slope

*S*= 0.07 ps/nm

^{2}/km,

*γ*= 1.2/W/km, and the EDFA noise figure was set to 5.0 dB. Polarization-mode dispersion was not included in our simulations.

### 3.3 Receiver

### 3.4 The WDM system

*System 1*with fourteen channels at 25 GHz spacing each carrying 112 Gbit/s of data (14 and 16 Gbaud for PM-16-QAM and 128-SP-QAM, respectively), and

*System 2*with seven 50 GHz-spaced channels operating at 224 Gbit/s (28 and 32 Gbaud for PM-16-QAM and 128-SP-QAM, respectively). To enable a fair performance comparison, the aggregate data rate was the same for the two systems. In addition, we included the case in which 128-SP-QAM has the same symbol rate as PM-16-QAM, to see how the format performs as a fall-back option. All channels were launched co-polarized, as there is no reason to believe that the launch polarization has a significant influence on the performance. For example, numerical comparisons of PM-QPSK and PS-QPSK have shown that the impact of launch polarization alignment is negligible [12

12. P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for nonlinear fiber propagation effects,” Opt. Express **18**(11), 11360–11371 (2010). [CrossRef] [PubMed]

## 4. Simulation results

### 4.1 Back-to-back OSNR

^{−3}is 1.4 dB and 2.0 dB in favor of 128-SP-QAM at the same data rate and the same symbol rate as PM-16-QAM, respectively. This is in good agreement with the 1.3 dB difference at 112 Gbit/s that was observed in [10]. The penalties for using differential coding are 0.5 dB and 0.4 dB at a receiver BER of 10

^{−3}for 128-SP-QAM and PM-16-QAM, respectively. Subsequently, the difference in required OSNR is reduced by only 0.1 dB compared to non-differential coding.

^{−3}is reduced by 1.3 dB and 1.9 dB for 128-SP-QAM with the same data rate and the same symbol rate, respectively. As this difference is very close to the one observed without the DSP and the filters, we conclude that the DSP algorithms work equally well for both formats.

### 4.2 Linewidth tolerance

^{−3}as a function of ∆ν∙

*T*and the block length

*N*in the phase estimation algorithm for 112 Gbit/s 128-SP-QAM and 112 Gbit/s PM-16-QAM, respectively. The formats have similar linewidth tolerance for the investigated block lengths.

*N*is used for each value of ∆ν∙

*T*, and both formats have a 0.5 dB OSNR penalty threshold, compared to the low phase noise regime, at ∆ν∙

*T*≈10

^{−4}, corresponding to a combined laser linewidth of 1.6 MHz and 1.4 MHz for 112 Gbit/s 128-SP-QAM and PM-16-QAM, respectively. This result is similar to what was achieved in [15

**27**(8), 989–999 (2009). [CrossRef]

### 4.3 Transmission performance

^{−3}. However, in WDM transmission the reach is similar for the two different systems. This is in good agreement with the results presented in [20

20. P. Poggiolini, G. Bosco, A. Carena, V. Curri, V. Miot, and F. Forghieri, “Performance dependence on channel baud-rate of PM-QPSK systems over uncompensated links,” IEEE Photon. Technol. Lett. **23**(1), 15–17 (2011). [CrossRef]

^{−3}for all span lengths for the latter format. For both single channel and WDM transmission, 128-SP-QAM can be transmitted 40-50% longer at both 112 Gbit/s and 224 Gbit/s. This shows that 128-SP-QAM achieves increased reach compared to PM-16-QAM and increased spectral efficiency compared to PM-QPSK. The increase in symbol rate of 14% to maintain a constant data rate is not as substantial as the 33% increase required to use PS-QPSK instead of PM-QPSK [6

6. M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express **17**(13), 10814–10819 (2009). [CrossRef] [PubMed]

## 5. Conclusion

^{−3}was 0.5 dB for 128-SP-QAM, compared to 0.4 dB for PM-16-QAM. We performed simulations including laser phase noise and found the phase noise tolerance to be similar for the two formats for the algorithms that we used. The algorithms also turned out to perform equally well for 128-SP-QAM and PM-16-QAM.

^{−3}for all investigated span lengths and for both single channel and WDM transmission. Furthermore, our results show that the transmission reach in the two WDM systems is similar, although the data rate per channel is two times higher in system 2.

## Acknowledgment

## References and links

1. | D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol. |

2. | H. Sun, K. T. Wu, and K. Roberts, “Real-time measurements of a 40 Gb/s coherent system,” Opt. Express |

3. | J.-X. Cai, Y. Cai, C. R. Davidson, D. G. Foursa, A. J. Lucero, O. V. Sinkin, W. W. Patterson, A. N. Pilipetskii, G. M. Mohs, and N. S. Bergano, “Transmission of 96x100-Gb/s bandwidth-constrained PDM-RZ-QPSK channels with 300% spectral efficiency over 10610 km and 400% spectral efficiency over 4370 km,” J. Lightwave Technol. |

4. | P. J. Winzer, A. H. Gnauck, C. R. Doerr, M. Magarini, and L. L. Buhl, “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM,” J. Lightwave Technol. |

5. | 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, “Ultra-high capacity WDM transmission using spectrally-efficient PDM 16-QAM modulation and C- and extended L-band wideband optical amplification,” J. Lightwave Technol. |

6. | M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express |

7. | E. Agrell and M. Karlsson, “Power-efficient modulation formats in coherent transmission systems,” J. Lightwave Technol. |

8. | X. Liu, T. H. Wood, R. W. Tkach and S. Chandrasekhar “Demonstration of record sensitivity in an optically pre-amplified receiver by combining PDM-QPSK and 16-PPM with pilot-assisted digital coherent detection,” in |

9. | H. Bülow, “Polarization QAM modulation (POLQAM) for coherent detection schemes,” in |

10. | L. D. Coelho and N. Hanik, “Global optimization of fiber-optic communication systems using four-dimensional modulation formats,” in |

11. | G. Ungerboeck, “Channel coding with multilevel/phase signals,” IEEE Trans. Inf. Theory |

12. | P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for nonlinear fiber propagation effects,” Opt. Express |

13. | M. Karlsson and E. Agrell, “Spectrally efficient four-dimensional modulation,” in |

14. | J. H. Conway and N. J. A. Sloane, “Fast quantizing and decoding algorithms for lattice quantizers and codes,” IEEE Trans. Inf. Theory |

15. | T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol. |

16. | P. Johannisson, M. Sjödin, M. Karlsson, H. Wymeersch, E. Agrell, and P. A. Andrekson, “Modified constant modulus algorithm for polarization-switched QPSK,” Opt. Express |

17. | C. Xie, “Local oscillator induced penalties in optical coherent detection systems using electronic chromatic dispersion compensation,” in |

18. | D. Wang and C. R. Menyuk, “Polarization evolution due to the Kerr nonlinearity and chromatic dispersion,” J. Lightwave Technol. |

19. | P. Serena, A. Vannucci, and A. Bononi, “The performance of polarization switched QPSK (PS-QPSK) in dispersion managed WDM transmissions,” in |

20. | P. Poggiolini, G. Bosco, A. Carena, V. Curri, V. Miot, and F. Forghieri, “Performance dependence on channel baud-rate of PM-QPSK systems over uncompensated links,” IEEE Photon. Technol. Lett. |

**OCIS Codes**

(060.0060) Fiber optics and optical communications : Fiber optics and optical communications

(060.1660) Fiber optics and optical communications : Coherent communications

(060.4080) Fiber optics and optical communications : Modulation

**ToC Category:**

Fiber Optics and Optical Communications

**History**

Original Manuscript: February 14, 2012

Revised Manuscript: March 21, 2012

Manuscript Accepted: March 21, 2012

Published: March 26, 2012

**Citation**

Martin Sjödin, Pontus Johannisson, Jianqiang Li, Erik Agrell, Peter A. Andrekson, and Magnus Karlsson, "Comparison of 128-SP-QAM with PM-16-QAM," Opt. Express **20**, 8356-8366 (2012)

http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-8-8356

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### References

- D.-S. Ly-Gagnon, S. Tsukamoto, K. Katoh, and K. Kikuchi, “Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation,” J. Lightwave Technol.24(1), 12–21 (2006). [CrossRef]
- H. Sun, K. T. Wu, and K. Roberts, “Real-time measurements of a 40 Gb/s coherent system,” Opt. Express16(2), 873–879 (2008). [CrossRef] [PubMed]
- J.-X. Cai, Y. Cai, C. R. Davidson, D. G. Foursa, A. J. Lucero, O. V. Sinkin, W. W. Patterson, A. N. Pilipetskii, G. M. Mohs, and N. S. Bergano, “Transmission of 96x100-Gb/s bandwidth-constrained PDM-RZ-QPSK channels with 300% spectral efficiency over 10610 km and 400% spectral efficiency over 4370 km,” J. Lightwave Technol.29(4), 491–498 (2011). [CrossRef]
- P. J. Winzer, A. H. Gnauck, C. R. Doerr, M. Magarini, and L. L. Buhl, “Spectrally efficient long-haul optical networking using 112-Gb/s polarization-multiplexed 16-QAM,” J. Lightwave Technol.28(4), 547–556 (2010). [CrossRef]
- 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, “Ultra-high capacity WDM transmission using spectrally-efficient PDM 16-QAM modulation and C- and extended L-band wideband optical amplification,” J. Lightwave Technol.29(4), 578–586 (2011). [CrossRef]
- M. Karlsson and E. Agrell, “Which is the most power-efficient modulation format in optical links?” Opt. Express17(13), 10814–10819 (2009). [CrossRef] [PubMed]
- E. Agrell and M. Karlsson, “Power-efficient modulation formats in coherent transmission systems,” J. Lightwave Technol.27(22), 5115–5126 (2009). [CrossRef]
- X. Liu, T. H. Wood, R. W. Tkach and S. Chandrasekhar “Demonstration of record sensitivity in an optically pre-amplified receiver by combining PDM-QPSK and 16-PPM with pilot-assisted digital coherent detection,” in National Fiber Optic Engineers Conference, OSA Technical Digest (CD) (Optical Society of America, 2011), paper PDPB1.
- H. Bülow, “Polarization QAM modulation (POLQAM) for coherent detection schemes,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OWG2.
- L. D. Coelho and N. Hanik, “Global optimization of fiber-optic communication systems using four-dimensional modulation formats,” in 37th European Conference and Exposition on Optical Communications, OSA Technical Digest (CD) (Optical Society of America, 2011), paper Mo.2.B.4. .
- G. Ungerboeck, “Channel coding with multilevel/phase signals,” IEEE Trans. Inf. Theory28(1), 55–67 (1982). [CrossRef]
- P. Poggiolini, G. Bosco, A. Carena, V. Curri, and F. Forghieri, “Performance evaluation of coherent WDM PS-QPSK (HEXA) accounting for nonlinear fiber propagation effects,” Opt. Express18(11), 11360–11371 (2010). [CrossRef] [PubMed]
- M. Karlsson and E. Agrell, “Spectrally efficient four-dimensional modulation,” in Optical Fiber Communication Conference, OSA Technical Digest (Optical Society of America, 2012), paper OTu2C.1
- J. H. Conway and N. J. A. Sloane, “Fast quantizing and decoding algorithms for lattice quantizers and codes,” IEEE Trans. Inf. Theory28(2), 227–232 (1982). [CrossRef]
- T. Pfau, S. Hoffmann, and R. Noé, “Hardware-efficient coherent digital receiver concept with feedforward carrier recovery for M-QAM constellations,” J. Lightwave Technol.27(8), 989–999 (2009). [CrossRef]
- P. Johannisson, M. Sjödin, M. Karlsson, H. Wymeersch, E. Agrell, and P. A. Andrekson, “Modified constant modulus algorithm for polarization-switched QPSK,” Opt. Express19(8), 7734–7741 (2011). [CrossRef] [PubMed]
- C. Xie, “Local oscillator induced penalties in optical coherent detection systems using electronic chromatic dispersion compensation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OMT4.
- D. Wang and C. R. Menyuk, “Polarization evolution due to the Kerr nonlinearity and chromatic dispersion,” J. Lightwave Technol.17(12), 2520–2529 (1999). [CrossRef]
- P. Serena, A. Vannucci, and A. Bononi, “The performance of polarization switched QPSK (PS-QPSK) in dispersion managed WDM transmissions,” in 2010 36th European Conference and Exhibition on Optical Communication (ECOC) (2010), paper Th.10.E.2.
- P. Poggiolini, G. Bosco, A. Carena, V. Curri, V. Miot, and F. Forghieri, “Performance dependence on channel baud-rate of PM-QPSK systems over uncompensated links,” IEEE Photon. Technol. Lett.23(1), 15–17 (2011). [CrossRef]

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