OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 26 — Dec. 10, 2012
  • pp: B141–B150
« Show journal navigation

Improved two-stage equalization for coherent Pol-Mux QPSK and 16-QAM systems

Chen Zhu, An V. Tran, Simin Chen, Liang B. Du, Trevor Anderson, Arthur J. Lowery, and Efstratios Skafidas  »View Author Affiliations


Optics Express, Vol. 20, Issue 26, pp. B141-B150 (2012)
http://dx.doi.org/10.1364/OE.20.00B141


View Full Text Article

Acrobat PDF (1260 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We report a two-stage blind frequency domain equalization method for long-haul coherent polarization-multiplexed (pol-mux) systems using quadrature phase shift keying (QPSK) and 16-quadrature amplitude modulation (16-QAM). In the first stage, blind CD parameter prediction is conducted prior to a CD equalizer. This supports flexible path switching in optical networks. In the second stage, a frequency-domain multi-modulus algorithm (MMA) equalizer is used to cope with the residual fiber impairments and perform polarization de-multiplexing. Compared with the conventional constant modulus algorithm (CMA), MMA shows advantages including better steady state performance and a faster convergence rate. Furthermore, all the estimation and equalization algorithms are implemented in the frequency domain which potentially provides the least complexity for the pol-mux optical coherent systems. The proposed algorithm is experimentally demonstrated with an 800-km 10 Gbaud coherent optical pol-mux system. For QPSK signal, the proposed method achieves error-free transmission and shows superior convergence speed against CMA, and for 16-QAM signals, the proposed MMA outperforms CMA with more than 1-dB improvement in Q-value.

© 2012 OSA

1. Introduction

Enabled by coherent detection technique, advanced modulation formats improve spectral efficiency to support the growing demand of capacity in fiber transmission systems [1

1. J. Yu and X. Zhou, “Ultra-high-capacity DWDM transmission system for 100G and beyond,” IEEE Commun. Mag. 48(3), S56–S64 (2010). [CrossRef]

]. Moreover, polarization multiplexing can be utilized to double bandwidth efficiency. 100 Gb/s pol-mux quadrature phase shift keying (QPSK) systems are becoming widely deployed for optical transport networks. More recently, high-order quadrature amplitude modulation (QAM) formats are being extensively investigated to pursue further compression of the signal spectrum [2

2. 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]

4

4. 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), 3042–3049 (2009). [CrossRef]

].

Coherent detection retains both the amplitude and phase information of the received signal, allowing digital signal processing (DSP) to compensate for linear optical channel impairments such as chromatic dispersion (CD) and polarization mode dispersion (PMD) [5

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

7

7. B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE Sel. Top. in Quantum Electron. 16(5), 1180–1192 (2010). [CrossRef]

]. Compared with time domain (TD) DSP approaches that use finite-impulse-response (FIR) filters, frequency domain (FD) implementations show advantages in terms of lower computational effort as they employ fast Fourier transforms (FFT) and block processing [8

8. R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent optical single carrier transmission using overlap frequency domain equalization for long-haul optical systems,” J. Lightwave Technol. 27(16), 3721–3728 (2009). [CrossRef]

]. Among existing single carrier frequency domain equalization (SC-FDE) systems, the data-aided (DA) approach has been proposed to allow impairment compensation in a single step [9

9. A. V. Tran, C. Zhu, C. C. Do, S. Chen, T. Anderson, D. Hewitt, and E. Skafidas, “8×40-Gb/s optical coherent pol-mux single carrier system with frequency domain equalization and training sequences,” IEEE Photon. Technol. Lett. 24(11), 885–887 (2012). [CrossRef]

]. However this technique requires the transmission of a long training sequence for channel estimation, which reduces spectral efficiency. The FD non-data-aided (NDA) method [10

10. M. S. Faruk and K. Kikuchi, “Adaptive frequency-domain equalization in digital coherent optical receivers,” Opt. Express 19(13), 12789–12798 (2011). [CrossRef] [PubMed]

] also achieves equalization in a single step, but uses a constant modulus algorithm (CMA), which exhibits large errors for high-order QAM systems and has only been demonstrated for short transmission distances.

This paper provides a comprehensive theoretical analysis and detailed experimental results of the two-stage FD equalization method for pol-mux NDA QPSK and 16-QAM systems proposed in [14

14. C. Zhu, A. V. Tran, S. Chen, L. B. Du, T. Anderson, A. J. Lowery, and E. Skafidas, “Dual-stage frequency domain equalization for long-haul coherent polarization-multiplexed QPSK and 16-QAM systems,” in European Conference on Optical Communication (ECOC), paper We.1.A.2 (2012).

]. In the first stage, a blind CD parameter prediction is conducted prior to the CD equalizer, which works over a wide range of dispersions. The second stage uses a FD multiple-input multiple-output (MIMO) equalizer, based on the multi-modulus algorithm (MMA), to provide better performance than CMA adaptation. In 800-km 40-Gb/s pol-mux QPSK and 80-Gb/s pol-mux 16-QAM transmission experiments, the proposed FD algorithms show much lower implementation complexity, without performance degradation, when compared with TD and CD parameter assisted systems. We show that our second-stage FD-MMA algorithm provides faster convergence and better steady-state error performance than a FD-CMA implementation.

The paper is organized as follows. Section 2 describes the system design including the system setup and DSP schedules. The experimental results are presented and discussed in Section 3. Finally, Section 4 summarizes the paper.

2. Two-stage NDA SC-FDE system design

2.1 Receiver DSP architecture

2.2 Blind CD compensation

2.3 Frequency domain blind equalization

The second stage equalizer can operate on shorter blocks of data, so for numerical efficiency the output samples of stage one are rearranged into shorter blocks. The output samples of the CD equalizer are first transformed back to TD. Then, to let the frequency domain equalizer use 2-fold oversampling, for each polarization the samples are split into two tributaries: one for even samples or odd samples. Blocks of 16 samples are collected and four FFTs are used to transform each of these four tributaries to the frequency domain.

In the following analysis, N equals to 8, which is half of the equalizer length, bold-face letters are used to represent vectors, []Hdenotes the Hermitian transpose, []* indicates the complex conjugate operation, []T stands for the matrix transpose, []e/o stands for even or odd tributary of the samples, i and j indicate either x or y polarization, 0Nand IN are defined as N×N zero and identity matrix, while P=[IN0N]T and Q=[0NIN]T are used to pad zeroes before and after the vector being multiplied, respectively.

With the conventional TD blind MIMO equalizer, the output can be formulated as [5

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

]:
xout=wxxHxin+wxyHyin
(2)
yout=wyxHxin+wyyHyin
(3)
where wij is a 2N×1 column vector representing the TD equalizer coefficients. As shown in Fig. 2(a)
Fig. 2 (a) Structure of FD adaptive equalizer, (b) tap weight update algorithm.
, the frequency domain implementation of the adaptive equalizer is based on the even-odd sub-equalizers approach [10

10. M. S. Faruk and K. Kikuchi, “Adaptive frequency-domain equalization in digital coherent optical receivers,” Opt. Express 19(13), 12789–12798 (2011). [CrossRef] [PubMed]

] and the 50% overlap-save sectioning method [18

18. J. J. Shynk, “Frequency-domain and multirate adaptive filtering,” IEEE Sig. Proc. Mag. 9(1), 14–37 (1992). [CrossRef]

]. Firstly we define the 2N×1 auxiliary coefficient vectorWije/o=FFT[Pwije/o], and Eije/o=FFT[P(wije/o)*] as the FD tap weight vector used for equalization, where P=[IN0N]T means adding zeroes after the vector being multiplied. The relationship between Wije/o and Eije/o can be expressed as [19

19. B. Porat, A Course in Digital Singal Processing (Wiley, 1997).

]:
Eije/o=(KWije/o)*
(4)
where K=(1zeros(1,2N1)zeros(2N1,1)fliplr(I2N1)) aims to convert between FFT of a vector and the FFT of its complex conjugate vector, and fliplr() means flipping the matrix from left to right. Then the FD equalizer output can be expressed as:
xout=GIFFT(XineExxe+XinoExxo+YineExye+YinoExyo)
(5)
yout=GIFFT(XineEyxe+XinoEyxo+YineEyye+YinoEyyo)
(6)
where: is the element-wise multiplication, Xout/Yout is the 2N × 1 FD input vector, G=[0NIN] is used to filter out the first N samples of the equalized output represented by the IFFT bracket, and xout/yout is the remaining N TD output samples.

The gradient estimation is shown in Fig. 2(b), where the TD error, hx/y, is first calculated using various algorithms which will be discussed in details later, and then the FD error can be estimated by taking the FFT of the zero-padding TD error [18

18. J. J. Shynk, “Frequency-domain and multirate adaptive filtering,” IEEE Sig. Proc. Mag. 9(1), 14–37 (1992). [CrossRef]

]:
Hx/y=FFT(Qhx/y)
(7)
where Q=[0NIN]T is used to pad N zeros before the original N TD errors.

Using the knowledge of the FD error, we can update the auxiliary coefficient vector to be:
Wix,newe/o=Wixe/o+μFFT(TIFFT[K{(Xine/o)*Hi}*])
(8)
Wiy,newe/o=Wiye/o+μFFT(TIFFT[K{(Yine/o)*Hi}*])
(9)
where μ is the step-size parameter, and T=(IN0N0N0N) is used to replace the last N samples with zeros. Finally the converted tap weight for equalization can be obtained by using Eq. (4).

Figure 3
Fig. 3 (a) Complexity comparison between FD and TD adaptive equalizer, (b) Zoomed-in version of (a).
compares the computational complexity for this FD second stage setup (with either CMA or MMA) with the conventional TD-CMA method. The computational complexity of TD-CMA is (12N+2) multiplications per symbol [10

10. M. S. Faruk and K. Kikuchi, “Adaptive frequency-domain equalization in digital coherent optical receivers,” Opt. Express 19(13), 12789–12798 (2011). [CrossRef] [PubMed]

], while FD-MMA requires(12log2(2N)+12) multiplications per symbol including 24 2N-point FFT/IFFT operations and 24N multiplications to output 2N symbols for two polarizations. From Fig. 3(b), it is clear that the implementation complexity of the FD method will be much lower than the TD technique when FFT size (2N) is ≥8; also, the complexity of FD-MMA is slightly higher than FD-CMA because the MMA needs to update the real and imaginary parts of the errors separately.

Regarding the error calculation, CMA is a well known approach and is well established in commercial systems. Its error function can be formulated as [5

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

]:
hCMA,x/y=xout/yout(RCMA2|xout/yout|2)
(10)
RCMA2=E[|sn|4]E[|sn|2]
(11)
where E[] is the expectation operation, sn indicates the transmitted symbol, e.g. [1+j,1j,1+j,1j] for QPSK modulation. For MMA, the real and imaginary parts of the TD error can be calculated as [20

20. J. Yang, J.-J. Werner, and G. A. Dumont, ““The multimodulus blind equalization and its generalized algorithms,” IEEE J. Sel. Areas on Commun. 20(5), 997–1015 (2002). [CrossRef]

]:
(hx/y)=(xout/yout)(RMMAL(xout/yout)L)
(12)
(hx/y)=(xout/yout)(RMMAL(xout/yout)L)
(13)
RMMAL=E[(sn)2L]E[(sn)L]=E[(sn)2L]E[(sn)L]
(14)
where [] and [] represent real and imaginary functions, respectively, and L is a positive integer constant and equals to 2 throughout the paper since this value provides the best compromise between performance and complexity of implementation.

Figure 4
Fig. 4 Convergence contour for: (a) CMA with QPSK; (b) CMA with 16-QAM; (c) MMA with QPSK; (d) MMA with 16-QAM.
shows the convergence principle of CMA and MMA for QPSK and 16-QAM modulation formats. As indicated in Figs. 4(a) and 4(b), the convergence contour of CMA can be thought as a single dimensional update following a constant modulus (a circle). While from Figs. 4(c) and 4(d) it is clear that the convergence principle of MMA can be considered as the sum of two one-dimensional error functions, therefore the convergence rate for MMA is expected to outperform CMA as it simultaneously minimizes the errors of the real and imaginary parts of the equalized signal around these two separate straight moduli (two moduli with same value when the QAM constellation is square).

3. Experimental results and discussion

Figure 5
Fig. 5 Experimental setup of coherent pol-mux SC-FDE system. ECL: external cavity laser, AWG: arbitrary waveform generator, LPF: low-pass filter, AMP: RF amplifier, PBC: polarization beam combiner, OBPF: optical band-pass filter, PBS: polarization beam splitter, LO: local oscillator.
shows the experimental setup for the NDA SC-FDE system. A pair of 10 Gsymbols/s arbitrary waveform generators are used to generate two independent baseband signal streams with a desired modulation format. The data modulates the output of a 100-kHz linewidth external cavity laser using two optical I/Q modulators, one for each polarization. After being polarization-multiplexed with a polarization beam combiner, the signals are transmitted through several 80-km amplified spans of standard single-mode fiber (SSMF), then through a commercial PMD emulator yielding 30-ps mean differential group delay (DGD). At the receiver end, the signal goes through an optical band-pass filter followed by a polarization beam splitter. The signal is then detected by an optical 90-degree hybrid with a local oscillator, balanced photodiodes and low-pass filters. An Agilent real-time oscilloscope is then used to sample the outputs of the receivers at 40 Gsamples/s, with 8 × 105 samples for each polarization stored for off-line processing.

Figure 6
Fig. 6 CD estimation error versus transmission distances.
shows the CD estimation errors versus transmission distance. The largest estimation error for both the QPSK and the 16-QAM systems is 400 ps/nm, which demonstrates that the FD CD estimation algorithm is precise enough for the 16-tap second stage FD blind equalizer to absorb the residual fiber impairments.

For back-to-back (B2B) QPSK transmission with only PMD impairments, the measured Q-values versus different step sizes with 5000 iterations for FD-CMA and FD-MMA are plotted in Fig. 7(a)
Fig. 7 QPSK system: (a) Measured Q-value for CMA and MMA with different step sizes for B2B transmission; (b) Equalization performance versus number of iterations after B2B and 800-km transmission; (c) Q-value with different transmission distances for different algorithms; (d) and (e): constellation diagrams of the equalized x/y-polarization signal after 800-km transmission.
. The optimum step size of 0.003 for CMA is slightly larger than that of 0.001 for MMA, while MMA can achieve similar performance with a step size of 0.003. We can get a better performance using very small step sizes combined with a large number of iterations; however, this is very time-consuming and has high implementation complexity. Thus we only consider solutions that are a good compromise between performance and convergence time. Figure 7(b) shows the Q-values versus iterations for FD-CMA and FD-MMA algorithms with different step sizes after B2B and 800-km transmission. It is clear that with the same step size, MMA converges much faster than CMA, although a smaller step size (μ = 0.001) slows down the MMA’s convergence speed. However, it is still faster then CMA with larger step size. This characteristic of MMA can save a large number of symbols and the time needed for the blind adaptation. Figure 7(c) shows the measured Q values versus transmission distance for QPSK. The results for both TD-CMA and FD-CMA are almost the same, which shows that the FD implementation achieves a similar performance to the TD one. Moreover, the measured Q values using FD-MMA do not depend on prior knowledge of the CD, which confirms that the proposed method works well. The Q value is close to 17 dB after 800-km transmission, which means error free transmission can be achieved using either CMA or MMA. However MMA can speed up the blind adaptation with its faster convergence rate. Figures 7(d) and 7(e) show the constellation diagrams of the equalized signal for x and y polarization after 800-km transmission, respectively.

Figure 8(a)
Fig. 8 16-QAM transmission system: (a) Measured Q-value for CMA and MMA with different step sizes for B2B transmission; (b) Equalization performance versus number of iterations after B2B and 800-km transmission; (c) Q-value with different transmission distances for different algorithms; (d) and (e): constellation diagrams of equalized x/y-polarization signal after 800-km of transmission.
shows the B2B transmission performance for 16-QAM with different step sizes for FD-CMA and FD-MMA. Similar to the QPSK case, the optimum step size for MMA is also smaller than for CMA. In this case we use the optimum step size for MMA and CMA separately in order to gain better equalization performance. Figure 8(b) shows the Q-value versus number of iterations for MMA and CMA after B2B and 800-km transmission. MMA converges similarly to CMA with a smaller step size, which again illustrates the faster converge feature of MMA. This time MMA outperforms CMA with a better steady-state Q-value performance. Figure 8(c) shows the transmission performance for 16-QAM system. FD-MMA outperforms FD-CMA for all transmission distances. The Q-value for 800 km is about 9 dB, which is larger than the 20% soft-decision FEC limit of 7 dB for error free transmission. By comparing the constellation diagrams of the equalized 16-QAM x and y polarization signals after 800-km transmission with the QPSK case in Fig. 6, we see that the degradation of Q-value for the 16-QAM signal is due to much denser constellations and less optical power obtained by the higher-order modulation formats.

4. Conclusions

Acknowledgments

NICTA is funded by the Australian Government as represented by the Department of Broadband, Communications and the Digital Economy and the Australian Research Council through the ICT Centre of Excellence program. The research is also partially supported by the Australian Research Council Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems.

References and links

1.

J. Yu and X. Zhou, “Ultra-high-capacity DWDM transmission system for 100G and beyond,” IEEE Commun. Mag. 48(3), S56–S64 (2010). [CrossRef]

2.

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]

3.

I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM coherent optical system,” J. Lightwave Technol. 27(15), 3042–3049 (2009). [CrossRef]

4.

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), 3042–3049 (2009). [CrossRef]

5.

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

6.

M. Kuschnerov, M. Chouayakh, K. Piyawanno, B. Spinnler, E. de Man, P. Kainzmaier, M. S. Alfiad, A. Napoli, and B. Lankl, “Data-aided versus blind single-carrier coherent receivers,” IEEE Photon. J. 2(3), 387–403 (2010). [CrossRef]

7.

B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE Sel. Top. in Quantum Electron. 16(5), 1180–1192 (2010). [CrossRef]

8.

R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent optical single carrier transmission using overlap frequency domain equalization for long-haul optical systems,” J. Lightwave Technol. 27(16), 3721–3728 (2009). [CrossRef]

9.

A. V. Tran, C. Zhu, C. C. Do, S. Chen, T. Anderson, D. Hewitt, and E. Skafidas, “8×40-Gb/s optical coherent pol-mux single carrier system with frequency domain equalization and training sequences,” IEEE Photon. Technol. Lett. 24(11), 885–887 (2012). [CrossRef]

10.

M. S. Faruk and K. Kikuchi, “Adaptive frequency-domain equalization in digital coherent optical receivers,” Opt. Express 19(13), 12789–12798 (2011). [CrossRef] [PubMed]

11.

S. Yamanaka, T. Kobayashi, A. Sano, H. Masuda, E. Yoshida, Y. Miyamoto, T. Nakagawa, M. Nagatani, and H. Nosaka, “11 × 117 Gb/s PDM 16-QAM Transmission over 1440 km with a spectral efficiency of 6.4 b/s/Hz using high-speed DAC,” in European Conference and Exhibition on Optical Communication (ECOC), paper We.8.C.1 (2010).

12.

A. H. Gnauck, P. J. Winzer, C. R. Doerr, and L. L. Buhl, “10 × 112-Gb/s PDM 16-QAM transmission over 630 km of fiber with 6.2-b/s/Hz spectral efficiency,” in Optical Fiber Communication Conference (OFC), paper PDPB8 (2009).

13.

X. Zhou, J. Yu, M.-F. Huang, Y. Shao, T. Wang, P. Magill, M. Cvijetic, L. Nelson, M. Birk, G. Zhang, S. Ten, H. B. Matthew, and S. K. Mishra, “Transmission of 32-Tb/s capacity over 580 km using RZ-shaped PDM-8QAM modulation format and cascaded multimodulus blind equalization algorithm,” J. Lightwave Technol. 28(4), 456–465 (2010). [CrossRef]

14.

C. Zhu, A. V. Tran, S. Chen, L. B. Du, T. Anderson, A. J. Lowery, and E. Skafidas, “Dual-stage frequency domain equalization for long-haul coherent polarization-multiplexed QPSK and 16-QAM systems,” in European Conference on Optical Communication (ECOC), paper We.1.A.2 (2012).

15.

T. Nakagawa, M. Matsui, T. Kobayashi, K. Ishihara, R. Kudo, M. Mizoguchi, and Y. Miyamoto, “Non-data-aided wide-range frequency offset estimator for QAM optical coherent receivers,” in Optical Fiber Communication Conference (OFC), paper OMJ1 (2011).

16.

S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Decision-aided carrier phase estimation for coherent optical communications,” J. Lightwave Technol. 28(11), 1597–1607 (2010). [CrossRef]

17.

F. N. Hauske, Z. Zhang, C. Li, C. Xie, and Q. Xiong, “Precise, robust and least complexity CD estimation,” in Optical Fiber Communication Conference (OFC), paper JWA32 (2011).

18.

J. J. Shynk, “Frequency-domain and multirate adaptive filtering,” IEEE Sig. Proc. Mag. 9(1), 14–37 (1992). [CrossRef]

19.

B. Porat, A Course in Digital Singal Processing (Wiley, 1997).

20.

J. Yang, J.-J. Werner, and G. A. Dumont, ““The multimodulus blind equalization and its generalized algorithms,” IEEE J. Sel. Areas on Commun. 20(5), 997–1015 (2002). [CrossRef]

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications
(060.4510) Fiber optics and optical communications : Optical communications

ToC Category:
Subsystems for Optical Networks

History
Original Manuscript: August 23, 2012
Manuscript Accepted: November 10, 2012
Published: November 28, 2012

Virtual Issues
European Conference on Optical Communication 2012 (2012) Optics Express

Citation
Chen Zhu, An V. Tran, Simin Chen, Liang B. Du, Trevor Anderson, Arthur J. Lowery, and Efstratios Skafidas, "Improved two-stage equalization for coherent Pol-Mux QPSK and 16-QAM systems," Opt. Express 20, B141-B150 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-26-B141


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. J. Yu and X. Zhou, “Ultra-high-capacity DWDM transmission system for 100G and beyond,” IEEE Commun. Mag.48(3), S56–S64 (2010). [CrossRef]
  2. 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]
  3. I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM coherent optical system,” J. Lightwave Technol.27(15), 3042–3049 (2009). [CrossRef]
  4. 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), 3042–3049 (2009). [CrossRef]
  5. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express16(2), 804–817 (2008). [CrossRef] [PubMed]
  6. M. Kuschnerov, M. Chouayakh, K. Piyawanno, B. Spinnler, E. de Man, P. Kainzmaier, M. S. Alfiad, A. Napoli, and B. Lankl, “Data-aided versus blind single-carrier coherent receivers,” IEEE Photon. J.2(3), 387–403 (2010). [CrossRef]
  7. B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE Sel. Top. in Quantum Electron.16(5), 1180–1192 (2010). [CrossRef]
  8. R. Kudo, T. Kobayashi, K. Ishihara, Y. Takatori, A. Sano, and Y. Miyamoto, “Coherent optical single carrier transmission using overlap frequency domain equalization for long-haul optical systems,” J. Lightwave Technol.27(16), 3721–3728 (2009). [CrossRef]
  9. A. V. Tran, C. Zhu, C. C. Do, S. Chen, T. Anderson, D. Hewitt, and E. Skafidas, “8×40-Gb/s optical coherent pol-mux single carrier system with frequency domain equalization and training sequences,” IEEE Photon. Technol. Lett.24(11), 885–887 (2012). [CrossRef]
  10. M. S. Faruk and K. Kikuchi, “Adaptive frequency-domain equalization in digital coherent optical receivers,” Opt. Express19(13), 12789–12798 (2011). [CrossRef] [PubMed]
  11. S. Yamanaka, T. Kobayashi, A. Sano, H. Masuda, E. Yoshida, Y. Miyamoto, T. Nakagawa, M. Nagatani, and H. Nosaka, “11 × 117 Gb/s PDM 16-QAM Transmission over 1440 km with a spectral efficiency of 6.4 b/s/Hz using high-speed DAC,” in European Conference and Exhibition on Optical Communication (ECOC), paper We.8.C.1 (2010).
  12. A. H. Gnauck, P. J. Winzer, C. R. Doerr, and L. L. Buhl, “10 × 112-Gb/s PDM 16-QAM transmission over 630 km of fiber with 6.2-b/s/Hz spectral efficiency,” in Optical Fiber Communication Conference (OFC), paper PDPB8 (2009).
  13. X. Zhou, J. Yu, M.-F. Huang, Y. Shao, T. Wang, P. Magill, M. Cvijetic, L. Nelson, M. Birk, G. Zhang, S. Ten, H. B. Matthew, and S. K. Mishra, “Transmission of 32-Tb/s capacity over 580 km using RZ-shaped PDM-8QAM modulation format and cascaded multimodulus blind equalization algorithm,” J. Lightwave Technol.28(4), 456–465 (2010). [CrossRef]
  14. C. Zhu, A. V. Tran, S. Chen, L. B. Du, T. Anderson, A. J. Lowery, and E. Skafidas, “Dual-stage frequency domain equalization for long-haul coherent polarization-multiplexed QPSK and 16-QAM systems,” in European Conference on Optical Communication (ECOC), paper We.1.A.2 (2012).
  15. T. Nakagawa, M. Matsui, T. Kobayashi, K. Ishihara, R. Kudo, M. Mizoguchi, and Y. Miyamoto, “Non-data-aided wide-range frequency offset estimator for QAM optical coherent receivers,” in Optical Fiber Communication Conference (OFC), paper OMJ1 (2011).
  16. S. Zhang, P. Y. Kam, C. Yu, and J. Chen, “Decision-aided carrier phase estimation for coherent optical communications,” J. Lightwave Technol.28(11), 1597–1607 (2010). [CrossRef]
  17. F. N. Hauske, Z. Zhang, C. Li, C. Xie, and Q. Xiong, “Precise, robust and least complexity CD estimation,” in Optical Fiber Communication Conference (OFC), paper JWA32 (2011).
  18. J. J. Shynk, “Frequency-domain and multirate adaptive filtering,” IEEE Sig. Proc. Mag.9(1), 14–37 (1992). [CrossRef]
  19. B. Porat, A Course in Digital Singal Processing (Wiley, 1997).
  20. J. Yang, J.-J. Werner, and G. A. Dumont, ““The multimodulus blind equalization and its generalized algorithms,” IEEE J. Sel. Areas on Commun.20(5), 997–1015 (2002). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited