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standard DFB lasers and real-time digital receiver," Electron. Lett. 42, 1175-1176 (2006).

D. Ly-Gagnon, K. Katoh, K. Kikuchi, "Unrepeatered optical transmission of 20 Gbit/s
quadrature phase-shift keying signals over 210 km using homodyne
phase-diversity receiver and digital signal processing," Electron. Lett. 41, 206-207 (2005).

G. Charlet, M. Salsi, J. Renaudier, O. Pardo, H. Mardoyan, S. Bigo, "Performance comparison of singly-polarised and
polarisation-multiplexed coherent transmission at 10 Gbauds under
linear impairments," Electron. Lett. 43, 1109-1111 (2007).

H. Zhao, E. Agrell, M. Karlsson, "Unequal bit error probability in coherent QPSK
fiber-optic systems using phase modulator based transmitters," Eur. Trans. Telecommun. 19, 895-906 (2007).

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J. Kahn, K.-P. Ho, "Spectral efficiency limits and modulation/detection
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S. Benedetto, A. Djupsjöbacka, B. Lagerström, R. Paoletti, P. Poggiolini, G. Mijic, "Multilevel polarization modulation using a
specifically designed LiNbO$_3$ device," IEEE Photon. Technol. Lett. 6, 949-951 (1994).

S. Benedetto, P. Poggiolini, "Theory of polarization shift keying modulation," IEEE Trans. Commun. 40, 708-721 (1992).

D. Saha, T. Birdsall, "Quadrature-quadrature phase-shift keying," IEEE Trans. Commun. 37, 437-448 (1989).

L. Zetterberg, H. Brändström, "Codes for combined phase and amplitude modulated
signals in a four-dimensional space," IEEE Trans. Commun. COM-25, 943-950 (1977).

M. K. Simon, R. Annavajjala, "On the optimality of bit detection of certain digital
modulations," IEEE Trans. Commun. 53, 299-307 (2005).

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J. R. Pierce, "Comparison of three-phase modulation with two-phase
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J. R. Pierce, "Optical channels: Practical limits with photon
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N. Ekanayake, T. Tjhung, "On ternary phase-shift keyed signaling," IEEE Trans. Inf. Theory IT-28, 658-660 (1982).

G. Welti, J. Lee, "Digital transmission with coherent four-dimensional
modulation," IEEE Trans. Inf. Theory IT-20, 497-502 (1974).

L. Xiao, X. Dong, "New analytical expressions for orthogonal,
biorthogonal, and transorthogonal signaling in Nakagami fading
channels with diversity reception," IEEE Trans. Wireless Commun. 4, 1418-1424 (2005).

J.-E. Porath, T. Aulin, "Design of multidimensional signal constellations," Inst. Electr. Eng. Proc.-Commun. 150, 317-323 (2003).

J. Renaudier, G. Charlet, M. Salsi, O. Pardo, H. Mardoyan, P. Tran, S. Bigo, "Linear fiber impairments mitigation of 40-Gbit/s
polarization-multiplexed QPSK by digital processing in a coherent
receiver," J. Lightw. Technol. 26, 36-42 (2008).

S. Betti, F. Curti, G. D. Marchis, and E. Iannone, "A novel multilevel coherent optical system:
4-quadrature signaling," J. Lightw. Technol. 9, 514-523 (1991).

S. Betti, G. De Marchis, E. Iannone, P. Lazzaro, "Homodyne optical coherent systems based on
polarization modulation," J. Lightw. Technol. 9, 1314-1320 (1991).

R. Cusani, E. Iannone, A. Salonico, M. Todaro, "An efficient multilevel coherent optical system:
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K. Kikuchi, S. Tsukamoto, "Evaluation of sensitivity of the digital coherent
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H. Sun, K. Wu, K. Roberts, "Real-time measurements of a 40 Gb/s coherent system," Opt. Exp. 16, 873-879 (2008).

E. Ip, A. Lau, D. Barros, J. Kahn, "Coherent detection in optical fiber systems," Opt. Exp. 16, 753-791 (2008) (erratum, vol. 16, no. p. 26, p. 21943, 2008).

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