We investigate, by means of simulations, polarization-multiplexed (PolMux) quadrature duobinary (QDB) as a modulation format for transmitting 111-Gb/s signals with high spectral efficiency (SE) and acceptable system complexity. We show that PolMux-QDB can be used to transmit 111-Gb/s signals with an SE of 4-b/s/Hz, but at the same time has a higher tolerance to nonlinear transmission effects and local oscillator laser linewidth compared to PolMux-16QAM at the same data rate and SE.
Machi, F., Alfiad, M. S., Kuschnerov, M., Wuth, T., Borne, van den, D., Hanik, N., & Waardt, de, H. (2010). 111-Gb/s PolMux-Quadrature Duobinary for Robust and Bandwidth Efficient Transmission. IEEE Photonics Technology Letters, 22(11), 751-753. https://doi.org/10.1109/LPT.2010.2045495