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Pilot tones based polarization rotation, frequency offset and phase estimation for polarization multiplexed Offset-QAM Multi-Subcarrier coherent optical systems

Multi-Subcarrier Offset-Quadrature Amplitude Modulation (OQAM) scheme has recently been proposed to increase the spectral efficiency (SE) of optical coherent communication systems. Thanks to the half symbol delay between the in-phase and quadrature components, OQAM signals can achieve channel spacing values as low as the symbol rate Rs, without suffering from inter channel interference (ICI) even in the condition of severe spectral overlapping. On the other hand, phase noise due to this half symbol delay not only results in the random rotation of the received constellation points but also introduces additional cross talk after equalization, when using standard equalization algorithms. Therefore, OQAM signals have very poor tolerance towards laser linewidth. In addition, for a polarization multiplexed (PM) system, polarization rotation between the x and y polarizations interacting with the phase noise can severely affect the performance. In this paper we propose a pilot tones (PTs) based approach for multi-subcarrier OQAM signals. The PTs are used to estimate frequency offset (FO), carrier phase (CP) and the polarization rotation matrix. An equalizer based on a modified constant modulus algorithm (CMA) is finally used to recover the signal.