Frequency, phase, and polarization-tracking algorithms for arbitrary four-dimensional signal constellations

The need for software-defined transmissions raises new challenges for the transceiver design: multiple modulation formats have to be supported to accommodate for varying bandwidth demand and physical characteristics of different optical paths. In order to keep the receiver complexity and cost low, most of the digital signal processing functionalities should be shared by the different formats. In this paper we address the problems of carrier frequency and phase recovery as well as the tracking of fast polarization rotations for arbitrary constellations in two or four dimensions. We first report how not only carrier frequency recovery but also ADC sampling error correction can be performed using a modulation-format independent frequency-domain approach. We then report a framework where the combined impact of phase error and fast polarization rotation can be described by a rotation in a four-dimensional space and show how to efficiently estimate and compensate this rotation. We finally investigate the dynamic performance of the reported algorithms for polarization-switched QPSK and 4D 32QAM constellations using numerical simulations.