Multicarrier Offset-QAM for Long-Haul Coherent Optical Communications

The availability of high-speed digital-to-analog converters for coherent optical communication systems makes it possible to digitally divide the available electrical bandwidth into subchannels on each optical carrier which enables a parallel implementation of the digital signal processing. In this paper, we use electrical multicarrier offset-QAM modulations to achieve crosstalk-free modulation for subcarriers spaced at the symbol rate. We show that in coherent optical communication systems, several impairments are bound to break the subcarrier orthogonality and cause failures of payload symbol recovering. We propose a dedicated digital signal processing architecture which implements channel estimation and a new algorithm to track the phase in multicarrier offset-QAM modulation. We experimentally compare the latter with conventional multicarrier and single carrier QAM modulation so as to assess the benefits of crosstalk mitigation via offset-QAM modulation. We carry out experiments to test the transmission performance of multicarrier offset-QAM modulation in a dispersion-unmanaged long-haul transmission link.