Digital signal processing for coherent UDWDM passive optical networks

In this contribution, we discuss digital signal processing (DSP) options for coherent ultra-dense wavelength-division multiplexing (UDWDM) passive optical networks (PON). In particular, we focus on the joint reception of several 2.488 Gb/s polarization-division multiplexed differential quadrature phase-shift keying (PDM-DQPSK) upstream signals by the optical line terminal (OLT). Due to the cost-sensitivity in access networks, joint heterodyne detection of multiple upstream channels in the OLT is an attractive solution to reduce the required hardware effort. The main challenge from a DSP perspective is the digital separation and down-conversion of potentially hundreds of channels simultaneously. Since each upstream signal originates from an independent source, the channels are also affected by different clock phase and -frequency offsets, as well as by strong phase noise from the use of tunable lasers. These effects need to be considered and compensated for by the DSP. We discuss algorithms which perform the required compensation and present strategies for real-time implementation of these algorithms on an FPGA platform.