Real-Time Software Implementation of Filter-Separated MIMO Adaptive Equalizer for Fully Virtualized Access Networks

Developing dedicated hardware for each application in future optical access network systems causes significant initial investment costs. Thus, these costs are alleviated by optical access networks using general-purpose devices related to software-defined networking (SDN) and network function virtualization (NFV). We have studied physical layer virtualization to bring the advantages of SDN and NFV in optical access networks. We have focused on digital coherent technology to enhance capacity and extend reach within optical access networks. However, the future virtualized coherent access faces stringent frequency characteristics and the impact of chromatic dispersion (CD). The number of taps of finite impulse response (FIR) filters in the receiver increases rapidly for the CD compensation. The previously proposed DSP algorithm may not operate in this situation because the processing time exceeds the requirement. Therefore, this paper proposes and implements software for a lightweight multi-input-to-multi-output (MIMO) adaptive equalizer that separates FIR filters depending on the tap update period. The proposed method uses two types of FIR filters; one is used for polarization demultiplexing with short taps, and the other is a filter with long taps for the compensation of the CD in the fiber channel and the frequency distortion from the transceiver. Owing to filter separation, the calculation cost can be reduced. We indicate the processing time meeting desired requirements through real-time transmission experiments, even with increased taps. Additionally, we conduct offline simulations and experiments at a higher symbol rate than the real-time experiment.