Subband Processing for OQAM Digital-Subcarrier-Multiplexing Optical Coherent Systems

High baud-rate signal with wide spectral width not only requires high-complexity digital signal processing (DSP) in coherent receiver, but also is susceptible to transmission impairments including optical filtering in the link. In this paper, we propose a novel subband-based DSP structure, which decomposes the signal into narrower-bandwidth subbands, performs chromatic dispersion compensation (CDC) and butterfly adaptive filtering (BAF) on a subband basis, and then synthesizes the processed subbands. In this way, the proposed structure achieves an arbitrary spectral processing resolution regardless of the transmitted signal spectral profile and reduces the required size of fast Fourier transform for CDC and the tap number for BAF. We investigate the proposed subband structure in offset-quadrature-amplitude-modulation digital subcarrier multiplexing systems. Simulations at 140 GBaud and experiments at 40 GBaud show that this scheme achieves a faster convergence rate, and enhanced tolerance to optical filtering, rotation of state of polarization and delay of the BAF feedback loop. It also reduces the complexity of CDC and BAF to 30% of that using conventional DSP structure, even taking subband decomposition and synthesis into account.