Circular-wrapped integration interference suppression-based channel estimation for WDM PDM CO-GFDM systems.

Due to the non-orthogonality of the generalized frequency division multiplexing (GFDM) modulation system, the fiber channel introduces intrinsic interference to the coherent optical GFDM (CO-GFDM), which significantly degrades the system performance. In the state of the art, there has been no detailed analysis of this intrinsic interference for CO-GFDM. Recent works have reported simulation studies and experimental demonstrations of CO-GFDM. However, theoretical discussion on the intrinsic interference suppression-based channel estimation (CE) methods for CO-GFDM systems has not been reported. In this paper, we analyzed the shifted-multiplied integration interference (SMI) and circular-wrapped integration interference (CWI) for CO-GFDM caused by the non-orthogonality for the first time. Theoretical analysis and simulation results indicate that SMI is negligible compared to CWI, which dominates the interference in CO-GFDM systems. We systematically derived the frequency-domain transmission model for wavelength division multiplexing (WDM) polarization division multiplexing (PDM) CO-GFDM theoretically, based on the CWI effect suppression. Specific pseudo pilot sequence for CO-GFDM is utilized to suppress the CWI effect. Both fully-loaded (FL) and half-loaded (HL) frequency-domain CE methods are discussed. The robustness against CWI for the pseudo pilot-based method and random sequence-based method is compared for both FL and HL scenarios. By employing our proposed method, the robustness of the WDM PDM CO-GFDM system against the CWI in both linear and non-linear transmission has been significantly enhanced, achieving a transmission rate at the T-bit/s level. The theoretical analysis is validated by numerical Monte Carlo simulations of WDM PDM CO-GFDM systems.