Transceiver impairment mitigation for digital subcarrier multiplexing signals by adaptive multi-layer filters with...

We propose an adaptive multi-layer (ML) filter architecture to compensate for linear impairments of digital subcarrier (DSC) multiplexing signals that occur in transmitter (Tx) and receiver (Rx) components. In this architecture, DC-symmetric SC pairs of a DSC signal are individually processed by adaptive ML filters that consist of strictly linear (SL) and widely linear (WL) filter layers, and the coefficients of the ML filters are adaptively controlled through gradient calculation using back propagation and stochastic gradient descent. Static chromatic dispersion compensation is performed on the received DSC signal and its complex conjugate before SC demultiplexing. After SC demultiplexing, each DC-symmetric SC pair is fed into the first 2 × 1 SL filter layer of the ML filters for compensation of in-phase (I) and quadrature (Q) impairments on the Rx side, taking into account the conjugate-image component. Then each SC is fed into the 2 × 2 SL filter for polarization demultiplexing with carrier phase recovery. Finally, the 2 × 1 WL filter is operated on each DC-symmetric SC pair to compensate for Tx-side I and Q impairments. We experimentally evaluated the proposed adaptive ML filter for an 8-DSC signal and compared it with a single-carrier signal in an 11-channel wavelength-division multiplexed transmission using 128-Gbaud polarization-multiplexed 16-quadrature amplitude modulation signals over a 1,200-km single-mode fiber (SMF). The proposed adaptive ML filter architecture for DSC-multiplexed signals effectively mitigates various IQ impairments and achieves performance that is nearly equivalent to the single-carrier case. For the 8-DSC signal, it also achieves performance comparable to that of a 16 × 4 WL filter with a 23% reduction in DSP complexity in terms of required complex-valued multiplications. It also remains effective with up to 5 ps IQ skew, an IQ gain imbalance ratio of 1.5, and a 10° IQ phase imbalance on the Tx and Rx sides. Moreover, robust performance was confirmed under simultaneous Tx and Rx IQ impairments, and stable adaptation was maintained even under polarization scrambling rates up to 10 krad/s.