Effective performance-enhanced CS-NFDM system employing general orthogonal circulant transform precoding

In the continuous spectrum nonlinear frequency division multiplexing (CS-NFDM) system, inter-symbol interference (ISI) and system noise exhibit non-uniform distribution characteristics on the nonlinear spectrum (NS), resulting in a significant reduction in the signal-to-interference-and-noise ratio (SINR) of subcarriers located at the edge of the spectrum, which seriously impairs the overall transmission performance of the system. This paper improves the general orthogonal circulant transform (GOCT) precoding technology and applies it to the CS-NFDM system for what we believe to be the first time, which effectively addresses the above issues, and we refer to this new system as the GOCT-NFDM system. This system employs the GOCT matrix to perform linear precoding on each subcarrier, which spreads the information on each subcarrier to all subcarriers, thereby achieving uniform SINR distribution across different NS components and effectively improving the system performance. The effectiveness of the proposed scheme is validated by building a GOCT-NFDM simulation system for 800 km transmission. Compared with the traditional CS-NFDM system, the GOCT-NFDM system achieves a 1.67 dB Q-factor gain at the optimal launch power under the transmission scenario with 128 subcarriers and a signal bandwidth of 52 GHz, and delivers a maximum Q-factor gain of 2.05 dB with 256 subcarriers and a signal bandwidth of 72 GHz. To further validate the feasibility of the proposed scheme in practical scenarios, a GOCT-NFDM experimental system for 806 km transmission is established. Compared with the traditional CS-NFDM system, the GOCT-NFDM system achieves Q-factor gains of 0.56 dB and 0.74 dB at the optimal launch power with 128 subcarriers and signal bandwidths of 40 GHz and 48 GHz, respectively. In terms of implementation complexity, the additional complexity introduced by the proposed scheme is <i>O</i>(<i>N</i>_c<i>log</i>⁡<i>N</i>_c). Given the significant performance improvement it brings to the CS-NFDM system, this slight increase in complexity is acceptable.