Continuous spectrum nonlinear frequency division multiplexing transmission based on subcarrier index modulation

In a continuous spectrum nonlinear frequency division multiplexing (CS-NFDM) system, the existence of a guard interval (GI) avoids burst interaction during propagation, but it also severely hampers the enhancement of spectral efficiency (SE). On the contrary, if the GI is simply reduced, the transmission quality of the CS-NFDM signal is significantly degraded. To solve the above problems, we first propose to introduce the subcarrier index modulation (SIM) technique in the CS-NFDM system, which is later referred to as the SIM-NFDM system. Firstly, at the transmitter side of the proposed scheme, all subcarriers are defined as either silent or activated states, where the position indexes of the silent subcarriers are utilized to carry extra bits of information, thus enhancing the transmission rate. Additionally, the introduction of silent subcarriers reduces the average signal power, decreases computational errors caused by processing noise, and improves transmission quality. The effectiveness of the proposed scheme is verified by SIM-NFDM simulation and experimental systems with signal bandwidths of 64 GHz and 16 GHz. The results indicate that the maximum <i>Q</i>-factor improvement of the SIM-NFDM system is 1.28 dB and 0.42 dB compared to the classic CS-NFDM system, respectively. In addition, this study calculates the maximum normalized transmission rate (NTR) enhancement of 14.5% for SIM-NFDM compared to the classic CS-NFDM system. It is worth noting that the additional complexity added by the proposed scheme is of the order of <i>O</i>(<i>m</i>). This order of magnitude of complexity increase is acceptable considering the performance advantages highlighted by this system.