Adaptive modulation scheme for DML-based IM/DD FOFDM systems using overlap frequency-domain equalization

To further investigate the optical fast orthogonal frequency-division multiplexing (FOFDM) with sub-channel spacing equal to half of the symbol rate, an adaptive modulation scheme for FOFDM is proposed and experimentally demonstrated in low-cost short-reach intensity-modulated and direct-detection (IM/DD) systems using a directed modulated distributed feedback laser (DML). The experimental results and analyses indicate, it is feasible to transmit an adaptively modulated optical FOFDM signal over 20-km and 50-km standard single mode fibers (SSMFs). Compared with a single modulation format of 16-PAM, after 20-km SSMF transmission, the receiver sensitivity of the adaptive modulation scheme can be improved by about 2.3 dB at the bit error ratio (BER) of 2.4x10(-2). Moreover, after 50-km SSMF transmission, the adaptive modulation scheme can still under 2.4x10(-2) when received optical power (ROP) is larger than -1 dBm, while the 16-PAM modulation scheme cannot. In addition, an overlap frequency-domain equalization (OFDE) scheme is employed after sub-carrier demultiplexing at the receiver to enable the effects of inter-symbol interference (ISI) to be effectively suppressed even though a cyclic prefix (CP) is not employed. Experimental results show that the FOFDM systems employing OFDE scheme have almost no performance degradation when compared to employing conventional FDE with an additional CP, but the spectral efficiency (SE) can be significantly improved by 12.37%.