Proposal and Demonstration of Subcarrier Index Modulation OFDM for RoF System With Enhanced Spectral Efficiency

A radio-over-fiber (RoF) system employing subcarrier index modulation (SIM) orthogonal frequency-division multiplexing (OFDM) signal is proposed and experimentally demonstrated in this paper. In the system, the SIM will add a new freedom to the conventional modulation techniques [i.e., ON-OFF keying or quadrature phase-shift keying (QPSK)], which in different subcarrier states (activated and silent) are employed to convey information, leading to an enhanced spectral efficiency. In the proof-of-concept experiments, a 10.5-GHz RoF system carrying QPSK-SIM-OFDM signal is established with a hybrid link of 10-km fiber and 1.5-m free space. The spectral efficiency of the SIM-OFDM can be enhanced by up to 14.5% for 128 subcarriers with 21 silent subcarriers or 15.6% for 512 subcarriers with 97 silent subcarriers, compared with that of conventional OFDM systems. The bit error rates are analyzed and estimated to be less than the forward error correction threshold.