Research on an M-Ary Frequency Shift Keying With Index Modulation System for Underwater Acoustic Communication

Underwater acoustic (UWA) communication provides an effective way for underwater devices to establish wireless links, which is crucial for the development of Internet of Underwater Things (IoUT). Because noncoherent orthogonal frequency division multiplexing with M-ary frequency shift keying (OFDM-MFSK) adopts energy detection methods without channel estimation and channel equalization, it is not sensitive to the challenges posed by UWA channels, including low signal-to-noise ratio (SNR), large Doppler effect, and time-varying multipath propagation. As a result, it exhibits strong robustness for UWA communication. However, its spectral efficiency is severely limited. In this article, inspired by index modulation (IM) technology, OFDM-MFSK with IM systems are proposed. Specifically, two schemes are proposed: separate spectrum band OFDM-MFSK with IM (SOMI) and time spectrum band OFDM-MFSK with IM (TOMI). SOMI aims to overcome frequency-selective fading by leveraging the performance gain of multiple subbands, while TOMI is suitable for the channels with extremely limited bandwidth at the expense of time slots. To further enhance performance, a soft-decision algorithm, which is easily combined with channel decoding, is adopted for the proposed systems. The combination probability matrices are utilized to calculate the log-likelihood ratio for each bit. Finally, the efficiency of the proposed systems is demonstrated through simulation and sea trial experiments. In particular, by using a single receiving array, the proposed systems achieve error-free communication with a data rate of 2688 bps under an average SNR of 11.88 dB.