## 1 Introduction  
A sound wave is the most effective information carrier for underwater detection and communication among the forms of energy that human beings have mastered so far. The underwater acoustic communication technique for high-speed and highly reliable information transmission in the ocean has become one of the popular research focuses facing the fast-growing information technology sector and the accelerating development of ocean resources [1]. However, an underwater acoustic channel in the ocean is complex since it may cause loss of transmitted acoustic signals, the multipath effect, phase fluctuation or other disturbances [2]. Its complexity is also revealed by its variation with the time, space, and signal frequency. Among them, the multipath effect is a key impact on the performance of an underwater acoustic communication system. Among the existing communication techniques, frequency shift keying has a strong resistance to the multipath effect and channel fluctuation [3]. It does not require accurate carrier synchronization. Hence, it is applicable to an underwater acoustic channel in harsh environments. Nevertheless, it is normally characterized by low bandwidth utilization, and requires a guard band between frequency bands. For this reason, channel frequency will not be fully utilized, which becomes a serious problem for an underwater acoustic channel with highly limited bandwidth [4]. In order to achieve a high information transmission rate with limited bandwidth for underwater acoustic channel communication, the acoustic communication technique based on orthogonal frequency division multiplexing (OFDM) has attracted much attention in recent years. The main idea of OFDM system is to divide the channel bandwidth into several sub-passband parts with equal spacing from the perspective of frequency domain. Carriers in different bandwidth channels meet the characteristics of mutual orthogonal, and the low-speed information flow is carried on different sub-carriers for transmission. At the receiving end, the received signal is integrated by using the same subcarriers, respectively, and the original sent signal can be obtained due to the orthogonal characteristics between carriers [5; 6]. The advantage of this method is that it can effectively weaken the interference of multipath effect on the sending signal. In addition, due to the mutually orthogonal characteristics between subcarriers, the frequency spectrum in their frequency domain can be spaced very close and overlapped according to certain requirements, so it has a very high frequency band utilization efficiency and will not cause mutual interference between subcarriers [7]. OFDM can be considered the finest choice which can effectively overcome the underwater acoustic channel effects. That is due to its ability to deal with the long multipath spread underwater acoustic channel with low-complex frequency-domain equalization; this means that, different from signal carrier systems, OFDM does not require a complicated time-domain equalization [8; 9]. While keeping the advantages of the existing frequency shift keying technique, the OFDM-based technique realizes a high frequency band utilization rate, so that it is more suitable for underwater acoustic channels. In [10], an OFDM-based system with an orthogonal pulse waveform was devised for the concurrent transmission of training sequence and information data. It performed very well in the simulation. However, the concurrent transmission led to a low bandwidth utilization rate, and the system had very high power consumption because of the training sequence. Reference [11] presented an OFDM-based system designed with frequency domain equalization. The system achieved the synchronization and channel estimation simultaneously by virtue of pseudorandom sequence, and delivered a low symbol error ratio. Nevertheless, the OFDM-based communication system was exposed to frequency selective fading because of the severe multipath effect in the underwater acoustic channel.