# Long-Range Acoustic Communication Using Differential Chirp Spread Spectrum
## 1 Introduction  
Recently, research on long-range acoustic communication (LRAC) has been actively underway [1; 2; 3; 4; 5; 6; 7], largely in the military arena, such as for autonomous underwater vehicles (AUVs) [4; 6]. Especially in the military arena, LRAC should not only have reliable communication, but also confidentiality. The spread spectrum (SS) technique is a good method in terms of confidentiality of communication, and research on the SS method has recently been underway [6; 8; 9; 10; 11]. When the SS technique is applied, the energy of the signal spreads widely in the frequency domain, making it difficult to detect signals in the interceptor. In addition, it is difficult to determine whether a signal is the signal itself or the sound generated by acoustic animals, because there are many living things in range [11]. These features make the use of SS in LRAC an outstanding method. In addition, underwater research is underway for not only SS but also Orthogonal Frequency Division Multiplexing (OFDM). OFDM has good performance in delay spreading and is good in channel variation because it deals with the channel block-to-block. However, over long distances, this is quite challenging, and we chose the SS method considering communication over 90 km or longer distances [12; 13; 14; 15; 16].  
First, SS communication can be divided into the direct sequence spread spectrum (DSSS) and frequency hopping (FH) technique [8; 9]. Recently, research on these two methods has been activein underwater communication [12; 17; 18; 19; 20; 21]. The disadvantage of these techniques stands out in certain channels. The loss of LRAC can occur in two main ways: one is the problem of having a low signal-to-noise ratio (SNR) by distance and another is Doppler effects of the channel by platform motions and ocean current. DSSS has good performance for problems caused by a low SNR [6; 8; 9; 12; 17; 18; 19]; however, the channel has high Doppler effects, so the performance of DSSS will be reduced, in which case the FH technique will have better performance [8; 9; 20; 21]. In the case of the chirp spread spectrum (CSS), because of the high matched-filter process gain, which seems to be good in the general channel, it has the advantage of always having a constant performance rather than being advantageous or disadvantageous on a particular channel [22; 23; 24].  
Underwater communication has five categories according to the distance [3], expressed in Table 1. We conducted the experiment at a long range of communication at a distance of 10-100 km, which has a higher SNR compared to very long-range communication and has fewer multipaths and low variance compared to medium-range communication. As a result of these characteristics, the CSS was adopted.  
The performance of CSS communication was determined by the process gain of the matched filter. The proposed method is to increase the process gain of the matched filter when the data rate is constant, and differential coding is used as a way to increase the time-bandwidth product cost. We increased the amount of time-bandwidth product by using two symbols to demodulate using the current symbol and previous symbol. Details of these methods are explained in Section 3.  
We analyzed performance according to SNR and Doppler through the use of a simple simulation [25; 26; 27]. First, we identified the performance according to the SNR of the proposed method through simulation, and then we identified the difference in performance between the conventional method and the proposed method through the lake trial. The descriptions related to the simulation and lake experiments are explained in Sections 4 and 5.1. Finally, the sea trial demonstrated that the proposed method is capable of communication at a distance of \(\sim\)90 km. A detailed description of each is given in Section 5.2.