# High-Reliability Underwater Acoustic Communication Using an M-ary Cyclic Spread Spectrum
## 1 Introduction  
Underwater acoustic (UWA) communication is a critical tool for gathering data for marine environmental monitoring and scientific research as well as for military purposes [1]. UWA communication technology has been the sole technique used to achieve long-distance underwater communication until now. Underwater radio transmission, on the other hand, is more complicated and challenging than terrestrial radio transmission. Multipath propagation, frequency selective fading, low-propagation velocity, and restricted bandwidth are all characteristic of underwater acoustic channels [2]. The low-propagation velocity of the acoustic signal, as well as the movement of the transceiver and ocean currents, can cause Doppler shifts and spread. Furthermore, frequency selective fading and excessive noise interference can continuously disrupt underwater acoustic communication. Spread spectrum communication was first used for military purposes and is a method of spreading and transmitting a broadband signal to generate a robust signal in an environment with a low signal-to-noise ratio (SNR) [3, 4]. Direct sequence, time hopping, and frequency hopping are examples of communication systems that use the spread spectrum methodology [5, 6, 7]. The direct sequence spread spectrum (DSSS) method, in particular, hides the signal in background noise, inhibits eavesdropping, and can help with multipathtransmission issues. Long-range underwater communication was carried out utilizing a single carrier in DSSS transmission [8; 9; 10; 11; 12; 13], and sea trials were carried out using a multicarrier spread spectrum [14]. However, because of the spreading process, the transmission speed is decreased, and the packet length is increased; as a result, Doppler spreading can have a significant impact. Because considerable information can be carried in a symbol, conventional cyclic shift keying (CSK) is suitable for a channel with a short coherence time at a high transmission rate to compensate for the decrease in the transmission rate [15; 16]. [PERSON] et al. proposed a method combining orthogonal frequency division multiplexing (OFDM) and CSK for underwater acoustic communication and exhibited its performance through a simulation based on a representative study linked to the CSK method for underwater acoustic communication [17]. The sequence overlapped and demodulated through the position of the peak and was demonstrated through sea trials, in addition to the method of creating orthogonality of codes by cyclic shifting [18; 19]. The M-ary spread spectrum (MSS) approach, which enhances transmission efficiency by using multiple orthogonal sequences instead of cycling a sequence such as the CSK method, is another method that can improve transmission efficiency [20; 21]. In addition, by combining these two methods, there is an M-ary cyclic shift keying (MCSK) method, which is a method for putting additional information into one symbol by the cyclic shifting of a sequence having orthogonality in common. The MCSK transmission scheme combines the orthogonality of MSS and CSK to transmit more information in one symbol. Therefore, it is possible to minimize the time that a communication signal occupies a channel and is less affected by an underwater channel having a short coherence time. [20]. This paper proposes an M-ary cyclic spread spectrum (MCSS) method that combines MSS and MCSK. The MSS method has the advantage of tracking Doppler per symbol [22], and the CSK method has an advantage in the underwater communication channel with a short coherence time by transmitting more information in one symbol [18; 19; 23]. An MSS system-based receiver processing method for tracking the Doppler factor for each symbol as well as a method for reducing the amount of computation are presented. Because using a Doppler bank requires a significant amount of computation, the maximum range of Doppler is calculated using the peak position of the MSS correlation characteristic and CSK's shifted correlation characteristic to save computation time and improve accuracy.