## 3 Types of Underwater Communication Systems
### Applied Underwater Modulation Schemes  
For the transmission of wireless signals, a technique known as modulation is widely implemented, which consists in wrapping the signal of interest around a carrier signal [99]. Furthermore, the modulated carrier (signal of interest plus carrier) is radiated through the medium from the transmitter to the receiver. The implementation of UWSNs employs various modulation techniques, with the choice of a specific type depending on the application requirements [100]. Common modulation methods used in underwater environments include Frequency-Shift Keying (FSK), Phase-Shift Keying (PSK), Orthogonal Frequency-Division Multiplexing (OFDM), spread spectrum techniques, pulse-based modulations, chirps, Coded Orthogonal Frequency-Division Multiple Access (COFDM), on-off keying, pulse position modulation, pulse width modulation, and digital pulse interval modulation. [PERSON] et al. [101] demonstrated the use of FSK modulation on a 40 kHz acoustic modem with ultrasonic frequency, where a microcontroller, amplifier, and transducer were used to implement a UWSNs. The proposed system achieved a transmission rate of 1200 bps for sending a \"hello world\" message from the transmitter to the receiver. However, FSK supports only low data rates, making it unsuitable for AUVs, as well as audio and video streaming applications [101]. [PERSON] et al. [101] tested their proposed system in a pool, transmitting the signal between two transducers which illustrate all system components. At distances between 100 cm and 130 cm, the resulting Bit Error Rate (BER) was 0%, but at 150 cm and 170 cm, the BER increased to 6.2% and 35%, respectively [101]. These results highlight significant limitations on transmission distance between nodes, as the BER rises with increasing distance and BER measures the quality of a digital communication channel. It is a crucial indicator for assessing the performance of various systems, including telecommunications, wireless networks, and data storage. A lower BER indicates fewer errors in transmitted data. By monitoring BER, engineers and researchers can optimise system performance and ensure reliable data transmission. [PERSON] et al. [102] conducted a test-tank experiment using UVLC with PSK; based on the results, [PERSON] et al. [102] proposed the use of UVLC drones employing PSK. Their approach utilised fast-blinking Light-Emmiting Diodes (LEDs) at the transmitter, with the pattern detected by the receiver's Complementary Metal-Oxide-Semiconductor (CMOS) image sensor using the rolling-shutter effect and subsequently demodulated [102]. Rolling shutter is a type of image sensor that captures a frame line by line, rather than all at once. [PERSON] et al. [102] stated that the rolling-shutter effect is employed for baseband transmission, while PSK enables passband transmission [102]. Their experiments demonstrated superior PSK performance over On-Off Keying (OOK) modulation, especially when the carrier frequency exceeds the Noise Frequency (fn) [102]. In tests where the fn was 0, both BERs of PSK and OOK remained 0; however, as fn increased between 5 Hz and 1000 Hz, OOK exhibited significant BER, while PSK maintained a BER of 0, confirming its superior performance [102]. At a carrier frequency of 4.5 kHz and fn between 0 and 5 kHz, the transmission rate was 300 bps over a distance of 400 mm between transmitter and receiver [102]. [PERSON] et al. [71] concurs that UVLC is a potential futuristic optical candidate for systems requiring high rate and low latency. However, short distance transmission, severe attenuation, and susceptibility to water turbulence are some drawbacks of Visible Light Communication (VLC) [71; 102]. Furthermore, application of UVLC in 5G IoUT technologies produced promising results--up to a 1 Gbps data transmission rate [71].