# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Underwater Acoustic Communications  
Developed in the late 50s of the 20 th century [26], acoustic communication is certainly the most mature underwater telecommunication technology available so far. It provides long transmission ranges, up to tens of kilometers depending on the carrier frequency and the environmental conditions [10]. The modem bandwidth and its consequent communication rate depend on the carrier frequency, on the characteristics of the modem transducers, as well as on external conditions, such as noise caused by ships, wind, and marine life; the multipath; and the Doppler effect caused by the movements of the submerged nodes. For these reasons, the communication rate of an acoustic link ranges from few tens of bits per second for long-range communications up to few tens of kilobits per second for short-range links. The main disadvantage of acoustic communications are the long propagation delay caused by the low sound speed, the time variation of noise and of the channel impulse response, the presence of asymmetric acoustic links, and the poor performance in shallow water (i.e., when the water column is less than 100 m) due to signal reflections. In a mobile network deployed in shallow water, the multipath caused by signal reflections often results in link disruption, where, for instance, the communication between two nodes deployed at a depth of 1 m is established in the range of 0 up to 110 m, lost in the range of 110 up to 220 m, and established again in the range of 220 up to 290 m (Figure 2a). The link, instead, is definitely more stable in a deep-water scenario, where communication can be established up to the maximum range of the modem without link disruption (Figure 2b). Depending on the expected conditions and user needs, there is a wide set of acoustic modems in the market that can be employed in a variety of specific scenarios. For example, to achieve a communication range of more than 4 km, a modem with a carrier frequency below 12 kHz should be used, such as Benthos ATM 960 in the low-frequency (LF) band [28], EvoLogics S2C 7/17 [29], AquaSeNT AM-D2000 [30], LinkQuest UWM3000 [31], AQUATEC AQUAModem1000 [32], Developic HAM.NODE [11], the Sercel MATS3G 12 kHz [33] modem, the WHOI MicroModem [12], and Kongsberg cNODE LF [34]. The transducer of most of these devices can be customized to the geometry of the channel2, and the modem bit rate can be adapted accordingly. For this reason, all LF modems can achieve a communication rate up to few kilobits per second in a vertical link in deep water, where the multipath is negligible, while in a horizontal link in very shallow  
Figure 2: Depiction of the Bellhop ray tracer [27] path loss to present the multipath effect in shallow (**a**) and deep water (**b**). water, they can reach a maximum rate of few hundreds of bits per second. Among the others, WHOI demonstrated that, in good channel conditions, it is possible to achieve a communication link of 70 to 400 km at 1 to 10 b/s in the arctic [35] by employing a carrier frequency of 900 Hz. For communication ranges from 1 to 3 km, instead, a medium frequency (MF) modem is most suitable, as it can provide a higher bit rate. The carrier frequency of a MF modem is, depending on the manufacturer, selected between 20 and 30 kHz. All aforementioned manufacturers that produce LF devices also develop MF acoustic modems. In addition to them, other companies also supply commercial off-the-shelf products in this range, such as the Popoto Modem [36], the Applicon Seamodem [37], Sonardyne 6G [38], DSPComm Aquacomm Gen2 [39], SubNero [40], and the Blueprint Subsea [41] acoustic modem. In low-noise scenarios, a vertical communication link with an MF modem can reach a bit rate up to tens of kbps [42], depending on the modem manufacturer, while in horizontal communications in very shallow water, they can provide a communication link with a bit rate spanning from 500 bps up to few kbps.