# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Modern Selection and Considerations
In this work, we cannot analyze the characteristics of all transducers of each modem (as most companies do not provide this information); thus, we assume that two modems can be used together if their bandwidths are spaced apart by at least 5 kHz. When designing deployment with an underwater vehicle, also the interference between modems and the acoustic localization systems, such as ultra-short baseline (USBL) and long baseline (LBL) acoustic positioning systems [104], used to track the vehicle position along the whole mission must be avoided. A USBL is composed of two components: a transceiver, usually deployed from a control station with a well-known position, such as a ship, and a transponder, installed on the underwater vehicle that needs to be tracked. The former is equipped with an array of at least four hydrophones, used to determine the target position from the range and bearing obtained from the acoustic signal received by the latter. Specifically, the transceiver sends an acoustic pulse (interrogation) to the transponder that responds with another acoustic pulse (reply) immediately, so the transceiver can triangulate the position of the transponder by means of its hydrophone array. In case of bandwidth overlap, the acoustic pulses sent by the USBL may interfere with the acoustic communications: to overcome this issue, some companies [41, 105] provide the possibility to perform low-rate communication (up to few hundreds of bits per second) with their USBL systems. Some modem manufactures, instead, provide a version of their modem that incorporates a USBL [28, 29, 31, 34, 38, 47, 55], where the transponder is just a normal unit of the modem programmed to answer the USBL request and the transceiver is a modified version of the modem that includes the hydrophone array into the modem transducer. An LBL system, instead, uses a network of sea-floor-mounted baseline transponders as the reference point for navigation. The exact coordinates of the baseline transponders are known, and they are used for determining target positions. Baseline transponders reply to acoustic interrogation signals from target-mounted transponders with their own acoustic pulses, allowing a target to calculate its own position by measuring the distance between itself and each transponder of the baseline array. Although the deployment of an LBL, that typically requires at least four baseline transponders plus the target transponder, is more expensive than the deployment of a USBL, it provides a higher positioning precision and is often used in oil and gas fields. In addition, the LBL transponder can either be specifically manufactured for positioning [105] or can be a regular acoustic modem with positioning capabilities [12, 28, 29, 31, 34, 38, 40, 55, 56, 57]. The modems equipped with USBL or LBL functionalities are able to switch between positioning and data modes or provide an automatic protocol that performs tracking of the vehicle along with the communication, at the price of a throughput reduction between 10% and 20%. In our design, the longest range acoustic modem should also provide either LBL or USBL capabilities. One of the main disadvantages of LF acoustic modems when used in ROV and AUV operations is the fact that they are strongly affected by noise caused by the propellers of ships and vessels [106]. From our tests, we indeed discovered that, in a network deployed 40 m from a cargo ship docked in a port, a long-range EvoLogics S2C 7/17 modem reaches only the same transmission range as an EvoLogics S2C HS (i.e., 200 m) because the noise level of the former is very close to the saturation level of its transducer while the latter is almost unaffected due to its high-frequency bands. Indeed, HF acoustic signals mainly suffer from the noise caused by wind-driven waves and not by shipping noise [10].