# Wireless Remote Control for Underwater Vehicles
## 2 Underwater Communication Technologies
### Underwater Acoustic Communications
Due to the high cost of good-quality LF and MF transducers with a wide bandwidth (the cost of the single transducer can easily exceed 2 kEUR, i.e., 2000 EURO), research and industrial prototypes of high-performance LF and MF acoustic modems are mostly developed by navy-related research institutes and companies, such as TNO [43], FOI [44], FFI [45], Wartsila ELAC [46], and L3 HARRIS [47], that are specifically interested in very-long-range communications for surveillance applications [48]. Also, the JANUS North Atlantic Treaty Organization (NATO) standard focuses on LF and MF frequency bands [49]. In this frequency domain, some universities and civil research institutes developed some low-cost low-power products for medium- and short-range (few hundreds of meters) low-rate (few hundreds of bits per second) modems for internet of things (IoT) applications [50; 51; 52] by employing low-cost narrow band transducers. Some commercial low-cost MF acoustic modems (with a price of less than 2 kEUR) are also available off the shelf. For instance, the modem recently launched by DSPComm [39] has a maximum transmission rate of 100 bit/s and a nominal range of 500 m. With the same range, the Micron Data Modem developed by Tritech [53] is a low-power compact modem with a maximum data rate of 40 bps. Similar performance can be obtained with the Desert Star SAM-1 modem [54]. Finaly, DeveNET, a company that mainly produces communication and localization equipment for divers, supplies Sealink [55], an affordable and low-power acoustic modem that provides a range up to 8 km at a data rate of 80 bps. To establish communication links of less than 500 m, a high-frequency (HF) acoustic modem with a carrier frequency of at least 40 kHz can be used. The market of HF modems is divided into two segments: low-rate acoustic modems and high-rate acoustic modems. While the former has a price below 2 kEUR and is suitable for low-rate (less than 200 bps) communication in shallow water up to a distance of 200 m [56; 57; 58], the latter has the same price as MF and LF acoustic modems (between 8 and 10 keUR, depending on depth rate requirements) and can be used for sending a still-frame slide-show-like video feedback from an underwater vehicle [43; 59; 60; 61; 62] to a control station, as they can perform transmissions with a bit rate of more than 30 kbps [29]. In this paper, we focus on this last type of modem in order to achieve the best performance possible despite the higher price. Indeed, they can easily achieve a bit rate of more than 30 kbps in vertical links, and in horizontal communications in very shallow water, they can still provide a rate of at least 2 kbps. Although several high-rate acoustic modems are available in the literature, only a few of them are commercial off-the-shelf products. The maximum rate of a LinkQuest [31] modem is 35.7 kbit/s (with a directional beam pattern), while EvoLogics S2 CM HS [29] is the off-the-shelf modem that provides the highest bit rate (62.5 kbit/s up to 300 m in good channel conditions, although its actual maximum throughput would typically be about 30 kbit/s). This last modem has been used in [61] to perform in-tank low-quality video streaming, where the transmission bit rate selected by the modem during its initial handshake phase [63] was 31.25 kbps and the actual throughput obtained 20 kbps. In good channel conditions, this performance can also be obtained with other EvoLogics models, such as S2C 48/78 and S2C 42/65. While the former is optimized for horizontal communications, the latter is suitable for vertical links. S2C HS, instead, has an omnidirectional transmitter and, therefore, is more suitable for being installed in an AUV or in an ROV. Modems that can provide a higher bit rate are either university noncommercial systems [59, 62, 64, 65] or company prototypes waiting for a bigger market demand before becoming available off the shelf [66].