Nowadays, the technologies used for underwater wireless communications are acoustics, RF, Magneto-Inductive (MI), and optical. In this section, we analyze these technologies by presenting the pro and cons of each system, some details on their expected performance, and the lessons learnt from the actual performance we experienced during sea trials.
2.1. Underwater Acoustic Communications
Developed in the late 50s of the 20th 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 2
a). 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 2
b). 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
], Develogic 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 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.
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
], Wärtsilä ELAC [46
], and L3HARRIS [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
] 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
], 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
] 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 S2CM 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
] or company prototypes waiting for a bigger market demand before becoming available off the shelf [66
]. The Marecomms Roboust Acoustic Modem (ROAM), recently developed in partnership with Geospectrum, achieves a throughput of 26.7 kbps at a distance of 600 m while operating in the HF band in shallow water. A test has been performed in the presence of Doppler as the nodes were floating randomly with a speed between 0.5 and 1 knot [67
]. With the new version of the modem, the manufacturer expects to achieve 50 kb/s within a range of 1 km. This modem is expected to become available on the market by late 2020 to early 2021. The ROAM modem can also operate in the MF band, providing a bit rate of 13 kbps.
The most representative underwater acoustic modems with omnidirectional beam patterns that can also be employed for communications in shallow-water scenarios are summarized in Table 1
2.2. Optical Communications
Although acoustic modems are the typical solution for underwater communications, their bandwidth is very limited. The need for high-speed communications in an underwater environment has pushed the realization of optical devices that can transmit data within short distances at a bit rate on the order of one or more Mbps (up to few Gbps at very short ranges, depending on the model and the water conditions). Indeed, unlike acoustic communications, optical communications are more suitable for ranges up to 100 m, especially in deep dark waters, and are not affected by multipath, shipping, and wind noise, as their performance mainly depends on water turbidity and sunlight noise [70
]. In fact, high turbidity scatters and attenuates the optical field, whereas ambient light may become a significant source of noise, making transmissions close to the sea surface more difficult. The turbidity coefficient, called the attenuation coefficient, is composed of the sum of scattering and absorption coefficients. The former depends on the quantity of particles, such as plankton, dissolved in the water. The plankton exists due to the chlorophyll effect, that happens only where solar light reaches the medium, i.e., in shallow water, up to a depth of 100 m. The latter, instead, is an inherent optical property of the medium. Blue and green lights, which have wavelengths of 470 and 530 nm, respectively, are the most widely used for underwater optical communication [71
], as these wavelengths are the least attenuated in deep and shallow water, respectively. Intuitively, in order to understand which of the two wavelengths is less attenuated in a certain scenario, we just need to observe the color of the water in the presence of white light (e.g., sunlight). If the color of the water is blue, the best wavelength to use is around 470 nm (that is the typical case of a deep water deployment); otherwise, if the water color is green, wavelengths around 530 nm should be employed (that is the typical case of deployment close to the surface).
Similar to acoustic modems, optical transceivers are also designed to perform best in some scenarios; therefore, the best optical modem that outperforms all others in all possible conditions does not exist. Specifically, we can divide the optical modems into models composed of a light emitting diode (LED)-based transmitter designed for hemispherical communications [8
] and models composed of a high-directional laser-based transmitter [74
]. Although the latter achieves a throughput from 10 to 100 times higher than the former, we focus on LED-based modems, as laser-based transmitters require perfect alignment between the transmitter and receiver, a condition that a mobile vehicle can fulfill only during a docking operation or if the modems are equipped with beam-steering capabilities, such as the SA Photonics Neptune modem [80
]. The latter, however, is not an off-the-shelf product, i.e., it is custom built to order, with a significantly higher price compared to other commercial products.
We can also divide the optical modems into models tailored for dark water medium range (MR) communications (up to 100 m) [7
] and devices designed for short-range (SR) communications in high ambient light environments [9
]. In the MR class, we can find Sonardyne BlueComm 200 [7
], equipped with a very sensitive receiver based on a photomultiplier. This modem achieves a hemispherical transmission rate of 10 Mbps up to a distance of 100 m, but only in deep, dark waters. The same modem would perform poorly in the presence of light noise due to saturation of the receiver: for this reason, Sonardyne designed an ultraviolet version of this modem, able to achieve a maximum range of 75 m even in the presence of some ambient light. Still, from our experience, both models are unable to establish a communication link when deployed a few centimeters above the sea surface during daytime. Similar issues have been experienced with Hydromea Luma 500ER [25
], able to cover, in good conditions, more than 50 m with a bit rate of 500 kbps (beam pattern 120
); the Ifremer optical modem [73
], that can communicate at a similar range with a bit rate of 3 Mbps; and the early-stage version of the ENEA proof of concept (PoC) prototype [81
]. Also, the AquaOptical modem developed by MIT [72
] can perform MR communications in low solar noise conditions by reaching a maximum range of 50 m with a rate of 4 Mbps. The beam patterns of both the Ifremer and MIT modems are 100
, while the ENEA optical modem is omnidirectional. Customized LED-based MR optical modems are developed by Penguin ASI [82
]; the maximum performance of their system is in the order of 100 s of Mbps at hundreds of meters but comes at the price of very bulky and expensive modems that are only suitable for extremely specialized applications, such as deployment in heavy-size working-class ROVs or similar vehicles.
Models designed for SR communications, instead, typically overcome the ambient light noise issue by employing a noise-compensating mechanism to avoid receiver saturation, at the price of lower bit rate and range. This mechanism typically consists of measuring the average noise at the receiver and of injecting a signal with equal intensity but opposite sign at the receiver unit. BlueComm 100 [7
], for instance, can be used in all water conditions, including shallow water in daytime to transmit at a rate of 5 Mbps in SR at a maximum distance of 15 m. Its beam pattern is 120
. Similarly, the Sant’Anna OptoCOMM modem [8
] can establish a 10-meter communication link at a speed of 10 Mbps, when both the receiver and transmitter are deployed just half a meter below the sea surface. They use optical lenses to reduce the beam aperture angle to 20
and to reduce the receiver field of view to 70
to limit sunlight noise. Also, ENEA developed a solar light noise-cancellation mechanism for their new version of the optical modem: preliminary results declared by ENEA proved that their new prototype can now communicate in high ambient light conditions, at the price of a reduced bit rate. Another commercial off-the-shelf optical modem for SR is the AQUAmodem Op1 [9
], which achieves 80 kbps at 1 m, with a beam pattern of 34
. The company declares that the modem is affected by direct sunlight noise but is generally robust to low sources of ambient light noise, as it can be used in the presence of ROV lights without compromising the communication link. The same happens for the CoSa optical modem [5
], able to reach 2 Mbps at up to 20 m, with a transmitting beam aperture angle of 45
and a receiver field of view of 90
. Another low-cost modem that is quite robust to sunlight noise is the optical modem developed by IST [84
] that can reach 200 kbps at a maximum distance of up to 10 m and, different from the other modems presented so far, uses green instead of blue LEDs, as it is specifically tailored for shallow-water operations. This modem uses optical lenses to reduce the beam aperture angle to 12
and an optical filter to reduce sunlight noise.
The most representative underwater optical modems are summarized in Table 2
2.3. Radio Frequency and Magneto-Inductive Communications
Also, electromagnetic radio frequency and magneto-inductive communications can be used underwater. Compared with acoustic and optical waves, RF waves can perform a relatively smooth transition through the air–water interface [85
]. This benefit can be used to achieve cross-boundary communication: for instance, the authors in [19
] used this concept to pilot an ROV deployed up to 45 cm below the water surface. Another advantage is that RF and MI are almost unaffected by water turbulence, turbidity, misalignment between transmitters and receivers, multipath, and acoustic and solar noise, that are the main causes of poor performance for either optical or acoustic modems when used in practical scenarios. For these reasons, when in range, RF and MI can provide a much more stable link than optical and acoustic communications, with less disruptions; thus, in our opinion, they should be preferred to the other media whenever the bit rate and the range required by the application can be supported. However, their communication range is usually limited to no more than a few meters. Inductive modems [86
], for instance, are often deployed in mooring systems [89
], as they enable communication over jacketed mooring lines and can be used to retrieve data from instruments such as Conductivity, Temperature, and Pressure sondes (CTDs) and Acoustic Doppler Current Profilers (ADCPs) by substituting physical connectors and the need for dedicated cables for communication. These modems generate a low-frequency signal that travels in the mooring line and can only substitute mechanical connectors for low-rate communications (up to 5 kbps). Also RF modems [5
] can be used to replace the mechanical connector of cables in very short ranges, but they provide broadband communications (up to 100 s of Mbps) and thus can support high-rate-demanding applications, such as real-time control and high-quality video streaming. For example, Hydromea uses an RF connector in the umbilical cable of the EXRAY ROV [24
], where the vehicle’s tether can be disconnected to perform an autonomous mission before being reconnected again. Also, in this case, the communication range is in the order of few centimeters. WiSub supplies the Maelstrom connector [92
], able to support both power and data transfer via RF. Communication based on a microwave link has a rate of 100 Mbps up to a distance of 5 cm between the connectors. Similar devices are sold also by Blue Logic [93
]. Broadband RF modems can also be employed in docking stations to quickly download data from an AUV [94
]. For instance, the WFS Seatooth S500 [91
] RF modem provides a bit rate up to 100 Mbps up to a range of 10 cm, and the Lubeck University of Applied Science developed the CoSa underwater WiFi [5
], with a rate of 10–50 Mbps up to 10 cm.
These examples prove how RF communications can achieve high transmission bit rates underwater, although their communication range is very limited. Indeed, RF communications suffer from RF interference and are prone to very strong attenuation in salty waters, where the conductivity of the medium is larger than in fresh waters. A range up to a few meters (SR) can be reached with RF modems, at the price of a lower bit rate. For example, INESC Tec developed a dipole antenna prototype [6
] to support 1 Mbps communication at 1 m, and the Lubeck University of Applied Sciences developed a dipole [5
] antenna to communicate with a rate of 0.2 to 1 Mbps and a range of 1-8 m, depending on water conditions (i.e., 1 meter in salty water and 8 m in fresh water).
Although in air MI communication is outperformed by RF modems, as the latter can achieve a higher bit rate and a longer range, underwater MI modems are almost unaffected by the change of medium while the electrical field is strongly attenuated. Indeed, MI modems are proven to reach a bit rate of a few kbps at tens of meters, both in air and underwater [95
]. Dalhousie University developed an MI prototype that achieves 8 kbps at 10 m [96
], to perform low-rate low-latency communications. With MI modems, longer distances can be achieved at the price of a lower bit rate. For instance, the authors in [95
] established a directional link with a maximum range of 41 m and an omnidirectional link with a range of 26 m. Both links provide data rates of 512 bps. With their new modem design recently presented in [97
], they were able to achieve 1 kbps at a 40-m distance with an omnidirectional beam pattern. Nearly 20 years ago, the authors in [98
], instead, demonstrated a 153-bps communication link at a distance of 250 m and a 40-bps communication link at a distance of 400 m.
Very-low (VLF) and extremely-low radio frequency (ELF) signals have been extensively used during the cold war to communicate from inland control stations to submarines [99
]. The drawbacks of these systems are the low rate and the need for a very large and high-power-consuming inland antennas. Indeed, VLF can provide a 300 bps one-way communication link from shore to the submarine up to a distance of 20 m below the sea surface and requires a broadcast inland antenna with a size between 300 m and 2 km [99
]. For example, the Sweden Grimeton Radio Station [100
] uses a set of antennas that are 1.9 km long, each with an RF power peak of 200 kW.
ELF can also be used to communicate from land to submarines (one way): they reach up to 1 bps [101
] at a range of several hundreds of meters below the sea surface but require a grounded wire inland antenna (ground dipole) with a size of several tens of kilometers and a transmission power in the order of millions of watts. Due to the high cost of deployment, US, Russia, India, and China are the only nations known to have constructed ELF communication facilities. For instance, the US ELF system employed a ground dipole antenna 52 km long [102
] while the Russian system used an antenna 60 km long [103
]. This system has been typically used to signal one-way coded messages to the submarine’s commander to resurface to receive more information via other means.
Both VLF and ELF technologies are not applicable to ROVs and AUVs but only to submarines due to their very large size and demanding power consumption.
The most representative underwater MI and RF communication systems are summarized in Table 3
2.4. Modem Selection and Considerations
According to the technology comparison presented in this section and summarized in Figure 3
, we can conclude that optical technologies are the preferred choice up to a distance of about few tens of meters (100 m in very good conditions) whereas acoustics would be the preferred choice from that point onward. We also note that RF and MI modems are consistently outperformed by optical or acoustic modems, although they have the advantage that their communication is not prone to environmental characteristics (unlike acoustic and optical).
The optical modem considered in this paper is BlueComm 200, that has a hemispherical beam pattern and is able to transmit at a speed of 10 Mbps at a range up to 100 m in good channel conditions, such as those considered in this paper. For different scenarios, when choosing which modem to use, it should be considered that the BlueComm 200 modem is strongly affected by noise due to sunlight and external lights, and for this reason, Sonardyne supplies it together with an ROV lighting system that does not affect the modem performance. Indeed, BlueComm 200 can be used only in deep-water scenarios or during night operations in shallow water. If the ROV needs to be operated in shallow water during daytime, a different model tailored for shorter ranges in these conditions should be selected, like BlueComm 100, that achieves a maximum range of 15 m and a maximum rate of 5 Mbps even in the presence of sunlight, or BlueComm 200 UV, that still suffers from direct sunlight but is robust against ROV lighting systems.
One of the most important things to take into account when selecting multiple acoustic modems to be used in the same network is to avoid interference between the different devices. For instance, if both LF and MF acoustic modems need to be used in the same system, the maximum working frequency of the LF modem must be smaller than the minimum working frequency of the MF modem to avoid bandwidth overlap. Moreover, some guard between the bandwidths of the modems should be provided, as the drop of the transducer sensitivity outside the modem bandwidth is usually not vertical: for example, EvoLogics S2C 7/17 should not be used along with EvoLogics S2C 18/34 because the bandwidths of the two modems are spaced apart only by 1 kHz. 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
] 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
], 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
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
]. Another reason why MF and HF modems are used more often in small AUVs and ROVs than LF modems is because the integration of LF modems in small vehicles can be complex or even impossible due to the large size of their transducer, that has a diameter of at least 12 cm and a total weight that can easily exceed 5 kg. MF and HF modems, instead, usually have a total weight of less than 2.5 kg and a transducer diameter of less than 6 cm.
The acoustic modems selected for the wireless remote control designed in this paper are the Subnero WNC and the EvoLogics S2C HS, both equipped with an omnidirectional transducer, the former operating in the MF bandwidth with LBL capabilities and the latter operating in the HF bandwidth.