Understanding Underwater Wireless Communication Systems

Ocean exploration is essential for several reasons:

• To monitor the environment (climate, pollution, oil and gas fields, and to predict natural disasters).
• Underwater exploration (discovery of resources, deep-sea archaeology).
• Scientific data collection (marine biology, oceanography).
• Search and survey to detect objects in the ocean using imaging/mapping techniques.

Radio frequency waves can propagate longer distances through seawater at very low frequencies (30 to 300 Hz). However, this requires larger antennas and higher transmit power, which is not feasible. Moreover, very high attenuation occurs using RF wave propagation in the ocean. Optical waves are also affected by scattering losses and can only be used for shorter distances.

As underwater wireless communication is not feasible using radio frequency (RF) and optical light-based communication systems, it is carried out using acoustic waves. As mentioned in Table 1 below, underwater acoustic communication links are classified based on range. The table also mentions bandwidths used for different ranges.

Acoustic links used in underwater communication systems are of two types: vertical and horizontal. This is based on the direction of the sound ray. Acoustic frequencies from 10Hz to 1MHz are used in underwater communication.

Table 1: Underwater acoustic communication range and bandwidth

There are two network topologies that can be used for underwater communication systems: centralized and decentralized. In a centralized architecture, all the nodes (i.e., underwater (UW) sink) communicate using a central station (onshore sink or surface sink/station). Centralized architecture is very similar to cellular network architecture. In a decentralized architecture, nodes communicate using their neighbors. Decentralized architecture is also known as an ad-hoc network.

Fig 1: Underwater wireless communication network using acoustic waves

Figure 1 depicts the centralized architecture of an underwater communication system. As shown, a group of sensor nodes are installed at the bottom of the ocean. These nodes communicate with one or more underwater-installed sinks (uw-sinks). These uw-sinks operate as relays between underwater nodes and the surface station. As shown, the surface station communicates with the surface sink and onshore sink using satellite links.

Like land mobile communication, the bottom area of the ocean is divided into clusters. One uw-sink is installed or anchored in each of the clusters. To achieve communication with both underwater nodes and the surface station, the uw-sink is equipped with two transceivers: horizontal and vertical. The horizontal transceiver provides communication between the uw-sink and sensor nodes. Commands/configuration data are sent from the uw-sink to sensors via these links. Moreover, sensors collect the monitored data from the uw-sink using these links. These horizontal transceivers are short-range transceivers. Vertical transceivers are used for long-range communication between the uw-sink and the surface station, as shown. These transceivers can cover a distance of up to 10 km.

Underwater Acoustic Communication Protocol Stack

The protocol stack of this system consists of layers, namely the physical layer, data link layer, network layer, and transport layer. These layers have the same functionality as OSI layers.

• Physical Layer: This layer takes care of modulation and Error Correction. As phase tracking is a tedious task underwater, non-coherent FSK modulation is used in modems for underwater communication systems. However, advancements in DSP have led to the development of PSK and QAM-based modems for these underwater applications.
• Data Link Layer: It is similar to the MAC layer used for providing access to common resources for multiple nodes. The common techniques used for multiple access are FDMA, TDMA, and CDMA.
• Network Layer: This layer basically takes care of routing messages within the network. The protocols depend on the network topology employed in the underwater network.
• Transport Layer: This layer provides reliable communication between two systems (i.e., transmitting and receiving). It also takes care of flow control as well as congestion control.

Types of Underwater Wireless Communication

These systems facilitate communication and data exchange in aquatic environments. They are essential for various applications such as environmental monitoring, oceanographic research, underwater robotics, and offshore exploration. The common types of underwater wireless systems are as follows:

• Acoustic communication system: It uses sound waves to convey data between underwater devices. It is used for various functions such as remote control of underwater vehicles, sensor networks, and to monitor underwater ecosystems. It is the most prevalent method due to its ability to transmit signals over long distances in water environments.
• RF communication system: It is less common due to the high attenuation of radio signals in water. It can be used for coastal monitoring or communication near the water surface or shallow water scenarios.
• Fiber optic communication system: It relies on light signals to transmit data through water.
• Hybrid communication system: It combines different communication methods.
• Underwater sensor network: It consists of interconnected underwater nodes which are equipped with sensors. These nodes collaborate to collect and relay data from the environment.

Conclusion

Underwater wireless communication faces many challenges, such as multipath propagation, signal attenuation, and water turbidity. These parameters impact the range, data rate, and reliability of these underwater systems. Advances in technology continue to drive improvements in such systems, which expand their capabilities and applications.