Our current deep-ocean expeditions generally require submarines controlled by human pilots or by robots which, if not physically tethered to a boat, must regularly surface to broadcast their findings. To make progress in this field, we need autonomously powered robots that can communicate wirelessly from the depth of the ocean and orient themselves underwater. This poses a few difficulties: high-frequency radio signals, which otherwise is often our go-to communication medium on land, are quickly submerged in water; lower frequencies, on the other hand, can travel further but don’t carry as much information.
This article continues the series of research on Programming and Electronics for AE: Communication.
Communication is sending and receiving information between two or more people. The person sending the message is referred to as the sender; the person receiving the information is called the receiver.
There are two types of communication between devices: wired and wireless communication.
Underwater Communication is a technique of sending and receiving messages below water.
There are a few factors affecting Underwater Communication:
- Multipath propagation: The multipath propagation resulting from a variety of signal paths that may exist between the transmitter and the receiver can increase the interference, such as the distortion of the signal, the loss of data and multipath fading.
- Time variations of the channel
- Small Bandwidth Available: Data transfer bit rate
- Signal Attenuation: Attenuation is a general term that refers to any reduction in the strength of a signal. Attenuation is a natural consequence of signal transmission over long distances. It can also be termed as signal loss.
- Noises: a) Man made Noise – Machinery noise (pumps, power plant), b) Ambient Noise – Hydrodynamics (currents, storms, wind).
Underwater wireless communication refers to the data transmission in unguided water environment through wireless carriers, i.e., radio-frequency (RF) wave, acoustic wave, and optical wave.
Different wireless underwater technologies
The transmission of electronic data underwater using electromagnetic radio waves which can travel through the vacuum of the outer space.
- Suitable for shorter-distance data transmission.
- Propagate under water at an extremely low frequency.
- Require a large antennae and a high transmission power.
- High attenuation in seawater.
Radio Communication is the transmission of signals by modulation of electromagnetic waves with frequencies.
In underwater communication, optical signals are used to support high data rate transmission.
It is capable of exceeding Gbps at a distance of a few hundreds of meters due to the high frequency of the optical carrier.
- Optical signals are rapidly absorbed in the water
- Scattering caused by suspended particles.
With this technology, high data rate transmission is possible by using optics. However, the distance between the transmitter and the receiver must be short, due to the extremely challenging underwater environments characterized by high multi-scattering and absorption. Multi-scattering causes the optical pulse to widen in the spatial, temporal, angular, and polarization domains. Optical signal gets scattered badly under water, and the absorption is also high. The available bandwidth is extremely limited for acoustic signal. For a very long distance (i.e., 1000 km), the available bandwidth falls below a kilohertz (kHz). On the other hand, for very short ranges below around 100m, more than a hundred kHz of bandwidth may be available.
Among different types of waves, acoustic waves are used as the primary carrier for underwater wireless communication systems thanks to its relatively low absorption in underwater environments. However, the present technology using acoustic waves for underwater communication has its performance undermined by low bandwidth, high transmission losses, time varying multipath propagation, high latency and Doppler spread.
The current available underwater acoustic communication can support data rate up to tens of kbps for long distances (ranging in kms) and up to hundreds of kbps for short distances (a few meters). An acoustic modem is used for long distance communication.
Protocols used for Communication
A protocol is a set of rules to govern the data transfer between the devices. The rules are applied for the following purposes:
- For compressing the data.
- For the sending device to notify it has finished sending a message.
- For the receiving device to notify it has received a message.
The Internet Protocol (IP) is a method or a protocol by which data is sent from one computer to another on the Internet. Each computer (known as a host) on the Internet has at least one IP address that uniquely identifies it from all other computers on the Internet.
When you send or receive data (for example, an e-mail note or a Web page), the message gets divided into little chunks called packets. Each of these packets contains both the sender’s Internet address and the receiver’s address. First, any packet is sent to a gateway computer that understands a small part of the Internet. The gateway computer reads the destination address and forwards the packet to an adjacent gateway. This adjacent gateway in turn reads the destination address and so forth across the Internet until one gateway recognizes the packet as belonging to a computer within its immediate neighborhood or domain. That gateway then forwards the packet directly to the computer whose address is specified.
Each packet can, if necessary, be sent by a different route across the Internet, because a message is divided into a number of packets. The packets can arrive in a different order than the order they are sent. The Internet Protocol just delivers them.
The following protocols’ job is to deliver the packets in the right order.
- TCP (The Transmission Control Protocol) is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error onto any other machines on the internet.
- UDP (User Datagram Protocol) is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing. It is widely used for one shot, client-server-type request-reply queries and applications in which prompt delivery is more important than an accurate delivery, such as transmitting speech or video.
- RTSP (Real Time Streaming Protocol) and RTMP (Real Time Messaging Protocol) are used to transfer audio and video content from a remote location to your computer. They are designed to stream media over the network.
- HTTP (Hypertext Transfer Protocol) is the set of rules for transferring files (text, graphic images, sound, video, and other multimedia files) on the World Wide Web. As soon as a Web user opens their Web browser, the user is indirectly making use of HTTP.
- HLS (HTTP Live Streaming) is a media streaming protocol that is used for delivering visual and audio media over the internet. HLS streams are generated on the fly and stored on an HTTP server.
- DASH (Dynamic Adaptive Streaming over HTTP), also known as MPEG-DASH, is an adaptive bitrate streaming technique that enables high quality streaming of media contents over the Internet. The contents are delivered from conventional HTTP web servers.
Architecture of underwater communication:
The Hardware Requirements (minimum) for Underwater Acoustic Communication:
- Dual/Quad Core Processor
- 4 GB RAM
- 512 MB VRAM (Video RAM)
- 1.2 Mbps- 1.5 Mbps – Upload speed
- Dedicated Video card
- Acoustic Modem(3rd version)
ROS (Robot Operating System)
ROS is an open-source, meta-operating system for your robot. It provides the services you would expect from an operating system, including hardware abstraction, low-level device control, implementation of commonly-used functionality, message-passing between processes, and package management. It also provides tools and libraries for obtaining, building, writing, and running codes across multiple computers.
For ROS Kinetic Installation : http://wiki.ros.org/kinetic/Installation.
The details on power management will be updated in the next articles. Don’t forget to sign up to our newsletters or join our pioneer group for further updates!