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Underwater Wireless Acoustic Video Communications Channel

The problem of real-time wireless video transmission from an underwater vehicle (AUV) to a vessel or surface platform represents one of the last milestones in the development of effective autonomous systems for ocean exploration and monitoring.

Underwater exploration has emerged as an area of great interest to many scientists and engineers, as well as to general population. Crucial components in the development of ocean-monitoring systems are the autonomous underwater vehicles and the means by which they can communicate to the surface.

While the majority of today’s underwater imaging is performed by transmitting signals from submersibles to the surface via optical cables, advances in acoustic underwater communications make it possible to conceive of a scenario in which video signals are transmitted in a wireless manner.

Till the moment the most offshore underwater production system for construction, its inspections and maintenance were based on the using of ROVs (Remotely Operated Vehicles). It means that ROV should be followed by the vessel that should deliver to ROV electricity and exchanged the control and telemetry data with ROV by the umbilical cord. The tether system of ROV decrease its maneuverability, effectiveness and enlarge of the risks of damages and failure of the operations.

Currently commercial available acoustic modems provide transmission rates up to several kilobits per second (kbps). While these rates may be sufficient for navigation and control, data rates that are at least ten, if not a hundred times higher are required for reasonable quality video transmission.

The key to achieving video transmission over the band limited underwater channels lies in two approaches: (1) efficient data compression and (2) use of highly bandwidth efficient modulation methods. The goal in combining these two approaches is to close the gap between the bit rate needed for video transmission and that supported by the acoustic channel.

BaltRobotics had resolved successfully the both tasks and had designed, manufactured and tested “Underwater Wireless Acoustic Video Communication Channel” with the following characteristics:

Characteristic	Value	Notes
*Modem characteristics*
1. Bandwidth	80 kHz
2. Bit Rate	128 kb/s	Bandwidth efficiency (“special efficiency”) – 1,8 b/s/Hz
3. Maximum Distance of Communication	200 m
*Video characteristics*
4. Video Standards (input)	SD Color Video
5. Frame size in pixels (W×H)	640×480
6. Frame Rate	15 frames per sec
7. Video options	Advanced 3D-Modelling and Objects Recognition
8. Stable recovering of Video with the Bit Rates	From 10 kb/s till 150 kb/s
9. Video Compression	At 0.02 bits per pixel

The main factors influencing underwater acoustic communications include:

-         Transmission loss: it consists of attenuation and geometric spreading, the first being mainly provoked by absorption due to conversion of acoustic energy into heat and the second being caused by the spreading of sound energy as a result of the expansion of the wavefronts;

-         Noise: It can be classified as man-made noise and ambient noise;

-         Multipath: Multipath propagation is responsible for severe degradation of the acoustic communication signal, since it generates Inter Symbol Interference (ISI);

-         High delay and delay variance: The propagation speed in the underwater channel is five orders of magnitude lower than in the radio channel;

-         Doppler spread: The Doppler frequency spread can be significant in underwater channels, causing a degradation in the performance of digital communications.

In motion environments (such as vessel/platform motion and scattering of the moving sea surface), the slow propagation speed of sound introduces large Doppler spread or shifts, which causes severe interference among different frequency components of the signal (also referred to as frequency spreading).

The objective of underwater acoustic communication is to overcome the performance limitations induced by the highly dispersive channel, while at the same time improve the bandwidth efficiency.

“Underwater Wireless Acoustic Video Communication Channel” designed by BaltRobotics operates reliably in the working depth of AUV X-3A till 200 m and in the range of velocities of AUV.

High Data Rate Ultrasonic full duplex Modem

High Data Rate Ultrasonic full duplex Modem

Acoustic communication is the most versatile and widely used technique in underwater environments due to the low attenuation (signal reduction) of sound in water.

This is especially true in thermally stable, deep water settings.

On the other hand, the use of acoustic waves in shallow water can be adversely affected by temperature gradients, surface ambient noise, and multipath propagation due to reflection and refraction.

The much slower speed of acoustic propagation in water, about 1500 m/s (meters per second), compared with that of electromagnetic and optical waves, is another limiting factor for efficient communication and networking. Nevertheless, the currently favorable technology for underwater communication is upon acoustics.

Free-space optical (FSO) waves used as wireless communication carriers are generally limited to very short distances because the severe water absorption at the optical frequency band and strong backscatter from suspending particles. Even the clearest water has 1000 times the attenuation of clear air, and turbid water has more than 100 times the attenuation of the densest fog. Nevertheless, underwater FSO, especially in the blue-green wavelengths (450-550 nm), offers a practical choice for high-bandwidth communication - 10-150 Mbps with negligible delay over moderate ranges - up to about 100 m.

On the front of using electromagnetic (EM) waves in radio frequencies, conventional radio does not work well in an underwater environment due to the conducting nature of the medium, especially in the case of seawater. However, if EM could be working underwater, even in a short distance, its much faster propagating speed is definitely a great advantage for faster and efficient communication among nodes.

Radio waves propagate at long distances through conductive sea water only at extra low frequencies (30-300 Hz), which require large antennae and high transmission power.

A short range broadband electromagnetic telemetry link can be established for 100 bps over 30m or 100 kbps over short distances up to 10 m.

Up to date and extending to the near future, acoustic waves will be staying as the major carrier of wireless communication in Underwater Wireless Communication Networks.

The major characteristics of acoustic, electromagnetic and optical carriers are sunnarized in Table 1.

Comparison of acoustic, electro-magnetic and optical waves in seawater environments

Table 1

System requirements

Control signals:
- data rate: up to 1 kbps; low BER.
Telemetry data:
- data rate: 1~10’s kbps; BER:     10^-3   ~ 10^-4
Speech signals:
- data rate: 3 kbps (SSB); BER:         10^-2
Video transmission:
- data rate: 10~100’s kbps; BER: 10^-3   ~ 10^-4
High Data Rate Ultrasonic Full Duplex Modem (HDR-U-FDM) had designed by BaltRobotics to transmit compressed video data from AUV X-3A to the vessel side.
HDR-U-FDM meet the requirements for data transmission mentioned above.

Comparison of acoustic, electro-magnetic and optical waves in seawater environments

Table 1