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