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
System requirementsControl signals:
- data rate: up to 1 kbps; low BER.
- data rate: 1~10’s kbps; BER: 10-3 ~ 10-4
- data rate: 3 kbps (SSB); BER: 10-2
- 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.