Table 1 Comparison of electromagnetic waves in the air and UWA waves
From: Adaptive modulation and coding in underwater acoustic communications: a machine learning perspective
Characteristic | Electromagnetic waves in the air | UWA waves |
|---|---|---|
Medium dependence | Propagate regardless of medium, even in vacuum | Must rely on medium vibration |
Propagation uniformity | Generally along a straight line, at a stable speed | Along a curve, with speed greatly affected by temperature |
Absorption loss under water | 3 dB/m@10kHz | 1.1 dB/km@10kHz |
Speed in the air | 3×108 m/s | 340 m/s |
Speed under water | 2.25×108 m/s | 1490 m/s |
Typical working frequency and wavelength | GSM—frequency, 900 MHz; wavelength, 0.33 m | Sonar—frequency, 5 kHz; wavelength, 0.3 m |
Communication latency | Small | Large |
Multipath delay | Small multipath delay | Large delay (>10 ms), across dozens of symbols. |
Doppler | Small scaling factor (≤10−5) | Large scaling factor (10−2) |
Variation in time and space | Related to change of communication scenarios and variation in short-wave ionospheric reflection | Related to rapid changes of waves and periodic changes of seawater |