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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