Sea trial results of a robust and spectral‐efficient OFDM underwater communication system

In single‐carrier, high data‐rate systems symbol durations are much smaller than channel delay spreads. Reliable equalization in such systems is extremely difficult to attain because the channel is time varying and highly frequency selective. One way to circumvent the need of using costly equalization schemes, while maintaining high data rates, is to use multicarrier modulation in which broadband data is transmitted in parallel on multiple narrowband subcarriers. Symbol duration T in such systems is usually larger than delay spreads such that each narrowband subcarrier experiences only nonfrequency‐selective flat fading. To improve spectral efficiency and eliminate the use of guard bands between subcarriers, each subcarrier can be chosen to be orthogonal to the others. Such a multicarrier system is referred to as orthogonal frequency division multiplexing (OFDM). Unfortunately, the orthogonality is destroyed in typical underwater channels due to large Doppler shifts and spreads (compared to the inverse of...