Channel Estimation and Equalization of Underwater Acoustic MIMO-OFDM Systems: A Review Estimation du canal et l’égalisation des systèmes MEMS-MROF acoustiques sous-marins: une revue

High-speed underwater acoustic (UWA) communication is challenging due to limited bandwidth, extended multipath spread, severe Doppler effect, and large transmission loss. The use of multiple transducers at both transmitter and receiver increases the spatial diversity of the system, and orthogonal frequency-division multiplexing (OFDM) solves the problem of limited bandwidth due to its high spectral efficiency. Thus, the combination of multiple-input multiple-output (MIMO) and OFDM shifts the UWA communication to a whole new level; however, at the same time, it also increases the design challenges for the development of such systems. Intersymbol interference, intercarrier interference, and cochannel interference are introduced in the received signal by the extended multipath and Doppler shifts along with different types of noises due to the noisy acoustic channel. Therefore, many channel estimation and equalization schemes are developed for the UWA MIMO-OFDM communication systems. In this paper, these schemes are thoroughly reviewed, summarized, categorized, and compared with other similar algorithms in terms of complexity and performance while pointing out the prevailing research problems.