Turbo Equalization for Underwater Acoustic Faster-than-Nyquist Signaling System

Underwater acoustic communications (UAC) is the main technical means for underwater cableless transmission. Due to the serious multi-path, time-varying and Doppler effects of underwater acoustic channel (UWAC), the received signals are overlapped and distorted, which brings serious inter symbol interference (ISI). On the other hand, the limited bandwidth and the low signal-to-noise ratio in UWAC seriously limit the transmission rate. Faster-than-Nyquist (FTN) signaling can break the restriction of Nyquist criterion and achieve higher transmission rate, but at the cost of introducing inherent ISI. Therefore, how to eliminate the serious ISI at the receiving end of UAC-FTN system with low complexity has become the main problem that restricts the application of FTN technology in UAC. To solve this problem, in the paper, a turbo receiver with tridirectional channel estimation scheme and low-complexity soft information interaction is proposed for the UAC-FTN system. The channel impulse response during one data block is obtained by estimating the channel parameters of the front, back and middle parts of this block. Lake experiment results verify that the new algorithm has better BER performance and lower complexity compared to some recently proposed algorithms in the literature.