Prefix-Free Frequency Domain Equalization for Underwater Acoustic Single Carrier Transmissions

Prefixes, in forms of cyclic-prefixes or zero-padding, have often been viewed as indispensable in underwater acoustic single-carrier transmissions to avoid inter-block interference (IBI) and to formulate circular convolution. However, the prefix introduces overhead and reduces spectral efficiency. This is especially true in fast time-varying channel conditions. Here, we propose a prefix-free scheme to facilitate frequency-domain equalization (FDE) in an underwater acoustic communication system, based on the time reversal processing. In the proposed scheme, three main procedures, block partitioning, IBI cancellation, and prefix reconstruction, precede the FDE operation. The block partitioning at the receiver provides flexibility to support channel equalization in different channel fluctuation rates. The utilized prefix reconstruction scheme requires a strong first arrival to minimize the noise enhancement. Our solution is to use the time-reversal processing, because the resultant equivalent impulse response, also known as the q-function, has a stable and compact peak. To further enhance receiver performance, we incorporate two strategies. One is overlapping partitioning. The other is iterative prefix reconstruction. The proposed schemes have been tested using the field measurements obtained from the Gulf of Mexico in August 2016. Communications over four ranges at the carrier frequency of 85 kHz with a symbol rate of 17 kHz have been demonstrated. The overlapping partitioning and iterative prefix reconstruction strategies have been shown to generate improvements in the receiver performance.