A key research area in wireless transmission is underwater communications. It has a vital role in applications such as underwater sensor networks (UWSNs) and disaster detection. The underwater channel is very unique as compared to other alternatives of transmission channels. It is characterized by path loss, multipath fading, Doppler spread and ambient noise. Thus, the bit error rate (BER) is increased to a large extent when compared to its counterpart of cellular communications. Acoustic signals are the current best solution for underwater communications. The use of electromagnetic or optical waves obviously entails a much higher data rate. However, they suffer from high attenuation, absorption or scattering. This paper proposes a novel fractional fast Fourier transform (FrFT)—orthogonal frequency division multiplexing (FrFT-OFDM) system for underwater acoustic (UWA) communication—which employs the amplitude shift keying (ASK) modulation technique (FrFT-ASK-OFDM). Specifically, ASK achieves a better bandwidth efficiency as compared to other commonly used modulation techniques, such as quadrature amplitude modulation (QAM) and phase shift keying (PSK). In particular, the system proposed in this article can achieve a very promising BER performance, and can reach higher data rates when compared to other systems proposed in the literature. The BER performance of the proposed system is evaluated numerically, and is compared to the corresponding M-ary QAM system in the UWA channel for the same channel conditions. Moreover, the performance of the proposed system is compared to the conventional fast Fourier transform (FFT)-OFDM (FFT-OFDM) system in the absence and presence of the effect of carrier frequency offset (CFO). Numerical results show that the proposed system outperforms the conventional FFT-based systems for UWA channels, even in channels dominated by CFO. Moreover, the spectral efficiency and data rate of the proposed system are approximately double the values of the corresponding conventional OFDM systems for the same parameters.

A key research area in wireless transmission is underwater communication. It has a vital role in applications like underwater sensor networks (UWSNs), disaster detection and surveillance. The underwater channel is considered as one of the most challenging communication channels due to the inherent unique characteristics of the medium. Acoustic signals are the current best candidates for underwater communication. Orthogonal frequency division multiplexing (OFDM) facilitates high data rate transmission over long underwater acoustic (UWA) channels. The main aim of this paper is to evaluate the BER performance of fractional Fourier transform (FRFT) based OFDM in a frequency selective Rician fading UWA channel in the presence of CFO. We also propose a novel 16 QAM FRFT-based OFDM to be used in the same channel conditions. The simulation results show that the BER performance of FRFT-OFDM system outperforms than that of the FFT-OFDM system at different values of FRFT angle parameter `α'. The plots also indicate that our proposed 16 QAM scheme can support high data rates and still can provide good BER performance.