Realization of Orthogonal Frequency Division Multiplexing (OFDM) in Underwater Acoustic (UWA) communication

Abstract

Orthogonal Frequency Division Multiplexing (OFDM) is an attractive modulation technique for underwater acoustic communications, where it has proven to be effective in combating the frequency-selective multipath distortion without the need for complex time-domain equalizers, thus offering simplicity of implementation. However, OFDM signals are characterized by large peak-to-average power ratios (PAR), which can reduce the system efficiency. To avoid the non-linear distortion in the transmit amplifier, the peak signal amplitude must be kept under a pre-specified limit, i.e. a back-off must be introduced, which reduces the average (useful) power. Acoustic systems are confined to low frequencies due to the propagation losses, while further band-limitingoccurs naturally in the transducer. The transducer thus acts as an analog filter in the final stage of transmission. While otherwise undesirable, this filtering effect can be exploited towards PAR reduction [26], also have considered the challenges in underwater and made a comparison with other communication techniques in attenuation, effective range, propagation delay, bandwidth and data rate, and looking for improvement of such technique to enhance the system.