Investigation of BER Improvement Techniques in Underwater Acoustic Communication

Abstract

In past few decades, researchers have delivered about great advances in the underwater wireless communication. Underwater communication has found an increasing use in an wide range of applications, such as environmental research, navigation technology via autonomous underwater vehicle (AUV), and civil missions in ocean. Since, electromagnetic waves propagates extremely short distance because of high attenuation, we use acoustic waves. The underwater acoustic channel has unique characteristics due to which there are no good synthetic channel models. Some of these characteristics are low speed of acoustic waves, Doppler distortion in the signal, frequency dependent path attenuation. Due time variability and multipath, estimation of channel is very difficult. In this thesis we consider frequency selective channel modelled by Nakagami-m distribution. There are many noise sources are present in underwater. Hence, standard Gaussian noise model is not applicable for underwater environment conditions. Depending upon the frequency of operation, noise will also change. There are several noise models are proposed, we consider Generalized Gaussian distribution model. Because of the frequency selective nature of the channel, intersymbol interference (ISI) is very high. In order to mitigate the ISI, we use the ZP-OFDM. The speed of acoustic wave is very slow, hence it induces extreme Doppler effect. So, before detecting the received data we need to mitigate the Doppler distortion.
In this thesis, we first model the channel by considering Nakagami-m distribution along with GG distribution noise model. Then, we estimated the Doppler scaling factor by appending the preamble and postamble to transmitting signal. Further, we design receiver algorithm when each path having common Doppler scaling factor as well as different Doppler scaling factor. Finally, we incorporate the receiver diversity by employing the maximal ratio combining where we observed improvement in the BER performance.