| The underwater environment is widely regarded as one of the most difficult communication channels, especially in shallow water conditions. Wind-generated bubbles near the surface can result in a significant attenuation of surface scattered signals. Orthogonal frequency division multiplexing (OFDM) is an attractive communication technique, which has received considerable attention in underwater acoustic (UWA) communications. In the past decade, a new technique called limited feedback was invented, which yielded the near optimal channel adaption by allowing the receiver to send a small number of bits about the channel state information (CSI) to the transmitter. Recently, the limited feedback technique has been widely employed in wireless communication systems and standards, such as multi-user multiple-input multiple-output (MU-MIMO) systems, IEEE 802.11n and 3GPP-LTE (3rd Generation Partnership Program-Long Term Evolution).;In this dissertation, we first investigate an OFDM communication system with pilot-aided channel estimation for time-varying shallow water acoustic channels. Secondly, we present both non-iterative and iterative bit and power loading algorithms for UWA OFDM communication systems with limited feedback, based on capacity and bandwidth efficiency criteria respectively. Finally, we analyze the effect of bubbles on UWA OFDM communication systems with limited feedback. Results reveal that limited feedback is a worthwhile technique to be deployed in UWA communication systems and that analysis of the effect of bubbles on acoustic signal propagation is significant for designing high speed and reliable UWA communication systems in shallow water environments. |
