Underwater acoustic modem using OFDM

Abstract

This thesis is about design, simulation and testing of an underwater acoustic modem using OFDM. The thesis work combines a theoretical part, whose objective is to understand the appropriate techniques to deal with the characteristics of the targeted channel, simulations and a practical part regarding the system deployment and experimental tests. There has been a great growing interest in transmitting real-time data and video. Unmanned underwater vehicles (UUVs) for military and scientific applications have become important. Building distributed and scalable underwater wireless sensor networks (UWSN) that will bring significant advantages and benefits to underwater applications, such as ocean observation for scientific exploration, commercial exploitation, coastline protection and target detection in military events has been in the scope of researchers. Based on these, designing a concrete system with high data rate will benefit many underwater acoustic (UWA) applications. The existing systems in literature use single carrier transmission and rely on linear or non-linear equalization techniques to suppress inter-symbol interference (ISI), however this requires complex equalizers and results in low data rates. Therefore we concentrate on multicarrier modulation. In this thesis ZP-OFDM (Zero Padded-Orthogonal Frequency Division Multiplexing) receiver is built, where CFO (Carrier Frequency Offset) compensation, pilot-tone based channel estimation, and data demodulation are carried out on the basis of each OFDM block. The implemented OFDM system has been developed in MATLAB. MATLAB scripts generate a data burst that contained OFDM blocks, and then they are transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are demodulated and data is detected. Simulations aim to provide correct implementation of all the algorithms by coupling the generated OFDM signal to a channel using Bellhop underwater channel model and noise addition algorithm, that artificially introduces some of the real channel effects into the signal. The method is tested in a shallow-water experiment at Bilkent Lake. Over a bandwidth of 12 kHz, the data rate is 13.92 kb/s with QPSK modulation, when the number of subcarriers was 1024. Bit-error-rate (BER) is less than 9x10−2 without any coding.