Design of high frequency FPGA acoustic modem for underwater communication

Abstract

Current underwater acoustic networks mostly rely on low frequency modems. While long range communication can be achieved with these modems, their low operating frequencies limit the available channel bandwidth, which results in low data rates. The low frequency acoustic channel also suffers from substantial multipath and Doppler effects, which constrain the signal quality at the receiver. On the other hand, strong, frequency dependent attenuation constrains the range of high frequency underwater acoustic links. High frequency signals, however, offer substantially greater signal bandwidth, and improved channel quality. The impact of short link range can be reduced by multi-hop networking. This dissertation addresses the lack of high data rate underwater acoustic modems and presents a novel design idea for a high frequency acoustic modem for underwater communication. Reconfigurable Field Programmable Gate Arrays (FPGAs) are used to implement computationally extensive algorithms and provide good system functionality at low cost. This reconfigurable, low cost modem can therefore play a significant role for development and testing of underwater modem techniques. Our work focuses on implementing a modem solely in an FPGA, whereas most existing modems are based on DSP processors. The modulation scheme used is BPSK as it is simple to implement and much less susceptible to noise than other modulation techniques frequently used under water. Our software defined modem provides flexibility by allowing modification with relative ease as the design evolves. This thesis demonstrates the feasibility of a high frequency FPGA based modem which is a valuable tool to provide a better understanding of the high frequency acoustic channel, and exploits the utility of absorption to enhance channel re-use rates in underwater acoustic networks. The modulator has been implemented in the FPGA and tested in laboratory and open water to generate recordings that conform to modelling. The Matlab model of the demodulator reconstructs the carrier and accomplishes code synchronization and data detection from recordings of both laboratory and open water tests. The demodulator has also been implemented in the FPGA. Test results from the implementation are covered in this work. Design and FPGA implementation of the individual blocks of the modem are also discussed.