Design of a high frequency FPGA acoustic modem for underwater communication

Contemporary underwater acoustic networks use low frequency modems. While these modems can provide long range communication, their low operating frequencies mean that only low channel bandwidth is available, which results in slow data rates. This motivates our development of a high frequency modem which offers the potential for large channel bandwidth, and hence greater link capacity. There is a range-frequency trade-off because absorption becomes very high at high frequency. Our intended operating frequencies from 100 kHz to 1 MHz would only support link ranges perhaps from 1 km down to under 100m, with communication ranges longer than this requiring forwarding over a network. Reconfigurable computing based Field Programmable Gate Arrays (FPGAs) are used to accelerate product development and support evolution of fielded systems. Given the immaturity of the field of underwater communication, a reconfigurable modem is a valuable tool for development and testing modem techniques. We present a design idea to implement an acoustic modem solely in FPGA, whereas most existing modems are implemented as a combination of FPGA and DSP processors. Aside from simple anti-aliasing filters, which could be incorporated in the preamplifier stage, the modem does all of its processing in the digital domain - maximising flexibility. In this work, we describe the initial design and architecture of our software based acoustic modem that avoids the monetary cost or time investment required to design a commercial modem or custom hardware for many applications. Our demodulator is implemented using a Costas loop which performs both suppressed carrier reconstruction and synchronous data detection within the loop. Results from initial implementation are also reported in this paper.