A reconfigurable software-defined acoustic modem (SDAM) allows the researchers concentrate on algorithmic developments, especially in the field of underwater communications featured with temporally and spatially varying channels. In this paper, we develop a real-time SDAM prototype based on Universal Software Radio Peripheral (USRP) for underwater communications. However, several hardware and software challenges, including hardware matching, special programming mechanism and fine frame synchronization, should be investigated for the real-time SDAM. We tackle these challenges and present the corresponding solutions. An oversampled baseband Orthogonal Frequency Division Multiplexing (OFDM) is presented to match the hardware requirements of USRP, and then a unique control signaling mechanism adaptive to underwater signal processing flow is developed. For real-time implementations, a fine frame synchronization method is adopted to cope with significant fading. Experimental results show that the newly developed SDAM prototype is reliable for real-time underwater communications with severe multipath environments.
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