Design of A Software-defined Underwater Acoustic Modem with Real-time Physical Layer Adaptation Capabilities

This article describes the design of a custom software-defined modem with adaptive physical layer for underwater acoustic (UWA) communications. The modem consists of a commercial software-defined radio (SDR) interfaced with a wideband acoustic transducer through amplifying circuitry. With this custom-built platform, we focus on the unique physical layer challenges of the underwater acoustic channel to demonstrate the benefits of real-time adaptation in such rapidly varying environments. We first focus on an Orthogonal-Frequency-Division-Multiplexing (OFDM) transmission scheme. In particular, for the forward link, we consider and implement a high-data rate Zero-Padded OFDM (ZP--OFDM) physical layer with a superimposed convolutional error-correction coding scheme. ZP--OFDM offers high re-configurability in terms of number of OFDM subcarriers, modulation type (e.g., BPSK, QPSK), and error-correction coding rate. Real-time adaptation at the transmitter is achieved through a robust feedback link based on a binary chirp spread-spectrum modulation (B-CSS). We demonstrate that joint real-time adaptation of system parameters such as modulation constellation and channel coding rate leads to significant data rate increase under preset bit-error-rate (BER) constraints. Moreover, in the same context, we present for the first time a seamless switch of our SDR transmitter between different signaling technologies such as OFDM and direct-sequence spread-spectrum (DS-SS).