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

Abstract

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 reconfigurability 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).

Chirp-based LPD/LPI underwater acoustic communications with code-time-frequency multidimensional spreading

“…Since then, chirp signals have been used as a communication technology that can enable low data rate, robust, low-power (LPD/LPI) wireless communications on simple-design, lowcost transceivers in different applications including indoor wireless communications [14], multiuser applications [15,16], and WLAN [17] and WPAN [18] applications. Chirp signals also have been proposed in the UWA communications literature as highly reliable but low data rate alternatives [10,19,20].…”

“…We conducted a performance evaluation study for the proposed transmission scheme on a multi-scale simulator that evaluates underwater chirp-based communications at two different levels, i.e., (i) at the wave level by modeling acoustic propagation in selected reference scenarios, (ii) at the bit level by simulating in detail the proposed chirp-based transmission schemes. In addition, we performed an experiment evaluation of the proposed scheme by implementing the proposed scheme on our custom software-defined acoustic platform [10][11][12] and conducted field experiments in a harbor in Los Angeles, CA.…”

SEANet G2

“…The modems were reported to provide support for software-defined physical and data-link layers that enable real-time parameter adaptation. With similar goals, in our previous work [15,16], we proposed an underwater acoustic networking platform based on USRP with external controller on a laptop running the protocol stack and offering real-time reconfiguration capability at the physical layer. In [17], we presented a first generation SEANet platform based on pure software-processing on a resource-constrained Cortex microcontroller, which offers adaptation at multiple layers through its software-defined functionalities that span physical, data-link, network, and application layer.…”

U-Wear

“…OFDM has been extensively used in underwater acoustic communications [35] because of its robustness against frequency-selective channels with long delay spreads. The idea behind OFDM is to use a large number of closely spaced orthogonal sub-carriers, such that for each sub-carrier the channel is subject to flat fading.…”

“…By leveraging the flexibility of the software-defined architecture, upper layer protocols can reconfigure on-thefly PHY layer parameters such as modulation, signal bandwidth and FEC coding rate, among others. Reconfiguration functionalities allow to develop reactive or proactive control algorithms to adapt the underlying communication link to the channel variations or to upper layer protocol requirements [25,35].…”