George Sklivanitis

Software-defined underwater acoustic networks: toward a high-rate real-time reconfigurable modem

We review and discuss the challenges of adopting software-defined radio principles in underwater acoustic networks, and propose a software-defined acoustic modem prototype based on commercial off-the-shelf components. We first review current SDR-based architectures for underwater acoustic communications. Then we describe the architecture of a new software-defined acoustic modem prototype, and provide performance evaluation results in both indoor (water tank) and outdoor (lake) environments. We present three experimental testbed scenarios that demonstrate the real-time reconfigurable capabilities of the proposed prototype and show that it exhibits favorable characteristics toward spectrally efficient cognitive underwater networks, and high data rate underwater acoustic links. Finally, we discuss open research challenges for the implementation of next-generation software-defined underwater acoustic networks.

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).

RcUBe: Real-time reconfigurable radio framework with self-optimization capabilities

Existing commercial wireless systems are mostly hardware-based, and rely on closed and inflexible designs and architectures. Moreover, despite recent significant algorithmic developments in cross-layer network adaptation and resource allocation, existing network architectures are unable to incorporate most of these advancements. While software-defined radio (SDR) was envisioned as a new paradigm promising radical runtime adaptation through all layers of the networking protocol stack, the reality of the state-of-the-art in wireless networking practice is far from having fulfilled such promise of fast and intelligent reconfigurability and adaptability. Networking research based on the “software-defined radio” paradigm has suffered almost invariably from the lack of adequate and coherently designed abstractions to (i) define networking protocols and their cross-layer interactions across all layers of the protocol stack; (ii) define decision-making algorithms to control such interactions. To address this need, we introduce RcUBe (Real-time Re-configurable Radio), a novel architectural radio framework based on abstractions that offer real-time reconfigurability and optimization capabilities at the PHY, MAC, and network layers of the protocol stack. Unlike state-of-the-art solutions, RcUBe offers a structured methodology at variable levels of abstraction to accommodate implementations of a wide range of network architectures and protocols and complex decision-making in a modular, platform-independent way. RcUBe provides these features through a design structured into four distinct, but interacting planes, namely decision, control, data, and register plane. The broad capabilities of the proposed framework are demonstrated on a network level software-defined radio setup through a range of experiments where RcUBe is used to implement various reconfigurable functionalities of a wireless system at the PHY, MAC, and network layer.

Addressing next-generation wireless challenges with commercial software-defined radio platforms

We review commercially available software- defined radio platforms and classify them with respect to their ability to enable rapid prototyping of next-generation wireless systems. In particular, we first discuss the research challenges imposed by the latest software-defined radio enabling technologies including both analog and digital processing hardware. Then we present the state-of-the-art commercial software-defined radio platforms, describe their software and hardware capabilities, and classify them based on their ability to enable rapid prototyping and advance experimental research in wireless networking. Finally, we present three experimental testbed scenarios (wireless terrestrial, aerial, and underwater) and argue that the development of a system design abstraction could significantly improve the efficiency of the prototyping and testbed implementation process.

Receiver configuration and testbed development for underwater cognitive channelization

We propose a receiver configuration and we develop a software-defined-radio testbed for real-time cognitive underwater multiple-access communications. The proposed receiver is fully reconfigurable and executes (i) all-spectrum cognitive channelization and (ii) combined synchronization, channel estimation, and demodulation. Online (real-time) experimental field studies using in-house built modems demonstrate our theoretical developments and show that cognitive channelization is a powerful proposition for underwater communications that leads to significant improvement of spectrum utilization. Even in the absence of interference, due to the noise characteristics of the acoustic channel, cognitive channelization offers significant performance improvements in terms of receiver pre-detection signal-to-interference-plus-noise-ratio and bit-error-rate.

All-Spectrum Cognitive Channelization around Narrowband and Wideband Primary Stations

In this paper we design, implement, and experimentally evaluate a wireless software-defined radio platform for cognitive channelization in the presence of narrowband or wideband primary stations. Cognitive channelization is achieved by jointly optimizing the transmission power and the waveform channel of the secondary users. The process of joint resource allocation requires no a-priori knowledge of the transmission characteristics of the primary user and maximizes the signal-to- interference-plus-noise ratio (SINR) at the output of the secondary receiver. This is achieved by designing waveforms that span the whole continuum of available/device-accessible spectrum, while satisfying a peak power constraint for the secondary users and an interference temperature (IT) constraint for the primary users. We build a four-node software-defined radio testbed and experimentally demonstrate in an indoor laboratory environment the theoretical concepts of all-spectrum cognitive channelization in terms of pre-detection SINR and bit- error-rate (BER) at both primary and secondary receivers.

Building a Low-Cost Digital Garden as a Telecom Lab Exercise

In an interdisciplinary, semester-long class, undergraduate students learn how to build a low-cost, multihop wireless sensor network from first principles for a digital garden. This type of course better prepares electrical engineering graduates for the sensor-rich, pervasive computing era.

Sparse waveform design for all-spectrum channelization

We introduce maximum-SINR sparse-binary waveforms that modulate data information symbols from any finite alphabet and span the whole continuum of the available/device-accessible spectrum. We offer an optimal algorithm that designs the proposed waveforms by maximizing the signal-to-interference-plus-noise ratio (SINR) at the output of the maximum-SINR linear receiver. In addition, we offer a suboptimal algorithm for the same problem with significantly reduced computational complexity. The post-filtering SINR improvements attained by the proposed waveforms in a single-input single-output (SISO) communication system with colored interference are presented analytically. Simulation studies compare the proposed waveforms with their conventional non-sparse counterparts and demonstrate their superior SINR performance.

Adaptive sparse-binary waveform design for all-spectrum channelization

We introduce maximum-SINR, sparse-binary waveforms that modulate data information symbols over the entire continuum of the available/device-accessible spectrum. We present an optimal algorithm that designs the proposed waveforms by maximizing the signal-to-interference-plus-noise ratio (SINR) at the output of the maximum- SINR linear filter at the receiver. In addition, we propose a suboptimal, computationally-efficient algorithm. Simulation studies compare the proposed sparse-binary waveforms with their conventional non-sparse binary counterparts and demonstrate their superior SINR performance. The post-filtering SINR and bit-error rate (BER) improvements attained by the proposed waveforms are also experimentally verified in a software-defined radio testbed operating in multipath laboratory environment, in the presence of colored interference.

Distributed MIMO Underwater Systems: Receiver Design and Software-Defined Testbed Implementation

We design, implement, and evaluate an acoustic receiver structure for distributed multi-input and multi-output (MIMO) underwater systems that accounts for multiple carrier frequency offsets (CFOs) and multiple timing offsets (TOs) encountered in real deployments of underwater communication systems. We focus on challenging practical issues that arise in underwater acoustic sensor network setups where co-located multi-antenna sensor deployment is not feasible due to power, computation, and hardware limitations. In this paper, we utilize distributed underwater sensors to form virtual MIMO underwa- ter systems without requiring frequency or time synchronization. The proposed receiver consists of a bank of matched filters (one per effective CFO) at each receive antenna, followed by an information symbol detector. Each filter in the bank is sampled at the symbol rate with sampling timing selected according to the corresponding TO. We evaluate in real-time the performance of our algorithmic developments in a software-defined underwater testbed that utilizes in-house built software-defined acoustic modems (SDAMs). Experimental studies in both indoor, lab-controlled (tank) and outdoor (lake) real-world environments demonstrate superior bit-error-rate (BER) receiver performance compared to receiver designs that are not able to accommodate multiple CFOs and multiple TOs.