Michael Zuba

Mobi-Sync: Efficient Time Synchronization for Mobile Underwater Sensor Networks

Time synchronization is an important requirement for many services provided by distributed networks. A lot of time synchronization protocols have been proposed for terrestrial Wireless Sensor Networks (WSNs). However, none of them can be directly applied to Underwater Sensor Networks (UWSNs). A synchronization algorithm for UWSNs must consider additional factors such as long propagation delays from the use of acoustic communication and sensor node mobility. These unique challenges make the accuracy of synchronization procedures for UWSNs even more critical. Time synchronization solutions specifically designed for UWSNs are needed to satisfy these new requirements. This paper proposes Mobi-Sync, a novel time synchronization scheme for mobile underwater sensor networks. Mobi-Sync distinguishes itself from previous approaches for terrestrial WSN by considering spatial correlation among the mobility patterns of neighboring UWSNs nodes. This enables Mobi-Sync to accurately estimate the long dynamic propagation delays. Simulation results show that Mobi-Sync outperforms existing schemes in both accuracy and energy efficiency.

Challenges and Opportunities of Underwater Cognitive Acoustic Networks

In oceans, both the natural acoustic systems (such as marine mammals) and artificial acoustic systems [like underwater acoustic networks (UANs) and sonar users] use acoustic signal for communication, echolocation, sensing, and detection. This makes the channel spectrum heavily shared by various underwater acoustic systems. Nevertheless, the precious spectrum resource is still underutilized temporally and spatially in underwater environments. To efficiently utilize the spectrum while avoiding harmful interference with other acoustic systems, a smart UAN should be aware of the surrounding environment and reconfigure their operation parameters. Unfortunately, existing UAN designs have mainly focused on the single network scenario, and very few studies have considered the presence of nearby acoustic activities. In this paper, we advocate cognitive acoustic as a promising technique to develop an environment-friendly UAN with high spectrum utilization. However, underwater cognitive acoustic networks (UCANs) also pose grand challenges due to the unique features of underwater channel and acoustic systems. In this paper, we comprehensively investigate these unique characteristics and their impact on the UCAN design. Finally, possible solutions to tackle such challenges are advocated.

JSL: Joint time synchronization and localization design with stratification compensation in mobile underwater sensor networks

Time synchronization and localization are basic services in a sensor network system. Although they often depend on each other, they are usually tackled independently. In this work, we investigate time synchronization and localization problems in underwater sensor networks. We propose a joint solution for localization and time synchronization, in which the stratification effect of underwater medium is considered, so that the bias in the range estimates caused by assuming sound waves travel in straight lines in water environments is compensated. By combining time synchronization and localization, the accuracy of both are improved jointly. Additionally, an advanced tracking algorithm IMM (interactive multiple model) is adopted to improve the accuracy of localization in the mobile case. Furthermore, by combining both services, the number of required exchanged messages is significantly reduced, which saves on energy consumption. Simulation results show that both services are improved and benefit from this scheme.

DA-Sync: A Doppler-Assisted Time-Synchronization Scheme for Mobile Underwater Sensor Networks

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). Although many time-synchronization protocols have been proposed for terrestrial wireless sensor networks, none of them can be directly applied to UWSNs. This is because most of these protocols do not consider long propagation delays and sensor node mobility, which are important attributes in UWSNs. In addition, UWSNs usually have high requirements in energy efficiency. To solve these new challenges, innovative time synchronization solutions are demanded. In this paper, we propose a pairwise, cross-layer, time-synchronization scheme for mobile underwater sensor networks, called DA-Sync. The scheme proposes a framework to estimate the doppler shift caused by mobility, more precisely through accounting the impact of the skew. To refine the relative velocity estimation, and consequently to enhance the synchronization accuracy, the Kalman filter is employed. Further, the clock skew and offset are calibrated by two runs of linear regression. Simulation results show that DA-Sync outperforms the existing synchronization schemes in both accuracy and energy efficiency.

TSMU: A Time Synchronization Scheme for Mobile Underwater Sensor Networks

Time synchronization plays a critical role in distributed network systems. In this paper, we investigate the time synchronization problem in the context of underwater sensor networks (UWSNs). We propose a pairwise, cross-layer, time synchronization scheme for mobile underwater sensor networks, called TSMU. Facilitated by the Kalman Filter, the proposed method greatly improves the dynamic propagation delay estimation by exploring the Doppler effect. Simulation results show that TSMU outperforms existing synchronization schemes in both accuracy and energy efficiency.

Toward practical MAC design for underwater acoustic networks

Recently, various medium access control (MAC) protocols have been proposed for underwater acoustic networks (UANs). These protocols have significantly improved the performance of MAC layer in theory. However, two critical characteristics, low transmission rates and long preambles, found in the commercial modem-based real systems, severely degrade the performance of existing MAC protocols in the real world. Thus, a new practical MAC design is demanded. Toward an efficient approach, this paper analyzes the impact of these two modem characteristics on the random access-based MAC and handshake-based MAC, which are two major categories of MAC protocols for UANs. We further develop the nodal throughput and collision probability models for representative solutions of these two MAC protocol categories. Based on the analyses, we believe time sharing-based MAC is very promising. Along this line, we propose a time sharing-based MAC and analyze its nodal throughput. Both analytical and simulation results show that the time sharing-based solution can achieve significantly better performance.

Launching denial-of-service jamming attacks in underwater sensor networks

Recent surges in the development of Underwater Sensor Networks (UWSNs) have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, there is limited work on secure communication mechanisms and techniques to protect these networks. Security mechanisms are widely studied in terrestrial networks and various defense mechanisms have been developed as safeguards. Due to the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UWSNs. In this paper, we study the effects of denial-of-service jamming attacks on UWSNs in real-world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on existing commercial brand acoustic modems and an OFDM modem prototype. We show that UWSNs can be easily jammed using carefully timed attacks which are energy efficient.

Vulnerabilities of underwater acoustic networks to denial-of-service jamming attacks

Recent surges in the development of underwater acoustic networks UANs have lead to a rapid acceptance of this technology in scientific, commercial, and military applications. However, limited work has been performed on developing secure communication mechanisms and techniques to protect these networks. Security mechanisms are wildly studied in terrestrial networks, and various defense mechanisms have been developed as safeguards. Because of the difference in communication mediums and physical environments, the existing solutions for terrestrial networks cannot be directly applied for UANs. In this paper, we study the effects of denial-of-service jamming attacks on UANs using real-world field tests. We develop our own jammer hardware and signals in order to analyze the characteristics of different jamming attack models on a network. Our tests are performed on multiple commercial brand acoustic modems and an orthogonal frequency division multiplexing modem prototype. We show that UANs can be easily jammed using carefully timed attacks, which are energy efficient. Copyright

Practical Coding-based Multi-Hop Reliable Data Transfer for underwater acoustic networks

In this paper, we investigate reliable data transfer for multi-hop underwater acoustic networks. Motivated by experiences from real-world field tests, we propose a Practical Coding-based Multi-hop Reliable Data Transfer (PCMRDT) protocol. For the per-hop reliable data transfer, PCMRDT combines random linear coding and selective repeat to achieve high reliability and efficiency. We analyze the data recovery capability for random linear coding so that we can set an appropriate coding rate. In addition, PCMRDT utilizes a multi-hop coordination mechanism to eliminate collisions and decreases average end-to-end delay over multiple hops. Simulation results show that PCMRDT can significantly reduce the network delay with high energy efficiency.

Cyber-physical systems

A retrospective analysis of cyber-physical systems theory is given, and its current state is characterized. A number of problems arising in the theory of hybrid automata is investigated. A semigroup transition system is considered, which underlies the extension of the algebraic theory of interaction of labeled transition systems to cyber-physical systems.