T. Owens Walker

Radio Tomography for Roadside Surveillance

Radio tomographic imaging (RTI) has recently been proposed for tracking object location via radio waves without requiring the objects to transmit or receive radio signals. The position is extracted by inferring which voxels are obstructing a subset of radio links in a dense wireless sensor network. This paper proposes a variety of modeling and algorithmic improvements to RTI for the scenario of roadside surveillance. These include the use of a more physically motivated weight matrix, a method for mitigating negative (aphysical) data due to noisy observations, and a method for combining frames of a moving vehicle into a single image. The proposed approaches are used to show improvement in both imaging (useful for human-in-the-loop target recognition) and automatic target recognition in a measured data set.

A secure wireless network for roadside surveillance using radio tomographic imaging

This paper proposes and demonstrates the novel application of a secure wireless sensor network for roadside surveillance and vehicular detection using radio tomographic imaging. Network architecture is based on the well-established Zigbee standard and medium access is provided through a time division multiple access scheme. Wireless security vulnerabilities are considered and a four-part security scheme is presented. Field test results are provided to validate both the baseline radio tomographic imaging functionality and the accompanying wireless sensor network security mechanisms.

A system of systems study of space-based networks utilizing picosatellite formations

Space-based networks have long been proposed to support a myriad of both space-based and terrestrial-based applications. While much research has been done on the individual systems that will comprise this network, it was not until recently that the system was examined as a whole utilizing the tools of systems of systems engineering to provide integration and optimization. In this work, we continue this effort by characterizing the specific independent systems that comprise the space-based networks and the associated interfaces to allow integration. We provide analysis of availability and capacity metrics for the space-based network. Additionally, our findings suggest that capacity constraints must be taken into account to arrive at realistic availability calculations.

Energy Efficiency of Centralized and Distributed Computation in Unattended Multi-Hop Wireless Sensor Networks for Battlefield Monitoring

The energy efficiency trade-offs available between centralized and distributed solutions in unattended wireless sensor network deployments such as those that support remote battlefield monitoring remain an open research question. In this paper, we compare the relative energy efficiency of these two approaches in multi-hop wireless sensor networks. We develop a framework that includes both total and per node energy efficiency expressions and apply it to the beamforming class of unattended battlefield monitoring solutions using Mica2, MicaZ and the latest generation Telos sensor motes. A performance threshold is shown to exist between these approaches which can be exploited through the use of preamble sampling.

Flow-Specific Medium Access for Networked Satellite System

Prior work in modeling the satellite-based detection and tracking components of the ballistic missile defense system as a large-scale, wireless sensor network relies on a medium access scheme that can accommodate the large propagation delays encountered in these networked satellite systems. While existing satellite-based systems typically employ a form of time division multiple access, recent efforts have begun to explore contention-based approaches. In this work, we quantify the effect of the large propagation delays on both contention-based and contention-free solutions and propose a flow-specific medium access solution that provides improved delay performance by dynamically adapting the networked satellite medium access scheme to changes in both individual flow and link characteristics. A comparison with CSMA and TDMA is provided through simulation results using a version of the traffic-adaptive cooperative wireless sensor medium access control protocol that has been modified to accommodate large propagation delays.

Energy consumption analysis of flow-specific medium access and the role of probabilistic preamble sampling in energy and delay performance

In this paper, we present a novel energy consumption analysis of the flow-specific Cooperative Wireless Sensor Network Medium Access Control (CWS-MAC) protocol utilizing a nine-state radio energy model that includes both steady state and transient state energy consumption behavior. CWS-MAC is a traffic-adaptive, energy-efficient medium access scheme that has been shown in the literature to outperform both traditional contention and non-contention-based approaches by providing medium access service on a per flow basis. We also formally define and quantitatively investigate the technique of probabilistic preamble sampling which provides the ability to trade-off delay performance and energy consumption in the operation of low-power medium access schemes.

Performance analysis of slotted ALOHA with periodic server vacations for energy-efficient medium access

Slotted ALOHA is one of the earliest proposed contention-based medium access solutions for wireless networks and it remains a common approach in literature. In this paper, we present the throughput and delay performance analysis for a slotted ALOHA system with periodic server vacations. By this, we mean that the service is governed by a fixed cycle composed of alternating active and inactive periods, representative of an energy-efficient medium access solution that implements sleep cycles. To our knowledge, ours is the first work to fully develop this analysis.

An off-the-shelf, low detectability, low data rate, timing-based covert channel for IEEE 802.11 wireless networks

In this paper, we propose and implement a novel covert channel for wireless networks employing the IEEE 802.11 standard. This timing-based covert channel trades off data rate to provide a covert channel that is detectable only at the physical layer (layer 1) and is compatible with all off-the-shelf versions of the current standard. Information is embedded in the interarrival times of probe request frames or beacon frames and data rates in excess of 50 bps with observed symbol error rates of less than 2% and 6.25% in a controlled environment and uncontrolled environment respectively.

Inferring read and write operations of solid-state drives based on energy consumption

The demand for Solid-State drives (SSDs) has risen as their speed, size, and architecture have improved. SSDs introduce a security risk because they contain a separate processor to optimize the SSDs lifespan. The lack of transparency of the operations conducted by this onboard processor creates a challenge to users, particularly in digital forensics settings. This paper utilizes non-destructive voltage measurement techniques to monitor the occurrence of read and write operations on an SSD. An automated cross-drive, cross-operating system classifier is presented which is capable of correctly identifying both read and write operations with up to 100 percent accuracy across four different experimental configurations.

Effects of channel environment on timing advance for mobile device positioning in long-term evolution networks

As part of the synchronization process between a tower and a cell phone in an LTE network, a timing advance (TA) that forces the cell phone to advance the transmission of its signal based upon the signal propagation time between the cell phone and the tower is calculated. The correlation between TA value and distance allows for a potential location scheme to be implemented. We present an improved model of the probability density function for the error associated with TA-based positioning. Additionally, we characterize the variance of TA in multiple channel environments to estimate accuracy of a TA-based positioning system.