Scott F. Midkiff

On energy provisioning and relay node placement for wireless sensor networks

Wireless sensor networks that operate on batteries have limited network lifetime. There have been extensive recent research efforts on how to design protocols and algorithms to prolong network lifetime. However, due to energy constraint, even under the most efficient protocols and algorithms, the network lifetime may still be unable to meet the missions requirements. In this paper, we consider the energy provisioning (EP) problem for a two-tiered wireless sensor network. In addition to provisioning additional energy on the existing nodes, we also consider deploying relay nodes (RNs) into the network to mitigate network geometric deficiencies and prolong network lifetime. We formulate the joint problem of EP and RN placement (EP-RNP) into a mixed-integer nonlinear programming (MINLP) problem. Since an MINLP problem is NP-hard in general, and even state-of-the-art software and techniques are unable to offer satisfactory solutions, we develop a heuristic algorithm, called Smart Pairing and INtelligent Disc Search (SPINDS), to address this problem. We show a number of novel algorithmic design techniques in the design of SPINDS that effectively transform a complex MINLP problem into a linear programming (LP) problem without losing critical points in its search space. Through numerical results, we show that SPINDS offers a very attractive solution and some important insights to the EP-RNP problem.

On renewable sensor networks with wireless energy transfer: The multi-node case

Wireless energy transfer based on magnetic resonant coupling is a promising technology to replenish energy to sensor nodes in a wireless sensor network (WSN). However, charging sensor node one at a time poses a serious scalability problem. Recent advances in magnetic resonant coupling shows that multiple nodes can be charged at the same time. In this paper, we exploit this multi-node wireless energy transfer technology to address energy issue in a WSN. We consider a wireless charging vehicle (WCV) periodically traveling inside a WSN and charging sensor nodes wirelessly. We propose a cellular structure that partitions the two-dimensional plane into adjacent hexagonal cells. The WCV visits these cells and charge sensor nodes from the center of a cell. We pursue a formal optimization framework by jointly optimizing traveling path, flow routing and charging time. By employing discretization and a novel Reformulation-Linearization Technique (RLT), we develop a provably near-optimal solution for any desired level of accuracy.

Prolonging sensor network lifetime with energy provisioning and relay node placement

Wireless sensor networks that operate on batteries have limited network lifetime. There have been extensive recent research efforts on how to design protocols and algorithms to prolong network lifetime. However, due to energy constraint, even under the most efficient protocols and algorithms, the network lifetime may still be unable to meet the missions requirements. In this paper, we consider the energy provisioning problem for a two-tier wireless sensor network. In addition to provisioning additional energy on the existing nodes, we also consider deploying relay nodes (RNs) into the network to mitigate network geometric deficiency and prolong network lifetime. We formulate the joint problem of energy provisioning and relay node placement (EP-RNP) into a mixed-integer nonlinear programming (MINLP) problem. Since an MINLP problem is NP-hard in general, and even the state-of-the-art software and techniques are unable of offer satisfactory solutions, we develop a heuristic algorithm, called SPINDS, to address this problem. We show a number of novel algorithmic design techniques in the design of SPINDS that effectively transforms a complex MINLP problem into linear programming (LP) problems without losing critical points in its search space. Through numerical results, we show that SPINDS offers very attractive solution and some important insights to the EP-RNP problem.

Bundling mobile base station and wireless energy transfer: Modeling and optimization

Wireless energy transfer is a promising technology to fundamentally address energy and lifetime problems in a wireless sensor network (WSN). On the other hand, it has been well recognized that a mobile base station has significant advantages over a static one. In this paper, we study the interesting problem of co-locating the mobile base station on the wireless charging vehicle (WCV). The goal is to minimize energy consumption of the entire system while ensuring none of the sensor nodes runs out of energy. We develop a mathematical model for this complex problem. Instead of studying the general problem formulation (OPT-t), which is time-dependent, we show that it is sufficient to study a special subproblem (OPT-s) which only involves space-dependent variables. Subsequently, we develop a provably near-optimal solution to OPT-s. The novelty of this research mainly resides in the development of several solution techniques to tackle a complex problem that is seemingly intractable at first glance. In addition to addressing a challenging and interesting problem in a WSN, we expect the techniques developed in this research can be applied to address other related networking problems involving time-dependent movement, flow routing, and energy consumption.

A real-time medium access control protocol for ad hoc wireless local area networks

We develop and analyze a simple, elegant medium access control (MAC) protocol for use in transmitting real-time data in point to point ad hoc wireless local area networks (WLANs). Our enhancement of IEEE 802.11, real-time MAC (RT-MAC), achieves dramatic reductions in mean delay, missed deadlines, and packet collisions by selectively discarding packets and sharing station state information. For example, in a 50 station network with a normalized offered load of 0.7, mean delay is reduced from more than 14 seconds to less than 45 ms, late packets are reduced from 76% to less than 1%, and packet collisions are reduced from 36% to less than 1%. Regression models are developed from simulation data to describe network behavior in terms of throughput, mean delay, ratio of late packets, and ratio of collisions. Stations using RT-MAC are interoperable with stations using IEEE 802.11.

Maximizing the Lifetime of Wireless Sensor Networks through Optimal Single-Session Flow Routing

Wireless sensor networks are becoming increasingly important in recent years due to their ability to detect and convey real-time, in-situ information for many civilian and military applications. A fundamental challenge for such networks lies in energy constraint, which poses a performance limit on the achievable network lifetime. We consider a two-tier wireless sensor network and address the network lifetime problem for upper-tier aggregation and forwarding nodes (AFNs). Existing flow routing solutions proposed for maximizing network lifetime require AFNs to split flows to different paths during transmission, which we call multisession flow routing solutions. If an AFN is equipped with a single transmitter/receiver pair, a multisession flow routing solution requires a packet-level power control at the AFN so as to conserve energy, which calls for considerable overhead in synchronization among the AFNs. In this paper, we show that it is possible to achieve the same optimal network lifetime by power control on a much larger timescale with the so-called single-session flow routing solutions, under which the packet-level power control and, thus, strict requirement on synchronization are not necessary. We also show how to perform optimal single-session flow routing when the bit-rate of composite flows generated by AFNs is time-varying, as long as the average bit-rate can be estimated

An analysis of Bluetooth security vulnerabilities

Bluetooth has been developed to provide mobile ad hoc connectivity between a wide range of portable and fixed devices. In this paper, the Bluetooth system is described with an emphasis on its security features and known vulnerabilities. Additional security vulnerabilities were discovered using a scheme called VERDICT. These vulnerabilities are compared to vulnerabilities found in the IEEE 802.11 wireless local area network standard with VERDICT. Vulnerabilities were found as a result of poor improper validation, exposure, and randomness. These vulnerabilities include device address validation, invalid states, and exposed keys.

Multipath routing for multiple description video in wireless ad hoc networks

As developments in wireless ad hoc networks continue, there is an increasing expectation with regard to supporting content-rich multimedia communications (e.g., video) in such networks, in addition to simple data communications. The recent advances in multiple description (MD) video coding have made it highly suitable for multimedia applications in such networks. In this paper, we study the important problem of multipath routing for MD video in wireless ad hoc networks. We follow an application-centric cross-layer approach and formulate an optimal routing problem that minimizes the application layer video distortion. We show that the optimization problem has a highly complex objective function and an exact analytic solution is not obtainable. However, we find that a meta-heuristic approach such as genetic algorithms (GAs) is eminently effective in addressing this type of complex cross-layer optimization problems. We provide a detailed solution procedure for the GA-based approach, as well as a tight lower bound for video distortion. We use numerical results to demonstrate the superior performance of the GA-based approach and compare it to several other approaches. Our efforts in this work provide an important methodology for addressing complex cross-layer optimization problems, particularly those involving application and network layers.

On Routing for Multiple Description Video Over Wireless Ad Hoc Networks

We study the problem of multipath routing for double description (DD) video in wireless ad hoc networks. We follow an application-centric cross-layer approach and formulate an optimal routing problem that minimizes the application layer video distortion. We show that the optimization problem has a highly complex objective function and an exact analytic solution is not obtainable. However, we find that a meta-heuristic approach such as genetic algorithms (GAs) is eminently effective in addressing this type of complex cross-layer optimization problems. We provide a detailed solution procedure for the GA-based approach. Simulation results demonstrate the superior performance of the GA-based approach versus several other approaches. Our efforts in this work provide an important methodology for addressing complex cross-layer optimization problems, particularly those involved in the application and network layers

Distributed channel assignment protocols: a cross-layer approach [wireless ad hoc networks]

Despite being the subject of many years of research, distributed channel assignment remains a challenging problem. In this paper, we use a cross-layer approach and present a family of efficient distributed channel assignment protocols for wireless ad hoc networks. These protocols seek to assign distinct channels to each node within all sets of interfering nodes on the same route, subject to a channel availability constraint. Because the proposed protocols combine channel assignment with routing, they exhibit significantly lower communication, computation, and storage complexity than existing channel assignment schemes. We present an example protocol that utilizes the AODV routing protocol. In addition, we prove the correctness of the algorithms and derive an upper bound on the number of channels required to both resolve collisions and mitigate interference. Simulation results show that the performance of the proposed protocols can approach that of centralized near-optimal algorithms when the number of active routes is small.