Qixin Wang

Cyber-Physical Systems: A New Frontier

The report of the Presidents Council of Advisors on Science and Technology (PCAST) has placed CPS on the top of the priority list for federal research investment [6]. This article first reviews some of the challenges and promises of CPS, followed by an articulation of some specific challenges and promises that are more closely related to the sensor networks, ubiquitous and trustworthy computing conference.

Acoustic target tracking using tiny wireless sensor devices

With the advancement of MEMS technologies, wireless networks consist of tiny sensor devices hold the promise of revolutionizing sensing in a wide range of application domains because of their flexibility, low cost and ease of deployment. However, the constrained computation power, battery power, storage capacity and communication bandwidth of the tiny devices pose challenging problems in the design and deployment of such systems. Target localization using acoustic signal with tiny wireless devices is a particularly difficult task due to the amount of signal processing and computation involved. In this paper, we provide an in-depth study of designing such wireless sensor networks for real-world acoustic tracking applications. We layout a cluster-based architecture to address the limitations of the tiny sensing devices. To achieve effective utilization of the scarce wireless bandwidth, a quality-driven paradigm to suppress redundant information and resolve contention is proposed. One instance of the quality-driven approach is implemented in the acoustic tracking system, where the quality of the tracking reports can be quantified numerically. We demonstrate the effectiveness of our proposed architecture and protocols using a sensor network testbed based on UCBerkeley mica motes. Considering the performance limitations of tiny sensor devices, the achieved acoustic target tracking accuracy is extraordinarily good. Our experimental study also shows that the acoustic target tracking quality can be indeed measured and used to assist resource allocation decisions. This application-driven design and implementation exercises also serve to identify important areas of further work in in-network processing and communications.

WiCop: Engineering WiFi Temporal White-Spaces for Safe Operations of Wireless Body Area Networks in Medical Applications

ZigBee and other wireless technologies operating in the (2.4GHz) ISM band are being applied in Wireless Personal Area Networks (WPAN) for many medical applications. However, these low duty cycle, low power, and low data rate medical WPANs suffer from WiFi co-channel interferences. WiFi interference can lead to longer latency and higher packet losses in WPANs, which can be particularly harmful to safety-critical applications with stringent temporal requirements, such as ElectroCardioGraphy (ECG). This paper exploits the Clear Channel Assessment (CCA) mechanism in WiFi devices and proposes a novel policing framework, WiCop, that can effectively control the temporal white-spaces between WiFi transmissions. Such temporal white-spaces can be utilized for delivering low duty cycle WPAN traffic. We have implemented and validated WiCop on SORA, a software-defined radio platform. Experimental results show that with the assistance of the proposed WiCop policing schemes, the packet reception rate of a ZigBee-based WPAN can increase by up to 116% in the presence of a heavy WiFi interferer. A case study on the medical application of WPAN ECG monitoring demonstrates that WiCop can bound ECG signal distortion within 2% even under heavy WiFi interference. An analytical framework is devised to model the CCA behavior of WiFi interferers and the performance of WPANs under WiFi interference with or without WiCop protection. The analytical results are corroborated by experiments.

Optimal real-time sampling rate assignment for wireless sensor networks

How to allocate computing and communication resources in a way that maximizes the effectiveness of control and signal processing, has been an important area of research. The characteristic of a multi-hop Real-Time Wireless Sensor Network raises new challenges. First, the constraints are more complicated and a new solution method is needed. Second, a distributed solution is needed to achieve scalability. This article presents solutions to both of the new challenges. The first solution to the optimal rate allocation is a centralized solution that can handle the more general form of constraints as compared with prior research. The second solution is a distributed version for large sensor networks using a pricing scheme. It is capable of incremental adjustment when utility functions change. This article also presents a new sensor device/network backbone architecture---Real-time Independent CHannels (RICH), which can easily realize multi-hop real-time wireless sensor networking.

Toward online hybrid systems model checking of cyber-physical systems' time-bounded short-run behavior

Many Cyber-Physical Systems (CPS) are highly nondeterministic. This often makes it impractical to model and predict the complete system behavior. To address this problem, we propose that instead of offline modeling and verification, many CPS systems should be modeled and verified online, and we shall focus on the systems time-bounded behavior in short-run future, which is more describable and predictable. Meanwhile, as the system model is generated/updated online, the verification has to be fast. It is meaningless to tell an online model is unsafe when it is already out-dated. To demonstrate the feasibility of our proposal, we study two cases of our ongoing projects, one on the modeling and verification of a train control system, and the other on a Medical Device Plug-and-Play (MDPnP) application. Both cases are about safety-critical CPS systems. Through these two cases, we exemplify how to build online models that describe the time-bounded short-run behavior of CPS systems; and we show that fast online modeling and verification is possible.

From Offline toward Real Time: A Hybrid Systems Model Checking and CPS Codesign Approach for Medical Device Plug-and-Play Collaborations

Hybrid systems model checking is a great success in guaranteeing the safety of computerized control cyber-physical systems (CPS). However, when applying hybrid systems model checking to Medical Device Plug-and-Play (MDPnP) CPS, we encounter two challenges due to the complexity of human body: 1) there are no good offline differential equation-based models for many human body parameters; 2) the complexity of human body can result in many variables, complicating the system model. In an attempt to address the challenges, we propose to alter the traditional approach of offline hybrid systems model checking of time-unbounded (i.e., infinite horizon, a.k.a., long run) future behavior to online hybrid systems model checking of time-bounded (i.e., finite horizon, a.k.a., short run) future behavior. According to this proposal, online model checking runs as a real-time task to prevent faults. To meet the real-time requirements, certain design patterns must be followed, which brings up the codesign issue. We propose two sets of system codesign patterns for hard real time and soft real time, respectively. To evaluate our proposals, a case study on laser tracheotomy MDPnP is carried out. The study shows the necessity of online model checking. Furthermore, test results based on real-world human subject trace show the feasibility and effectiveness of our proposed codesign.

A Switch Design for Real-Time Industrial Networks

The convergence of computers and the physical world is the theme for next generation networking research. This trend calls for real-time network infrastructure, which requires a high-speed real-time WAN to serve as its backbone. However, commercially available high-speed WAN switches (routers) are designed for best-effort Internet traffic. A real-time switch design for the aforementioned networks is missing. We propose a real-time switch design using a crossbar switching fabric. The proposed switch can be implemented by making minimal modification, or even simplification, to the widely implemented iSLIP crossbar switch scheduler. Our real-time switch serves periodic and aperiodic traffic with real-time virtual machine tasks, which simplifies analysis, provides isolation, and facilitates future hierarchical scheduling and flow aggregation. Taking advantage of the fact that most industrial real-time network flows rarely change, our switch is better adapted to providing high bandwidths and low latencies.

Optimal QoS sampling frequency assignment for real-time wireless sensor networks

How to allocate computing and communication resources in a way that maximizes the effectiveness of control and signal processing has been an important area of research. The characteristic of a multi-hop real-time wireless sensor network raises new challenges. First, the constraints are more complicated and a new solution method is needed. Second, we need a distributed solution to achieve scalability. This paper presents solutions to both of the new challenges. The first solution to the optimal frequency allocation is a centralized solution that can handle the more general form of constraints as compared with prior research. The second solution is a distributed version for large networks using a pricing scheme. It is capable of incremental adjustment when utility functions change.

Lightning: A Hard Real-Time, Fast, and Lightweight Low-End Wireless Sensor Election Protocol for Acoustic Event Localization

We present the Lightning Protocol, a hard real-time, fast, and lightweight protocol to elect the sensor closest to an impulsive sound source. This protocol can serve proximity-based localization or leader election for sensor collaboration. It utilizes the fact that electromagnetic waves propagate much faster than acoustic waves to efficiently reduce the number of contending sensors in the election. With simple RF bursts, most basic comparison operations, no need of clock synchronization, and a memory footprint as small as 5,330 bytes of ROM and 187 bytes of RAM, the protocol incurs O(1) transmissions, irrespective of the sensor density, and guarantees hard real-time (O(1)) localization time cost. Experiment results using UC Berkeley Motes in a common office environment demonstrate that the time delay for the Lightning Protocol is on the order of milliseconds. The simplicity of the protocol reduces memory cost, computation complexity, and programming difficulty, making it desirable for low-end wireless sensors.

ORTEGA: An Efficient and Flexible Software Fault Tolerance Architecture for Real-Time Control Systems

Fault tolerance is an important aspect in real-time computing. In real-time control systems, tasks could be faulty due to various reasons. Faulty tasks may compromise the performance and safety of the whole system and even cause disastrous consequences. In this paper, we describe ORTEGA (On-demand Real-TimE GuArd), a new software fault tolerance architecture for real-time control systems. ORTEGA has high fault coverage and reliability. Compared with existing real-time fault tolerance architectures, such as Simplex, ORTEGA allows more efficient resource utilizations and enhances flexibility. These advantages are achieved through the on-demand detection and recovery of faulty tasks. ORTEGA is applicable to most industrial control applications where both efficient resource usage and high fault coverage are desired.