Stephen B. Wicker

The impact of data aggregation in wireless sensor networks

Sensor networks are distributed event-based systems that differ from traditional communication networks in several ways: sensor networks have severe energy constraints, redundant low-rate data, and many-to-one flows. Data-centric mechanisms that perform in-network aggregation of data are needed in this setting for energy-efficient information flow. In this paper we model data-centric routing and compare its performance with traditional end-to-end routing schemes. We examine the impact of source-destination placement and communication network density on the energy costs and delay associated with data aggregation. We show that data-centric routing offers significant performance gains across a wide range of operational scenarios. We also examine the complexity of optimal data aggregation, showing that although it is an NP-hard problem in general, there exist useful polynomial-time special cases.

Design challenges for energy-constrained ad hoc wireless networks

Ad hoc wireless networks enable new and exciting applications, but also pose significant technical challenges. In this article we give a brief overview of ad hoc wireless networks and their applications with a particular emphasis on energy constraints. We then discuss advances in the link, multiple access, network, and application protocols for these networks. We show that cross-layer design of these protocols is imperative to meet emerging application requirements, particularly when energy is a limited resource.

Game theory in communications: motivation, explanation, and application to power control

Game theory is a set of tools developed to model interactions between agents with conflicting interests, and is thus well-suited to address some problems in communications systems. We present some of the basic concepts of game theory and show why it is an appropriate tool for analyzing some communication problems and providing insights into how communication systems should be designed. We then provided a detailed example in which game theory is applied to the power control problem in a CDMA-like system.

Applications of error-control coding

An overview of the many practical applications of channel coding theory in the past 50 years is presented. The following application areas are included: deep space communication, satellite communication, data transmission, data storage, mobile communication, file transfer, and digital audio/video transmission. Examples, both historical and current, are given that typify the different approaches used in each application area. Although no attempt is made to be comprehensive in the coverage, the examples chosen clearly illustrate the richness, variety, and importance of error-control coding methods in modern digital applications.

Stability of multipacket slotted Aloha with selfish users and perfect information

Aloha is perhaps the simplest and most-studied medium access control protocol in existence. Only in the recent past, however, have researchers begun to study the performance of Aloha in the presence of selfish users. In this paper, we present a game-theoretic model of multipacket slotted Aloha with perfect information. We show that this model must have an equilibrium and we characterize this equilibrium. Using the tools of stochastic processes, we then establish the equilibrium stability region for some well-known channel models.

On the behavior of communication links of a node in a multi-hop mobile environment

In this work, we develop an analytical framework to investigate the behavior of the communication links of a node in a random mobility environment. Analytical expressions characterizing various properties related to the formation, lifetime and expiration of links are derived. The derived framework can be used to design efficient algorithms for medium access, routing and transport control, or to analyze and optimize the performance of existing network protocols. A number of applications of the characteristics investigated, such as selection of stable routes, route cache lifetime optimization, providing Quality-of-Service (QoS) data communication and analysis of route lifetime are discussed. In particular, we focus on designing an efficient updating strategy for proactive routing protocols based on the derived statistics. Using simulations, we show that the proposed strategy can lead to significant performance improvements in terms of reduction in routing overhead, while maintaining high data packet delivery ratio and acceptable latency.

Selfish users in Aloha: a game-theoretic approach

Past studies of Aloha have emphasized system-wide goals such as achieving maximum throughput or minimum delay. We use game theory to analyze Aloha from the perspective of a selfish user. we construct an Aloha game and examine the optimal behavior of individual users. We show that the Aloha game has an equilibrium and that an Aloha system in which the users are selfish will be stable provided the attempt rate is sufficiently low. We then compare the performance of a selfish Aloha system with the performance of a centrally controlled slotted Aloha system. With some system parameters performance is near the optimum performance obtained by a centrally-controlled system. By utilizing a selfish-user assumption, it is possible to build systems which are robust and scalable.

An upper bound on the sum-rate distortion function and its corresponding rate allocation schemes for the CEO problem

We consider a distributed sensor network in which several observations are communicated to the fusion center using limited transmission rate. The observation must be separately encoded so that the target can be estimated with minimum average distortion. We address the problem from an information theoretic perspective and establish the inner and outer bound of the admissible rate-distortion region. We derive an upper bound on the sum-rate distortion function and its corresponding rate allocation schemes by exploiting the contra-polymatroid structure of the achievable rate region. The quadratic Gaussian case is analyzed in detail and the optimal rate allocation schemes in the achievable rate region are characterized. We show that our upper bound on the sum-rate distortion function is tight for the quadratic Gaussian CEO problem in the case of same signal-to-noise ratios at the sensors.

Packet combining systems based on the Viterbi decoder

Type-I hybrid-ARQ protocols can be used to construct powerful adaptive rate algorithms through the use of packet combining techniques. In this paper several packet combining schemes are presented for use in conjunction with the Viterbi decoder over stationary and time-varying channels. The first technique presented is an averaged diversity combiner, which is shown to be identical in performance to an interleaved code combiner over an AWGN channel. The averaged diversity combiner is then generalized to make use of packet weights based on either ideal channel state information or weights derived from side information generated by the Viterbi decoder. It is shown that the weighted diversity combiner using decoder side information performs almost as well as the system using ideal channel state information. All of the packet combining schemes discussed in this paper provide improved throughput and reliability performance relative to that provided by the standard type-I hybrid-ARQ or EEC systems. This performance improvement is obtained at the expense of negligible to moderate modifications to the transmitter and receiver. Performance bounds are derived for each of the combining schemes and their tightness verified through simulation results. >

CareNet: an integrated wireless sensor networking environment for remote healthcare

CareNet is an integrated wireless sensor environment for remote healthcare that uses a two-tier wireless network and an extensible software platform. CareNet provides both highly reliable and privacy-aware patient data collection, transmission and access. This paper describes our system architecture, software development, and the results of our field studies.