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Mobile Computing, IEEE Transactions on

Issue 2 • Date Feb. 2011

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Displaying Results 1 - 15 of 15
  • [Front cover]

    Page(s): c1
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    Freely Available from IEEE
  • [Inside front cover]

    Page(s): c2
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  • A Spectrum Decision Framework for Cognitive Radio Networks

    Page(s): 161 - 174
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    Cognitive radio networks have been proposed as a solution to both spectrum inefficiency and spectrum scarcity problems. However, they face to a unique challenge based on the fluctuating nature of heterogeneous spectrum bands as well as the diverse service requirements of various applications. In this paper, a spectrum decision framework is proposed to determine a set of spectrum bands by considering the application requirements as well as the dynamic nature of spectrum bands. To this end, first, each spectrum is characterized by jointly considering primary user activity and spectrum sensing operations. Based on this, a minimum variance-based spectrum decision is proposed for real-time applications, which minimizes the capacity variance of the decided spectrum bands subject to the capacity constraints. For best-effort applications, a maximum capacity-based spectrum decision is proposed where spectrum bands are decided to maximize the total network capacity. Moreover, a dynamic resource management scheme is developed to coordinate the spectrum decision adaptively dependent on the time-varying cognitive radio network capacity. Simulation results show that the proposed methods provide efficient bandwidth utilization while satisfying service requirements. View full abstract»

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  • Distributed Fault-Tolerant Quality of Wireless Networks

    Page(s): 175 - 190
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    A mobile ad hoc network (MANET) consists of a group of communicating hosts that form an arbitrary network topology by means of any of several wireless communication media. MANET communications represent a diversification in communication technology necessary to solve the stringent end-to-end requirements of QoS-based communication networks. Of the many challenges in this complex distributed system, the problem of routing based on a predefined set of customer preferences, critical to guaranteeing quality-of-service, is the focus of this research. Specifically, this paper modifies a cluster-based QoS routing algorithm for mobile ad hoc networks with the aim of providing fault tolerance, which is a critical feature in providing QoS in the link failure-prone environment of mobile networks. Performance of this new fault-tolerant cluster-based QoS wireless algorithm is evaluated according to failure recovery time, dropped packets, throughput, and sustained flow bandwidth via simulations involving node failure scenarios along QoS paths. View full abstract»

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  • A Medium Access Control Scheme for Wireless LANs with Constant-Time Contention

    Page(s): 191 - 204
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    In today's wireless networks, stations using the IEEE 802.11 Standard contend for the channel using the Distributed Coordination Function (DCF). Research has shown that DCF's performance degrades especially with the large number of stations. This becomes more concerning due to the increasing proliferation of wireless devices. In this paper, we present a Medium Access Control (MAC) scheme for wireless LANs and compare its performance to DCF and to other efficient schemes. Our scheme, which attempts to resolve the contention in a constant number of slots (or constant time), is called CONTI. The contention resolution happens over a predefined number of slots. In a slot, the stations probabilistically send a jam signal on the channel. The stations listening retire if they hear a jam signal. The others continue to the next slot. Over several slots, we aim to have one station remaining in the contention, which will then transmit its data. We find the optimal parameters of CONTI and present an analysis on its performance. More comprehensive evaluation is presented in the simulation results where we compare CONTI, DCF, and other efficient schemes from the literature. We consider the number of slots used, the collision rate, the throughput, the delay, and the fairness. The highest throughput was achieved by CONTI. Moreover, our results provide measurements from each of the schemes that we consider and provide the insight on each scheme's operation. View full abstract»

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  • Secret Key Establishment Using Temporally and Spatially Correlated Wireless Channel Coefficients

    Page(s): 205 - 215
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    When implementing data encryption and decryption in a symmetric cryptosystem, secure distribution of the secret key to legitimate nodes can be a challenge. In this paper, we consider establishing secret keys using the common wireless channel, with particular emphasis on the spatial and temporal correlations of the channel coefficients. Specifically, we investigate the influence of channel correlation on the bound of the key size generated from the common channel using a simple single-input single-output channel model, and we verify the existence of a sampling approach able to generate a key using the minimum possible sampling window. We also explore decorrelation of the channel coefficients in a multiple-input multiple-output channel, and we use a statistical independence test to demonstrate that this process cannot be separated into spatial and temporal decorrelation processes. The insights gained from these studies assist in the development of a practical key generation protocol based on a published channel coefficient quantization method and incorporating flexible quantization levels, transmission of the correlation eigenvector matrix, and LDPC coding to improve key agreement in an authenticated public channel. Finally, we present simulations with real channel measurements that solidify the fundamental conclusions. View full abstract»

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  • Rendezvous for Cognitive Radios

    Page(s): 216 - 227
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    Cognitive radios have been touted as a solution to communicating in a Dynamic Spectrum Access environment. This paper examines how cognitive radios initially find one another among the expanse of ever-changing open spectrum, termed the rendezvous problem. Specifically, it addresses the problem of rendezvous under varying levels of system capabilities, spectrum policies, and environmental conditions. The focus is on rendezvous when there are are no control channels or centralized controllers, which we term the blind rendezvous problem. Under these conditions, a sequence-based and modular clock blind rendezvous algorithms are proposed, and it is shown that the performance of these algorithms compares favorably to that of a random blind rendezvous algorithm. Specifically, the sequence-based algorithm provides a bounded Time To Rendezvous (TTR) and the ability to prioritize channels where rendezvous is more likely to occur; the modular clock algorithm reduces the expected TTR, requires little precoordination among radios attempting to rendezvous, and is robust to radios sensing different sets of available channels. View full abstract»

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  • OFDM-Based Common Control Channel Design for Cognitive Radio Ad Hoc Networks

    Page(s): 228 - 238
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    Cognitive radio (CR) technology allows devices to opportunistically use the vacant portions of the licensed wireless spectrum. However, the available spectrum changes dynamically with the primary user (PU) activity, necessitating frequent PU sensing coordination and exchanging network topology information in a multihop CR ad hoc network. To facilitate these tasks, an always-on, out-of-band common control channel (CCC) design is proposed that uses noncontiguous OFDM subcarriers placed within the guard bands separating the channels of the licensed spectrum. First, the task of choosing the OFDM-specific parameters, including the number, power, and bandwidth of the subcarriers is formulated as a feasibility problem to ensure that the CCC does not adversely interfere with the PU operation. Second, for unicast messaging between a given pair of users, a subset of the guard bands may be chosen, which allows an additional measure of protection for the adjacent PU spectrum. For this, the multiarm bandit algorithm is used that allows the guard band selection to evolve over time based on the observed interference from the PU. Results reveal that our proposed CCC ensures connectivity and improved PU protection with a limited trade-off in data rate when compared to frequency-hopping and cluster-based CCC schemes. View full abstract»

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  • Cognitive Medium Access: Exploration, Exploitation, and Competition

    Page(s): 239 - 253
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    This paper considers the design of efficient strategies that allow cognitive users to choose frequency bands to sense and access among multiple bands with unknown parameters. First, the scenario in which a single cognitive user wishes to opportunistically exploit the availability of frequency bands is considered. By adopting tools from the classical bandit problem, optimal as well as low complexity asymptotically optimal solutions are developed. Next, the multiple cognitive user scenario is considered. The situation in which the availability probability of each channel is known is first considered. An optimal symmetric strategy that maximizes the total throughput of the cognitive users is developed. To avoid the possible selfish behavior of the cognitive users, a game-theoretic model is then developed. The performance of both models is characterized analytically. Then, the situation in which the availability probability of each channel is unknown a priori is considered. Low-complexity medium access protocols, which strike an optimal balance between exploration and exploitation in such competitive environments, are developed. The operating points of these low-complexity protocols are shown to converge to those of the scenario in which the availability probabilities are known. Finally, numerical results are provided to illustrate the impact of sensing errors and other practical considerations. View full abstract»

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  • An Improved Geocast for Mobile Ad Hoc Networks

    Page(s): 254 - 266
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    Geographic addressing of packets within mobile ad hoc networks enables novel applications, including hard real-time engagement simulation in military training systems, geographic command and control functions in training and emergency communications, and commercial messaging applications as well. The most scalable implementation of geoaddressing is via a geocast protocol, where nodes selectively retransmit packets based on local decision rules. Well-designed retransmission heuristics yield scalable geographic flooding that outperforms alternative geoaddressing approaches. However, previous geocast implementations, while effective, fall into two categories. Approaches based on flooding are unscalable due to the high load they generate. Scalable approaches, on the other hand, have trouble in complex environments, lacking sufficient intelligence about the necessary directionality of packet flow. The present paper defines a novel geocast heuristic, the Center Distance with Priority (CD-P) Heuristic, which both significantly improves on reliability of existing scalable geocasts and yet also remains scalable as scenario complexity increases. This paper describes the new technique as well as an evaluation study comparing it to previous approaches. View full abstract»

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  • Anticollision Protocols for Single-Reader RFID Systems: Temporal Analysis and Optimization

    Page(s): 267 - 279
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    One of the major challenges in the use of Radio Frequency-based Identification (RFID) on a large scale is the ability to read a large number of tags quickly. Central to solving this problem is resolving collisions that occur when multiple tags reply to the query of a reader. To this purpose, several MAC protocols for passive RFID systems have been proposed. These typically build on traditional MAC schemes, such as aloha and tree-based protocols. In this paper, we propose a new performance metric by which to judge these anticollision protocols: time system efficiency. This metric provides a direct measure of the time taken to read a group of tags. We then evaluate a set of well-known RFID MAC protocols in light of this metric. Based on the insights gained, we propose a new anticollision protocol, and show that it significantly outperforms previously proposed mechanisms. View full abstract»

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  • Security Games for Vehicular Networks

    Page(s): 280 - 290
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    Vehicular networks (VANETs) can be used to improve transportation security, reliability, and management. This paper investigates security aspects of VANETs within a game-theoretic framework where defensive measures are optimized with respect to threats posed by malicious attackers. The formulations are chosen to be abstract on purpose in order to maximize applicability of the models and solutions to future systems. The security games proposed for vehicular networks take as an input centrality measures computed by mapping the centrality values of the car networks to the underlying road topology. The resulting strategies help locating most valuable or vulnerable points (e.g., against jamming) in vehicular networks. Thus, optimal deployment of traffic control and security infrastructure is investigated both in the static (e.g., fixed roadside units) and dynamic cases (e.g., mobile law enforcement units). Multiple types of security games are studied under varying information availability assumptions for the players, leading to fuzzy game and fictitious play formulations in addition to classical zero-sum games. The effectiveness of the security game solutions is evaluated numerically using realistic simulation data obtained from traffic engineering systems. View full abstract»

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  • Discriminant Minimization Search for Large-Scale RF-Based Localization Systems

    Page(s): 291 - 304
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    In large-scale fingerprinting localization systems, fine-grained location estimation and quick location determination are conflicting concerns. To achieve finer grained localization, we have to collect signal patterns at a larger number of training locations. However, this will incur higher computation cost during the pattern-matching process. In this paper, we propose a novel discriminant minimization search (DMS)-based localization methodology. Continuous and differentiable discriminant functions are designed to extract the spatial correlation of signal patterns at training locations. The advantages of the DMS-based methodology are threefold. First, with through slope of discriminant functions, the exhaustive pattern-matching process can be replaced by an optimization search process, which could be done by a few quick jumps. Second, the continuity of the discriminant functions helps predict signal patterns at untrained locations so as to achieve finer grained localization. Third, the large amount of training data can be compressed into some functions that can be represented by a few parameters. Therefore, the storage space required for localization can be significantly reduced. To realize this methodology, two algorithms, namely, Newton-PL and Newton-INT, are designed based on the concept of gradient descent search. Simulation and experiment studies show that our algorithms do provide finer grained localization and incur less computation cost. View full abstract»

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  • TMC Information for authors

    Page(s): c3
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    Freely Available from IEEE
  • [Back cover]

    Page(s): c4
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Aims & Scope

Mobile Computing, as proposed in this Transactions, focuses on the key technical issues related to (a) architectures, (b) support services, (c) algorithm/protocol design and analysis, (d) mobile environments, (e) mobile communication systems, (f) applications, and (g) emerging technologies.

Full Aims & Scope

Meet Our Editors

Editor-in-Chief
Prasant Mohapatra
Interim Vice-Provost and CIO
Professor, Dept. Computer Science
University of California, Davis, USA
pmohapatra@ucdavis.edu