http://www.computer.org/tmc The IEEE Transactions on Mobile Computing 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, (g) components, including devices, hardware, and software, (h) implementation issues, including interference, power, and software constraints of mobile devices, and (i) emerging technologies. en-us Wed, 4 Jan 2012 11:00:01 GMT http://csdl.computer.org/common/images/logos/tmc.gif List of recently published journal articles http://www.computer.org/tmc http://doi.ieeecomputersociety.org/10.1109/TMC.2011.129 Directional antennas can divide the transmission range into several sectors. Thus, through switching off sectors in unnecessary directions in wireless networks, we can save bandwidth and energy consumption. In this paper, we will study a directional virtual backbone (VB) in the network where directional antennas are used. When constructing a VB, we will take routing and broadcasting into account since they are two common operations in wireless networks. Hence, we will study a VB with guaranteed routing costs, named α Minimum rOuting Cost Directional VB (α-MOC-DVB). Besides the properties of regular VBs, α-MOC-DVB also has a special constraint — for any pair of nodes, there exists at least one path all intermediate directions on which must belong to α-MOC-DVB and the number of intermediate directions on the path is smaller than α times that on the shortest path. We prove that construction of a minimum α-MOC-DVB is an NP-hard problem in a general directed graph. A heuristic algorithm is proposed and theoretical analysis is also discussed in the paper. Extensive simulations demonstrate that our α-MOC-DVB is much more efficient in the sense of VB size and routing costs compared to other VBs. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.129 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.127 For real-time video broadcast where multiple users are interested in the same content, mobile-to-mobile cooperation can be utilized to improve delivery efficiency and reduce network utilization. Under such cooperation, however, real-time video transmission requires end-to-end delay bounds. Due to the inherently stochastic nature of wireless fading channels, deterministic delay bounds are prohibitively difficult to guarantee. For a scalable video structure, an alternative is to provide statistical guarantees using the concept of effective capacity/bandwidth by deriving quality of service exponents for each video layer. Using this concept, we formulate the resource allocation problem for general multihop multicast network flows and derive the optimal solution that minimizes the total energy consumption while guaranteeing a statistical end-to-end delay bound on each network path. A method is described to compute the optimal resource allocation at each node in a distributed fashion. Furthermore, we propose low complexity approximation algorithms for energy-efficient flow selection from the set of directed acyclic graphs forming the candidate network flows. The flow selection and resource allocation process is adapted for each video frame according to the channel conditions on the network links. Considering different network topologies, results demonstrate that the proposed resource allocation and flow selection algorithms provide notable performance gains with small optimality gaps at a low computational cost. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.127 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.124 Network Mobility (NEMO) efficiently manages the mobility of multiple nodes that moves together as a mobile network. A major limitation of the basic protocol in NEMO is the inefficient route between end hosts. A number of prefix delegation-based schemes have been proposed in the literature to solve the route optimization problem in NEMO. Approaches used by the schemes trade off delivery of packets through the partially-optimized route with signaling and other processing overheads. Cost of delivering packets through the partially-optimized route along with signaling and other processing cost need to be measured to find out the gain from tradeoff. However, cost analysis performed so far on NEMO protocols consider only the cost of signaling. In this paper, we have developed analytical framework to measure the costs of the basic protocol for NEMO, and four representative prefix delegation-based schemes. Our results show that cost of packet delivery through the partially-optimized route dominates over other costs. Therefore, optimizing the route completely is preferable to reduction of signaling as far as cost of network mobility is concerned. Our cost analysis framework will help in decision making to select the best route optimization scheme depending on the load imposed by the scheme on the infrastructure. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.124 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.121 Extending the network lifetime is one of the primary concerns in the ubiquitous deployment of wireless sensor networks. One approach to efficiently utilize the limited energy supplies of the sensors is to have the medium access control (MAC) protocol periodically put the sensors to sleep and wake them up to reduce idle listening, which is energy intensive. For protocol designers, it is important to understand which type of duty-cycled MAC protocol should be chosen (synchronized or asynchronous), as well as what values should be assigned to the protocol parameters under a given network scenario in order to achieve a desirable performance. However, previous work to analyze the performance of different duty-cycled MAC protocols is either protocol-specific, or limited to one aspect of the performance metric. Hence, we model and analyze the throughput, delay and energy consumption of both synchronized and asynchronous duty-cycled MAC protocols, with applications to S-MAC and X-MAC. Our method provides comprehensive performance estimation and comparison for different duty-cycled MAC protocols, and provides flexibility to trade off different performance metrics by optimizing the protocol parameters. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.121 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.114 This paper describes the design and implementation of a file system based distributed authoring system for documents for which changes by different group members are harder to automatically reconcile into a single version. Our system relies on users to moderate and reconcile updates by other group members. Our file system based approach also allows group members to modify any document type. We maintain one updateable copy of the shared content on each group member's node. We also hoard read-only copies of each of these updateable copies in any interested group member's node. All these copies are propagated to other group members at a rate that is solely dictated by the wireless user availability. The various copies are reconciled using the moderation operation; each group member manually incorporates updates from all the other group members into their own copy. The system assists with the convergence process by using the made-with knowledge of all causal file system reads of contents from other replicas. An analysis using a long term wireless user availability traces from a university shows the strength of our asynchronous and distributed update propagation mechanism. Our user space file system prototype exhibits acceptable file system performance. A subjective evaluation showed that the moderation operation was intuitive for students. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.114 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.110 In this paper, we define an ad hoc network with multi-sources and only one destination node as Converge-Cast network. Our aim is to study the performance of this kind of network by studying the capacity and delay of it. Assume $n$ wireless nodes are deployed in a square region with uniform size. For each session $k$ nodes are randomly selected as active sources and start to send one packet to another certain, unique destination node, which are also randomly selected, via relays. We first consider the stationary case, capacity is mainly discussed and delay entirely depends on average number of hops. We find per-node capacity is $\Theta(1/\sqrt{n\log n})$, which is the same as that of unicast, presented in \cite{Kumar}. Then mobility (i.i.d) is introduced to the network in order to increase network capacity. We divide our study into three steps. The first step is to learn the delay in single-session transmission. We find that the delay is $\Theta(n\log k)$ with 1-hop strategy, and $\Theta(n\log k/m)$ with 2-hop strategy with redundancy, where $m$ denotes the number of replicas for each packet. The special case for Non-redundancy for $m=1$, which shows the fact that adding non-redundancy relays do not improve the performance. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.110 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.53 Recent developments in nanotechnology have enabled the fabrication of nanomachines with very limited sensing, computation, communication, and action capabilities. The network of communicating nanomachines is envisaged as nanonet- works that are designed to accomplish complex tasks such as drug delivery and health monitoring. For the realization of future nanonetworks, it is essential to develop novel and efficient communication and networking paradigms. In this paper, the first step towards designing a mobile ad hoc molecular nanonetwork (MAMNET) with electrochemical communication is taken. MAMNET consists of mobile nanomachines and infostations that share nanoscale information using electrochemical communication whenever they have a physical contact with each other. In MAMNET, the intermittent connectivity introduced by the mobility of nanomachines and infostations is a critical issue to be addressed. An analytical framework that incorporates the effect of mobility into the performance of electrochemical communication among nanomachines is presented. Using the analytical model, numerical analysis for the performance evaluation of MAMNET is obtained. Results reveal that MAMNET achieves adequately high throughput to enable frontier nanonetwork applications with acceptable communication latency. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.53 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.33 In recent years, there has been a significant amount of work done in developing low-complexity scheduling schemes to achieve high performance in multi-hop wireless networks. A centralized sub-optimal scheduling policy, called Greedy Maximal Scheduling (GMS) is a good candidate because its empirically observed performance is close to optimal in a variety of network settings. However, its distributed realization requires high complexity, which becomes a major obstacle for practical implementation. In this paper, we develop simple distributed greedy algorithms for scheduling in multi-hop wireless networks. We reduce the complexity by relaxing the global ordering requirement of GMS, up to near-zero. Simulation results show that the new algorithms approximate the performance of GMS, and outperform the state-of-the-art distributed scheduling policies. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.33 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.279 Our work is motivated by geographical forwarding of sporadic alarm packets to a base station in a WSN, where the nodes are sleep-wake cycling. Our approach is to solve, at each forwarding node enroute to the sink, the local forwarding problem of minimizing one-hop waiting delay subject to a lower bound constraint on a suitable reward offered by the next-hop relay. The forwarding node, to begin with, is uncertain about the number of relays, their wake-up times, and the reward values. At each relay wake-up instant the forwarding node's problem is to forward the packet or to wait for further relays to wake-up. We formulate our local forwarding problem as a POMDP and obtain inner and outer bounds for the optimal policy. Motivated by the computational complexity involved in the policies derived out of these bounds, we formulate an alternate simplified model, the optimal policy for which is a simple threshold rule. We provide simulation results to compare the performance of the inner and outer bound policies against the simple policy, and also against the optimal policy when the source knows the exact number of relays. We finally compare the end-to-end performance obtained by the simple policy against that obtained by the globally optimal forwarding algorithm proposed by Kim-et-al. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.279 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.278 Recent advances in the application field increasingly demand the use of wireless camera sensor networks (WCSNs), for which localization is a crucial task to enable various location-based services. Most of the existing localization approaches for WCSNs are essentially interactive, i.e. require the interaction among the nodes throughout the localization process. As a result, they are costly to realize in practice, vulnerable to sniffer attacks, inefficient in energy consumption and computation. In this paper we propose LISTEN, a non-interactive localization approach. Using LISTEN, every camera sensor node only needs to silently listen to the beacon signals from a mobile beacon node and capture a few images until determining its own location. We design the movement trajectory of the mobile beacon node, which guarantees to locate all the nodes successfully. We have implemented LISTEN and evaluated it through extensive experiments. Both the analytical and experimental results demonstrate that it is accurate, cost-efficient, and especially suitable for WCSNs that consist of low-end camera sensors. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.278 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.277 Due to high mobility of nodes in mobile ad hoc networks (MANETs), there exist frequent link breakages which lead to frequent path failures and route discoveries. The overhead of a route discovery can not be neglected. In a route discovery, broadcasting is a fundamental and effective data dissemination mechanism, where a mobile node blindly rebroadcasts received route request packets until they reach the destinations, and thus it causes the broadcast storm problem. In this paper, we propose a neighbor coverage based probabilistic rebroadcast protocol for reducing routing overhead in MANETs. In order to effectively exploit the neighbor coverage knowledge, we propose a novel rebroadcast delay scheme to determine the rebroadcast order, and then we can obtain the more accurate additional coverage ratio by sensing neighbor coverage knowledge. We also define a connectivity factor to provide the node density adaptation. By combining the additional coverage ratio and connectivity factor, we set a reasonable rebroadcast probability. Our approach combines the advantages of the neighbor coverage knowledge and the probabilistic mechanism, which can significantly decrease the number of retransmissions so as to reduce the routing overhead, and can also improve the routing performance. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.277 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.276 The development of handheld mobile terminals (MT) capable of operating over both Wireless Local Area Networks (WLAN) and the cellular media is an important step towards the evolution of next-generation integrated networks. For real-time applications like voice, efficient vertical handover (VH) algorithms are required in maintaining a seamless connectivity and an acceptable level of quality for mobile users in heterogeneous environments. This paper proposes an analytical method for evaluating the performance of a VH algorithm that relies on the Received Signal Strength Indicator (RSSI) samples. The system model is portrayed upon addressing relevant factors that affect the quality and continuity of a voice call, and a set of performance metrics are proposed. A rigorous signal strength time-series is utilized in characterizing the instantaneous decision metrics, and a novel intermediary system model, namely the N-model, is proposed to capture the large-scale shadowing effects. The performance of a generic algorithm that relies on the RSSI, which itself is susceptible to estimation error, is evaluated for an MT roaming in and out of the WLAN coverage area. Results obtained using the analytical expressions are validated by comparing them to that obtained through Monte Carlo simulation. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.276 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.275 Ubiquitous smartphones are increasingly becoming the dominant platform for collaborative sensing. Smartphones, with their ever richer set of sensors, are being used to enable collaborative driver-assistance services like traffic advisory and road conditions monitoring. To enable such services, the smartphones' GPS, accelerometer and gyro sensors have been widely used. On the contrary, smartphone cameras, despite being very powerful sensors, have largely been neglected. In this paper, we introduce a collaborative sensing platform that exploits the cameras of windshield-mounted smartphones. To demonstrate the potential of this platform, we propose several services that it can support, and prototype SignalGuru, a novel service that leverages windshield-mounted smartphones and their cameras to collaboratively detect and predict the schedule of traffic signals, enabling Green Light Optimal Speed Advisory (GLOSA) and other novel applications. Results from two deployments of SignalGuru, using iPhones in cars in Cambridge (MA, USA) and Singapore, show that traffic signal schedules can be predicted accurately. On average, SignalGuru comes within 0.66s, for pre-timed traffic signals and within 2.45s, for traffic-adaptive traffic signals. Feeding SignalGuru's predicted traffic schedule to our GLOSA application, our vehicle fuel consumption measurements show savings of 20.3%, on average. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.275 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.274 Ad-hoc low-power wireless networks are an exciting research direction in sensing and pervasive computing. Prior security work in this area has focused primarily on denial of communication at the routing or medium access control levels. This paper explores resource depletion attacks at the routing protocol layer, which permanently disable networks by quickly draining nodes' battery power. These "Vampire" attacks are not specific to any specific protocol, but rather rely on the properties of many popular classes of routing protocols. We find that all examined protocols are susceptible to Vampire attacks, which are devastating, difficult to detect, and are easy to carry out using as few as one malicious insider sending only protocol-compliant messages. In the worst case, a single Vampire can increase network-wide energy usage by a factor of O(N), where N in the number of network nodes. We discuss methods to mitigate these types of attacks, including a new proof-of-concept protocol that provably bounds the damage caused by Vampires during the packet forwarding phase. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.274 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.273 Cognitive radio, which enables dynamic access of under-utilized licensed spectrums, is a promising technology for more efficient spectrum utilization. Since cognitive radio enables the access of larger amount of spectrum, it can be used to build wireless mesh networks with higher network capacity, and thus provide better quality of services for high bit-rate applications. In this paper, we study the multi-source video on-demand application in multi-interface cognitive wireless mesh networks. Given a video request, we find a joint multi-path routing and spectrum allocation for the session to minimize its total bandwidth cost in the network, and therefore maximize the number of sessions the network can support. We propose both distributed and centralized routing and channel allocation algorithms to solve the problem. Simulation results show that our algorithms increase the maximum number of concurrent sessions that can be supported in the network, and also improve each session's performance with regard to spectrum mobility. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.273 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.272 In cognitive radio networks, an attacker transmits signals mimicking the characteristics of primary signals, in order to prevent secondary users from transmitting. Such an attack is called primary user emulation (PUE) attack. TV towers and wireless microphones are two main types of primary users in white space. Existing work on PUE attack detection only focused on the first category. For the latter category, primary users are mobile and their transmission power is low. These properties introduce great challenges on PUE detection and existing methods are not applicable. In this paper, we propose a novel method to detect the emulation attack of wireless microphones. We exploit the relationship between RF signals and acoustic information to verify the existence of wireless microphones. The effectiveness of our approach is validated through real-world implementation. Extensive experiments show that our method achieves both false positive rate and false negative rate lower than 0.1 even in a noisy environment. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.272 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.271 We propose to establish a secure communication channel among devices based on similar audio patterns. Features from ambient audio are used to generate a shared cryptographic key between devices without exchanging information about the ambient audio itself or the features utilised for the key generation process. We explore a common audio-fingerprinting approach and account for the noise in the derived fingerprints by employing error correcting codes. This fuzzy-cryptography scheme enables the adaptation of a specific value for the tolerated noise among fingerprints based on environmental conditions by altering the parameters of the error correction and the length of the audio samples utilised. In this paper we experimentally verify the feasibility of the protocol in four different realistic settings and a laboratory experiment. The case-studies include an office setting, a scenario where an attacker is capable of reproducing parts of the audio context, a setting near a traffic loaded road and a crowded canteen environment. We apply statistical tests to show that the entropy of fingerprints based on ambient audio is high. The proposed scheme constitutes a totally unobtrusive but cryptographically strong security mechanism based on contextual information. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.271 http://opac.ieeecomputersociety.org/opac?year=2012&volume=11&issue=02&acronym=tmc IEEE Transactions on Mobile Computing http://www.computer.org/portal/site/tmc/ http://doi.ieeecomputersociety.org/10.1109/TMC.2011.270 Broadcast scheduling in multi-hop Wireless Sensor Networks (WSNs) is an effective mechanism to perform interference-aware broadcasting. Existing works provide centralized solutions, which cannot be implemented locally. Additionally, they consider very elementary network and interference models, in which, either all sensor nodes have the same transmission range or their transmission ranges are equal to their interference ranges. These assumptions are not very practical. Furthermore, they entirely ignore the existence of WSNs in 3D. In this paper, we study the broadcast scheduling in 2D and 3D WSNs. We consider that sensor nodes may have different transmission ranges and their interference ranges are times of their transmission ranges (where alpha > 1). We devise efficient coloring methods for coloring a hexagonal tiling in 2D plane and a truncated octahedron tiling in 3D space, based on which we propose O(1)-centralized approximation algorithms and the first O(1)-localized approximation algorithms for the broadcast scheduling problem in 2D and 3D WSNs respectively. Finally, we present an efficient greedy heuristic to study the effect of various priority metrics for greedily scheduling multiple interfering transmissions. Theoretical analysis and experimental results are provided to evaluate the performance of our algorithms. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.270 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.269 WiFi continues to be a prime source of energy consumption in mobile devices. This paper observes that, despite a rich body of research in WiFi energy management, there is room for improvement. Our key finding is that WiFi energy optimizations have conventionally been designed with a single AP in mind. However, network contention among different APs can dramatically increase a client's energy consumption. Each client may have to keep awake for long durations before its own AP gets a chance to send it packets to it. As AP density increases, the waiting time inflates, resulting in a proportional decrease in battery life. We design SleepWell, a system that achieves energy efficiency by evading network contention. The APs regulate the sleeping window of their clients in a way that different APs are active/inactive during non-overlapping time windows. The solution is analogous to the common wisdom of going late to office and coming back late, thereby avoiding the rush hours. We evaluate SleepWell on a testbed of laptops and Android phones over a wide variety of scenarios and traffic patterns. Results show a median gain of up to 2x when WiFi links are strong; when links are weak and the network density is high, the gains can be even more. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.269 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.267 In certain applications, the locations of events reported by a sensor network need to remain anonymous. Known as the source anonymity problem, this problem has emerged as an important topic in the security of wireless sensor networks, with variety of techniques based on different adversarial assumptions being proposed. In this work, we present a new framework for modeling, analyzing and evaluating anonymity in sensor networks. The novelty of the proposed framework is twofold: first, it introduces the notion of "interval indistinguishability" and provides a quantitative measure to model anonymity in wireless sensor networks; second, it maps source anonymity to the statistical problem of binary hypothesis testing with nuisance parameters. We then analyze existing solutions for designing anonymous sensor networks using the proposed model. We show how mapping source anonymity to binary hypothesis testing with nuisance parameters leads to converting the problem of exposing private source information into searching for an appropriate data transformation that removes or minimize the effect of the nuisance information. By doing so, we transform the problem from analyzing real-valued sample points to binary codes, which opens the door for coding theory to be incorporated into the study of anonymous sensor networks. Finally, we discuss how existing solutions can be modified to improve their anonymity. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.267 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.266 Wireless Sensor Networks (WSNs) are increasingly used in data-intensive applications such as micro-climate monitoring, precision agriculture, and audio/video surveillance. A key challenge faced by data-intensive WSNs is to transmit all the data generated within an application's lifetime to the base station despite limited power supply at the sensors. We propose using low-cost disposable mobile relays to reduce the energy consumption of data-intensive WSNs. Our approach differs from previous work in two main aspects. First, it does not require complex motion planning of mobile nodes, so it can be implemented on a number of low-cost mobile sensor platforms. Second, we integrate the energy consumption due to both mobility and wireless transmissions into a holistic optimization framework. Our framework consists of three main algorithms. The first algorithm computes an optimal routing tree assuming no nodes can move. The second algorithm improves the topology of the routing tree by greedily adding new nodes to the tree. The third algorithm improves the routing tree by relocating its nodes without changing its topology. We present efficient distributed implementations that require only limited, localized synchronization. Our simulation results show that our algorithms significantly outperform the best existing solutions. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.266 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.264 The latest generation of Wireless Local Area Networks (WLANs) is based on IEEE 802.11n-2009 Standard. The standard provides very high data rates at the physical layer and aims to achieve a throughput at the Medium Access Control (MAC) layer that is higher than 100 Mbps. To do that, the standard introduces several mechanisms to improve the MAC efficiency. The most notable ones are the use of frame aggregation and Block-ACK frames. The standard, however, doesn't introduce a mechanism to reduce the probability of collision. This issue is significant because, with a high data rate, an AP would be able to serve a large number of stations, which would result in a high collision rate. In this paper, we propose a Group-based MAC (GMAC) scheme that reduces the probability of collision and also uses frame aggregation to improve the efficiency. The contending stations are divided into groups. Each group has one station that is the group leader. Only the leader stations contend, hence, reducing the probability of a collision. We evaluate the performance of our scheme with analytic and simulation results. The results show that GMAC achieves a high throughput, high fairness, low delay and maintains a high performance with high data rates. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.264 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.263 Social network applications are becoming increasingly popular on mobile devices. A mobile presence service is an essential component of a social network application because it maintains each mobile user's presence information, such as the current status (online/offline), GPS location and network address, and also updates the user's online friends with the information continually. If presence updates occur frequently, the enormous number of messages distributed by presence servers may lead to a scalability problem in a large-scale mobile presence service. To address the problem, we propose an efficient and scalable server architecture, called PresenceCloud, which enables mobile presence services to support large-scale social network applications. PresenceCloud organizes presence servers into a quorum-based server-to-server architecture for efficient presence searching. It also leverages a directed search algorithm and a one-hop caching strategy to achieve small constant search latency. We analyze the performance of PresenceCloud in terms of the search cost and search satisfaction level. The search cost is defined as the total number of messages generated by the presence server when a user arrives; and search satisfaction level is defined as the time it takes to search for the arriving user's friend list. The results of simulations demonstrate that PresenceCloud achieves performance gains in the search cost without compromising search satisfaction. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.263 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.262 This work studies the problem of tracking signal-emitting mobile targets using navigated mobile sensors based on signal reception. Since the mobile target's maneuver is unknown, the mobile sensor controller utilizes the measurement collected by a wireless sensor network in terms of the mobile target signal's time of arrival (TOA). The mobile sensor controller acquires the TOA measurement information from both the mobile target and the mobile sensor for estimating their locations before directing the mobile sensor's movement to follow the target. We propose a min-max approximation approach to estimate the location for tracking which can be efficiently solved via semidefinite programming (SDP) relaxation, and apply a cubic function for mobile sensor navigation. We estimate the location of the mobile sensor and target jointly to improve the tracking accuracy. To further improve the system performance, we propose a weighted tracking algorithm by using the measurement information more efficiently. Our results demonstrate that the proposed algorithm provides good tracking performance and can quickly direct the mobile sensor to follow the mobile target. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.262 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.261 Many efforts have been devoted to maximizing network throughput in a multi-channel multi-radio wireless mesh network. Most current solutions are based on either purely static or purely dynamic channel allocation approaches. In this paper, we propose a hybrid multi-channel multi-radio wireless mesh networking architecture, where each mesh node has both static and dynamic interfaces. We first present an Adaptive Dynamic Channel Allocation protocol (ADCA), which considers optimization for both throughput and delay in the channel assignment. In addition, we also propose an Interference and Congestion Aware Routing protocol (ICAR) in the hybrid network with both static and dynamic links, which balances the channel usage in the network. Our simulation results show that compared to previous works, ADCA reduces the packet delay considerably without degrading the network throughput. The hybrid architecture shows much better adaptivity to changing traffic than purely static architecture without dramatic increase in overhead, and achieves lower delay than existing approaches for hybrid networks. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.261 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.260 We propose a model-based description and analysis framework for the design of wireless system architectures. Its aim is to address the shortcomings of existing approaches to system verification and the tracking of anomalies in safety-critical wireless systems. We use AADL (Architecture Analysis and Description Language) to describe an analysis-oriented architecture model with highly modular components. We also develop the cooperative tool chains required to analyze the performance of a networked system by simulating the system. We show how this framework can support a detailed and largely automated analysis of a complicated networked wireless system by applying it to examples from wireless healthcare and video broadcasting. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.260 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.259 We present a tool to estimate the interference between nodes and links in a live wireless network by passive monitoring of wireless traffic. This tool does not require any controlled experiments, injection of probe traffic in the network, or even access to the network nodes. Our approach requires deploying multiple sniffers across the network to capture wireless traffic traces. These traces are then analyzed using a machine learning approach to infer the carrier-sense relationship between network nodes. This coupled with an estimation of collision probabilities helps us to deduce the interference relationships. We also demonstrate an important application of this tool-detection of selfish carrier-sense behavior. This is based on identifying any asymmetry in carrier-sense behavior between node pairs and finding multiple witnesses to raise confidence. We evaluate the effectiveness of the tool for both the applications using extensive experiments and simulation. Experimental and simulation results demonstrate that the proposed approach of estimating interference relations is significantly more accurate than simpler heuristics and quite competitive with active measurements. Evaluations using a real testbed as well as ns2 simulation studies demonstrate excellent detection ability of the selfish behavior. On the other hand, the metric of selfishness used to estimate selfish behavior matches closely with actual degree of selfishness observed. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.259 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.258 A growing number of ad hoc networking protocols and location-aware services require that mobile nodes learn the position of their neighbors. However, such a process can be easily abused or disrupted by adversarial nodes. In absence of a-priori trusted nodes, the discovery and verification of neighbor positions presents challenges that have been scarcely investigated in the literature. In this paper, we address this open issue by proposing a fully-distributed cooperative solution that is robust against independent and colluding adversaries, and can be impaired only by an overwhelming presence of adversaries. Results show that our protocol can thwart more than 99% of the attacks under the best possible conditions for the adversaries, with minimal false positive rates. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.258 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.257 We design an efficient sensing order selection strategy for a distributed cognitive radio (CR) network, where two or more autonomous CRs sense the channels sequentially (in some sensing order) for spectrum opportunities. We are particularly interested in the case where CRs with false alarms autonomously select the sensing orders in which they visit channels, without coordination from a centralized entity. We propose an adaptive persistent sensing order selection strategy and show that this strategy converges and reduces the likelihood of collisions among the autonomous CRs as compared to a random selection of sensing orders. We also show that, when the number of CRs is less than or equal to the number of channels, the proposed strategy enables the CRs to converge to collision-free channel sensing orders. The proposed adaptive persistent strategy also reduces the expected time of arrival at collision-free sensing orders as compared to the randomize after every collision strategy, in which a CR, upon colliding, randomly selects a new sensing order. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.257 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.256 The study and design of Joint Radio Resource Management (JRRM) techniques is a key and challenging aspect in future heterogeneous wireless systems where different Radio Access Technologies will physically coexist. In these systems, the total available radio resources need to be used in a coordinated way to guarantee adequate satisfaction levels to all users, and maximize the system revenues. In addition to carry out an efficient use of the available radio resources, JRRM algorithms need to exhibit good computational performance to guarantee their future implementation viability. In this context, this paper proposes novel JRRM techniques based on linear programming techniques, and investigates their computational cost when implemented in DSP platforms commonly used in mobile base stations. The obtained results demonstrate the feasibility to implement the proposed JRRM algorithms in future heterogeneous wireless systems. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.256 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.255 Mobility management in Long Term Evolution (LTE) is different from that in the third generation mobile telecom networks. In LTE, the Mobility Management Entity (MME) is responsible for the mobility management function. The MME is connected to a large number of evolved Node Bs (cells) that are grouped into the Tracking Areas (TAs). The TAs are further grouped into TA Lists (TALs). When a User Equipment (UE) moves out of the current TAL, it reports its new location to the MME. If the LTE network attempts to connect to the UE, the MME asks the cells in the TAL to page the UE. In LTE paging, the MME may sequentially page a cell, the TA of the cell, and/or TAL of the cell. This paper investigates the performance of LTE paging, and provides the guidelines for the best paging sequence of cells. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.255 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.254 Wireless networks and personalized mobile devices are deeply integrated and embedded in our lives. Such wide adoptions of new technologies will impact user behavior and in turn will affect network performance. It is imperative to characterize the fundamental structure of wireless user behavior in order to model, manage, leverage and design efficient mobile networks. One major challenge in characterizing user behavior stems from the significant size and complexity of user behavioral data. Without summarization and dimension reduction, the sheer amount of data does not provide much useful information. The key contribution of the paper is a novel similarity metric based on a matrix representation of mobility preferences and its decomposition. This method provides an efficient way to reduce important spatiotemporal dynamics in user mobility into a few eigen-behavior vectors. This also facilitates nodes to exchange their mobility summaries and determine their mutual similarity locally. Without any assumption on the properties of user population, we use unsupervised learning (clustering) techniques to classify WLAN users. Such a user grouping scheme based on learned user behavior is crucial for applications relying on the usage context of each mobile device (e.g., participatory sensing, social-relationship-aware message forwarding). http://doi.ieeecomputersociety.org/10.1109/TMC.2011.254 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.253 In a cognitive radio network, primary service providers (PSPs) set prices for the vacant licensed bands and sell/lease them for pecuniary gains while the secondary service provider (SSP) can buy/rent the bands and support opportunistic spectrum accessing (OSA). However, due to the unpredictable activities of primary services, the SSP may suffer the monetary risk or failure to meet the traffic demands from the secondary users. It is challenging for the SSP to measure the risk for OSA, to choose the bands to use, and to split the traffic on the band-mix, when there are multiple vacant bands from PSPs. In this paper, we first introduce the X loss, an intuitive measurement for the risk for OSA. Although the X loss is attractively simple, it underestimates the potential risk for OSA and is mathematically not sub-additive, which makes it difficult to support the band-mix selection for traffic splitting. To overcome this problem, we propose a more suitable risk measurement, which is sub-additive and consistent with the SSP's perception of the risk. Based on the proposed risk metric, we formulate the band-mix selection problem as an optimization problem and solve it by linear programming. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.253 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.252 Compared to single-hop networks such as WiFi, multi-hop infrastructure wireless mesh networks (WMNs) can potentially embrace the broadcast benefits of a wireless medium in a more flexible manner. Rather than being point-to-point, links in the WMNs may originate from a single node and reach more than one other node. Nodes located farther than a one-hop distance and overhearing such transmissions may opportunistically help relay packets for previous hops. With multiple radios, a node can also improve its capacity by transmitting over multiple radios simultaneously using orthogonal channels. Capitalizing on these potential advantages requires effective routing and efficient mapping of channels to radios (channel assignment). While efficient channel assignment can greatly reduce interference from nearby transmitters, effective routing can potentially relieve congestion on paths to the infrastructure. Routing however requires that only packets pertaining to a particular connection be routed on a pre-determined route. Random network coding breaks this constraint by allowing nodes to randomly mix packets overheard so far before forwarding. We mathematically formulate the joint problem of random network coding, channel assignment and broadcast link scheduling, taking into account opportunistic overhearing, the interference, radio and channel constraints, and fairness among unicast connections. Based on this formulation, we develop a suboptimal, auction-based solution for overall network throughput optimization. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.252 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.251 Cooperative packet delivery can improve the data delivery performance in wireless networks by exploiting the mobility of the nodes, especially in wireless mobile delay-tolerant networks (DTNs). For such a network, we study the problem of rational coalition formation among mobile nodes to cooperatively deliver packets to other mobile nodes in a coalition. The mobile nodes can be either well-behaved or misbehaving in the sense that the well-behaved nodes always help each other for packet delivery, while the misbehaving nodes act selfishly and may not help the other nodes. A Bayesian coalitional game model is developed to analyze the behavior of mobile nodes in presence of this uncertainty of node behavior (i.e., type). Given the beliefs about the other mobile nodes' types, each mobile node makes a decision to form a coalition, and thus the coalitions in the network vary dynamically. We present a distributed algorithm and a discrete-time Markov chain (DTMC) model to find the Nash-stable coalitional structures. We also consider another solution concept, namely, the Bayesian core. The Bayesian game model is extended to a dynamic game model for which we propose a method for each mobile node to update its beliefs about other mobile nodes' types when the coalitional game is played repeatedly. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.251 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.250 This paper proposes the first resource allocation scheme in the literature to support scalable-video multicast for WiMAX relay networks. We prove that when the available bandwidth is limited, the bandwidth allocation problems of (1) maximizing network throughput and (2) maximizing the number of satisfied users are NP-hard. To find the near-optimal solutions to this type of maximization problem in polynomial time, this study first proposes a greedy weighted algorithm, GWA, for bandwidth allocation. By incorporating with table-consulting mechanisms, the proposed GWA can intelligently avoid redundant bandwidth allocation and thus accomplish high network performance (such as high network throughput or large number of satisfied users). To maintain the high performance gained by GWA and simultaneously improve its worst case performance, this study extends GWA to a bounded version, BGWA, which guarantees that its performance gains are lower bounded. This study shows that the computational complexity of BGWA is also in polynomial time and proves that BGWA can provide at least 1/ρ times the performance of the optimal solution, where ρ is a finite value no less than one. Finally, simulation results show that the proposed BGWA bandwidth allocation scheme can effectively achieve different performance objectives with different parameter settings. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.250 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.249 Unpredictable node mobility, low node density and lack of global information make it challenging to achieve effective data forwarding in Delay Tolerant Networks (DTNs). Most of the current data forwarding schemes choose the nodes with the best cumulative capability of contacting others as relays to carry and forward data, but these nodes may not be the best relay choices within a short time period due to the heterogeneity of transient node contact characteristics. In this paper, we propose a novel approach to improve the performance of data forwarding with a short time constraint in DTNs by exploiting the transient social contact patterns. These patterns represent the transient characteristics of contact distribution, network connectivity and social community structure in DTNs, and we provide analytical formulations on these patterns based on experimental studies of realistic DTN traces. We then propose appropriate forwarding metrics based on these patterns to improve the effectiveness of data forwarding. When applied to various data forwarding strategies, our proposed forwarding metrics achieve much better performance compared to existing schemes with similar forwarding cost. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.249 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.248 The application of complex network models to communication systems has led to several important results: nonetheless, previous research has often neglected to take into account their temporal properties, which in many real scenarios play a pivotal role. At the same time, network robustness has come extensively under scrutiny. Understanding whether networked systems can undergo structural damage and yet perform efficiently is crucial to both their protection against failures and to the design of new applications. In spite of this, it is still unclear what type of resilience we may expect in a network which continuously changes over time. In this work we present the first attempt to define the concept of temporal network robustness: we describe a measure of network robustness for time-varying networks and we show how it performs on different classes of random models by means of analytical and numerical evaluation. Finally, we report a case study on a real-world scenario, an opportunistic vehicular system of about 500 taxicabs, highlighting the importance of time in the evaluation of robustness. Particularly, we show how static approximation can wrongly indicate high robustness of fragile networks when adopted in mobile time-varying networks, while a temporal approach captures more accurately the system performance. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.248 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.247 In conventional wireless mesh networks (WMNs), multi-hop relays are performed in the backbone comprising of interconnected mesh routers (MRs) and this causes capacity degradation. This paper proposes a hybrid WMN architecture that the backbone is able to utilize random connections to access points (APs) of wireless LAN (WLAN). In such a proposed hierarchal architecture, capacity enhancement can be achieved by letting the traffic take advantage of the wired connections through APs. Theoretical analysis has been conducted for the asymptotic capacity of three-tier hybrid WMN, where per-MR capacity in the backbone is firstly derived and per-MC capacity is then obtained. Besides related to the number of MR cells as a conventional WMN, the analytical results reveal that the asymptotic capacity of a hybrid WMN is also strongly affected by the number of cells having AP connections, the ratio of access link bandwidth to backbone link bandwidth, etc. Appropriate configuration of the network can drastically improve the network capacity in our proposed network architecture. It also shows that the traffic balance among the MRs with AP access is very important to have a tighter asymptotic capacity bound. The results and conclusions justify the perspective of having such a hybrid WMN utilizing widely deployed WLANs. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.247 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.246 Vehicular Ad Hoc Networks (VANETs) adopt the Public Key Infrastructure (PKI) and Certificate Revocation Lists (CRLs) for their security. In any PKI system, the authentication of a received message is performed by checking if the certificate of the sender is included in the current CRL, and verifying the authenticity of the certificate and signature of the sender. In this paper, we propose an Expedite Message Authentication Protocol (EMAP) for VANETs, which replaces the time-consuming CRL checking process by an efficient revocation checking process. The revocation check process in EMAP uses a keyed Hash Message Authentication Code (HMAC), where the key used in calculating the HMAC is shared only between non-revoked On-Board Units (OBUs). In addition, EMAP uses a novel probabilistic key distribution, which enables non-revoked OBUs to securely share and update a secret key. EMAP can significantly decrease the message loss ratio due to the message verification delay compared with the conventional authentication methods employing CRL. By conducting security analysis and performance evaluation, EMAP is demonstrated to be secure and efficient. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.246 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.245 Estimating link capacity in a wireless network is a complex task because the available capacity at a link is a function of not only the current arrival rate at that link, but also of the arrival rate at links which interfere with that link as well as of the nature of interference between these links. Models which accurately characterize this dependence are either too computationally complex to be useful or lack accuracy. Further, they have a high implementation overhead and make restrictive assumptions, which makes them inapplicable to real networks. In this paper, we propose CapEst, a general, simple yet accurate, measurement-based approach to estimating link capacity in a wireless network. To be computationally light, CapEst allows inaccuracy in estimation; however, using measurements, it can correct this inaccuracy in an iterative fashion and converge to the correct estimate. Our evaluation shows that CapEst always converged to within 5% of the correct value in less than 18 iterations. CapEst is model-independent, hence, is applicable to any MAC/PHY layer and works with auto-rate adaptation. Moreover, it has a low implementation overhead, can be used with any application which requires an estimate of residual capacity on a wireless link. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.245 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.244 The temporal correlation of interference is a key performance factor of several technologies and protocols for wireless communications. A comprehensive understanding of interference correlation is especially important in the design of diversity schemes, whose performance can severely degrade in case of highly-correlated interference. Taking into account three sources of correlation -- node locations, channel, and traffic -- and using common modeling assumptions -- random homogeneous node positions, Rayleigh block fading, and slotted ALOHA traffic -- we derive closed-form expressions and calculation rules for the correlation coefficient of the overall interference power received at a certain point in space. Plots give an intuitive understanding as to how model parameters influence the interference correlation. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.244 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.243 Fingerprint-based methods are widely adopted for indoor localization purpose because of their cost-effectiveness compared to other infrastructure-based positioning systems. However, the popular location fingerprint, Received Signal Strength (RSS), is observed to differ significantly across different devices' hardware even under the same wireless conditions. We derive analytically a robust location fingerprint definition, the Signal Strength Difference (SSD), and verify its performance experimentally using a number of different mobile devices with heterogeneous hardware. Our experiments have also considered both Wi-Fi and Bluetooth devices, as well as both access-point-based localization and mobile-node-assisted localization. We present the results of two well-known localization algorithms (K Nearest Neighbor and Bayesian Inference) when our proposed fingerprint is used, and demonstrate its robustness when the testing device differs from the training device. We also compare these SSD based localization algorithms' performance against that of two other approaches in the literature that are designed to mitigate the effects of mobile node hardware variations, and show that SSD based algorithms have better accuracy. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.243 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.242 Adaptation of modulation and transmission bit-rates for video multicast in a multi-rate wireless network is a challenging problem because of network dynamics, variable video bit-rates, and heterogeneous clients who may expect differentiated video qualities. Prior work on the leader-based schemes selects the transmission bit-rate that provides reliable transmission for the node that experiences the worst channel condition. However, this may penalize other nodes that can achieve a higher throughput by receiving at a higher rate. In this work, we investigate a rate-adaptive video multicast scheme that can provide heterogeneous clients differentiated visual qualities matching their channel conditions. We first propose a rate scheduling model that selects the optimal transmission bit-rate for each video frame to maximize the total visual quality for a multicast group subject to the minimum-visual-quality-guaranteed constraint. We then present a practical and easy-to-implement protocol, called QDM, which constructs a cluster-based structure to characterize node heterogeneity and adapts the transmission bit-rate to network dynamics. Since QDM selects the rate by a sample-based technique, it is suitable for real-time streaming even without any pre-process. We show that QDM can adapt to network dynamics and variable video-bit rates efficiently, and produce a gain of 2-5dB in terms of the average video quality as compared to the leader-based approach. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.242 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.240 In this paper, we focus on a scheme that supports mobility for IPv6 over Low power Wireless Personal Area Network (6LoWPAN) sensor nodes. We define a protocol for 6LoWPAN mobile sensor node, named 6LoMSN, based on Proxy Mobile IPv6 (PMIPv6). The conventional PMIPv6 standard supports only single-hop networks and cannot be applied to multi-hop-based 6LoWPAN. It does not support the mobility of 6LoMSNs, and 6LoWPAN gateways, named 6LoGW, cannot detect the PAN attachment of the 6LoMSN. Therefore, we define the movement notification of a 6LoMSN in order to support its mobility in multi-hop-based 6LoWPAN environments. The attachment of 6LoMSNs reduces signaling costs over the wireless link by using router solicitation and router advertisement messages. Performance results show that our proposed scheme can minimize the total signaling costs and handoff latency. Additionally, we present the design and implementation of the 6LoMSN mobility based on PMIPv6 for a healthcare system. According to the experimental results, the 6LoMSN of the proposed PMIPv6-based 6LoWPAN can be expected to use more of the battery lifetime. We also verify that the 6LoMSN can maintain connectivity, even though it has the freedom of being able to move between PANs without a mobility protocol stack. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.240 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.239 A substantial body of literature exists addressing the capacity of wireless networks. However, it is commonly assumed that all nodes in the network are identical. The issue of heterogeneity has not been embraced into the discussions. In this paper, we investigate the throughput capacity of heterogeneous wireless networks with general network settings. Specifically, we consider an extended network with $n$ normal nodes and $m=n^b$ more powerful helping nodes in a rectangular area with width $s(n)$ and length $n/s(n)$, where $s(n)=n^w$ and $0\leq w\leq 1/2$. We assume that there are $n$ flows in the network. All the $n$ normal nodes are sources while only randomly chosen $n^d$ ($0\leq d\leq 1$) normal nodes are destinations. We further assume that the $n$ normal nodes are uniformly and independently distributed, while the $m$ helping nodes are either regularly placed or uniformly and independently distributed, resulting in two different kinds of networks called Regular Heterogeneous Wireless Networks and Random Heterogeneous Wireless Networks, respectively. We show that network capacity is determined by the shape of the network area, the number of destination nodes, the number of helping nodes, and the bandwidth of helping nodes. We also find that heterogeneous wireless networks can provide throughput exponentially higher than traditional homogeneous wireless networks only under certain conditions. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.239 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.238 Estimating the number of RFID tags in the region of interest is an important task in many RFID applications. In this paper we propose a novel approach for efficiently estimating the approximate number of RFID tags. Compared with existing approaches, the proposed Probabilistic Estimating Tree (PET)protocol achieves O(loglogn) estimation efficiency, which remarkably reduces the estimation time while meeting the accuracy requirement. PET also largely reduces the computation and memory overhead at RFID tags. As a result, we are able to apply PET with passive RFID tags and provide scalable and inexpensive solutions for large-scale RFID systems. We validate the efficacy and effectiveness of PET through theoretical analysis as well as extensive simulations. Our results suggest that PET outperforms existing approaches in terms of estimation accuracy, efficiency, and overhead. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.238 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.237 Today's location-sensitive service relies on user's mobile device to determine the current location. This allows malicious users to access a restricted resource or provide bogus alibis by cheating on their locations. To address this issue, we propose A Privacy-Preserving LocAtion proof Updating System (APPLAUS) in which co-located Bluetooth enabled mobile devices mutually generate location proofs and send updates to a location proof server. Periodically changed pseudonyms are used by the mobile devices to protect source location privacy from each other, and from the untrusted location proof server. We also develop user-centric location privacy model in which individual users evaluate their location privacy levels and decide whether and when to accept the location proof requests. In order to defend against colluding attacks, we also present betweenness ranking based and correlation clustering based approaches for outlier detection. APPLAUS can be implemented with existing network infrastructure, and can be easily deployed in Bluetooth enabled mobile devices with little computation or power cost. Extensive experimental results show that APPLAUS can effectively provide location proofs, significantly preserve the source location privacy, and effectively detect colluding attacks. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.237 http://doi.ieeecomputersociety.org/10.1109/TMC.2011.236 In wireless networks, simultaneously active transmitters typically operate in separate communication channels to avoid mutual interference. This study focuses on the challenge of increasing the capacity of a wireless network by enabling multiple transmissions in each available channel. Active transmitters are assumed to maintain the receiver signal-to-noise-and-interference ratio (SINR) at a predetermined target value via power control to promote the quality of wireless connections. To this end, we propose distributed medium access algorithms that allow every transmitter-receiver pair to determine whether a target SINR is physically achievable through iterative power control in a given shared channel. The proposed algorithms are shown by theoretical analysis to be fast, accurate, and energy-efficient. Numerical simulations demonstrate their ability to outperform related medium access schemes based on random access, carrier sensing, controlled power-up, or invariant channel probing. Our major contribution consists of solving the open problem of accurate real-time computation of the spectral radius of an unknown network information matrix. This makes our framework applicable not only to testing target SINR achievability, but also to other aspects of wireless engineering such as energy-efficiency, power control stability, and handover prioritization, in which knowledge of the spectral radius plays a key role. http://doi.ieeecomputersociety.org/10.1109/TMC.2011.236