Scott C.-H. Huang

Physical layer security in wireless networks: a tutorial

Wireless networking plays an extremely important role in civil and military applications. However, security of information transfer via wireless networks remains a challenging issue. It is critical to ensure that confidential data are accessible only to the intended users rather than intruders. Jamming and eavesdropping are two primary attacks at the physical layer of a wireless network. This article offers a tutorial on several prevalent methods to enhance security at the physical layer in wireless networks. We classify these methods based on their characteristic features into five categories, each of which is discussed in terms of two metrics. First, we compare their secret channel capacities, and then we show their computational complexities in exhaustive key search. Finally, we illustrate their security requirements via some examples with respect to these two metrics.

Minimum-latency aggregation scheduling in multihop wireless networks

Minimum-latency aggregation schedule (MLAS) in synchronous multihop wireless networks seeks a shortest schedule for data aggregation subject to the interference constraint. In this paper, we study MLAS under the protocol interference model in which each node has a unit communication radius and an interference radius ρ ≥ 1. All known aggregation schedules assumed ρ = 1, and the best-known aggregation latency with ρ = 1 is 23<i>R</i> + Δ - 18 where <i>R</i> and Δ are the radius and maximum degree of the communication topology respectfully. In this paper, we first construct three aggregations schedules with ρ = 1 of latency 15<i>R</i> + Δ - 4, 2<i>R</i> + <i>O</i>(log <i>R</i>) + Δ and (1 + <i>O</i>(log <i>R</i>/3√<i>R</i>)) <i>R</i> + Δ respectively. Then, we obtain two aggregation schedules with ρ > 1 by expanding the first two aggregation schedules with ρ = 1. Both aggregation schedules with ρ > 1 have latency within constant factors of the minimum aggregation latency.

Minimum connected dominating sets and maximal independent sets in unit disk graphs

In ad hoc wireless networks, a connected dominating set can be used as a virtual backbone to improve the performance. Many constructions for approximating the minimum connected dominating set are based on the construction of a maximal independent set. The relation between the size mis(G) of a maximum independent set and the size cds(G) of a minimum connected dominating set in the same graph G plays an important role in establishing the performance ratio of those approximation algorithms. Previously, it is known that mis(G) ≤ 4ċcds(G) + 1 for all unit disk graphs G. In this paper, we improve it by showing mis(G) ≤ 3.8ċcds(G) + 1.2.

Nearly Constant Approximation for Data Aggregation Scheduling in Wireless Sensor Networks

Data aggregation is a fundamental yet time-consuming task in wireless sensor networks. We focus on the latency part of data aggregation. Previously, the data aggregation algorithm of least latency [1] has a latency bound of (Delta - 1)R, where Delta is the maximum degree and R is the network radius. Since both Delta and R could be of the same order of the network size, this algorithm can still have a rather high latency. In this paper, we designed an algorithm based on maximal independent sets which has an latency bound of 23R + Delta - 18. Here Delta contributes to an additive factor instead of a multiplicative one; thus our algorithm is nearly constant approximation and it has a significantly less latency bound than earlier algorithms especially when Delta is large.

Minimum-Latency Broadcast Scheduling in Wireless Ad Hoc Networks

A wide range of applications for wireless ad hoc networks are time-critical and impose stringent requirement on the communication latency. This paper studies the problem Minimum-Latency Broadcast Scheduling (MLBS) in wireless ad hoc networks represented by unit-disk graphs. This problem is NP-hard. A trivial lower bound on the minimum broadcast latency is the radius R of the network with respect to the source of the broadcast, which is the maximum distance of all the nodes from the source of the broadcast. The previously best-known approximation algorithm for MLBS produces a broadcast schedule with latency at most 648 R. In this paper, we present three progressively improved approximation algorithms for MLBS. They produce broadcast schedules with latency at most 24 R -23, 16 R -15, and R + O (log R) respectively.

Opportunistic Sensing in Wireless Sensor Networks: Theory and Application

In real world, wireless heterogeneous sensor network (HSN) design and information integration are necessary in different applications. Traditionally, wireless sensor networks information integration is set up to passively fuse all received data. Such an approach is computationally challenging and operationally ineffective because improvements in information accuracy are not guaranteed. Opportunistic Sensing (OS) refers to a paradigm for signal and information processing in which a network of sensing systems can automatically discover and select sensor platforms based on an operational scenario. In this paper, we propose theory and algorithms of OS to simplify the HSN design and promote more efficient information integration. We propose an information theoretical criterion for opportunistic sensing in HSN, and show that HSN with correlated modalities needs less number of codewords than that with independent modalities. Our OS algorithm advances autonomous sensing that not only ensures effective utilization of sensing assets but also provides robust optimal performance. We apply our OS algorithm to radar sensor networks for surveillance and monitoring, and show that our approach works very well and much better than other approaches.

Broadcast Scheduling in Interference Environment

Broadcast is a fundamental operation in wireless networks and naive flooding is not practical because it cannot deal with interference. Scheduling is a good way to avoid interference, but previous studies on broadcast scheduling algorithms all assume highly theoretical models such as the unit disk graph model. In this work, we re-investigate this problem using the 2-disk and the signal-to-interference-plus-noise-ratio (SINR) model to realize it. We first design a constant approximation algorithm for the 2-disk model and then extend it to the SINR model. This result is the first result on broadcast scheduling algorithms in SINR model, to the best of our knowledge.

Fault-tolerant and reliable computation in cloud computing

Cloud computing, with its great potentials in low cost and on-demand services, is a promising computing platform for both commercial and non-commercial computation clients. In this work, we investigate the security perspective of scientific computation in cloud computing. We investigate a cloud selection strategy to decompose the matrix multiplication problem into several tasks which will be submitted to different clouds. In particular, we propose techniques to improve the fault-tolerance and reliability of a rather general scientific computation: matrix multiplication. Through our techniques, we demonstrate that fault-tolerance and reliability against faulty and even malicious clouds in cloud computing can be achieved.

SAP: seamless authentication protocol for vertical handoff in heterogeneous wireless networks

802.11 standards support high data rates for a low price and thus provides an economical way for WLANs. On the other hand, 3G standards offer a much wider area of coverage that enables ubiquitous connectivity. The integration of them takes advantages from both sides and offers the possibility of achieving anywhere, anytime cost-efficient Internet access. To facilitate such integration, seamless vertical handoff is one of the major challenges because it needs to make physical movement transparent to mobile users and preserves application-level connectivity. Previous works did not consider the impact of authentication mechanisms on the performance of vertical handoff, especially on its delay. In a 3G-WLAN integration environment, since 3G and WLAN may use different authentication servers, when a mobile terminal hands over across them, certain authentication procedure needs to be performed. According to the literature, such authentication delay may be as high as hundreds of milliseconds, which is intolerable for delay-sensitive applications. We present seamless authentication protocols (SAPs) for vertical handoff in wireless heterogeneous networks, to reduce this delay. Simulation results show that SAP significantly reduces the delay caused by authentication procedures in vertical handoff.

Timer-Based CDS Construction in Wireless Ad Hoc Networks

The connected dominating set (CDS) has been extensively used for routing and broadcast in wireless ad hoc networks. While existing CDS protocols are successful in constructing CDS of small size, they either require localized information beyond immediate neighbors, lack the mechanism to properly handle nodal mobility, or involve lengthy recovery procedure when CDS becomes corrupted. In this paper, we introduce the timer-based CDS protocols, which first elect a number of initiators distributively and then utilize timers to construct a CDS from initiators with the minimum localized information. We demonstrate that our CDS protocols are capable of maintaining CDS in the presence of changes of network topology. Depending on the number of initiators, there are two versions of our timer-based CDS protocols. The Single-Initiator (SI) generates the smallest CDS among protocols with mobility handling capability. Built on top of SI, the Multi-Initiator (MI) version removes the single point of failure at single-initiator and possesses most advantages of SI. We evaluate our protocols by both the ns-2 simulation and an analytical model. Compared with the other known CDS protocols, the simulation results demonstrate that both SI and MI produce and maintain CDS of very competitive size. The analytical model shows the expected convergence time and the number of messages required by SI and MI in the construction of CDS, which match closely to our simulation results. This helps to establish the validity of our simulation.