Existing work on placing additional relay nodes in wireless sensor networks to improve network connectivity typically assumes homogeneous wireless sensor nodes with an identical transmission radius. In contrast, this paper addresses the problem of deploying relay nodes to provide fault-tolerance with higher network connectivity in heterogeneous wireless sensor networks, where sensor nodes possess different transmission radii. Depending on the level of desired fault-tolerance, such problems can be categorized as: (1) full fault-tolerance relay node placement, which aims to deploy a minimum number of relay nodes to establish k (k ≥ 1) vertex-disjoint paths between every pair of sensor and/or relay nodes; (2) partial fault-tolerance relay node placement, which aims to deploy a minimum number of relay nodes to establish k (k ≥ 1) vertex-disjoint paths only between every pair of sensor nodes. Due to the different transmission radii of sensor nodes, these problems are further complicated by the existence of two different kinds of communication paths in heterogeneous wireless sensor networks, namely two-way paths, along which wireless communications exist in both directions; and one-way paths, along which wireless communications exist in only one direction. Assuming that sensor nodes have different transmission radii, while relay nodes use the same transmission radius, this paper comprehensively analyzes the range of problems introduced by the different levels of fault-tolerance (full or partial) coupled with the different types of path (one-way or two-way). Since each of these problems is NP-hard, we develop O(σk 2 )-approximation algorithms for both one-way and two-way partial fault-tolerance relay node placement, as well as O(σk 3 )-approximation algorithms for both one-way and two-way full fault-tolerance relay node placement (σ is the best performance ratio of existing approximation algorithms for finding a minimum k-vertex connected spanning graph). To facilitate the applications in higher dimensions, we also extend these algorithms and derive their performance ratios in d-dimensional heterogeneous wireless sensor networks (d ≥ 3). Finally, heuristic implementations of these algorithms are evaluated via simulations.

The global Electronic Health Record (EHR) market is growing dramatically and expected to reach $39.7 billions by 2022. To safe-guard security and privacy of EHR, access control is an essential mechanism for managing EHR data. This paper proposes a hybrid architecture to facilitate access control of EHR data by using both blockchain and edge node. Within the architecture, a blockchain-based controller manages identity and access control policies and serves as a tamper-proof log of access events. In addition, off-chain edge nodes store the EHR data and apply policies specified in Abbreviated Language For Authorization (ALFA) to enforce attribute-based access control on EHR data in collaboration with the blockchain-based access control logs. We evaluate the proposed hybrid architecture by utilizing Hyperledger Composer Fabric blockchain to measure the performance of executing smart contracts and ACL policies in terms of transaction processing time and response time against unauthorized data retrieval.

Using the technology of wireless energy transfer, the paper proposes Qi-ferry which physically carries energy, roves a wireless sensor network, and wirelessly charges sensor nodes to extend their lifetime. To optimize the usage of the entire energy reserve on a Qi-ferry, both the movement of the Qi-ferry itself and its wireless charging of sensor nodes share the same pool of battery energy reserve, resulting in a tradeoff between how many sensors the Qi-ferry could charge and how far it could travel. The paper formulates an energy-constrained Qi-ferry wireless charging problem in wireless sensor networks, which maximizes the number of sensors wirelessly charged by a Qi-ferry subject to an energy constraint limiting the total energy consumed by the Qi-ferry for both moving and charging. Due to the NP-hardness of the problem, the paper proposes heuristic solutions based on the meta-heuristics of Particle Swarm Optimization. Evaluation results validate the effectiveness of the solutions.Index Terms-Qi-Ferry, wireless energy transfer, wireless sensor networks, tour planning, heuristic algorithm, particle swarm optimization.

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Abstract-In contrast to existing work on the connected coverage problem in wireless sensor networks which assumes omnidirectional sensors with disk-like sensing range, this paper investigates a suite of novel problems related to connected coverage in directional sensor networks where sensors only sense directionally and have a sector-like sensing range. We first consider the problems of deploying a minimum number of directional sensors to form a connected network to cover either a set of pointlocations (Connected Point-Coverage Deployment (CPD)) or the entire target sensing area (Connected Region-Coverage Deployment (CRD)). CPD is NP-hard as its subproblem of Geometric Sector Cover (GSC) is NP-hard. We present two approximation algorithms for GSC as subroutines, and develop a general solution framework for CPD with approximation ratio σ + O(1), where σ is the approximation ratio of the selected GSC subroutine. We also describe two efficient deployment patterns with guaranteed covering density for CRD, and analyze their performance bounds with respect to arbitrary non-crossing deployment patterns. Extensive simulation results validate the correctness and merits of the presented algorithms and analysis.