Hung-Chang Chen

CollECT: Collaborative event detection and tracking in wireless heterogeneous sensor networks

Tracking is an important application in wireless sensor networks (WSNs), especially for the urgent event of interest. Recent research has paid much attention to the WSN wherein all sensor nodes are identical in sensing units, but the utilization of different types of sensor nodes has not been widely explored. In the paper, we propose a fully distributed protocol, CollECT, to event detection and tracking in wireless heterogeneous sensor networks (WHSNs), which consists of various types of sensor nodes with different sensing units. The main idea of CollECT is collaboration, by which the same type of sensor nodes construct the attribute region, represented by a convex polygon, whereas the different types of sensor nodes determine whether the event occurs. In CollECT, three procedures, vicinity triangulation, event determination, and border sensor node selection, are proposed to construct the attribute region, to determine the occurrence of the event, and to select border sensor nodes to stand for the event boundary, respectively. Simulation results validate the performance of CollECT in terms of accuracy of event tracking and fitness of the border sensor nodes selected. Approximately 87% sensor nodes within the event region can be correctly identified on average. Additionally, the border sensor nodes selected by CollECT are beneficial to efficiently stand for the event boundary.

On Connected Target Coverage for Wireless Heterogeneous Sensor Networks with Multiple Sensing Units

The paper considers the connected target coverage (CTC) problem in wireless heterogeneous sensor networks (WHSNs) with multiple sensing units, termed MU-CTC problem. MU-CTC problem can be reduced to a connected set cover problem and further formulated as an integer linear programming (ILP) problem. However, the ILP problem is an NP-complete problem. Therefore, two distributed heuristic schemes, REFS (remaining energy first scheme) and EEFS (energy efficiency first scheme), are proposed. In REFS, each sensor considers its remaining energy and its neighbors’ decisions to enable its sensing units and communication unit such that all targets can be covered for the required attributes and the sensed data can be delivered to the sink. The advantages of REFS are its simplicity and reduced communication overhead. However, to utilize sensors’ energy efficiently, EEFS is proposed. A sensor in EEFS considers its contribution to the coverage and the connectivity to make a better decision. To our best knowledge, this paper is the first to consider target coverage and connectivity jointly for WHSNs with multiple sensing units. Simulation results show that REFS and EEFS can both prolong the network lifetime effectively. EEFS outperforms REFS in network lifetime, but REFS is simpler.

On avoiding RTS collisions for IEEE 802.11-based wireless ad hoc networks

The paper proposes an RCA (RTS collision avoidance) MAC protocol to reduce RTS collisions for IEEE 802.11-based mobile ad hoc networks (MANETs). RTS/CTS exchanging is used for the resolution of the hidden terminal problem. However, the paper shows that, even the backoff counters of two stations are different, RTS frames are also collided to each other due to the hidden terminal problem. The situation would be getting worse in high traffic load or in a dense network. RTS collisions not only result in the following CTS or ACK collisions, but also induce false blocking problem, even dead locks of transmissions. To address the above problems, an RCA MAC protocol is proposed to reduce RTS collisions. The RCA protocol utilizes a narrow band, called the tone channel, to announce the RTS transmission in advance in order to preclude the RTS transmissions of two-hop neighbors. To reduce the channel and hardware overhead, an improvement to the RCA protocol is also devised, which only uses a single channel and one transceiver to reduce RTS collisions. The RCA protocol provides a type of fast collision detection and decreases the probability of RTS collisions, which is benefit for RTS/CTS exchange scheme. Meanwhile, the RCA protocol can reduce the retransmission cost and have lower control overhead than that of IEEE 802.11 DCF. In addition, simulation results verify the advantages of the RCA protocol in comparison with IEEE 802.11 DCF.

A location-aware multicasting protocol for Bluetooth Location Networks

Bluetooth Location Network (BLN) is a Bluetooth radio network that is composed of some mobile Bluetooth devices and static Bluetooth units, and is established at the system initialization to form a spontaneous network topology. In a BLN, a multicast service is defined as the periodical delivering of messages from a Service Server to a set of mobile devices which are the multicast members predefined by the Service Server. Several multicast protocols have been proposed for the Ad-Hoc networks, but they create an inefficient multicast tree for the BLN due to the existing differences in the radio characteristics between Ad-Hoc and Bluetooth radio networks. The present paper analyzes these differences and proposes a novel multicasting protocol for constructing an efficient multicast tree in a BLN. The proposed protocol constructs a multicast tree with good features which include the shortest path, a higher degree of path sharing, and fewer forwarding nodes. Simulation results reveal that the proposed multicast protocol outperforms the existing multicast protocols in the BLN.

PALM: A Partition Avoidance Lazy Movement Protocol for Mobile Sensor Networks

The paper proposes a distributed partition avoidance lazy movement (PALM) protocol for mobile sensor networks (MSNs). In general, connectivity and coverage are two major factors to the success of a sensor network. Therefore, PALM takes both connectivity and coverage into account to avoid network partition and keep high sensing quality. Since sensor movement is the major source of energy consumption, thus, in order not to cause frequent movement, PALM triggers sensor movement only when the network has a risk of partition, but not when coverage holes appear. The paper proposes a sufficient condition of keeping a network connected. Based on the condition, PALM adopts the lazy movement policy for a sensor to determine when to move and uses the principles of an effective movement for a sensor to decide where to move. Accordingly, PALM can keep the network connected and can make the effective coverage as large as possible to maintain high sensing quality. In comparison with the related work, PALM can reduce the energy consumption and further extend the network lifetime due to the lazy movement policy and the principles of an effective movement. Simulation results also verify the advantages of the proposed protocol.

Channel-Aware Subchannel Renumbering and Downlink Burst Allocation for IEEE 802.16 OFDMA Systems

This paper investigates the downlink burst allocation problem in IEEE 802.16 OFDMA system. In order to increase the subchannel utilization and to improve the network throughput, a channel-aware renumbering (CSR) scheme and a channel-aware downlink burst allocation (CDBA) scheme are proposed in this paper. CSR considers the difference among subchannel qualities of MSs and logically renumbers the subchannels so that each MS can have a group of subchannels supported the same and more efficient modulation and coding (MC) scheme. CDBA is to decide the size, shape and the position of each burst. Because of the rectangular burst allocation constraint, some slots in or among rectangular bursts will be wasted. These two kinds of slot wastage problems are defined as internal and external fragmentation problems in the paper. Therefore, in order to comply with the rectangular burst allocation constraint, to overcome the fragmentation problems and to increase the network throughput, CDBA is to determine the size and the position of each MSs burst based on the subchannel qualities of MSs and the bandwidth requirements. The simulation results show that CDBA with CSR outperforms other related approaches with or without CSR in terms of the throughput, the service ratio, the subchannel utilization and the DL-MAP overhead.

On Avoiding RTS Collisions for IEEE 802.11-BasedWireless Ad Hoc Networks

The paper proposes an RCA MAC protocol to reduce RTS collisions. The protocol utilizes a narrow band, called the tone channel, to announce the RTS transmission in advance to preclude the RTS transmissions of two-hop neighbors. To reduce the channel and hardware overhead, an improvement to the RCA protocol is also devised, which only uses a single channel and one transceiver to reduce RTS collisions. The RCA protocol provides a type of fast collision detection and decreases the probability of RTS collisions. Meanwhile, the RCA protocol can reduce the retransmission cost and has lower control overhead than that of IEEE 802.11 DCF. In addition, simulation results verify the advantages of the RCA protocol in comparison with IEEE 802.11 DCF

Integrating Target Coverage and Connectivity for Wireless Heterogeneous Sensor Networks with Multiple Sensing Units

The paper considers the target coverage and connectivity problem in wireless heterogeneous sensor networks (WHSNs) with multiple sensing units. The paper reduces the problem to a connected set cover problem and further formulates it as integer programming (IP) constraints. Two heuristic but distributed schemes, remaining energy first scheme (REFS) and energy efficient first scheme (EEFS), are proposed to solve the target coverage and connectivity problem. Simulation results show that REFS and EEFS can prolong the network lifetime effectively. Furthermore, EEFS outperforms against REFS in network lifetime.

Development and Evaluation of a Self-Regulatory-Learning-Cycle-Based System for Self-Regulated e/m-Learning

The paper proposes a self-regulatory-learning- cycle-based system to not only provide a pleasant learning environment, but also effectively enhance the performance of self-regulated learning (SRL) of learners. Learners utilizing the proposed system can repeatedly experience SRL processes and establish their patterns of SRL gradually. Experiment results show that the proposed system can help learners possess SRL skills, especially for unskillful learners.

A decentralized minislot scheduling protocol (DMSP) in TDMA-based wireless mesh networks

Wireless mesh network is a promising network topology that can provide high data rate backhaul network access. For achieving high data rate backhaul network access, a well-designed bandwidth scheduling protocol is necessary for wireless mesh networks. This paper takes minislot scheduling problem for IEEE 802.16 mesh networks as an example and formulates the problem as an integer linear programming model in this paper, where minislot is an atomic bandwidth allocation unit for data transmissions among subscriber stations and base station. Due to the high computational complexity for solving integer linear programming model at subscriber stations and the degradation of bandwidth utilization resulted from data collision problems and minislot insufficient problems, this paper proposes a decentralized minislot scheduling protocol to make subscriber stations, rather than base station, schedule minislot usage for throughput gains in the IEEE 802.16 mesh networks. The decentralized minislot scheduling protocol includes minislot usage constraints and minislot decision strategies to alleviate data collisions and minislot insufficient problems as well as to increase bandwidth utilization. The proposed protocol can not only accommodate to the IEEE 802.16 standard, but also makes subscriber stations schedule minislots with the latest minislot usage information. Besides IEEE 802.16 mesh networks, the proposed protocol also can apply to any wireless mesh networks with less or no modifications. From the simulation results, the performance of the proposed protocol outperforms the other related contributions in terms of the transmission delay, control overhead, minislot utilization and the network throughput.