Guey-Yun Chang

Novel Channel-Hopping Schemes for Cognitive Radio Networks

Recently, cognitive radio (CR) has become a key technology for addressing spectrum scarcity. In CR networks, spectrum access should not interfere the colocate incumbent networks. Due to the requirement above, common control channel approaches, which are widely used in traditional multichannel environments, may face serious CR long-time blocking problem and control channel saturation problem. Although channel-hopping-based approaches can avoid these two problems, existing works still have significant drawbacks including long time-to-rendezvous, unbalance channel loading, and low channel utilization. In this paper, we introduce three channel-hopping approaches, RCCH, ARCH, and SARCH for synchronous and asynchronous environments, respectively. Compared with previous works, our schemes outperform the state of the art in terms of these metrics.

A Distributed Taxi Hailing Protocol in Vehicular Ad-Hoc Networks

In this paper, a distributed taxi hailing protocol in vehicular ad-hoc networks (VANET) is suggested. Our protocol consists of two parts: taxi booking and taxi de- blocking. Taxi booking part ensures that a vacant taxi with shortest driving distance to passenger under real traffic regulations is booked. Taxi de-blocking part aims to de-block blocked vacant taxis as soon as possible. Simulation results show that compared with previous results, at least 40% of booking time, 50% of waiting time of passengers and 50% of driving distance from booked taxi to passenger, is reduced in our protocol.

Scalable continuous object detection and tracking in sensor networks

With the advancement of MEMS technologies, sensor networks have opened up broad application prospects. An important issue in wireless sensor networks is object detection and tracking, which typically involves two basic components, collaborative data processing and object location reporting. The former aims to have sensors collaborating in determining a concise digest of object location information, while the latter aims to transport a concise digest to sink in a timely manner. This issue has been intensively studied in individual objects, such as intruders. However, the characteristic of continuous objects has posed new challenges to this issue. Continuous objects can diffuse, increase in size, or split into multiple continuous objects, such as a noxious gas. In this paper, a scalable, topology-control-based approach for continuous object detection and tracking is proposed. Extensive simulations are conducted, which show a significant improvement over existing solutions.

Anti-Jamming Rendezvous Scheme for Cognitive Radio Networks

In cognitive radio networks (CRNs), channel hopping-based communications are widely used to improve channel utilization. However, channel hopping schemes for CRNs are usually vulnerable to jamming attacks, especially when jammers have cognitive radios to perform channel sensing and fast channel switching. Many mitigating approaches for coping with jamming attacks in wireless communications rely on pre-shared secrets (e.g., pre-shared hopping sequences). In CRNs, pre-sharing secrets between senders and receivers is usually impractical (because neighborhood dynamically changes, and receivers of a broadcast may be unknown to the sender). Hence, anti-jamming channel hopping approaches without pre-shared secrets have gained more and more research interests. However, existing approaches either have unbounded time to rendezvous on an available channel (even no signals of jammers and PUs appear), or require role pre-assignment (SUs should be pre-assigned as a sender or receiver). Role pre-assignment is not applicable to environments where each SU may play a sender and a receiver, simultaneously. In this paper, we propose an anti-jamming channel hopping scheme, Sec-CH. Sec-CH has bounded time to rendezvous and can work without role pre-assignment.

A Region-Based Hierarchical Location Service with Road-Adapted Grids for Vehicular Networks

In VANETs, it is very important to communicate between two vehicles, but how to get the correct position of a vehicle is not easy. Due to vehicles are moving fast, topology in VANETs changes rapidly. As a result, location services processed in VANETs are more difficult than in MANETs. In our thesis, we propose a hierarchical location service system, it provides a low cost and rapid service. First, we select the main arteries to divide network into grids because of there are more vehicles than in normal roads, and then design a mechanism that when vehicles need to send update packets. This mechanism can decrease the number of update packets and still gain correct vehicles??? location. Second, we design grids with three levels, the higher the level, the larger the area. Each level stores update packets sent within its area. Vehicles using our system can find the destination vehicle distributedly within a small area; if the target is not within this area, then find within a larger area. Besides, we propose a packets collection method, it can be adjusted with different size of collection area. The simulation results show that our scheme could decrease the number of location update packets effectively, and keep high success rate of location service.

Asynchronous Quorum-Based Blind Rendezvous Schemes for Cognitive Radio Networks

In cognitive radio networks, unlicensed users [secondary users (SU)] need to rendezvous on licensed channels before establishing communication links. Dedicated common control channel is the simplest way to achieve rendezvous. However, due to the absolute priority of licensed users [primary users (PU)] on accessing licensed channels, a dedicated common control channel may cause the PU long-time blocking problem, and the control channel saturation problem in a high SU density environment. Channel hopping schemes have been proposed to avoid the problems mentioned above. In this paper, we introduce two quorum-based channel hopping schemes. Our schemes outperform in terms of the four metrics: maximum time to rendezvous, channel loading, degree of rendezvous, and maximum conditional time to rendezvous.

A Novel Approach for k-Coverage Rate Evaluation and Re-Deployment in Wireless Sensor Networks

Coverage problem is a fundamental issue in wireless sensor networks. In this paper, we consider two sub-problems: k-coverage rate evaluation and k-coverage rate deployment. The former aims to evaluate the ratio of k-covered area relative to the monitored area, while the latter aims to determine the minimum number of sensors required and their locations to guarantee that k-coverage rate of the monitored area meets application requirements. For k-coverage rate evaluation problem, a non-uniform-grid- based approach for random deployments is proposed. For k-coverage rate deployment problem, a greedy-based approach is suggested to meet the requirement of k-coverage rate. Simulation results show that both our schemes are more time efficient than previous work.

Zooming: A Zoom-Based Approach for Parking Space Availability in VANET

In this paper, we propose a zoom-based approach for parking space availability. The main idea of our scheme lies on the fact that drivers near the queried locations are interested in detail (zoomed-in) parking space information, while drivers in a distant place are interested in rough (zoomed-out) information about free parking spaces. Our zooming technique is based on discrete cosine transform. Besides, a winner-take-all scheme is also proposed to further reduce communication overhead in packet loss environment. As compared to previous work, simulation results show that our scheme reduces 40% to 50% communication overhead.

A Hole Avoiding Routing Protocol in Wireless Sensor Networks

In wireless sensor networks, an important challenge often faced in geographic greedy forwarding is the local minimum phenomenon which is caused by holes. For solving this problem, most hole-avoiding protocols route packets along the perimeter of holes or forbidden regions. Thus, sensor nodes on the perimeter of holes and forbidden regions exhaust their energy faster than others, which enlarge the hole size, called growing hole problem. In this paper, we propose an energy-efficient hole avoiding routing protocol (HARP) for growing hole problem in multi-hole environments.

Adaptive k -coverage contour evaluation and deployment in wireless sensor networks

The problem of coverage is a fundamental issue in wireless sensor networks. In this article, we consider two subproblems: k-coverage contour evaluation and k-coverage rate deployment. The former aims to evaluate, up to k, the coverage level of any location inside a monitored area, while the latter aims to determine the locations of a given set of sensors to guarantee the maximum increment of k-coverage rate when they are deployed into the area. For the k-coverage contour evaluation problem, a nonuniform-grid-based approach is proposed. We prove that the computation cost of our approach is at most the square root of existing solutions. Based on our k-coverage contour evaluation scheme, a greedy k-coverage rate deployment scheme (k-CRD) is proposed, which is shown to be an order faster than existing studies for k-coverage rate deployment. The k-CRD can incorporate two different heuristics to further reduce its running time. Simulation results show that k-CRD with these heuristics can be significantly more time efficient without causing much degradation in the coverage rate of final deployment.