Seungjae Shin

A Smart Handover Decision Algorithm Using Location Prediction for Hierarchical Macro/Femto-Cell Networks

To reduce the number of unnecessary handover in hierarchical macro/femto-cell networks, it is necessary to avoid macro -> femto cell handovers of temporary femtocell visitors who stay in the femtocell for a relatively short time. In this paper, we propose a smart handover decision algorithm exploiting future mobility pattern prediction scheme to prevent macro -> femto cell handovers of such temporary femtocell visitors. Our simulation result shows that the proposed algorithm effectively reduces the number of unnecessary handovers.

Bandwidth Efficient Key Distribution for Secure Multicast in Dynamic Wireless Mesh Networks

In the near future, various multicast based services will be provided over wireless mesh networks. For secure multicast services, various tree based group key management schemes have been introduced until now. Traditional tree based approaches mainly focus on reducing the number of rekeying messages transmitted by the key distribution center. However, they do not consider the network bandwidth used for transmitting each rekeying message.We propose a bandwidth efficient key tree management scheme for dynamic wireless mesh networks where membership changes occur frequently. Simulation results show that our scheme effectively reduces the bandwidth consumption used for rekeying compared to existing key tree schemes.

Analysis of Cell Sojourn Time in Heterogeneous Networks With Small Cells

Recently, heterogeneous networks with small cells have been widely used to increase the capacity of mobile systems. In such environments, accurate estimation of the mean cell sojourn time is critical for evaluating the performance of the network and its applications. It is especially important to analyze the cell sojourn times of mobile users as they reside in different network tiers: either macro-cell-only or small-cell-covered areas. But because small cells are deployed in an irregular manner, it is difficult to derive the analytical mean cell sojourn time in a macro-cell-only area. In this letter, we propose a novel approach to resolve this difficulty. We developed a simple but effective trick that approximates the heterogeneous network to a discrete grid so that it becomes tractable, making it possible to derive the analytical mean sojourn time in the macro-cell-only area. Simulation results confirm that the proposed method has excellent accuracy for general random walk mobility models including random waypoint, Brownian motion, tailored Brownian motion, and truncated Levy walk.

Person Wide Web: Active location based web service architecture using wireless infrastructure

Due to the recent advancements of Wi-Fi technologies, people can receive various types of data services via their portable devices such as laptop, smart-phone, and PDA. Especially, as the localization technologies are well developed, the LBS (Location Based Service) is recognized as a promising technology in the upcoming ubiquitous era. In most of LBS applications, users generally tend to visit shops, restaurant and public facilities in his surroundings. To exploit this feature, we propose Person Wide Web (PWW), a new type of location based web service architecture that effectively recognizes the location specific web resources to the mobile device user based on his geographical position. Among so-many resources in the WWW, only the things closely located to the user, make up his PWW. By using the PWW, various location specific data services such as shop advertisement, restaurant menu, and discount coupon, are expected to form a new type of business model in which marketers can actively promote the mobile device users to gladly use their services. We implemented a prototype of PWW system on Android platform and saw that our prototype has reasonable performance on service establishment and message delivery time. We believe the PWW has a potential to open up a new market area of provider triggered wireless data services.

Heterolayered 2D nanohybrids of uniformly stacked transition metal dichalcogenide–transition metal oxide monolayers with improved energy-related functionalities

Heterolayered 2D nanohybrids of uniformly stacked transition metal dichalcogenide (TMD)–transition metal oxide (TMO) monolayers are synthesized by a self-assembly of exfoliated nanosheets (NSs) with charge-balancing cations. This is the first example of a layer-by-layer-ordered 2D nanohybrid of exfoliated TMD and TMO nanosheets. The homogeneous incorporation of MnO2 NSs into restacked MoS2 NSs is quite effective in improving the multifunctionality of MoS2 as an electrocatalyst for the hydrogen evolution reaction (HER) and electrode for Li-ion batteries, underscoring the universal merit of layer-by-layer ordering of TMD and TMO NSs. In situ spectroscopic analysis of the present TMD–TMO nanohybrid during the electrocatalytic reaction highlights the efficient interfacial charge transfer of the restacked nanosheets with enhanced porosity. The beneficial effect of incorporated TMO NSs on the performance of TMD NSs is attributable to an efficient interfacial charge transfer and increased porosity with depressed self-stacking of NSs and the stabilization of the metallic 1T TMD phase, resulting in the improvement of charge transfer kinetics, the enhancement of reactivity, and the provision of many reaction sites upon hybridization. Density functional theory (DFT) calculations further support the stabilization of the 1T TMD structure over the 2H TMD one via the interaction with neighboring TMO NSs. The present study clearly demonstrates that the restacking of the colloidal mixtures of TMO and TMD NSs can provide an effective way to explore novel multifunctional heterostructures.

A Secure Self-Encryption Scheme for Resource Limited Mobile Devices

Recently, IT and mobile technology are developed rapidly. Mobile devices such as a smartphone and tablet PC have become widely popular, and many people store their private information into the devices. However, the portability and mobility of devices take risks such as being lost or stolen. These devices are the one of main cause to leak the sensitive and confidential information. To protect the information leakage from devices, the encryption algorithm is required. The existing encryption algorithms take long delay time and heavy battery consumption in mobile devices with restricted resources. Previously, the Self-Encryption (SE) which is a lightweight encryption algorithm for mobile devices was proposed, which had critical weaknesses. This paper proposes a novel SE scheme with a random permutation and bit-flipping process. Our proposed scheme derives the keystream from the plaintext itself, but the statistical correlations are effectively removed from the novel randomization process. It gives a solution to overcome weaknesses of the original SE scheme and the complexity to make adversaries it difficult to launch a brute force attack, and satisfies a 0/1 uniformity of key and cipher stream, which is an important property of the secure stream cipher. Moreover, it is faster and more energy-efficient than other ciphers such as AES and RC4. The evaluation is performed by the Android platform. The delay time and the battery consumption are measured and analyzed, which show that the proposed scheme is secure and best suited to the mobile platform.