Designing a fair scheduling mechanism for IEEE 802.11 wireless LANs

A fair scheduling mechanism called distributed elastic round robin (DERR) is proposed in this letter for IEEE 802.11 wireless LANs operated in a distributed manner. To quantify the fairness, we not only derive its fairness bound, but also observe the fairness through ratios of throughput and weight using a simulation approach. By numerical comparisons among DERR, distributed deficit round robin (DDRR), and IEEE 802.11e, we demonstrate that DERR outperforms the other two mechanisms in performance and fairness.     

Exploitation of a microfluidic device capable of generating size-tunable droplets for gene delivery

This study presents a new microfluidic chip that generates micro-scale emulsion droplets for gene delivery applications. Compared with conventional methods of droplet formation, the proposed chip can create uniform droplets (size variation <7.1%) and hence enhance the efficiency of the subsequent gene delivery. A new microfluidic chip was developed in this study, which used a new design with a pneumatic membrane chamber integrated into a T-junction microchannel. Traditionally, the size of droplets was controlled by the flow rate ratio of the continuous and disperse phase flows, which can be controlled by syringe pumps. In this study, a pneumatic chamber near the intersection of the T-junction channel was designed to locally change the flow velocity and the shear force. When the upper air chamber was filled with compressed air, the membrane was deflected and then the droplet size could be fine-tuned accordingly. Experimental data showed that using the new design, the higher the air pressure applied to the active tunable membrane, the smaller the droplet size. Finally, droplets were used as carriers for DNA to be transfected into the Cos-7 cells. It was also experimentally found that the size of the emulsion droplets plays an important role on the efficiency of the gene delivery. The preliminary results of this paper have been presented at the 2007 IEEE International Conference of Nano/Molecular Medicine and Engineering (IEEE NANOMED 2007), Macau, China, 6–9 August, 2007.     

Concentric Distributed Localization Based on the Tripodal Anchor Structure and Grid Scan for Wireless Sensor Networks

Achieving high accuracy with minimum reference nodes, anchor nodes, and computation and communication costs is a goal for the localization in wireless sensor networks. Targeting at this goal, a localization scheme called concentric distributed localization with the tripodal anchor structure and grid scan (CDL-TAGS) requiring two reference nodes and a few anchor nodes is proposed in this paper. Under the precondition that the system has randomly distributed normal sensor nodes, a tripodal anchor structure is first designed. With this structure, the localization process is started from the centroid node and then stretched outward to the farthest normal nodes. Based on the two best reference nodes, a virtual point is generated to serve as the third reference node. In the CDL-TAGS scheme, a grid scan algorithm is employed to estimate the position of a normal node. Finally, we show that the communication overhead and time and space complexities among sensor nodes for CDL-TAGS can be kept at a low level. In addition, CDL-TAGS can achieve better accuracy with minimum anchor nodes as compared to some closely related localization schemes in the literature through simulation results.     

Efficient Simulation of Value-at-Risk Under a Jump Diffusion Model: A New Method for Moderate Deviation Events

Importance sampling is a powerful variance reduction technique for rare event simulation, and can be applied to evaluate a portfolio’s Value-at-Risk (VaR). By adding a jump term in the geometric Brownian motion, the jump diffusion model can be used to describe abnormal changes in asset prices when there is a serious event in the market. In this paper, we propose an importance sampling algorithm to compute the portfolio’s VaR under a multi-variate jump diffusion model. To be more precise, an efficient computational procedure is developed for estimating the portfolio loss probability for those assets with jump risks. And the tilting measure can be separated for the diffusion and the jump part under the assumption of independence. The simulation results show that the efficiency of importance sampling improves over the naive Monte Carlo simulation from 9 to 277 times under various situations.     

A microfluidic device for chemical and mechanical stimulation of mesenchymal stem cells

Recently, there has been considerable interest in stem cells which have the potential to differentiate into multiple lineages for cell therapy. Special attention has been paid, in particular, to the use of alternative sources of stem cells with fewer ethical issues. For example, mesenchymal stem cells (MSCs) from bone marrow have been proven to be multipotent for transplantation and tissue engineering. In this study, an integrated microfluidic chip capable of chemically and mechanically stimulating human mesenchymal stem cells (hMSCs) for adipogenic differentiation was presented. It was composed of a dilution module for controlling the insulin concentrations, and pneumatically-modulated membrane structures for precisely applying shear stresses on cells to perform chemical and mechanical stimulation, respectively. With this approach, a long-term culture and differentiation of MSCs were performed in an automatic manner. The accumulation of lipids that represented the results of insulin simulation was evaluated by Oil Red O staining. The measurements including lipid droplet numbers, optical density (OD) values, and expression of the PPARγ2 gene were used to assess the level of differentiation of the MSCs. The experimental result showed that the maximum oil droplets were induced under an optimal insulin concentration of 10 μg/ml. It was also revealed that, under mechanical stimulation, adipogenesis was inhibited under stronger levels of applied shear stresses and at higher pulsation frequencies. This proposed microfluidic chip has great potential as a powerful tool for MSC studies.     

Energy-Efficient Routing Protocol for Wireless Sensor Networks with Static Clustering and Dynamic Structure

Data gathering is an essential operation in wireless sensor networks. For periodic data gathering applications, each sensor node has data that must be sent to a distant base station in a round of communication. Due to the limited battery power of sensor nodes, each sensor node transmitting its sensed data to the base station directly significantly consumes its energy. This work presents a hierarchical ring-based data gathering (HRDG) scheme for dense wireless sensor networks. A hierarchical grid structure is constructed, and only some sensor nodes are elected as grid heads for gathering data, subsequently reducing the total energy consumption per round. Grid heads are then organized into hierarchical rings to decrease the transmission delay of a round. The proposed HRDG scheme focuses on reducing the energy × delay cost in a round of data gathering. Moreover, the energy × delay cost of HRDG is analyzed. Simulation results indicate that the proposed HRDG scheme outperforms other data gathering schemes in terms of the number of rounds, the energy × delay cost and coverage ratio.     

Enhancing WLAN location privacy using mobile behavior

Recently wireless network is massively used in the daily life, but user’s location privacy can be threatened. In a wireless local area network (WLAN), an adversary can track a user through his/her unchanged MAC address. Many correlation researches have been proposed to combat this issue, but they have not included mobile behavior of neighboring nodes as a key factor; therefore, their solutions may miss the opportune moment to update MAC address to improve the user’s location privacy. In other words, the existing schemes in the opportune moment to update MAC address may not be the best one. Furthermore, they will have many unnecessary MAC addresses to be updated; it then causes the network throughput being reduced. In this paper, we are going to enhance user’s location privacy with the relative positioning scheme. We analyze the mobile behavior of neighboring nodes to decide which mobile nodes are going to update their MAC addresses. The experiment results show that our scheme can decrease the time of changing MAC address, and also to enhance the user’s location privacy, although the network throughput is a little decrease.     

Cooperative Location Management for a Cellular Network with Ad Hoc Communication

The cellular network with ad hoc communication has been foreseen in the next-generation mobile communication system. For such a network, a distance-based (DB) location management scheme cooperated with the ad hoc assistance node (DBCAN) is proposed in this paper. Allowing a mobile terminal to pass its mobility information to the ad hoc assistance node (AAN) with appropriate location updating, DBCAN can effectively reduce the paging cost and limit the paging delay. Moreover, DBCAN includes a method to determine whether AANs should be utilized or not to make cost effective. Our simulation results show that DBCAN can effectively reach cost reduction and still get performance improvement even if the probability to successfully deliver mobility information to AANs falls to the median level.     

Design of Fair Scheduling Schemes for the QoS-Oriented Wireless LAN

How to simultaneously achieve fairness and quality-of-service (QoS) guarantee in QoS-oriented wireless local area networks (LANs) is an important and challenging issue. Targeting at this goal and jointly taking priority setting, fairness, and cross-layer design into account, four scheduling schemes designed for the QoS-oriented wireless LAN mainly based on concepts of deficit count and allowance are proposed in this paper to provide better QoS and fairness. Using multiple deficit count to interframe space (IFS) and allowance to IFS mappings for different priorities, enhanced distributed deficit round robin (EDDRR) and enhanced distributed elastic round robin (EDERR) schemes are designed to reduce (or even eliminate) possible collisions, while EDDRR with backoff interval and EDERR with backoff interval schemes still keep the backoff procedure but dynamically adjust backoff intervals for nonfailure events (the events excluding collisions and failed transmissions) depending on the priority setting and deficit count or allowance with a cross-layer design. Through extensive numerical examples, we show that the proposed schemes outperform the closest scheduling schemes in the literature and exhibit much better QoS as well as station-level and flow-level fairness.