Fixed collision rate back-off scheme for collision resolution in wireless networks

This investigation proposes a fixed collision rate (FCR) back-off algorithm for wireless networks. The proposed scheme takes advantage of the central unit (CU) in a wireless network to broadcast a common back-off window size to all the users, significantly alleviating the unfairness of bandwidth utilization in conventional binary exponential back-off (BEB) algorithms. It is shown that, when maximum throughput is achieved, collision rate is almost a constant for any traffic load. In the operation of the FCR, the CU dynamically adjusts the back-off window size to keep the collision rate at a constant level for maximum throughput. Simulation results demonstrate that the unfairness of bandwidth utilization in the BEB is significantly lessened and the throughput can be maintained at e^-1≈0.368 when the number of users approaches infinity. The capture effect even further improves system performance.     

Stability analysis and controller synthesis for discrete uncertain singular fuzzy systems with distinct difference matrices in the rules

This paper mainly studies an extended discrete singular fuzzy model incorporating the multiple difference matrices in the rules and discusses its stability and design issues. By embracing additional algebraic constraint, traditional discrete Takagi-Sugeno (T-S) fuzzy model can be extended to a generalised discrete singular Takagi-Sugeno (GDST-S) model with individual difference matrices E_i in the locally singular models, where it can describe a larger class of physical or non-linear systems. Based on the linear matrix inequality (LMI) approach, we focus on deriving some explicit stability and design criteria expressed by the LMIs for the regarded system. Thus, the stability verification and controller synthesis can be performed by the current LMI tools. Finally, some illustrative examples are given to illustrate the effectiveness and validity of the proposed approach.     

Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid

This study proposes a method to form ultrafine α-Al₂O₃ powders. Oleic acid is mixed with Al(OH)₃ gel. The gel is the precursor of the Al₂O₃. After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al₂O₃. The phase transformation in this process is as follows: emulsion →γ-Al₂O₃→δ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃. This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al₂O₃ from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations.     

Modeling and evaluating IPTV applications in WiMAX networks

Internet Protocol-based Television (IPTV) is a digital television service which delivers television content via an IP network. The rapid growth of wireless network technology in recent years has changed, the way people access the Internet. Adding mobility to IPTV can create a truly compelling ubiquitous service which spans different network domains and varied IP-enabled terminals and devices, such as set-top boxes, PCs and cell phones. However, extending IPTV service to wireless networks requires overcoming bandwidth bottlenecks and high packet loss rates. Following the IEEE 802.16 standard, worldwide interoperability for microwave access (WiMAX) features high data rates and large service coverage, offering a wireless broadband solution for IPTV services. While previous research has focused on creating a broadband IPTV service few studies have practically evaluated IPTV applications in a wireless broadband network environment. In this paper, we model and evaluate a common constant bit rate (CBR)¹ based IPTV application and an IPTV live streaming (PPStreaming)² application while retrieving IPTV content via a WiMAX network. We also use the NS2 simulation tool to evaluate the performance of these two IPTV applications. The evaluation metrics include latency, packet loss, data rate and throughput statistics when the two IPTV applications are run in the WiMAX network. ¹The simplest IPTV solution is to convey video content by CBR. IPTV operators and content delivery networks relay CBR streaming content to control the demand for network capacity. Broadcasters prefer CBR video as it conserves bandwidth resources, but CBR delivery can degrade video quality result in higher overall demand on network capacity. ²PPStreaming (also referred to as P2P streaming Internet TV) is a network for live media streaming. In principle it’s similar to BitTorrent (BT) in that it provides stable and smooth broadcast of TV programs to broadband users. Unlike traditional streaming media, PPStreaming adopts P2P-streaming technology and supports full-scale visits with tens of thousands of simultaneous users. Its client software can be used in the browser or as a standalone executable.     

On the Power Allocation and System Capacity of OFDM Systems Using Superimposed Training Schemes

Channel estimation in multipath environments is typically performed using the pilot-symbol-assisted modulation (PSAM) scheme. However, the traditional PSAM scheme requires the use of dedicated pilot subcarriers and therefore leads to a reduction in the bandwidth utilization. Accordingly, this paper investigates a channel-estimation approach for orthogonal frequency-division multiplexing (OFDM) systems using a superimposed training (ST) scheme, in which the pilot symbols are superimposed onto the data streams prior to transmission. By using equally spaced pilot symbols of equal power and assuming that the number of pilots is larger than the channel order, it is shown that the channel-estimation performance is independent of the number of pilots used. The optimal ratio of the pilot symbol power to the total transmission power is analyzed to maximize the lower bound of the channel capacity. Overall, the current results show that the ST-based channel estimation schemes have a slightly poorer performance than the PSAM scheme but yield higher system capacity.     

Miniaturization of Electrostatic Fluid Accelerators

Existing thermal-management methods for electronics do not meet the technology needs and remain a major bottleneck in the evolution of computing, sensing, and information technology. The decreasing size of microelectronic components and the resulting increasing thermal output density require novel cooling solutions. Electrostatic fluid accelerators (EFAs), also known as electrohydrodynamic ionic wind pumps, have the potential of becoming a critical element of electronic thermal-management solutions. In order to take full advantage of EFA-based thermal management, it is essential to miniaturize EFA technology. This paper demonstrates the successful operation of a mesoscale microfabricated silicon EFA. Several cantilever structures fabricated in bulk silicon with radii of tip curvature ranging from 0.5 to 25 mum are used as the corona electrode. The device was fabricated using the combination of deep reactive ion etching (DRIE) and reactive ion etch (RIE) microfabrication processes. Forced convection cooling is demonstrated using infrared imaging, showing a 25degC surface temperature reduction over an actively heated substrate. The fabrication and test results of a mesoscale microfabricated EFA are presented in this paper.     

Controller synthesis for uncertain fuzzy systems with integrating multiple time-varying delays

This article mainly investigates the controller synthesis for uncertain fuzzy systems with integrating multiple time-varying delays, which can describe a much larger class of nonlinear systems or uncertain time-delay systems. First, a sufficient stability condition for the unforced Takagi–Sugeno (T-S) fuzzy models with multiple time-varying delays and uncertainties are derived. By involving the parallel distributed compensator (PDC), some design conditions for the resulting closed-loop fuzzy systems are further presented. These proposed conditions are all expressed in terms of linear matrix inequalities (LMIs), and we can readily perform the PDC synthesis from current LMI solvers. Numerical examples are given to demonstrate the validity and superiority of the proposed approach.     

Sintering of aluminum nitride by using alumina crucible and MoSi2 heating element at temperatures of 1650 °C and 1700 °C

Aluminum nitride (AlN) ceramics, prepared with Y₂O₃ and CaO sintering additives, have been densified in an Al₂O₃ crucible at temperatures of up to 1650 °C and 1700 °C using a conventional MoSi₂ heating element furnace. The results of this study show that relative densities in excess of 99% of theoretical and a relatively high-thermal conductivity of 147 W m⁻¹ K⁻¹ have been achieved for feedstock materials prepared with combined addition of 1 wt.% Y₂O₃ and 1 wt.% CaO. All of the phases in sintered samples have been shown to be crystalline AlN and minor amount of secondary phases, were detected such as enriched Y- and Ca-aluminates by the XRD patterns, back-scattered imagery and microprobe analysis. The advantage of using the particular experimental system and sintering condition is considered to be amenable to lower production cost and enhance the feasibility of mass production. Critical temperature for AlN densification to obtain the highest density is about 1650 °C.     

Variations in a Boehmite Gel and Oleic Acid Emulsion under Calcination

In this paper, we describe variations in a boehmite (AlOOH) and oleic acid emulsion during the process of forming superfine α-Al₂O₃ crystallites from the mixture of oleic acid and a boehmite gel precursor. We also propose that the oleic acid decomposes under calcination, generating carbon, which can effectively prevent agglomeration of Al₂O₃ particles. Calcination for the present study was conducted under a reduced oxygen atmosphere, in the temperature range 25°–1100°C. Phase variations of the mixture under calcination were identified by Fourier transform infrared spectrometry (FTIR), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). The FTIR spectra were used when the mixed emulsion of oleic acid and boehmite gel was heated, to investigate the adsorption reaction of the aluminum oleate; the C—O—C cross-linking structure of oxygenation, which aided in carbon formation; and the ability of the carbon generated with α-Al₂O₃ during phase transformation to prevent agglomeration (vermicularity). The products were analyzed by XRD at different temperatures, and TEM was used to examine the individual diameters of the α-Al₂O₃ crystallites.