Cross‐layer and cognitive QoS management system for next‐generation networking

Owing to limited wireless network resources, network applications must provide an adaptive quality‐guaranteed service to satisfy user requirements. Different applications are associated with different quality of service (QoS) concerns, as well as different QoS control parameters. This work presents an adaptive QoS algorithm by discussing the QoS specifications of three wireless access technologies, i.e. 3G, WiMAX and WiFi. Based on cross‐layer and cognition concepts, these environmental parameters are integrated with the sensing of spectral and received signal strength from a cognitive radio paradigm. An adaptive QoS algorithm is then proposed to select the optimal access network for services. Simulation results indicate that the proposed adaptive QoS algorithm outperforms available ones in real‐time applications. Compared with traditional algorithms, the proposed algorithm reduces not only the average delay time and jitter for VoIP services to 0.16 s and 0.09 ms, respectively, but also the packet loss ratio for high‐definition video streaming by 3.4%. Copyright © 2011 John Wiley & Sons, Ltd.     

A cross‐layer TCP for providing fairness in wireless mesh networks

The fair allocation of resources among different nodes is one of the critical problems in wireless mesh networks. Existing solutions mainly focus on rate‐limitation policies or distributed fair MAC schemes at the potential expense of total network utilization. This paper investigates a special starvation problem among TCP flows that are different hops away from the BS, as well as its recently proposed solution, the ‘Minimum Content Window’ policy based on IEEE 802.11e. It is found that the aggregate throughput degrades sharply because the effect of this policy on the TCP congestion mechanism has been overlooked. This paper proposes a priority‐based congestion control by using ‘Cross‐Layer Explicit Congestion Notification’. Analysis and simulation results demonstrate that our scheme can improve the fairness of TCP flows while the aggregate throughput is at least 20% higher than the ‘Minimum Content Window’ policy. Copyright © 2011 John Wiley & Sons, Ltd.     

Design of Two‐Stage Class AB CMOS Buffers: A Systematic Approach

A systematic approach for the design of two‐stage class AB CMOS unity‐gain buffers is proposed. It is based on the inclusion of a class AB operation to class A Miller amplifier topologies in unity‐gain negative feedback by a simple technique that does not modify quiescent currents, supply requirements, noise performance, or static power. Three design examples are fabricated in a 0.5 µm CMOS process. Measurement results show slew rate improvement factors of approximately 100 for the class AB buffers versus their class A counterparts for the same quiescent power consumption (< 200 µW).     

Performance enhancement of III–V compound multijunction solar cell incorporating transparent electrode and surface treatment

To enhance the performance of tandem‐type III–V compound multijunction solar cells, the transparent indium‐tin‐oxide (ITO) film was used to replace conventional metal electrode for increasing the incident light area. For performing ohmic contact between the n‐AlInP window layer and the ITO film, a transition layer of Au/AuGeNi thin metals was used and investigated. Besides, to improve ohmic performance and to passivate the surface states, (NH₄)₂S_x surface treatment was used. The conversion efficiency of the (NH₄)₂S_x‐treated triple‐junction solar cells was increased more than 3.09%. Furthermore, an improved oblique SiO₂/SiO₂/ITO triple antireflection structure was designed to reduce the reflectivity of illuminating sunlight. The conversion efficiency of the (NH₄)₂S_x‐treated triple‐junction solar cell with improved antireflection structure could be improved more than 4.23%. Simple and effective approaches were designed to improve the performances of tandem‐type III–V triple‐junction solar cells. Copyright © 2010 John Wiley & Sons, Ltd.     

Efficiency of Light Outcoupling Structures in Organic Light‐Emitting Diodes: 2D TiO2 Array as a Model System

Organic light‐emitting diodes (OLEDs) are widely used in research and are established in the industry. The building block nature of organic compounds enables a vast variety of materials. On top of that, there exist many strategies to improve the light outcoupling of OLEDs making a direct comparison of outcoupling technologies difficult. Here, a novel approach is introduced for the evaluation of light outcoupling structures. The new defined “efficiency of light outcoupling structures” (ELOS) clearly determines the effectiveness of the light outcoupling structure by weighting the experimental efficiency enhancement over the theoretical outcoupling gain. It neither depends on cavity design nor on the chosen organic material. The methodology is illustrated for red phosphorescent OLEDs comprising internal and external light outcoupling structures. Assumptions and further uses are discussed with respect to experimental and theoretical handling. In addition, the ELOS is calculated for various outcoupling techniques from literature to demonstrate the universality. Finally, most suitable reference OLEDs are discussed for application of light outcoupling structures. The presented approach enables new possibilities for studying light outcoupling structures and improves their comparability in a highly material‐driven research field.     

Sub‐Nanometer Level Size Tuning of a Monodisperse Nanoparticle Array Via Block Copolymer Lithography

The fabrication and catalytic application of a size‐tunable monodisperse nanoparticle array enabled by block copolymer lithography is demonstrated. Highly uniform vertical cylinder nanodomains are achieved in poly(styrene‐block‐4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer thin‐films by solvent annealing. The prominent diffusion of the anionic metal complexes into the protonated P4VP cylinder nanodomains occurs through specific electrostatic interactions in a weakly acidic aqueous solution. This well‐defined diffusion with nanoscale confinement enables preparation of the laterally ordered monodisperse nanoparticle array with sub‐nanometer level precise size tuning. The controlled growth of monodisperse nanoparticle arrays is proven by their catalytic use for vertical carbon nanotube (CNT) growth via plasma enhanced chemical vapor deposition (PECVD). Since the size of the catalyst particles is the decisive parameter for the diameters and wall‐numbers of CNTs, the highly selective growth of double‐walled or triple‐walled CNTs could be accomplished using monodisperse nanoparticle arrays.     

Time‐resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se₂ solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real‐time synchrotron‐based X‐ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross‐sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe₂ starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga‐rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti‐reflective coating). The evolution of the Cu(In,Ga)Se₂ diffraction signals reveals that the minimum process time for high‐quality Cu(In,Ga)Se₂ absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance analysis of opportunistic nonregenerative relaying

Opportunistic relaying in cooperative communication depends on careful relay selection. However, the traditional centralized method used for opportunistic amplify‐and‐forward protocols requires precise measurements of channel state information at the destination. In this paper, we adopt the max–min criterion as a relay selection framework for opportunistic amplify‐and‐forward cooperative communications, which was exhaustively used for the decode‐and‐forward protocol, and offer an accurate performance analysis based on exact statistics of the local signal‐to‐noise ratios of the best relay. Furthermore, we evaluate the asymptotical performance and deduce the diversity order of our proposed scheme. Finally, we validate our analysis by showing that performance simulation results coincide with our analytical results over Rayleigh fading channels, and we compare the max–min relay selection with their centralized channel state information‐based and partial relay selection counterparts. Copyright © 2013 John Wiley & Sons, Ltd.     

Opportunistic interference management: a new approach for multiantenna downlink cellular networks

A new approach for multiantenna broadcast channels in cellular networks based on multiuser diversity concept is introduced. The technique called opportunistic interference management achieves dirty paper coding capacity asymptotically with minimum feedback required. When there are K antennas at the base station with M mobile users in the cell, the proposed technique only requires K integer numbers related to channel state information between mobile users and base station. The encoding and decoding complexity of this scheme is the same as that of point‐to‐point communications, which makes the implementation of this technique easy. An antenna selection scheme is proposed at the base station to reduce the minimum required mobile users significantly at the expense of reasonable increase in feedback. In order to guarantee fairness, a new algorithm is presented that incorporates opportunistic interference management into existing Global System for Mobile communications (GSM) standard. Copyright © 2014 John Wiley & Sons, Ltd.