Scheduled WiFi using distributed contention in WLANs: algorithms,experiments, and case-studies

The ubiquitous adoption of WiFi introduces large diversity in types of application requirements and topological characteristics. Consequently, considerable attention is being devoted to making WiFi networks controllable without compromising their scalability. However, the main MAC protocol of WiFi, distributed coordination function (DCF), is a contention-based protocol using random backoff. Thus, operating under DCF, the access of channel is hard to control and nonpredictable. In order to provide controllability of channel access in WiFi, we propose Rhythm, a MAC protocol that achieves scheduled WiFi efficiently using distributed contention. By achieving scheduled WiFi, channel access can be controlled by manipulating the schedule decision. We evaluate the performance of Rhythm through analysis, experiments, and case-studies.     

Utilization of self-injection Fabry–Perot laser diode for long-reach WDM-PON

In this investigation we propose and demonstrate a wavelength widely tunable laser source employing a self-injected Fabry–Perot laser diode (FP-LD) for long-reach wavelength-division-multiplexed passive optical network (WDM-PON). By using a tunable bandpass filter and an optical circulator inside the gain cavity, a stable and single-longitudinal-mode (SLM) laser output is achieved. Besides, the proposed laser sources are directly modulated at 2.5 Gb/s for both downlink and uplink transmissions of 85 km single mode fiber (SMF) in PON without dispersion compensation.     

Variance plus bias optimal response surface designs with qualitative factors applied to stem choice modeling

This paper explores the issue of model misspecification, or bias, in the context of response surface design problems involving quantitative and qualitative factors. New designs are proposed specifically to address bias and compared with five types of alternatives ranging from types of composite to D‐optimal designs using four criteria including D‐efficiency and measured accuracy on test problems. Findings include that certain designs from the literature are expected to cause prediction errors that practitioners would likely find unacceptable. A case study relating to the selection of science, technology, engineering, or mathematics majors by college students confirms that the expected substantial improvements in prediction accuracy using the proposed designs can be realized in relevant situations. Copyright © 2011 John Wiley & Sons, Ltd.     

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.     

Synthesis and photoluminescence of bright water-soluble CdSe/ZnS quantum dots overcoated by hybrid organic shell

Photoluminescence properties from water soluble CdSe/ZnS QDs encapsulated with hybrid trioctylphosphine-poly(acrylamide-co-acrylic acid)-ethanolamine (TOPO-PSMA-EA) shell have been investigated. It was found that PL efficiency of CdSe/ZnS QDs in water was increased 5–30% after introducing PSMA-EA polymers to encapsulate CdSe/ZnS-TOPO QDs. Higher PSMA concentrations were found to enhance the PL efficiency of QDs up to 1.8 folds, which is ascribed to a better packing and passivation of the TOPO-PSMA-EA shell over the QDs. Time-resolved photoluminescence suggested that the mean lifetime of photoexcited carriers in the water-soluble CdSe/ZnS-TOPO-PSMA-EA QDs elongated 2–17 ns compared with that of uncoated samples, indicating that PL quenching defects were effectively removed for CdSe/ZnS QDs with hybrid TOPO-PSMA-EA shell.     

Effect of silanes on the morphology,hydrophobicity, and dynamical mechanical properties of polyimide/silica hybrid membranes

Polyimide (PI)/silica hybrid membranes with high contact angles were prepared through the in situ sol–gel process. The precursor, poly(amic acid) with controlled block chain length, was synthesized using 4,4′‐diaminodiphenyl ether (ODA), 3,3′,4,4′‐benzophenone‐tetracarboxylic dianhydride (BTDA) and 3‐aminopropyl‐trimethoxysilane (APrTMOS) or 3‐aminopropyldimethylethoxysilane (APDiMOS). And then, phenyltrimethoxysilane (PTS) or tetramethoxysilane (TMOS) or methyltrimethoxysilane (MTrMOS) was respectively, added to the above polyamic acid and mixed thoroughly. Following curing reaction, the PI/silica hybrid membranes with different cross‐linkages, silica content, and hydrophobic properties were prepared. The effect on the formation of PI imide ring during imidization reaction is increased as the increase of silanes content and characterized by frequency shiftment and absorbance ratio of Fourier transform infrared (FTIR) measurements. All the hybrid membranes show high transparency though with high silica contents. The storage modulus, tan δ, and damping intensity by DMA measurements are all correlated with silane content or block chain length. And all these membranes with silane content possess high contact angle as compared to pure PI without any silanes added and the contact angles increase with increasing the silane content. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011