Low Roll‐Off and High Efficiency Orange Organic Light Emitting Diodes with Controlled Co‐Doping of Green and Red Phosphorescent Dopants in an Exciplex Forming Co‐Host

An exciplex forming co‐host is introduced in order to fabricate orange organic light‐emitting diodes (OLEDs) with high efficiency, low driving voltage and an extremely low efficiency roll‐off, by the co‐doping of green and red emitting phosphorescence dyes in the host. The orange OLEDs achieves a low turn‐on voltage of 2.4 V, which is equivalent to the triplet energy gap of the phosphorescent‐green emitting dopant, and a very high external quantum efficiency (EQE) of 25.0%. Moreover, the OLEDs show low efficiency roll‐off with an EQE of over 21% at 10 000 cdm^?2. The device displays a very good orange color (CIE of (0.501, 0.478) at 1000 cdm^?2) with very little color shift with increasing luminance. The transient electroluminescence of the OLEDs indicate that both energy transfer and direct charge trapping takes place in the devices.     

One‐Step Fabrication of Multifunctional Core‐Shell Fibres by Co‐Electrospinning

In the present study, multifunctional core‐shell fibre mats were designed by co‐electrospinning. These core‐shell fibre mats have three different functionalities: 1) they are magnetic, 2) they change their optical properties with the pH of the media, and 3) they are sensitive to O₂. The shell is formed by a fluorescent pH‐sensitive co‐polymer which was previously synthesised and characterized by our research group. The core is a suspension formed by magnetic nanoparticles in a solution made up by a lipophilic indicator dye (oxygen indicator; PtOEP) and, poly‐methyl methacrylate, in THF. The magnetic nanoparticles were prepared by encapsulation of magnetite within a cross‐linked polymeric matrix (MMA‐co‐EDMA). To our knowledge, this is the first time that three functionalities (magnetic properties, sensitivity to pH, and response to O₂ concentration) were successful conjugated on the same micro‐ or nano‐material via a facile one‐step process with high yield and cost effectiveness. The morphology of the well‐organized core‐shell fibres were characterized by high resolution scanning electron microcopy (HRSEM), transmission electron microcopy (TEM), and confocal laser microscopy. The luminescent properties of core‐shell fibre mats were analysed and successfully used for simultaneously monitoring pH (from 6 to 8) and O₂, showing complete reversibility, high sensitivity (i.e., K_sv = 7.07 bar^?1 for determining O₂ in aqueous media), high magnetic susceptibility, and short response times.     

Statistical analysis of co‐channel interference in wireless communications systems

Co‐channel interference is recognized as one of the major factors that limits the capacity and link quality of a wireless communications system. An appropriate understanding of the statistical behavior of the co‐channel interference is therefore required when analyzing and designing techniques that mitigate its undesired effects. The total co‐channel interference in a wireless communications system is usually modeled as the sum of lognormally distributed signals, and is generally assumed to be itself lognormally distributed. Based on this assumption, several methods for estimating the moments of the resulting lognormal distribution have been proposed. The accuracy of these methods has been studied in previous works, under the assumption of having all summand signals (individual interference signals) identically distributed. Such an assumption rarely holds in practical cases of emerging wireless communications systems, where co‐channel interference may stem from far‐away macrocells and nearby transmitters, causing the interference signals to have different moments. In this paper we present an analysis of the accuracy of two popular methods for computing the moments of a sum of lognormal random variables, namely Wilkinson's method and Schwartz and Yeh's method, for the general case when the summands have different mean values and standard deviations in decibel units. We show that Schwartz and Yeh's method provides better accuracy than Wilkinson's method and is virtually invariant with the difference of the mean values and standard deviations of the summands. Copyright © 2001 John Wiley & Sons, Ltd.     

Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency

Phosphorescent organic light‐emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum efficiency (EQE), driving voltage, and efficiency roll‐off are reported, making use of an exciplex‐forming co‐host. This exciplex‐forming co‐host system enables efficient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge‐injection barrier from the charge‐transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn‐on voltage of 2.4 V, a very high EQE of 29.1% and a very high power efficiency of 124 lm W^?1. In addition, the OLEDs achieve an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m^?2.     

Highly Efficient Quantum‐Dot‐Sensitized Solar Cell Based on Co‐Sensitization of CdS/CdSe

Cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dots (QDs) are sequentially assembled onto a nanocrystalline TiO₂ film to prepare a CdS/CdSe co‐sensitized photoelectrode for QD‐sensitized solar cell application. The results show that CdS and CdSe QDs have a complementary effect in the light harvest and the performance of a QDs co‐sensitized solar cell is strongly dependent on the order of CdS and CdSe respected to the TiO₂. In the cascade structure of TiO₂/CdS/CdSe electrode, the re‐organization of energy levels between CdS and CdSe forms a stepwise structure of band‐edge levels which is advantageous to the electron injection and hole‐recovery of CdS and CdSe QDs. An energy conversion efficiency of 4.22% is achieved using a TiO₂/CdS/CdSe/ZnS electrode, under the illumination of one sun (AM1.5,100 mW cm^?2). This efficiency is relatively higher than other QD‐sensitized solar cells previously reported in the literature.     

HFC系统的业务量模型及分析

在HFC系统结构中，终端用户可以通过网络请求不同的业务服务，对于用户的网络接口单元（NIU），同轴电缆是其共用的传输介质。因此同轴电缆的业务传送能力，对于确定系统的规模，保证阻塞性能达到一个令人满意的水平，都是至关重要的。利用分析和仿真两种方法对同轴电缆的业务传送能力进行了研究。     

Game‐theoretic modeling and analysis of relay selection in cooperative wireless networks

In this paper, distributed single relay selection in cooperative wireless networks is modeled as a Chinese restaurant game (CRG). Specifically, the CRG is used to model strategic relay selection decisions of source nodes, taking into account negative network externality due to the potential sharing of relay nodes among source nodes. Two cases are studied as follows: (i) perfect relay transmit power (RTP) knowledge and (ii) imperfect RTP knowledge. Under the first case, a distributed relay selection algorithm is proposed and shown to converge to a Nash equilibrium grouping. Under the second case, a reinforcement learning algorithm is proposed and combined with the distributed relay selection algorithm to allow network nodes to select rate‐maximizing relays. Simulation results verify the efficiency of the proposed distributed relay selection algorithm when compared with other relay selection schemes and demonstrate that it yields a network sum‐rate that is comparable with that of centralized relay selection. Copyright © 2014 John Wiley & Sons, Ltd.     

Modeling and analysis of WAP performance over wireless links

In this paper, an analytical model for studying the performance behavior of wireless application protocol (WAP) over wireless links is proposed. A Rayleigh fading channel model is used to characterize the behavior of the wireless channel. Mathematical expressions that represent the performance of WAP as a function of the protocol and the channel parameters are derived. Computer simulation results are presented to validate the analytical results. It is shown that WAP performs better in a bursty error environment than in a random error environment. The goodput of WAP can be improved by increasing the WAP packet group size, but the significance of the improvement depends on the underlying channel condition.     

Co‐deposited Cu(I) Complex for Tri‐layered Yellow and White Organic Light‐Emitting Diodes

Four compounds 4‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]isoquinoline (TCIQ), 3‐[3,6‐di(carbazol‐9‐yl)carbazol‐9‐yl]pyridine (TCPy), 4‐(carbazol‐9‐yl)isoquinoline (4CIQ), and 3‐(carbazol‐9‐yl)pyridine (CPy) containing pyridyl or isoquinolyl were designed and synthesized to co‐deposition with copper iodide (CuI) to form luminescent Cu(I) complex doped film in situ, which could be utilized as the emissive layer in organic light‐emitting diodes (OLEDs). It is found that simple tri‐layered yellow and white OLEDs can be achieved by co‐depositing CuI and TCIQ with tuning ratios. The compound TCIQ serves a dual role as both a ligand for forming the emissive Cu(I) complex and as a host matrix for the formed emitter in yellow OLEDs, and a third role as a blue emitter in white OLEDs.     

Wi‐Fi‐based modeling and hybrid routing scheme for delay‐tolerant public bus network

Delay‐ or Disruption‐Tolerant Networking (DTN) is a communications approach that is utilized in easily disrupted or delayed networks. Examples of such networks are often found in heterogeneous networks, mobile or extreme terrestrial networks, and planned networks in space. In this paper, we examine the metropolitan bus network as a research target of DTN for a public transport network. We analyze the metropolitan bus network through spatial and temporal modeling using an existing Bus Information System (BIS) database. On the basis of the results of our analysis, we propose and design an appropriate DTN routing scheme called Hybrid Position‐based DTN Routing. This scheme uses position‐based routing instead of address‐based routing by soliciting infrastructural help from nearby Access Points for the real‐time BIS location service. We simulated our scheme using a WLAN for the wideband DTN communication and evaluated it by comparing it with traditional Ad hoc flooding, Epidemic routing, and strategic protocol steps in our own algorithm. The results indicate that our scheme achieves reasonably high performance in terms of packet delivery ratio, latency, and resource usage. Copyright © 2013 John Wiley & Sons, Ltd.     

Empirical analysis of the reliability of low‐rate wireless u‐healthcare monitoring applications

In this paper, we demonstrate an empirical analysis of the reliability of low‐rate wireless u‐healthcare monitoring applications. We have considered the performance analysis of the IEEE 802.15.4 low‐rate wireless technologies for u‐healthcare applications. For empirical measurement, we considered three scenarios in which the reliability features of the low‐rate wireless u‐healthcare monitoring applications have been measured: (i) distance between sensor nodes and base station; (ii) deployment of the number of sensor nodes in a network; and (iii) data transmission by different time intervals. The experimental results show that received data are used to calculate BER and analyze the performance according to the scenarios. The BER is affected when varying the distance between sensor node and base station, the number of nodes, and time interval. Copyright © 2011 John Wiley & Sons, Ltd.     

An Interface‐Induced Co‐Assembly Approach Towards Ordered Mesoporous Carbon/Graphene Aerogel for High‐Performance Supercapacitors

Hierarchically porous composites with mesoporous carbon wrapping around the macroporous graphene aerogel can combine the advantages of both components and are expected to show excellent performance in electrochemical energy devices. However, the fabrication of such composites is challenging due to the lack of an effective strategy to control the porosity of the mesostructured carbon layers. Here an interface‐induced co‐assembly approach towards hierarchically mesoporous carbon/graphene aerogel composites, possessing interconnected macroporous graphene networks covered by highly ordered mesoporous carbon with a diameter of ≈9.6 nm, is reported. And the orientation of the mesopores (vertical or horizontal to the surface of the composites) can be tuned by the ratio of the components. As the electrodes in supercapacitors, the resulting composites demonstrate outstanding electrochemical performances. More importantly, the synthesis strategy provides an ideal platform for hierarchically porous graphene composites with potential for energy storage and conversion applications.     

Monte Carlo–based channel estimation and performance evaluation for UWOC links under geometric losses

Underwater wireless optical communication has been studied and proposed as a potential technology for various underwater applications including monitoring and surveillance using wireless sensor networks. In this context, we present a statistical model using an exhaustive Monte Carlo approach for an underwater wireless optical communication link between sensor nodes in a data mulling environment. This work provides insights into the science of photon transport underwater, its utilization in a communication system, evaluation, and optimization of underwater communication. We characterize the temporal channel behavior taking into account multiple scattering of photons involving the constraints of detector aperture diameter, field‐of‐view (FOV), and link distance for an aligned point‐to‐point link. We analyze the designed model in terms of total received power and bit error rate for clear ocean waters. Our simulation results show that the detector aperture and FOV has an impact towards a successful communication in the distance from 5 to 20 m. In addition, results show that the FOV is an affecting factor only in the presence of larger aperture sizes of the detector.     

BER minimization for dual‐polarized wireless systems with polarization‐domain rotation

We consider the problem of bit error rate (BER) degradation because of the power gain imbalance between horizontal (H)‐polarization and vertical (V)‐polarization components in an orthogonal dual‐polarization transmission system. To alleviate the aforementioned BER degradation problem, we propose a non‐orthogonal polarization‐domain rotation scheme where the axes of H‐polarization and V‐polarization components are rotated with different angles at the transmitter and de‐rotated at the receiver. In addition, in order to assess the effectiveness of the polarization‐domain rotation scheme, we derive the closed‐form BER expression under a practical dual‐polarized channel model, which is represented by cross‐polarization ratio and co‐polarization ratio (CPR). We also derive the approximated BER expressions for the two asymptotic values of CPR: balanced CPR and infinite CPR. With the derived BER expressions, we find the optimal rotation angles that jointly minimize the BER. According to the numerical results, it is shown that about 3dB E_b/N₀ gain is obtained at the BER of 10^?4 and the CPR of 10dB by the polarization‐domain rotation scheme with optimal rotation angles compared with the conventional orthogonal dual‐polarization transmission. Copyright © 2015 John Wiley & Sons, Ltd.     

An end‐to‐end channel allocation scheme for a wireless mesh network

Co‐channel interference seriously influences the throughput of a wireless mesh network. This study proposes an end‐to‐end channel allocation scheme (EECAS) that extends the radio‐frequency‐slot method to minimize co‐channel interference. The EECAS first separates the transmission and reception of packets into two channels. This scheme can then classify the state of each radio‐frequency‐slot as transmitting, receiving, interfered, free, or parity. A node that initiates a communication session with a quality of service requirement can propagate a channel allocation request along the communication path to the destination. By checking the channel state, the EECAS can determine feasible radio‐frequency‐slot allocations for the end‐to‐end path. The simulation results in this study demonstrate that the proposed approach performs well in intra‐mesh and inter‐mesh communications, and it outperforms previous channel allocation schemes in end‐to‐end throughput. Copyright © 2013 John Wiley & Sons, Ltd.     

Analysis on Co‐channel Interference of Human Body Communication Supporting IEEE 802.15.6 BAN Standard

Human body communication (HBC) is being recognized as a new communication technology for mobile and wearable devices in a body area network (BAN). This paper presents co‐channel interference experienced by HBC supporting the physical layer in the IEEE 802.15.6 BAN standard. To analyze the co‐channel interference, a co‐channel interference model is introduced, and space‐domain and time‐domain parameters representing an interference environment are generated using the co‐channel interference model. A new signal‐to‐interference ratio (SIR) parameter depending on the peak amplitudes of the data signals causing co‐channel interference is defined; co‐channel interference can be easily analyzed and modelled using the newly defined SIR. The BER degradation model derived using the co‐channel interference model and SIR in this paper can be effectively used to estimate the performance.     

Co‐channel interference cancelation at the user terminal in multibeam satellite systems

We study the applicability of soft interference cancelation in the forward link of multibeam satellite systems with focus on mobile terminals. We adopt a standard currently used in commercial satellite systems as a reference. The multibeam satellite antenna radiation diagram has been generated using a physical optics reflector model while a widely adopted channel model has been used for the land mobile satellite channel. The interference pattern has been derived using a system simulator developed by the European Space Agency. Starting from the analysis of the interference pattern, we study the application of a low‐complexity soft interference cancelation scheme for commercial applications. Our results show that, under realistic conditions, a two‐colors frequency reuse scheme can be employed while guaranteeing service availability across the coverage and keeping the complexity at the user terminals relatively low. Copyright © 2015 John Wiley & Sons, Ltd.