Walsh coded training signal aided time domain channel estimation for MIMO-OFDM systems

This letter proposes a novel Walsh coded training signal design and decoding method to estimate the channel response in MIMO-OFDM systems. The Walsh coded training signals, designed to be orthogonal in the time domain, facilitate the separation of the desired training signal from the received mixed signal and the estimation of the channel response. The proposed channel estimation method is directly applicable to practical MIMO-OFDM systems with null subcarriers and exhibits nearly the same performance as Li?s original channel estimator [5] at a much reduced computational complexity.     

Small molecule interlayer for solution processed phosphorescent organic light emitting device

Using a 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) small molecule interlayer, we have fabricated efficient green phosphorescent organic light emitting devices by solution process. Significantly a low driving voltage of 3.0 V to reach a luminance of 1000 cd/m² is reported in this device. The maximum current and power efficiency values of 27.2 cd/A and 17.8 lm/W with TCTA interlayer (thickness 30 nm) and 33.7 cd/A and 19.6 lm/W with 40 nm thick interlayer are demonstrated, respectively. Results reveal a way to fabricate the phosphorescent organic light emitting device using TCTA small molecule interlayer by solution process, promising for efficient and simple manufacturing.     

A posteriori joint detection and discrimination of pulses in a quasiperiodic pulse train

The problem of a posteriori joint detecting and discriminating pulses in a quasiperiodic pulse train is studied. By the quasiperiodic pulse train, we mean any pulse train in which the time lapse between the beginning instants of two consecutive pulses varies over time within a certain fixed interval. We consider the kind of quasiperiodic pulse train in which the beginning instants of pulses are deterministic (nonrandom). We analyze the case when all pulses in a pulse train belong to an alphabet of reference pulses having identical duration. It is assumed that the observed interval of the pulse train contains the complete pulses (no parts of pulses are missing at the observation) and that the unobservable pulse train is distorted by an additive white Gaussian noise. Up until this time, there has been no exact algorithm to solve this a posteriori problem under these very simple assumptions because of enormous combinatorial complexity. We derive and prove an efficient (polynomial) computing algorithm for the exact solution to this problem. The recursive equations for step-by step discrete optimization are obtained under the maximum-likelihood criterion. The same formulas hold under the least-squares criterion. The computational load of the algorithm is evaluated, and its dependency on the parameters of the problem is proven.     

Analysis of the reservoir effect on electromigration reliability

The reservoir effect on electromigration reliability is analyzed using the normalized vacancy concentration distribution in the reservoir region of multi-level Al–0.5%Cu interconnect structure. With the assumption of steady state for the vacancy concentration and the fact that no current flow conducts in the reservoir region during electromigration test, a simple equation for calculation of the vacancy concentration is induced. Then direct calculation of the equation is carried out utilizing the hydrostatic stress distribution computed from finite element method to estimate the probability of initial void formation in the reservoir region. Finally, three multi-level Al–0.5%Cu interconnect structures with different reservoir lengths are constructed and electromigration lifetime for the structures is measured to clarify these computational results. From the results of this study, we conclude that the normalized vacancy concentration under the assumption of steady state can be regarded as a quantitative parameter to analyze the reservoir effect on electromigration reliability.     

An enhanced channel-quality indication (CQI) reporting scheme for HSDPA systems

A major evolution of UMTS standard is the high speed downlink packet access (HSDPA). One of key techniques supporting HSDPA is the adaptive modulation and coding (AMC) in which the modulation scheme and the coding rate are adaptively changed according to the downlink channel quality reported by the user equipment (UE). Therefore, the channel quality indication (CQI) reporting scheme is directly related to the accuracy of AMC and the performance of HSDPA. This letter proposes an enhanced CQI reporting scheme that can be used when a proportional fair scheduling algorithm (PFA) is used as a packet scheduling algorithm. The proposed scheme uses a dynamic threshold to filter off redundant CQI reports. With the proposed scheme, the battery capacity of UK can be conserved, maintaining the performance of traditional CQI reporting scheme.     

Synergistic Effect of Excited State Property and Aggregation Characteristic of Organic Semiconductor on Efficient Hole-Transportation in Perovskite Device

Intrinsic characteristics of organic semiconductor-based hole transport materials (HTMs) such as facile synthesizability, energy level tunability, and charge transport capability have been highlighted as crucial factors determining the performances of perovskite photovoltaic (PV) cells. However, their properties in the excited state have not been actively studied, although PVs are operated under solar illumination. Here, the characteristics of organic HTMs in their excited state such as transition dipole moment can be a decisive factor that can improve built-in potential of PVs, consequently enhancing their charge extraction property as well as reducing carrier recombination. Moreover, the aggregation property of organic semiconductors, which has been an essential factor for high-performance organic HTMs to improve their carrier transport property, can induce a synergistic effect with their excited state property for the high-efficiency perovskite PVs. Additionally, it is also confirmed that their optical bandgaps, manipulated to have their absorption in the UV region, are beneficial to block UV light that degrades the quality of perovskite, consequently improving the stability of perovskite PV in p–i–n configuration. As a proof-of-concept, a model system, composed of triarylamine and imidazole-based organic HTMs, is designed, and it is believed that this strategy paves a way toward high-performance and stable perovskite PV devices.     

Immunoregulation of Macrophages by Controlling Winding and Unwinding of Nanohelical Ligands

Developing materials with the capability of changing their innate features can help to unravel direct interactions between cells and ligand-displaying features. This study demonstrates the grafting of magnetic nanohelices displaying cell-adhesive Arg-Gly-Asp (RGD) ligand partly to a material surface. These enable nanoscale control of rapid winding (“W”) and unwinding (“UW”) of their nongrafted portion, such as directional changes in nanohelix unwinding (lower, middle, and upper directions) by changing the position of a permanent magnet while keeping the ligand-conjugated nanohelix surface area constant. The unwinding (“UW”) setting cytocompatibility facilitates direct integrin recruitment onto the ligand-conjugated nanohelix to mediate the development of paxillin adhesion assemblies of macrophages that stimulate M2 polarization using glass and silicon substrates for in vitro and in vivo settings, respectively, at a single cell level. Real time and in vivo imaging are demonstrated that nanohelices exhibit reversible unwinding, winding, and unwinding settings, which modulate time-resolved adhesion and polarization of macrophages. It is envisaged that this remote, reversible, and cytocompatible control can help to elucidate molecular-level cell–material interactions that modulate regenerative/anti-inflammatory immune responses to implants.     

Continuous Control of Charge Transport in Bi‐Deficient BiFeO3 Films Through Local Ferroelectric Switching

It is demonstrated that electric transport in Bi‐deficient Bi_1‐δFeO₃ ferroelectric thin films, which act as a p‐type semiconductor, can be continuously and reversibly controlled by manipulating ferroelectric domains. Ferroelectric domain configuration is modified by applying a weak voltage stress to Pt/Bi_1‐δFeO₃/SrRuO₃ thin‐film capacitors. This results in diode behavior in macroscopic charge‐transport properties as well as shrinkage of polarization‐voltage hysteresis loops. The forward current density depends on the voltage stress time controlling the domain configuration in the Bi_1‐δFeO₃ film. Piezoresponse force microscopy shows that the density of head‐to‐head/tail‐to‐tail unpenetrating local domains created by the voltage stress is directly related to the continuous modification of the charge transport and the diode effect. The control of charge transport is discussed in conjunction with polarization‐dependent interfacial barriers and charge trapping at the non‐neutral domain walls of unpenetrating tail‐to‐tail domains. Because domain walls in Bi_1‐δFeO₃ act as local conducting paths for charge transport, the domain‐wall‐mediated charge transport can be extended to ferroelectric resistive nonvolatile memories and nanochannel field‐effect transistors with high performances conceptually.     

Controlling resonance frequencies in antennas to achieve wideband operation

A novel antenna design method for controlling resonance frequencies is presented. This method can be realised by simply inserting a spiral inductance and a gap capacitance into a conventional loop-type antenna, and this technique remarkably enhances an impedance bandwidth without increasing the antenna size. The proposed antenna has been designed to have 530 MHz (975?1505 MHz) and 150 MHz (2410?2560 MHz) dual bands (VSWR = 2.5), and can be effectively used as an internal antenna for global system for mobile communications (GSM) and Bluetooth bands. These wideband characteristics provide a critical advantage because the shift of an antenna operation frequency increases when it is near various other components.     

Work‐Function Engineering of Graphene Anode by Bis(trifluoromethanesulfonyl)amide Doping for Efficient Polymer Light‐Emitting Diodes

Graphene has been considered to be a potential alternative transparent and flexible electrode for replacing commercially available indium tin oxide (ITO) anode. However, the relatively high sheet resistance and low work function of graphene compared with ITO limit the application of graphene as an anode for organic or polymer light‐emitting diodes (OLEDs or PLEDs). Here, flexible PLEDs made by using bis(trifluoromethanesulfonyl)amide (TFSA, [CF₃SO₂]₂NH) doped graphene anodes are demonstrated to have low sheet resistance and high work function. The graphene is easily doped with TFSA by means of a simple spin‐coating process. After TFSA doping, the sheet resistance of the TFSA‐doped five‐layer graphene, with optical transmittance of ≈88%, is as low as ≈90 Ω sq^?1. The maximum current efficiency and power efficiency of the PLED fabricated on the TFSA‐doped graphene anode are 9.6 cd A^?1 and 10.5 lm W^?1, respectively; these values are markedly higher than those of the PLED fabricated on pristine graphene anode and comparable to those of an ITO anode.