High-Efficiency Power Amplifier Using Novel Dynamic Bias Switching

A novel bias-switching scheme for a high-efficiency power amplifier is proposed. Two voltage levels for the drain bias of the RF power amplifier are generated using a combination of a class E dc/ac inverter and a class E rectifier with offset voltage. When signal peaks occur, the output of the class E dc/ac inverter is rectified and the rectified dc is added to the offset voltage by the class E rectifier, which boosts the drain bias of the RF power amplifier. Except during peaks, the drain bias of the RF power amplifier is connected to the offset voltage directly. Since the efficiency when there are no peaks is very high due to the direct connection between the offset voltage and drain bias, the overall efficiency of the RF power amplifier can be improved dramatically in high peak-to-average power ratio (PAPR) systems. The measured results show that the drain bias of the RF power amplifier is boosted up to approximately 1.8 times the offset voltage when the RF peaks generate. The overall efficiency of the proposed bias-switching amplifier is improved by 62% compared to that of the fixed bias amplifier in high PAPR systems     

20% External Quantum Efficiency in Solution‐Processed Blue Thermally Activated Delayed Fluorescent Devices

Highly efficient solution‐processed blue thermally activated delayed fluorescent (TADF) devices are developed by designing soluble blue TADF emitters. The solubility and emission color could be managed by introducing F as an electron withdrawing unit instead of CN. Two soluble blue TADF emitters are synthesized and show a high external quantum efficiency of 20.0% with a color coordinate of (0.16,0.26), and it is the best quantum efficiency reported in solution‐processed TADF devices. The device performances of the solution‐processed blue TADF devices are comparable to those of vacuum‐processed blue TADF devices.     

Speech Enhancement Using Phase‐Dependent A Priori SNR Estimator in Log‐Mel Spectral Domain

We propose a novel phase‐based method for single‐channel speech enhancement to extract and enhance the desired signals in noisy environments by utilizing the phase information. In the method, a phase‐dependent a priori signal‐to‐noise ratio (SNR) is estimated in the log‐mel spectral domain to utilize both the magnitude and phase information of input speech signals. The phase‐dependent estimator is incorporated into the conventional magnitude‐based decision‐directed approach that recursively computes the a priori SNR from noisy speech. Additionally, we reduce the performance degradation owing to the one‐frame delay of the estimated phase‐dependent a priori SNR by using a minimum mean square error (MMSE)‐based and maximum a posteriori (MAP)‐based estimator. In our speech enhancement experiments, the proposed phase‐dependent a priori SNR estimator is shown to improve the output SNR by 2.6 dB for both the MMSE‐based and MAP‐based estimator cases as compared to a conventional magnitude‐based estimator.     

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.