Design and Performance of Space–Time Precoder With Hybrid ARQ Transmission

Multiple-input-multiple-output (MIMO) technology can efficiently increase the system capacity in rich scattering environments without increasing the bandwidth or transmission power. The precoder for MIMO transmission is a processing technique that exploits the channel state information (CSI) by operating on the signal before transmission to effectively improve link performance. A hybrid automatic repeat request (HARQ) scheme can be incorporated with the linear precoder to ensure highly reliable communication. To fully utilize the type-I HARQ diversity gain, particularly in slow-fading channels, we propose the optimal design principle of linear precoders whose column vectors are correspondingly orthogonal to each other. In addition, the practical solution based on codebook is given in this paper. Finally, simulation results demonstrate the effectiveness of the proposed precoders in reducing the detection of bit error rate (BER) and in improving normalized throughput.     

A Multisubscriber Variable-Rate Sampling HCDM System with Dynamic Buffer Control

In this paper we present a multisubscriber variable-rate sampling hybrid companding delta modulation (HCDM) system for simultaneous transmission of several speech signals. This system employs both the statistical multiplexing and variable-rate sampling schemes. It transmits speech signals synchronously at a fixed rate using a buffer. In this system the sampling rate of each subscriber is varied according to the speech activity and the status of buffer occupancy, and only the speech portion is coded for transmission. To optimize the system performance within the allowed maximum transmission delay (300 ms), an efficient dynamic buffer control algorithm is proposed. When the number of subscribers is six and the transmission rate for each subscriber is 16 kbits/s, the proposed system yields a performance improvement of about 10 dB over the conventional single-subscriber HCDM system. The buffer delay in this case is 150 ms, which gives a perceptually negligible effect.     

Structural and Morphological Investigation of Pendeo-Epitaxy 3C-SiC on Si Substrates

Successful pendeo-epitaxy growth of cubic silicon carbide (3C-SiC) on off-axis Si(001) substrates was achieved. The structural and morphological characteristics of pendeo-epitaxy 3C-SiC were strongly affected by underlying stripes and seed layer thickness. Stripes perpendicular to the Si substrate off-axis provide about three times faster lateral growth rate compared with parallel oriented stripes. Root-mean-square (RMS) measurements using atomic force microscope (AFM) indicate that the surface morphology of Pendeo-epitaxy 3C-SiC films remarkably improves with increasing seed layer thickness: from 9.8 nm for 3 μm thickness to 0.5 nm for 10 μm thickness. These effects on pendeo-epitaxy 3C-SiC are discussed with scanning electron microscopy (SEM) and AFM investigation.     

Monolithic SiGe HBT Feedforward Variable Gain Amplifiers for 5 GHz Applications

Monolithic SiGe heterojunction bipolar transistor (HBT) variable gain amplifiers (VGAs) with a feedforward configuration have been newly developed for 5 GHz applications. Two types of the feedforward VGAs have been made: one using a coupled‐emitter resistor and the other using an HBT‐based current source. At 5.2 GHz, both of the VGAs achieve a dynamic gain‐control range of 23 dB with a control‐voltage range from 0.4 to 2.6 V. The gain‐tuning sensitivity is 90 mV/dB. At V_CTRL= 2.4 V, the 1 dB compression output power, P_1‐dB, and dc bias current are 0 dBm and 59 mA in a VGA with an emitter resistor and ‐1.8 dBm and 71mA in a VGA with a constant current source, respectively.     

Novel Fairness Improved Radio Resource Management Schemes for Best-Effort Service in OFDMA Systems

In this paper, we propose a complete radio resource management procedure for best-effort service in OFDMA systems, which improves the system fairness with graceful throughput degradation compared to the upper bound of the system throughput. By the proposed bandwidth and power allocation algorithms, the user in the worst channel environment has almost the same probability of accessing the system as the user in the best channel environment. Furthermore, a novel sub-channel allocation algorithm is proposed to exploit frequency selectivity and multi-user diversity gains simultaneously in OFDMA systems, which is able to achieve the highest system throughput given each user’s channel environment.     

Mitigation and Verification Methods for Sn Whisker Growth in Pb-Free Automotive Electronics

This work describes mitigation methods against Sn whisker growth in Pb-free automotive electronics using a conformal coating technique, with an additional focus on determining an effective whisker assessment method. We suggest effective whisker growth conditions that involve temperature cycling and two types of storage conditions (high-temperature/humidity storage and ambient storage), and analyze whisker growth mechanisms. In determining an efficient mitigation method against whisker growth, surface finish and conformal coating have been validated as effective means. In our experiments, the surface finish of components comprised Ni/Sn, Ni/SnBi, and Ni/Pd. The effects of acrylic silicone, and rubber coating of components were compared with uncoated performance under high-temperature/humidity storage conditions. An effective whisker assessment method during temperature cycling and under various storage conditions (high temperature/humidity and ambient) is indicated for evaluating whisker growth. Although components were finished with Ni/Pd, we found that whiskers were generated at solder joints and that conformal coating is a useful mitigation method in this regard. Although whiskers penetrated most conformal coating materials (acrylic, silicone, and rubber) after 3500 h of high-temperature/humidity storage, the whisker length was markedly reduced due to the conformal coatings, with silicone providing superior mitigation over acrylic and rubber.     

Optimization of thermally reduced graphene oxide for an efficient hole transport layer in polymer solar cells

In this work, graphene oxide (GO) reduced by thermal annealing was employed as a hole transport layer (HTL) in bulk heterojunction (BHJ) solar cells. Considering the insulating property of nonreduced GO, the annealing temperature plays an important role in recovering the conjugated structure of the graphene sheet, and thereby the conductivity of GO. BHJ solar cells with high-temperature (e.g., 230 °C) reduced GO as the HTL showed much larger fill factor (FF) than devices with low-temperature (e.g., 130 °C) reduced GO as the HTL, indicating the better conductivity of GO annealed at an elevated temperature due to the removal of oxygen functional groups from the graphene sheet to a much-higher level. On the other hand, the work function of GO may be lowered toward that of graphene (4.5 eV) with increasing the reduction temperature, which results in a decreased open-circuit voltage (V_oc) for the high-temperature reduced GO devices. By further optimizing the concentration and spin-coating speed of GO dispersion, we achieved a power conversion efficiency (PCE) that is 26% higher than devices without any HTL. This is mainly attributed to the increase in FF as a result of the decreased series resistance (R_s). In addition, the PCE of the optimized GO device was ～85% of the PCE of the conventional device with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. We anticipate that further optimization of the reduction conditions (e.g., using chemical reductants) will lead to the better performance of GO solar cells.     

A printed OTFT-backplane for AMOLED display

An AMOLED panel driven by an OTFT-backplane is an attractive display because OTFTs and OLEDs use organic materials with unique characteristics such as low temperature and solution processing ability, and thus are able to implement the key features of future displays. In this study we applied some printing technologies to fabricate an OTFT-backplane for AMOLEDs. Screen printing combined with photolithography with Ag ink was used for the gate electrodes and scan bus lines and contact pads. Ag metal lines with a width of 20 μm and thickness of 60 nm and resistivity of 3.0 × 10^?5 Ω cm were achieved. Inkjet printing was applied to deposit TIPS-pentacene as an organic semiconductor. The OTFT-backplane using the Ag gate electrodes and TIPS-pentacene exhibited uniform performance over 17,500 pixels on a 7 in. panel. The mobility was 0.31 ± 0.05 cm²/V s with a deviation of 17%. The AMOLED panel successfully demonstrated its ability to display patterns.     

Improved photovoltaic performance by enhanced crystallinity of poly(3-hexyl)thiophene

The packing effects induced by the hydrophilic ethylene glycol oligomer side chains occurred in a P3HT:PC₆₁BM mixture upon the addition of small amounts of BP93 (containing 7 mol% PEGT blocks), leading to an enhanced crystallinity among the P3HT molecules, even in a P3HT:PC₆₁BM blend. The enhanced crystallinity improved the charge transport and current density (8.3–11.1 mA/cm²) and increased the power conversion efficiency (3.1–3.9%) in an organic solar cell.     

Ultrathin Sticker‐Type ZnO Thin Film Transistors Formed by Transfer Printing via Topological Confinement of Water‐Soluble Sacrificial Polymer in Dimple Structure

To create ultrathin sticker‐type electronic devices that can be attached to unconventional substrates, it is highly desirable to develop printable membrane‐type electronics on a handling substrate and then transfer the printing to a target surface. A facile method is presented for high‐efficiency transfer printing by controlling the interfacial adhesion between a handling substrate and an ultrathin substrate in a systematic manner under mild conditions. A water‐soluble sacrificial polymer layer is employed on a dimpled handling substrate, which enables the topological confinement of the polymer residue inside and near the dimples during the etching and drying processes to reduce the interfacial adhesion gently, creating a high yield of transfer printing in a deterministic manner. As an example of an electronic device that was created using this method, a highly flexible sticker‐type ZnO thin film transistor was successfully developed with a thickness of 13 μm including a printable ultrathin substrate, which can be attached to various substrates, such as paper, plastic, and stickers.