Inter‐carrier Interference Reduction Scheme for SFBC‐OFDM Systems

In this paper, we first analyze carrier‐to‐interference ratio performance of the space–frequency block coded orthogonal frequency‐division multiplexing (SFBC‐OFDM) system in the presence of phase noise (PHN) and residual carrier frequency offset (RCFO). From the analysis, we observe that conventional SFBC‐OFDM systems suffer severely in the presence of PHN and RCFO. Therefore, we propose a new inter‐carrier interference (ICI) self‐cancellation method — namely, ISC — for SFBC‐OFDM systems to reduce the ICI caused by PHN and RCFO. Through the simulation results, we show that the proposed scheme compensates the ICI caused by PHN and RCFO in Alamouti SFBC‐OFDM systems and has a better performance than conventional schemes.     

Femtojoule-Power-Consuming Synaptic Memtransistor Based on Mott Transition of Multiphasic Vanadium Oxides

To realize the potential of Mott transition of multiphasic vanadium oxides (VO_x) for memory applications, the development of VO_x memtransistors on SiO₂ wafer is introduced. Through electrical characterizations, the volatile memory behaviors of the VO_x memtransistors are observed in both two- and three-terminal measurements. Their capacitive memory and resistive switching mechanisms are strongly related to the mixed VO_x/SiO₂ interface (called VSiO_x). VSiO_x supports the Mott transition in VO_x at low bias voltages (<0.5 V), leading to the low power consumption of the memtransistor. Moreover, the fast switching time (≈35 ns) and tunable memory retention with the synaptic functions (potentiation and depression) of the memtransistors (by using the gate and drain biases) are demonstrated. Overall, the findings open up major opportunities for constructing ultrafast and femto-joule power-consuming neuromorphic devices.     

Direct Graphene Transfer and Its Application to Transfer Printing Using Mechanically Controlled,Large Area Graphene/Copper Freestanding Layer

Direct graphene transfer is an attractive candidate to prevent graphene damage, which is a critical problem of the conventional wet transfer method. Direct graphene transfer can fabricate the transferred graphene film with fewer defects by using a polymeric carrier. Here a unique direct transfer method is proposed using a 300 nm thick copper carrier as a suspended film and a transfer printing process by using the polydimethylsiloxane (PDMS) stamp under controlled peeling rate and modulus. Single and multilayer graphene are transferred to flat and curved PDMS target substrate directly. With the transfer printing process, the transfer yield of a trilayer graphene with 1000 µm s^?1 peeling rate is 68.6% of that with 1 µm s^?1 peeling rate. It is revealed that the graphene transfer yield is highly related to the storage modulus of the PDMS stamp: graphene transfer yield decreases when the storage modulus of the PDMS stamp is lower than a specific threshold value. The relationship between the graphene transfer yield and the interfacial shear strain of the PDMS stamp is studied by finite‐element method simulation and digital image correlation.     

Intercell Interference Coordination Using Threshold-Based Region Decisions

IMT-Advanced mobile communication systems make it possible for any devices to access high-speed networks anytime and anywhere. To meet the needs of IMT-Advanced systems, cellular systems must solve the problem of intercell interference caused by frequency reuse. Intercell interference problems become severe when orthogonal frequency division multiplexing (OFDM) transmission, which is a key technology for 4G communication systems, is used in a cellular system. In this paper, a zone-based intercell interference coordination (ICIC) scheme with high flexibility and low cost is proposed, and its performance is evaluated through multicell system-level simulations carried out according to the simplified 3GPP (3rd Generation Partnership Project) Long Term Evolution (LTE) system parameters. In the proposed algorithm, each cell is divided into several regions based on threshold values. Each region reuses frequencies in different ways, and the regions have different maximum transmit (TX) powers according to the interference environment. Even though the proposed scheme can be implemented with low complexity by using only the existing user equipment (UE) measurement, simulation results have confirmed that it provides significant improvements in geometry distribution.     

Charge‐Generating Mode Control in High‐Performance Transparent Flexible Piezoelectric Nanogenerators

In this work, we demonstrate the mode transition of charge generation between direct‐current (DC) and alternating‐current (AC) from transparent flexible (TF) piezoelectric nanogenerators (NGs), which is dependent solely on the morphology of zinc oxide (ZnO) nanorods without any use of an AC/DC converter. Tilted ZnO nanorods grown on a relatively low‐density seed layer generate DC‐type piezoelectric charges under a pushing load, whereas vertically aligned ZnO nanorods on a relatively high‐density seed layer create AC‐type charge generation. The mechanism for the geometry‐induced mode transition is proposed and characterized. We also examine the output performance of TF‐NGs which employ an indium zinc tin oxide (IZTO) film as a TF electrode. It is demonstrated that an IZTO film has improved electrical, optical, and mechanical properties, in comparison with an indium tin oxide (ITO) film. Enhanced output charge generation is observed from IZTO‐based TF‐NGs when TF‐NGs composed of only ITO electrodes are compared. This is attributed to the higher Schottky barrier and the lower series resistance of the IZTO‐based TF‐NGs. Thus, by using IZTO, we can expect TF‐NGs with superior mechanical durability and power generating performance.     

Optimized Hash Tree for Authentication in Sensor Networks

Authentication based on the Merkle tree has been proposed as an energy efficient approach in a resource constrained sensor network environment. It replaces complicated certificate verification with more power efficient hash computations. While previous works assumed complete binary Merkle tree structures, which can be used efficiently only in sensor networks with a specific number of sensor nodes, we investigate incomplete Merkle trees to support any number of sensors. For the incomplete Merkle tree, we demonstrate that an optimal structure can be found through mathematical analysis and simulation. A novel tree indexing scheme is also proposed to reduce communication overhead and save sensors resources during authentication     

Controlled assembly of single SWNTs bundle using dielectrophoresis

We present a mass productive and reproducible assembly technique of a single bundle of single-walled carbon nanotubes (sb-SWNTs) using dielectrophoresis (DEP). Gold electrodes with 10 gaps made via microlithography were used to align the carbon nanotubes (CNTs). The magnitude and type of applied electric field were investigated to verify their effects on CNT assembly. The optimum assembling conditions in which sb-SWNTs could be positioned at a desired site were experimentally identified, and the characteristics of the assembled sb-SWNTs were evaluated from AFM, Raman spectroscopy, and I-V curve. This assembly method has potential for applications, such as gas sensors or electronic devices.     

Poly(diketopyrrolopyrrole‐benzothiadiazole) with Ambipolarity Approaching 100% Equivalency

As a characteristic feature of conventional conjugated polymers, it has been generally agreed that conjugated polymers exhibit either high hole transport property (p‐type) or high electron transport property (n‐type). Although ambipolar properties have been demonstrated from specially designed conjugated polymers, only a few examples have exhibited ambipolar transport properties under limited conditions. Furthermore, there is, as yet, no example with ‘equivalent’ hole and electron transport properties. We describe the realization of an equivalent ambipolar organic field‐effect transistor (FET) by using a single‐component visible–near infrared absorbing diketopyrrolopyrrole (DPP)‐benzothiadiazole (BTZ) copolymer, namely poly[3,6‐dithiene‐2‐yl‐2,5‐di(2‐decyltetradecyl)‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione‐5’,5’’‐diyl‐alt‐benzo‐2,1, 3‐thiadiazol‐4,7‐diyl] ( PDTDPP‐alt‐BTZ ). PDTDPP‐alt‐BTZ shows not only ideally balanced charge carrier mobilities for both electrons (?_e = 0.09 cm²V^?1s^?1) and holes (?_h = 0.1 cm²V^?1s^?1) but also its inverter constructed with the combination of two identical ambipolar FETs exhibits a gain of ～35 that is much higher than usually obtained values for unipolar logic.     

Joint Optimization of User Set Selection and Transmit Power Allocation for Orthogonal Random Beamforming in Multiuser MIMO Systems

When the number of users is finite, the performance improvement of the orthogonal random beamforming (ORBF) scheme is limited in high signal‐to‐noise ratio regions. In this paper, to improve the performance of the ORBF scheme, the user set and transmit power allocation are jointly determined to maximize sum rate under the total transmit power constraint. First, the transmit power allocation problem is expressed as a function of a given user set. Based on this expression, the optimal user set with the maximum sum rate is determined. The suboptimal procedure is also presented to reduce the computational complexity, which separates the user set selection procedure and transmit power allocation procedure.