Efficient non-coherent demodulation scheme for IEEE 802.15.4 LR-WPAN systems

An efficient non-coherent demodulation algorithm to compensate for the frequency offset effectively is proposed for IEEE 802.15.4 LR-WPAN systems working in the 868/915 MHz band. In the proposed algorithm, the decision criterion is changed depending on the frequency offset estimated in the preamble, and the received signal is differentially demodulated by using the phase difference between two consecutive bits. Simulation results show that the proposed method achieves better performance compared to conventional non-coherent algorithms, without increasing hardware costs significantly.     

Optimized Asymmetric Double Quantum Well for High Electric-Field-Sensitivity Electroabsorption: Excitonic Mixing Effects

Asymmetric double quantum wells (ADQWs) are optimized to exhibit maximum optical modulation sensitivity by varying the barrier width, barrier position, and well width. Anticrossing of the two lowest excitons in ADQWs significantly enhances the modulation sensitivity. Consideration of exciton mixing is crucial to obtain accurate estimates of the effects. For a given linewidth of the excitonic peaks, we find optimum structural parameters that exhibit maximum modulation sensitivity. Values dalpha / dE ~ 6.51 x 10⁴ kV^-1of of at 4.2 K and ~1.25 x 10³ kV^-1of at 298 K are predicted in GaAs-based ADQWs. The result provides new design guidelines in fabricating high-sensitivity QW electroabsorption modulators.     

Chiral Stereoisomer Engineering of Electron Transporting Materials for Efficient and Stable Perovskite Solar Cells

A series of chiral stereoisomers of electron transporting materials with two chiral substituents is rationally designed and synthesized, and the influence of stereoisomerism on their physical and electronic properties is investigated to demonstrate highly efficient and stable perovskite solar cells (PSCs). Compared to mesomeric naphthalene diimide (NDI) derivatives, which have heterochiral side groups with centrosymmetric molecular packing of symmetric‐shaped conformers in the crystalline state, enantiomeric NDI derivatives have homochiral side groups that exhibit non‐centrosymmetric molecular packing of asymmetric‐shaped conformers in the crystalline state and exhibit better solution processability based on one order of magnitude higher solubility. A similar trend is observed in different rylene diimide stereoisomers based on larger semiconducting core perylene diimide. The PSCs based on NDI enantiomers with good film‐forming ability and a very high lowest phase transition temperature (T_lowest) of 321 °C exhibit a high and uniform average power conversion efficiency (PCE) of 19.067 ± 0.654%. These PSCs also have a high temporal device stability, with less than 10% degradation of the PCE at 100 °C for 1000 h without encapsulation. Therefore, chiral stereoisomer engineering of charge transporting materials is a potential approach to achieve high solution processability, excellent performance, and significant temporal stability in organic electronic devices.     

Optical Characteristics of Stretchable Chiral Liquid Crystal Elastomer under Multiaxial Stretching

Chiral liquid crystal elastomers (CLCEs) are soft photonic materials that exhibit both the photonic characteristics of nanoscale periodic helical structures and mechanical properties of rubber. Owing to its elasticity, the structural color of CLCEs can be tuned through mechanical deformations known as mechanochromism. Thus far, there is significant research attention to exploring the mechanochromism of CLCEs. However, most studies have only discussed the color shifting of CLCEs under uniaxial deformation. Therefore, the optical and chiral structural deformation behaviors of CLCEs under multiaxial stress are not well understood. This study investigates multiaxial (uniaxial, biaxial, and out-of-plane) stretching-induced helical structure change and the resulting optical properties of CLCEs. The results confirm that uniaxial stretching leads to a loss of intrinsic circular polarization selectivity in CLCEs due to helix unwinding deformations, while biaxial and out-of-plane stretching maintain circular polarization.     

Relations among Security Models for Authenticated Key Exchange

Usually, key‐establishment protocols are suggested in a security model. However, there exist several different security models in the literature defined by their respective security notions. In this paper, we study the relations between the security models of key establishment. For the chosen security models, we first show that some proven key‐establishment protocols are not secure in the more restricted security models. We then suggest two compilers by which we can convert a key‐establishment protocol that is secure in a specific security model into a key‐establishment protocol that is still secure in a more restricted security model.     

Monolithic distributed amplifier with active control schemes for optimum gain and group-delay flatness, bandwidth, and stability

In this paper, active control schemes are presented to optimize the performance of the distributed amplifier (DA) subject to the process variation. A detailed analysis of the DA with mismatched termination loads has been performed, which reveals that pronounced gain and group-delay ripple arises at the low-frequency end from the reflected waves in the artificial transmission line. To solve this problem, an active variable resistor is proposed as the gate-line termination load. The gain and stability of the cascode DA has also been analyzed, which identifies the most critical component determining the tradeoff between the gain-bandwidth product (GBP) and the stability to be the gate feedback resistor of common-gate field-effect transistor. It is also replaced with the active resistor to maximize GBP, while avoiding oscillations. A nine-section cascode DA with active control features is designed and fabricated using commercial GaAs pseudomorphic high electron-mobility transistor foundry. The measurement shows that the gain and group-delay ripple can be minimized, and GBP can be maximized without oscillations by the active bias controls. Active control schemes allow the monolithic DAs to be fine tuned after the fabrication and, thus, can be a robust DA design methodology against process variation and inaccurate device models.     

Functionally Antagonistic Hybrid Electrode with Hollow Tubular Graphene Mesh and Nitrogen‐Doped Crumpled Graphene for High‐Performance Ionic Soft Actuators

Ionic soft actuators, which exhibit large mechanical deformations under low electrical stimuli, are attracting attention in recent years with the advent of soft and wearable electronics. However, a key challenge for making high‐performance ionic soft actuators with large bending deformation and fast actuation speed is to develop a stretchable and flexible electrode having high electrical conductivity and electrochemical capacitance. Here, a functionally antagonistic hybrid electrode with hollow tubular graphene meshes and nitrogen‐doped crumpled graphene is newly reported for superior ionic soft actuators. Three‐dimensional network of hollow tubular graphene mesh provides high electrical conductivity and mechanically resilient functionality on whole electrode domain. On the contrary, nitrogen‐doped wrinkled graphene supplies ultrahigh capacitance and stretchability, which are indispensably required for improving electrochemical activity in ionic soft actuators. Present results show that the functionally antagonistic hybrid electrode greatly enhances the actuation performances of ionic soft actuators, resulting in much larger bending deformation up to 620%, ten times faster rise time and much lower phase delay in a broad range of input frequencies. This outstanding enhancement mostly attributes to exceptional properties and synergistic effects between hollow tubular graphene mesh and nitrogen‐doped crumpled graphene, which have functionally antagonistic roles in charge transfer and charge injection, respectively.     

A fully integrated switched-capacitor DC–DC converter with hybrid output regulation

In this paper, we propose a fully integrated switched-capacitor (SC) DC–DC converter with hybrid output regulation that allows a predictable switching noise spectrum. The proposed hybrid output regulation method is based on the digital capacitance modulation for fine regulation and the automatic frequency scaling for coarse regulation. The automatic frequency scaler and on-chip current sensor are implemented to adjust the switching frequency at one of the frequencies generated by a binary frequency divider with change in load current. Thus, the switching noise spectrum of the proposed SC DC–DC converter can be predicted over the entire load range. In addition, the bottom-plate losses due to the parasitic capacitances of the flying capacitors and the gate-drive losses due to the gate capacitances of switches are reduced at light load condition since the switching frequency is automatically adjusted. The proposed SC DC–DC converter was implemented in a 0.13 µm CMOS process with 1.5 V devices, and its measurement results show that the peak efficiency and the efficiency at light load condition are 69.2% and higher than 45%, respectively, while maintaining a predictable switching noise spectrum.     

Analysis of TFCI coding performance of WCDMA systems

In WCDMA system, transport format combination indicator (TFCI) is encoded by an encoder based on Reed-Muller code and transmitted in a time-multiplexed fashion. When the number of TFCI information bits is less than the number of input bits to encoder, subset of basis sequences should be used. In this paper, we analyze the upper bound of word error rate (WER) of TFCI coding and find the optimal subset of basis sequences that minimize the bound. Since the bound of the WER is dependent on the channel correlation, the basis sequences to mitigate the effect of channel correlation are selected. From the result, a basis sequence with uniformly distributed bit of '1' is firstly chosen in optimal subset of basis sequences. The choice of basis sequences is more significant for high mobile speed.     

Effect of tap spacing on the performance of direct-sequencespread-spectrum RAKE receiver

The effect of tap spacing on the performance of a RAKE receiver is analyzed analytically in a frequency-selective fading channel. A continuous time multipath fading channel model is used for the analysis, and the expression of the correlation between the desired signals, interference signals, and noise signals at the output of each branch of the RAKE receiver is derived for various chip waveforms. Since the noise components of each branch signal are correlated to each other, an optimum combining rule based on the maximum-likelihood criterion is derived to gain utmost performance. It is shown that the performance of the system can be improved by setting the tap spacing of the RARE receiver below the chip duration when the bandwidth of the transmitted signal is larger than the inverse of the chip duration. Also, it is shown that the normalized capacity of the system can be increased by using a chip waveform occupying wider bandwidth, which takes advantage of the increased diversity gain merits of a wide-band code-division multiple-access system at the same chip rate. It is noted that the derived combining rule gives diversity gain against the fading process as well as noise whitening processing gain against multiple-access interference at the same time