Combined Use of Magnetic and Electrically Conductive Fillers in a Polymer Matrix for Electromagnetic Interference Shielding

The combined use of a highly magnetic filler (mumetal) in low proportion and a highly conductive filler in high proportion in a polymer matrix provides a composite material that is more effective for electromagnetic interference shielding than the use of a highly magnetic filler alone or the use of a highly conductive filler alone. Mumetal is effective (due to absorption) when it is in a composite material of DC electrical resistivity below 10 Ω cm, as provided by conductive fillers, which contribute to shielding by reflection and allow paths for eddy current.     

Tunable Capacitor Based on Polymer-Dispersed Liquid Crystal for Power Harvesting Microsystems

A tunable capacitor based on polymer-dispersed liquid-crystal (PDLC) technology is presented in this paper. Its application for robust power harvesting microsystems was investigated. The power harvesting device utilized a piezoelectric microcantilever excited by ambient random vibrations to convert mechanical energy into electric power. For improving the power harvesting efficiency, the PDLC tunable capacitor was used to adjust the resonance frequency of the piezoelectric microcantilever beam to match the frequency of the ambient vibrations in real time. The fabrication process and measurement results of the PDLC tunable capacitor are detailed. The measured tuning ratio of the PDLC tunable capacitor was 63% at 300-Hz excitation frequency when a 25-V driving voltage was applied. The dielectric and optical properties of the fabricated PDLC tunable capacitor have been examined thoroughly. Based on the results of the experiment, an equivalent lumped-element model of the PDLC tunable capacitor has been developed. The simulation results showed that the impedance of the developed model agreed well with that of the fabricated tunable capacitor. This model can be incorporated into the equivalent circuit of the integrated power harvesting system for efficiency optimization.     

Coherent frequency/phase modulation scheme

An L-phase-levelled coherent frequency/phase modulation (LCFPM) scheme is proposed and analysed for additive white Gaussian noise channels. The optimised LCFPM scheme with L>2 is found to have superior bandwidth efficiency to the conventional dual-frequency quadrature frequency/phase modulation scheme     

Optimal operation of coreless PCB transformer-isolated gate drivecircuits with wide switching frequency range

Gate drive circuits for power MOSFETs and insulated gate bipolar transistors (IGBTs) often require electrical isolation. Coreless printed circuit board (PCB) transformers have been shown to have desirable characteristics from a few hundreds of hertz to a few megahertz and can be used for both power and signal transfer at low-power level. At low operating frequency, the magnetizing inductance has such low impedance that the driving power requirement could become excessive. This paper describes the use of a coreless PCB transformer for isolated gate drive circuits over a wide-frequency range. Based on a resonance concept, the optimal operating condition that minimizes the power consumption of the gate drive circuits is developed and verified with experiments. The coreless PCB transformer demonstrated here confirms a fundamental concept that the size and volume of a magnetic core could approach zero and become zero if the operating frequency is sufficiently high. Coreless PCB transformers do not require the manual winding procedure and thus simplify the manufacturing process of transformer-isolated gate drive circuits and low-power converters. Their sizes can be much smaller than those of typical core-based pulse transformers. The electrical isolation of a PCB is much higher than that of an optocoupler     

Isometry-Based Shape-Adaptive Fractal Coding for Images

Fractal image coding is an effective method to eliminate the image redundancy through piecewise self-transformability. The fractal code consists of a set of contractive affine transforms. To improve the performance when a range block experiences large error, we usually partition the range block into square or nonsquare subrange blocks for two- or multilevel fractal coding. In this paper, we find an inherent property of fractal coding that can be used to decide the edge orientation of a range block. Then this property is used for shape-adaptive fractal coding (SAFC). In SAFC, the top-level range block is partitioned into square or nonsquare (rectangle or triangle) subrange blocks for multilevel fractal encoding. Here, the maximum size of the range block can be the same as that of the whole image size while the minimum size is 4×4. In SAFC, no additional computations are required to obtain the edge orientation of a range block. Instead, we propose an edge-orientation detector, where the edge orientation of a range block is obtained during the fractal encoding process. According to our simulation results, SAFC can reduce the bit rate requirement of the conventional fractal coding scheme.     

Satellite UMTS/IMT2000 W-CDMA air interfaces

The third-generation IMT-2000 initiative of the International Telecommunication Union is aiming at the provision of a limited family of global standards providing worldwide multimedia wireless services in a host of environments encompassing indoor picocells to satellite megacells. The ITU IMT-2000 initiative has been supported by several regional standardization bodies such as the European Telecommunication Standard Institute with its Universal Mobile Telecommunication System. In this article a few air interface proposals for the satellite component of UMTS/IMT-2000 based on adaptation of the emerging terrestrial wideband CDMA standards are reviewed. In particular, S-UMTS requirements are examined together with W-CDMA applicability to the satellite environment. It is shown that with minor adaptations, the terrestrial W-CDMA air interface provides an efficient solution for the satellite UMTS component. This commonality will certainly help in the realization of low-cost low-size dual-mode user terminals encompassing both terrestrial and satellite components     

Effect of IQ Mismatch Compensation in an Optical Coherent OFDM Receiver

The performance enhancement using the Gram–Schmidt orthogonalization procedure (GSOP) for compensating transmitter (Tx) and receiver (Rx) in-phase/quadrature (IQ) mismatch in the coherent optical orthogonal frequency-division-multiplexing system is investigated. The analytic result containing frequency offset is presented to describe the different behavior of Tx and Rx IQ mismatch. Bit-error-rate curves measured at back-to-back and 1040-km transmission over single-mode fiber indicate that the GSOP is more efficient to compensate of Rx IQ mismatch than Tx IQ mismatch.      

Direct Physical Imaging and Chemical Probing of LiFePO4 for Lithium‐Ion Batteries

The control of unexpectedly rapid Li intercalation reactions without structural instability in olivine‐type LiFePO₄ nanocrystals is one of the notable scientific advances and new findings attained in materials physics and chemistry during the past decade. A variety of scientific studies and technological investigations have been carried out with LiFePO₄ to elucidate the origins of many peculiar physical aspects as well as to develop more effective synthetic processing techniques for better electrochemical performances. Among the several features of LiFePO₄ that have attracted much interest, in this article we address four important issues—regarding doping of aliovalent cations, distribution of Fe‐rich secondary metallic phases, nanoparticle formation during crystallization, and antisite Li/Fe partitioning—by means of straightforward atomic‐scale imaging and chemical probing. The direct observations in the present study provide significant insight into alternative efficient approaches to obtain conductive LiFePO₄ nanocrystals with controlled defect structures.     

Secure group communications using key graphs

Many emerging network applications are based upon a group communications model. As a result, securing group communications, i.e., providing confidentiality, authenticity, and integrity of messages delivered between group members, will become a critical networking issue. We present, in this paper, a novel solution to the scalability problem of group/multicast key management. We formalize the notion of a secure group as a triple (U,K,R) where U denotes a set of users, K a set of keys held by the users, and R a user-key relation. We then introduce key graphs to specify secure groups. For a special class of key graphs, we present three strategies for securely distributing rekey messages after a join/leave and specify protocols for joining and leaving a secure group. The rekeying strategies and join/leave protocols are implemented in a prototype key server we have built. We present measurement results from experiments and discuss performance comparisons. We show that our group key management service, using any of the three rekeying strategies, is scalable to large groups with frequent joins and leaves. In particular, the average measured processing time per join/leave increases linearly with the logarithm of group size