Analysis of low bit rate image transform coding

Calculations based on high-resolution quantizations prove that the distortion rate D(R¯) of an image transform coding is proportional to 2^-2R when R¯ is large enough. In wavelet and block cosine bases, we show that if R¯<1 bit/pixel, then D(R¯) varies like R¯^1-2γ, where γ remains of the order of 1 for most natural images. The improved performance of embedded codings in wavelet bases is analyzed. At low bit rates, we show that the compression performance of an orthonormal basis depends mostly on its ability to approximate images with a few nonzero vectors     

Cultural adaptation and information design: two contrasting views

Approaches to using visual language in a cultural context can be placed on a continuum, with global (universal) on one end and culture-focused on the other. Each approach reveals contrasting assumptions about three central design issues: perception, aesthetics and pragmatics. The global approach is characterized by attempts to invent an objective, universal visual language or to define such a language through perceptual principles and empirical research. The culture-focused perspective is founded on the principle that visual communication is intimately bound to experience and hence can function only within a given cultural context, to which designers must be sensitive. While the modernist, universal approach has been losing ground to the postmodern, culture-focused approach, the two complement each other in a variety of ways and, depending on the rhetorical situation, offer pragmatic benefits and drawbacks     

UHF频段卫星移动通信系统若干问题的探讨

本文根据我国陆海空移动用户的需求与特点，借鉴国外经验，提出了建设ＵＨＦ频段卫星移动通信系统的建议；对照已建成的Ｃ波段卫星固定通信系统，分析了其优缺点；最后深入探讨了应用中的若干技术问题。     

Optimization of modulation bandwidth in DBR lasers with detunedBragg reflectors

The dynamical behavior of distributed Bragg reflector lasers with detuned Bragg reflectors is investigated theoretically. The model is based on the traveling wave equations, which are solved by expanding the solution in terms of the longitudinal eigenmodes. It is shown that for a drastic enhancement of the modulation bandwidth the interplay of the dominant mode and one side mode has to be enforced. Under these conditions, the modulation bandwidth can be enhanced to more than 70 GHz     

Distributed autonomous wireless channel assignment algorithm withpower control

Local autonomous dynamic channel allocation (LADCA) including power control is essential to accommodating the anticipated explosion of demand for wireless. The authors simulate call performance for users accessing channels in a regular cellular array with a base located at the center of each hexagon. The computer model includes stochastic channel demand and a propagation environment characterized by attenuation with distance as well as shadow fading. The study of LADCA shows that distributed power control and channel access can be combined in an access management policy that achieves satisfactory system capacity and provides desired call performance. The authors report: LADCA/power control is observed to be stable alleviating a major concern about users unaware of the signal to interference problems their presence on a channel might cause to others. There can be substantial inadvertent dropping of calls in progress caused by originating calls. Modeling user time dynamics is essential. LADCA contrasts very favorably with fixed channel allocation (FCA) in a comparative example     

A poincaré-bendixson theorem for analytic families of vector fields

We provide a characterization of the limit periodic sets for analytic families of vector fields under the hypothesis that the first jet is non-vanishing at any singular point. Also, applying the family desingularization method, we reduce the complexity of some of these sets.     

Self and mutual admittance of slot antennas on a dielectric half-space

In this paper, an efficient implementation of the spectral domain moment technique is presented for computing the self and mutual coupling between slot antennas on a dielectric half-space. It is demonstrated that by the proper selection of the weighting functions in the method of moments, the analytic evaluation or simplification of the transverse moment integrals is enabled. This results into a significant reduction of the required computational labor. The method is then utilized in order to provide design data for the self and mutual admittances between two slot antennas on a dielectric substrate lens in the case of fused quartz (∈ _r =3.80), crystal quartz (∈ _r =4.53), silicon (∈ _r =11.9) and GaAs (∈ _r =12.8). The presented technique and associated results are useful when designing twin slot quasi-optical receivers, imaging arrays, phased arrays or power-combining arrays of slot elements at millimeter-wave frequencies.     

Influence of the sulfonate countercation on the thermal stability of nafion perfluorosulfonate membranes

We report a strong dependence of the thermal stability of Nafion® perfluorosulfonate ionomer on the nature of the counterion associated with the fixed sulfonate site. These results were obtained using thermal gravimetric analysis on a series of alkali metal and alkyl ammonium cation-exchanged Nafion films. We have found that the temperature of decomposition of Nafion is inversely dependent on the size of the exchanged cation; i.e., Nafion films show improved thermal stability as the size of the counter cation decreases. We attribute this inverse relationship of thermal stability with counterion size to an initial decomposition reaction which is strongly influenced by the strength of the sulfonate-coun-terion interaction. © 1993 John Wiley & Sons, Inc.     

Experimental studies of microwave amplification in the ion-focusedregime

Studies of microwave amplification with an in-focused electron beam drawn from an induction injector are reported. A free-electron laser (FEL) operating at 9.4 GHz and employing ion-focusing within the interaction region has achieved power in excess of 30 MW at 9.4 GHz, with a beam energy of 0.8 MeV and current of 0.7 kA. Peak gain is 20 dB/m, with no saturation after 15 wiggler periods. Also reported are the first evolution and detuning data for an ion-channel laser/maser (ICL). Two shortcomings of the prematurely halted ICL studies are poor frequency discrimination and a large axial plasma gradient. Prospects for operation with an upgraded 1.6 MeV accelerator are discussed     

Pseudo-epitaxial C60 films prepared by a hot-wall method

We have used both reflection-geometry and grazing-incidence-geometry X-ray scattering to study thin films of C₆₀ evaporated onto mica substrates via a hot-wall technique. The growth mode yields close-packed C₆₀ planes, which are parallel to the substrate surface and which exhibit out-of-plane correlation lengths of 850 Å. In the film plane the C₆₀ is at best pseudo-epitaxial, with a 0.9° distribution of crystallite orientations, a 450 Å in-plane correlation length, and a 3.7% lattice mismatch, better than obtained by other thin film techniques but far from the accepted definition of single crystal thin film epitaxy.