Spreading code protocols for distributed spread-spectrum packetradio networks

Spreading code protocols for a distributed spread-spectrum packet radio network are presented. A distributed single-hop system (i.e. each terminal can hear all other terminals) with the users approximately synchronized and a set of prespecified spreading codes are presented. The spreading code protocol is a policy for choosing a spreading code to be used, given that a terminal has a packet to send, and a policy for monitoring spreading codes, given that a terminal is idle. A slotted system where a packet occupies a number of slots is considered, and two protocols that involve changing the spreading code of a transmission after an initial header is transmitted are presented. In one protocol, the header is transmitted on a common code, and in the other it is transmitted on a receiver-based code, the rest of the packet being transmitted on a transmitter-based code. In the receiving mode, a terminal monitors either a common code, in the first case, or a receiver-based code in the latter. Upon recognizing its own address and the source address, the receiver dynamically switches to a despreading code corresponding to the source. Throughput results are obtained for the case of geometrically distributed packet lengths     

Simultaneous compression and amplification of ultrashort pulses atMHz repetition rates

The authors report simultaneous compression and chirped pulse amplification at megahertz repetition rates of ultrashort pulses using a cavity-dumped dye laser with a pumping argon ion laser as the amplifier. In a single stage, the pulse is compressed by a factor of 25 and the energy per pulse is simultaneously increased by a factor of 2, without reducing the repetition rate. Starting with a pulse that has a duration of 4.3 ps and a peak power of 5.8 kW, a pulse with a duration of 170 fs and a peak power of 294 kW is obtained. A second stage of pulse compression is used to obtain a broadband pulse with a duration >~50 fs and a peak power of ~100 kW at 2 MHz     

La synthèse d’antennes à deux réflecteurs conformés à alimentation décalée

A potentially economic method for upgrading the gain of the large earth reflector antenna Cassegrain system to a gain comparable to that obtainable with a dualshaped reflector antenna system is presented herein. It involves a redesign of only the subreflector portion of a Cassegrain antenna or the introduction of a subreflector feed system for a paraboloid. A pair of offset subreflectors are synthesized which will give a controllable high gain amplitude distribution in the aperture of the large paraboloid. The synthesis method that is used is based on an approximate formulation for an offset dual shaped high gain antenna where the geometrical optics energy was scattered from a subreflector and then from a second large reflector which reflected a uniform phase distribution. In the present offset dual shaped subreflector (DSS) antenna, the second reflection is from a smaller subreflector and it scatters a spherical wave that feeds a hyperboloid or feeds a large paraboloid directly. Excellent results are shown for the approximate synthesis of the DSS.     

Synthesis of the fields of a transverse feed for a spherical reflector

The problem of designing a transverse feed for a spherical reflector is considered and a method is presented for synthesizing the fields on a surface of a sphere enclosing a feed that will produce a specified reflected field at the surface of a spherical reflector. The method identifies the reflector and a spherical surface enclosing the feed as a boundary value problem and uses a finite set of spherical waves to approximate the boundary conditions. A feed designed to excite this field will in turn produce the desired reflected field at the surface of the reflector, under the condition that that portion of the reflected field which is scattered by the feed may be neglected. It is shown that the feed need produce only a small part of the synthesized field to obtain an antenna efficiency of more than 70 percent. Some typical field distributions will be shown so as to indicate a method for designing a feed and to point out the correlation between the polarization of the synthesized field and the polarization of the reflected field at the surface of the reflector.     

The Matrix Formulation of Scattering Problems

Two regions in space are coupled through an opening in a perfectly conducting surface. By using a complete set of eigenvectors in the opening, each region can be represented by an equivalent Norton circuit involving a short-circuit current (a vector) and a generator admittance (a matrix). The particular case of a cavity at resonance is investigated. Application to a cavity terminated in a waveguide is considered, and the transformation of the equivalent circuit resulting from the shift of the terminal plane is analyzed. After solving the example of a slotted waveguide, a possible set of eigenvectors for an arbitrary opening is proposed.     

Equivalent transformations for the mixed lumped Brune-type sectionand uniform transmission line

As an analytical method for nonuniform transmission lines (NTLs) equivalent transformations are extended to a more general case, namely a mixed lumped Brune-type section and a uniform transmission line (unit element, UE). Circuits consisting of a cascade connection of a lumped Brune section and a UE are equivalent to one consisting of a cascade connection of a nonuniform transmission line whose characteristic impedance distribution is expressed with a trigonometric function and a lumped Brune section. This equivalent transformation method is easily applied to a circuit consisting of a lumped C section and a UE. The equivalent circuit is a circuit consisting of an NTL and a lumped C section. In this case, the characteristic impedance distribution of the NTL may be expressed in terms of a hyperbolic function. Exact network functions of the NTLs are easily obtained from the equivalent circuits without solving the telegrapher's equation. By considering the limiting case of these equivalent transformations, equivalent transformations for circuits consisting of a cascade connection of a lumped resonance circuit and a circuit and a uniform transmission line are derived     

Unifying probabilistic and variational estimation

A maximum a posteriori (MAP) estimator using a Markov or a maximum entropy random field model for a prior distribution may be viewed as a minimizer of a variational problem.Using notions from robust statistics, a variational filter referred to as a Huber gradient descent flow is proposed. It is a result of optimizing a Huber functional subject to some noise constraints and takes a hybrid form of a total variation diffusion for large gradient magnitudes and of a linear diffusion for small gradient magnitudes. Using the gained insight, and as a further extension, we propose an information-theoretic gradient descent flow which is a result of minimizing a functional that is a hybrid between a negentropy variational integral and a total variation. Illustrating examples demonstrate a much improved performance of the approach in the presence of Gaussian and heavy tailed noise. In this article, we present a variational approach to MAP estimation with a more qualitative and tutorial emphasis. The key idea behind this approach is to use geometric insight in helping construct regularizing functionals and avoiding a subjective choice of a prior in MAP estimation. Using tools from robust statistics and information theory, we show that we can extend this strategy and develop two gradient descent flows for image denoising with a demonstrated performance.     

New Address-While-Display Driving Method for High Contrast Ratio and Wide Operation Margin PDP Using Short Ramp Reset (SRR) and Short Pulse Erase (SPE) Schemes

We propose a new address-while-display (AWD) driving method for a plasma display panel to obtain a high contrast ratio and a wide operation margin, which is composed of a short ramp reset (SRR) period, a short erase period, a sustain period, and an address period as the basic units. The SRR pulse and the short pulse erase period make it possible to obtain a wide operating voltage margin and minimize the background luminance by redistributing the wall charges in a short initialization time between the address and the scan electrode. As a result, a high darkroom contrast ratio of 10 000 : 1 could be achieved with a wide operating voltage margin of 40 V for a stable address.      

A bi-directional transceiver PIC for ping-pong local loop configurations operating at 1.3-/spl mu/m wavelength

We propose and demonstrate a simple, potentially low-cost, photonic integrated circuit that can operate as a transceiver in a ping-pong optical data-link configuration. The device is designed for uncooled operation, with a gain section, a detector, and a beam expander inside a Fabry-Perot cavity. The detector has a 1.4 /spl mu/m quaternary layer with a broad spectral responsivity range extending to 33 nm higher than the lasing wavelength of the device. The transmitter can provide 8 dBm inside a single-mode fiber, and the beam expander relaxes the alignment tolerance to a single-mode fiber.     

Plane wave propagation in a uniaxial bianisotropic medium with an application to a polarization transformer

Uniaxial bianisotropic medium is a generalization of the well-studied bi-isotropic and chiral media. It is obtained, for example, when microscopic helices with parallel axes are positioned in a host dielectric in random locations. Plane wave propagation in such a medium is studied and a simple solution for the dispersion equation and for the eigenwaves are found. As a numerical example, polarization properties of a transverse wave propagating in a uniaxial bianisotropic medium is considered. The results give a simple possibility to construct a polarization transformer with a transversely uniaxial chiral medium for changing the polarization of a propagating plane wave.     

nc-Ge/Si岛基光子晶体单点缺陷腔的数值模拟与分析

阐述了利用光子晶体单点缺陷微腔来提高Ge/Si纳米岛发光效率的机理.通过3D FDTD方法计算出在平板厚度为300nm时,谐振波长随a和r/a变化的规律,即当给定r/a,h时,波长随晶格常数成次线性增加;当给定a,h时,波长随r/a的增加而减小.并从理论上给予分析.     

Noncontact AE measurement system using acoustic microscope

The surface of a slide glass (a specimen) with a notch was set in water at the focal region of a point-focus-beam lens to detect the AE (acoustic emission) signals radiating from cracks around the notch during a three point bending test. Output signals received by a transducer with a centre frequency around 50 MHz were amplified and A/D converted at a sampling rate of 200 MHz by a digital oscilloscope. The power spectra of the output signals were compared for the two cases with and without the weight used for the application of fracturing stress on the glass specimen. AE signals with a frequency range from a few megahertz to 100 MHz were successfully detected with this system for the glass specimen mounted on a small three point bending test bench.<>     

Performance analysis by simulation of floating-gate MOSFETs applied on a bidirectional associative memory architecture

Floating-gate MOSFETs (FGMOSFETs) are devices that can be electrically programmable and have a non-volatile characteristic. This feature can be adopted to configure a basic cell performing as a variable resistance that can be applied in artificial neural networks as a synapse. Based on a simple model and considering the coupling coefficient of the structure as the gain of a voltage controlled voltage source, the electrical characteristics of a floating-gate MOSFET can be simulated in PSpice and an artificial neural net, such as the bidirectional associative memory (BAM), can be implemented. Therefore a performance analysis of the net may be done with different sets of threshold voltages for the FGMOSFETs configured as a CMOS inverter used as a synapse. The objective is to know pattern pairs in a bidirectional way. The result is a correlation matrix for the BAM as a function of an electrical parameter of the devices, which is directly related to the respective matrix calculated by the matrix dot product, using the method outlined by Kosko.     

MEMS红外探测器响应时间测量方法研究

传统地,MEMS红外探测器响应时间的测量需要基于黑体辐射源、斩波器、水冷装置等设备搭建一套复杂的测量系统,然而斩波器的遮挡区域和透光区域具有一定的面积,其按某频率工作时会消耗一定的时间,而测试所得的器件响应时间无法排除斩波器的工作耗时,导致测试结果存在较大误差,所测响应时间为14.46ms。为解决这一问题,提出了一种以钛宝石激光器为辐射光源,利用声光调制器构建纳秒级激光脉冲,MEMS红外探测器响应激光脉冲的作用输出脉冲电信号,很好地规避了测量系统中设备工作耗时引入的时间参数,所测响应时间仅为3.13ms。由此可见,传统方法中斩波器工作耗时引入的时间误差甚至超过器件响应时间的300%,充分证明了此方法可以有效解决这一问题,进而为MEMS红外探测器以及其它光学探测器性能参数的测试与计量提供了一种新的方法。     

Electrically steerable passive array radiator (ESPAR) antennas

A novel ground-plane shape to improve the horizontal gain for electrically steerable passive array radiator (ESPAR) antennas is investigated. The peak directivity of a monopole antenna with a finite ground plane, such as a circular or rectangular ground plane is at a higher angle from the horizon. Thus, the horizontal gain is lower than that of a dipole antenna. Using a circular ground plane of a half-wavelength radius with a skirt of a quarter wavelength wound around it, the angle of the peak directivity and the horizontal gain for the ground-based communication system were improved. For an ESPAR antenna - one of the monopole arrays - the horizontal gain was also improved. It was confirmed that it was possible to form the main-beam radiation and a beam in the horizontal plane, each in an arbitrary direction, by changing the control voltage to the passive element. This was shown by an experiment with an ESPAR antenna with a finite reflector, a feed element, and seven passive elements.     

A Study on Straight-Line Tracking and Posture Control in Electric Bicycle

The development of automatic control for driving a bicycle is a challenging theme and is expected to be a human assist system. Previously, an acceleration-based method for stabilizing bicycle posture was proposed by the authors. In the experiments with this controller, the posture of the bicycle might be stabilized, but it is impossible to run on the desired trajectory, because there is no consideration with respect to a trajectory control. For the sake of expanding this system into more sophisticated equipment, a realization of the trajectory control for the bicycle is important. From the viewpoint of an assist system for human motion, a unified control of posture and trajectory brings a sophisticated function to a bicycle, and a high-performance bicycle is expected to be a convenient vehicle, similar to a small car. This paper proposes two strategies to stabilize bicycle posture and trajectory control that realizes a straight-line tracking: one is a lateral velocity controller, and the other is a steering function controller. The validity of the proposed approaches is evaluated by simulations and experiments.     

Adaptive transmission with finite code rates

This work examines a transmission system which adapts a finite set of code rates and a continuously varying transmit power. We propose a technique for finding the average reliable throughput (ART)-maximizing policy satisfying an average power constraint for a slow fading additive white Gaussian noise (AWGN) channel. ART is a measure motivated by the information outage and can, for example, be argued to characterize the long-term average throughput of a data packet transmission system with a transmit queue and a feedback protocol which requests retransmission of erroneously received packets. Given the size of the code rate set L, the ART-maximizing policy has the following properties. 1. For a given set of code rates, the optimum allocation policy suggests quantizing the fading state space into a set of L+1 corresponding intervals. For each quantization interval the optimal policy specifies a minimum transmitted power assignment which guarantees zero information outage. The optimum average power assignments across quantization intervals have a waterfilling relationship with respect to the interval channel quality measure. 2. The joint optimization of quantization intervals and the corresponding rate assignments are shown to have multiple local maxima. Nevertheless, this optimization problem can be reduced to a simple one-dimensional search over a parameter which determines the outage interval. Numerical results show that, in a Rayleigh-fading channel, there is only a 1-dB gap between the ergodic capacity and the throughput of a two-rate adaptive transmission system when the throughput is less than 6 bits/s/Hz. A special case of our optimal policy assignment is the optimal power and rate policy for an adaptive M-QAM system.     

Remote-controlled platoon merging via coder-estimator sequence algorithm for a communication network

In this paper, a platoon-merging control system is considered as a remotely located system with a state represented by a stochastic process. In this system, it is common to encounter situations where a single decision maker controls a large number of subsystems, and observation and control signals are sent over a communication channel with finite capacity and significant transmission delays. Unlike a classical estimation problem where the observation is a continuous process corrupted by additive noise, there is a constraint that the observation must be coded and transmitted over a digital communication channel with finite capacity. A recursive coder-estimator sequence is a state estimation scheme based on observations transmitted with finite communication capacity constraint. In this paper, we introduce a stochastic model for the lead vehicle in a platoon of vehicles in a lane considering the angle between the road surface and a horizontal plane as a stochastic process. In order to merge two platoons, the lead vehicle of the following platoon is controlled by a remote control station. Using the coder-estimator sequence, the remote control station designs the feedback controller. The simulation results show that the inter-vehicle distance and the deviation from the desired inter-vehicle distance are well regulated.     

A Picture Browsing System

We describe a system for storing pictures in a database and retrieving them from a remote location over a low bit-rate channel. Even with compression techniques, it takes a long time to transmit a picture, making search for a picture of desired attributes time-consuming. Our system improves the human interaction with the picture database by constructing an auxiliary text database containing a list of attributes of each picture, a hierarchical encoder-decoder, and a light pen to select the areas of picture buildup. Picture searching (or browsing) takes place in two stages: in the first stage, knowing the required picture attributes, a user selects a subset of pictures by matching attributes to the text database; in the second stage, these selected pictures are displayed hierarchically so that a low resolution picture is reproduced first and made sharper gradually. A light pen allows the user to give priority to upgrade selected areas or reject a picture. Our techniques of hierarchical coding are simple to implement. Informal tests indicate that it is much easier to browse through a picture database using this system and that the time to retrieve a picture of given attributes decreases considerably compared to sequential picture presentation.