共查询到20条相似文献,搜索用时 46 毫秒
1.
Energy transfer from selectively formed metastable states of SiO is used to pump sodium-atom laser amplifiers at λ≈569 nm (4d 2D -3p 2P ), λ≈616 nm (5s 2S -3p 2 P ), and λ≈819 nm (3d 2D -3p 2P ). The a 3Σ+ and b 3Π states of SiO are generated in high yield from the Si+N2O→SiO+N2 reaction. The energy stored in the triplet states is transferred in a highly efficient collisional process to pump sodium atoms to their lowest excited 3d 2 D , 4d 2D , and 5s 2S states. Adopting a sequence in which high concentrations of silicon and sodium atoms are mixed and oxidized, a continuous amplification (gain condition) is monitored which suggests the creation of a population inversion among the receptor sodium-atom energy levels and forms the basis for full cavity oscillation on the Na4 d 2D -3p 2P transition at 569 nm 相似文献
2.
Performance limits of coded multilevel differential PSK (MDPSK) in multipath Rayleigh fading channels are described. The simple Gaussian metric is assumed for reasons for practicality even though it is not the maximum likelihood. The channel cutoff rate, R 0 of MDPSK is analyzed based on the metric. Account is taken of additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread. For the analysis of the spectrum efficiency of a cellular mobile radio system employing coded MDPSK, its service area is defined as the area in which, with a bit rate of R information bit/symbol (R ⩽R 0), reliable communications are possible. Three optimal information bit rates are determined from the channel cutoff rate to minimize the required average signal energy per information bit-to-noise power spectral density ratio (E b/N 0) to maximize the tolerable r.m.s. delay spread τr.m.s. and to maximize the spectrum efficiency 相似文献
3.
Janssen A.J.E.M. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1992,38(1):168-171
It is shown how the Zak transform can be used to find nontrivial examples of functions f , g ∈L 2(R ) with f ×g ≡0≡F ×G , where F , G are the Fourier transforms of f , g , respectively. This is then used to exhibit a nontrivial pair of functions h , k ∈L2(R ), h ≠k , such that |h |=|k |, |H |=|K |. A similar construction is used to find an abundance of nontrivial pairs of functions h , k ∈L2 (R ), h ≠k , with |A h |=|A k| or with |W h|=|W k| where A h, A k and W h, W k are the ambiguity functions and Wigner distributions of h , k , respectively. One of the examples of a pair of h , k ∈L 2(R ), h ≠k , with |A h|=|A k| is F.A. Grunbaum's (1981) example. In addition, nontrivial examples of functions g and signals f 1≠f 2 such that f 1 and f 2 have the same spectrogram when using g as window have been found 相似文献
4.
A Kalman filter for optimal restoration of multichannel images is presented. This filter is derived using a multichannel semicausal image model that includes between-channel degradation. Both stationary and nonstationary image models are developed. This filter is implemented in the Fourier domain and computation is reduced from O (Λ 3N 3M 4) to O (Λ3N 3M 2 ) for an M ×M N -channel image with degradation length Λ. Color (red, green, and blue (RGB)) images are used as examples of multichannel images, and restoration in the RGB and YIQ domains is investigated. Simulations are presented in which the effectiveness of this filter is tested for different types of degradation and different image model estimates 相似文献
5.
Percival R.M. Phillips M.W. Hanna D.C. Tropper A.C. 《Quantum Electronics, IEEE Journal of》1989,25(10):2119-2123
The fluorescence spectrum of praseodymium in a silica host has been investigated through the direct excitation of the 3P 0, 1D 2, and 1G 4 absorption bands, which occur at around 488, 590, and 974 nm, respectively. The observed spontaneous emission consists of four main bands with peaks at 633, 707, 888, and 1080 nm. In each case the level principally responsible for these emissions was found to be 1D 2. Stimulated emission has been seen for the first time at 888 nm for a threshold of 10 mW absorbed power. In addition, the lasing characteristics at 1080 nm have been investigated 相似文献
6.
A report is presented of the thermal shifts of eleven of the twelve lines from the 4F 3/2 Stark energy levels to the 4I 11/2 energy levels in an Nd:YAG laser for a temperature change from 20-200°C. The thermal shift difference between the Stark sublevels R 1, R 2 in 4F 3/2 is found to be about -0.6±0.6 cm-1/100°C. Within experimental uncertainty, all of the lasing lines either moved to longer wavelength or remained unchanged with increasing temperature 相似文献
7.
Amari S.-I. Han T.S. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1989,35(2):217-227
A statistical inference problem for a two-terminal information source emitting mutually correlated signals X and Y is treated. Let sequences X n and Y n of n independent observations be encoded independently of each other into message sets M X and M Y at rates R 1 and R 2 per letter, respectively. This compression causes a loss of the statistical information available for testing hypotheses concerning X and Y . The loss of statistical information is evaluated as a function of the amounts R 1 and R 2 of the Shannon information. A complete solution is given in the case of asymptotically complete data compression, R 1, R 2→0 as n →∞. It is shown that the differential geometry of the manifold of all probability distributions plays a fundamental role in this type of multiterminal problem connecting Shannon information and statistical information. A brief introduction to the dually coupled e -affine and m -affine connections together with e -flatness and m -flatness is given 相似文献
8.
A routing architecture applying the concept of multichannel transmission groups (MCTGs) for ATM systems is proposed. A queuing analysis of an internally nonblocking ATM switch employing this MCTG concept with partially shared output buffers is presented. The analysis is based on the discrete-time D A///D /c /B queuing model. Both bulk input traffic bulk-size distribution (A ) and deterministic traffic (D 1 +. . .+D N) are considered. The impact of switch speedup on the performance is also taken into account. It is shown that the MCTG architecture yields better performance in terms of delay and cell loss probability than its single channel counterpart. It is also found that the switch speedup required to closely approximate the optimal performance obtained by having the switch fabric run N times as fast as the input and output channels, where N is the size of the switch, is rather small compared to N . This makes the practical realization of the proposed switch architecture feasible 相似文献
9.
The reliability function of a component whose lifetime is exponentially distributed with a known parameter λ>0 is R (t |λ)=exp (-λt ). If an environmental effect multiplies the parameter by a positive factor η, then the reliability function becomes R (t |η,λ)=exp(-ηλt ). The authors assume that η itself is random, and its uncertainty is described by a Dirichlet process prior D (α) with parameter α=MG 0, where M >O represents an intensity of assurance in the prior guess, G 0, of the (unknown) distribution of η. Under squared error loss, the Bayes estimator of R (t |η,λ) is derived both for the no-sample problem and for a sample of size n . Using Monte Carlo simulation, the effects of n , M , G 0 on the estimator are studied. These examples show that: (a) large values of n lead to estimates where the data outweigh the prior, and (b) large values of M increase the contribution of the prior to the estimates. These simulation results support intuitive ideas about the effect of environment and lifetime parameters on reliability 相似文献
10.
Woodward J.R. Cobb S.H. Shen K. Gole J.L. 《Quantum Electronics, IEEE Journal of》1990,26(9):1574-1587
Evidence is obtained which demonstrates the potential for developing purely chemical visible lasers based on rapid near-resonant energy transfer from metastable excited triplet states of germanium and silicon monoxide (a 3Σ+,b 3Π) to select metal atoms. In this study, the Group IIIA metal atoms were chosen as the energy receptors for the energy transfer-pump sequence. Excited triplet states were generated from the Ge-O3, Ge-N2, Si-N2O, and Si-NO2 reactions; the bulk of the experimental results was obtained with a germanium-based system. The energy stored in the long-lived triplet states is transferred to pump X 2P 1/2 thallium, indium, and gallium atoms to their lowest lying 2 S 1/2 states. The authors observe a system of temporal behavior which suggests the creation of a population inversion producing a gain condition and forming the basis for full cavity oscillation on the Tl 72S 1/2-6P 3/2 transition at 535 nm 相似文献
11.
12.
It is well known that on-off keying (OOK), i.e. the transmission of a pulse for a data one (mark) and transmission of no signal for a data zero (space), exhibits a 3 dB inferiority to antipodal signalling, in terms of the signal-to-noise ratio required to achieve a specified bit error probability. When the channel is characterized by the cutoff rate R 0, this 3 dB inferiority persists, in the sense that the required E b/N 0 to operate at a given value of R 0 is 3 dB greater for the OOK channel than for the antipodal channel. However, it is shown that, if the definition of cutoff rate is generalized in an appropriate way to reflect the fact that signal energy is expended only in the transmission of marks, part of this penalty is recovered. In fact, it is shown that, in the limit of low R 0, there is no penalty in signal-to-noise ratio. Even at rates of realistic interest, the penalty is significantly less than the expected 3 dB 相似文献
13.
Ashley T. Dean A.B. Elliott C.T. McConville C.F. Whitehouse C.R. 《Electronics letters》1988,24(20):1270-1272
Molecular-beam epitaxy has been used for the first time to fabricate np junctions in InSb grown onto p -type InSb (100) substrates. Diodes formed by the epitaxial growth of a silicon-doped layer on undoped homoepitaxial material exhibited a bulk generation-recombination-limited R 0A value of 105 Ω cm2 and D λpk * of 3×1012 cm Hz1/2 W-1 at liquid nitrogen temperature 相似文献
14.
Stimulated anti-Stokes Raman scattering at 145.7 nm in inverted Se is generated by photolysis of OCSe at 193 nm and the subsequent up-conversion of 205.1-nm pump radiation. The tuning profile is characterized with OCSe pressure, the energies and relative timing of the pump and photodissociation lasers, and CO and Ar buffer-gas pressure. The use of CO as a buffer gas is discovered to be detrimental to the anti-Stokes output. Sixteen competing stimulated emission channels are found to suppress anti-Stokes emission on exact 4p 4 1S 0-4p 3 5s 3 P 01 pump resonance 相似文献
15.
Shamai S. Bar-David I. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1989,35(5):1079-1084
A low-pass and a bandpass additive white Gaussian noise channel with a peak-power constraint imposed on otherwise arbitrary input signals are considered. Upper bounds on the capacity of such channels are derived. They are strictly less than the capacity of the channel when the peak-power constrain is removed and replaced by the average-power constraint, for which the Gaussian inputs are optimum. This provides the answer to an often-posed question: peak-power limiting in the case of bandlimited channels does reduce capacity, whereas in infinite bandwidth channels it does not, as is well known. For an ideal low-pass filter of bandwidth B , the upper bound is B log 0.934P /(N 0B ) for P/( N 0B )≫1, where P is the peak power of the input signal and N 0/2 is the double-sided power spectral density of the additive white Gaussian noise 相似文献
16.
Nonequiprobable signaling on the Gaussian channel 总被引:1,自引:0,他引:1
Calderbank A.R. Ozarow L.H. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1990,36(4):726-740
Signaling schemes for the Gaussian channel based on finite-dimensional lattices are considered. The signal constellation consists of all lattice points within a region R , and the shape of this region determines the average signal power. Spherical signal constellations minimize average signal power, and in the limit as N →∞, the shape gain of the N -sphere over the N -cube approaches πe /6≈1.53 dB. A nonequiprobable signaling scheme is described that approaches this full asymptotic shape gain in any fixed dimension. A signal constellation, Ω is partitioned into T subconstellations Ω0 , . . ., Ωτ-1 of equal size by scaling a basic region R . Signal points in the same subconstellation are used equiprobably, and a shaping code selects the subconstellation Ωi with frequency f i. Shaping codes make it possible to achieve any desired fractional bit rate. The schemes presented are compared with equiprobable signaling schemes based on Voronoi regions of multidimensional lattices. For comparable shape gain and constellation expansion ratio, the peak to average power ratio of the schemes presented is superior. Furthermore, a simple table lookup is all that is required to address points in the constellations. It is also shown that it is possible to integrate coding and nonequiprobable signaling within a common multilevel framework 相似文献
17.
Analysis is made of the effects of Doppler on the error rate performance of a low data rate binary FSK frequency hopping receiver, employing a discrete Fourier transform (DFT) technique for baseband detection. Bit detection decision is made by locating the maximum of the DFT outputs which, in the frequency domain, are assumed to be separated by 1/T where T is the bit period. Both the worst case and average error performances are obtained and presented as a function of E b/N 0 for various values of M where E b/N 0 is the signal bit energy-to-noise density ratio and M is the degree of freedom associated with the Doppler uncertainty window. The E b/N 0 degradation as a function of M is also presented 相似文献
18.
Roberts J.H. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》1988,34(6):1483-1490
A probability density function Pm(R 1,R 2,Δ) is presented for a narrowband noise process in which R 1 and R 2 are two envelope samples and Δ is the phase difference. For m =1 the process is Gaussian, but for m =2,3, etc., it is non-Gaussian. New second-order statistical properties are identified for it as well as the density function for the resulting envelope when a signal is added to the noise. These results are given, though the major concern is with the density of the phase difference Δ and the density of &thetas;, the response of an FM detector fed with the noise 相似文献
19.
Dau-Sing Wang 《Antennas and Propagation, IEEE Transactions on》1988,36(11):1594-1601
The asymptotic behavior of the scattering solutions for a point source radiating in the presence of a N -layered sphere is studied. A spectral integral representation is used to represent the potentials and the fields. The scattering solutions are approximated by the residue series arising from the poles of the integrand in the complex-order plane (v-plane). A diffraction factor D 0 (vp) is used to determine the significance of each pole in the evaluation of the scattering solutions. Numerical results illustrate the dependence of D 0(νp) on the geometric parameters and material properties. Results for multilayered cylinders are included for comparison 相似文献
20.
The author addresses the problem of computing the eigensystem of the modified Hermitian matrix, given the prior knowledge of the eigensystem of the original Hermitian matrix. Specifically, an additive rank-k modification corresponding to adding and deleting blocks of data to and from the covariance matrix is considered. An efficient and parallel algorithm which makes use of a generalized spectrum-slicing theorem is derived for computing the eigenvalues. The eigenvector can be computed explicitly in terms of the solution of a much-reduced (k ×k ) homogeneous Hermitian system. The overall computational complexity is shown to be improved by an order of magnitude from O (N 3) to O (N 2k ), where N ×N is the size of the covariance matrix. It is pointed out that these ideas can be applied to adaptive signal processing applications, such as eigen-based techniques for frequency or angle-of-arrival estimation and tracking. Specifically, adaptive versions of the principal eigenvector method and the total least squares method are derived 相似文献