Bell Labs

About: Bell Labs is a based out in . It is known for research contribution in the topics: Laser & Optical fiber. The organization has 36499 authors who have published 59862 publications receiving 3190823 citations. The organization is also known as: Bell Laboratories & AT&T Bell Laboratories.

Effect of fading correlation on adaptive arrays in digital mobile radio

Abstract: In this paper, we investigate the effect of correlations among the fading signals at the antenna elements of an adaptive array in a digital wireless communication system. With an adaptive array, the signals received by multiple antennas are optimally weighted and combined to suppress interference and combat desired signal fading. Previous results for flat and frequency-selective fading assumed independent fading at each antenna. Here, we present a model of local scattering around a mobile where the received multipath signals arrive at the base station within a given beamwidth, and derive a closed-form expression for the correlation as a function of antenna spacing. Results show that the degradation in performance with correlation in an adaptive array that combats fading and suppresses interference is only slightly larger than that for combating fading alone, i.e., with maximal ratio combining. This degradation is small even with correlation as high as 0.5. >

Simple models for reading neuronal population codes

Abstract: In many neural systems, sensory information is distributed throughout a population of neurons. We study simple neural network models for extracting this information. The inputs to the networks are the stochastic responses of a population of sensory neurons tuned to directional stimuli. The performance of each network model in psychophysical tasks is compared with that of the optimal maximum likelihood procedure. As a model of direction estimation in two dimensions, we consider a linear network that computes a population vector. Its performance depends on the width of the population tuning curves and is maximal for width, which increases with the level of background activity. Although for narrowly tuned neurons the performance of the population vector is significantly inferior to that of maximum likelihood estimation, the difference between the two is small when the tuning is broad. For direction discrimination, we consider two models: a perceptron with fully adaptive weights and a network made by adding an adaptive second layer to the population vector network. We calculate the error rates of these networks after exhaustive training to a particular direction. By testing on the full range of possible directions, the extent of transfer of training to novel stimuli can be calculated. It is found that for threshold linear networks the transfer of perceptual learning is nonmonotonic. Although performance deteriorates away from the training stimulus, it peaks again at an intermediate angle. This nonmonotonicity provides an important psychophysical test of these models.

Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films

Abstract: Calculations are made which explain qualitatively the multiphoton-assisted electron tunneling recently observed in superconducting diodes by Dayem and Martin. It seems to us that the microwave field is much too weak to cause any nonlinearities in the conduction current in the superconductors. Thus, the interaction does not cause transitions between electron states with different wave numbers. Rather, the energies of the electrons are varied adiabatically by the microwave fields. This gives rise to effective changes in the density of states versus energy which are dramatically illustrated in the tunneling current.Calculations are performed for three different possible forms of the field interaction. Qualitatively, the theory fits the experimental observations very well, but, as in the somewhat similar case of phonon-assisted tunneling, the largest postulated interaction seems about an order of magnitude too small to explain the observations on a quantitative basis.

Kinetics of Radiative Recombination at Randomly Distributed Donors and Acceptors

Abstract: The kinetics of the recombination of holes trapped on acceptors with electrons trapped on donors have been treated theoretically and the results compared with the observed low-temperature fluorescent decay of GaP doped with sulphur donors and silicon acceptors. The assumptions are made that the distribution of donors and acceptors is random and that the recombination rate depends exponentially on the donor and acceptor separation. The theoretical problem can be exactly solved when either the donor or acceptor is in excess, and an approximate solution is given for the case of exact compensation. The decay of the total pair emission has been measured over many factors of 10 of intensity and time and can be satisfactorily accounted for by the theory using only two adjustable parameters. If the decay occurs from a system in which all the donors and acceptors are not initially neutral, different decay curves are obtained which can be explained by assuming that the capture cross section of a pair for a hole or electron depends approximately on the square of the internuclear pair separation. The spectra have also been measured after flash excitation, and they are found to change in shape and position as a function of time. These changes can be quantitatively explained in terms of the simple theory, provided residual broadening effects from phonon interactions and other sources are included. The effects of a magnetic field at low temperature on the decay kinetics have been predicted and observed and allow the determination of certain $g$ values. At high-impurity concentrations there are deviations from simple behavior, evidently caused by complications in the chemical doping of the crystals. Experimental evidence for pair effects in the edge emission of CdS is also briefly reported. It is likely that similar decay phenomena observed in other seniconductors and phosphors can be explained on a similar basis without invoking arbitrary trap distributions. It is only necessary to assume a random distribution of donors and acceptors throughout the crystal.