Fundamental limits for information retrieval

The fundamental limits of performance for a general model of information retrieval from databases are studied. In the scenarios considered, a large quantity of information is to be stored on some physical storage device. Requests for information are modeled as a randomly generated sequence with a known distribution. The requests are assumed to be “context-dependent,” i.e., to vary according to the sequence of previous requests. The state of the physical storage device is also assumed to depend on the history of previous requests. In general, the logical structure of the information to be stored does not match the physical structure of the storage device, and consequently there are nontrivial limits on the minimum achievable average access times, where the average is over the possible sequences of user requests. The paper applies basic information-theoretic methods to establish these limits and demonstrates constructive procedures that approach them, for a wide class of systems. Allowing redundancy greatly lowers the achievable access times, even when the amount added is an arbitrarily small fraction of the total amount of information in the database. The achievable limits both with and without redundancy are computed; in the case where redundancy is allowed the limits essentially coincide with lower limits for more general storage systems     

Fundamental limits on EMC

Both fundamental and state-of-the-art limits are treated, with emphasis on the former. Fundamental limits result from both natural and man-made electromagnetic noise which then affect two basic ratios, signal-to-noise (S/N) and extraneous-input-to-noise (I/N). Tolerable S/N values are discussed for both digital and analog communications systems. These lead to tolerable signal-to-extraneous-input (S/I) ratios, again for digital and analog communications systems, as well as radar and sonar. State-of-the-art limits for transmitters include RF noise emission, spurious emissions, and intermodulation. Receiver limits include adjacent-channel interactions, image, IF, and other spurious responses, including cross modulation, intermodulation, and desensitization. Unintentional emitters and receivers are also discussed. Coupling limitations between undesired sources and receptors are considered from mechanisms including radiation, induction, and conduction.     

Fundamental limits and possibilities for future telecommunications

Using fundamental physical and information theoretical relations, we have considered fundamental capacity limits and possibilities of fiber optical, cellular radio, and satellite communication systems. In a fiber to the home scenario more than 1 Gb/s equivalent circuit-switched capacity may well be feasible in the future. In a microwave cellular or satellite radio network for mobile subscribers it may well be feasible, although requiring low-cost very advanced electronics, to reach several tens of megabits per second     

Fundamental limits of silicon technology

Measures of the performance of digital electronics have increased steadily for four decades. The essential ingredient of progress has been miniaturization. The road to advances beyond a decade into the future has always been obscure and has stimulated much speculation as to where miniaturization must end. Thus far, new ideas have regularly met the challenges posed by new problems and have allowed the trends to continue. The persistence of silicon as the basis of information technology is the one constant. The problems and solutions arising from changing physics caused by miniaturization are examined in the perspective of the environment in which millions of closely packed devices must function and the external world that information processing technology serves. The complex structure of devices offers considerable room for ingenuity and novel approaches. The large number of devices collected on a single chip of silicon must communicate with one another via a complex array of wires. Miniaturization of the wires poses a different set of questions. The simple structure of a wire offers less scope for invention than is possible with devices and it is more difficult to see solutions to problems. Nevertheless, as has happened in the past, it is difficult to find obviously insuperable barriers to progress in fundamental physics     

片状固体激光器的基模

片状固体激光器片内的光束在上下表面产生全内反射,按之字形路径传输,提高了激活介质的利用率,从而可获得较高的能量输出.更重要的是,与传统的棒状固体激光器相比,它能补偿激光介质内部因温度不均匀而造成的光学畸变,因而引起了人们的广泛注意.本文试图用矩阵光学来分析它的补偿性.同时对这种结构的基模进行分析.由于片状固体激光器的菲涅耳数N(?)1,因而适于用矩阵光学来进行分析.     

Fundamental limits in optical communication

This paper addresses fundamental limits on the ability to communicate over optical fiber and optical free-space channels. The underlying limits on the sensitivity of optical receivers dictated by quantum effects are reviewed. Receiver sensitivity limitations caused by background light, dark current, post detection amplifier noise, and transmitter imperfection are then considered. Finally the limitations of the channels related to input coupling, loss, and delay distortion are reviewed. Throughout, comparisons are made between the optical systems and their microwave counterparts to highlight differences and the origins of those differences.     

Fundamental limits in information theory

Shannon information theory is in essence a study of "fundamental limits." In this paper we attempt to give the reader a taste of the nature of these limits and of how they relate to real situations in communication.     

Criterion for improved linearity of 1.3-µm InGaAsP-InP buried-heterostructure lasers

Nonlinearities in tile light-current characteristics of planar-active buried-heterostructure (BH) lasers are associated with higher order mode transitions, spectral broadening and, in some cases, the onset of TM polarized stimulated emission. Measurements and calculations are presented which show that these nonlinearities appear at higher power in devices with reduced active volume. These results provide a practical guide to the fabrication of "kink-free" 1.3-μm InGaAsP buried-heterostructure lasers for use in high-bit-rate fiber communication systems.     

增益压缩因子及对半导体激光器的线性输出的影响

在简单地介绍了增益压缩因子是如何被人们引入速率方程组中去的之后，讨论了该因子对半导体激光器稳态特性的影响。根据压缩因子的大小重新考虑那些在速率方程组近似求解时曾被忽略的参量，对速率方程组进行了解析求解。结果表明，增益压缩因子只会使输出功率下降而不改变其线性，并且预言了腔内载流子密度高于阈值密度的可能性，而自发辐射的贡献才使激光器的输出功率与电流之间的线性关系被破坏。     

Fundamental performance limits in image registration

The task of image registration is fundamental in image processing. It often is a critical preprocessing step to many modern image processing and computer vision tasks, and many algorithms and techniques have been proposed to address the registration problem. Often, the performances of these techniques have been presented using a variety of relative measures comparing different estimators, leaving open the critical question of overall optimality. In this paper, we present the fundamental performance limits for the problem of image registration as derived from the Cramer-Rao inequality. We compare the experimental performance of several popular methods with respect to this performance bound, and explain the fundamental tradeoff between variance and bias inherent to the problem of image registration. In particular, we derive and explore the bias of the popular gradient-based estimator showing how widely used multiscale methods for improving performance can be explained with this bias expression. Finally, we present experimental simulations showing the general rule-of-thumb performance limits for gradient-based image registration techniques.     

Fundamental limits in digital information processing

Computing and information processing are limited by a variety of factors. Some of these, such as the need to dissipate energy, the high voltage requirement, and the difficulty of removing heat are fundamental in that they have a basis in physical theory and in knowledge of the properties of materials. Limits arising from the physics of devices are also in this class. It appears that many of these physical limits could be relaxed by operating circuits at low temperature. Another class of limits is based on the experiences of many independent workers who find that a large amount of communication among various parts of a system is required. The many long interconnections utilized create great complexity and play a dominant role in the power dissipation required in information processing.     

Dependence of threshold current and linearity on Bragg wavelengthfor grating-surface-emitting lasers

We report on a systematic experimental study of the dependence of the threshold current and power-current linearity on the detuning of the Bragg wavelength from the gain peak for grating-surface-emitting lasers. Minimum threshold currents were obtained for lasers emitting at wavelengths 2 to 9 nm on the long wavelength side of the gain peak, depending on the cavity length. Threshold currents as low as 98 A/cm² were obtained at the optimum Bragg wavelength. A strong dependence of the linearity on the detuning was found and the lasers showed good linearity at the wavelengths yielding optimum low threshold currents     

Fundamental limits of sub-ps pulse generation by active modelocking of semiconductor lasers: the spectral gain width and the facetreflectivities

The authors present a numerical simulation of active mode locking of a semiconductor laser amplifier (SCLA) in an external cavity, which includes the finite spectral gainwidth. This is shown to be essential for introducing a lower limit of the pulsewidth, although the gain linewidth is about a factor of 10-50 broader than the inverse pulsewidth. This numerical treatment consistently explains the experimental findings that even SCLA facet reflectivities as low as 10 ^-4 lead to trailing pulses with an intensity almost of the same order of magnitude as the leading one. This theory makes detailed predictions about the influence of the facet reflectivities, the spectral gainwidth, and the injection current on the picosecond pulse generation     

CPM 编码调制系统的基本性能限

针对CPM系统的信道容量研究结果进行了综述,并推导计算了Rayleigh衰落信道下CPM的容量分析方法与计算结果。首先回顾了CPM系统的基本概念及其有限状态马尔可夫模型,并对AWGN信道下CPM的可达信息速率计算方法进行了讨论;然后将其推广到Rayleigh衰落信道,推导出在该信道下计算CPM容量的算法。考虑了接收端已知信道状态信息及未知信道状态信息2种情况。在导出容量计算的算法后,通过Monte Carlo仿真给出了在AWGN及Rayleigh衰落信道下几种典型CPM方案的容量曲线。     

Fundamental limit on number of channels in subcarrier-multiplexed lightwave CATV system

A fundamental limit on the number of channels in an SCM lightwave CATV system is derived by considering an idealised link consisting of a shot-noise-limited receiver and a laser transmitter with a linear light/current characteristic. For VSB-AM TV, the limit is only 50 channels per milliwatt of received optical power.<>     

Fundamental limits on optical pulse detection and digitalcommunication

In this paper, the techniques of statistical decision theory are applied to direct detection of optical pulses. A realistic and general model of a classical photodetector/amplifier receiver is considered. Optimum decision strategies and corresponding receiver sensitivities are derived both for single-pulse detection and for continuous digital communication in which the intersymbol interference is constrained to be zero. A comparison is made to the classic work of Personick and Smith. Their results are shown to be remarkably close to the fundamental limits at high bit rates. At low bit rates, the optimal filter for small duty-cycle pulses gives much better performance than the filters analyzed by Personick and Smith. Optimum values for the Personick integrals are also given. Results are given for a number of cases of practical importance including a photodiode/FET receiver     

Fundamental limits to power consumption of lc subthreshold oscillators

The performance of different LC oscillator topologies is evaluated, showing that their current and power requirements are fundamentally equivalent for many commonly employed single-stage circuits, and are subject to a practical lower limit dictated by the characteristics of the active devices and inductors used. Two fundamental quality indicators are defined, which can summarise their current and energy performance. An example of a two-stage circuit is provided, which is not subject to that limit.     

Semiconductor lasers versus external modulators: a comparison ofnonlinear distortion for lightwave subcarrier CATV applications

Second- and third-order distortion measurements performed on various buried-heterostructure InGaAsP DFB (distributed feedback) lasers and on LiNbO₃ external modulators are discussed. It is shown that by optimally biasing the external modulator, the second-order distortion can be eliminated but the third-order distortion is 25 dB greater than that of direct modulation. It is also shown that these discrete two-tone measurements can be used to predict the system performance of a 42-channel CATV trunk system. The increased composite-triple beat and lower carrier-to-noise ratio, due to modulator loss, are the disadvantages of the external modulator. If it is possible to improve third-order distortion by modifying the structure of the modulator or by electronic compensation without increasing the second-order distortion, then these modulators may be useful for CATV applications