Look-ahead Schemes Research Articles

Look‐ahead distribution power restoration analysis based on integrated operation of distribution automation and advanced metring infrastructure systems

Power restoration schemes incorporated into Distribution Automation (DA) system helps in quickly restoring the power during emergency situations like faults. Due to the numerous advancements made in the distribution system operation in recent times, power restoration analysis needs to be done in a look-ahead manner based on system wide operational & monitored data apart from just fault data processing during the outages. With the evolution of Advanced Metering Infrastructure (AMI) during recent times, it is now possible to design the look-ahead restoration schemes by integrating the operation of DA with AMI system using internet protocol based communications, preferably. In this paper, we propose a novel method for power restoration analysis which execute all pre-defined restoration schemes in a look-ahead manner by representing distribution system as a weighted graph and computing graph weights based on representation of DA & AMI system’s current operational/measured/constraint data from each distribution load segment. Computed graph weights are converted to restoration indices with associated ranking based mechanism to indicate criticality & considerations for each power restoration scheme in case of fault occurrence in real-time. A case study on the standard RBTS test system has been demonstrated for the proposed method.

Dynamic decision making for graphical models applied to oil exploration

We present a framework for sequential decision making in problems described by graphical models. The setting is given by dependent discrete random variables with associated costs or revenues. In our examples, the dependent variables are the potential outcomes (oil, gas or dry) when drilling a petroleum well. The goal is to develop an optimal selection strategy of wells that incorporates a chosen utility function within an approximated dynamic programming scheme. We propose and compare different approximations, from naive and myopic heuristics to more complex look-ahead schemes, and we discuss their computational properties. We apply these strategies to oil exploration over multiple prospects modeled by a directed acyclic graph, and to a reservoir drilling decision problem modeled by a Markov random field. The results show that the suggested strategies clearly improve the naive or myopic constructions used in petroleum industry today. This is useful for decision makers planning petroleum exploration policies.

Probabilistic carry state estimate for improved asynchronous adder performance

The paper presents a new type of simple adder, suitable for asynchronous digital circuits and implementation in VLSI technology, which has either speed and/or area advantages over existing designs. It is based on the concept of predicting the carry from least to most significant halves of a 32 or 64 bit adder in such a way that it has a high probability of being correct, while introducing only a low area overhead from the required early completion control circuitry. Detailed design and simulation of the adder at the gate level is presented, together with its evaluation by comparing detailed performance with equivalent ripple, carry lookahead, and carry select designs. Since the objective is improved asynchronous circuits, it is average rather than worst-case delays that are the significant measures. In comparison to other adder networks it is demonstrated that by using the important metrics of area, speed and delay-area product the proposed adder can outperform the 32 bit and 64 bit adders cited in the literature. Delay-area product results show that the proposed approach gives a saving of over 14% and 24% on the carry-select lookahead schemes for 32 bit and 64 bit adders respectively.

Conflict-Directed Backjumping Revisited

In recent years, many improvements to backtracking algorithms for solving constraint satisfaction problems have been proposed. The techniques for improving backtracking algorithms can be conveniently classified as look-ahead schemes and look-back schemes. Unfortunately, look-ahead and look-back schemes are not entirely orthogonal as it has been observed empirically that the enhancement of look-ahead techniques is sometimes counterproductive to the effects of look-back techniques. In this paper, we focus on the relationship between the two most important look-ahead techniques---using a variable ordering heuristic and maintaining a level of local consistency during the backtracking search---and the look-back technique of conflict-directed backjumping (CBJ). We show that there exists a ``perfect'' dynamic variable ordering such that CBJ becomes redundant. We also show theoretically that as the level of local consistency that is maintained in the backtracking search is increased, the less that backjumping will be an improvement. Our theoretical results partially explain why a backtracking algorithm doing more in the look-ahead phase cannot benefit more from the backjumping look-back scheme. Finally, we show empirically that adding CBJ to a backtracking algorithm that maintains generalized arc consistency (GAC), an algorithm that we refer to as GAC-CBJ, can still provide orders of magnitude speedups. Our empirical results contrast with Bessiere and Regin's conclusion (1996) that CBJ is useless to an algorithm that maintains arc consistency.

Scheduling Video Streams in Video-on-Demand Systems: A Survey

Developments in multimedia technology over the past decade has caused video-on-demand services to emerge as a new paradigm in home entertainment. Because of the large volume of data involved in the process and stringent continuity and real-time constraints, these services pose challenges that are different from the standard file transfer operations in the network. The necessity of efficient usage of scarce resources like network bandwidth and server capacity (in terms of I/O bandwidth) demands novel and easy-to-use schemes for scheduling continuous video streams. This paper presents an overview of the major scheduling policies that have emerged in the recent past. In particular, the paper provides detailed discussion on policies based on principles of broadcasting, batching, caching, and piggybacking or merging. Policies like look-ahead scheduling schemes that are designed exclusively to provide certain interactive VCR-like control operations are also covered. A conceptual comparison between the various classes of scheduling policies is carried out to identify common threads and key concepts. Performance of these policies in terms of bandwidth demand reduction, customer waiting time reduction, provision of interactive control by the user, and fairness of service are given special emphasis. The paper concludes with a discussion on the possible avenues of further reasearch and development in this potentially interesting and challenging field.

Pipelined recursive digital filters: a general look-ahead scheme and optimal approximation

A new look-ahead (LA) scheme, distributed look-ahead (DLA), is proposed for pipelined implementation of recursive digital filters. It is established that, in the case of many recursive filters, DLA can provide equivalent and stable implementation with reduced multiplication and latch complexities, and higher area-efficiency. It also produces outputs with reduced delays, when compared with some existing LA schemes. Stability regions for the proposed scheme are also well defined. Next, a time-domain approximation method is proposed for direct design of pipelined recursive filters that can achieve superior match with reduced pipelined filter orders.

A linear system solver based on a modified Krylov subspace method for breakdown recovery

Despite its usefulness in solving eigenvalue problems and linear systems of equations, the nonsymmetric Lanczos method is known to suffer from a potential breakdown problem. Previous and recent approaches for handling the Lanczos exact and near-breakdowns include, for example, the look-ahead schemes by Parlett-Taylor-Liu [23], Freund-Gutknecht-Nachtigal [9], and Brezinski-Redivo Zaglia-Sadok [4]; the combined look-ahead and restart scheme by Joubert [18]; and the low-rank modified Lanczos scheme by Huckle [17]. In this paper, we present yet another scheme based on a modified Krylov subspace approach for the solution of nonsymmetric linear systems. When a breakdown occurs, our approach seeks a modified dual Krylov subspace, which is the sum of the original subspace and a new Krylov subspaceK m (w j ,A T ) wherew j is a newstart vector (this approach has been studied by Ye [26] for eigenvalue computations). Based on this strategy, we have developed a practical algorithm for linear systems called the MLAN/QM algorithm, which also incorporates the residual quasi-minimization as proposed in [12]. We present a few convergence bounds for the method as well as numerical results to show its effectiveness.

Fuzzy constraints in job-shop scheduling

This paper proposes an extension of the constraint-based approach to job-shop scheduling, that accounts for the flexibility of temporal constraints and the uncertainty of operation durations. The set of solutions to a problem is viewed as a fuzzy set whose membership function reflects preference. This membership function is obtained by an egalitarist aggregation of local constraint-satisfaction levels. Uncertainty is qualitatively described in terms of possibility distributions. The paper formulates a simple mathematical model of job-shop scheduling under preference and uncertainty, relating it to the formal framework of constraint-satisfaction problems in artificial intelligence. A combinatorial search method that solves the problem is outlined, including fuzzy extensions of well-known look-ahead schemes.

Heuristics and look-ahead integration to solve constraint satisfaction problems efficiently

Logic programming languages, such asProlog, allow a declarative specification of Constraint Satisfaction Problems (CSPs), freeing the user from specifying more or less complex control directives. However, the price to pay for such flexibility is a loss of efficiency, which makes Logic Programming inadequate to solve CSPs of even moderate size and complexity. To extend the range of applicability of logic programming, several improvements have been proposed. The use of heuristics is one such improvement. Although this usually forces the user to specify some form of control (thus abandoning the pure declarative nature of a logic program), these specifications can be made declarative by making use of some appropriate meta-predicates. Another extension to logic programming that improves its efficiency, is the active use of constraints, as done in the various formulations of constraint logic programming languages. In particular, the use of finite domains is quite adequate to implement look-ahead schemes to efficiently solve several types of CSPs. In this paper, we discuss the complementary nature of heuristics and look-ahead schemes and present a constraint logic programming framework that integrates both these techniques. Results obtained with a time-tabling problem executed on a prototype that implements such a framework are presented, and show that significant efficiency improvements can be achieved when compared with the separate use of the two techniques.

Declination of fundamental frequency (F) in speech production

Declarative utterances in English are typically accompanied by a gradual long-term fall in F0. It has been suggested in previous literature that this fall may be represented as a linear function of time, on which various inflections are superimposed. This study was designed to determine the form of F0 declination in phonetically matched sentence materials. Ten practiced speakers read the sentences, which were subsequently analyzed for F0 and amplitude using computerized techniques. The results indicated that, for a variety of declarative sentences, the declination could be approximated by a piecewise linear function in which two-thirds of the F0 fall occurred during the first one-half of the utterance's duration. This piecewise linear function provided a close fit to the data for each of the ten speakers, including both males and females. In additional experiments, F0 declination was examined as a function of systematic variation in syntactic structure, constituent length, and speaking rate. The results of these experiments permit inferences about the organization of the speaker's coding operations, including look-ahead schemes. Implications for speech synthesis-by-rule will also be discussed. [Supported by NIH Grant NS-13028 and an NIH postdoctoral fellowship.]

An Investigation of Alternative Cache Organizations

An investigation of the various cache schemes that are practical for a minicomputer has been found to provide considerable insight into cache organization. Simulations are used to obtain data on the performance and sensitivity of organizational parameters of various writeback and lookahead schemes. Hardware considerations in the construction of the actual cache-minicomputer are also noted and a simple cost/performance analysis is presented.