Serial Search Strategy Research Articles

On the Performance of Wide-Bandwidth Signal Acquisition in Dense Multipath Channels

This paper investigates important properties of acquisition receivers that employ commonly used serial-search strategies. In particular, we focus on the properties of the mean acquisition time (MAT) for wide bandwidth signals in dense multipath channels. We show that a lower bound of the MAT over all possible search strategies is the solution to an integer programming problem with a convex objective function. We also give an upper bound expression for the MAT over all possible search strategies. We demonstrate that the MAT of the fixed-step serial search (FSSS) does not depend on the timing delay of the first resolvable path, thereby simplifying the evaluation of the MAT of the FSSS. The results in this paper can be applied to design and analysis of fast acquisition systems in various wideband scenarios.

A new approach to rapid PN code acquisition using iterative message passing techniques

Iterative message passing algorithms on graphs, which are generalized from the well-known turbo decoding algorithm, have been studied intensively in recent years because they can provide near-optimal performance and significant complexity reduction. In this paper, we demonstrate that this technique can be applied to pseudorandom code acquisition problems as well. To do this, we represent good pseudonoise (PN) patterns using sparse graphical models, then apply the standard iterative message passing algorithms over these graphs to approximate maximum-likelihood synchronization. Simulation results show that the proposed algorithm achieves better performance than both serial and hybrid search strategies in that it works at low signal-to-noise ratios and is much faster. Compared with full parallel search, this approach typically provides significant complexity reduction.

Performance evaluation of serial‐search strategies for the acquisition of DS/CDMA signals

AbstractSearch strategies for initial synchronization of direct sequence (DS) spread spectrum communication systems are compared, modelled and implemented using MATLAB then tested in the presence of additive white Gaussian noise (AWGN) and narrowband interference. The performance of these search strategies, namely, offset‐z‐search strategies and expanding alternate search strategies, in terms of the mean value of the search length is evaluated for different probabilities of detection. A truncated Gaussian probability phase distribution is assumed (as a priori information about the correct phase) and the performance of these search strategies is tested for different standard deviations (σ) of the phase.A novel search strategy, namely the offset‐z3, based on the offset z‐search strategy is introduced. Among all search strategies investigated the non‐uniform expanding alternate (NUEA) search strategy is found to perform best at high probability of detection. However, for a more peaked phase distribution, the offset‐z3‐search strategy is shown to offer better performance at low probability of detection. Outcomes of the simulated system support trends predicted by theory for the performance of different search strategies. Copyright © 2003 John Wiley & Sons, Ltd.

Discrimination of spatial relations and features in faces: Effects of inversion and viewing duration

We studied discrimination of changes in eye position, mouth position, and eye colour at viewing durations ranging from 1 second to unlimited time. With upright faces, perception was rapid and did not improve above 2 seconds viewing time. Face inversion impaired discrimination of mouth position significantly, eye position slightly, but not eye colour. The 'inversion effect' for mouth position decreased with increasing stimulus duration, and disappeared when the subject knew that the only change in a trial was in mouth position. A subsequent experiment showed that the inversion impairment in the mouth region was not specific to spatial position but affected mouth colour to a lesser degree. When the mouth region was made more salient by increasing the frequency of mouth change trials, the inversion effect for mouth position decreased, and correlated with an increase in inversion effect for eye position but not eye colour. We conclude that the dominant effect of face inversion upon perception is decreased discrimination in less salient facial regions, that this impairment lessens with increasing viewing time, and that it affects both features and their spatial relations, though the effect on the latter is greater. These results are consistent with greater dependence on a serial component search strategy in inverted faces.

Search strategies in syntactic reanalysis.

We first consider the nature of syntactic reanalysis, paying particular attention to questions about whether and how it differs from the construction of an initial analysis, such as whether it is encapsulated or whether alternatives are considered in parallel or not. We then outline different strategies that the processor may use in reanalysis, and review the relevant evidence. We show that our experimental evidence is compatible with an encapsulated serial search strategy.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task while learning it and after learning it. Here we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task during and after learning it. Here, we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

A multiple-tree search procedure for the resource-constrained project scheduling problem

For over three decades, researchers have sought effective solution procedures for PERT/CPM types of scheduling problems under conditions of limited resource availability. This paper presents a procedure for this problem which takes advantage of the emerging technology provided by multiple parallel processors to find and verify an optimal schedule for a project under conditions of multiple resource constraints. In our approach, multiple solutions trees are searched simultaneously in the quest for a minimum duration schedule. Global upper and lower bound information in common memory is shared among processors, enabling one or several processors to prune potentially significant portions of its search tree based upon bounds discovered by a processor using a different search tree. Computational experience is reported both for problems in which resources are available in constant amounts per period, as well as the much more difficult problem in which the resources available are allowed to vary over the schedule horizon (e.g., travel, sick leave, assignment to other tasks or projects, and so forth). The modular multiple-tree search procedure described in this paper is quite general, permitting most types of existing serial search strategies to be adapted to this approach where multiple processors are available.

Electrophysiological evidence for parallel and serial processing during visual search

Event-related potentials were recorded from young adults during a visual search task in order to evaluate parallel and serial models of visual processing in the context of Treisman's feature integration theory. Parallel and serial search strategies were produced by the use of feature-present and feature-absent targets, respectively. In the feature-absent condition, the slopes of the functions relating reaction time and latency of the P3 component to set size were essentially identical, indicating that the longer reaction times observed for larger set sizes can be accounted for solely by changes in stimulus identification and classification time, rather than changes in post-perceptual processing stages. In addition, the amplitude of the P3 wave on target-present trials in this condition increased with set size and was greater when the preceding trial contained a target, whereas P3 activity was minimal on target-absent trials. These effects are consistent with the serial self-terminating search model and appear to contradict parallel processing accounts of attention-demanding visual search performance, at least for a subset of search paradigms. Differences in ERP scalp distributions further suggested that different physiological processes are utilized for the detection of feature presence and absence.

In search of depth

Four experiments designed to investigate a visual search task are reported. In each experiment, subjects searched for either a two-dimensional shape rotated in depth among frontal-parallel distractors or a frontal-parallel shape among distractors that were rotated in depth. The principal independent variable was search-set size. In addition, over the four experiments, a variety of spatial arrangements and two axes of rotation in depth were sampled. The chief aim of the experiments was to adduce evidence bearing on the attentional demands of searching for depth. The slopes of the reaction-time (RT) functions were taken as diagnostic. Experiments 1-3 exhibited positive slopes for the RT-set-size function. These slopes appear to be due to a conscious adoption of a serial search strategy by the subjects. When this tendency was suppressed by the procedures of Experiment 4, the slope of the RT-set-size functions did not differ significantly from zero. We conclude that, in agreement with the findings of other studies, slant-in-depth can be detected preattentively.

Personality style differences in visual search performance

The present study continues the examination of the personality styles in normal populations that have been described in the clinical literature. Earlier work has focused on behavioral, attitudinal, and cognitive components of the personality styles. The present study extends our exploration of the cognitive domain by examining the performance of different personality groups on a visual scanning task. Hysterics searched a stimulus array for letter targets at a mean rate of 0.04 ± SD 0.177 seconds per unit of information, which was slower than the control rate of 0.0225 ± SD 0.0078 seconds. The results indicated that hysterics used a serial search strategy to compensate for an inability to process information efficiently and thus differed from compulsives who conformed to task demands and performed similarly to controls.

Evidence for a Right Hemisphere Superiority and for a Serial Search Strategy in a Dot Detection Task

In Experiment I the subjects manually signaled whether a solid dot was present or not in an array of empty dots presented either in the right or in the left visual field. The dots were either three or six and the target could appear in any of the possible locations. The results showed (a) a left visual field superiority; (b) a left hand superiority; (c) faster responses to three-dot than to six-dot displays, and (d) an effect of the location of the target in the display. Experiment II exactly replicated the previous experiment with the exception that the stimuli were centrally projected. The results showed: (a) a left hand superiority; (b) an effect of the number of dots in the display. These results can be interpreted to indicate: (a) a right hemisphere specialization in a simple feature detection task without any spatial component; (b) the compatibility of a right-hemisphere advantage with a serial search strategy.

Mnemonic Retrieval Strategies in Serial to Paired-Associate Transfer

42 subjects were assigned to either of three rates of paired-associate presentation after having learned a nine-item serial list. Group I received a 4:4-sec. rate of presentation and Groups II and III 6:6 sec. and 8:8 sec., respectively. Half of the 16 paired associates were words previously learned in the serial list; half were control pairs. Contrary to previous studies, positive transfer was found for the serial-list-derived word pairs. This difference is attributed to the slower rates of presentation which allow the subjects to use a serial search strategy in retrieving contiguous pairs from the serial list. Serial search is viewed as a more parsimonious explanation than the associative one and the present results seem to add some support to the serial search hypothesis. It is also recognized that the subjects' serial search strategy is effective only under certain conditions.