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1.
We consider a system comprising a finite number of nodes, with infinite packet buffers, that use unslotted ALOHA with Code
Division Multiple Access (CDMA) to share a channel for transmitting packetised data. We propose a simple model for packet
transmission and retransmission at each node, and show that saturation throughput in this model yields a sufficient condition
for the stability of the packet buffers; we interpret this as the capacity of the access method. We calculate and compare
the capacities of CDMA‐ALOHA (with and without code sharing) and TDMA‐ALOHA; we also consider carrier sensing and collision
detection versions of these protocols. In each case, saturation throughput can be obtained via analysis of a continuous time
Markov chain. Our results show how saturation throughput degrades with code‐sharing. Finally, we also present some simulation
results for mean packet delay. Our work is motivated by optical CDMA in which “chips” can be optically generated, and hence
the achievable chip rate can exceed the achievable TDMA bit rate which is limited by electronics. Code sharing may be useful
in the optical CDMA context as it reduces the number of optical correlators at the receivers. Our throughput results help
to quantify by how much the CDMA chip rate should exceed the TDMA bit rate so that CDMA‐ALOHA yields better capacity than
TDMA‐ALOHA.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
2.
P. K. Khare J. M. Keller M. S. Gaur Ranjeet Singh S. C. Datt 《Polymer International》1994,35(4):337-343
The electrical conductivity of solution-grown ethyl cellulose (EC) films, 5–30 μm thick, has been studied in the sandwich configuration (metal–EC–metal) as a function of iodine concentration from 0.5 to 5.0 wt% ratio. The studies were conducted in the temperature range 333–383 K, while the field was varied over the range (3.0–5.5) × 104V/cm. Aluminium was used as the lower electrode, while the upper electrode was of Al, Ag, Cu, Au or Sn. Certain transient effects such as a large burst of current immediately after the application of field were observed. An attempt was made to identify the nature of the current by comparing the observed dependence on electric field, electrode material and temperature with the respective characteristic features of the existing theories of electrical conduction. The results show that the electrical conduction follows Ohm's law at lower fields, while at higher fields, space-charge limited current (SCLC) was observed. It was also found that Richardson–Schottky emission was responsible, to some extent, for the transport of charge carriers in the polymer. The conductivity of the films increased on doping with iodine. The dopant molecules are considered to act as additional trapping centes and provide links between the polymer molecules in the amorphous region, thus resulting in the formation of charge transfer complexes. 相似文献
3.
4.
A role for isothiocyanates in plant resistance against the specialist herbivore Pieris rapae 总被引:8,自引:1,他引:8
We experimentally reanalyzed the classic interaction between Pieris rapae, a specialist lepidopteran herbivore, and isothiocyanates (mustard oils) that are characteristic phytochemicals of the Brassicaceae. Previous investigations have suggested that P. rapae is unaffected by isothiocyanates. Using whole plants, root extracts, and a microencapsulated formulation of allyl isothiocyanate, we now show that isothiocyanates reduce herbivore survival and growth, and increase development time, each in a dose-dependent manner. Neither the substrate allyl glucosinolate, nor myrosinase, the enzyme that results in the breakdown of glucosinolates, negatively affected P. rapae. Thus, we present strong evidence for a role for isothiocyanates in plant resistance against the specialist herbivore P. rapae. 相似文献
5.
The network scenario is that of an infrastructure IEEE 802.11 WLAN with a single AP with which several stations (STAs) are associated. The AP has a finite size buffer for storing packets. In this scenario, we consider TCP-controlled upload and download file transfers between the STAs and a server on the wireline LAN (e.g., 100 Mbps Ethernet) to which the AP is connected. In such a situation, it is well known that because of packet losses due to finite buffers at the AP, upload file transfers obtain larger throughputs than download transfers. We provide an analytical model for estimating the upload and download throughputs as a function of the buffer size at the AP. We provide models for the undelayed and delayed ACK cases for a TCP that performs loss recovery only by timeout, and also for TCP Reno. The models are validated in comparison with NS2 simulations. 相似文献
6.
Robert Jay Milewski Venu Govindaraju Anurag Bhardwaj 《International Journal on Document Analysis and Recognition》2009,11(4):203-218
A new paradigm, which models the relationships between handwriting and topic categories, in the context of medical forms,
is presented. The ultimate goals are: (1) a robust method which categorizes medical forms into specified categories, and (2)
the use of such information for practical applications such as an improved recognition of medical handwriting or retrieval
of medical forms as in a search engine. Medical forms have diverse, complex and large lexicons consisting of English, Medical
and Pharmacology corpus. Our technique shows that a few recognized characters, returned by handwriting recognition, can be
used to construct a linguistic model capable of representing a medical topic category. This allows (1) a reduced lexicon to
be constructed, thereby improving handwriting recognition performance, and (2) PCR (Pre-Hospital Care Report) forms to be
tagged with a topic category and subsequently searched by information retrieval systems. We present an improvement of over
7% in raw recognition rate and a mean average precision of 0.28 over a set of 1,175 queries on a data set of unconstrained
handwritten medical forms filled in emergency environments.
This work was supported by the National Science Foundation. 相似文献
7.
Huaigu Cao Venu Govindaraju Anurag Bhardwaj 《International Journal on Document Analysis and Recognition》2011,14(2):145-157
With the ever-increasing growth of the World Wide Web, there is an urgent need for an efficient information retrieval system
that can search and retrieve handwritten documents when presented with user queries. However, unconstrained handwriting recognition
remains a challenging task with inadequate performance thus proving to be a major hurdle in providing robust search experience
in handwritten documents. In this paper, we describe our recent research with focus on information retrieval from noisy text
derived from imperfect handwriting recognizers. First, we describe a novel term frequency estimation technique incorporating
the word segmentation information inside the retrieval framework to improve the overall system performance. Second, we outline
a taxonomy of different techniques used for addressing the noisy text retrieval task. The first method uses a novel bootstrapping
mechanism to refine the OCR’ed text and uses the cleaned text for retrieval. The second method uses the uncorrected or raw
OCR’ed text but modifies the standard vector space model for handling noisy text issues. The third method employs robust image
features to index the documents instead of using noisy OCR’ed text. We describe these techniques in detail and also discuss
their performance measures using standard IR evaluation metrics. 相似文献
8.
Research on fine tuning stabilization properties has received attention for more than a decade. This paper presents probabilistic algorithms for fault containment. We demonstrate two exercises in fault containment in a weakly stabilizing system, which expedite recovery from single failures, and confine the effect of any single fault to the constant-distance neighborhood of the faulty process. The most significant aspect of the algorithms is that the fault gap, defined as the smallest interval after which the system is ready to handle the next single fault with the same efficiency, is independent of the network size. We argue that a small fault gap increases the availability of the fault-free system. 相似文献
9.
Most existing solutions to group security in Mobile Ad Hoc Networks (MANETs) rely on a multicast Core Based Tree (CBT) for key distribution. Such solutions, although suitable for systems with low mobility and static characteristics, are unsuitable for dynamic and sparse groups with changing neighborhoods. In this paper, we propose an entirely decentralized key generation mechanism, employing a central trusted entity only during initialization. Using our approach, keys can be established between group members with absolutely no prior communication. The solution relies on threshold cryptography and introduces a novel concept of Node-Group-Key (NGK) mapping. We have provided an extensive analytical model for the computations involved and communication costs and have also provided a lie detection mechanism. Simulation results show appreciable performance improvement and enhanced robustness. 相似文献
10.
M2Tracker: A Multi-View Approach to Segmenting and Tracking People in a Cluttered Scene 总被引:1,自引:0,他引:1
When occlusion is minimal, a single camera is generally sufficient to detect and track objects. However, when the density of objects is high, the resulting occlusion and lack of visibility suggests the use of multiple cameras and collaboration between them so that an object is detected using information available from all the cameras in the scene.In this paper, we present a system that is capable of segmenting, detecting and tracking multiple people in a cluttered scene using multiple synchronized surveillance cameras located far from each other. The system is fully automatic, and takes decisions about object detection and tracking using evidence collected from many pairs of cameras. Innovations that help us tackle the problem include a region-based stereo algorithm capable of finding 3D points inside an object knowing only the projections of the object (as a whole) in two views, a segmentation algorithm using bayesian classification and the use of occlusion analysis to combine evidence from different camera pairs.The system has been tested using different densities of people in the scene. This helps us determine the number of cameras required for a particular density of people. Experiments have also been conducted to verify and quantify the efficacy of the occlusion analysis scheme. 相似文献