Sharif Digital Repository

It's well known that "hidden- and exposed-terminal" are among the main problems in ad-hoc WLAN networks. In addition, there are scenarios where the desired destination is located in the range of other transmitters, so that the efforts on setting up communication with this terminal will fail due to collisions occurred between desired control packets and unwanted received data packets at destination. In such scenarios, conventional protocols can not address the problem efficiently, resulting in throughput and channel utilization degradation. In this paper, by using the same PHY of IEEE 802.11 and making slight changes in its MAC layer, a new MAC protocol is presented to address such problems.... 

Assume that we are given a set of points some of which are black and the rest are white. The goal is to find a set of convex polygons with maximum total area that cover all white points and exclude all black points. We study the problem on three different settings (based on overlapping between different convex polygons): (1) In case convex polygons are permitted to have common area, we present a polynomial algorithm. (2) In case convex polygons are not allowed to have common area but are allowed to have common vertices, we prove the NP-hardness of the problem and propose an algorithm whose output is at least (Formula presented.). (3) Finally, in case convex polygons are not allowed to have... 

This paper introduces a novel approach for power system fault diagnosis based on synchronized phasor measurements during the fault. The synchronized measurements are obtained in real time from Phasor Measurement Units (PMUs) and compared with offline thresholds determined by decision trees (DTs) to diagnose the fault. The DTs have already been trained offline using detailed power system analysis for different fault cases. While the traditional methods for fault diagnosis use the status of protective relays (PRs) and circuit breakers (CBs) to infer the fault section in the power system, the proposed method uses the available signals following the fault and thus can be trusted even in case of... 

In this paper, we have proposed a fast and easy-to-develop FPGA-based fault injection technique. This technique uses the Altera FPGAs debugging facilities in order to inject SEU fault model in both flip-flops and memory units. Since this method uses the FPGAs built-in facilities, it imposes a negligible performance and area overhead on the system. The experimental results on Leon2 processor shows that the proposed technique is on average four orders of magnitude faster than a simulation-based fault injection  

Let P be a set of points in the plane, each equipped with a directional antenna that can cover a sector of angle α and range r. In the symmetric model of communication, two antennas u and v can communicate to each other, if and only if v lies in u’s coverage area and vice versa. In this paper, we introduce the concept of fault-tolerant spanners for directional antennas, which enables us to construct communication networks that retain their connectivity and spanning ratio even if a subset of antennas are removed from the network. We show how to orient the antennas with angle α and range r to obtain a k-fault-tolerant spanner for any positive integer k. For α ≥ π, we show that the range 13 for... 

In this paper, a system wide scheme is proposed to arrest voltage instability. This scheme consists of two stabilizing and securing parts. For post- contingency situations, where there are no equilibrium solutions, first, the stabilizing scheme finds the closest solvable operating point, and then passes this point to the securing scheme. The securing scheme brings the solvable, but insecure operating point, back to a secure and stable one. Also, a voltage stability indicator is proposed to monitor the available margin to the collapse point. The proposed stabilizing and securing model are solved using the sequential quadratic programming technique. ©2009 IEEE  

Symmetric relaying is a method of relaying in which the relays can decode the message of other relays in the network in addition to the source message. In this paper an achievable rate is presented for a symmetric two-relay network based on partial decoding. The strategy make use of familiar techniques such as product binning, regular encoding/sliding window decoding and regular encoding/backward decoding. The proposed rate is shown to subsume the previously proposed rate for feed-forward relay network based on decode-and-forward. This rate is also used to establish the capacity of a generalisation of Aref network called 'semi-deterministic relay network with no interference at the relays'... 

The problem of surveillance for intrusion detection in a camera sensor network is addressed in this paper. In order to save limited resources, a sensing task should involve just the right number of sensors. For a wide enough coverage area random and uniform distribution can be applied. We propose a novel method which allows reduction of number of sensors and guarantees a desired surveillance against aerial intrusion. Enhancement of the method is also offered to further reduce the number of sensors and improve the performance. ©2008 IEEE  

Since DCF is the main protocol of accessing other mobile stations in ad hoc networks such as IEEE 802.11 WLANs, a self-controlling method for each station is necessary, called the backoff algorithm. An exponential backoff method has been used in WLANs and significant efforts have been made to analyze its throughput and other important properties such as delay and jitter. In this article, we propose a new backoff algorithm and model it with a discrete-time Markov chain; measuring its saturation throughput under several conditions and several set of parameters which are to be adjusted according to the network condition, with the aim of approaching maximum throughput when stations are... 

In this paper, the extended finite element method (XFEM) is employed to model the presence of discontinuities caused by frictional contact. The method is used in modeling strong discontinuity within a standard finite element framework. In extended finite element method (XFEM) technique, the special functions are included in standard FEM to simulate discontinuity without considering the boundary conditions in meshing the domain. In this study, the classical finite element approximation is enriched by applying additional terms to simulate the frictional behavior of contact between two bodies. These terms, which are included for enrichment of nodal displacements, depend on the contact condition... 

There is a need to find cheaper and simpler techniques for rural water filtration system in developing countries. Using a filter made of blocks for the water treatment enables one to make vertical filters. It is expected that the amount of land utilized would decrease by more than 70 % if vertical filters were used. The operation and washing would be simpler than using horizontal sand filters as filters made of blocks would be used instead. The feasibility study focused on finding adequate materials, compositions and methods of making a block with enough resistance to water pressure, sufficient porosity for water transformation, and using inexpensive, readily available materials. A pilot... 

Hidden-terminal is one of the main problems in ad hoc wireless networks. In addition, there are scenarios where the desired destination is located in the range of other transmitters, so that the efforts on setting up communication with this terminal will fail due to collisions that may occur between transmitted control frames and undesired received control and data frames. This phenomenon becomes a bottleneck when most of data transmissions experience packet fragmentation. In such scenarios, the desired destination becomes unreachable during the data transfer of neighboring nodes. Using the same PHY of IEEE 802.11 and making slight modifications in its MAC, a new medium access control scheme... 

An 8-bit CMOS folding-interpolating analog-todigital converter is presented. A new method for designing optimized averaging circuit is also described. Careful circuit design and layout leads to a high-speed (160 MSPS) and low power (70 mW in 2.5 V supply voltage) ADC. The ADC is successfully implemented in 0.25um CMOS digital process and it takes 1x1.4 mm2 silicon area. © 2005 IEEE  

This study is an attempt to investigate and analyze uncertainty through the Glue method in parts of Tabriz, Iran, using the SWMM urban flood model. In order to quantify the uncertainty, the initial range of input parameters including curve number, impenetrability percentage, penetrability level’s coarseness coefficient, and impenetrability level’s coarseness coefficient are defined and the GLUE algorithm is used to conduct primary sampling operation from parametric space using Latin hypercube sampling method. Considering the simulation results and the observed values of synchronized events, about %20 of the total outputs and generated parameters series have been excluded as acceptable... 

Communication wire delay between multiple blocks is becoming a critical issue in System on Chip (SoC) design. Scheduling-based Latency-Insensitive Design (LID) is a method to alleviate wire delays by utilizing a central scheduling scheme for periodic clock gating of the blocks. The scheduling scheme resides in shift registers as sequences of '1' and '0' bits. In many systems, these sequences are too long, and have large area overhead. This problem indisposes the implementation of the scheduling based protocol. This paper proposes an algorithm that finds sequences with shorter lengths in comparison with the prior algorithm. On synthetic/random test cases, the algorithm gives 45% reduction on... 

This paper enhances the observability of power networks by taking into consideration random component outages. The architecture of wide-area measurement system (WAMS) is analyzed in order to identify components that would affect the network observability. An iterative framework is devised to calculate a bus index in power networks equipped with phasor measurement units (PMUs) and conventional measurements. The average of bus indices represents a system index which provides an overall insight on the power network observability. The system index is utilized as a criterion to distinguish among multiple optimal PMU placements. Conventional bus injection and line flow measurements and the effect...