The Dark Side of Micro-Task Marketplaces: Characterizing Fiverr and Automatically Detecting Crowdturfing

As human computation on crowdsourcing systems has become popular and powerful for performing tasks, malicious users have started misusing these systems by posting malicious tasks, propagating manipulated contents, and targeting popular web services such as online social networks and search engines. Recently, these malicious users moved to Fiverr, a fast-growing micro-task marketplace, where workers can post crowdturfing tasks (i.e., astroturfing campaigns run by crowd workers) and malicious customers can purchase those tasks for only $5. In this paper, we present a comprehensive analysis of Fiverr. First, we identify the most popular types of crowdturfing tasks found in this marketplace and conduct case studies for these crowdturfing tasks. Then, we build crowdturfing task detection classifiers to filter these tasks and prevent them from becoming active in the marketplace. Our experimental results show that the proposed classification approach effectively detects crowdturfing tasks, achieving 97.35% accuracy. Finally, we analyze the real world impact of crowdturfing tasks by purchasing active Fiverr tasks and quantifying their impact on a target site. As part of this analysis, we show that current security systems inadequately detect crowdsourced manipulation, which confirms the necessity of our proposed crowdturfing task detection approach

Enabling Quality-Driven Scalable Video Transmission over Multi-User NOMA System

Recently, non-orthogonal multiple access (NOMA) has been proposed to achieve higher spectral efficiency over conventional orthogonal multiple access. Although it has the potential to meet increasing demands of video services, it is still challenging to provide high performance video streaming. In this research, we investigate, for the first time, a multi-user NOMA system design for video transmission. Various NOMA systems have been proposed for data transmission in terms of throughput or reliability. However, the perceived quality, or the quality-of-experience of users, is more critical for video transmission. Based on this observation, we design a quality-driven scalable video transmission framework with cross-layer support for multi-user NOMA. To enable low complexity multi-user NOMA operations, a novel user grouping strategy is proposed. The key features in the proposed framework include the integration of the quality model for encoded video with the physical layer model for NOMA transmission, and the formulation of multi-user NOMA-based video transmission as a quality-driven power allocation problem. As the problem is non-concave, a global optimal algorithm based on the hidden monotonic property and a suboptimal algorithm with polynomial time complexity are developed. Simulation results show that the proposed multi-user NOMA system outperforms existing schemes in various video delivery scenarios.Comment: 9 pages, 6 figures. This paper has already been accepted by IEEE INFOCOM 201

On Balancing Event and Area Coverage in Mobile Sensor Networks

In practice, the mobile sensor networks have two important tasks: firstly, sensors should be able to locate themselves close to where major events are happening so that event tracking becomes possible; secondly, the sensor networks should also maintain a good area coverage over the environment in order to detect new events. Because these two tasks are usually conflicting with each other, a coverage control policy should be able to balance the event and area coverage of the environment. However, most existing work is to achieve either optimal event coverage or optimal area coverage over the environment. In this thesis, a Voronoi-based coverage control with task assignment is introduced: each sensor is allowed to switch between event and area coverage depending on the intensity of events within its Voronoi cell, and both continuous-time and discrete-time control for sensor positions are discussed

Study of stress in microelectronic materials by photoelasticity

The study of stress is playing an important role in microelectronic technology. In comparison with other techniques, the photoelasticity technique has the advantages of having high spatial resolution and high sensitivity. It can be used in qualitative observation in real-time and quantitative determination of stress distribution in the microelectronic materials and devices. This dissertation presents a systematic study of photoelastic stress analysis in microelectronic materials and devices, ranged from theoretical study to practical system setup, from measurement methods to their applications. At first, based on the detailed survey on the piezo-optic properties of the crystals used in microelectronics, we apply the stress-optic law of engineering mechanics to study the stress in crystals, such as silicon, gallium arsenide, and diamond. We, for the first time, derive the relationship between the stress ellipsoid and the refractive index ellipsoid, and, derive the matrix forms of piezo-optic coefficient tensor for several commonly used coordinates. These theoretical results have laid a firm ground for the photoelastic stress analysis of microelectronic materials and devices. Second, based on exclusive experiments, we develop several effective methods of photoelastic analysis to determine the stress state in the samples. Some of them are successfully borrowed from the classic photoelastic mechanics, such as the Semarmont compensation which is used to determine the decimal fringe of the isochromatic line, and the shearing stress difference method which is used to separate two principal stresses and obtain the stress distribution of a sample. We also provide our origination to the photoelastic techniques, such as the three-direction observation method which is used to abstract the principal stresses from the secondary principal stresses; the Fourier analysis method and the intensity analysis method, both of which are especially suitable for precisely and automatically determining the principal stress distribution in a large area. Combining computer and digital image processing techniques with photoelastic techniques, we set up a photoelastic measurement system, which has the capability of qualitative observation of the photoelastic patterns and quantitative measurement of the stress distribution. We apply the photoelasticity principles and methods to investigate the stress state in microelectronic materials, including developing a series of feasible methods to measure the stress distribution of the microelectronic materials and devices, investigating the mechanisms of stress induction and stress change. For example, we investigate the stress distribution of a synthetic diamond substrate, study the stress induced during the typical impurity diffusion processes for manufacturing power diodes, and analyze the stress induced in the thin film/substrate structures. We also develop some models to explain the measurement results and provide theoretical discussions of the results