Does Stationarity Characterize Real GDP Movements? Results from Non-Linear Unit Root Tests

Using non-linear unit root tests this paper investigates non- stationarity of real GDP per capita for seven OECD countries over the period 1900-2000. Non-linear unit root tests are more powerful than traditional ADF statistics in rejecting the null unit root hypothesis. To this end we adopt a first order Fourier approximation that may capture many features of non-linear adjustment. Empirical results show that, contrary to what the linear ADF statistics suggest, stationarity characterizes six out of the seven countries. This finding stands at variance with other recent studies which conclude that movements in real GDP per capita can be characterized as a non-stationary process.Unit root tests;non-linear model;real GDP

Efficiency measurement with nonstationary variables: an application of panel cointegration techniques

In this paper, we apply panel cointegration tests and estimation techniques to obtain efficiency measures when it is uncertain whether the underlying technological relationship is structural or spurious due to possible non-stationarity of the data. We illustrate the dangers of efficiency measurement with panel data when integration and cointegration are not taken into account. We apply these techniques to efficiency measurement in U.S. airlines and find striking differences compared to results obtained with the traditional approach.Cointegration

Electronegativity and doping in Si1-xGex alloys

Silicon germanium alloys are technologically important in microelectronics but also they are an important paradigm and model system to study the intricacies of the defect processes on random alloys. The key in semiconductors is that dopants and defects can tune their electronic properties and although their impact is well established in elemental semiconductors such as silicon they are not well characterized in random semiconductor alloys such as silicon germanium. In particular the impact of electronegativity of the local environment on the electronic properties of the dopant atom needs to be clarified. Here we employ density functional theory in conjunction with special quasirandom structures model to show that the Bader charge of the dopant atoms is strongly dependent upon the nearest neighbor environment. This in turn implies that the dopants will behave differently is silicon-rich and germanium-rich regions of the silicon germanium alloy

Cellular-Broadcast Service Convergence through Caching for CoMP Cloud RANs

Cellular and Broadcast services have been traditionally treated independently due to the different market requirements, thus resulting in different business models and orthogonal frequency allocations. However, with the advent of cheap memory and smart caching, this traditional paradigm can converge into a single system which can provide both services in an efficient manner. This paper focuses on multimedia delivery through an integrated network, including both a cellular (also known as unicast or broadband) and a broadcast last mile operating over shared spectrum. The subscribers of the network are equipped with a cache which can effectively create zero perceived latency for multimedia delivery, assuming that the content has been proactively and intelligently cached. The main objective of this work is to establish analytically the optimal content popularity threshold, based on a intuitive cost function. In other words, the aim is to derive which content should be broadcasted and which content should be unicasted. To facilitate this, Cooperative Multi- Point (CoMP) joint processing algorithms are employed for the uni and broad-cast PHY transmissions. To practically implement this, the integrated network controller is assumed to have access to traffic statistics in terms of content popularity. Simulation results are provided to assess the gain in terms of total spectral efficiency. A conventional system, where the two networks operate independently, is used as benchmark.Comment: Submitted to IEEE PIMRC 201

Objects, worlds, and students: virtual interaction in education

The main aim of this study is to form a complete taxonomy of the types of interactions that relate to the use of a virtual world for engaging learning experiences, when blended and hybrid learning methods are to be used. In order to investigate this topic more accurately and effectively, we distinguish four dimensions of interactions based on the context in which these occur, and the involved parts: in-world and in-class, user-to-user and user-to-world interactions. In order to conduct investigation into this topic and form a view of the interactions as clear as possible, we observed a cohort of 15 undergraduate Computer Science students while using an OpenSim-based institutionally hosted virtual world. Moreover, we ran a survey where 50 students were asked to indicate their opinion and feelings about their in-world experience. The results of our study highlight that educators and instructors need to plan their in-world learning activities very carefully and with a focus on interactions if engaging activities are what they want to offer their students. Additionally, it seems that student interactions with the content of the virtual world and the in-class student-to-student interactions, have stronger impact on students’ engagement when hybrid methods are used

Multicast Multigroup Precoding and User Scheduling for Frame-Based Satellite Communications

The present work focuses on the forward link of a broadband multibeam satellite system that aggressively reuses the user link frequency resources. Two fundamental practical challenges, namely the need to frame multiple users per transmission and the per-antenna transmit power limitations, are addressed. To this end, the so-called frame-based precoding problem is optimally solved using the principles of physical layer multicasting to multiple co-channel groups under per-antenna constraints. In this context, a novel optimization problem that aims at maximizing the system sum rate under individual power constraints is proposed. Added to that, the formulation is further extended to include availability constraints. As a result, the high gains of the sum rate optimal design are traded off to satisfy the stringent availability requirements of satellite systems. Moreover, the throughput maximization with a granular spectral efficiency versus SINR function, is formulated and solved. Finally, a multicast-aware user scheduling policy, based on the channel state information, is developed. Thus, substantial multiuser diversity gains are gleaned. Numerical results over a realistic simulation environment exhibit as much as 30% gains over conventional systems, even for 7 users per frame, without modifying the framing structure of legacy communication standards.Comment: Accepted for publication to the IEEE Transactions on Wireless Communications, 201

Smooth Breaks and Nonlinear Mean Reversion: Post-Bretton Woods Real Exchange Rates

The recent literature on Purchasing Power Parity (PPP) has emphasized the role of two phenomena that may lead to the rejection of the PPP hypothesis: structural breaks and nonlinear adjustment induced by transaction costs. These two hypotheses are analyzed separately in the literature. We develop tests for unit roots that account jointly for structural breaks and nonlinear adjustment. Structural breaks are modeled by means of a Fourier function that allows for infrequent smooth temporary mean changes and is hence compatible with long-run PPP. Nonlinear adjustment is modeled by means of an ESTAR model. Our tests present good finite sample properties. The tests are applied to a set of 15 OECD countries’ RERs and are able to reject the null of a unit root in 14 cases. The breaks are usually associated with the great appreciation and later depreciation of the dollar in the 1980s and the ESTAR adjustment appears to play an important role.Fourier model; ESTAR; nonlinear adjustment; PPP;