Growth of layered Lu2_2Fe3_3O7_7 and Lu3_3Fe4_4O10_{10} single crystals exhibiting long-range charge order via the optical floating-zone method

We report the controlled growth of single crystals of intercalated layered Lu$_{1+n}$Fe$_{2+n}$O$_{4+3n-\delta}$ ($n$=1,2) with different oxygen stoichiometries ${\delta}$. For the first time crystals sufficiently stoichiometric to exhibit superstructure reflections in X-ray diffraction attributable to charge ordering were obtained. The estimated correlation lengths tend to be smaller than for not intercalated LuFe$_2$O$_4$. For Lu$_2$Fe$_3$O$_7$, two different superstructures were observed, one an incommensurate zigzag pattern similar to previous observations by electron diffraction, the other an apparently commensurate pattern with ($\frac{1}{3}\frac{1}{3}0$) propagation. Implications for the possible charge order in the bilayers are discussed. Magnetization measurements suggest reduced magnetic correlations and the absence of an antiferromagnetic phase.Comment: 7pages, 7figure

Effective Capacity in Cognitive Radio Broadcast Channels

In this paper, we investigate effective capacity by modeling a cognitive radio broadcast channel with one secondary transmitter (ST) and two secondary receivers (SRs) under quality-of-service constraints and interference power limitations. We initially describe three different cooperative channel sensing strategies with different hard-decision combining algorithms at the ST, namely OR, Majority, and AND rules. Since the channel sensing occurs with possible errors, we consider a combined interference power constraint by which the transmission power of the secondary users (SUs) is bounded when the channel is sensed as both busy and idle. Furthermore, regarding the channel sensing decision and its correctness, there exist possibly four different transmission scenarios. We provide the instantaneous ergodic capacities of the channel between the ST and each SR in all of these scenarios. Granting that transmission outage arises when the instantaneous transmission rate is greater than the instantaneous ergodic capacity, we establish two different transmission rate policies for the SUs when the channel is sensed as idle. One of these policies features a greedy approach disregarding a possible transmission outage, and the other favors a precautious manner to prevent this outage. Subsequently, we determine the effective capacity region of this channel model, and we attain the power allocation policies that maximize this region. Finally, we present the numerical results. We first show the superiority of Majority rule when the channel sensing results are good. Then, we illustrate that a greedy transmission rate approach is more beneficial for the SUs under strict interference power constraints, whereas sending with lower rates will be more advantageous under loose interference constraints.Comment: Submitted and Accepted to IEEE Globecom 201

Effective Capacity in Multiple Access Channels with Arbitrary Inputs

In this paper, we consider a two-user multiple access fading channel under quality-of-service (QoS) constraints. We initially formulate the transmission rates for both transmitters, where the transmitters have arbitrarily distributed input signals. We assume that the receiver performs successive decoding with a certain order. Then, we establish the effective capacity region that provides the maximum allowable sustainable arrival rate region at the transmitters' buffers under QoS guarantees. Assuming limited transmission power budgets at the transmitters, we attain the power allocation policies that maximize the effective capacity region. As for the decoding order at the receiver, we characterize the optimal decoding order regions in the plane of channel fading parameters for given power allocation policies. In order to accomplish the aforementioned objectives, we make use of the relationship between the minimum mean square error and the first derivative of the mutual information with respect to the power allocation policies. Through numerical results, we display the impact of input signal distributions on the effective capacity region performance of this two-user multiple access fading channel

Performance Analysis of Energy-Detection-Based Massive SIMO

Recently, communications systems that are both energy efficient and reliable are under investigation. In this paper, we concentrate on an energy-detection-based transmission scheme where a communication scenario between a transmitter with one antenna and a receiver with significantly many antennas is considered. We assume that the receiver initially calculates the average energy across all antennas, and then decodes the transmitted data by exploiting the average energy level. Then, we calculate the average symbol error probability by means of a maximum a-posteriori probability detector at the receiver. Following that, we provide the optimal decision regions. Furthermore, we develop an iterative algorithm that reaches the optimal constellation diagram under a given average transmit power constraint. Through numerical analysis, we explore the system performance

Design of a Cognitive VLC Network with Illumination and Handover Requirements

In this paper, we consider a cognitive indoor visible light communications (VLC) system, comprised of multiple access points serving primary and secondary users through the orthogonal frequency division multiple access method. A cognitive lighting cell is divided into two non-overlapping regions that distinguish the primary and secondary users based on the region they are located in. Under the assumption of equal-power allocation among subcarriers, each region is defined in terms of its physical area and the number of allocated subcarriers within that region. In this paper, we provide the lighting cell design with cognitive constraints that guarantee fulfilling certain illumination, user mobility, and handover requirements in each cell. We further argue that, under some conditions, a careful assignment of the subcarriers in each region can mitigate the co-channel interference in the overlapping areas of adjacent cells. Numerical results depict the influence of different system parameters, such as user density, on defining both regions. Finally, a realistic example is implemented to assess the performance of the proposed scheme via Monte Carlo simulations

Global Trade Models and Economic Policy Analyses: Relevance, Risks and Repercussions for Africa

Computable general equilibrium (CGE) models are widely used for trade policy analyses and recommendations. Simulation results from these models have also been used as a basis for offering advice to African countries on what positions to take in multilateral trade negotiations. There is however increasing discomfort with the use of these models for policy recommendations, especially in Africa. In this paper we compare the results of several CGE studies that examined the impact of potential Doha Round reforms on Africa and demonstrate that the results differ drastically both in terms of magnitude and direction. Part of the discrepancies in results can be explained by differences in database, model structure, and choice of parameters. Others are, however, difficult to explain because several studies either do not report key assumptions made or do not provide a clear description of how their framework differs from those in the literature. We also show that the modelling approach and the database used in most CGE studies do not take account of key features of African economies that have serious implications for the impact of trade reforms on Africa. Finally, we outline potential consequences of the misuse of CGE models for policy evaluation and suggest pitfalls to avoid if CGE model results are to be taken seriously by policy makers in Africa.Trade Reforms; CGE Models; Doha Round; Africa

Financing Development in Africa: Trends, Issues and Challenges

Access to finance is critical to successful development in Africa. This paper presents recent trends in various aspects of development finance and provides a critical assessment of the costs of meeting the Millennium Development Goals in the region. It also examines recent proposals for financing the MDGs. Furthermore, it examines the key international commitments made to Africa as well as the extent to which donors have fulfilled these commitments. Finally, it examines issues and challenges arising from recent initiatives on aid and debt.Development; Finance; MDGs; Africa; Trends