Maize Production and Agricultural Policies in Central America and Mexico

This paper reviews trends in maize production and consumption in Central America and Mexico in the context of the political and economic changes taking place in the region since the 1970s. The authors focus on the effects of the structural adjustment programs in the 1980s and 1990s. The analysis begins by reviewing the economic context in which maize production occurs in the region and the main economic policy instruments affecting the maize economy. Next, trends in maize consumption and production are analyzed, along with the main factors influencing maize production, including trends in the public financing of maize research and extension. The authors find that several factors related to structural adjustment have defined--and are still defining--the course of agriculture, including maize production, in the countries of the region. The impact of these factors on maize production, consumption, and import trends has been different in Central America and in Mexico. In particular, the reduction or complete elimination of production incentives, the reduction of trade barriers, the liberalization of input and product prices, the deregulation of the currency exchange rate, the control of inflation, and the restructuring of agricultural research systems between the public and the private sectors have determined how basic grains are produced in the region and how they will be produced in the future. Furthermore, the visible and increasing deterioration of the natural resource base has raised great concern about the need to promote more sustainable, environmentally friendly uses of production systems and natural resources.Agricultural and Food Policy, Crop Production/Industries,

On the distribution of an effective channel estimator for multi-cell massive MIMO

Accurate channel estimation is of utmost importance for massive MIMO systems to provide significant improvements in spectral and energy efficiency. In this work, we present a study on the distribution of a simple but yet effective and practical channel estimator for multi-cell massive MIMO systems suffering from pilot-contamination. The proposed channel estimator performs well under moderate to aggressive pilot contamination scenarios without previous knowledge of the inter-cell large-scale channel coefficients and noise power, asymptotically approximating the performance of the linear MMSE estimator as the number of antennas increases. We prove that the distribution of the proposed channel estimator can be accurately approximated by the circularly-symmetric complex normal distribution, when the number of antennas, M, deployed at the base station is greater than 10

Radar perspective on the variability of tropical convection characteristics over the southwestern Amazon and eastern Pacific regions

Spring 2003.Includes bibliographical references.The focus of this study is to evaluate the intra-regional and inter-regional variability of tropical convection characteristics in the southwestern Amazon and eastern Pacific warm-pool regions. Convection is examined using radar data collected during two tropical field experiments: TRMM-LBA (Tropical Rainfall Measuring Mission - Large Scale Biosphere-Atmosphere in Amazonia) and EPIC (Eastern Pacific Investigation of Climate Processes in the Coupled Ocean-Atmosphere System). In each of the two tropical regions studied, two distinct wind regimes have been observed to occur. In the southwest Amazon, one wind regime was called easterly regime and the other westerly regime. These changes in wind regime have been previously shown to be associated with the passage of baroclinic waves in the subtropical parts of Brazil and the formation of a synoptic-scale feature known as the South Atlantic Convergence Zone. In the east Pacific, one wind regime was called the northerly regime and the other southerly regime. The changes in wind regime over the east Pacific have been observed to be associated with the passage of easterly waves. The variability of the convective characteristics is evaluated in each region as a function of time of day and wind circulation. Some of the features used to evaluate the characteristics of convection include convective area, convective fraction, reflectivity profiles, rain rates, warm rain statistics and ice fraction. The results presented in this thesis showed that the easterly and northerly wind regimes more frequently featured characteristics of stronger convection: greater rain rates, greater reflectivities and convective fractions, deeper convective cores and smaller fractions of warm-rain-producing areas. The results also showed that the easterly regime was associated with higher ice fractions. The diurnal cycle results indicated that convection initiates in the morning and peaks in the afternoon over the southwest portion of the Amazon, whereas in the east Pacific the convection initiates after sunset and peaks just before sunrise. Variations associated with wind regime were shown to be more intense in the southwest Amazon. It was also found that the east Pacific region presented larger convective areas and convective fractions than the southwest portion of the Amazon. The eastern Pacific region was also associated with larger fractions of warm rain areas, but overall the fraction of rainfall owed to warm processes was very small and approximately the same in both regions.Sponsored by the National Science Foundation ATM-0002256

Channel estimation for massive MIMO TDD systems assuming pilot contamination and frequency selective fading

Channel estimation is crucial for massive multiple-input multiple-output (MIMO) systems to scale up multi-user MIMO, providing significant improvement in spectral and energy efficiency. In this paper, we present a simple and practical channel estimator for multipath multi-cell massive MIMO time division duplex systems with pilot contamination, which poses significant challenges to channel estimation. The proposed estimator addresses performance under moderate to strong pilot contamination without previous knowledge of the inter-cell large-scale fading coefficients and noise power. Additionally, we derive and assess an approximate analytical mean square error (MSE) expression for the proposed channel estimator. We show through simulations that the proposed estimator performs asymptotically as well as the minimum MSE estimator with respect to the number of antennas and multipath coefficients

On the application of massive mimo systems to machine type communications

This paper evaluates the feasibility of applying massive multiple-input multiple-output (MIMO) to tackle the uplink mixed-service communication problem. Under the assumption of an available physical narrowband shared channel, devised to exclusively consume data traffic from machine type communications (MTC) devices, the capacity (i.e., number of connected devices) of MTC networks and, in turn, that of the whole system, can be increased by clustering such devices and letting each cluster share the same time-frequency physical resource blocks. Following this research line, we study the possibility of employing sub-optimal linear detectors to the problem and present a simple and practical channel estimator that works without the previous knowledge of the large-scale channel coefficients. Our simulation results suggest that the proposed channel estimator performs asymptotically, as well as the MMSE estimator, with respect to the number of antennas and the uplink transmission power. Furthermore, the results also indicate that, as the number of antennas is made progressively larger, the performance of the sub-optimal linear detection methods approaches the perfect interference-cancellation bound. The findings presented in this paper shed light on and motivate for new and exciting research lines toward a better understanding of the use of massive MIMO in MTC networks

Channel estimation for massive MIMO TDD systems assuming pilot contamination and flat fading

Channel estimation is crucial for massive massive multiple-input multiple-output (MIMO) systems to scale up multi-user (MU) MIMO, providing great improvement in spectral and energy efficiency. This paper presents a simple and practical channel estimator for multi-cell MU massive MIMO time division duplex (TDD) systems with pilot contamination in flat Rayleigh fading channels, i.e., the gains of the channels follow the Rayleigh distribution. We also assume uncorrelated antennas. The proposed estimator addresses performance under moderate to strong pilot contamination without previous knowledge of the cross-cell large-scale channel coefficients. This estimator performs asymptotically as well as the minimum mean square error (MMSE) estimator with respect to the number of antennas. An approximate analytical mean square error (MSE) expression is also derived for the proposed estimator