Fernando Collantes Gutiérrez

Declive demográfico y cambio económico en las Áreas de montaña españolas, 1860–2000

espanolEste trabajo analiza el declive demografico experimentado por las 84 principales comarcas de montana espanolas, que representan mas del 20 por cien del territorio nacional y que, en torno a 1860, concentraban el 15 por cien de la poblacion del pais. La despoblacion de la montana espanola fue mas tardia que la de las montanas de los paises europeos mas desarrollados, si bien, una vez arrancada, fue muy intensa en perspectiva comparada. La despoblacion se generalizo durante la segunda mitad del siglo XX como consecuencia de importantes carencias en la calidad de vida de las montanas, carencias que a su vez estaban vinculadas al escaso grado de diversificacion de su economia y a las dificultades encontradas por la poblacion a la hora de acceder a equipamientos y servicios basicos. La despoblacion actuo ademas como mecanismo de seleccion economica, en la medida en que contribuyo a la desaparicion de la economia campesina tradicional y a la emergencia «por defecto» de formas mas diversificadas. EnglishThis paper analyses the demographic decline experienced by Spain�s 84 main mountainous districts, which cover more than 20 per cent of the national surface and had 15 per cent of the country�s total population in 1860. In Spain, mountain depopulation took place later than in other, more advanced European countries. However, once it started, the process was very intense in comparative perspective. Depopulation became a general phenomenon during the second half of the twentieth century as a consequence of significant shortcomings in rural welfare � these shortcomings were related in turn to low degrees of economic diversification and high levels of penalty on the access to basic equipment and services. Furthermore, depopulation acted as a mechanism for economic selection because it contributed to the disappearance of the traditional peasant economy and the «default» emergence of more diversified patterns.

A* Based Algorithm for Reduced Complexity ML Decoding of Tailbiting Codes

The A* algorithm is a graph search algorithm which has shown good results in terms of computational complexity for Maximum Likelihood (ML) decoding of tailbiting convolutional codes. The decoding of tailbiting codes with this algorithm is performed in two phases. In the first phase, a typical Viterbi decoding is employed to collect information regarding the trellis. The A* algorithm is then applied in the second phase, using the information obtained in the first one to calculate the heuristic function. The improvements proposed in this work decrease the computational complexity of the A* algorithm using further information from the first phase of the algorithm. This information is used for obtaining a more accurate heuristic function and finding early terminating conditions for the A* algorithm. Simulation results show that the proposed modifications decrease the complexity of ML decoding with the A* algorithm in terms of the performed number of operations.

Performance analysis of turbo decoding algorithms in wireless OFDM systems

Turbo codes are well known to be one of the error correction techniques which achieve closer results to the Shannon limit. Nevertheless, the specific performance of the code highly depends on the particular decoding algorithm used at the receiver. In this sense, the election of the decoding algorithm involves a trade off between the gain introduced by the code and the complexity of the decoding process. In this work we perform a thorough analysis of the different iterative decoding techniques and analyze their suitability for being implemented in the user terminals of new cellular and broadcast systems which are based on orthogonal frequency division multiplexing (OFDM). The analyzed iterative decoding algorithms are the max-log-MAP and the soft output Viterbi algorithm (SOVA), since both of them have a relative low computational complexity, simplifying their implementation in cost efficient terminals. Simulation results have been obtained for different encoder structures, block sizes and considering realistic channel conditions (an OFDM transmission over a wireless channel).

Performance analysis of diversity transmission modes in UTRA FDD under time-varying multipath channels

This paper analyses the effect of the propagation environment on the theoretical performance of the downlink transmit diversity techniques specified in the standard of the UTRA FDD mode. These techniques are classified as open loop and closed loop modes. Simulation results, obtained via a C simulator of the physical layer of UMTS, show the negative effect of mobility. The diversity gain obtained with the closed loop techniques, which are the best ones, drops drastically when the Doppler frequency rises. In that case, the open loop mode of diversity performs well. Nevertheless, it is highly dependent on the correlation of the antennas, which reduces the theoretical improvement.

Cartesian loop transmitter for TETRA

This paper presents the design process of a Cartesian loop transmitter and the simulated results include the effects of IQ modulator and demodulator, the requirements for the low-pass filter, open loop gain and loop delay, to verify the TETRA (Terrestrial European Trunked Radio) ACI mask and stability. The Cartesian feedback will suppose an algebraic equation for each sample at the input of the system. The modulator and demodulator errors are simulated using a matricial model for the gain and phase imbalances and DC-offset. The low-pass filter is simulated with a first order infinite impulsional response filter. The AM/AM and AM/PM power amplifier conversion is simulated with polynomic regression from Mitsubishi (M68749) measurements.

Inverted-F antenna with parasitic elements for TETRA handset

The paper presents the VSWR (voltage standing wave ratio) performance and radiation pattern for a low-profile IFA (inverted-F antenna) matched at 450 MHz. It is known that when the IFA height from a conducting ground plane is approximately one-tenth of the wavelength, the frequency bandwidth for a VSWR=2 criterion is approximately 8%. Nakano et al. (1998) proposed two L-figured parasitic elements added to the IFA to widen the VSWR bandwidth. It should be a large value for handset systems if this criterion is applied to structures working at lower frequencies (450 MHz) such as TETRA. A design of IFA is presented with different lengths for parasitic elements to generate two resonance frequencies near the central frequency of design. Antenna characteristics; mounted on the handset, are calculated using the method of moments (MoM) with wire grid models using numerical electromagnetic code NEC2. The mean effective gain (MEG) for different statistical distribution models of incident waves are presented too.

Two step SOVA-based decoding algorithm for tailbiting codes

In this work we propose a novel decoding algorithm for tailbiting convolutional codes and evaluate its performance over different channels. The proposed method consists on a fixed two-step Viterbi decoding of the received data. In the first step, an estimation of the most likely state is performed based on a SOVA decoding. The second step consists of a conventional Viterbi decoding that employs the state estimated in the previous step as the initial and final states of the trellis. Simulations results show a performance close to that of maximum-likelihood decoding.

Channel Independent Precoder for OFDM-Based Systems Over Fading Channels

In this paper we propose an independent channel precoder for orthogonal frequency division multiplexing (OFDM) systems over fading channels. The design of the precoder is based on the information redistribution of the input modulated symbols among the output precoded symbols. The proposed precoder decreases the variance of the instantaneous noise power at the receiver produced by the channel variability. The employment of an interleaver together with a precoding matrix whose size does not depend on the number of data carriers in an OFDM symbol allows different configurations of time-frequency diversity which can be easily adapted to the channel conditions. The precoder is evaluated with a modified Zero Forcing (ZF) equalizer whose maximum gain is constrained by means of a clipping factor. Thus, the clipping factor limits the noise power transfer in the receiver deprecoding block in low SNR conditions.

Evaluation of a Reduced Complexity ML Decoding Algorithm for Tailbiting Codes on Wireless Systems

Tailbiting convolutional codes will be used for several applications in new cellular mobile radio systems. This encoding method does not reset the encoder memory at the end of each data block, avoiding the overhead of the zero tail and improving the efficiency. Nevertheless, the absence of a known tail highly increases the complexity of the decoding process. Recently, the use of the A* algorithm has highly decreased the computational complexity of Maximum Likelihood (ML) decoding of tailbiting convolutional codes. The decoding proccess in this case requires two steps. In the first step, a typical Viterbi decoding is employed to collect information regarding the trellis. The A* algorithm is then applied in the second step, using the information obtained in the first step to calculate the heuristic function. The improvements proposed in this work decrease the computational complexity of the A* algorithm using further information from the first step of the algorithm. This information is used for finding early terminating conditions for the A* algorithm. Simulation results show that the proposed modification decreases the complexity of ML decoding with the A* algorithm in terms of the performed number of operations.

Performance analysis of downlink transmit diversity system applied to the UTRA FDD mode

This paper analyses the performance of the downlink transmit diversity techniques specified in the standard of the UTRA FDD mode. The Downlink DPCH can use either closed loop or open loop transmit diversity (space time block coding based transmit diversity) or closed loop transmit diversity to improve performance. In order to evaluate how these techniques work, a simulator of the physical layer of UMTS has been implemented in the C language. Simulation results show that the closed loop methods are the best way of getting diversity gain in radio environments with a low Doppler frequency, especially mode 2. Nevertheless, when a correct channel knowledge (high Doppler frequency) does not exist, the open loop transmit diversity mode is the best option.