Showing papers by "Instituto Politécnico Nacional published in 1997"

Bit-interleaved coded modulation

Abstract: It has been recognized by Zehavi (1992) that the performance of coded modulation over a Rayleigh fading channel can be improved by bit-wise interleaving at the encoder output, and by using an appropriate soft-decision metric as an input to a Viterbi (1990) decoder. The paper presents in a comprehensive fashion the theory underlying bit-interleaved coded modulation, provides tools for evaluating its performance, and gives guidelines for its design.

Serial concatenation of interleaved codes: performance analysis, design and iterative decoding

Abstract: A serially concatenated code with interleaver consists of the cascade of an outer encoder, an interleaver permuting the outer codewords bits, and an inner encoder whose input words are the permuted outer codewords. The construction can be generalized to h cascaded encoders separated by h-1 interleavers. We obtain upper bounds to the average maximum-likelihood bit error probability of serially concatenated block and convolutional coding schemes. Then, we derive design guidelines for the outer and inner encoders that maximize the interleaver gain and the asymptotic slope of the error probability curves. Finally, we propose a new, low-complexity iterative decoding algorithm. Throughout the paper, extensive comparisons with parallel concatenated convolutional codes known as "turbo codes" are performed, showing that the new scheme can offer superior performance.

Compression of high-energy laser pulses below 5 fs

Abstract: High-energy 20-fs pulses generated by a Ti:sapphire laser system were spectrally broadened to more than 250 nm by self-phase modulation in a hollow fiber filled with noble gases and subsequently compressed in a broadband high-throughput dispersive system. Pulses as short as 4.5 fs with energy up to 20-microJ were obtained with krypton, while pulses as short as 5 fs with energy up to 70 microJ were obtained with argon. These pulses are, to our knowledge, the shortest generated to date at multigigawatt peak powers.

Asymptotic zero-transition activity encoding for address busses in low-power microprocessor-based systems

Abstract: In microprocessor-based systems, large power savings can be achieved through reduction of the transition activity of the on- and off-chip buses. This is because the total capacitance being switched when a voltage change occurs on a bus line is usually sensibly larger than the capacitive load that must be charged/discharged when internal nodes toggle. In this paper, we propose an encoding scheme which is suitable for reducing the switching activity on the lines of an address bus. The technique relies on the observation that, in a remarkable number of cases, patterns traveling onto address buses are consecutive. Under this condition it may therefore be possible, for the devices located at the receiving end of the bus, to automatically calculate the address to be received at the next clock cycle; consequently, the transmission of the new pattern can be avoided, resulting in an overall switching activity decrease. We present analytical and experimental analyses showing the improved performance of our encoding scheme when compared to both binary and Gray addressing schemes, the latter being widely accepted as the most efficient method for address bus encoding. We also propose power and timing efficient implementations of the encoding and the decoding logic, and we discuss the applicability of the technique to real microprocessor-based designs.

Nonlinear observer design to synchronize hyperchaotic systems via a scalar signal

Abstract: In this work control theory is used to formalize hyperchaos synchronization as a nonlinear observer design issue. Following this approach, a new systematic tool to synchronize a class of hyperchaotic systems via a scalar transmitted signal is developed. The proposed technique has been applied to synchronize two well-known hyperchaotic systems.

The effect of surface roughness on multifrequency polarimetric SAR data

Abstract: In this paper, multifrequency, polarimetric SAR data acquired during the second SIR-C/XSAR mission over the Matera (Italy) test site are analyzed. The main objective of the study is to assess the possibility of extracting relevant information about surface roughness using polarimetry. The methodology is expected to be of interest either for deriving maps of surface roughness or for setting up soil moisture retrieving procedure based on a preliminary estimation of soil roughness. After a description of ground data, experimental, and theoretical backscattering coefficients are investigated. In a further step, the dependence of copolarized correlation coefficient on the roughness and moisture states is addressed. In order to investigate the polarization effect, the correlation coefficient is described for any set of orthogonal polarizations. The results indicate an enhanced sensitivity of the correlation coefficient on roughness using circular polarizations. The analysis is subsequently extended to additional data sets acquired in the frame of SIR C/XSAR experiment (Les Landes forest, France) and AIRSAR experiment (Chickasha, USA). These further investigations confirm the trend observed on Matera data.

A novel-high energy pulse compression system: generation of multigigawatt sub-5-fs pulses

Abstract: Powerful techniques for spectral broadening and ultrabroadband dispersion control, which allow the compression of high-energy femtosecond pulses to a duration of a few optical cycles, are presented. Spectral broadening by propagation along hollow-core fused silica fiber filled with atomic and molecular gases is studied under two excitation regimes with high-energy input pulses of 140 fs and 20 fs duration respectively. Conditions for optimum pulse compression are outlined considering the role of self-phase modulation and gas dispersion in the two regimes. With 20 fs input pulses and under optimum compression conditions we demonstrate a pulse shortening down to 4.5 fs with output energy up to 70 μJ using a high-throughput prism-chirped-mirror delay line. These pulses are the shortest generated to date at multigigawatt peak power. PACS: 42.65.Re; 42.65.Vh Ultrashort-pulse lasers are the most important experimental tools for investigating fast-evolving atomic and molecular dynamics in physics, chemistry, and biology. In the last few years, great technological advances have been made in the field of ultrafast pulse generation. New mode-locking techniques such as additive-pulse mode-locking and Kerr-lens mode-locking have been successfully used for femtosecond pulse generation from a wide range of solid-state laser oscillators [1]. Using chirped mirrors [2] for intracavity dispersion control, pulses down to 7.5 fs have been directly generated by a Kerr-lens mode-locked Ti:sapphire oscillator [3] and, more recently, 6.5-fs pulses have been obtained using broadband semiconductor saturable absorbers for self-starting [4]. Ti:sapphire amplifiers seeded by femtosecond laser oscillators can now generate pulses of 20–30 fs with gigawatt [5, 6] or terawatt [7–9] peak power at repetition rates in the kHz and 10 Hz regimes, respectively. Ultrashort pulses can also be generated by extracavity compression techniques, in which the pulses are spectrally broadened upon propagation in a suitable nonlinear waveguide and subsequently compressed in a carefully designed optical dispersive delay line. Spectral broadening of laser pulses by self-phase modulation (SPM) in a single-mode optical fiber is a well-established technique: pulses down to 6 fs were obtained in 1987 from 50-fs pulses from a mode-locked dye laser [10]. More recently 13-fs pulses from a cavity-dumped Ti:sapphire laser were compressed to 5 fs with the same technique [11]. However, the use of single-mode fibers limits the pulse energy to a few nanojoules. A powerful pulse compression technique based on spectral broadening in an hollow fiber filled with noble gases has demonstrated the capability of handling highenergy pulses (sub-mJ range) [12]. This technique presents the advantages of a guiding element with a large diameter mode and of a fast nonlinear medium with high threshold for multiphoton ionization. New concepts in the construction of dispersive delay lines have been applied in the development of specially designed chirped mirrors for fine control of cubic and quartic phase dispersion terms over a large spectral bandwidth [3]. The implementation of the hollow-fiber technique using 20-fs seed pulses from a Ti:sapphire system [5] and a high-throughput broadband dispersive delay line consisting of prisms and chirped mirrors has recently permitted the generation of multigigawatt sub-5 fs pulses [13]. In this paper we present a comprehensive analysis of compression experiments with high-energy femtosecond pulses performed using gas-filled hollow fibers. Spectral broadenings obtained in different gases are compared for 140-fs and 20-fs input pulses generated by Ti:sapphire laser systems, and the optimum conditions for pulse compression are outlined considering the role of SPM and gas dispersion. A new ultrabroadband prism-chirped-mirror dispersive delay line, characterized by a high throughput and dispersion control up to the fourth order, is described in detail. The paper is organized as follows. In Sect. 1 we provide a description of hollow fiber modes and discuss the major advantages of this device compared to optical fibers. Sect. 2 reports on typical spectral broadenings achieved under different excitation conditions. In Sect. 3 we report on the characteristics of the prism-chirped-mirror compressor and discuss the experimental results obtained with 20-fs input pulses. Under optimum compression conditions we show a pulse shortening down to 4.5 fs with output energy up to 70 μJ. These pulses are the

MACA-BI (MACA By Invitation)-a receiver oriented access protocol for wireless multihop networks

Abstract: A novel wireless MAC protocol named MACA-BI (MACA By Invitation) is introduced. MACA-BI is a simplified version of the well known MACA (Multiple Access Collision Avoidance) protocol, which is based on the request to send/clear to send (RTS/CTS) handshake and which has inspired the IEEE 802.11 wireless LAN standard. In MACA-BI, the RTS part of the RTS/CTS handshake is suppressed, leaving only the clear to send a control message which can be viewed as an "invitation" by the receiver to transmit. This reduction greatly improves the efficiency when radio turn-around time is significant with respect to packet transmission time. Yet, it preserves the "data" collision free property of MACA. Simulation results for various multihop topologies show that, when the traffic characteristics are stationary or predictable, MACA-BI outperforms several known multiple access protocols, especially when "hidden terminal" conditions are predominant.

Polymer blends based on polyolefin elastomer and polypropylene

Abstract: A new family of homogeneous polyolefin polymers that exhibit unique molecular and rheological properties designated polyolefin elastomers (POEs) are characterized by a narrow molecular weight and high degrees of comonomer distribution. Because these copolymers are often elastomeric in nature, one of the uses for these materials is as impact properties improver for brittle polymers such as polypropylene at low temperatures. In this work a study was carried out about the effectiveness of the polyethylene elastomer (POE) as an impact modifier for polypropylene in relation to the traditional modifier EPDM. In this study the flow properties of of the POE/PP and EPDM/PP blends were also evaluated. The blends were analyzed by solid-state 13C nuclear magnetic resonance (13C-NMR) spectroscopy, scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The results showed that PEE/PP and EPDM/PP blends present a similar crystalline behavior, which resulted in a similar mechanical performance of the blends, on the composition analyzed. It was also verified that the POE/PP blend presents lower torque values than the EPDM/PP blend, which indicates a better processability when POE is used as an impact modifier. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2005–2014, 1997

MACA-BI (MACA by invitation). A wireless MAC protocol for high speed ad hoc networking

Abstract: This paper introduces a new wireless MAC protocol, MACA-BI (MACA by invitation). The protocol is a simplified version of the well known MACA (multiple access collision avoidance) based on the request to send/clear to send (RTS/CTS) handshake. The clear to send (CTS) control message is retained, while the request to send (RTS) part of the RTS/CTS handshake is suppressed. MACA-BI, preserving the data collision free property, is more robust than MACA to problems such as protocol failures (control packet collision and corruption) and finite turn-around time. Analytic results for a 1 Mbps single-hop far-field wireless network, and simulation results for a 10 Mbps multi-hop near-field ATM wireless indoor network, show that MACA-BI outperforms other multiple access protocols in high speed, steady traffic environments (e.g. ATM VBR and CBR) and where the propagation delay can be neglected (typically indoor).

Impact of diversity reception on fading channels with coded modulation. I. Coherent detection

Abstract: We address the problem of designing and analyzing the performance of a coded modulation scheme for the fading channel when space diversity is used. Under fairly general conditions, a channel affected by fading can be turned into an additive white Gaussian noise (AWGN) channel by increasing the number of diversity branches. Consequently, it can be expected (and is indeed verified by our analyses and simulations) that a coded modulation scheme designed to be optimal for the AWGN channel also will perform asymptotically well on a fading channel with diversity. This paper presents bounds on the bit-error probability of a system with coded modulation and diversity for space- and time-correlated Rician flat fading. Specifically, we derive a new method which allows evaluation of the pairwise error probability extremely easily, as well as accurately and computationally fast. The accuracy achieved improves considerably on the widely used, but rather loose Chernoff bound. Starting from this analysis, we study the asymptotic behavior of the fading channel with diversity as the number of diversity branches increases, and we address the effects of diversity on coded modulation performance and design criteria, including the effect on interleaver depth (which affects the total delay of the system).

Biodiversity of Bradyrhizobia Nodulating Lupinus spp.

Abstract: The genetic structure of Bradyrhizobium isolates recovered from three Lupinus species (Lupinus campestris, Lupinus montanus, and Lupinus exaltatus) grown in Mexico was examined. Among 41 Bradyrhizobium isolates, 18 electrophoretic types (ETs) were distinguished by multilocus enzyme electrophoresis of five metabolic enzymes. The mean genetic diversity, 0.64, indicated that there was great genetic diversity in the population sampled. Most isolates (63%) fell into two closely related clusters (clusters I and II) and were the types most frequently isolated from the root nodules of L. montanus and L. campestris. ET cluster III isolates were frequent nodule occupants of L. exaltatus. The isolates also were assigned to three main groups by using Curie point pyrolysis mass spectrometry. In general, the multilocus enzyme electrophoretic data and pyrolysis mass spectrometric data agreed. We determined the 16S rRNA sequences of representative Lupinus isolates and of Bradyrhizobium japonicum USDA 6T and found that the lupine isolates were highly related to the B. japonicum type strain, although not all B. japonicum type strains (subcultures maintained in different bacterial collections) had identical small-subunit rRNA.

New Methods of Free-Radical Perfluoroalkylation of Aromatics and Alkenes. Absolute Rate Constants and Partial Rate Factors for the Homolytic Aromatic Substitution by n-Perfluorobutyl Radical.

Abstract: New methods of free-radical perfluoroalkylation of aromatics and alkenes are reported. n-C4F9I has been utilized as source of C4F9• radical through iodine abstraction by phenyl or methyl radical. The reaction with alkenes, carried out in the presence of catalytic amount of Cu(OAc)2, leads to substitution by a mechanism substantially identical to the aromatic substitution and not to the usual chain addition of perfluoroalkyl group and iodine atom to the double bond. This has allowed to measure for the first time the absolute rate constants and the partial rate factors for the homolytic aromatic perfluoroalkylation by competition kinetics. The C4F9• radical shows a clear-cut electrophilic character in the aromatic substitution, as already reported for the addition to alkenes, but the low regio- and chemoselectivities suggest that the polar effect is not the main factor in determining the high reactivity of perfluoroalkyl radicals toward aromatics (105−106 M-1 s-1, 2−3 orders of magnitude more reactive than ...

The influence of the preparation method on the surface structure of ZnAl2O4

Abstract: The effect of preparation method (sol-gel, wet mixing or coprecipitation) on the surface structure of zinc aluminate is reported. Coprecipitated and sol-gel prepared ZnAl2O4 were found to be present spinel structure. If calcium was added during synthesis, it was found to be deposited on top of the spinel surface while, if tin was added, the surface of ZnAl2O4 was reconstructed. These aluminates were impregnated with platinum and tested in the isobutane dehydrogenation. The characterization techniques used were LEIS and X-ray diffraction.

A time-domain optical transmission system simulation package accounting for nonlinear and polarization-related effects in fiber

Abstract: The fast-paced evolution of long-haul and high-bit-rate terrestrial and submarine optical transmission links requires powerful analysis tools that take into account all the relevant phenomena in the fiber. To provide such a tool, we developed a time-domain optical system simulation package, integrated in the TOPSIM simulation environment. The fiber simulation module makes use of the vector form of the propagation equations to account for the quasi-degenerate two-mode (the two polarizations) medium propagation characteristics. This way, all polarization-related effects and their interplay with the other linear and nonlinear phenomena in the fiber can be accurately modeled. In particular, the fiber third-order susceptivity, responsible for all major nonlinear effects, is expressed in its actual vector form, so that nonlinear polarization mode coupling could be accounted for. Conventional birefringence and PMD are generated using appropriate random models. A novel feature of the simulator is that it uses time-domain digital filters to simulate dispersion effects, as opposed to the usual FFT-based algorithms. This approach leads to more efficient computing for a wide range of bandwidth and dispersion values. We present the fiber simulation module in detail. As an example of the use of the simulation package, the analysis of a long-haul two-channel transoceanic WDM transmission system is presented.

Ingold−Fischer “Persistent Radical Effect”, Solvent Effect, and Metal Salt Oxidation of Carbon-Centered Radicals in the Synthesis of Mixed Peroxides from tert-Butyl Hydroperoxide

Abstract: Mixed peroxides are formed from tert-butyl hydroperoxide (TBH), tert-butyl peroxalate (TBP), and a variety of substrates (p-cresol, cyclohexene, styrene, α-methylstyrene, acrylonitrile, 2-methylcyclohexanone). Also, the oxidation of THF in the presence of acrylonitrile under the same conditions gives the mixed peroxide, generated by addition of the tetrahydrofuranyl radical to the double bond and the cross-coupling of the radical adduct with the tert-butylperoxyl radical. Similarly, benzoyl peroxide, TBH, and acrylonitrile give the mixed peroxide by oxidative arylation of the double bond. Paradoxically, TBH acts as effective inhibitor of the polymerization of vinyl monomers (acrylonitrile, styrene). An overall kinetic evaluation suggests that the conditions for the Ingold−Fischer “persistent radical effect”, characterized by the simultaneous formation of a persistent and a transient radical, are fulfilled in all cases. The reactions are strongly affected by solvents, which form hydrogen bonds with TBH. Ca...

Germination of Nosema algerae (Microspora) Spores: Conditional Inhibition by D2O, Ethanol and Hg2+ Suggests Dependence of Water Influx upon Membrane Hydration and Specific Transmembrane Pathways

Abstract: The germination of microsporidian spores under conditions expected to affect water flow across the plasma membrane-wall complex was studied by assessing their responses to in vitro stimulation with Na+ or K+. Partial or full substitution of common water with D2O, which more effectively coats ions and electrostatically-charged cell surfaces with relatively stable hydration layers, delayed and inhibited spore germination in a concentration-dependent manner; yet, preincubation in 100% D2O did not change the normal response to standard stimulation. Water structure-breaking conditions, such as an increase in temperature (within the 15 degrees C to 40 degrees C range) or in ionic strength (1- to 10-fold normal), opposed the inhibition by D2O and allowed significant stimulation by Li+, the monovalent cation with the largest hydration diameter and a usually weak stimulant action on the spores. Ethanol, known to reduce water permeation across cell membranes and phospholipid bilayers, also caused a powerful and dose-dependent (1% to 4% v/v) inhibition of spore germination, but pretreatment with ethanol did not affect the normal response. HgCl2, an inhibitor of specific water channels, blocked spore germination at just 250 microM in the normal stimulation solution irrespective of the temperature, and permitted only a delayed response in high salt stimulation solutions. However,the inhibition by Hg2+ was abolished by the simultaneous presence of 2-mercaptoethanol in the medium. These results suggest (1) that spore germination is keenly dependent upon the hydration states of both the plasma membrane-wall complex and the stimulant ions, and (2) that osmotic water flows into the spores through specific transmembrane pathways with critical sulfhydryl groups, i.e., analogous to the water channels that facilitate water movements across the plasma membranes of highly permeable cells.