Two-dimensional Modulation Research Articles

Identification of Hydrogen Bonds to the Rieske Cluster through the Weakly Coupled Nitrogens Detected by Electron Spin Echo Envelope Modulation Spectroscopy

The interaction of the reduced[2Fe-2S] cluster of isolated Rieske fragment from the bc1 complex of Rhodobacter sphaeroides with nitrogens (14N and 15N) from the local protein environment has been studied by X- and S-band pulsed EPR spectroscopy. The two-dimensional electron spin echo envelope modulation spectra of uniformly 15N-labeled protein show two well resolved cross-peaks with weak couplings of approximately 0.3-0.4 and 1.1 MHz in addition to couplings in the range of 6-8 MHz from two coordinating Ndelta of histidine ligands. The quadrupole coupling constants for weakly coupled nitrogens determined from S-band electron spin echo envelope modulation spectra identify them as Nepsilon of histidine ligands and peptide nitrogen (Np), respectively. Analysis of the line intensities in orientation-selected S-band spectra indicated that Np is the backbone N-atom of Leu-132 residue. The hyperfine couplings from Nepsilon and Np demonstrate the predominantly isotropic character resulting from the transfer of unpaired spin density onto the 2s orbitals of the nitrogens. Spectra also show that other peptide nitrogens in the protein environment must carry a 5-10 times smaller amount of spin density than the Np of Leu-132 residue. The appearance of the excess unpaired spin density on the Np of Leu-132 residue indicates its involvement in hydrogen bond formation with the bridging sulfur of the Rieske cluster. The configuration of the hydrogen bond therefore provides a preferred path for spin density transfer. Observation of similar splittings in the 15N spectra of other Rieske-type proteins and [2Fe-2S] ferredoxins suggests that a hydrogen bond between the bridging sulfur and peptide nitrogen is a common structural feature of [2Fe-2S] clusters.

Large penalty of hybrid diversity with uncoded modulation in slow rayleigh fading

In hybrid selection/maximal ratio combining (H-S/MRC) the receiver selects the L branches with the largest signal-to-noise ratios (SNRs) from N available diversity branches and performs maximal ratio combining (MRC). A penalty is incurred with respect to MRC which can be defined as the increase in SNR required for H-S/MRC to achieve the same symbol error probability as MRC. Recently, this penalty was investigated specifically for two-dimensional modulations. In this paper, we derive the asymptotic SNR penalties in slow Rayleigh fading for small and large values of SNR for general uncoded signaling constellations.

Two- and one-dimensional magnetic modulations of low-field-induced phases in TbRu 2Si 2 at low temperatures

Pulsed-neutron diffraction studies have been performed on a TbRu 2Si 2 single crystal applying magnetic field along the c-axis. We have determined long-period commensurate magnetic structures for low-field-induced phases at 1.6 K below the lowest transition point T t ′ = 4.2 K . We find a two-dimensional spin modulation with major magnetic vectors of (3/13 0 0) and (3/13±3/13 0) with the sequence ○44¯54¯44¯ containing one ferrimagnetic plane (mixed plane) for every 26 ferromagnetic (2 0 0)/(1 0 0) planes along the a-axis for the first-observed low-field phase. This magnetic modulation is nearly one-dimensional but definitely two-dimensional with a magnetic unit cell of 13 a × 13 a × c , where a and c are the lattice constants of the original chemical cell. For a further increase of magnetic field secondly one-dimensional spin modulation appears with (3/13 0 0) of the sequence 54¯54¯44¯ of 26 ferromagnetic (2 0 0)/(1 0 0) planes without any mixed plane along the a-axis, whose magnetic unit cell is 13 a × a × c .

Characterization of the Exchangeable Protons in the Immediate Vicinity of the Semiquinone Radical at the QH Site of the Cytochrome bo3 from Escherichia coli

The cytochrome bo3 ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O2 to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. Two-dimensional electron spin echo envelope modulation has been applied to explore the exchangeable protons involved in hydrogen bonding to the semiquinone by substitution of 1H2O by 2H2O. Three exchangeable protons possessing different isotropic and anisotropic hyperfine couplings were identified. The strength of the hyperfine interaction with one proton suggests a significant covalent O-H binding of carbonyl oxygen O1 that is a characteristic of a neutral radical, an assignment that is also supported by the unusually large hyperfine coupling to the methyl protons. The second proton with a large anisotropic coupling also forms a strong hydrogen bond with a carbonyl oxygen. This second hydrogen bond, which has a significant out-of-plane character, is from an NH2 or NH nitrogen, probably from an arginine (Arg-71) known to be in the quinone binding site. Assignment of the third exchangeable proton with smaller anisotropic coupling is more ambiguous, but it is clearly not involved in a direct hydrogen bond with either of the carbonyl oxygens. The results support a model that the semiquinone is bound to the protein in a very asymmetric manner by two strong hydrogen bonds from Asp-75 and Arg-71 to the O1 carbonyl, while the O4 carbonyl is not hydrogen-bonded to the protein.

Exact Evaluation of Bit- and Symbol-Error Rates for Arbitrary Two-Dimensional Modulation and Nonuniform Signaling in AWGN Channel

Exact evaluation of the bit- and symbol-error rates in a 2-D constellation has closed solutions for particular modulations and/or bits-to-symbol mapping. To solve this fundamental problem of digital communications, we propose a general method which yields the exact results for arbitrary modulation symbols' set, arbitrary bits-to-symbol mapping, and deals with the case of nonuniform signaling. These three conditions define any digital transmission using memoryless modulation, so the proposed method is a general tool solving all problems tackled in the literature, under constraints imposed on one or more of the parameters defining the modulation or signaling type. Such an evaluation tool is of practical importance during the design of the modulation. Our analysis and numerical simulations show the advantages offered by the new method when compared with the bounding techniques.

Magnetotransport measurements on overgrown cleaved edge heterostructures in Hall bar geometry

We present new methods to pattern and characterize the overgrown cleaved egde (CE) of GaAs/AlGaAs heterostructures. Four point measurements, which allow a direct measurement of the magnetotransport coefficients ρ xx and ρ xy of a two-dimensional electron system on the CE, have been out of reach so far. By means of novel preparation techniques a contacted Hall bar structure can be created on the edge of the cleavage plane. The potential of the new method is first tested on a system which is density modulated in a direction parallel to the current flow due to an underlying GaAs/AlGaAs superlattice. To create a two-dimensional electric modulation we managed to pattern the active area of the Hall bar with periodically arranged lines in the direction perpendicular to the MBE-grown superlattice. The resulting unit cells are reflected in magnetoresistance oscillations associated with the most prominent one-dimensional Fermi contours along x- and y-direction.

Checkerboard-Like Charge Ordering in Underdoped Cuprates

Recent scanning tunneling microscopy measurements which indicate the formation of two-dimensional density modulations at some doping levels in cuprates were reviewed. A model of hard-core bosons which represent bound hole pairs in the two-dimensional doped antiferromagnet was discussed in the context of these experimental results. By means of an exact numerical diagonalization it was shown that the Coulomb repulsion between bosons brings about the formation of charge modulations.

Cluster N1 of complex I from Yarrowia lipolytica studied by pulsed EPR spectroscopy

After reduction with nicotinamide adenine dinucleotide (NADH), NADH:ubiquinone oxidoreductase (complex I) of the strictly aerobic yeast Yarrowia lipolytica shows clear signals from five different paramagnetic iron-sulfur (FeS) clusters (N1-N5) which can be detected using electron paramagnetic resonance (EPR) spectroscopy. The ligand environment and the assignment of several FeS clusters to specific binding motifs found in several subunits of the complex are still under debate. In order to characterize the hyperfine interaction of the surrounding nuclei with FeS cluster N1, one- and two-dimensional electron spin echo envelope modulation experiments were performed at a temperature of 30 K. At this temperature only cluster N1 contributes to the overall signal in a pulsed EPR experiment. The hyperfine and quadrupole tensors of a nitrogen nucleus and the isotropic and dipolar hyperfine couplings of two sets of protons could be determined by numerical simulation of the one- and two-dimensional spectra. The values obtained are in perfect agreement with a ferredoxin-like binding structure by four cysteine amino acid residues and allow the assignment of the nitrogen couplings to a backbone nitrogen nucleus and the proton couplings to the beta-protons of the bound cysteine residues.

Channel Data Retrieval in Page-Based Holographic Data Storage

A channel data retrieval scheme for page-based holographic data storage has been developed. It includes several signal-processing blocks for retrieving data reliably under noisy condition, namely, a frame grabber for capturing data adaptively in a rotating disk, a mark detector for tracing the data location, an equalizer for compensating interference, a two-dimensional modulation decoder for suppressing errors, and a three-dimensional error correction decoder for correcting errors generated by the holographic storage channel. The proposed scheme has been implemented with a field-programmable gate array chip and applied to actual data retrieval from holographic data storage. With the proposed channel data retrieval hardware, the user video data is successfully reconstructed from the holographic disk, which contains video data as holograms.

Binary phase only filters for rotation and scale invariant pattern recognition with the joint transform correlator

The joint transform correlator (JTC) is one of the two main optical image processing architecture which provides a highly effective way of comparing images in a wide range of applications. Traditionally, an optical correlator is used to compare an unknown input scene with a pre-captured reference image library, to detect if the reference occurs within the input. Strength of the correlation signal decreases rapidly as the input object rotates or varies in scale relative to the reference object. The aim of this paper is to overcome the intolerance of the JTC to rotation and scale changes in the target image. Many JTC systems are constructed with the use of ferroelectric liquid crystal (FLC) spatial light modulators (SLMs) as they provide fast two-dimensional binary modulation of coherent light. Due to the binary nature of the FLC SLMs used in the JTC systems, any image addressed to the device need to have some form of thresholding. Carefully thresholding the grey scale input plane and the joint power spectrum (JPS) has significant effect on the quality of correlation peaks and zero order (DC) noise. A new thresholding technique to binarise the JPS has been developed and implemented optically. This algorithm selectively enhances the desirable fringes in the JPS which provide correlation peaks of higher intensity. Zero order noise is further reduced when compared to existing thresholding techniques. Keeping in mind the architecture of the JTC and limitations of FLC SLMs, a new technique to design rotation and scale invariant binary phase only filters for the JTC architecture is presented. Filers design with this technique have limited dynamic range, higher discriminability among target and non-target objects, and convenience for implementation on FLC SLMs. Simulation and experiments shows excellent results of various rotation and scale invariant filters designed with this technique. A rotation invariant filter is needed for various machine vision applications of the JTC. By fixing the distance between camera and input object, the scale sensitivity of the correlator can be avoided. In contrast to the industrial machine vision applications, scale factor is very important factor for the applications of the JTC systems in defence and security. A security system using a scale invariant JTC will be able to detect a target object well in advance and will provide more time to take a decision.

Peak power reduction for OFDM systems with orthogonal pilot sequences

In this paper, a novel peak-to-average power reduction approach for orthogonal frequency division multiplexing (OFDM) has been addressed. Two-dimensional pilot-symbol assisted modulation (2D-PSAM) is employed in coherent OFDM for channel estimation, and it is based on inserting known symbols spread throughout the 2D time-frequency grid. These pilot symbols are employed to simultaneously perform distortionless peak power reduction with a suboptimum technique named orthogonal pilot sequences (OPS), which reduces additional system complexity and side information compared to optimum pilot values. This proposal attains a further step over other previous works, since this set of sequences allows blind detection at the receiver without prior knowledge of any side information.

Estimating errors in transmission systems due to impulse noise

Impulse noise is a major limiting factor to the performance of digital subscriber lines, powerline communication systems, and digital TV. Research has shown that the impulse noise in all those transmission media has very similar statistical properties. Using noise distribution parameters for the specific case of telephone lines, this paper suggests a Bernoulli–Weibull model of impulse noise in the local loop at symbol level. This model is then applied to develop closed-form expressions for the error probability of PAM and single carrier QAM. A novel extension of the distribution of the impulse noise amplitudes to two dimensions is introduced to enable the analysis of two-dimensional modulations, such as QAM. It is shown that earlier noise models, which assume Gaussian distributed impulse amplitudes or Rayleigh distributed powers, produce overly optimistic results for the errors owing to impulse noise in comparison with the Bernoulli–Weibull model presented here. The error performance of multicarrier QAM (DMT) is also evaluated based on the Bernoulli–Weibull model, however this is done numerically owing to the lack of an analytical solution. It is shown that multicarrier QAM performs better than single carrier systems but only for low impulse power and low impulse probability. The analytical framework presented here is also directly applicable to powerline transmission and DVB-T systems as they have very similar impulse noise statistics to those in telephone lines.

Two-Dimensional Modulation Transfer Function Model Based on Visual Characteristics for Assimilation Phenomenon

This paper describes an assimilation phenomenon on monochrome images divided from the size of patterns, and proposes an analytical model about this phenomenon. The frequency spectrum of a sample pattern by the discrete Fourier Transform and human visual characteristics by the Butterworth type Low Pass Filter (LPF) are formulized. A two-dimensional model of the relation between the samples and human visual characteristics, which is additionally introduced the concept of a noise level is proposed, and discussed with the assimilation phenomenon qualitatively and quantitatively. Then, a comparative examination of this model is carried out by subjective evaluation, using the visual distance, and the validity is verified. As a result, the analysis using this proposal model, the cut-off frequency of this LPF is set to 20 cycle/degree, its attenuance is set to -20 dB/decade, and the noise level presumes -50 dB. The results almost conformed qualitatively to the results of evaluation experiments in this example.

Error Performance of Selection Combining and Switched Combining Systems in Rayleigh Fading Channels With Imperfect Channel Estimation

In this paper, we present a general analysis of the performance of selection combining (SC), switch-and-stay combining (SSC), and switch-and-examine combining (SEC) systems in Rayleigh fading channels with imperfect channel estimation (ICE). The complex channel estimate and the actual fading are modeled as jointly Gaussian random variables. For SC systems with channel estimation error, closed-form expressions are obtained for the error rates of M/sub s/-ary pulse amplitude modulation (PAM) and rectangular-quadrature amplitude modulation (QAM), and simple single integral formulas with finite integration limits are derived for the symbol error probability of arbitrary two-dimensional (2-D) modulation formats. These error probability expressions are then applied to three types of channel estimation errors potentially encountered in practical systems to study their impact on the performance of selection diversity. Moreover, single integral formulas with finite integration limits are derived for the performance of SSC and SEC systems with minimum mean square error (MMSE) channel estimation. Optimum switching thresholds for 2-D modulation formats with MMSE based switched combining are acquired through numerical computation.

Mn2+-adenosine nucleotide complexes in the presence of the nitrogenase iron-protein: detection of conformational rearrangements directly at the nucleotide binding site by EPR and 2D-ESEEM (two-dimensional electron spin-echo envelope modulation spectroscopy).

Both ATP and a bivalent nucleotide-bound metal activator, normally Mg2+, are required for nitrogenase activity. EPR and ESEEM (electron spin-echo envelope modulation) measurements have been carried out on adenosine nucleotides in which the Mg2+ ion that is usually bound is replaced by Mn2+ in the presence of Kp2 (nitrogenase Fe-protein from Klebsiella pneumoniae). The Mn2+ zero-field splitting parameters have been determined from the EPR-spectrum to be |D|=0.0125 cm(-1) with a rhombicity lambda=E/D=0.31 by direct diagonalization of the complete spin Hamiltonian. ESEEM spectra of the Fe-protein with MnADP and MnATP both show an ESEEM line pair with one signal component at about 3.6 MHz and a relatively broad resonance at 8 MHz originating from a superhyperfine coupling to a 31P nuclear spin from one or more directly co-ordinated phospho group(s) of the nucleotide. A pronounced resonance overlapping the low-frequency component of the (31)P-signal at about 3.5 MHz is attributed to an interaction of Mn2+ with univalent 23Na nuclei. ESEEM lines at frequencies <3.5 MHz have been ascribed to interactions with 14N nuclei. Differences in the 14N features that depend on the type of nucleotide are consistent with substantial conformational rearrangements at the nucleotide-binding site upon hydrolysis. In addition, four-pulse HYSCORE (hyperfine sublevel correlation spectroscopy) experiments not only confirm the three-pulse ESEEM results, but also achieve significantly better spectral deconvolution, especially of the 31P-couplings, and demonstrate that the nucleotide is at least a unidentate ligand of Mn2+. Moreover it was also possible to identify peaks from an 14N interaction more clearly; these most probably arise from outer-sphere interactions with nitrogen atom(s) of non-co-ordinated residues which are affected by conformational rearrangements upon nucleotide hydrolysis. In addition, different redox states of the [4Fe-4S] cluster of the Fe-protein show disparate conformations of the metal-nucleotide co-ordination environment, demonstrating that also the cluster site communicates with the nucleotide binding site.

The Ternary Rare‐Earth Polychalcogenides LaSeTe2, CeSeTe2, PrSeTe2, NdSeTe2, and SmSeTe2: Syntheses, Crystal Structures, Electronic Properties, and Charge‐Density‐Wave‐Transitions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Mechanism of lateral ordering of InP dots grown on InGaP layers

The mechanisms leading to the spontaneous formation of a two-dimensional array of InP∕InGaP dots grown by chemical-beam epitaxy are discussed. Samples where the InGaP buffer layer was grown at different conditions were characterized by transmission electron microscopy. Our results indicate that a periodic strain field related to lateral two-dimensional compositional modulation in the InGaP buffer layer determines the dot nucleation positions during InP growth. Although the periodic strain field in the InGaP is large enough to align the InP dots, both their shape and optical properties are effectively unaltered. This result shows that compositional modulation can be used as a tool for in situ dot positioning.

Measurements of laser-imprinting sensitivity to relative beam mistiming in planar plastic foils driven by multiple overlapping laser beams

In a direct-drive, inertial confinement fusion implosion, a spherical target is irradiated by a large number of overlapped laser beams. Imprinting of laser modulations depends on the relative arrival time of laser beams and their angles of incidence. This dependence was measured in planar plastic targets using six overlapping beams on the OMEGA laser system [T. R. Boehly, D. L. Brown, R. S. Craxton, R. L. Keck, J. P. Knauer, J. H. Kelly, T. J. Kessler, S. A. Kumpan, S. J. Loucks, S. A. Letzring, F. J. Marshall, R. L. McCrory, S. F. B. Morse, W. Seka, J. M. Soures, and C. P. Verdon, Opt. Commun. 133, 495 (1997)]. One of the beams (the imprint beam) had a special phase plate that produced two-dimensional modulations on the target, easily distinguishable from the features imprinted by the other five drive beams. The timing of the imprint beam was varied with respect to the drive beams to study imprinting sensitivity to beam mistiming. Shifting the imprint beam to arrive before the other beams significantly increased the imprint efficiency. The results are in very good agreement with the model predictions.

Performance analysis of three-branch selection combining over arbitrarily correlated Rayleigh-fading channels

The recent literature has thoroughly treated two-branch selection combining (SC) over correlated Rayleigh fading and three-branch SC over exponentially correlated Rayleigh fading. However, a long-standing open problem involves the three-branch SC performance over arbitrarily correlated Rayleigh fading. We solve this problem completely by deriving new infinite series expressions for the cumulative distribution function, the probability density function, and the moment generating function (mgf) of the three-branch SC output signal-to-noise ratio (SNR). The output mgf can be used to derive the average symbol-error rate for any two-dimensional digital modulations. The outage probability and the higher moments of the SC output SNR are also derived. These analytical results are canonical, in that the three-branch SC performance is now completely solved for arbitrary correlation. Some previous results are shown to be special cases of our new results.

The ternary rare-earth polychalcogenides LaSeTe 2, CeSeTe 2, PrSeTe 2, NdSeTe 2, and SmSeTe 2: Syntheses, crystal structures, electronic properties, and charge-density-wave-transitions

Phase transitions have been observed for LaSeTe 2, CeSeTe 2, PrSeTe 2, and NdSeTe 2 during temperature dependent single-crystal structure determinations and measurements of the electrical transport properties. The high temperature structure of all compounds is an ordered ternary variant of the NdTe 3 type and can be described in the space group Cmcm (no. 63). The low temperature structures are incommensurately modulated due to the occurrence of a charge density wave (CDW). The LT structures of LaSeTe 2, PrSeTe 2, and NdSeTe 2 can be described in the ( 3 + 1 ) -dimensional superspace group Cmcm(00 γ)s00 with the following lattice parameters and translational parts γ of the modulation vectors: a = 4.295 ( 1 ) Å , b = 25.371 ( 4 ) Å , c = 4.306 ( 1 ) Å , γ = 0.29 ( 1 ) for LaSeTe 2 (at T = 173 K ); a = 4.262 ( 1 ) Å , b = 25.034 ( 4 ) Å , c = 4.272 ( 1 ) Å , γ = 0.29 ( 1 ) for PrSeTe 2 (at 120 K); a = 4.256 ( 1 ) Å , b = 24.993 ( 3 ) Å , c = 4.258 ( 1 ) Å , γ = 0.30 ( 1 ) for NdSeTe 2 (at 173 K). The square tellurium nets of the basic structure are distorted at lower temperatures to give zigzag chains of tellurium atoms. SAED patterns show that CeSeTe 2 exhibits a two-dimensional modulation with modulation vectors of q 1 = ( − 0.12 ( 1 ) , 0 , 0.12 ( 1 ) ) and q 2 = ( 0.12 ( 1 ) , 0 , 0.12 ( 1 ) ) , and can be described in the superspace group C 2 / c ( s 0 q , r 0 p ) 00 with lattice parameters of a = 4.257 ( 1 ) Å , b = 25.176 ( 5 ) Å , c = 4.259 ( 1 ) Å (at T = 120 K ). Resistance measurements show that the non-modulated HT phases exhibit a metal-like temperature dependence of the resistance whereas the modulated LT phases behave like semiconductors. The transition temperatures T CDW have been found to be 491(5) K for LaSeTe 2, 226(3) K for PrSeTe 2, and 207(3) K for NdSeTe 2 by X-ray diffraction and 275(3) K for CeSeTe 2 by measurements of the electrical resistance.