Flat Power Spectrum Research Articles

Conformal universe as false vacuum decay

We point out that the (pseudo)conformal universe scenario may be realized as the decay of a conformally invariant, metastable vacuum, which proceeds via spontaneous nucleation and subsequent growth of a bubble of a putative new phase. We study perturbations about the bubble and show that their leading late-time properties coincide with those inherent in the original models with homogeneously rolling backgrounds. In particular, the perturbations of a spectator dimension-zero field have a flat power spectrum.

Statistical model of OFDM and its application in nonlinearity analysis of LTE-Advanced systems

In this letter, to study the nonlinearity of RF power amplifiers (PAs), we showed that LTE-Advanced signals with large number of subcarriers can be expressed as a white Gaussian noise statistical model with flat power spectrum. This characteristic enables us to express the spectrum regrowth of LTE-Advanced signals up to an arbitrarily high order in an explicit form, in terms of the traditional PA nonlinearity parameter intercept points without complicated convolution. This general analytic expression would help RF designers to predict the PA distortion with simple intercept point expression, and provide insight for pre-distortion.

A Novel Class of Zero-Correlation Zone Sequence Set Having a Low Peak-Factor and a Flat Power Spectrum

A Novel Class of Zero-Correlation Zone Sequence Set Having a Low Peak-Factor and a Flat Power Spectrum

Bremsstrahlung from Relativistic Heavy Ions in a Fixed Target Experiment at the LHC

We calculate the emission of bremsstrahlung from lead and argon ions in ultraperipheral collisions in a fixed target experiment (AFTER) that uses the LHC beams. With nuclear charges ofZeequal to 82eand 18e, respectively, these ions are accelerated to energies of 7 Tev ×Z. The bremsstrahlung peaks around ≈100 GeV and the spectrum exposes the nuclear structure of the incoming ion. The peak structure is significantly different from the flat power spectrum pertaining to a point charge. Photons are predominantly emitted within an angle of 1/γto the direction of ion propagation. Our calculations are based on the Weizsäcker-Williams method of virtual quanta with application of existing experimental data on photonuclear interactions.

On the modeling and nonlinear dynamics of autonomous Silva-Young type chaotic oscillators with flat power spectrum.

The nonlinear dynamics of fourth-order Silva-Young type chaotic oscillators with flat power spectrum recently introduced by Tamaseviciute and collaborators is considered. In this type of oscillators, a pair of semiconductor diodes in an anti-parallel connection acts as the nonlinear component necessary for generating chaotic oscillations. Based on the Shockley diode equation and an appropriate selection of the state variables, a smooth mathematical model (involving hyperbolic sine and cosine functions) is derived for a better description of both the regular and chaotic dynamics of the system. The complex behavior of the oscillator is characterized in terms of its parameters by using time series, bifurcation diagrams, Lyapunov exponents' plots, Poincaré sections, and frequency spectra. It is shown that the onset of chaos is achieved via the classical period-doubling and symmetry restoring crisis scenarios. Some PSPICE simulations of the nonlinear dynamics of the oscillator are presented in order to confirm the ability of the proposed mathematical model to accurately describe/predict both the regular and chaotic behaviors of the oscillator.

The spectral diversity of resting-state fluctuations in the human brain.

In order to assess whole-brain resting-state fluctuations at a wide range of frequencies, resting-state fMRI data of 20 healthy subjects were acquired using a multiband EPI sequence with a low TR (354 ms) and compared to 20 resting-state datasets from standard, high-TR (1800 ms) EPI scans. The spatial distribution of fluctuations in various frequency ranges are analyzed along with the spectra of the time-series in voxels from different regions of interest. Functional connectivity specific to different frequency ranges (<0.1 Hz; 0.1–0.25 Hz; 0.25–0.75 Hz; 0.75–1.4 Hz) was computed for both the low-TR and (for the two lower-frequency ranges) the high-TR datasets using bandpass filters. In the low-TR data, cortical regions exhibited highest contribution of low-frequency fluctuations and the most marked low-frequency peak in the spectrum, while the time courses in subcortical grey matter regions as well as the insula were strongly contaminated by high-frequency signals. White matter and CSF regions had highest contribution of high-frequency fluctuations and a mostly flat power spectrum. In the high-TR data, the basic patterns of the low-TR data can be recognized, but the high-frequency proportions of the signal fluctuations are folded into the low frequency range, thus obfuscating the low-frequency dynamics. Regions with higher proportion of high-frequency oscillations in the low-TR data showed flatter power spectra in the high-TR data due to aliasing of the high-frequency signal components, leading to loss of specificity in the signal from these regions in high-TR data. Functional connectivity analyses showed that there are correlations between resting-state signal fluctuations of distant brain regions even at high frequencies, which can be measured using low-TR fMRI. On the other hand, in the high-TR data, loss of specificity of measured fluctuations leads to lower sensitivity in detecting functional connectivity. This underlines the advantages of low-TR EPI sequences for resting-state and potentially also task-related fMRI experiments.

Improvement of Power Spectrum in Ultrashort Pulse Reflectometry Signals Using Three Chirp Configuration

The flat power spectrum of the transmitter output signal for the desired frequency range is ideal to achieve the best performance of ultrashort pulse reflectometry. However, the power spectrum of a typical pulse generator decreases significantly as frequency increases. A configuration of three chirped waveforms was employed to improve the power spectrum of the transmitter signal at higher frequencies. To determine the amplification gain required for higher frequency components, three chirped waveforms were theoretically generated and their power spectra were measured using numerical band-pass filters. Based on the results of numerical computations, the three chirp configuration was successfully applied to the design of the transmitter for a broadband system..

Power spectra of infragravity waves in a deep ocean

AbstractInfragravity waves (IGWs) play an important role in coupling wave processes in the ocean, ice shelves, atmosphere, and the solid Earth. Due to the paucity of experimental data, little quantitative information is available about power spectra of IGWs away from the shore. Here we use continuous, yearlong records of pressure at 28 locations on the seafloor off New Zealand's South Island to investigate spectral and spatial distribution of IGW energy. Dimensional analysis of diffuse IGW fields reveals universal properties of the power spectra observed at different water depths and leads to a simple, predictive model of the IGW spectra. While sources of IGWs off New Zealand are found to have a flat power spectrum, the IGW energy density has a pronounced dependence on frequency and local water depth as a result of the interaction of the waves with varying bathymetry.

A SIGNAL MODEL BASED ON COMBINATION CHAOTIC MAP FOR NOISE RADAR

We propose a combination chaotic map (CCM) signal model to resolve the limited-word-length problem in digitally realizing chaotic signals used for noise radar. The proposed CCM has approximated infinite dimension, much more complicated phase space structure as well as better chaotic properties. The radar signal based on CCM presents much lower PSLR of auto-correlation as well as much flatter power spectrum, so it is very suitable for generating wide-band radar signal. Simulation experiments are conducted to show the good performance of the signal.

Background reionization history from omniscopes

The measurements of the 21-cm brightness temperature fluctuations from the neutral hydrogen at the Epoch of Reionization should inaugurate the next generation of cosmological observables. In this respect, many works have concentrated on the disambiguation of the cosmological signals from the dominant reionization foregrounds. However, even after perfect foregrounds removal, our ignorance on the background reionization history can significantly affect the cosmological parameter estimation. In particular, the interdependence between the hydrogen ionized fraction, the baryon density and the optical depth to the redshift of observation induce nontrivial degeneracies between the cosmological parameters that have not been considered so far. Using a simple but consistent reionization model, we revisit their expected constraints for a futuristic giant 21-cm omniscope by using for the first time Markov Chain Monte Carlo methods on multiredshift full sky simulated data. Our results agree well with the usual Fisher matrix analysis on the three-dimensional flat sky power spectrum but only when the above-mentioned degeneracies are kept under control. In the opposite situation, Fisher results can be inaccurate. We show that these conditions can be fulfilled by combining cosmic microwave background measurements with multiple observation redshifts probing the beginning of the Epoch of Reionization. This allows a precise reconstruction of the total optical depth, reionization duration and maximal spin temperature. Finally, we discuss the robustness of these results in presence of unresolved ionizing sources. Although most of the standard cosmological parameters remain weakly affected, we find a significant degradation of the background reionization parameter estimation in presence of nuisance ionizing sources.

Flat Broadband Chaos in Vertical-Cavity Surface-Emitting Lasers Subject to Chaotic Optical Injection

Four schemes for the generation of chaos in vertical-cavity surface-emitting lasers (VCSELs) by optical feedback and/or optical injection have been evaluated experimentally. The experimental results show that the preferred scheme is to effect chaotic optical injection into a stable operating VCSEL. This is an effective method to generate a flat broad-bandwidth chaotic signal with the bandwidth of the chaotic signal being enhanced by more than four times compared with that obtained using optical feedback. In addition, the chaotic signal generated by chaotic optical injection has a flat power spectrum. If a continuous wave (CW) signal is injected into a chaotic VCSEL, the bandwidth of chaos can also be enhanced, but less than that using the preferred scheme. Moreover, the spectrum in this case is not very flat. For CW optical injection into a CW VCSEL, the power spectrum of the chaos has many peaks and so is not suitable for applications in chaotic optical communication and random number generation. Injecting chaotic light into a chaotic VCSEL provides little enhancement of the chaos bandwidth.

Crab nebula gamma-ray flares as relativistic reconnection minijets

The unusually short durations, high luminosities, and high photon energies of the Crab Nebula gamma-ray flares require relativistic bulk motion of the emitting plasma. We explain the Crab flares as the result of randomly oriented relativistic "minijets" originating from reconnection events in a magnetically dominated plasma. We develop a statistical model of the emission from Doppler boosted reconnection minijets and find analytical expressions for the moments of the resulting nebula light curve (e.g. time average, variance, skewness). The light curve has a flat power spectrum that transitions at short timescales to a decreasing power-law of index 2. The flux distribution from minijets follows a decreasing power-law of index ~ 1, implying the average flux from flares is dominated by bright rare events. The predictions for the flares' statistics can be tested against forthcoming observations. We find the observed flare spectral energy distributions (SEDs) have several notable features: A hard power-law index of p ~< 1 for accelerated particles that is expected in various reconnection models, including some evidence of a pile-up near the radiation reaction limit. Also, the photon energy at which the SED peaks is higher than that implied by the synchrotron radiation reaction limit, indicating the flare emission regions' Doppler factors are ~> few. We conclude that magnetic reconnection can be an important, if not dominant, mechanism of particle acceleration within the nebula.

Sensitivity of cochlear nucleus neurons to spatio-temporal changes in auditory nerve activity

The spatio-temporal pattern of auditory nerve (AN) activity, representing the relative timing of spikes across the tonotopic axis, contains cues to perceptual features of sounds such as pitch, loudness, timbre, and spatial location. These spatio-temporal cues may be extracted by neurons in the cochlear nucleus (CN) that are sensitive to relative timing of inputs from AN fibers innervating different cochlear regions. One possible mechanism for this extraction is "cross-frequency" coincidence detection (CD), in which a central neuron converts the degree of coincidence across the tonotopic axis into a rate code by preferentially firing when its AN inputs discharge in synchrony. We used Huffman stimuli (Carney LH. J Neurophysiol 64: 437-456, 1990), which have a flat power spectrum but differ in their phase spectra, to systematically manipulate relative timing of spikes across tonotopically neighboring AN fibers without changing overall firing rates. We compared responses of CN units to Huffman stimuli with responses of model CD cells operating on spatio-temporal patterns of AN activity derived from measured responses of AN fibers with the principle of cochlear scaling invariance. We used the maximum likelihood method to determine the CD model cell parameters most likely to produce the measured CN unit responses, and thereby could distinguish units behaving like cross-frequency CD cells from those consistent with same-frequency CD (in which all inputs would originate from the same tonotopic location). We find that certain CN unit types, especially those associated with globular bushy cells, have responses consistent with cross-frequency CD cells. A possible functional role of a cross-frequency CD mechanism in these CN units is to increase the dynamic range of binaural neurons that process cues for sound localization.

The signature of the first stars in atomic hydrogen at redshift 20

Dark and baryonic matter moved at different velocities in the early Universe, which strongly suppressed star formation in some regions. This was estimated to imprint a large-scale fluctuation signal of about two millikelvin in the 21-centimetre spectral line of atomic hydrogen associated with stars at a redshift of 20, although this estimate ignored the critical contribution of gas heating due to X-rays and major enhancements of the suppression. A large velocity difference reduces the abundance of haloes and requires the first stars to form in haloes of about a million solar masses, substantially greater than previously expected. Here we report a simulation of the distribution of the first stars at redshift 20 (cosmic age of around 180 million years), incorporating all these ingredients within a 400-megaparsec box. We find that the 21-centimetre hydrogen signature of these stars is an enhanced (ten millikelvin) fluctuation signal on the hundred-megaparsec scale, characterized by a flat power spectrum with prominent baryon acoustic oscillations. The required sensitivity to see this signal is achievable with an integration time of a thousand hours with an instrument like the Murchison Wide-field Array or the Low Frequency Array but designed to operate in the range of 50-100 megahertz.

Non-Gaussian features of primordial magnetic fields in power-law inflation

We show that a conformal-invariance violating coupling of the inflaton to electromagnetism produces a cross correlation between curvature fluctuations and a spectrum of primordial magnetic fields. According to this model, in the case of power-law inflation, a primordial magnetic field is generated with a nearly flat power spectrum and rms amplitude ranging from nG to pG. We study the cross correlation, a three-point function of the curvature perturbation and two powers of the magnetic field, in real and momentum space. The cross-correlation coefficient, a dimensionless ratio of the three-point function with the curvature perturbation and magnetic field power spectra, can be several orders of magnitude larger than expected as based on the amplitude of scalar metric perturbations from inflation. In momentum space, the cross-correlation peaks for flattened triangle configurations, and is three orders of magnitude larger than the squeezed triangle configuration. These results suggest likely methods for distinguishing the observational signatures of the model.

Performance analysis of LTE multiuser flat downlink power spectrum and radio resources scheduling

Long Term Evolution LTE technology is becoming the de-facto NGN mobile access network. In order to deliver multimedia contents and services, higher data rates and low latency is required. The new planned LTE base stations can deliver on high data throughput at lower latency than available cellular networks. However, this deployment comes at an increased complexity of radio resources scheduling and power allocations. More than one half of the network operating cost comes from electrical energy. Therefore, one aim of the network operators is to reduce cellular networks power consumption. In this paper, a simulation study of three proposed scheduling algorithms are studied under base station flat power spectrum constraints. LTE cell deployment is evaluated and analyzed to maximize cell throughput, while preventing cell edge customers from starvation. Studying the performance of flat cell power requirements, cell coverage, resource scheduling and maximum throughput is another aim of this paper. Average service mean time is a concept we introduce as a measure of the system efficiency.

Generation of Broadband Chaotic Laser Using Dual-Wavelength Optically Injected Fabry–Pérot Laser Diode With Optical Feedback

Chaotic laser with a flat power spectrum up to 32.3 GHz has been generated by using a dual-wavelength optically injected Fabry-Perot laser diode with optical feedback. The Fabry-Perot laser diode with fiber ring cavity is utilized to generate the chaotic light. The bandwidth of the chaotic laser, due to dual-wavelength optical injection, is enhanced roughly four times as much as that of the chaotic laser without optical injection.

Autonomous Silva–Young Type Chaotic Oscillator with Flat Power Spectrum

A novel Silva-Young type fourth-order autonomous chaotic oscillator is described. In comparison with the original nonautonomous Silva-Young circuit it lacks the external periodic driving oscillator, but includes a passive internal second-order resonant LRC circuit, inserted in the positive feedback loop. In contrast to many other nonautonomous and autonomous chaotic oscillators, the suggested circuit exhibits fairly flat power spectrum of the output signal. Hardware experimental results and numerical results are presented. Good agreement between numerical and experimental results is observed. Ill. 5, bibl. 22 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.115.9.761

Scalar perturbations in conformal rolling scenario with intermediate stage

Scalar cosmological perturbations with nearly flat power spectrum mayoriginate from perturbations of the phase of a scalar field conformallycoupled to gravity and rolling down negative quartic potential. Weconsider a version of this scenario whose specific property is a longintermediate stage between the end of conformal rolling and horizon exitof the phase perturbations. Such a stage is natural, e.g., in cosmologieswith ekpyrosis or genesis. Its existence results in small negative scalartilt, statistical anisotropy of all even multipoles starting fromquardupole of general structure (in contrast to the usually discussedsingle quadrupole of special type) and non-Gaussianity of a peculiar form.

Heat Conduction in a Three-Dimensional Momentum-Conserving Anharmonic Lattice

Heat conduction in three-dimensional anharmonic lattices was numerically studied by the Green-Kubo theory. For a given lattice width W, a dimensional crossover is generally observed to occur at a W-dependent threshold of the lattice length. Lattices shorter than W will display a 3D behavior while lattices longer than W will display a 1D behavior. In the 3D regime, the heat current autocorrelation function was found to show a power-law decay as a function of the time lag τ as τ^{β} with β=-1.2. This indicates normal heat conduction. However, the decay exponent deviates significantly from the conventional theoretical value of β=-1.5. A flat power spectrum S(ω) of the global heat current in the low-frequency limit was also observed in the 3D regime. This provides not only an alternative verification of normal heat conduction but also a clear physical insight into its origin.