Channel Spectrum Research Articles

Interstitial Ti for intermediate band formation in Ti-supersaturated silicon

We have analyzed by means of Rutherford backscattering spectrometry (RBS) the Ti lattice location and the degree of crystalline lattice recovery in heavily Ti implanted silicon layers subsequently pulsed laser melted (PLM). Theoretical studies have predicted that Ti should occupy interstitial sites in silicon for a metallic-intermediate band (IB) formation. The analysis of Ti lattice location after PLM processes is a crucial point to evaluate the IB formation that can be clarifyied by means of RBS measurements. After PLM, time-of-flight secondary ion mass spectrometry measurements show that the Ti concentration in the layers is well above the theoretical limit for IB formation. RBS measurements have shown a significant improvement of the lattice quality at the highest PLM energy density studied. The RBS channeling spectra reveals clearly that after PLM processes Ti impurities are mostly occupying interstitial lattice sites.

Modeling and Validation of Channel Idleness and Spectrum Availability for Cognitive Networks

The potential of successful cognitive radio networks operating in TV White Spaces (and other future bands re-allocated for unlicensed operation) has led to significant upsurge of interest in their design optimization - particularly those that are cross-layer in nature, involving both MAC protocols as well as physical layer aspects such as channel sensing. Typically, these seek to optimize a network-level metric (notably, aggregate throughput) of secondary (cognitive) network subject to interference constraints on the primary. In turn, this requires suitable sensing by cognitive users to detect availability of primary channels (currently unused by the protected incumbents) for opportunistic usage. To date, most studies have used largely hypothetical assumptions regarding channel idleness and resulting spectrum availability due to primary user dynamics. For example, idleness of channels over any spectrum are typically assumed to be an independent and identically distributed Bernoulli sequence. In contrast, nearly all real-time measurements suggest that channel idleness is frequency dependent, i.e., the probability that a channel is idle depends on the channel location. Cognitive radio research thus increasingly needs more realistic and validated models for channel idleness as the foundation of credible cross-layer analysis; this is the primary contribution of our work. We use two sets of real-time measurements conducted in disparate geographic locations over four distinct time intervals to show that channel idleness is appropriately modeled as independent but non-identical (i.n.i.d.) Bernoulli variables characterized by p_i, the probability of idleness for the i-th channel. We validate that Beta distribution can be used for modeling the variations in channel idleness probabilities; the Beta distribution parameters are estimated from the data to produce the best model fit. Based on the validated i.n.i.d. model, we build a predictive model by computing the availability probability of k channels, i.e, P{N_{idle} = k}, where N_{idle} denotes the number of idle channels over the spectrum of N channels. However, the combinatorial complexity inherent in the computation of P{N_{idle} = k} suggests the need for efficient approximations. We accomplish this by classifying idleness of channels based on the magnitude of p_i, and propose a novel Poisson-normal approximation for computing P{N_{idle} = k}. For validation, the distribution obtained from our technique is compared with the exact distribution and normal approximation using the approximation error criterion.

Turbulent channel flow simulations in convecting reference frames

We perform finite-difference numerical simulations of turbulent channel flow in stationary and moving reference frames. The flow statistics computed in the stationary frame exhibit significant discrepancies with respect to reference spectral data in terms of energy spectra and convection velocity of disturbances, especially in the near-wall region. On the other hand, simulations performed in a reference frame moving with the bulk flow velocity suffer from this shortcoming to a much lesser extent. We propose an explanation of these observations by assuming the scalar Burgers’ equation as working model. The analysis highlights the lack of Galilean invariance of finite-difference schemes, which exhibit a leading dispersive error in the presence of bulk advection. The numerically computed energy spectra exhibit the same features as the channel spectra, supporting the notion that dispersive effects are responsible for the blockage of the mechanism of energy cascade to high wavenumbers.

Defect microstructure in heavy-ion-bombarded (0 0 0 1) ZnO

Radiation defects in oxides are complex and remain poorly understood. Here, we use transmission electron microscopy to study ZnO crystals bombarded at room temperature with heavy ions (500keV Xe). Results reveal that the damage evolution proceeds via the formation of a band of cavities centered on ∼7nm from the sample surface. With further irradiation, a layered structure is formed, with alternating near-stoichiometric and Zn-rich layers. The anomalous intermediate peak and step in ion channeling spectra are attributed to a Zn-rich defect band and an interface between stoichiometric and Zn-rich layers, respectively. To explain these observations, we propose a damage build-up scenario involving vacancy clustering, loss of O from the surface, and peculiarities of point-defect transport through a Zn-rich defect band toward the surface.

Fourier domain optical coherence tomography system with balance detection

A Fourier domain optical coherence tomography system with two spectrometers in balance detection is assembled using each an InGaAs linear camera. Conditions and adjustments of spectrometer parameters are presented to ensure anti-phase channeled spectrum modulation across the two cameras for a majority of wavelengths within the optical source spectrum. By blocking the signal to one of the spectrometers, the setup was used to compare the conditions of operation of a single camera with that of a balanced configuration. Using multiple layer samples, balanced detection technique is compared with techniques applied to conventional single camera setups, based on sequential deduction of averaged spectra collected with different on/off settings for the sample or reference beams. In terms of reducing the autocorrelation terms and fixed pattern noise, it is concluded that balance detection performs better than single camera techniques, is more tolerant to movement, exhibits longer term stability and can operate dynamically in real time. The cameras used exhibit larger saturation power than the power threshold where excess photon noise exceeds shot noise. Therefore, conditions to adjust the two cameras to reduce the noise when used in a balanced configuration are presented. It is shown that balance detection can reduce the noise in real time operation, in comparison with single camera configurations. However, simple deduction of an average spectrum in single camera configurations delivers less noise than the balance detection.

Optimal Myopic Sensing and Dynamic Spectrum Access in Cognitive Radio Networks with Low-Complexity Implementations

Cognitive radio techniques allow secondary users (SU's) to opportunistically access underutilized primary channels that are licensed to primary users. We consider a group of SU's with limited spectrum sensing capabilities working cooperatively to find primary channel spectrum holes. The objective is to design the optimal sensing and access policies that maximize the total secondary throughput on primary channels accrued over time. Although the problem can be formulated as a Partially Observable Markov Decision Process (POMDP), the optimal solutions are intractable. Instead, we find the optimal sensing policy within the class of myopic policies. Compared to other existing approaches, our policy is more realistic because it explicitly assigns SU's to sense specific primary channels by taking into account spatial and temporal variations of primary channels. Contributions: (1) formulation of a centralized spectrum sensing/access architecture that allows exploitation of all available primary spectrum holes; and (2) proposing sub-optimal myopic sensing policies with low-complexity implementations and performance close to the myopic policy. We show that our proposed sensing/access policy is close to the optimal POMDP solution and outperforms other proposed strategies. We also propose a Hidden Markov Model based algorithm to estimate the parameters of primary channel Markov models with a linear complexity.

Optimization of MAC Frame Structure for Opportunistic Spectrum Access

Sensing-throughput tradeoff is involved in opportunistic spectrum access (OSA). At the physical layer it's expressed as the tradeoff between false alarm and misdetection, while at the MAC layer it's between maximizing the throughput of secondary users and reducing collisions with primary users. To balance both tradeoffs together drives the optimization of MAC frame structure for OSA. Since channel handoff results in a time overhead for MAC frame, it's first researched by modeling OSA dynamics in the paper. Three handoff cases and their probabilities are deduced, which lead to three application scenarios of MAC frame. Thus the throughput model of secondary network involving channel handoff is proposed. By balancing the misdetection and false alarm probabilities and maximizing the throughput of secondary network subject to the sensing quality and collision avoidance constraints, the optimal sensing time and frame duration are deduced as closed forms. The characteristics of the optimal MAC frame structure are researched as well. Theoretical and simulated results disclose the impacts of channel handoff and spectrum sensing on MAC frame structure and the achievable throughput of secondary network, which provide an insight for OSA design and improvement.

Spectrally Resolved White-Light Interferometric Sensor for Absolute Position Measurement Based on Hilbert Transform

A spectrally resolved white-light interferometric sensor for absolute position measurement based on Hilbert transform is presented. In order to avoid the possible singularities in the standard method obtained during the division of the measured spectral fringes and their Hilbert transform, a novel method is developed. The method is based on the simple mathematical apparatus where the division with zero is avoided by the division of the measured channeled spectrum and its Hilbert transform with envelope of both functions. Both of the divided functions are further differentiated, squared, and summed in order to find the optical path difference in the interferometer and thus the absolute position. The proposed sensing system currently provides unambiguous measurement over a range of 200 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> with a mid range error of better than 17 nm.

Proton Transport in a Membrane Protein Channel: Two-Dimensional Infrared Spectrum Modeling

We model the two-dimensional infrared (2DIR) spectrum of a proton channel to investigate its applicability as a spectroscopy tool to study the proton transport process in biological systems. Proton transport processes in proton channels are involved in numerous fundamental biochemical reactions. However, probing the proton transport process at the molecular level is challenging, because of the limitation in both spatial and time resolution of the traditional experimental approaches. In this paper, we perform proton transport molecular dynamics simulations and model the amide I region of the 2DIR spectrum of a proton channel to examine its sensitivity to the proton transport process. We first report the position dependent proton transfer rates along the channel. The rates in the middle of the channel are larger than those in the entrance. In the presence of protons, we find that the antidiagonal line width of the 2DIR spectrum is larger, and the time evolution of the 2DIR spectrum is slower than that without proton. The time evolution of the 2DIR spectrum with different isotope-labeled residues is similar, even if the local proton transfer rates are different. This results from the proton hopping and the channel water rotation being collective mechanisms, and these effects are convoluted in the spectra.

DEPTH-DEPENDENT TETRAGONAL DISTORTION STUDY OF AlGaN EPILAYER THIN FILM USING RBS AND CHANNELING TECHNIQUE

An epilayer of wide-band gap Al x Ga 1-x N was grown on sapphire by metal organic chemical vapor deposition (MOCVD) method with a GaN buffer layer of thickness slightly greater than 1 μm. Average composition of Al in the AlGaN layer is determined by Rutherford backscattering spectroscopy (RBS). Simulation of the random spectra reveals that the AlGaN layer has a composition of Al 0.05 Ga 0.95 N and a thickness of 200 nm. Low ratio of channeling and random spectra yields suggests good crystalline quality of ternary AlGaN epilayer. Tetragonal distortion (eT) caused by elastic strain in AlGaN is measured from channeling angular scans taken around an off-normal [Formula: see text] axis in the [Formula: see text] plane of the AlGaN layer. The resulting AlGaN is identified to be showing compressive strain at the interfacial layer and the magnitude of the stress reduces towards the near-surface layer. It can be expected from the results that a 550 nm thick, epitaxially grown, AlGaN layer on sapphire can be relaxed completely (eT = 0).

A simple low-coherence interferometric sensor for absolute position measurement based on central fringe maximum identification

In this paper, a simple low-coherence interferometric sensor for absolute position measurement based on central fringe maximum identification is presented. The channeled spectrum, captured by a commercial spectrometer, is analyzed by using an algorithm based on the fitting of the calculated autocorrelation function of the captured optical power spectrum with the sum of two Gaussian functions. The position of the central fringe is obtained directly from the separation between two fitted Gaussian functions. In order to boost the precision of such a built sensing system, the position of the maximum of the central fringe is identified by a simple algorithm. The system currently provides unambiguous measurement over a range of 200 μm with a mid-range error less than 1.4 nm. In addition to this, the sensor is characterized by a very small sensing head (bare single-mode fiber with a diameter of 125 μm) and by very high resistance to environmental influences, thus enabling the possibility of using a very long down-lead sensing fiber.

Multiwavelength erbium-doped fiber laser using twin-core photonic crystal fiber

We propose a novel multiwavelength erbium-doped fiber laser incorporating a twin-core photonic crystal fiber (TC-PCF) based Mach-Zehnder interferometer (MZI). TC-PCF based Mach-Zehnder interferometer acts as a wavelength-selective comb filter in the laser cavity and shows polarization dependent spectral response. The obtained channeled spectrum can be switched between two operation modes with two different multiwavelength intervals. The lasing wavelength spacing could be switched by adjusting internal polarization controllers in the ring cavity. The proposed fiber laser can operate with two typical lasing wavelength spacing of 1.60 and 3.27 nm with a signal-to-noise ratio (SNR) over 30 dB. The temporal stability of laser output was also investigated.

Susceptor-assisted microwave annealing for activation of arsenic dopants in silicon

Microwave annealing of arsenic-doped silicon was employed to achieve nearly complete dopant activation and repair of damage caused by ion implantation. Analysis of Rutherford backscattering spectra suggested that volumetric heating from microwaves can repair ion-implantation damage. Secondary ion mass spectroscopy depth profiling revealed that even with high damage due to implanted arsenic, microwave annealing achieves repair of lattice damage, and electrical activation of dopants without allowing any significant dopant diffusion into the silicon substrate. Surface temperatures greater than 700°C were achieved within ~100s with assisted microwave heating, marking this as a quick annealing technique when compared to un-assisted annealing. This temperature was sufficient for solid phase epitaxial growth in Si. The temperature profile recorded by a thermocouple-calibarated IR pyrometer was explained based upon the type of losses the sample undergoes while heating. The mechanism for susceptor-assisted microwave heating was dominated by dipole polarization losses in the initial stages of anneal and by Ohmic conduction losses at higher temperatures. Cross-section transmission electron microscopy, along with ion channeling spectra indicated that the silicon lattice regained nearly all of its crystallinity during the microwave anneal. Hall measurement and sheet resistance characterization were used to assess the extent of dopant activation.

Cognitive Femtocell: A Cost-Effective Approach Towards 4G Autonomous Infrastructure Networks

We study a method that facilitates autonomous base station deployment in a 4G long term evolution cellular network. Small cellular base stations, or femtocells, are co-channel deployed by network users in an underlay macrocellular network. Autonomous deployment allows the network to grow in an organic manner and meanwhile introduces new challenges for interference management. We propose a cost-effective scheme in order to manage the downlink interference from user-deployed femtocells to macrocell users. We name the approach cognitive femtocell in a sense that a cognitive radio function is enabled on femtocell access points, which facilitates autonomous radio resource management. Our analyses show largely improved performance in terms of channel throughput and spectrum reuse efficiency.

Synthesis, crystal growth and characterization of nonlinear optical organic crystal: p-Toluidinium p-toluenesulphonate

p-Toluidinium p-toluenesulphonate (p-TTS) an organic nonlinear optical crystal has been grown from the aqueous solution by slow evaporation solution growth technique. Single crystal X-ray diffraction analysis reveals that p-TTS crystallizes in monoclinic crystal system. The structural perfection of the grown p-TTS single crystal has been analyzed by high-resolution X-ray diffraction rocking curve measurements. Fourier transform infrared spectral studies have been performed to identify the functional groups. The optical transmittance window and the lower cutoff wavelength of the grown crystals have been identified by UV–vis–IR studies. Birefringence of p-TTS crystal has been studied using channel spectrum measurement. The laser damage threshold value was measured using Nd:YAG laser. The second harmonic conversion efficiency of p-TTS has been determined using Kurtz powder technique. Thermo gravimetric and differential thermal analyses were used to study its thermal properties. Dielectric constant, dielectric loss and AC conductivity of the grown p-TTS single crystal has been studied.

Channel Assignment Algorithms for OSA-Enabled WLANs Exploiting Prioritization and Spectrum Heterogeneity

SUMMARY Allowing WLANs to exploit opportunistic spectrum ac-cess (OSA) is a promising approach to alleviate spectrum congestion prob-lems in overcrowded unlicensed ISM bands, especially in highly denseWLAN deployments. In this context, novel channel assignment mecha-nisms jointly considering available channels in both unlicensed ISM andOSA-enabled licensed bands are needed. Unlike classical schemes pro-posed for legacy WLANs, channel assignment mechanisms for OSA-enabled WLAN should face two distinguishing issues: channel prioriti-zation and spectrum heterogeneity. The first refers to the fact that addi-tional prioritization criteria other than interference conditions should beconsidered when choosing between ISM or licensed band channels. Thesecond refers to the fact that channel availability might not be the same forall WLAN Access Points because of primary users’ activity in the OSA-enabled bands. This paper firstly formulates the channel assignment prob-lem for OSA-enabled WLANs as a Binary Linear Programming (BLP)problem. The resulting BLP problem is optimally solved by means ofbranch and bound algorithms and used as a benchmark to develop morecomputationally efficient heuristics. Upon such a basis, a novel channel as-signment algorithm based on weighted graph coloring heuristics and ableto exploit both channel prioritization and spectrum heterogeneity is pro-posed. The algorithm is evaluated under different conditions of AP densityand primary band availability.

Spectrum Environment Machine Learning in Cognitive Radio

Using the theory of machine learning to spectrum cognition and management is a necessary requirement of realizing cognitive radio technology. In order to evaluate the performance of wireless channel spectrum sensing strategy effectively, the statistical mean value is put forward as evaluation criteria for the cognitive user with fixed rate service. Study shows that, for the objective license channel vacancy and occupancy state time with exponentially distribution as well as the same distribution parameter, vacancy state probability follows uniform distribution. Then, the paper analyses how the channel time detection threshold have an influence on throughput. With the increasing of sample size, the estimate of throughput converges in probability to its statistical mean value. Simulation indicates that the spectrum sensing throughput performance of the cognitive user with fixed rate service based on machine learning achieves better performance compared with random spectrum sensing. Ultimately, the channel time detection threshold is achieved when maximizing the statistical mean throughput.

LSP 파라미터의 분포특성을 이용한 EVRC의 음질개선에 관한 연구

EVRC에서는 채널 스펙트럼의 효율을 높이고 시스템의 소비 전력을 줄이기 위하여, 통화시간 중에서 사용자가 말을 할 때만 음성신호를 압축하여 전달하고, 말을 하지 않을 때는 음성신호를 전달하지 않는다. 또한, EVRC에서는 음성 프레임을 1, 1/2, 1/8의 세 가지 전송률로 구분하여 다르게 처리 하고 있으며, 예를 들어, 1/8 전송률은 입력 신호가 묵음구간인 것을 의미한다. 본 연구에서는 LSP 파라미터의 분포특성을 이용한 유성음 구간, 무성음 구간, 묵음 구간을 구분하는 방법을 사용하여, 유성음인 경우에 대해 1 rate으로 부호화하고, 무성음 구간의 경우는 1/2 rate, 묵음의 경우에는 1/8 rate으로 전송하는 방법에 대하여 제안하였다. 즉, EVRC에서 full rate으로 보내는 부분에 대해서는 기존의 방식을 그대로 적용하며, half rate은 유성음, 무성음을 구분하여 유성음일 경우 full rate으로 바꾸어 전송하였고, 묵음에 대해서는 EVRC 기본 rate을 적용하였다. 실험과정에서는, SNR, ASDM, 전송률을 측정하였으며, 제안한 알고리즘을 사용하는 경우 EVRC에 비해 음성품질이 향상됨을 증명하였다. To improve the efficiency of the channel spectrum and to reduce the power consumption of the system in EVRC, the voice signal is compressed and transmitted only when the user speaks to. In addition to this, voice frames are divided into three rates 1, 1/2 and 1/8 and each frame is handled differently. For example, we assumed that the input is silence region if the 1/8 rate is used. In this paper, the sections are firstly separated into the voiced speech signal region, unvoiced speech signal region, and silence region by using distribution characteristics of LSP parameters. Then the paper suggested to encode 1 rate for the voiced speech signal, 1/2 rate for the unvoiced speech signal region, 1/8 rate for the silence region. In other words, traditional way of transmission is used when sending full rate in the EVRC. However, when sending half rate, the voice is firstly distinguished between voiced and unvoiced. If the voice is distinguished as voiced, voice is converted into full rate before the transmission. If it is distinguished as silence, EVRC's basic rate is applied. In the experimental results with SNR, ASDM, transmission bit rate measurement, we have demonstrated that voice quality was improved by using the proposed algorithm.

Inhibition of hERG Potassium Channels by Celecoxib and Its Mechanism

BackgroundCelecoxib (Celebrex), a widely prescribed selective inhibitor of cyclooxygenase-2, can modulate ion channels independently of cyclooxygenase inhibition. Clinically relevant concentrations of celecoxib can affect ionic currents and alter functioning of neurons and myocytes. In particular, inhibition of Kv2.1 channels by celecoxib leads to arrhythmic beating of Drosophila heart and of rat heart cells in culture. However, the spectrum of ion channels involved in human cardiac excitability differs from that in animal models, including mammalian models, making it difficult to evaluate the relevance of these observations to humans. Our aim was to examine the effects of celecoxib on hERG and other human channels critically involved in regulating human cardiac rhythm, and to explore the mechanisms of any observed effect on the hERG channels.Methods and ResultsCelecoxib inhibited the hERG, SCN5A, KCNQ1 and KCNQ1/MinK channels expressed in HEK-293 cells with IC50s of 6.0 µM, 7.5 µM, 3.5 µM and 3.7 µM respectively, and the KCND3/KChiP2 channels expressed in CHO cells with an IC50 of 10.6 µM. Analysis of celecoxib's effects on hERG channels suggested gating modification as the mechanism of drug action.ConclusionsThe above channels play a significant role in drug-induced long QT syndrome (LQTS) and short QT syndrome (SQTS). Regulatory guidelines require that all new drugs under development be tested for effects on the hERG channel prior to first administration in humans. Our observations raise the question of celecoxib's potential to induce cardiac arrhythmias or other channel related adverse effects, and make a case for examining such possibilities.

Spectroscopic Birefringence Mapping by Channeled Spectrum

Birefringence mapping of objects has attracted great interest in the fields of material technology and biomedical science. This paper describes spectroscopic birefringence mapping using a channeled spectrum. To perform spectroscopic birefringence mapping using a microscope, an acousto-optical tunable filter (AOTF) was employed as a spectrometer. The AOTF can select a specific wavelength from a white light source. The optical arrangement consists of a white light source, the AOTF, a polarizer, two retarders with high-order retardance, two objective lenses, an analyzer, and a CCD camera. The polarizer and the first retarder can modulate the polarization state and they function as polarizing optics, while the second retarder and the analyzer can analyze the polarization state and they function as analyzing optics. A sample is inserted between the polarizing and analyzing optics. Images obtained at a specific wavelength generate 128 data points at visible wavelengths. A channeled spectrum (i.e., the intensity distribution) can be generated from the images obtained. A Fourier spectrum can be obtained by the fast Fourier transform (FFT) method. The birefringence properties of the sample can be derived by calculating the inverse FFT. We used this technique to measure the polarization properties of a nematic liquid crystal droplet.