Hopping Frequency Research Articles

Optimal Frequency Hopping Sequences of Odd Length

In this paper, a new generalized cyclotomy with respect to a positive odd integer is introduced, and a construction of frequency hopping sequence sets and two constructions of frequency hopping sequences are proposed as its applications. The frequency hopping sequence sets and frequency hopping sequences obtained in this paper can be optimal with respect to the Peng-Fan bound and Lempel-Greenberger bound, respectively. Further, the length of sequences in the optimal frequency hopping sequence sets can be any odd integer larger than 3. Some of them have new parameters.

다상 DFT 필터뱅크를 이용한 도약신호 검출에 관한 연구

시간에 따라 중심주파수를 바꾸는 도약신호를 도약주기, 도약 주파수 등에 대한 정보 없이 검출하는 것은 대단히 어렵다고 알려져 있다. 본 논문에서는 도약 신호가 존재하는 광대역 샘플링 신호로부터 도약신호의 중심주파수, 도약 주기 등의 정보를 검출하는 알고리즘을 제안하였다. 도약 신호를 검출하기 위한 일반적인 방법으로는 다수의 협대역 필터가 필요하지만 이러한 구현은 비효율적이므로 본 논문에서는 다상 DFT 필터뱅크를 도입하였다. 또한 다상 DFT 필터뱅크의 출력으로부터 도약신호를 검출하는 알고리즘을 제안하였다. 제안하는 검출 알고리즘은 메모리 사이즈나 구현 복잡도를 줄이기 위해 이진 이미지 신호처리에 기반하여 개발되었다. 제안하는 알고리즘의 성능은 모의실험과 FPGA (field programmable gate array) 구현을 통하여 확인하였다. It is known that the detection of hopping signals without any information about hopping duration and hopping frequency is rather difficult. This paper considers the blind detection of hopping signal's information such as hopping duration and hopping frequency from the sampled wideband signals. In order to find hopping information from the wideband signals, multiple narrow-band filters are required in general, which leads to huge implementation complexity. Instead, this paper employs the polyphase DFT(discrete Fourier transform) filterbank to reduce the implementation complexity. This paper propose hopping signal detection algorithm from the polyphase DFT filterbank output. Specifically, based on the binary image processing, the proposed algorithm is developed to decrease the memory size and H/W complexity. The performance of the proposed algorithm is evaluated through the computer simulation and FPGA (field programmable gate array) implementation.

Synthesis, Electrical And Dielectrical Properties Of Lithium Iron Oxide

Lithium iron oxide (LiFeO2) cathode material was prepared by using hydrothermal synthesis. The XRD spectrum exhibited predominant (200) orientation peak at 2q= 43.63o corresponding to cubic rock-salt structure with Fm3m space group and the estimated lattice parameter of the sample is 4.176 Å. Electric and dielectric properties were studied over a frequency range of 1 Hz – 1 MHz and in the temperature range from 300 K to 573 K. The ionic conductivity of the sample was found to be 1.9 x 10-6 S/m at 373 K. The temperature dependent conductivity was conformed from the Arrhenius relation and the activation energy was found to be 0.39 eV. A mixed, ionic and electronic conduction was observed from the analysis. The electrical conductivity was found to be decreased with increasing temperature. The dielectric properties were analyzed in the framework of complex dielectric permittivity and complex electric modulus formalisms. The evolution of the complex permittivity as a function of frequency and temperature was investigated. Several important parameters such as activation energy, ionic hopping frequency, carrier concentration, ionic mobility and diffusion coefficient etc, were determined.

Metal–insulator transition and hopping conduction mechanisms in the La0.7Ba0.3MnO3 compound

The La0.7Ba0.3MnO3 compound, obtained in volume by sol–gel with a cubic crystal lattice, exhibits metal–insulator phase transitions near 336K and a ferromagnetic–paramagnetic transition near 375K. The correlation between the magnetic and transport properties is evidenced by AC susceptibility measurements, impedance spectroscopy and DC conductivity. The activation energy values obtained by impedance spectroscopy and by the four-point method differed slightly. The real part of the conductivity σ′(ω) for different temperatures showed a plateau (DC regime) for various frequency decades followed by a dispersive region following a Jonscher's power law tendency. The value of the exponent n, which was less than one, showed a distribution of relaxation times. The cross-frequency ωp, which was associated with the carrier hopping frequency, showed a thermally activated tendency with an almost constant carrier density for the entire range of temperatures. The relaxation time distribution was also verified by the dielectric modulus M(ω).

IMPEDANCE SPECTROSCOPY STUDIES OF NANO-CRYSTALLINE LiFePO4 CATHODE SYNTHESIZED BY HYDROTHERMAL METHOD

LiFePO 4 has become of great interest as storage cathodes for rechargeable lithium batteries because of their high energy density, low raw materials cost, environmental friendliness, high thermal stability. In the present investigation, lithium iron phosphate ( LiFePO 4) cathode material has been prepared by using hydrothermal synthesis. The structure of the sample has been studies by using X-ray diffraction. The XRD spectrum exhibited different characteristic peaks along with (311) predominant orientation corresponding to an orthorhombic crystal structure with Pnma space group. Electrical and dielectric properties were studied over a frequency range of 1 Hz – 1 MHz at different temperatures. The electrical conductivity was found to be increased with increasing temperature following Arrhenius relation with an estimated activation energy of 0.44 eV. The dielectric properties were analyzed in the framework of complex dielectric permittivity and complex electric modulus formalisms. The evolution of the complex permittivity as a function of frequency and temperature was investigated. Several important parameters such as activation energy, ionic hopping frequency, carrier concentration, ionic mobility and diffusion coefficient etc, were determined.

Structure and kinetics in the freezing of nearly hard spheres

We consider homogeneous crystallisation rates in confocal microscopy experiments on colloidal nearly hard spheres at the single particle level. These we compare with Brownian dynamics simuations by carefully modelling the softness in the interactions with a Yukawa potential, which takes account of the electrostatic charges present in the experimental system. Both structure and dynamics of the colloidal fluid are very well matched between experiment and simulation, so we have confidence that the system simulated is close to that in the experiment. In the regimes we can access, we find reasonable agreement in crystallisation rates between experiment and simulations, noting that the larger system size in experiments enables the formation of critical nuclei and hence crystallisation at lower supersaturations than the simulations. We further examine the structure of the metastable fluid with a novel structural analysis, the topological cluster classification. We find that at densities where the hard sphere fluid becomes metastable, the dominant structure is a cluster of m=10 particles with five-fold symmetry. At a particle level, we find three regimes for the crystallisation process: metastable fluid (dominated by m=10 clusters), crystal and a transition region of frequent hopping between crystal-like environments and other (m\neq10) structures

Differences in Spring-Mass Characteristics Between One- and Two-Legged Hopping

Although many athletic activities and plyometric training methods involve both unilateral and bilateral movement, little is known about differences in the leg stiffness (K(leg)) experienced during one-legged hopping (OLH) and two-legged hopping (TLH) in place. The purpose of this study was to investigate the effect of hopping frequencies on differences in K(leg) during OLH and TLH. Using a spring-mass model and data collected from 17 participants during OLH and TLH at frequencies of 2.0, 2.5, and 3.0 Hz, K(leg) was calculated as the ratio of maximal ground reaction force (F(peak)) to the maximum center of mass displacement (ΔCOM) at the middle of the stance phase measured from vertical ground reaction force. Both K(leg) and F(peak) were found to be significantly greater during TLH than OLH at all frequencies, but type of hopping was not found to have a significant effect on ΔCOM. These results suggest that K(leg) is different between OLH and TLH at a given hopping frequency and differences in K(leg) during OLH and TLH are mainly associated with differences in F(peak) but not ΔCOM.

Interface ferromagnetism in a SrMnO3/LaMnO3superlattice

Resonant soft x-ray absorption measurements at the O K edge on a SrMnO3/LaMnO3 superlattice show a shoulder at the energy of doped holes, which corresponds to the main peak of resonant scattering from the modulation in the doped hole density. Scattering line shape at the Mn L3,2 edges has a strong variation below the ferromagnetic transition temperature. This variation has a period equal to half the superlattice superperiod and follows the development of the ferromagnetic moment, pointing to a ferromagnetic phase developing at the interfaces. It occurs at the resonant energies for Mn3+ and Mn4+ valences. A model for these observations is presented, which includes a double-exchange two-site orbital and the variation with temperature of the hopping frequency tij between the two sites.

Dispersion in AC conductivity of fragile glass melts near glass transition temperature

We analyzed scaling behaviour of conductivity spectra of two fragile glass melts 2Ca(NO3)2.3KNO3 (CKN) and 2Ca(NO3)2.3RbNO3 (CRN) below and above their glass transition temperature (TG). Spectra were described in terms of power law. Conductivity isotherms of these systems were found to follow time temperature superposition principle (TTSP) below and above TG and can be scaled to a master curve using hopping frequency as scaling factor. However, for both the systems, a deviation from TTSP was discerned in the vicinity of TG. We also observed, for both CKN and CRN, a change in exponent factor of the power law and in number density of the mobile charge carriers around TG. The experimental data were also analyzed in terms of electric modulus mechanism. Both, conductivity and modulus formulism revealed the similar trend of TTSP behaviour. The deviation from TTSP near TG has been correlated with the changes observed in these two quantities. Further, it has been proposed that the magnitude of change in dispersion of conductivity spectra is directly related with the decoupling and fragility of the glass melt systems.

Dielectric relaxation and electrical conductivity in ferroelectric ceramic/polymer composites around the glass transition

The electrical conductivity behavior of the [(Pb0.88Sm0.08)(Ti0.99Mn0.01)O3]/polyetherketoneketone ceramic/polymer composite is studied in a wide temperature and frequency ranges around the glass transition of the polymer phase (Tg). The universal relaxation law is applied to model the experimental response. The dc conductivity (σdc) and the hopping frequency (ωH) follow an Arrhenius dependence. The activation energy values for σdc are associated to oxygen migration. The contribution of the conductive processes to the dielectric relaxation is analyzed, considering the oxygen vacancies concentration in the ceramic phase. It is also considered the structural change around Tg and its influence on the dielectric relaxation.

Imperfection of the clustered perovskite structure, phase transitions, and magnetoresistive properties of ceramic La0.6Sr0.2Mn1.2 − x Ni x O3 ± δ (x = 0–0.3)

Ceramic samples of lanthanum strontium manganite perovskites La0.6Sr0.2Mn1.2 − xNixO3 ± δ (0 ≤ x ≤ 0.3) have been investigated using the X-ray diffraction, magnetic (χac), 55Mn NMR, resistive, and magnetoresistive methods. The specific features of the influence of the composition on the structure and properties of nonstoichiometric manganite perovskites have been established. It has been found that the rhombohedrally (R\(\bar 3\)c) distorted perovskite structure contains cation and anion vacancies, as well as nanostructured clusters with Mn2+ ions in the A-positions. The substitution of Ni3+ ions (r = 0.74 A) for Mn3+ ions (r = 0.785 A) leads to a decrease in the lattice parameter a, the ferromagnetic-paramagnetic phase transition temperature TC, and the metal-semiconductor phase transition temperature Tms due to the disturbance of the superexchange interactions between heterovalent manganese ions Mn3+ and Mn4+. The observed anomalous magnetic hysteresis at 77 K has been explained by the antiferromagnetic effect of the unidirectional exchange anisotropy of the ferromagnetic matrix structure on the magnetic moments of the superstoichiometric manganese Mn2+ ions located in nanostructured planar clusters. An analysis of the asymmetrically broadened 55Mn NMR spectra of the compounds has revealed a high-frequency electronic superexchange of the ions Mn3+ ⟷ O2− ⟷ Mn4+; a local heterogeneity of their surrounding by other ions, vacancies, and clusters; and a partial localization of Mn4+ ions. The local hyperfine interaction fields on 55Mn nuclei have been determined. The concentration dependences of the activation energy and charge hopping frequency have confirmed that the Ni ions decrease the electrical conductivity due to the weakening of the electronic superexchange Mn3+ ⟷ O2− ⟷ Mn4+. Two types of magnetoresistive effects have been found: one effect, which is observed near the phase transition temperatures TC and Tms, is caused by scattering at intracrystalline nanostructured heterogeneities, and the other effect, which is observed in the low-temperature range, is induced by tunneling through intercrystalline mesostructured boundaries. The phase diagram has demonstrated that there is a strong correlation between magnetic and electrical properties in rare-earth manganites.

Conductivity and dielectric studies of Li2SnO3

Lithium stannate (Li2SnO3) has been prepared by solution evaporation method. The precursor obtained is sintered at 800°C for 5, 6, and 7 h, respectively. X-ray diffractogram confirmed that the sample obtained after sintering is Li2SnO3. The pelletized Li2SnO3 after heating at 500 °C for 3 h is used for electrochemical impedance spectroscopy characterization. Impedance measurements have been carried out over frequency range from 50 Hz to 1 MHz and temperature range from 563 to 633 K. The conductivity–temperature relationship is Arrhenian. Several important parameters such as activation energy, ionic hopping frequency and its rate, carrier concentration term, mobile ion number density, ionic mobility, and diffusion coefficient have been determined. The characteristics of log conductivity and log ionic hopping rate against temperature for the system suggest that the conduction and ionic hopping processes are thermally activated. The values of activation energy for conduction and relaxation processes as well as activation enthalpy for ionic hopping are about the same.

Influence of europium doping on conductivity of LiNiPO4

Europium doped LiNiPO4 and undoped LiNiPO4 were prepared by Pechini method. Compound formation temperature was confirmed from thermogravimetry and differential thermal analysis (TG/DTA). Powder X-ray diffraction (XRD) pattern confirmed the formation of pure LiNiPO4 compound with an orthorhombic structure. The conductivity and modulus analyses of the samples were carried out at different temperatures and frequencies using the complex impedance spectroscopy technique. The conductivity parameters such as ion hopping frequency and the charge carrier concentration term were calculated using Almond and West formalisms. An increase of one order of magnitude in the ionic conductivity has been observed for 1.0% Eu-doped LiNiPO4. (mole fraction). The complex modulus studies suggest the presence of non-Debye type of relaxation in the materials.

Joint signal parameter estimation of frequency-hopping communications

In the problem of the parameter estimation of frequency-hopping (FH) signals, most of existing works can only provide the unauthorised detection of the FH signals or estimate the part of parameters of FH signals, therefore cannot provide sufficient information to demodulate signals for message deciphering applications in a non-cooperative communications. This study proposes a new method based on reassigned smoothed pseudo Wigner-Ville distribution (SPWVD) and maximum-likelihood estimation for joint signal parameter estimation of FH communications with M -ary frequency-shift-keyed (MFSK) orthogonal modulation. With the good time frequency concentration and restraining cross-term ability of reassigned SPWVD, this algorithm can efficiently estimate the parameters of FH signals which include hopping frequencies, hopping rate, hopping sequence and modulation type without making any assumption about the alphabet of hopping frequencies or the synchronisation. The algorithm has improved estimation accuracy, reduced estimation root-mean-squared error and obtained better performance than that of the approach based on conventional time-frequency distribution. The simulation results are presented to evaluate the performances of the proposed algorithm. The root-mean-squared error of hopping frequency estimation is <;10 -3 for signal-to-noise ratio (SNR) of >-2 dB. The percentage of correct modulation recognition goes to 90% when SNR >3 dB.

基于小波变换的多分辨率分析分割算法的研究

跳频信号以其自身优良的抗截获、抗干扰能力，在军事通信中得到了广泛的应用，因此，对跳频信号的截获展开研究具有非常重要的现实意义。本文采用时频联合分析的方法，深入研究了跳频信号在线自动盲检测和频点搜索捕获问题，并对其特征参数提取算法进行了研究。针对跳频信号的特征提取，研究了一种基于小波脊线的特征提取算法，通过matlab仿真实验对方法的性能进行了验证，实验表明基于小波脊线的特征提取算法有很大的性能优势，进一步提高了频率估计的准确性和时效。 Frequency hopping signals in their own excellent anti-interception, anti-interference ability, in the military communication has been widely applied, therefore, it has a very important practical significance for the research of interceptting the frequency hopping signals. In this paper, the joint time-frequency analysis, in-depth study of the frequency hopping signal line automatic detection and frequency blind search acquisition issues, and its feature extraction algorithm is studied. Frequency hopping signals for feature extraction of a feature based on wavelet ridge extraction algorithm, the matlab simulation results the performance of the method was validated by experimental results show that the characteristics of wavelet ridge extraction algorithm is very performance advantages, and further improve the frequency estimation accuracy and timeliness.

항재밍을 위한 위성데이터링크 웨이브폼 및 중계기 구조

In this paper, satellite data link waveform is proposed which is based on the waveform of Link-16 but LDPC code is studied instead of CCSK code in order to be optimized to satellite nonlinear channel environment and transmission characteristics. And the DSM (Demux, Selective CH switch, and Mux) transponder structure is suggested which can remove all of the jamming signal out of the transmission signal band and convert uplink hopping frequency to desired ones of downlink. The results of BER and anti-jamming performance analysis shows that the required Eb/No and processing gain in the worst case partial band jammer of the proposed waveform are 2.5dB and 52dB respectively and the anti-jamming capacity improvement of DSM transponder is maximum 2dB.

Charge carrier dynamics and relaxation in (polyethylene oxide-lithium-salt)-based polymer electrolyte containing 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide as ionic liquid

In this paper we report the dynamics of charge carriers and relaxation in polymer electrolytes based on polyethylene oxide (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI) ionic liquid prepared by solution cast technique. It has been observed that the incorporation of BMPTFSI into PEO-LiTFSI electrolyte is an effective way for increasing the amorphous phase to a large extent. It has also been observed that both the glass transition and melting temperatures decrease with the increase of BMPTFSI concentration. The ionic conductivity of these polymer electrolytes increases with the increase of BMPTFSI concentration. The highest ionic conductivity obtained at 25 °C is ~3×10(-4) S cm(-1) for the electrolyte containing 60 wt % BMPTFSI and ethylene oxide (EO)/Li ratio of 20. The temperature dependence of the dc conductivity and the hopping frequency show Vogel-Tamman-Fulcher type behavior indicating a strong coupling between the ionic and the polymer chain segmental motions. The frequency dependence of the ac conductivity exhibits a power law with an exponent n which decreases with the increase of temperature. The scaling of the ac conductivity indicates that relaxation dynamics of charge carriers follows a common mechanism for all temperatures and BMPTFSI concentrations. We have also presented the electric modulus data which have been analyzed in the framework of a Havriliak-Negami equation and the shape parameters obtained by the analysis show slight temperature dependence, but change sharply with BMPTFSI concentration. The stretched exponent β obtained from Kohlrausch-Williams-Watts fit to the modulus data is much lower than unity signifying that the relaxation is highly nonexponential. The decay function obtained from analysis of experimental modulus data is highly asymmetric with time.

Ac conductivity and relaxation in CdO doped poly ethylene oxide-LiI nanocomposite electrolyte

We have studied the ac conductivity and relaxation in PEO-LiI electrolytes in which different concentrations of CdO nanoparticles (ranging from 0.05 to 0.2 wt. %) have been introduced. The ac conductivity data have been discussed in the framework of power law and electric modulus formalisms. The hopping frequency obtained from the power law analysis obeys the Vogel–Tamman–Fulcher relation and the sample possessing the highest hopping frequency shows the highest dc conductivity. Furthermore, the frequency exponent decreases with the increase of temperature, suggesting a weaker correlation among the Li+ ions. Scaling of the conductivity spectra has also been performed in order to obtain insight into the relaxation mechanisms. We have observed that the imaginary modulus spectra are much broader than the Debye peak-width, but are asymmetric and skewed toward the high frequency sides of the maxima. The modulus data have been fitted to the non-exponential Kohlrausch–Williams–Watts function and the value of the stretched exponent is fairly low, suggesting a wide distribution of relaxation times and cooperative motion of the ions in the nanocomposites.

Ionic transport properties of LiCoPO 4 cathode material

LiCoPO 4 has been prepared by the Pechini-type polymerizable precursor method. The formation of the compound is confirmed by XRD analysis. Ion transport mechanism has been analyzed using the AC impedance technique. The conductivity parameters such as ion hopping frequency ( ω p) and the charge carrier concentration term (K) have been calculated using Almond and West formalisms. The conductivity relaxation time ( τ c) is calculated from the peak frequency of the modulus spectra.

Structural and electrical properties of nonstoichiometric semiconducting pyrochlores in Ca–Ce–Ti–Nb–O system

A series of quaternary nonstoichiometric cubic pyrochlores with varying cerium content in Ca–Ce–Ti–Nb–O system has been synthesized through solid state route. The powder X-ray diffraction and Rietveld analysis indicate that smaller Ti cation occupies the A site by displacing the A and O′ ions causing displacive disorder in the cubic pyrochlore structure. The electrical conductivity measurements reveal that they exhibit semiconductor behavior in a broad temperature range (30–600 °C) and the conductivity increases with increase in Ce content. The X-ray photoelectron spectroscopy (XPS) analysis corroborates the presence of Ce in 3 + state. The impedance analysis reveals two types of conduction processes from the frequency dependent conductivity plots, i.e. low frequency conduction due to short range hopping and high frequency conduction due to the localized reorientation hopping mechanism. The relaxational properties indicate that hopping frequency is dependent on cerium concentration and is a measure of degree of disorder in the system.