Frequency Selective Propagation Research Articles

Распространение невзаимных спиновых волн в многослойном магнонном кристалле

The possibility of frequency-selective propagation of spin waves in a magnonic microwave guide with a magnonic crystal consisting of two layers with different values of the saturation magnetization in the layers is demonstrated. It is shown that multimode propagation of spin waves can occur inside a two-layer structure in two frequency ranges, while the presence of a magnonic crystal on the surface of the structure leads to the manifestation of a band gap in one of the frequency ranges. At the same time, the process of propagation of a spin-wave signal is accompanied by a strong nonreciprocity, which manifests itself in a change in the amplitude-frequency characteristics when the direction of the external magnetic field is reversed, while the frequency range of the band gap differs depending on the direction of the field. The proposed concept of a two-layer spin-wave waveguide can underlie the manufacture of multichannel filters or magnonic logic devices.

Propagation of nonreciprocal spin waves in a multilayer magnonic crystal

The possibility of frequency-selective propagation of spin waves in a magnonic microwave guide with a magnonic crystal consisting of two layers with different values of the saturation magnetization in the layers is demonstrated. It is shown that multimode propagation of spin waves can occur inside a two-layer structure in two frequency ranges, while the presence of a magnonic crystal on the surface of the structure leads to the manifestation of a band gap in one of the frequency ranges. At the same time, the process of propagation of a spin-wave signal is accompanied by a strong nonreciprocity, which manifests itself in a change in the amplitude-frequency characteristics when the direction of the external magnetic field is reversed, while the frequency range of the band gap differs depending on the direction of the field. The proposed concept of a two-layer spin-wave waveguide can underlie the manufacture of multichannel filters or magnonic logic devices. Keywords: magnonics, nonlinearity, nonlinear systems, multilayer systems, spin waves.

Nonreciprocal propagation of spin waves in a bilayer magnonic waveguide based on yttrium-iron garnet films

Objectives. Nonreciprocal spin wave effects can manifest themselves in metalized films of ferrite garnets. By studying the dynamics of spin waves in micro- and nano-scale magnetic films, the possibility of using multilayer dielectric films of yttrium iron garnet (YIG) to ensure the manifestation of the nonreciprocity effect is demonstrated. This approach offers advantages compared to the use of a layered YIG/metal structure due to significantly lower spin-wave losses in the two-layer YIG film consisting of layers with different values of magnetization. Such films can be used in logical elements to create controllable Mach-Zehnder interferometers based on magnonic principles. The purpose of this work is to reconcile the concept of nonreciprocal spin-wave propagation of a signal with the simultaneous manifestation of the effects arising from the propagation of spin waves in microwave guides formed by finite-width YIG films.Methods. We used an experimental microwave spectroscopy method based on a vector network analyzer along with a finite difference method to perform a numerical simulation of the dispersion characteristics of spin waves in two-layer magnonic microwave guides. An analytical model was also used to obtain a dispersion equation based on the magnetostatic approximation.Results. Based on measurements of the amplitude and phase responses, the possible coexistence of two frequency ranges for the propagation of a spin-wave signal in a two-layer magnon microwave guide based on a YIG film formed by two layers with different values of saturation magnetization was demonstrated. Regimes of nonreciprocal propagation of a spin-wave signal were revealed. A numerical model was using to study the formation mechanisms of spin wave modes in the spectrum of a two-layer structure formed due to the finite dimensions of the microwave guide. An analytical model was used to evaluate the transformation of the mode spectrum. The experimental data are in good agreement with the results of the proposed numerical and analytical models.Conclusions. The possibility of frequency-selective propagation of spin waves in a magnon microwaveguide consisting of two layers with different saturation magnetization values is demonstrated. Multimode propagation of spin waves can occur inside a two-layer structure in two frequency ranges. At the same time, this process is accompanied by a strong nonreciprocity of spin-wave signal propagation, which manifests itself in a change in the amplitude and phase responses when the direction of the external magnetic field is reversed. The proposed two-layer spin-wave waveguide concept can be used in the manufacture of magnon interconnects and magnon interferometers with the support of multiband regimes of operation.

Frequency-selective propagation of spin waves in a three-dimensional magnon T-shaped splitter

Using numerical and experimental methods, the mechanism of control of the transmission of a spin-wave signal in a three-dimensional magnon splitter, formed by an orthogonal junction of magnetic strips of yttrium iron garnet, has been investigated. It is shown that by variation the size of the air gap between the spin-waveguide sections, it is possible to control the selection of the signal propagating in the output sections of the structure. From an applied point of view, the results obtained can be used to create an interconnection element in multilevel magnon information processing devices for the formation of multilayer magnon network topologies and miniaturization of computing devices based on the principles of magnonics. Key words: spin waves, magnonics, three-dimensional interconnections, micromagnetic modeling. Keywords: spin waves, magnonic, micromagnetic simulations, three-dimensional structure.

Эффекты нелинейности при распространении спиновых волн в двуслойном магнонном волноводе

The possibility of frequency-selective propagation of spin waves in a linear and nonlinear mode in a magnon microwave medium consisting of two layers with different values of the magnetization saturation of each layer is demonstrated. It is shown that multimode propagation of spin waves can be carried out inside the two-layer structure in two frequency ranges, with an increase in the power of the input microwave signal leading to a change in the boundaries of both frequency ranges. At the same time, this process is accompanied by a strong nonreciprocity of the spin-wave signal propagation, which manifests itself in a change in the amplitude-frequency characteristics when the direction of the external magnetic field is reversed, with a nonlinear mode change in the frequency bandwidth borders can change with increasing pumping power. The proposed concept of a two-layer spin-wave waveguide can form the basis for the fabrication of nonlinear magnon elements demonstrating interconnection functions with support for multiband modes of operation

Nonlinear effects of spin wave propagation in a bilayer magnon waveguide

The possibility of frequency-selective propagation of spin waves in a linear and nonlinear mode in a magnon microwave medium consisting of two layers with different values of the magnetization saturation of each layer is demonstrated. It is shown that multimode propagation of spin waves can be carried out inside the two-layer structure in two frequency ranges, with an increase in the power of the input microwave signal leading to a change in the boundaries of both frequency ranges. At the same time, this process is accompanied by a strong nonreciprocity of the spin-wave signal propagation, which manifests itself in a change in the amplitude-frequency characteristics when the direction of the external magnetic field is reversed, with a nonlinear mode change in the frequency bandwidth borders can change with increasing pumping power. The proposed concept of a two-layer spin-wave waveguide can form the basis for the fabrication of nonlinear magnon elements demonstrating interconnection functions with support for multiband modes of operation. Keywords: magnonics, nonlinearity, nonlinear systems, multilayer systems, spin waves.

Channel Estimation Based on IOTA Filter in OFDM/OQPSK and OFDM/OQAM Systems

In this paper, we present a study of bit error rate (BER) for orthogonal frequency division multiplexing/offset quadrature phase shift keying (OFDM/OQPSK) and OFDM/offset quadrature amplitude modulation (OQAM) systems with an isotropic orthogonal transfer algorithm (IOTA) filter. The novel noise suppression method based on an IOTA filter is proposed to reduce the error of channel estimation caused by additive white Gaussian noise (AWGN). The OFDM/OQPSK and OFDM/OQAM systems do not insert the guard interval (GI) and pilots in the signal frames, thus they possess transmission efficiency. An analysis was carried out for convolutional coded OFDM/OQPSK and OFDM/OQAM systems in Rayleigh fading channels with generator polynomials and constraint lengths. Compared with conventional OFDM/QPSK and OFDM/QAM systems with the insertion of comb-type pilots, the proposed IOTA filter-based channel estimation method can provide significant energy per bit to time-varying noise power spectral density ratio gains over time and frequency-selective propagation Rayleigh fading channels in OFDM/OQPSK and OFDM/OQAM systems.

Unsupervised Clustering for Nonlinear Equalization in Indoor Millimeter-Wave Communications

Millimeter-wave (mmWave) systems have been considered as a promising candidate for 5G networks because of their potential advances in significant bandwidth enhancement. However, due to the extremely high operating frequency of mmWave systems, they generally suffer from severe frequency-selective propagation and nonlinear distortion in the power amplifier, which introduces unfavorable impact on the signal detection process. We discover that for indoor mmWave communications, the constellation of signals becomes much more “clean and tidy” at the receiver side compared with the current wireless systems (e.g., LTE and WiFi), thanks to the channel sparsity characteristic of mmWave communications. Motivated by this observation, we propose in this paper several detection algorithms. Specifically, K-means clustering (KMC) algorithm is first introduced into clustering signal detection due to its advantage in the circle or spherical cluster shape. Then, an improved KMC detector is proposed to avoid the deficiencies of KMC for the error floor and high complexity. Moreover, a density and distance-based clustering detector, a non-iterative algorithm, is proposed and it does not need to preset the number of clusters. The above-proposed algorithms do not require any prior information about the power amplifier and the channel state information at the receiver end, which presents noticeable practical achievements on cost, complexity, and hardware constraints. The simulation results verify the effectiveness of the proposed schemes.

Enhanced Widely Linear Filtering to Make Quasi-Rectilinear Signals Almost Equivalent to Rectilinear Ones for SAIC/MAIC

Widely linear (WL) receivers have the capability to perform single antenna interference cancellation (SAIC) of one rectilinear (R) or quasi-rectilinear (QR) co-channel interference (CCI), a function which is operational in global system for mobile communications (GSM) handsets in particular. Moreover, SAIC technology for QR signals is still required for voice services over adaptive multi-user channels on one slot (VAMOS) standard, a recent evolution of GSM/EDGE standard, to mitigate legacy GSM CCI in particular. It is also required for filter bank multi-carrier offset quadrature amplitude modulation (FBMCOQAM) networks, which are candidate for 5G mobile networks, to mitigate inter-carrier interference (ICI) at reception for frequency selective propagation channels in particular. In this context, the purpose of this paper is twofold. The first one is to get more insights into the existing SAIC technology, and its extension to multiple antenna called MAIC, by showing analytically that, contrary to what is accepted as true in the literature, SAIC/MAIC implemented from standard WL filtering may be less efficient for QR signals than for R ones. From this result, the second purpose of the paper is to propose and to analyze, for QR signals and frequency selective fading channels, a SAIC/MAIC enhancement based on a three-input WL frequency shift (FRESH) receiver, making QR signals always almost equivalent to R ones for WL filtering in the presence of CCI. The results of the paper, completely new, may contribute to develop elsewhere new powerful WL receivers for QR signals and for both VAMOS and FBMC-OQAM networks in particular.

Frequency-selective propagation of localized spoof surface plasmons in a graded plasmonic resonator chain.

Localized spoof surface plasmon polaritons (spoof-SPPs) in a graded spoof-plasmonic resonator chain with linearly increasing spacing are experimentally investigated at microwave frequencies. Transmission measurements and direct near-field mappings on this graded chain show that the propagation of localized spoof-SPPs can be cutoff at different positions along the graded chain under different frequencies due to the graded coupling between adjacent resonators. This mechanism can be used to guide localized spoof-SPPs in the graded chain to specific positions depending on the frequency and thereby implement a device that can work as a selective switch in integrated plasmonic circuits.

Novel Transmit Diversity Scheme for TDS-OFDM System With Frequency-Shift m-Sequence Padding

This contribution proposes a design method for training sequence (TS) defined as frequency-shift m-sequence (FSMS) and a novel transmit diversity scheme with FSMS padding as guard interval (GI) for the further evolution of time domain synchronous orthogonal frequency domain multiplexing (TDS-OFDM) systems. The good auto-correlation characteristic of FSMS can facilitate synchronization and channel estimation at the receiver. Due to the cross-correlation characteristics of FSMSs, the accuracy of channel estimation is affected by the mutual interference among the transmit antennas. To further improve the accuracy of channel estimation, the channel impulse response (CIR) should be estimated in an iterative manner to suppress the effects of the mutual interference and additive white Gaussian noise. Compared with the transmit diversity schemes based on the space-frequency coded TS and space-time coded TS for TDS-OFDM system, the proposed transmit diversity scheme with FSMS padding can provide diversity gain and superior system performance over time-varying and/or frequency-selective propagation conditions.

Frequency Selective Propagation by Employing Fabry—Perot Nanocavities in a Subwavelength Double-slit Structure

By embedding a nanocavity adjacent to one or both of slits in a subwavelength double-slit structure, frequency selective propagation through the slits is demonstrated. When the incident light wavelength corresponds to the cavity resonance mode, the electromagnetic wave passing through the slit will be trapped within the nanocavity. Therefore, the double slit operates as a single slit and light propagation is solely allowed through the partner slit. These wavelengths are determined by applying the Fabry—Perot resonance condition for the nanocavities. Various geometrical structures result in different effective refractive indexes. Thus, the effective refractive index and consequently the attenuation wavelength can be adjusted by choosing the appropriate parameters of the nanocavity. Our theoretical predictions are in good agreement with 2D finite-difference time-domain simulation.

Blind Channel Equalization Using Constrained Generalized Pattern Search Optimization and Reinitialization Strategy

We propose a global convergence baud-spaced blind equalization method in this paper. This method is based on the application of both generalized pattern optimization and channel surfing reinitialization. The potentially used unimodal cost function relies on higher- order statistics, and its optimization is achieved using a pattern search algorithm. Since the convergence to the global minimum is not unconditionally warranted, we make use of channel surfing reinitialization (CSR) strategy to find the right global minimum. The proposed algorithm is analyzed, and simulation results using a severe frequency selective propagation channel are given. Detailed comparisons with constant modulus algorithm (CMA) are highlighted. The proposed algorithm performances are evaluated in terms of intersymbol interference, normalized received signal constellations, and root mean square error vector magnitude. In case of nonconstant modulus input signals, our algorithm outperforms significantly CMA algorithm with full channel surfing reinitialization strategy. However, comparable performances are obtained for constant modulus signals.

Energy-Based TOA Estimation

This paper investigates time of arrival estimation via impulse radio ultra-wideband technology. A dense multipath channel is assumed and frequency-selective propagation effects are taken into account that make the individual received pulses different in shape from the transmitted monocycles. In these conditions a conventional correlation-based estimator cannot be used and other schemes must be resorted to. Assuming only an approximate knowledge of the received pulses' duration, a TOA estimator is derived making use of least mean square techniques. The algorithm operates on energy measurements made on a folded version of the received signal. Its performance is investigated by simulation under implementation conditions of different complexity. In particular, sub-Nyquist sampling rates are considered and analog-to-digital conversion with limited resolution is explored. Comparisons are made with other available schemes.

Optimal training for MIMO frequency-selective fading channels

High data rates give rise to frequency-selective propagation effects. Space-time multiplexing and/or coding offer attractive means of combating fading and boosting capacity of multi-antenna communications. As the number of antennas increases, channel estimation becomes challenging because the number of unknowns increases, and the power is split at the transmitter. Optimal training sequences have been designed for flat-fading multi-antenna systems or for frequency-selective single transmit antenna systems. We design a low-complexity optimal training scheme for block transmissions over frequency-selective channels with multiple antennas. The optimality in designing our training schemes consists of maximizing a lower bound on the ergodic (average) capacity that is shown to be equivalent to minimizing the mean square error of the linear channel estimator. Simulation results confirm our theoretical analysis that applies to both single- and multicarrier transmissions.

Effect of a Multi-Band OFDM-MIMO System on Frequency-Selective Propagation Characteristics

A multi-band OFDM-MIMO system, in which the multi-band OFDM method is combined with the MIMO concept, has been developed. In this system, the spectra of multi-band OFDM signals are adaptively shared to improve the path gain for MIMO multiple transmission. The OFDM signal is divided into multi-bands, and the divided signals are transmitted on an appropriate beam in each multi-band. Multi-band transmission improves the overall transmission capacity by reducing the degradation of frequency-selective channels. In this paper, we report on our evaluation of the multi-band OFDM-MIMO system with frequency-selective fading channels.

Optimal training for block transmissions over doubly selective wireless fading channels

High data rates give rise to frequency-selective propagation, whereas carrier frequency-offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. To mitigate the resulting time- and frequency-selective (or doubly selective) channels, optimal training sequences have been designed only for special cases: pilot symbol assisted modulation (PSAM) for time-selective channels and pilot tone-assisted orthogonal frequency division multiplexing (OFDM) for frequency-selective channels. Relying on a basis expansion channel model, we design low-complexity optimal PSAM for block transmissions over doubly selective channels. The optimality in designing our PSAM parameters consists of maximizing a tight lower bound on the average channel capacity that is shown to be equivalent to the minimization of the minimum mean-square channel estimation error. Numerical results corroborate our theoretical designs.

Chip-interleaved block-spread code division multiple access

A novel multiuser-interference (MUI)-free code division multiple access (CDMA) transceiver for frequency-selective multipath channels is developed. Relying on chip-interleaving and zero padded transmissions, orthogonality among different users' spreading codes is maintained at the receiver even after frequency-selective propagation. As a result, deterministic multiuser separation with low-complexity code-matched filtering becomes possible without loss of maximum likelihood optimality. In addition to MUI-free reception, the proposed system guarantees channel-irrespective symbol detection and achieves high bandwidth efficiency by increasing the symbol block size. Filling the zero-gaps with known symbols allows for perfectly constant modulus transmissions. Important variants of the proposed transceivers are derived to include cyclic prefixed transmissions and various redundant or nonredundant precoding alternatives. (Semi-) blind channel estimation algorithms are also discussed. Simulation results demonstrate improved performance of the proposed system relative to competing alternatives.

Frequency selective propagation effects on spread-spectrum receiver tracking

This paper presents results of investigations of frequency selective scintillation effects, due to transionosphetic propagation, on the performance of spread-spectrum receivers. Emphasis is placed on pseudonoise (PN) direct sequence systems. Pertinent features of the disturbed ionospheric propagation channel are described together with the characteristics of resulting satellite signal scintillations. Effects of frequency selective scintillations on PN spread-spectrum code correlation and tracking are illustrated in some detail. The design of the receiver PN code tracking loop is an important factor in determining the significance of these effects. Of the designs that have been investigated, a dedicated (non-time-shared) noncoherent code tracking loop appears to be least susceptible to losing code lock in the presence of frequency selective scintillation. Possible design modifications to further enhance the robustness of PN code tracking are discussed.

The correlation between frequency-selective fading and multipath propagation over an ionospheric path

When continuous-wave high-frequency radio transmission within a band several kilocycles per second wide is examined it is found that signals fade differently throughout the band. The phenomenon has been known for many years as selective fading. In this report the relationship between selective fading and the relative time delays of the various propagating modes is studied over the 2450-km path from Mayaguez, Puerto Rico, to Smyrna, Georgia. Selective fading was measured by propagating a frequency-modulated CW transmission over the path while the modes propagating, and their structure, were recorded by means of oblique step-frequency soundings. It was confirmed that, when two modes were propagating at the FM-CW frequency, selective-fading nulls were separated in the frequency spectrum by an amount that is the reciprocal of the group time delay between the modes. The FM-CW technique represents an inexpensive means for determining the degree of multipath that exists on a given circuit at a given time. Such soundings, which can be made by simple modification of existing equipment, allow the degree of multipath to be quickly estimated and can also give clear advance warning of impending circuit failure resulting from falling MUF.