Multiband Scenario Research Articles

Adaptive Pilot Patterns for CA-OFDM Systems in Nonstationary Wireless Channels

In this paper, we investigate the performance gains of adapting pilot spacing and power for carrier aggregation orthogonal frequency-division multiplexing (CA-OFDM) systems in nonstationary wireless channels. In current multiband CA-OFDM wireless networks, all component carriers use the same pilot density, which is designed for poor channel environments. This leads to unnecessary pilot overhead in good channel conditions and performance degradation in the worst channel conditions. We propose adaptation of pilot spacing and power using a codebook-based approach, where the transmitter and the receiver exchange information about the fading characteristics of the channel over a short period of time, which are stored as entries in a channel profile codebook. We present a heuristic algorithm that maximizes the achievable rate by finding the optimal pilot spacing and power from a set of candidate pilot configurations. We also analyze the computational complexity of our proposed algorithm and the feedback overhead. We describe methods to minimize the computation and feedback requirements for our algorithm in multiband CA scenarios and present simulation results in typical terrestrial and air-to-ground/air-to-air nonstationary channels. Our results show that significant performance gains can be achieved when adopting adaptive pilot spacing and power allocation in nonstationary channels. We also discuss important practical considerations and provide guidelines to implement adaptive pilot spacing in CA-OFDM systems.

Multigap Superconductivity at Extremely High Temperature: A Model for the Case of Pressurized H2S

It is known that in pressurized H2S the complex electronic structure in the energy range of 200 meV near the chemical potential can be separated into two electronic components, the first characterized by steep bands with a high Fermi velocity and the second by flat bands with a vanishing Fermi velocity. Also the phonon modes interacting with electrons at the Fermi energy can be separated into two components: hard modes with high energy around 150 meV and soft modes with energies around 60 meV. Therefore we discuss here a multiband scenario in the standard BCS approximation where the effective BCS coupling coefficient is in the range 0.1- 0.32. We consider a first (second) BCS condensate in the strong (weak) coupling regime 0.32 (0.15). We discuss different scenario segregated in different portions of the material. The results show the phenomenology of unconventional superconducting phases in this two-gap superconductivity scenario where there are two electronic components in two Fermi surface spots, the pairing is mediated by either by a soft or a hard phonon branch where the inter-band exchange term, also if small, plays a key role for the emergence of high temperature superconductivity in pressurized sulfur hydride.

Superconducting gap structure of the skutteruditeLaPt4Ge12probed by specific heat and thermal transport

We investigated the superconducting order parameter of the filled skutterudite LaPt4Ge12, with a transition temperature of Tc = 8.3 K. To this end, we performed temperature and magnetic-field dependent specific-heat and thermal-conductivity measurements. All data are compatible with a single superconducting s-wave gap. However, a multiband scenario cannot be ruled out. The results are discussed in the context of previous studies on the substitution series Pr1-xLaxPt4Ge12. They suggest compatible order parameters for the two end compounds LaPt4Ge12 and PrPt4Ge12. This is not consistent with a single s-wave gap in LaPt4Ge12 considering previous reports of unconventional and/or multiband superconductivity in PrPt4Ge12.

Computational complexity reduction in nonuniform compressed sensing by multi-coset emulation

Single-channel nonuniform sampling (SNS) is a Compressed Sensing (CS) approach that allows sub-Nyquist sampling of frequency sparse signals. The relatively simple architecture, comprising one wide-band sampling channel, makes it an attractive solution for applications such as signal analyzers and telecommunications. However, a high computational cost of the SNS signal reconstruction is an obstacle for real-time applications. This paper proposes to emulate multi-coset sampling (MCS) in SNS acquisition as a means to decrease the computational costs. Such an emulation introduces performance-complexity tradeoffs due to the difference of the SNS and MCS models. We investigate these tradeoffs with numerical simulations and theoretical assessments of the reconstruction complexity in multi-band signal scenarios. These scenarios include different numbers, different widths and positions of the frequency bands and different levels of noise in the signals. For the SNS reconstruction, we consider the accelerated iterative hard thresholding algorithm; for the MCS reconstruction, the multiple signal classification and focal underdetermined system solver algorithms are used. The proposed emulation reduces the computational complexity up to several orders of magnitude. For one of the scenarios, the reconstruction quality slightly decreases. For the other scenarios, the reconstruction quality is either preserved or improved.

Non-Fermi-liquid behavior from partial nesting in multiorbital superconductors

Partial nesting between two connected or disconnected regions of the Fermi surface leads to fractional powers of the Coulomb scattering lifetime as a function of temperature and frequency. This result is first demonstrated for a toy band structure where partial nesting occurs within a single band and between different regions of the Brillouin zone. A comparison is then made to a multiband scenario by studying the scattering rate of an effective two orbital model that was proposed in the context of multi-orbital superconductors. In the process, various model independent factors affecting the temperature exponent, n, are identified. The logarithmically divergent contributions of the lowest order vertex correction to the multi-orbital susceptibility, and the role played by nesting in suppressing these divergences is analyzed. The relevance of these results is discussed keeping the recently observed anomalous resistivity in the Co doped Iron superconductor LiFeAs as a backdrop.

Multiband superconductivity in Ta4Pd3Te16 with anisotropic gap structure

We carried out measurements of the magnetoresistance, magnetic susceptibility and specific heat on crystals of the low-dimensional transition metal telluride Ta4Pd3Te16. Our results indicate that Ta4Pd3Te16 is an anisotropic type-II superconductor in the clean limit with the extracted Ginzburg–Landau parameter 84. The upper critical field Hc2(T) shows an anomalous temperature dependence at low temperatures and the anisotropy of Hc2(T) is strongly T-dependent, both of which indicate a multiband scenario. The electronic specific heat (T) can be consistently described by a two-gap (s + d waves) model from the base temperature up to . Our results suggest nodal and multiband superconductivity in Ta4Pd3Te16.

Improving dynamic range of software‐defined radio receivers for multi‐carrier wireless systems

This study presents an architecture to increase the instantaneous dynamic range of multi-carrier wideband digital receivers. The proposed topology includes a directional coupler followed by two parallel analog-to-digital converters and digital signal processing to reconstruct the signal. It will be shown that the proposed technique is able to increase the dynamic range of software-defined radio (SDR) receivers for multiband signals presenting certain statistical patterns. The concept is validated by simulations and measurements of a real SDR front-end receiver for multiband communications scenario, which will demonstrate improvements in the order of 6 dB when compared to existing alternatives.

Optimal Sequential Channel Estimation and Probing for Multiband Cognitive Radio Systems

In this paper, we propose a novel sequential channel estimation approach for multiband cognitive radio (CR) systems. We introduce a general model and test two scenarios of practical interest. The two scenarios are as follows: 1) CR users optimally estimate all the available bands; and 2) CR users find one good channel with a large gain. In particular, we use a sequential search in which the CR users estimate the available channels one by one. During the search, the CR users determine whether to terminate the current channel estimation process and switch to the next channel based on the training symbols received so far. Our objective is to design a switch function, an estimator, and a stopping rule that minimize a combination of estimation time and error. For the multiband estimation scenario, we show that the optimal rule is to find the optimal number of symbols required for each channel in a joint optimization problem. For the good channel search problem, we show that the optimal decision rules that minimize a properly chosen cost function have a simple structure. In particular, both the termination and switching rules are threshold based. Numerical results are provided to illustrate the effectiveness of the proposed algorithms.

Evolution of London penetration depth with scattering in single crystals of K1−x Nax Fe2 As2

London penetration depth, $\lambda (T)$, was measured in single crystals of K$_{1-x}$Na$_x$Fe$_2$As$_2$, $x$=0 and 0.07, down to temperatures of 50~mK, $\sim T_c/50$. Isovalent substitution of Na for K significantly increases impurity scattering, with $\rho(T_c)$ rising from 0.2 to 2.2 $\mu \Omega$cm, and leads to a suppression of $T_c$ from 3.5~K to 2.8~K. At the same time, a close to $T$-linear $\Delta \lambda (T)$ in pure samples changes to almost $T^2$ in the substituted samples. The behavior never becomes exponential as expected for the accidental nodes, as opposed to $T^2$ dependence in superconductors with symmetry imposed line nodes. The superfluid density in the full temperature range follows a simple clean and dirty $d$-wave dependence, for pure and substituted samples, respectively. This result contradicts suggestions of multi-band scenarios with strongly different gap structure on four sheets of the Fermi surface.

Frequency-Agile Multiband Quadrature Sigma-Delta Modulator for Cognitive Radio: Analysis, Design and Digital Post-Processing

A quadrature ΣΔ analog-to-digital converter (ADC) is a promising solution for intermediate frequency digitizing software defined cognitive radio (CR) receivers because of, e.g., multiband capability and power efficiency. However, inherent coefficient mismatches between the in-phase and quadrature rails can severely damage the performance of such receiver by creating mirror-frequency interference (MFI). In this article, a novel frequency-agile and reconfigurable transfer function design, allowing digital post-compensation of the MFI, is proposed. The design is based on a novel closed-form transfer function model for higher-order quadrature ΣΔ modulators (QΣΔMs) under implementation inaccuracies, proposed herein. By doing the compensation in digital domain, it is possible to take into account all error-sources of the receiver chain at once, including, e.g., a quadrature mixer before the ADC. This capability is obtained by preserving the mirror-band signal information and using the noise transfer function of a QΣΔM to remove quantization noise from therein. This is demonstrated in a multiband scenario aimed for CR receivers, where a number of frequency channels can be received and detected in parallel. Practical examples of the transfer function analysis under implementation inaccuracies and the post-compensation performance are given with a two-stage QΣΔM, having stage-orders of four, allowing eighth-order noise shaping.

Coherence Lengths for Superconductivity in the Two-Orbital Negative-U Hubbard Model

Multi-component superconductivity models, developed more than fty years starting from the papers [1 3], include rather varied physics. In connection with the presence of interacting order parameters the main properties of the multi-band systems are quite di erent from the corresponding characteristics in single-band superconductors. The examination of theses peculiarities has been an object of growing interest. The various multi-component theoretical scenarios have been applied for a number of superconducting materials (see [4 7] and references therein). The research activity in this direction has been especially stimulated by the acceptance of the multi-gap superconductivity in MgB2 [8], cuprates [9] and iron arsenic compounds [10]. In particular, the derivation of high-quality superconducting regions from oxygen ordering, observed recently in La2CuO4+y [11], supports the multi-band theoretical scenario of superconductivity in cuprates. In the present contribution we study the coherency of the superconducting ordering of a two-band (two-orbital) system with the negative-U Hubbard intra-orbital pairing and inter-orbital pair-transfer interaction. One can distinguish here two characteristic length scales [12] in the spatial behaviour of superconducting uctuations. One of these lengths as a function of temperature behaves critically diverging at the phase transition point. The other one remains nite and its temperature dependence is weaker. The formation of these length scales is caused by the interband interaction mixing the superconducting order parameters of initially non-interacting bands. The critical and non-critical coherence lengths associate also with critical and non-critical uctuations (see e.g. Refs. [13, 14]) which appear as the certain linear combinations of the deviations from the equilibrium band superconducting orders. Consequently, these length scales cannot be attributed to di erent bands involved [15] (see also Refs. [16 18]). Our results have been obtained using the superconducting negative-U Hubbard model [19] for a two-orbital system on a two-dimensional lattice.

Evolution of transport properties of BaFe2−xRuxAs2in a wide range of isovalent Ru substitution

The effects of isovalent Ru substitution at the Fe sites of BaFe2-xRuxAs2 are investigated by measuring resistivity and Hall coefficient on high-quality single crystals in a wide range of doping (0 < x < 1.4). Ru substitution weakens the antiferromagnetic (AFM) order, inducing superconductivity for relatively high doping level of 0.4 < x < 0.9. Near the AFM phase boundary, the transport properties show non-Fermi-liquid-like behaviors with a linear-temperature dependence of resistivity and a strong temperature dependence of Hall coefficient with a sign change. Upon higher doping, however, both of them recover conventional Fermi-liquid behaviors. Strong doping dependence of Hall coefficient together with a small magnetoresistance suggest that the anomalous transport properties can be explained in terms of anisotropic charge carrier scattering due to interband AFM fluctuations rather than a conventional multi-band scenario.

Experimental evidence for a two-band superconducting state ofNbSe2single crystals

We report on measurements and a detailed analysis of the reversible magnetization of superconducting ${\text{NbSe}}_{2}$ single crystals. By comparing the experimental data with Ginzburg Landau theory we show that superconductivity in ${\text{NbSe}}_{2}$ cannot be explained by an anisotropic single-band but by a multiband scenario. Applying a simple two-band model reveals the basic mixed-state parameters, which are quite different in the two bands. We identify a strongly anisotropic band that determines the properties at high magnetic fields and a second almost isotropic band that dominates at low fields. Our method is well suited for distinguishing anisotropic single-band from multiband superconductivity in various materials.

Quasiparticle relaxation dynamics in spin-density-wave and superconductingSmFeAsO1−xFxsingle crystals

We investigate the quasiparticle relaxation and low-energy electronic structure in undoped SmFeAsO and near-optimally doped SmFeAsO_{0.8}F_{0.2} single crystals - exhibiting spin-density wave (SDW) ordering and superconductivity respectively - using pump-probe femtosecond spectroscopy. In the undoped single crystals a single relaxation process is observed, showing a remarkable critical slowing down of the QP relaxation dynamics at the SDW transition temperature T_{SDW}\simeq125{K}. In the superconducting (SC) crystals multiple relaxation processes are present, with distinct SC state quasiparticle recombination dynamics exhibiting a BCS-like T-dependent superconducting gap, and a pseudogap (PG)-like feature with an onset above 180K indicating the existence of a pseudogap of magnitude 2\Delta_{\mathrm{PG}}\simeq120 meV above T_{\mathrm{c}}. From the pump-photon energy dependence we conclude that the SC state and PG relaxation channels are independent, implying the presence of two separate electronic subsystems. We discuss the data in terms of spatial inhomogeneity and multi-band scenarios, finding that the latter is more consistent with the present data.

Orthogonal Frequency Division Multiplexing Timing Synchronization in Multi-Band Orthogonal Frequency Division Multiplexing Ultra-Wideband Systems

Problem statement: MB-OFDM UWB systems have attractive features and received a great attention. It has the capability of sending/receiving data at rates in access of 100 Mbits sec-1. Owing to the sensitivity of OFDM systems to synchronization errors, a fast synchronization algorithm is required to discern for the start of OFDM symbol. Existing approaches to OFDM time synchronization rely on their proposed preamble. Approach: In this study, a comprehensive algorithm for Orthogonal Frequency Division Multiplexing (OFDM) timing synchronization suitable for Multi-Band OFDM Ultra-Wideband (MB-OFDM UWB) systems was proposed. Firstly, we proposed a technique for packet detection in the multi-band scenario. Secondly, a new timing metric for timing synchronization is proposed. Then, we propose a technique for frame synchronization. Finally, time frequency code identification and low-complexity timing refinement technique was proposed. Results: The performance of the timing synchronization (coarse synchronization and timing refinement) is evaluated in terms of Mean Squared Error (MSE) in MB-OFDM UWB Channel Models (CM1, CM2, CM3 and CM4). It was found that the proposed method performs better than existing approaches. Conclusion: A suitable timing synchronization steps for MB-OFDM UWB systems was proposed. The proposed method can be applied on any preamble signal.

A Technique for Frequency Synchronization in Multi-Band OFDM in Realistic UWB Channel

This paper presents a highly accurate frequency offset estimation algorithm for multi-band orthogonal frequency division multiplexing (MB-OFDM) systems effective for realistic ultra-wideband (UWB) environment. The proposed algorithm derives its estimates based on phase differences in the received subcarrier signals of several successive OFDM symbols in the preamble. We consider different carrier frequency offsets and different channel responses in different bands to keep the analysis and simulation compatible for practical multi-band UWB scenario. Performance of the proposed algorithm is studied by means of bit error rate (BER) performance of MB-OFDM system. In order to compare the variance of the synchronizer to that of the theoretical optimum, we derive the Cramer---Rao lower bound (CRLB) of the estimation error variance and compare it with the simulated error variance both in additive white Gaussian noise and UWB channel model (CM) environments, CM1---CM4. Next, we modify the estimation algorithm by proposing a multi-band averaging frequency offset synchronization (MBAFS) scheme. We establish superior BER performance with MBAFS compared to our first scheme. We calculate modified CRLB for MBAFS and compare it with simulation results for CM1---CM4. Both analysis and simulation show that MBAFS algorithm can estimate the carrier frequency offset effectively and precisely in UWB fading channels for MB-OFDM applications. We also analyze the computational complexity of both the proposed algorithms in order to verify their feasibility of implementation in practical UWB receiver design.

Multiband Scheduler for Future Communication Systems

Operation in multiple frequency bands simultaneously is an important enabler for future wireless communication systems. This article presents a new concept for scheduling transmissions in a wireless radio system operating in multiple frequency bands: the Multiband Scheduler (MBS). The MBS ensures that the operation in multiple bands is transparent to higher network layers. Special attention is paid to achieving low delay and latency when operating the system in the multiband mode. In particular, we propose additions to the ARQ procedures in order to achieve this. Deployment details and assessment results are presented for two multiband deployment scenarios. The first scenario is operation in a spectrum sharing context where multiple bands are used: one dedicated band for basic service and one shared extension band for extended services. In the second scenario we consider multiband operation in a relay environment, where the two bands have different propagation properties and relays provide extra coverage and capacity in the whole cell.

Superconductivity in PrOs4Sb12: On the double transition and multiband effects

The superconducting state of the heavy fermion PrOs4Sb12 was studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The temperature and field dependence of the thermal conductivity confirm the recently suggested multiband superconductivity scenario and point to fully open gaps on the whole Fermi surface.

Evidence of multiband superconductivity in the quaternary borocarbide superconductor YNi2B2C using directional point-contact spectroscopy

In this paper, we review our recent investigations on the gap anisotropy of the quaternary borocarbide superconductor YNi2B2C using directional point-contact spectroscopy. Through a detailed study of the temperature and magnetic field dependence of the superconducting energy gaps we show that the gap anisotropy in this material originates from electrons on different Fermi sheets having very different Fermi velocities. The gap anisotropy in this material is therefore well explained through a multiband scenario where electrons in different k-directions have very different electron–phonon coupling strength.

Superconductivity in [formula omitted]: On the double transition and multiband effects

The superconducting state of the heavy fermion PrOs 4 Sb 12 was studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The temperature and field dependence of the thermal conductivity confirm the recently suggested multiband superconductivity scenario and point to fully open gaps on the whole Fermi surface.