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Dileep Kumar

Publications and source records attributed to Dileep Kumar.

35 records · Page 2Linked to original sources
arXiv2023

Polarization Diversity-enabled LOS/NLOS Identification via Carrier Phase Measurements

The provision of accurate localization is an increasingly important feature of wireless networks. To this end, a reliable distinction between line-of-sight (LOS) and non-LOS (NLOS) radio links is necessary to avoid degenerative localization estimation biases. Interestingly, LOS and NLOS transmissions affect the polarization of the received signals differently. In this work, we leverage this phenomenon to propose a threshold-based LOS/NLOS classifier exploiting weighted differential carrier phase measurements over a single link with different polarization configurations. Operation in either full or limited polarization diversity systems is possible. We develop a framework for assessing the performance of the proposed classifier, and show through simulations the performance impact of the reflecting materials in NLOS scenarios. For instance, the classifier is far more efficient in NLOS scenarios with wooden reflectors than in those with metallic reflectors. Numerical results evince the potential performance gains from exploiting full polarization diversity, properly weighting the differential carrier phase measurements, and using multi-carrier/tone transmissions. Finally, we show that the optimum decision threshold is inversely proportional to the path power gain in dB, while it does not depend significantly on the material of potential NLOS reflectors.

cs.IT↗
arXiv2022

Latency-Aware Multi-antenna SWIPT System with Battery-Constrained Receivers

Power splitting (PS) based simultaneous wireless information and power transfer (SWIPT) is considered in a multi-user multiple-input-single-output broadcast scenario. Specifically, we focus on jointly configuring the transmit beamforming vectors and receive PS ratios to minimize the total transmit energy of the base station under the user-specific latency and energy harvesting (EH) requirements. The battery depletion phenomenon is avoided by preemptively incorporating information regarding the receivers' battery state and EH fluctuations into the resource allocation design. The resulting time-average sum-power minimization problem is temporally correlated, non-convex (including mutually coupled latency-battery queue dynamics), and in general intractable. We use the Lyapunov optimization framework and derive a dynamic control algorithm to transform the original problem into a sequence of deterministic and independent subproblems, which are then solved via two alternative approaches: i) semidefinite relaxation combined with fractional programming, and ii) successive convex approximation. Furthermore, we design a low-complexity closed-form iterative algorithm exploiting the Karush-Kuhn-Tucker optimality conditions for a specific scenario with delay bounded batteryless receivers. Numerical results provide insights on the robustness of the proposed design to realize an energy-efficient SWIPT system while ensuring latency and EH requirements in a time dynamic mobile access network.

eess.SP↗
arXiv2022

Enhancing the limit of uniaxial magnetic anisotropy induced by ion beam erosion

The artificial tailoring of magnetic anisotropy by manipulation of interfacial morphology and film structure are of fundamental interest from application point of view in spintronic and magnetic memory devices. This letter reports an approach of engineering and enhancing the strength of oblique incidence ion beam erosion (IBE) induced in-plane uniaxial magnetic anisotropy (UMA) by simultaneous modification of film morphology as well as film texture. To meet this objective, Cobalt film and Si substrate have been taken as a model system. Unlike conventional post growth IBE of film, we direct our effort to the sequential deposition and subsequent IBE of the film. Detailed in-situ investigation insights that the film grows in highly biaxially textured polycrystalline state with formation of nanometric surface ripples. The film also exhibits pronounced UMA with easy axis oriented parallel to the surface ripple direction. Remarkably, the induced UMA is about one order of magnitude larger than the reported similar kind of earlier studies. The possibility of imposing in-plane crystallographic texture giving rise to magneto-crystalline anisotropy, along with long-range dipolar interaction throughout ripple crests enhances the strength of the UMA. The present findings can be further extended to systems characterized by different crystallographic structure and magnetic properties and show the general applicability of the present method.

cond-mat.mtrl-sci↗
arXiv2022

Study of Asymmetric Magnetization Reversal and Exchange Bias in FePt(L10)/FeCo/CoO/FeCo Magnetic Multilayer

The effect of the saturation field on the magnetization reversal of FePt(L10)/FeCo/CoO/FeCo multilayer (ML) has been investigated to understand the origin of asymmetric magnetization reversal and its correlation with exchange bias (EB). In the ML structure, the bottom FeCo layer is coupled to the hard FePt(L10) layer, and the top FeCo layer is comparatively free due to the relatively more distance from it. The ML has been deposited under UHV conditions and characterized at each stage of growth using magneto-optical Kerr effect and x-ray reflectivity techniques. Magnetization reversal is further studied through domain imaging using the Kerr microscopy technique. The experimental findings reveal that ML exhibits asymmetrical magnetization reversal for a certain range of azimuthal angles for both 1.5kOe and 50kOe saturation fields; however, this angular range of asymmetry decreases with the increase in the saturation field. Furthermore, EB was absent at the low saturation field, whereas, EB, in addition to asymmetry, is observed at the large saturation field. The origin of asymmetry is attributed to non-collinearity between magnetic anisotropy axes of both FeCo layers. It results from the proximity effect through short-range Heisenberg exchange interaction via the CoO barrier layer. On the other hand, EB arises due to unidirectional anisotropy induced in the FePt layer due to the high saturation field. It is further proposed that asymmetry would disappear when unidirectional anisotropy is strong enough to align both the FeCo layers in the saturation direction leading to loss of the non-collinearity between them.

cond-mat.mtrl-sci↗
arXiv2022

Origin of exchange bias in [Co/Pt]ML/Fe multilayer with orthogonal magnetic anisotropies

Magnetization reversal of soft ferromagnetic Fe layer, coupled to [Co/Pt]ML multilayer [ML] with perpendicular magnetic anisotropy (PMA), has been studied in-situ with an aim to understand the origin of exchange bias (EB) in orthogonal magnetic anisotropic systems. The interface remanant state of the ML is modified by magnetic field annealing, and the effect of the same on the soft Fe layer is monitored using the in-situ magneto-optical Kerr effect (MOKE). A considerable shift in the Fe layer hysteresis loop from the centre and an unusual increase in the coercivity, similar to exchange bias phenomena, is attributed to the exchange coupling at the [Co/Pt]ML and Fe interface. The effect of the coupling on spin orientation at the interface is further explored precisely by performing an isotope selective grazing incident nuclear resonance scattering (GINRS) technique. Here, the interface selectivity is achieved by introducing a 2 nm thick Fe57 marker between [Co/Pt]ML and Fe layers. Interface sensitivity is further enhanced by performing measurements under the x-ray standing wave conditions. The combined MOKE and GINRS analysis revealed the unidirectional pinning of the Fe layer due to the net in-plane magnetic spin at the interface caused by magnetic field annealing. Unidirectional exchange coupling or pinning at the interface, which may be due to the formation of asymmetrical closure domains, is found responsible for the origin of EB with an unusual increase in coercivity.

cond-mat.mtrl-sci↗
arXiv2022

Massive MIMO with Radio Stripes for Indoor Wireless Energy Transfer

Radio frequency wireless energy transfer (WET) is a promising solution for powering autonomous Internet of Things (IoT) deployments. In this work, we leverage energy beamforming for powering multiple user equipments (UEs) with stringent energy harvesting (EH) demands in an indoor distributed massive multiple-input multiple-output system. Based on semi-definite programming, successive convex approximation (SCA), and maximum ratio transmission (MRT) techniques, we derive optimal and sub-optimal precoders aimed at minimizing the radio stripes' transmit power while exploiting information of the power transfer efficiency of the EH circuits at the UEs. Moreover, we propose an analytical framework to assess and control the electromagnetic field (EMF) radiation exposure in the considered indoor scenario. Numerical results show that i) the EMF radiation exposure can be more easily controlled at higher frequencies at the cost of a higher transmit power consumption, ii) training is not a very critical factor for the considered indoor system, iii) MRT/SCA-based precoders are particularly appealing when serving a small number of UEs, thus, especially suitable for implementation in a time domain multiple access (TDMA) scheduling framework, and iv) TDMA is more efficient than spatial domain multiple access (SDMA) when serving a relatively small number of UEs. Results suggest that additional boosting performance strategies are needed to increase the overall system efficiency, thus making the technology viable in practice.

cs.IT↗
arXiv2021

Latency-Constrained Highly-Reliable mmWave Communication via Multi-point Connectivity

The sensitivity of millimeter-wave (mmWave) radio channel to blockage is a fundamental challenge in achieving low-latency and ultra-reliable connectivity. In this paper, we explore the viability of using coordinated multi-point (CoMP) transmission for a delay bounded and reliable mmWave communication. We propose a novel blockage-aware algorithm for the sum-power minimization problem under the user-specific latency requirements in a dynamic mobile access network. We use the Lyapunov optimization framework, and provide a dynamic control algorithm, which efficiently transforms a time-average stochastic problem into a sequence of deterministic subproblems. A robust beamformer design is then proposed by exploiting the queue backlogs and channel information, that efficiently allocates the required radio and cooperation resources, and proactively leverages the multi-antenna spatial diversity according to the instantaneous needs of the users. Further, to adapt to the uncertainties of the mmWave channel, we consider a pessimistic estimate of the rates over link blockage combinations and an adaptive selection of the CoMP serving set from the available remote radio units (RRUs). Moreover, after the relaxation of coupled and non-convex constraints via the Fractional Program (FP) techniques, a low-complexity closed-form iterative algorithm is provided by solving a system of Karush-Kuhn-Tucker (KKT) optimality conditions. The simulation results manifest that, in the presence of random blockages, the proposed methods outperform the baseline scenarios and provide power-efficient, high-reliable, and low-latency mmWave communication.

eess.SP↗
arXiv2021

Synthesis and study of fcc-Co derived from isostructural Co4N

This work demonstrates synthesis and study of fcc-Co derived from an isostructural Co4N. Diffusion measurements carried out in this work, reveal that N self-diffusion is the swiftest in Co4N compared to other transition metal nitrides or even the mononitride CoN. By the application of a high substrate temperature (Ts) growth or thermal annealing temperature (Ta); N diffuses out from the fcc-Co4N above 573 K leaving behind a high purity fcc-Co phase. Generally, Co grows in a hcp structure and a (partial) hcp to fcc-Co transformation takes place around 700 K or above 70 GPa. The proposed route through nitridation and diffusion of N not only bring down the phase transition temperature, an impurity present in the form of hcp-Co can be avoided altogether. Oriented Co4N(111) thin films were grown using a CrN(111) template on a quartz substrate using a dc magnetron sputtering. Samples were grown at different Ts or room temperature grown Co4N samples were annealed at different Ta. Analysis using x-ray diffraction, N K-edge x-ray absorption, x-ray photoelectron and secondary ion mass spectroscopy confirmed the formation of fcc-Co4N or fcc-Co phases. It was found that Co-N bonding and N concentration get significantly reduced at a high Ts or Ta. Magnetization measurements combining ex-situ and in-situ magneto-optical Kerr effect showed differences in anisotropy and coercivity of Co4N and fcc-Co samples. Combining structural, electronic and magnetization measurements, it has been observed that a high purity fcc-Co can be conveniently derived from the isostructural Co4N aided by an exceptionally high N self-diffusion in Co4N.

cond-mat.mtrl-sci↗
arXiv2021

Blockage-aware Reliable mmWave Access via Coordinated Multi-point Connectivity

The fundamental challenge of the millimeter-wave (mmWave) frequency band is the sensitivity of the radio channel to blockages, which gives rise to unstable connectivity and impacts the reliability of a system. To this end, multi-point connectivity is a promising approach for ensuring the desired rate and reliability requirements. A robust beamformer design is proposed to improve the communication reliability by exploiting the spatial macro-diversity and a pessimistic estimate of rates over potential link blockage combinations. Specifically, we provide a blockage-aware algorithm for the weighted sum-rate maximization (WSRM) problem with parallel beamformer processing across distributed remote radio units (RRUs). Combinations of non-convex and coupled constraints are handled via successive convex approximation (SCA) framework, which admits a closed-form solution for each SCA step, by solving a system of Karush-Kuhn-Tucker (KKT) optimality conditions. Unlike the conventional coordinated multi-point (CoMP) schemes, the proposed blockage-aware beamformer design has, per-iteration, computational complexity in the order of RRU antennas instead of system-wide joint transmit antennas. This leads to a practical and computationally efficient implementation that is scalable to any arbitrary multi-point configuration. In the presence of random blockages, the proposed schemes are shown to significantly outperform baseline scenarios and result in reliable mmWave communication.

eess.SP↗
arXiv2019

Multidimensional Shifts And Finite Matrices

Let $X$ be a $2$-dimensional subshift of finite type generated by a finite set of forbidden blocks (of finite size). We give an algorithm for generating the elements of the shift space using sequence of finite matrices (of increasing size). We prove that the sequence generated yields precisely the elements of the shift space $X$ and hence characterizes the elements of the shift space $X$. We extend our investigations to a general $d$-dimensional shift of finite type. In the process, we prove that that elements of $d$-dimensional shift of finite type can be characterized by a sequence of finite matrices (of increasing size).

math.DS↗
arXiv2019

Evolution of magnetic anisotropy in cobalt film on nanopatterned silicon substrate studied in situ using MOKE

Evolution of magnetization behaviour of cobalt film on nano patterned silicon substrate, with film thickness, has been studied. In situ magneto-optical Kerr effect measurements during film deposition allowed us to study genuine thickness dependence of magnetization behaviour, all other parameters like surface topology, deposition conditions remaining invariant. The film exhibits uniaxial magnetic anisotropy, with its magnitude decreasing with increasing film thickness. Analysis shows that anisotropy has contributions from both, i) exchange energy which is volume dependent and, ii) stray dipolar fields at the surface/interface. This suggests that local magnetization follows only partially the topology of the rippled surface. As expected from energy considerations, for small film thickness, the local magnetization closely follows the surface contour of the ripples making the volume term as the dominant contribution. With increasing film thickness, the local magnetization gradually deviates from the local slope and approaches towards a uniform magnetization along the macroscopic film plane making the surface term as the dominant contribution. Significant deviation from the anisotropy energy expected on the basis of theoretical considerations can be attributed to several factors like, deviation of surface topology from an ideal sinusoidal wave, breaks of continuity along the ripple direction, defects like pattern dislocations, and possible decrease in surface modulation depth with increasing film thickness.

cond-mat.mtrl-sci↗
arXiv2018

Fractional Crank-Nicolson-Galerkin Finite Element Scheme for the Time-Fractional Nonlinear Diffusion Equation

This article presents a finite element scheme with Newton's method for solving the time-fractional nonlinear diffusion equation. For time discretization, we use the fractional Crank-Nicolson scheme based on backward Euler convolution quadrature. We discuss the existence-uniqueness results for the fully-discrete problem. Discrete fractional Gronwall type inequality for the backward Euler convolution quadrature which is used to approximate the Riemann-Liouville fractional derivative is established by using the idea of D.Li et al. given in [11]. A priori error estimate for the fully-discrete problem in $L^2(Ω)$ norm is derived. Numerical results based on finite element scheme are provided to validate the theoretical estimates.

math.AP↗
arXiv2016

High resolution Kerr microscopy study of exchange bias phenomena in FePt/Fe exchange spring magnets

Magnetization and magnetic microstructure of top soft magnetic layer (Fe), which is exchange spring coupled to bottom hard magnetic layer ($L1_0$ FePt) is studied using high resolution Kerr microscopy. When the sample (FePt/Fe) is at remanent condition of hard magnetic layer, considerable shifting of Fe layer hysteresis loop from centre i.e., exchange bias phenomena is observed. It is observed that one can tune the magnitude of exchange bias shift by reaching the remanent state from different saturating fields ($H_{SAT}$) and also by varying the angle between measuring field and $H_{SAT}$. The M-H loops and domain images of top soft Fe layer demonstrates unambiguously that soft magnetic layer at remanent state in such exchange coupled system is having unidirectional anisotropy. An analogy is drawn and the observations are explained in terms of the mostly accepted models of exchange bias phenomena exhibited by bilayers consisting of ferromagnetic(FM) and anti-ferromagnetic (AFM) layers, when the AFM layer is field cooled across $N\acute{e}el$ transition temperature.

cond-mat.mtrl-sci↗
arXiv2016

Matrix Characterization of Multidimensional Subshifts of Finite Type

Let $X\subset A^{Z^d}$ be a $2$-dimensional subshift of finite type. We prove that any $2$-dimensional multidimensional subshift of finite type can be characterized by a square matrix of infinite dimension. We extend our result to a general $d$-dimensional case. We prove that the multidimensional shift space is non-empty if and only if the matrix obtained is of positive dimension. In the process, we give an alternative view of the necessary and sufficient conditions obtained for the non-emptiness of the multidimensional shift space. We also give sufficient conditions for the shift space $X$ to exhibit periodic points.

math.DS↗
arXiv2012

Interface induced perpendicular magnetic anisotropy in Co/CoO/Co thin film structure: An in-situ MOKE investigation

Co /CoO/Co polycrystalline film was grown on Si (001) substrate and magnetic properties have been investigated using in-situ magneto-optic Kerr effect during growth of the sample. Magnetic anisotropy with easy axis perpendicular to the film surface has been observed in top Co layer, whereas bottom layer was found to be soft with in-plane magnetization without any influence of top layer. Ex-situ in-plane and out-of-plane diffraction measurements revealed that the growth of Co on oxidized interface takes place with preferential orientation of c-axis perpendicular to the film plane, which results in the observed perpendicular magnetic anisotropy. Texturing of the c-axis is expected to be a result of minimization of the interface energy due to hybridization between Co and oxygen at the interface.

cond-mat.mtrl-sci↗
arXiv2009

Asymmetric diffusion at the interfaces in multilayers

Nanoscale diffusion at the interfaces in multilayers plays a vital role in controlling their physical properties for a variety of applications. In the present work depth-dependent interdiffusion in a Si/Fe/Si trilayer has been studied with sub-nanometer depth resolution, using x ray standing waves. High depth-selectivity of the present technique allows one to measure diffusion at the two interfaces of Fe namely, Fe-on-Si and Si-on-Fe, independently, yielding an intriguing result that Fe diffusivity at the two interfaces is not symmetric. It is faster at the Fe-on-Si interface. While the values of activation energy at the two interfaces are comparable, the main difference is found in the pre-exponent factor suggesting different mechanisms of diffusion at the two interfaces. This apparently counter-intuitive result has been understood in terms of an asymmetric structure of the interfaces as revealed by depth selective conversion electron Mossbauer spectroscopy. A difference in the surface free energies of Fe and Si can lead to such differences in the structure of the two interfaces.

cond-mat.mes-hall↗