The Orpheus C. Kerr Papers

UANL

Studies on Spatial and Temporal solitons with varying dispersion,diffraction and nonlinearity

Department of Physics, Cochin University of Science and Technology

Untersuchung von Schichtsystemen für die magnetooptische Datenspeicherung mit Hilfe der Kerr-Mikroskopie

Untersuchung von Schichtsystemen für die magneto-optische Datenspeicherung mit Hilfe der Kerr-Mikroskopie Dissertation von Stephan Knappmann, Universität Gh Kassel, 2000 Im Rahmen dieser Arbeit wurde ein modifiziertes Kerr-Mikroskop aufgebaut und für eine neuartige Untersuchungsmethode von gekoppelten magnetischen Schichten angewendet. Damit können Schichtsysteme untersucht werden, bei denen die eine Schicht eine temperaturabhängige Reorientierung der Magnetisierung von in-plane zu senkrecht zeigt. Derartige Schichten sind für zukünftige magnetooptische Speichertechniken interessant, da sie für die magnetisch induzierte Superauflösung (MSR) eingesetzt werden können. Zunächst wurde ein Ganzfeld-Kerr-Mikroskop aufgebaut und durch einen zusätzlichen Strahlengang erweitert. Da die Proben lokal geheizt werden sollten, wurde der Strahl eines Diodenlasers (l = 780 nm) in den Strahlengang eingekoppelt, durch das Mikroskopobjektiv auf die Probe fokussiert und nach der Reflexion vollständig aus dem Strahlengang entfernt. Dies war deshalb wichtig, da Domänenbilder aufgenommen werden sollten, während die Schichten lokal geheizt werden. Mit diesem Aufbau ist es möglich, den magnetooptischen Ausleseprozeß in MSR Systemen mikroskopisch zu simulieren. Dies wurde am Beispiel einer Doppelschicht demonstriert. Dieses Schichtsystem besteht aus einer Speicherschicht mit senkrechter Magnetisierung (TbFeCo) und einer Ausleseschicht mit temperaturabhängiger Reorientierung (GdFeCo). Damit ist dieses Schichtsystem für eine spezielle MSR Technik (CAD-MSR) geeignet. Dabei wird im heißen Bereich des laserinduzierten Temperaturprofils die Information aus der Speicherschicht in die Ausleseschicht kopiert, so daß eine Apertur gebildet wird, die nur von der Temperaturverteilung abhängt. Bei diesem Schichtsystem konnte gezeigt werden, daß sich durch ein Magnetfeld die Aperturwirkung deutlich verbessern läßt. Dieses Ergebnis läßt sich auf alle Schichten übertragen, bei denen die Reorientierung der Magnetisierung durch allmähliche Drehung erfolgt. Die wesentlichen Ergebnisse der Domänenbeobachtungen an der TbFeCo/GdFeCo-Doppelschicht konnten durch Simulationen mit Hilfe eines einfachen Modells bestätigt werden. Ein weiterer Schwerpunkt der Arbeit lag in der Untersuchung von Vielfachschichten. Es wurden Tb/Fe-Vielfachschichten mit einer Modulationswellenlänge von 1,5 - 5 nm durch Kathodenzerstäubung hergestellt und magnetisch charakterisiert. Die natürlichen Domänen-strukturen zeigten eine starke Abhängigkeit von dem Verhältnis der Tb- und Fe-Schichtdicken. Die Erklärung kann durch das Wechselspiel von magnetostatischer Energie EMS und Domänenwandenergie EDW gegeben werden. In der Nähe des Kompensationspunktes ist EMS klein und EDW dominiert. Als Folge bilden sich zirkulare Domänen. Mit der Entfernung vom Kompensationspunkt steigt EMS und es wird eine verzweigte Domänenstruktur favorisiert. Thermomagnetische Schreibexperimente an einer ausgewählten Tb/Fe-Vielfachschicht haben ergeben, daß sich bei kleinen Schreib-Magnetfeldern Subdomänenstrukturen ausbilden können. In der Praxis ist ein solcher Subdomänenzustand unerwünscht, da er zu einem erhöhten Ausleserauschen führen würde. Der Effekt läßt sich durch ein höheres Magnetfeld oder eine höhere Kompensationstemperatur (höhere Tb-Konzentration) vermeiden. Schließlich wurde demonstriert, daß nach kleinen Änderungen mit dem Mikroskop Domänen in longitudinaler Konfiguration abgebildet werden können. Da die Kerr-Drehungen hier kleiner sind als im polaren Fall, mußten verschiedene Verfahren der Bildverarbeitung angewendet werden, um den magnetischen Kontrast zu vergrößern. Neben der bekannten Subtraktion eines Referenzbildes wurde ein neues Verfahren zur weiteren Verbesserung der Domänenbilder angewendet, wobei zwei Referenzbilder mit entgegengesetzter Magnetisierung benötigt werden.

The effect of nonlinearity on the variational assimilation of satellite observations using a simple column model

Cloud imagery is not currently used in numerical weather prediction (NWP) to extract the type of dynamical information that experienced forecasters have extracted subjectively for many years. For example, rapidly developing mid-latitude cyclones have characteristic signatures in the cloud imagery that are most fully appreciated from a sequence of images rather than from a single image. The Met Office is currently developing a technique to extract dynamical development information from satellite imagery using their full incremental 4D-Var (four-dimensional variational data assimilation) system. We investigate a simplified form of this technique in a fully nonlinear framework. We convert information on the vertical wind field, w(z), and profiles of temperature, T(z, t), and total water content, qt (z, t), as functions of height, z, and time, t, to a single brightness temperature by defining a 2D (vertical and time) variational assimilation testbed. The profiles of w, T and qt are updated using a simple vertical advection scheme. We define a basic cloud scheme to obtain the fractional cloud amount and, when combined with the temperature field, we convert this information into a brightness temperature, having developed a simple radiative transfer scheme. With the exception of some matrix inversion routines, all our code is developed from scratch. Throughout the development process we test all aspects of our 2D assimilation system, and then run identical twin experiments to try and recover information on the vertical velocity, from a sequence of observations of brightness temperature. This thesis contains a comprehensive description of our nonlinear models and assimilation system, and the first experimental results.

The effect of nonlinearity in CO2 heating rates on the attribution of stratospheric ozone and temperature changes

An analysis of the attribution of past and future changes in stratospheric ozone and temperature to anthropogenic forcings is presented. The analysis is an extension of the study of Shepherd and Jonsson (2008) who analyzed chemistry-climate simulations from the Canadian Middle Atmosphere Model (CMAM) and attributed both past and future changes to changes in the external forcings, i.e. the abundances of ozone-depleting substances (ODS) and well-mixed greenhouse gases. The current study is based on a new CMAM dataset and includes two important changes. First, we account for the nonlinear radiative response to changes in CO2. It is shown that over centennial time scales the radiative response in the upper stratosphere to CO2 changes is significantly nonlinear and that failure to account for this effect leads to a significant error in the attribution. To our knowledge this nonlinearity has not been considered before in attribution analysis, including multiple linear regression studies. For the regression analysis presented here the nonlinearity was taken into account by using CO2 heating rate, rather than CO2 abundance, as the explanatory variable. This approach yields considerable corrections to the results of the previous study and can be recommended to other researchers. Second, an error in the way the CO2 forcing changes are implemented in the CMAM was corrected, which significantly affects the results for the recent past. As the radiation scheme, based on Fomichev et al. (1998), is used in several other models we provide some description of the problem and how it was fixed.

On the power amplifier nonlinearity in MIMO transmit beamforming systems

In this paper, single-carrier multiple-input multiple-output (MIMO) transmit beamforming (TB) systems in the presence of high-power amplifier (HPA) nonlinearity are investigated. Specifically, due to the suboptimality of the conventional maximal ratio transmission/maximal ratio combining (MRT/MRC) under HPA nonlinearity, we propose the optimal TB scheme with the optimal beamforming weight vector and combining vector, for MIMO systems with nonlinear HPAs. Moreover, an alternative suboptimal but much simpler TB scheme, namely, quantized equal gain transmission (QEGT), is proposed. The latter profits from the property that the elements of the beamforming weight vector have the same constant modulus. The performance of the proposed optimal TB scheme and QEGT/MRC technique in the presence of the HPA nonlinearity is evaluated in terms of the average symbol error probability and mutual information with the Gaussian input, considering the transmission over uncorrelated quasi-static frequency-flat Rayleigh fading channels. Numerical results are provided and show the effects on the performance of several system parameters, namely, the HPA parameters, numbers of antennas, quadrature amplitude modulation modulation order, number of pilot symbols, and cardinality of the beamforming weight vector codebook for QEGT.

Analysis and compensation of power amplifier nonlinearity in MIMO transmit diversity systems

The nonlinearity of high-power amplifiers (HPAs) has a crucial effect on the performance of multiple-input-multiple-output (MIMO) systems. In this paper, we investigate the performance of MIMO orthogonal space-time block coding (OSTBC) systems in the presence of nonlinear HPAs. Specifically, we propose a constellation-based compensation method for HPA nonlinearity in the case with knowledge of the HPA parameters at the transmitter and receiver, where the constellation and decision regions of the distorted transmitted signal are derived in advance. Furthermore, in the scenario without knowledge of the HPA parameters, a sequential Monte Carlo (SMC)-based compensation method for the HPA nonlinearity is proposed, which first estimates the channel-gain matrix by means of the SMC method and then uses the SMC-based algorithm to detect the desired signal. The performance of the MIMO-OSTBC system under study is evaluated in terms of average symbol error probability (SEP), total degradation (TD) and system capacity, in uncorrelated Nakagami-m fading channels. Numerical and simulation results are provided and show the effects on performance of several system parameters, such as the parameters of the HPA model, output back-off (OBO) of nonlinear HPA, numbers of transmit and receive antennas, modulation order of quadrature amplitude modulation (QAM), and number of SMC samples. In particular, it is shown that the constellation-based compensation method can efficiently mitigate the effect of HPA nonlinearity with low complexity and that the SMC-based detection scheme is efficient to compensate for HPA nonlinearity in the case without knowledge of the HPA parameters.

Performance analysis of AF cooperative systems with HPA nonlinearity in semi-blind relays

In this paper, dual-hop amplify-and-forward (AF) cooperative systems in the presence of high-power amplifier (HPA) nonlinearity at semi-blind relays, are investigated. Based on the modified AF cooperative system model taking into account the HPA nonlinearity, the expression for the output signal-to-noise ratio (SNR) at the destination node is derived, where the interference due to both the AF relaying mechanism and the HPA nonlinearity is characterized. The performance of the AF cooperative system under study is evaluated in terms of average symbol error probability (SEP), which is derived using the moment-generating function (MGF) approach, considering transmissions over Nakagami-m fading channels. Numerical results are provided and show the effects of some system parameters, such as the HPA parameters, numbers of relays, quadrature amplitude modulation (QAM) order, Nakagami parameters, on performance.

Analysis and compensation for the joint effects of HPA nonlinearity, I/Q imbalance and crosstalk in MIMO beamforming systems

In this paper, we investigate the joint effects of high-power amplifier (HPA) nonlinearity, in-phase/quadrature-phase (I/Q) imbalance and crosstalk, on the performance of multiple-input multiple-output (MIMO) transmit beamforming (TB) systems, and propose a compensation method for the three impairments together. The performance of the MIMO TB system equipped with the proposed compensation scheme is evaluated in terms of average symbol error probability and capacity when transmissions are performed over uncorrelated Rayleigh fading channels. Numerical results are provided and show the effects on performance of several system parameters, namely, the HPA parameters, image-leakage ratio, crosstalk, numbers of antennas, length of pilot symbols and phase-shift keying modulation order.

Compensation for HPA nonlinearity and I/Q imbalance in MIMO beamforming systems

In this paper, we investigate the effects of high-power amplifier (HPA) nonlinearity and in-phase and quadrature-phase (I/Q) imbalance on the performance of multiple-input multiple-output (MIMO) transmit beamforming (TB) systems. Specifically, we propose a compensation method for HPA nonlinearity and I/Q imbalance together in MIMO TB systems. The performance of the MIMO TB system under study is evaluated in terms of the average symbol error probability (SEP) and system capacity, considering transmission over uncorrelated frequency-flat Rayleigh fading channels. Numerical results are provided and show the effects of several system parameters, such as the HPA parameters, image-leakage ratio, numbers of transmit and receive antennas, length of pilot symbols, and modulation order of phase-shift keying (PSK), on performance.

Optimal beamforming in MIMO systems with HPA nonlinearity

In this paper, multiple-input multiple-output (MIMO) transmit beamforming (TB) systems under the consideration of nonlinear high-power amplifiers (HPAs) are investigated. The optimal beamforming scheme, with the optimal beamforming weight vector and combining vector, is proposed for MIMO systems with HPA nonlinearity. The performance of the proposed MIMO beamforming scheme in the presence of HPA nonlinearity is evaluated in terms of average symbol error probability (SEP), outage probability and system capacity, considering transmission over uncorrelated quasi-static frequency-flat Rayleigh fading channels. Numerical results are provided and show the effects of several system parameters, namely, parameters of nonlinear HPA, numbers of transmit and receive antennas, and modulation order of phase-shift keying (PSK), on performance.

Impact of HPA nonlinearity on MIMO systems with quantized equal gain transmission

In this paper, we investigate the performance of multiple-input multiple-output (MIMO) transmit beamforming (TB) systems in the presence of nonlinear high-power amplifiers (HPAs). Due to the suboptimality of maximal ratio transmission/maximal ratio combining (MRT/MRC) under HPA nonlinearity, quantized equal gain transmission (QEGT) is suggested as a feasible TB scheme. The effect of HPA nonlinearity on the performance of MIMO QEGT/MRC is evaluated in terms of the average symbol error probability (SEP) and system capacity, considering transmission over uncorrelated quasi-static frequency-flat Rayleigh fading channels. Numerical results are provided and show the effects of several system parameters, such as the parameters of nonlinear HPA, cardinality of the beamforming weight vector codebook, and modulation order of quadrature amplitude modulation (QAM), on performance.

On the effect of power amplifier nonlinearity on MIMO transmit diversity systems

Nonlinearity of high-power amplifier (HPA) plays a crucial role in the performance of multiple-input multiple-output (MIMO) systems. In this paper, we investigate the performance of MIMO orthogonal space-time block coding (STBC) systems in the presence of nonlinear HPA. Specifically, we assess the impact of HPA nonlinearity on the average symbol error probability (SEP), total degradation (TD), and system capacity of orthogonal STBC in uncorrelated Nakagami-m fading channels. Numerical results are provided and show the effects of several system parameters, such as the output back-off (OBO) of nonlinear HPA, numbers of transmit and receive antennas, and modulation order of quadrature amplitude modulation (QAM), on performance.