UPTIME: an IMS-based mobility framework for next generation mobile networks

Seamless inter-technology mobility is one of the fundamental requirements of next generation mobile networks. For seamless mobility, handover delay and packet loss should be minimized. However, existing solutions suffer from a number of shortcomings in satisfying these requirements: first, handover preparation schemes fail to minimize the handover delay as much as possible. Second, minimizing packet loss which is usually using soft handover (SHO) schemes are excessively wasteful of scarce resources. In this paper, we propose the uninterrupted proactive connection transfer for IMS mobility enhancement (UPTIME) mobility framework which achieves seamless mobility while minimizing excessive power and radio resource consumption. UPTIME incorporates two mechanisms; a proactive handover preparation method and an optimized SHO technique for handover execution. We demonstrate the benefits of the proposed framework through both analysis and simulation. Our simulation results for typical LTE/WiMAX handovers show that the handover preparation delay can be reduced by 70 %, and good packet loss performance can be achieved whilst saving 43 % of radio resources and 48 % of battery power.     

Characterization of in-situ terahertz detection by optical interaction in a periodically poled stoichiometric lithium tantalate nonlinear crystal

Terahertz waves are generated using a femtosecond laser pulse in a periodically poled stoichiometric lithium tantalate crystal and simultaneously detected via a non-collinear optical parametric interaction inside the same crystal. Real time up-conversion signal between the generated THz and an optic probe pulses is measured depending on the beam overlapped conditions using a general silicon-photodiode for the THz detection. The non-collinear geometry is to facilitate manipulated property of the position-dependent bandwidth at narrow and broad bandwidths of 45 GHz and 3.3 THz, respectively at the one crystal. Furthermore, an aperture effect at the detection part is characterized as the function of size and position owing to the spatial distribution of the frequency conversion signal and it is applied in optimization of the in-situ detection scheme.     

Terahertz polarization beam splitter based on photonic crystal and multimode interference

We design a compact terahertz （THz） polarization beam splitter. Both plane wave expansion method and fi- nite-difference time-domain method are used to calculate and analyze the characteristics of the proposed device. The designed polarization beam splitter can split TE-polarized and TM-polarized THz waves into different propagation di- rections. The simulation results show that the extinction ratios are larger than 18.36 dB for TE polarization and 13.35 dB for TM polarization in the frequency range from 1.86 THz to 1.91 THz, respectively. The designed polarization beam splitter has the advantages of small size and compact structure with a total size of 4.825 mm×0.400 mm.     

Improved Hydrogen Capping Effect in n-Type Crystalline Silicon Solar Cells by SiN(Si-Rich)/SiN(N-Rich) Stacked Passivation

The effect of hydrogen capping of SiN(Si-rich)/SiN(N-rich) stacks for n-type c-Si solar cells was investigated. Use of a passivation layer consisting of Si-rich SiN with a refractive index (n) of 2.7 and N-rich SiN with a refractive index of 2.1 improved the thermal stability. A single SiN passivation layer with a refractive index of 2.05 resulted in an initial lifetime of 200 μs whereas the layer with a refractive index of 2.7 resulted in a high initial lifetime of 2 ms, but the layer degraded rapidly after firing. A stacked passivation layer with refractive indices of 2.1 and 2.7 had a stable lifetime of 1.5 ms with an implied open-circuit voltage (iV _oc) of 720 mV after firing. The thermally stable passivation mechanism with changing amounts of Si–N and Si–H bonding was analyzed by Fourier-transform infrared (FTIR) spectroscopy. Incorporation of the SiN _x stack layer (2.7 + 2.1) into the passivated rear of n-type Cz silicon screen-printed solar cells resulted in energy conversion efficiency of 19.69%. Improved internal quantum efficiency in the long-wavelength range above 900 nm, with V _oc of 630 mV, is mainly because of superior passivation of the rear surface compared with conventional solar cells.     

On the kinetics of electron processes in 60Co γ-irradiated n-Ge single crystals

Variations in the main electrical parameters of n-Ge single crystals with different doping levels under the effect of 60Co γ-irradiation are investigated. A noticeable increase in the carrier mobility in the irradiated samples is observed in some dopant concentration range and the nature of this effect is explained. It is demonstrated that the electron-mobility variation under the action of γ-irradiation in the initial n-Ge crystals with an oxygen impurity and in the annealed crystals have opposite signs. It is established that the oxygen complexes formed in the samples during annealing and the local lattice stresses near these complexes play a key role in the radiation-induced mobility increase. It is clarified that the radiation resistance of the electron mobility significantly depends on the crystallographic orientation of the investigated samples.     

Electrical Characteristics of Al/Poly(methyl methacrylate)/p-Si Schottky Device

Al/Poly(methyl methacrylate)(PMMA)/p-Si organic Schottky devices were fabricated on a p-Si semiconductor wafer by spin coating of PMMA solution. The capacitance–voltage (C–V) and conductance–voltage (G–V) characteristics of Al/PMMA/p-Si structures have been investigated in the frequency range of 1 kHz–10 MHz at room temperature. The diode parameters such as ideality factor, series resistance and barrier height were calculated from the forward bias current–voltage (I–V) characteristics. In order to explain the electrical characteristics of metal–polymer–semiconductor (MPS) with a PMMA interface, the investigation of interface states density and series resistance from C–V and G–V characteristics in the MPS structures with thin interfacial insulator layer have been reported. The measurements of capacitance (C) and conductance (G) were found to be strongly dependent on bias voltage and frequency for Al/PMMA/p-Si structures. The values of interface state density (D _it) were calculated. These values of D _it and series resistance (R _s) were responsible for the non-ideal behavior of I–V and C–V characteristics.     

A model for the complex permittivity of water at frequencies below 1 THz

Experimental permittivity data of liquid water, compiled from the open literature, were selectively applied to support a modeling strategy. Frequencies up to 1 THz and atmospheric temperatures are covered with an expression made up by two relaxation (Debye) terms. The double-Debye model reduces to one term when the high frequency limit is set at 100 GHz, and the model can be extended to 30 THz by adding two resonance (Lorentzian) terms. The scheme was carried out by employing nonlinear least-squares fitting routines to data we considered reliable.     

Robust MMSE Design With Asynchronous Interference Mitigation in Cooperative Base Station Systems

This paper investigates minimum mean square error (MMSE) with asynchronous interference mitigation in cooperative base station systems. We consider the asynchronous transmission because of the different propagation times between the base station (BS) and mobile stations (BSs). Meanwhile, the channel quantization errors duo to channel quantization is taken into account in our analysis. The proposed scheme is robust to asynchronous interference and channel quantization errors in BSs cooperation systems. Simulations results show that proposed MMSE scheme achieve an improved performance compared with the conventional MMSE in BSs cooperative systems.     

Sample Thickness Measurement with THz-TDS: Resolution and Implications

The accuracy of any measurement with terahertz time-domain spectroscopy (THz-TDS) depends strongly on knowing and supplying the precise sample thickness when processing the raw terahertz data. Sample thickness usually is estimated using other (non-THz) metrology and invariably involves some degree of uncertainty. It turns out that the terahertz data itself typically also contains information regarding sample thickness. However, there is limited systematic work addressing the following questions: What is the best method for extracting sample thickness from THz-TDS data? And, what is the thickness resolution obtainable with THz-TDS? In this study we demonstrate how these questions can be answered in general by answering them for a specific example: undoped silicon wafers. We determine the accuracy with which the exact thickness of nominally 500 μm silicon wafers can be measured using transmission mode THz-TDS. We analyze and compare the resolution of 5 different approaches for determining sample thickness using THz-TDS data, including two new methods and three methods proposed in the literature. The quantitative results and analyses of methods we present will be useful in developing far-infrared optical metrology. Conversely the quantitative results presented can be used to relate uncertainty in sample thickness to uncertainty in the measured terahertz data both in the time domain and frequency domain (phase and amplitude). Finally, a precise understanding of the relationship between sample thickness and THz-TDS data can be used to formulate superior THz-TDS data work-up methodologies.     

Sixtieth order harmonic mixing to 4.25 THz using a Schottky diode

Harmonic mixing of mm-waves with radiation of 3.7 THz with different Schottky diodes is reported. The highest mixing order is 60 producing a beat between 72 GHz and 4.25 THz radiation. Such high frequency mixing has so far only been possible using cryogenic Josephson mixers. The present result permits substantial simplification of measurements of optical frequencies.     

Investigation of GaAs Schottky barrier diodes in the THz range

The properties of GaAs Schottky barrier diodes as video detectors and mixing elements were investigated in the frequency range from 0.8–2.5 THz. For the most sensitive diode, the video responsivity and system noise temperature were measured as a function of incident laser power. The highest video responsivity was 2,000 V/W at 214μm and 60 V/W at 118μm. For five diodes differing in doping, capacitance, series resistance and anode diameter, the system noise temperature was measured at 214μm and 118μm. The best single sideband (SSB) values are 12,300 K and 24,200 K at 214μm and 118μm, respectively. The system noise temperature versus frequency is given over the range from 0.5–3 THz for two specific diodes demonstrating that the sharpness of the I–V characteristics is only of secondary importance for mixer perfomance at such high frequencies.     

Virtualized Execution and Management of Hardware Tasks on a Hybrid ARM-FPGA Platform

Emerging hybrid reconfigurable platforms tightly couple capable processors with high performance reconfigurable fabrics. This promises to move the focus of reconfigurable computing systems from static accelerators to a more software oriented view, where reconfiguration is a key enabler for exploiting the available resources. This requires a revised look at how to manage the execution of such hardware tasks within a processor-based system, and in doing so, how to virtualize the resources to ensure isolation and predictability. This view is further supported by trends towards amalgamation of computation in the automotive and avionics domains, where such properties are essential to overall system reliability. We present the virtualized execution and management of software and hardware tasks using a microkernel-based hypervisor running on a commercial hybrid computing platform (the Xilinx Zynq). The CODEZERO hypervisor has been modified to leverage the capabilities of the FPGA fabric, with support for discrete hardware accelerators, dynamically reconfigurable regions, and regions of virtual fabric. We characterise the communication overheads in such a hybrid system to motivate the importance of lean management, before quantifying the context switch overhead of the hypervisor approach. We then compare the resulting idle time for a standard Linux implementation and the proposed hypervisor method, showing two orders of magnitude improved performance with the hypervisor.     

Structural Characterization and Thermoelectric Properties of Hot-Pressed CoSi Nanocomposites

Fabrication of nanocomposites by introduction of SiO₂ metal oxide nanoparticles into a cobalt silicide thermoelectric matrix is studied. The CoSi matrix material was prepared through solid-state synthesis, and the nano-SiO₂ metal oxide was introduced by mechanical grinding. The mixed powders were hot pressed to fabricate nanocomposites. The structural and morphological modifications were studied by powder x-ray diffraction analysis and scanning electron microscopy. The thermoelectric properties of the materials were followed through the Hall effect, Seebeck coefficient, and electrical and thermal conductivities in the temperature range from 300 K to 1000 K.     

Charge transport in Si-SiO2 and Si-TiO2 nanocomposite structures

The experimental data on dispersed Si-SiO₂ and Si-TiO₂ nanocomposite structures different in terms of physical, chemical, and insulator properties of oxide components are reported. The parameters of the nanocomposite structures are studied by FTIR spectroscopy and impedance spectroscopy. It is shown that, in such structures, the mechanisms of charge-carrier transport are defined by the properties of Si nanocrystallites and the corresponding oxide as well as by interaction processes at interfaces between grains.     

Design and fabrication of 0.5 micron GaAs Schottky barrier diodes for low-noise terahertz receiver applications

Recent technological advances have made possible the development of heterodyne receivers with high sensitivity and high spectral resolution for frequencies in the range 1,000–3,000 GHz (1–3 THz). These receivers rely on GaAs Schottky barrier mixer diodes to translate the high-frequency signal to a lower frequency where amplification and signal processing are possible. At these frequencies, the diode quality is a major limitation to the performance of the receiver. The design, fabrication and DC evaluation of a diode for this frequency range is presented. A figure-of-merit cut-off frequency of over 10 THz is achieved with a record low zero biased capacitance of 0.5 fF. Results from RF tests are also given.     

Effects of Propagation Models on AODV in Mobile Ad-hoc Networks

Mobile ad-hoc networks (MANET) operate like self organizing entity. The mobile nodes are operate in a host as well as in a router and dynamic topology. In MANET every network node operates autonomously. It has limited resources like power, bandwidth and storage capacity. Selection of a propagation model plays the vital role of application possibilities in MANET. In this paper, calculations have been performed for three propagation models: Two-ray ground, COST 231 and Okumura–Hata model. The paper shows the impact of these propagation models based on Ad-hoc on-demand distance vector (AODV) protocol at 1.5 GHz frequency, varying transmitted power and number of nodes. This paper also studies effects of the propagation models with the conclusion of choosing the most accurate propagation model. Okumura–Hata propagation model shows better results at 1.5 GHz frequency as compared to COST 231 and Two-ray ground model in open space using Network Simulator (NS 2.35). Two-ray ground propagation model shows better results with varying transmitted power and number of nodes as compared to COST 231 and Okumura–Hata model.     

Capacity and interference modeling of CSMA/CA networks using SSI point processes

The relative location of simultaneous transmitters, i.e. the set of nodes transmitting a frame at a given time, has a crucial impact on the performance of multi hop wireless networks. Two fundamental aspects of wireless network performances are related to these locations: capacity and interference. Indeed, as interference results from the summation of signals stemmed by concurrent transmitters, it directly depends on the transmitters’ location. On the other hand, the network capacity is proportional to the number of simultaneous transmitters. In this paper, we investigate original point processes that can be used to model the location of transmitters that comply with the CSMA/CA policies, i.e. the Medium Access Control protocol used in 802.15.4 and 802.11 families of wireless technologies. We first propose the use of the Simple Sequential Inhibition point process to model CSMA/CA networks where clear channel assessment depends on the strongest emitter only. We then extend this point process to model a busy medium detection based on the strength of all concurrent signals. We finally compare the network capacity obtained through realistic simulations to a theoretical capacity estimated using the intensity of the SSI point process. It turns out that the proposed model is validated by the simulations.     

Theoretical performance of novel multipliers at millimeter and submillimeter wavelengths

A theoretical comparison of various low and high order multipliers for 200 GHz and 1 THz has been carried out. Novel diodes including single barrier varactors, barrier-intrinsic-n⁺ diodes and high electron mobility varactors are shown to have excellent theoretical performance, comparable or better than the conventional Schottky varactors for single and double diode frequency multipliers at millimeter and submillimeter wavelengths, whereas quantum well diodes, since they suffer from high resistive losses, are shown to be less attractive. In comparison to the conventional Schottky varactor, these new diodes have some potential advantages in their characteristics such as nonlinearity or a special symmetry. For future optimization some general comments on these advantages as well as other factors affecting multiplication is given.     

Terahertz Spectra of L-Ascorbic Acid and Thiamine Hydrochloride Studied by Terahertz Spectroscopy and Density Functional Theory

We have investigated the terahertz spectra of L-ascorbic acid and thiamine hydrochloride measured by terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FTIR). The measured absorption spectra were demonstrated to be in good agreement with the results simulated by Density Functional Theory (DFT) using hybrid functional B3LYP with basis set of 6-31G (d), except with slight frequency shift and few peaks missing. We presented the comparison of measured spectra by the FTIR spectroscopy employing low temperature silicon bolometer as detector and the TDS system. The measured spectra of the L-ascorbic acid showed shoulder bands at 0.25, 1.1, 1.5, 1.82, 2.03, 2.30, 2.44, 2.67, 2.97, 3.12, and 3.40 THz, respectively. The spectra of the thiamine hydrochloride show shoulder bands at 0.48, 1.11, 1.57, 1.75, 1.92, 2.08, 2.31, 2.53, 2.69, 2.85, 3.12, 3.22, and 3.31 THz. Most absorption peaks of the two samples agree with the results simulated by Density Function Theory (DFT) method of Gaussian 09 software. In our work, more spectral peaks based on experimental and theoretical results were found in comparison to that of other groups, since we employed higher sensitive FTIR measurement system and considered the effect of number of molecule unit in simulation. The study suggests that the effect of intermolecular vibration is stronger than intramolecular interaction on the absorption bands in THz region.