Performance analysis of dynamic spectrum handoff scheme with variable bandwidth demand of secondary users for cognitive radio networks

Cognitive radio (CR) has attracted considerable attention as a promising technology for solving the current inefficient use of spectrum. In CR networks, available sub-channels are dynamically assigned to secondary users (SUs). However, when a primary user accesses a primary channel consisting of multiple sub-channels, data transmissions of the SUs already using the sub-channels may be terminated. In this paper, we analyze the performance of dynamic spectrum handoff scheme with channel bonding, in which the number of sub-channels used by an SU are variable. We model the multichannel CR network as a multiserver priority queueing system without waiting facility, deriving the blocking probability, the forced termination probability and the throughput for SUs. In terms of the way of forced termination, we consider two policies; one is that SUs using the largest number of sub-channels are forced to terminate their transmissions, and the other is that SUs using the smallest number of sub-channels are chosen for termination. The analysis is also validated by simulation. Numerical examples show that in both forced-termination policies, the throughput of SUs that are forced to terminate their transmissions degrades as the offered load to the system increases.     

Photocatalytic activity of TiO2/Ti composite coatings fabricated by mechanical coating technique and subsequent heat oxidation

Titanium (Ti) coatings were fabricated on alumina (Al₂O₃) balls by mechanical coating technique (MCT) with Ti powder. The Ti coatings were then oxidized to titanium dioxide (TiO₂) coatings at different temperatures. The oxidation behavior and microstructure evolution of these coatings were investigated. The results showed that the inner and surface layers of the Ti coatings were oxidized simultaneously. When oxidizing at a relatively low temperature for a short time, TiO₂/Ti composite coatings were obtained. Increasing the oxidation temperature or time increased the thickness of the TiO₂ layer and eventually Ti coatings were totally oxidized to TiO₂ coatings. During oxidation, TiO₂ needles formed at a lower temperature grew to generate columnar crystals. The photocatalytic activity of these coatings was examined. Compared with TiO₂ coatings, the TiO₂/Ti composite coatings showed much higher photocatalytic activity. The highest activity was observed for the TiO₂/Ti composite coatings prepared by MCT and subsequent oxidation at 1073 K for 15 h and then the thickness of the TiO₂ layer was 27 μm.     

Performance improvement and analysis of a 1.6 μm continuous-wave modulation laser absorption spectrometer system for CO2 sensing

In a previous study, we developed a 1.6 μm continuous-wave (cw) modulation laser absorption spectrometer system for CO(2) sensing and demonstrated the measurement of small fluctuations in CO(2) corresponding to a precision of 4 parts per million (ppm) with a measurement interval of 32 s. In this paper, we present the process to achieve this highly specific measurement by introducing important points, which have not been shown in the previous study. Following the results of preliminary experiments, we added a function for speckle averaging on the optical antenna unit. We additionally came up with some ideas to avoid the influences of etalon effects and polarization dependence in optical components. Because of the new functions, we realized a calibration precision of 0.006 dB (rms), which corresponds to a CO(2) concentration precision of less than 1 ppm for a 2 km path. We also analyzed the CO(2) sensing performance after the improvements described above. The measured short time fluctuation of the differential absorption optical depth was reasonably close to that calculated using the carrier-to-noise ratio of the received signal.     

Ray-tracing method for isotropic inhomogeneous refractive-index media from arbitrary discrete input

We have developed a ray-tracing simulation procedure for optically isotropic gradient refractive-index media. The procedure can take discrete points of arbitrary distribution for the definition of refractive-index distributions and lens surfaces. It is useful for simulating ray trajectories in real lens systems. The procedure is applied to a ray-tracing simulation of the Luneburg lens and a radial gradient optical fiber. The simulation results are compared with the analytical solutions, and it is shown that they are in precise agreement.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

Evaluations of pure and ytterbium doped transparent ceramic complex perovskite scintillators

Optical and scintillation properties of complex perovskite transparent ceramic scintillators of pure and Yb³⁺ doped Ba(ZrMgTa)O₃ and (LaSr)(AlTa)O₃ ceramics are reported. The materials were produced by Murata Manufacturing. Their optical properties including transmittance and photoluminescence spectra were evaluated. The ceramics demonstrated high transparency (50–70%) in visible wavelength region. Additionally, strong emission peaks were observed at 470 nm in Ba(ZrMgTa)O₃ under 284 nm excitation and at 500 nm in (LaSr)(AlTa)O₃ under 324 nm excitation. The photoluminescence decay times of Ba(ZrMgTa)O₃ and (LaSr)(AlTa)O₃ samples were 14 and 16 μs, respectively. Judging from these optical properties, Yb³⁺ emission was not observed in these materials. In radio luminescence spectra, all specimens exhibited the same emission peaks with relatively higher emission intensity. ²⁴¹Am 5.5 MeV α-ray induced pulse height spectra of the samples were also measured, and (LaSr)(AlTa)O₃ demonstrated ～500 ph/5.5 MeV α with 10 μs shaping time constant.     

Flow visualization and numerical analysis of a new mixing equipment with vibratory fins

The mixing performance of a new type of mixing equipment which has several fin oscillators on a pair of shafts with a vibrating motor was investigated. This mixing equipment, which is mainly used for industrial plating processes, was usually operated at a vibrating frequency of about 40 Hz with the amplitude 1 mm. The flow visualization in this equipment showed that the flow in the vessel at laminar flow region was vertically divided into two distinct symmetric regions. The numerical simulation of the flow and the mixing patterns agreed well with the visualization result at laminar flow region.     

Comparison of CW THz Wave Spectrometer with Laser Diode Excitation and Pulsed THz Wave Spectrometer with Cr:forsterite Sources Based on Difference Frequency Generation of Near-infrared Lasers in GaP

We constructed two types of terahertz (THz) spectrometers with automatic scanning control based on the difference frequency generation method by the excitation of the phonon-polariton mode in GaP. The pulsed THz wave spectroscopic systems were based on an optical parametric oscillator and Nd: YAG laser sources having a frequency resolution of 1.5 GHz, and on Cr:fosterite sources with a resolution of 20 GHz. Following these, we recently constructed a continuous wave (CW) THz wave spectroscopic system with laser diode excitation. One of the advantages of the CW THz wave spectrometer is its wide frequency tuning range with fine frequency resolution of < 8 MHz. In this study, we compare both types of spectrometers (pulsed versus CW) to show the characteristics of each system in terms of frequency resolution. The absorption spectra of a non-deformed white polyethylene crystal and ultra-high molecular weight polyethylene with/without deformation are measured by using the CW THz wave spectrometer and pulsed THz wave spectrometer. The effect of the high-resolution CW THz wave spectrometer is shown based on the THz spectroscopic results.     

Crystal modulus of a new semiaromatic polyamide 9‐T

The elastic modulus (E_l) of crystalline regions in the direction parallel to the chain axis in a new semiaromatic polyamide 9‐T (PA9‐T) containing a long aliphatic chain unit was measured by X‐ray diffraction and the derived E_l value was discussed in terms of its molecular conformation and mechanical properties. For low and high molecular weight PA9‐Ts, the E_l value of 40 GPa was obtained. The low E_l value for PA9‐T is due to the presence of aromatic groups with long crankshaft arms, which result in a greater moment of force during deformation compared with those polymers with planner zigzag molecular conformation like polyethylene and nylon 6 α‐form. Another factor for the low E_l is due to the contraction of its molecular chains that is likely originated from the existence of a torsional conformation. The presence of contraction causes some internal rotation in the polymer chain during extensional deformation, which results in low E_l. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers