Multi-Channel Assignment Schemes and Handoff Study in CDMA Cellular Systems

The CDMA system promisingly provides more capacity than AMPS. However, providing multi-channels for a CDMA system is requisite for satisfying the demand of capacity. In this paper, three schemes of multi-channel assignment are proposed. Based on computer simulation, performance measures, e.g. blocking probabilities, the overall capacity, the capacity of each channel, and the number of re-accesses, are obtained. Since the same frequency channel may be assigned to two adjacent cells, the effect of soft handoff is also considered. Numerical results demonstrate that scheme 3 (i.e., the random re-selection scheme) performs the best of the proposed three schemes because it provides the highest system capacity while keeping the cost of accessing channels (i.e., in terms of number of channel accesses) to a minimum.     

Intermodulation distortion in a multiple-quantum-well semiconductor optical amplifier

The measurement of intermodulation distortion (IMD) induced by carrier-density modulation in a multiple-quantum-well (MQW) semiconductor amplifier is reported. The results show that MQW amplifiers have 15 dB less IMD than conventional buried-heterostructure semiconductor amplifiers. The IMD is dependent on the output power of the amplifiers, which confirms that the carrier-density modulation is the dominant nonlinear mechanism in MQW amplifiers. In addition, the results show that, unlike conventional buried-heterostructure amplifiers, MQW amplifiers have at least two time constants (200-250 ps and <10 ps) for the gain recovery process.<>     

Infrared detection by Tl−Ba−Ca−Cu−O superconducting films

The response of polycrystalline Tl?Ba?Ca?Cu?O superconducting thin films on short laser pulses has been investigated for radiation between 10 μm and 500 μm wavelength. Fast signals with time constants less than 1 ns were observed for wavelengths longer than about 100 μm whereas for shorter wavelengths only a bolometric signal could be detected.     

A model for hot-electron-induced MOSFET linear-current degradationbased on mobility reduction due to interface-state generation

A simple model for the hot-electron degradation of MOSFET linear-current drive is developed on the basis of the reduction of the inversion-layer mobility due to the generation of interface states. The model can explain the observed dependence of the device hot-electron lifetime on the effective channel length and oxide thickness by taking into account both the relative nonscalability of the localized damage region and the dependence of the linear-current degradation on the effective vertical electric field E_eff. The model is verified for deep-submicrometer non-LDD n-channel MOSFETs with L_eff=0.2-1.5 μm and T_ox=3.6-21.0 nm. From the correlation between linear-current and charge-pumping degradation, the scattering coefficient α, which relates the number of generated interface states to the corresponding amount of inversion-layer mobility reduction, can be extracted and its dependence on E_eff determined. Using this linear-current degradation model, existing hot-electron lifetime prediction models are modified to account explicitly for the effects of L_eff and T _ox     

Luminescence decay and injected carrier lifetime in the high injection region of AlGaAs lasers

Luminescence decays following short current pulse excitation of an antireflection coated AlGaAs laser diode have been measured. Using an optical gating technique, decays in the high injection region (simgthreshold density of uncoated lasers) were measured with a 100 ps time resolution. The observed luminescence decay is shown to be strongly affected by net gain in the active region. It is also shown that both monomolecular and bimolecular carrier recombination must be considered. A model has been developed that takes these effects into account and is shown to accurately describe steady-state and decay spontaneous emission intensities from laser diodes. A procedure is outlined for determining the necessary device and material parameters for interpreting laser diode characteristics.     

Anisotropic properties of aromatic polyamides containing non-rigid rodlike units

Summary Three kinds of aromatic polyamides were synthesized by Higashi reaction: aromatic polyamides with even, odd number of methylenes and a meta phenylene as a joint in the main chain. Each polymer consisted of 7 p-phenylene-amide elements as a rigid segment. The polymer with even number of methylenes showed a lyotropic liquid crystalline behavior and a broader biphasic region than the fully para aromatic polyamide such as poly(p-phenylene terephthalamide). Most polymers with odd number of methylenes did not form a liquid crystalline phase and showed a crystallo-solvate. Exceptionally, the polymer with glutaryl unit was observed to have the swollen gel of colored pattern by the polarized microscope. The zigzag polymer, characterized by the highly kinetic rigidity, revealed an anisotropic gel phase in a solution.     

Asymptotic loss of real-time traffic in wireless mobile networks with selective-repeat ARQ

We study the issue of quality of service (QoS) for real-time traffic over a wireless channel deploying automatic repeat request (ARQ) error control. An analytical model has been derived to evaluate the queueing related loss and the wireless channel related loss. In contrast to previous work, this model quantifies the interaction between the network layer and the physical layer, and then it enables the admission controllers of wireless networks to improve utilization while satisfying the traffic QoS constraints through cross-layer design techniques.     

Luminescence,charge mobility,and optical waveguiding of two-dimensional organic rubrene nanosheets: Comparison with one-dimensional nanorods

Intrinsic characteristics of organic and inorganic nanostructures depend on their physical dimensions (i.e., size and shape) and crystallinity. Here, we compared the nanoscale optical and electrical properties of organic rubrene one-dimensional (1-D) nanorods (NRs) and two-dimensional (2-D) nanosheets (NSs). From high-resolution laser confocal microscope photoluminescence (PL) measurements, the light-emission characteristics of 2-D rubrene NSs varied with the crystalline domain direction, indicating intrinsic PL anisotropy, which was distinguishable from 1-D rubrene single NRs, because of anisotropy π–π stacking molecular arrangements. We also observed the variation of charge carrier mobility depending on the measured directions (i.e., anisotropy of charge transport) in rubrene NS-based field-effect transistors. The optical waveguiding properties of rubrene nanostructures were strongly correlated to the dimensionality of materials and PL anisotropy.