Study of Phosphorus Doped Micro/Nano Crystalline Silicon Films Deposited by Filtered Cathodic Vacuum Arc Technique

Phosphorus doped micro/nano crystalline silicon thin films have been deposited by the filtered cathodic vacuum arc technique at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 ^°C. The films have been characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, secondary ion mass spectroscopy, dark conductivity ( σ _D), activation energy ( ΔE) and optical band gap (E _g). The XRD patterns show that the RT grown film is amorphous in nature but high T_s (225 and 350 ^°C) deposited films have a crystalline structure with (111) and (220) crystal orientation. The crystallite size of the higher T_s grown silicon films evaluated was between 17 to 31 nm. Raman spectra reveal the amorphous nature of the film deposited at RT whereas higher T_s deposited films show a higher crystalline nature. The crystalline volume fraction of the silicon film deposited at higher T_s was estimated as 65.7 % and 74.4 %. The values of σ _D, ΔE and E _g of the silicon films deposited at different T_s were found to be in the range of 8.84 x 10 ^?4? 0.98 ohm ^?1 cm ^?1, 0.06 - 0.31 eV and 1.31-1.93 eV, respectively. A n-type nc-Si/p-type c-Si heterojunction diode was fabricated which showed the diode ideality factor between 1.1 to 1.5.     

Performance analysis of TCM with generalized selection combining on Rayleigh fading channels

We derive the computational cutoff rate, R/sub 0/, for coherent trellis-coded modulation (TCM) schemes on independent identically distributed (i.i.d.) Rayleigh fading channels with (K, L) generalized selection combining (GSC) diversity, which combines the K paths with the largest instantaneous signal-to-noise ratios (SNR) among the L available diversity paths. The cutoff rate is shown to be a simple function of the moment generating function (MGF) of the SNR at the output of the (K, L) GSC receiver. We also derive the union bound on the bit error probability of TCM schemes with (K, L) GSC in the form of a simple, finite integral. The effectiveness of this bound is verified through simulations.     

Toward secure and private service discovery anywhere anytime

With the advances in and convergence of Internet technologies, embedded computers, and wireless communication, computing devices have become part of our daily life. Hand-held devices and sensors with wireless connections create opportunities for many new nomadic applications. Service discovery is an essential component for cognitive science to discover existing network services just-in-time. Unlike many other approaches, we propose a service discovery model supporting nomadic users and services in public environments. Our model emphasizes secure and private service discovery in such environments. Location sensing is integrated for location dependent service discovery and is used to lessen service discovery network infrastructure requirements. We analyze the system performance and show our formal verification of the protocols. Our implementation shows that our model is feasible.     

Performance of closed-loop power control in DS-CDMA cellularsystems

In situations where the round-trip delay between the mobile and the base stations is smaller than the correlation time of the channel, power control schemes using feedback from the base station can effectively compensate for the fast fading due to multipath. We study several closed-loop power control (CLPC) algorithms by analysis and detailed simulation. We introduce a new loglinear model for analyzing the received power correlation statistics of a CLPC scheme. The model provides analytical expressions for the temporal correlation of the power controlled channel parameterized by the update rate, loop delay, and vehicle speed. The received power correlation statistics quantify the ability of closed-loop power control to compensate for the time-varying channel. To study more complex update strategies, detailed simulations that estimate the channel bit-error performance are carried out. Simulation results are combined with coding bounds to obtain quasi-analytic estimates of the reverse link capacity in a direct-sequence code-division multiple-access (DS-CDMA) cellular system. The quasi-analytic approach quantifies the performance improvements due to effective power control in both single-cell and multicell DS-CDMA systems operating over both frequency-nonselective and frequency-selective fading channels. The effect of nonstationary base stations on the system performance is also presented     

SNR estimation in Nakagami-m fading with diversity combining and its application to turbo decoding

We propose an online signal-to-noise ratio (SNR) estimation scheme for Nakagami-m (1960) fading channels with L branch equal gain combining (EGC) diversity. We derive the SNR estimate based on the statistical ratio of certain observables over a block of data, and use the SNR estimates in the iterative decoding of turbo codes on Nakagami-m fading channels with L branch EGC diversity. We evaluate the turbo decoder performance using the SNR estimate under various fading and diversity scenarios (m = 0.5, 1, 5 and L = 1, 2, 3) and compare it with the performance using perfect knowledge of the SNR and the fade amplitudes.     

Performance analysis of UDP with energy efficient link layer on Markov fading channels

In this paper, we analyze the throughput and energy efficiency performance of user datagram protocol (UDP) using linear, binary exponential, and geometric backoff algorithms at the link layer (LL) on point-to-point wireless fading links. Using a first-order Markov chain representation of the packet success/failure process on fading channels, we derive analytical expressions for throughput and energy efficiency of UDP/LL with and without LL backoff. The analytical results are verified through simulations. We also evaluate the mean delay and delay variation of voice packets and energy efficiency performance over a wireless link that uses UDP for transport of voice packets and the proposed backoff algorithms at the LL. We show that the proposed LL backoff algorithms achieve energy efficiency improvement of the order of 2-3 dB compared to LL with no backoff, without compromising much on the throughput and delay performance at the UDP layer. Such energy savings through protocol means will improve the battery life in wireless mobile terminals.     

Optimization of stereolithography process parameters for part strength using design of experiments

Rapid prototyping (RP) is an emerging technology that has been implemented in many spheres of industry – particularly in the area of new product development. Growth of this field has been rapid in recent years. Stereolithography (SL) is one of the most popular RP process used for rapid tooling applications. There are several process parameters contributing to the strength of an SL product. The contribution of three parameters; namely, layer thickness, post curing time and orientation are most significant. In light of this concern, an attempt has been made to study and optimize these process parameters for maximum part strength, and develop an empirical relationship between process parameters and part strength through design of experiments (DOE). The proposed DOE is verified with the data of experiments conducted under standard conditions.     

Ni0.5Zn0.5Fe2O4-dispersed In2O3-spotted ZnO nanoparticles: Ammonia-source and surface and photocatalytic properties

Ammonia-source, used to attain the desired pH during synthesis, is conceived to influence the physical characteristics of ZnO-based nanomaterials, and the catalytic activity is susceptible to surface characteristics of semiconductor–photocatalyst. In this context, Ni_0.5Zn_0.5Fe₂O₄-dispersed In₂O₃-spotted ZnO nanoparticles have been obtained by using either tetramethyl ammonium hydroxide or ammonium carbonate as ammonia-source at identical pH (9) using identical quantities of the precursors following identical synthetic procedure. The nanoparticles have been characterized using energy dispersive X-ray spectroscopy, elemental mapping, selected area electron and X-ray diffractometries, transmission electron microscopy, etc. The nanoparticles obtained using ammonium carbonate possess larger (1) pore width, (2) pore volume, and (3) surface area compared with nanoparticles prepared employing tetramethyl ammonium hydroxide. Although the electrical properties of both the samples do not differ remarkably, the violet light-absorption of the sample prepared using the carbonate is slightly larger than that of the other sample. Further, the In₂O₃-spotting is slightly larger on using ammonium carbonate than using tetramethyl ammonium hydroxide. To degrade dye under visible light, the sample obtained using ammonium carbonate shows larger catalytic activity compared with nanoparticles prepared using tetramethyl ammonium hydroxide. The observed photocatalytic activities are explained based on the surface characteristics.     

Crystal plasticity with Jacobian-Free Newton–Krylov

The objective of this work is to study potential benefits of solving crystal plasticity finite element method (CPFEM) implicit simulations using the Jacobian-Free Newton–Krylov (JFNK) technique. Implicit implementations of CPFEM are usually solved using Newton’s method. However, the inherent non-linearity in the flow rule model that characterizes the crystal slip system deformation on occasions would require considerable effort to form the exact analytical Jacobian needed by Newton’s method. In this paper we present an alternative using JFNK. As it does not require an exact Jacobian, JFNK can potentially decrease development time. JFNK approximates the effect of the Jacobian through finite differences of the residual vector, allowing modified formulations to be studied with relative ease. We show that the JFNK solution is identical to that obtained using Newton’s method and produces quadratic convergence. We also find that preconditioning the JFNK solution with the elastic tensor provides the best computational efficiency.