Closed-form expressions for computer-aided design of microstrip ring antennas

This article presents closed-form expressions for the resonant frequency and radiation characteristics of microstrip ring antennas to assist in the computer-aided design of such antennas. Radiation fields are obtained using the vector electric potential calculated from the magnetic currents flowing along the inner and outer edges of the planar waveguide model for the microstrip ring structure. Expressions for radiated power, conductor and dielectric loss factors, resonant resistance, efficiency, and directivity are derived. The smaller the radius of curvature of the ring, the larger is the spacing between the magnetic currents flowing along the inner and outer edges and thus the radiation efficiency is higher. The effect of a dielectric layer covering the ring antenna is also included. Experimental results for the resonant frequency and the radiation patterns agree well with the theoretical predictions. © 1992 John Wiley & Sons, Inc.     

Development of preforming pitch for carbon-carbon composites from coal-based precursors

Preforming pitch, a special type of pitch, used for the matrix formation of carbon-carbon composites, has been developed from suitable coal-based precursors, using the techniques of distillation, condensation and polymerization. The effects of various processing parameters, namely the temperature and period of heat treatment, and the atmosphere (inert or partial vacuum) during the heat treatment, on the characteristics of the resulting pitch have been studied. Some of the pitches were subjected to field trials from which the characteristics of a good preforming pitch leading to a carbon-carbon composite of density around 1.8 g cm^–3 have been identified.     

Breakdown voltage enhancement from channel quantization inInAl/As/n+-InGaAs HFET's

In_0.52Al_0.48As/n⁺-In_0.53 Ga_0.47As HFETs have been fabricated with different channel thicknesses. It is shown that by reducing the channel thickness from 350 to 100 Å, the reverse gate breakdown voltage improves from 9 to 19 V. This is partially attributed to the increased effective bandgap that results from energy quantization in the channel. This bandgap enhancement is directly confirmed by photoluminescence (PL) measurements on the same heterostructures. Channel quantization emerges as a promising approach for exploiting the excellent transport properties of InGaAs with high InAs mole fraction. The principle behind the work should be applicable to other narrow-gap semiconductors     

Low-noise gaas MESFET optimization using an orthogonal array approach

This article presents an experimental optimization of the noise figure of small-signal self-aligned FETs. An L₁₈ orthogonal array has been used to find the main effects of specific device parameters on noise performance at 18 GHz. This knowledge has been used to find an alternative device which shows a ～0.7-dB improvement in noise measure without requiring major process changes. Hybrid low-noise amplifiers built with the improved FETs confirm the noise performance and show record power performance for high dynamic range applications. © 1994 John Wiley & Sons, Inc.     

2005

In recent years, wireless Internet service providers (WISPs) have established Wi-Fi hotspots in increasing numbers at public venues, providing local coverage to traveling users and empowering them with the ability to access email, Web, and other Internet applications on the move. In this paper, we observe that while the mobile computing landscape has changed both in terms of number and type of hotspot venues, there are several technological and deployment challenges remaining before hotspots can become an ubiquitous infrastructure. These challenges include authentication, security, coverage, management, location services, billing, and interoperability. We discuss existing research, the work of standards bodies, and the experience of commercial hotspot providers in these areas, and then describe compelling open research questions that remain. Anand Balachandran has been a member of the research staff at Intel Research, Seattle since October 2003. His research interests include wireless networking systems, wireless Internet, infrastructure and ad-hoc networks, and mobile and ubiquitous computing. He received his Bachelor of Technology degree from the Indian Institute of Technology, Madras in 1995, his Master’s degree from Columbia University, in 1997, and his Ph.D. degree in Computer Science and Engineering from the University of California at San Diego in 2003. Geoffrey M. Voelker is an assistant professor at the University of California at San Diego. His research interests include operating systems, distributed systems, networking, and mobile computing. He received a BS degree in Electrical Engineering and Computer Science from the University of California at Berkeley in 1992, and the M.S. and Ph.D. degrees in Computer Science and Engineering from the University of Washington in 1995 and 2000, respectively. In 2000, he received the first Computing Research Association (CRA) Digital Government Fellowship, and in 2002 he received the Hellman Young Faculty Fellowship at UCSD. Victor Bahl is a Senior Researcher and the Manager of the Networking Group in Microsoft Research. His research interests span a variety of problems in wireless networking. In addition to making many product contributions, he has authored over 65 scientific papers, 44 issued and pending patent applications and several book chapters. He is the co-founder and Chairman of the ACM Special Interest Group in Mobility (SIGMOBILE); the founder and past Editor-in-Chief of ACM Mobile Computing and Communications Review, and the founder and Steering Committee Chair of ACM/USENIX Mobile Systems Conference (MobiSys); He has served on the editorial board of IEEE Journal on Selected Areas in Communications, and is currently serving on the editorial boards of Elsevier’s Adhoc Networking Journal, Kulwer’s Telecommunications Systems Journal, and ACM’s Wireless Networking Journal. He has served as a guest editor for several IEEE and ACM journals and on networking review panels organized by the National Science Foundation (NSF), the National Research Council (NRC) and European Union’s COST. He has served as the General Chairman, Program Chair and Steering Committee member of several IEEE and ACM conferences and on the Technical Program Committee of over 45 international conferences and workshops. He is the recipient of Digital’s Doctoral Engineering Award (1994) and ACM SIGMOBILE’s Distinguished Service Award (2001). He is a Fellow of the ACM, a Senior Member of the IEEE and a past president of the electrical engineering honor society Eta kappa Nu-Zeta Pi. Dr. Bahl received his Ph. D in Computer Systems Engineering from the University of Massachusetts Amherst.This revised version was published online in August 2005 with a corrected cover date.     

Average power handling capability of multilayer microstrip lines

This article describes the average power handling capability (APHC) of multilayer microstrip lines, including the effect of mismatch at the terminations. The data presented herein are validated by considering an example of a 12‐W monolithic microwave integrated circuit power amplifier fabricated using multilayer low‐loss microstrip technology. The calculated value of APHC for a 50‐Ω line of a 75‐μm‐thick GaAs substrate is 1445 W at 10 GHz, whereas the corresponding value for a multilayer microstrip that has 10‐μm‐thick polyimide is only 44 W. At 40 GHz, these values are reduced by a factor of 2. © 2001 John Wiley & Sons, Inc. Int J RF and Microwave CAE 11: 385–395, 2001.     

The design and implementation of HomeRF: a radio frequency wirelessnetworking standard for the connected home

The HomeRF Working Group (WG) is a consortium of more than 100 companies from the computer, telecommunications, and consumer electronics industries. This group has developed an open specification called the Shared Wireless Access Protocol-Cordless Access (SWAP-CA) that enables radio frequency (RF) wireless connectivity between a diverse set of devices and computing resources in and around a typical home. Built around an RF spectrum with worldwide availability, SWAP-CA includes operational support for both managed and ad hoc networks of devices. It combines and extends wireless networking and cordless telephony into a single unified protocol allowing mobile devices to communicate via both voice and data traffic simultaneously over the Internet and/or over the public switched telephone network (PSTN). For batter-operated devices, it includes a power management mechanism than ensures connection longevity. The technology has been specifically optimized for consumer applications and price points, and, consequently, the HomeRF WG has the broad backing of the major corporate stakeholders interested in enabling tetherless networking within the home     

Performance of 4- and 8-state TCM schemes with asymmetric 8-PSK infading channels

For Rayleigh fading channels, Ricean fading channels with a small parameter and shadowed Rician channels with the code design criteria are to maximize the effective length as well as the minimum product distance of the code. In this paper, we use two different asymmetric 8-PSK signal sets in 4-state and 8-state rate 2/3 trellis-coded modulation (TCM) schemes and show performance gain over TCM schemes with symmetric 8-PSK constellations. The performance gain is due to an increase in the minimum product distance compared to that of the best known TCM schemes of comparable states using symmetric 8-PSK signal sets while the effective length remains same. Simulation is performed over the Rayleigh, Ricean, and shadowed Rician fading channels to demonstrate the performance gain of the asymmetric 8-PSK TCM schemes over the symmetric 8-PSK TCM     

Cuckoo and krill herd‐based k‐means++ hybrid algorithms for clustering

Clustering algorithms can be optimized using nature‐inspired techniques. Many algorithms inspired by nature, namely, firefly algorithm, ant colony optimization algorithm, and so forth, have improved clustering results. k‐means is a popular clustering technique but has limitations of local optima, which have been overcome using its various hybrids. k‐means++ is a hybrid k‐means clustering algorithm that gives the procedure to initialize centre of the clusters. In the proposed work, hybrids of nature‐inspired techniques using cuckoo and krill herd algorithm are implemented on k‐means++ algorithm to enhance cluster quality and generate optimized clusters. The designed algorithms are implemented, and the results are compared with their counterparts. Performance parameters such as accuracy, f‐measure, error rate, standard deviation, CPU time, cluster quality check, and so forth are used to measure the clustering capabilities of these algorithms. The results indicate the high performance of newly designed algorithms.