Technologies for Wireless Applications: Past, Present, and Future
Microwave Journal Guest Editor
This special section contains articles based on papers originally presented at the IEEE MTT-S International Topical Symposium on Technologies for Wireless Applications held in Vancouver, BC in February in conjunction with INTER COMM® '99. The MTT-S Topical Symposium focused on specific areas of engineering and scientific interest aimed at products and services associated with the wireless communications industry. The five articles presented here have been expanded to provide additional material of interest to Microwave Journal readers.
The number of papers presented at this meeting, held every other year in conjunction with INTER COMM®, has doubled since its inception in 1995 - well behind Moore's Law, but just short of the rate of increase of the S&P 500. While the growth in the number of papers at one conference may not have the credibility of these other indicators in measuring the growth of the microwave wireless field, the topics discussed clearly indicate the milestones achieved.
Commercial millimeter-wave systems, including automotive sensor suites at 77 GHz and local multipoint distribution service at 30 GHz, are experiencing explosive demand and represent substantial progress from the dual-use technology transfer activities that took place during the early to mid-1990s. Today, millimeter-wave manufacturing issues are high volume module assembly and development of low cost, high frequency packaging methods and materials to meet the demands of the marketplace. The lead paper in this section by Geller et al. discusses advances in ceramic multilayer packaging techniques that allow dimensional control necessary for high frequency operation, integrated heat spreading to include power devices and process developments allowing buried passive components.
Meanwhile, research continues on new devices and materials that show promise for use over a broad frequency range in commercial wireless applications. Kobayashi et al. review the features of the InP material system and applications of heterojunction bipolar transistor devices based on this material. In the four years since the first topical conference, the InP material system has demonstrated outstanding performance, and efforts are now directed toward volume production.
At the lower end of the commercial microwave system spectrum, handset size (weight) appears to have leveled off after nearly two decades of steady reduction as accommodation of multiple standards occupies designer energy. At least one of these emerging standards, wideband CDMA, is forcing a detailed understanding of linearity and its relation to device design and process parameters. In the past four years, measures of linearity have moved from simple single-tone to system-level adjacent-channel power ratio (ACPR) descriptions and are related directly to device parameters. In the first of two papers on distortion in power FETs, Tkachenko et al. link process parameter variation to measured values of intermodulation distortion. In the second paper, Hau et al. demonstrate the relationship between this intermodulation distortion and ACPR.
Finally, in the area of antennas for microwave systems, low cost and form factor requirements continue to rely upon planar techniques. In the past four years, faster, more accurate designs have been facilitated by improved optimization and modeling techniques. Examples of a number of novel designs are presented in the final paper by Deal et al.
So what's up for the future? Some safe bets include increasing digital (and, therefore, silicon or silicon-based) content in microwave systems as processing speed becomes both faster and cheaper. Moore's Law is good for at least another 15 years and will enable the demise of analog functions such as filters and mixers and the growth of "software" handsets and radios whose modulation formats and other architectural details are programmable. Continued advances in modeling capability, including brute force numerical techniques with speedier execution that will allow rigorous solutions of complex structures in shorter times can be expected. Expanding the scope of the modeling activity to the system level, maintaining traceability to the physics of key analog devices or components will allow faster system design and validation of new system architectures.
After that, my crystal ball becomes foggy. Check back in another four years, and let's make another measurement.