WIRELESS PERFORMANCE DEMANDS FOR HTS DEVICES

For filters in receivers, the high-quality factors achievable in compact HTS structures can translate to the following:

  1. Lower insertion loss (and resultant lower noise floor) for increased channel sensitivity. The lower noise is greatly aided by use of cooled low-noise semiconductor amplifiers. This sensitivity improvement is particularly useful in range extension for sparsely populated rural cells or for satellite links. HTS upgrades have already been accomplished in several rural, large-area cells in the United States.
  2. Sharper frequency rejection skirts for increased channel selectivity. The crowded urban cells will benefit substantially by improved ability to reject close-range interference from adjacent bands. The interfering signal can emanate either from mobile or base-station transmitters, and rejections of more than 100 dB are often required. Fractional bandwidths of only a few tenths of a percent and very precise filter specifications place a premium on both the quality factor and accuracy of implementation, with many (10-20) poles being required to achieve the proper shape factor.

For filters in transmitters, the lower insertion loss and sharper skirts achievable in HTS structures could provide improved channelization of the radiated power to the desired band with suppression of spurious signals generated in adjacent bands by final-stage amplifier nonlinearities. This application area is particularly challenging, as it requires the handling of high circulating powers (many kW) within the resonators, and can lead to both intermodulation distortion and thermal degradation.

HTS technology has the obvious barrier to use of requiring proper thermal packaging and cooling. U.S. firms have demonstrated that the multiple filters required in a base station (for multipath diversity and subdivision of cells into sectors) can be packed compactly in a cold space. The cooler must be reliable, with infrequent (or no) preventative maintenance required for more than 5 years. As will be described in Chapter 7, a variety of machines are being developed for this purpose both in the United States and Japan.

There appear to be no first-order market differences for wireless between Japan, the United States, and Europe. All have a large installed base of wired lines and a rapidly expanding wireless infrastructure. And in all cases, the wireless market is being penetrated by a variety of competing systems and providers. This global marketplace is both highly competitive and being propelled by large investments and corporate alliances. It is a hotbed of competing technologies, standards, and protocols. As the costs rise and roll out of new networks approaches, the time window contracts for insertion of any new technology. The second- order differences that exist within the developed countries, such as spectral allocations, customer density, and signaling protocols make no significant difference in mitigating the barriers for insertion of HTS technology.


Published: July 1998; WTEC Hyper-Librarian