Despite careful work in both compact cryocoolers and microwave filters, no true "fieldable" subsystem demonstration has been reported in Japan. However, individual filters and coolers have been integrated for in-laboratory tests. One such test was described at AMTEL (Fig. 4.3).

Fig. 4.3. HTS filter system (AMTEL).

Such laboratory milestones were reached by the small U.S. firms in about 1995. These companies have today reached the stage where multiple filters packaged with ancillary electronics in a single system have been tower-mounted for a myriad of "drive-around" demonstrations of improved link margin for reception of the mobile signals. Such tests were reported in the technical and business press, and HTS subsystems are now functioning in several operational base stations handling real customer calls. Although unsubstantiated, tests may also be underway in Japan using subsystems obtained from the U.S. commercial sector.

It is important to note that what is typically demonstrated in field tests is an approximately 1 dB lower noise figure and hence range extension capability. These improvements result from the reduced noise floor achievable with cold preamplifiers and low-loss filters. While reducing the number of dropped calls near a cell boundary, this benefit of range extension may be of only limited interest in the commercial marketplace. Of course, the greatest value will obtain for the dense channelization, in transmit and receive, of the link signals. The market is being fueled by the expectation of myriad services to be offered to a large and dense customer base. During initial roll-out, cells may have a small subscriber base, but in Japan and Europe and in the most lucrative U.S. markets, dense-packed subscribers will be the target group of the service providers. Hence, the greatest market opportunity exists for tightest use and reuse of the frequency spectrum through sharp-skirted receive and transmit filters, rather than through extended range. The interlacing of the two (A and B) provider band allocations for the conventional cellular Advanced Mobile Phone System (AMPS) in the United States is an extreme example of the adjacent channel interference that is a worldwide concern of service providers.

The U.S. R&D effort, while not yet experiencing a true wireless "market pull," is certainly driving toward technical requirements set by the advanced development segment of the wireless community. While not yet a real market pull, this setting of requirements for the filter subsystem performance has served to focus and accelerate the pace of HTS microwave development in the United States in much the same way as the High Temperature Superconducting Space Experiment (HTSSE) program did earlier. This pull in the United States has resulted in not only well packaged and ruggedized cryoassemblies, but a diversity of designs in which complex filter responses are synthesized by both lumped- and distributed-component techniques. There is also a heightened appreciation for the power- handling issues related to distortion products created by out-of-band and edge-of-band interferers. Finally, the market pull has given U.S. HTS researchers a calibration on the pace and diversity of conventional technology with which engineers are seeking to solve the system problems.

Published: July 1998; WTEC Hyper-Librarian