Commercial Applications
Wireless Communications
Superconductors offer the unique advantages of ultra-low dissipation and distortion as well as intrinsic (quantum) accuracy. These advantages combine to enable what researchers with the US Army have called the "most significant change to satellite communications worldwide in 30 years." Advanced filters are already deployed in commercial wireless base stations, enabling wider range and fewer dropped calls. A more significant development involves a complete transition to all-digital receivers and transmitters, a migration currently in the prototype phase which promises dramatic improvements in efficiency and cost for both military and commercial wireless communication systems.
HTS Filters
Cellular telephone base stations process signals by first detecting them with the antennas mounted at the top of the tower and then dividing those signals into channels dedicated to specific conversations. This division, when implemented with conventional technology, reduces the range of communication because of dissipation or loss of signal strength in the signal processing, and imposes limits on the number of channels due to lack of sharp filters and the resulting necessity to avoid overlap between the filtered outputs of different conversations handled. By using High Temperature Superconducting (HTS) filters, both limitations are addressed, and the service providers can increase the range of the base stations as well as enable a much larger number of simultaneous channels.
This technology is now commercially deployed, with over 10,000 such systems in use in commercial systems and a total of over 200 million hours of run time. In an industry where reliability and uptime are the number one goal, these HTS filters have proven to be highly advantageous to the system.
All-Digital Receivers
While HTS filters have demonstrated the unique positive impact of superconductors on the analog (non-digital) elements in wireless communications, the true revolution under development is one that takes advantage of the intrinsic linearity and quantum accuracy of superconductors to produce the world’s best analog-to-digital converters. Much like digital CDs and digital television provide a superior experience as well as improved efficiency, all-digital receivers carry the same benefits to wireless communications. The crux of the improvement is in the unique ability of superconducting analog-to-digital converters to digitize a wide band of signal without the need for analog pre-processing.
As a result, significant portions of the system, which usually add weight, volume, cost, and distortion, are completely eliminated. In addition, the manipulation of the digital data enables full flexibility in accommodating any protocol through the use of software, thus leading to an effectively universal system where the same hardware is adapted, by software, - to "translate" and decode any incoming signal from any other system.
Compatibility with legacy systems as well as "future-proofing" are ensured. While this technology is not yet commercially available, its development is proceeding at an impressive pace, supported in part by the US Government and in part by commercial ventures.
“Most Significant Change to Satellite Communications Worldwide in 30 years”
Recently, a demonstration of the capability of superconducting All-Digital Receivers was carried out by the US Army, in which an x-band satellite communications link was closed using such a receiver. The ability to directly digitize the x-band RF signal was proven and led to the proclamation that the “most significant change to satellite communications worldwide in 30 years” had been achieved. The heart of the system is the superconducting integrated circuit (IC) shown in the figure to the right. This circuit is made similarly to semiconductor ICs, but the key material here, instead of being Silicon, is the low temperature superconductor Niobium. The chip - less than half the size of a penny - contains about 11,000 Josephson junctions laid out to form superconducting Rapid Single Flux Quantum (RSFQ) circuits that move picosecond-duration magnetic pulses. After many years of research, the superconductivity community worldwide has determined that such digital applications can only be harnessed using low temperature superconductors, a fact that simplifies design and fabrication of the circuits but adds the more difficult constraint of cooling the IC to 4 degrees Kelvin (by contrast to the more easily reachable 70 degrees Kelvin for HTS materials). [1]
Nevertheless, advances in cryogenics, including recent breakthroughs in pulse-tube technology, fully provide the enabling cooling platform for packaging and deploying All-Digital Receivers. In addition, the same technology can be applied to All-Digital Transmitters (and hence, All-Digital Transceivers), resulting in similar gains in performance, cost, and efficiency.
Issues and Recommendations
Both military and commercial applications stand to benefit from the use of superconducting All-Digital Receivers (and Transmitters and Transceivers). The more difficult technical problems to address are in military applications, due to the much wider spectrum of frequencies, protocols, and applications involved.
On the commercial front, commodity-level cost is such a competitive factor that initially, the better performance afforded by superconducting All-Digital technology is relegated to future plans pending its further evolution into a fully developed low-cost alternative. The much needed exploitation of supercomputing All-Digital technology will be enabled, perhaps uniquely, by a dedicated Governmental support to produce and demonstrate field prototypes with full performance. These demonstrations will be the needed catalyst to engage procurement plans from the Government and to place this technology on the roadmap for commercial wireless communications companies.
- [1] Photoillustration by Stephen Larsen and Hypres. At the heart of the X-Band All-Digital Receiver is one-centimeter superconducting niobium chip - less than half the size of a penny - that contains about 11,000 Josephson junctions laid out to form superconducting Rapid Single Flux Quantum (RSFQ) circuits that move picosecond-duration magnetic pulses.