STI: Improving the Quality of Wireless®

When cooled to extremely low temperatures, certain materials experience zero electrical resistance at DC. They conduct current with no heat loss and are called superconductors.

Zero electrical resistance logically provides better circuit performance for electronic components. In 1986, a new class of materials called high-temperature superconductors (HTS) was discovered that made the necessary cooling more cost-effective.

Superconductors have many different applications, ranging from levitating trains to ultra-efficient power lines. Superconductor Technologies Inc. (STI) has been developing high-quality superconducting materials and systems since 1987, focusing on the radio frequency segment of the wireless communications market with the thin-film microelectronics in its SuperLink® systems.

A History of Superconductivity
Superconductivity was first observed in mercury in 1911 at the University of Leiden in the laboratory of H. Kamerlingh Onnes, the first person to liquefy helium and reach temperatures as low as 1.7 Kelvin (K). Zero Kelvin is referred to as absolute zero, or zero thermal energy--the point at which all molecular motion in the conducting medium stops.

For example, the molecules in a gas experience a great deal of motion, or heat, such as water when it becomes steam at 212°F, 100°C, or 373K. When the steam cools to a liquid state, its molecules are still in motion, but not as fast. Water becomes a solid in the form of ice at 32°F, O°C--and 273K. Even the molecules in ice experience a great deal of motion compared to those in superconductors.

A major milestone in the history of superconductivity occurred in 1957 with the BCS Theory, named for its developers--Bardeen, Cooper, and Schrieffer. Their work provided the first complete physical description of the phenomenon of superconductivity. In 1972, Dr. Robert Schrieffer won the Nobel Prize for his role in developing the theory.

First Applications of Superconductors
The first real applications for superconductors were developed in the 1960s with the nearly coincident discoveries of NbTi (which made superconducting wire practical) and the Josephson junction (which made possible many unique electronic devices). The first applications were high-field electromagnets, ultrasensitive magnetometers, and high-Q RF cavities. The largest use of superconductivity to date has been in electromagnets for Magnetic Resonance Imaging (MRI) systems.

High-Temperature Superconductors (HTS)
HTS material was discovered in late 1986 when Müller and Bednorz of IBM's Zurich Lab announced a superconducting oxide at 30K. In 1987, Paul Chu of the University of Houston announced the discovery of a compound, Yttrium Barium Copper Oxide (YBCO), that became superconducting at 90K. The next months saw a race for even higher temperatures that produced bismuth compounds (BSCCO) superconductive up to 110K and thallium compounds (TBCCO) superconductive up to 127K.

HTS materials above 77K allowed the use of a readily available, low-cost, easy-to-use coolant--liquid nitrogen. Before that, superconductivity had required liquid helium (at 4K), which is both more expensive and more difficult to handle. In addition, these higher superconducting temperatures above 77K made superconductivity possible for the first time using compact, closed--cycle refrigerators, as light as a few ounces and running on a few watts of electricity, compared to the multi-kilowatt refrigerators weighing hundreds of pounds required for superconductors at 4K.

STI Obtains Exclusive Worldwide License to HTS Material
In 1992, STI obtained an exclusive worldwide license from the University of Arkansas for use of the superconducting thallium compound (TBCCO). TBCCO has the highest superconducting transition temperature among the HTS materials with superior microwave properties. STI's earliest work with these materials was in the development of thin-film microelectronics to reduce signal interference in U.S. government aircraft.

The National Bureau of Standards suggests that the term cryogenics be applied to all temperatures below -150°Celsius (-238°Fahrenheit or 123 Kelvin). Cryogenic temperatures are reached either by the rapid evaporation of volatile liquids or by the expansion of gases confined initially at pressures of 150 to 200 atmosphere. The expansion may be simply through a valve to a region of lower pressure, or it may occur in the cylinder of a reciprocating engine, with the gas driving the piston of the engine, as in the cryogenic coolers offered by STI.

Cryogenics History
Between 1823 and 1845, British chemists Sir Humphry Davy and Michael Faraday experimented in low-temperature physics. Davy and Faraday generated gases by heating an appropriate mixture in one end of a sealed, conical tube. The other end was chilled in a mixture of salt and ice. The combination of reduced temperature and increased pressure caused the evolved gas to liquefy. When the tube was opened, the liquid evaporated rapidly and cooled to its normal boiling point. By evaporating solid carbon dioxide mixed with ether at low pressure, Faraday succeeded in reaching a temperature of about 163K (about -110 °C or -166°F).

If a gas, initially at a moderate temperature, is expanded through a valve, its temperature increases. But if its initial temperature is below the inversion temperature, the expansion instead will cause a reduction in temperature. The inversion temperatures of hydrogen and helium, two primary cryogenic gases, are extremely low. These gases first must be cooled below their inversion temperatures--the hydrogen by liquid air, and the helium by liquid hydrogen.

By cascading these processes, French physicist Louis Paul Cailletet and Swiss scientist Raoul Pierre Pictet produced droplets of liquid oxygen in 1877. Their success pointed to the possibility of liquefying any gas by moderate compression at temperatures below the critical temperature.

Dutch physicist Heike Kamerlingh Onnes founded the first liquid-air plant in 1894 using the cascade principle. British chemist Sir James Dewar first liquefied hydrogen in 1898 and Kamerlingh Onnes liquefied helium, the most difficult of the gases to liquefy, in 1908.

How to Produce Such Low Temperatures
The evaporation of liquid helium at reduced pressures produces temperatures as low as 0.7K (-272.44°C or -458.4°F). Even lower temperatures can be attained by adiabatic, or heatless, demagnetization. This procedure requires that a magnetic field be established around a substance made of paramagnetic ions while the substance is cooled in liquid helium. The field aligns the ionic magnets. When the field is removed, the tiny magnets resume their random alignments, reducing the thermal energy of the whole sample. The temperature, therefore, falls as low as 0.002K (-273.15°C or -459.67°F).

Cryogenic temperatures can be reached using a specially designed refrigerator, referred to as a cryocooler, or by submersing the device to be cooled in a fluid which boil at a low temperature. Liquids that are commonly used to achieve cryogenic temperature are Nitrogen, which boils at 77K, (-321°F, -196°C), and Helium, which boils at 4K, (-452°F or -269°C). Cryocoolers achieve their cooling capability by either controlled evaporation of volatile liquids (using the heat of vaporization as the means to cool), by controlled expansion of gasses confined initially at high pressure (such as 150 to 200 atmospheres), or by acting as a heat-pump by alternatively expanding a gas near the area to be cooled (absorbing heat by the so-called heat of expansion), then compressing at another location (removing the heat to the ambient) in a closed-cycle. STI uses a closed-cycle cryocooler based upon the Stirling cycle, an example of the hot approach that provides the highest efficiency and smallest size and weight of all known cryocooler cycles.

Measuring Extremely Low Temperatures
One procedure is to measure the pressure of a known quantity of hydrogen or helium, although this procedure fails at the lowest temperatures. The vapor pressure of helium-4--that is, helium of atomic mass 4--or of helium-3 supplements the preceding method. Determinations of the electrical resistance of metals or semiconductors and their magnetic measurements extend the range still further.

Storage of Liquids at Extremely Low Temperatures
Dewars (named for Sir James Dewar) consist of two flasks, one within the other, separated by an evacuated space. The outside of the inner flask and the inside of the outer flask are both silvered to prevent radiant heat from passing across the vacuum.

How do cryogenic temperatures affect superconductivity?
The electrical resistance of many, but not all, metals and metalloids decreases abruptly to zero at temperatures of a few degrees Kelvin. If an electric current is introduced into a ring of metal that has been cooled to the superconductive state, the current will continue to travel around the ring and may be detected hours later.

What are some applications for cryogenics technology?
One of the most important applications of this technology is the large-scale production of liquid nitrogen and liquid oxygen from air. Liquid oxygen has a number of uses, including rocket engines, cutting and welding torches, life support in space and deep-sea vehicles, and blast furnace operations. Liquid nitrogen is used in the manufacture of ammonia for fertilizers, as well as in the rapid cooling of frozen foods to prevent damage to cell tissues.

Nuclear physicists require liquid hydrogen and liquid helium in particle detectors, particle accelerators, and nuclear fusion research. Infrared devices, masers, and lasers can employ cryogenic temperatures as well. Cryogenics already has made possible the commercial transportation of liquefied natural gas. Natural gas is liquefied at 110K, causing it to contract to 1/600th of its volume at room temperature and making it sufficiently compact for swift transport in specially insulated tankers.

Cryogenic surgery, or cryosurgery, is being used for the treatment of Parkinson's Disease by selectively destroying tissue by freezing it with a probe or scalpel cooled with liquid nitrogen. The dead cells then are removed through normal bodily processes. The advantage to this method is that freezing the tissue, rather than cutting it, produces less bleeding. Cryosurgery has been successful in removing tonsils, hemorrhoids, warts, cataracts, and some tumors.

The wireless industry has evolved tremendously in recent years. At STI, we believe the best is yet to come.

The wireless industry continues to prepare for a major shift in focus from voice communications to one that utilizes high-speed data as well. This shift will occur through the deployment of standards referred to as third generation, or 3G, wireless communications.

STI's superconducting microelectronics are making significant advances in 3G communications transmitting data over a wireless network. Because the wireless channel does not provide a consistent channel quality, the 3G air interfaces are much more advanced in how they deliver data. While specific techniques vary considerably, they all share a common characteristic. Data rate throughput (the speed at which data is conveyed) is proportional to channel quality. In cases where the channel is noisy, the 3G air interfaces actually decrease the rate at which data is sent to ensure that the message is delivered correctly. Higher data rate channels require better quality airlink channels.

Data rate vs. power trade-off
One way to improve the communications channel quality is to increase the power of the transmitter. The receiver then can interpret the higher-powered signal more easily. As the desired data rate increases, however, the RF power required from the user's device must increase. While this appears to be a simple solution, the consequences are increased interference to other users, increased exposure to RF radiation, and shorter handset battery life.
Data rate vs. distance trade-off
For users relatively close to the cellsite, communications channels usually are quite good. As a result, these users enjoy a high data rate. As users travel away from the cellsite, however, communications channels tend to degrade. The direct consequence is that users experience a lower data rate as they move farther from the cellsite.
Data rate vs. noise trade-off
As the noise and interference on a given RF channel increases, the data rate for all users on that channel decreases.

SuperLink™ in 3G communications
SuperLink provides a more robust channel, making it easier to detect and interpret signals accurately from a wireless subscriber. This translates directly into lower handset power and higher data rates over a longer distance. By significantly reducing the level of interfering signals operating at nearby frequencies and providing a low-noise amplifier, SuperLink provides a powerful channel-enhancing option to 3G systems. This improvement translates directly into a potentially higher data throughput for all users.

STI believes that 2.5G and 3G systems will derive even greater benefits from superconductor filter technology than do present-generation networks. In future CDMA networks, for example:

Uplink will become the limiting factor. CDMA2000 introduces several enhancements for both the uplink and downlink. Capacity involvement for the downlink will increase two to three times, while the uplink will increase one-and-a-half times. This ratio puts greater demands on uplink performance requiring improved receiver sensitivity.
Higher data rates require better quality links. For a set bandwidth, higher user data rates result in a lower processing gain in CDMA2000 and WCDMA This increases the need for superconductors to improve interference protection and increase receiver sensitivity.
Higher data rates require more power. Higher data rates require more power from handsets, decreasing battery life dramatically. This can be offset by the lower handset transmitter power, enabled by the use of superconductors. This is true for TDMA EDGE networks as well.

For example, a SuperLink providing a 3 db improvement to the channel quality increases the speed of data transmission to twice the original rate. To a service provider, this increases the size of the "sweet spot" where users can enjoy the highest speed connection and can increase the average data rate throughout the network.

For data-intensive applications like e-mail attachments, music, pictures, video stills, or even video conferencing, this change is substantial. It also enables applications that otherwise might not be practical, such as remote server/router applications where wireline services are not available. The sensitivity increase also decreases handset power required to establish a good link.

The history of 3G communications
The first generation, or 1G, wireless systems, such as AMPS, were analog-based communications systems that provided basic telephone-like functionality. Second generation, or 2G, systems such as TDMA, CDMA, and GSM used digital standards--that is, information transmitted was encoded into bits. Current standards providing intermediate data performance and an evolutionary path from existing 2G systems toward 3G systems are referred to as 2.5G.

One of the first goals of 3G systems was to eliminate existing different standards worldwide and create one unified standard that would allow a user to carry one phone for world travel. Another goal was to provide high data rates of up to 384 kb/s over a packet-based wireless system, enabling wireless data services to be connected to the network anywhere at any time.

The current 3G standard, however, is actually a family of standards--three based on CDMA standards and one based on TDMA standards. The primary reason for this divergence is to provide an evolutionary path to the higher data rate services for operators and service providers who already have invested heavily in their infrastructures.

CDMA2000 1xRTT and 3xRTT provide an evolutionary path from CDMA TIA/EIA-95 for existing operators that use CDMA, such as carriers in the U.S. and Korea.
EDGE (Enhanced Data [Rate] for GSM Evolution) provides an evolutionary path for operators using TDMA TIA/EIA-136 and for operators using GSM.
Finally, WCDMA, so-called Wideband CDMA, has two variants that are a completely new air interface, but use the same infrastructure as GSM. Japan has been the most aggressive in making plans to deploy WCDMA. Spectrum auctions for WCDMA deployment are completed in the U.K. and in Germany, with other European countries following.

In addition to the work being done by the standards bodies, individual corporations have proposed their own approaches. Qualcomm developed the HDR (High-speed Data Rate) approach, which is optimized for packet data-only transmission and reception (that is, it doesn't support voice). Motorola and Nokia have proposed a migration path from CDMA2000 1xRTT to what they call 1XTREME, which promises voice and data speeds up to 5.2 Mb/s.

Moreover, 2.5G standards aim to increase the data rate of existing systems and/or provide packet data functionality, but didn't necessarily meet the goal of the 3G data rate, CDMA IS-95B and GPRS (General Packet Radio Service) are specifications deployed currently. Standards under development to provide additional voice capacity to existing TIA/EIA-136 networks include IS-136+ and TDMA-6.