Telecommunications Research and Engineering at the Institute for Telecommunication Sciences of the Department of Commerce: Meeting the Nation's Telecommunications Needs (2015)

D

The Value and Use of Wireless Technology

Wireless is a short-cut terminology for electromagnetic (EM) waves transmitted through free space (without wires). EM waves are fundamental in nature. As waves, they are sinusoidal fields defined by their frequency, which can have any value from very slow vibrations to ultra-rapid vibrations. The EM waves engineers create are radiated by vibrating electrons, electrically driven to emit at the characteristic frequencies needed for wireless technology. Table D.1 gives the range of frequencies used for wireless and provides names for the ranges of frequencies that engineers use. The wavelengths of EM waves are related to the reciprocal of their frequencies. Those EM waves with wavelengths between 1 m and 1 mm are called microwaves. Millimeter waves are the name given to the highest frequencies used for wireless, with wavelengths from 0.10 to 1 cm. The terms airwaves and radio waves are sometimes used for EM waves, and the entire range of EM frequencies available for applications is called spectrum. Governments all over the world control their own airwaves by allocating separate bands of spectrum for separate applications.

EM waves can be used to transmit information if the waves are modulated (their characteristics changed with time). Radio waves, particularly microwaves and millimeter waves, form the basis of today’s wireless technology. For certain applications, light waves can also be used to establish short-range wireless links, although most of their high-speed applications require transmission in optical fibers/waveguides.

Communication requires modulating the EM waves, resulting in a spread of frequencies (bandwidth) that carry the information surrounding the carrier frequency that defines the unmodulated wave. Because the bandwidth is a well-defined fraction of the carrier frequency, it can be seen that higher carrier frequencies can have larger bandwidth and can transmit more information. Modulated EM waves form the basis of today’s wireless telecommunication systems. Today, much of the modulation has a digital format.

CONTROL OF THE AIRWAVES

The first public awareness of the importance of wireless telecommunication was the distress signals broadcast by the sinking Titanic in 1912. These signals were picked up by a nearby ship, beginning the famous rescue operation. The resulting publicity demonstrated to the world that safety required specific frequencies to be reserved for specific applications, such as SOS emergencies. Thus began government control of airwaves. The military buildup due to World War I caused wireless frequencies to be reserved primarily for government use. In the United States, the federal assignment of frequencies to commercial entities began in 1922, when 833 kHz was defined as the frequency to be used for “Entertainment” and 619 kHz as the frequency for “Market and Weather.” These frequencies were in the kHz regime because wireless systems at that time could only operate at very low frequencies. As technology rapidly improved, higher frequencies could be used. By the following year, the spectrum (range of available frequencies) was assigned out to 1350 kHz (1.350 MHz).

TABLE D.1 Spectrum Frequency and Wavelength Ranges for Different Wireless Applications

NOTE: Shaded areas are microwave.

Because EM waves easily travel across boundaries between countries, beginning in the 1920s, governments around the globe have negotiated within the International Telecommunications Union (ITU)¹ to allocate different frequency ranges for different applications while providing for compatibility. Radio services that have been allocated spectrum include fixed service (e.g., from one cell tower to another), mobile service (e.g., aeronautical and marine communications), land mobile service (cell phones), and broadcast service (TV), as well as standard frequency and time signals, discrete frequencies that require exclusivity for radio astronomy, and amateur radio. Through extensive negotiation, the entire radio spectrum has been divided into specific blocks of frequencies for each application; most often, several blocks at different carrier frequencies can service the same application. These blocks of frequencies are then allocated by most countries.

Spectrum Auctions

In the U.S. spectrum is allocated by the Federal Communication Commission (FCC) for nongovernmental applications and by the National Telecommunications and Information Administration (NTIA) allocates spectrum for governmental applications. When the same band is shared for different applications, both entities need to agree. Initially, there was enough spectrum for everyone, and allocations were made by determining the best use for each band based on both availability and the state of the relevant technology at the time. However, in 1994 the rapid growth in personal communication systems and the impending explosion of mobile cell phone usage forced re-consideration of band allocation within the spectrum. The FCC introduced the idea of auctioning these frequency allocations to the highest bidders, with the understanding that profits could be returned to the federal budget.

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¹ The International Telecommunication Union coordinates the shared global use of radio spectrum. Internationally harmonized global spectrum allocations allow manufacturers to develop worldwide markets, as demonstrated through the widespread adoption and commercial success of IEEE) standards for wireless local area networks (WLANs) in 1999, and more recent agreements that all WLANs operate in the same globally allocated spectrum. WLAN succeeded because there was universal international agreement for the use of the 2.4 GHz industry, science, and medical (ISM) band and 5 GHz Unlicensed National Information Infrastructure (UNII) bands, which allowed major manufacturers to devote significant resources to create products that could be sold and used globally. Without international spectrum agreements, new wireless technologies will founder for lack of a global market. Countries whose engineers participate in the standard-setting of ITU have more political clout to persuade the global community to choose standards that are consistent with the technology used in their country.

The first auction assigned 50 to 100 kHz bandwidth blocks within the 901-941 MHz range of carrier frequencies nationwide. Ten national licenses were provided for a total bandwidth of 0.78 MHz, for which the FCC and the U.S. government received $617 million. A few days later, the FCC auctioned off the blocks of higher bandwidth near the 218 MHz carrier frequency to 297 metropolitan service areas with the idea that these blocks could be reused by different metropolitan areas. The FCC planned for them to be used for interactive video and data services. In fact, these blocks were broken up by the carriers to enable them to handle multiple mobile phones operating simultaneously in the same cell. Almost $214 million was raised.

The rapid adoption of cell phones by the general population made apparent the value of owning spectrum bandwidth assignments. Only 4 months later, a similar auction assigning carrier frequencies of 1.8-.9 GHz (known as the A and B blocks) with 60 MHz bandwidth each sold for more than $7 billion. One year later (1995) more than $10 billion was raised in an auction to assign the higher 1.9 GHz frequency (the so-called C-block). The rapid rise in commercial value points to the rapid growth in bandwidth requirements for the burgeoning cell phone industry; the second year of FCC spectrum auctions brought in $17 billion dollars. A few of the relevant later auctions are described below.

The FCC Auction 8 in 1996 assigned frequencies for satellite TV (DBS, or Direct Broadcast Satellite). A very high carrier frequency of 12.2-12.7 GHz was chosen to provide the needed bandwidth: 28 channels at 24 MHz for a total bandwidth of 672 MHz. This was designed for direct-to-home satellite service to permit delivery of digitally compressed TV signals to individual households by means of an external receiving antenna. The external antenna was required because waves at this high frequency do not readily pass through walls. MCI paid $682 million for the nationwide license, less than the bidding on lower-frequency spectrum because of the limited market due to the need for an external antenna and the growing availability of directly connected cable.

By 2000, Auction 30 was designed for outdoor fixed point-to-point and point-to-multipoint communications, such as between cell towers. The 39 GHz carrier frequency was designated with 14 channels each with bandwidth of 100 MHz. Usage of these frequencies requires highly directional antennas to counteract propagation losses while enabling point-to-point communication.

As TV moved from analog to digital, more bandwidth previously used for analog TV transmissions could be repurposed for wireless telecommunications applications. The frequency band from 470-806 MHz had originally been reserved for broadcasting TV. In 2008, bands with carrier frequencies between 698 and 806 MHz were auctioned off, bringing in a record $19 billion.

By 2012, concerns about improving homeland security pointed to the need for a network on a single frequency band over which all first-responders could wirelessly communicate; at the time, different responders typically used different frequency bands. Congress created the First Responder Network Authority (FirstNet) with the mission to provide a single interoperable platform for emergency and daily public safety communications. FirstNet will build, operate, and maintain the first high-speed, nationwide wireless broadband network dedicated to public safety. The spectrum license issued to FirstNet is for two 10 MHz channels of paired spectrum at 758-768 MHz and 788-798 MHz, plus guard bands at 768-769 MHz and 798-799 MHz to reduce interference from adjacent channels.

To pay for FirstNet, the FCC arranged Auction 97 to allocate mid-band spectrum between 1700 and 2100 MHz frequencies (1.7-2.1 GHz). This so-called AWS-3 band has become very valuable, because increasing numbers of Americans use Internet-enabled wireless devices to do more things that require faster networks, such as watching streaming video. The auction set a new record, netting $41.3 billion for 12-year licenses. This AWS-3 band already had incumbent users, and new users would have to share it. Box D.1 outlines the requirements on new licensees designed to protect incumbent operations. Furthermore, the licensees will have to wait before they can use their new spectrum because the Department of Defense (DOD) is using some of the frequencies for missile guidance systems, drone training programs, and similar activities. DOD expects these programs to take 5-10 years to relocate to other spectrum bands. As noted in Chapter 4, the Boulder telecommunications laboratories can provide considerable value to operation of this new shared allocation and have the potential to act as an

independent agent to resolve disagreements between parties that share the same spectrum band, determining whether or not the AWS-3 requirements have been met.

Auction 97 was authorized to raise money for the federal government, including the funding of FirstNet. The FCC set a goal of raising at least $10.6 billion for the sale of 1,600 licenses. Because the auction generated $41 billion, FirstNet has been fully funded. The aim of this newly defined band is for all safety officers (police and fire fighters) to migrate to this new band.

Not all bands in this spectrum are equally valuable for all applications. The lower frequencies can be transmitted through walls and are useful for mobile phones, for example. But they have smaller bandwidth so they cannot carry large amounts of information. They also require larger antennas than do the higher frequencies. Higher carrier frequencies designated with larger bandwidths can carry more information, but walls and other obstacles may block it. The highest frequencies—millimeter waves (mm-band) (30 GHz and above)—are also attenuated by foliage and absorbed in the atmosphere. The propagation distance through the atmosphere generally decreases as the frequency goes up. This has advantages because the frequencies can be reused easily with the limited propagation range in this band. Thus, millimeter-waves may be useful for line-of-sight transmission for short distances. While they use smaller antennas, the electrical components become more expensive at higher frequencies. However, the cost per bit of information will decrease. Furthermore, licenses for services at higher frequencies are typically less expensive than at lower frequencies because they are not suitable for mobile phone applications. Also, mm-wave bands can be made available quickly because there are no incumbent users. All these factors point to an expected increase in mm-wave wireless telecommunications.

Typical systems split the carrier frequency into a number of channels, each with a specific transmission bandwidth that determines the rate of information that can be transmitted. Ten times the frequency can mean either ten times the bandwidth per channel or 10 times the number of channels. As transmission frequencies move into the multi-gigahertz range, larger bandwidth channels are possible, and continuous innovations in technology promise to deliver ever-increasing bandwidths and capabilities.

APPLICATIONS IN UNLICENSED BANDS

Some of the spectrum is set aside for unlicensed applications, meaning one does not need a license to use the spectrum as long as one abides by defined constraints associated with the specific bands. These include the ISM radio bands, reserved internationally for the use of EM energy for industrial, scientific and medical purposes, and the U-NII band, used extensively for wireless local networks.

Industrial, Scientific, and Medical Band

The ISM band is a part of the radio spectrum that can be used without a license in most countries.² In the U.S., the bands were initially used for machines that used or emitted radio frequencies but not for radio communications.

• Industrial applications. Radio frequency energy is used for a variety of industrial welding, heating, and drying applications, including ceramics, foam, fiberglass, composites, textiles, food tempering and pasteurizing, wood, and paper. Industrial RF heaters can have powers in the 100 kW range. As just one example, RF energy is useful for sealing plastics. An RF heat sealer heats a plastic part to the point at which it can bond with another plastic part or to another surface. The technique is faster and cleaner than conventional thermal welding and also produces a stronger bond. There are more than 100,000 RF heat sealers in operation in the United States used in a variety of industries. The power generated by an RF heat sealer ranges from about 1.5 kW to more than 60 kW—a power level comparable with the highest power radio and television transmitters. However, the power is concentrated into the area that needs sealing. Nonetheless, there is considerable possibility of RF signals leaking from the equipment into the airwaves where it may interfere with RF communications applications in the same frequency band. In most cases, this equipment is qualified and monitored for non-intentional radiation during the course of its use, but the Boulder telecommunications laboratories should be aware of these possible sources of microwave interference.

  RF generators also excite plasmas for materials processing such as plasma vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced chemical vapor deposition (PECVD) and etching. These processes are used for applications such as semiconductor manufacturing, integrated circuit (IC) fabrication, thin-film heads for disks, CDs, hard disk coatings, and other industrial uses. RF is also used in the development of IC devices at geometries from 0.5 to 0.25 microns for digital random-access memory, logic, application-specific integrated circuits, and other such devices. Other major application for RF-generated plasmas is industrial-scale plasma welding and cutting.

• Scientific applications. There is a multitude of scientific users of radio spectrum including radio astronomers and Earth scientists using remote sensing.³ Satellite-based sensing is used extensively in weather prediction and for meteorological and climate-sensing applications, typically at frequencies spread across the EM spectrum, sometimes up to 60 GHz. Remote sensing can also be terrestrial or from airplanes. Measurement of absorption and scattering of RF beams can provide water vapor profiles, snow and ice coverage, cloud liquid water, and rain rate to monitor the state of the environment.

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² International Telecommunication Union, Article 1, Section 1.15.

³ National Academies of Science, Technology, and Medicine, A Strategy for Active Remote Sensing Amid Increased Demand for Radio Spectrum (2015, The National Academies Press, Washington, D.C.) explores the continued need for spectrum for scientific research and encourages NASA to engage with regulatory agencies, including the NTIA, to limit interference and develop sharing mechanisms.

• Some scientific applications require specific RF frequencies that may not be within ISM bands but must be protected from interference. Radio astronomy is an example. In order for radio astronomers to detect the faint signals from cosmic sources, certain scientifically important RF bands are kept clear of radio transmissions. Nevertheless, there is still some interference due to legal, high-power transmitters outside but close in frequency to the radio astronomy band. This is because all practical transmitters radiate a very small fraction of power outside their designated frequency, which can swamp the signals radio astronomers are trying to detect. Any frequency for which the atmosphere is transparent can be used for radio astronomy. However, other frequencies are assigned to other services which might cause interference. For example, Jupiter’s most interesting radiation is between 15 MHz and 30 MHz. To study Jupiter’s radiation in this band, the radio astronomer has to contend with transmissions from all over the world as well as computer- and television interference in the nearby 88 - 108 MHz FM broadcast band. The radio astronomer has to learn to distinguish between all kinds of noise and that coming from Jupiter. In the future, their work may become harder if their bands are not protected from emerging commercial demands on broad airwave access. Other scientific applications include microwave spectroscopy. For example, many electron paramagnetic resonance (EPR) spectrometers operate near 9.8 GHz.

• Medical applications. High frequency microwaves can be used to create therapeutic localized heating, particularly at frequencies absorbed by water. Unlike other forms of electromagnetic frequencies that cause a “surface effect,” wherein the skin feels the heat application, RF energy can penetrate the body and be absorbed in deep body locations with a lower amount of heat sensation. RF medical devices transmit radio waves to increase the temperature of tissue. A sharp heat boundary is created between the affected tissue and that surrounding it allowing for surgeons to operate with a high level of precision and control, without much sacrifice to the adjacent normal tissue. The lower operating temperatures of RF, as compared to traditional electrosurgical or laser surgery tools, enables surgeons to remove, shrink, or sculpt soft tissue while simultaneously sealing blood vessels. RF works particularly well on connective tissue, which is primarily comprised of collagen and shrinks when exposed to heat.

• Commercial use of ISM band. Because ISM frequencies are available for unlicensed devices without paying for access, They are often used for commercial applications, including short-range communication devices. Communications equipment operating in these bands must accept any interference generated by ISM equipment, and users do not have regulatory safeguard from ISM equipment operation. More recently, because of high demand for usage for wireless connections to the Internet through mobile devices, the ISM bands have also been used through local access points to provide an alternative to connection via a traditional cell tower or access point. These connections must follow the existing rules for non-interference with other users of the ISM bands.

  The ISM bands at 2.45 GHz and 900 MHz are important for historical reasons and have become the frequency most used by cordless phones, Bluetooth-enabled devices, baby monitors, and RF remotes. The 2.45 GHz band is also assigned to the microwaves generated by tubes within microwave ovens because this frequency is strongly absorbed in water, making it useful for microwave heating within the oven. The electrical components within the transmitters and receivers at this frequency are highly developed and inexpensive.

  The other major commercial ISM application at the present time is RFID (radio frequency identification) readers, although most of these operate at lower frequencies than 2.4 GHz in order to restrict their read ranges. RFID is a technology that incorporates the use of electrostatic or electromagnetic coupling in the RF portion of the electromagnetic spectrum to uniquely identify an object, animal, or person. An early demonstration of reflected-power RFID tags took place at the Los Alamos National Laboratory in 1973. The portable system operated at 915 MHz and used 12-bit tags. This technique is used by the majority of today's UHF-ID and microwave RFID tags.

Unlicensed National Information Infrastructure

In 1997, the FCC developed regulation for additional bands in the 5 GHz range, known as the U-NII.⁴ The band is significant in that it is commonly used for local wireless networks, under the IEEE 802.11 specifications. The upper band (5.725-5.825 GHz) overlaps with the ISM band and is used by wireless internet service providers. U-NII devices are unlicensed intentional radiators that use wideband digital modulation techniques to provide high-data-rate mobile and fixed communications used by individuals, businesses, and institutions, particularly for wireless local area networking—including Wi-Fi—and broadband access.

To promote use of U-NII, the FCC removed the indoor-only restriction and increase the permitted power for certain frequencies to accommodate the next generation of Wi-Fi technology. To ensure that all such devices comply with U-NII requirements intended to protect authorized users from harmful interference, the FCC set rules applicable to all digitally modulated devices operating across this 125 MHz of the spectrum. The FCC required that all U-NII device software be secured to prevent modification so that the devices will operate only as authorized, reducing the potential for harmful interference to authorized users. Finally, to protect Terminal Doppler Weather Radar (TDWR) systems and other radar systems operating in nearby frequencies from harmful interference, the FCC technical rules and compliance measurement procedures for U-NII devices were modified.

MILLIMETER WAVES

The mm-band lies in the 30-300 GHz range (with wavelengths from 10-1 mm, respectively).⁵ There is significant diversity in the types of equipment and applications using this spectrum, but investments in commercial technology have been slower due to unknowns in operational conditions and unpredictable variations in propagation environments. However, giving the increasing demand for spectrum, swiftly increasing applications and research into the transmission of these high frequencies is imperative. Current applications include the following:

• Radio astronomy and remote sensing including temperature measurements in the upper atmosphere;
• Weapons systems, including short-range fire-control radar in tanks and aircraft, and automated guns on naval ships;
• Security screening commonly used by the Transportation Safety Administration for airport screening; and
• Telecommunication applications for unlicensed short-range data links.⁶

Governments and the ITU are just now generating standards for global spectrum bands are “frequencies that are at least an order of magnitude greater than today’s fourth-generation (4G) Long Term Evolution (LTE) and WiMax mobile networks.”⁷ Similar to the WLAN unlicensed products moving from 1 to 5 GHs frequencies in early generation to 60 GHz today, the cellular industry is moving

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⁴ United States CFR Title 47, Part 15—Radio Frequency Devices, Subpart E—Unlicensed National Information Devices, Paragraph 15.407—General technical requirements.

⁵ European Telecommunications Standards Institute, “Millimetre Wave Transmission,” Technology Leaflet, 2015, http://www.etsi.org/images/files/ETSITechnologyLeaflets/MillimetreWaveTransmission.pdf.

⁶ The upcoming IEEE Wi-Fi standard is expected to run on the 60 GHz band with data transfer rates of up to 7 Gbit/s.

⁷ T.S. Rappaport, R.C. Daniels, R.W. Heath, and J.N. Murdock, Introduction to Millimeter Wave Wireless Communications, October 6, 2014, http://www.informit.com/articles/article.aspx?p=2249780.

to mm-wave bands that support massive data rates. This immense increase in available spectrum bandwidth could lead to new capabilities. For example, the unlicensed band at 60 GHz contains more bandwidth capacity “than has been used by every satellite, cellular, Wi-Fi, AM radio, FM radio, and television station in the world.”⁸ Developing technologies to support the use of mm-wave frequencies to augment the currently densely allocated and assigned spectrum bands for wireless communications could provide a massive amount of bandwidth.

Research Challenges in the Use of High-Frequency Waves

Much research is needed to support the use of the mm-wave for telecommunication. Due to the higher carrier frequency, propagation characteristics of promising mm-wave frequencies show path loss is larger in non-line-of-sight conditions compared to UHF and microwave bands. The scattering effects also cause weak signals to become an important source of diversity, and non-line-of-sight paths are weaker, making blockage and coverage holes more pronounced.⁹ Directional beamforming will be needed to allow high-quality links both at the base station and at the handset where propagation can be improved.

It is anticipated that the combination of cost-effective complementary metal-oxide semiconductor (CMOS) technology operating efficiently in the mm-wave frequency bands and high-gain steerable antennas at the mobile and base station strengthens the future practicability of mm-wave wireless communications. Rapid advancements and price reductions in integrated mm-wave (>30 GHz) analog circuits, baseband digital memory, and processors have enabled progress as well. Recent work in integrating mm-wave transmitters and receivers with advanced circuitry and new phased array and beamforming techniques also support telecommunication use.

It is expected that the semiconductor industry is poised to produce cost-effective, mass-market products for mm-wave communication. Operation at 60 GHz at reasonable costs will also be enabled through a continuation of advancements in CMOS and silicon-germanium technologies. Packaging the analog components along with the digital hardware necessary to process massive bandwidths has only been possible in the past decade.¹⁰

Additionally, the increased absorption and scattering loss at higher frequencies that shifts the technology away from long-range communications actually aids close-range communications. Thus, it permits aggressive frequency reuse while simultaneously operating networks that do not hinder each other.¹¹ Highly directional antennas needed for path loss mitigation actually work to promote security as long as network protocols and front-end hardware enabled antenna arrays are flexibly steered. Many communication networks are now residing at the 60 GHz range for distances less than 100 m. In addition, the 20 dB/km oxygen attenuation at 60 GHz disappears at other mm-wave bands, such as 28, 38, or 72 GHz. This development provides alternatives to today’s cellular bands for longer-range outdoor mobile communications. Recent research in use of smart antennas, beamforming, and spatial processing has found that urban environments provide rich multipath, especially reflected and scattered energy at or above 28 GHz. It is anticipated that this rich multipath could be exploited to increase received signal power in non-line of sight propagation environments.¹²

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⁸ T.S. Rappaport, R.C. Daniels, R.W. Heath, and J.N. Murdock, Introduction to Millimeter Wave Wireless Communications, October 6, 2014, http://www.informit.com/articles/article.aspx?p=2249780.

⁹ Ibid.

¹⁰ Ibid.

¹¹ Ibid.

¹² Ibid.

It is worth mentioning that work is already under way to understand what might be done in the Terahertz bands. This is a wide open research area at this point, and the Boulder telecommunications laboratories could be engaged to think about this very unique spectrum and how it might be measured and used in the future.

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¹³ T.S. Rappaport, Robert C. Daniels, Robert W. Heath, and James N. Murdock, Introduction to Millimeter Wave Wireless Communications, October 6, 2014, http://www.informit.com/articles/article.aspx?p=2249780.