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Spectrum Sensing and Sharing

INTRODUCTION

Research into spectrum sensing and sharing is housed within the Spectrum Technology and Research Division of the Communications Technology Laboratory (CTL). Within the sensing realm, the primary research focus areas are microwave noise metrology, radio frequency spectrum sensing, and atmospheric spectroscopy. Within the sharing realm, the focus areas are wireless coexistence and “black box” spectrum sensing. This research spans a very wide range of frequencies from radio (including millimeter wave and terahertz) to infrared and optical. The application space is equally broad, ranging from spectrum sharing between federal and commercial wireless systems to femtosecond-level time transfer.

Current research programs include the following:

• Wireless coexistence;
• Monitoring changes in radio spectrum use during the COVID-19 pandemic;
• The use of Wi-Fi signal propagation changes to monitor patient breathing for medical applications;
• Test and measurement lead for the National Advanced Spectrum and Communications Test Network (NASCTN), including Advanced Wireless Services (AWS)-3 and Citizens Broadband Radio Service (CBRS) projects;
• Digital radiometry;
• Infrared spectroscopy using optical frequency comb for atmospheric sensing;
• Ultrahigh accuracy time transfer using optical frequency comb pulses; and
• Calibration of blackbody targets for use in remote sensing.

Research challenges include the following:

• Use of artificial intelligence and machine learning to generate waveforms simulating real-world signals, to be used for spectrum coexistence studies without complications related to off-air capture, such as personally identifiable information;
• Receiver performance characterization; and

• Measuring skyward emissions from 5G systems for use in compatibility studies with, for example, passive remote sensing satellites.

ASSESSMENT OF TECHNICAL PROGRAMS

Laser Combs

In response to the 2019 National Academies of Sciences, Engineering, and Medicine’s assessment of CTL,¹ some components of CTL were reorganized. One result of this reorganization was the assimilation of the National Institute of Standards and Technology’s (NIST’s) Fiber Sources and Applications Group into CTL’s Spectrum Technology and Research Division from its previous home in the Physical Measurement Laboratory. This group’s principal research thrust in the spectrum sensing domain is related to the application of laser combs for precision spectroscopy, a technology whose development at NIST earned a share of the 2005 Nobel Prize in Physics.

Laser combs have multiple applications that are being developed. For example, the combs can be used for exceptionally precise time transfer and ranging over free space links. The group is working at expanding the distance over which such transfer can take place, up to hundreds of kilometers (while using eye-safe power levels).

The large number of laser frequencies across the comb can be used to create a hyperspectral atmospheric transmission sensor, which can be used to accurately detect and quantify molecular species in the atmosphere. The technology has been used to detect, for example, methane leaks across oil fields and abundances of molecular species relevant to climate change across a city-scale environment. The technology has also been used for in situ atmospheric measurements that can be used to calibrate or validate space-based atmospheric sounders.

NIST and CTL have been successful in developing and refining laser comb technology and helping to transfer the technology to commercial ventures. This is a valuable contribution both to the industrial community and to climate science at large. While CTL’s work on laser combs is important and impressive, the panel notes that it is not clear that this work fits within the definition of communications.

Recommendation 8-1: CTL should continue its goal of refining the laser comb technology and helping to transfer the technology for both commercial and scientific purposes.

Wireless Coexistence

The Spectrum Technology and Research Division is actively involved in various studies related to coexistence of wireless systems, and spectrum sharing at large. The chances and actual occurrences of conflicts in spectrum use are also increasing, such as the example of the potential interference between 5G deployments and the nearby aeronautical radio altimeter systems in the 3 and 4 GHz ranges.

The wireless coexistence work is linked to the group’s work on wireless signal characterization, as coexistence depends on the nature and strength of the interfering signals. The Spectrum Technology and Research Division cited several examples of past or current work in this domain, including the following:

• Wi-Fi and Bluetooth coexistence as a testbed for general coexistence studies;
• Characterization of user terminal emissions for predicting coexistence between Advanced Wireless Services (AWS) and Department of Defense (DoD) systems in the 1.7 GHz range;

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¹ National Academies of Sciences, Engineering, and Medicine, 2019, An Assessment of the Communications Technology Laboratory at the National Institute of Standards and Technology: Fiscal Year 2019, Washington, DC: The National Academies Press, https://doi.org/10.17226/25602.

• Initial efforts at the use of artificial intelligence and machine learning for test waveform generation, so that realistic interfering signals can be generated without recording real radio signals off air, which can create privacy issues;
• 5G skyward emissions to measure the impact of 5G signals on aeronautical and satellite systems; and
• Issues of fairness among coexisting systems.

A Spectrum Technology and Research Division’s research effort has applied machine learning to determine which transmission parameters are most impactful to coexistence between disparate systems (such as Wi-Fi and Bluetooth). It has shown that restricting attention to those parameters can significantly reduce (by as much as 30 percent) the number of measurements needed to parameterize coexistence criteria. One of the practical impacts is to speed up analyses of coexistence between two or more systems that may be under consideration to share spectrum.

The fair coexistence work done by the group aims to develop a framework to objectively answer the question of whether a given scenario achieves a desired level of fairness, where fairness is intended as an input to the model and measurement method being developed.

Overall, the wireless coexistence work is relevant, of high quality, and important to the management of spectrum.

Spectrum Sensing

The Spectrum Technology and Research Division made a pivot of resources to address the COVID-19 pandemic. The group used machine learning to infer a person’s breathing from variations of the Wi-Fi received signal (the physical movement of respiration caused measurable changes in signal), which may have applications in other diseases or future pandemics.

The group also took advantage of the unique shift in home, school, and work arrangements during the pandemic to perform detailed studies of changing radio spectrum use during this time.

National Advanced Spectrum and Communications Test Network

The Spectrum Technology and Research Division in CTL hosts the program office for NASCTN. As part of this role, they are a government-wide resource for undertaking technical studies related to NASCTN projects. Examples include the past and ongoing work on understanding coexistence between AWS-3 systems and DoD’s aeronautical telemetry system that operates in the same band, and a newly launched project to study CBRS. Specifically, the new study will provide data-driven insight into the CBRS-sharing ecosystem’s effectiveness between commercial and DoD radar systems, track changes in the spectrum environment over time, and assess whether the CBRS industry’s network of radar sensors is performing its job to detect and avoid DoD radar when in use, according to the Part 96 rules.

The division has moved beyond current coexistence issues between radar and 4G LTE communications systems, or incumbent and secondary communication systems, and plans to study 5G- and-beyond systems. Such future 5G, 6G, and beyond systems will operate at higher frequencies with narrow beams and large available bandwidth.

The division reported about the work done at the beginning of the pandemic to allow measurements to be run remotely—a capability that is expected to automate measurement campaigns, making them more efficient and more extensive in the future. It also voiced concerns about (1) the amount of time required for purchases above $10,000 and (2) the availability of advanced equipment for spectrum sharing and sensing research, some of which is not yet commercially available.

The Spectrum Technology and Research Division disseminates its work through presentations and publications in professional conference proceedings and journals. It contributes to standards-developing organizations such as the Institute of Electrical and Electronics Engineers (IEEE), the American National

Standards Institute (ANSI), the 3rd Generation Partnership Project, the Technology Innovation Program, and the International Electrotechnical Commission. It has ongoing collaborations with industry and other agencies, including joint publications and standard contributions. Technology transfer happens through NIST programs such as the Technology Maturation Accelerator Program and the NIST-Science and Technology Entrepreneurship Program. Technical notes, reports, and data sets are made available online through NIST repositories and through the NASCTN webpage.

The group has received NIST awards for service, including Presidential Rank Award, Distinguished Associate Award, various medals, etc.

The Spectrum Technology and Research Division collaborates with other groups and federal agencies (i.e., through NASCTN) to share lessons learned. It shares detailed reports and data, with specifications, rationales for decisions, and challenges. Testbeds are open to others.

NIST has participated in industry standards groups. For example, NIST helped guide the standards for CBRS within the Wireless Innovation Forum. From the adoption of CBRS rules by the Federal Communications Commission (FCC) in 2015, NIST representatives participated in Wireless Innovation Forum standards meetings and made contributions on a wide range of standards items, including propagation models, interference statistics, protocols, and certification test code.

CHALLENGES AND OPPORTUNITIES

The Spectrum Technology and Research Division has moved beyond current coexistence issues between radar and 4G LTE communications systems, or incumbent and secondary communication systems, and plans to study 5G-and-beyond systems. Such future 5G, 6G, and beyond systems will operate at higher frequencies, where coexistence issues among active systems may be not relevant because beams are narrow and the available bandwidth is large. Instead, there may be potential coexistence issues with passive, possibly non-terrestrial, systems. In addition, next-generation wireless systems will include mixed industrial Internet of Things (IoT) applications and smart grid applications. Issues of trustable measurements and coexistence in such environments are expected to be rather challenging, given that IoT devices are expected to be heterogeneous, power limited, computationally limited, and geographically distributed in areas difficult, if not impossible, to access. In addition, medical devices will also need to be secure and guarantee privacy. Advances in these challenging areas of outdoor, dynamic, and distributed sensing are expected to have great practical impact.

Recommendation 8-2: CTL should continue its work on spectrum coexistence.

The use of spectrum to monitor atmospheric emissions is very interesting and timely. Upward skyward measurements are an opportunity for collaboration with the channel-modeling group in the next-generation wireless research focus area.

Recommendation 8-3: CTL should reach out to and collaborate with industry partners to develop tools that employ technologies being researched at CTL, where appropriate, to address climate change challenges.

Key Recommendation 10: CTL should engage actively with industry, including wireless standards groups, to remain aware of upcoming and pressing spectrum coexistence issues. The Spectrum Technology and Research Division should develop a framework that allows fast pivoting of resources (and techniques) as new bands become available to be able to contribute timely, accurate, and unbiased assessments of coexistence among increasingly dense and disparate spectrum uses.

A concern shared by the panel members is that spectrum sharing and spectrum sensing activities are not integrated, and thus their synergies are not clear. This is understandable at this stage, as the group is

very new and some of its members have not worked together before. An immediate and concentrated effort appears needed to identify common goals and projects going forward. The panel positively assessed the ongoing efforts of the researchers of this new division, aiming to identify common challenges in terrestrial-to-non terrestrial measurements, emissions at higher frequencies, and also applications for rural and agricultural uses of wireless systems.

The panel heard from another division within CTL working in the Core Networks research focus area about the very exciting ongoing work on the Rydberg Atom-based Radio Frequency Field Probe, a technology that, if possible to develop to its full commercial potential, could revolutionize radio communications and metrology.

Key Recommendation 11: The Spectrum Sharing and Sensing team should collaborate closely with the Rydberg Atom-based Radio Frequency Field Probe team to examine the probe’s potential application to spectrum sensing and metrology and identify key technology opportunities in the space of spectrum metrology.

PORTFOLIO OF SCIENTIFIC EXPERTISE

Personnel Expertise

The Spectrum Technology and Research Division is well staffed with many of its team members recognized as experts in their field. There are only a couple of vacancies at the time of writing this report. The team has been able to attract amazing talent that matches well to its mission and work and appears to be on track to achieve its goals.

Challenges and Opportunities

The Spectrum Technology and Research Division competes with industry and academia for the best people. The CTL director reports that they have implemented many aspects of workplace flexibility (in terms of work from home, schedule flexibility, and split shifts) to compensate for what they may not be able to offer in terms of salary. The team members attested to a very respectful and team-oriented work environment.

EFFECTIVE DISSEMINATION OF OUTPUTS

Accomplishments

The Spectrum Technology and Research Division has a good publication record. The group disseminates its work through presentations and publications in professional conference proceedings and journals. It contributes to standards-developing organizations, such as IEEE, ANSI, the 3rd Generation Partnership Project, the Technology Innovation Program, and the International Electrotechnical Commission. It has ongoing collaborations with industry and other agencies, including joint publications and standard contributions. Technology transfer happens through NIST programs, such as the Technology Maturation Accelerator Program and the NIST-Science and Technology Entrepreneurship Program. Technical notes, reports, and data sets are made available online through NIST repositories and the NASCTN webpage.

The group has a track record of receiving NIST awards for service, including the Presidential Rank Award, the Distinguished Associate Award, and various medals.

The Spectrum Technology and Research Division collaborates with other groups and federal agencies (i.e., through NASCTN) to share lessons learned. They share detailed reports with specifications, rationales for decisions, and challenges. Testbeds are open to others. This knowledge is beneficial to those who plan to set up similar or related experiments, as it reduces the design time, allows them to readily identify key challenges, and focuses on improving performance.

Challenges and Opportunities

NIST has successfully participated in industry standards groups. For example, NIST helped guide the standards for CBRS within the Wireless Innovation Forum. Since the adoption of CBRS rules by FCC in 2015, NIST representatives participated in Wireless Innovation Forum standards meetings and made important contributions to a wide range of standards items, including propagation models, interference statistics, protocols, and certification test codes. Similar opportunities will present themselves in, for example, the 3.1 to 3.45 GHz band, which is currently under study for shared use.

Recommendation 8-4: Where possible, the Spectrum Technology and Research Division should engage with industry standards groups to both understand the ongoing needs of the wireless industry and to offer CTL’s expertise in specific areas.