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Next-Generation Wireless
The NextG Wireless Research program within the Communications Technology Laboratory (CTL) is comprised of 31 experienced and highly skilled radio frequency engineers, physicists, software engineers, and computer scientists based in Gaithersburg, Maryland, and Boulder, Colorado. The team is involved in a variety of inter-disciplinary wireless research projects, including, but not limited to, millimeter-wave wireless propagation modeling and performance evaluation of next-generation waveforms and communication protocols. Publicly sharing the models and disseminating data sets to the research community are some of the key highlights of the group’s accomplishments, along with their high-impact conference and journal publications. The collaboration extends beyond U.S. academic institutions to include foreign research partners, who have been doing mid-band work for use with 5G that the United States is just beginning to take up, as part of their cutting-edge, next-generation research and Channel Model Alliance work.
ASSESSMENT OF TECHNICAL PROGRAMS
The following discussion focuses on three major projects that the Next-Generation Wireless research group is currently involved in, and two collaborative support projects to highlight their key accomplishments, new opportunities, and this panel’s recommendations. We discuss the projects in the bottom-up order of open systems interconnect layers starting with the physical layer work, then protocols (medium access and networking), and then the application layer work.
Millimeter Wave Instrumentation (Physical Layer)
Metrology for Next-Generation Wireless Networks
This project includes (1) precision channel propagation measurements and modeling, (2) wireless system performance, and (3) communications for Internet of Things (IoT) applications. The channel measurement work relies on three successive generations of CTL radio frequency channel sounders, the
first two covering 28, 60, and 83 GHz and the third covering 140 GHz. The group has developed a comprehensive measurement-based modeling and analysis tools suite. Device-to-device communication is being investigated under Long-Term Evolution (LTE) and New Radio protocols via link-level simulations and system-level evaluations. The work is of the highest quality, reflecting the NIST tradition of performing high-precision, reproducible, and traceable measurements.
The group indicated that the sub-6 GHz bands are increasingly important and agreed that the United States is somewhat behind the rest of the world in this regime. While the United States focused on millimeter-wave technology with regards to 5G research, the rest of the world, including but not limited to Europe and Asia, focused on sub-6GHz band research.1 Now that the United States has started to use the sub-6 GHz band for deploying 5G networks, it is important that CTL expands its research to include the sub-6 GHz band. The CTL group noted, however, that the inclusion of sub-6 GHz into their channel measurement work would be dependent on the availability of increased resources.
Recommendation 5-1: CTL should partner with foreign researchers on sub-6 GHz work as a way to get a quick jump-start on the research by leveraging existing work on the topic, then work to identify gaps and focus their research on those topics with respect to the future sub-6 GHz research work.
Next-Generation (NextG) Channel Model Alliance
The group has actively participated in the Next-Generation (NextG) Channel Model Alliance since 2015, providing data sets and models to the open research community, including making them publicly available via MATLAB toolbox. The focus and the energy spent are apparent and contribute to the success of the effort. But there are new frontiers beyond the NextG Channel Model Alliance work, including, but not limited to, power management and dynamic spectrum sharing at the edge that will need a concerted effort from the larger community, including CTL from the measurement perspective with respect to next-generation wireless deployment.
Key Recommendation 6: CTL should create a roadmap of what the next 5 years of NextG Channel Model Alliance’s work will look like. The group should use this roadmap to determine where they can leverage CTL’s unique capabilities to focus on the key gaps and make the biggest impacts on next-generation wireless technology and its deployment in the next 5 years.
5G New Radio Direct Mode Communication (Protocol)
The Next-Generation Wireless group conducted a study on 5G New Radio direct mode communication in collaboration with the public safety research group since direct mode—also called device-to-device in 4G LTE and side link in 5G—is a key functional capability for public safety communications. The study focused on resource management, physical layer capacity, quality of service, and LTE/New Radio coexistence. If feasible and successfully deployed, the device-to-device mode will keep first responders connected in situations where there is no cellular infrastructure to provide a 4G/5G network service. Moreover, unlike the 5G non-standalone mode, which focuses on the 4G/5G network coexistence, the 5G standalone mode side link deployment is more likely to happen as large-scale IoT and vehicle-to-everything applications demands are only met by the 5G New Radio standard. The Office of the Under Secretary of Defense for Research and Engineering (OUSD (R&E)) recently funded a 5G
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1 For more information see https://media.defense.gov/2019/Apr/03/2002109302/-1/1/0/DIB_5G_STUDY_04.03.19.pdf.
standalone side link study that could be leveraged to get a head-start on a standalone mode side link study.2
Recommendation 5-2: CTL should develop a partnership with OUSD (R&E) and collaborate on OUSD (R&E)-funded projects, like the 5G side link study. CTL should also leverage the ongoing work to update their 4G/5G non-standalone mode study with the relevant findings from the OUSD (R&E) study, especially regarding the public safety use case.
Spectrum Sharing for Citizens Broadband Radio Service (Application)
The Next-Generation Wireless group has contributed significantly to Citizens Broadband Radio Service band spectrum sharing by creating tools, models, and data sets, along with developing standard techniques to achieve dynamic and efficient spectrum sharing between commercial and federal users in the 3.5 GHz frequency range. The team developed synthetic waveform generation tools and contributed to the test harnesses that were essential for the testing of industry-developed environmental sensing capability sensors. These sensors are widely used by industry and academia today and enable shared use of the CBRS band. Their work on this topic, including the reference implementation and publications, has been recognized with a major award within NIST and by the community at large. Given the increasing demand for the more shared spectrum between federal and commercial users, it is important that CTL extend their spectrum sharing application work to include the multi-band spectrum sharing between federal users and commercial users in what was traditionally Department of Defense bands. The National Science Foundation has started a set of Spectrum Innovation Initiative projects that creates dynamic radio access zones for spectrum sharing between federal and commercial users at the edge instead of using centralized databases.
Recommendation 5-3: CTL should create a roadmap for the application-related work as applied to multi-band spectrum sharing deployment at the user device. CTL should also collaborate on National Science Foundation–funded Spectrum Innovation Initiative projects to align with the ongoing work on this topic and identify gaps that can be included in the roadmap.
Collaborative Project
Open Radio Access Network (RAN) Research
The Next-Generation Wireless research group, in collaboration with the Core Network research group, is looking into acquiring an Open Radio Access Network (RAN) 5G testbed to use for their future 5G/6G research work. The testbed would be key to combining the Next-Generation Wireless research group’s ongoing physical layer, protocol layer, and application layer work described above so that a cross-layer, full-stack analysis of next-generation wireless network systems can be done and the impact with respect to full-stack 5G/6G performance can be understood. Many testbeds are available and in use today across industry, open-source communities, and national laboratories. The selection process of the right 5G testbed for CTL’s use will need to be well documented so that other laboratories can apply similar thought processes and criteria to acquire 5G testbeds for their use in the near future.
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2 3GPP TR 23.700-33 V0.3.0 (2022-05). 3GPP TR 23.700-33 V0.3.0 (2022-05) 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; Study on system enhancement for Proximity based Services (ProSe) in the 5G System (5GS); Phase 2 (Release 18).
Key Recommendation 7: The Next-Generation Wireless group should be actively involved in the Open RAN testbed selection process in collaboration with the Core Networks group. The selection process and criteria of the Open RAN testbed process should be well documented and disseminated to the other national laboratories that are also looking to acquire similar testbeds in the near term.
ACCOMPLISHMENTS, CHALLENGES, AND OPPORTUNITIES
Collaboration with International Researchers
Because NIST performs a variety of work that touches on national security and export control, foreign nationals cannot work as permanent employees at NIST. This is a challenge because there is a lot of international graduate student talent that NIST cannot engage with as a part of their permanent workforce. However, 6-month visiting appointments for international researchers can be arranged. Despite the challenges in engaging with international researchers, CTL has used this visiting appointment authority effectively to engage in international collaboration. This is commendable. Further expanding ongoing international collaborations is a great opportunity to advance the Next-Generation Wireless group’s future sub-6GHz work.
Millimeter Wave Work
Millimeter wave measurements and modeling are the most significant activity of the group. CTL’s millimeter-wave and terahertz channel sounder are recognized by the research community as the best of breed. CTL’s open-source millimeter-wave Raytracer and measurement data are used by commercial industries as means of evaluating their product’s performance. The combined work of radio frequency, LIDAR (light detection and ranging), and camera systems to tell where radiation emanates from a body, for precision channel modeling, has proliferated applications in the area of human presence detection, human activity tracking, and augmented reality/virtual reality.
The challenges with millimeter-wave operations remain, however. The blockage problem with millimeter-wave communication—due to the frequency range and its high attenuation due to mobile blockers and the user’s own body—has not been solved. The perennially suggested solution of network densification and small-cells to scale millimeter-wave deployment has not been shown to make business sense except in niche applications. In March 2020, the Federal Communications Commission auctioned millimeter-wave spectrum for $2.24/Hz; in January 2021 mid-band spectrum (e.g., C-band) was auctioned for $290/Hz. Innovation in the sub-6 GHz band is likely to be much more profitable, and thus more relevant than innovation in the millimeter-wave band in the near term. There is also ever-increasing demand to share more spectrum between the federal and commercial users in the sub-6 GHz band. Therefore, instead of focusing on millimeter wave for 5G deployment, the sub-6GHz band deployment makes more sense both technically and economically. A shift to sub-6 GHz 5G deployment brings with it the need to accurately study the impact of this mid-band spectrum sharing on military and existing federal system operations. The Next-Generation Wireless researchers at CTL have a vital role to play in using high-fidelity experiments to quantify the technical impacts of sub-6 GHz deployment in the near future.
Recommendation 5-4: CTL should extend its next-generation wireless systems propagation measurement program to include the sub-6 GHz bands because they are technically and economically appealing and are being adopted in the United States. CTL should conduct high-fidelity experiments to quantify the technical impacts of sub-6 GHz 5G deployment in the near future.
NextG Channel Alliance
The Next-Generation Wireless research program is a key contributor to the NextG Channel Alliance, an established user community for wireless signal propagation measurements and modeling. The group helps maintain the signal propagation modeling data repository as part of the alliance’s working group members. CTL’s active participation in Alliance activities and the quality of seminars and workshops held by the Alliance are evident in the yearly increase in industry participation and the collaborative work published by the Next-Generation Wireless group in collaboration with the larger industry groups and stakeholders. Furthermore, the attendance of researchers from CTL’s Next-Generation Wireless group at different Institute of Electrical and Electronics Engineers (IEEE) conferences and events—such as the IEEE Conference on Vehicular Technology, the International Conference on Communications, and the Global Communications Conference—have benefited the Alliance meetings and vice versa.
There are newer and harder pressing problems beyond channel modeling that relate to the scalability and feasibility of deploying 5G/6G networks in near future. For example, power management, and intelligent surface reflectors that jointly adjust and reconfigure wireless signals for longer-range transmission are some of the areas that need measurement study, modeling, and characterization. There is an opportunity for CTL to consider what beyond the NextG Channel Alliance work could benefit from their attention to make an equally significant impact on 5G/6G network deployment as the Alliance work had on 4G/5G deployment via their millimeter-wave study that has been ongoing for over 6 years.
Recommendation 5-5: CTL should build a roadmap for what is next beyond the NextG Channel Alliance work and align the roadmap with the emerging 6G network problems that will be critical to mitigate before making any large-scale deployment of 6G in any type of setting.
5G New Radio Side Link
5G New Radio Side Link enables direct communication between devices without packets going through the network. While this direct communication can occur both in network and out of network (i.e., out of coverage or off network), CTL has mainly focused its research on the out of coverage scenario to address critical communication for first responders in areas where there is no network or where the network has been damaged. A study report published in 2021 by CTL evaluated the state of New Radio Side Link based on 3GPP Release 16, which was developed to support V2X (i.e., vehicle-to-everything) communications. The report covered issues or gaps within the protocols to support New Radio Proximity Services, direct device discovery, direct communication, and user equipment-based relays. The report further recommended enhancements to the New Radio Side Link power efficiency, reliability, and latency. CTL has also been developing link-level and system-level simulation capabilities to evaluate the performance of New Radio Side Link, including capacity and range. The challenge is that the community and the efforts funded by the Office of the Under Secretary of Defense for Research and Engineering (OUSD (R&E)) have already released Proximity Services architectural advancements based on Release 18. Release 18 highlights Non-standalone mode of user equipment to 5G Radio Access Network interface and support for user equipment-to- user equipment Unicast, user equipment-to-Network Relay, and path switching between user equipment-to-RAN and user equipment-to-user equipment communication paths for relay and non-relay use cases, which provide significant resiliency to networking capability for first responders.
Recommendation 5-6: CTL should review the 5G New Radio device-to-device side link instantiation recommended by the Release 18 study as it pertains to public safety application use case and confirm that their output from their 2021 study, which is based on Release 16, agrees with the more recent OUSD (R&E) work based on Release 18.
Spectrum Sharing
The dynamic spectrum sharing application is a great use case study of the group’s combined full-stack performance and modeling work. As successful as this group has been in the deployment of database-based Citizens Broadband Radio Service spectrum sharing tools and techniques, dynamic radio access zone research is a great opportunity for the group to make spectrum sharing using machine learning algorithms between federal and commercial users more dynamic and edge-based. Furthermore, commercial off-the-shelf lightweight hardware and software components make deploying these machine learning algorithms possible at the user edge. The Open RAN has made it easy to deploy such artificial intelligence and machine learning orchestration algorithms at the edge by defining a standard real-time and near-real-time application (called xAPP and rAPP) and application programming interface via their specification. Furthermore, there is an opportunity to delve into the security of these interfaces and how original equipment manufacturers adopt the Open RAN standard for commercial deployment.
Recommendation 5-7: The CTL Next-Generation Wireless group should collaborate with the Spectrum Sharing group and continue the study of dynamic spectrum sharing as the key application performance study of their full-stack 5G physical layer and network (protocol) layer work.
Resources
Overall, given the inherent importance of next-generation wireless research and development, it is somewhat surprising that the Next-Generation Wireless program has the smallest budget of any of the other groups within the Wireless Networks Division ($6.51 million, 5.6 percent of the overall budget). Out of necessity, this group has had to limit its scope and research focus. It is good to hear that a $12 million budget increase has been requested for the advanced communications research (including the next-generation waveform and the spectrum sharing work).
PORTFOLIO OF SCIENTIFIC EXPERTISE
The Next-Generation Wireless group has the world-class expertise that it needs to accomplish its current missions and to develop its work in different directions as future circumstances dictate. Their multi-disciplinary team of radio frequency electrical and computer engineers, physicists, and computer scientists covers the full range of expertise needed to do hands-on hardware and software development and mathematical modeling to achieve the organization’s mission and program objectives. One improvement would be to hire a telecommunications engineer with expertise in 5G RAN after CTL acquires its Open RAN testbed.
EFFECTIVE DISSEMINATION OF OUTPUTS
CTL has been highly effective in disseminating its research findings via publications, foreign and domestic academic collaborations, the release of design documents, data sharing, and reference implementation code releases via GitHub. Over the last 3 to 5 years, they have published in over 35 combined journals and high-impact conference papers, over three patents, and a follow-on entrepreneurial pursuit to commercialize some of the Internet protocol, machine learning models, and data shared via Matlab toolbox and reference implementation code published out in CTL’s GitHub account.
