Taking Stock of Spectrum Sharing

Abstract

Shared access to airwaves dates back to the dawn of radio. Yet, recent spectrum policy in the United States has articulated the idea of “spectrum sharing” as a distinct form of radio regulation. In this paper we take stock of spectrum sharing as a public policy, with a focus on four questions. 1. What does it mean to have a spectrum sharing “policy”? We consider different ways of looking at spectrum sharing, including as a form of traditional radio coexistence and as a set of new technologies. We observe: * Sharing technologies can be grouped according to a simple framework, based on whether they use coordinating, sensing, or informing techniques and whether the implementation is centralized or decentralized. * Spectrum sharing as a public policy is not the same as spectrum sharing technology. Spectrum sharing as public policy is necessary when different categories of spectrum users are governed by overlapping, heterogeneous regulatory models. 2. What have we learned from recent spectrum sharing policy approaches? We consider several different spectrum sharing initiatives from recent years, including coordinated sharing in the licensed AWS-3 Band, automated sharing in the UHF, 5 GHz, and 6 GHz Bands (Television White Space, Dynamic Frequency Selection, and Automatedic Frequency Coordination, respectively), and multi-tiered sharing in the 3.5 GHz Band (the Citizens Broadband Radio Service). Key takeaways include the following: * White House leadership is critical when federal spectrum interests are at stake. Convening stakeholders in an organized forum to align expectations and share information is also essential. Sharing scenarios that involve both unlicensed operators and more traditional licensed users generally require an automated sharing technology to operate at scale. This automation can take the different technological forms described above. Enforcement is an important function that can be facilitated through centralized systems. * Multi-tier sharing can be complex but has been shown to work, with over 150,000 network nodes deployed in CBRS and no reported cases of interference. The CBRS rules promote sharing among a wide and heterogeneous range of users and use cases, including public and private networks, mobile and fixed wireless access, and consumer and industrial applications. Rule changes after the original order prevented experiential learning from some of the more innovative aspects of the CBRS regime and limited access to the Priority Access License tier in major markets to large network service providers. 3. What principles should guide future spectrum sharing policies? From these past experiences we distill several principles that are essential to a successful spectrum sharing policy: * Develop interference expectations. Sharing regimes should be informed by reasonable interference expectations, developed using modern analytics, rather than worst-case analysis. * Encourage standards and technologies that facilitate sharing. Sharing can be facilitated through private sector development of cross-industry “meta standards,” designed to be agnostic as to the underlying radio technologies used for transmission. Promote sharing by design, particularly in bands in which there are no incumbents. * Avoid artificial scarcity. Spectrum “scarcity” is often a byproduct of regulation. Wherever possible, the default should be rules that promote the abundance of spectrum through spectrum re-use. * Avoid “Trojan Horses” for market power. No single private entity should hold the key to enabling a centralized shared spectrum policy. * Build in enforcement mechanisms. Enforcement is necessary to ensure various stakeholders trust the sharing mechanism. 4. What spectrum sharing policies might the current Administration pursue? We see two main opportunities for spectrum sharing policy in the near term: * 3 GHz Band. The 3 GHz Band has evolved into a patchwork of different exclusive and shared regimes. We see an opportunity to consolidate “like” types of spectrum access in different parts of the band, reserving the upper 3 GHz for more traditional cellular licensing and extending the multi-tier CBRS framework farther into the lower portion of the band to facilitate sharing with federal systems. * Lower 37 GHz Band. This band is currently allocated for both federal government and commercial use, with little or no incumbent use. It presents a unique opportunity to develop “two-way” sharing from the ground up. There is no magic formula for spectrum sharing policy. A dynamic landscape of heterogeneous spectrum uses, multiple spectrum authorities, technology advances, and evolving national objectives requires case-by-case assessments. However, such assessments should be informed by the experiences and principles described above. This paper was published with support from Schmidt Futures. Note: following initial publication of this working paper, Qualcomm clarified that its proposed approach to licensing the Lower 37 GHz band would accommodate both fixed and mobile operations for federal as well as commercial users.