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CHAPTER V - Technical Elements of a General Purpose Spectrum Regime

A general purpose spectrum regime that accommodates all three models of spectrum management will require sophisticated technology and an enlightened regulatory underpinning. Thus, the Roundtable devoted considerable attention to the technical elements of such a regime. The discussion focused largely on what is needed to implement the spectrum-sharing model, because it is the newest and most technically demanding of the three models.

Minimum Requirements

Participants wrestled first with the question of just how flexible the regulatory rules could be under a general purpose spectrum regime. Stated differently, what are the minimum requirements that regulators would need to impose? The group concluded that, strictly from an engineering perspective, a general purpose spectrum regime would need to impose just two requirements: “operating rights” and “admission control.” The group’s analysis of these requirements drew heavily on the work of the FCC’s Technological Advisory Council (TAC). The TAC is a formal advisory committee established under the Federal Advisory Committee Act, and several of the Roundtable participants are active members of the TAC.

Operating Rights. Operating rights consist of transmit rights and interference protection rights. Transmit rights, or “transmission permissions,” refer to the amount of electromagnetic energy a spectrum-using device is allowed to deliver. This could be a measure of the energy a device transmits (transmit power) or, alternatively, the energy that gets received by other devices in the field (field strength).

As the technical experts in the group envisioned them, transmission permissions will have several novel features. First, the defining parameters will be expressed as probabilities rather than absolute values. For example, the license might require that the power density not exceed a specified level more than five percent of the time at more than 95 percent of locations for a given time window and test area. Second, a transmission permission will not entail an obligation to prevent “harmful interference” to other concurrent operations, as is the case with traditional FCC licenses. Rather, if resulting energy levels meet the (probabilistic) requirements specified in the license, the licensee will not be liable for harm to other operators.i

Interference rights, or “interference protections,” answer the question of how much protection from interference a receiving system is entitled to. Specifically, interference protections (also known as reception limits, interference limits and harm claim thresholds) will quantify the level of interference from a third party that any particular receiver will be expected to tolerate before the radio system can have a claim of harmful interference. As with transmission permissions, the relevant parameters of interference protections will be defined probabilistically (e.g., a rule might say that a specified field strength or power flux density is not to be exceeded more than a set percentage of times and at more than a set percentage of locations within a particular service area).

Two basic points about interference protections are worth noting. First, as discussed in Section III, a reception-oriented interference requirement represents a new element of spectrum management—one designed to avoid cases such as Nextel-public safety and LightSquared-GPS, in which the performance of receivers limits the potential for adjacent bands to support valuable new services. Having clearly defined interference-protection rights should help to achieve more efficient trade-offs between the rights of transmitters and receivers, with the goal of maximizing concurrent operations as opposed to minimizing harmful interference. Moreover, operators should be able to negotiate these arrangements privately and routinely, with much less need for regulatory intervention.

Second, interference protections represent a performance (or output) measure, as distinct from receiver standards, which are a design (or input) measure. Although mandated design standards would recognize the importance of receivers in managing interference, they could have unintended consequences, e.g., such standards often become tied to a specific technology, which impedes innovation. Thus, spectrum experts have come to believe that, in addressing the need for a receiver-oriented interference requirement, it is more efficient to specify a performance requirement and leave it up to device manufacturers to figure out the best way to achieve it.

Admission Control. The second technical requirement for a general purpose spectrum regime, admission control, refers to the process or mechanism for deciding who can get access (admission) to a spectrum band at any given time. Access can include not just permission to transmit but also permission to operate a receive-only radio station, such as a satellite receive station or a radio telescope.

Admission control can be accomplished in a number of ways, beginning with the use of licensing. Another approach is ex ante device certification; for example, in unlicensed bands, any device can operate as long as it is compliant with the FCC’s Part 15 rules. A third approach to admission control is an SAS, which uses some combination of geolocation/database and spectrum sensing techniques to limit the access of devices to shared or unlicensed spectrum. Yet a fourth approach is ex post removal of a non-compliant device. (Ex post admission control is part of an enforcement regime, which is discussed more below.)

No Usage Requirement. Participants stressed that a general purpose spectrum regime could function with no requirements other than operating rights and admission control. Specifically, there is no need to limit a particular band to a specific use, such as public safety radio services or satellite communication of broadcast signals. This flexibility is possible because, as noted earlier, spectrum uses differ only in their operating rights, and under the conditions associated with spectrum sharing, a single band of spectrum will be able to accommodate multiple sets of operating rights.

Role of the SAS and the Emergence of Assured Coexistence Engines (ACE)

Participants emphasized the critical role that the SAS will play in a general purpose spectrum regime—at least in shared-use bands. In addition to their admission-control role, such systems can facilitate interference resolution and system management, thus allowing for more effective utilization of shared-use spectrum (as measured in terms of efficiency, resilience or other criteria). By contrast to shared-use bands, exclusive-use bands will have less need for an SAS because the spectrum operator already performs the same function. In traditional unlicensed bands, the strict limits on power will obviate much of the need for access control and system management, although an SAS can nevertheless add value by providing a tool with which the rights of unlicensed devices can be modified.

Participants noted that there are several emerging technologies for SAS management. One is the approach used in the TV white space, which combines geolocation and database techniques. With this approach, the system stores information on spectrum utilization by incumbents and other authorized users in a central database. New users are required to communicate with the database and to dynamically select channels, times and/or locations that will avoid interference. An alternative approach is spectrum sensing, which requires new users to monitor the actual usage of spectrum by incumbents and other authorized users, and restricts them from selecting channels, times and/or locations that would cause interference.

A major fault line in the debate over spectrum sharing has to do with the appropriate number of tiers. A number of industry stakeholders, including members of the Wi-Fi community, prefer the three-tier approach proposed by PCAST (and recently approved by the FCC in its 3.5 GHz proceeding). As noted earlier, some wireless carriers and vendors initially expressed a preference for a two-tier approach to sharing, such as ASA/LSA, although that may be changing, and some stakeholders are now discussing the use of LTE-U/LAA in the third (GAA) tier of a three-tier system.

Significantly, Roundtable participants concluded that an individual band of spectrum could accommodate both a two-tier approach (LSA/ASA) and a three-tier approach (PCAST/CBRS) to shared access. Other approaches, such as the one used for unlicensed access to TV white space, could also be accommodated. The group coined the term “Assured Coexistence Engines,” or ACE, to describe this vision. The implication of ACE is that operators and regulators do not need to choose one approach to protected shared access over another—they can have multiple approaches in a single band (although, as discussed below, regulators will need to provide for interoperability).

The group discussed at length how the SAS admission control function should handle excess demand. Specifically, if a shared-use band is nearing the ceiling on aggregate interference, as defined by the interference protection standard, how will the SAS decide which GAA devices to admit and which ones to turn away? Some participants pointed out that, while this is not an issue in unlicensed bands because there is no guarantee of quality of service, with the approach envisioned in the PCAST report, in which PA users have protected access, there will need to be an “etiquette” or “protocol” for limiting GAA devices. Peter Pitsch said he saw the issue of who determines that protocol as a red flag: “To the extent you are creating a legal advantage and [there is] more than one person who wants it, then you’re going to have policy questions that arise inevitably out of that.”

Pierre de Vries from the University of Colorado’s Silicon Flatirons Center defined that as a “fairness issue” and said there was a variety of algorithms for making spectrum access decisions on the grounds of “fairness.” He pointed out that, traditionally, the spectrum community has not wanted to see the FCC select the fairness algorithm because (as with design standards) it inevitably becomes technology-specific, which stifles innovation. Consistent with that view, a number of participants felt that the technical community could develop an access protocol, much as it develops equipment standards. One participant pointed to the 3650–3700 MHz band, which the FCC set aside for contention-based use by unlicensed devices; the technical community came up with a contention-based protocol that users have followed in that band.

However, other participants stressed that this was something the FCC needed to bless, if not design. This is particularly the case because there will be multiple SASs operating in a single band, and they will need to be synchronized. As Jon Peha put it, “You can’t have rough consensus” on something this challenging and important. “You really need to codify it…as a form of etiquette, and etiquettes are something that the FCC at least has to bless.”

Technical Advances that Would Facilitate a General Purpose Spectrum Regime

Participants also considered what technological changes are needed to enable a general purpose spectrum regime, which in this context referred to wide-scale spectrum sharing. The group concluded that existing technology is capable of supporting this goal at the pilot stage. (As one participant put it, “the technology is ready to go from lab to street.”) Thus, technology should not be a reason to delay. At the same time, there is a “chicken-and-egg problem”: technology companies need rules in order to build a fully functional system, but the FCC needs evidence that the technology will work (and that there will be a demand for it) before it can write the rules.
There are four concrete areas where additional technical advances are needed to take wide-scale spectrum sharing beyond the pilot stage:

  • Wide-band sensors: Spectrum sharing requires sensors that can scan a large spectral bandwidth—several hundred megahertz or even several gigahertz—to detect unused spectrum. Current technology does not allow sensors to sample the spectrum at a rate high enough to provide the needed resolution. To get the desired resolution, the technology needs to incorporate advanced signal sensing and sampling techniques.
  • Self Organizing Networks (SON): Wireless networks consist of thousands of base stations, each with hundreds of settings. Self-organizing networks are an automation tool, designed to allow a network operator to organize, manage and “heal” its network more efficiently—for example, by managing neighbor cell relations (known as Automatic Neighbor Relations, or ANR). The massive deployment of small cells that is likely to occur in shared-use bands will require much more sophisticated ANR functionality.
  • Software to manage the co-existence of old and new devices: Wide-scale spectrum sharing will require the software to ensure that older, less spectrum-efficient devices do not crowd out newer, more spectrum-efficient ones. One approach is to “reflash” older devices so that they have more advanced capabilities. Another is to “brick up” such devices, in effect, rendering them unusable.
  • Filters, power amplifiers and antennas to allow for dynamic spectrum access: The “front end” environment for a mobile device is increasingly complex and can include 10–20 components, such as a filter, power amplifiers, antenna tuners and switches. With more sophisticated front-end technologies, devices will be able to tolerate higher levels of interference and take better advantage of shared access bands.

Participants observed that the correlation between the price of a good and the scale of production poses its own chicken-and-egg problem. To elaborate, to get this technology into the market, it needs to be very low cost. That is relatively easy for a manufacturer, once it is producing at high volume. The challenge is to develop ways to “flatten this curve so that the technology is cheap even at small scale.”

Government Responsibilities

Participants looked next at what the federal government needs to do to enable a general purpose spectrum regime. The group identified three key steps that it wants to see regulators take in the near future. By “regulators,” the group had in mind NTIA as well as the FCC.

First, the government needs to define the two sets of requirements discussed above: operating rights (transmit rights and interference-protection rights) and admission control. Regulators should give priority to interference-protections rights, because they represent a new element of spectrum management and thus are an area in which regulators have less experience. The FCC’s 3.5 GHz proceeding specifies the amount of interference protection to which Tier 2 licensees (PA users) are entitled. Separately, the FCC needs to specify protection limits for Tier 1 licensees, such as C-band earth stations. Ideally, NTIA will set similar limits for federal spectrum users.

Second, the FCC needs to set the requirements necessary to allow SASs to operate and interoperate. As noted above, there is no need to select one over another, because the systems can co-exist. However, the government needs to set performance specifications for these systems as well as requirements for synchronization and interoperability.

Third, the government needs to facilitate “markets in interference protection,” referring to the ability of spectrum users to modify their transmit rights and interference protection rights through bilateral negotiations. The key action required is the definition of these rights (step one above) and a specification of the mechanism for their enforcement.

Enforcement

Finally, participants addressed the question of how a general purpose spectrum regime would identify and punish bad behavior. Using automobile transportation as an analogy, the group identified four stages to the enforcement process:

  1. Observation (watching drivers, measuring their speed to see if they're driving recklessly);
  2. Allegation (e.g., a police officer writes a ticket);
  3. Adjudication (the individual who got the ticket disputes the allegation, so the two sides must go to court); and
  4. Remediation (depending on the outcome of the court proceeding, the government takes some action, such as imposing a fine, putting points on the driver’s license or taking the license away altogether).

To promote efficient and innovative use of spectrum technologies, the goal is to jump directly from the observation of a problem to remediation wherever possible. That is, although mechanisms for allegation and adjudication would exist as a backstop, it is preferable to invoke them infrequently. For example, when an electronic sign on the road shows a car’s current driving speed (and the speed limit), most drivers slow down to conform to the speed limit.

Participants also considered the role of technology in the enforcement process and concluded that technology can facilitate enforcement at every stage. Observation will benefit from the use of tools like crowdsourcing (using the vast number of deployed devices to track problems) as well as the SASs themselves, which are a form of “big data.” Those same tools will be critically important at the allegation stage, where it will be necessary to have an “audit trail” and, in effect, “prove that your radar gun was accurately calibrated.” Stage three, adjudication, requires judgment, which is an inherently human function; nevertheless, technology can streamline the process (and obviate it in some cases). Finally, at the remediation stage, as an alternative to taking punitive action, an SAS can redirect an interference-causing user to a non-interfering spectrum position. Moreover, by tracking the trends in ad hoc interference resolution, technology can help to flag emerging remediation problems.

ENDNOTES
i Jean Pierre de Vries, “How I Learned to Stop Worrying and Love Interference: Using Well-Defined Radio Rights to Boost Concurrent Operation,” September 2010. Available online: http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1672375.
 
 
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