Is CBRS for everybody? – growing pains and progress towards a practical solution
CBRS – Where are we today?
When launched, the CBRS band was dubbed the innovation band. It was intended to allow non-CSPs access to 150MHz of prime mid-band spectrum that was only lightly used by federal government incumbents and Private Land Mobile Radio (Part 90 wireless) users. The aim was to unlock untapped markets, new use cases, and new revenue streams.
The CBRS band consists of three tiers. Spectrum allocation for commercial operations and incumbent protections are managed by a spectrum allocation service (SAS).
The top tier consists of incumbents (federal government users) that occupy the top of the pyramid and are afforded full protection from interference by commercial operations within the band. After federal incumbents, the middle part of the tri-layered pyramid is reserved for the Priority Access Licensees (PAL). PAL licenses consume up to 70MHz in the lower 100MHz of the band, are licensed across US counties, are renewable every 10 years, and receive interference protection from generally authorized access (GAA) users at the bottom of the CBRS shared spectrum pyramid.
The PAL auction was completed in September 2020 with the major service providers taking a majority stake in those licenses, as evident in the graph below.
PAL was officially launched in April 2021 and service providers, with the support of their vendors and SAS providers, have been steadily deploying to capitalize on their investments.
As of March 2022, the OnGo Alliance reported that there were 208,000 CBRS base station devices (CBSDs) deployed in the CBRS band, a 100% increase over the previous year. This statistic alone points to a significant uptick in activities within the band. With PAL being consumed by many of the big CSPs, and the secondary market not quite off the ground for PAL spectrum leasing, as little as 80MHz may remain in the GAA part of the band. At first glance, 80MHz would seem sufficient to satisfy the needs of new entrants eager to take advantage of this valuable mid band spectrum; but is it?
Over the last year, I’ve witnessed an increase in the number of opportunities and engagements by non-CSPs wanting to make use of the band. These include non-CSPs utilizing CBRS for innovative use cases such as wireless video surveillance, semi-autonomous trucking, remote health services and manufacturing automation. A significant number use GAA spectrum.
Another use case that recently has trigged additional interest in this band is multi-operator neutral host leveraging the Multi Operator Core Network (MOCN) configuration; the benefits of sharing both infrastructure and spectrum for small and low-capacity venues could enable CSPs to extend their coverage to venues that otherwise would present a Total Cost of Ownership (TCO) challenge as a single operator system.
Recall that all CBRS PAL and GAA users must defer to incumbents, particularly in coastal areas. This causes occasional temporary loss of spectrum. In some situations, the paucity of alternative PAL resources drives the PAL licensees to seek alternative GAA spectrum when impacted by incumbent activity.
During trials, proofs of concept (PoC), and early deployments with customers, Ericsson has recorded congestion of GAA spectrum. In many cases, this congestion has caused service outages. The example below is a spectrum scan showing the signal levels seen across the CBRS band in an area of a city where Ericsson was conducting a trial with a customer in the GAA portion of the band. The results show that during the 2 min sampling periods, most of the band is experiencing very high RSSI levels (i.e., >-99dBm).
The GAA band is not given any type of interference protection by the SAS. Without any protection from interference, those operators will have to work with their SAS providers to develop a channel sharing plan to mitigate interference issues. Usually, channel sharing plans mean that operator A gets half of the available spectrum and operator B gets the other half.
In the short term, this mitigates the immediate problem, but long term, channel sharing plans have limitations:
- It takes time to create the plan
- It takes time to agree and implement
- Available spectrum is reduced
- It is only valid until the next operator deploys in the band in the same geographical area
After a few iterations, the viable portions of the spectrum are completely consumed. Don’t forget that GAA needs to yield to incumbent activity, which can further minimize spectrum availability.
What can be done to mature the band?
Standards bodies such as OnGo Alliance and WinnForum have provided specifications that outline methodologies aimed at managing interference in the GAA band through GAA spectrum coordination and TDD frame configuration guidance.
While most users deploy 3GPP technologies, the band does not require a single technology, so there are a number of WiFi, WiMAX, and other proprietary technologies in use. This hampers coordination solutions such as TDD frame alignment.
The OnGo Alliance solution is Coexistence Managers (CxM) that can apportion spectrum to compatible users in Coexistence Groups. However, the regulatory implications around recertification of SAS equipment and a lack of interest in conducting interoperability testing has impeded progress. While a step in the right direction, SAS and CBSD vendors have been slow to adopt these specifications, largely because the functionality provided is not mandated by FCC rules.
Some of the SAS providers have implemented proprietary (“bitsa”) versions of the GAA Spectrum coordination specifications as an initial phase of coexistence support. It should be noted that the SAS does not have the right to deny spectrum that is not restricted by incumbents or PAL users; the authority provided by regulation is limited.
What is the solution for CBRS spectrum?
Long term success requires a means to assign spectrum dynamically in the GAA part of the band. While specifications to do this have been agreed upon by thought leaders in the industry, they have yet to be put to the test in a real-world scenario.
Ericsson is spearheading a proposal, in collaboration with SAS providers and OnGo Alliance, on a “Practical GAA coexistence solution” as the first phase of GAA coexistence support. The result of this collaboration was a change of direction by OnGo Coexistence Task Group to develop TS 2003 “Collaborative Coexistence Framework”. If approved, the new TS would be included in OnGo Release 5 specifications. The goal of these efforts is to simplify the approach to coexistence, reduce SAS computational load, and enhance adoption of GAA coexistence support within the CBRS band.
This new GAA coexistence framework will provide support to those operators who are partially or solely reliant on the GAA spectrum for their wireless networking endeavors and ensuring the value of CBRS GAA spectrum is a much higher value proposition than unlicensed spectrum options.
So, is the GAA portion of the CBRS band viable to deliver on its intended promises? So far, we’ve seen some interesting early use cases and innovative applications of the spectrum. But these experiments have also exposed some practical limitations.
CSPs and non-CSPs desire CBRS band spectrum have higher quality and availability in comparison to unlicensed spectrum. In addition, the spectrum has to be affordable and mostly free from interference.
CBRS is still only three years old, and so any teething problems should be taken in that light. With effective coexistence solutions on their way, CBRS has the potential to be an innovative and useful tool in the next phase of spectrum delivery.
For more information about Ericsson’s CBRS solutions go here:
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