Google updates engineers’ group on 3.5 GHz CBRS

A recent meeting of the Association of Federal Communications Consulting Engineers (AFCCE) hosted Dr. Andrew Clegg from Google. He updated the group on the status of the 3550-3700 MHz Citizens Broadcast Radio Service (CBRS). Google has been an active proponent, including through work in the Wireless Innovation Forum (WinnForum), which is developing spectrum-sharing standards.

Not all at the luncheon were familiar with CBRS so Dr. Clegg provided an overview. CBRS resides in the new Part 96 of the FCC’s rules. There’s a three-tier architecture. Tier 1 includes protected incumbents. Tier 2 is for auctioned Priority Access Licenses (PALs). Tier 3 is for “free” General Authorized Access (GAA). A Spectrum Access System (SAS) protects Tier 1 and actively manages Tiers 2 and 3.

Tier 1 incumbents include Department of Defense radar systems and fixed satellite earth stations. The main incumbent of concern is in-band radar on aircraft carriers. There are only 17 ships in the world with this radar, but most are in U.S. coastal waters and docked at Navy bases in those waters. An Environmental Sensing Capability (ESC), yet to be developed and approved, will sense these emissions and inform the SAS. Because the radar is narrowband, if it’s detected, the CBRS radio should be able to move to another frequency. The Department of Defense is sensitive to the geolocation of ships, so they’ll be sensed but not located. Dr. Clegg discussed several adjacent-band incumbent operations that require protection. As they are not in-band, they’re easier to protect.

The Department of Defense is sensitive to the geolocation of ships, so they’ll be sensed but not located. Dr. Clegg discussed several adjacent-band incumbent operations that require protection. As they’re not in-band, they’re easier to protect.

Tier 2 PALs will take up the lower 70 MHz of the 3550-3650 MHz band (in seven 10-megahertz channels). PALs are licensed to census tracts. There are about 4,000 persons in each tract. Since they’re so small, there are over 70,000 census tracts. That times seven PAL channels per tract adds complexity to the auction. There will be more licenses to keep track of (over 500,000) than in any previous FCC spectrum auction.

Tier 3 GAA users will take up the remaining 30 MHz, and can use PAL spectrum when it’s inactive. GAA is roughly analogous to Wi-Fi, but it’s licensed by rule (and not unlicensed like Wi-Fi).

Wireless Internet Service Providers (WISPs) now in the 3650-3700 MHz band are grandfathered for five years; then, they will have to comply with the new Part 96 rules or cease operation.

Two use cases are expected to dominate. The first is capacity augmentation for mobile broadband (offload from mobile broadband spectrum). Look for the larger carriers to be involved here. The second is the neutral host model, an example being a node in a building that would provide capacity to any operator, perhaps with the building owner sharing in revenue.

As for hardware, commercial base stations, or as the rules call them, Citizens Broadband Radio Service Devices (CBSDs), should appear later this year, with client equipment (called End User Devices (EUDs)) soon after. Implementation in handsets is in the works. 3GPP has band classes covering CBRS spectrum, and LTE is expected to dominate initally as the air interface. There are higher and lower power CBSDs; lower-power ones are analogous to Wi-Fi access points, and higher-power ones are more comparable to mobile broadband base stations.

The rules require that higher power CBSDs be professionally installed (which is a business opportunity in itself). There was some skepticism in the audience over the professional installation provisions (from experience in TV white space), and a preference for not relying on this and instead having devices sense their environment and behave appropriately.  Dr. Clegg suspects this is the way things will move.

As the SAS is supposed to manage and authorize transmissions, the issue arises of how a user first requests a connection from the SAS. What’s being worked on is a method whereby a short unmanaged transmission might be made initially to gain access to the SAS.

Propagation prediction is a major issue. Obstructions (buildings and foliage) greatly reduce 3.5 GHz signal strength and will be a “very big factor” in network planning. This is seen as a good thing, however, as it eases the task of filling in small areas without causing interference to existing service. Google has geospatial resources to bring to this problem. It has detailed building and foliage databases, some with detail down to individual trees.

Current RF planning tools are missing a “3D” capability, which would be helpful for 3.5 GHz; at one address, there may be more than one base station vertically accress several floors.

FCC movement on the PAL auction is expected later this year or early next year.

See also:

Understanding the New U.S. 3.5 GHz Band (WinnForum)

FCC documents related to the CBRS docket (12-354)