October 1, 2020

A Closer Look at Spectrum Allocation for CBRS

There has been a lot in the news recently about CBRS – Citizens Broadband Radio Service.  Some say that this will herald in a new world of innovation.  Other say that it is just an extension of their current business.  As we will see, it is both of these views … as well as others!

The recent auctions that have been in the news for CBRS spectrum are referred to as Priority Access Licenses or PAL.  But what is important to understand about CBRS is that these Priority Access Licenses are only part of the story.

PAL Auction

The PAL auction news is often dominated by the big players, such as Verizon, Cox Communications, Windstream, etc.

While these high-profile auctions captured most of the media attention, the real important players in the CBRS auctions were the middlemen who won 10 to 100 licenses in 5 to 20 counties.  This is significant because the PAL licenses are county-based.  Most of the auction winners were looking for a way to augment their business in their local areas, many of which encompass 5 to 20 counties.  For example, 70 of the Wireless Internet Service Providers Association (WISPA) members won 3,600 licenses in more than 1,350 counties, or an average of 2.6 licenses per county — about 50 licenses per member.

If you won the Priority Access License auction in your area, do you own or control all of the spectrum in your area?  Unfortunately, the answer is no.  Likewise, if someone else won the PAL auction in your counties, does that spell doom for your business?  Fortunately, this answer is also no.  There are several business and technical nuances that must be considered.

General Authorized Access Spectrum

The General Authorized Access (GAA) spectrum is free for anyone to use, assuming that they play by the rules.  It is important to note that this GAA spectrum is the biggest of the bunch.  Anyone can use this spectrum as long as they do not interfere with either the PAL users, incumbent users or the Military/Satellite users.

An example of a GAA use is the city of McAllen, Texas, which sits near the Mexico border where more than 25% of residents live below the poverty line.  The city wanted to provide free Wi-Fi access to the many of the children for their online education.  To do this they used the GAA spectrum to provide a connection backhaul to Wi-Fi access points placed around the city.

Spectrum Access System

If the CBRS frequency systems seem complicated, it is because of this idea of spectrum sharing.  While spectrum sharing is not a new idea, this particular implementation is new.  Geographic spectrum sharing has been going on for many, many years.  Not just in telecommunications, but in AM/FM broadcast all the way to specific military and satellite applications.  In geographic spectrum sharing, the same frequency is used over and over again but in disparate geographic areas.

With this new CBRS auction, time sharing has been brought into the equation.   With time sharing, the same channels of the CBRS frequency can be used in the same geography, as long as they are not used at the same time.  The role of the newly created Spectrum Access System, SAS, is to coordinate the use of the new frequencies.  The SAS will also make sure that the each of the users has good and appropriate access.  The SAS also provides frequency and service protection for their respective users.

However, this presents a challenge to WISPs who must constantly monitor SINR (signal-to-interference-ratio (SNIR). This metric is used to give theoretical upper bounds on channel capacity (or the rate of information transfer) in wireless communication systems. But if the failover frequency given to you by the SAS causes sites miles away to suddenly have unacceptable, SINR how will you know?

ETI Software’s device management platform, Beamfly, can help. Beamfly gives you the ability to coordinate operations between known SAS info and real RF stats. Beamfly tracks SINR by cross referencing radio stats with EARFCN over time. No matter how the SAS moves you, Beamfly can help see where you are going.

CBRS Migration and monitoring is one of many examples where remote device management be a time and cost saver for WISPs.