Our airwaves are congested. We have all manner of radio transmitters broadcasting into a huge number of different frequencies and a great many technologies designed to efficiently use this spectrum. Currently, different technologies are utilizing different frequencies. In the case of the carriers, they are allocated various chunks of spectrum with which to run their services. There are certain limits and restrictions, but the lower the frequency, the better the building penetration and therefore the effective range of the mast, but the slower the data transfer rate. Thus, low frequency spectrum is great at providing something of a backbone, blanket coverage with busier areas utilizing high frequency masts. We must also consider that not all frequency bands are exclusively reserved for the cellular data networks.

The Federal Communications Commission, or FCC, has the unenviable task of keeping all parties happy when it comes to dividing up spectrum across the competing industries and agencies. This leads to today's topic of discussion, the unlicensed 3.5 GHz band, which is currently being considered for use by three users: Incumbent Access, Priority Access Licensees and General Authorized Access users. The Incumbent Access primarily consists of Department of Defense radar systems and fixed-satellite service earth stations. Incumbent users are protected from interference from Priority Access and General Authorized Access users, which consist of a variety of different users. The core of the rules system is that these users may license a 10 MHz block of spectrum but must effectively give way should an Incumbent user wish to use the spectrum. At the recent International Symposium on Advanced Radio Technologies (ISART 2015), Google discussed some of the work that it has been doing with the 3.5 GHz frequency around the Spectrum Access System, or SAS. Google have pushed their Spectrum Access System to the third generation and it is now capable of dynamically managing the relationships between the 3.5 GHz frequency users. Google's idea is that SAS can be used to build an ecosystem with their software at the heart of it. This should keep the airwaves free for the military but also keeps other users' services running cleanly and quickly.


The reason why Google is interested in the 3.5 GHz spectrum is simple: it's because this unlicensed spectrum may be used for Internet of Things devices. Many Internet of Things, or IoT, devices will need low latency, high-performance wireless networks but typically over a very short range, so the high frequency 3.5 GHz spectrum is ideal for either 802.11 WiFi or LTE purposes. Google's current SAS can handle up to ten million devices under management, although it estimates that only one to five million devices will end up using the service. Google's system appears to have considerable headroom, but is currently still undergoing testing. The project is currently collecting data from sensors in Virginia Beach and is expanding to San Diego, San Francisco and the Florida Keys areas in the coming months. The plan is to be collecting data from six sites in the coming months.

Although Google's Spectrum Access Service is a technically interesting project, the ramifications of this run deeper than just the 3.5 GHz spectrum. President Obama has announced plans to release up to 500 MHz of spectrum for commercial mobile technologies and a presidential technical advisory committee concluded that the traditional way of clearing spectrum used by Government agencies and auctioning it off for private sector use was too costly and disruptive, but that the way forward is in sharing the existing spectrum. And this requires technology to work smarter, which is one of Google's fortes.