A radio frequency band covering the range of 3550 - 3700 MHz in the United States.
3550 - 3700 MHz is a relatively high frequency range, meaning this band does not penetrate buildings and terrain well, and is unsuitable for long distances over land.
It operates in a unique three-tier sharing system. Tier 1 is for US Military coastal and Navy RADAR, as well as certain satellite communications. These users always have priority, but are rarely using all of the band in any given location. Therefore Tier 2 and 3 users can use the band when- and where-ever it's not being used by Tier 1 users. A central database controls, in real time, who can use which part of the band, when, and where. Companies can pay the FCC for licenses to operate in Tier 2, which simply means they have priority over Tier 3 (anyone without a license.)
Tier 3 operates as unlicensed spectrum, much like Wi-Fi. Tier 2 provides more reliable service.
Mobile companies can deploy 4G LTE technology in band 48, using it augment their existing LTE networks. They can do this using both indoor and outdoor antennas, which have different specific rules for this band.
There are several different ways LTE can be deployed in band 48.
Companies can deploy relatively standard LTE in this band with Tier 2 licenses. It is an unpaired band, meaning there are not separate frequencies for transmitting versus receiving. Therefore, in order to deploy standard LTE it must be the TDD variety of LTE.
Alternately, companies can use asymmetric Carrier Aggregation together with other LTE bands. In this configuration, another band is the main band, and band 48 is used to add capacity to the downlink (download) direction only.
See: Carrier Aggregation
Where entities are operating band 48 networks under Tier 3 (unlicensed) rules, they can used LTE-U, which lets LTE technology operate in unlicensed radio spectrum much like Wi-Fi.