LTE on License-Exempt Spectrum

The radio frequency spectrum is classified as licensed and license-exempt/unlicensed spectrum. A traditional cellular communications system (e.g., LTE) operates unexceptionally on licensed spectrum. This paper explains the concept of cellular communications on both licensed and license-exempt carriers under a unified cellular architecture. It addresses the key challenges and describes a baseline communication framework that enables such operation, including carrier sensing based on listen-before-talk, discontinuous transmissions with limited channel occupation time, synchronization between licensed and license-exempt carriers, and coexistence with other incumbent systems like WiFi. These concept and techniques are further exemplified by a practical system, the LTE license-assisted access, including the downlink featured in Release 13 LTE and the most recent addition of uplink in Release 14 LTE.