Abstract
LTE Unlicensed extends LTE to unlicensed spectrum by aggregating unlicensed carriers with licensed ones through carrier aggregation or solely operating in unlicensed bands. It can create a unified network, increase network capacity, provide ubiquitous mobility, enhance user experience, and lower operational costs. However, to harvest these potential benefits, many technical challenges have to be addressed for LTE Unlicensed, including harmonious coexistence between LTE Unlicensed and other incumbent unlicensed systems, quality-of-service guarantee in unreliable unlicensed spectrum, as well as maintenance of ecosystem balance in unlicensed spectrum. In this article, we provide an overview of the current studies in LTE Unlicensed. In particular, we introduce regulation requirements and current standardization progress. We further outline three main challenges, describe possible deployment scenarios, and summarize technical solutions. The impact of LTE Unlicensed on the incumbent unlicensed systems is also examined by leveraging stochastic geometry, and finally, some potential research topics are identified.