The growing popularity of mobile Internet and massive MTC with special traffic characteristics and locations have imposed huge challenges to current cellular networks. Deploying new base stations, however, becomes difficult and expensive, especially for complicated urban scenarios and MTC traffic. The UAV-assisted heterogeneous cellular solution is proposed in this article. It utilizes UAV-based floating relay (FR) to deploy FR cells inside the macrocell, and thus achieves dynamic and adaptive coverage. Comprehensive analyses on FR cells' deployment including frequency reuse, interference, backhaul resource allocation, and coverage are given.