According to www.faa.gov/uas -
"Unmanned aircraft systems (UAS) are inherently different from manned airspace. Introducing UAS into the nations airspace is challenging for both the Federal Aviation Administration (FAA) and aviation community, because the U.S. has the busiest most complex airspace in the world. The FAA is taking an incremental approach to safe UAS integration."
Please use the following links as references below.
Different Types of UAS Operations