  Ultra dense cloud small cell network (UDCSNet), which combines cloud
computing and massive deployment of small cells, is a promising technology for
the fifth-generation (5G) LTE-U mobile communications because it can
accommodate the anticipated explosive growth of mobile users' data traffic. As
a result, fronthauling becomes a challenging problem in 5G LTE-U UDCSNet. In
this article, we present an overview of the challenges and requirements of the
fronthaul technology in 5G \mbox{LTE-U} UDCSNets. We survey the advantages and
challenges for various candidate fronthaul technologies such as optical fiber,
millimeter-wave based unlicensed spectrum, Wi-Fi based unlicensed spectrum, sub
6GHz based licensed spectrum, and free-space optical based unlicensed spectrum.
