  Due to the exponentially increased demands of mobile data traffic, e.g., a
1000-fold increase in traffic demand from 4G to 5G, network densification is
considered as a key mechanism in the evolution of cellular networks, and
ultra-dense heterogeneous network (UDHN) is a promising technique to meet the
requirements of explosive data traffic in 5G networks. In the UDHN, base
station is brought closer and closer to users through densely deploying small
cells, which would result in extremely high spectral efficiency and energy
efficiency. In this article, we first present a potential network architecture
for the UDHN, and then propose a generalized orthogonal/non-orthogonal random
access scheme to improve the network efficiency while reducing the signaling
overhead. Simulation results demonstrate the effectiveness of the proposed
scheme. Finally, we present some of the key challenges of the UDHN.
