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Due to the exponentially increased demands of mobile data traffic, for example, 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 UDHN is a promising technique to meet the requirements of explosive data traffic in 5G networks. In the UDHN, the base station is brought closer and closer to users through densely deploying small cells, which results 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.