This chapter investigates the application non-orthogonal multiple access (NOMA) in heterogeneous ultra-dense networks (HUDNs).Particularly, we propose a unified NOMA framework first. Then the applications of the proposed unified NOMA framework in HUDNs will be discussed. With the fact that small cells are densely deployed and the non-orthogonality of resource sharing, the system suffers severe interference. In this chapter, we identify the key challenges in the unified NOMA enabled HUDNs, especially for user association and resource allocation. In addition, we carry out the related case studies for the proposed unified NOMA enabled HUDNs including the user association based on matching theory and resource allocation based on optimization techniques. Furthermore, some critical insights will be provided for the design of NOMA enabled HUDNs, which can promote network access capacity in the next generation of communication systems.

In recent years, the overall network data traffic is dramatic increasing,a promising solution is the deployment of ultra dense networks (UDNs) combined with millimeter wave (mmWave) communication technology, which is expected to enhance the overall performance of the network in terms of energy efficiency and load balancing. In this chapter,user association and power allocation inmmWave-based UDNs is considered with attention to load balance constraints, energy harvesting by base stations, user quality of service requirements, energy efficiency, and cross-tier interference limits. This chapter not only establish the system utility optimal function model in the limitations of power and QoS, but also gives an iterative gradient user association and power allocation algorithm to resolve the optimization issue. This algorithm provides a best ratio of convergence and can get a near optimal scheme. In addtion, through utilizing Lagrangian dual decomposition, the dual optimization issue is disintegrated to two sub-problems, we can resolve them respectively. The simulation datum indicate that our method is effective.