n recent years, BS-assisted D2D emerged. The idea is to allow the BS to assist user and resource coordination, but the level of assistance is less than that in cellular networks. Nevertheless, to better take advantage of scarce spectrum, DUs can be supported in an underlaid mode, in which they can share the same spectrum with cellular users. In this way, careful interference coordination mechanism is required. 2.1.3 MU-MIMO and D2D in 5G In recent years, several research have explored the combination of MU-MIMO and D2D in the same system. Early work [100] studied a pair of DUs in the presence of cellular net- works, and a resource allocation problem was formulated to maximize network throughput. Later in [204], a more general scheme with multiple pairs of DUs underlying a MU-MIMO cellular network was investigated. With the opportunistic D2D feature, Karakus and Dig- gavi [93] proved that D2D and MU-MIMO cooperation can boost signal-to-noise ratio (SNR) performance, especially for users at network edge. As we mentioned in Chapter 1, NOMA can further improve spectral and power efficiency. A more interesting yet challenging question is how to jointly consider NOMA, D2D, and MU-MIMO. The key part of MU-MIMO is to design a suitable precoding matrix for trans- mitters based on various objective functions, such as overall system capacity or minimum power consumption. When jointly considering MU-MIMO, NOMA, and D2D, a tight coor- dination among these three mechanisms should be carefully designed SO that the overall system performance can be maximized. In [97], NOMA and MU-MIMO are jointly designed to improve the total system throughput. In this chapter, we propose a new mechanism that jointly coordinates beamforming-based MU-MIMO, NOMA, and D2D communications in a downlink cellular network. By supporting DUs in a NOMA/MU-MIMO cellular network, more complicated interference scenarios arise. To address that, we develop two different precoding schemes. One aims to cancel out the BS to DUs interference, while the other one a