MIMO technology for wireless communication, in which multiple spatially distributed users with one or more antennas can be transmitted at the same time and frequency by the base station with multiple antennas, at the cost of compli- cated signal processing and large overhead. Nevertheless, MU-MIMO can greatly improve the system capacity by exploiting the spatial diversity gain among multiple users. The major benefits are improved system throughput (sum rate of users), power, and spectral efficiency. It is expected that massive MIMO will be an important component in 5G cellular networks. 2.1.2 D2D Communication D2D communication is proposed as another 5G enabler. Compared with traditional BS-centric communication, D2D allows users to initiate and communicate directly, with little or no BS intervention. Intuitively, D2D can reduce signal overhead from/to the BS, lower their transmission power if the receiver is in close proximity, and reduce communication latency. Due to its advantages on power and spectral efficiency, as well as the latency improvement, D2D seeks for larger roles in 5G. However, D2D scheme faces many practical challenges. One is the spectrum coordination. In BS-centric networks, spectrum is allocated by BS, and each user in the system is also synchronized such that they can only use spectrum at their designated slot. D2D is more autonomous and lacks central coordination; hence, spectrum sharing among different users can be problematic. More importantly, D2D will co-exist with cellular users. To ensure cellular users (CUs) per- formance, usually D2D users (DUs) access spectrum in an opportunistic approach, similar to cognitive networks. The other challenge is QoS. It is difficult to achieve guaranteed QoS, 5G and Beyond Wireless Communication Networks, First Edition. Haijian Sun, Rose Qingyang Hu, and Yi Qian. © 2024 John Wiley & Sons Ltd. Published 2024 by John Wiley & Sons Ltd. 2 5G Wireless Networks with Underlaid D2D Communications primarily due to the lack of centralized resource management. I