evices, such as phones, tablets, and home appliances, is the main driving force for the ever-increasing mobile traffic today. It is not surprising that mobile traffic increased 10-fold between 2014 and 2019 globally. The mobile data traffic is expected to grow much faster than fixed IP traffic in the upcoming years [34]. Wireless technologies dramatically changed the way people interact, communicate, and collaborate, especially at post-Covid era. The need for faster, more efficient and secure, and intelligent communication tech- nique remains strong. While the current wireless communication systems such as 4G long term evolution (LTE) have been pushed to their theoretic capacity limit, different air inter- face and radio access technologies including heterogeneous network (HetNet) [76, 77], multiuser multi-input multi-output (MU-MIMO) [105], and device-to-device (D2D) com- munication [51] have become potential paradigms to fulfill the gap between demands from end users and the capacity that current air interface can provide. 5G and Beyond System Requirements In their pioneering work [10], Andrews et al. evaluated the requirements for 5G. In short, 5G wireless communication system should provide 1,000 times aggregate data improve- ment over 4G, support for as low as 1 ms round-trip latencies, 10 times longer battery life for low-power devices, and also support 10,000 times or more low-rate devices in single macro cell, see Figure 1.1 for a brief illustration. Due to those high requirements, the transformation from 4G to 5G cannot be simply fulfilled by extensions of current technologies. In general, 5G and beyond system should support or deliver the following aspects. Notably, (i) more bandwidth. Currently commercial cellular systems use frequen- cies below 6 GHz (sub-6 GHz); in fact, there is abundant bandwidth in the millimeter-wave (mmWave) band, for example in 28 GHz and above, which can provide more bandwidth that previously have not been applied in cellular networks. (ii) More antennas. Higher frequency also