multi-user diversity effect. The distance with each DU pair is fixed at 30 m. The wireless channel consists of pathloss, shadowing, and Rayleigh fading with a pathloss exponent 2. PMBS and PD are set to 30 Watt and 1 Watt, respectively. For comparison purpose, instead of using NOMA in each beam, we apply a traditional TDMA scheme here to support these 2 users in each beam. Specifically, we allocate an equal number of time slots to 2 TDMA users. The scheme is also referred as "Naive TDMA." (2.34) Figure 2.2 presents the system capacity of two proposed ZF precoding methods as the num- ber of users grows, and the results are scaled over the highest achievable rate. Here we 2.6 Summary NOMA, ZF1 Naive TDMA, ZF1 NOMA, ZF2 Naive TDMA, ZF2 Number of CUs Figure 2.2 System capacity of two proposed ZF precoding methods VS. TDMA as the number of user grows (R0 = 0.5 bit/s/Hz). set R = 0.5 bit/s/Hz. We can see that NOMA outperforms naive TDMA in both precod- ing schemes when the number of CUs is large. However, when the number is small, lim- ited number of CUs can be chosen to perform NOMA; thus, the performance gain is not obvious, even worse than TDMA. We also find that using ZF2 leads to a higher overall system throughput than ZF1. Because with ZF2, DUs experience a much lower interfer- ence than with ZF1 SO that the throughput elevation from DUs exceeds the throughput degradation from CUs due to inter-beam interference, which results a net gain on overall system throughput. Moreover, as the user number increases, the system benefits more from NOMA+MU-MIMO due to a higher multiuser diversity gain. DUs normally are considered as a complementary communication method. So we are particularly interested in the performance of CUs. In Figure 2.3, the throughput of CUs is calculated. NOMA shows a superior spectral efficiency compared with naive TDMA. In this case, ZF1 has a much better performance than ZF2 since ZF1 precoding eliminates inter-beam interference for CUs, while ZF2 aims to eliminate interference from CUs to DUs. But