 if we combine results from both Figures 2.2 and 2.3, we can see that the overall throughput is higher with ZF2 since DUs are configured with a very good channel setting SO that they contribute to overall throughput significantly. 2.6 Summary In this chapter, we study the performance of a cellular network that supports NOMA, MU-MIMO, and D2D communications. Specifically, we use NOMA and MU-MIMO for the cellular downlink users to improve overall system spectrum efficiency. D2D users are further supported in the underlay mode to exploit the frequency reuse again. Two different precoding mechanisms are defined. We formulate an optimization problem 2 5G Wireless Networks with Underlaid D2D Communications CUs with NOMA, ZF1 CUs with Naive TDMA, ZF1 CUs with NOMA, ZF2 CUs with Naive TDMA, ZF2 Number of CUs Figure 2.3 CUs capacity of two proposed ZF precoding methods vs. TDMA as the number of user grows (RO = 0.5 bit/s/Hz). aiming to maximize the system performance and develop a suboptimal approach to solve the problem in two steps. Simulation results show NOMA and MU-MIMO altogether will improve the overall cellular user throughput significantly. When underlay D2D users are added, two different precoding schemes lead to different performance, with one favoring CUEs and one favoring DUEs. But both lead to a net system gain. 5G NOMA-Enabled Wireless Networks 3.1 Background In Chapter 2, we incorporated NOMA with MU-MIMO in a D2D underlaid system. The advantages of applying NOMA in such a scheme are in two folds: (i) it can support more users simultaneously; (ii) overall system performance in terms of total throughput is also improved. Intuitively, NOMA will create a win-win situation for a pair of users with strong and weak channel condition. The reason is that the stronger user is typically bandwidth-limited, while the weaker user is interference-limited In NOMA, signals for both users are set to transmit simultaneously, SO the bandwidth-limited user can get more spectrum resources, while the interference-limited user c