## 3 Non-Orthogonal Multiple Access  
By exploiting different power levels, NOMA aims to serve multiple users using the same time and frequency [28; 29; 30; 31; 32; 33], leading to an improved spectral efficiency when compared with OFDMA [29; 30; 31; 32]. Since MIMO systems are a key feature of 5G, the inclusion of NOMA always has to be incorporated with MIMO [33], or more precisely with m-MIMO. NOMA is an effective mechanism to accommodate a higher number of users without a spectrum increase. This leads to a higher channel capacity, especially useful in scenarios with extremely high numbers of mobile terminals, such as in 5G use cases with mMTC or URLLC [29; 31; 32]. Due to the near-far problem and power control, the transmission power of different users suffers variations. By employing successive interference cancellation (SIC) and detecting users' signals in descending order, NOMA can detect different users that share the same time and frequency. Two different types of NOMA exist: conventional NOMA and cooperative NOMA. As can be viewed in Figure 5 with conventional NOMA, the SIC of a reference user only detects, regenerates, and cancels users' signals with powers higher than the reference user. Therefore, those user signals closer to the base station (with less power, due to power control) are not cancelled, representing interference, and degrading the performance (see User 1 in Figure 5). On the other hand, cooperative NOMA allows the cancellation of all interfering users' signals and provides diversity. Cooperative NOMA considers that users closer to the base station, that have previously detected and subtracted more powerful users (farther from the base station), send over the air copies of signals of said more powerful users. Let us focus on the example depicted in Figure 5. User 2, the user closer to the base station (strong user, due to stronger channel conditions), using SIC, detects User 1 first (a weak user, i.e., a more powerful user) and subtracts this signal from the received signal before detecting its own signal (User 2's signal). Assuming cooperative NOMA, User 2 sends over the air the previously detected signal of User 1. While with conventional NOMA User 1's signal detection would be corrupted by User 2's signal (less powerful and therefore not detected by SIC), with cooperative NOMA, User 1 also receives the copy of its signal sent by User 2, and then employs an efficient algorithm to combine the two versions of User 1's signal. Note that the version of User 1's signal detected by User 2 should be subject to the cancellation of User 2's signal before it is transmitted. This is an action that allows such a copy of User 1's signal to be clean of interference. Therefore, cooperative NOMA brings special added value for users that are farther from the base station) i.e., those with higher powers). Figure 5: Illustration of a non-orthogonal multiple access (NOMA) scheme, considering a two-user scenario. With the aim of explaining the NOMA concept, rather than performing a detailed scientific study, Figure 6 shows the performance results for conventional NOMA (designated in the figure as NOMA) and cooperative NOMA (designated in the figure as COOP NOMA) with \(4\times 32\) MIMO, considering the zero forcing receiver, in the downlink direction. Similar to Figure 5, two NOMA users were considered in the simulation with received powers of (1, 0.5), where the first value (1 in this case) corresponded to the power of the reference user and the other value was the power of the interfering user (0.5 in this case, i.e., 3 dB below). Considering the downlink transmission, the base station sent the two user signals superimposed, but with different powers. With NOMA, users with poorer channel conditions are allocated more power. Therefore, due to the near-far problem, a user closer to the base station tends to receive signals with less power (0.5 in this case) than those farther from it (1 in this case).