## 4 Performance Results
On the other hand, even with the noise enhancement typical of the ZF receiver [17], it performs better than that of the MRC. Cooperative NOMA comprises the cancellation of the interfering signals associated with all users and exploits diversity. Cooperative NOMA considers that the lower power users retransmit the symbols detected by the SIC of higher power users (typically using decode and forward, in Time Division Multiplexing). These additional signals are utilized by higher power users to exploit diversity, as the same signals are also received directly  
Figure 6: Results for 2 NOMA users with powers [1 0.5], with 4 \(\times\) 32 MIMO. from the base station (assuming downlink) and a copy of them (clean of interferences) is relayed by the lower power users. These signals are combined to improve performance. Figure 6 shows that such combination of signals performed with the Cooperative NOMA results in a good performance improvement, when compared with conventional NOMA, for both MRC and ZF, whose performances are close to that of the Match Filter Bound (MFB). Note that the MFB curve is a way to measure the channel modelled by the sum of delayed and independently Rayleigh-fading rays, which can be viewed as a lower bound. Figure 6 also evidences that, with Cooperative NOMA, the MRC performs better than the ZF. This derives from the fact that Cooperative NOMA allows the cancellation of all NOMA interference (not only of users with higher receive power levels) and, additionally, allows exploiting diversity. This lower level of interference allows the iterative MRC receiver to improve the symbol estimates in different iterations. On the other hand, the performance of the ZF is limited, as this receiver presents noise enhancement. Furthermore, the level of complexity of the ZF is substantially higher than that of the MRC, as it requires the computation of the pseudo-inverse of the channel matrix for each frequency component. Figure 7 shows the BER performance in the same scenario as that of Figure 6 with the difference that the power of users is \([0.5\,1]\). In this scenario, the reference user tends to be closer to the base station (power 0.5), while the interfering user tends to be further from the base station (power 1). As previously mentioned, we refer to the distance from the base station only for explanation purposes, as the fading or the power control can also make the received power suffer variations. Noteworthy is that the conventional NOMA receiver comprises the detection, regeneration and cancellation of the users' signals by their descending order of powers (up to the power of the reference user), before the detection of the reference user takes place. Consequently, the detection of the reference user tends to be clean of interferences and the performance achieved with conventional NOMA is already good. In this scenario, since the reference user presents a power level (e.g., 0.5) which is 3 dB lower than the interfering user (e.g., 1), such signal detection is not carried out in the SIC receiver of the interfering user and Cooperative NOMA is not implemented, as it does not bring any added value. Moreover, note due to its lower power, the SIC of the reference user, using the mode of conventional NOMA, is now able to cancel the interfering signal, as the power of the interfering user is higher, making the effectiveness of cooperative NOMA useless. Figure 7: Results for 2 NOMA users with powers \([0.5\,1]\), with \(4\times 32\) MIMO. Figure 8 shows the performance results for conventional NOMA against those of Cooperative NOMA, for the scenario with four users sharing the spectrum with NOMA, with powers [1 0.5 2 4] (1 stands for the power of the reference user, while 0.5, 2 and 4 stands for the power of the interfering users). The MIMO with 4 transmitting antennas and 32 receiving antennas is considered.