## 4 Performance Results  
This section studies the performance results, namely the Bit Error Rate (BER) obtained with NOMA, associated to m-MIMO. Two types of NOMA are studied: conventional NOMA and Cooperative NOMA. We assume ideal channel estimation and the block transmission technique SC-FDE. The BER is evaluated as a function of \(E_{b}/N_{0}\), where \(E_{b}\) is the energy of the received bits and \(N_{0}\) is the one-sided power spectral density of the noise. This performance was evaluated using Monte Carlo simulations, with QPSK modulation and with a block length of \(N=256\) symbols (similar results were observed for other values of \(N\), provided that \(N>>1\)). A Rayleigh fading channel was considered with 16 uncorrelated equal power paths. The duration of the useful part of the blocks (N symbols) is \(1\upmu\)s and the cyclic prefix has a duration of \(0.125\upmu\)s. In our paper we considered a carrier frequency of 5 GHz, but the adopted signal processing schemes could be successfully employed regardless of the adopted carrier frequency. Four iterations of the MRC receiver were assumed to detect the MIMO signals using SC-FDE transmission technique (detection and interference cancellation) [17]. Beyond four iterations, the performance improvement was almost negligible. MIMO using spatial multiplexing (multi-layer transmission) is adopted. In this sense, results with \(T\)x\(R\) (\(T\) transmitting antennas and \(R\) receiving antennas) mean that there are \(T\) parallel flows of symbols (the symbols rate increases \(T\) times). As in the case of the MIMO multi-layer transmission, \(R\) needs to be equal to or higher than \(T\), for the detection to be possible. It is worth noting that the SIC, as part of the NOMA receiver, focuses on received power levels, which depend on several factor, such as the near-far problem, fading and power control. Consequently, the reference to distances is only used for description purposes of different scenarios, but the same applies to the other factors, such as fading and power control, that influences the variation of the received power levels. Figure 6 shows the performance results for conventional NOMA (designated in figures as \"NOMA\") and Cooperative NOMA (designated in figures as \"COOP NOMA\"), with 4 \(\times\) 32 MIMO, considering two receivers: ZF and MRC. Two NOMA users were considered in the simulation, with receive power levels [1 0.5], where the first value in the vector [1 0.5] (1, in this case) corresponds always to the power of the reference user, being the other value the power of the interfering user (0.5, in this case). In this scenario, the power of the interfering user is 3 dB lower than that of the reference user. Weak users are those that tends to be further from the base station (can also be due to e.g., fading) and, therefore, the propagation losses are higher. With NOMA, this is mitigated by employing higher transmit power. Therefore, it is assumed that the interfering user with transmit power 0.5 tends to be closer to the base station than the reference user, whose transmit power is 1. As can be seen, the results of Figure 6 obtained with conventional NOMA are quite limited, due to the existence of residual interference. Noteworthy is that the SIC that is part of the receiver only detects, regenerates and cancels users' signals with powers higher than those of the reference user, which is not the case here (the interfering user has power 0.5, which is not cancelled). This explains the low performance achieved with conventional NOMA, for both ZF and MRC. It is also viewed that, with conventional NOMA, the ZF performs better than the MRC. However, the MRC consists of an iterative receiver that estimates the transmitted symbols and aims to improve such estimate in each iteration. With high level of interference associated with the non-cancelled interfering NOMA user, the symbol estimates performed in each iteration of the MRC receiver is poor and therefore, it is not able to perform well.