## 4 Numerical Results and Discussion  
In the section, the simulation results of the two-hop mmWave MIMO downlink RN architecture are presented. To assess the key performance indicators of the considered design, we present the results of Monte-Carlo computer simulations, aimed at BER and throughput versus Signal-to-Noise Ratio (SNR) of the proposed two-hop Relay cooperative system. We consider a network encompassing one gNodeB equipped with \(\rm N_{T}^{S}=\{1,2,4\}\) transmitter antennas, and transmitting 64-QAM and 256-QAM symbols. The RN (A&F or D&F) is equipped with \(\rm N_{R}^{R}=N_{T}^{R}=\{1,2,4\}\) receiver and transmitter antennas. Besides, the NR-UE is equipped of \(\rm N_{R}^{R}=\{1,2\}\) receiver antennas. Note that, when performing MIMO simulations, you will have as many transmitter and receiver chains as the number of antennas used in the simulation. The KPIs are evaluated by carrying out \(10^{3}\) independent Monte-Carlo trials, which in each run is used independent sets of channel realizations and noise. For each SNR transmission, \(10^{3}\) frames were sent, where the Relay processes the  
Figure 8: Spectrum of the input and output of the mmWave RF transmitter chain. Figure 9: Spectrum of the input and output of the mmWave RF receiver chain. signal and forwards it. In Table 2, the simulation parameters are summarized, and the correspondent distances to calculate of the path loss in the scenario are shown in Table 3.  
Two types of NR-RNs are considered: A&F and D&F. In the case of the NR D&F RN two situations are developed: (a) an exact knowledge of the channel is available, and (b) an estimation of it is made. Furthermore, we have denoted \(\rm{N_{T}^{S}\times N_{R}^{R}=N_{T}^{R}\times N_{R}^{P}}\), like the considered MIMO scheme in our mmWave cooperative system. Figure 10 shows the corresponding BER performance comparison of the mmWave MIMO cooperative network of the implemented relaying strategies, considering the 64-QAM constellation. It may be seen that A&F protocol with SISO achieved a performance discrete. Besides, the D&F strategy with the knowledge of the channel (PCE) and SISO reached the best performance among the implemented NR-RN with SISO technology. However, the mmWave NR-D&F protocol with LS channel estimation presented a close BER in comparison when was considered the knowledge of the channel. From Figure 10, it can be verified that the D&F SISO with PCE reached the BER level of 1.8 \(\times\) 10\({}^{-2}\) at around \(\rm{SNR}=20\) dB. In this context, the MIMO technique was introduced in the cooperative system, which specifically (2 \(\times\) 2) or (4 \(\times\) 4) MIMO and (2 \(\times\) 2) or (4 \(\times\) 2) MIMO in the Backhaul Link and Access Link, respectively, was considered. With the MIMO schemes, it can be seen that the cooperative system considerably increased the BER performance, in comparison with SISO technique. In this regard, we note that the (2 \(\times\) 2 \(\times\) 2) NR-A&F protocol achieves the same BER level of the D&F SISO with PCE at around \(\rm{SNR}=14\) dB, so that a performance gain of 6 dB is obtained. Furthermore, when (4 \(\times\) 4 \(\times\) 2) NR-A&F strategy is taken into account, a BER level of 1.1 \(\times\) 10\({}^{-3}\) is reached, which supposes an enhancement of 7 dB in comparison with (2 \(\times\) 2 \(\times\) 2) NR-A&F. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameters** & **Backhaul Link** & **Access Link** \\ \hline Signal Bandwidth & 25 MHz & 25 MHz \\ Carrier Frequency & 3.5 GHz & 26.1 GHz \\ Tx/Rx schemes & SISO, 2 \(\times\) 2 and 4 \(\times\) 4 & SISO, 2 \(\times\) 2 and 4 \(\times\) 2 \\ SSB Configuration & Case B & Case B \\ transmitted SSB & 4 blocks & 4 blocks \\ Subcarrier Spacing & 30 kHz & 30 kHz \\ SSB periodicity & 20 ms & 20 ms \\ Modulations & 64-QAM and 256-QAM & 64-QAM and 256-QAM \\ Channel Models & CDL-D & CDL-D \\ Light-Vision & LOS & LOS \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Main parameters of simulation.