## 7 Conclusions  
In this paper, the use of relay nodes in co-operative 5G networks has been studied. For this purpose, a decode-and-forward (D&F) relay node has been designed and implemented according to the latest 3 GPP standard for co-operative 5G networks. The developed hardware platform is based on the use of software defined radio (SDR) and the MatlabTM tool, which presents great flexibility and reconfigurability for the implementation of potential 5G applications. Furthermore, the hardware developed for the implementation of the D&F relay node is deeply described by means of a flowchart following the algorithms implemented in MatlabTM software. A study of 5G NR downlink, in SISO and 2 \(\times\) 2 MIMO configurations, has been carried out with signals complying with 3 GPP standardized requirements. To evaluate the performance of the proposed architecture and its benefits, a set of measurements in a real outdoor-to-indoor environment have been carried out. The obtained results clearly show that the D&F protocol co-operative network for 5G mobile wireless communications outperforms the non-co-operative architecture. The key performance indicators (KPIs), EVM, BER, and throughput obtained for the co-operative network are significantly better compared to those obtained for the direct link (gnodeB-to-UE). In this context, the proposed SISO D&F protocol co-operative network represents a substantial improvement, in terms of KPIs over 2 \(\times\) 2 MIMO non-co-operative network. In addition, MIMO technology was able to reduce the bit error rate and increase the average capacity at the D&F relay node for 64-QAM and 256-QAM modulation schemes, demonstrating higher throughput and spectral efficiency of the co-operative network. For example, the maximum throughput of the proposed non-co-operative 5G network was 6.12 Mbps for the 256-QAM modulation scheme and MIMO antenna technique. Nevertheless, with D&F protocol, the obtained results from \(\text{P}_{\text{BS}}=9\) dBm are higher than the direct link for the same configuration. On the other hand, we believe that the designed platform provides a cost-effective, scalable, and easy-to-upgrade solution to enable other 3 GPP standardized 5G solutions and the application to wireless communications in millimeter bands, which is the future work to be addressed. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{4}{c}{**Direct Link**} & \multicolumn{4}{c}{**D\&F Protocol**} \\ \cline{2-9}
**KPIs** & \multicolumn{2}{c}{**SISO**} & \multicolumn{2}{c}{**2 \(\times\) 2 MIMO**} & \multicolumn{2}{c}{**SISO**} & \multicolumn{2}{c}{**2 \(\times\) 2 MIMO**} \\ \cline{2-9}  & **64-QAM** & **256-QAM** & **64-QAM** & **256-QAM** & **64-QAM** & **256-QAM** & **64-QAM** & **256-QAM** \\ \hline BER & \(1.5\times 10^{-1}\) & \(1.9\times 10^{-1}\) & \(9.6\times 10^{-2}\) & \(1.4\times 10^{-1}\) & \(2.6\times 10^{-2}\) & \(5.2\times 10^{-2}\) & \(1.2\times 10^{-3}\) & \(1.4\times 10^{-3}\) \\ Throughput & 3.27 & 1.87 & 4.77 & 6.12 & 8.95 & 10.47 & 11.06 & 15.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of results: With D&F relay node vs. direct link for \(\text{P}_{\text{BS}}=15\) dBm. **Author Contributions:** [PERSON] conceived the paper, designed the experiments, performed experiments, analyzed the data, and wrote the paper; [PERSON] and [PERSON] reviewed and edited the paper. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was supported by the Spanish Ministry of Science, Innovation and Universities within the projects TEC2017-87061-C3-1-R (CIENCIA/AEI/FEDER, UE) and PID2020-113979 RB-C21 funded by MCIN/AEI/ 10.13039/501100011033. The work of [PERSON] is supported by a pre-doctoral Contract PRE2018-085032 from the Ministry of Science and Innovation. **Institutional Review Board Statement:** Not Applicable. **Informed Consent Statement:** Not Applicable. **Data Availability Statement:** Data sharing not applicable. **Conflicts of Interest:** The authors declare no conflicts of interest.